Periodic Reporting for period 4 - QUEST (QUantum Hall Edge State Tunnelling spectroscopy)
Periodo di rendicontazione: 2020-04-01 al 2020-09-30
The quantum nature of an electronic fluid is ubiquitous in many solid-state systems subjected to correlations or confinement. This is particularly true for two-dimensional electron gases (2DEGs) in which fascinating quantum states of matter, such as the integer and fractional quantum Hall (QH) states, arise under strong magnetic fields. The understanding of QH systems relies on the existence of one-dimensional (1D) conducting channels that propagate unidirectionally along the edges of the system, following the confining potential. Due to the buried nature of 2DEG commonly built in semiconducting heterostructures, the considerable real space structure of this 1D electronic fluid and its energy spectrum remain largely unexplored. This project consists in exploring at the local scale the intimate link between the spatial structure of QH edge states, coherent transport and the coupling with superconductivity at interfaces. We use graphene as a surface-accessible 2DEG to perform a pioneering local investigation of normal and superconducting transport through QH edge states. A new and unique hybrid Atomic Force Microscope and Scanning Tunneling Microscope (STM) operating in the extreme conditions required for this physics, i.e. below 0.1 kelvin and up to 14 teslas, has been developed and enable unprecedented access to the edge of a graphene flake where QH edge states propagate. With this new microscope, we have carried out pioneering tunneling spectroscopy of the ground state of the zeroth Landau level in graphene under strong magnetic field, and have investigated the edge of a graphene crystal where the quantum Hall channels propagate. Our study of the Landau level spectroscpy upon approaching the graphene edge uncovered that quantum Hall edge channels are located in a very narrow region of few magnetic lengths from the graphene crystal edge, in stark contrast with usual quantum Hall systems in semiconducting heterostructures. Besides, we succeeded to couple those quantum Hall channels to superconducting electrodes and measure superconducting devices. This sets an important milestone for more advance superconducting devices taking advantages of the topological nature of the quantum Hall effect. Overall, our original combination of magnetotransport measurements with scanning tunnelling spectroscopy have solved fundamental questions on the considerable real-space structure of integer and fractional QH edge states impinged by either normal or superconducting electrodes.
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).
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.
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.
This project led to several breakthrough works at the crossroad of several fields, namely graphene quantum Hall effect, topological insulators and disordered superconductivity. We performed pioneering investigation of tunneling between or into quantum Hall edge channels via the use of gate-defined quantum point contacts or via the newly developed high-field low-temperature scanning tunneling microscope. This two tunneling approaches allowed us to demonstrate quantum Hall interferometry in devices where the tunneling acts as partioning mirrors for electron edge channels, and also performed unprecedented scanning tunneling spectroscopy of the quantum Hall physics in graphene. The novel knowledge acquired with this work, especially on the exact spatial structure of the quantum Hall edge channel is beyond expectation and shows unexpected behavior that are essential to understanding the physics of graphene quantum Hall and its application in various fields. We also demonstrated that we can use special high-dielectric constant substrate to screen Coulomb interaction in the graphene quantum Hall regime and thus yields an unusual topological phase of graphene that exhibits the quantum spin Hall effect. This latter work opens new avenue for spintronics and topological quantum computing in hybrid superconducting devices.