Effect of normal processes on thermal conductivity of germanium,silicon and diamond

The effect of normal scattering processes is considered to redistribute the phonon momentum in (a) the same phonon branch — KK-S model and (b) between different phonon branches — KK-H model. Simplified thermal conductivity relations are used to estimate the thermal conductivity of germanium, silicon and diamond with natural isotopes and highly enriched isotopes. It is observed that the consideration of the normal scattering processes involving different phonon branches gives better results for the temperature dependence of the thermal conductivity of germanium, silicon and diamond with natural and highly enriched isotopes. Also, the estimation of the lattice thermal conductivity of germanium and silicon for these models with the consideration of quadratic form of frequency dependences of phonon wave vector leads to the conclusion that the splitting of longitudinal and transverse phonon modes, as suggested by Holland, is not an essential requirement to explain the entire temperature dependence of lattice thermal conductivity whereas KK-H model gives a better estimation of the thermal conductivity without the splitting of the acoustic phonon modes due to the dispersive nature of the phonon dispersion curves.      

Effects of surface charges on phonon properties and thermal conductivity in GaN nanofilms

Surface charges can modify the elastic modulus of nanostructure, leading to the change of the phonon and thermal properties in semiconductor nanostructure. In this work, the influence of surface charges on the phonon properties and phonon thermal conductivity of GaN nanofilm are quantitatively investigated. In the framework of continuum mechanics,the modified elastic modulus can be derived for the nanofilm with surface charges. The elastic model is presented to analyze the phonon properties such as the phonon dispersion relation, phonon group velocity, density of states of phonons in nanofilm with the surface charges. The phonon thermal conductivity of nanofilm can be obtained by considering surface charges. The simulation results demonstrate that surface charges can significantly change the phonon properties and thermal conductivity in a GaN nanofilm. Positive surface charges reduce the phonon energy and phonon group velocity but increase the density of states of phonons. The surface charges can change the size and temperature dependence of phonon thermal conductivity of GaN nanofilm. Based on these theoretical results, one can adjust the phonon properties and temperature/size dependent thermal conductivity in GaN nanofilm by changing the surface charges.     

表面粗糙度对薄膜面向热导率的影响研究

通过求解声子辐射输运方程(EPRT)计算得到了薄膜的面向晶格热导率.在薄膜界面采用与声子波长相关的镜反射率模型,考虑了薄膜的厚度、温度和表面粗糙度等对其热导率的影响.结果表明,界面粗糙度对薄膜热导率的影响很大.减小界面粗糙度,会使得薄膜热导率大大增加.另外,薄膜厚度减小使得热导率峰值对应的温度增加.     

Theory of the phonon Hall effect in paramagnetic dielectrics

Based upon Raman spin-lattice interaction, we propose a theoretical model for the phonon Hall effect in paramagnetic dielectrics, which was discovered recently in an experiment [C. Strohm, G. L. J. A. Rikken, and P. Wyder, Phys. Rev. Lett. 95, 155901 (2005).]. The phonon Hall effect is revealed to be a phonon analogue to the anomalous Hall effect in electron systems. The thermal Hall conductivity is calculated by using the Kubo formula. Our theory reproduces the essential experimental features of the phonon Hall effect, including the sign, magnitude, and linear magnetic field dependence of the thermal Hall conductivity.     

具有分离门电抽运石墨烯中电子-空穴等离子体的冷却效应

本文研究了室温条件下具有分离门的电诱导石墨烯n-i-p结构中, 与电子和空穴注入有关的粒子数反转效应. 考虑n区横向电场的屏栅效应, 计算了电子-空穴的有效温度与门电压以及光声子的有效温度与门电压的关系, 结果表明注入可以导致n区中电子-空穴等离子体显著冷却, 直至低于晶格温度; 计算了电流-电压特性以及与频率有关的动态电导率, 在一定的电压下, 动态电导率在太赫兹频段可以为负值. 研究表明电子-空穴等离子体冷却能够加强负动态电导率效应, 提高实现太赫兹激射的可行性. 关键词： 石墨烯 n-i-p结构 有效温度 动态电导率     

Resonant phonon scattering in a coupled spin-phonon system

From a perturbation treatment of phonon scattering by a spin system of arbitrary concentration, we derive the frequency and concentration dependence of the phonon relaxation time, which are important for the analysis of thermal conductivity experiments. Comparison is made with theories of coupled spin-phonon modes based on Green-function methods.     

