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 ρ_s/ρ and specific heat c_p 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 ρ_s/ρ and c _p due to the presence of the boxes in the temperature region where the film orders. The enhancement in ρ_s/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-T_c superconductors (HTSC)[3].