Negative-mass hydrodynamics in a spin-orbit coupled Bose-Einstein condensate

Negative effective mass is peculiar; whereas objects usually accellerate away from a push, negative-mass objects will accelerate $\textit{towards}$ the push. This strange behaviour can be realized in spin-orbit coupled (SOC) BECs where the dispersion relationship can be engineered to exhibit negative curvature. In this talk we will describe an experiment, where trapped $^{87}$Rb atoms expand in the presence of a spin-orbit coupling, demonstrating an interesting array of dynamical phenomena, including the breaking of Galilean covariance, dynamical instabilities, and a slowing down consistent with negative acceleration. We show that these features can be described with a simple theory of negative-mass hydrodynamics, and argue that this also explains a related phenomena of self-trapping seen in optical lattices.