Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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34118 Publications

Practice patterns of image guided particle therapy in Europe: a 2016 survey of the European Particle Therapy Network (EPTN)

Bolsi, A.; Peroni, M.; Amelio, D.; Dasu, A.; Stock, M.; Toma-Dasu, I.; Witt Nyström, P.; Hoffmann, A. L.

Background and Purpose: Image guidance is critical in achieving accurate and precise radiation delivery in particle therapy, even more than in photon therapy. However, equipment, quality assurance procedures and clinical workflows for image-guided particle therapy (IGPT) may vary substantially between centres due to a lack of standardization. A survey was conducted to evaluate the current practice of IGPT in European particle therapy centres.

Material and Methods: In 2016, a questionnaire was distributed among 19 particle therapy centres in 12 European countries. The questionnaire consisted of 30 open and 37 closed questions related to image guidance in the general clinical workflow, for moving targets, current research activities and future perspectives of IGPT.

Results: All centres completed the questionnaire. The IGPT methods used by the 10 treating centres varied substantially. The 9 non-treating centres were in the process to introduce IGPT. Most centres have developed their own IGPT strategies, being tightly connected to their specific technical implementation and dose delivery methods.

Conclusions: Insight into the current clinical practice of IGPT in European particle therapy centres was obtained. A variety in IGPT practices and procedures was confirmed, which underlines the need for harmonisation of practice parameters and consensus guidelines.

Keywords: Particle therapy; image guidance; survey

Permalink: https://www.hzdr.de/publications/Publ-26912
Publ.-Id: 26912


Realizing the insulator-to-metal transition in Se-hyperdoped Si via non-equilibrium material processing

Liu, F.; Prucnal, S.; Berencén, Y.; Zhang, Z.; Yuan, Y.; Liu, Y.; Heller, R.; Böttger, R.; Rebohle, L.; Skorupa, W.; Helm, M.; Zhou, S.

We report on the insulator-to-metal transition in Se-hyperdoped Si layers driven by manipulating the Se concentration via non-equilibrium material processing, i.e. ion implantation followed by millisecond-flash lamp annealing. Electrical transport measurements reveal an increase of the carrier concentration and conductivity with the increasing Se concentration. For the semi-insulating sample with Se concentrations below the Mott limit, quantitative analysis of the temperature dependence of the conductivity indicates a variable-range hopping mechanism with an exponent of s  =  1/2 rather than 1/4, which implies a Coulomb gap at the Fermi level. The observed insulator-to-metal transition is attributed to the formation of an intermediate band in the Se-hyperdoped Si layers.

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Permalink: https://www.hzdr.de/publications/Publ-26911
Publ.-Id: 26911


PT-symmetry and related geometrical structures

Günther, U.

In non-relativistic quantum mechanics, the dynamics of closed quantum systems is described by Hamiltonians which are self-adjoint in appropriately chosen Hilbert spaces. For PT-symmetric quantum systems, the Hamiltonians are in general no longer self-adjoint in standard Hilbert spaces, but rather they are self-adjoint in Krein spaces, Hilbert spaces endowed with indefinite metric structures. Moreover, the spectra of PT-symmetric Hamiltonians are symmetric with regard to the real axis in the spectral plane. Apart from Hamiltonians with purely real spectra this includes also Hamiltonians whose spectra may contain sectors of pairwise complex-conjugate eigenvalues. Considering families of parameter-dependent Hamiltonians one can arrange for parameter-induced passages from sectors of purely real spectra to sectors of complex-conjugate spectral branches. Corresponding passages can be regarded as PT-phase transitions from sectors of exact PT-symmetry to sectors of spontaneously broken PT-symmetry. Approaching a PT-phase transition point, the eigenvectors of the Hamiltonian tend toward their isotropic limit --- an, in general, infinite-dimensional (Krein-space) generalization of the light-cone limit in Minkowski space. At a phase transition, the Hamiltonian is no longer diagonalizable, but similar to an arrangement of nontrivial Jordan-blocks. The interplay of these structures is briefly reviewed with special emphasis on the related Lie-algebraic and Lie-group aspects. With the help of Cartan-decompositions associated hyperbolic structures and Lie-triple-systems are discussed for finite-dimensional setups as well as for their infinite-dimensional generalizations (Hilbert-Schmidt (HS) Lie groups, HS Lie algebras, HS Grassmannians). The interconnection of Krein-space structures and PT-phase transitions is demonstrated on two exactly solvable models: PT-symmetric Bose-Hubbard models and PT-symmetric plaquette arrangements.

Keywords: PT symmetry; PT phase transitions; Krein spaces; Jordan blocks; Lie algebras; Lie triple systems; Hilbert-Schmidt Lie groups; HS Lie algebras; HS Grassmannians; PT-symmetric Bose-Hubbard models; plaquette arrangements

  • Invited lecture (Conferences)
    Symmetry 2017 - The First International Conference on Symmetry, 16.-18.10.2017, Barcelona, Spain
  • Open Access Logo Abstract in refereed journal
    Proceedings / MDPI AG 2(2018), 25
    DOI: 10.3390/proceedings2010025

Permalink: https://www.hzdr.de/publications/Publ-26910
Publ.-Id: 26910


PT quantum mechanics: finite-dimensional and infinite-dimensional matrix models and their Lie group structures

Günther, U.

Structural features of PT-symmetric quantum mechanical matrix models are discussed: hidden group theoretical aspects, Lie triple systems following from Cartan decompositions of the corresponding Lie algebras, projectivization embeddings to resolve singularities at PT phase transitions. Starting from these structural findings for finite-dimensional PT-symmetric matrix setups, possible technically feasible extensions toward infinite-dimensional Hilbert-Schmidt Lie groups, Fredholm groups and PT-symmetry related Hilbert-Schmidt Grassmannians are sketched. Concrete physical setups where these features show up are briefly discussed.

Keywords: PT symmetry; phase transitions; Lie algebras; Lie triple systems; PT phase transitions; Hilbert-Schmidt Lie groups; Fredholm groups; Hilbert-Schmidt Grassmannians

  • Invited lecture (Conferences)
    15th Conference “Mathematics in Technical and Natural Sciences”, 17.-22.09.2017, Zakopane, Poland

Permalink: https://www.hzdr.de/publications/Publ-26909
Publ.-Id: 26909


ESUO activities: an update

Froideval, A.

The European Synchrotron and free-electron laser (FEL) User Organisation (ESUO) established in 2010, is now representing about 30.000 users of the European synchrotron and FEL radiation facilities. This user community is distributed over 30 European countries and is represented within the ESUO board by 1 up to 4 national delegate(s) per country, depending on the size of the user community in that country. In the present talk, the past, recent and future ESUO activities are presented.

Keywords: European synchrotron and FEL user organisation (ESUO); European projects; scientific collaborations; European synchrotron and FEL radiation facilities

  • Lecture (others)
    3rd Meeting of the European User Offices, 23.-24.10.2017, Lund, Sweden

Permalink: https://www.hzdr.de/publications/Publ-26908
Publ.-Id: 26908


Unusual Coulomb effects in graphene

Winnerl, S.

In many semiconductors Coulomb scattering plays an essential role in the thermalization process of a non-equilibrium carrier distribution. Here we discuss three surprising and fascinating manifestations of Coulomb scattering in graphene.
The first observation concerns a double-bended saturation behavior of bleaching induced by near-infrared radiation. The complete bleaching at high fluences is related to Pauli blocking. At much lower fluences, however, the balance between scattering into or out of the optically probed regions in k-space via Coulomb interaction results in a qualitatively similar behavior [1].
The second phenomenon is the optically induced anisotropy in k-space for excitation with linearly polarized radiation and its relaxation to a Fermi-Dirac distribution. Polarization resolved pump-probe experiments at different photon energies provide strikingly direct insights into role of individual processes: Carrier-phonon scattering rapidly transforms the initial anisotropic non-equilibrium distribution into an isotropic one [2]. When carrier-phonon scattering is quenched by applying photon energies below the optical phonon energy, however, Coulomb interaction is the only strong source of scattering [3]. As Coulomb scattering in graphene is predominantly collinear, the anisotropy persists for fairly long times (a few ps).
The third set of experiments tackles the dynamics of graphene in a magnetic field perpendicular to the graphene layer. In this case, the band structure beaks up into a series of non-equidistant Landau levels (LLs). We study in detail the population and polarization dynamics of the levels with index -1, 0 and 1. Applying circularly polarized radiation allows one to selectively excite the energetically degenerate transitions LL-1  LL0 and LL0  LL1, respectively. Applying all four combinations of pumping and probing with left and right circularly polarized radiation reveals a surprising behavior: The possibility to deplete the zeroth Landau level while it is optically pumped at the same time [4]. This is caused by strong Auger scattering, the Coulomb process that thermalizes the carrier distribution in Landau quantized graphene. It also causes a fast dephasing of the microscopic polarization, as evidenced in four-wave-mixing experiments [5]. We discuss the possibility to apply Landau quantized graphene as a gain medium in a tunable laser and as a tunable nonlinear optical material.
We are grateful to a number of people, most importantly, from the experimental side, to M. Mittendorff, J. C. König-Otto, H. Schneider and M. Helm. Furthermore to E. Malic, A. Knorr and A. Belyanin for microscopic theory, and to C. Berger and W. A. de Heer for sample growth.
[1] T. Winzer, M. Mittendorff, S. Winnerl, H. Mittenzwey, R. Jago, M. Helm, E. Malic, and A. Knorr, Nature Commun. 8, 15042 (2017).
[2] M. Mittendorff, T. Winzer, E. Malic, A. Knorr, C. Berger, W. A. de Heer, H. Schneider, Manfred Helm, and S. Winnerl, Nano Lett. 14, 1504 (2014).
[3] J. C. König-Otto, M. Mittendorff, T. Winzer, F. Kadi, E. Malic, A. Knorr, C. Berger, W. A. de Heer, A. Pashkin, H. Schneider, M. Helm, and S. Winnerl, Phys. Rev. Lett. 117, 087401 (2016).
[4] M. Mittendorff, F. Wendler, E. Malic, A. Knorr, M. Orlita, M. Potemski, C. Berger, W. A. de Heer, H. Schneider, M. Helm, and S. Winnerl, Nature Physics 11, 75 (2015).
[5] J. C. König-Otto, Yongrui Wang, Alexey Belyanin, C. Berger, W. A. de Heer, M. Orlita, A. Pashkin, H. Schneider, M. Helm, S. Winnerl, Nano Lett. 17, 2184 (2017).

Keywords: graphene; ultrafast dynamics; Coulomb scattering

  • Invited lecture (Conferences)
    Finite Systems in Nonequilibrium: From Quantum Quench to the Formation of Strong Correlations, 10.-30.09.2017, Natal, Brasil

Permalink: https://www.hzdr.de/publications/Publ-26907
Publ.-Id: 26907


Landau-quantized graphene as a nonlinear THz material

König-Otto, J. C.; Wang, Y.; Belyanin, A.; Pashkin, A.; Schneider, H.; Helm, M.; Winnerl, S.

Graphene, a gapless two-dimensional semiconductor, features constant optical absorption in a wide spectral range. In presence of a magnetic field, the linear band structure of graphene at low energies splits up into a series of non-equidistant Landau levels (LLs). Consequently, the optical absorption is redistributed into Landau-level resonances. Population inversion [1, 2] and strong optical nonlinearities [3] have been predicted for Landau-quantized graphene. Experimentally the population dynamics has been studied and direct evidence for strong Auger scattering in the time domain has been found [4]. In this presentation we show first experiments on the polarization dynamics and the scaling behavior of the four-wave mixing (FWM) signal.
The experiments were performed on almost intrinsic layers of epitaxial multilayer graphene grown on the C-terminated side of SiC. The sample was kept at 10 K in a split coil magnet with optical access. Using linearly polarized radiation at a frequency of 19 THz we investigated the LL-1 → LL0 and LL0 → LL1 transition, which were tuned into resonance by a magnetic field of 4.5 T. Employing radiation pulses with a duration of 4 ps from the free-electron laser FELBE, the degenerate FWM signals were recorded and compared to pump-probe signals. The FWM signal is essentially symmetric and reflects the pulse duration of radiation pulses. This indicates that the dephasing time of the microscopic polarization is faster than the pulse duration. The excited population, on the other hand, is present on much longer timescales. At low intensities, the FWM scales quadratically with the power of the incident beam, that delivers two photons for the FWM process. At incident fields above ~10 kV/cm saturation is observed. Furthermore, the magnetic field was tuned while keeping the photon energy fixed. This reveals a considerably smaller linewidth of the third-order susceptibility resonance as compared to the linewidth of the linear absorption measured by Fourier transform spectroscopy. This is consistent with the nonlinear scaling of the FWM signal. Our experimental results, in particular also the deduced value for the surface susceptibility of the order of 10-19 m3/V2, are in accord with theoretical predictions based on the density matrix formalism.
In summary, Landau-quantized graphene represents a strong nonlinear medium with a resonance tunable by the magnetic field. This may be interesting for nonlinear THz applications, such as frequency mixing and parametric generation.
We are grateful to C. Berger and W. A. de Heer from Georgia Tech and M. Orlita from LNCMI-CNRS in Grenoble for sample growth and linear magneto-spectroscopy measurements, respectively.

References
[1] F. Wendler and E. Malic, Sci. Rep. 5, 12646, 2015.
[2] Y. Wang, M. Tokman, and A. Belyanin Phys. Rev. Lett. 91, 033821 (2015).
[3] X. Yao and A. Belyanin, Phys. Rev. Lett. 108, 255503 (2012).
[4] M. Mittendorff, F. Wendler, E. Malic, E. Knorr, M. Orlita, M. Potemski, C. Berger, W. A. de Heer, H. Schneider, M. Helm, and S. Winnerl, Nature Phys. 11, 75 (2015).

Keywords: graphene; nonlinear optics; Landau quantization

  • Lecture (Conference)
    Optical Terahertz Science and Technology, 02.-07.04.2017, London, UK

Permalink: https://www.hzdr.de/publications/Publ-26906
Publ.-Id: 26906


Teilchenphysik in Bleistiftstaub: das Wundermaterial Graphen

Winnerl, S.

Kohlenstoff ist in Form von Graphit und Diamant den Menschen seit Jahrtausenden bekannt. Die erste Kohlenstoff Nanostruktur wurde in Form von Fullerenen („Fußballmoleküle“) in den 1980er Jahren entdeckt, es folgten Kohlenstoff-Nanoröhren in den 1990er Jahren. Im Jahr 2004 wurde schließlich Graphen, eine einzelne Schicht aus bienenwabenförmig angeordneten Kohlenstoffatomen, von Geim und Novoselov entdeckt und untersucht. Für diese Forschung wurden die beiden im Jahr 2010 mit dem Physik-Nobelpreis ausgezeichnet.
Graphen besitzt faszinierende grundlegende physikalische Eigenschaften. Insbesondere weisen Elektronen in Graphen eine Energie-Impuls Beziehung auf, wie man sie von Teilchen aus der Hochenergiephysik kennt. Wir werden diskutieren, wie sich dies auf weitere physikalische Eigenschaften auswirkt und einen kurzen Überblick geben, wie sich die mechanischen, optischen und elektrischen Eigenschaften von Graphen von anderen Materialien unterscheiden. Daraus lassen sich Schlüsse ziehen, für welche Anwendungen Graphen interessant ist.
Schließlich geben wir einen kurzen Einblick in unsere eigene Forschung an Graphen, die sich besonders mit der Dynamik von Graphen-Elektronen auf ultrakurzen Zeitskalen beschäftigt und dafür das Großgerät FELBE (Freie-Elektronen Laser am HZDR) verwendet.

  • Lecture (others)
    Seniorenakademie Dresden, 26.11.2017, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-26905
Publ.-Id: 26905


Magneto-structural correlations in Fe60Al40 thin films

Ehrler, J.; Potzger, K.; Grenzer, J.; Zhou, S.; Böttger, R.; Bali, R.

Magnetic materials become more significant for future data storage devices and spintronic applications. In certain alloy thin films like Fe60Al40, nano-sized ferromagnetic structures can be created by means of focused ion irradiation.[1,2,3] Fe60Al40 shows a disorder induced phase transition from the thermodynamically stable, chemically ordered B2 to the metastable A2 phase going along with an evolution of ferromagnetism and an increase of the lattice parameter (Figure 1). This can be explained with the higher local number of Fe-Fe nearest neighbors in the disordered state. The correlation between structural and magnetic properties in the phase transition regime, consisting of A2 and B2 phases, is uncertain as well as the influence of the ion type or the temperature treatment.
The effects of ion implantation on the structural and magnetic properties of 250 nm thick Fe60Al40 films, possessing A2 and B2 structure respectively, have been investigated by means of X-ray diffraction (XRD) and Vibrating sample magnetometry. From XRD measurements, the order parameter S and the peak shift due to the change of the lattice parameter have been derived and correlated with the magnetization. The irradiation of paramagnetic B2 Fe60Al40 with H+, He+ or Ne+ ions with different fluences at low temperatures led to an increase of the saturation magnetization (MS) which was expected to be directly related to the number of displacements per atom (dpa) by using the simulation program TRIM [4], independent on the ion species. However, unlike than expected, the induced magnetization differed but correlated directly with the measured lattice parameter. A significant change of lattice parameter and MS did not appear for proton irradiation at elevated temperatures (250 °C) where the ordered B2 phase was retained. Upon low temperature (LN2) hydrogen implantation of disordered A2 Fe60Al40 films, on the other hand, unlike for helium or neon irradiation, the lattice parameter and the saturation magnetization decreased indicating a little ordering. This might offer the possibility of H+ irradiation induced ordering of chemically disordered alloy thin films well below the ordering temperature.
Furthermore, the studies show that the structural and magnetic properties of 250 nm thick Fe60Al40 films are directly linked with each other (Figure 2) and do not depend on the type of treatment. The chemical disorder induced evolution of ferromagnetism comes along with an abrupt disappearance of the (100)-superlattice peak represented by the order parameter dropping to 0. Nevertheless, the role of defects remains uncertain since ion irradiation leads besides the structural disordering also to an increase of the defect concentration and a temperature treatment to structural ordering and an annealing of defects. However, as described beforehand, H+ implantation causes little ordering but also an increase of the open volume defect concentration, which was characterized by means of Positron annihilation spectroscopy. This offers the opportunity to differentiate between structural disorder and defect concentration.
Given the fact that the proton implanted films follow the shown general behavior a dependence on the structural order only can be assumed.

  • Lecture (Conference)
    Seminar der Professur Werkstofftechnik, 12.-13.01.2018, Meißen, Germany

Permalink: https://www.hzdr.de/publications/Publ-26904
Publ.-Id: 26904


Tuning the magnetic and structural properties of Fe60Al40 thin films by ion irradiation

Ehrler, J.; Bali, R.; Böttger, R.; Zhou, S.; Grenzer, J.; Potzger, K.

Magnetic materials become more significant for future data storage devices and spintronic applications. In certain alloy thin films like Fe60Al40, nano-sized ferromagnetic structures can be created by means of focused ion irradiation.[1,2,3] Fe60Al40 shows a disorder induced phase transition from the thermodynamically stable, chemically ordered B2 to the metastable A2 phase going along with an evolution of ferromagnetism and an increase of the lattice parameter (Figure 1). This can be explained with the higher local number of Fe-Fe nearest neighbors in the disordered state. The correlation between structural and magnetic properties in the phase transition regime, consisting of A2 and B2 phases, is uncertain as well as the influence of the ion type or the temperature treatment.
The effects of ion implantation on the structural and magnetic properties of 250 nm thick Fe60Al40 films, possessing A2 and B2 structure respectively, have been investigated by means of X-ray diffraction (XRD) and Vibrating sample magnetometry. From XRD measurements, the order parameter S and the peak shift due to the change of the lattice parameter have been derived and correlated with the magnetization. The irradiation of paramagnetic B2 Fe60Al40 with H+, He+ or Ne+ ions with different fluences at low temperatures led to an increase of the saturation magnetization (MS) which was expected to be directly related to the number of displacements per atom (dpa) by using the simulation program TRIM [4], independent on the ion species. However, unlike than expected, the induced magnetization differed but correlated directly with the measured lattice parameter. A significant change of lattice parameter and MS did not appear for proton irradiation at elevated temperatures (250 °C) where the ordered B2 phase was retained. Upon low temperature (LN2) hydrogen implantation of disordered A2 Fe60Al40 films, on the other hand, unlike for helium or neon irradiation, the lattice parameter and the saturation magnetization decreased indicating a little ordering. This might offer the possibility of H+ irradiation induced ordering of chemically disordered alloy thin films well below the ordering temperature.
Furthermore, the studies show that the structural and magnetic properties of 250 nm thick Fe60Al40 films are directly linked with each other (Figure 2) and do not depend on the type of treatment. The chemical disorder induced evolution of ferromagnetism comes along with an abrupt disappearance of the (100)-superlattice peak represented by the order parameter dropping to 0. Nevertheless, the role of defects remains uncertain since ion irradiation leads besides the structural disordering also to an increase of the defect concentration and a temperature treatment to structural ordering and an annealing of defects. However, as described beforehand, H+ implantation causes little ordering but also an increase of the open volume defect concentration, which was characterized by means of Positron annihilation spectroscopy. This offers the opportunity to differentiate between structural disorder and defect concentration.
Given the fact that the proton implanted films follow the shown general behavior a dependence on the structural order only can be assumed.

