Quantum ergodicity in the many-body localization problem

TL;DR

This paper extends Page's entanglement entropy results to disordered many-body systems, revealing that eigenstates are thermally distributed over energy shells and challenging the notion of non-ergodic extended states.

Contribution

It generalizes entanglement entropy analysis to realistic disordered systems, showing eigenstates fill only part of Fock space and are thermally distributed before localization.

Findings

Eigenstates occupy only a fraction of Fock space with increasing disorder.

Eigenstates are thermally distributed over energy shells prior to localization.

Results challenge the concept of non-ergodic extended states in many-body systems.

Abstract

We generalize Page's result on the entanglement entropy of random pure states to the many-body eigenstates of realistic disordered many-body systems subject to long range interactions. This extension leads to two principal conclusions: first, for increasing disorder the "shells" of constant energy supporting a system's eigenstates fill only a fraction of its full Fock space and are subject to intrinsic correlations absent in synthetic high-dimensional random lattice systems. Second, in all regimes preceding the many-body localization transition individual eigenstates are thermally distributed over these shells. These results, corroborated by comparison to exact diagonalization for an SYK model, are at variance with the concept of "non-ergodic extended states" in many-body systems discussed in the recent literature.