Spectroscopy of NbSe Using Energy-Tunable Defect-Embedded Quantum Dots.

Quantum dots have sharply defined energy levels, which can be used for high resolution energy spectroscopy when integrated in tunneling circuitry. Here we report dot-assisted spectroscopy measurements of the superconductor NbSe, using a van der Waals device consisting of a vertical stack of graphene-MoS-NbSe. The MoS tunnel barriers host naturally occurring defects which function as quantum dots, allowing transport via resonant tunneling. The dot energies are tuned by an electric field exerted by a back-gate, which penetrates the graphene source electrode. Scanning the dot potential across the superconductor Fermi energy, we reproduce the NbSe density of states which exhibits a well-resolved two-gap spectrum. Surprisingly, we find that the dot-assisted current is dominated by the lower energy feature of the two NbSe gaps, possibly due to a selection rule which favors coupling between the dots and the orbitals which exhibit this gap.