arXiv subjects

Peter Reimann

Publications and source records attributed to Peter Reimann.

At least 19 recordsLinked to original sources

arXiv2026

Absence of thermalization after a local quench and strong violation of the eigenstate thermalization hypothesis

Absence of thermalization after a global quantum quench is a well-established numerical observation in integrable many-body systems, and can be empirically related to a violation of the eigenstate thermalization hypothesis (ETH) in such models. Still, in many of those examples a weaker version of the conventional ETH (wETH) has been numerically reported or even rigorously proven. In this paper we show analytically and illustrate numerically that the absence of thermalization is already possible after a local quench. A closely related finding is a strong violation of the ETH, meaning that not even the wETH is fulfilled anymore. In our analytical explorations we focus on XX-spin-chain models with open boundary conditions, where the local quench is generated by initiating the system in thermal equilibrium and then suddenly switching on (or slightly changing) a single-spin impurity either at the end or in the center of the chain. Numerically we observe qualitatively similar phenomena also for more general XXZ-models in the case of an end-impurity, but not in the case of a central impurity.

cond-mat.stat-mech↗

arXiv2026

Basis dependence of eigenstate thermalization

Eigenstate thermalization refers to the property that an energy eigenstate of a many-body system is indistinguishable from a thermal equilibrium ensemble at the same energy as far as expectation values of local observables are concerned. In systems with degeneracies, the choice of an energy eigenbasis is not unique and the fraction of basis states exhibiting eigenstate thermalization can vary. We present a simple example where this fraction vanishes in the thermodynamic limit for one basis choice, but remains nonzero for another choice. In other words, the weak eigenstate thermalization hypothesis is satisfied in the first, but violated in the second basis. We furthermore prove that degeneracies must abound whenever a system is simultaneously symmetric under spatial translations and reflection. Finally, we derive general bounds on how strongly eigenstate thermalization may depend on the choice of the basis, and we reveal some interesting implications regarding the temporal relaxation properties of such systems.

cond-mat.stat-mech↗

arXiv2025

Dynamical typicality in classical lattice systems

Considering deterministic classical lattice systems with continuous variables, we show that, if the initial conditions are sampled according to a probability distribution in which the dynamical variables are statistically independent, the dynamical trajectory of any macroscopic observable is approximately the same for the vast majority of the states in the sample. Our proof relies on general concentration of measure results which provide tight bounds for the deviation from typical behavior in the case of large system sizes. The only condition that we assume for the dynamics is that the influence of a local perturbation in the initial state decays sufficiently fast with distance at any finite time. Our results are relevant, in particular, to classical Hamiltonian systems on a lattice. We apply our general results to a system of coupled rotors with long-range interactions, and report dynamical simulations which verify our findings.

cond-mat.stat-mech↗

arXiv2025

Similarities and differences of typicality in quantum and classical systems

Typicality is a well-established and very general property of quantum many-body systems, referring to the phenomenon that the expectation values of any given observable are practically indistinguishable for the overwhelming majority of all pure states (normalized vectors) in a sufficiently high-dimensional Hilbert (sub-)space. Here, we provide very simple and general arguments that analogous typicality properties of pure states (phase space points) in classical many-body systems are still expected to hold true for macroscopic observables, but not any more for microscopic (few-body) observables.

cond-mat.stat-mech↗

arXiv2025

Thermalization in a simple spin-chain model

We consider the common spin-1/2 XX-model in one dimension with open boundary conditions and a large but finite number of spins. The system is in thermal equilibrium at times t<0, and is subject to a weak local perturbation (quantum quench) at t=0. Focusing mainly on single-spin perturbations and observables, we show that the system re-thermalizes for sufficiently large times t>0 without invoking any unproven assumptions besides the basic laws of quantum mechanics. Moreover, the time-dependent relaxation behavior is obtained in quantitative detail and is found to exhibit a wealth of interesting features.

cond-mat.stat-mech↗

arXiv2024

Random-matrix approach to time-dependent forcing in many-body quantum systems

Changing some of its parameters over time is a paradigmatic way of driving an otherwise isolated many-body quantum system out of equilibrium, and a vital ingredient for building quantum computers and simulators. Here, we further develop a recently proposed nonlinear response theory which is based on typicality and random-matrix methods, and which is applicable to a wide variety of such parametrically perturbed systems in and out of equilibrium: We derive analytical approximations of the characteristic response function for the two limiting cases of fast driving and of strong and short-ranged-in-energy driving. Furthermore, we work out implications and predictions for common applications, including finite-time quenches and time-dependent forcing that breaks conservation laws of the underlying undriven system. Finally, we verify all predictions by numerical examples and discuss the theory's scope and limitations.

