Two characteristic contributions to the superconducting state of 2$H$-NbSe$_2$

Abstract: Multiband superconductivity arises when multiple electronic bands contribute to the formation of the superconducting state, allowing distinct pairing interactions and gap structures. Here, we present field- and temperature-dependent data on the vortex lattice structure in 2$H$-NbSe$2$ as a contribution to the ongoing debate on the nature of the superconductivity in this material. The field-dependent data clearly show that there are two distinct superconducting bands, and the contribution of one of them to the vortex lattice signal is completely suppressed for magnetic fields above $\sim$ 0.8 T, well below $B\mathrm{{c2}}$. By combining the temperature and field scans, we can deduce that there is a moderate degree of interband coupling. From the observed temperature dependences, we find that at low field and zero temperature, the two gaps in temperature units are 13.1 $\pm$ 0.2 and 6.5 $\pm$ 0.3 K ($\Delta_{0}$ = 1.88 and 0.94 $k\mathrm{{B}} T\mathrm{{c}} $); the band with the larger gap gives just under two-thirds of the superfluid density. The penetration depth extrapolated to zero field and zero temperature is 160 $\pm$ 2 nm.