Publications
Publications of Institute of Theoretical Physics  Year 2023 to 2025
Involved institute: Institute of Theoretical Physics (from 2018)
Year 2023 to 2025
Articles ref. in Journals
"Online First" included
Relaxation to persistent currents in a Hubbard trimer coupled to fermionic baths
Szpak, N.; Schaller, G.; Schützhold, R.; König, J.
Abstract
We consider a ring of fermionic quantum sites, modeled by the FermiHubbard Hamiltonian, in which electrons can move and interact strongly via the Coulomb repulsion. The system is coupled to fermionic cold baths, which by the exchange of particles and energy induce relaxation in the system. We eliminate the
environment and describe the system effectively by Lindblad master equations in various versions valid for different coupling parameter regimes. The early relaxation phase proceeds in a universal way, irrespective of the relative couplings and approximations. The system settles down to its lowenergy sector and is consecutively well approximated by the Heisenberg model. We compare different Lindblad approaches, which, in the late relaxation, push the system towards different final states with opposite, extreme spin orders, from ferromagenetic to antiferromagnetic. Due to spin frustration in the trimer (a three site ring), degenerate ground states are formed by spin waves (magnons). The system described by the global coherent version of the Lindblad operators relaxes
towards the final states carrying directed persistent spin currents. We numerically confirm these predictions.
Keywords: Lindblad equation; FermiHubbard Hamiltonian; relaxation dynamics; Heisenberg model; coherent approximation

Physical Review B 110(2024), 115131
DOI: 10.1103/PhysRevB.110.115131
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 Open Access Version from arxiv.org
 Secondary publication expected from 17.09.2025
Permalink: https://www.hzdr.de/publications/Publ39778
Momentum space separation of quantum path interferences between photons and surface plasmon polaritons in nonlinear photoemission microscopy
Dreher, P.; Janoschka, D.; Giessen, H.; Schützhold, R.; Davis, T. J.; HornVon Hoegen, M.; Meyer Zu Heringdorf, F.J.
Abstract
Quantum path interferences occur whenever multiple equivalent and coherent transitions result in a common final state. Such interferences strongly modify the probability of a particle to be found in that final state, a key concept of quantum coherent control. When multiple nonlinear and energydegenerate transitions occur in a system, the multitude of possible quantum path interferences is hard to disentangle experimentally. Here, we analyze quantum path interferences during the nonlinear emission of electrons from hybrid plasmonic and photonic fields using timeresolved photoemission electron microscopy.We experimentally distinguish quantum path interferences by exploiting the momentum difference between photons and plasmons and through balancing the relative contributions of their respective fields. Our work provides a fundamental understanding of the nonlinear photon–plasmon–electron
interaction. Distinguishing emission processes in momentum space, as introduced here, could allow nanooptical quantumcorrelations to be studied without destroying the quantum path interferences.
Keywords: PEEM; surface plasmon polaritons; photoemission

Contribution to WWW
https://arxiv.org/abs/2310.11936
DOI: 10.48550/arXiv.2310.11936
arXiv: 10.48550/arXiv.2310.11936 
Nanophotonics 13(2024)9, 15931602
DOI: 10.1515/nanoph20230776
Permalink: https://www.hzdr.de/publications/Publ39131
Searching for axion resonances in vacuum birefringence with threebeam collisions
Abstract
We consider birefringent (i.e., polarization changing) scattering of xray photons at the superposition of two optical laser beams of ultrahigh intensity and study the resonant contributions of axions or axionlike particles, which could also be short lived. Applying the specifications of the Helmholtz International Beamline for Extreme Fields (HIBEF), we find that this setup can be more sensitive than previous lightbylight scattering (birefringence) or lightshiningthroughwall experiments in a certain domain of parameter space. By changing the pump and probe laser orientations and frequencies, one can even scan different axion masses, i.e., chart the axion propagator.

Contribution to WWW
https://arxiv.org/abs/2307.08345
DOI: 10.48550/arXiv.2307.08345
arXiv: https://doi.org/10.48550/arXiv.2307.08345 
Physical Review D 109(2024)L091901
DOI: 10.1103/PhysRevD.109.L091901
Cited 1 times in Scopus
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Permalink: https://www.hzdr.de/publications/Publ39130
Critical behavior of the dimerized Si(001) surface: Continuous orderdisorder phase transition in the twodimensional Ising universality class
Brand, C.; Hucht, A.; Mehdipour, H.; Jnawali, G.; Fortmann, J. D.; Tajik, M.; Hild, R.; Sothmann, B.; Kratzer, P.; Schützhold, R.; HornVon Hoegen, M.
Abstract
The critical behavior of the orderdisorder phase transition in the buckled dimer structure of the Si(001) surface is investigated both theoretically by means of firstprinciples calculations and experimentally by spot profile analysis lowenergy electron diffraction (SPALEED). We use density functional theory (DFT) with three different functionals commonly used for Si to determine the coupling constants of an effective lattice Hamiltonian describing the dimer interactions. Experimentally, the phase transition from the lowtemperature c(4×2) to the hightemperature p(2×1)reconstructed surface is followed through the intensity and width of the superstructure spots within the temperature range 78–400K. Near the critical temperature Tc = 190.6K, we observe universal critical behavior of spot intensities and correlation lengths, which falls into the universality class of the twodimensional (2D) Ising model. From the ratio of correlation lengths along and across the dimer rows we determine effective nearestneighbor couplings of an anisotropic 2D Ising model,
J = (−24.9 ± 0.9stat ± 1.3sys )meV and J⊥ = (−0.8 ± 0.1stat )meV.We find that the experimentally determined coupling constants of the Ising model can be reconciled with those of the more complex lattice Hamiltonian
from DFT when the critical behavior is of primary interest. The anisotropy of the interactions derived from the
experimental data via the 2D Ising model is best matched by DFT calculations using the PBEsol functional.
The trends in the calculated anisotropy are consistent with the surface stress anisotropy predicted by the
DFT functionals, pointing towards the role of surface stress reduction as a driving force for establishing the
c(4×2)reconstructed ground state.

