Phonon Transport and Thermal Conductivity in a Four-Terminal Structure

With the help of scattering-matrix method, the acoustic phonon ballistic transmission and the thermal conductivity are studied detailedly in a four-terminal structure. We find that the transmission coefficients and the reduced thermal conductance for each region sensitively depend on geometric parameters, and are of quantum character, but the reduced total thermal conductance for all regions seems independent of structure parameters when the temperature is not very low. Our results show that one can control the thermal conductivity for each region to match practical requirements in devices by adjusting the geometric parameters.     

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.     

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     

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.     

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     

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.     

Thermal conductance associated with six types of vibration modes in quantum wire modulated with quantum dot

We study the ballistic phonon transport and thermal conductance of six low-lying vibration modes in quantum wire modulated with quantum dot at low temperatures. A comparative analysis is made among the six vibrational modes. The results show that the transmission rates of the six vibrational modes relative to reduced frequency display periodic or quasi-periodic oscillatory behavior. Among the four acoustic modes, the thermal conductance contributed by the torsional mode is the smallest, and the thermal conductances of other acoustic modes have adjacent values. It is also found that the thermal conductance of the optical mode increases from zero monotonously. Moreover, the total thermal conductance in concavity-shaped quantum structure is lower than that in convexity-shaped quantum structure. These thermal conductance values can be adjusted by changing the structural parameters of the quantum dot.     

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.     

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.     

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.     

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     

Thermal transport in four-terminal graphene nano-junctions

The thermal transport properties of four-terminal graphene nano-junctions (FGNJs) consisting of semi-infinite armchair-edged nanoribbon and zigzag-edged nanoribbon were calculated. The thermal transport in FGNJs is sensitive to their geometric shape. The thermal conductance of FGNJs depends on the width of semi-infinite graphene nanoribbons and center region. These thermal transport phenomena can be explained by analyzing the phonon transmission coefficient. Compared with previous thermal rectifiers, reverse modulation can be obtained by changing the width of the thermal terminal. The results provide significant physical models and theoretical validity in designing the thermal devices based on the graphene nano-junctions.     

内嵌定向高导热层疏导式结构热防护机理分析

针对飞行器高超声速飞行时严重的气动加热环境, 提出内嵌定向高导热层的疏导式热防护系统. 运用数值方法分析了特定条件下内嵌定向高导热层的疏导式系统的防热效果, 外壁面最高温度下降了9.1%, 内壁面最高温度下降了31.5%, 高温区和低温区都被封闭在外层区域, 内层温度更加均匀, 实现了热流由高温区向低温区的转移, 削弱了高温区的热载荷, 强化了整体结构的热防护能力. 研究表明, 随着气动热流密度比与辐射散热面积比的增大, 疏导结构的冷却效果增强. 本文还对疏导防热系统的结构参数和材料参数对冷却效果的影响进行了分析, 为结构的设计和材料的选取提供一定的依据.     

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.     

Material properties dependent on the thermal transport in a cylindrical nanowire

Using the elastic wave continuum model, we investigate the effect of material properties on ballistic thermal transport in a cylindrical nanowire. A comparative analysis for the convexity-shaped and concavity-shaped structure is made. It is found that in the convexity-shaped structure, the material with higher wave velocity in the convexity region can increase the thermal conductance at the lower temperature range; the thermal conductance of the nanowire with higher wave velocity in the convexity region is lower than that of the nanowire with lower wave velocity in the convexity region at the higher temperature range. However, in the concavity-shaped structure, the material properties of the concavity region have less effect on the thermal conductance at the lower temperature range; the material with higher wave velocity in the concavity region can reduce the thermal conductance at the higher temperature range. A brief analysis of these results is given.     

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.     

Modulated thermal transport for flexural and in-plane phonons in double-stub graphene nanoribbons

Thermal transport properties are investigated for out-of-plane phonon modes(FPMs) and in-plane phonon modes(IPMs) in double-stub graphene nanoribbons(GNRs). The results show that the quantized thermal conductance plateau of FPMs is narrower and more easily broken by the double-stub structure. In the straight GNRs, the thermal conductance of FPMs is higher in the low temperature region due to there being less cut-off frequency and more low-frequency excited modes. In contrast, the thermal conductance of IPMs is higher in the high temperature region because of the wider phonon energy spectrum. Furthermore, the thermal transport of two types of phonon modes can be modulated by the double-stub GNRs, the thermal conductance of FPMs is less than that of IPMs in the low temperatures, but it dominates the contribution to the total thermal conductance in the high temperatures. The modulated thermal conductance can provide a guideline for designing high-performance thermal or thermoelectric nanodevices based on graphene.     

Four-terminal thermal conductance of mesoscopic dielectric systems

A four-terminal thermal conductance formula for a mesoscopic dielectric system with arbitrary central scattering region is derived. Similar to four-terminal electric conductance, the four-terminal thermal conductance also has a set of Onsager relations. In the temperature T-->0 limit, in contrast to the two-terminal thermal conductance which is a monotonic function of T and tends to zero, the four-terminal thermal conductance is nonmonotonic and tends to infinity. We also find that temperatures of the two terminals without thermal flux become very close to each other at low temperatures. Rather different behaviors are found for systems satisfying fractional exclusion statistics.     

Low-temperature anisotropy of the thermal conductivity of single-crystal nanofilms and nanowires

The effect of phonon focusing on the phonon transport in single-crystal nanofilms and nanowires is studied in the boundary scattering regime. The dependences of the thermal conductivity and the free path of phonons on the geometric parameters of nanostructures with various elastic energy anisotropies are analyzed for diffuse phonon scattering by boundaries. It is shown that the anisotropies of thermal conductivity for nanostructures made of cubic crystals with positive (LiF, GaAs, Ge, Si, diamond, YAG) and negative (CaF₂, NaCl, YIG) anisotropies of the second-order elastic moduli are qualitatively different for both nanofilms and nanowires. The single-crystal film plane orientations and the heat flow directions that ensure the maximum or minimum thermal conductivity in a film plane are determined for the crystals of both types. The thermal conductivity of nanowires with a square cross section mainly depends on a heat flow direction, and the thermal conductivity of sufficiently wide nanofilms is substantially determined by a film plane orientation.