Direct Evidence for Density Wave Order in 3He Doped Two Dimensional 4He Supersolid

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.
Comment: Main body of the paper plus supplementary material