  • Poster
    HZDR PhD Seminar, 16.-18.10.2017, Seiffen, Germany

Permalink: https://www.hzdr.de/publications/Publ-26903
Publ.-Id: 26903


Tuning the magnetic and structural properties of Fe60Al40 thin films by ion irradiation

Ehrler, J.; Bali, R.; Böttger, R.; Zhou, S.; Grenzer, J.; Potzger, K.

Magnetic materials are significant for future data storage device and spintronic applications.
Recently it has been shown that nano-sized ferromagnetic structures can be generated within non-ferromagnetic B2-Fe60Al40 thin films via local ion-irradiation. Ion-irradiation leads to disordering, forming the ferromagnetic A2 phase. The mechanism of this disorder induced effect has been variously attributed to the increase in the number of Fe-Fe nearest neighbors due to disorder and the corresponding increase of the lattice parameter (a0) [1,2,3]. Considering a0 as an indicator for the disorder as the driving parameter for the ferromagnetic order, 250 nm thick B2-Fe60Al40 films irradiated with various ion species, i.e. H+, He+ and Ne+, have been investigated with respect to the correspondence between a0 and the saturation magnetization (MS).
The films have been prepared by magnetron sputtering and irradiated at the Ion Beam Center at the Helmholtz-Zentrum Dresden - Rossendorf. The irradiation-induced displacements per atom (dpa) were calculated with the help of the program TRIM [4]. The films have been irradiated at low
temperature (LN2) with an energy of 17 keV and a fluence of 4E17 ions/cm² for H+ ions and with 3.52E16 ions/cm² at 27 keV for He+. According to
TRIM, these implantation parameters induce the same dpa for the H+ and He+ irradiations. MS was measured by Vibrating sample magnetometry and the order parameter (S) as well as a0 were derived from X-ray diffraction.
The non-irradiated B2-ordered Fe60Al40 films (S=1) possess an MS of 5 kA/m and a0 of 2.89 Å. Despite similar ion-induced disorder, films irradiated with H+ and He+ showed an increased MS of 314 kA/m and 551 kA/m respectively. This suggests deviations from the TRIM calculations, in particular for the lighter H+ ion. Nevertheless, study of a vast number of films irradiated with H+, He+ as well as Ne+ at low and room temperature respectively shows a fundamental correlation between structural and magnetic properties of the Fe60Al40 films, independent of the implanted ion species or temperature treatment (see Figure 1).
The films exhibit a sharp transition at a0=2.91 Å, where MS rapidly increases, tracked exactly by a decrease of S, suggesting that MS depends on the increase of the Fe-Fe nearest neighbor interactions (as indicated by decreasing S) only being independent on a0. DFT calculations will help further understanding of this correlation.

  • Lecture (Conference)
    Moscow International Symposium on Magnetism 2017, 01.-06.07.2017, Moscow, Russia

Permalink: https://www.hzdr.de/publications/Publ-26902
Publ.-Id: 26902


Tuning the magnetic and structural properties of Fe60Al40 thin films by ion irradiation

Ehrler, J.; Bali, R.; Böttger, R.; Zhou, S.; Grenzer, J.; Potzger, K.

Magnetic materials become more significant for future data storage devices and spintronic applications. In certain alloy thin films like Fe60Al40, nano-sized ferromagnetic structures can be created by means of focused ion irradiation.[1,2,3] Fe60Al40 shows a disorder induced phase transition from the thermodynamically stable, chemically ordered B2 to the metastable A2 phase going along with an evolution of ferromagnetism and an increase of the lattice parameter (Figure 1). This can be explained with the higher local number of Fe-Fe nearest neighbors in the disordered state. The correlation between structural and magnetic properties in the phase transition regime, consisting of A2 and B2 phases, is uncertain as well as the influence of the ion type or the temperature treatment.
The effects of ion implantation on the structural and magnetic properties of 250 nm thick Fe60Al40 films, possessing A2 and B2 structure respectively, have been investigated by means of X-ray diffraction (XRD) and Vibrating sample magnetometry. From XRD measurements, the order parameter S and the peak shift due to the change of the lattice parameter have been derived and correlated with the magnetization. The irradiation of paramagnetic B2 Fe60Al40 with H+, He+ or Ne+ ions with different fluences at low temperatures led to an increase of the saturation magnetization (MS) which was expected to be directly related to the number of displacements per atom (dpa) by using the simulation program TRIM [4], independent on the ion species. However, unlike than expected, the induced magnetization differed but correlated directly with the measured lattice parameter. A significant change of lattice parameter and MS did not appear for proton irradiation at elevated temperatures (250 °C) where the ordered B2 phase was retained. Upon low temperature (LN2) hydrogen implantation of disordered A2 Fe60Al40 films, on the other hand, unlike for helium or neon irradiation, the lattice parameter and the saturation magnetization decreased indicating a little ordering. This might offer the possibility of H+ irradiation induced ordering of chemically disordered alloy thin films well below the ordering temperature.
Furthermore, the studies show that the structural and magnetic properties of 250 nm thick Fe60Al40 films are directly linked with each other (Figure 2) and do not depend on the type of treatment. The chemical disorder induced evolution of ferromagnetism comes along with an abrupt disappearance of the (100)-superlattice peak represented by the order parameter dropping to 0. Nevertheless, the role of defects remains uncertain since ion irradiation leads besides the structural disordering also to an increase of the defect concentration and a temperature treatment to structural ordering and an annealing of defects. However, as described beforehand, H+ implantation causes little ordering but also an increase of the open volume defect concentration, which was characterized by means of Positron annihilation spectroscopy. This offers the opportunity to differentiate between structural disorder and defect concentration.
Given the fact that the proton implanted films follow the shown general behavior a dependence on the structural order only can be assumed.

  • Poster
    IEEE International Magnetics Conference 2017, 24.-28.04.2017, Dublin, Ireland

Permalink: https://www.hzdr.de/publications/Publ-26901
Publ.-Id: 26901


Tuning of magnetic and structural properties of Fe60Al40 thin films by ion irradiation

Ehrler, J.; Bali, R.; Böttger, R.; Grenzer, J.; Potzger, K.

The effects of ion implantation on the structural and magnetic properties of Fe60Al40 films, possessing A2 and B2 structure respectively, have been investigated by means of X-ray diffraction and Vibrating sample magnetometry. The studies show that the magnetic properties of the 250 nm thick Fe60Al40 films, showing a phase transition from para- to ferromagnetic state, depend on the structural order only. The evolution of ferromagnetism comes along with an abrupt disappearance of the (100)-superlattice peak.
The irradiation of superparamagnetic B2 Fe60Al40 with H+, He+ or Ne+ ions with different fluences at low temperatures led to an increase of saturation magnetization (MS) going along with a lattice expansion induced by structural disorder. This effect didn’t appear for proton irradiation at elevated temperatures (250 ∘C) where the ordered B2 was retained. The low temperature hydrogen implantation of disordered A2 Fe60Al40 films on the other hand decreased, unlike helium or neon irradiation, the lattice parameter and MS indicating a little ordering.
This might offer the possibility of H+ irradiation induced ordering of A2 Fe60Al40 thin films well below the ordering temperature.

  • Lecture (Conference)
    DPG Spring Meeting 2017, 19.-24.03.2017, Dresden, Germany

Permalink: https://www.hzdr.de/publications/Publ-26900
Publ.-Id: 26900


Modeling Laser-Plasma Interaction under Extreme Conditions Towards In-Situ Pump-Probe Simulations

Garten, M.; Hübl, A.; Widera, R.; Burau, H.; Grund, A.; Metzkes, J.; Kluge, T.; Schramm, U.; Cowan, T.; Bussmann, M.

Laser-driven solid density plasmas can be used to generate highly energetic electrons and ions. Diagnosing properties within those plasmas at nm length scales and down to fs timescales is crucial in understanding the involved processes. This has recently become feasible through the advent of X-Ray Free Electron Lasers (XFELs). For instance, XFELs allow imaging the electron density distribution within plasmas via Small Angle X-Ray Scattering (SAXS). We present a scalable GPU-based software framework for simulating photon scattering processes of X-ray beams in matter using Monte-Carlo methods. These simulations enable us to produce synthetic SAXS signals from the interaction of a modeled X-ray pulse with an arbitrarily complex, 3D electron density distribution obtained e.g. from detailed particle-in-cell simulations. Additionally, we present radiation transport methods in our 3D3V fully-relativistic PIC code PIConGPU. These methods enhance modeling of self-imaging of solid-density plasmas and lay the foundation for in-situ simulations of pump-probe experiments. Our new framework allows for single and multiple scattering and is extendable to include complex physics processes like ionization, atomic excitation, and de-excitation along the photon path to further enhance its predictive capability.

Keywords: Laser-ion acceleration; simulation; HPC; PIConGPU; ParaTAXIS; particle-in-cell; X-Ray; XFEL; SAXS

  • Lecture (others)
    HZDR PhD Seminar, 16.-18.10.2017, Seiffen, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-26899
Publ.-Id: 26899


Modeling Multiple Coherent and Incoherent Photon Scattering in Solid-Density Plasmas with Particle-In-Cell Simulations

Garten, M.; Grund, A.; Hübl, A.; Burau, H.; Widera, R.; Pausch, R.; Debus, A.; Kluge, T.; Fortmann-Grote, C.; Schramm, U.; Cowan, T.; Bussmann, M.

Laser-driven solid density plasmas can be used to generate highly energetic electrons and ions. Diagnosing properties within those plasmas at nm length scales and down to fs timescales is crucial in understanding the involved processes. This has recently become feasible through the advent of X-Ray Free Electron Lasers (XFELs). For instance, XFELs allow imaging the electron density distribution within plasmas via Small Angle X-Ray Scattering (SAXS). We present a scalable GPU-based software framework for simulating photon scattering processes of X-ray beams in matter using Monte-Carlo methods. These simulations enable us to produce synthetic SAXS signals from the interaction of a modeled X-ray pulse with an arbitrarily complex, 3D electron density distribution obtained e.g. from detailed particle-in-cell simulations. Additionally, we present radiation transport methods in our 3D3V fully-relativistic PIC code PIConGPU. These methods enhance modeling of self-imaging of solid-density plasmas and lay the foundation for in-situ simulations of pump-probe experiments. Our new framework allows for single and multiple scattering and is extendable to include complex physics processes like ionization, atomic excitation and de-excitation along the photon path to further enhance its predictive capability.

Keywords: X-Ray; XFEL; SAXS; simulation; HPC; PIConGPU; ParaTAXIS; particle-in-cell

  • Lecture (Conference)
    3rd European Advanced Accelerator Concepts Workshop, 25.-29.09.2017, La Biodola, Isola d'Elba, Italia

Permalink: https://www.hzdr.de/publications/Publ-26898
Publ.-Id: 26898


inSPECtor - integrated spectroscopy sensor system for laser-induced fluorescence and hyperspectral imaging

Seidel, P.; Lorenz, S.; Fuchs, M.; Gloaguen, R.

With the recently intensified development in key technologies such as renewable energy, electric mobility as well as high-tech computer and telecommunication systems, the robust detection and characterization of rare earth elements (REE) simultaneously gained in importance. It is essential not only for the safeguarding of a continuous supply with crucial REE by characterizing new and existing deposits, but also for the future recycling of today’s high-tech products and plants. Until now, hyperspectral absorption as well as emission spectroscopy have been proven to be capable of REE detection in different materials. While absorption spectroscopy has been successfully used for complex and natural samples, existing studies in laser-induced emission spectroscopy (or laser fluorescence) often focus mainly on synthetic crystals and single REE standards. Within this contribution, we focus on the qualitative and quantitative characterization of REE in natural minerals and complex rocks for their possible use as sensors in the mining industry. Based on the integration of emission and absorption spectroscopy techniques we propose a simple yet robust pathway for detection of REE in rock samples.

Keywords: Laser-induced fluorescence; mineral analysis; sensors in mining

  • Invited lecture (Conferences)
    Workshop on innovative technologies in exploration and mineral detection, 14.12.2017, Barcelona, Espana

Permalink: https://www.hzdr.de/publications/Publ-26897
Publ.-Id: 26897


Combination of Hyperspectral Absorption and Photoluminescence Spectroscopy for the Detection of Rare Earth Elements in Natural Minerals

Seidel, P.; Sharma, S.; Lorenz, S.; Fuchs, M.; Beyer, J.; Heitmann, J.; Gloaguen, R.

With the recently intensified development in key technologies such as renewable energy, electric mobility as well as high-tech computer and telecommunication systems, the robust detection and characterization of rare earth elements (REE) simultaneously gained in importance. It is essential not only for the safeguarding of a continuous supply with crucial REE by characterizing new and existing deposits, but also for the future recycling of today’s high-tech products and plants. Until now, hyperspectral absorption as well as emission spectroscopy have been proven to be capable of REE detection in different materials [1,2]. While absorption spectroscopy has been successfully used for complex and natural samples, existing studies in laser-induced emission spectroscopy (or laser fluorescence) often focus mainly on synthetic crystals and single REE standards. Within this contribution, we focus on the qualitative and quantitative characterization of REE in natural minerals and complex rocks for their possible use as sensors in the mining industry. Based on the integration of emission and absorption spectroscopy techniques we propose a simple yet robust pathway for detection of REE in rock samples.

[1] D. J. Turner, B. Rivard, L. Groat American Mineralogist, 99, 1335–1346 (2014)
[2] M. Gaft, G. Panczer, R. Reisfeld, E. Uspensky Phys. Chem. Minerals, 28, 347-363 (2001)

Keywords: Laser-induced fluorescence; mineral analysis; sensors in mining

  • Lecture (Conference)
    Photoluminescence in Rare Earths: Photonic Materials and Devices (PRE'17), 02.12.2017, Roma, Italien

Permalink: https://www.hzdr.de/publications/Publ-26896
Publ.-Id: 26896


p-type co-doping effect of (Ga,Mn)P: Magnetic and magneto-transport properties

Xu, C.; Yuan, Y.; Wang, M.; Hentschel, H.; Böttger, R.; Helm, M.; Zhou, S.

In this paper, we perform a comparison of magnetic and electrical properties between Mn-doped and (Mn, Zn) co-doped GaP dilute ferromagnetic semiconductors. Due to the shallow Zn impurity level (20–40 meV above the top of the III-V compounds valence band), the Zn co-doping leads to the increase of conductivity of (Ga,Mn)P, however both the Curie temperature and magnetization reduce, which is probably due to the suppression of active Mn substitution by Zn co-doping.

Keywords: Dilute ferromagnetic semiconductors; The Curie temperature; Magnetization; Co-doping; Magneto-transport

Downloads:

Permalink: https://www.hzdr.de/publications/Publ-26895
Publ.-Id: 26895


Improved background suppression for radiative capture reactions at LUNA with HPGe and BGO detectors

Boeltzig, A.; Best, A.; Imbriani, G.; Junker, M.; Aliotta, M.; Bemmerer, D.; Broggini, C.; Bruno, C. G.; Buompane, R.; Cavanna, A. C. F.; Chillery, T.; Ciani, G. F.; Corvisiero, P.; Csedreki, L.; Davinson, T.; Deboer, R. J.; Depalo, R.; Di Leva, A.; Elekes, Z.; Ferraro, F.; Fiore, E. M.; Formicola, A.; Fülöp, Z.; Gervino, G.; Guglielmetti, A.; Gustavino, C.; Gyürky, G.; Kochanek, I.; Menegazzo, R.; Mossa, V.; Pantaleo, F. R.; Paticchio, V.; Perrino, R.; Piatti, D.; Prati, P.; Schiavulli, L.; Stöckel, K.; Straniero, O.; Strieder, F.; Szücs, T.; Takács, M. P.; Trezzi, D.; Wiescher, M.; Zavatarelli, S.

Direct measurements of small nuclear reaction cross sections require a low background in the signal region of interest to achieve the necessary sensitivity. We describe two complementary detector setups that have been used for studies of reactions with solid targets at the Laboratory for Underground Nuclear Astrophysics (LUNA): a high-purity germanium detector and a bismuth germanate (BGO) detector. We present the effect of a customised lead shielding on the measured background spectra in the two detector setups at LUNA. We developed a model to describe the contributions of environmental and intrinsic backgrounds in the BGO detector measurements. Furthermore we present an upgrade of the data acquisition system for our BGO detector, which allows us to exploit the features of the segmented detector and overcome some of the limitations encountered in previous experiments. We conclude with a discussion on the improved sensitivity of the presented setups, and the benefits for ongoing and possible future measurements.

Permalink: https://www.hzdr.de/publications/Publ-26894
Publ.-Id: 26894


Casting technology for ODS steels - dispersion of nanoparticles in liquid metals

Sarma, M.; Grants, I.; Kaldre, I.; Bojarevics, A.; Gerbeth, G.

Dispersion of particles to produce metal matrix nanocomposites (MMNC) can be achieved by means of ultrasonic vibration of the melt using ultrasound transducers. However, a direct transfer of this method to produce steel composites is not feasible because of the much higher working temperature. Therefore, an inductive technology for contactless treatment by acoustic cavitation was developed. This report describes the samples produced to assess the feasibility of the proposed method for nano-particle separation in steel. Stainless steel samples with inclusions of TiB2, TiO2, Y2O3, CeO2, Al2O3 and TiN have been created and analyzed. Additional experiments have been performed using light metals with an increased value of the steady magnetic field using a superconducting magnet with a field strength of up to 5 T.

Keywords: Photovoltaic silicon; ribbon-growth on substrate; numerical simulation

  • Open Access Logo IOP Conference Series: Materials Science and Engineering 228(2017), 012020

Permalink: https://www.hzdr.de/publications/Publ-26893
Publ.-Id: 26893


Helium and Neon ion based microscopy and nanofabrication

Hlawacek, G.

HIM nanofabrication overview

  • Lecture (Conference)
    SiC workshop, 08.12.2017, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-26892
Publ.-Id: 26892


Structuring magnetic and electronic materials using GFIS noble gas focused beams

Hlawacek, G.

New device concepts envisioned to solve some of the pressing problems in todays computing technology require new methodological approaches—both for manufacturing but also during initial research. In this presentation I want to highlight the potential and limitations of ion beams and in particular gas field ion sources based (GFIS) noble gas beams for this demanding development process.
In an first example I want to present results of low fluence ion beam structuring of alloys with interesting magnetic properties such as FeAl. This material undergoes a phase transition upon ion irradiation that converts the initially paramagnetic material into a ferromagnetic one. Using the highly localized irradiation possible in the helium ion microscope and low fluencies of only 1-5 Ne + per nm 2 we can locally change the properties and this create arbitrary shaped nano magnets. The fundamental properties of these electron spin controlling structures with critical dimensions as small as 20 nm can be studied by TEM holography or scanning transmission x-ray microscopy.
Other device concepts require the control of currents at the single electron level. In the second part of the talk I will present first results of the realization of a CMOS compatible single electron transistor (SET) that works at room temperature (RT). We employ a focused GFIS Ne beam to locally mix silicon into a thin silicon dioxide layer. During a subsequent thermal treatment a single silicon cluster with a diameter of only 2-3 nm forms in the oxide. The cluster is separated from the surrounding silicon by only 2 nm providing optimum tunnel distances for RT SET operation. This process is based on the small size of collision cascade in the HIM. A more CMOS compatible restriction of the mixed volume can be achieved by using broad beam irradiation and nano pillars. The first is a well established technique in semiconductor fabrication and latter can be mass fabricated using advanced lithography. In the so achieved restricted mixed volume a single cluster forms during the subsequent annealing.
Both examples highlight the flexibility of the GFIS technique and its potential for the rapid prototyping of new device concepts based on ion beam techniques.
This work has been partially funded by the European Unions Horizon 2020 Research and Innovation Program under grant agreement No. 688072 IONS4SET.

