Superfluid toroidal currents in atomic condensates

Abstract

The dynamics of toroidal condensates in the presence of condensate flow and dipole perturbation have been investigated. The Bogoliubov spectrum of condensate is calculated for an oblate torus using a discrete-variable representation and a spectral method to high accuracy. The transition from spheroidal to toroidal geometry of the trap displaces the energy levels into narrow bands. The lowest-order acoustic modes are quantized with the dispersion relation $ω\sim |m| ω_s $ with $m=0,\pm 1,\pm 2, ...$. A condensate with toroidal current $κ$ splits the $|m|$ co-rotating and counter-rotating pair by the amount: $ΔE \approx 2 |m|\hbar^2 κ< r^{-2}>$. Radial dipole excitations are the lowest energy dissipation modes. For highly occupied condensates the nonlinearity creates an asymmetric mix of dipole circulation and nonlinear shifts in the spectrum of excitations so that the center of mass circulates around the axis of symmetry of the trap. We outline an experimental method to study these excitations.