Optical properties of unconventional superconductors

TL;DR

This paper reviews optical conductivity measurements in unconventional superconductors, discusses measurement techniques like the R-T method, and analyzes the optical spectra of specific superconductors to understand their gap symmetries.

Contribution

It provides a comprehensive analysis of optical properties in unconventional superconductors, including new insights into anisotropic and multiband gap structures.

Findings

Optical conductivity spectra support d-wave pairing in Nd_{2-x}Ce_xCuO_4.

The R-T method yields precise far-infrared conductivity measurements.

Optical properties of MgB_2 suggest specific superconducting gap symmetry.

Abstract

The optical conductivity measurements give a powerful tool to investigate the nature of the superconducting gap for conventional and unconventional superconductors. In this article, first, general analyses of the optical conductivity are given stemmed from the Mattis-Bardeen formula for conventional BCS superconductors to unconventional anisotropic superconductors. Second, we discuss the reflectance-transmittance (R-T) method which has been proposed to measure far-infrared spectroscopy. The R-T method provides us precise measurements of the frequency-dependent conductivity. Third, the optical conductivity spectra of the electron-doped cuprate superconductor Nd_{2-x}Ce_xCuO_4 are investigated based on the anisotropic pairing model. It is shown that the behavior of optical conductivity is consistent with an anisotropic gap and is well explained by the formula for d-wave pairing in the far-infrared region. The optical properties of the multiband superconductor MgB_2, i n which the existence of superconductivity with relatively high-T_c (39K) was recently announced, is also examined to determine the symmetry of superconducting gaps.