Dynamical properties of ultracold bosons in an optical lattice

TL;DR

This paper investigates the excitation spectrum of strongly correlated ultracold bosons in optical lattices, identifying gapped and gapless modes in different phases, and relates theoretical predictions to experimental observations of finite frequency absorption.

Contribution

The study introduces a Schwinger-boson mean-field approach to analyze excitation modes and proposes experimental extensions to verify the Higgs mode in superfluid phases.

Findings

Identification of two gapped modes in the Mott insulator.

Observation of a Goldstone sound mode and a gapped Higgs mode in the superfluid.

Explanation of finite frequency absorption via Higgs mode excitation.

Abstract

We study the excitation spectrum of strongly correlated lattice bosons for the Mott-insulating phase and for the superfluid phase close to localization. Within a Schwinger-boson mean-field approach we find two gapped modes in the Mott insulator and the combination of a sound mode (Goldstone) and a gapped (Higgs) mode in the superfluid. To make our findings comparable with experimental results, we calculate the dynamic structure factor as well as the linear response to the optical lattice modulation introduced by Stoeferle et al. [Phys. Rev. Lett. 92, 130403 (2004)]. We find that the puzzling finite frequency absorption observed in the superfluid phase could be explained via the excitation of the gapped (Higgs) mode. We check the consistency of our results with an adapted f-sum-rule and propose an extension of the experimental technique by Stoeferle et al. to further verify our findings.