Infrared properties of exotic superconductors

TL;DR

This paper reviews the infrared spectral features of various exotic superconductors, highlighting commonalities like the absence of a gap signature and the presence of strong mid-infrared absorption linked to unconventional electronic interactions.

Contribution

It provides a comparative analysis of infrared properties across different classes of non-traditional superconductors, emphasizing the role of d-wave symmetry and strong coupling effects.

Findings

Lack of a gap signature at 2Δ in spectra

Presence of residual conductivity due to d-wave order parameter

Strong mid-infrared absorption indicating non-Fermi liquid behavior

Abstract

The infrared spectra of the non-traditional superconductors share certain common features. The lack of a gap signature at $2\Delta$ and the residual conductivity are the consequence of a d-wave order parameter. The high $T_c$ materials, the organic conductors and the heavy Fermion materials have a strong mid-infrared absorption band which can be interpreted as strong coupling of the carriers to electronic degrees of freedom which leads to a breakdown of the Fermi liquid picture. The cuprates and the organic charge transfer salts are unique in possessing an intrinsic low dimensionality. The charge transport normal to the highly conducting direction is incoherent down to the lowest temperatures and frequencies.