Phonon Transport In The Curved Nanobelts

The development of microelectronics technology has led to the size of devices be reduced to micro-nano scale. Many novel phenomena have been found in nanostructures, which led the research of nano-scale be the hot issues. Using Formally Scattering Theory, the scalar phonon transport in a helix nanobelt and a twisted nanobelt under stress-free boundary condition are studied respectively. The effects of bending and torsion on the thermal conductance are also discussed. By employing coordinate transformation, the phonon transport in bending nanobelt is transformed into the phonon transport in plane-straight nanobelts with an effective potential caused by curvature and torsion, and then we use the scattering theory to solve the transmission probability and the thermal conductance. Conclusions can be obtained by theoretical derivations and numerical calculations:curvature and torsion of the structure induce quantum interferences between phonon modes, which suppresses thermal conductance to some extent. For the case of phonon transport in a helix nanobelt, transmission mainly occurs between the phonon modes with the same channel indexes in the small curvature approximation; Compared to the plane-straight nanobelt, the perfect stair of total phonon transmission spectrum shows oscillation behavior caused by quantum interference, and with the increases of bending degree, the oscillation is more intense and complex; curvature suppresses the thermal conductance in some extent, and the suppression effect becomes stronger when the curvature increases. For the phonon transport in a twisted nanobelt, numerical results shows that phonon transport in this system with a certain selective-law rule, which shows that the transmission probabilities are zero when the absolute difference of the two different transverse quantum numbers are odd. For the specific coupling case, the oscillation amplitude of the transmission probability increases with the increase of torsion when the belt twisted light, but with the further increase of torsion, the transmission probability mainly behaves decreasing with the increase of torsion accompanied by the appearance of some peaks. In addition, the calculation results also show that the existence of torsion in a twisted nanobelt hardly affects the thermal conductivity. These results show that we can control the thermal transmission by adjusting the parameters of nanostructure, which can provide theoretical references for designing and applying quantum devices with specific functions.First, the new progress in nanotechnology and theoretical method in studying the phonon transport and thermal conductivity are introduced in Chapter 1.In Chapter 2, the method dealing with phonon transport problems in bent structures is described. By employing moving coordinate system along the structure, the problem is transformed into the phonon transport in plane-straight nanobelts with an effective potential caused by curvature and torsion, and then we use the Formally Scattering Theory to solve the transmission probability which links with the thermal conductance. In Chapter 3 and 4 we specifically study the heat transport properties in helical nanobelts and twisted nanobelt respectively, and the effects of bending on the thermal conductivity are also discussed. The summary and outlook for this paper are arranged in Chapter 5.