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Zoom of a 1D simulation matching the lower middle frame of Fig. 2 showing the total density n=n↑+n↓ as a function of time in the region where the dynamic instability first appears. Dashed lines are the three group velocities vg=E−′(k) at the quasimomenta k where the inverse effective mass m−1∗=E−′′(k) first becomes negative (steepest line), the point of maximum m−1∗ (middle), and the point where the m−1∗ returns to positive (least steep line). Red points demonstrate where the local quasimomentum lies in the negative-mass region. Note that, as described in the text, the pileup initially contains many density fluctuations, but sharpens as solitons and phonons “radiate” energy away from the wall.

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Negative-Mass Hydrodynamics in a Spin-Orbit–Coupled Bose-Einstein Condensate

ABSTRACT

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\char21{}Coupled Bose-Einstein Condensate

Author:

M. A. Khamehchi et al.

Publication:

Physical Review Letters

Publisher:

American Physical Society

Date:

Apr 10, 2017