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

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 $\hat{I}¼$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.

Main body of the paper plus supplementary material