Theoretical Study On Electronic Transport Of Multi-Quantum Dots Interferometer System

The manipulation of electrical transport in semiconductor quantum dot systems has long been an important research topic,which can bring about many promising applications in the field of electronic technology.Semiconductor quantum dots have been widely exploited in single electronic electrometers,static memory components,quantum information processing and quantum computing.Basically,coupled multi-quantum dot systems have more tunable degrees of freedoom than single quantum dot systems,which is helpful to control the charge transport,spin-polarized transport and thermoelectric transport of systems.Coupled multi-quantum dots can be used as basic electronic components on integrated quantum chips.The study of electrical transport and thermoelectric conversion in coupled multi-quantum dot systems can provide necessary fundamental support for the design and fabrication of novel quantum functional devices.In the thesis,the Keldysh nonequilibrium Green's function is used to establish Dyson's equation.Combining with Langreth's theorem,the charge transport,photon-assisted electric transport,spin transport and thermoelectric transport of several coupled multi-quantum dot systems with different configurations are studied theoretically.Firstly,on the basis of the parallel double quantum dot interferometer structure,a quantum dot is suspended laterally to constructe an asymmetrically-coupled triple-quantum dots interferometer.The corresponding photon-assisted transport model is developed.By adjusting the inter-dot coupling strength and the time-dependent external field,the average current of the system can be controlable to realize the rectification function of mesoscopic device.The conversion between the peak and valley of average current can be performed by adjusting the amplitude of the time-dependent external field.Changing the asymmetry parameter of the coupling strength between quantum dots and electrodes can distinguish easily the main and sideband peaks of average current.This can help to determine the separation of system's energy levels.Secondly,in order to effectively control spin-polarized current in mesoscopic quantum devices,the “T-type three-quantum dot molecule” embedded in the arms of Aharonov-Bhom interferometer is designed.The photon-assisted charge and spin transport characteristics in the quantum devices are analyzed.Aharonov-Bhom effect of average current can be observed when transverse flux passes through the system.By adjusting the Rashba spin-orbit coupling strength,pure spin-up(-down)polarization transmission can be achieved in the whole region of quantum dot energy level.In the action of symmetrical time-dependent external field,the magnitude and direction of spin polarization can be effectively controlled by changing the DC bias,flux and Rashba spin-orbit coupling phase factor.The function of multiple photon-electron pump can be achieved by applying time-modulated asymmetric external field in the system.Thirdly,the photon-assisted electrical transport of “linear double-quantum dot molecule” Aharonov-Bohm interferometer is designed and studied theoretically.Further,the electrical transport of the “linear three-quantum dot molecule” Aharonov-Bohm interferometer is analyzed.In the “linear double-quantum dot molecules” AB interferometer,the splitting of conductive resonance peak can be induced by adjusting the inter-dot coupling strength or magnetic flux.By controlling the flux passing through the AB interferometer,the digital conversion between 0 and 1 of the conductive resonance peak value is realized.This provides a new physical solution for the manufacture of quantum switches.With the help of magnetic flux and Rashba spin-orbit interaction,the system can be used as a spin filtering.For the “linear three-quantum dot molecule” Aharonov-Bohm interferometer,when the numerical difference of inter-dot coupling strengths in two “linear three-quantum dot molecules” becomes small,Fano anti-resonance peak can be observed in the conductance spectrum.By adjusting structural parameters of the system,two bound states can be formed simultaneously in the continuum.With the increase of magnetic flux,the anti-resonance point in conductance gradually evolves into an anti-resonance band.The results provide new insights for the further understanding on the electrical transport of “linearly coupled multi-quantum dot molecular chain” embedded in Aharonov-Bohm interferometer system.Finally,to improve the thermoelectric conversion performance of mesoscopic quantum devices,a new scheme that two quantum dots are seperately suspended in each arm of parallel coupled double quantum dots interferometer is presented.Due to the coupling of symmetrically-suspended quantum dots,double Fano resonance can be produced in the spectra of conductivity and thermal conductivity at low temperatures.This can greatly enhance the thermoelectric conversion performance of the system.Benefited from the coexistence of local bipolar effect and Fano resonance,the thermoelectric figure of merit of the system can be enhanced two times as large as that of the parallel coupled double quantum dot system.By exploring the unusual property,the symmetrically coupled four quantum dots interferometer system can be helpful to design quantum devices with efficient thermoelectric conversion.At the relatively high temperature,the thermoelectric figure of merit can be optimized by adjusting the dot-lead coupling strengths and the energy levels of quantum dots.In summary,the thesis presents theoretically a systemetical study on the electron transport,photon-assisted transport,spin-polar transport and thermoelectric transport in several typical schemes of coupled multi-quantum dot interferometers.It turns out that the effective control of conductance,average current,spin-polarized current and thermoelectric parameters can be achievable with the adjustment of the structural parameters of the system.These typical systems of multi-quantum dot-coupled interferometer can be implementable in experiment by precisely designing ohmic contact electrodes.The results given by our study would be of interest in the design of spin quantum devices and thermoelectric conversion quantum devices.