Kondo Effect In Quantum Dot Coupled To Ferromagnetic Leads

As the typical representation of exhibiting quantum effect in low dimensional mesoscopic system, the quantum dot system becomes the hot topic recently. Especially, after finding the clear Kondo phenomenon in tunnelling conductance of quantum dot, the Kondo effect was widely studied in condensed matter physics. It is a well-known physics phenomenon. The effect arises from the interactions between a single magnetic impurity and the electrons of the bulk mental under low temperature. Recent advances in nanofabrication technology have made it possible to investigate various aspects of this effect. One used Quantum Dots coupled to circuit by tunneling barriers under controlled circumstances experimentally, which has aroused new interest in this phenomenon. In contrast to the enhancement of the resistivity of the Kondo effect in a bulk metal, the Kondo resonance near the Fermi level localized at the quantum dot provides a new channel for the mesoscopic current and leads to an increase of the conductance in a quantum dot. Recently, a great effort has been dedicated to the study of Kondo effect in quantum dot coupled to ferromagnetic leads. In this thesis, we use the slave-boson mean-filed approximation and equation of motion technique to solve the Green function. On one hand, we investigate the Kondo effect in a quantum dot coupled to ferromagnetic leads and a mesoscopic ring. We have found that: (â…°) for antiparallel spin alignment the Kondo resonances for spin up and spin down configurations appear at the same position. (â…±) For parallel spin alignment, the Kondo resonance splits for spin up and spin down configurations. At the same time, the Kondo resonance at the Fermi level of the dot presents the periodic change along with the aggrandizement of the magnetic flux and the number of lattice sites N R in the mesoscopic ring. (â…²) Both the Kondo resonance and the spin-splitting of the single electron levels can be controlled by internal magnetization of the electrodes, which can be used to generate a spin-valve effect. It clearly demonstrates the mesoscopic nature of the Kondo scattering. On the other hand, we study the spin-flip process through double quantum dots coupled to ferromagnetic leads. We find that the strength of the spin-flip process is one of the key parameters to control transport phenomena via the modified Kondo resonances. Due to the large spin-flip process on the dots, when the magnetic configuration is parallel, the original spin-up Kondo peak of both dots is split into two peaks and the spin-down peak is suppressed or changed to one peak from the splitting peaks, in both the equilibrium and the nonequilibrium cases. When the magnetic configuration is antiparallel, the phenomena of the left dot are the same as those of the P configuration while those of the right dot are reversed. These novel results are helpful in exploring the electronic correlation in spintronics.