Keywords: HIM

  • Invited lecture (Conferences)
    MRS Fall Meeting, 28.11.2017, Boston, USA

Permalink: https://www.hzdr.de/publications/Publ-26891
Publ.-Id: 26891


Analytic approaches with focused ion beams

Hlawacek, G.; Klingner, N.; Heller, R.; Facsko, S.

Focused ion beams have become the number one tool for localized materials modification at the nanometer scale. While initially limited to Ga ions only the last decade has broadened the field by introducing commercial columns based on plasma sources, liquid metal alloy sources (LMAIS) and gas field ion sources (GFIS). The range of available ions now includes among others Li, Cs, many transition metals and notably several gases such H2, N2, He, Ne and Xe. While all of those have their benefits He has the highest potential to be used in high resolution analytical applications.

In this presentation I would like to present available solutions for FIB based nano-analytics that do not require an additional electron beam for the actual analysis, but where the ion is directly or indirectly responsible for the creation of the signal of interest. I will focus on the most wide spread methods which include Secondary Ion Mass Spectrometry (SIMS) [1], Backscatter Spectrometry (BS) [2], channeling [3], as well as Ionoluminescence [4].

Keywords: HIM

  • Invited lecture (Conferences)
    FIBiB, 06.11.2017, Berlin, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-26890
Publ.-Id: 26890


Spatially controlled formation of nanostructures for magnetic and electronic applications

Hlawacek, G.

Gas field ion source (GFIS) based focused ion beam techniques historically also known as Helium Ion Microscopy (HIM) is recognized for its high resolution imaging and nanofabrication capabilities[1]. Over the last decade the tool has been utilized in many different ways. Applications include classical semiconductor materials, magnetic materials, 2D materials, nuclear materials but also biological materials.
In this presentation I want to highlight the potential and limitations of gas field ion sources based (GFIS) noble gas beams for new magnetic and electronic device concepts. In an first example I want to present results of low fluence ion beam structuring of alloys with interesting magnetic properties such as FeAl.
This material undergoes a phase transition upon ion irradiation that converts the initially paramagnetic material into a ferromagnetic one. Using the highly localized irradiation possible in the helium ion microscope and low fluencies of only 1-5 Ne per nm we can locally change the properties and this create arbitrary shaped nano magnets. The fundamental properties of these electron spin controlling structures with critical dimensions as small as 20 nm can be studied by TEM holography or scanning transmission x-ray microscopy.
Other device concepts require the control of currents at the single electron level. In the second part of the talk I will present first results of the realization of a CMOS compatible single electron transistor (SET) that works at room temperature (RT). We employ a focused GFIS Ne beam to locally mix silicon into a thin silicon dioxide layer. During a subsequent thermal treatment a single silicon cluster with a diameter of only 2-3nm forms in the oxide. The cluster is separated from the surrounding silicon by only 2nm providing optimum tunnel distances for RT SET operation. This process is based on the small size of collision cascade in the HIM. A more CMOS compatible restriction of the mixed volume can be achieved by using broad beam irradiation and nano pillars. The first is a well established technique in semiconductor fabrication and latter can be mass fabricated using advanced lithography. In the so achieved restricted mixed volume a single cluster forms during the subsequent annealing.
Both examples highlight the flexibility of the GFIS technique and its potential for the rapid prototyping of new device concepts based on ion beam techniques.
This work has been partially funded by the European Union’s Horizon 2020 Research and Innovation Program under grant agreement No. 688072 “IONS4SET”.
[1] Gregor Hlawacek and Armin Gölzhäuser, editors. Helium Ion Microscopy. Springer International Publishing, Cham, 2016.

Keywords: HIM

  • Invited lecture (Conferences)
    JASP, 07.09.2017, Fukuoka, Japan

Permalink: https://www.hzdr.de/publications/Publ-26889
Publ.-Id: 26889


Nanofabrication with the Helium Ion Microscope

Hlawacek, G.

Helium Ion Microscopy (HIM) [1,2] is best known for its high resolution imaging capabilities of both conductive as well as insulating samples. However, since the introduction of Ne as an imaging gas for the gas field ion source (GFIS) an increasing number of nano-fabrication applications are realized.
While the use of Neon as an imaging gas results in a somewhat lower lateral resolution (1.8 nm for 25 keV Ne compared to 0.5 nm for 30 keV He) the user usually benefits from the much higher cross section for nuclear stopping. The latter results in a larger number of sputtered atoms and bonds broken directly by small impact parameter collisions.
Here, I first want to summarize results obtained over the last years using focused ion beam induced deposition (FIBID) using the HIM [3]. In addition I will show results on resist writing using the HIM.
Both approaches benefit from the negligible proximity effect in the HIM. This is related to the different energy distribution of the electrons created by the ion beam as well as the near absence of second or higher generation electrons. Consequently, line patterns with a half pitch of 4 nm have been reported [4]. For high aspect pillars this often results in narrow structures created using FIBID as compared to FEBID.
In the second part I will present results obtained using direct write milling low fluence ion beam irradiation and ion beam based mixing. In all three cases the electronic or magnetic properties of the target material will be altered at the nano-scale in a controlled way to achieve new functionality. The examples comprise • The fabrication of semiconducting graphene nano-ribbons by direct milling [5]
• The fabrication of a lateral spin valve structure using low fluence ion irradiation [6]
• The formation of individual 3 nm Si clusters for a room temperature single electron transistor
For all presented examples the critical length scale of the nanostructure is smaller or in the range of collision cascade. This size regime can not be accessed with traditional broad beam based ion irradiation and holds many promises but also challenges that need to be overcome to enable new device concepts and new functional materials on the nano-scale.

Keywords: HIM

  • Invited lecture (Conferences)
    CELINA, 15.09.2017, Porto, Portugal

Permalink: https://www.hzdr.de/publications/Publ-26888
Publ.-Id: 26888


Helium and Neon ion based microscopy and nanofabrication

Hlawacek, G.

Gas field ion source (GFIS) based focused ion beam techniques historically also known as Helium Ion Microscopy (HIM) is recognized for its high resolution imaging and nanofabrication capabilities [1]. Over the last decade the tool has been utilized in many different ways. Applications include classical semiconductor materials, magnetic materials, 2D materials, nuclear materials but also biological materials.
I will introduce the technique and discusse the different instrumentation add ons we utilize and develop in our lab. This includes ionoluminescence [2], in-situ electrical probing [3], and in-situ irradiation at elevated temperatures. Finally, I want to present how we achieve a world record lateral resolution for backscatter spectrometry using time-of-flight [4]. On the materials science part I will present examples from our own lab covering the various application fields, including patterning of magnetic [5] and 2D materials [3] but also structural characterization of epitaxial metal layers [6]. The capability to investigate insulating or biological samples without compromising the performance of the machine will be presented in the second part [7]. This unique feature of the helium ion microscope is possible thanks to the use of an electron flood gun for charge compensation.

Keywords: HIM

  • Invited lecture (Conferences)
    Physik Kolloquium, 03.07.2017, Leoben, Österreich

Permalink: https://www.hzdr.de/publications/Publ-26887
Publ.-Id: 26887


Spatially controlled ripple formation in the HIM using low voltages and high temperatures

Sottili, L.; Hlawacek, G.; Engler, M.; Facsko, S.

Ripple formation is a well known phenomenon that is observed
Conclusion
•Ripple formation has been facilitate by using a new home
Localized FIB based ripple formation
Temperature dependence Broad beam ripple formation with Ne
•Large area HIM patterning for many materials under low energy ion bombardment.
Often broad beam ion irradiation using energies of only a few keV is employed to create these self-organized patterns. We present for the first time ripple patterns that have been created in GaAs(001) using 5 keV Ne ions and elevated temperatures of up to 590 K in a Helium Ion Microscope (HIM). HIM is well known for its outstanding imaging and micro and nano fabrication capabilities.
However, most results so far have been achieved at room temperature and by using energies between 25 keV and 35 keV.
For this work we lowered the acceleration voltage to below 5 keV while maintaining an acceptable lateral resolution in the nm range .

Keywords: HIM; ripples

  • Poster
    Nanopatterning2017, 26.-30.06.2017, Helsinki, Finland

Permalink: https://www.hzdr.de/publications/Publ-26886
Publ.-Id: 26886


Spatially controlled fabrication of individual silicon nano clusters using ion beam mixing and thermal treatment

Hlawacek, G.; Xu, X.; Prüfer, T.; Wolf, D.; Hübner, R.; Bischoff, L.; Möller, W.; Facsko, S.; von Borany, J.; Heinig, K. H.

The miniaturization of computing devices and the introduction of the internet of things creates an increasing demand for the development of low power devices.
Single electron transistors (SETs) are very low power dissipation devices and thus ideally suited for this demand. Combined with existing CMOS technology which is characterized by high speed and driving the existing draw backs of SETs are compensated. The development of such hybrid SET-CMOS devices is currently hindered by missing large scale manufacturing routes. For room temperature (RT) operation it is necessary to create a single nanocluster with a diameter below 5 nm exactly positioned between source and drain at a tunnel distance of only a few nanometers.
We show the first results on the way to a CMOS compatible fabrication process based on ion beam mixing and self-assembly to form a Si cluster with a size below 5 nm. Our process ensures that (a) the cluster size is small enough to allow RT operation, (b) the cluster is located at the correct tunnel distance between source and drain, (c) the clusters form at predetermined locations, and (d) the process is CMOS compatible. These goals are reached by a combination of localized ion beam mixing and a carefully tuned thermal treatment that leads to a self-assembly process that guarantees (a) and (b). In this initial demonstration of the single cluster formation process we utilize a Helium Ion Microscope to locally mix Si into a buried oxide layer. The highly focused Ne beam available in this instrument allows point like irradiation and hence reduces the mixing volume to (10nm) 3 . Subsequent annealing results in the formation of a single 2 nm Si cluster located less than 3 nm from the adjacent Si/SiO 2 interfaces.
Energy-filtered TEM (EFTEM) (Figure 1) has been utilized to reveal the presence of individual clusters. For a CMOS compatible fabrication process the restriction of the collision cascade and therefore the ion beam mixed volume will be realized by using broad beam irradiation of nanopillars with an embedded oxide layer and a diameter below 20 nm. The aim is to fabricate gate-all-around nanopillar RT-SETs together with state-of-the-art FETs. TRI3DYN and kMC simulations (Figure 2) are used to study these future works and compare them to the above discussed study based on focused ion beam irradiation.

This work is being funded by the European Union’s Horizon 2020 research and innovation program under Grant Agreement No 688072 (Project IONS4SET).

Keywords: HIM

  • Lecture (Conference)
    EIPBN, 30.05.-02.06.2017, Orlando, USA
  • Lecture (Conference)
    eu-f-n, 04.07.2017, Graz, Österreich

Permalink: https://www.hzdr.de/publications/Publ-26885
Publ.-Id: 26885


Scanning Transmission Ion Microscopy

Hlawacek, G.; Krasheninnikov, A.; Klingner, N.; Möller, W.; Munnik, F.; Facsko, S.

Project presentation npSCOPE

Keywords: STIM; HIM

  • Lecture (others)
    npSCOPE kick off meeting, 16.-17.01.2017, Belvaux, Luxembourg

Permalink: https://www.hzdr.de/publications/Publ-26884
Publ.-Id: 26884


Strain doping driven metal-insulator transition in LaNiO3 films

Changan, W.; Parul, P.; Pei-Chun, W.; Ping-Chun, W.; Mao, W.; Chi, X.; Yujia, Z.; Roman, B.; Manfred, H.; Ying-Hao, C.; Shengqiang, Z.

Complex oxides are fascinating materials, in which the manipulation of charge, orbital and lattice degrees of freedom leads to numerous exciting phenomena. We report here the use of He ion irradiation to control the out-of-plane lattice constant of epitaxial LaNiO3 (LNO) thin films independently without a change of the in-plane lattice constant. All the LNO films with the fluence less than 1×1015 He/cm2 exhibit metallic behaviors along with a slight resistivity upturn at low temperature, whereas the film with 2.5×1015 He/cm2 shows metallicity at high temperature and insulator-like behavior at low temperature. Further, the fitting for the temperature dependent resistance indicates that electrical-conductivity carriers are mainly scattered by electron-boson interactions rather than electron-electron interactions. These results suggest that He ion irradiation can be an alternative route to tune the functionality of complex oxides.

  • Lecture (Conference)
    European Materials Research Society, 18.09.2017, Warsaw, Poland

Permalink: https://www.hzdr.de/publications/Publ-26883
Publ.-Id: 26883


Coupling of magnetism and structural phase transition in V2O3/Co bilayers

Changan, W.; Chi, X.; Ye, Y.; Yu-Jia, Z.; Shengqiang, Z.

Exchange couplings across interfaces of hybrid magnetic heterostructures are being considered as unique opportunity for functional materials design. In this study, we show that both coercivity and magnetization of V2O3/Co bilayers are affected by the stress associated with structural phase transition across metal-insulator phase transition in V2O3. The change in coercivity is as large as 59% in a very narrow temperature range. The magnetic properties can be controlled by stress, which is significant for future multiferroic and spintronics applications.

  • Poster
    Deutsche Physikalische Gesellschaft, 19.03.2017, Dresden, Germany

Permalink: https://www.hzdr.de/publications/Publ-26882
Publ.-Id: 26882


Performance Enhancement of Reconfigurable Field Effect Transistors (RFETs)

Khan, M. B.; Deb, D.; Georgiev, Y. M.; Erbe, A.

Scaling down of CMOS faces strong challenges due to which advanced fabrication techniques, advanced materials, new device and logic concepts have gained importance. These concepts include undoped silicon nanowire based reconfigurable devices, which can be programmed as p- or n-channel FETs by controlling the electrostatic potential applied at gate electrodes. In this talk, fabrication and electrical characterization of undoped sub-20 nm silicon nanowires (SiNWs) will be reported. SiNWs are fabricated on intrinsic silicon-on-insulator (SOI) substrates in <110> and <100> crystal orientations using a top down approach. Hydrogen silsesquioxane (HSQ), a negative tone electron beam resist, is used for nano-patterning and as a hard mask for etching. Nanowire etching process is optimized using an inductively coupled plasma (ICP) source and C4F8/SF6/O2 mixed gas recipe at 18◦C. These NWs are subsequently silicidized to form Scottky junctions. Electrical characterization shows different charge carrier transport in <110> and <100> crystal orientations. Control over silicide formation to enhance the performance of these devices will be discussed.

  • Lecture (Conference)
    IHRS NanoNet Annual Workshop 2017, 16.-18.08.2017, Klingenberg-Colmnitz, Germany

Permalink: https://www.hzdr.de/publications/Publ-26881
Publ.-Id: 26881


Electrical characterization of sub-20 nm silicon nanowires fabricated using electron beam lithography and inductively coupled plasma etching

Khan, M. B.; Deb, D.; Georgiev, Y. M.; Erbe, A.

Scaling down of CMOS faces strong challenges due to which advanced fabrication techniques, advanced materials, new device and logic concepts have gained importance. These concepts include undoped silicon nanowire based reconfigurable devices, which can be programmed as p-FET or n-FET by controlling the electrostatic potential applied at gate electrodes. In this work, fabrication and electrical characterization of undoped sub-20 nm silicon nanowires (SiNWs) is reported. SiNWs are fabricated on intrinsic silicon-on-insulator (SOI) substrates in <110> and <100> crystal directions using a top down approach. Hydrogen silsesquioxane (HSQ), a negative tone electron beam resist, is used for nano-patterning and as a hard mask for etching. Nanowire etching process is optimized using an inductively coupled plasma (ICP) source and C4F8/SF6/O2 mixed gas recipe at 18 oC. These NWs are oxidized to form a SiO2 shell and subsequently silicidized. Final observations include different charge carrier transport in <110> and <100> crystal directions.

Keywords: Silicon nanowire; etching; lithography; schottky barrier devices

  • Poster
    Materials for Advanced Metallization, 26.-29.03.2017, Dresden, Germany

Permalink: https://www.hzdr.de/publications/Publ-26880
Publ.-Id: 26880


A Cytomic Approach Towards Genomic Individuality of Neurons

Arendt, T.; Belter, B.; Brückner, M. K.; Ueberham, U.; Morawski, M.; Tarnok, A.

Here, we describe an approach for the DNA quantification of single cells in brain slices based on image cytometry (IC) that allows mapping the distribution of neurons with DNA content variation (DCV) in the context of preserved tissue architecture. The method had been optimized for DNA quantification of identified neurons but could easily be adapted to other tissues. It had been validated against chromogenic in situ hybridization (CISH) with chromosome-specific probes and laser microdissection followed by quantitative PCR (qPCR) of alu repeats. It can be combined with immunocytochemical detection of specific marker proteins which allow for further specification of cellular identity in the context of defined brain pathology. The method can be applied in a high-throughput mode where it allows analyzing 500,000 neurons per brain in a reasonable time. The combination of cytometry with molecular biological characterization of single microscopically identified neurons as outlined here might be a promising approach to study molecular individuality of neurons in the context of its physiological or pathophysiological environment. It reflects the concept of cytomics and will forward our understanding of the molecular architecture and functionality of neuronal systems.

Keywords: Ageing; Alzheimer’s disease; Aneuploidy; Cell death; Cellular individuality; Cytomics; DNA content variation; Genomic mosaic; Neurodegeneration; Polyploidy; Single-cell analysis

  • Book chapter
    Frade, Jose Maria, Gage, Fred H.: A Cytomic Approach Towards Genomic Individuality of Neurons, Paris: Springer, 2017, 978-1-4939-7279-1, 81-106
    DOI: 10.1007/978-1-4939-7280-7

Permalink: https://www.hzdr.de/publications/Publ-26879
Publ.-Id: 26879


Magneto-optical spectroscopy of diluted magnetic semiconductors GaMnAs prepared by ion implantation and further impulse laser annealing

Gan'Shina, E. A.; Golik, L. L.; Kun'Kova, Z. E.; Zykov, G. S.; Bykov, I. V.; Rukovishnikov, A. I.; Yuan, Y.; Böttger, R.; Zhou, S.

Ga1−xMnxAs layers prepared by ion implantation and subsequent pulsed laser annealing with the planned Mn concentrations of x = 0.01–0.08 have been studied using the magneto-optical transversal Kerr effect (TKE) and spectral ellipsometry. The spectral dependences of the diagonal and nondiagonal components of the permittivity tensor (PT), as well as the spectrum of magnetic circular dichroism (MCD) have been calculated for the layers. The obtained spectra of the diagonal PT components show that the layers under study maintain the zinc-blende crystal structure of the parent GaAs semiconductor. All studied samples reveal a strong TKE response at low temperatures with a dependence of an effective Curie temperature (at which TKE appears) on the Mn concentration. A number of extrema in the low-temperature TKE and MCD spectra are close to the energies of transitions in the Γ and L critical points of the parent semiconductor band structure that confirms the intrinsic ferromagnetism of the Ga1−xMnxAs layers. The MCD spectra shape and its change with Mn concentration increasing are discussed on a base of the valence-band model of ferromagnetism in DMS.

Downloads:

Permalink: https://www.hzdr.de/publications/Publ-26878
Publ.-Id: 26878


Transition Metal Dichalcogenides under Ion Irradiation: From Defects to Atomic Structure Engineering

Ghorbani-Asl, M.; Kretschmer, S.; Krasheninnikov, A. V.

