@article{oai:oist.repo.nii.ac.jp:00000376,
 author = {Khamehchi, M. A. and Hossain, Khalid and Mossman, M. E. and Zhang, Yongping and Busch, Th. and Forbes, Michael McNeil and Engels, P.},
 issue = {15},
 journal = {Physical Review Letters},
 month = {Apr},
 note = {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.},
 title = {Negative-Mass Hydrodynamics in a Spin-Orbit–coupled Bose-Einstein Condensate},
 volume = {118},
 year = {2017}
}