Institut des
NanoSciences de Paris

Thesis Defense - Gerbold Ménard - Friday september 16 2016 at 2 pm

UPMC - 4 place Jussieu. 75005 Paris - Amphithéâtre Charpak (tour 22-23 niveau Saint Bernard)

Gerbold Ménard, team « Spectroscopy of Novel Quantum States »

2D superconductors perturbed by local magnetism : from Yu-Shiba-Rusinov bound states to Majorana quasiparticles

a. Yu-Shiba-Rusinov bound states imaged by scanning tunneling spectroscopy at the Fermi level around an iron impurity in 2H-NbSe2.
b. Radial dependence of a Majorana dispersive bound state in a Pb/Si(111) monolayer..

One of the present days goals of condensed matter physics is to create new systems with topological properties, especially in the field of superconductivity. One of the ways envisioned to create topological superconductors is to locally induce a magnetic interaction in the form of chains of magnetic impurities, vortices or magnetic clusters of ordered magnetic impurities. In this thesis we studied a set of effects from individual impurities to organized clusters interacting with two-dimensional superconductors. Using scanning tunneling microscopy and spectroscopy we considered two systems, monocrystals of 2H-NbSe2 and monolayers of Pb/Si(111). Thanks to the two-dimensional electronic behavior of these two systems we show how the spatial extent of the bound states induced by magnetic impurities is considerably enhanced compared to the case of a three-dimensional superconductor. By combining these magnetic atoms using a self-assembly method we were able to create ferromagnetic clusters that lead to a topological superconductivity in Pb monolayers. In particular we present here measurement of topological edge states at the interface Pb/Si(111) and Pb/Co/Si(111). We also present the measurement of zero bias peaks in the center of larger magnetic clusters that sign the presence of Majorana fermions in these systems. Our results show that an adequate patterning of surfaces could realize topological patches and call for a pursuit of the efforts in the subject in order to be able to control Majorana fermions that could eventually lead to breakthrough in quantum computation.


Jean-Marc Berroir, President
Jean-Pascal Brison, Examinator
Tristan Cren, Thesis director
Laurent Limot, Protractor
Vincent Repain, Examinator
Dimitri Roditchev, Thesis director
Peter Wahl, Protractor