The Role of Spin Anisotropy in the Unbinding of Interfaces

We study the ground state of a classical X-Y model with $p \ge 3$-fold spin anisotropy $D$ in a uniform external field, $H$. An interface is introduced into the system by a suitable choice of boundary conditions. For large $D$, as $H \to 0$, we prove using an expansion in $D^{-1}$ that the interface unbinds from the surface through an infinite series of layering transitions. Numerical work shows that the transitions end in a sequence of critical end points.