Thermalization and its absence in the XX spin chain model

by Christian Eidecker-Dunkel

Thursday Aug 27, 2026, 1:30 PM → 2:00 PM Europe/Berlin

D5-153 (UHG)

A quantum quench often consists in the following setup: Some system is initially prepared in an equilibrium state of Hamiltonian Hg=H−gV, subject to a (small) parameter change at time t=0, and evolves in time with Hamiltonian H for times t>0. In such a setup, [1] rigorously established that the relaxation behavior of (the expectation values of) some observable A can be approximated by a series of correlations of A and V in the equilibrium pertaining to H. More precisely speaking, it was shown that the approximation is exact up to a finite error independent of the time or system size, and analytical upper bounds are available for many important cases. With this properties, said approximation forms a mighty tool for analytical studies of thermalization and its absence. Taking the paradigmatic XX spin chain model with a magnetic field as an example, it could be shown that the XX model thermalizes whenever it is subject to a homogeneous magnetic field [2]. Turning away from the homogeneous case, it was shown that even one single inhomogeneity in the magnetic field might be sufficient to prevent thermalization [3]. In my presentation, I will briefly introduce the general framework of [1], before presenting the results and implications of [2] and [3] in some detail. 

[1] P. Reimann and C. Eidecker-Dunkel, Onsager’s regression hypothesis adjusted to quantum systems, Phys. Rev. B 110, 014306 (2024)
[2] P. Reimann and C. Eidecker-Dunkel, Thermalization in a simple spin-chain model, Phys. Rev. B 111, 054312 (2025)
[3] P. Reimann and C. Eidecker-Dunkel, Absence of thermalization after a local quench and strong violation of the eigenstate thermalization hypothesis, Phys. Rev. B 113, 144304 (2026)