Non-universal Casimir Effect in Saturated Superfluid $^4$He Films at T$_\lambda$

TL;DR

This study investigates the Casimir effect in superfluid helium films near the transition temperature, revealing a non-universal Casimir effect linked to viscous suppression of second sound modes.

Contribution

It presents the first observation of a non-universal Casimir effect in superfluid helium films caused by viscous effects on second sound modes.

Findings

Casimir dip in film thickness coincides with superfluid onset.

A new film-thickening effect occurs at T_λ during slow temperature sweeps.

Evidence suggests a non-universal Casimir effect due to viscous suppression.

Abstract

Measurements of Casimir effects in $^4$He films in the vicinity of the bulk superfluid transition temperature $T_\lambda$ have been carried out, where changes in the film thickness and the superfluid density are both monitored as a function of temperature. The Kosterlitz-Thouless superfluid onset temperature in the film is found to occur just as the Casimir dip in the film thickness from critical fluctuations becomes evident. Additionally, a new film-thickening effect is observed precisely at $T_\lambda$ when the temperature is swept extremely slowly. We propose that this is a non-universal Casimir effect arising from the viscous suppression of second sound modes in the film.