X. Huang, W. Dietsche, M. Hauser, K. von Klitzing
The condensation of excitons in GaAs bilayers results in the quantum Hall effect and Josephson tunneling between the layers. These phenomena are a consequence of the Cooper-pair properties of excitons formed by occupied and vacant electron states in two closely spaced quantum wells, each containing a half-filled Landau level. The correlation between the layers gives rise to a gapped BCS-like groundstate leading to the quantum Hall effect. It also leads to a nearly dissipationless Josephson tunnel current between the layers which is equivalent to the creation of additional ground-state excitons by an Andreev-reflection like process and which prevails until a critical current is reached. While Josephson junctions in conventional superconductors are coupled by the coherence of the groundstate wavefunction, it is unknown if there is any exciton mediated interaction between different Josephson contacts in the bilayer system. Here we report that it is indeed possible to prepare two Josephson junction at the same device with almost no electric current flowing between them but which are intricately coupled by the exchange of groundstate excitons. In ring-shaped (Corbino-) samples, groundstate excitons can flow nearly dissipationless between the inner and the outer edge while virtually no charge current is carried between them. Using contact pairs at both the inner and the outer edge of a Corbino ring, we apply Josephson currents to both edges simultaneously and find that the critical Josephson current at one edge can be more than doubled if the Josephson current at the other edge has opposite polarity. This coupling of the Josephson contacts follows from the two different possible polarities of the Cooper-pairs (excitons) in a bilayer system and distinguishes it from conventional superconductors.
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http://arxiv.org/abs/1204.3295
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