The Electronic Transport And Quantum Coherence Effect In The Coupled Quantum Dot Structure

A semiconductor quantum dot(QD) is a quasi-zero-dimensional mesoscopicstructure, which presents discrete electron energy spectrum and strong electroninteraction. Because of such electronic characteristics, a single QD is viewed asan artificial atom. Accordingly, a structure consisting of several coupled QDsconfines electrons in a way as an artificial molecule. QDs can be incorporatedinto a mesoscopic circuit. Thus, the electron structure in QDs can be revealedby the observation of the electronic transport spectrum. Meanwhile, the control-lable electronic transport properties through a variety of QD structures suggestmany promising device applications. For example, some multiple QD structureshave been recently considered as the device prototype for the realization of quan-tum computation. In contrast to a single QD, multiple QD structures possessmore structural parameters that can be used to tune their electronic transportproperties. Therefore, the investigation on the multiple QD structures are thehighlight in the field relevant to QD systems. In this thesis we report our theo- retical investigations about electronic transport through several typical multipleQD structures, by means of non-equilibrium Green function technique.Quantum interference plays a dominant role in the electronic transport pro-cess through a mesoscopic structure. As a typical mesoscopic structure, coupledmultiple QD structures provide a variety of Feynman paths to take part in thequantum interference. For instance, Feynman paths are allowed to include theKondo level due to electron correlation, as well as the realistic single-particlelevels in QDs. As a result, quantum interference among these distinct Feynmanpaths brings about novel electronic transport properties. We will focus on quan-tum interference which is the underlying mechanism for the electronic transportproperties through several di?erent multiple QD structures. Below we outlineour works brie?y.First, we consider a QD chain, two arbitrary component QDs can be selectedto couple directly to two leads. We have calculated the linear conductance spec-trum and the I-V spectrum of such a multiple QD structure. It is well-knownthat the resonant peaks in the linear conductance spectrum of any multiple QDstructure coincide with the eigen energy spectrum of the total QD molecule. Andthe conductance peaks arise from the constructive quantum interference amongany possible Feynman paths. For the present QD chain, we find that the destruc-tive quantum interference can give rise to conductance zeros, in contrast to theresonant peaks. Such a negative quantum interference is called antiresonance.We find several interesting features about antiresonance: The positions of theantiresonance in the linear conductance spectrum(the conductance zeros) re?ectthe eigen-energies of the dangling QDs outside the leads. The antiresonance isindependent of the QD-lead couplings. This is in contrast to the resonant peakwhich vanishes at a su?ciently strong QD-lead coupling. The antiresonance be-haves as a series of staircases in the I-V spectrum. In addition, we find that theantiresonance survives in the presence of many-body e?ect. With an appropri-ate energy, the incident electron with a specific spin can pass through the QD chain resonantly, while the electron with the opposite spin is completely inhib-ited due to antiresonance. Based on such an interplay of the resonance and theantiresonance, we propose a device application for spin filtering.Secondly, we find an interesting phenomena in the transport properties ofthe serially coupled quantum dot chain although antiresonance can't occur insuch a structure. It is well-known that the electron transport spectrum re?ectsthe eigen-energy spectrum of the QD chain. Therefore, the linear conductancespectrum is often used to explore the eigen-energy spectrum of the coupled QDsexperimentally. However, such a mapping is only correct in the limit of weak dot-lead coupling. And when such coupling is su?ciently strong, the resonant peaksin the conductance spectrum no longer coincide with the eigen-energies of the QDchain. To our knowledge, a comprehensive analysis of the conductance spectrumof a QD chain in this regime is still lacking. We wonder whether this implies thatthe resonant quantum interference is invalid in such a situation. To clarify thisissue we study the linear conductance spectrum of a QD chain with relativelystrong dot-lead coupling. Of course, in such a situation the linear conductancespectrum deviates greatly from the eigen-energy spectrum of the N-QD chain.The interesting thing we find is that the linear conductance spectrum turns intothe eigen-energy spectrum of the N ? 2 or N ? 1 QDs. As the dot-lead couplingincreases, such a mapping becomes more and more accurate. It seems that theperipheral QDs do not exist any more. We view the peripheral QD, togetherwith the lead connecting to it, as a composite lead. We can then establish arenormalized QD-lead coupling, by which the linear conductance spectrum inthe strong coupling limit can be well explained. According to this result, wecan conclude that the quantum interference can not be destroyed by the strongQD-lead coupling. However, it behaves in a di?erent way.Thirdly, we pay attention to the quantum interference between a Kondo leveland a realistic single-electron level in QDs. When a QD level is far lower thanthe Fermi level, and a strong Coulomb repulsion prohibits the double electron occupancy in the QD, Kondo resonance occurs which establishes a Kondo levelat the Fermi level. This is as a result of the spin single state between the localspin in the QD and continuous state in the reservoirs. In order to study thequantum interference among the Kondo level and any realistic QD levels, weconsider a triplet QD chain coupling to two leads serially. We arrange the middleQD in Kondo region which provides a Kondo level, and the two peripheral QDsin the mixed-valence region which provide two single-electron levels. Then, bycalculating the conductance spectrum, we can discuss the quantum interferencearising from Kondo level and the single-electron levels. In so doing, we employthe slave-boson mean field approximation(SBMFA). We find that the conductancespectrum can re?ect the molecular energy spectrum of the triplet QDs very well.This result indicates that a Kondo level can be treated in the same way as arealistic single electron level. In addition, such a triplet QD chain can be viewedas a Kondo QD coupling to the leads indirectly, via two discrete levels. We findthat the Kondo peak in the density of states in this structure becomes narrowerwith the increase of the QD-lead coupling, which is just the opposite to the resultof the case whereby the Kondo QD couples directly to the leads.Finally, we turn to study the quantum interference between two Kondo levels.To do so, we consider a Kondo QD coupled to two leads directly. Attached toone lead, there is another QD which is also in Kondo region. Thus we establishtwo Kondo levels in a coupled QD structure. Of course, quantum interferencebetween such two Kondo levels will in?uence the conductance spectrum to someextent. In addition, we would like to point out that this model can well mimic apractical structure, i.e. a QD coupled to dilute magnetic impurity doped leads.Magnetic impurity doped lead is currently employed to implement the highlyspin-polarized electron injection. By calculation, we find that the Kondo peakin the conductance spectrum totally vanishes. A conductance zero comes intobeing at the same position instead. This is certainly due to the destructivequantum interference between the two Kondo levels, namely, the antiresonance e?ect. We develop a Feynman path expansion technique, by which the nature ofquantum interference among the two Kondo levels can be clearly seen. Togetherwith the RKKY interaction which can also cause the suppression of the Kondoresonance, our theoretical result can explain the recent experimental work aboutthe quenched Kondo e?ect.