Liquid-to-gas phase transitions in two-dimensional quantum systems at zero temperature

Abstract

Two-dimensional systems of bosons and fermions are studied at zero temperature by means of variational calculations. When viewed as a function of the quantum parameterη = ℏ2/mɛΣ in the variational context, bosons are found to undergo a second-order liquid-to-gas transition, whereas, contrary to expectations, fermions are found to undergo a first-order transition with a region in which liquid and gaseous phases can coexist. Although these results are qualitatively the same as those in three dimensions, it is emphasized that, for two-dimensional fermions, the underlying physics is quite different. These results are examined critically in view of the special properties of two-dimensional systems. It is found that critical values of η are rigorously lower bounds, and it is further argued that the essential features of these results remain valid. Finally, the implications of these results for experiments on submonolayer physisorbed helium and spin-aligned hydrogen systems are discussed.