quant-ph↗

arXiv2024

Onsager's regression hypothesis adjusted to quantum systems

Onsager's regression hypothesis connects the temporal relaxation of close-to-equilibrium systems with their dynamical correlation functions at thermal equilibrium. While the hypothesis is provably correct in classical systems, it is known to fail in the quantum regime. Here, we derive a suitably adjusted quantum version of Onsager's original hypothesis. Rigorous analytical results are complemented by a variety of numerical examples.

cond-mat.stat-mech↗

arXiv2024

Stalled response near thermal equilibrium in periodically driven systems

The question of how systems respond to perturbations is ubiquitous in physics. Predicting this response for large classes of systems becomes particularly challenging if many degrees of freedom are involved and linear response theory cannot be applied. Here, we consider isolated many-body quantum systems which either start out far from equilibrium and then thermalize, or find themselves near thermal equilibrium from the outset. We show that time-periodic perturbations of moderate strength, in the sense that they do not heat up the system too quickly, give rise to the following phenomenon of stalled response: While the driving usually causes quite considerable reactions as long as the unperturbed system is far from equilibrium, the driving effects are strongly suppressed when the unperturbed system approaches thermal equilibrium. Likewise, for systems prepared near thermal equilibrium, the response to the driving is barely noticeable right from the beginning. Numerical results are complemented by a quantitatively accurate analytical description and by simple qualitative arguments.

cond-mat.stat-mech↗

arXiv2023

Allosteric impurity effects in long spin chains

Allosterism traditionally refers to local changes in an extended object, for instance the binding of a ligand to a macromolecule, leading to a localized response at some other, possibly quite remote position. Here, we show that such fascinating effects may already occur in very simple and common quantum many-body systems, such as an anisotropic Heisenberg spin chain: Introducing an impurity at one end of a sufficiently long chain may lead to quite significant changes of the observable behavior near the other end, but not in the much larger region in between. Specifically, spin autocorrelation functions at thermal equilibrium are found to exhibit a pronounced allosterism of this type.

cond-mat.stat-mech↗

arXiv2022

Thermalization of locally perturbed many-body quantum systems

Deriving conditions under which a macroscopic system thermalizes directly from the underlying quantum many-body dynamics of its microscopic constituents is a long-standing challenge in theoretical physics. The well-known eigenstate thermalization hypothesis (ETH) is presumed to be a key mechanism, but has defied rigorous verification for generic systems thus far. A weaker variant (weak ETH), by contrast, is provably true for a large variety of systems, including even many integrable models, but its implications with respect to the problem of thermalization are still largely unexplored. Here we analytically demonstrate that systems satisfying the weak ETH exhibit thermalization for two very natural classes of far-from-equilibrium initial conditions: the overwhelming majority of all pure states with a preset non-equilibrium expectation value of some given local observable, and the Gibbs states of a Hamiltonian which subsequently is subject to a quantum quench in the form of a sudden change of some local system properties.

cond-mat.stat-mech↗

arXiv2021

Symmetry-prohibited thermalization after a quantum quench

The observable long-time behavior of an isolated many-body system after a quantum quench is considered, i.e., an eigenstate (or an equilibrium ensemble) of some pre-quench Hamiltonian $H$ serves as initial condition which then evolves in time according to some post-quench Hamiltonian $H_p$. Absence of thermalization is analytically demonstrated for a large class of quite common pre- and post-quench spin Hamiltonians. The main requirement is that the pre-quench Hamiltonian must exhibit a $Z_2$ (spin-flip) symmetry, which would be spontaneously broken in the thermodynamic limit, though we actually focus on finite (but large) systems. On the other hand, the post-quench Hamiltonian must violate the $Z_2$ symmetry, but for the rest may be non-integrable and may obey the eigenstate thermalization hypothesis for (sums of) few-body observables.

cond-mat.stat-mech↗

arXiv2021

Refining Deutsch's approach to thermalization

The ground breaking investigation by Deutsch [Phys. Rev. A 43, 2046 (1991)] of how closed many-body quantum systems approach thermal equilibrium is revisited. It is shown how to carry out some important steps which were still missing in that paper. Moreover, the class of admitted systems is considerably extended.

