Liquid and solid phases of He-3 on graphite

Abstract

Recent heat-capacity experiments show quite unambiguously the existence of a liquid He-3 phase adsorbed on graphite. This liquid is stable at an extremely low density, possibly one of the lowest found in nature. Previous theoretical calculations of the same system, and in strictly two dimensions, agree with the result that this liquid phase is not stable and the system is in the gas phase. We calculated the phase diagram of normal He-3 adsorbed on graphite at T = 0 using quantum Monte Carlo methods. Considering a fully corrugated substrate, we observe that at densities lower than 0.006 angstrom(-2) the system is a very dilute gas that, at that density, is in equilibrium with a liquid of density 0.014 angstrom(-2). Our prediction matches very well the recent experimental findings on the same system. On the contrary, when a flat substrate is considered, no gas-liquid coexistence is found, in agreement with previous calculations. We also report results on the different solid structures, and on the corresponding phase transitions that appear at higher densities.