Nicolas MOREAU has received the 2022 IMCN Best Thesis Award on 26 May 2023.
His work was entitled "Scanning gate imaging and tuning of quantum electronic transport in graphene"
This Prize, which is granted yearly by the IMCN Institute, rewards the most outstanding PhD work among those who graduated during the previous civil year (2022 in the present case). This initiative aims at promoting excellence in scientific research within the Institute.
The jury, chaired by Prof. Xavier GONZE, emphasizes that in his PhD thesis, Nicolas Moreau successfully tackled difficult problems, that have been investigated by some of the best physicists in the world. He managed combine excellent experimental achievements with advanced simulations techniques to propose a consistent picture explaining the whole phenomenology.
Abstract
In graphene, charge carriers behave as massless (Dirac) fermions, yielding unprecedented electrical properties.
In this thesis, the electronic transport of Dirac fermions is studied in graphene quantum point contacts thanks to low-temperature scanning gate microscopy (SGM), a technique consisting in recording the sample resistance while changing locally the charge carrier density with a biased metallic tip.
The focus has been put on backscattering mechanisms and three main directions are explored.
First, real space signatures of Klein tunneling, preventing the backscattering of Dirac fermions, are investigated by creating a movable pn junction with the SGM movable top gate. The experimental results are discussed in the light of simulations, revealing that the SGM conductance maps yield an image of the current density around and through the lens, with a direct evidence of Klein tunneling.
Second, SGM characterizations are reported under a large magnetic field, in the quantum Hall regime where charge carriers flow in topologically protected quantum Hall edge channels (QHECs). The recorded SGM signatures indicate that counterpropagating QHECs, separated by a few hundreds of nanometers, exist along the same edges in graphene and that charge carriers backscattering is achieved by coupling these QHECs through the localized states of antidots located between them.
Third, these antidots are shown to act as nano-sized Fabry-Perot interferometers. A simple model is used to reproduce the experimental results, showing that the signatures associated to two distinct regimes, namely the Aharonov-Bohm and Coulomb dominated regimes, can be explained in a single framework ignoring the Coulomb interactions.