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

TL;DR

This paper investigates the ratio of the spin gap to the critical temperature in a 2D spin-pseudospin model undergoing a Peierls-like transition, revealing its universal and renormalized nature independent of microscopic details.

Contribution

It demonstrates the universality and renormalization of the gap versus critical temperature ratio in Peierls-type phase transitions with charge and spin order.

Findings

The ratio is renormalized from the classical value.

The ratio is universal across different Peierls-like transitions.

The ratio does not depend on microscopic parameters.

Abstract

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.