Distribution of the superconducting gap in an YNi2B2C film studied by point contact spectroscopy

The differential resistances $R_d=dV/dI(V)$ of point contacts between a normal metal and a c axis oriented YNi2B2C film ($T_c$ = 15.2K) in the superconducting (SC) state have been investigated. $R_d(V)$ contains clear "gap" features connected with processes of Andreev reflection at the boundary between normal metal and superconductor that allow the determination of the SC gap $\Delta$ and its temperature and magnetic field dependence. A distribution of $\Delta$ from $\Delta_min\approx$ 1.5 meV to $\Delta_max\approx$ 2.4 meV is revealed; however the critical temperature $T_c$ in all cases corresponded to that of the film. The value 2$\Delta_max/k_BT_c\approx$3.66 is close to the BCS value of 3.52, and the temperature dependence $\Delta(T)$ is BCS-like, irrespective of the actual $\Delta$ value. It is supposed that the distribution of $\Delta$ can be attributed to a gap anisotropy or to a multiband nature of the SC state in YNi2B2C, rather than to the presence of nodes in the gap.Comment: 6 two-column pages, 7 figs; V2: as published, Fig.4 is modifie

Magnetic field dependence of superconducting energy gaps in YNi2B2C: Evidence of multiband superconductivity

We present results of in field directional point contact spectroscopy (DPCS) study in the quaternary borocarbide superconductor YNi2B2C, which is characterized by a highly anisotropic superconducting gap function. For I||a, the superconducting energy gap (D), decreases linearly with magnetic field and vanishes around 3.25T which is well below the upper critical field (Hc2~6T) measured at the same temperature (2.2K). For I||c, on the other hand, D decreases weakly with magnetic field but the broadening parameter (G) increases rapidly with magnetic field with the absence of any resolvable feature above 3.5T. From an analysis of the field variation of energy gaps and the zero bias density of states we show that the unconventional gap function observed in this material could originate from multiband superconductivity.Comment: 19 pages including figures (final version