Phase Diagram of a Two-Dimensional Neutral Classical Coulomb Gas from a Non-perturbative sine-Gordon Expansion

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Within the sine-Gordon formalism of a two-dimensional neutral classical Coulomb gas, a convergent expansion with non-perturbative nature is performed to calculate the thermodynamic potential and construct the phase diagram. It is shown that truncation at the first order yields the Gaussian approximation. The second- and third-order corrections are analyzed for the case of small fugacity and are shown that they substantially improve the Gaussian-approximation phase diagram. In particular, these corrections introduce a new conducting phase and make the insulator-conductor coexistence phase end at the conducting phase. The latter result is in agreement with the prediction made by generalized renormalization-group calculations.