Grand Thermodynamic Potential in A Two-Band Unconventional Superconductor

Shestakov, V.A., Korshunov, M.M.// Journal of Superconductivity and Novel Magnetism//

https://doi.org/10.1007/s10948-025-07074-8

In a two-band system, both conventional sign-preserving $s_{+ +}$ and unconventional sign-changing $s_{\pm}$ superconducting state may appear at low temperatures. Moreover, they may transform from one to another due to the impurity scattering. To study the details of such a transition here we derive the expression for the Grand thermodynamic potential $Ω$ for a two-band model with nonmagnetic impurities considered in a $T$ -matrix approximation. For the iron-based materials within the multiband Eliashberg theory, we show that the $s_{\pm} \to s_{+ +}$ transition in the vicinity of the Born limit is a first order phase transition.