Researches On Elastic Phonon Transport And Thermal Conductance In Quantum Structures At Low Temperature

In recent years, the nanoscale quantum structures and devices have already becomeone of the research hotspots of the material science and condensed matter physics due totheir novel physical properties and prospective and potential applications. In this thesis,the effects of geometrical shapes, structure parameters and evanescent modes on ballisticphonon transport and thermal conductance in low-dimensional quantum structures areinvestigated. The achievement made in this thesis are summarized as follows:Using scattering-matrix method, we investigate the thermal transport associated withballistic phonon in Young two-slit and Fabry-Pabry quantum structures. The numericalresults show that the transmission coefficient and thermal conductance are very small dueto the multiple re?ection in the quantum structures. By adjusting the geometric param-eters in the two-slit structure, we can modulate the phonon transport and thermal con-ductance. The transmission and the thermal conductance may show the maximum valueand minimum value at the center of the structure. In Fabry-Pabry quantum structure, thetransmission coefficient show approximate periodicity as a function of geometric param-eters. The quantized thermal conductance does not appears due to the fact that hard-wallboundary conditions are applied in the structure. The comparison between phonon andelectron transport properties in the two structures and the optical experiment is given.We investigate the in?uence of the evanescent modes on the acoustic phonon trans-port probability and thermal conductance in quantum structures at low temperatures. Themechanism for the effect of the evanescent modes on the transport of phonon and thermalconductance is revealed . The evanescent modes play different roles on the transport pos-sibility and thermal conductance in the narrow-wide-narrow and wide-narrow-wide quan-tum structures. For narrow-wide-narrow structure, the interaction between the evanescentmodes and propagating modes suppresses the propagating of acoustic phonon modes andmake the transmission and thermal conductance decrease notably. However, for wide-narrow-wide structure, evanescent modes would provide the channel for the propagatingmodes and lead to the increase of the transmission probability and the thermal conduc-tance. It is found that the evanescent modes with different mode index have differentattenuation length, the lowest several modes with the largest attenuation length play im-portant role on the transport of phonon propagating modes.We calculate the transmission probability and thermal conductance in a double-bend quantum structure with finite thickness (3D) and compare with 2D geometries. The quan-tum steps of the acoustic phonon transport probability are presented when assumptionthat the quantum wire is perfect, and the characteristics of quantum steps are differentfrom those of 2D systems, also different from those of electronic transport in quantumwire. In a 2D model, the perfect transmission quantum steps is always located just at aninteger-reduced frequency and the magnitude of the step is always one where a new modeis starting to be excited. However, in the 3D model, the location of the new modes startingto be excited is not always at the integer reduced frequency and depend on the geometricparameters. The magnitude of the step is either one or two. The double-bend quantumwire show the resonate transport behavior, and the transmission and thermal conductanceare also very sensitive to the geometric parameters. It is suggested that adjusting thegeometric parameters is an effective way to control the transmission properties and thethermal conductance of the structure to match practical requirements in devices.We investigate the acoustic phonon thermal transport and thermal conductance of aasymmetric three-terminal ballistic junctions with finite thickness. The results show thatthe phonon modes have selective transport behavior, the phonon modes with differentmode index transport into different channels selectively. We can obtain the some phononmodes with certain energy from different channel or forbid some phonon mode transportby tuning the geometric parameters and incidence frequency. Based on the phonon trans-port character, we obtain the mode split condition and design the acoustic phonon modessplitter and phonon mode switchable device theoretically. We observed the nonintegerquantized thermal conductance at very low temperature, the thermal conductances of dif-ferent channels show the different properties with the change of geometric parameters andshow the obviously dependence on geometric effect.We investigate the transport property of the symmetry and asymmetry three-terminalquantum structures at non-uniform temperature. The results show that, when the temper-ature difference between the left and right thermal reservoirs is existed, the temperatureof the center terminal TC is always higher than the average temperature T0 of the leftand right terminals in the symmetry three-terminal quantum structure. In the asymmetrythree terminal quantum structure, the TC always deviates from the averaging temperatureof the left and right terminals. The lower the averaging temperature of the left and rightterminals, the more obviously the difference between the TC and T0. Moreover, the neg-ative thermal conductance is observed in the structure, which suggest the three-terminalquantum structures have the thermal rectification effect.