From Continuous to First-Order Transition in a Simple XY Model

摘要: An $\mathrm{XY}$ model of classical spins, on a two-dimensional square lattice, with nearest-neighbor interaction $V(\ensuremath{\theta})=2J[1\ensuremath{-}{(\frac{{cos}^{2}\ensuremath{\theta}}{2})}^{{p}^{2}}]$ is studied by Monte Carlo simulation that also monitors the vortex excitations. Both continuous and first-order transitions are found. In latter density vortices sharply increases at transition point. The turns first order, because shape $V$ such vortex-antivortex pair formation suppressed low temperatures for $p\ensuremath{\gtrsim}3$.