Giant Proximity Effect in Superfluid Helium

Recently, it was shown that two confined regions of liquid 4He exhibit a proximity effect over distances much larger than the correlation length ξ[1; 2]. Here we report measurements of the superfluid fraction ρs/ρ and specific heat cp of a 33.6 nm film. Comparison with previous data from a 31.7 nm film in contact with an array of 34 × 106 (2 μm)3 boxes of 4He allows us to show quantitatively the enhancement in ρs/ρ and cp due to the presence of the boxes in the temperature region where the film orders. The enhancement in ρs/ρ is observed up to distances 650 times the bulk correlation length. This anomalously large length scale is analogous to a giant proximity effect observed in High-Tc superconductors (HTSC)[3].