Contribution to WWW
https://arxiv.org/abs/2310.10488
arXiv: 10.48550/arXiv.2310.10488 
Physical Review B 109(2024)
DOI: 10.1103/PhysRevB.109.134104
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Permalink: https://www.hzdr.de/publications/Publ39119
Nonperturbative Floquet engineering of the toriccode Hamiltonian and its ground state
Petiziol, F.; Wimberger, S.; Eckardt, A.; Mintert, F.
Abstract
We theoretically propose a quantum simulation scheme for the toriccode Hamiltonian, the paradigmatic model of a quantum spin liquid, based on timeperiodic driving. We develop a hybrid continuousdigital strategy that exploits the commutativity of different terms in the target Hamiltonian. It allows one to realize the required fourbody interactions in a nonperturbative way, attaining strong coupling and the suppression of undesired processes. In addition, we design an optimal protocol for preparing the topologically ordered ground states with high fidelity. A proofofprinciple implementation of a topological device and its use to simulate the topological phase transition are also discussed. The proposed scheme finds natural implementation in architectures of superconducting qubits with tunable couplings.

Physical Review B 109(2024), 075126
DOI: 10.1103/PhysRevB.109.075126
Cited 2 times in Scopus
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Permalink: https://www.hzdr.de/publications/Publ38801
Universal definition of the nonconformal trace anomaly
Ferrero, R.; FranchinoVinas, S.; Frob, M. B.; Lima, W. C. C.
Abstract
We show that there exists a generalized, universal notion of the trace anomaly for theories which are not conformally invariant at the classical level. The definition is suitable for any regularization scheme and clearly states to what extent the classical equations of motion should be used, thus resolving existing controversies surrounding previous proposals. Additionally, we exhibit the link between our definition of the anomaly and the functional Jacobian arising from a Weyl transformation.
Keywords: Conformal anomaly; Trace anomaly; Nonconformal theory; Conformal theory

Contribution to WWW
https://arxiv.org/abs/2312.07666
arXiv: 2312.07666 [hepth] 
Physical Review Letters 132(2024), 071601
DOI: 10.1103/PhysRevLett.132.071601
Cited 4 times in Scopus
Permalink: https://www.hzdr.de/publications/Publ38287
Attraction Versus Repulsion Between Doublons or Holons in MottHubbard Systems
Queißer, F.; Schaller, G.; Schützhold, R.
Abstract
For the Mott insulator state of the FermiHubbard model in the strongcoupling limit, we study the interaction between quasiparticles in the form of doublons and holons. Comparing different methods – the hierarchy of correlations, strongcoupling perturbation theory, and exact analytic solutions for the Hubbard tetramer – we find an effective interaction between doublons and/or holons to linear order in the hopping strength which can display attractive as well as repulsive contributions, depending on the involved momenta. Finally, we speculate about the implications of our findings for hightemperature uperconductivity.
Keywords: Hubbard model; Quasiparticles; Boltzmann equations; Superconductivity

Contribution to WWW
https://arxiv.org/abs/2304.03034
DOI: 10.48550/arXiv.2304.03034
arXiv: 2304.03034 
International Journal of Theoretical Physics 62(2023), 239
DOI: 10.1007/s10773023054849
Cited 1 times in Scopus
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Permalink: https://www.hzdr.de/publications/Publ38133
Energy transfer between gravitational waves and quantum matter
Gräfe, J.; Adamietz, F.; Schützhold, R.
Abstract
We study the interaction between gravitational waves and quantum matter such as BoseEinstein condensates, superfluid helium, or ultracold solids, explicitly taking into account the changes of the trapping potential induced by the gravitational wave. As a possible observable, we consider the change of energy due to the gravitational wave, for which we derive rigorous bounds in terms of kinetic energy and particle number. Finally, we discuss implications for possible experimental tests.

