The infrared spectra of the non-traditional superconductors share certain common features. The lack of a gap signature at 2Δ and the residual conductivity are the consequence of a d-wave order parameter. The high Tcmaterials, 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.
Timusk, T. (1999). Infrared properties of exotic superconductors. Physica C: Superconductivity and Its Applications, 317–318, 18–29. https://doi.org/10.1016/S0921-4534(99)00042-8