Point-contact spectroscopy of the nickel borocarbide superconductor YNi2B2C in the normal and superconducting state

TL;DR

This study uses point-contact spectroscopy to analyze the electron-phonon interaction and superconducting gap anisotropy in YNi2B2C, revealing a multiband superconducting state with directional gap variation.

Contribution

It provides the first detailed directional analysis of the superconducting gap and electron-phonon interaction in YNi2B2C using point-contact spectroscopy.

Findings

Electron-phonon spectral function peaks at 12 meV and 50 meV.

Superconducting gap varies with direction: 1.5 meV along [100], 2.3 meV along [001].

Superconducting gap behavior suggests multiband superconductivity.

Abstract

Point-contact (PC) spectroscopy measurements of YNi2B2C single crystals in the normal and superconducting (SC) state (T_c=15.4K) 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 \Delta_[100]=1.5 meV for the a direction and \Delta_[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 2\Delta_[001]/kT_c=3.6 is close to the BCS value of 3.52, and the temperature dependence \Delta_[001](T) is BCS-like, while the for small gap \Delta_[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 \Delta can be attributed to a multiband nature of the SC state in YNi2B2C.