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

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 Tc*≅5.6 K<Tc≅8.5 K, where a specific magnetic ordering starts to play a role. For R = Tm (Tc ≅ 10.5 K) a decrease of the SC gap is observed below 5 K