Vortex generation in stirred binary Bose-Einstein condensates

Abstract

The dynamical vortex production, with a trap-confining time-dependent stirred potential, is studied by using mass-imbalanced cold-atom coupled Bose-Einstein condensates (BEC). The vortex formation is explored by considering that both coupled species are confined by a pancake-like harmonic trap, slightly modified elliptically by a time-dependent periodic potential, with the characteristic frequency enough larger than the transversal trap frequency. The approach is applied to the experimentally accessible binary mixtures $^{85}$Rb-$^{133}$Cs and $^{85}$Rb-$^{87}$Rb, which allow us to verify the effect of mass differences in the dynamics. For both species, the time evolutions of the respective energy contributions, together with associated velocities, are studied in order to distinguish turbulent from non-turbulent flows. By using the angular momentum and moment of inertia mean values, effective classical rotation frequencies are suggested, which are further considered within simulations in the rotating frame without the stirring potential. Spectral analysis is also provided for both species, with the main focus being the incompressible kinetic energies. In the transient turbulent regime, before stable vortex patterns are produced, the characteristic $k^{-5/3}$ Kolmogorov behavior is clearly identified for both species at intermediate momenta $k$ above the inverse Thomas-Fermi radial positions, further modified by the universal $k^{-3}$ scaling at momenta higher than the inverse of the respective healing lengths. Emerging from the mass-imbalanced comparison, relevant is to observe that, as larger is the mass difference, much faster is the dynamical production of stable vortices.