Influences Of Impurity On Electron Transport Properties In A Double-Quantum-Dot Aharonov-Bohm Ring

The study on electron transport through the mesoscopic quantum dot systems is a hot topic in the field of condensed matter physics and materials physics. Because it not only is the complement of basic theory, but also provides theoretical basis for the development of quantum device. Current experimental developments and advances at the nanometer scale have allowed realizing and manipulating quantum dots (QD) in a controlled way, which motivated researchers to devote themselves to the electron properties in such atom-like QD systems. Despite these works, so far it is still a formidable challenge to fabricate two clean QDs in the experiments due to irregularities and defects in the QD system. Some localized states often appear in QD systems, which are hybridized with the QD levels. But they are not coupled directly to the leads, which, thereby, are called the impurity states. The existence of these impurities can affect the electron transport properties. And it will also have certain effect on the quality of the nano device manufactured. So it is very necessary to investigate the influence of impurities on electron transport through coupled-QD structures.In this thesis, by means of the non-equilibrium Green function technique, the impurity-modulated electron transport properties in a double quantum dot Aharonov-Bohm ring are theoretically studied, by considering impurities locally and nonlocally coupled to the QDs in the ring arms, respectively. Under different structural and environmental parameters, to compare and analyse the results obtained is main work, and eventually we get some meaningful results.It is found that the impurity influence the electron transport in a nontrivial way through the study. In the case of zero magnetic flux, a single-level impurity leads to the appearance of Fano line shapes in the conductance spectra. The positions of Fano antiresonances are determined by both the impurity-QD couplings and the QD levels separated from the Fermi level, and the positions of Fano antiresonances meet certain algebraic relation. Whereas when a magnetic flux is introduced with the phase factorΦ=π, we found that the applied magnetic flux can change the role of imputity. Accordingly, the Breit-Wigner lineshapes appear in the conductance curves instead of the Fano lineshapes. Compared with the local impurity case, the nonlocal impurity’s effects on quantum interference are more obvious. Nonlocal coupling of impurities to the QDs alters the conductance period versus the magnetic flux. In addition, the multilevel impurities further transform the electron transport with the complication of conductance curves.It is necessary to make a remark about the electron interaction during investigating the impurity-modulated electron teansport. When electron interaction is considered within the second-order approximation, we find the important role of the electron interaction in modifying the electron transport process. In the two cases of local and nonlocal impurities, with the change in electron interaction strength, the impurities induce different Fano line-shapes in the conductance spectra which possess the opposite symmetries. Another difference between the results in such two configurations is that the Fano zero is robust in the nonlocal impurity case even though the change in electron interaction.