Measurement of collective excitations in a spin-orbit-coupled   Bose-Einstein condensate
  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. 
M. A. Khamehchi, Yongping Zhang, Chris Hamner, Thomas Busch, and Peter   Engels
cond-mat.quant-gas