Reverse process of usual optical analysis of boson-exchange superconductors: impurity effects on $s$- and $d$-wave superconductors

TL;DR

This study reverses the typical optical analysis of boson-exchange superconductors to understand impurity effects on optical properties of s- and d-wave superconductors, providing insights into their gap features and electron-boson interactions.

Contribution

It introduces a reverse analysis method to examine impurity effects on optical constants in superconductors, highlighting differences between s- and d-wave pairing symmetries.

Findings

Impurities reduce superfluid density and enhance gap features.

s-wave superconductors show clearer gap features than d-wave.

Impurity effects are similar in s- and d-wave superconductors.

Abstract

We performed a reverse process of a usual optical data analysis of boson-exchange superconductors. We calculated the optical self-energy from two (MMP and MMP+peak) input model electron-boson spectral density functions using Allen's formula for one normal and two ($s$- and $d$-wave) superconducting cases. We obtained the optical constants including the optical conductivity and the dynamic dielectric function from the optical self-energy using an extended Drude model, and finally calculated reflectance spectrum. Furthermore to investigate impurity effects on optical quantities we added various levels of impurities (from the clean to the dirty limit) in the optical self-energy and performed the same reverse process to obtain the optical conductivity, the dielectric function, and reflectance. We observed that impurities give similar effects on various optical constants of $s$- and $d$-wave superconductors; the more impurities give the more distinct gap feature and the less superfluid density. However, the $s$-wave superconductor gives the superconducting gap feature more clearly than the $d$-wave superconductor because in the $d$-wave superconductors the optical quantities are averaged over the anisotropic Fermi surface. Our results also supply helpful information to see how characteristic features of the electron-boson spectral function and the $s$- and $d$-wave superconducting gaps appear in various optical constants including raw reflectance spectrum. Our study also may help to understand the usual optical analysis process thoroughly. Further systematic study of experimental data collected at various conditions using the optical analysis process will help to reveal the origin of the mediated boson in the boson-exchange superconductors.