Confined ⁴He Near T_λ: Scaling and Giant Proximity Effects

This is a brief review of the experimental results for the critical behavior near T_λ of ⁴He confined in a well-defined geometry. This is realized using cells formed with patterned Si wafers which are directly bonded. The thrust of these experiments is to verify finite-size scaling near T_λ for different dimensionality crossover and to explore the coupling and proximity effects between two adjoining regions of confined ⁴He. Predictions of finite-size scaling are verified for film confinement in the case of the specific heat. However, there are issues in the temperature region where the film becomes superfluid. In the case of coupling and proximity effects, one finds that remarkably and unexpectedly these extend orders of magnitude beyond the range of the correlation length ξ. A more complete discussion of these results and techniques, as well as other relevant experiments in helium, and the connection to observations with superconductors can be found in (FM. Gasparini et al. Rev. Mod. Phys.80:1009–1059, 2008: JK Perron et al. Rep. Prog. Phys.82: 1–29, 2019).