Exotic superfluidity in spin-orbit coupled Bose-Einstein condensates

TL;DR

This paper investigates the unique superfluid properties of spin-orbit coupled Bose-Einstein condensates, revealing novel features like frame-dependent critical velocity and dependence on cross helicity due to broken Galilean invariance.

Contribution

It introduces the analysis of Bogoliubov excitations in spin-orbit coupled BECs, uncovering new superfluidity characteristics absent in conventional superfluids.

Findings

Presence of two Bogoliubov excitation branches: gapless and gapped.

Superfluidity depends on cross helicity and is frame-dependent.

Critical velocity is not well-defined independently of the reference frame.

Abstract

We study the superfluidity of a spin-orbit coupled Bose-Einstein condensate (BEC) by computing its Bogoliubov excitations, which are found to consist of two branches: one is gapless and phonon-like at long wavelength; the other is typically gapped. These excitations imply a superfluidity that has two new features: ({\it i}) due to the absence of the Galilean invariance, one can no longer define the critical velocity of superfluidity independent of the reference frame; ({\it ii}) the superfluidity depends not only on whether the speed of the BEC exceeds a critical value, but also on {\it cross helicity} that is defined as the direction of the cross product of the spin and the kinetic momentum of the BEC.