Exotic superfluidity in spin-orbit coupled Bose-Einstein condensates

The authors study the superfluidity of a spin-orbit coupled Bose-Einstein condensate (BEC) by computing its Bogoliubov excitations, which are found to have two branches: one is gapless and phonon-like at long wavelength; the other is typically gapped. These excitations imply superfluidity that has two new features: 1. Due to the absence of the Galilean invariance, one can no longer define the critical velocity of superfluidity independent of the reference frame. 2. The superfluidity depends not only on whether the speed of the BEC exceeds a critical value, but also on cross-helicity that is defined as the direction of the cross-product of the spin and the kinetic momentum of the BEC.