Physical Review D 108(2024), 064056
DOI: 10.1103/PhysRevD.108.064056
Cited 1 times in Scopus 
Contribution to WWW
https://arxiv.org/abs/2302.14694
DOI: 10.48550/arXiv.2302.14694
arXiv: 2302.14694
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Permalink: https://www.hzdr.de/publications/Publ38131
Dynamically Assisted Tunneling in the Floquet Picture
Ryndyk, D.; Kohlfürst, C.; Queißer, F.; Schützhold, R.
Abstract
We study how tunneling through a potential barrier V(x) can be enhanced by an additional harmonically oscillating electric field E(t)=E0 cos(ωt). To this end, we transform into the KramersHenneberger frame and calculate the coupled Floquet channels numerically. We find distinct signatures of resonances when the incident energy E equals the driving frequency ω=E which clearly shows the breakdown of the timeaveraged potential approximation. As a simple model for experimental applications (e.g., in solid state physics), we study the rectangular potential, which can also be benchmarked with respect to analytical results. Finally, we consider the truncated Coulomb potential relevant for nuclear fusion.
Keywords: Dynamically Assisted Quantum Tunneling; Assisted Hydrogen Fusion; HighIntensity Electric Fields; CoupledChannel Equations

Contribution to WWW
https://arxiv.org/abs/2309.12205
arXiv: doi.org/10.48550/arXiv.2309.12205 
Physical Review Research 6(2024), 023056
DOI: 10.1103/PhysRevResearch.6.023056
Cited 2 times in Scopus
Permalink: https://www.hzdr.de/publications/Publ38124
Pion and kaon electromagnetic and gravitational form factors
Xu, Y.Z.; Ding, M.; Raya, K.; Roberts, C. D.; RodríguezQuintero, J.; Schmidt, S. M.
Abstract
A unified set of predictions for pion and kaon elastic electromagnetic and gravitational form factors is obtained using a symmetrypreserving truncation of each relevant quantum field equation. A key part of the study is a description of salient aspects of the dressed graviton + quark vertices. The calculations reveal that each meson’s mass radius is smaller than its charge radius, matching available empirical inferences; and meson core pressures are commensurate with those in neutron stars. The analysis described herein paves the way for a direct calculation of nucleon gravitational form factors.

Contribution to WWW
https://arxiv.org/abs/2311.14832 
European Physical Journal C 84(2024), 191
DOI: 10.1140/epjc/s1005202412518x
Cited 6 times in Scopus
Permalink: https://www.hzdr.de/publications/Publ38114
Sieving parton distribution function moments via the moment problem
Abstract
We apply a classical mathematical problem, the moment problem, with its related mathematical achievements, to the study of the parton distribution function (PDF) in hadron physics, and propose a strategy to sieve the moments of the PDF by exploiting its properties such as continuity, unimodality, and symmetry. Through an errorinclusive sifting process, we refine three sets of PDF moments from Lattice QCD. This refinement significantly reduces the errors, particularly for higher order moments, and locates the peak of PDF simultaneously. As our strategy is universally applicable to PDF moments from any method, we strongly advocate its integration into all PDF moment calculations.

Contribution to WWW
https://arxiv.org/abs/2308.14871 
Physics Letters B 851(2024), 138568
DOI: 10.1016/j.physletb.2024.138568
Permalink: https://www.hzdr.de/publications/Publ38113
Constraining the pion distribution amplitude using DrellYan reactions on a proton
Xing, H.Y.; Ding, M.; Cui, Z.F.; Pimikov, A. V.; Roberts, C. D.; Schmidt, S. M.
Abstract
Using a reaction model that incorporates pion bound state effects and continuum results for proton parton distributions and the pion distribution amplitude, $\varphi_\pi$, we deliver parameterfree predictions for the $\mu^+$ angular distributions in $\pi N \to \mu^+ \mu^ X$ reactions on both unpolarised and polarised targets. The analysis indicates that such angular distributions are sensitive to the pointwise form of $\varphi_\pi$ and suggests that unpolarised targets are practically more favourable. The precision of extant data is insufficient for use in charting $\varphi_\pi$; hence, practical tests of this approach to charting $\varphi_\pi$ must await data with improved precision from newgeneration experiments. The reaction model yields a nonzero singlespin azimuthal asymmetry, without reference to $T$odd parton distribution functions (DFs). This may necessitate additional care when attempting to extract such $T$odd DFs from data.

Contribution to WWW
https://arxiv.org/abs/2308.13695 
Physics Letters B 849(2024), 138462
DOI: 10.1016/j.physletb.2024.138462
Cited 4 times in Scopus
Permalink: https://www.hzdr.de/publications/Publ38111
Optimal control with a multidimensional quantum invariant
Modesto, O.R.; Selwyn, S.; Sahra, A. K.; Samuel, J. H.; Winfried, K. H.; Mintert, F.
Abstract
Optimal quantum control of continuous variable systems poses a formidable computational challenge because of the highdimensional character of the system dynamics. The framework of quantum invariants can significantly reduce the complexity of such problems, but it requires the knowledge of an invariant compatible with the Hamiltonian of the system in question. We explore the potential of a Gaussian invariant that is suitable for quadratic Hamiltonians with any given number of motional degrees of freedom for quantum optimal control problems that are inspired by current challenges in groundstate to groundstate shuttling of trapped ions.

