Grazing Incidence Infrared Reflectivity of La$_{1.85}$Sr$_{0.15}$CuO$_4$ and NbN

TL;DR

This study uses grazing incidence infrared reflectivity to investigate the optical properties of superconductors NbN and La$_{1.85}$Sr$_{0.15}$CuO$_4$, confirming BCS theory for NbN and revealing d-wave symmetry in the cuprate.

Contribution

It demonstrates the effectiveness of grazing incidence IR reflectivity in probing superconducting gap symmetries and provides experimental evidence for d-wave pairing in La$_{1.85}$Sr$_{0.15}$CuO$_4$.

Findings

NbN results agree with BCS theory

La$_{1.85}$Sr$_{0.15}$CuO$_4$ shows non-isotropic gap

Optical conductivity decreases below 20 meV

Abstract

Infrared reflectivity measurements, using p-polarized light at a grazing angle of incidence, show an increased sensitivity to the optical conductivity of highly reflecting superconducting materials. We demonstrate that when this measurement technique is applied to the conventional s-wave superconductor NbN, the results are in perfect agreement with BCS theory. For the in-plane response of a La$_{1.85}$Sr$_{0.15}$CuO$_4$ single crystal, in the superconducting state, we find a reduction of the optical conductivity in the frequency range below 20 meV. The observed frequency dependence excludes an isotropic s-wave gap, but agrees well with model calculations assuming a d-wave order parameter.