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.