Stability of Half-Quantum Vortices in p_x+ip_y Superconductors

We consider the stability conditions for half-quantum vortices in a quasi-two-dimensional p_x+ip_y superconductor (such as Sr₂RuO₄ is believed to be). The predicted exotic nature of these excitations has recently attracted much attention, but they have not been observed yet. We emphasize that an isolated half-quantum vortex has a divergent energy cost in the bulk due to its unscreened spin current, which requires two half-quantum vortices with opposite spin winding to pair. We show that the stability of such a pair is enhanced when the ratio of spin superfluid density to superfluid density ρ_sp/ρ_s is small. We propose using various mesoscopic geometries to stabilize and observe these exotic excitations.