arXiv subjects

Ralf Schützhold

Publications and source records attributed to Ralf Schützhold.

At least 37 records · Page 2Linked to original sources

arXiv2023

Search for axion resonances in vacuum birefringence with three-beam collisions

We consider birefringent (i.e., polarization changing) scattering of x-ray photons at the superposition of two optical laser beams of ultra-high intensity and study the resonant contributions of axions or axion-like particles, which could also be short-lived. Applying the specifications of the Helmholtz International Beamline for Extreme Fields (HIBEF), we find that this set-up can be more sensitive than previous light-by-light scattering (birefringence) or light-shining-through-wall 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.

hep-ph↗

arXiv2023

Attraction versus repulsion between doublons or holons in Mott-Hubbard systems

For the Mott insulator state of the Fermi-Hubbard model in the strong-coupling limit, we study the interaction between quasi-particles in the form of doublons and holons. Comparing different methods -- the hierarchy of correlations, strong-coupling 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 $T$ which can display attractive as well as repulsive contributions, depending on the involved momenta. Finally, we speculate about the implications of our findings for high-temperature superconductivity.

cond-mat.str-el↗

arXiv2023

Dynamically Assisted Tunneling in the Floquet Picture

We study how tunneling through a potential barrier $V(x)$ can be enhanced by an additional harmonically oscillating electric field ${\mathfrak E}(t)={\mathfrak E}_0\cos(ωt)$. To this end, we transform into the Kramers-Henneberger 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 time-averaged 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.

quant-ph↗

arXiv2023

Higher-harmonic generation in the driven Mott-Hubbard model

Using Floquet theory and the hierarchy of correlations, we study the non-equilibrium dynamics of the Mott insulator state in the Fermi-Hubbard model under the influence of a harmonically oscillating electric field representing the pump laser. After deriving the associated Floquet exponents, we consider higher-harmonic generation where the strongest signal is obtained if the driving frequency equals one third of the Mott gap.

cond-mat.str-el↗

arXiv2023

Assisted neutrino pair production in combined external fields

Neutrino--antineutrino ($ν\barν$) pair production is one of the main processes responsible for the energy loss of stars. Apart from the collision of two ($γγ\toν\barν$) or three ($γγγ\toν\barν$) real photons, pair creation from a photon and photon collisions in the presence of nuclear Coulomb fields or external magnetic fields have been considered previously. Here, we study a pair production of neutrino and antineutrino from a low-energy photon in the presence of a combined homogeneous magnetic field and the Coulomb field of a nucleus with charge number $Z$.

hep-ph↗

arXiv2023

Doublon-holon pair creation in Mott-Hubbard systems in analogy to QED

Via the hierarchy of correlations, we study doublon-holon pair creation in the Mott state of the Fermi-Hubbard model induced by a time-dependent electric field. Special emphasis is placed on the analogy to electron-positron pair creation from the vacuum in quantum electrodynamics (QED). We find that the accuracy of this analogy depends on the spin structure of the Mott background. For Ising type anti-ferromagnetic order, we derive an effective Dirac equation. A Mott state without any spin order, on the other hand, does not explicitly display such a quasi-relativistic behavior.

cond-mat.str-el↗

arXiv2023

Energy transfer between gravitational waves and quantum matter

We study the interaction between gravitational waves and quantum matter such as Bose-Einstein condensates, super-fluid Helium, or ultra-cold 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.

quant-ph↗

arXiv2023

Dimer Coupling Energies of the Si(001) Surface

The coupling energies between the buckled dimers of the Si(001) surface were determined through analysis of the anisotropic critical behavior of its order-disorder phase transition. Spot profiles in high-resolution low-energy electron diffraction as a function of temperature were analyzed within the framework of the anisotropic two-dimensional Ising model. The validity of this approach is justified by the large ratio of correlation lengths, $ξ_\parallel^+/ξ_\perp^+ = 5.2$ of the fluctuating $c(4 {\times} 2)$ domains above the critical temperature $T_\mathrm{c} = (190.6 \pm 10)$ K. We obtain effective couplings $J_\parallel = (-24.9 \pm 1.3)$ meV along the dimer rows and $J_\perp = (-0.8 \pm 0.1)$ meV across the dimer rows, i.e., antiferromagnetic-like coupling of the dimers with $c(4 {\times} 2)$ symmetry.

cond-mat.mes-hall↗

arXiv2022

Sauter-Schwinger effect for colliding laser pulses

Via a combination of analytical and numerical methods, we study electron-positron pair creation by the electromagnetic field ${\bf A}(t,{\bf r})=[f(ct-x)+f(ct+x)]{\bf e}_y$ of two colliding laser pulses. Employing a generalized WKB approach, we find that the pair creation rate along the symmetry plane $x=0$ (where one would expect the maximum contribution) displays the same exponential dependence as for a purely time-dependent electric field ${\bf A}(t)=2f(ct){\bf e}_y$. The pre-factor in front of this exponential does also contain corrections due to focusing or de-focusing effects induced by the spatially inhomogeneous magnetic field. We compare our analytical results to numerical simulations using the Dirac-Heisenberg-Wigner method and find good agreement.

hep-ph↗

arXiv2022

Ginzburg effect in a dielectric medium with dispersion and dissipation

As a quantum analog of Cherenkov radiation, an inertial photon detector moving through a medium with constant refractive index $n$ may perceive the electromagnetic quantum fluctuations as real photons if its velocity $v$ exceeds the medium speed of light $c/n$. For dispersive Hopfield type media, we find this Ginzburg effect to extend to much lower $v$ because the phase velocity of light is very small near the medium resonance. In this regime, however, dissipation effects become important. Via an extended Hopfield model, we present a consistent treatment of quantum fluctuations in dispersive and dissipative media and derive the Ginzburg effect in such systems. Finally, we propose an experimental test.

