Hard superconducting gap and vortex-state spectroscopy in NbSe$_2$ van der Waals tunnel junctions

Device-based tunnel spectroscopy of superconductors was first performed by Giaever, whose seminal work provided clear evidence for the spectral gap in the density of states (DOS) predicted by the Bardeen-Cooper-Schrieffer (BCS) theory. Since then, tunnel-barrier-based heterostructure devices have revealed myriad physical phenomena and found a range of applications. Most of these devices rely on a limited number of oxides, which form high-quality insulating, non-magnetic barriers. These barriers, however, do not grow well on all surfaces. Promising alternatives are van der Waals (vdW) materials, ultrathin layers of which can be precisely positioned on many surfaces; they have been shown to form tunnel barriers when engaged with graphene. Here we demonstrate that vdW semiconductors MoS$_2$ and WSe$_2$ deposited on the superconductor NbSe$_2$ form high quality tunnel barriers, with transparencies in the $10^{-8}$ range. Our measurements of the NbSe$_2$ DOS at 70mK show a hard superconducting gap, and a quasiparticle peak structure with clear evidence of contributions from two bands, with intrinsic superconductivity in both bands. In both perpendicular and parallel magnetic fields, we observe a sub-gap DOS associated with vortex bound states. The linear dependence of the zero-bias signal on perpendicular field allows us to confirm the s-wave nature of superconductivity in NbSe$_2$. As vdW tunnel barriers can be deployed on many solid surfaces, they extend the range of superconducting and other materials addressable not only by high resolution tunneling spectroscopy but also non-equilibrium and/or non-local transport.