Sara Loucif : Tunneling spectroscopy and Josephson effect in few-layer NbSe2

Reducing a superconductor to the atomic-thickness limit can profoundly alter its properties and reveal phenomena absent in the bulk. The transition-metal dichalcogenide NbSe2 is a striking example: in its few-layer limit, the breaking of inversion symmetry combined with a strong spin-orbit coupling gives rise to Ising superconductivity, in which the electron spins are locked out of the atomic plane and the Cooper pairs are protected against inplane magnetic fields. Few-layer NbSe2 also displays several signatures of unconventional superconductivity especially under an applied in-plane magnetic field, and whether its superconductingstate is best described by a single or a multi-band order parameter remains debated. In this work, we probe the superconducting nature of few-layer NbSe2 through superconductor–insulator–superconductor (SIS) NbSe2/MoS2/Al junctions, combining tunneling

spectroscopy and Josephson effect measurements. The samples are fabricated by van der Waals assembly. We probe here the properties of a three-layer NbSe2 flake. The high energy resolution of the SIS junctions allows us to resolve a two-gap structure whose temperature dependence indicates that the two gaps are strongly coupled. Under an in-plane magnetic field, the NbSe2 density of states remains almost unchanged up to 4 T, as expected for an Ising superconductor, while the field evolution of the spectra below the critical field of aluminum points to a k selectivity of the tunneling process. We then study the Josephson effect and the dynamics of the switching and retrapping currents, and finally the effect of quasiparticle injection on the density of states and the distribution function of the NbSe2 flake.