Optical Conductivity of the Superconductors $\mathit{L}{\mathrm{Ni}}_2{B}_{2}C$ ( $\mathit{L}=Lu$ and Y)

We have measured the optical reflectivity of LuN${\mathrm{i}}_2$${\mathrm{B}}_2$C and YN${\mathrm{i}}_2$${\mathrm{B}}_2$C compounds, and have evaluated the optical conductivity both below and above the superconducting transition temperature. The normal state optical properties suggest that these superconductors are almost in the clean limit. Our results below $T_c$ give, however, evidence of a superconducting gap signature, and are in agreement with a BCS singlet ground state. Our experimental findings also indicate a moderate-to-strong coupling limit for the pairing mechanism.