Spectroscopic evidence for two-gap superconductivity in the quasi-one dimensional chalcogenide Nb$_{2}$Pd$_{0.81}$S$_{5}$

Abstract

We present the first direct evidence for two-band superconductivity in the quasi-one dimensional chalcogenide Nb$_{2}$Pd$_{0.81}$S$_{5}$. Soft point contact spectroscopic measurements reveal Andreev reflection in the differential conductance $G$ in the zero-resistance superconducting (SC) state below $T_c$(=6.6~K). Multiple peaks in $G$ were clearly observed at 1.8~K and were successfully explained by the two-band Blonder-Tinkham-Klapwijk model with two gaps $\Delta_{1}=0.48$~meV and $\Delta_{2}=1.05$~meV. Their evolution in temperature and magnetic field is consistent with the conventional BCS theory. The SC gap to $T_c$ ratio for $\Delta_1$ and $\Delta_2$ is 1.7 and 3.7, which is similar to the weak coupling BCS prediction of 3.5. These results demonstrate that the newly discovered niobium chalcogenide is a two-gap superconductor in the weak coupling limit.