Phase transitions driven by single-vortex excitations in confined geometries

We propose a new class of phase transitions associated with the spontaneous generation of single vortices and antivortices in confined superfluid systems. Nucleation of single vortices from the boundary drives the systems to a topological phase transition at a temperature lower than the critical Berezinskǐ-Kosterlitz-Thouless temperature. This phenomenon is universal and nearly independent of the system geometry. We establish a universal expression for the critical temperature of the new transition. This theoretical prediction for the variation of the critical temperature with system size is in excellent agreement with available experimental data.