Liquid-solid phase transition in Bose systems: instability of the variational liquid ground state

We investigate conditions under which the variational ground state of a one component quantum Bose fluid becomes unstable in the vicinity of the liqui d-solid phase transition. We find a first critical density where a non-sphe rical two-body structure of the fluid will lower the energy and propose tha t this structure has the point-group symmetry of the emerging crystal. If t he fluid density is increased further the energy has a minimum only with re spect to spherically symmetric perturbations. This minimum disappears at a second critical density and no solution is found at higher densities. We gi ve results for the two instability densities both in liquid He-4 and in Yuk awa Bose fluid. We propose that the two densities approximately correspond to the equilibrium freezing and melting densities. (C) 2000 Elsevier Scienc e B.V. All rights reserved.