Monte Carlo simulation for square planar model with a small fourfold symmetry-breaking field

Monte Carlo (MC) simulations on the square planar rotator model with small fourfold symmetry-breaking field h₄ points out the existence of a Berezinskii-Kosterlitz-Thouless phase at intermediate temperatures between the low-temperature ferromagnetic phase and the high-temperature paramagnetic phase. This result contrasts with the expectation of a single order-disorer phase transition characterized by nonuniversal critical exponents for any h₄ as suggested by a renormalization group analysis and confirmed by MC simulations for intermediate and large symmetry-breaking fields.