arXiv subjects

Ralf Schützhold

Publications and source records attributed to Ralf Schützhold.

At least 19 recordsLinked to original sources

arXiv2026

Evolution of terahertz third harmonic response across rare-earth nickelate phase-diagram

High harmonic generation (HHG) is a sensitive probe for investigating electronic structures and dynamics of materials and a source for attosecond pulses. In particular, HHG with terahertz (THz) light can enable probing of nonlinear responses in correlated materials arising from low-energy many-body interactions. However, THz HHG studies have so far largely focused on topological materials and superconductors, leaving out other potential material systems which could also become efficient THz HHG sources. Here, we report THz third harmonic generation (THG) in rare-earth nickelates -- a prototype material for exploring the Mott insulator-metal transition and related technological applications. We find that the THG amplitude is highly sensitive to the strengths of electronic and magnetic phases of nickelates. In films with sharp phase-transitions, the local maximum and minimum in the temperature-dependent THG amplitude coincide with insulator-metal and magnetic transition temperatures, respectively. While in films with weaker transitions, these features shift toward lower temperatures or even monotonous THG enhancement is observed down to low temperatures. We developed a generalized theory for THz harmonic generation in negative charge-transfer insulators and outlined strategies to enhance the THz nonlinearities further. Our study broadens the scope of THz HHG studies and related applications to strongly correlated materials.

cond-mat.str-el↗

arXiv2026

Quantum Zeno effect versus adiabatic quantum computing and quantum annealing

For the adiabatic version of Grover's quantum search algorithm as proposed by Roland and Cerf, we study the impact of decoherence caused by a rather general coupling to some environment. For quite generic conditions, we find that the quantum Zeno effect poses strong limitations on the performance (quantum speed-up) since the environment effectively measures the state of the system permanently and thereby inhibits or slows down quantum transitions. Generalizing our results, we find that similar restrictions should apply universally to adiabatic quantum algorithms and quantum annealing schemes which are based on analogous isolated Landau-Zener type transitions at avoided level crossings (similar to first-order phase transitions). As a possible resort, more gradual changes of the quantum state (as in second-order phase transitions) or suitable error-correcting schemes such as the spin-echo method may alleviate this problem.

quant-ph↗

arXiv2026

Disorder-induced localisation in the Mott-Hubbard model

For the Fermi-Hubbard model in the Mott insulator phase, we employ the hierarchy of correlations to study how doublon and holon quasi-particle excitations are affected by adding disorder to the system. We study two types of disorder: charge disorder, in the form of on-site potential randomness; and spin disorder, in the form of a fixed, randomly generated background spin arrangement. By analysing the quasi-particle eigen-spectra and quantifying the degree to which the corresponding eigen-states localise, we find both an energetic and spatial separation between localised and delocalised states in the charge disorder. In contrast, the spin disorder results in localised states throughout the quasi-particle bands. Finally, we repeat our calculations using strong-coupling perturbation theory, and compare the results obtained from both methods.

cond-mat.str-el↗

arXiv2026

Higher harmonics in Mott-Hubbard insulators as sensors

Using strong-coupling time-dependent perturbation theory, we study the response of Mott and charge-transfer insulators to an oscillating electric field. We derive analytical expressions for the resulting higher-harmonic currents and show that they encode information about spin order and microscopic hopping pathways. The results demonstrate that higher harmonics can serve as probes of correlated materials and as sensors of the applied driving field.

cond-mat.str-el↗

arXiv2025

Magneto-photoelectric effect in graphene via tailored potential landscapes

We consider the propagation of charge carriers in planar graphene under the combined influence of a constant transversal magnetic field $B$ and an in-plane varying electric potential $ϕ(x)$. By suitably designing the potential landscape $ϕ(x)$, we may effectively steer charge carriers generated by photo-excitation, for example, in order to achieve an efficient charge separation. These finding may pave the way for transport schemes or photoelectric/photovoltaic applications.

cond-mat.mes-hall↗

arXiv2025

Kibble-Zurek dynamics in the anisotropic Ising model of the Si(001) surface

As a simplified description of the non-equilibrium dynamics of buckled dimers on the Si(001) surface, we consider the anisotropic 2D Ising model and study the freezing of spatial correlations during a cooling quench across the critical point. Depending on the cooling rate, we observe a crossover from 1D to 2D behavior. For rapid cooling, we find effectively 1D behavior in the strongly coupled direction, for which we provide an exact analytic solution of the non-equilibrium dynamics. For slower cooling rates, we start to see 2D behavior where our numerical simulations show an approach to the usual Kibble-Zurek scaling in 2D.

