Experimental evidence for a two-gap structure of superconducting NbSe2: A specific-heat study in external magnetic fields

To resolve the discrepancies of the superconducting order parameter in quasi-two-dimensional NbSe2, comprehensive specific-heat measurements have been carried out. By analyzing both the zero-field and mixed-state data with magnetic fields perpendicular (H perpendicular to c) to and parallel (H parallel to 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 NbSe2 due to the thermodynamic consistency; (2) Delta(L)=1.26 meV and Delta(S)=0.73 meV are obtained; (3) N-S(0)/N(0)=11%-20%; (4) the observation of the kink in gamma(H) curve suggests that the two-gap scenario is more favorable than the anisotropic s-wave model to describe the gap structure of NbSe2; and (5) Delta(S) is more isotropic and has a three-dimensional-like feature and is located either on the Se or the bonding Nb Fermi sheets.