Structure of metastable 2D liquid helium

We present diffusion Monte Carlo (DMC) results on a novel, superfluid phase in two-dimensional 4 He at densities higher than 0.065 angstrom(-2), which is very close to the freezing density. The new phase has anisotropic, hexatic orbital order, but the single-particle density remains constant. By increasing density the hexatic superfluid forms a metastable state, which lies above the crystal ground state in energy. This implies that the liquid-solid phase transition takes place in two stages: a second-order phase transition from the isotropic superfluid to the hexatic superfluid, followed by a first-order transition that localizes atoms into the triangular crystal order. Copyright (C) EPLA, 2008