Itinerant-localized dual character of a strongly-correlated superfluid Bose gas in an optical lattice

TL;DR

This paper explores the dual itinerant and localized excitation characteristics of a strongly-correlated superfluid Bose gas in an optical lattice, revealing a coexistence of gapless and gapped modes near the superfluid-insulator transition.

Contribution

It extends the standard-basis operator method to analyze excitation spectra in a boson Hubbard model, uncovering the dual nature of excitations in the superfluid phase.

Findings

In the Mott phase, excitation spectrum has a finite energy gap.

In the superfluid phase, gapless Bogoliubov mode coexists with a gapped band.

Rf-tunneling current measurements can reveal the duality near the transition.

Abstract

We investigate a strongly-correlated Bose gas in an optical lattice. Extending the standard-basis operator method developed by Haley and Erdos to a boson Hubbard model, we calculate excitation spectra in the superfluid phase, as well as in the Mott insulating phase, at T=0. In the Mott phase, the excitation spectrum has a finite energy gap, reflecting the localized character of atoms. In the superfluid phase, the excitation spectrum is shown to have an itinerant-localized dual structure, where the gapless Bogoliubov mode (which describes the itinerant character of superfluid atoms) and a band with a finite energy gap coexist. We also show that the rf-tunneling current measurement would give a useful information about the duality of a strongly-correlated superfluid Bose gas near the superfluid-insulator transition.