Physical Review A 108(2023), 22601
DOI: 10.1103/PhysRevA.108.022601
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Permalink: https://www.hzdr.de/publications/Publ38102
Fast optomechanical photon blockade
Yuxun, L.; Sofia, Q.; Mintert, F.
Abstract
The photon blockade effect is commonly exploited in the development of singlephoton sources. While the photon blockade effect could be used to prepare highfidelity singlephoton states in idealized regimes, practical implementations in optomechanical systems suffer from an interplay of competing processes. Here we derive a control scheme that exploits destructive interference of Fock state amplitudes of more than one photon. The resulting preparation time for photonblockaded quantum states is limited only by the optomechanical interaction strength and can thus be orders of magnitude shorter than in existing schemes that achieve photon blockade in the steady state.

Physical Review Research 5(2023), 23148
DOI: 10.1103/PhysRevResearch.5.023148
Cited 3 times in Scopus
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Permalink: https://www.hzdr.de/publications/Publ38100
Momentum spectrum of Schwinger pair production in fourdimensional edipole fields
Degli Esposti, G.; Torgrimsson, G.
Abstract
We calculate the momentum spectrum of electronpositron pairs created via the Schwinger mechanism by a class of fourdimensional electromagnetic fields called edipole fields. To the best of our knowledge, this is the first time the momentum spectrum has been calculated for 4D, exact solutions to Maxwell’s equations. Moreover, these solutions give fields that are optimally focused, and are hence particularly relevant for future experiments. To achieve this we have developed a worldline instanton formalism where we separate the process into a formation and an acceleration region.

Contribution to WWW
https://arxiv.org/pdf/2308.01659.pdf 
Physical Review D 109(2024)1, 016013
DOI: 10.1103/PhysRevD.109.016013
Cited 2 times in Scopus
Permalink: https://www.hzdr.de/publications/Publ37992
Floquet analysis of a superradiant manyqutrit refrigerator
Kolisnyk, D.; Queißer, F.; Schaller, G.; Schützhold, R.
Abstract
We investigate superradiant enhancements in the refrigeration performance in a set of N threelevel systems that are collectively coupled to a hot and a cold thermal reservoir and are additionally subject to collective periodic (circular) driving. Assuming the systemreservoir coupling to be weak, we explore the regime of stronger periodic driving strengths by comparing collective weakdriving, FloquetLindblad, and FloquetRedfield master equations. We identify regimes where the power injected by the periodic driving is used to pump heat from the cold to the hot reservoir and derive analytic sufficient conditions for them based on a cycle analysis of the FloquetLindblad master equation. In those regimes, we also argue for which parameters collective enhancements like a quadratic scaling of the cooling current with N can be expected and support our arguments by numerical simulations.
Keywords: Floquet theory; periodic driving; refrigeration; superradiance; cooling condition; collective effects

Contribution to WWW
https://arxiv.org/abs/2310.18126
DOI: 10.48550/arXiv.2310.18126 
Physical Review Applied 21(2024), 044050
DOI: 10.1103/PhysRevApplied.21.044050
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 Open Access Version from arxiv.org
 Secondary publication expected from 26.04.2025
Permalink: https://www.hzdr.de/publications/Publ37731
Study of QED singular properties for variable gyromagnetic ratio g≃2
Rafelski, J.; Evans, S.; Labun, L.
Abstract
Using the external field method, {\it i.e.\/} evaluating the effective action $V_{\mathrm{eff}}$ for an arbitrarily strong constant and homogeneous field, we explore nonperturbative properties of QED allowing arbitrary gyromagnetic ratio $g$. We find a cusp at $g = 2$ in: a) The QED $b_0$renormalization group coefficient, and in the infinite wavelength limit in b) a subclass containing the pseudoscalar ${\cal P}^{2n}= (\vec E\cdot\vec B)^{2n} $ of lightlight scattering coefficients. Properties of $b_0$ imply for certain domains of $g$ asymptotic freedom in an Abelian theory.
Keywords: Schwinger effect; vacuum stability; magnetic moment

Contribution to WWW
https://arxiv.org/abs/2212.13165
DOI: 10.48550/arXiv.2212.13165
arXiv: 2212.13165 
Physical Review D 107(2023), 076002107600212
DOI: 10.1103/PhysRevD.107.076002
Cited 2 times in Scopus
Permalink: https://www.hzdr.de/publications/Publ37455
Improving EulerHeisenbergSchwinger Effective Action with Dressed Photons
Abstract
We implement a longstanding proposal by Weisskopf to apply virtual polarization corrections to
the in/out external fields in study of the EulerHeisenbergSchwinger effective action. Our approach
requires distinguishing the electromagnetic and polarization fields based on mathematical tools
developed by Bia lynickiBirula, originally for the BornInfeld action. Our solution is expressed
as a differential equation where the oneloop effective action serves as input. As a first result of
our approach, we recover the higherorder onecut reducible loop diagrams discovered by Gies and
Karbstein.
Keywords: Euler–Heisenberg–Schwinger (EHS); nonperturbative vacuum structure; quantum electrodynamics (QED)

