NUMERICAL SIMULATIONS OF ROUGHENING TEMPERATURES IN A HEXAGONAL VAN-DER-WAALS HE-4 CRYSTAL

We have carried out numerical simulations to investigate roughening on He-4 crystal surfaces. Algorithms were constructed for an h.c.p, crys tal, incorporating van-der-Waals interatomic interactions. The Wulff p lot was calculated at 0 K. Also, by sectioning the crystal in several symmetry directions, roughening temperatures, T-R, have been calculate d for these orientations using a lattice-gas model. In particular, the se have been determined for the c, a and s facets, as well as for lowe r-symmetry facets which involve interactions between higher-than-neare st-neighbours. On one of these facets, we have observed the formation of anisotropic clusters below a certain critical temperature. The valv es for the high-symmetry facets are more precise but in broad agreemen t with those calculated by Touzani and Wortis(1) and do not agree part icularly well with the observed valves. We are therefore investigating quantum corrections to the calculations.