Quantum chaos and ensemble inequivalence of quantum long-range Ising chains

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.