NMR measurements on 3He adsorbed on Grafoil at submonolayer coverages and millikelvin temperatures.

Abstract

Precise nuclear magnetic susceptibility results on $^{3}\mathrm{He}$ adsorbed on Grafoil are presented for submonolayer coverages in the temperature reange 5--600 mK. At low areal densities, the film is in a two-dimensional homogeneous liquid state. Some $^{3}\mathrm{He}$ atoms, however, are localized by the substrate heterogeneity; our quantitative measurements show that their amount increases from 2% to 4% of a monolayer thereby leading to significant corrections of the fluid-phase density. The Fermi tempreature of the liquid has been determined as a function of coverage; it decreases continuously for densities larger than 0.01 atoms/A${\mathrm{\r{}}}^{2}$. At high fluid densities, the susceptibility is enchanced by one order of magnitude with respect to the ideal-gas value. At \ensuremath{\rho}=0.0449 atoms/A${\mathrm{\r{}}}^{2}$, a two-phase coexistence region begins on a Grafoil sample; \ensuremath{\rho}=0.0434 atoms/A${\mathrm{\r{}}}^{2}$ is estimated for a perfect graphite surface. The coexistence is characterized by a linear increase (linear decrease) of the solid (liquid) fraction and a constant Fermi temperature of the liquid phase. Finally, at \ensuremath{\rho}=0.0646 atoms/A${\mathrm{\r{}}}^{2}$, the liquid phase disappears completely. Our results are compared to previous heat capacity and NMR measurements at submonolayer coverages.