Spin-Polarized Transport Through The Double Quantum Dots System Coupled To Ferromagnetic Leads

Abstract: 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. This effect originally arises from the physically abnormal phenomenon under low temperature in dilute magnetic alloy.Recent developments and advances in nanofabrication technology have made it possible to investigate various aspects of Fano versus Kondo resonance by means of Quantum Dots, which has greatly aroused new interest in both the effects of the mesoscopic system. 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 the one hand, we investigate the phenomenon of spin-polarized transport of the parallel double quantum dots (DQD) coupled to ferromagnetic leads. We have found that: the Kondo resonance of this system at the Fermi energy is relevant to the value of the spin-polarized strength and the magnetic flux. Moreover, in the antiparallel spin alignment, the Kondo resonances for spin up and spin down configurations appear at the same position, and it isn't influenced by the value of the spin-polarized strength and the magnetic flux in the ferromagnetic leads. However, for parallel spin alignment, the Kondo resonance splits for spin up and spin down configurations. On the other hand, we study the Fano-Kondo effect in the T-shaped double quantum dots coupled to ferromagnetic leads. Contrasting with the usual series and parallel DQD systems, the T-shaped DQD systems show some different properties. On the one hand, the T-shaped DQD systems provide an idea model system for studying the two impurity effects and the related experiment can be performed under controlled circumstance. On the other hand, the T-shaped DQD systems are another prototype of correlated systems, for which the special arrangement of the DQD provides two paths for the electrons to go through, one is through the central QD and the other is through the side QD. Besides, the Kondo effect arises from the interactions between a single magnetic impurity and the electrons of the bulk mental under low temperature, so the observation of the Kondo effect in QD systems opened a new path for the investigation of quantum correlation between localized spin in QD and the free lead. The Fano effect appears as a result of quantum interference between a discrete single energy level and a direct channel characterized by its continuous spectrum. It is thus interesting to study how the Kondo versus Fano effect affect the characteristic transport properties in the T-shaped DQD system. Through our study, we find the following results: (â…°) For antiparallel spin alignment, the Kondo resonances for spin up and spin down configurations appear at the same position, and it isn't influenced by the value of the spin-polarized strength in the ferromagnetic leads, which is the same as the case of the parallel double quantum dots discussed before. (â…±) For parallel spin alignment, the Kondo resonance splits obviously for spin up and spin down configurations. The up-spin resonance is suppressed with the increase of the value of the spin-polarized strength, and its position no longer stay at the Feimi energy and shifts towards lower energy. However the down-spin resonance is enhanced and shifts towards higher energy. (â…²) Due to the existence of the side coupled dots, the Kondo resonance and Fano interference coexist. And the interdot coupling strength greatly influence the Fano-Kondo effect of the central quantum dot. These novel results are helpful in exploring the electronic correlation in spintronics.