Fully-gapped superconductivity in single crystalline NbC and TaC probed by point-contact spectroscopy

We report our point-contact spectroscopy (PCS) study on the single-crystalline transition metal carbide NbC and TaC with a superconducting transition temperature T-c = 12.0 and 10.2 K, respectively. The differential conductance curves from both soft-PCS and mechanical-PCS (MPCS) on NbC at 1.9 K can be well fitted by a single-gap s-wave Blonder-Tinkham-Klapwijk model, and the temperature dependent gap follows a standard Bardeen-Cooper-Schrieffer (BCS) behavior, yielding ?(NbC)(0) = 2.0 +/- 0.3 meV and 2?(NbC)(0)/k(B)T(c) = 3.9 +/- 0.3 in the intermediate coupling regime. Similarly, our MPCS data on TaC crystals also show its superconducting gap as a function of temperature follows the BCS behavior, yielding ?(TaC)(0) = 1.7 +/- 0.1 meV and 2?(TaC)(0)/k(B)T(c )= 3.9 +/- 0.1. Our results thus support that both NbC and TaC are fully gapped s-wave superconductors.