Calculation of van der Waals Effects on Correlation-length Scaling Near

We report calculations of the superfluid density, p_s of helium confined between Si surfaces, and of the onset temperature for helium films formed on a Mylar substrate. The calculations are done within λ-theory and include the effect of helium-surface interaction. We find that this interaction precludes size scaling with the bulk correlation length. The calculated effect however is not enough to explain the experimental results. In the case of the onset temperature, the influence of the van der Waals field causes the effective shift exponent to be less than that of the correlation length.