Phonon-mediated anisotropic superconductivity in the Y and Lu nickel borocarbides

We present scanning tunneling spectroscopy and microscopy measurements at low temperatures in the borocarbide materials $R{\mathrm{Ni}}_2{\mathrm{B}}_2\mathrm{C}$ $(R=\mathrm{Y},$ Lu). The characteristic strong-coupling structure due to the pairing interaction is unambiguously resolved in the superconducting density of states. It is located at the superconducting gap plus the energy corresponding to a phonon mode identified in previous neutron-scattering experiments. These measurements also show that this mode is coupled to the electrons through a highly anisotropic electron-phonon interaction originated by a nesting feature of the Fermi surface. Our experiments, from which we can extract a large electron-phonon coupling parameter $\lambda$ (between 0.5 and 0.8), demonstrate that this anisotropic electron-phonon coupling has an essential contribution to the pairing interaction. The tunneling spectra show an anisotropic s-wave superconducting gap function.