Measurement of collective excitations in a spin-orbit-coupled Bose-Einstein condensate

TL;DR

This study measures the collective excitation spectrum of a spin-orbit-coupled Bose-Einstein condensate, revealing mode softening and finite excitation gaps that shed light on phase transitions and fluid dynamics.

Contribution

It provides the first detailed measurement of collective excitations in a spin-orbit-coupled BEC, highlighting mode softening and symmetry properties related to phase transitions.

Findings

Mode softening at finite momentum observed

Finite excitation gap prevents complete mode softening

Symmetry under sign change of Raman detuning confirmed

Abstract

We measure the collective excitation spectrum of a spin-orbit coupled Bose-Einstein condensate using Bragg spectroscopy. The spin-orbit coupling is generated by Raman dressing of atomic hyperfine states. When the Raman detuning is reduced, mode softening at a finite momentum is revealed, which provides insight towards a supersolid-like phase transition. We find that for the parameters of our system, this softening stops at a finite excitation gap and is symmetric under a sign change of the Raman detuning. Finally, using a moving barrier that is swept through the BEC, we also show the effect of the collective excitation on the fluid dynamics.