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

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 $T_λ$ where the effect is the greatest. $^4$He films of 238, 285, and 340 Åthickness are adsorbed on N-doped silicon substrates with roughness $\approx 8 Å$. 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 $x=td^{1/ν}=-9.7\pm 0.8 Å^{1/ν}$. 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 $\sim 2 Å$ thinner than the normal film.