Onset of Superfluidity in 4He Films Adsorbed on Disordered Substrates

TL;DR

This study investigates the onset of superfluidity in 4He films on disordered substrates, revealing a transition akin to a superconductor-insulator transition, but with discrepancies from existing Bose glass theories.

Contribution

It provides experimental evidence linking superfluid onset to a mobility edge and challenges current scaling theories of the Bose glass phase.

Findings

Superfluid transition occurs at a critical coverage related to a mobility edge.

Heat capacity indicates the non-superfluid phase is gapped and insulating.

Superfluid density measurements do not align with Bose glass scaling predictions.

Abstract

We have studied 4He films adsorbed in two porous glasses, aerogel and Vycor, using high precision torsional oscillator and DC calorimetry techniques. Our investigation focused on the onset of superfluidity at low temperatures as the 4He coverage is increased. Torsional oscillator measurements of the 4He-aerogel system were used to determine the superfluid density of films with transition temperatures as low as 20 mK. Heat capacity measurements of the 4He-Vycor system probed the excitation spectrum of both non-superfluid and superfluid films for temperatures down to 10 mK. Both sets of measurements suggest that the critical coverage for the onset of superfluidity corresponds to a mobility edge in the chemical potential, so that the onset transition is the bosonic analog of a superconductor-insulator transition. The superfluid density measurements, however, are not in agreement with the scaling theory of an onset transition from a gapless, Bose glass phase to a superfluid. The heat capacity measurements show that the non-superfluid phase is better characterized as an insulator with a gap.