Superconducting order parameter in nonmagnetic borocarbides RNi${}_2$B${}_2$C (R=Y, Lu) probed by point-contact Andreev reflection spectroscopy

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.

10 pages, 8 figures, submitted to Phys. Rev. B