XXIX International Symposium "Nanophysics and Nanoelectronics", Nizhny Novgorod, March 10-14, 2025
Superconductivity

On the abrupt change in the superconducting order parameter in the vicinity of the $s_\pm\to s_{++}$ transition in the Born limit

V. A. Shestakov, M. M. Korshunov

L. V. Kirensky Institute of Physics, Siberian Branch of the Russian Academy of Sciences, Krasnoyarsk

Abstract: In multiband systems like iron pnictides and chalcogenides, the unconventional superconducting state can emerge. Effect of disorder on this state may have unexpected consequences, such as, for instance, change of the $s_\pm$ superconducting order parameter structure to the $s_{++}$ one. Studying of a system behavior near such a transition is very important. For this purpose, the grand thermodynamic potential for the normal, $\Omega_{\mathrm{N}}$, and superconducting, $\Omega_{\mathrm{S}}$, states is calculated, as well as their difference $\Delta\Omega=\Omega_{\mathrm{S}}-\Omega_{\mathrm{N}}$. An expression for $\Delta\Omega$ is derived for a two-band model of iron-based superconductors with the nonmagnetic impurities. The presence of disorder is considered within the T-matrix approximation for a multiband Eliashberg theory. Near the Born limit in the vicinity of $s_\pm\to s_{++}$ transition, two sets of solutions, which are obtained for opposite directions in changing of the impurity scattering rate, are found to exist. Based on $\Delta\Omega$, a phase diagram showing energetically favorable solutions for $s_\pm$ and $s_{++}$ states as well as the transition between them is plotted. At low temperatures within the region where two sets of solutions coexist, the transition is abrupt and barely dependent on temperature. At higher temperatures, the Eliashberg equations have a single set of solutions, with the transition between $s_\pm$ and $s_{++}$ states being smooth.

Keywords: unconventional superconductivity, iron pnictides, iron chalcogenides, grand thermodynamic potential, impurity scattering, Eliashberg equations.

Received: 06.03.2025
Revised: 06.03.2025
Accepted: 05.05.2025

DOI: 10.61011/FTT.2025.07.61183.35HH-25

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