Thermodynamic Evidence for Density Wave Order in Two Dimensional 4He Supersolid

TL;DR

This study provides thermodynamic evidence for density wave order in a two-dimensional $^{4}$He supersolid, using doping with $^{3}$He impurities and measuring heat capacity, magnetization, and NMR relaxation times across a broad temperature range.

Contribution

It offers direct experimental evidence for density wave order in a 2D $^{4}$He supersolid, advancing understanding of its intertwined density wave and superfluid phases.

Findings

Evidence of density wave order from thermodynamic measurements.

Identification of a solid second layer film matching superfluid response regimes.

Changes in ground state correlated with $^{4}$He concentration and $^{3}$He doping.

Abstract

We previously reported the discovery of a two dimensional $^{4}$He supersolid; a state with intertwined density wave and superfluid order, observed in the second layer of $^{4}$He adsorbed on graphite. In this Letter we provide direct evidence for the density wave order, obtained by doping the layer with a small concentration of $^{3}$He atoms (impuritons). The heat capacity, magnetization and NMR relaxation times of the $^{3}$He were measured over a wide temperature range from 200 $\mu$K to 500 mK. They provide evidence for changes in the ground state in the second layer film as the amount of $^{4}$He is increased at various fixed $^{3}$He doses. Clear evidence is obtained for a solid second layer film, matching the r\a'egime of superfluid response previously observed in pure $^{4}$He films.