Contribution to WWW
https://arxiv.org/abs/2306.07887
DOI: 10.12693/APhysPolA.143.S13
arXiv: 2306.07887
Cited 2 times in Scopus 
Acta Physica Polonica A 6(2023)143, S13S17
DOI: 10.12693/APhysPolA.143.S13
Cited 2 times in Scopus
Permalink: https://www.hzdr.de/publications/Publ37453
Dimer Coupling Energies of the Si(001) Surface
Brand, C.; Hucht, A.; Jnawali, G.; Fortmann, J. D.; Sothmann, B.; Mehdipour, H.; Kratzer, P.; Schützhold, R.; HornVon Hoegen, M.
Abstract
The coupling energies between the buckled dimers of the Si(001) surface were determined through analysis of the anisotropic critical behavior of its orderdisorder phase transition. Spot profiles in highresolution lowenergy electron diffraction as a function of temperature were analyzed within the framework of the anisotropic twodimensional Ising model. The validity of this approach is justified by the large ratio of correlation lengths, ζ +/ζ∥+=5.2 of the fluctuating c(4×2) Domains above the critical temperature Tc=(190.6±10) K. We obtain effective couplings J∥=(24.9±1.3) meV along the dimer rows and J⊥=(0.8±0.1) meV across the dimer rows, i.e., antiferromagneticlike coupling of the dimers with c(4×2) symmetry.
Keywords: Dimers; Electrons; Ising model; Correlation lengths; Coupling energies; Critical behaviour; Critical temperatures; Effective coupling; High resolution; Lowenergy electron diffraction; Order/disorder phase transition; Spot profile; Twodimensional; Anisotropy

Physical Review Letters 130(2023), 126203
DOI: 10.1103/PhysRevLett.130.126203
Cited 1 times in Scopus 
Contribution to WWW
arXiv:2302.01715 [condmat.meshall]: https://arxiv.org/abs/2302.01715
DOI: 10.48550/arXiv.2302.01715
Permalink: https://www.hzdr.de/publications/Publ37064
Fresh look at experimental evidence for odderon exchange
Schmidt, S. M.; Cui, Z.F.; Binosi, D.; Roberts, C. D.; Triantafyllopoulos, D. N.
Abstract
Theory suggests that in highenergy elastic hadron+hadron scattering, tchannel exchange of a family of colourless crossingodd
states – the odderon – may generate differences between pp¯ and pp crosssections in the neighbourhood of the
diffractive minimum. Using a mathematical approach based on interpolation via continued fractions enhanced by statistical
sampling, we develop robust comparisons between pp¯ elastic differential crosssections measured at √s=1.96 TeV by the
D0 Collaboration at the Tevatron and functionformunbiased extrapolations to this energy of kindred pp measurements at
√s/TeV=2.76,7,8,13 by the TOTEM Collaboration at the LHC and a combination of these data with earlier crosssection
measurements at √s/GeV=23.5,30.7,44.7,52.8,62.5 made at the intersecting storage rings. Focusing on a domain that
straddles the diffractive minimum in the pp¯ and pp crosssections, we find that these two crosssections differ at the
(2.2−2.6)σ level; hence, supply evidence with this level of significance for the existence of the odderon. If combined with
evidence obtained through different experimenttheory comparisons, whose significance is reported to lie in the range
(3.4−4.6)σ, one arrives at a (4.0−5.2)σ signal for the odderon.
Keywords: Diffractive processes; Highenergy hadron; Odderon; Regge phenomenology; Schlessinger point; Strong interactions in the standard model of particle physics; hadron interactions

Physics Letters B B(2023)839, 137826
DOI: 10.1016/j.physletb.2023.137826
Cited 4 times in Scopus 
Contribution to WWW
https://arxiv.org/abs/2205.15438
DOI: 10.48550/arXiv.2205.15438
arXiv: Fresh look at experimental evidence for odderon exchange
Permalink: https://www.hzdr.de/publications/Publ37063
Generalized GelfandDikii equation and solitonic electric fields for fermionic Schwinger pair production
Ahmadiniaz, N.; Fedotov, A. M.; Gelfer, E. G.; Pyo Kim, S.; Schubert, C.
Abstract
Schwinger pair creation in a purely timedependent electric field can be reduced to an effective quantum mechanical problem using a variety of formalisms. Here we develop an approach based on the GelfandDikii equation for scalar QED, and extent it to spinor QED. We discuss some solvable special cases from this point of view. It was previously shown how to use the wellknown solitonic solutions of the KdV equation to construct “solitonic” electric fields that do not create scalar pairs with an arbitrary fixed momentum. We show that this construction can be adapted to the fermionic case in two inequivalent ways, both leading to the vanishing of the paircreation rate at certain values of the P ̈oschlTeller like index of the associated Schr ̈odinger equation. Thus for any given momentum, we can construct electric fields that create scalar particles but not spinor particles, and also the other way round. Therefore, while often spin is even neglected in Schwingerpair creation, in such cases it becomes decisive.
Keywords: Schwinger mechanism; Pair production; GelfandDikii equation