We study the effects of ion irradiation on suspended MoS₂ monolayer (ML) by using molecular dynamics (MD) combined with density-functional theory (DFT) calculations. We systematically study the production of defects in a free-standing MoS₂ ML under noble gas ions bombardment for a broad range of incident angles and ion energies and determine the probabilities of producing single Mo and S vacancies. By comparing MD trajectories and analytical models for binary collision, we identified both direct and indirect mechanisms for defect production. Our results demonstrate that a selective sputtering of S atoms from the upper or lower layer can be achieved by choosing ion energy and incidence angle. The probability of producing S vacancy from upper layer increases by tilting the ion beam from the normal direction. The results showed that the defects cross section for both S and Mo vacancy grows with ion mass while the values for S vacancy are much higher than Mo vacancy. We further show the possibility of producing stable mixed MoSX (X from group V or VII) compounds with different electronic properties using ion irradiation. These findings suggest a promising route for post-growth processing of these materials for engineering electronic devices.

Keywords: transition metal dichalcogenides; ion irradiation; defects; molecular dynamics; density-functional theory

  • Lecture (Conference)
    MRS Spring Meeting, 17.-21.04.2017, Phoenix, Arizona, United States

Permalink: https://www.hzdr.de/publications/Publ-26877
Publ.-Id: 26877


Nearest-neighbor Kitaev exchange blocked by charge order in electron-doped α-RuCl3

Koitzsch, A.; Habenicht, C.; Müller, E.; Knupfer, M.; Büchner, B.; Kretschmer, S.; Richter, M.; van den Brink, J.; Börrnert, F.; Nowak, D.; Isaeva, A.; Doert, T.

A quantum spin liquid might be realized in α-RuCl3, a honeycomb-lattice magnetic material with substantial spin-orbit coupling. Moreover, α-RuCl3 is a Mott insulator, which implies the possibility that novel exotic phases occur upon doping. Here, we study the electronic structure of this material when intercalated with potassium by photoemission spectroscopy, electron energy loss spectroscopy, and density functional theory calculations. We obtain a stable stoichiometry at K0.5RuCl3. This gives rise to a peculiar charge is proportionation into formally Ru2+ (4d6) and Ru3+ (4d5). Every Ru 4d5 site with one hole in the t2g shell is surrounded by nearest neighbors of 4d6 character, where the t2g level is full and magnetically inert. Thus, each type of Ru site forms a triangular lattice, and nearest-neighbor interactions of the original honeycomb are blocked.

Keywords: Kitaev exchange; quantum spin liquid; charge order

Downloads:

Permalink: https://www.hzdr.de/publications/Publ-26876
Publ.-Id: 26876


Structural transformations in two-dimensional transition-metal dichalcogenide MoS2 under electron beam: insights from first-principles calculations

Kretschmer, S.; Komsa, H.-P.; Bøggild, P.; Krasheninnikov, A. V.

Recently a phase transition from the hexagonal 1H to trigonal distorted 1T'-phase in two-dimensional (2D) MoS2 has been induced by electron irradiation [1]. Using density functional theory calculations, we study the energetics of these stable and metastable phases when electric charge, mechanical strain and vacancies are present. Based on the results of our calculations, we propose an explanation for this phenomenon which is likely promoted by charge redistribution in the monolayer combined with vacancy formation due to electron beam and associated mechanical strain in the sample. Considering variations of the total energy difference between both phases the described mechanism can be extended to other transition-metal dichalcogenides.

Keywords: phase transition; two-dimensional; transition-metal dichalcogenides; electron irradiation; electron beam

  • Poster
    17th International Conference on Density-Functional Theory and its Applications, 21.-25.08.2017, Tällberg, Sweden

Permalink: https://www.hzdr.de/publications/Publ-26875
Publ.-Id: 26875


The mystery of semiconductor to metal phase transition in MoS2 under electron beam

Kretschmer, S.; Komsa, H.-P.; Krasheninnikov, A. V.

Recently a phase transition from the hexagonal 1H to trigonal distorted 1T' - phase in two-dimensional (2D) MoS2 has been induced by electron irradiation [1]. Using density functional theory calculations, we study the energetics of these stable and metastable phases when electric charge, mechanical strain and vacancies are present. Based on the results of our calculations, we propose an explanation for this phenomenon which is likely promoted by charge redistribution in the monolayer combined with vacancy formation due to electron beam and associated mechanical strain in the sample.

Keywords: semiconductor-metal transition; phase transition; two-dimensional; electron beam

  • Lecture (Conference)
    Trends in NanoTechnology, 05.06.2017, Dresden, Germany
  • Lecture (Conference)
    22nd International Workshop on Inelastic Ion-Surface Collissions, 17.-22.09.2017, Dresden, Germany

Permalink: https://www.hzdr.de/publications/Publ-26874
Publ.-Id: 26874


Ion-beam mediated patterning of MoS₂ monolayers

Ghorbani-Asl, M.; Kretschmer, S.; Spearot, D.; Krasheninnikov, A. V.

Using analytical potential molecular dynamics combined with first-principles calculations, we study the production of defects in free-standing MoS₂ monolayers under ion irradiation for a wide range of ion energies when nuclear stopping dominates. The probabilities of defect production have been studied for various types of defects. We show that depending on the incident angle, ion type, and energy, sulfur atoms can be sputtered away predominantly from the top or bottom layers, providing unique opportunities for ion-beam mediated patterning of MoS₂. As an example, we study the stability and electronic properties of mixed MoSX compounds where X are chemical elements from group V or VII. We demonstrate that such systems can show metallic character (e.g. MoSF) and further be used to design metal/semiconductor/metal junctions, which exhibit negative differential resistance.

Keywords: ion-irradiation; MoS₂ monolayer; molecular dynamics; first-principles calculations

  • Lecture (Conference)
    DPG Spring Meeting, 19.-24.03.2017, Dresden, Germany
  • Lecture (Conference)
    Flatlands 2017, 29.08.-01.10.2017, EPFL, Switzerland

Permalink: https://www.hzdr.de/publications/Publ-26873
Publ.-Id: 26873


Phase Transitions in Two-Dimensional Transition Metal Dichalcogenides under Electron Beam

Kretschmer, S.; Komsa, H.-P.; Bøggild, P.; Krasheninnikov, A. V.

Recently a phase transition from the hexagonal 1H to trigonal distorted 1T’-phase in two-dimensional (2D) MoS2 has been induced by electron irradiation [1]. Using density functional theory calculations, we study the energetics of these stable and metastable phases when electric charge, mechanical strain and vacancies are present. Based on the results of our calculations, we propose an explanation for this phenomenon which is likely promoted by charge redistribution in the monolayer combined with vacancy formation due to electron beam and associated mechanical strain in the sample. We further show that this mechanism can be extended to other 2D transition metal dichalcogenides.

[1] Y.-C. Lin, D. O. Dumcenco, Y.-S. Huang, and K. Suenaga, Nature Nanotechnology 9, 391 (2014)

Keywords: phase transition; two-dimensional; transition metal dichalcogenides; electron beam

  • Lecture (Conference)
    Towards Reality in Nanoscale Materials IX, 13.-16.02.2017, Levi, Finland
  • Lecture (Conference)
    MRS Spring Meeting, 17.-21.04.2017, Phoenix, USA
  • Lecture (others)
    Seminar, 15.-19.05.2017, Helsinki, Finland

Permalink: https://www.hzdr.de/publications/Publ-26872
Publ.-Id: 26872


Mechanosensing at the nanoscale: the influence of thermoplastic nanostructures on neural cells

Masciullo, C.; Tonazzini, I.; Dell’Anna, R.; Sonato, A.; Böttger, R.; Pepponi, G.; Romanato, F.; Cecchini, M.

It is well established that the behavior of neural cells is influenced by geometrical patterns in the micrometric and sub-micrometric range. Here we present two different types of periodical patterns in the nanometric range (i.e. with a typical features having a lateral size ≤ 100 nm) and their impact on cell contact guidance. In the first case, hierarchical periodic nano-rippled structure (i.e. nano-ripples) made by ion-bombardment technique were replicated on top of polyethylene terephtalate (PET) films. We demonstrated that Schwann cells actively interact with these nanorippled surfaces, showing perpendicular contact guidance and improved adhesion and proliferation with respect to standard flat substrates. The second type of scaffolds here presented consist in cyclic-olefin-copolymer (COC) nanogratings with periodicity (down to 200nm -50% duty cycle), obtained by hot embossing from photoresist molds fabricated by interference lithography. In this case, we coupled the substrates with the PC12 neuronal cell line and measured the neurite alignment and focal adhesion (FA) morphometric parameters. We show optimal contact guidance in the case of periodicity > 400nm, while a progressive degradation of polarized alignment appears by further decreasingthe grating lateral dimensions, correlating with FA shaping. These results set for the first time a lower limit in grating periodicity for effective neurite contact guidance. Altogether thesestudies provide interesting elements for regenerative medicine applications and for developing artificial neural interfaces.

Keywords: Mechanobiology; Nanotechnology

  • Poster
    Nanoengineering for Mechanobiology, 26.-29.03.2017, Camogli, Genova, Italy

Permalink: https://www.hzdr.de/publications/Publ-26871
Publ.-Id: 26871


Ions irradiation on bi-layer coatings

Tessarolo, E.; Corso, A. J.; Böttger, R.; Martucci, A.; Pelizzo, M. G.

Future space missions will operate in very harsh and extreme environments. Optical and electronics components need to be optimized and qualified in view of such operational challenges. This work focuses on the effect of low alpha particles irradiation on coatings. Low energy He+ (4 keV and 16 keV) ions have been considered in order to simulate in laboratory the irradiation of solar wind (slow and fast components) alpha particles. Mono- and proper bi-layers coatings have been investigated. The experimental tests have been carried out changing doses as well as fluxes during the irradiation sessions. Optical characterization in the UV-VIS spectral range and superficial morphological analysis have performed prior and after irradiation.

Keywords: ion Irradiation; coatings; space applications

  • Contribution to proceedings
    SPIE Optical Engineering + Applications, 06.-10.08.2017, San Diego, United States
    Proc. SPIE 10401, Astronomical Optics: Design, Manufacture, and Test of Space and Ground Systems
    DOI: 10.1117/12.2274795

Permalink: https://www.hzdr.de/publications/Publ-26870
Publ.-Id: 26870


Curvature-induced asymmetry of spin-wave dispersion

Otálora, J. A.; Yan, M.; Schultheiss, H.; Lindner, J.; Fassbender, J.; Hertel, R.; Kákay, A.

Spin waves (SWs), or magnons, are dynamic eigen-oscillations of spins in ferromagnetic materials. Analogous to the electron currents in electronics, spin-based currents are proposed to be used to carry, transport and process information in the research field of magnonics. Since SWs, with frequencies between GHz to THz range propagate over macroscopic distances without electron charges being displaced, information technologies based on magnon computation are expected to achieve low power consumption, fast operative rates and small packing sizes. Accordingly, remarkable progress has been made both theoretically and experimentally, leading to prototype building blocks of spin-wave-based logics.
Due the their stable magnetisation states and small sizes magnetic nanotubes are perfect candidates for magnonic waveguides. Such novel structures can nowadays be very well produced [1,2], motivated by the broad range of applications for magnetoresistive devices, optical metamaterials, cell-DNA separators, and drug delivery vectors [3,4]. The high stability of their equilibrium state [5,6] against external perturbations and their robust domain walls propagating with velocities faster than the SW phase velocity [7] promote MNTs as appealing candidates for racetrack memory devices [8,9] and information processing [7].
We show using micromagnetic simulations and analytical calculations that spin-wave propagation in ferromagnetic nanotubes is fundamentally different than in flat thin films. The dispersion relation is asymmetric regarding the sign of the wave vector for both the zeroth and first order azimuthal modes. This is a purely curvature induced effect and its origin is identified to be the classical dipole-dipole interaction. In certain cases the Damon-Eshbach modes in nanotubes behave as the volume-charge-free backward volume modes in flat thin films. Such non-reciprocal spin-wave propagation [10] is known for flat thin films with Dzyalonshiinsky-Moriya interaction, an antisymmetric exchange due to spin-orbit coupling. The analytical expression of the dispersion relation has the same mathematical form as in flat thin films with DMI. The influence of curvature on spin waves is thus equivalent to an effective dipole-induced Dzyalonshiinsky-Moriya interaction [11].
We also derive the dispersion relation for the limiting cases k=0 and k much larger than 1/R, where k is the wave vector of the spin wave and R the nanotube radius. For the first case, the mathematical formula of the dispersion relation resembles the well-known Kittel formula for the ferromagnetic resonance of a thin film with the in-plane magnetization parallel to the applied field, and both oriented perpendicularly to the in-plane easy axis of the shape anisotropy field. In the latter case the expression is identical to the exchange-dominated dispersion relation of a planar thin film in the Damon-Esbach configuration with the in-plane magnetization oriented perpendicularly to the in-plane easy axis.

Keywords: spin waves; dipole-induced DMI; broken inversion symmetry

  • Lecture (Conference)
    Hysteresis Modelling and Magnetism - HMM, 29.-31.05.2017, Barcelona, Spain

Permalink: https://www.hzdr.de/publications/Publ-26869
Publ.-Id: 26869


From plasma acceleration to accelerators ? Status of laser and plasma accelerator development in Dresden

Schramm, U.

Seminar Talk on laser plasma accelerators

Keywords: Laser plasma acceleration; high power laser development

  • Invited lecture (Conferences)
    Plasmaphysik Seminar der GSI, 23.01.2018, Darmstadt, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-26868
Publ.-Id: 26868


Curvature-Induced Asymmetry of Spin-Wave Dispersion in Nanotubes

Kakay, A.; Otalora, J. A.; Schultheiss, H.; Lindner, J.; Hertel, R.; Fassbender, J.

Due the their stable magnetisation states and small sizes magnetic nanotubes are perfect candidates for magnonic waveguides. Such novel structures can nowadays be very well produced [1,2], motivated by the broad range of applications for magnetoresistive devices, optical metamaterials, cell-DNA separators, and drug delivery vectors [3,4].
The high stability of their equilibrium state [5,6] against external perturbations and their robust domain walls propagating with velocities faster than the SW phase velocity [7] promote MNTs as appealing candidates for racetrack memory devices [8,9] and information processing [7].
We show using micromagnetic simulations and analytical calculations that spin-wave propagation in ferromagnetic nanotubes is fundamentally different than in flat thin films. The dispersion relation is asymmetric regarding the sign of the wave vector.
As shown in figures 1 and 2, the spin-wave dispersion is asymmetric for both the zeroth and first order azimuthal modes. This is a purely curvature induced effect and its origin is identified to be the classical dipole-dipole interaction.
In certain cases the Damon-Eshbach modes in nanotubes behave as the volume-charge-free backward volume modes in flat thin films. Such non-reciprocal spin-wave propagation [10] is known for flat thin films with Dzyalonshiinsky-Moriya interaction (DMI), an antisymmetric exchange due to spin-orbit coupling. The analytical expression of the dispersion relation has the same mathematical form as in flat thin films with DMI. The influence of curvature on spin waves is thus equivalent to an effective dipole-induced Dzyalonshiinsky-Moriya interaction [11].
We also derive the dispersion relation for the limiting cases k=0 and k much larger than 1/R, where k is the wave vector of the spin wave and R the nanotube radius. For the first case, the mathematical formula of the dispersion relation resembles the well-known Kittel formula for the ferromagnetic resonance of a thin film with the in-plane magnetization parallel to the applied field, and both oriented perpendicularly to the in-plane easy axis of the shape anisotropy field. In the latter case the expression is identical to the exchange-dominated dispersion relation of a planar thin film in the Damon-Esbach configuration with the in-plane magnetization oriented perpendicularly to the in-plane easy axis.

Keywords: asymmetric dispersion; spin waves; DMI

  • Lecture (Conference)
    Intermag Dublin, 24.04.2017, Dublin, Ireland

Permalink: https://www.hzdr.de/publications/Publ-26867
Publ.-Id: 26867


Curvature-Induced Asymmetry of Spin-Wave Dispersion

Otalora, J. A.; Yan, M.; Lindner, J.; Schultheiss, H.; Fassbender, J.; Hertel, R.; Kakay, A.

We show using micromagnetic simulations and analytical calculations that spin-wave propagation in ferromagnetic nanotubes is fundamentally different than in flat thin films. The dispersion relation is asymmetric regarding the sign of the wave vector. This is a purely curvatureinduced effect and its fundamental origin is identified to be the classical dipole-dipole interaction. In certain cases the Damon-Eshbach modes in nanotubes behave as the volume-charge-free backward volume modes in flat thin films. Such non-reciprocal spin-wave propagation [1] is known for flat thin films with Dzyalonshiinsky-Moriya interaction (DMI), an antisymmetric exchange due to spin-orbit coupling.
The analytical expression of the dispersion relation has the same mathematical form as in flat thin films with DMI. The influence of curvature on spin waves is thus equivalent to an effective dipole-induced Dzyalonshiinsky-Moriya interaction [2].
[1] K. Zakeri, et. al., Phys. Rev. Lett. 104, 137203 (2010).
[2] J.A. Otálora, et. al., Phys. Rev. Lett. 117, 227203 (2016).

Keywords: spin waves; asymmetric dispersion; broken inversion symmetry

  • Lecture (Conference)
    DPG Dresden, 19.-24.03.2017, Dresden, Germany

Permalink: https://www.hzdr.de/publications/Publ-26866
Publ.-Id: 26866


EGFR Amplification, Tumor Microenvironment and Gene Expression as potential Biomarkers for EGFR-directed Radiotherapy in HNSCC Xenograft Models

Koi, L.; Linge, A.; Löck, S.; Thurow, C.; Baumann, M.; Krause, M.; Gurtner, K.

Fragestellung: Zur Etablierung neuer spezifischer Targets und Biomarker zur Evaluierung neuer Therapieansätze in Kombination mit der Strahlentherapie wurden insgesamt 10 verschiedene Plattenepithelkarzinomzelllinien des Kopf-/Halsbereiches in vivo und ex vivo untersucht. Die Ergebnisse der ersten 5 Tumormodelle wurden bereits publiziert (Gurtner et al., Radiother Oncol 2011 (99): 323–330). Die Wirkung einer Kombination von fraktionierter Bestrahlung und EGFR-Inhibition auf die lokale Tumorkontrolle wurde mit Parametern, wie z.B. EGFR-Amplifizierungsstatus und Tumormikromilieubedingungen, korreliert.
Methodik: Die Bestimmung der lokalen Tumorkontrolle 120 Tage nach Ende der Bestrahlung erfolgte nach alleiniger fraktionierter Strahlentherapie (30 f/6 Wo) oder nach simultaner Applikation des monoklonalen Antikörpers Cetuximab. Der Amplifikationsstatus wurde durch die Fluoreszenz-in situ-Hybridisierung (FISH) untersucht (EGFR-CEP-7 ratio) und mit dem Ansprechen auf eine EGFR-gerichtete Kombinationstherapie verglichen. Zusätzlich wurden unbehandelte und mit Cetuximab +/- Strahlentherapie behandelte Tumoren entnommen, die zur Untersuchung der Tumormikromilieueigenschaften (Hypoxie, Perfusion) sowie Genexpressionsanalysen verwendet wurden.
Ergebnisse: Von den insgesamt 10 untersuchten Tumormodellen wurde durch die Kombination mit dem molekularen Antikörper Cetuximab und fraktionierter Bestrahlung bei 6 Modellen eine signifikante Verbesserung der lokalen Tumorkontrolle im Vergleich zur alleinigen Bestrahlung erreicht. Bei drei dieser Responder konnte eine Amplifikation des EGF-Rezeptors nachgewiesen werden. Alle sechs Respondermodelle zeigten eine höhere Perfusionsrate im Vergleich zu den Tumormodellen, die nicht auf eine EGFR-gerichtete Therapie ansprachen. In Genexpressionsanalysen an unbehandelten Tumoren konnten signifikante Unterschiede sowohl zwischen Responder und Non-Responder als auch zwischen EGFR-amplifizierten und nicht EGFR-amplifizierten Tumoren nachgewiesen werden.
Schlussfolgerung: Tumoren mit einer guten Perfusion zeigten in diesen Experimenten ein besseres Ansprechen auf die EGFR-gerichtete Kombinationstherapie. Zudem scheinen neben der EGFR-Amplifikation auch spezielle Stammzellmarker eine vielversprechende Rolle bei der kombinierten Strahlentherapie zu spielen.

