First-order transition in the dense two-dimensional classical Coulomb gas.

The phase diagram of the two-dimensional classical neutral Coulomb gas (CG) at finite temperature has been investigated via the mapping to the ground-state properties of the (1 + 1)-dimensional quantum sine-Gordon model. On the basis of self-consistent-field theory recently proposed by the authors for the quantum sine-Gordon model, we show that the CG undergoes two distinct classes of Kosterlitz-Thouless phase transitions between the dielectric gas and conductive plasma: one is the usual continuous transition of infinite order at very small fugacity, the other is a first-order discontinuous transition starting at the fugacity z* = 0. 08 and the temperature T* = 0. 25. Our results further corroborate the existence of a first-order phase transition in the dense CG system predicted by Minnhagen's self-consistent linear screening renormalization theory and Monte Carlo simulations.