Critical behaviour of the compact 3d U(1) gauge theory at finite temperature

Critical properties of the compact three-dimensional U(1) lattice gauge theory are explored at finite temperatures. The critical point of the deconfinement phase transition, critical indices and the string tension are studied numerically on lattices with temporal extension N_t = 8 and spatial extension ranging from L = 32 to L = 256. The critical indices, which govern the behaviour across the deconfinement phase transition, are generally expected to coincide with the critical indices of the two-dimensional XY model. It is found that the determination of the infinite volume critical point differs from the pseudo-critical coupling at L = 32, found earlier in the literature and implicitly assumed as the onset value of the deconfined phase. The critical index $\nu$ computed from the scaling of the pseudocritical couplings agrees well with the value $\nu$ = 1/2 of the XY model. The computation of the index $\eta$ brings to a value larger than expected. The possible reasons for such behaviour are discussed.