A negative effective mass can be realized in quantum systems by engineering the dispersion relation. A powerful method is provided by spin-orbit coupling, which is currently at the center of intense research efforts. Here we measure an expanding spin-orbit coupled Bose-Einstein condensate whose dispersion features a region of negative effective mass. We observe a range of dynamical phenomena, including the breaking of parity and of Galilean covariance, dynamical instabilities, and self-trapping. The experimental findings are reproduced by a single-band Gross-Pitaevskii simulation, demonstrating that the emerging features - shock waves, soliton trains, self-trapping, etc. - originate from a modified dispersion. Our work also sheds new light on related phenomena in optical lattices, where the underlying periodic structure often complicates their interpretation.
CITATION STYLE
Khamehchi, M. A., Hossain, K., Mossman, M. E., Zhang, Y., Busch, T., Forbes, M. M. N., & Engels, P. (2017). Negative-Mass Hydrodynamics in a Spin-Orbit-Coupled Bose-Einstein Condensate. Physical Review Letters, 118(15). https://doi.org/10.1103/PhysRevLett.118.155301
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