Magnetoresistance, Gating and Proximity Effects in Ultrathin NbN-Bi2Se3 Bilayers

Ultrathin Bi2Se3-NbN bilayers comprise a simple proximity system of a topological insulator and an s-wave superconductor for studying gating effects on topological superconductors. Here we report on 3 nm thick NbN layers of weakly connected superconducting islands, overlayed with 10 nm thick Bi2Se3 film which facilitates enhanced proximity coupling between them. Resistance versus temperature of the most resistive bilayers shows insulating behavior but with signs of superconductivity. We measured the magnetoresistance (MR) of these bilayers versus temperature with and without a magnetic field H normal to the wafer (MR=[R(H)-R(0)]/\{[R(H)+R(0)]/2\}), and under three electric gate-fields of 0 and ±2 MV/cm. The MR results showed a complex set of gate sensitive peaks which extended up to about 30 K. The results are discussed in terms of vortex physics, and the origin of the different MR peaks is identified and attributed to flux-flow MR in the isolated NbN islands and the different proximity regions in the Bi2Se3 cap-layer. The dominant MR peak was found to be consistent with enhanced proximity induced superconductivity in the topological edge currents regions. The high temperature MR data suggest a possible pseudogap phase or a highly extended fluctuation regime.

Vortices in superconducting nano-networks with anti-dots array

Vortices (magnetic flux quanta) in the superconducting networks perforated with anti-dots (holes) arrays behave as electrons in atomic lattice of crystals. Repulsive and attractive interaction among vortices and anti-dots resemble to those among electrons and atoms in crystals. To confirm the variety of the vortex physics similar to the solid state physics, we have fabricated such superconducting networks with antidots array in metallic, inter-metallic and high-T c superconductors (HTSCs), and have measured magneto-resistance of vortex-flow. In these materials, we have observed integer-matching at the matching fields and fractional-matching effect between them. Most of them are well explained by commensurability between Abrikosov vortex lattice and anti-dots array. Furthermore, the effect of the anti-dots array in HTSCs appears as another kind of phase transitions instead of to the first-order melting transition of vortex lattice in pristine samples.

Three-dimensional spatially resolved neutron diffraction from a disordered vortex lattice

The vortex matter in bulk type II superconductors serves as a prototype system for studying the random pinning problem in condensed matter physics. Since the vortex lattice is embedded in an atomic lattice, small-angle neutron scattering (SANS) is the only technique that allows for direct structural studies. In traditional SANS methods, the scattering intensity is a measure of the structure factor averaged over the entire sample. Recent studies in vortex physics have shown that it is highly desirable to develop a SANS technique that is capable of resolving the spatial inhomogeneities in the bulk vortex state. This article reports a novel slicing neutron diffraction technique using atypical collimation and an areal detector, which allows for observing the three-dimensional disorder of the vortex matter inside an as-grown Nb single crystal.