Spectroscopy of $NbSe_2$ using Energy-Tunable Defect-Embedded Quantum Dots

TL;DR

This study uses defect-embedded quantum dots within a van der Waals heterostructure to perform high-resolution spectroscopy of the superconductor NbSe2, revealing a two-gap density of states and a dominance of the lower energy gap in tunneling current.

Contribution

It demonstrates a novel quantum dot-assisted spectroscopy method for probing NbSe2's electronic structure using a layered van der Waals device.

Findings

Reproduced the two-gap spectrum of NbSe2.

Observed dominance of the lower energy gap in tunneling current.

Identified a possible selection rule affecting coupling to the gaps.

Abstract

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 $NbSe_2$, using a van der Waals device consisting of a vertical stack of $graphene-MoS_2-NbSe_2$. The $MoS_2$ 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 $NbSe_2$ 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 $NbSe_2$ gaps, possibly due to a selection rule which favors coupling between the dots and the orbitals which exhibit this gap.