Hilbert-space correlations beyond multifractality and bipartite entanglement in many-body localised systems

TL;DR

This paper investigates the unique Hilbert-space correlations in many-body localized systems that reconcile their area-law entanglement with multifractal eigenstate statistics, revealing fundamental differences from ergodic and fractal states.

Contribution

It identifies and computes specific Hilbert-space correlations that explain how MBL eigenstates exhibit both area-law entanglement and multifractality, a novel insight into their structure.

Findings

Hilbert-space correlations differ from ergodic states

Correlations distinguish MBL eigenstates from fractal states

Correlations underpin coexistence of entanglement and multifractality

Abstract

Eigenstates of many-body localised (MBL) systems are characterised by area-law bipartite entanglement along with multifractal statistics of their amplitudes on Hilbert space. At the same time, sparse random pure states with fractal statistics are not compatible with area-law and necessarily exhibit volume-law entanglement. This raises the question that what correlation functions of Hilbert-space amplitudes MBL eigenstates must possess for their area law entanglement to be compatible with their multifractality. In this work, we identify and compute such appropriate Hilbert-space correlations which carry information of entanglement. We find that, for MBL eigenstates, these correlations are qualitatively different not only from those of ergodic states but also of sparse random states with fractal statistics. This enables us to show that indeed the said correlations lie at the heart of the coexistence of area-law entanglement and multifractality for MBL eigenstates.