Contribution to WWW
2205.159 [hepth]: https://arxiv.org/pdf/2205.15946.pdf
arXiv: https://arxiv.org/pdf/2205.15946.pdf 
Physical Review D 108(2023)3, 036019
DOI: 10.1103/PhysRevD.108.036019
Cited 1 times in Scopus
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Permalink: https://www.hzdr.de/publications/Publ36279
Precision Storage Rings for Electric Dipole Moment Searches: A Tool En Route to Physics BeyondtheStandardModel
Ströher, H.; Schmidt, S. M.; Lenisa, P.; Pretz, J.
Abstract
Electric Dipole Moments (EDM) of particles (leptons, nucleons and light nuclei) are currently deemed one of the best indicators for new physics, i.e. phenomena, which lie outside the Standard Model (SM) of elementary particle physics – so called physics “BeyondtheStandardModel” (BSM). Since EDMs of the SM are vanishingly small, a finite permanent EDM would indicate chargeparity symmetry (CP) violation in addition to the wellknown sources of the SM and could explain the baryon asymmetry of the Universe, while an oscillating EDM would hint at a possible Dark Matter (DM) field comprising axions or axionlike particles (ALPs). A new approach exploiting polarized charged particles (proton, deuteron, 3He) in precision storage rings offers the prospect to push current experimental EDM upper limits significantly further including the possibility of an EDM discovery. In this paper, we describe the scientific background and the steps towards the realization of a precision storage ring, which will make such measurements possible.
Keywords: Baryon Asymmetry; Dark Matter; Electric Dipole Moments; Storage Rings; Polarized Beams

Particles 6(2023)1, 385398
DOI: 10.3390/particles6010020
Cited 1 times in Scopus
Permalink: https://www.hzdr.de/publications/Publ36278
Detection schemes for quantum vacuum diffraction and birefringence
Ahmadiniaz, N.; Cowan, T.; Grenzer, J.; FranchinoVinas, S.; Laso García, A.; Smid, M.; Toncian, T.; Trejo Espinosa, M. A.; Schützhold, R.
Abstract
Motivated by recent experimental initiatives, such as at the
Helmholtz International Beamline for Extreme Fields (HIBEF)
at the European Xray Free Electron Laser (XFEL), we calculate
the birefringent scattering of xrays at the combined field of
two optical (or nearoptical) lasers and compare various scenarios.
%
In order to facilitate an experimental detection of quantum vacuum diffraction and
birefringence, special emphasis is placed on scenarios where the initial
and final xray photons differ not just in polarization, but also in
propagation direction (corresponding to scattering angles in the mrad regime)
and possibly energy.
Keywords: Strong Field QED; Vacuum qirefringence; Quantum vacuum diffraction; EulerHeisenberg Lagrangian; XFEL; Laser; Lightbylight scattering

Contribution to WWW
https://arxiv.org/abs/2208.14215 
Physical Review D 108(2023)7, 076005
DOI: 10.1103/PhysRevD.108.076005
Cited 3 times in Scopus
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Permalink: https://www.hzdr.de/publications/Publ36020
Resummed heatkernel and form factors for surface contributions: Dirichlet semitransparent boundary conditions
FranchinoVinas, S.
Abstract
In this article we consider resummed expressions for the heatkernel's
trace of a Laplace operator, the latter including a potential and imposing Dirichlet semitransparent boundary conditions on a surface of codimension one in flat space.
We obtain resummed expressions that correspond to the first and second order expansion of the heatkernel in powers of the potential.
We show how to apply these results to obtain the bulk and surface form factors of a scalar quantum field theory in $d=4$ with a Yukawa coupling to a background.
A characterization of the form factors in terms of pseudodifferential operators is given.

Contribution to WWW
https://arxiv.org/abs/2208.11979 
Journal of Physics A 56(2023), 115202
DOI: 10.1088/17518121/acbd26
Cited 2 times in Scopus
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Permalink: https://www.hzdr.de/publications/Publ36019
Geometrizing the Klein–Gordon and Dirac equations in Doubly Special Relativity
FranchinoVinas, S.; Relancio, J. J.
Abstract
In this work we discuss the deformed relativistic wave equations, namely the KleinGordon and Dirac equations in a Doubly Special Relativity scenario.
We employ what we call a geometric approach, based on the geometry of a curved momentum space, which should be seen as complementary to the more spread algebraic one.
In this frame we are able to rederive wellknown algebraic expressions, as well as to treat yet unresolved issues, to wit, the explicit relation between both equations, the discrete symmetries for Dirac particles, the fate of covariance, and the formal definition of a Hilbert space for the KleinGordon case.
Keywords: Doubly Special Relativity; KleinGordon equation; Dirac equation; Curved momentum space

Contribution to WWW
https://arxiv.org/abs/2203.12286 
Classical and Quantum Gravity 40(2023), 054001
DOI: 10.1088/13616382/acb4d4
Cited 12 times in Scopus
Downloads
 Open Access Version from arxiv.org
 Secondary publication expected
Permalink: https://www.hzdr.de/publications/Publ36017
Worldline instantons for the momentum spectrum of Schwinger pair production in spacetime dependent fields
Degli Esposti, G.; Torgrimsson, G.
Abstract
We show how to use the worldlineinstanton formalism to calculate the momentum spectrum of the electronpositron pairs produced by an electric field that depends on both space and time. Using the LSZ reduction formula with a worldline representation for the propagator in a spacetime field, we make use of the saddlepoint method to obtain a semiclassical approximation of the pairproduction spectrum. In order to check the final result, we integrate the spectrum and compare with the results obtained using a previous instanton method for the imaginary part of the effective action.

