NBIA Condensed Matter Theory Seminar: Guido Burkard

Spin and valley coherence in quantum dots

Graphene and other two-dimensional solids have emerged as interesting materials for coherent spin physics and spin qubits. In graphene, due to the low concentration of nuclear spins and relatively weak spin- orbit coupling, long spin coherence times can be expected [1]. However, the localization of electrons in quantum dots in graphene is a non-trivial task due to the absence of a band gap and the related effect of Klein tunneling. Among the possible solutions to this problem are electrostatically defined quantum dots in graphene nanoribbons or in bulk graphene with an induced band gap, for which we have calculated the spin relaxation time [2,3]. We found an interesting interplay between the valley degeneracy present in graphene and other materials and the spin-orbit induced spin relaxation which originates from the absence of a van Vleck cancellation known from GaAs quantum dots [2]. The valley degeneracy also affects the exchange interaction between tunnel-coupled quantum dots by entangling the spin and valley degrees of freedom. This spin-valley entanglement necessitates special procedures for spin-based quantum information processing and indicates the possibility of valley qubits [4]. Two-dimensional semiconducting transition metal dichalcogenides (TMDCs) share many properties of graphene, but comprise a band gap and strong spin-orbit coupling [5] which makes them interesting as a host material for spin and valley qubits.

[1] B. Trauzettel, D. Bulaev, D. Loss, and GB, Nature Phys. 3, 192 (2007).
[2] P. R. Struck and GB, Phys. Rev. B 82, 125401 (2010).
[3] M. Droth and GB, Phys. Rev. B 84, 155404 (2011).
[4] N. Rohling and GB, New Journal of Physics 14, 083008 (2012).
[5] A. Kormányos, V. Zólyomi, N. D. Drummond, P. Rakyta, GB, and V. I. Fal'ko, Phys. Rev. B 88, 045416 (2013).