Tunneling spectroscopy in van-der-Waals superconducting devices

I will describe our experiments probing the tunneling spectra of the layered superconductor NbSe₂ using van-der-Waals tunnel junctions. The junctions, characterized by a hard gap,enable the measurement of high resolution spectra and the probing of sub-gap states. Usingthis method, we make a number of key findings: (i) We find a clear signature of a 2nd , low-energy gap, in NbSe₂ . By measuring the spectra vs. in-plane and out-of-plane fields, we can place values on its diffusion constant and coherence length. (ii) We probe NbSe₂ spectra at the ultrathin limit. We find that thin NbSe₂ retains its gap up well above the Pauli limit, a consequence of Ising protection. (iii) Finally, we find that van-der-Waals tunnel barriers host defects which undergo proximity with the underlying superconductor, giving rise to Andreev bound states. As NbSe₂ survives very high in-plane fields, we can track the defect state energy which exhibits a field-dependent singlet-doublet transition. I will discuss possible origins of the singlet-ground state behavior of defect-related Andreev bound states.