cond-mat.stat-mech↗

arXiv2021

Typical relaxation of perturbed quantum many-body systems

We substantially extend our relaxation theory for perturbed many-body quantum systems from [Phys. Rev. Lett. 124, 120602 (2020)] by establishing an analytical prediction for the time-dependent observable expectation values which depends on only two characteristic parameters of the perturbation operator: its overall strength and its range or band width. Compared to the previous theory, a significantly larger range of perturbation strengths is covered. The results are obtained within a typicality framework by solving the pertinent random matrix problem exactly for a certain class of banded perturbations and by demonstrating the (approximative) universality of these solutions, which allows us to adopt them to considerably more general classes of perturbations. We also verify the prediction by comparison with several numerical examples.

cond-mat.stat-mech↗

arXiv2020

Modification of quantum many-body relaxation by perturbations exhibiting a banded matrix structure

We investigate how the observable relaxation behavior of an isolated quantum many-body system is modified in response to weak-to-moderate perturbations within a nonperturbative typicality framework. A key role is played by the so-called perturbation profile, which characterizes the dependence of the perturbation matrix elements in the eigenbasis of the unperturbed Hamiltonian on the difference of the corresponding energy eigenvalues. In particular, a banded matrix structure is quantitatively captured by a perturbation profile which approaches zero for large energy differences. The temporal modification of the relaxation is linked to the perturbation profile via a nonlinear integral equation, which admits approximate analytical solutions for sufficiently weak and strong perturbations, and for which we work out a numerical solution scheme in the general case. As an example, we consider a spin lattice model with a pronounced banded matrix structure, and we find very good agreement of the numerics with our analytical predictions without any free fit parameter.

cond-mat.stat-mech↗

arXiv2020

Why are macroscopic experiments reproducible? Imitating the behavior of an ensemble by single pure states

Evidently, physical experiments are practically reproducible even though the fully identical preparation of initial state wave functions is often far beyond experimental possibilities. It is thus natural to explore if and in which sense specific, uncontrollable features of initial wave functions are irrelevant for the observable course of an experiment. To this end we define ensembles of pure states which are then shown to generate extremely similar non-equilibrium dynamics of the expectation values of practically all standard observables. The ensembles are constructed to comply with some reduced, coarse a priori information on the state of the system, like, e.g. a few specific expectation values, etc. However, different types of ensembles with different additional properties are possible. We discuss some of them.

cond-mat.stat-mech↗

arXiv2020

Persistent many-body quantum echoes

We consider quantum many-body systems evolving under a time-independent Hamiltonian $H$ from a nonequilibrium initial state at time $t=0$ towards a close-to-equilibrium state at time $t=τ$. Subsequently, this state is slightly perturbed and finally propagated for another time period $τ$ under the inverted Hamiltonian $-H$. The entire procedure may also be viewed as an imperfect time inversion or "echo dynamics". We unravel a remarkable persistence of such dynamics with respect to the observable deviations of the time-dependent expectation values from the equilibrium expectation value: For most perturbations, the deviations in the final state are essentially independent of the inversion time point $τ$. Our quantitative analytical predictions compare very well with exact numerical results.

cond-mat.stat-mech↗

arXiv2020

Predicting Imperfect Echo Dynamics in Many-Body Quantum Systems

Echo protocols provide a means to investigate the arrow of time in macroscopic processes. Starting from a nonequilibrium state, the many-body quantum system under study is evolved for a certain period of time $\tau$. Thereafter, an (effective) time reversal is performed that would -- if implemented perfectly -- take the system back to the initial state after another time period $\tau$. Typical examples are nuclear magnetic resonance imaging and polarization echo experiments. The presence of small, uncontrolled inaccuracies during the backward propagation results in deviations of the "echo signal" from the original evolution, and can be exploited to quantify the instability of nonequilibrium states and the irreversibility of the dynamics. We derive an analytic prediction for the typical dependence of this echo signal for macroscopic observables on the magnitude of the inaccuracies and on the duration $\tau$ of the process, and verify it in numerical examples.

cond-mat.stat-mech↗

arXiv2020

Relaxation theory for perturbed many-body quantum systems versus numerics and experiment

An analytical prediction is established of how an isolated many-body quantum system relaxes towards its thermal long-time limit under the action of a time-independent perturbation, but still remaining sufficiently close to a reference case whose temporal relaxation is known. This is achieved within the conceptual framework of a typicality approach by showing and exploiting that the time-dependent expectation values behave very similarly for most members of a suitably chosen ensemble of perturbations. The predictions are validated by comparison with various numerical and experimental results from the literature.

cond-mat.stat-mech↗