Anderson localization: A view from Krylov space

TL;DR

This paper investigates Anderson localization using Krylov space methods, constructing local integrals of motion and analyzing their properties across different phases and dimensions, revealing insights into the nature of the Anderson transition.

Contribution

It introduces a Krylov space approach to construct and analyze local integrals of motion in the Anderson model, providing analytical and numerical insights into localization phenomena.

Findings

Edge states localized on a vanishing fraction of Krylov space.

Disorder induces power-law decaying dimerization in the Krylov hopping problem.

Critical LIOMs exhibit a power-law decay with an exponent of approximately 0.29.

Abstract

The Krylov subspace expansion is a workhorse method for sparse numerics that has been increasingly explored as source of physical insight into many-body dynamics in recent years. In this work we revisit the venerable Anderson model of localization in dimensions $d=1, 2, 3, 4$ to construct local integrals of motion (LIOM) in Krylov space. These appear as zero eigenvalue edge states of an effective hopping problem in the Krylov superoperator subspace and can be analytically constructed given the Lanczos coefficients. We exploit this idea, focusing on $d=3$, to study the manifestation of the disorder driven Anderson transition in the anatomy of LIOMs. We find that the increasing complexity of the Krylov operators results in a suppression of the fluctuations of the Lanczos coefficients. As such, one can study the phenomenology of the integrals of motion in the disorder averaged Krylov chain. We find edge states localized on vanishing fraction of Krylov space (of dimension $D_K=V^2$ for cubes of volume $V$), both in localized and extended phases. Importantly, in the localized phase, disorder induces powerlaw decaying dimerization in the (Krylov) hopping problem, producing stretched exponential decay of the LIOMs in Krylov space with a stretching exponent $1/2d$. Metallic LIOMs are completely delocalized albeit across only $\propto \sqrt{D_K}$ states. Critical LIOMs exhibit powerlaw decay with an exponent matching the expected value of $0.29$.