Effect of structural defect on phonon transmission quantization in low-dimensional superlattices

Using the scattering-matrix cascading method, we investigate the effect of structural defect on the acoustic phonon transmission quantization in a low-dimensional superlattice quantum waveguide. In the present system, the phonon transmission exhibits rather complex resonant behaviors. It is found that a lateral defect in an otherwise periodic structure modifies the transversal phonon transmission spectra nontrivially and completely washes away the transmission quantization due to the strong additional scattering by defect. However, the appreciable quantization survives in the presence of a longitudinal defect. Our results also show that by adjusting the geometric parameters of the structural defect, one can control the phonon transport of the structure to match practical requirements in devices.     

Phonon Transmission and Thermal Conductance in Fibonacci Wire at Low Temperature

We investigate the phonon transmission and thermal conductance in a general Fibonacci quasicrystal by the model of lattice dynamics and the technique of transfer matrix. It is found that quasiperiodic distribution of masses may greatly destroy the phonon transport at both low and high frequencies and thus may affect the thermal conductance. The thermal conductance increases with temperature at low temperatures and displays saturation with further increase of the temperature. Such saturation behaviour is preserved even when the mass ratio of atoms in the Fibonacci chain is changed.     

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.     

Phonon Transport and Thermal Conductivity in an Acoustic Filter

We investigate the phonon ballistic transmission and the thermal conductivity in a dielectric quantum structure. It is found that these observable quantities sensitively depend on geometric parameters, and are of quantum character. The total transmission coetfficient as a function of the reduced waveguide-length exhibits periodical behaviour and the reduced thermal conductance decreases below the ideal universal value for the low temperature. Our results show that one can control the thermal conductivity of the structure and make all kinds of acoustic filters to match practical requirements in devices by adjusting the geometric parameters.     

Nonequilibrium Green's function approach to phonon transport in defective carbon nanotubes

We have developed a new theoretical formalism for phonon transport in nanostructures using the nonequilibrium phonon Green's function technique and have applied it to thermal conduction in defective carbon nanotubes. The universal quantization of low-temperature thermal conductance in carbon nanotubes can be observed even in the presence of local structural defects such as vacancies and Stone-Wales defects, since the long wavelength acoustic phonons are not scattered by local defects. At room temperature, however, thermal conductance is critically affected by defect scattering since incident phonons are scattered by localized phonons around the defects. We find a remarkable change from quantum to classical features for the thermal transport through defective carbon nanotubes with increasing temperature.     

嵌入线型缺陷的石墨纳米带的热输运性质

采用非平衡格林函数方法研究了嵌入有限长、半无限长、 无限长线型缺陷的锯齿型石墨纳米带 (ZGNR)的热输运性质.结果表明, 缺陷类型和缺陷长度对ZGNR的热导有重要影响. 当嵌入的线型缺陷长度相同时, 包含t5t7线型缺陷的石墨纳米带比包含Stone-Wales线型缺陷的条带热导低. 对于嵌入有限长、同种缺陷的ZGNR, 其热导随线型缺陷的长度增加而降低, 但是当线型缺陷很长时, 其热导对缺陷长度的变化不再敏感.通过比较嵌入有限长、半无限长、无限长线型缺陷的ZGNR, 我们发现嵌入无限长缺陷的条带比嵌入半无限长缺陷的条带热导高, 而后者比嵌入有限长线型缺陷的条带热导高. 这主要是因为在这几种结构中声子传输方向的散射界面数不同所导致的. 散射界面越多, 对应的热导就越低. 通过分析透射曲线和声子局域态密度图, 解释了这些热输运现象. 这些研究结果表明线型缺陷能够有效地调控石墨纳米带的热输运性质. 关键词： 石墨烯 线型缺陷 热导     

Thermal properties of carbon nanotubes and nanotube-based materials

The thermal properties of carbon nanotubes are directly related to their unique structure and small size. Because of these properties, nanotubes may prove to be an ideal material for the study of low-dimensional phonon physics, and for thermal management, both on the macro- and the micro-scale. We have begun to explore the thermal properties of nanotubes by measuring the specific heat and thermal conductivity of bulk SWNT samples. In addition, we have synthesized nanotube-based composite materials and measured their thermal conductivity. The measured specific heat of single-walled nanotubes differs from that of both 2D graphene and 3D graphite, especially at low temperatures, where 1D quantization of the phonon bandstructure is observed. The measured specific heat shows only weak effects of intertube coupling in nanotube bundling, suggesting that this coupling is weaker than expected. The thermal conductivity of nanotubes is large, even in bulk samples: aligned bundles of SWNTs show a thermal conductivity of >200 W/m K at room temperature. A linear K(T) up to approximately 40 K may be due to 1D quantization; measurement of K(T) of samples with different average nanotube diameters supports this interpretation. Nanotube–epoxy blends show significantly enhanced thermal conductivity, showing that nanotube-based composites may be useful not only for their potentially high strength, but also for their potentially high thermal conductivity. Received: 17 October 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

固-固接触热传导的声子传递系数

声子传递系数是影响低温接触界面热传导的重要因素,文中对中间低温(20K~200K)接触传导模型的声子传递系数进行了讨论,分析了接触界面温差、弹性镜面传递和散射传递下的声子传递系数;还讨论了热流方向对声子传递系数的影响;指出了声失配理论预测值与实验值间存在差别的可能原因。该讨论对分析接触热传导有一定意义。     

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。     

Effects of contact shape on ballistic phonon transport in semiconductor nanowires

We study the effects of contact shape on ballistic phonon transport in semiconductor nanowires at low temperatures using an approximative scalar model of continuum elasticity. Five different contacts connected to two semiconductor nanowires with different transverse widths are discussed. Numerical results show that the contact shape acts as an ‘acoustic impedance adaptor’, playing a crucial role on the ballistic phonon transmission and thermal conductance. The phonon coupling in the contacts with certain length facilitates ballistic phonon transmission compared to the abrupt interface, in which the phonon scattering is the strongest. It is found that the more the contact is abrupt, the smaller the thermal conductance is. The catenoidal contact rather than the abrupt interface is also the competitive candidate to obtain bigger thermal conductance. These results indicate that choosing an appropriate contact shape is one of the most critical factors to accurately measure the thermal conductance with a very high precision and reliability in different temperature ranges at low temperatures.     

