Variational analysis of deconfinement in compact U(1) gauge theory

A variational method is used to analyse compact U(1) gauge theory in 2+1-dimensions at finite temperature, T, weak coupling, g and where the fundamental magnetic monopoles have magnetic charge 2\pi n/g. The theory undergoes a critical transition from a confining phase at temperatures below T_c=2g^2/n^2\pi to a deconfined phase at temperatures above T_c. The free energy and all its derivatives are continuous at T_c, indicative of the BKT phase transition. The relevant gauge-invariant correlation functions decay exponentially at large distances. The spatial Wilson loop obeys the area law at all finite temperatures, even for the non-compact theory. The case n=2 corresponds to the compact U(1) theory considered as a low energy effective theory for the spontaneously broken Georgi--Glashow model. The results in this case agree with those derived previously for compact U(1) in this model using dimensional reduction of the Lagrangian.