An operator theoretical proof for the second-order phase transition in the BCS-Bogoliubov model of superconductivity

TL;DR

This paper provides an operator-theoretic proof that the transition in the BCS-Bogoliubov superconductivity model is second-order, analyzing the gap equation's properties and behavior near the critical temperature.

Contribution

It introduces a novel operator-theoretic approach to prove the second-order nature of the phase transition in the BCS-Bogoliubov model.

Findings

Transition is second-order in the BCS-Bogoliubov model.

Explicit expression for the specific heat gap at transition temperature.

Solution properties like smoothness and monotonicity are established.

Abstract

We first show some properties such as smoothness and monotone decreasingness of the solution to the BCS-Bogoliubov gap equation for superconductivity. Moreover we give the behavior of the solution with respect to the temperature near the transition temperature. On the basis of these results, dealing with the thermodynamic potential, we then show that the transition from a normal conducting state to a superconducting state is a second-order phase transition in the BCS-Bogoliubov model of superconductivity from the viewpoint of operator theory. Here we have no magnetic field and we need to introduce a cutoff $\varepsilon>0$, which is sufficiently small and fixed (see Remark \ref{rmk:varepsilon}). Moreover we obtain the exact and explicit expression for the gap in the specific heat at constant volume at the transition temperature.