Superconducting order parameter in nonmagnetic borocarbides RNi$_{2}$B$_{2}$C (R=Y, Lu) probed by point-contact Andreev reflection spectroscopy

TL;DR

This study investigates the superconducting gap structure in nonmagnetic borocarbides LuNi₂B₂C and YNi₂B₂C using point-contact Andreev reflection spectroscopy, revealing BCS-like behavior with small anisotropy and addressing previous controversies.

Contribution

The paper provides the first comprehensive analysis of the superconducting order parameter in these materials using multiple crystallographic orientations and confirms BCS-like behavior with minor anisotropy.

Findings

Gap values follow BCS predictions with temperature and magnetic field.

Small anisotropy observed in the superconducting gap.

Results are consistent across different crystal sets and orientations.

Abstract

We report on the measurements of the superconducting order parameter in the nonmagnetic borocarbides LuNi$_2$B$_2$C and YNi$_2$B$_2$C. Andreev conductance spectra are obtained from nanoscale metallic junctions on single crystal surfaces prepared along three major crystallographic orientations: [001], [110], and [100]. The gap values extracted by the single-gap Blonder-Tinkham-Klapwijk model follow the BCS predictions as a function of temperature and magnetic field and exhibit a small anisotropy. These observations are robust and reproducible among all the measurements on two different sets of LuNi$_2$B$_2$C crystals and one set of YNi$_2$B$_2$C crystals. We suggest possible explanations for the small gap anisotropy based on the recent detailed gap measurements by angle-resolved photoemission spectroscopy and the tunneling cone effect. Our results provide a consistent picture of the superconducting gap structure in these materials, addressing the controversy particularly in the reported results of point-contact Andreev reflection spectroscopy.