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.     

Universal features of quantized thermal conductance of carbon nanotubes

The universal features of quantized thermal conductance of carbon nanotubes (CNTs) are revealed through a theoretical analysis based on the Landauer theory of heat transport. The phonon-derived thermal conductance of semiconducting CNTs exhibits a universal quantization in the low-temperature limit, independent of the radius or atomic geometry. The temperature dependence follows a single curve given in terms of temperature scaled by the phonon energy gap. The thermal conductance of metallic CNTs has an additional contribution from electronic states, which also exhibits quantized behavior up to room temperature.     

Phonon transmission and thermal conductance in one-dimensional system with on-site potential disorder

The role of on-site potential disorder on phonon transmission and thermal conductance of one-dimensional system is investigated. We found that the on-site potential disorder can lead to the localization of phonons, and has great effect on the phonon transmission and thermal conductance of the system. As on-site potential disorder W increases, the transmission coefficients decrease, and approach zero at the band edges. Corresponding, the thermal conductance decreases drastically, and the curves for thermal conductance exhibit a series of steps and plateaus. Meanwhile, when the on-site potential disorder W is strong enough, the thermal conductance decreases dramatically with the increase of system size N. We also found that the efficiency of reducing thermal conductance by increasing the on-site potential disorder strength is much better than that by increasing the on-site potential?s amplitude.     

螺旋纳米带中的声子输运

运用微分几何方法及形式散射理论,研究螺旋纳米带中的标量声子输运问题,计算了声子透射系数及热导率.数值结果表明,弯曲导致了声子模式之间的量子干涉,使总透射系数随能量变化的量子化台阶呈现振荡行为,有效地抑制了热导率. 关键词： 螺旋纳米带 声子输运 形式散射理论 微分几何方法     

含半圆弧形腔的量子波导中声学声子输运和热导特性

采用散射矩阵方法,研究了在应力自由和硬壁两种典型的边界条件下含半圆弧形腔的量子波导中声学声子输运和热导性质.结果表明在两种边界条件下声子透射谱和热导有着不同的特征.在应力自由边界条件下,能观察到普适的量子化热导现象,当结构为一理想的量子线时,在低温区域有一个量子化平台出现,而当半圆弧形结构存在时,非均匀横向宽度引发的弹性散射使得量子化平台被破坏;在硬壁边界条件下,不可能观察到量子化热导现象,热导随温度的增加单调上升;计算结果表明还可以通过调节半圆弧形结构的半径来调控声子的输运概率和热导. 关键词： 声学声子输运 热导 量子体系     

多通道石墨纳米带中弹性声学声子输运和热导特性

采用非平衡格林函数方法, 在保持总的能量输出通道中石墨链数不变的条件下, 研究并比较了并列的石墨纳米带通道中弹性声学声子输运和热导特性. 结果表明, 能量输出通道的增加能降低每个能量输出通道的热导; 与能量输入热库最近的能量输出通道热导最大, 最远的能量输出通道热导最小; 中间能量输出通道的热导性质与并列的各输出通道的结构参数密切相关, 最近和最远的能量输出通道的热导性质仅与各自能量输出通道的结构参数有关; 粗糙边缘结构能有效调节各通道的热导; 总的热导性质与能量输出通道石墨链数、能量输出通道数以及边缘结构粗糙程度密切相关.     

Phonon thermal conductance of disordered graphene strips with armchair edges

Based on the model of lattice dynamics together with the transfer matrix technique, we investigate the thermal conductances of phonons in quasi-one-dimensional disordered graphene strips with armchair edges using Landauer formalism for thermal transport. It is found that the contributions to thermal conductance from the phonon transport near von Hove singularities is significantly suppressed by the presence of disorder, on the contrary to the effect of disorder on phonon modes in other frequency regions. Besides the magnitude, for different widths of the strips, the thermal conductance also shows different temperature dependence. At low temperatures, the thermal conductance displays quantized features of both pure and disordered graphene strips implying that the transmission of phonon modes at low frequencies are almost unaffected by the disorder.     

Ballistic phonon thermal transport across topologically structured nanojunctions on Gold wires

We investigate the coherent phonon thermal transport at low temperatures in Gold nanowires, in order to study the effects of scattering on the lattice thermal conductivity. Three types of shaped joint nanostructures are employed in our calculation. We present a detailed study of the thermal conductance as a function of the temperature for different shaped joint. This is done by solving the phonon Boltzmann transport equation in the ballistic regime and calculating the transmission rates of the vibration modes through the consideration of the phonon group velocity modification in the system. The transmission properties are calculated by use of the matching method in the harmonic approximation with nearest and next nearest neighbor force constants. The results show that the transmission probabilities depend on the type of joint nanostructure. The pronounced fluctuations of the transmission spectra as a function of the frequency can be understood as Fano resonances. It is also found that the behavior of the thermal conductance versus temperature is qualitatively different for different nanostructures and depends sensitively on the width of the shaped joint.     

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     

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.