We have studied of a key mesoscopic graphene device that provides local control on quantum Hall edge channels transmission with gate electrodes. This device is a high mobility graphene layer encapsulated between boron nitride flakes and equipped with split-gate electrodes, which operates as a quantum point contact and enable to control the individual transmission of integer and fractional quantum Hall edge channels (Nature Communications (2017)). This work enabled us to move towards more advanced devices and develop quantum Hall interferometry in graphene. We demonstrated the first graphene quantum Hall interferometer of the Fabry-Pérot type that shows unprecedented agreement with theoretical expectations (
https://arxiv.org/abs/2008.11222(odnośnik otworzy się w nowym oknie)).
In the course of actions focusing on the study of quantum Hall physics and edge states in graphene, we investigate an original idea of the PI to induce at the charge neutrality of graphene a topological phase. We studied the effect of dielectric screening of the Coulomb interaction in the quantum Hall regime by placing graphene in very close proximity to a substrate of high-dielectric constant, namely SrTiO3. We demonstrated that this Coulomb screening yields a topological phase with helical edge transport at charge neutrality. This ground-breaking result published in Science 367, 781 (2020) provides a new platform for spintronics and for topological superconductivity.
These quantum Hall edge channels were thoroughly investigated with our AFM-STM microscope. We performed STM spectroscopy of Landau levels at low temperature and up to 14 teslas on graphene samples. We succeeded to perform tunneling spectroscopy on the edge of the graphene crystal. We directly measured the evolution of the density of states at the edge and observed a localization of the quantum Hall edge channels in the very vicinity of the graphene edge, on the scale of few magnetic length. These major results that are the chief objective of QUEST were obtained during the last months of the project, after the Covid-19 lock-down, and will be submitted to high-profile journals in the coming months.
Another achievement concerns disordered superconductors that are key for the second aspect of this project, which consists in studying the coupling between superconductivity and quantum Hall physics. We performed systematic study of the magnetotransport properties of nanowires of amorphous MoGe that could be used as high field superconducting electrodes, and revealed a new quantum phase transition occurring at the critical field. This work is published in Nature Physics 14, 912 (2018) and is accompanied by a second work in Nature Physics 15, 48 (2019) that addresses the peculiar physics of the upper critical field in 2D films of amorphous indium oxide.
To address the interplay between quantum Hall effect and superconductivity, we used this expertise on MoGe that proved to give highly transmittive, superconducting contacts on BN-encapsulated graphene. We demonstrated that the resilience of the MoGe superconductivity to high magnetic field permits to operate Josephson junctions up to remarkably high magnetic field. We demonstrated supercurrent carried by helical quantum Hall edge channels in graphene devices screened by SrTiO3 substrate. Those results obtained in the last year of QUEST constitutes the second main objective that we achieved successfully.
