Electron Transport Through Single-molecular Magnet Manipulated By Mascroscopic Quantum Effect

The macroscopic quantum coherence (MQC) of single-molecule magnets(SMMs) has attracted a great deal of interest. One expects a rich interplay between quantum tunneling, phase coherence, and electronic correlations in the transport properties of SMMs. In this paper, we make a detailed investigation of the electron transport through the SMM manipulated by the MQC, which owns great potential application on nanoelctronics in the future. Our purpose is to explore the effect of the MQC on the electron transport throuth SMM; and on the other hand, to provide the theoretical support in the design of molecular devices. After brief review of the mescoscopic transport, MQC in SMM and electron transport through SMM, we present a detailed introduction of the method in this paper—nonequilibrium Green's function and slave boson mean field approach(SBMFA) in the chapter 2. The dissertation is devoted a theoretical analysis of electron transport through a quantum dot with an embedded biaxial SMM and Kondo effect in SMM.First, We report a theoretical analysis of electron transport through a quantum dot with an embedded biaxial single-molecule magnet, which is coupled to ferromagnetic electrodes of parallel and antiparallel magnet-configurations. For the antiparallel configuration of complete polarization, it is shown that the originally prohibited electron transport can be opened up by the macroscopic quantum coherence of the molecular magnet, which provides a spin-flipping mechanism. The charge-current and differential conductance are controllable by variation of the magnitude and orientation of an external magnetic field, which in turn manipulates the macroscopic quantum coherence of the molecular magnet. Moreover, the transport can be switched off at particular values of the magnetic field. A new transport channel is found in the completely polarized parallel-configuration induced by the tunnel splitting of molecular magnet. For non-completely polarized leads, the tunneling effect of the SMM can not be ignored in high magnetic field regime.Then, we investigate a theoretical analysis of electron transport through a quantum dot with an embedded SMM based on mapping of the many-body interaction-system onto a one-body problem by means of the non-equilibrium Green's function technique. It is found that the conducting current exhibits a stepwise behavior and the nonlinear differential conductance displays additional peaks with variations of the sweeping speed and the magnitude of magnetic field. This observation can be interpreted by the interaction of electron-spin with the SMM and the quantum tunneling of magnetization. The inelastic conductance and the corresponding tunneling processes are investigated with normal as well as ferromagnetic electrodes. In the case of ferromagnetic configuration, a sudden TMR-switch with the variations of magnetic field is observed, which is seen to be caused by the inelastic tunneling. Moreover, the shot noise and Fano factor are strongly dependent on the magnetic field, exchange coupling and sweeping speed of the field, and be enhanced by the coupling between electron and SMM. We also observe a suppression of shot noise is exhibited in the S-V diagram due to the suppression of inelastic tunneling.Finally, a Kondo-effect in single-molecule magnet is investigated based on combination of slave boson mean-field theory and the non-equilibrium Green function technique. It is found that the macroscopic quantum coherence of molecule-magnet results in the Kondo peak-split of nonlinear differential conductance with the help of interaction between electron-spin and molecular magnet. the peak height and position is sensitive to the sweeping speed of the applied magnetic field:In the fast sweeping field, the Kondo-peak shifts due to Zeeman effect. In the slow sweeping field, the quantum tunneling of magnetization results in a satellite peak from the 10-th channel at higher bias. It is also demonstrated that the Kondo peaks can be controlled by the electron-molecule coupling and the polarization parameter in the case of ferromagnetic electrodes.