  • Open Access Logo Abstract in refereed journal
    Strahlentherapie und Onkologie 193(2017), S123-S124

Permalink: https://www.hzdr.de/publications/Publ-26864
Publ.-Id: 26864


Scaling laser plasma acceleration to the Petawatt level

Schramm, U.

Invited talk on laser plasma acceleration

Keywords: laser plasma accelerators

  • Invited lecture (Conferences)
    International Conference on Extreme Light (ICEL), 06.-09.11.2017, Szeged, Ungarn
  • Invited lecture (Conferences)
    IBS Conference on Laser Plasma Accelerators 2017 (LPAW), 28.08.-01.09.2017, Jeju Island, Korea

Permalink: https://www.hzdr.de/publications/Publ-26863
Publ.-Id: 26863


High-field study of UCo2Si2 : Magnetostriction at metamagnetic transition and influence of Fe substitution

Andreev, A. V.; Skourski, Y.; Gorbunov, D. I.; Prokes, K.

UCo2Si2 (tetragonal crystal structure) is antiferromagnet below TN = 83 K with ferromagnetic basal-plane layers of U magnetic moments oriented parallel to the c axis. The layers are coupled in +-+- sequence along this axis. In fields of 45 T applied along the c axis, UCo2Si2 exhibits very sharp metamagnetic transition to ++- uncompensated antiferromagnetic state. The transition is accompanied by pronounced magnetostriction effects. The crystal expands along the c axis by 1 * 10-4 and shrinks in the basal plane by 0.5 * 10-4 (at 1.5 K) resulting in negligible volume effect. Between 20 K and 40 K the transition changes from the first- to the second-order type. The Fe doping in UCo2Si2 reduces TN from 83 K to 80 K at x = 0.2 in U(Co1-xFe)2Si2. Metamagnetic transition shifts to higher fields (from 45 T at x = 0 - 56 T for x = 0.2). Magnetization jump over the transition remains practically the same which is in agreement with uranium magnetic moment determined by neutron diffraction on crystal with x = 0.1 as 1.29 μB, i.e. only slightly lower than that in UCo2Si2.

Permalink: https://www.hzdr.de/publications/Publ-26862
Publ.-Id: 26862


Transition from itinerant metamagnetism to ferromagnetism in UCo1-xOsxAl solid solutions

Andreev, A. V.; Sebek, J.; Shirasaki, K.; Danis, S.; Gorbunov, D. I.; Yamamura, T.; Vejpravova, J.; Havela, L.; de Boer, F. R.

The influence of substitution of a small amount of Os (< 2 %) on the Co sublattice on the magnetism of the itinerant metamagnet UCoAl is studied on single-crystalline UCo1-xOsxAl compounds with x = 0.002, 0.005 and 0.01. For x = 0.002, the ground state is still paramagnetic, like in UCoAl. The metamagnetic-transition field is 0.37 T, twice lower than in UCoAl. The compound with x = 0.005 is at the border between the paramagnetic and the ferromagnetic ground state. At T = 2 K, it is ferromagnetic, at elevated temperatures a magnetic field is necessary to maintain the magnetic state. In zero field, the ferromagnetic state vanishes at T = 8 K. The compound with x = 0.01 is a ferromagnet with strong uniaxial magnetic anisotropy similar to the previously studied compounds with x = 0.02-0.20.

Permalink: https://www.hzdr.de/publications/Publ-26861
Publ.-Id: 26861


Magnetisation process in the rare earth tetraborides, NdB4 and HoB4

Brunt, D.; Balakrishnan, G.; Mayoh, D. A.; Lees, M. R.; Gorbunov, D.; Qureshi, N.; Petrenko, O. A.

A field-induced magnetisation process in the frustrated antiferromagnets is often much richer compared to the materials without competing interactions. The applied field tends to stabilise unusual spin configurations which frequently results in the appearance of magnetisation plateaux. Here we report a study into the field-induced magnetisation of the two frustrated rare earth tetraborides, HoB4 and NdB4. NdB4 shows a fractional magnetisation plateau occurring at M/Msat ≈ 1/5 before saturating in a field of 33 kOe. On cooling down to 0.5 K the temperature dependent susceptibility of NdB4 shows an unconventional transition where the system returns to the zero field antiferromagnetic state from a higher-temperature ferrimagnetic state. We are able to reconstruct the magnetic phase diagram of NdB4 from the magnetisation, susceptibility and resistivity measurements for both H ‖ c and H ⊥ c. For HoB4, the most interesting behaviour is found at the lowest temperature of 0.5 K, where the field dependent magnetisation demonstrates a new fractional ½-magnetisation plateau. Further insight into the relations between the exchange interactions and single ion effects is gained through high-field magnetisation measurements in both HoB4 and NdB4.

Permalink: https://www.hzdr.de/publications/Publ-26860
Publ.-Id: 26860


Mechanical Properties of Metal Oxide Aerogels

Benad, A.; Jürries, F.; Vetter, B.; Klemmed, B.; Hübner, R.; Leyens, C.; Eychmüller, A.

In this study we report on mechanical properties of molded, single component Al2O3, Ga2O3, Fe2O3, and ZrO2 as well as mixed aerogels, made from yttrium stabilized zirconia, yttrium aluminum garnet, and zinc aluminum spinel. Initially all aerogels were produced equally in molded bodies by a facile epoxy method and were annealed afterward at 300 °C. Then we performed uniaxial pressure tests on cylindrical aerogel monoliths to gain Young's modulus which depends on composition, density, and post-treatment. Already pure aerogels like ZrO2 show well-promising Young's modulus of 10.7 MPa, whereas most popular SiO2 materials display a modulus between 2 and 3 MPa at comparable densities. Moreover we focused on Al2O3 aerogels which exhibit high stability and interesting densification behavior depending on the annealing temperature. On the basis of this observation, we combined the toughness of the Al2O3 scaffold with the extraordinary hardness of ZrO2, by adding up to 20 atom % Zr, to increase the specific Young's modulus. For the mixed material with a Zr content of 20 atom %, we reach a record value for compressible aerogels of 125 MPa mL g-1.

Permalink: https://www.hzdr.de/publications/Publ-26859
Publ.-Id: 26859


Magnetic properties of DyFe5-xCoxAl7: Suppression of exchange interactions and magnetocrystalline anisotropy by Co substitution

Gorbunov, D. I.; Andreev, A. V.; Neznakhin, D. S.; Henriques, M. S.; Sebek, J.; Skourski, Y.; Danis, S.; Wosnitza, J.

In 3d - 4f intermetallic compounds, a Co substitution for Fe usually strengthens the exchange interactions due to a shift of the Fermi energy to a region of the 3d band with higher density of states. Here, we study the influence of Co on the magnetism of ferrimagnetic DyFe5Al7 using magnetization measurements in static (up to 14 T) and pulsed (up to 58 T) magnetic fields. We find that the homogeneity range of DyFe5-xCoxAl7 is limited to x ≤ 2:5. The Co substitution for Fe produces a strong detrimental effect on the 3d - 3d intrasublattice exchange interactions, which leads to a pronounced decrease of the Curie temperature and of the 3d - 4f intersublattice exchange field. A reduction of the density of states at the Fermi level might occur due to a broadening of the 3d band. A decrease of the rare-earth contribution to the magnetic anisotropy energy is also observed with increasing Co content. This is attributed to a competition between the Fe and Co contributions to the crystal electric field at the Dy site.

Downloads:

  • Secondary publication expected

Permalink: https://www.hzdr.de/publications/Publ-26858
Publ.-Id: 26858


Research facility for radiobiological studies at the University Proton Therapy Dresden

Beyreuther, E.; Baumann, M.; Enghardt, W.; Helmbrecht, S.; Karsch, L.; Krause, M.; Pawelke, J.; Schreiner, L.; Schürer, M.; von Neubeck, C.

Purpose: In order to take full advantage of proton radiotherapy the biological effect of protons in normal and tumor tissue as well as the interaction with concomitant therapies should be investigated and understood in detail. Dedicated and systematic in vitro trials are needed to resolve the underlying mechanisms and processes that are necessary to prepare the translation into the clinics. For this purpose, a setup for radiobiological studies and the corresponding dosimetry should be established that enables in vitro experiments at a horizontal proton beam and, as a reference, a clinical 6 MV photon linear accelerator (Linac).
Methods and results: The experimental proton beam is characterized by high beam availability and reliability throughout the day in parallel to patient treatment. For cell irradiation, a homogeneous 10 × 10 cm² proton field with an optional spread-out Bragg-peak can be formed. A water-filled phantom was installed that allows for precise positioning of different cell sample geometries along the proton path. The depth-dose profiles within the phantom and the dose homogeneity over different cell samples were characterized for the proton beam and the photon reference source. A daily quality assurance protocol was implemented that provides absolute dose information required for significant and reproducible in vitro trials.
Conclusion: In the experimental room of the University Proton Therapy Dresden, clinically relevant conditions for proton in vitro experiments have been realized. The established cell phantom and dosimetry, which facilitate irradiation in an aqueous environment, could easily be transferred to other proton, photon or even ion accelerators. Precise positioning and easy exchange of cell samples, monitor unit-based dose delivery, and high beam availability allow for systematic in vitro trials. The close vicinity to the radiotherapy and radiobiology departments provides access to a clinical linacs as well as the interdisciplinary basis for further translational steps.

Permalink: https://www.hzdr.de/publications/Publ-26857
Publ.-Id: 26857


Irradiation of mouse brains at University Proton Therapy Dresden (UPTD): Treatment planning, dose verification and biological assessment of radiation damage

Müller, J.; Suckert, T.; Beyreuther, E.; Tillner, F.; Krause, M.; Enghardt, W.; Dietrich, A.; Bütof, R.; Lühr, A.; von Neubeck, C.

Proton therapy holds the potential to spare tumor-surrounding tissue and is therefore frequently used in brain tumor treatment. Clear indications from in vitro experiments show that the clinically applied relative biological effectiveness of 1.1 is higher towards the distal edge of the proton Bragg peak. Due to planning margins and range uncertainties the Bragg peak is typically placed in normal tissue and might increase neurological toxicities. Clinical evidence for radiation induced brain necrosis is yet scarce, nonetheless, needs critical evaluation. Sophisticated in vivo studies might be able to close this knowledge gap when designed to mirror the clinical exposure scenario.
Here, the setup for in vivo experiments together with the first results of mouse brain irradiation at the experimental beam line [1] of the UPTD will be presented. Absolute dosimetry in treatment position was done with ionization chambers and EBT3 radiochromic films. The anesthetized mice were fixed in an in-house developed multi-modality bed suitable for imaging and irradiation. Following CT for target delineation and proton radiography [2] for treatment positioning, mice were irradiated with a lateral collimated proton beam of 2 mm in diameter to the proximal hemisphere of the brain. Following radiation, mouse brains were excised and analyzed for DNA and tissue damage. Matching photon experiments with SAIGRT [3] are currently underway allowing in the future a direct comparison of treatment related side effects in the brain.
[1] Helmbrecht et al. J Instrum 2016
[2] Müller et al. Acta Oncologica 2017
[3] Tillner et al. Phys Med Biol 2016

  • Contribution to proceedings
    PTCOG 57 - 57th Annual Conference of the Particle Therapy Co-operative Group, 21.-26.05.2018, Cincinnati, USA

Permalink: https://www.hzdr.de/publications/Publ-26856
Publ.-Id: 26856


Establishment of small animal irradiation at University Proton Therapy Dresden

Müller, J.; Beyreuther, E.; Suckert, T.; Neubert, C.; Karsch, L.; von Neubeck, C.; Pawelke, J.; Schürer, M.; Krause, M.; Lühr, A.

Introduction: It is a common practice to use a fixed relative biological effectiveness (RBE) of 1.1 when planning treatments and analyzing outcomes for proton therapy. In contrast, a multitude of in vitro experiments demonstrate variable RBE values. Also, some clinical evidence of RBE variability is emerging, especially at the distal edge of proton treatment fields, showing increased risk of normal tissue complications. However, only a limited number of in vivo trials have been performed to confirm such results. This contribution presents an irradiation setup to study adverse effects in mouse brains induced at proton field edges.
Methods: The mouse is fixated (teeth, ears) in a closed sterile 3D printed holder specifically designed for CT and MR imaging as well as for irradiation with X-rays and protons. Target delineation based on CT and MR imaging can be performed before irradiation. Image-guided positioning of the target volume is achieved by proton radiography [1] with the mouse in treatment position.
In a first brain toxicity study, the distal edge of a laterally collimated clinical proton field (150 MeV) will be positioned in the proximal hemisphere of the mouse brain by inserting polycarbonate plates in front of the mouse holder. For different beam settings, dose distributions in treatment position were obtained with radiochromic EBT3 films placed in plastic phantoms within the mouse holder. Variation of the proton beam range and lateral shape with the amount of decelerating material and collimator size, respectively, were analyzed and used to build a proton beam model. The beam intensity, measured with an ionization chamber, was correlated with the EBT3 film dose measurements at treatment position. This allows for a controlled irradiation of the brain volume with predefined and absolute dose values.
Results and Conclusion: All requirements for systematic proton irradiation experiments in vivo are established at the University Proton Therapy Dresden including target volume delineation, mouse positioning, and dosimetry. First experiments comparing brain toxicity after proton irradiation of one hemisphere relative to photon treatment are in progress.
Reference: [1] Müller et al. Acta Oncologica 2017

  • Lecture (Conference)
    Fourth Symposium on Precision Image-Guided Small Animal RadioTherapy, 12.-14.03.2018, Lisbon, Portugal

Permalink: https://www.hzdr.de/publications/Publ-26855
Publ.-Id: 26855


Ion beam based methods for materials research and investigation of pipe organ metallic materials

Skorupa, W.; Pelic, B.; Werner, H.; Eule, D.

Pipe organ instruments contain mostly a considerable number of metallic pipes (flute and reed types), which are sometimes prone to heavy corrosion attack, resulting finally in a loss of their voice. Under certain conditions, the atmospheric corrosion of reed pipe tongues as well as flute pipe foots consisting of Cu-Zn alloys (brass) and PbSn-based alloys, respectively, is strongly enhanced by traces of volatile organic compounds (especially acetic acid vapor) and other corrosive gases.
Experiments have been undertaken to explore the corrosion resistance of CuZn and PbSn-based alloys against vapour from an aqueous solution containing high acetic acid concentration (2 – 5 v/v%), by deposition of protective films of either Al2O3 or Al on the nanoscale using pulsed laser deposition (PLD) and magnetron sputtering (MS). Afterwards, in order to improve the adhesion between the deposited layer and the substrate as well as to perform a kind of nitridation of the coatings, the samples were implanted with nitrogen ions using the plasma immersion ion implantation (PI3) process. Such a nanoscale coating (~50 nm) is then able to withstand stresses and vibrations due to sound generation in organ pipes. Moreover it produces a barrier to volatile organic acids and water vapour. The laboratory corrosion test of the applied protective treatment for lead-tin and brass samples were combined with the field work studies to approach the best conditions for the samples research in real environment.
Moreover, ion beam analysis with the Rossendorf external beam facility was used to determine corrosion products on extremely valuable organ pipes from the early 18th century of the famous organ builder Gottfried Silbermann.

Keywords: Ion beam based methods; pipe organ; brass; lead-tin-alloys; antocorrosive protection; plasma immersion ion implantation; nitridation; pulsed laser deposition; magnetron sputtering

  • Lecture (Conference)
    20th International Conference on Surface Modification of Materials by Ion Beams, 09.-14.07.2017, Lisboa, Portugal

Permalink: https://www.hzdr.de/publications/Publ-26854
Publ.-Id: 26854


New approaches for studying radiobiological effects of kilovoltage X-rays in vivo and in vitro

Hunger, A.; Burger, K.; Porth, A.-K.; Dierolf, M.; Günther, B.; Bartzsch, S.; Achterhold, K.; Gleich, B.; Beyreuther, E.; Pfeiffer, F.; Combs, S. E.; Wilkens, J. J.; Schmid, T. E.

Introduction: X-ray microbeam radiation therapy (MRT) as a novel tumor treatment strategy deposits high doses in spatially fractionated X-ray beamlets promising reduced normal tissue toxicity, compared to conventional irradiation, and a better tumor control. Radiobiological studies of MRT with kilovoltage X-rays are mainly performed at synchrotron radiation facilities with high costs and space requirements. The Munich Compact Light Source (MuCLS) is a laboratory-sized and cost-effective source based on inverse Compton scattering of infrared laser photons [1]. Currently the most widely accepted method for assessment of treatment efficiencies is tumor growth delay with subcutaneous tumors in the hind leg of small animals. However, a new model is required for MRT with kilovoltage X-ray beams which only allow for short penetration depths. Therefore, we successfully developed a setup for a growth delay study in a tumor-bearing mouse ear model for investigation of MRT at the MuCLS. In addition, we successfully established a protocol to isolate tumor cells from irradiated tumors for evaluation of radiobiological effect on cellular level.
Materials & Methods: The dose rate of the MuCLS was improved with the installation of a polycapillary collimation optic. A W-Air collimator was inserted to get a collimated X-ray beam with 50 μm wide microbeams and a center-to-center distance of 350 μm. We implemented the mouse ear tumor model with a human head and neck cancer cell line FaDu [2] suspended in extracellular matrix and subcutaneously injected into the right ear of NMRI (nu/nu) mice. After reaching a size of 2x2 mm2 tumors were irradiated using doses of either 3 and 5 Gy with 25 keV X-rays at the MuCLS. Tumor growth delay was determined with a caliper over a follow-up period of 30 days and compared between MRT, homogeneous and control mice. Animals were sacrificed when tumors reached the 15-fold initial volume. A single tumor cell suspension was prepared from excised tumors for in-vitro studies. The analysis of radiosensitivity by colony formation assay and stable chromosomal aberrations by two-color fluorescence in-situ hybridization is still on-going.
Results: We successfully installed a setup at the MuCLS which allows irradiation of tumors in small animals and implemented a xenograft tumor model in mouse ears. Homogeneously irradiated tumors showed a growth delay at 5 Gy compared to control mice. There was no tumor growth delay after MRT and homogeneous irradiation at 3 Gy. Tumor cells from irradiated tumors were successfully isolated and cultured. Preliminary data shows an increased radiosensitivity of tumor cells originating from homogeneously and MRT-irradiated tumors compared to control tumor cells.
Conclusion: This innovative approach allowed the irradiation of tumors in a mouse ear model at a novel laser-based X-ray source, the MuCLS. Homogeneous irradiation at MuCLS induced a tumor growth delay at 5 Gy. In addition, we successfully validated a protocol for tumor cell isolation for investigations of radiation-induced effects.
Supported by the DFG Cluster of Excellence: Munich-Centre for Advanced Photonics.
References:
[1] Eggl et al., J. Synchrotron Rad. (2016) 23: 1137
[2] Beyreuther et al., PLos One (2017)

  • Lecture (Conference)
    Fourth Symposium on Precision Image-Guided Small Animal RadioTherapy, 12.-14.03.2018, Lisbon, Portugal

Permalink: https://www.hzdr.de/publications/Publ-26853
Publ.-Id: 26853


Nanoscale anticorrosive protection of pipe organ metallic materials

Skorupa, W.; Pelic, B.; Werner, H.; Eule, D.