Thermal conductivity of NbSe3

The thermal conductivity, κ, of NbSe₃ has been measured by novel self-heating techniques that allowed the electric field dependence of κ to also be measured. Measurements were made from 35 K to room temperature. Above the charge density wave transitions, the phonon thermal conductivity is 4–7 times the electron thermal conductivity, and it rises smoothly below the transitions, indicating that phonon-phonon scattering predominates. Phonon mean free parths have been estimated at 187 A° at 60 K and 60 A° at 150 K. No clear anomalies were observed at the phase transitions, giving upper limits to changes in the phonon mean free path. No field dependence of κ was observed.     

Study of ac hopping conductivity on one—dimensional nanometre systems

In this paper, we establish a one-dimensional random nanocrystalline chain model, we derive a new formula of ac electron－phonon－field conductance for electron tunnelling transfer in one-dimensional nanometre systems. By calculating the ac conductivity, the relationship between the electric field, temperature and conductivity is analysed, and the effect of crystalline grain size and distortion of interfacial atoms on the ac conductance is discussed. A characteristic of negative differential dependence of resistance and temperature in the low-temperature region for a nanometre system is found. The ac conductivity increases linearly with rising frequency of the electric field, and it tends to increase as the crystalline grain size increases and to decrease as the distorted degree of interfacial atoms increases.     

Low-field carrier transport properties in biased bilayer graphene

Based on a semiclassical Boltzmann transport equation in random phase approximation, we develop a theoretical model to understand low-field carrier transport in biased bilayer graphene, which takes into account the charged impurity scattering, acoustic phonon scattering, and surface polar phonon scattering as three main scattering mechanisms. The surface polar optical phonon scattering of carriers in supported bilayer graphene is thoroughly studied using the Rode iteration method. By considering the metal–BLG contact resistance as the only one free fitting parameter, we find that the carrier density dependence of the calculated total conductivity agrees well with that observed in experiment under different temperatures. The conductivity results also suggest that in high carrier density range, the metal–BLG contact resistance can be a significant factor in determining the BLG conductivity at low temperature, and both acoustic phonon scattering and surface polar phonon scattering play important roles at higher temperature, especially for BLG samples with a low doping concentration, which can compete with charged impurity scattering.     

Temperature and concentration dependences of thermal conductivity of solid solutions of gadolinium and dysprosium sulfides

Thermal conductivity of a number of solid solutions of gadolinium and dysprosium sulfides has been studied experimentally within the temperature range 80-400 K. The work offers the data on thermal conductivity coefficient and lattice thermal conductivity of the studied samples. It was found that replacement of gadolinium ions by dysprosium ions leads to significant decrease of the samples?? thermal conductivity and changes its temperature dependence character due to the resonance scattering of phonons by paramagnetic ions of dysprosium. Influence of this mechanism of phonon scattering conditions the area of anomalous change observed on the concentration dependence of thermal conductivity coefficient.     

金属纳米颗粒的热导率

本文使用统计模拟方法对金属纳米颗粒的电子平均自由程进行了计算,并考察了纳米颗粒的晶格比热和声子平均群速度,最后应用动力学理论对纳米颗粒的电子热导率和声子热导率分别进行了求解.研究结果表明:具有相同特征尺寸的方形、球形纳米颗粒的无量纲电子(或声子)平均自由程比较接近.金属纳米颗粒的电子热导率远大于声子热导率;电子、声子热导率随着直径减小呈现降低趋势,而电子热导率的颗粒尺度依赖性比声子热导率更为明显;随着颗粒直径进一步减小,声子热导率与电子热导率趋于同一数量级.当纳米颗粒特征尺寸大于4倍块材电子(或声子)平均自由程,其电子(或声子)热导率的颗粒尺度依赖性将减弱.     

