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One of the greatest challenges for the development of spin-based devices is the understanding of mechanisms that give rise to spin-polarized currents. Advances in the field have been achieved by studying the effect of different types of spin-orbit interactions in semiconductor materials. Rashba spin-orbit interactions are of particular interest for systems with surfaces since they present a natural breaking of space-inversion symmetry, a condition for its existence. Graphene, a monolayer of graphite, is not expected to exhibit large Rashba couplings in isolation, but recently, several groups have successfully produced samples with band-splittings consistent with large values of the Rashba coupling. Two important effects result from large Rashba interactions: the linear dispersion at the Dirac points becomes quadratic (with the consequent change in the density of states) and new Dirac points are generated around the (K; K') points.
Because Rashba changes the density of states in graphene, it is natural to wonder about its consequences on the Kondo effect. The role of spin-orbit interactions on the Kondo effect, an issue posed for the first time by D. Gainon and A. Hegger in 1969, has been the topic of much controversy in later years, with many partial (and sometimes contradictory) answers. From a two-dimensional Anderson impurity model with Rashba interactions, we have obtained the effective Hamiltonian at low-temperatures. We carried out the complete analysis of the Kondo regime, with and without particle-hole symmetry. The main features are: a two-channel Kondo regime with antiferro- and ferromagnetic couplings, and the presence of Dzyaloshiinski-Moriya interactions when particle-hole symmetry is broken. Interesting results are also obtained for graphene where an exponential enhanced Kondo temperature is predicted. |
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