Historical pipe organs with their unique sound and beautiful housing are important objects of the European cultural heritage dating back to the 15th century for the oldest ones being playable yet. But new instruments are built permanently up to the present time. The instruments contain mostly a considerable number of metallic pipes (flute and reed types), which are sometimes prone to heavy corrosion attack from their environment, resulting finally in a loss of their voice. Under certain conditions, the atmospheric corrosion of reed pipe tongues as well as flute pipe foots consisting of Cu-Zn alloys (brass) and PbSn-based alloys, respectively, is strongly enhanced by traces of volatile organic compounds (especially acetic acid vapor) and other corrosive gases.
Investigations have been undertaken to explore the corrosion resistance of CuZn and PbSn-based alloys against vapour from an aqueous solution containing high acetic acid concentration (2 – 5 v/v%), by deposition of protective films of either Al2O3 or Al on the nanoscale using pulsed laser deposition (PLD) and magnetron sputtering (MS). Afterwards, in order to improve the adhesion between the deposited layer and the substrate as well as to perform a kind of nitridation of the coatings, the samples were implanted with nitrogen ions using the plasma immersion ion implantation (PI3) process. Such a nanoscale coating (~50 nm) is then able to withstand stresses and vibrations due to sound generation in organ pipes. Moreover it produces a barrier to volatile organic acids and water vapour. The laboratory corrosion test of the applied protective treatment for lead-tin and brass samples were combined with the field work studies to approach the best conditions for the samples research in real environment. Some of the samples were exposed for 15 months in a small North-German church with a harmful (corrosive) indoor environment.
Modification and analysis of the surface of metals and thin film properties on the nanoscale using fundamental phenomena based on ion-solid interactions as well as standard conventional methods can create new technological applications in restoration and conservation to protect our historical and modern cultural heritage in regard to environmental attacks.

Keywords: nanoscale coating; anticorrosive protection; historical organs; pulsed laser deposition; magnetron sputtering; acetic acid; plasma immersion ion implantation; cultural heritage

  • Lecture (Conference)
    3rd International Conference NANOAPP (Nanomaterials & Application), 14.-18.06.2017, Bled, Slovenia

Permalink: https://www.hzdr.de/publications/Publ-26852
Publ.-Id: 26852


Infrared nanoscopy on Si-doped GaAs-InGaAs core-shell nanowires

Lang, D.; Balaghi, L.; Dimakis, E.; Hübner, R.; Kehr, S. C.; Eng, L. M.; Winnerl, S.; Schneider, H.; Helm, M.

We present nanoscopic studies on MBE-grown GaAs-InGaAs core-shell nanowires (NWs) with various Si-doped shells. For higher dopings and charge carrier densities, the plasmonic resonance shifts into the mid-infrared wavelength range, that can be fully probed using IR radiation from the FELBE free-electron laser source at the Helmholtz-Center Dresden-Rossendorf. Exploring these plasmonic resonance peaks at different wavelengths allows mapping the local charge carrier density and distribution along the NW with a high spatial resolution of better than 50 nm. Preliminary results using a CO2 laser scanned from 9.7-11.4 μm indicated the resonant behavior for the highest shell doping, in clear contrast to lower or undoped NWs. In the resonant case, the near-field (NF) emerging from the NW is strongly increased as compared to the substrate, in accordance with theory. The NF profiles of particular NWs show characteristic modifications at different wavelengths, which indicate an inhomogeneous distribution of the charge carrier density.

Keywords: FEL; mid-infrared; nanowires; near-field microscopy; plasmonics

  • Poster
    WIRMS 2017 - 9th International Workshop on Infrared Microscopy and Spectroscopy with Accelerator Based Sources, 25.-28.09.2017, Oxford, United Kingdom
  • Lecture (Conference)
    DPG-Frühjahrstagung, 11.-16.03.2018, Berlin, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-26851
Publ.-Id: 26851


Low-temperature scattering scanning near-field optical micropscopy (LT-s-SNOM)

Lang, D.; Döring, J.; Kuschewski, F.; Kehr, S. C.; Eng, L. M.; Winnerl, S.; Schneider, H.; Helm, M.

We present a combination of a versatile low-temperature scattering-type near-field optical microscope (LT-s-SNOM [1]) with a tunable infrared free-electron laser (FEL [2]). Our s-SNOM operates over a broad temperature range from 15 - 300 K [1,3,4] and is unique in being tunable over a broad frequency range, thanks to the FEL. The overall LT-s-SNOM functionality down to lowest temperature was tested on both standard Au and structured Si-SiO2 samples, revealing net near-field contrasts and no topography cross-talk. Secondly, we investigated several ferroelectric phase transitions in barium titanate single crystals at 273 K [1] and 193 K [5], allowing to associate clear near-field resonances to every phase and each ferroelectric domain; here, the clear benefit of our LT-s- SNOM pays off, being able to record s-SNOM, PFM, KPFM and topographic data with one and the same tip from every sample surface spot. Thirdly, we used these piezoelectrics to quantify the local temperature increase under the AFM tip upon IR irradiation. [1] Döring et al., Appl. Phys. Lett. 105, 053109 (2014). [2] Kuschewski et al., Appl. Phys. Lett. 108, 113102 (2016). [3] Yang et al., Rev. Sci. Instrum. 84, 023701 (2013). [4] McLeod et al., Nature Phys. (2016); DOI: 10.1038/NPHYS3882. [5] Döring et al., J. Appl. Phys. 120, 084103 (2016).

Keywords: FEL; near-field micropscopy; mid-infrared; phase transition; low temperature

  • Poster
    DPG-Frühjahrstagung der Sektion Kondensierte Materie, 19.-24.03.2017, Dresden, Deutschland
  • Poster
    OTST2017 - Optical Terahertz Science and Technology, 02.-07.04.2017, London, United Kingdom

Permalink: https://www.hzdr.de/publications/Publ-26850
Publ.-Id: 26850


WP4 : Highly selective metal recovery techniques for complex metal mixtures including by-product and critical metals

Scharf, C.; Kelly, N.; Helbig, T.

WP4 : Highly selective metal recovery techniques for complex metal mixtures including by-product and critical metals

  • Lecture (others)
    CHROMIC – M6 Meeting, 10.-11.05.2017, Freiberg, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-26849
Publ.-Id: 26849


GA - WP4 meeting (M12)

Scharf, C.; Kelly, N.

GA - WP4 meeting (M12)

  • Lecture (others)
    CHROMIC M12 General Assembly Meeting, 22.-23.11.2017, Orléans, Frankreich

Permalink: https://www.hzdr.de/publications/Publ-26848
Publ.-Id: 26848


Helmholtz Institute Freiberg for Resource Technology - FWGM - Metallurgy & Recycling

Scharf, C.; Kelly, N.

Helmholtz Institute Freiberg for Resource Technology FWGM - Metallurgy & Recycling

  • Lecture (others)

Permalink: https://www.hzdr.de/publications/Publ-26847
Publ.-Id: 26847


Exploring the Metabolism of (+)-[18F]Flubatine in vitro and in vivo: LC-MS/MS aided Identification of Radiometabolites in a Clinical PET Study

Ludwig, F.-A.; Fischer, S.; Smits, R.; Deuther-Conrad, W.; Hoepping, A.; Tiepolt, S.; Patt, M.; Sabri, O.; Brust, P.

Both (+)-[18F]flubatine and its enantiomer (-)-[18F]flubatine are radioligands for the neuroimaging of a4ß2 nicotinic acetylcholine receptors (nAChRs) by positron emission tomography (PET). Within a clinical study in patients with early Alzheimer’s disease, (+)-[18F]flubatine ((+)-[18F]1) was examined regarding its metabolic fate, in particular by identification of degradation products detected in plasma and urine. The investigations included an in vivo study of (+)-flubatine ((+)-1) in pig and structural elucidation of formed metabolites by LC-MS/MS. Incubations of (+)-1 and (+)-[18F]1 with human liver microsomes were performed to generate in vitro metabolites as well as radiometabolites, which enabled an assignment of their structures by comparison of LC-MS/MS and radio-HPLC data. Plasma and urine samples taken after administration of (+)-[18F]1 into human were examined by radio-HPLC and, on the basis of results obtained in vitro and in vivo, formed radiometabolites were identified.
In pig, (+)-1 was monohydroxylated at different sites of the azabicyclic ring system of the molecule. Additionally, one intermediate metabolite underwent glucuronidation, as also demonstrated in vitro. In human, 95.9 ? 1.9% (N = 10) of unchanged tracer remained in plasma, 30 min after injection. However, despite the low metabolic degradation, both radiometabolites formed could be characterized as i.) a product of C-hydroxylation at the azabicyclic ring system and ii.) a glucuronide conjugate of previously N8-hydroxylated (+)-[18F]1.

Keywords: [18F]flubatine; NCFHEB; [18F]FLBT; radiometabolites; glucuronides; liquid chromatrography-tandem mass spectrometry (LC-MS/MS); liver microsomes; positron emission tomography (PET); nicotinic acetylcholine receptors (nAChRs)

Permalink: https://www.hzdr.de/publications/Publ-26846
Publ.-Id: 26846


A spectroscopic and computational study of Cm3+ incorporation in lanthanide phosphate rhabdophane (LnPO4·0.67H2O) and monazite (LnPO4)

Huittinen, N.; Scheinost, A. C.; Ji, Y.; Kowalski, P. M.; Arinicheva, Y.; Wilden, A.; Neumeier, S.; Stumpf, T.

This study investigates the incorporation of the minor actinide curium (Cm3+) in a series of synthetic La1-xGdxPO4 (x = 0, 0.24, 0.54, 0.83, 1) monazite and rhabdophane solid solutions. To obtain information of the incorporation process on the molecular scale and to understand the distribution of the dopant in the synthetic phosphate phases, combined time-resolved laser fluorescence spectroscopy (TRLFS) and x-ray absorption fine structure (XAFS) spectroscopy investigations have been conducted and complemented with ab initio atomistic simulations. We found that Cm3+ is incorporated in the monazite endmembers (LaPO4 and GdPO4) on one specific, highly ordered lattice site. The intermediate solid solutions, however, display increasing disorder around the Cm3+ dopant as a result of random variations in nearest neighbor distances. In hydrated rhabdophane, and especially its La-rich solid solutions, Cm3+ is preferentially incorporated on non-hydrated lattice sites. This site occupancy is not in agreement with the hydrated rhabdophane structure, where two thirds of the lattice sites are associated with water of hydration (LnPO4·0.67H2O), implying that structural substitution reactions cannot be predicted based on the structure of the host matrix only.

Keywords: Cm3+; incorporation; monazite; rhabdophane; spectroscopy; ab initio simulations

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Permalink: https://www.hzdr.de/publications/Publ-26844
Publ.-Id: 26844


Thermodynamic evaluations using the law of mass action under consideration of the activity coefficients in the system NdCl₃-HCl (or NaOH)-H₂O-DEHPA-kerosene

Scharf, C.; Ditze, A.

Rare earth elements, including neodymium, are in widespread use today. They serve, for example, as alloying elements in magnesium (WE 43, WE 54, AE44, AE42), in permanent magnets (neodymium-iron-boron) or as luminescent materials. In addition, they are amongst the most important commodities in Europe. The demand for their use is growing and the recycling of these elements is indispensable. Their recycling potential can be increased through detailed scientific studies and to this end this article presents a thermodynamic evaluation of equilibrium data in the system neodymium-chloride-hydrochloric acid (or sodium hydroxide)-water-di-(2-ethylhexyl) phosphoric acid (DEHPA)-kerosene. Considering the relationship between the activity coefficients γ(Nd³⁺(aq)), γ(Nd(org)) and γ((DEHPA)₂), which arise from the law of mass action, the deviations of the experimental results from the ideal behaviour can be explained. From the calculation of γ(Nd³⁺(aq)) with the expanded Debye-Hückel equation and from literature data for γ((DEHPA)₂), indications arise for the development of the functions of the activity coefficients of DEHPA and neodymium in the organic phase.

Keywords: Law of mass action; Thermodynamic evaluation

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Permalink: https://www.hzdr.de/publications/Publ-26843
Publ.-Id: 26843


Spectroscopic investigations of actinide incorporation in LnPO4 ceramic solid solutions

Huittinen, N.

Introduction to luminescence spectroscopy will be presented using Eu3+ and Cm3+ as examples. Examples of how luminescence spectroscopy can be used to probe the local environment of lanthanide and actinide doped monazite ceramics will be given.

Keywords: incorporation; luminescence spectroscopy; lanthanides; actinides; monazite ceramics

  • Invited lecture (Conferences)
    ThUL School 2017, 16.10.2017, Forschungszentrum Jülich, Germany

Permalink: https://www.hzdr.de/publications/Publ-26842
Publ.-Id: 26842


Environmental radioactivity studies in Kabul and northern Afghanistan

Tanha, M. R.; Ikeda-Ohno, A.; Schulze, M.; Khalid, F. R.; Storai, M. A.; Walther, C.

From earlier surveys conducted by soviet researchers, the Kabul area was identified as a region of high natural radioactivity. However, only fragmentary maps on dose rates (often only given in relative units) are available. No detailed information of, e.g., uranium and thorium distributions in the upper soil and rock exists. In recent years, residential houses have been built in some of these places known for their elevated radiation dose rate. In order to assess possible radiological risk, 51 soil and rock samples as well as 51 all-purpose water samples were collected and measured with regard to radioisotope content and contamination by other pollutants such as, e.g. heavy metals. For the rocks and soil samples, gamma spectroscopy was used as main technique, while ICP-MS and ICP-OES was used as main technique for water analysis. Furthermore, alpha spectroscopy, μ-XRF, PXRD, TOF-SIMS and LSC were used to verify the gamma spectroscopy and ICP-MS results. Activity concentrations in soil and rocks ranged between 160 to 28600 Bq/kg, 73 to 383000Bq/kg, and 270 to 24600 Bq/kg for uranium, thorium, and potassium, respectively. While none of the samples showed any anomalies of the radioactive equilibria some of the samples contained remarkably high levels of thorium and uranium (and their daughter nuclides). Thorium was bound in a cheralite mineral structure. Not all of the investigated waters are safe for drinking, exceeding the national and international recommended values.

Keywords: Radioecology; environmental radioactivity; natural samples; soils; rocks; drinking water

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Permalink: https://www.hzdr.de/publications/Publ-26841
Publ.-Id: 26841


Magnetorotational instability in Taylor-Couette flows between cylinders with finite electrical conductivity

Rüdiger, G.; Schultz, M.; Stefani, F.; Hollerbach, R.

The nonaxisymmetric azimuthal magnetorotational instability is studied for hydromagnetic Taylor-Couette flows between cylinders of finite electrical conductivity. We find that the magnetic Prandtl number Pm determines whether perfectly conducting or insulating boundary conditions lead to lower Hartmann numbers for the onset of instability. Regardless of the imposed rotation profile, for small Pm the solutions for perfectly conducting cylinders become unstable for weaker magnetic fields than the solutions for insulating cylinders. The critical Hartmann and Reynolds numbers form monotonic functions of the ratio sigma of the electrical conductivities of the cylinders and the fluid, such that sigma = O(10) provides a very good approximation to perfectly conducting cylinders, and sigma = O(0.1) a very good approximation to insulating cylinders. These results are of particular relevance for the super-rotating case where the outer cylinder rotates faster than the inner one; in this case the critical onset values are substantially different for perfectly conducting versus insulating boundary conditions. An experimental realization of the super-rotating instability, with liquid sodium as the fluid and cylinders made of copper, would require an electric current of at least 33.5 kA running along the central axis.

Keywords: astrophysical fluid dynamics; Taylor-Couette flow

Permalink: https://www.hzdr.de/publications/Publ-26840
Publ.-Id: 26840


Nanopatterning during ion sputtering: The reverse epitaxy mechanism on crystalline surfaces

Erb, D.; Malsch, G.; Engler, M.; Ou, X.; Facsko, S.

Normal-incidence low energy ion irradiation is known to amorphize and smoothen semiconductor surfaces via ballistic redistribution of atoms by the impacting ions [1]. However, intriguing nanoscale surface patterns can form spontaneously, if the semiconductor substrate is heated above its recrystallization temperature during ion irradiation [2].

Above the recrystallization temperature the surface remains crystalline even under ion irradiation: On the one hand, bulk defects are annealed instantly. On the other hand, on a crystalline surface the diffusing surface vacancies and ad-atoms encounter the Ehrlich-Schwoebel barrier, the energy barrier for crossing terrace steps. In analogy to epitaxial growth, this can lead to the formation of well-defined faceted surface structures for ion irradiations performed in a specific energy and temperature range [2]. The resulting surface morphology is strongly dependent on the crystalline structure and surface orientation of the semiconductor substrate. For instance, Ge(001) exhibits inverse pyramids with a square base and Ge(111) shows inverse pyramids with a threefold symmetry, while GaAs(001) and InAs(001) develop regular ripple structures with a saw tooth profile [3]. The periodicity and the regularity of the surface pattern can be influenced by external process parameters such as substrate temperature, ion energy, and ion fluence.

In this contribution, we outline the reverse epitaxy mechanism, highlight the diversity of resulting surface patterns, and present possible applications, for instance in templated metal nanostructure growth or in the fabrication of semiconductor nanostructures.

[1] C S Madi et al., Physical Review Letters 106, 066101 (2011)
[2] X Ou et al., Physical Review Letters 111, 016101 (2013)
[3] X Ou et al., Nanoscale 7, 18928 (2015)

  • Poster
    IISC, 17.-22.09.2017, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-26839
Publ.-Id: 26839


Bottom-up fabrication of periodic nanostructure arrays based on reverse epitaxy

Erb, D.; Ou, X.; Dimakis, E.; Hübner, R.; Lenz, K.; Schlage, K.; Röhlsberger, R.; Lindner, J.; Facsko, S.

Potential technological applications of periodic nanostructure arrays range from photovoltaics [1] to biomolecule detection using plasmonic signal enhancement [2] and information technology based on magnonic crystals [3]. Industrial-scale fabrication of such devices requires nanopatterning processes which are cost-effective, scalable, and highly reproducible. These demands can be met by a versatile bottom-up approach based on ion irradiation of semiconductor surfaces and well-established thin film deposition techniques.
Reverse epitaxy, i.e. the self-assembly of vacancies and ad-atoms under ion irradiation, leads to nanoscale surface patterning with well-defined lateral periodicity on semiconductor substrates [4]. Among these, GaAs(001) and InAs(001) surfaces exhibit regular faceting and thus lend themselves to transferring this pattern regularity to other materials. The nanofaceted surfaces can for instance be employed as substrates for molecular beam epitaxy under grazing incidence, producing periodic arrays of nanowires, periodically corrugated thin films, or combinations thereof by geometrical shading. They can also induce long-range chemical ordering in diblock copolymer thin films, which may then serve as highly ordered chemical templates for metal nanostructure growth in a variety of pattern morphologies [5]. Furthermore, separated semiconductor nanostructures can be fabricated by introducing an interlayer before ion irradiation.
In this contribution, we outline the reverse epitaxy mechanism and present examples of how it can be employed in the fabrication of large-area nanostructure arrays. We hope to stimulate discussion of further applications by emphasizing the simplicity and versatility of this bottom-up approach.
[1] H.A. Atwater and A. Polman, Nature Materials 9 (2010)
[2] J. Vogt et al., Phys. Chem. Chem. Phys. 17 (2015)
[3] D. Grundler, Nature Physics 11 (2015)
[4] X. Ou et al., Nanoscale 7 (2015)
[5] D. Erb et al., Science Advances 1 (2015)

  • Poster
    SMMIB17 - International Conference on Surface Modification of Materials by Ion Beams, 09.-14.07.2017, Lisboa, Portugal

Permalink: https://www.hzdr.de/publications/Publ-26838
Publ.-Id: 26838


Ion-induced nanopatterning of GaAs and InAs (001) surfaces and possible applications in bottom-up nanostructure fabrication

Erb, D.; Malsch, G.; Engler, M.; Ou, X.; Facsko, S.

Periodic nanostructure arrays are sought-after for advanced photovoltaics, high-sensitivity biomolecule detection, and future information technology. One cost-effective bottom-up approach to fabricate such nanostructure arrays is templated growth on spontaneously nanopatterned surfaces, which can be achieved on semiconductors by low energy ion irradiation [1]. We studied the influence of process parameters such as ion energy and fluence, substrate temperature, and ion incidence angles on the resulting nanoscale morphologies of GaAs and InAs surfaces.

