Berlin 2008 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 21: Transport: Graphene and Carbon Nanotubes

TT 21.8: Talk

Tuesday, February 26, 2008, 17:45–18:00, EB 202

Long-range spin interaction between Quantum dots in a Graphene nanoribbon — •Matthias Braun and Guido Burkard — Institut für Theoretische Physik C, RWTH-Aachen, Germany

Recently it was proposed to realize spin qubits in graphene ribbons with semiconducting armchair boundaries [1]. The band gap arising in ribbons does avoid the Klein tunneling i.e., enable a confinement of electrons. Since the band gap is significantly smaller than in conventional semiconductors, RKKY is not necessarily the dominant spin-spin interaction between the qubits. Indeed, we show that a superexchange mechanism can lead to a tunable long-range spin-spin interaction. We model the system by two Anderson impurities coupled by a common lead. We remove all first and second order tunneling processes by a two-stage Schrieffer-Wolff transformation, and integrate out the lead degrees of freedom. At the end we arrive at an effective fourth order spin Hamiltonian. We can identify different kinds of interactions such as RKKY or superexchange. Which interaction dominates the spin dynamics depends on system parameters. We are particularly interested in the superexchange process as this spin interaction can not only couple adjacent quantum dots but also next-to-nearest neighbors.

[1] Nature Phys. 3, 192 (2007).