Spectroscopy of barrier defects coupled to superconductors in Van der Waals tunneling devices

Superconductor-Quantum Dot (SC-QD) coupling is a subject of an intense research effort. We report on SC-QD devices realized in Van der Waals tunneling heterostructures consisting of transition metal dichalcogenide (TMD) semiconducting barrier and the 2-band superconductor NbSe₂. The QDs in our study are naturally occurring defects in the TMD barrier. As reported earlier by Dvir et. al. [1], Andreev Bound states may form due to proximitized defects in an NbSe₂-WSe₂ system. Here we focus on two device structures. In the first, tunneling through an MoS₂ barrier into a graphene-NbSe₂ stack reveals zero-energy Kondo features which arise due to the presence of carriers associated with the NbSe₂ low-energy band turning normal. In the 2^nd device, defect-dot energies are tunable by use of a graphene tunneling electrode which permits the penetration of electric field from an electrostatic gate. This allows the QD to be used as spectroscopic probe to study resonant tunnelling into the superconductor, allowing low-energy spectroscopy of the inner band of NbSe₂.

References
T. Dvir, M. Aprili, C. H. L. Quay, H. Steinberg, arxiv:1906.01215