The Electron And Spin Properties In The AB Interferometer With Embodied Quantum Dot Molecules

Semiconductor quantum dot (QD) is a quasi-zero-dimensional mesoscopic structure, which presents discrete electron energy spectrum and strong electron interaction. Because of such electronic characteristics, a single QD is viewed as an artificial atom. Accordingly, a structure consisting of several coupled QDs confines electrons in a way as an artificial molecule. QDs can be incorporated into a mesoscopic circuit. Thus, the electron structure in QDs can be revealed by the observation of the electronic transport spectrum. Meanwhile, the controllable electronic transport properties through a variety of QD structures suggest many promising device applications. For example, some multiple QD structures are recently considered as the device prototype to realize the quantum computation. In contrast to a single QD, multiple QD structures possess more structural parameters to tune their electronic transport properties. Therefore, the investigations on the multiple QD structures are the current focus in the field of mesoscopic physics.In this thesis we report our theoretical investigations about the electron and spin transport through several Aharonov-Bohm (AB) interferometers with typical embodied multiple QD structures, by means of non-equilibrium Green function technique, thereby, some interesting results are obtained. It is well-known that quantum interference plays a dominant role in the electronic transport process through a mesoscopic structure. As a typical mesoscopic structure, coupled multiple QDs and the structure of AB interferometer provide a variety of Feynman paths to take part in the quantum interference. As a result, the quantum interference among these distinct Feynman paths brings about novel electronic transport properties. We will focus on the quantum interference which is the underlying mechanism for the electronic transport properties through several different QD molecule structures. Below we outline our works briefly from two aspects:On one hand, we established the three following models: The interferometer with arbitrary neighbor QDs of one-dimensional QD chain embodied in its two arms, the interferometer with the peripheral QDs of one-dimensional QD chain in its two arms, and the interferometer with a QD ring embodied in it. We theoretically investigated the electronic transport through three structures. For the first model, we found that when the QD chain is symmetrically placed some of its molecular states decouple from the leads. Namely, in the absence of magnetic flux all odd molecular states decouple from the leads, but all even molecular states decouple from the leads when an appropriate magnetic flux is introduced. Interestingly, the antiresonance position in the electron transport spectrum is independent of the change of the decoupled molecular states.For the case of the peripheral QDs of one-dimensional QD chain embodied in the arms of AB interferometer, it was found that, in the absence of magnetic flux, all the even molecule states of odd-numbered QD structures decouple from the leads and in even-numbered QD systems all the odd molecule states decouple from the leads, which indicates the formation of remarkable bound states in the continuum. Meanwhile, what's interesting is that apparent antiresonance occurs in electron transport through this structure, the positions of which are accordant with all even (odd) eigenenergies of the sub-molecule of the even (odd) -numbered QDs without the peripheral dots. All these results are efficiently modified by the presence of magnetic flux through this system.With the above results, one can understand that the decouple phenomena are tightly dependent on the symmetries of the considered structures. So it can be predicted that when a QD ring is embodied in the AB interferometer, there will be remarkable decouple results, since the symmetry from both the QD ring and the AB interferometer, respectively. Consequently, the occurrence of decouple results in electron transport through the QD ring in the interferometer is remarkable, independent of the number of quantum dots in the ring. Furthermore, by the presence of an appropriate magnetic flux through the interferometer, the linear conductance spectrum of the (2n+l) -quantum-dot ring (n∈integer) is the same as that of the 2n-quantum-dot chain, on account of the occurrence of decouple inelectron transport through these two kinds of systems.When incorporating the many-body effect by only considering the Hubbard term andtruncating the motion equation of the Green functions to the second-order, we show that the emergence of decoupling gives rise to the apparent destruction of electron-hole symmetry. In addition, by adjusting the magnetic flux through either subring of the AB interferometer, some molecular states decouple from one lead but still couple to the other, and then some new antiresonances occur. In addition, we investigated the negative differential capacitance caused by the decoupling mechanism with a finite bias voltage between two leads. As a consequence, the decoupling phenomenon can be demolished since the structure parameters that take some specific values are destroyed by the variation of the bias voltage. Thus, the electrons located in QDs due to the decoupling effect, driven by the bias, will take part in electron tunneling and then enter the drain of the system.On the other hand, the electron and spin properties in the complicated AB interferometers were studied. First, electron transport properties of a triple-terminal Aharonov-Bohm interferometer were theoretically studied. By applying a Rashba spin-orbit coupling to a quantum dot locally, we found that remarkable spin polarization comes about in the electron transport process with tuning the structure parameters, i.e, the magnetic flux or quantum dot levels. When the QD levels are aligned with the Fermi level, there only appear spin polarization in this structure by the presence of an appropriate magnetic flux. However, in absence of magnetic flux spin polarization and spin separation can be simultaneously realized with the adjustment of QD levels, namely, an incident electron from one terminal can select a specific terminal to depart from the QDs according to its spin state.In the following, spin-dependent electron transport properties in a parallel double quantum dot structure with four terminals were theoretically investigated by means of the nonequilibrium Green function technique. As a result, when applying a local Rashba spin-orbit interaction on an individual QD and introducing a finite bias between the transverse terminals, we showed that in the presence of appropriate structure parameters, apparent pure spin currents come into being in the longitudinal terminals with the same amplitude and opposite polarization directions. Besides, the polarization directions of such spin currents can be efficiently inverted by the adjustment of structure parameters.