cond-mat.stat-mech↗

arXiv2025

Ultra-cold atoms as quantum simulators for relativistic phenomena

The goal of this article is to review developments regarding the use of ultra-cold atoms as quantum simulators. Special emphasis is placed on relativistic quantum phenomena, which are presumably most interesting for the audience of this journal. After a brief introduction into the main idea of quantum simulators and the basic physics of ultra-cold atoms, relativistic quantum phenomena of linear fields are discussed, including Hawking radiation, the Unruh effect, cosmological particle creation, the Gibbons-Hawking and Ginzburg effects, super-radiance, Sauter-Schwinger and Breit-Wheeler pair creation, as well as the dynamical Casimir effect. After that, the focus is shifted to phenomena of non-linear fields, such as the sine-Gordon model, the Kibble-Zurek mechanism, false-vacuum decay, and quantum back-reaction. In order to place everything into proper context, the basic underlying mechanisms of these phenomena are briefly recapitulated before their simulators are discussed. Even though effort is made to provide a review as fair as possible, there can be co claim of completeness and the selection as well as the relative weights of the topics may well reflect the personal view and taste of the author.

quant-ph↗

arXiv2025

Relaxation to persistent currents in a Hubbard trimer coupled to fermionic baths

We consider a ring of fermionic quantum sites, modeled by the Fermi--Hubbard 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 low--energy 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.

cond-mat.str-el↗

arXiv2025

Stimulated emission or absorption of gravitons by light

We study the exchange of energy between gravitational and electromagnetic waves in a Sagnac type geometry, in analogy to an ``optical Weber bar.'' In the presence of a gravitational wave (such as the ones measured by LIGO), we find that it should be possible to observe signatures of stimulated emission or absorption of gravitons with present day technology. Apart from marking the transition from passively observing to actively manipulating such a natural phenomenon, this could also be used as a complementary detection scheme. Non-classical photon states may improve the sensitivity and might even allow us to test certain quantum aspects of the gravitational field.

gr-qc↗

arXiv2025

Effective time-dependent temperature for fermionic master equations beyond the Markov and the secular approximations

We consider a fermionic quantum system exchanging particles with an environment at a fixed temperature and study its reduced evolution by means of a Redfield-I equation with time-dependent (non-Markovian) coefficients. We find that the description can be efficiently reduced to a standard-form Redfield-II equation, however, with a time-dependent effective bath temperature obeying a universal law. At early times, after the system and environment start in a product state, the effective temperature appears to be very high, yet eventually it settles down towards the true environment value. In this way, we obtain a time-local master equation, offering high accuracy at all times and preserving the crucial properties of the density matrix. It includes non-Markovian relaxation processes beyond the secular approximation and time-averaging methods and can be further applied to various types of Gorini-Kossakowski-Sudarshan-Lindblad equations. We derive the theory from first principles and discuss its application using a simple example of a single quantum dot.

cond-mat.mes-hall↗

arXiv2024

Quasi-particle propagation across semiconductor-Mott insulator interfaces

As a prototypical example for a heterostructure combining a weakly and a strongly interacting quantum many-body system, we study the interface between a semiconductor and a Mott insulator. Via the hierarchy of correlations, we derive and match the propagating or evanescent (quasi) particle solutions on both sides and assume that the interactions among the electrons in the semiconducting regions can be absorbed by an effective potential. While the propagation is described by a band-like dispersion in both the weakly and the strongly interacting case, the inverse decay length across the interface follows a different dependence on the band gap in the Mott insulator and the semiconductor. As one consequence, tunnelling through a Mott insulating layer behaves quite differently from a semiconducting (or band insulating) layer. For example, we find a strong suppression of tunnelling for energies in the middle between the upper and lower Hubbard band of the Mott insulator.

cond-mat.str-el↗

arXiv2024

Space-time toy model for Hawking radiation

By gluing together two sections of flat space-time in different metric representations (with the Minkowski metric representing the region far away from the black hole and the Rindler metric modeling the vicinity of the horizon), we construct a simplified toy model for black-hole evaporation. The simple structure of this toy model allows us to construct exact analytic solutions for the two-point functions in the various vacuum states (Israel-Harte-Hawking, Unruh and Boulware states) in an easy way and thus helps to understand and disentangle the different ingredients for Hawking radiation better.

