Impurity effects on s+g-wave superconductivity in borocarbides Y(Lu)Ni2B2C

Recently a hybrid s+g-wave pairing was proposed to describe the experimental observation for a nodal structure of the superconducting gap in borocarbide YNi2B2C and possibly LuNi2B2C. In this paper the impurity effects on the s+g-wave superconductivity are studied in both Born and unitarity limits. The quasiparticle density of states and thermodynamics are calculated. It is found that the nodal excitations in the clean system are immediately prohibited by impurity scattering and a finite energy gap increases quickly with the impurity scattering rate. This leads to an activated behavior in the temperature dependence of the specific heat. Qualitative agreement with the experimental results is shown. Comparison with d wave and some anisotropic s waves studied previously is also made.