On the Novel Superfluidity in the Second Layer of $^4$He on Graphite

TL;DR

This paper reports the discovery of a new superfluid phase in second-layer $^4$He on graphite, characterized by superfluidity and viscoelasticity, supported by combined torsional oscillator and heat capacity measurements at ultra-low temperatures.

Contribution

It provides direct experimental evidence for a novel superfluid phase in $^4$He on graphite and introduces a superfluid liquid-crystal hypothesis, addressing substrate effects.

Findings

Identification of a new superfluid phase in $^4$He on graphite.

Superfluid phase exhibits both superfluidity and viscoelasticity.

The phase is stable over a finite density range.

Abstract

Evidence for a new type of superfluid phase in second-layer $^4$He on graphite has been obtained from simultaneous measurements of torsional-oscillator response and heat-capacity on exactly the same sample down to 30 mK, which resolve substrate-related uncertainties in the previous studies. The new phase hosts both superfluidity and enhanced viscoelasticity, and is stable over a finite density interval, strongly supporting the proposed superfluid liquid-crystal hypothesis. A random-Josephson-network analysis shows that the widely reported log-$T$ dependence of the superfluid density is likely due to substrate imperfections.