Abstract
By developing a hydrodynamic formalism, we investigate the expansion dynamics of the single-minimum phase of a binary spin–orbit coupled Bose–Einstein condensate, after releasing from an external harmonic trap. We find that the expansion of the condensate along the direction of the spin–orbit coupling is dramatically slowed down near the transition between the single-minimum phase and the plane-wave phase. Such a slow expansion, resembling a form of an effective localization, is due to the quenching of the superfluid motion which results in a strong increase of the effective mass. In the single-minimum phase the anisotropic expansion of the Bose gas, which is spin balanced at equilibrium, is accompanied by the emergence of a local spin polarization. Our analytic scaling solutions emerging from hydrodynamic picture are compared with a full numerical simulation based on the coupled Gross–Pitaevskii equations.
Highlights
Since the first experimental realization of BoseEinstein condensation in 1995 the expansion of quantum gases, after release of the confining trap, has systematically provided crucial information on the physical properties of such systems
In many cases the condensate is in the so-called Thomas-Fermi limit, where the equation of state of uniform matter can be directly employed in the local density approximation and the mechanism of the expansion is well described using the hydrodynamic formalism of superfluids
In usual superfluids at low temperature, the hydrodynamic picture is based on the irrotationality constraint for the velocity field which takes the most famous expression v = ( /m)∇φ where φ is the phase of the order parameter, is the reduced Plank constant and m is the atomic mass
Summary
Since the first experimental realization of BoseEinstein condensation in 1995 the expansion of quantum gases, after release of the confining trap, has systematically provided crucial information on the physical properties of such systems. The expansion of the condensate has been employed to characterize the emergence of the superfluid to Mott insulator transition in the presence of a periodic optical lattice [5], the effects of localization in the presence of disorder [6], the occurrence of Bloch oscillations [7] and a large variety of physical phenomena in both Bose and Fermi quantum gases [8]. For values of the Raman coupling close to phase transition between the single-minimum phase and the plane-wave phase, the expansion along the direction of spin-orbit coupling is dramatically quenched exhibiting an effective localization caused by the lowering of the superfluid flow.
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