Non-universal critical Casimir force in confined ⁴He near the superfluid transition

We present the results of a one-loop calculation of the effect of a van der Waals type interaction potential $\sim | {\bf x} |^{-d-\sigma}$ on the critical Casimir force and specific heat of confined $^4$He near the superfluid transition. We consider a $^4$He film of thickness $L$. In the region $L \gtrsim \xi$ (correlation length) we find that the van der Waals interaction causes a leading non-universal non-scaling contribution of $O (\xi^2 L^{-d-\sigma})$ to the critical temperature dependence of the Casimir force above $T_\lambda$ that dominates the universal scaling contribution $\sim e^{- L/\xi}$ predicted by earlier theories. For the specific heat we find subleading non-scaling contributions of $O(L^{-1})$ and $O(L^{-d-\sigma})$.