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Condensed Matter > Statistical Mechanics
Title: Quantum chaos and ensemble inequivalence of quantum long-range Ising chains
(Submitted on 11 Dec 2020 (v1), last revised 29 Sep 2021 (this version, v3))
Abstract: We use large-scale exact diagonalization to study the quantum Ising chain in a transverse field with long-range power-law interactions decaying with exponent $\alpha$. We numerically study various probes for quantum chaos and eigenstate thermalization {on} the level of eigenvalues and eigenstates. The level-spacing statistics yields a clear sign towards a Wigner-Dyson distribution and therefore towards quantum chaos across all values of $\alpha>0$. Yet, for $\alpha<1$ we find that the microcanonical entropy is nonconvex. This is due to the fact that the spectrum is organized in energetically separated multiplets for $\alpha<1$. While quantum chaotic behaviour develops within the individual multiplets, many multiplets don't overlap and don't mix with each other, as we analytically and numerically argue. Our findings suggest that a small fraction of the multiplets could persist at low energies for $\alpha\ll 1$ even for large $N$, giving rise to ensemble inequivalence.
Submission history
From: Angelo Russomanno [view email][v1] Fri, 11 Dec 2020 17:16:56 GMT (17394kb,D)
[v2] Mon, 2 Aug 2021 15:23:18 GMT (3809kb,D)
[v3] Wed, 29 Sep 2021 16:25:41 GMT (3548kb,D)
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