Impurities-contributed Abnormal Thermoelectric Effect In A Parallel Double Quantum Dot Structure

The study of the electron transport properties in the mesoscopic quantum dots system has become an important field of condensed matter physics and materials physics. Because it’s not only an addition to the basic theory, but also it provides a theoretical basis for the development of quantum devices, with the constant progress and sustainable development of experiment technology, within the scope of the nanometer scale, has allowed to implement and control dots by controllable method, it’s also promoted even more people to commit to research the characteristics of electrons in a quantum dot system. although there has the above work, but because of the irregularity and defection of quantum dot system, the existence of impurities is unavoidable. So far, it’s a big challenge to make two pure quantum dots. Some local states often appeared in the quantum dot system,which makes the energy of a quantum dot hybridizate, but because they are not directly coupled with electrode;, so they are called the state of impurities. The presence of these impurities will affect the transport properties of electrons, so it is very necessary to study the influence of the electron transport properties by the impurity within the coupled quantum dot structure.By means of the nonequilibrium green’s function method,This thesis aim to discuss the abnormal thermoelectric effect in a parallel double quantum dot structure coupled to a impurity,We consider the impurities coupling quantum dots with three forms:the first kind of circumstance is one impurity coupled with quantum dots in resonance path, we found that, in this case, where kBT=0.5Γ,φ=0, with the increase of impurity-QD coupling, the value of the thermal power,the value of the figure of merit ZT, the conductance and thermal conductance oscillates. Except such oscillation, the extremum of coefficient enhances and shifts to the high-energy direction. If a local magnetic flux increases to φ=π,the effect of impurity is more apparent, If the flux is in the case φ=π,there will be two branches of the oscillation, a branch in Fano area, and the other branch near the area λ2≈-2ε0. In addition, when the temperature enhanced to kBT=0.1Γ,the impurity effect is still clearly seen. Namely, the impurity can efficiently enhance the magnitude of the thermoelectric effect. Instead in the case of φ=π, with the increase of λ1, the oscillation don’t appeared in the figure.Similar to the analysis way above, we discuss the thermoelectric effect in the presence of a local impurity in the nonresonant channel, in general, in the case of no magnetic flux, the extreme of the S, the case of merit ZT, the conductance and thermal conductance is only seeable in the case of λ2>0.5Γ,which are located in the vicinity of ε0=0. Alternatively, in the case of φ=π, we see that the extremum near the point of ε0=0.5Γ survives in the case of weak Impurity-QD coupling. When λ2>0.5Γ,the extremum of S appears beside the energy zero point. Up to now, we have known that when the nonresonant channel is coupled to an additional impurity, the change of the thermoelectric quantities is more distinct compared with the results of a resonant-channel impurity. At last, we would like to present the thermoelectric properties by the coexistence of impurities in both channels, We see that at the low-temperature case,ie, in the case of kBT=0.05F, the effects of the impurities in the two channels can be observed, respectively. To be specific, the increased resonant-channel impurity pushes the extremum of S and ZT which exist in the Fano region to the high-energy direction, accompanied by the oscillation of them. The nonresonant-channel impurity just leads to the enhancement of S and ZT near the energy zero point.It is noteworthy that independent of the magnetic flux, the resonant channel in the low-energy region can be always observed. On the other hand, when temperature increases to kBT=0.1Γ,the resonant-channel impurity contributes little to the change of the S and ZT. Especially in the case0=0,we do not see the appearance of S (or ZT) extremum in the Fano region., as for the case φ=π, the effect of the resonant-channel is just to shift the extremum of S (or ZT) to the high-energy direction but almost not to vary its value.Second, we found that whatever situation the impurity coupled with,The thermoelectric effect can be enhanced. This means that the destruction of the Fano interference isn’t the necessary conditions to restrain thermoelectric effect. Therefore,we hope that different results can help us to understand the role of the impurities in regulating the thermoelectric properties in a parallel double quantum dots structure.