Point-contact spectroscopy of the borocarbide superconductor YNi2B2C

Point-contact (PC) spectroscopy measurements on YNi2B2C single crystals in the normal and superconducting (SC) state (Tc ≅ 15.4 K) for the main crystallographic directions are reported. The PC study reveals the electron-phonon interaction (EPI) function with a dominant maximum around 12 meV and a further weak structure (kink or shallow broad maximum) at higher energy at about 50 meV. Other phonon maxima at 20, 24 and 32 meV specified in the phonon DOS of YNi2B2C by neutron measurements [F. Gompf et al., Phys. Rev. B 55 (1997) 9058] are not resolved in the PC spectra pointing out to the main role of the low energy phonon modes in EPI. Directional study of the SC gap results in Δ[100] ∼ 1.5 meV for the a-direction and Δ[001] ∼ 2.4 meV along the c-axis which may point to anisotropic and/or multiband behavior. Noteworthy, the critical temperature Tc in all cases corresponds to that of bulk samples. The value 2Δ[001]/kBTc ∼ 3.6 is close to the BCS one of 3.52, and the temperature dependence Δ(T) is BCS-like, while for the a-direction Δ(T) deviates from mean-field BCS behavior above Tc/2. The directional variation in Δ can be attributed to the multiband nature of the SC state in YNi2B2C predicted 10 years ago [S.V. Shulga et al., Phys. Rev. Lett. 80 (1998) 1730]