Charge And Spin Transport Properties Of Coupled Triple Quantum Dot System

Quantum dot is similar to atom in energy level structure, and quantum dotenergy level can be changed by adjusting its size. Quantum dots system own someunique physical properties, such as quantum tunneling effect, quantum interferece,Coulomb blockade effect, Fano effect and Kondo effect and so on, these effect withcharge and spin transport properties are physical basis for designing new typenano-device, thus quantum dot system own broad application prospect.Coupled-quantum-dot system possesses more tunable parameters than a singlequantum dot to manipulate charge and spin transport behavior, which can beconsidered as an applied quantum device. Theoretical study on charge and spintransport through coupled-quantum-dot system can provide essential theoreticalsupport for designing quantum device, and is important to force the design andapplication of mesoscopic device. It is important to study theoretically charge andspin transport through coupled quantum dot system in theory and deviceapplications. Employing nonequilibrium Green’s function method, we study chargeand spin transport through coupled triple quantum dot system with externalmagnetic flux, RSOI（Rashba spin orbit interaction）, and intradot Coulombinteraction taken into consideration. Tnfluence of quantum dot level, intradotCoulomb interaction, RSOI and external magnetic flux on charge and spin transportproperties of coupled triple quantum dot system is theoretically investigated.Firstly, charge and spin transport properties of a quantum dot—“quantum dotmolecule” Aharonov-Bohm interferometer have been investigated. Employing theDyson equation and the equation of motion for nonequilibrium Green’s function,and adopting Hartree-Fock truncate approximation, the expression of conductancecan be obtained. Numerical results indicate the conductances of two different spincomponents for eletron are respective resonance and anti-resonance simultaneouslyat the same location, which is the basis for the design of our spin filter. Theconversion between100%polarization and unpolarization can be realized bycontrolling coupling or uncoupling between two quantum dots in “quantum dotmolecule”. This can be used as a principle for the design of a polarization pulse.Besides, the direction of spin polarization and the magnitude of spin polarizabilitycan be changed by adjusting coupling strength between two quantum dots in“quantum dot molecule”.Secondly, charge transport properties through a coupled triple quantum dotAharonov-Bohm interferometer have been investigated. In the numerical analysis, the dot levels are taken three differet values and the coupling strengths between thedots are assumed to be the same. It is shown that the intradot Colomb interactioninfluences strongly charge transport properties of the system. Compared with thecase in the absence of the intradot Coulomb interactions, more conductance peaksoccur in the conductance spectrum. With the increase of intradot Coulombinteractions, three groups of peaks begin into two groups of peaks in theconductance spectrum. The lineshapes of two groups of conductance peaks aredifferent. By controlling the intradot Coulomb interaction strength, the transitionbetween the resonance and Fano antiresonance peaks is observed. The combinedeffect of intradot Coulomb interaction and magnetic flux may lead to degeneracy ofthe conductance peaks. When three quantum dots have the same energy level andinterdot coupling, two groups of conductance peaks in conductance spectrum aresimilar. As intradot Coulomb interaction is unchanged, magnetic flux influencesnotably charge transport properties of the system.Thirdly, charge and spin transport properties through triple Rashba quantumdot system have been investigated. Three cases for charge transport through thesystem:（1）the case for a quantum dot couples weakly with another quantum dot butuncoupled with the leads （the quantum dot can be seen as a dangling quantum dot）,two sharp resonance peaks appear at energy levelε=ε₀andε=ε₀+U, respectively;with the increase of the coupling strength between them, two sharp resonance peakschange into two Fano resonance peaks; for sufficiently large coupling strengthbetween them, two Fano resonance peaks change into the resonance peaks again.（2）the case for a quantum dot only couples with two leads （the coupling strengthbetween them is assumed as Γ₃）, compared with conductance spectrum throughcoupled double Rashba quantum dot, two new Fano resonance peaks appear inconductance spectrum. With the increase of the coupling strength Γ₃, two Fanoresonance peaks change into two resonance peaks;（3） when three quantum dotsform linear structure and each quantum dot couples with the leads, two new Fanoresonance peaks are also observed. The conductance is spin polarized under thecombined effect of magnetic flux and RSOI.Finally, we design a coupled three-quantum-dot ring, in which three quantumdots couples with the leads, simultaneously, the influence the dot levels and interdotcoupleing strength on Fano effect is investigated. It is found that, as one takessymmetrical （t_i=t） and large interdot tunneling couplings, different levelsarrangement (ε_1σ≠ε_2σ=ε_3σ orε_1σ≠ε_2σ≠ε_3σ) can lead to Fano effect in theconductance spectrum. When one quantum dot level is different from the others two same quantum dot levels, one Fano anti-resonance peak appears in the conductancespectrum; when three quantum dot levels are different each other, two Fanoanti-resonance peaks appear in the conductance spectrum. When three quantum dotlevels are the same (ε_iσ=ε₀) and different interdot coupling strength arrangement（t₁=t₂≠t₃or t₁≠t₂≠t₃）, Fano resonance peaks also occur in the conductancespectrum. When one interdot coupling strength is different from the others two sameinterdot coupling strengths, one Fano anti-resonance peak appears in theconductance spectrum; when three interdot coupling strengths are different eachother, two Fano anti-resonance peaks appear in the conductance spectrum. Inaddition, three quantum dot levels and three interdot coupling strengths are assumedas the same value, respectively, Fano anti-resonance peaks disappear, and only oneBreit-Wigner resonance peak appears in the conductance spectrum.In conclusion, we investigate charge and spin transport properties throughcoupled triple quantum dot system. It is clarified how the intradot Coulombinteraction, Rashba spin-orbit interaction, and structure parameters change chargeand spin transport properties. Model we design is simple and can come true by usingthe existing technical means in experiment. These results we obtain are important todesign future quantum devices.