Fragile ergodic phases in logarithmically-normal Rosenzweig-Porter model

TL;DR

This paper introduces the LN-RP model, an extension of the Rosenzweig-Porter model with log-normal distributed off-diagonal elements, revealing a fragile weakly ergodic phase and new phase transition criteria relevant to many-body localization.

Contribution

The paper proposes the LN-RP model with log-normal off-diagonal elements, identifying a fragile weakly ergodic phase and establishing new stability criteria and phase transition points.

Findings

Discovery of a fragile weakly ergodic phase in LN-RP model.

Identification of a transition between ergodic phases (FWE transition).

Full phase diagram including localization, ergodic, and weakly ergodic phases.

Abstract

In this paper we suggest an extension of the Rosenzweig-Porter (RP) model, the LN-RP model, in which the off-diagonal matrix elements have a wide, log-normal distribution. We argue that this model is more suitable to describe a generic many body localization problem. In contrast to RP model, in LN-RP model a fragile weakly ergodic phase appears that is characterized by broken basis-rotation symmetry which the fully-ergodic phase, also present in this model, strictly respects in the thermodynamic limit. Therefore, in addition to the localization and ergodic transitions in LN-RP model there exists also the transition between the two ergodic phases (FWE transition). We suggest new criteria of stability of the non-ergodic phases which give the points of localization and ergodic transitions and prove that the Anderson localization transition in LN-RP model involves a jump in the fractal dimension of the eigenfunction support set. We also formulate the criterion of FWE transition and obtain the full phase diagram of the model. We show that truncation of the log-normal tail shrinks the region of weakly-ergodic phase and restores the multifractal and the fully-ergodic phases.