Fractional Vortices, $\mathbb{Z}_2$ Gauge Theory, and the Confinement-Deconfinement Transition

In this paper we discuss the classical 3D XY model whose nearest-neighbor interaction is a mixture of $\cos(\theta_i-\theta_j)$ (ferromagnetic) and $\cos2(\theta_i-\theta_j)$ (nematic). This model is dual to a theory with integer and half-integer vortices. While both types of vortices interact with a non-compact $U(1)$ gauge field (the "EM" interaction), the half-vortices interact with an extra interaction mediated by a $\mathbb{Z}_2$ gauge field. We shall discuss the confinement-deconfinement transition of the half-integer vortices, the Wilson and the 't Hooft loops and their mutual statistics in path integral language. In addition, we shall present a quantum version of the classical model which exhibits these physics.