Tunneling spectroscopy of the superconducting energy gap inRNi2B2C(R=YandLu)

Superconducting energy gap in $R{\mathrm{Ni}}_2{\mathrm{B}}_2\mathrm{C} (R=\mathrm{Y}\mathrm{and}\mathrm{Lu})$ has been investigated using a break-junction tunneling technique. The observed differential conductance $\frac{\mathrm{dI}}{\mathrm{dV}}$ is well described by the BCS density of states with a small lifetime broadening parameter. We have found small gap values for the local low-$T_c$ phases in addition to the bulk $T_c$ of 15.6-16.6 K. The gap value of $2\Delta =4.5\mathrm{}$ meV with $T_c=15\mathrm{}$ K gives the BCS ratio of $\frac{2\Delta }{k_B{T}_c}=3.5\mathrm{}$, which is kept constant down to the observed lowest $T_c$ of 4.6 K. These results give clear evidence for the BCS mechanism of superconductivity for these compounds.