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Full electrostatic control over polarized currents through spin-orbital Kondo effect

Numerical calculations indicate that by suitably controlling the individual gate voltages of a capacitively coupled parallel double quantum dot, with each quantum dot coupled to one of two independent non-magnetic channels, this system can be set into a spin-orbital Kondo state by applying a magnetic field. This Kondo regime, closely related to the SU(4) Kondo, flips spin from one to zero through cotunneling processes that generate almost totally spin-polarized currents with opposite spin orientation along the two channels. Moreover, by appropriately changing the gate voltages of both quantum dots, one can simultaneously flip the spin polarization of the currents in each channel. As a similar zero magnetic field Kondo effect has been recently observed by Y. Okazaki {\it et al.} [Phys. Rev. B {\bf 84}, (R)161305 (2011)], we analyze a range of magnetic field values where this polarization effect seems robust, suggesting that the setup may be used as an efficient bipolar spin filter, which can generate electrostatically reversible spatially separated spin currents with opposite polarizations.