Tunneling into the Vortex State of NbSe ₂ with van der Waals Junctions

We have performed device-based tunneling spectroscopy of NbSe ₂ in the vortex state with a magnetic field applied both parallel and perpendicular to the a-b plane. Our devices consist of layered semiconductors placed on top of exfoliated NbSe ₂ using the van der Waals transfer technique. At zero field, the spectrum exhibits a hard gap, and the quasiparticle peak is split into low- and high-energy features. The two features, associated with the effective two-band nature of superconductivity in NbSe ₂ , exhibit markedly distinct responses to the application of magnetic field, suggesting an order-of-magnitude difference in the spatial extent of the vortex cores of the two bands. At energies below the superconducting gap, the hard gap gives way to vortex-bound Caroli-de Gennes-Matricon states, allowing the detection of individual vortices as they enter and exit the junction. Analysis of the subgap spectra upon application of parallel magnetic field allows us to track the process of vortex surface formation and spatial rearrangement in the bulk.