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

TL;DR

This study uses tunneling spectroscopy to reveal that phonon-mediated, highly anisotropic electron-phonon interactions significantly contribute to superconductivity in Y and Lu nickel borocarbides, with evidence of anisotropic s-wave gaps.

Contribution

It provides direct experimental evidence linking anisotropic electron-phonon coupling to superconductivity in borocarbides, highlighting the role of specific phonon modes.

Findings

Strong coupling features align with phonon energies from neutron scattering.

Electron-phonon coupling parameter lambda is between 0.5 and 0.8.

Superconducting gap exhibits anisotropic s-wave symmetry.

Abstract

We present scanning tunneling spectroscopy and microscopy measurements at low temperatures in the borocarbide materials RNi2B2C (R=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.