Pseudo-Goldstone Excitations in a Striped Bose-Einstein Condensate

TL;DR

This paper distinguishes between spontaneous and forced stripe states in Bose-Einstein condensates by analyzing their excitation spectra, highlighting the presence of gapped pseudo-Goldstone modes in the latter.

Contribution

It introduces a method to identify forced stripe phases via their gapped excitation spectra, contrasting with gapless modes in spontaneous symmetry breaking.

Findings

Forced stripe phase exhibits gapped pseudo-Goldstone modes.

Excitation spectra can differentiate between spontaneous and forced symmetry breaking.

Static structure factor characterizes the pseudo-Goldstone mode.

Abstract

Significant experimental progress has been made recently for observing long-sought supersolid-like states in Bose-Einstein condensates, where spatial translational symmetry is spontaneously broken by anisotropic interactions to form a stripe order. Meanwhile, the superfluid stripe ground state was also observed by applying a weak optical lattice that forces the symmetry breaking. Despite of the similarity of the ground states, here we show that these two symmetry breaking mechanisms can be distinguished by their collective excitation spectra. In contrast to gapless Goldstone modes of the \textit{spontaneous} stripe state, we propose that the excitation spectra of the \textit{forced} stripe phase can provide direct experimental evidence for the long-sought gapped pseudo-Goldstone modes. We characterize the pseudo-Goldstone mode of such lattice-induced stripe phase through its excitation spectrum and static structure factor. Our work may pave the way for exploring spontaneous and forced/approximate symmetry breaking mechanisms in different physical systems.