Transition Metal Dichalcogenide Superconductor Tunneling Devices: A Review

We review recent progress in the study of transition metal dichalcogenide (TMD) superconductors—such as NbSe2 and TaS2—-in tunnel devices where the barrier is also a van der Waals material. Superconducting TMDs are of particular interest due to their lack of in-plane centrosymetry, leading to Ising superconductivity: conventional s-wave superconductivity where the internal spin axis of Cooper pairs is held out of plane by the Ising spin-orbit field. The devices reviewed are fabricated by placing ultrathin barriers—typically few-layer insulating TMDs—on top of an exfoliated superconductor and subsequent patterning of tunneling electrodes. This results in high-quality normal-insulator-superconductor (NIS) tunnel junctions, which enable the measurement of superconducting spectra with fine energy resolution, down to dilution refrigerator temperatures (below 100 mK). As reported by the authors and others, these spectra reveal intricate signatures of TMD physics from the bulk to the 2D limit. We report on studies showing the two-band superconducting character of (bulk) NbSe2, revealing different depairing processes in the two different NbSe2 bands. These two bands also appear in the spectral weight of vortex-bound subgap states. At high in-plane magnetic fields, many unconventional superconducting phases have been predicted, which are not necessarily mutually exclusive: triplet, orbital Fulde-Ferrell-Larkin-Ovchinnikov (FFLO), striped FFLO, pair density wave, nematicity etc. We report on the spectral evolution of NbSe2 at high in-plane magnetic fields, which we interpret as evidence for odd-parity equal-spin triplet superconductivity. Finally, we present our vision for addressing these and other open questions with vdW tunneling devices.