Critical Casimir force in $^4$He films: confirmation of finite-size scaling

TL;DR

This study provides experimental confirmation of finite-size scaling in the critical Casimir force in $^4$He films near the superfluid transition, demonstrating universal behavior and identifying additional fluctuation forces.

Contribution

The paper experimentally confirms finite-size scaling of the critical Casimir force in $^4$He films and characterizes the universal scaling function.

Findings

Scaling function collapses onto a universal curve

Minimum of the scaling function at -1.30

Goldstone surface fluctuation force detected down to 2.13 K

Abstract

We present new capacitance measurements of critical Casimir force-induced thinning of $^4$He films near the superfluid/normal transition, focused on the region below $T_{\lambda}$ where the effect is the greatest. $^4$He films of 238, 285, and 340 \AA thickness are adsorbed on N-doped silicon substrates with roughness $\approx 8 {\AA}$. The Casimir force scaling function $\vartheta $, deduced from the thinning of these three films, collapses onto a single universal curve, attaining a minimum $\vartheta = -1.30 \pm 0.03$ at $x=td^{1/\nu}=-9.7\pm 0.8 {\AA}^{1/\nu}$. The collapse confirms the finite-size scaling origin of the dip in the film thickness. Separately, we also confirm the presence down to $2.13 K$ of the Goldstone/surface fluctuation force, which makes the superfluid film $\sim 2 {\AA}$ thinner than the normal film.