On Reduction of Critical Velocity in a Model of Superfluid Bose-gas with Boundary Interactions

TL;DR

This paper investigates how boundary interactions influence the critical velocity in a superfluid Bose-gas, showing that certain boundary excitations can destabilize the superfluid flow, with implications for more realistic models and soft matter systems.

Contribution

It introduces a simple model with boundary excitations affecting superfluid stability and discusses how realistic interactions modify the critical velocity.

Findings

Critical velocity vanishes in the semiclassical approximation due to boundary excitations.

Boundary interactions can destabilize superfluid flow in Bose-gases.

Surface modes may exist in soft matter containers with flexible walls.

Abstract

The existence of superfluidity in a 3D Bose-gas can depend on boundary interactions with channel walls. We study a simple model where the dilute moving Bose-gas interacts with the walls via hard-core repulsion. Special boundary excitations are introduced, and their excitation spectrum is calculated within a semiclassical approximation. It turns out that the state of the moving Bose-gas is unstable with respect to the creation of these boundary excitations in the system gas + walls, i.e. the critical velocity vanishes in the semiclassical (Bogoliubov) approximation. We discuss how a condensate wave function, the boundary excitation spectrum and, hence, the value of the critical velocity can change in more realistic models, in which ``smooth'' attractive interaction between the gas and walls is taken into account. Such a surface mode could exist in ``soft matter'' containers with flexible walls.