Quantum Ergodicity in the Many-Body Localization Problem

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 nonergodic extended states in many-body systems discussed in the recent literature.

Automated summary

The generalization of Page's result to many-body eigenstates of realistic disordered systems reveals that for increasing disorder, eigenstates only fill a fraction of the Fock space and exhibit intrinsic correlations. Prior to the many-body localization transition, individual eigenstates are thermally distributed over these shells, contradicting the concept of nonergodic extended states in many-body systems.