If the semiconductor surface is irradiated with low-energy ions above the recrystallization temperature of the material, the surface remains crystalline. In the so-called reverse epitaxy regime, the diffusion of the ion-induced vacancies and ad-atoms on the crystalline surface is subject to the Ehrlich-Schwoebel barrier, an energy barrier for crossing terrace steps. In analogy to epitaxial growth, for ion irradiations performed in a specific energy and temperature range this can lead to the formation of well-defined faceted surface structures [2]. The resulting surface morphology is strongly dependent on the crystalline structure of the semiconductor substrate. For instance, GaAs(001) and InAs(001) surfaces exhibit regular ripple structures with a saw tooth profile oriented along the [1-10] direction. For this pattern formation to take place, InAs must be kept in a temperature range between 160°C and 430 °C, while GaAs requires sample temperatures of at least 430 °C. Increasing the ion energy increases the ripple periodicity, and so does increasing the sample temperature at lower ion energies. The order of the pattern increases with increasing ion fluence and, for InAs, with increasing ion energy.

Such nanorippled surfaces can for example be employed as substrates for physical vapor deposition under various incidence angles, producing periodic arrays of nanowires, periodically corrugated thin films, or combinations thereof. Furthermore, epitaxial growth is expected on these crystalline surfaces for materials with low lattice mismatch.


[1] A. Keller and S. Facsko, Materials 3, 4811 (2010).
[2] X. Ou et al., Nanoscale 7 (2015).

  • Poster
    Nanopatterning / FOR3Nano Workshop, 26.-30.06.2017, Helsinki, Finnland

Permalink: https://www.hzdr.de/publications/Publ-26837
Publ.-Id: 26837


Bottom-up fabrication of periodic magnetic nanostructures based on ion-induced spontaneous surface nanopatterning

Erb, D.; Ou, X.; Schlage, K.; Lenz, K.; Röhlsberger, R.; Lindner, J.; Facsko, S.; Helm, M.; Fassbender, J.

Large-area nanopatterning is a key requirement in diverse applications ranging from photovoltaics to computing and biomolecule detection. We present a simple and scalable bottom-up nanopatterning approach based on ion irradiation of semiconductor surfaces and wellestablished thin film deposition techniques: On crystalline semiconductor substrates, nanoscale surface patterns with well-defined lateral periodicity form via the mechanism of reverse epitaxy, i.e. the nonequilibrium self-assembly of vacancies and ad-atoms under ion irradiation [1]. The nanopatterned surfaces can for instance be employed as substrates for MBE under grazing incidence, producing periodic metal nanostructures by geometrical shading. They can also be the basis for metal nanostructure growth in a variety of pattern morphologies by hierarchical self-assembly [2]. In this contribution, we outline the reverse epitaxy mechanism and present examples of periodic magnetic nanostructures based on the resulting surface patterns. We hope to stimulate discussion of further applications in magnetism by emphasizing the simplicity and versatility of this bottom-up approach.
[1] Ou et al., Nanoscale 7 (2015); [2] Erb et al., Science Advances 1 (2015)

  • Poster
    DPG Frühjahrstagung Sektion Kondensierte Materie, 19.-24.03.2017, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-26836
Publ.-Id: 26836


Bottom-up fabrication of periodic nanostructure arrays based on reverse epitaxy

Erb, D.; Ou, X.; Dimakis, E.; Hübner, R.; Lenz, K.; Schlage, K.; Röhlsberger, R.; Lindner, J.; Facsko, S.

Potential technological applications of periodic nanostructure arrays range from photovoltaics augmented by light trapping [1] to high-sensitivity biomolecule detection using plasmonic signal enhancement [2] and high-speed low-energy information technology based on magnonic crystals [3]. Industrial-scale fabrication of such devices requires nanopatterning processes which are cost-effective, scalable, and highly reproducible. These demands can be met by a versatile bottom-up approach based on ion irradiation of semiconductor surfaces and well-established thin film deposition techniques.
Reverse epitaxy, i.e. the non-equilibrium self-assembly of vacancies and ad-atoms under ion irradiation, leads to nanoscale surface patterning with well-defined lateral periodicity on semiconductor substrates [4]. The GaAs(001) and InAs(001) surfaces exhibit highly regular faceting and therefore lend themselves to transferring this pattern regularity to other materials. The nanofaceted surfaces with a sawtooth profile can for instance be employed as substrates for molecular beam epitaxy under grazing incidence, producing periodic arrays of nanowires, periodically corrugated thin films, or combinations thereof by geometrical shading. They can also induce long-range chemical ordering in diblock copolymer thin films, which may then serve as highly ordered chemical templates for metal nanostructure growth in a variety of pattern morphologies [5]. Furthermore, separated semiconductor nanostructures can be fabricated by introducing an interlayer before ion irradiation.

In this contribution, we outline the reverse epitaxy mechanism and present examples of how it can be employed in the fabrication of large-area nanostructure arrays. We hope to stimulate discussion of further applications by emphasizing the simplicity and versatility of this bottom-up approach.

[1] H.A. Atwater and A. Polman, Nature Materials 9 (2010); [2] J. Vogt et al., Phys. Chem. Chem. Phys. 17 (2015); [3] D. Grundler, Nature Physics 11 (2015); [4] X. Ou et al., Nanoscale 7 (2015); [5] D. Erb et al., Science Advances 1 (2015)

  • Poster
    Ionentagung 2017, 12.-15.02.2017, Göttingen, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-26835
Publ.-Id: 26835


Inter-patient variations of radiation-induced normal-tissue changes in Gd-EOB-DTPA-enhanced hepatic MRI scans during fractionated proton therapy

Richter, C.; Andronesi, O.; Borra, R.; Voigt, F.; Löck, S.; Duda, D.; Guimaraes, A.; Hong, T.; Bortfeld, T.; Seco, J.

Background and purpose
Previous MRI studies have shown a substantial decrease in normal-tissue uptake of a hepatobiliary-directed contrast agent 6–9 weeks after liver irradiation. In this prospective clinical study, we investigated whether this effect is detectable during the course of proton therapy.

Material and methods
Gd-EOB-DTPA enhanced MRI was performed twice during hypo-fractionated proton therapy of liver lesions in 9 patients (plus two patients with only one scan available). Dose-correlated signal changes were qualitatively scored based on difference images from the two scans. We evaluated the correlation between the MRI signal change with the planned dose map. The GTV was excluded from all analyses. In addition, were examined timing, irradiated liver volume, changes in liver function parameters as well as circulating biomarkers of inflammation.

Results
Strong, moderate or no dose-correlated signal changes were detected for 2, 3 and 5 patients, respectively. Qualitative scoring was consistent with the quantitative dose to signal change correlation. In an exploratory analysis, the strongest correlation was found between the qualitative scoring and pretreatment IL-6 concentration. For all patients, a clear dose-correlated signal decrease was seen in late follow-up scans.

Conclusion
Radiation-induced effects can be detected with Gd-EOB-DTPA enhanced MRI in a subgroup of patients within a few days after proton irradiation. The reason for the large inter-patient variations is not yet understood and will require validation in larger studies.

Keywords: Gd-EOB-DTPA; liver; MRI; radiation-induced changes; inflammatory response; in vivo verification

Permalink: https://www.hzdr.de/publications/Publ-26834
Publ.-Id: 26834


Evaluation of interfacial heat transfer models for flashing flow with two-fluid cfd

Liao, Y.; Lucas, D.

The complexity of flashing flows is increased vastly by the interphase heat transfer as well as its coupling with mass and momentum transfers. A reliable heat transfer coefficient is the key in the modelling of such kinds of flows with the two-fluid model. An extensive literature survey on computational modelling of flashing flows has been given in previous work. The present work is aimed at giving a brief review on available theories and correlations for the estimation of interphase heat transfer coefficient, and evaluating them quantitatively based on computational fluid dynamics simulations of bubble growth in superheated liquid. The comparison of predictions for bubble growth rate obtained by using different correlations with the experimental as well as direct numerical simulation data reveals that the performance of the correlations is dependent on the Jakob number and Reynolds number. No generally applicable correlations are available. Both conduction and convection are important in cases of bubble rising and translating in stagnant liquid at high Jakob numbers. The correlations combining the analytical solution for heat diffusion and the theoretical relation for potential flow give the best agreement.

Permalink: https://www.hzdr.de/publications/Publ-26833
Publ.-Id: 26833


Neutron irradiation test of Hamamatsu, SensL and AdvanSiD UV-extended SiPMs

Cordelli, M.; Diociaiuti, E.; Donghia, R.; Ferrari, A.; Miscetti, S.; Müller, S.; Sarra, I.

In this paper, we report the measurement of the neutron radiation hardness of custom Silicon Photomultipliers arrays (SiPMs) manufactured by three companies: Hamamatsu (Japan), AdvanSiD (Italy) and SensL (Ireland). These custom SiPMs consist of a 2 × 3 array of 6 × 6 mm^2 monolithic cells with pixel sizes of respectively 50 μm (Hamamatsu and SensL) and 30 μm (AdvanSid).
A sample from each vendor has been exposed to neutrons generated by the Elbe Positron Source facility (Dresden), up to a total fluence of ∼ 8.5 × 10^11 n 1MeV /cm 2 . Test results show that the dark current increases almost linearly with the neutron fluence.
The room temperature annealing was quantified by measuring the dark current two months after the irradiation test. The dependence of the dark current on the device temperature and on the applied bias have been also evaluated.

Keywords: SiPM; Neutron damage; annealing

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Permalink: https://www.hzdr.de/publications/Publ-26832
Publ.-Id: 26832


Annealing effect on ferromagnetic properties, hole concentration and electronic band structure of GaMnAs epitaxial layers

Zhu, J. J.; Li, L.; Chen, L.; Prucnal, S.; Grenzer, J.; Zhao, J. H.; Helm, M.; Zhou, S. Q.

In this paper, we analyze the correlation of the magnetism and the carrier concentration with the shift of the spectroscopic critical points for low compensated GaMnAs samples with a high Curie temperature of around 150 K. The GaMnAs layers were grown by low-temperature molecular beam epitaxy. The low-temperature annealing leads to a reduction of Mn interstitials from 0.8 to 0.4% and an enhancement in the hole concentration. The saturation magnetization is 51 emu/cm and the Curie temperature is 150 K after post-growth annealing, while those of as-grown layers are 37 emu/cm and 80 K. The resistivity dropped significantly after the post-growth annealing, due to the fact that the number of Mn, which act as double donors and compensate holes, was significantly reduced by the low-temperature and long-time annealing. The electronic band structure is investigated by spectroscopic ellipsometry. The transition energies of critical points show redshift after post-growth annealing due to the annealing-induced enhancement of the hole concentration. Our results support the valence band picture (the Zener model) in ferromagnetic GaMnAs.

Keywords: GaMnAs; DMS; doping; magnetic properties

Permalink: https://www.hzdr.de/publications/Publ-26831
Publ.-Id: 26831


Structural Analysis of Germanium (Ge)-Containing Ferrous Calcium Silicate Magnesia Slag for Applications of Black Copper Smelting

Shuva, M. A. H.; Rhamdhani, M. A.; Brooks, G. A.; Masood, S. H.; Reuter, M. A.

Ferrous-calcium-silicate (FCS) based slag has been used in the copper industry over the last two decades. The FCS slag has also been used in a secondary copper processing such as in black copper smelting which has different operation conditions compared to that of typical primary copper processing. In the process, a variety of copper bearing scraps from sources such as industrial waste, consumer waste and electronic waste (e-waste) are used. The valuable metals in these secondary resources are distributed in different phases during the process. Understanding the behaviour of the valuable elements at the relevant conditions is vital for optimizing the process and to maximize the recovery of the elements. In this study, we investigated the structure of Ge-containing FCS-based slag at different conditions (oxygen partial pressure and temperature) using Fourier-Transform Infrared (FTIR) spectroscopy to explore the effect of the polymerization on the behaviour of the valuable element Ge in the slag. It was found that experimental parameters significantly influence the slag structure, and therefore, the partitioning of Ge in the slag. A correlation between the distribution ratio of Ge and the slag structure has developed in the current study.

Keywords: Copper processing Slag structure Distribution ratio

  • Contribution to proceedings
    9th International Symposium on High-Temperature Metallurgical Processing organized in conjunction with the Annual Meeting and Exhibition - TMS 2018, 11.-15.03.2017, Phoenix, USA
    Minerals, Metals and Materials Series, Cham: Springer, 978-3-319-72137-8, 295-304
    DOI: 10.1007/978-3-319-72138-5_30

Permalink: https://www.hzdr.de/publications/Publ-26830
Publ.-Id: 26830


Diodengepumpte hochenergetische Laserverstärker für ultrakurze Laserpulse Das PENELOPE Lasersystem

Loeser, M.

Öffentlicher Verteidigungsvortrag der Dissertation "Diode-Pumped High-Energy Laser Amplifiers for Ultrashort Laser Pulses - The PENELOPE Laser System" von Herrn Markus Löser.

  • Lecture (others)
    Öffentliche Verteidigung/Disputation, 16.11.2017, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-26829
Publ.-Id: 26829


Structural impact of chromium incorporation in as-grown and flash-lamp-annealed sputter deposited titanium oxide films

Gago, R.; Prucnal, S.; Perez-Casero, R.; Caretti, I.; Jimenez, I.; Lungwitz, F.; Cornelius, S.

We address the impact of chromium (Cr) incorporation (< 15 at.%) in the structure of titanium dioxide (TiO2: Cr) films for as-grown and after flash-lamp-annealing (FLA) states. Samples were produced by DC magnetron sputtering on either unheated or heated (400 degrees C) substrates. Complementary medium-and local-order information was extracted by X-ray diffraction and absorption near-edge structure, respectively. TiO2: Cr grown on unheated substrates are amorphous with the major contribution from Cr3+ and progressive formation of Cr6+ with Cr. On heated substrates, anatase phase is dominant for low Cr levels (<= 7 at.%) and the structure evolves with Cr towards a disordered mixed-oxide with rutile structure. By tuning the FLA energy density, customized (single or mixed) phase formation is achieved from (initially amorphous) Cr-free TiO2. For amorphous TiO2: Cr with low Cr (<= 7 at.%), FLA induces a short-range rutile structure but structural ordering is not observed at higher Cr levels. Nonetheless, FLA annihilates Cr6+ sites and promotes Cr4+, which is associated to the mixed-oxide rutile. FLA also improves the pristine structure of anatase TiO2: Cr grown on heated substrates. These results provide relevant information about the atomic structure of mixed oxides and the use of FLA for the synthesis of band-gap engineered TiO2-based materials.

Keywords: TiO2; anatase; doping; flash lamp annealing; XRD

Permalink: https://www.hzdr.de/publications/Publ-26828
Publ.-Id: 26828


Doping of Ge via nonequilibrium processing

Prucnal, S.

Here an overview of different doping techniques will be presented. Special attention will be focused on the use of ion implantation followed by flash-lamp (FLA) annealing for the fabrication of heavily doped Ge. In contrast to conventional annealing procedures, rear-side FLA leads to full recrystallization of Ge and dopant activation independently of pre-treatment. The maximum carrier concentration is well above 10^20 cm-3 for n-type and above 10^21 for p-type doping. The recrystallization mechanism and the dopant distribution during rear-side FLA are discussed in detail.

Keywords: ion implantation; flash lamp annealing; Ge; doping

  • Invited lecture (Conferences)
    18th AGATA week and 2nd Position Sensitive Germanium Detectors and Application Workshop, 11.-15.09.2017, Milano, Italy

Permalink: https://www.hzdr.de/publications/Publ-26827
Publ.-Id: 26827


Pulse-Resolved Data Acquisition at High Repetition Rates: Getting the Most Out of Modern Light Sources

Green, B.; Kovalev, S.; Golz, T.; Stojanovic, N.; Kampfrath, T.; Fisher, A. S.; Gensch, M.

Sub cycle dynamics on THz driven Phenomena are an emerging class of experiments in ultra-fast science. Hence THz pump laser probe experiments are an essential class of experiments at the new TELBE THz facility. Even the most modern accelerators can be synchonised only down to 80 fs (peak to peak) by active feedbacks [1]. For that reason a unique high-rep-rate arrivaltime monitoring has been developed at TELBE that provides timing down to (in theory) 12 fs by post-mortem arrivaltime-jitter correction.

  • Poster
    XFEL User's Meeting, 26.01.2018, DESY, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-26826
Publ.-Id: 26826


Bonding analysis of tetravalent actinide complexes with salen

Kloditz, R.; Radoske, T.; Patzschke, M.; Stumpf, T.

The bonding situation of tetravalent actinide-complexes with salen is investigated by using tools of real-space bonding-analysis. These tools, involving plots of density-differences and non-covalent interactions (NCI), reveal covalent interactions of Th(IV) with the O-donor of salen. The contribution of the two f-electrons of U(IV) even enhance this interaction.

Keywords: bonding analysis; covalency; density difference; non-covalent interactions; actinides; salen; theoretical chemistry

  • Lecture (others)
    FENABIUM-Projekttreffen, 04.12.2017, Dresden-Rossendorf, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-26825
Publ.-Id: 26825


Bonding analysis of tetravalent actinide complexes with N-/O-donor ligands

Kloditz, R.; Radoske, T.; Patzschke, M.; Stumpf, T.

By using real-space bonding-analysis tools, like non-covalent interactions and density-difference plots, the bonding situation in tetravalent actinide (Th(IV), U(IV)) complexes with salen is investigated.

Keywords: bonding analysis; theoretical chemistry; actinides; salen; non-covalent interactions

  • Lecture (Conference)
    Theoretical User-Lab (ThUL School), 16.-20.10.2017, Jülich, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-26824
Publ.-Id: 26824


Defect engineering in SiC for quantum spintronics

Zhou, S.; Zhang, Z.; Liu, Y.; Hübner, R.; Gemming, S.; Helm, M.

Silicon carbide (SiC) is a wide band-gap semiconductor (6H-SiC with Eg of 3.05 eV) with unique mechanical, electrical, and thermal properties, which make the material suitable for many demanding applications in extreme conditions, such as high temperature, high power, high frequency and high radiation exposure. Two recently reported phenomena related to the defects in SiC are opening the door for semiconductor spintronics and quantum computing:
(1) Room temperature ferromagnetism has been observed in neon ion or neutron irradiated both 4H- and 6H-SiC [1, 2]. This is somehow surprising since the materials are transition-metal free, which also gives rise to the term ‘‘d0 ferromagnetism’’.
(2) Some defect (including the neutral carbon–silicon divacancy) spin states in 4H-SiC can be optically addressed and coherently controlled up to room temperature [3]. These defect spin states are ideal information carriers for quantum computing.

Particle irradiation provides a way to engineer defects in crystalline materials regarding the defect concentration and type. In this contribution, we made a comprehensive investigation on the structural and magnetic properties of ion implanted and neutron irradiated SiC samples. In combination with X-ray absorption spectroscopy, high-resolution transmission electron microscopy and first-principles calculations, we try to understand the mechanism in a microscopic picture.

For neon or xenon ion implanted SiC, we identify a multi-magnetic-phase nature [2, 4]. The magnetization of SiC can be decomposed into paramagnetic, superparamagnetic and ferromagnetic contributions. The ferromagnetic contribution persists well above room temperature and exhibits a pronounced magnetic anisotropy. By combining X-ray magnetic circular dichroism and first-principles calculations, we clarify that p-electrons of the nearest-neighbor carbon atoms around divacancies are mainly responsible for the long-range ferromagnetic coupling [5]. Thus, we provide a correlation between the collective magnetic phenomena and the specific electrons/orbitals.

For neutron irradiated SiC, we observe a strong paramagnetism, scaling up with the neutron fluence [6]. A weak ferromagnetic contribution only occurs in a narrow fluence window or after annealing. The interaction between the nuclear spin and the paramagnetic defect can effectively tune the spin-lattice relaxation time (T1) as well as the nuclear spin coherent time (T2). For the sample with the largest neutron irradiation fluence, T1 and T2 are determined to be around 520 s and 1 ms at 2K, respectively.

[1] Y. Liu, et al., Phys. Rev. Lett. 106, 087205 (2011).
[2] L. Li, et al., Appl. Phys. Lett. 98, 222508 (2011).
[3] W. Koehl, et al., Nature 479, 84 (2011).
[4] Y. Wang, et al., Phys. Rev. B 89, 014417 (2014).
[5] Y. Wang, et al., Scientific Reports, 5, 8999 (2015).
[6] Y. Wang, et al., Phys. Rev. B 92, 174409 (2015).

