Theory of the Critical Temperature of Superfluid Phase Transitions in Two Dimensions

An argument is given that a model field theory with an $n$-component order parameter has a nonzero critical temperature for large $n$ in two dimensions. A simple dependence of the depression of the $\lambda$ point below $T_{\lambda }$ (bulk) on film thickness is found which correlates with properties of the heat capacity for multilayer He films on graphite observed by Bretz.