We report dielectric constant measurements showing critical fluctuation-induced thinning of 4 He films near the superfluid transition. The films are adsorbed on a stack of copper electrodes suspended at different heights above bulk liquid. We calibrate the measurements by assuming that the film thickness away from the transition region at different heights is accurately given by theory. The thinning is found to be consistent with finite-size scaling, if the value of the scaling function for each thickness is normalized by its value at the minimum.

We present new capacitance measurements of critical Casimir force-induced thinning of 4He films near the superfluid transition, focused on the region below Tlambda where the effect is the greatest. 4He films of 238, 285, and 340 A 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=td1/nu=-9.7+/-0.8 A1/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 approximately 2 A thinner than the normal film.

We present capacitance measurements of the equilibrium thickness of (3)He-(4)He mixture films as a function of temperature and concentration. The films are adsorbed on a Cu substrate situated above bulk liquid mixture. As we scan across the tricritical point, we observe a thickening of the film indicating the presence of a repulsive critical Casimir force.

A new heterostructure is developed in which fractional monolayers of magnetic ions are introduced "digitally" within a semiconductor quantum well. Rearranging moments within two-dimensional (2D) planes provides additional control over their interactions, yielding tunable two-level electronic systems in which spin splittings are significantly larger than inhomogeneous linewidths in modest magnetic fields.Femtosecond-resolved electronic and magnetic spectroscopies reveal spin-flip scattering dependent only on the Zeeman energy but not the local magnetic environment, and long-lived dynamic magnetization s.

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