Study of the superconducting gap in $R{\mathrm{Ni}}_2{\mathrm{B}}_2\mathrm{C}(R=\mathrm{Y},\mathrm{Lu})$ single crystals by inelastic light scattering

Superconductivity-induced changes in the electronic Raman-scattering response were observed for the $R{\mathrm{Ni}}_2{\mathrm{B}}_2\mathrm{C}(R=\mathrm{Y},\mathrm{Lu})$ system in different scattering geometries. In the superconducting state, $2\Delta$-like peaks were observed in $A_{1g},$ $B_{1g},$ and $B_{2g}$ spectra from single crystals. The peaks in $A_{1g}$ and $B_{2g}$ symmetries are significantly sharper and stronger than the peak in $B_{1g}$ symmetry. The temperature dependence of the frequencies of the $2\Delta$-like peaks shows typical BCS-type behavior, but the apparent values of the $2\Delta$ gap are strongly anisotropic for both systems. In addition, for both ${\mathrm{YNi}}_2{\mathrm{B}}_2\mathrm{C}$ and ${\mathrm{LuNi}}_2{\mathrm{B}}_2\mathrm{C}$ systems, there exists reproducible scattering strength below the $2\Delta$ gap which is roughly linear to the frequency in $B_{1g}$ and $B_{2g}$ 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.