Superfluidity of $^4$He Confined in Nano-Porous Media

TL;DR

This study investigates superfluidity of helium-4 confined in nano-porous media, revealing a quantum phase transition at high pressure and emphasizing the role of phase fluctuations in superfluid behavior.

Contribution

It provides new insights into superfluid phase transitions in nano-confined helium-4, highlighting the importance of phase fluctuations and identifying a localized Bose condensation state.

Findings

Superfluid transition temperature approaches zero at 3.4 MPa.

The nonsuperfluid phase is a localized Bose condensate.

Superfluid density exhibits a T-linear term at high pressures.

Abstract

We have examined superfluid properties of $^4$He confined to a nano-porous Gelsil glass that has nanopores 2.5 nm in diameter. The pressure-temperature phase diagram was determined by torsional oscillator, heat capacity and pressure studies. The superfluid transition temperature $T_{\mathrm c}$ approaches zero at 3.4 MPa, indicating a novel "quantum" superfluid transition. By heat capacity measurements, the nonsuperfluid phase adjacent to the superfluid and solid phases is identified to be a nanometer-scale, localized Bose condensation state, in which global phase coherence is destroyed. At high pressures, the superfluid density has a $T$-linear term, and $T_{\mathrm c}$ is proportional to the zero-temperature superfluid density. These results strongly suggest that phase fluctuations in the superfluid order parameter play a dominant role on the phase diagram and superfluid properties.