【作者】 彭小芳；

【导师】 陈克求；

【作者基本信息】 湖南大学 ， 凝聚态物理， 2010， 博士

【摘要】 近年来,随着微／纳制造技术的飞速发展,电子器件的尺寸已经进入纳米尺度。纳米尺度量子器件由于其优良的性能以及重要的应用前景已经受到了人们的广泛的关注。量子体系中物理性质的研究已经成为凝聚态物理一个非常重要的研究方向,在当今世界的高新科技领域起着关键性的作用。本文对低维量子体系中声学声子输运和热导性质进行了系统的研究,取得了一些有意义的研究结果。研究了弹性和硬壁两种边界条件下三维T型量子结构中弹性声子输运和热导性质,详细地比较了两维和三维纳米模型的热输运性质。结果表明,对于弹性边界条件,当温度趋近于0k时,能够观察到量子化热导现象,量子化热导与结构的几何细节无关,三维情形与二维情形具有相似的热导性质。然而,对于硬壁边界条件,三维情形与二维情形中的热导性质是不同的。利用散射距阵方法,进一步研究了低温下两个耦合的量子点调制的量子线中的弹性声子热输运性质。研究结果表明,在极低的温度下,仅仅最低模被激发,热导随着结构参数的变化而单调变化。随着温度的升高,更多的模被激发,导致热导随着结构参数的变化而非线性变化。声子的输运几率与热导依赖于量子点相对量子线对称轴的相对位置。当量子线对称轴偏离量子点中心时,热导单调增加。研究表明,调节两量子点之间的距离能有效调节热导。此外,我们也观察到非均匀量子化输运台阶。前面的研究主要是针对单支振动模热输运性质的研究。实际上,对低温量子热导的贡献来自于最低的四支声学模。本文研究了具有catenoidal结构的量子线中最低的六支弹性声子模(压缩模,扭转模,两支弯曲模,两支光学模)的热导性质。研究结果表明最低的四支声学模的截止频率为0,而两支光学模的截止频率大于0。在理想的量子线中存在量子化的热导平台,当结构为具有catenoidal结构的量子线时,量子热导平台消失,被下降的热导曲线所取代。结果表明,来自不同振动模的热导具有不同的特征。研究了低温下被量子点调制的量子线中六支振动模的输运几率和热导的特点。同时,比较了六支振动模热导的性质。结果表明六支模的输运几率表现为周期或准周期的行为。不同振动模的热导具有不同的输运性质。通过改变量子点的结构参数能够有效调节热导的大小。此外,我们研究了三维超晶格中非谐相互作用的三声子散射、边界散射、界面散射、杂质散射、电声相互作用这五种散射机制对热导率的影响。研究结果表明在这五种散射机制中,边界散射和界面散射对减少热导率起着主要作用。改变横截面尺寸和超晶格横向周期的长度能够有效调节热导率。同时,研究表明超晶格横截面能有效约束声子的群速,从而导致热导率的变化。更多还原

【Abstract】 Recently, with the rapid devlepment of micro-/nano-fabrication techniques, the feature sizes of electronic device and thermal pathway are scaling down to nanodimenssion. There has increasing attention to nano-dimenssional electronic device due to their novel physical properties and extensive application prospects. The study of low-dimensional quantum system and correlative work has been a very important development direction of condensed. matter physics and study embranchment of new technology and new devices, which plays a crucial role in new and high-tech domain. In this paper, we investigate the property of quantum thermal conductance in low-dimensional quantum system and the influence for nanostructure dimension to thermal transport widely, and we find some significative results.Ballistic thermal conductance in a three-dimensional quantum wire with a stub structure is presentedunder both stress-free and hard wall boundary conditions at low temperatures. A comparative analysis for two-dimensional and three-dimensional models is made. The results show that whenstress-free boundary conditions are applied, the universal quantum thermal conductance can beobserved regardless of the geometry details in the limit T→0, and the behavior of the thermalconductance is qualitatively similar to that calculated by two-dimensional model. However, when hard wall boundary conditions are applied, the thermal conductance displays different behaviors inboth two-dimensional and three-dimensional models.Using scattering-matrix method,ballistic thermal transport properties at low temperatures in a quantum wire modulated withtwo coupling quantum dots are studied. The results show that when the temperature is low enoughwhere only the lowest mode can be excited, the reduced thermal conductance displays monotonicbehavior with the change of structural parameters. At higher temperature, more modes can beexcited and the reduced thermal conductance displays a nonlinear behavior with the change of thestructural parameters. It is also found that the phonon transmission and thermal conductance sensitively depend on the relative position of quantum dots and symmetric axis of the quantumwire. When the symmetry axis of quantum wire is away from the center of the quantum dots, thethermal conductance increases monotonously, and is different when the symmetric axis of quantumwire is away from from the center of quantum dot along different routes. It is also found that the thermal conductance can be modulated by the magnitude of the quantum dots and the length between the two quantum-dots. Moreover, inhomogeneous quantum transport steps and quantizedthermal conductance plateau can be observed in such structure.In the previous work, our studies foucs on the thermal transport of only single vibrational mode. Actually, the quantum thermal conductance relies on four low-lying acoustic vibrational modes. The thermal conductance associated with the lowest six types of ballistic phonon modes in quantum wire with catenoidal contacts is investigated. The results show that the cutoff frequency for the four types of acoustic modes is zero, while two types of optical modes are of nonzero cutoff frequency. For a perfect quantum wire, a quantized thermal conductance plateau can be observed. While for the structure with catenoidal contacts, the thermal conductance plateau is broken and a decrease in thermal conductance appears. The results also show that the reduced thermal conductance contributed from different vibrational modes has different characteristics.The ballistical phonon transport and thermal conductance of the six low-lying vibration modes at low temperatures in quantum wire modulated with quantum dot is investigated. A comparative analysis for the thermal conductance for the six vibrational modes is made. The results show that the transmission possibility of the six vibrational modes displays periodic or quasi-periodic oscillatory behavior. The thermal conductance contributed by different vibrational modes is of different characteristics, and can be adjusted by changing the structural parameters of the quantum dot.Moreover, we investigate the influence for five kides of scattering mechanisms to thermal conductivity, which are three-phonon Umklapp scattering, boundary scattering, interfacial scattering, mass difference(impurities) scattering, and phonon-electron scattering. Among the five kides of scattering mechanisms, the boundary scattering and the interfacial scattering are dominant resistive process for the decrease of the thermal conductivity. It is effective to adjust thermal conductivity by changing the dimension of the cross-section and the thickness of the constituent layers of the unit cell of the superlattice. The study also shows that the group velocity of phonons is dependent on the dimension of the cross-section, which can change the thermal conductivity effectively.更多还原