Contribution to WWW
https://arxiv.org/pdf/2212.11578.pdf 
Physical Review D 107(2023)5, 056019
DOI: 10.1103/PhysRevD.107.056019
Cited 5 times in Scopus
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Permalink: https://www.hzdr.de/publications/Publ36015
Identifying Time Scales in Particle Production from Fields
Diez, M.; Alkofer, R.; Kohlfürst, C.
Abstract
Particle production through ultrastrong electric fields is a wellstudied research field. Nevertheless, despite repeated attempts to relate the production rate within the field to the formation time of a particle, the latter is still shrouded in mystery. We provide an interpretation of a particle
distribution at finite times enabling us to isolate and, therefore, identify the relevant time scales
regarding particle formation in quantum physics within and beyond perturbation theory.
Keywords: Nonequilibrium Quantum Field Theory; StrongField Quantum Electrodynamics; Schwinger effect; Time scales

Contribution to WWW
arXiv:2211.07510 [hepph]: https://arxiv.org/pdf/2211.07510.pdf
DOI: 10.48550/arXiv.2211.07510 
Physics Letters B 844(2023), 138063
DOI: 10.1016/j.physletb.2023.138063
Cited 9 times in Scopus
Permalink: https://www.hzdr.de/publications/Publ35628
A glimpse into pion gravitational form factor
Abstract
We provide a novel approach to calculate the gravitational form factor of pion under the ladder approximation of the BetheSalpeter equation, with contact interactions. Central to this approach is a symmetrypreserving treatment of the dressed ππ amplitude, which shows explicitly the contributions from intrinsic quarks and bound states, the latter being necessary to produce the Dterm of pion in the softpion limit. The approach we provide in this work can be applied to many processes of physical significance.

Contribution to WWW
arXiv:2211.06635 [hepph]: https://arxiv.org/abs/2211.06635
arXiv: 2211.06635 
Physical Review D 107(2023), L031502
DOI: 10.1103/PhysRevD.107.L031502
Cited 8 times in Scopus
Permalink: https://www.hzdr.de/publications/Publ35627
Emergence of Hadron Mass and Structure
Ding, M.; Roberts, C. D.; Schmidt, S. M.
Abstract
Visible matter is characterised by a single mass scale; namely, the proton mass. The proton’s existence and structure are supposed to be described by quantum chromodynamics (QCD); yet, absent Higgs boson couplings, chromodynamics is scale invariant. Thus, if the Standard Model is truly a part of the theory of Nature, then the proton mass is an emergent feature of QCD; and emergent hadron mass (EHM) must provide the basic link between theory and observation. Nonperturbative tools are necessary if such connections are to be made; and in this context, we sketch recent progress in the application of continuum Schwinger function methods to an array of related problems in hadron and particle physics. Special emphasis is given to the three pillars of EHM – namely, the running gluon mass, processindependent effective charge, and running quark mass; their role in stabilising QCD; and their measurable expressions in a diverse array of observables.
Keywords: confinement of gluons and quarks; continuum Schwinger function methods; Dyson Schwinger equations

Particles 1(2023)6, 50120
DOI: 10.3390/particles6010004
Cited 38 times in Scopus
Permalink: https://www.hzdr.de/publications/Publ35519
Performance boost of a collective qutrit refrigerator
Abstract
A single qutrit with transitions selectively driven by weaklycoupled reservoirs can implement one of the world's smallest refrigerators. We analyze the performance of N such fridges that are collectively coupled to the reservoirs. We observe a quantum boost, manifest in a quadratic scaling of the steadystate cooling current with N. As N grows further, the scaling reduces to linear, since the transitions responsible for the quantum boost become energetically unfavorable. Finetuned interqutrit interactions may be used to maintain the quantum boost for all N and also for notperfectly collective scenarios.
Keywords: open quantum systems; Lindblad equation; Redfield equation; collective effects; quantum absorption refrigerator; qutrits; HolsteinPrimakoff transform; quadratic boost; nonequilibrium steady state

