Researches On Phonon Thermal Transport And Nonlinear Thermal Phenomena In Low-dimension Nanostructures At Low Temperature

With the fast development of nano technology, many micro devicesbecome smaller and smaller year by year. When a device reduces to nano scale,its thermal conductivity no longer is the nature of material but depends on shape,dimensions and boundaries of the device, no matter what way the thermaltransport of phonons in. Then, it is important to study the influence of shapesand structure parameters and so on on thermal transport and nonlinear thermalphenomena in nanostructures. But classical theory of thermal transport is nolonger apply to nanostructure in low temperature. Quantum-mechanical theory isused to research phonon modes in the system and features of thermal transport.And this is a important method to design and develop new nano devices. In thispaper, we investigate quantized thermal conductance of an asymmetric nanowireand thermal rectification in an asymmetric ballistic four-terminal nanostructuresin low-dimension and at low temperature.Firstly, we investigate the ballistic phonon thermal conductance in anasymmetric Z-shaped nanowire at low temperatures. We introduce the structureof the model of Z-shaped nanowire and derive the transmission coefficient. Then,we discuss the features of transmission coefficient and thermal conductance. Inthe temperatureâ†'0limit, similar to the previous results in symmetricnanowires, the universal quantum of thermal conductance (2k2B3h) could also beobserved in an antisymmetric nanowire, but could not be obtained in otherasymmetric ones. So, we find the ratea1a2of transverse sizes of the left andright terminals in the nanowire determines whether we could observe the universal value for the ballistic phonon thermal conductance. Only for, theuniversal value could be measured availably.Secondly, we study thermal rectification theoretically in a dielectricasymmetric ballistic four-terminal structure (DABFTS) at low temperature. Inthis paper, thermal rectifying behavior in DABFTS, the nonlinear characteristicof transporting phonons, can be displayed and, meanwhile, the reasons forthermal rectification are analyzed. Different from previous studies, we find twonecessary factors that cause thermal rectification in the structure:1) theexistence of the unequal density transverse phonon modes among energy inputterminal and energy output terminal;2) the existence of thermal reservoirsconnected to the other two terminals. More importantly, importantly, only thecombination of two necessary factors can produce the thermal rectification, andany of two factors is not possible to independently induce thermal rectifyingbehavior in the ballistic system.