Point-Contact Spectroscopy of the Borocarbide Superconductor YNi2B2C in the Normal and Superconducting State

摘要

Point-contact (PC) spectroscopy measurements of YNi2B2C single crystals in the normal and superconducting (SC) state (T c ≃ 15.4 K) for the main crystallographic directions are reported. The PC study reveals the electron–phonon interaction (EPI) spectral function with dominant phonon maximum around 12 meV and further weak structures (hump or kink) at higher energy at about 50 meV. No “soft” modes below 12 meV are resolved in the normal state. The PC EPI spectra are qualitatively similar for the different directions. Contrary, directional study of the SC gap results in Δ{[100] ≈ 1.5 meV for the a direction and Δ{[001] ≈ 2.3 meV along the c axis; however the critical temperature T c in PC in all cases is near to that in the bulk sample. The value 2Δ[001]/k B T c ≈ 3.6 is close to the BCS value of 3.52, and the temperature dependence Δ[001](T) is BCS-like, while the for small gap Δ[100](T) is below BCS behavior at T > T c /2 similarly as in the two-gap superconductor MgB2. It is supposed that the directional variation Δ can be attributed to a multiband nature of the SC state in YNi2B2C.