Bose glass transition and spin-wave localization for 2D bosons in a random potential

TL;DR

This paper investigates the transition between superfluid, Bose-glass, and insulator phases in 2D hard-core bosons with disorder, revealing a rich phase diagram influenced by quantum fluctuations and localization phenomena.

Contribution

It introduces a spin-wave approach to analyze the superfluid-insulator transition in disordered 2D bosons, uncovering the emergence of a Bose-glass phase and a mobility edge in the excitation spectrum.

Findings

Disappearance of Bose-condensed fraction before superfluidity with increasing disorder

Emergence of a finite Bose-glass phase between superfluid and insulator

Presence of a mobility edge in the spin-wave spectrum that vanishes in the Bose-glass phase

Abstract

A spin-wave approach of the zero temperature superfluid-insulator transition for two-dimensional hard-core bosons in a random potential $\mu=\pm$ W is developed. While at the classical level there is no intervening phase between the Bose-condensed superfluid (SF) and the gapped disordered insulator, the introduction of quantum fluctuations leads to a much richer physics. Upon increasing the disorder strength W, the Bose-condensed fraction disappears first, before the SF. Then a gapless Bose-glass phase emerges over a finite region until the insulator appears. Furthermore, in the strongly disordered SF regime, a mobility edge in the spin-wave excitation spectrum is found at a finite frequency $\Omega_c$ decreasing with W, and presumably vanishing in the Bose-glass phase.