quant-ph↗

arXiv2022

Environment-induced decay dynamics of anti-ferromagnetic order in Mott-Hubbard systems

We study the dissipative Fermi-Hubbard model in the limit of weak tunneling and strong repulsive interactions, where each lattice site is tunnel-coupled to a Markovian fermionic bath. For cold baths at intermediate chemical potentials, the Mott insulator property remains stable and we find a fast relaxation of the particle number towards half filling. On longer time scales, we find that the anti-ferromagnetic order of the Mott-Néel ground state on bi-partite lattices decays, even at zero temperature. For zero and non-zero temperatures, we quantify the different relaxation time scales by means of waiting time distributions which can be derived from an effective (non-Hermitian) Hamiltonian and obtain fully analytic expressions for the Fermi-Hubbard model on a tetramer ring.

cond-mat.str-el↗

arXiv2021

Decay of quantum sensitivity due to three-body loss in Bose-Einstein condensates

In view of the coherent properties of a large number of atoms, Bose-Einstein Condensates (BECs) have a high potential for sensing applications. Several proposals have been put forward to use collective excitations such as phonons in BECs for quantum enhanced sensing in quantum metrology. However, the associated highly non-classical states tend to be very vulnerable to decoherence. In this article, we investigate the effect of decoherence due to the omnipresent process of three-body loss in BECs. We find strong restrictions for a wide range of parameters and we discuss possibilities to limit these restrictions.

quant-ph↗

arXiv2021

Optical absorption and carrier multiplication at graphene edges in a magnetic field

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.

cond-mat.mes-hall↗

arXiv2021

Dynamically assisted tunneling in the impulse regime

We study the enhancement of tunneling through a potential barrier $V(x)$ by a time-dependent electric field with special emphasis on pulse-shaped vector potentials such as $A_x(t)=A_0/\cosh^2(ωt)$. In addition to the known effects of pre-acceleration and potential deformation already present in the adiabatic regime, as well as energy mixing in analogy to the Franz-Keldysh effect in the non-adiabatic (impulse) regime, the pulse $A_x(t)$ can enhance tunneling by ``pushing'' part of the wave-function out of the rear end of the barrier. Besides the natural applications in condensed matter and atomic physics, these findings could be relevant for nuclear fusion, where pulses $A_x(t)$ with $ω=1~\rm keV$ and peak field strengths of $10^{16}~\rm V/m$ might enhance tunneling rates significantly.

quant-ph↗

arXiv2020

Quantum radiation in dielectric media with dispersion and dissipation

By a generalization of the Hopfield model, we construct a microscopic Lagrangian describing a dielectric medium with dispersion and dissipation. This facilitates a well-defined and unambiguous $\textit{ab initio}$ treatment of quantum electrodynamics in such media, even in time-dependent backgrounds. As an example, we calculate the number of photons created by switching on and off dissipation in dependence on the temporal switching function. This effect may be stronger than quantum radiation produced by variations of the refractive index $Δn(t)$ since the latter are typically very small and yield photon numbers of order $(Δn)^2$. As another difference, we find that the partner particles of the created medium photons are not other medium photons but excitations of the environment field causing the dissipation (which is switched on and off).

quant-ph↗

arXiv2020

Relaxation dynamics in a Hubbard dimer coupled to fermionic baths: phenomenological description and its microscopic foundation

We study relaxation dynamics in a strongly-interacting two-site Fermi-Hubbard model that is induced by coupling each site to a local fermionic bath. To derive the proper form of the Lindblad operators that enter an effective description of the system-bath coupling in different temperature regimes, we employ a diagrammatic real-time technique for the time evolution of the reduced density matrix. In spite of a local coupling to the baths, the found Lindblad operators are non-local in space. We compare with the local approximation, where those non-local effects are neglected. Furthermore, we propose an improvement on the commonly-used secular approximation (rotating-wave approximation), referred to as coherent approximation, which turns out superior in all studied parameter regimes (and equivalent otherwise). We look at the relaxation dynamics for several important observables and compare the methods for early and late times in various temperature regimes.

cond-mat.str-el↗

arXiv2020

Comment on "Enhanced deuterium-tritium fusion cross sections in the presence of strong electromagnetic fields"

In their article [Phys.\ Rev.\ C {\bf 100}, 064610 (2019)], Lv, Duan, and Liu study the enhancement of deuterium-tritium fusion reactions by the electromagnetic field of an x-ray free-electron laser (XFEL). While we support the general idea (which was put forward earlier in our rapid communication [Phys.\ Rev.\ C {\bf 100}, 041601(R) (2019)]), we find that the time-averaged potential approximation used by Lv, Duan, and Liu is not justified in this regime and does not take into account important effects. Due to those effects, the enhancement mechanism may actually be more efficient than predicted by Lv, Duan, and Liu.

nucl-th↗

arXiv2019

Boltzmann relaxation dynamics of strongly interacting spinless fermions on a lattice

Motivated by the recent interest in non-equilibrium phenomena in quantum many-body systems, we study strongly interacting fermions on a lattice by deriving and numerically solving quantum Boltzmann equations that describe their relaxation to thermodynamic equilibrium.The derivation is carried out by inspecting the hierarchy of correlations within the framework of the 1/Z-expansion. Applying the Markov approximation, we obtain the dynamic equations for the distribution functions. Interestingly, we find that in the strong-coupling limit, collisions between particles and holes dominate over particle-particle and hole-hole collisions -- in stark contrast to weakly interacting systems. As a consequence, our numerical simulations show that the relaxation time scales strongly depend on the type of excitations (particles or holes or both) that are initially present.

quant-ph↗