Point-contact spectroscopy of the nickel borocarbide superconductors RNi2B2C (R=Y, Dy, Ho, Er, Tm, Lu)

TL;DR

This paper reviews point-contact spectroscopy studies on nickel borocarbide superconductors, revealing phonon interactions, crystal-field effects, and evidence for multi-band superconductivity with complex gap behaviors.

Contribution

It provides new insights into the electron-boson interactions, phonon and CEF excitations, and multi-band superconductivity in RNi2B2C compounds through detailed PC spectroscopy analysis.

Findings

Phonon maxima and CEF excitations observed in spectra.

Evidence for multi-band superconductivity in R=Y, Lu.

Complex temperature dependence of the superconducting gap.

Abstract

An overview of the recent efforts in point-contact (PC) spectroscopy of the nickel borocarbide superconductors RNi2B2C in the normal and superconducting (SC) state is given. The results of measurements of the PC electron- boson(phonon) interaction spectral function are presented. Phonon maxima and crystalline-electric-field (CEF) excitations are observed in the PC spectra of compounds with R=Dy, Ho, Er and Tm, while for R=Y a dominant phonon maximum around 12 meV is characteristic. Additionally, non-phonon and non-CEF maxima are observed near 3 meV in R=Ho and near 6 meV in R=Dy. Directional PC study of the SC gap gives evidence for the multi-band nature of superconductivity in R=Y, Lu. At low temperature the SC gap in R=Ho exhibits a standard single-band BCS-like dependence, which vanishes above T_c^*= 5.6K< T_c=8.5K, where a specifc magnetic ordering starts to play a role. For R=Tm (T_c=10.5 K) a decrease of the SC gap is observed below 5 K.