The gap vs critical temperature ratio in Peierls-type phase transitions

We analyze a 2D spin-pseudospin model, where the pseudospins represents the charge degrees of freedom. The model is known to undergo a phase transition with the simultaneous appearance of the long-range charge order and the spin gap. We show how the gap vs critical temperature ratio (also called the BCS ratio) gets renormalized from the classical non-interacting value. This value is also universal in the sense that is does not depend on the microscopic parameters of the model, and must be the same for various types of the Peierls-like transitions where the spin gap is accompanied by the structural, orbital or charge order.