| Abstract: | Superconductors of the transition metal dichalcogenide (TMD) family have seen a revival of interest subsequent to developments in device fabrication by mechanical exfoliation. Recent studies1 show that at the ultrathin limit, NbSe2 and similar TMDs can sustain superconductivity at very high in-plane magnetic fields, well beyond the Pauli limit2. This apparent stability is associated with Ising spin-orbit coupling, which keeps spins oriented out of the sample plane, thereby providing strong protection against depairing. In my talk, I will report our recent spectroscopy measurements of NbSe2 using vdW tunnel devices3. Our devices are fabricated by placing insulating barriers on top of exfoliated NbSe2 using the mechanical transfer technique. The resulting tunnel junctions exhibit extremely stable currents, and are characterized by a hard gap. At milli-Kelvin temperatures, the tunneling spectra exhibit a well-resolved separation into a two-gap structure. We show that by applying in-plane magnetic fields to bulk devices (20-50 nm thick), it is possible to distinguish between the kinematics of quasiparticles which belong to different gaps. When probing ultra-thin devices (3-4 layers), we find the larger energy gap to be almost fully protected to depairing, an effect consistent with transport studies. Finally, I will discuss the implications of our technique to vortex-bound state spectroscopy.
- X. Xi, Z. Wang, W. Zhao, J.-H. Park, K. T. Law, H. Berger, L. Forró, J. Shan & K. F. Mak. Ising pairing in superconducting NbSe2 atomic layers. Nature Physics 12, 139-143 (2016).
- J. M. Lu, O. Zheliuk, I. Leermakers, N. F. Q. Yuan, U. Zeitler, K. T. Law & J. T. Ye. Evidence for two-dimensional Ising superconductivity in gated MoS2. Science 350, 1353-1357 (2015).
- T. Dvir, F. Massee, L. Attias, M. Khodas, M. Aprili, C. H. L. Quay & H. Steinberg. Spectroscopy of bulk and few-layer superconducting NbSe2 with van der Waals tunnel junctions. Nature Communications 9, 598 (2018).
|