Surface tension of helium at the superfluid critical end -point

The singularity in the vapor-liquid interfacial tension, Σ(T), of helium at the transition to superfluidity is analyzed theoretically. The universal amplitude ratio Rσξ+ = K+(ξ0+)d-1/kBTλ, where K+ and K- are the amplitudes of the |T-Tλ|μ singularity in Σ, with μ = 1.343, is known from recent work to first order in ε=4-d for the general n-vector model in d dimensions. Extrapolation to d=3 for n=2 indicates Rσξ+=0.05-0.08, which is shown to be consistent with the experimental data. Further analysis of the experiments establishes that the universal ratio Q=K+/K- exceeds 0.35, and is consistent with the recent prediction Q≅0.9; this demonstrates the inadequacies of earlier theoretical treatments. The existence in the observed surface tension of an anomalous, negative contribution of unknown origin at a few millikelvin beneath Tλ is stressed. 32 refs., 6 figs