Critical point coupling and proximity effects in He4 at the superfluid transition

We report measurements of the superfluid fraction ρ_s/ρ and specific heat c_p near the superfluid transition of He4 when confined in an array of (2μm)3 boxes at a separation of S=2μm and coupled through a 32.5 nm film. We find that c_p is strongly enhanced when compared with data where coupling is not present. An analysis of this excess signal shows that it is proportional to the finite-size correlation length in the boxes ξ(t,L), and it is measurable as far as S/ξ∼30-50. We obtain ξ(0,L) and the scaling function (within a constant) for ξ(t,L) in an L3 box geometry. Furthermore, we find that ρ_s/ρ of the film persists a full decade closer to the bulk transition temperature T _λ than a film uninfluenced by proximity effects. This excess in ρ_s/ρ is measurable even when S/ξ>100, which cannot be understood on the basis of mean field theory.