gr-qc↗

arXiv2024

Continuous Dimer Angles on the Silicon Surface: Critical Properties and the Kibble-Zurek Mechanism

Langevin dynamics simulations are used to analyze the static and dynamic properties of an {XY} model adapted to dimers forming on Si(001) surfaces. The numerics utilise high-performance parallel computation methods on GPUs. The static exponent $ν$ of the symmetry-broken XY model is determined to $ν= 1.04$. The dynamic critical exponent $z$ is determined to $z = 2.13$ and, together with $ν$, shows the behavior of the Ising universality class. For time-dependent temperatures, we observe frozen domains and compare their size distribution with predictions from Kibble-Zurek theory. We determine a significantly larger quench exponent that shows little dependence on the damping or the symmetry-breaking field.

cond-mat.stat-mech↗

arXiv2024

Axion-induced Casimir force between nuclei and dynamical axion pair creation

We study the interaction between axions and nuclei by combining the Peccei-Quinn mechanism with results from quantum chromo-dynamics (QCD) which imply that the QCD condensates are reduced within nuclear matter. Thus, the effective axion mass is also reduced, yielding a finite axion-nucleon scattering cross section. Even in the absence of real axions, this interaction would manifest itself in a Casimir type attraction between two nuclei. Finally, accelerated nuclei can create entangled pairs of axions via the dynamical Casimir effect (or as signatures of the Unruh effect).

hep-ph↗

arXiv2024

Floquet analysis of a superradiant many-qutrit refrigerator

We investigate superradiant enhancements in the refrigeration performance of a set of $N$ three-level systems that are collectively coupled to a hot and a cold thermal reservoir and are additionally subject to collective periodic (circular) driving. Assuming the system-reservoir coupling to be weak, we explore the regime of stronger periodic driving strengths by comparing collective weak driving, Floquet-Lindblad, and Floquet-Redfield 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 Floquet-Lindblad 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.

quant-ph↗

arXiv2024

Critical behavior of the dimerized Si(001) surface: Continuous order-disorder phase transition in the two-dimensional Ising universality class

The critical behavior of the order-disorder phase transition in the buckled dimer structure of the Si(001) surface is investigated both theoretically by means of first-principles calculations and experimentally by spot profile analysis low-energy electron diffraction (SPA-LEED). 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 low-temperature $c(4 {\times} 2)$- to the high-temperature $p(2 {\times} 1)$-reconstructed surface is followed through the intensity and width of the superstructure spots within the temperature range 78-400 K. Near the critical temperature $T_c = 190.6\,\mathrm{K}$, we observe universal critical behavior of spot intensities and correlation lengths which falls into the universality class of the two-dimensional (2D) Ising model. From the ratio of correlation lengths along and across the dimer rows we determine effective nearest-neighbor couplings of an anisotropic 2D Ising model, $J_\parallel = (-24.9 \pm 0.9_\mathrm{stat} \pm 1.3_\mathrm{sys})\,\mathrm{meV}$ and $J_\perp = (-0.8 \pm 0.1_\mathrm{stat})\,\mathrm{meV}$. 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 $c(4 {\times} 2)$-reconstructed ground state.

cond-mat.mes-hall↗

arXiv2023

Momentum space separation of quantum path interferences between photons and surface plasmon polaritons in nonlinear photoemission microscopy

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 energy-degenerate 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 time-resolved 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, will ultimately allow nano-optical quantum-correlations to be studied without destroying the quantum path interferences.

cond-mat.mes-hall↗

arXiv2023

Back-reaction and correlation effects on pre-thermalization in Mott-Hubbard systems

For the Fermi-Hubbard model in the strongly interacting Mott insulator state, we study the pre-thermalization dynamics after a quench. To this end, we employ the method of the hierarchy of correlations and compare different levels of accuracy. To leading order, the usual free quasi-particle dynamics (as encoded in the two-point correlation functions) yields the standard picture of pre-thermalization. Taking into account the back-reaction of these quasi-particle fluctuations onto the mean-field background as the first next-to-leading order effect, we observe a strong degradation of pre-thermalization, especially in low dimensions. In contrast, the inclusion of three-point correlations enhances pre-thermalization.

cond-mat.str-el↗