F. Pazmandi et Z. Domanski, GROUND-STATE OF XY MODELS OF INTERACTING SPINS IN A TRANSVERSE FIELD, Journal of physics. A, mathematical and general, 26(15), 1993, pp. 689-692
We investigate the ground state of an X Y model with a transverse fiel
d, characterized by an exchange matrix, whose highest eigenvalue, lamb
da(max) is supposed to be positive and finite. We prove rigorously tha
t for a spin S the ground state changes at the transverse field value
K(c) = Slambda(max).