低温下多通道量子结构中的弹性声子输运和热导

利用弹性近似模型和散射矩阵方法,研究了低温下多通道量子结构中的弹性声学声子输运的性质. 计算结果表明,对于低频声学声子,只要通道的横向宽度相同,各通道中最低阶模的透射概率几乎不受其他结构参数的影响,且其数值都接近于0.25;而高频声学声子在各通道中的透射概率与结构参数密切相关,不同通道中的透射概率不同;当温度非常低时,各通道的热导都接近于量子化热导π²k²_BT/(3h)的四分之一;随着温度的升高,各通道的热导增减 关键词： 声学声子输运 热导 量子结构     

含双T形量子结构的量子波导中声学声子输运和热导

采用散射矩阵方法, 研究了含双T形量子结构的量子波导中声学声子输运和热导性质. 结果表明: 在极低温度, 双T形量子结构能增强低温热导; 相反地, 在相对较高的温度范围, 双T形量子结构能降低低温热导. 而在整个低温范围内, 增加散射区域最窄处的宽度能增强低温热导. 计算结果表明可以通过调节含双T形量子结构的量子波导结构来调控声子的输运概率和热导. 关键词： 声学声子输运 热导 量子结构     

量子点调制的一维量子波导中声学声子输运和热导

利用散射矩阵方法,比较了被一维凸形量子点、凹形量子点调制的量子线中膨胀模的声子输运和热导性质. 研究结果表明: 声子的输运概率与热导受制于量子点几何结构,具有凸形量子点结构的量子线中声子输运概率与热导K_CV大于具有凹形量子点结构的量子线中声子输运概率与热导K_CC. 两者热导之比K_CV/K_CC依赖于一维量子点的具体结构,且随着温度及主量子线与量子点横截面的边长差Δ_SL的增加而增加. 两种具有不同散射结构的一维量子线中热输运性质的区别在于凸形量子点结构中膨胀模数量总是大于凹形量子点结构中膨胀模数量的缘故. 关键词： 声学声子输运 热导 量子结构     

边界条件对介电量子波导中声子输运性质的影响

运用散射矩阵方法，研究了Neumann边界条件和Dirichlet边界条件在低温下对结构不连续的纳米结构中的声学声子输运系数的影响.数值结果表明，当边界条件不同时，声学声子输运系数会有极大的不同；在一定的结构条件下，由于声子模与模的耦合作用，出现了共振透射和禁止频带.     

结构缺陷对量子波导腔中热导的调控

利用散射矩阵方法和标量模型,研究了低温下可调缺陷对量子波导腔的热导的影响. 改变缺陷的参数能控制热导,缺陷的尺寸和位置能导致热导的改变,而且不同种类的缺陷也能导致热导随温度的变化. 关键词： 声学声子输运 热导 量子结构     

低维纳米材料量子热输运与自旋热电性质 ——非平衡格林函数方法的应用

低维材料不断涌现的新奇性质吸引着科学研究者的目光. 除了电子的量子输运行为之外, 人们也陆续发现和确认了热输运中显著的量子行为, 如 热导低温量子化、声子子带、尺寸效应、瓶颈效应等. 这些小尺度体系的热输运性质可以很好地用非平衡格林函数来描述. 本文首先介绍了量子热输运的特性、声子非平衡格林函数方法及其在低维纳米材料中的研究进展; 其次回顾了近年来在 一系列低维材料中发现的热-自旋输运现象. 这些自旋热学现象展现了全新的热电转换机制, 有助于设计新型的热电转换器件, 同时也给出了用热产生自旋流的新途径; 最后介绍了线性响应理论以及在此理论框架下结合声子、电子非平衡格林函数方法进行的一些有益的探索. 量子热输运的研究对热效应基础研究以及声子学器件、能量转换器件的发展有着不可替代的重要作用.     

多空穴错位分布对石墨纳米带中热输运的影响

利用非平衡格林函数方法研究了石墨纳米带中三空穴错位分布对热输运性质的影响.研究结果发现:三空穴竖直并排结构对低频声子的散射较小,导致低温区域三空穴竖直并排时热导最大,而在高频区域,三空穴竖直并排结构对高频声子的散射较大,导致较高温度区域三空穴竖直并排时热导最小;三空穴的相对错位分布仅能较大幅度地调节面内声学模高频声子的透射概率,而三空穴的相对错位分布能较大幅度地调节垂直振动膜高频声子和低频声子的透射概率,导致三空穴的相对错位分布不仅能大幅调节面内声学模和垂直振动模的高温热导,也能大幅调节垂直振动模的低温热导.研究结果阐明了空穴位置不同的石墨纳米带的热导特性,为设计基于石墨纳米带的热输运量子器件提供了有效的理论依据.     

