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∗≃5.6K