Institut de la Matière Condensée et des Nanosciences (IMCN),
Université catholique de Louvain (UCL), Louvain-la-Neuve, Belgique
Quantum transport in nanodevices is usually probed thanks to measurements of the electrical resistance or conductance, which lack the spatial resolution necessary to probe local-scale electron behaviour. Here, we will discuss how to get real-space local information on peculiar quantum transport phenomena inside graphene constrictions and graphene devices in general. The results were obtained using low temperature scanning gate microscopy (SGM), which consists in mapping the electrical conductance of a device as an electrically-biased sharp metallic tip scans in its vicinity . If the tip-induced perturbation is relatively small, SGM is an imaging technique giving access to the local density of states inside mesoscopic devices . Here, in contrast, we will focus on the regime of large tip-induced perturbation. This regime is particularly interesting in the case of graphene: depending on tip voltage and tip-sample distance, one can tune the shape, size, and smoothness of the moving scattering region associated with the tip, and eventually create a moving circular pn-junction within the device. We use this moving pn-junction to probe peculiar aspects of relativistic charge carrier dynamics within graphene devices: Klein tunnelling and current focusing and defocusing , Fabry-Pérot interferometry , signatures of whispering gallery modes . This work unlocks new possibilities in the field of electron optics in graphene.
In the quantum Hall regime, the same setup allows to evidence the mechanism of topological protection breakdown. It occurs as tunneling between counterpropagating quantum Hall edge states takes place through antidots located in the vicinity of device edges . The process is imaged in real space thanks to SGM, and comparisons with tight-binding model simulations allow to disentangle the details of the breakdown mechanism, and the role of electrical contacts .
 M.A. Eriksson et al., Appl. Phys. Lett. 69, 671 (1996); M. Topinka et al., Science 289, 671 (2000).
 F. Martins et al., Phys. Rev. Lett. 99, 136807 (2007) ; M. Pala et al., Phys. Rev. B 77, 125310 (2008).
 B. Brun et al., Phys. Rev. B 100, 041401 (2019).
 B. Brun et al., 2D Materials, 7, 025037 (2020).
 B. Brun et al., Nano Letters 22, 128 (2022).
 N. Moreau et al., Nature Comm. 12, 4265 (2021).
 N. Moreau et al., Phys. Rev. B 20, L201406 (2021).