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The Superconducting Proximity Effect through graphene from zero field to the Quantum Hall regime

15/05/2012 Sophie Guéron, LPS, Orsay, France

The celebrated electronic band structure of graphene leads to many interesting features. Among them is the possibility to tune its carrier density from electron to hole, with the consequence that the Integer Quantum Hall effect is observed over a wide range of magnetic fields. Another consequence is the fact that transport can proceed via carriers of either the conduction or the valence band, depending on the doping, and may even proceed via a conversion of one type of carrier into the other, across regions of different doping, the so called Klein tunneling effect. It was suggested (Beenakker 2006) that a superconductor/graphene interface should also reveal the fact that the valence and conduction band touch at the so called Dirac point. Indeed, transport across a Superconductor/Normal metal (S/N) interface at subgap energy implies extracting two electrons from the superconductor and injecting them into the N, which produces a correlated Andreev pair in the normal metal. In a usual (highly doped) normal metal, both electrons are injected in the conduction band of the N. The two injected members of the Andreev pair then follow the same, albeit time-reversed, diffusive path in the normal conductor, so that coherent propagation can occur over several micrometers (the phase coherence length at low temperature), leading to the flow of a supercurrent.
 

In contrast, at a superconductor/graphene (S/G) interface, if the superconductor’s Fermi level is aligned with the graphene Dirac point, the two electrons of a Cooper pair must split into an electron in the conduction band and the other in the valence band. The two members of the injected pair in the graphene now have the same velocity (rather than opposite) parallel to the S/G interface and thus do not follow the same diffusive path. The experimental detection of this special type of pair injection, called specular Andreev reflexion, has so far remained elusive.

In this talk I will suggest that diffusive transport of Andreev pairs through quantum coherent graphene reveals an analog of specular Andreev reflexion at an S/G interface, in the form of specular reflexions of Andreev pairs at the interface between a doped charge puddle and a zero density region. These processes result in the destruction of counter-propagation upon specular reflexion, and therefore suppress the critical current near the Dirac point in our quantum coherent, long and diffusive SGS junctions. In the second part of the talk, I will report our attempts at injecting Cooper pairs in graphene in the Quantum Hall regime. In contrast to the low field proximity effect, the supercurrent is no longer carried by many diffusing pairs, but must be carried exclusively by the chiral edge states. Thus the two injected electrons must propagate on opposite edges of the graphene sheet. We present a long SGS junction which sustains a tunable supercurrent at low magnetic field. The superconducting electrodes, made of a high critical field superconductor, remain superconducting at fields such that the graphene exhibits integer quantum Hall plateaus, indicating that transport proceeds via edge states. The non linear transport features we measure hint to the existence of interference, controlled by gate voltage or magnetic field, between the electrons propagating along different edges of the graphene.