| authors: | Jan Martinek (a,b,c) , Y. Utsumi (d), H. Imamura (d), M. Sindel (e), L.
Borda (e)
J. Barnas (c,f), S. Maekawa (b), J. König (a), G. Schön (a), and J. von
Delft (e) a) Institut für Theoretische Festkörperphysik, Universität Karlsruhe, Germany b) Institute for Materials Research, Tohoku University, Sendai, Japan c) Institute of Molecular Physics, Polish Academy of Sciences, Poznan, Poland d) Graduate School of Information Sciences, Tohoku University, Sendai, Japan e) Sektion Physik and Center for Nanoscience, LMU München, München, Germany f) Department of Physics, Adam Mickiewicz University, Poznan, Poland |
We study the Kondo effect in a quantum dot (QD), which is coupled to ferromagnetic leads, and analyze its properties. The new key questions, which emerge are: (i) does the Kondo effect survive and how does the spin-asymmetry affect the effect, (ii) how are the transport properties modified, and (iii) what is the ground state of the system? We analyzed these and related questions using scaling arguments, the equation of motion (EOM) technique, and numerical renormalization group (NRG) approach. It is shown that the Kondo effect is suppressed because of a spin splitting of the QD level. We find that the Kondo effect can be restored by the compensation of the splitting by an external magnetic field. The Kondo effect has then unusual spin asymmetry of the resonance amplitude with a reduced Kondo temperature, the ground state is still a locally screened state, describable by Fermi liquid theory and a generalized Friedel sum rule. Surprisingly, we find that the transport in the unitary limit is not spin dependent. We compare the analytical results by the exact NRG calculations, which confirm the obtained results for the equilibrium situations. We also analyze within an EOM approach the nonlinear transport through the QD. We find that for parallel alignment of the lead magnetizations the zero-bias anomaly is split. This splitting can be removed by appropriately tuning of a magnetic field. In the antiparallel configuration of the lead magnetizations no splitting occurs. A conceivable realization of proposed system might be carbon nanotubes in contact to ferromagnetic leads, or magnetic tunnel junctions with magnetic impurities in the barrier.
1. J. Martinek, Y. Utsumi, H. Imamura, J. Barnas, S. Maekawa, J. König, and G. Schön, cond-mat/0210006. 2. J. Martinek, M. Sindel, L. Borda, J. Barnas, J. König, G. Schön, and J. von Delft, preprint.