When: May 30, 16:00
Where: Skoltech, MR-408 (TROC-3, Blue building)
Scanning tunneling spectroscopy of the superconducting proximity effect
Institut des Nanosciences de Paris, Sorbonne Université and CNRS
Superconducting correlations can propagate at low temperature in a normal metal at an S-N interface when the interface transparency is high enough [1. In the case of diffusive metals, the Usadel theory is a very powerful theoretical framework to describe proximity effect . In particular when the length of the N part is smaller than the electronic phase coherence length, a minigap exists in N which acquire genuine superconducting properties. On the other hand, if the length of the N part is infinite there is no minigap but superconducting correlations affect the local density-of-states in an energy-dependent manner. We have studied by scanning tunneling microscopy/spectroscopy in ultrahigh vacuum what happens to such finite  and infinite  S-N junction using both lateral and vertical geometries. In the finite S-N junction in perpendicular magnetic field, we showed that it exhibits superconducting proximity vortices . The local density-of-states probed by STS is in very good agreement with self-consistent 3D Usadel equations modeling our S-N geometry. We also studied a lateral network of S-N-S Josephson junctions, built in situ using self-organized Pb islands grown on Si(111). Using perpendicular magnetic field to create phase gradients inside each S island we could study and map in real space the cores of Josephson vortices existing in the diffusive N parts . Finally we also could study locally the proximity effect between two superconductors of different energy gap values, one of them being a Pb atomic monolayer, the second one being a thin Pb island grown on the Pb monolayer .
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Christophe Brun got his PhD in Orsay University (France) for Scanning Tunneling Microscope (STM) study of charge density waves in quasi-1D conductors. Then Christophe joined Prof. Schneider’s group at EPFL (Switzerland), where he spent 4 years working on characterization of metallic and superconducting nanostructures with STM/STS and surface science related techniques. Since 2011 he is a CNRS researcher at Institut des Nanosciences de Paris, in Sorbonne University. Christophe’s research interests include disordered superconducting thin films and the problem of the superconductor-insulator transition, superconductivity in 2D (weak disorder effects, effect of spin-orbit coupling), superconducting proximity effect, interaction between local magnetic impurities and topological superconductivity, correlated electronic systems and unconventional superconductivity.