Giant coupling effects in confined ⁴He near T _λ

Superfluid ⁴He shares with superconductors a transition into a low temperature state where the order parameter is a wave function. For the low temperature superconductors, which have a large zero temperature correlation length, this results in well known Josephson effects reflecting the overlap of the wave function across barriers and weak links. Similar phenomena are harder to realize for ⁴He because the zero temperature correlation length is of the order of interatomic dimensions. The fact that for ⁴He the critical region, where the correlation length diverges, is accessible experimentally leads to a possible new kind of coupling. This differs from that of a superconductor in the sense that critical fluctuations are important. We have seen such coupling whereby two regions of confined ⁴He interact and influence their respective thermodynamic behavior (Perron et al. in Nat. Phys. 6:499-502, 2010). This interaction extends over length scales which are much larger than the correlation length. We describe measurements of heat capacity and superfluid density which illustrate this behavior.