Self-trapping in Cold Atoms from Negative Effective Mass

Self-trapping phenomena have been observed in optical lattices but the expalanation is complicated because of the underlying lattice geometry. In this talk, we will describe a simple theory based on negative-mass hydrodynamics in a spin-orbit coupled (SOC) Bose-Einstien condensate (BEC) of $^{87}\text{Rb}$, and argue that self-trapping can be explained solely in terms of negative effective mass without any complications of lattice geometry. We have engineered the underlying dispersion relation with a negative curavature region exhibiting exquisite control over atomic interactions, which leads to a number of interesting effects, such as breaking of Galilean covariance, modulational instability and a slow down demonstrating negative acceleration.