Phase transitions and accidental degeneracy in nonlinear spin systems

Abstract

A characterization of phase transitions and related accidental degeneracies is presented for a general spin Hamiltonian containing linear and quadratic terms. The existence of first-, second-, and third-order phase transitions is exhibited. Regions in the control parameters space are found where crossings and anticrossings of quantum levels take place. The possibility of closely reproducing the ground and first excited states using even- and odd-spin coherent states is emphasized.