Superconducting germanium discovered
Superconductors are an important subject in materials research. They loose their electrical resistance if cooled down to low temperatures, which causes them to conduct current without losses. FZD scientists were able for the first time to create superconducting germanium, which actually belongs to the group of semiconductors, by implanting gallium ions. This result was made possible by the close collaboration of Dr. Viton Heera of the Institute of Ion Beam Physics and Materials Research and Dr. Thomas Herrmannsdörfer of the High Magnetic Field Laboratory Institute, who joined their knowhow in ion implantation and the theory of superconductors. Both scientists were awarded the FZD Research Prize for their achievement which could have an impact on the semiconducting industry, replacing silicon as basic material by germanium.
T. Herrmannsdörfer, V. Heera et al., „Superconducting state in a gallium-doped germanium layer at low temperatures”, in: Physical Review Letters 102 / 21, 217003 (2009), doi: 10.1103/PhysRevLett.102.217003
Germanium could inherit silicon in semiconducting industry
Semiconducting germanium could enable faster electronic circuits than silicon, but there are still a number of problems that have to be solved. One of them is the implantation of foreign atoms into the semiconductors - a crucial process for producing transistors. As the material is damaged by this, it has to undergo thermal treatment, which is called annealing. Since this process leads to a strong distribution of the implanted phosphor atoms in germanium, industry has failed so far to produce n-channel transistors based on germanium. FZD scientists were now able to avoid the distribution, applying two new methods: firstly, by short-time annealing using a flash lamp, and, secondly, by thermal treatment of the semiconductor with subsequent proton beam irradiation.
C. Wündisch, M. Posselt et al., „Millisecond flash lamp annealing of shallow implanted layers in Ge“, in: Applied Physics Letters 95, 252107 (2009), doi: 10.1063/1.3276770
H. Bracht et al., „Interstitial-mediated diffusion in germanium under proton irradiation“, in: Physical Review Letters 103, 255501 (2009), doi: 10.1103/PhysRevLett.103.255501
New insights into superconductors
The FZD’s High Magnetic Field Laboratory offers high magnetic fields to international users. Investigations conducted last year with scientists of the Walther Meißner Institute of the Bavarian Academy of Sciences shed a completely new light on the prevailing theories of superconductors. The researchers investigated cuprates - i.e. compounds of copper, oxygen and other elements - which conduct current without losses already in the easily accessible temperature range between -150 °C and -200 °C. The cuprates had been doped prior to the experiments by varying concentrations of foreign atoms.
Measuring the electrical resistance, the scientists found out that high-temperature superconductors behave like metals in the normal-conducting state, refuting an assumption scientists hitherto believed in. The High Magnetic Field Laboratory offers unique conditions to suppress superconductivity and to investigate materials in the normal-conducting state. Future experiments are supposed to explain the transitions between both states which are still enigmatic. Only then will it be possible to produce tailored high-temperature superconductors for broad technological use.
T. Helm1 et al., „Evolution of the Fermi surface of the electron-doped high-temperature superconductor Nd2-xCexCuO4 revealed by Shubnikov-de Haas oscillations“, in: Physical Review Letters 103, 157002 (2009), doi: 10.1103/PhysRevLett.103.157002
1Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, Garching
Prof. Dr. Wosnitza, Joachim (High Magnetic Field Laboratory Institute)
First detection of rare Xi particle
All over the world scientists are using large particle accelerators to look into the secrets of matter, trying to find new particles that would fill the gap to understanding the structure of all matter. The GSI Helmholtzzentrum für Schwerionenforschung owns such large accelerators as well as corresponding detector systems like the HADES system, which was built using contributions of FZD scientists; this detector system is now used for experiments by the HADES collaboration that is made of about 400 international researchers. Last year, the rare Xi particle was detected there for the first time. FZD scientists found strong deviations between its formation as observed in the experiments and theoretical predictions.
The heavy Xi particles are formed at relatively low energies and exist only for a very short moment. The HADES detector only counted about 140 particles during 700 million particle collisions; nevertheless, this is one order of magnitude higher than expected. Investigations of the rare, heavy and strange Xi particles - they are not made up of up and down quarks that form the majority of the world we are familiar with, but consist of two strange quarks - can contribute to our understanding of the early stages of the evolution of matter in the big bang and the formation of chemical elements.
HADES Collaboration, „Deep subthreshold Xi-production in Ar+KCl reactions at 1.76A GeV“, in: Physical Review Letters 103, 132301 (2009), doi: 10.1103/PhysRevLett.103.132301
Prof. Dr. Kämpfer, Burkhard (Institute of Radiation Physics)
Dr. Kotte, Roland (Institute of Radiation Physics)
Neptunium retained by clay
Regarding its clay, granite and salt deposits, Germany has a wider choice than other countries for a repository for highly nuclear waste. Yet a final decision requires knowledge on how safe the waste can be locked in any of these rocks, especially if it gets in contact with water. Investigations at the FZD now proved that neptunium, a radioactive heavy metal that occurs in the waste produced by nuclear power stations, is retained by clay if water comes into play. Neptunium only emerges in low concentrations, but has a long half life and is difficult to analyze, which is why reliable information on the behavior of neptunium was still missing. FZD scientists examined the reactions of an aqueous solution of neptunium with several mineral oxide samples, which can count as models for authentic clay. They found out that neptunium mainly forms stable complexes on the surface of the oxides so that it is likely to be retained by clay in the environment, too.
