Phase transition in magnetically coupled spins on a ring (SOR) model
  We have considered a new type of 'XY' model where spins are placed on concentric ring with constant spin density in every ring. The spin executes continuous rotation under a modified Shore-Zwanzig Hamiltonian (J. Chem. Phys. 63, 5445 (1975)). We have performed Monte Carlo simulation using Glauber acceptance criteria. Computations of Binder's cumulant, specific heat and magnetic susceptibility all show presence of a finite temperature order-disorder phase transition in this spin system. The system size dependence of Binder's cumulant suggests the existence of a phase transition with a transition temperature of T* = 1.2. However, we have found no signature of the occurrence of vortex in our SOR model. The absence of hysteresis rules out the possibility of first order phase transition. We have found two "stable" states for T* = 0 phase. The perfectly ordered true ground state is obtained by gradual cooling of the system, while the other is obtained by starting the simulation with a random configuration at T* = 0. This second state has higher energy than the perfectly aligned ground state. 
Rajib Biswas and Biman Bagchi
cond-mat.stat-mech