Critical Casimir Effect in superfluid wetting films

TL;DR

This paper investigates the critical Casimir effect in superfluid wetting films of helium isotopes, providing theoretical scaling functions that explain experimental observations near phase transition points.

Contribution

It calculates the scaling functions of the critical Casimir force for superfluid helium films within relevant universality classes, explaining experimental data and predicting crossover behaviors.

Findings

Identification of the maximum of f_C below the tricritical point.

Explanation of the rich behavior of the scaling function vartheta from experimental data.

Prediction of crossover behavior between tricritical and lambda-transition points.

Abstract

Recent experimental data for the complete wetting behavior of pure 4He and of 3He-4He mixtures exposed to solid substrates show that there is a change of the corresponding film thicknesses L upon approaching thermodynamically the lambda-transition and the tricritical end point, respectively, which can be attributed to critical Casimir forces f_C. We calculate the scaling functions vartheta of f_C within models representing the corresponding universality classes. For the mixtures our analysis provides an understanding of the rich behavior of vartheta deduced from the experimental data and predicts the crossover behavior between the tricritical point and the lambda-transition of pure 4He which are connected by a line of critical points. The formation of a 'soft-mode' phase within the wetting films gives rise to a pronounced maximum of f_C below the tricritical point as observed experimentally. Near the tricritical point we find logarithmic corrections ~L^(-3)(ln L)^(1/2) for the leading behavior of vartheta dominating the contributions from the background dispersion forces.