New critical behavior in the dense two-dimensional classical Coulomb gas

We carry out Monte Carlo simulations of the two-dimensional classical neutral Coulomb gas of integer charges on a square lattice, as a function of temperature and chemical potential, and find new critical behavior. At low temperature, as density increases, the system has a first-order transition to an ordered-charge lattice. As temperature increases, this dense phase has a Kosterlitz-Thouless transition followed by an Ising transition. The first-order line joins the line of Ising transitions at a new tricritical point.