Berlin 2012 – scientific programme

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O: Fachverband Oberflächenphysik

O 62: [TT] Transport: Graphene 1 (jointly with MA, HL, DY, DS, O)

O 62.12: Talk

Thursday, March 29, 2012, 12:30–12:45, BH 334

Quantum Hall effect in graphene with superconducting electrodes — •Markus Weiss, Peter Rickhaus, and Christian Schönenberger — Departement Physik, Universität Basel, Klingelbergstrasse 82, CH-4056 Basel

We report on the realization of an integer quantum Hall system with superconducting electrodes. Graphene was contacted to niobium electrodes that show a critical field of about 4 tesla, where electronic transport passes mainly through quantum Hall edge-states and bulk transport is largely suppressed. We find a magnetic field range of more than one tesla where well developed quantum Hall plateaus coexist with superconductivity in the leads. In high magnetic fields with the electrodes in the normal state we observe plateaus at G=νe²/h for ν=2, 4, and 10. Reducing the magnetic field to below the upper critical field of the electrodes, the conductance on the plateaus shows a sudden increase. Whereas the conductance on the ν=2 plateau increases only by 10%, the increase on the ν=6 and ν=10 plateau is considerably larger with 60% and 80%, respectively. We attribute this conductance enhancement to multiple Andreev reflection processes along the graphene-superconductor interface, that lead to the formation of Andreev edge-states. The observed conductance enhancement of the ν=6 and 10 plateaus is consistent with a doubling of the conductance contribution of the second and third edge-states. We attribute the small conductance increase on the ν=2 plateau to the special nature of the zero energy Landau level, that makes the corresponding edge-state sensitive to the structure of the graphene edge.