Prof. Jan Martinek (Institute of Molecular Physics, Polish Academy of Sciences)
"Kondo effect in single-molecule spintronic devices"
We study the Kondo effect in a single molecule and quantum dot (QD), which is coupled to ferromagnetic leads, and analyze its properties.
The main goal of our work is to investigate how ferromagnetic leads influence the Kondo effect. At the beginning, based on a poor man's
scaling analysis we show that a splitting of the Kondo resonance similar to the usual magnetic-field-induced splitting will appear due to
exchange interaction with leads [1]. The most important result is that this splitting can be fully compensated by an appropriately tuned
external magnetic field and the strong coupling limit of the Kondo effect can be restored.
Moreover, we adapt the NRG method to the case of a QD coupled to ferromagnetic leads. We show that the Kondo effect in the presence of ferromagnetic
leads has unique properties such as a strong spin polarization of the density of states (DOS) at the Fermi level [2]. In order to account the effect
of a gate voltage on the spin splitting of an electronic level in a QD we have generalized the numerical renormalization group technique for an
arbitrary DOS shape. The gate voltage can be used to control the magnitude and sign of the spin splitting so a local exchange magnetic field in QDs.
We also analyze the nonlinear transport through the QD using a real-time diagrammatic technique that provides a systematic description of the
nonequilibrium dynamics of a system with strong local electron correlations [1]. We evaluate the theory in an extension of the resonant tunneling
approximation. Recently Kirchner at al. [3] analyzed the effect of spin waves in our system ? collective low energy excitations arising due to the
spontaneously broken spin symmetry of the ferromagnetic leads. One can then describe this system in terms of the Bose-Fermi Kondo model, which
couples the local moment to both a fermionic bath of conduction electrons and a bosonic one of spin waves. This model indicates possibility of
quantum critical behavior and related to it the non-Fermi liquid state. A conceivable realization of proposed system might be carbon nanotubes or
other molecular systems in contact to ferromagnetic leads, (ferromagnetic single-molecule transistor). New experimental results for a single C60
molecule attached to nickel electrodes confirm most of our theoretical predictions [4].
[1] J. Martinek, Y. Utsumi, H. Imamura, J. Barnas, S. Maekawa, J. Konig, and G. Schon, Phys. Rev. Lett. 91, 127203 (2003); Y. Utsumi, J. Martinek, G. Schon, H. Imamura, S. Maekawa, Phys. Rev. B 71, 245116 (2005).
[2] J. Martinek, M. Sindel, L. Borda, J. Barnas, J. Konig, G. Schon, and J. von Delft, Phys. Rev. Lett. 91, 247202 (2003); J. Martinek, M. Sindel, L. Borda, J. Barnas, R. Bulla, J. Konig, G. Schon, S. Maekawa, and J. von Delft, Phys. Rev. B 72, 121302 (2005).
[3] S. Kirchner, L. Zhu, Q. Si, D. Natelson, Proc. Natl. Acad. Sci. USA 102, 18824 (2005).
[4] A. N. Pasupathy, R. C. Bialczak, J. Martinek, J. E. Grose, L. A. K. Donev, P. L. McEuen, and D. C. Ralph, Science 306, 86 (2004).