微微秒时间间隔的晶格动力学

声学声子和光学声子的寿命以及寿命随温度、频率的变化影响固体的许多性质.例如,声学声子寿命对绝缘休、半导体和非晶固体的热输运性质的影响是极为重要的.光学声子的衰减动力学过程和光学声子与其它激发的耦合,对半导体的热电子输运,绝缘体中非平衡高频声学声子的产生等问题都起十分重要的作用.因此对声学及光学声子寿命的研究已成为固体物理中基本的和极感兴趣的课题. 高温下或在有短的平均自由程的材料中,声学声子激发的寿命通常是微微秒的范围.而频率为5—10THz的光学声子的寿命则是毫微微秒量级.最近两年,用微微秒与毫微微秒激光脉冲…     

复杂势场量子弹球中疤痕态的量子化条件

量子疤痕是波函数在经典不稳定周期轨道周围反常凝聚的一种量子或波动现象.人们对疤痕态的量子化条件进行了大量研究,对深入理解半经典量子化起到了一定的促进作用.之前大部分研究工作主要集中在硬墙量子弹球上,即给定边界形状的无穷深量子势阱系统.本文研究具有光滑复杂势场的二维量子弹球系统,考察疤痕态的量子化条件及其重复出现的规律,得到了与硬墙弹球不一样的结果,对理解这类现象是一个有益的补充.这些结果将有助于理解具有无规长程杂质分布的二维电子系统的态密度谱和输运行为.     

异侧非重叠三封闭端量子波导中的声子输运与热导

研究了异侧非重叠三封闭端量子波导中的声学声子传输和热导率性质.结果表明:由于激发模的产生,总传输系数在整数约化频率的时发生跳跃;各个激发模所产生的温度条件不一样,温度越高,被激发的模越多,并且高阶模对热导的影响较小;声子传输和热导性质与不连续结构的形状和位置有直接的关系,声子传输和热导性质对量子线的温度环境相当敏感.     

Thermal conductance in a quantum waveguide modulated with quantum dots

We investigate the thermal conductance in a quantum waveguide modulated with quantum dots at low temperatures. It is found that the thermal conductance sensitively depends on the geometrical parameters of the structure and boundary conditions. When the stress-free boundary conditions are applied in the structure, the universal quantum of thermal conductance can be found regardless of the geometry details in the limit T→0. For an uniform quantum waveguide, a thermal conductance plateau can be observed at very low temperatures; while for the quantum waveguide modulated with quantum dots, the plateau disappears, instead a decrease of the thermal conductance can be observed as the temperature goes up in the low temperature region, and its magnitude can be adjusted by the radius of the quantum dot. Moreover, it is found that the quantum waveguide with two coupling quantum dots exhibits oscillatory decaying thermal conductance behavior with the distance between two quantum dots. However, when the hard-wall boundary conditions are applied, the thermal conductance displays different behaviors.     

The influence of boundary conditions on thermal conductance in semiconductor quantum wire with structural defect

Using the scattering-matrix method, we investigate the influences of boundary conditions on thermal conductance in quantum wire with structural defect. A comparison between the thermal conductances is made when stress-free, hard-wall, and mixed boundary conditions are applied for acoustic transport leads. The results show that the quantized thermal conductance plateau at very low temperature can be observed only in transport lead with stress-free boundary condition. For hard-wall or mixed boundary conditions, qualitatively different thermal conductance characteristics are found. Moreover, we find that the behavior of the thermal conductance sensitively depend on the geometric parameters and the position of the defect in quantum wire.     

