Day / Time
Date(s) - 16/11/2015
11 h 00 min - 12 h 00 min
Superconducting Vortices in Mesoscopic Nanostructures
Superconductivity is characterized by two important length scales: the London penetration depth λ, and the coherence length ξ. In so-called type II superconductors, the length ξ is smaller than λ which results in the collapse of the magnetic field penetrating the material in quanta of flux called vortices. Each vortex is constituted by a nanometric core, with ξ wide, where superconductivity is destroyed, while around the core and in a typical radius of λ, superconducting currents circulate.
In this talk, I will show how the confinement of vortices to a scale comparable to ξ substantially modifies the superconducting properties inside the nanostructures as well as at proximity.
These studies were carried out with two systems, lead deposited in-situ on a silicon (111) substrate (in the group in France), and molybdenum germanium alloy deposited ex-situ on a germanium substrate and covered by a gold layer (in the group in Belgium). We studied the superconductivity by scanning tunneling spectroscopy under high vacuum at low temperatures down to 300 mK, and under magnetic field.
I will show how vortices are organized according to the lateral confinement and the applied magnetic field. In extremely confined systems (having the lateral size less than 10 ξ), we observed a quantum object predicted 45 years ago: the giant vortex. In weakly confined systems (having the lateral size bigger than 10 ξ), we demonstrated that triangular Abrikosov lattice is recovered while strong edge effect dominates in nanostructures with symmetry (squares in our case). We also observed effect of defects on the condensate such as vortex pinning, current crowding, vortex repulsion or attraction, and symmetry breaking, depending on its size and location. Finally, surface superconductivity appears close to a high critical field.
In the case of Pb/Si, the crystalline superconducting islands are connected by a disordered non-superconducting wetting layer of Pb. In the vicinity of each superconducting island, the wetting layer acquires specific tunneling characteristics which reflect the interplay between the proximity induced superconductivity and the inherent electron correlations of this ultimate diffusive two-dimensional metal. We reproduced the spatial evolution of the tunnel spectra by combining the Usadel equations and the theory of dynamic Coulomb blockade.
With reducing the distance between the islands, the proximity effect around each overlaps and forms a Josephson junction. We probed these junctions and we observed Josephson vortices under magnetic field. Thanks to the tunneling spectroscopy, we were able to study in detail the spectrum and geometry of the Josephson vortex cores, their number and positions, for a large variety of junctions. Not only we observed the existence of states inside the vortex core, but we have also shown that they result of an interference phenomenon, which explains the Fraunhofer figures observed in transport measurements.