Spectroscopy of NbSe2 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 NbSe2, using a van der Waals device consisting of a vertical stack of graphene-MoS2-NbSe2. The MoS2 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 NbSe2 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 NbSe2 gaps, possibly due to a selection rule which favors coupling between the dots and the orbitals which exhibit this gap.