Acoustic phonons transport in a quantum waveguide embedded double defects

By using scattering matrix method, we investigate the acoustic phonons transport in a quantum waveguide embedded double defects at low temperatures. When acoustic phonons propagate through the waveguide, the total transmission coefficient versus the reduced phonon frequency exhibits a series of resonant peaks and dips, and acoustic waves interfere with each other in the waveguide to form standing wave with particular wavelengths. In the waveguide with void defects, acoustic phonons whose frequencies approach zero can transport without scattering. The acoustic phonons propagating in the waveguide with clamped material defects, the phonons frequencies must be larger than a threshold frequency. It is also found that the thermal conductance versus temperature is qualitatively different for different types of defects. At low temperatures, when the double defects are void, the universal quantum thermal conductance and a thermal conductance plateau can be clearly observed. However, when the double defects consist of clamped material, the quantized thermal conductance disappears but a threshold temperature where mode 0 can be excited emerges. The results can provide some references in controlling thermal conductance artificially and the design of phonon devices.     

Acoustic phonons transport in a quantum waveguide embedded double defects

By using scattering matrix method, we investigate the acoustic phonons transport in a quantum waveguide embedded double defects at low temperatures. When acoustic phonons propagate through the waveguide, the total transmission coefficient versus the reduced phonon frequency exhibits a series of resonant peaks and dips, and acoustic waves interfere with each other in the waveguide to form standing wave with particular wavelengths. In the waveguide with void defects, acoustic phonons whose frequencies approach zero can transport without scattering. The acoustic phonons propagating in the waveguide with clamped material defects, the phonons frequencies must be larger than a threshold frequency. It is also found that the thermal conductance versus temperature is qualitatively different for different types of defects. At low temperatures, when the double defects are void, the universal quantum thermal conductance and a thermal conductance plateau can be clearly observed. However, when the double defects consist of clamped material, the quantized thermal conductance disappears but a threshold temperature where mode 0 can be excited emerges. The results can provide some references in controlling thermal conductance artificially and the design of phonon devices.     

Heat transport in a four-perpendicularity-bend quantum waveguide

Using the scattering matrix method, we investigate acoustic phonon transmission and thermal conductance in a four-perpendicularity-bend quantum waveguide at low temperatures. The transmission spectrum of the quantum waveguide displays a series of resonant peaks and dips; and when one of the bend heights is larger than or equal to the minimum of the dimensions of the phonon channel in the quantum waveguide, a stop-frequency gap will appear; and some single four-perpendicularity-bend quantum waveguides with larger bend heights exhibit narrower width or smaller number of the stop-frequency gaps than that with smaller bend heights. The thermal conductivity is much sensitive to the change of the smaller heights and longitudinal lengths of the bend section; and the thermal conductivity decreases with the increasing of the temperature first, then increases after it reaches a minimum. The investigations of multiple four-perpendicularity-bend waveguides connected in series indicate that the first additional waveguide suppresses the transmission coefficient and forms the stop-frequency gap; and two additional resonance peaks will be formed when each four-perpendicularity-bend waveguide is added in the series. The results could be useful for controlling thermal conductance artificially and the design of phonon devices.     

异侧非重叠三封闭端量子波导中的声子输运与热导

研究了异侧非重叠三封闭端量子波导中的声学声子传输和热导率性质。结果表明：由于激发模的产生,总传输系数在整数约化频率的时发生跳跃;各个激发模所产生的温度条件不一样,温度越高,被激发的模越多,并且高阶模对热导的影响较小;声子传输和热导性质与不连续结构的形状和位置有直接的关系,声子传输和热导性质对量子线的温度环境相当敏感。     

Enhancement of ballistic thermal conductance in three dimensional double stub structures

We investigate ballistic thermal conductance in three dimensional double stub structures by using the scattering-matrix method. It is found that in such structures the thermal conductance can be clearly enhanced by inhomogeneities despite of the phonon scattering. When the stress-free boundary condition is employed, the enhancement can be observed only at very low temperature, and such the enhancement is strongly sensitive to the coupling length between two stubs. However, when the hall-wall boundary condition is employed, the enhancement can be observed in all explored temperature range, and becomes insensitive to the coupling length. A brief analysis of these results is given.     

Different effect of evanescent modes on acoustic phonon transport in different types of a three-dimensional quantum wire

By the use of the scattering matrix method, we investigate the effect of evanescent modes on acoustic phonon transport and thermal conductance in both convex and concave type three-dimensional quantum wire. Our results show that the evanescent modes can enhance the transmission coefficient and the thermal conductance in the concave type three-dimensional quantum wire. However, for the convex type three-dimensional quantum wire, the evanescent modes can play adverse effect on the phonon transport. When the length of scattering region is large enough, for all types of three-dimensional quantum wire, the influence of evanescent modes on phonon transport becomes very weak.