To resolve the discrepancies of the superconducting order parameter in quasi-two-dimensional NbSe_2, comprehensive specific-heat measurements have been carried out. By analyzing both the zero-field and mixed-state data with magnetic fields perpendicular to and parallel 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 NbSe_2 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 NbSe_2; 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.
@article{arxiv.0712.2939,
title = {Experimental evidence for a two-gap structure of superconducting NbSe_2: a specific heat study in external magnetic fields},
author = {C. L. Huang and J. -Y. Lin and Y. T. Chang and C. P. Sun and H. Y. Shen and C. C. Chou and H. Berger and T. K. Lee and H. D. Yang},
journal= {arXiv preprint arXiv:0712.2939},
year = {2009}
}