Contribution to WWW
arXiv:2210.07844 [quantph]: https://arxiv.org/abs/2210.07844
DOI: 10.48550/arXiv.2210.07844 
Physical Review Applied 19(2023), 034023
DOI: 10.1103/PhysRevApplied.19.034023
Cited 11 times in Scopus
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 Open Access Version from arxiv.org
 Secondary publication expected
Permalink: https://www.hzdr.de/publications/Publ35273
A correspondence between the free and interacting field theories
Gao, F.; Ding, M.; Liu, Y.; Schmidt, S. M.
Abstract
We discover a correspondence between the free field and the interacting states. This correspondence is firstly given from the fact that the free propagator can be converted into a tower of propagators for massive states, when expanded with the Hermite function basis. The equivalence of propagators reveals that in this particular case the duality can naturally be regarded as the equivalence of one theory on the plane wave basis to the other on the Hermite function basis. More generally, the Hermite function basis provides an alternative quantization process with the creation/annihilation operators that correspond directly to the interacting fields. Moreover, the Hermite function basis defines an exact way of dimensional reduction. As an illustration, we apply this basis on 3+1 dimensional YangMills theory with three dimensional space being reduced through the Hermite function basis, and if with only the lowest order Hermite function, the equivalent action becomes the BanksFischlerShenkerSusskind (BFSS) matrix model.

Contribution to WWW
https://arxiv.org/abs/2202.03191 
European Physical Journal C 83(2023), 144
DOI: 10.1140/epjc/s10052023112784
Permalink: https://www.hzdr.de/publications/Publ34235
The QED four – photon amplitudes offshell: part 1
Ahmadiniaz, N.; LopezArcos, C.; LopezLopez, M. A.; Schubert, C.
Abstract
The QED fourphoton amplitude has been wellstudied by many authors, and onshell is treated in many textbooks. However, a calculation with all four photons offshell is presently still lacking, despite of the fact that this amplitude appears offshell as a subprocess in many different contexts, in vacuum as well as with some photons connecting to external fields. The present paper is the first in a series of four where we use the worldline formalism to obtain this amplitude explicitly in terms of hypergeometric functions, and derivatives thereof, for both scalar and spinor QED. The formalism allows us to unify the scalar and spinor loop calculations, to avoid the usual breaking up of the amplitude into three inequivalent Feynman diagrams, and to achieve manifest transversality as well as UV finiteness at the integrand level by an optimized version of the integrationbyparts procedure originally introduced by Bern and Kosower for gluon amplitudes. The full permutation symmetry is maintained throughout, and the amplitudes get projected naturally into the basis of five tensors introduced by Costantini et al. in 1971. Since in many applications of the “fourphoton box” some of the photons can be taken in the lowenergy limit, and the formalism makes it easy to integrate out any such leg, apart from the case of general kinematics (part 4) we also treat the special cases of one (part 3) or two (part 2) photons taken at low energy. In this first part of the series, we summarize the application of the worldline formalism to the Nphoton amplitudes and its relationto Feynman diagrams, derive the optimized tensordecomposed integrands of the fourphoton amplitudes in scalar and spinor QED, and outline the computational strategy tobe followed in parts 2 to 4. We also give an overview of the applications of the fourphoton amplitudes, with an emphasis on processes that naturally involve some offshell photons, either because external fields are involved or we use the amplitude as a building block for higherorder process. The case where all photons are taken at low energy (the “EulerHeisenberg approximation”) is simple enough to be doable for arbitrary photon numbers,and we include it here for completeness
Keywords: Fourphoton amplitude; Worldline formalism; BernKosower formalism

Contribution to WWW
https://arxiv.org/pdf/2012.11791.pdf 
Nuclear Physics B 991(2023), 116216
DOI: 10.1016/j.nuclphysb.2023.116216
Cited 8 times in Scopus
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Permalink: https://www.hzdr.de/publications/Publ34076
Optical absorption and carrier multiplication at graphene edges in a magnetic field
Queisser, F.; Lang, S.; Schützhold, R.
Abstract
We study optical absorption at graphene edges in a transversal magnetic field. The magnetic field bends the trajectories of particle and hole excitations into antipodal direction which generates a directed current. We find a rather strong amplification of the edge current by impact ionization processes. More concretely, the primary absorption and the subsequent carrier multiplication is analyzed for a graphene fold and a zigzag edge. We identify exact and approximate selection rules and discuss the dependence of the decay rates on the initial state.
Keywords: Graphene; Auger processes; optical absorption

Contribution to WWW
arXiv:2107.14524 [condmat.meshall]: https://arxiv.org/abs/2107.14524 
Physical Review B 108(2023)045403
DOI: 10.1103/PhysRevB.108.045403
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Permalink: https://www.hzdr.de/publications/Publ33886
Hierarchy of doubletime correlations
Abstract
The hierarchy of correlations is an analytical approximation method which allows us to study nonequilibrium phenomena in strongly interacting quantum manybody systems on lattices in higher dimensions. So far, this method was restricted to equaltime correlators ⟨A ^ μ (t)B ^ ν (t)⟩ . In this work, we generalize this method to doubletime correlators ⟨A ^ μ (t)B ^ ν (t ′ )⟩ , which allows us to study effective light cones and Green functions and to incorporate finite initial temperatures.

Contribution to WWW
arXiv:1909.10938: https://arxiv.org/abs/1909.10938 
Journal of Statistical Mechanics: Theory and Experiment 5(2023), 053101
DOI: 10.1088/17425468/acccde
Cited 3 times in Scopus
Permalink: https://www.hzdr.de/publications/Publ29916