Critical casimir force in 4He films:: Confirmation of finite-size scaling

We present new capacitance measurements of critical Casimir force-induced thinning of He-4 films near the superfluid transition, focused on the region below T-lambda where the effect is the greatest. He-4 films of 238, 285, and 340 angstrom thickness are adsorbed on atomically smooth, N-doped silicon substrates. The Casimir force scaling function theta, deduced from the thinning of these three films, collapses onto a single universal curve, attaining a minimum theta=-1.30 +/- 0.03 at x=td(1/nu)=-9.7 +/- 0.8 angstrom(1/nu). The collapse confirms the finite-size scaling origin of the dip in the film thickness. Separately, we also confirm the presence down to 2.13 K of the Goldstone or surface fluctuation force, which makes the superfluid film similar to 2 angstrom thinner than the normal film.