Properties Of Quantum Transport In Quasi-one-dimensional Semiconducting Nanostructures

Electronic transport and thermal transport in semiconductor quantum structures are important issues in condensed matter physics and have attracted considerable attention in recent years. Many interesting physical properties have been found in those structures such as quantized electronic conductance and thermal conductance, which are very useful properties in device application. In this thesis we present some detailed theoretical investigations on electronic transport and thermal transport in semiconductor quantum structures. The achievements made in this thesis are summaried as:We investigate transport properties of electrons through an asymmetric T-shaped magnetic quantum structure with the use of the mode-mathcing technique. The results show that the changes of both structural factors and magnetic field can affect the electronic scattering behavior and result in various patterns of electronic transmission. When different magnetic configurations and structural factors are applied, the transmission exhibits various patterns such as wide valley, sharp peak, resonant reflect, resonant transmission, and so on. Our results show that one may control the transmission property to design interferential quantum devices by adjusting magnetic configurations or structural parameters.The effects of magnetic field and bias voltage on spin polarized electron transport through a dual magnetic barriers quantum structure are investigated in this paper。The results show that: (i) the spin-polarization of electron can not be produced in the anti-parallel magnetic barriers structure at zero bias voltage; (ii) the energy threshold of electronic transmission are increase with the increases of magnetic field or bias voltage; (iii) the comparison of the electron spin polarization degree is made between the electron transport through GaAs and through InAs quantum structure, and find that though the spin polarization degree oscillation decrease with the increase of the incidence energy, the spin polarization degree of InAs is an order of magnitude higher than that of GaAs because of the higher Lande effective factor。By using the scattering matrix method, the properties of two-dimensional electron gas spin polarized transport through step-magnetic barriers structure at different bias voltage are investigated. The results show that the degree of spin polarization (i) is oscillation reduce if the incidence energy increase at zero bias voltage and (ii) reduce slower and the maximum of spin polarization decreases when the number of steps increase in this structure and (iii) enhance significantly in wider energy region at applied bias voltage, and more distinct spin-filtering properties are shown in the step-magnetic barriers.We investigate the effect of boundary conditions on acoustic phonon transport in a quantum waveguide with structural defects at low temperatures by using the scattering-matrix method. Here both Neumann and Dirichlet boundary conditions are considered. The results indicate that the transmission coefficients versus reduced phonon frequency is qualitatively different for different of boundary conditions. The stop-frequency gap and the transmission resonance can be observed for certain structural parameters due to mode-mode coupling.By using the scattering matrix method, we investigate the thermal conductance in a double-stub quantum wire at low temperatures. It is found that the quantum structures exhibit oscillatory decaying thermal conductance with the width between two stubs. When the stress-free boundary condition in used for the structures, thr universal quantized thermal conductance can be clearly observed. As the hard-wall boundary condition is considered, the quantized conductance can not be observed. The coupling effect between two stubs is qualitatively different for the different boundary conditions. This appears to be important for applications in devices.We investigate acoustic phonon transport and thermal conductance at low temperatures in a quantum wire with rectangle scatters periodically placed in the quantum channel. It is found that the transmission spectra of zero mode exhibits a series of resonant peaks-valleys structures, and the transport valley gradually develop into stop-frequency gap with increasing the number of the period. The number of resonant peaks or valleys between two nearest gaps is just twice as large as the number of the period. The result also indicates that the thermal conductance is sensitive to the number of the period and structural parameters, and the change is more pronounced in the structure with smaller number of period.