Experimental evidence for a two-gap structure of superconducting Nb Se2: A specific-heat study in external magnetic fields

To resolve the discrepancies of the superconducting order parameter in quasi-two-dimensional Nb Se2, comprehensive specific-heat measurements have been carried out. By analyzing both the zero-field and mixed-state data with magnetic fields perpendicular (H c) to and parallel (H c) to the c axis of the crystal and using the two-gap model, we conclude that (1) more than one energy scale of the order parameter is required for superconducting Nb Se2 due to the thermodynamic consistency; (2) ΔL =1.26 meV and ΔS =0.73 meV are obtained; (3) NS (0) N (0) =11%-20%; (4) the observation of the kink in γ (H) curve suggests that the two-gap scenario is more favorable than the anisotropic s -wave model to describe the gap structure of Nb Se2; and (5) ΔS is more isotropic and has a three-dimensional-like feature and is located either on the Se or the bonding Nb Fermi sheets.