Critical Point Coupling and Proximity Effects inHe4at the Superfluid Transition

Abstract

We report measurements of the superfluid fraction ρ(s)/ρ and specific heat c(p) near the superfluid transition of 4He when confined in an array of (2 μm)3 boxes at a separation of S=2 μm and coupled through a 32.5 nm film. We find that c(p) is strongly enhanced when compared with data where coupling is not present. An analysis of this excess signal shows that it is proportional to the finite-size correlation length in the boxes ξ(t,L), and it is measurable as far as S/ξ∼30-50. We obtain ξ(0,L) and the scaling function (within a constant) for ξ(t,L) in an L3 box geometry. Furthermore, we find that ρ(s)/ρ of the film persists a full decade closer to the bulk transition temperature T(λ) than a film uninfluenced by proximity effects. This excess in ρ(s)/ρ is measurable even when S/ξ>100, which cannot be understood on the basis of mean field theory.

Superfluid nanomechanical resonator for quantum nanofluidics

“…The coherent motion of the superfluid state is described by an order parameter, whose spatial fluctuations are correlated over a length scale given by the coherence length, ξ. This coherence length diverges at the superfluid transition and reaches a finite value in the low temperature limit (ξ 0 = 20 − 80 nm in 3 He 1 and ξ 0 = 0.1 nm in 4 He 2 ). By confining these quantum fluids in well-defined structures of size comparable to the coherence length, non-bulk phenomena can be revealed.…”

“…At low temperatures, liquid 3 He and 4 He transition into superfluids, which exhibit exotic properties -such as dissipationless flow -as a result of macroscopic quantum coherence. The coherent motion of the superfluid state is described by an order parameter, whose spatial fluctuations are correlated over a length scale given by the coherence length, ξ.…”

“…Despite the fact that these superfluids are well studied, only a few experiments are capable of measuring such tiny volumes, or small surface effects at the edge of bulk volumes. The most powerful experimental techniques to date are nuclear magnetic resonance NMR 6 in 3 He and heat capacity 3 in 4 He, although new techniques using nanomechanical structures are promising [11][12][13] . Yet numerous theoretical predictions go untested because the right experimental probes do not exist.…”

“…By confining these quantum fluids in well-defined structures of size comparable to the coherence length, non-bulk phenomena can be revealed. For instance, nanofluidic confinement has allowed the study of the order parameter fluctuation spectrum 3 , as well as proximity coupling 4 , near the superfluid transition of 4 He. In 3 He, confinement approaching the coherence length is predicted to result in new superfluid phases 5,6 due to geometrically induced distortion of the order parameter.…”

“…We present a model to interpret the dynamics of our superfluid nanomechanical resonator, and we show how it can be used for probing confined geometry effects on thermodynamic functions. We report isobaric measurements of the superfluid fraction in liquid 4 He at various pressures, and the onset of quantum turbulence in restricted geometry. …”