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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - QUEST (QUantum Hall Edge State Tunnelling spectroscopy)

Teaser

Summary

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, is being developed and will enable unprecedented access to the edge of a graphene flake where QH edge states propagate. Overall, our original combination of magnetotransport measurements with scanning tunnelling spectroscopy strives to solve fundamental questions on the considerable real-space structure of integer and fractional QH edge states impinged by either normal or superconducting electrodes. We are aiming at providing the first STM imaging and spectroscopy of QH edge channels, which promises to open a new field of investigation of the local scale physics of the QH effect.

Work performed

During the first reporting period of 18 months, our efforts have focused on the setup of our two major equipment. The quantum transport setup has been installed and equipped with low temperature filtering and proper wiring. It is now operating providing a base temperature of 0.01 kelvin and a maximum magnetic field of 18 teslas. The second setup is the low-temperature high magnetic field UHV scanning tunneling microscope (STM). The STM head has been mounted and tested. The UHV transfer chamber is now operational. We are waiting for the commercial UHV dilution refrigerator to be delivered in the coming weeks.
Intense work has also been done on high mobility graphene sample fabrication. We have recently realize a quantum point contact device operating in the integer and fractional quantum Hall regime of graphene. This breakthrough, published in Nature Communications 8:14983 (2017), opens an avenue for future works on tunneling between fractional quantum Hall edge channels in graphene and quantum Hall interferometry.

Final results

Our work on quantum point contact in high mobility graphene provides a new conceptual advance beyond the state of the art of graphene physics: It demonstrates for the first time gate-tunable control of the transmission of quantum Hall edge channels with electrostatic gates. Such gate-control were indeed thought to be impossible in graphene as gates usually induce holes states and conducting pn junctions. In the quantum Hall, however, an energy gap between hole and electron states can open, providing that the sample mobility is high enough. Owing to the high mobility of our samples we have been able to demonstrate quantum point contact operation in graphene, that is gate tunable transmission of integer and fractional quantum Hall edge channels, and decipher the delicate coupling between some of the quantum Hall edge channels. This work will impact the graphene community as it opens an avenue for a wealth of future works on tunneling between fractional quantum Hall edge channels, quantum Hall interferometry or shot noise experiments in graphene.