Emergence of damped-localized excitations of the Mott state due to disorder

TL;DR

This paper investigates how disorder affects low-energy excitations in the Mott insulating phase of ultracold bosonic gases, revealing the emergence of damped-localized states linked to the Bose-glass phase.

Contribution

It provides a field-theoretical analysis showing disorder-induced damping and localization of quasiparticle excitations in the Mott insulator.

Findings

Disorder increases the effective mass of excitations.

Damped states emerge and dominate the spectral band with strong disorder.

Damped-localized states are associated with the Bose-glass phase.

Abstract

A key aspect of ultracold bosonic quantum gases in deep optical lattice potential wells is the realization of the strongly interacting Mott insulating phase. Many characteristics of this phase are well understood, however little is known about the effects of a random external potential on its gapped quasiparticle and quasihole low-energy excitations. In the present study we investigate the effect of disorder upon the excitations of the Mott insulating state at zero temperature described by the Bose-Hubbard model. Using a field-theoretical approach we obtain a resummed expression for the disorder ensemble average of the spectral function. Its analysis shows that disorder leads to an increase of the effective mass of both quasiparticle and quasihole excitations. Furthermore, it yields the emergence of damped states, which exponentially decay during propagation in space and dominate the whole band when disorder becomes comparable to interactions. We argue that such damped-localized states correspond to single-particle excitations of the Bose-glass phase.