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

Abstract

We report our point-contact spectroscopy (PCS) study on the single-crystalline transition metal carbide NbC and TaC with a superconducting transition temperature = 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 = 2.0 ± 0.3 meV and 2/ = 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 = 1.7 ± 0.1 meV and 2/ = 3.9 ± 0.1. Our results thus support that both NbC and TaC are fully gapped s-wave superconductors.