Nonlinear Lattice Waves in Random Potentials

TL;DR

This paper reviews recent computational and theoretical advances in understanding how nonlinearity affects wave localization in disordered systems, highlighting the transition from Anderson localization to subdiffusive spreading.

Contribution

It provides a comprehensive overview of nonlinear effects on wave localization, including new insights into subdiffusive spreading and extensions to various localized systems.

Findings

Nonlinearity can induce wave spreading in localized systems.

Universal subdiffusive laws describe nonlinear wave spreading.

Extensions include quasiperiodic and dynamical localization cases.

Abstract

Localization of waves by disorder is a fundamental physical problem encompassing a diverse spectrum of theoretical, experimental and numerical studies in the context of metal-insulator transition, quantum Hall effect, light propagation in photonic crystals, and dynamics of ultra-cold atoms in optical arrays. Large intensity light can induce nonlinear response, ultracold atomic gases can be tuned into an interacting regime, which leads again to nonlinear wave equations on a mean field level. The interplay between disorder and nonlinearity, their localizing and delocalizing effects is currently an intriguing and challenging issue in the field. We will discuss recent advances in the dynamics of nonlinear lattice waves in random potentials. In the absence of nonlinear terms in the wave equations, Anderson localization is leading to a halt of wave packet spreading. Nonlinearity couples localized eigenstates and, potentially, enables spreading and destruction of Anderson localization due to nonintegrability, chaos and decoherence. The spreading process is characterized by universal subdiffusive laws due to nonlinear diffusion. We review extensive computational studies for one- and two-dimensional systems with tunable nonlinearity power. We also briefly discuss extensions to other cases where the linear wave equation features localization: Aubry-Andre localization with quasiperiodic potentials, Wannier-Stark localization with dc fields, and dynamical localization in momentum space with kicked rotors.