Two-Band Superconductivity in Few-Layer NbSe2 and TaS2

The nature of superconductivity in the transition metal dichalcogenide family, particularly 2H-NbSe2 and 2H-TaS2 in the ultrathin limit, is not fully understood. While tunneling measurements apparently show a single superconducting gap, its detailed shape cannot be reproduced by simple single-band theories of superconductivity. In this work, we present tunneling data into thin exfoliated samples of both materials and show excellent agreement with the McMillan two-band model of superconductivity when the scattering parameters are large with respect to the superconducting gaps, even when only a single gap is apparent. We propose that in thin samples, these highly cross-scattering gaps reside on the Γ and K Fermi surfaces derived from the transition metal atoms. Additionally, we quantitatively analyze pair breaking induced by magnetic field, and find that it is also well described by the two-band McMillan theory. Our results thus suggest that in its bulk thickness, 2H-NbSe2 is likely a three-band superconductor.