Critical velocities in superfluids and the nucleation of vortices

TL;DR

This paper investigates the mechanisms behind superfluidity breakdown, focusing on vortex nucleation and phase slips, and analyzes how environmental damping influences macroscopic quantum tunnelling in superfluid helium.

Contribution

It provides a detailed analysis of vortex nucleation as a quantum tunnelling process and examines environmental effects on this phenomenon in superfluid helium.

Findings

Vortex nucleation acts as a macroscopic quantum tunnelling process.

Damping influences the quantum tunnelling rate in superfluid helium.

Experimental data shows environmental effects on superfluid critical velocities.

Abstract

The problem of critical velocities in superfluids, that is the comprehension of superfluidity breakdown by flow, has been long standing. One difficulty stems from the existence of several breakdown mechanisms. A major advance has come from the observation of single $2\pi$ phase slips, which arise from the nucleation of quantised vortices, that is, their creation {\it ex nihilo}. The statistical properties of the nucleation process in both the thermal regime and the quantum regime are identified and analysed: vortex nucleation provides a well-documented case of macroscopic quantum tunnelling (MQT). In particular, a close scrutiny of the experimental data obtained on ultra-pure $^4$He reveals the influence of damping on tunnelling, a rare occurrence where the effect of the environment on MQT can be studied.