Experimental evidence for a two-gap structure of superconducting NbSe_2: a specific heat study in external magnetic fields

TL;DR

This study provides experimental evidence for a two-gap superconducting structure in NbSe_2 through detailed specific heat measurements under various magnetic field orientations, supporting a multi-gap model over anisotropic s-wave models.

Contribution

The paper offers the first comprehensive specific heat analysis confirming a two-gap superconducting structure in NbSe_2, including precise gap values and their anisotropic properties.

Findings

Identification of two distinct superconducting gaps: delta_L=1.26 meV and delta_S=0.73 meV.

Observation of a kink in gamma(H) indicating a two-gap scenario.

The smaller gap delta_S is more isotropic and possibly located on Se or bonding Nb Fermi sheets.

Abstract

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.