Non-ergodic extended states in $\beta$-ensemble

TL;DR

This paper investigates the eigenvector properties of the $eta$-ensemble, revealing non-ergodic extended states and phase transitions relevant to many-body localization, with implications for understanding ergodicity breaking in complex quantum systems.

Contribution

It uncovers the existence of non-ergodic extended states in the $eta$-ensemble and characterizes the phase transitions related to ergodicity and localization.

Findings

Anderson transition at $eta$ parameter $ ightarrow$ $eta = N^{- ext{gamma}}$

Non-ergodic extended states observed for $0< ext{gamma}<1$

Chaotic-integrable transition coincides with ergodicity breaking in $eta$-ensemble

Abstract

Matrix models showing chaotic-integrable transition in the spectral statistics are important for understanding Many Body Localization (MBL) in physical systems. One such example is the $\beta$-ensemble, known for its structural simplicity. However, eigenvector properties of $\beta$-ensemble remain largely unexplored, despite energy level correlations being thoroughly studied. In this work we numerically study the eigenvector properties of $\beta$-ensemble and find that the Anderson transition occurs at $\gamma = 1$ and ergodicity breaks down at $\gamma = 0$ if we express the repulsion parameter as $\beta = N^{-\gamma}$. Thus other than Rosenzweig-Porter ensemble (RPE), $\beta$-ensemble is another example where Non-Ergodic Extended (NEE) states are observed over a finite interval of parameter values ($0 < \gamma < 1$). We find that the chaotic-integrable transition coincides with the breaking of ergodicity in $\beta$-ensemble but with the localization transition in the RPE or the 1-D disordered spin-1/2 Heisenberg model where this coincidence occurs at the localization transition. As a result, the dynamical time-scales in the NEE regime of $\beta$-ensemble behave differently than the later models.