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arXiv:1908.10685

Prethermalization of density-density correlations after an interaction quench in the Hubbard model

Manuel Kreye, Stefan K. Kehrein

Published Aug 28, 2019• 2 citations
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Abstract

In weakly perturbed systems that are close to integrability, thermalization can be delayed by the formation of prethermalization plateaus. We study the build-up of density-density correlations after a weak interaction quench in the Hubbard model in $d > 1$ dimensions using unitary perturbation theory. Starting from a pre-quench state at temperature $T$, we show that the prethermalization values of the post-quench correlations are equal to the equilibrium values of the interacting model at the same temperature $T$. This is explained by the local character of density-density correlations.

DOI
10.48550/arxiv.1908.10685

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In weakly perturbed systems that are close to integrability, thermalization can be delayed by the formation of prethermalization plateaus. We study the build-up of density-density correlations after a weak interaction quench in the Hubbard model in d>1d > 1d>1 dimensions using unitary perturbation theory. Starting from a pre-quench state at temperature TTT, we show that the prethermalization values of the post-quench correlations are equal to the equilibrium values of the interacting model at the same temperature TTT. This is explained by the local character of density-density correlations.
We advance perturbative approaches for out-of-equilibrium quantum many-body systems by applying unitary perturbation theory to the real-time evolution of the weakly interacting Hubbard model in d > 1 dimensions. We derive general expressions for the fermionic operators that allow us to perturbatively calculate the time evolution of observables to second-order expansion in the interaction strength. We find that the results are stable up to and including the prethermalization regime. We apply our findings to the nonequilibrium build-up of density-density correlations after a weak interaction quench and compare the prethermalization values to equilibrium correlations. Furthermore, we analyze the light-cone-like structure of the density-density susceptibility and the equal-time correlation function in the context of the Lieb-Robinson bounds.
We study the Bose-Hubbard and Fermi-Hubbard models in the (formal) limit of large coordination numbers $Z\ensuremath{\gg}1$. Via an expansion into powers of 1/Z1/Z1/Z, we establish a hierarchy of correlations which facilitates an approximate analytical derivation of the time evolution of the reduced density matrices for one and two sites, etc. With this method, we study the quantum dynamics (starting in the ground state) after a quantum quench, i.e., after suddenly switching the tunneling rate JJJ from zero to a finite value, which is still in the Mott regime. We find that the reduced density matrices approach a (quasi)equilibrium state after some time. For one lattice site, this state can be described by a thermal state (within the accuracy of our approximation). However, the (quasi)equilibrium state of the reduced density matrices for two sites including the correlations can not be described by a thermal state. Thus, real thermalization (if it occurs) should take a much longer time. This behavior has already been observed in other scenarios and is sometimes called ``prethermalization''. Finally, we compare our results to numerical simulations for finite lattices in one and two dimensions and find qualitative agreement.
After a sudden disruption, weakly interacting quantum systems first relax to a prethermalized state that can be described by perturbation theory and a generalized Gibbs ensemble. Using these properties of the prethermalized state we perturbatively derive a kinetic equation which becomes a quantum Boltzmann equation in the scaling limit of vanishing interaction. Applying this to interaction quenches in the fermionic Hubbard model we find that the momentum distribution relaxes to the thermal prediction of statistical mechanics. For not too large interaction, this two-stage scenario provides a quantitative understanding of the time evolution leading from the initial pure via a metastable prethermal to the final thermal state.
We study the Bose and Fermi Hubbard model in the (formal) limit of large coordination numbers Z≫1Z\gg1Z≫1. Via an expansion into powers of 1/Z1/Z1/Z, we establish a hierarchy of correlations which facilitates an approximate analytical derivation of the time-evolution of the reduced density matrices for one and two sites etc. With this method, we study the quantum dynamics (starting in the ground state) after a quantum quench, i.e., after suddenly switching the tunneling rate JJJ from zero to a finite value, which is still in the Mott regime. We find that the reduced density matrices approach a (quasi) equilibrium state after some time. For one lattice site, this state can be described by a thermal state (within the accuracy of our approximation). However, the (quasi) equilibrium state of the reduced density matrices for two sites including the correlations cannot be described by a thermal state. Thus, real thermalization (if it occurs) should take much longer time. This behavior has already been observed in other scenarios and is sometimes called ``pre-thermalization.'' Finally, we compare our results to numerical simulations for finite lattices in one and two dimensions and find qualitative agreement.
We calculate the dynamics of local and non-local correlation functions of one and threedimensional weakly interacting Bose gas after an interaction quench. Within the Bogoliubov approximation we discuss the resulting quasi-steady prethermal state and relaxation to it. We discuss the deacay rates of Bogoliubov quasi-particles characterizing the expected departure from the prethermal state towards a fully thermalized one. We conclude that prethermalization in this situation manifest as a crossover.