Thinning of superfluid films below the critical point.

Experiments on 4He films reveal an attractive Casimir-like force at the bulk lambda point and in the superfluid regime. Previous work has explained the magnitude of this force at the lambda transition and deep in the superfluid region but not the substantial attractive force immediately below the lambda point. Utilizing a simple mean-field calculation renormalized by critical fluctuations we obtain an effective Casimir force that is qualitatively consistent with the scaling function vartheta obtained by collapse of experimental data.