Localization transition on the Random Regular Graph as an unstable tricritical point in a log-normal Rosenzweig-Porter random matrix ensemble

TL;DR

This paper introduces a log-normal extension of the Rosenzweig-Porter random matrix ensemble, revealing a tricritical point where the multifractal phase collapses and demonstrating the discontinuous nature of the Anderson transition across the model.

Contribution

It extends the GRP model with a log-normal distribution, identifying a tricritical point and analyzing the stability and nature of the Anderson transition in this new framework.

Findings

Identifies a tricritical point at p=1 where the multifractal phase collapses.

Shows the Anderson transition is discontinuous for all p>0.

Demonstrates the instability of the multifractal phase under distribution truncation.

Abstract

Gaussian Rosenzweig-Porter (GRP) random matrix ensemble is the only one in which the robust multifractal phase and ergodic transition have a status of a mathematical theorem. Yet, this phase in GRP model is oversimplified: the spectrum of fractal dimensions is degenerate and the mini-band in the local spectrum is not multifractal. In this paper we suggest an extension of the GRP model by adopting a logarithmically-normal (LN) distribution of off-diagonal matrix elements. A family of such LN-RP models is parametrized by a symmetry parameter $p$ and it interpolates between the GRP at $p\rightarrow 0$ and Levy ensembles at $p\rightarrow\infty$. A special point $p=1$ is shown to be the simplest approximation to the Anderson localization model on a random regular graph.We study in detail the phase diagram of LN-RP model and show that $p=1$ is a tricritical point where the multifractal phase first collapses. This collapse is shown to be unstable with respect to the truncation of the log-normal distribution. We suggest a new criteria of stability of the non-ergodic phases and prove that the Anderson transition in LN-RP model is discontinuous at all $p>0$.