05 April 2017 Superconducting Correlation in a Quantum Hall system in Nature Physics

A NbN superconducting electrode (green) contacts a graphene Quantum Hall bar (blue).

Scientists realized crossed Andreev conversion in a graphene Quantum Hall system. The quantum Hall effect (QH) supports a set of chiral, topological edge states at the boundary of a 2-dimensional electron gas, which form dissipationless 1-dimensional quasi-particles at the physical boundaries of the device. When electronically coupled to a superconductor (SC) that is narrower than the SC coherence length, the incoming QH electrons are correlated to the outgoing QH holes along the chiral edge state by the Andreev process, resulting in the so-called crossed Andreev process (CAC) across a QH edge. This electron-hole conversion mechanism is of paramount importance since this hybrid SC/QH system represents a novel route to creation and understanding of hybrid systems for quantum information processing, which is a promising pathway to create isolated non-Abelian anyonic zero modes that are predicted as potential building blocks of a topological quantum computer in the future.

In a recent paper published in Nature Physics, G.H. Lee, A. Yacoby, P. Kim et. al. at Harvard University (USA) and ICFO Professor Dmitri Efetov, leader of the newly founded Low Dimensional Quantum Materials group, fabricated highly transparent and nanometer-scale superconducting junctions to a graphene QH system. When subjected to low temperatures and high magnetic fields they observed that the chemical potential of the Quantum edge states across the superconducting electrode exhibited a sign reversal, which provided direct evidence of CAC.

While the research was mainly conducted at Harvard University and Massachusetts Institute of Technology, where Dmitri Efetov pursued his postdoctoral work, he will continue this line of research in his newly founded group at ICFO.