Vortices in Spatially Inhomogeneous Superfluids

TL;DR

This paper investigates vortex behavior in inhomogeneous superfluids, revealing anisotropic superflows, vortex precession, and a nearly uniform vortex density in rapid rotation, with results aligning with recent experimental observations.

Contribution

It provides a detailed analysis of vortex distributions and superflows in radially inhomogeneous superfluids, highlighting effects of inhomogeneity on vortex arrangements and dynamics.

Findings

Vortex precession occurs without external rotation due to superflow anisotropy.

Vortex density becomes nearly uniform at high rotation speeds.

Enhanced vortex density is observed at the trap center, matching experiments.

Abstract

We study vortices in a radially inhomogeneous superfluid, as realized by a trapped degenerate Bose gas in a uniaxially symmetric potential. We show that, in contrast to a homogeneous superfluid, an off-axis vortex corresponds to an anisotropic superflow whose profile strongly depends on the distance to the trap axis. One consequence of this superflow anisotropy is vortex precession about the trap axis in the absence of an imposed rotation. In the complementary regime of a finite prescribed rotation, we compute the minimum-energy vortex density, showing that in the rapid-rotation limit it is extremely uniform, despite a strongly inhomogeneous (nearly) Thomas-Fermi condensate density $\rho_s(r)$. The weak radially-dependent contribution ($\propto \nabla^2\ln\rho_s(r)$) to the vortex distribution, that vanishes with the number of vortices $N_v$ as $\frac{1}{N_v}$, arises from the interplay between vortex quantum discretness (namely their inability to faithfully support the imposed rigid-body rotation) and the inhomogeneous superfluid density. This leads to an enhancement of the vortex density at the center of a typical concave trap, a prediction that is in quantitative agreement with recent experiments (cond-mat/0405240). One striking consequence of the inhomogeneous vortex distribution is an azimuthally-directed, radially-shearing superflow.