  • Lecture (Conference)
    Gadest2017: Gettering and Defect Engineering in Semiconductor Technology, 01.-06.10.2017, Lopota, Georgia

Permalink: https://www.hzdr.de/publications/Publ-26823
Publ.-Id: 26823


Tetravalent actinides – from polymeric complexes to nanoparticles

Hennig, C.

Tetravalent actinides show a strong tendency towards hydrolysis. This forces the formation of polynuclear species. However, the polynuclear species undergo further polymerization and form colloids. One way to isolate polynuclear species is to introduce terminating groups which hinder further polymerization. We found that carboxylates are well suited for this task. Larger clusters can form either highly ordered nanoparticles, complete amorphous structures, or transitions between them. Nanoparticles with an ordered structure can be stabilized by the introduction of termination functions, whereas amorphous nanoparticles can be potentially stabilized by introducing ions that modify the surface charge. These modifications support the structure analysis of the species. The nanoparticles of tetravalent actinides can form colloid suspensions which are stable over years. Due to their low solubility, tetravalent actinides are considered as nearly immobile in the nature. The surface charge may significantly enhance their migration.

Keywords: tetravalent actinides; polynuclear species; nanoparticles; colloids

  • Invited lecture (Conferences)
    225th American Chemical Society Meeting, 18.-22.03.2018, New Orleans, USA

Permalink: https://www.hzdr.de/publications/Publ-26822
Publ.-Id: 26822


Tuning ferromagnetic semiconductors by ion irradiation

Zhou, S.; Yuan, Y.; Xu, C.; Rushforth, A.; Chen, L.; Sawicki, M.; Helm, M.

Ion irradiation has been widely used to render a semiconductor layer highly resistive through the creation of carrier-trapping centers. In Mn doped III-V compound semiconductors, which are proposed for spintronic applications, free carriers play deterministic roles for the magnetic properties and material functionalities. However, by substituting the cation, Mn in III-V acts as an acceptor, resulting in the difficulty to independently change the local-moment and hole concentration. In this contribution, we show how the carrier concentration in (III,Mn)V and the consequent magnetic properties can be precisely tuned by ion irradiation [1, 2].
On one hand, we investigate fundamentally how magnetic properties change upon shifting the Fermi level by hole compensation via ion irradiation. We monitor the change of Curie temperature (TC) and conductivity. For a broad range of samples including (Ga,Mn)As and (Ga,Mn)(As,P) with various Mn and P concentrations, we observe a smooth decrease of TC with carrier compensation over a wide temperature range while the conduction is changed from metallic to insulating. The existence of TC below 10 K is also confirmed in heavily compensated samples. Our experimental results are naturally explained by assuming that the Fermi level resides in the valence band being merged with the Mn-derived impurity band [2].
On the other hand, we explore the application potential of ion irradiation in semiconductor spintronics. We show that the magnetic easy axis of (Ga,Mn)(As,P) can be gradually changed between in-plane and out-of-plane directions [3]. Combined with the possibility of lateral patterning [4], such an approach allows for developing new concepts for spintronic devices.
[1] L. Li, S. Yao, S. Zhou, et al., J. Phys. D: Appl. Phys. 44, 099501 (2011).
[2] S. Zhou, et al., Phys. Rev. B 94, 075205 (2016).
[3] Y. Yuan, ..., S. Zhou, Phys. Rev. Mater. 1, 054401 (2017).
[4] L. Li, ..., S. Zhou, Nucl. Instr. Meth. B 269, 2469-2473 (2011).

  • Lecture (Conference)
    19th International Conferences on Radiation Effects in Insulators, 30.06.-07.07.2017, Versallies, France

Permalink: https://www.hzdr.de/publications/Publ-26821
Publ.-Id: 26821


Propagation of spin wave under time dependent magnetic fields

Nishida, N.; Matthies, P.

The spin-wave dispersion relation strongly depends on the effective magnetic field acting on the magnetic moments. In order to investigate how spin waves behave if the magnetic field changes in time during their propagation, we applied pulsed magnetic fields changing on the nanosecond time scale.
Spin waves were excited in a NiFe micro stripe. A static bias magnetic field and a time dependent pulse field were applied to the stripe. The spin-wave intensity was measured by time-resolved Brillouin light scattering microscopy. We observed a frequency shift of spin waves at the rising edge of the pulse field. The change in frequency can be understood by the shift of the dispersion relation induced by the pulse field. We succeeded to manipulate the spin-wave frequency during their propagation by temporally inhomogeneous magnetic fields.

  • Poster
    Bad Honnef Physics School on Magnetism: From Fundamentals to Spin based Nanotechnology, 17.-22.09.2017, Bad Honnef, Germany

Permalink: https://www.hzdr.de/publications/Publ-26820
Publ.-Id: 26820


Defect induced magnetism in SiC characterized by magnetometry

Zhou, S.

Silicon carbide (SiC) is a wide band-gap semiconductor with unique mechanical, electrical, and thermal properties, which make the material suitable for many demanding applications in extreme conditions, such as high temperature, high power, high frequency and high radiation exposure. The spin states related with defects in SiC can be optically addressed and coherently controlled up to room temperature [1] or can be ferromagnetically coupled [2, 3], opening the door for semiconductor spintronics and quantum computing.



In this contribution, we present a comprehensive investigation on defects in SiC by using magnetometry [2-6]. In combination with X-ray absorption spectroscopy, high-resolution transmission electron microscopy and first-principles calculations, we try to understand the mechanism of defect induced magnetism in SiC in a microscopic picture.



For neon or xenon ion implanted SiC, we identify a multi-magnetic-phase nature [3, 4]. The magnetization of SiC can be decomposed into paramagnetic, superparamagnetic and ferromagnetic contributions. The ferromagnetic contribution persists well above room temperature and exhibits a pronounced magnetic anisotropy. By combining X-ray magnetic circular dichroism and first-principles calculations, we clarify that p-electrons of the nearest-neighbor carbon atoms around divacancies are mainly responsible for the long-range ferromagnetic coupling [5]. Thus, we provide a correlation between the collective magnetic phenomena and the specific electrons/orbitals.



For neutron irradiated SiC, we observe a strong paramagnetism, scaling up with the neutron fluence [6]. A weak ferromagnetic contribution only occurs in a narrow fluence window or after annealing. The interaction between the nuclear spin and the paramagnetic defect can effectively tune the spin-lattice relaxation time (T1) as well as the nuclear spin coherent time (T2). For the sample with the largest neutron irradiation fluence, T1 and T2 are determined to be around 520 s and 1 ms at 2K, respectively.



[1] W. Koehl, et al., Nature 479, 84 (2011).

[2] Y. Liu, et al., Phys. Rev. Lett. 106, 087205 (2011).

[3] L. Li, et al., Appl. Phys. Lett. 98, 222508 (2011).

[4] Y. Wang, et al., Phys. Rev. B 89, 014417 (2014).

[5] Y. Wang, et al., Scientific Reports, 5, 8999 (2015).

[6] Y. Wang, et al., Phys. Rev. B 92, 174409 (2015).

  • Lecture (Conference)
    2017 MRS SPRING MEETING, 17.-21.04.2017, Phoenix, US

Permalink: https://www.hzdr.de/publications/Publ-26819
Publ.-Id: 26819


Facile Preparation of Multifunctionalisable ‘Stealth’ Upconverting Nanoparticles for Biomedical Applications

Nsubuga, A.; Sgarzi, M.; Zarschler, K.; Kubeil, M.; Hübner, R.; Steudtner, R.; Graham, B.; Joshi, T.; Stephan, H.

Pure hexagonal (β-phase) NaYF4-based hydrophobic upconverting nanoparticles (UCNPs) were surface-modified with O-phospho-L-threonine (OPLT), alendronic acid, and PEG-phosphate ligands to generate water-dispersible UCNPs. Fourier-transform infrared (FTIR) spectroscopy was used to establish the presence of the ligands on the UCNP surface. These UCNPs exhibit great colloidal stability and a near-neutral surface at physiological pH, as confirmed by dynamic light scattering (DLS) and zeta potential (ζ) measurements, respectively. The particles also display excellent long-term stability, with no major adverse effect on the size of UCNPs when kept at pH 7.4. Upon exposure to human serum, PEG-phosphate- and alendronate-coated UCNPs showed no formation of biomolecular corona, as confirmed by SDS-PAGE analysis. The photophysical properties of water-dispersible UCNPs were investigated using steady-state as well as time-resolved luminescence spectroscopy, under excitation at ca. 800 nm. The results clearly show that the UCNPs demonstrate bright upconversion (UC) luminescence. Furthermore, the presence of reactive groups on the NPs, such as, free amine group in alendronate-coated UCNPs, enables further functionalisation of UCNPs with, for example, small molecules, peptides, proteins, and antibodies. Overall these protein corona resistant UCNPs show great biocompatibility and are worthy of further investigation as potential new biomaging probes.

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Permalink: https://www.hzdr.de/publications/Publ-26818
Publ.-Id: 26818


Low energy dynamics in charge ordered R0.5Sr0.5MnO3 (R = Nd and Pr) manganite thin films

Rana, R.; Schmidt, J.; Grenzer, J.; Schneider, H.; Helm, M.; Pashkin, A.

Transition metal oxides represent an exotic class of correlated systems in which a complex interplay between the spin, charge, orbital and lattice degrees of freedom may result in colossal magnetoresistance, superconductivity; charge ordered (CO) phases, etc. The half-doped Pr0.5Sr0.5MnO3 manganite represents a unique stripe type CO-orbital order that induces transport and magnetic anisotropy whereas the CO in Nd0.5Sr0.5MnO3 is charge-exchange (CE)-type which is isotropic in nature.

We have systematically explored ~ 200nm epitaxial manganite thin films grown on (100), (110), and (111) oriented (LaAlO3)0.3(Sr2TaAlO6)0.7 substrates by pulsed laser deposition technique. Our Terahertz (THz) time-domain spectroscopic data reveal charge density wave (CDW) resonance centered around 5-6 meV for (110) oriented films and Drude-like conductivity for (100) and (111) oriented films. The CDW resonance in the optical conductivity spectra can be tuned from 4 meV to 6 meV for (110) oriented films and depends on the amount of ferromagnetic phase fraction in the CO matrix and corroborates well with the magnetization measurements. The nonlinear conductivity related to the sliding of the pinned CDW character makes the studied systems promising candidates for ultrafast coherent control of charge transport by resonant THz pumping.

Keywords: Manganites; charge order; terahertz

  • Poster
    DPG Spring Meeting 2017, 19.-24.03.2017, Dresden, Germany

Permalink: https://www.hzdr.de/publications/Publ-26817
Publ.-Id: 26817


Propagation of spin wave pulses during time dependent magnetic fields

Nishida, N.; Matthies, P.; Wagner, K.; Schultheiss, K.; Schultheiss, H.

Spin wave dispersion relation depends on a magnetic field. The resonance frequency is higher for higher magnetic fields. While it was previously reported how spin waves adapt to spatially inhomogeneous magnetic fields [1], we investigated spin-wave propagation under the influence of nanosecond magnetic field pulses.
We fabricated a 2 µm-wide spin-wave waveguide from NiFe with an antenna for spin wave excitation and a dc line below the spin-wave conduit. An external magnetic field was applied perpendicular to the stripe. In order to modulate the internal magnetic field in the stripe, a dc pulse was injected into the dc line, because the dc pulse generates Oersted field. We measured a magnon density on the stripe using time-resolved Brillouin light scattering microscopy, and investigated spin-wave dynamics when the dc pulse came in. We succeeded to observe the temporal magnon density when the dc pulse came in and went out. For a fixed excitation frequency observed a decrease (increase) of the spin-wave frequency at the rising (falling) edge of the dc pulse. Since Oersted field is in the opposite direction to the external magnetic field, the internal magnetic field is lower than the external field while the dc pulse is on. The dispersion relation shifts to lower frequency, and it matches to the resonance magnetic field temporarily. At that time spin wave pulse is excited under the antenna. We also observed the position dependency of the excited spin wave pulse. The spin wave pulse propagated, and the frequency shifted lower at the rising edge of the dc pulse, or shifted higher at the falling edge of the dc pulse. We succeeded to observe the excited spin wave pulse following the change of the dispersion relation, and demonstrated the modulation of spin wave by the change of the magnetic field.

[1] V. E. Demidov et. al., Appl. Phys. Lett. 99, 082507 (2011).

  • Poster
    Magnonics 2017, 07.-10.08.2017, Oxford, United Kingdom

Permalink: https://www.hzdr.de/publications/Publ-26816
Publ.-Id: 26816


Magnetic anisotropy in antiferromagnetic hexagonal MnTe

Kriegner, D.; Reichlova, H.; Grenzer, J.; Schmidt, W.; Ressouche, E.; Godinho, J.; Wagner, T.; Martin, S.; Shick, A.; Volobuev, V.; Springholz, G.; Holy, V.; Wunderlich, J.; Jungwirth, T.; Vyborny, K.

Antiferromagnetic hexagonal MnTe is a promising material for spintronic devices relying on the control of antiferromagnetic domain orientations. Here we report on neutron diffraction, magnetotransport, and magnetometry experiments on semiconducting epitaxial MnTe thin films together with density functional theory (DFT) calculations of the magnetic anisotropies. The easy axes of the magnetic moments within the hexagonal basal plane are determined to be along < 1-100 > directions. The spin-flop transition and concomitant repopulation of domains in strong magnetic fields is observed. Using epitaxially induced strain the onset of the spin-flop transition changes from similar to 2 to similar to 0.5 T for films grown on InP and SrF2 substrates, respectively.

Keywords: MOLECULAR-BEAM EPITAXY; NEUTRON-DIFFRACTION; MANGANESE TELLURIDE; THERMAL-EXPANSION; TEMPERATURE; BEHAVIOR; MEMORY; FILM

Permalink: https://www.hzdr.de/publications/Publ-26815
Publ.-Id: 26815


Synthesis and characterization of tetravalent actinide complexes with amidinates and guanidinates

Schöne, S.; März, J.; Kaden, P.

Recent results of ongoing research of tetravalent actinde complexes with amidinates and guadinates is presented. Thereby the presentation fucuses on solid state characterisations with SC-XRD and paramagnetic NMR studies in organic solvents.

  • Lecture (others)
    FENABIUM Projekttreffen, 04.12.2017, Dresden-Rossendorf, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-26814
Publ.-Id: 26814


Site-controlled Si quantum dots in buried SiO2 layer formed by ion-beam implantation and phase separation

Xu, X.; Wolf, D.; Hlawacek, G.; Engler, M.; Hübner, R.; Bischoff, L.; Pruefer, T.; Heinig, K.-H.; von Borany, J.

The increased use of personal computing devices and the Internet of Things (IoT) is accompanied by a demand for a computation unit with extra low energy dissipation. The single electron transistor (SET), which uses a Coulomb island to manipulate the movement of single electrons, is a candidate device for future low power electronics. However, so far its success is hindered by low temperature requirements and the missing CMOS-compatible fabrication route. By combining standard top-down lithography with bottom-up self-assembly of Si nanodots we will overcome this barrier.
In this work, Si nanodots--suitable for RT operation of SETs--are formed in a CMOS compatible way inside a buried SiO2 layer, providing the basic structure of an SET. This is achieved via phase separation induced by ion beam mixing in a geometrical restricted volume, followed by a thermal treatment. Guided by 3DkMC and TRI3DYN simulations, we utilize Helium Ion Microscopy (HIM) to irradiate continuous layers with Ne+, and Si+ broad beam irradiation of pillars. Both attempts lead to a restriction of the size of the collision cascade and hence the mixed volume. The size and position of the formed Si nanodots are studied with transmission electron microscopy, SIMS, and various electrical characterization techniques.

  • Lecture (Conference)
    Ionenstrahltreffen 2017, 13.02.2017, Göttingen, Germany

Permalink: https://www.hzdr.de/publications/Publ-26813
Publ.-Id: 26813


Site-controlled formation of Si nanodots in a buried SiO2 layer via ion-beam implantation and phase separation

Xu, X.; Wolf, D.; Hlawacek, G.; Hübner, R.; Gharbi, A.; Prüfer, T.; Bischof, L.; Heinig, K.-H.; Facsko, S.; von Borany, J.

The increased use of personal computing devices and the Internet of Things (IoT) is accompanied by a demand for a computation unit with extra low energy dissipation. The single electron transistor (SET), which uses a Coulomb island to manipulate the movement of single electrons, is a candidate device for future low power electronics. However, so far its success is hindered by low temperature requirements and the missing CMOS compatible fabrication route. By combining standard top-down lithography with bottom-up self-assembly of Si nanodots we will overcome this barrier.
In this work, Si nanodots--suitable for RT operation of SETs--are formed in a CMOS compatible way inside a buried SiO2 layer, providing the basic structure of an SET. This is achieved via phase separation induced by ion beam mixing in a geometrical restricted volume, followed by a thermal treatment. Guided by 3DkMC and TRI3DYN simulations, we utilize Helium Ion Microscopy (HIM) to irradiate continuous layers with Ne+, and Si+ broad beam irradiation of pillars. Both attempts lead to a restriction of the size of the collision cascade and hence the mixed volume. The size and position of the formed Si nanodots are studied with transmission electron microscopy, SIMS, and various electrical characterization techniques.

  • Lecture (Conference)
    81. DPG-Jahrestagung und DPG-Frühjahrstagung der Sektion Kondensierte Materie, 22.03.2017, Dresden, Germany

Permalink: https://www.hzdr.de/publications/Publ-26812
Publ.-Id: 26812


Site-controlled Si Nanodot Formation for a RT-SET via Ion Beam Mixing and Phase Separation

Xu, X.; Wolf, D.; Prüfer, T.; Hlawacek, G.; Hübner, R.; Bischoff, L.; Perego, M.; Gharbi, A.; Heinig, K.-H.; von Borany, J.

The increased use of personal computing devices and the Internet of Things (IoT) is accompanied by a demand for a computation unit with extra low energy dissipation. The Single Electron Transistor (SET), which uses a Coulomb island to manipulate the movement of single electrons, is a candidate device for future low-power electronics. However, so far its development is hindered by low-temperature requirements and the absence of CMOS compatibility. By combining advanced top-down lithography with bottom-up self-assembly of Si nano dots (NDs) we will overcome this barrier.
In this work, Si NDs – suitable as RT Coulomb islands – are formed via ion beam mixing followed by thermally stimulated phase separation. Broad-beam Si+ and Ne+ beams followed by a rapid thermal annealing (RTA) treatment were utilized to create a layer of NDs, which are subsequently visualized by Energy-Filtered Transmission Electron Microscopy (EFTEM). The conditions for ND formation, namely the dependence on ion type, primary energy, irradiation fluence, layer thickness and thermal budget during RTA, are optimized based on an extensive survey of this multidimensional parameter space. The presented work is guided by TRIDYN simulations of the Si excess in a SiO2 layer due to ion beam mixing and 3D Kinetic Monte-Carlo (3DkMC) simulation for the phase separation during the thermal treatment. To tailor towards a single Si ND, the focused Ne+ beam from the Helium Ion Microscope (HIM) is utilized to create user defined patterns of NDs in planar layer stacks. This allows achieving a mixing volume small enough for restricted Ostwald ripening and successful single ND formation. The existence of the formation of spatially controlled single NDs with a diameter of only 2.2 nm is confirmed by comparing the EFTEM Si plasmon-loss intensity with simulated plasmon loss images.
In the future – by combining conventional lithography, direct self-assembly (DSA) and ion beam mixing – nanopillars with a single embedded ND will be integrated in a CMOS-compatible way. EFTEM and electrical characterization techniques will be used for realizing this novel pathway towards a room-temperature SET device.

This work has been funded by the European Union’s Horizon2020 research program ‘IONS4SET’ under Grant Agreement No. 688072.

  • Lecture (Conference)
    FOR3NANO: Formation of 3D Nanostructures by Ion Beams, 29.06.2017, Helsinki, Finnland

Permalink: https://www.hzdr.de/publications/Publ-26811
Publ.-Id: 26811


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