Dynamical mechanisms of vortex pinning in superfluid thin films

TL;DR

This paper investigates vortex pinning mechanisms in superfluid thin films using analytical models and numerical simulations, identifying two distinct dynamical pinning processes and their dependence on obstacle size.

Contribution

It introduces a hydrodynamic approximation for vortex trajectories and validates it with Gross-Pitaevskii simulations, revealing two dynamical pinning mechanisms with distinct phonon signatures.

Findings

Pinning potentials of size comparable to the healing length are most effective.

Two dynamical pinning mechanisms identified: acoustic radiation and vortex dipole nucleation.

Analytical model accurately predicts vortex trajectories and unpinning velocities.

Abstract

We characterize the mechanisms of vortex pinning in a superfluid thin film described by the two-dimensional Gross-Pitaevskii equation. We consider a vortex "scattering experiment" whereby a single vortex in a superfluid flow interacts with a circular, uniform pinning potential. By an analogy with linear dielectrics, we develop an analytical hydrodynamic approximation that predicts vortex trajectories, the vortex fixed point and the unpinning velocity. We then solve the Gross-Pitaevskii equation to validate this model, and build a phase portrait of vortex pinning. We identify two different dynamical pinning mechanisms marked by distinctive phonon emission signatures: one enabled by acoustic radiation and another mediated by vortex dipoles nucleated within the pin. Relative to obstacle size, we find that pinning potentials on the order of the healing length are more effective for vortex capture. Our results could be useful in mitigating the negative effects of drag due to vortices in superfluid channels, in analogy to maximising supercurrents in type-II superconductors.