Eliashberg approach to superconductivity-induced infrared anomalies in Ba0.68K0.32Fe2As2

TL;DR

This study uses spectroscopic ellipsometry and Eliashberg theory to analyze superconductivity-induced infrared anomalies in Ba0.68K0.32Fe2As2, revealing multiband effects and spin-fluctuation contributions to optical conductivity suppression.

Contribution

It provides a detailed microscopic explanation of infrared anomalies in a high-temperature superconductor using a multiband Eliashberg framework.

Findings

Identification of two distinct superconducting gaps.

Observation of optical conductivity suppression up to 14kBTc.

Attribution of anomalies to spin-fluctuation--assisted processes.

Abstract

We report the full complex dielectric function of high-purity $\textrm{Ba}_{0.68}\textrm{K}_{0.32}\textrm{Fe}_2\textrm{As}_2$ single crystals with $T_{\mathrm{c}}=38.5\ \textrm{K}$ determined by wide-band spectroscopic ellipsometry at temperatures $10\leq T\leq300\ \textrm{K}$. We discuss the microscopic origin of superconductivity-induced infrared optical anomalies in the framework of a multiband Eliashberg theory with two distinct superconducting gap energies $2\Delta_{\mathrm{A}}\approx6\ k_{\mathrm{B}}T_{\mathrm{c}}$ and $2\Delta_{\mathrm{B}}\approx2.2\ k_{\mathrm{B}}T_{\mathrm{c}}$. The observed unusual suppression of the optical conductivity in the superconducting state at energies up to $14\ k_{\mathrm{B}}T_{\mathrm{c}}$ can be ascribed to spin-fluctuation--assisted processes in the clean limit of the strong-coupling regime.