Graphene in the quantum Hall regime

Quantum point contacts in high-mobility graphene

Staff: Benjamin Sacépé, Hermann Sellier
Postdoc: Louis Veyrat
Student: Alexis Coissard, Corentin Deprez, Marco Guerra, Hadrien Vignaud

The absence of energy gap in graphene at zero magnetic field makes the fabrication of nanostructures difficult using metallic surface gates. In large magnetic fields however, the formation of Landau levels creates gaps in the spectrum, and the conducting edge channels follow the electrostatic potential of the gates. In this regime, quantum point contacts can be realized to control the number of channels transmitted through the device. The graphene flake can also be encapsulated between two boro-nitride flakes to boost its electron mobility. In such an heterostructure, the four-fold degeneracy of the Landau levels is lifted, and the fractional quantum Hall effect has been observed. Thanks to this unprecedented graphene quality, we achieved a precise control of the transmitted current through a quantum point contact, and more complex devices can now be studied, such as quantum Hall interferometers.

Quantum point contact (narrow gates in yellow) on a graphene flake (honeycomb lattice) encapsulated between two BN flakes (region in blue). Two ohmic contacts are used to inject current in the graphene layer, and four others are used to measure the longitudinal and transverse voltages.

Edge channels in a graphene quantum point contact at high perpendicular magnetic field. Blue (red) lines indicate electron (hole) edge channels. In this example, the filling factor is 3 in the bulk, -3 below the gates, 1 in the constriction, allowing a single edge channel to be transmitted through the device.

Scanning electron micrograph of a QPC device. The white dotted lines show the edges of the graphene flake buried below the larger BN top flake in blue. The scale bar is 1 micron. The longitudinal and Hall resistances are obtained from the measured voltages VL and VH. The carrier density in the bulk is controlled by a backgate (not shown) and the QPC operation is controlled by the split gate voltage Vsg.

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References :
Tunable transmission of quantum Hall edge channels with full degeneracy lifting in split-gated graphene devices
K. Zimmermann, A. Jordan, F. Gay, K. Watanabe, T. Taniguchi, Z. Han, V. Bouchiat, H. Sellier, and B. Sacépé
Nature Communications 8, 14983 (2017)
Low-magnetic-field regime of a gate-defined constriction in high-mobility graphene
L. Veyrat, A. Jordan, K. Zimmermann, F. Gay, K. Watanabe, T. Taniguchi, H. Sellier, and B. Sacépé
Nano Letters 19, 635 (2019)

Positions :
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Fundings :
ERC grant "QUEST" (2015-2020)

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