K. Müller et al., „Sorption of Np(V) onto TiO2, SiO2, and ZnO2: An ATR-FTIR spectroscopic study“, in: Environmental Science & Technology 43 / 20, 7665-7670 (2009), doi: 10.1021/es901256v
Dr. Foerstendorf, Harald (Institute of Radiochemistry)
Dr. Katharina Müller (Institute of Radiochemistry)
Chemical milestone for tumor treatment
External radiation therapy, which makes use of the capability of radiation to destroy cells, is one of the three pillars in cancer treatment. As doctors cannot avoid irradiating healthy tissue, scientists are developing new concepts for therapy that include irradiation of the cancer from inside with the help of radionuclides. This could spare the healthy tissue in the best possible way. What the scientists need is carrier molecules which transport the radionuclides to the tumor.
FZD scientists are exploring the fundamentals for such new concepts from a chemical point of view. Last year, chemists managed to synthesize compounds of bio molecules, like proteins, peptides and nucleic acid components, and radionuclides (yttrium-90, luthetium-177) without changing or damaging the sensitive carrier molecules. The rough conditions normally required by reactions with radionuclides, like high temperatures or the use of unphysical substances, would actually damage bio molecules. FZD scientists found a way to keep them alive, producing radioactive components in the first step and connecting them to the carrier molecules via vacant binding sites afterwards.
J. Schlesinger et al., „Radiosynthesis of new [90Y]-DOTA-based maleimide reagents suitable for the prelabeling of thiol-bearing L-oligonucleotides and peptides“, in: Bioconjugate Chemistry 20, 1340-1348 (2009), doi: 10.1021/bc900095k
Dr. Pietzsch, Hans-Jürgen (Institute of Radiopharmacy)
The absolute instability
The magnetorotational instability (MRI) provides an explanation for the birth of stars and black holes from the accretion disks surrounding them. According to this theory the rotational flows in these disks, which are stable from a hydrodynamic point of view, are destabilized by magnetic fields. Not before 2006 scientists of the FZD and the Astrophysical Institute Potsdam were able for the first time to simulate MRI in the lab, even though it been predicted already 20 years before. The researchers destabilized a liquid metal flow rotating between two copper cylinders by applying a helical magnetic field.
The experiment received attention by scientists all over the world, but also led to a debate about the quality of this so called helical MRI and its relation with the classical MRI in the cosmos, for which a vertical magnetic field is assumed. Adding skilful changes at the top and bottom of the experiment, the scientists were now able to generate a more distinct instability which travels as a wave through the whole length of the cylinder. Moreover, the absolute instability of the wave observed was verified by the scientists through comparisons with numerical results. In addition, the researchers have been able to provide a theoretical explanation for the transition between classical and helical MRI.
F. Stefani et al., „Helical magnetorotational instability in a Taylor-Couette flow with strongly reduced Ekman pumping”, Physical Review E 80, 066303 (2009), doi: 10.1103/PhysRevE.80.066303
O. Kirillov, F. Stefani, „On the relation of helical and standard magnetorotational instability”, The Astrophysical Journal 712, 52-68 (2010), doi: 10.1088/0004-637X/712/1/52
Dr. Stefani, Frank (Institute of Safety Research)
Three-dimensional insights into fast flows
The world’s fastest electron-beam tomograph ROFEX is used at the FZD to investigate complex flow mixtures, for instance made up of water and air, with high temporal and spatial resolution. An electron beam is focused onto a wolfram surface so that X-ray radiation is generated which penetrates the object to be investigated. The information currently gained this way is limited to temporal changes of cross sections of the flows. As the scientists want to obtain three-dimensional information allowing conclusions with regard to the size and velocity of gas bubbles in the flow, they are currently developing a dual-plane tomographic technique.
Last year, the method was successfully tested at the Universität Stuttgart using an electron-beam welding facility where the application of electron-beam tomography for investigations of flow mixtures had been verified at first. The dual-plane tomographic technique is based on the fact that an electron beam penetrates the object under investigation in two planes. This enables the researchers to determine the time that it takes the flow to move from one plane to the other as well as deducing the velocity of the gas bubbles. In addition, the scientists are able to figure out the volume of the gas bubbles in the flow.
M. Bieberle et al., „Experimental two-phase flow measurement using ultra fast limited-angle-type electron beam X-ray computed tomography”, in: Experiments in Fluids 47, 369-378 (2009), doi: 10.1007/s00348-009-0617-6
M. Bieberle et al., „Dual-plane ultrafast limited-angle electron beam x-ray tomography”, Flow Measurement and Instrumentation (2009), doi: 10.1016/j.flowmeasinst.2009.12.001
Bieberle, Martina (Institute of Safety Research)