Study of the superconducting gap in single crystals by inelastic light scattering

Superconductivity-induced changes in the electronic Raman-scattering response were observed for the (Formula presented) system in different scattering geometries. In the superconducting state, (Formula presented)-like peaks were observed in (Formula presented) and (Formula presented) spectra from single crystals. The peaks in (Formula presented) and (Formula presented) symmetries are significantly sharper and stronger than the peak in (Formula presented) symmetry. The temperature dependence of the frequencies of the (Formula presented)-like peaks shows typical BCS-type behavior, but the apparent values of the (Formula presented) gap are strongly anisotropic for both systems. In addition, for both (Formula presented) and (Formula presented) systems, there exists reproducible scattering strength below the (Formula presented) gap which is roughly linear to the frequency in (Formula presented) and (Formula presented) symmetries. This discovery of scattering below the gap in nonmagnetic borocarbide superconductors, which are thought to be conventional BCS-type superconductors, is a challenge for current understanding of superconductivity in this system.