Electron-phonon scattering and conductivity of a quantum cylinder in a magnetic field

The contribution of electron-phonon scattering to conductivity of a quantum cylinder in a longitudinal magnetic field has been studied. It has been shown that the conductivity of the nanotube undergoes Aharonov-Bohm oscillations with variations in the magnetic flux through the nanotube cross section. The formulas describing the temperature dependence of the resistance of the nanostructure both in the case of an isotropic phonon spectrum and with allowance for the effects of phonon confinement have been obtained in the analytical form.     

Negative differential conductance and hot phonons in suspended nanotube molecular wires

Freely suspended metallic single-walled carbon nanotubes (SWNTs) exhibit reduced current carrying ability compared to those lying on substrates, and striking negative differential conductance at low electric fields. Theoretical analysis reveals significant self-heating effects including electron scattering by hot nonequilibrium optical phonons. Electron transport characteristics under strong self-heating are exploited for the first time to probe the thermal conductivity of individual SWNTs (approximately 3600 W m-1 K-1 at T=300 K) up to approximately 700 K, and reveal a 1/T dependence expected for umklapp phonon scattering at high temperatures.     

Absence of quantum diffusion in a disordered elastic chain

A relation between localization of phonon modes in a disordered elastic chain and absence of quantum heat conductivity is shown. The frequency dependence of the localization length of normal modes is found at various degrees of disorder. The similarity of the problem of phonon localization and of electron localization is established.     

The effect of normal phonon-phonon scattering processes on the maximum thermal conductivity of isotopically pure silicon crystals

The effect of normal phonon-phonon scattering processes on the thermal conductivity of silicon crystals with various degrees of isotope disorder is considered. The redistribution of phonon momentum in normal scattering processes is taken into account within each oscillation branch (the Callaway generalized model), as well as between different oscillation branches of the phonon spectrum (the Herring mechanism). The values of the parameters are obtained that determine the phonon momentum relaxation in anharmonic scattering processes. The contributions of the drift motion of longitudinal and transverse phonons to the thermal conductivity are analyzed. It is shown that the momentum redistribution between longitudinal and transverse phonons in the Herring relaxation model represents an efficient mechanism that limits the maximum thermal conductivity in isotopically pure silicon crystals. The dependence of the maximum thermal conductivity on the degree of isotope disorder is calculated. The maximum thermal conductivity of isotopically pure silicon crystals is estimated for two variants of phonon momentum relaxation in normal phonon-phonon scattering processes.     

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.     

The Fano anti-resonance and IR-properties of organic conductors

At high temperatures, i.e. in the metallic state excitation of the IR-forbidden symmetric intramolecular modes is shown to become effective. Near the threshold of emission for such a phonon there appears a fine structure as a result of the acoustic phonon assisted excitation of the intramolecular mode by energetic band electrons. The absorption reveals the behaviour typical of the Fano anti-resonance. The infrared frequency dependence of the organic compounds conductivity is discussed.     

Lattice thermal conductivity of δ-graphyne — A molecular dynamics study

Thermal conductivity of δ-graphyne was investigated using reverse non-equilibrium molecular dynamics simulations. The dependence of the thermal conductivities with the temperature, acetylenic linkages, and external strain were explained by the phonon density of states. Our simulations revealed that as the temperature increased, the thermal conductivity of graphene first increased and then decreased, whereas that of δ-graphyne monotonically decreased. Owing to the presence of the acetylenic linkages, a significant reduction was found in the thermal conductivity of δ-graphyne, which resulted in a phonon vibration mismatch or weakened coupling. Moreover, the temperature profile changed from mono linear to the ladder the number of acetylenic linkages increased. These results play a guidance role in the design and application of thermoelectrics devices using 2D carbon materials.     

Plateau in the temperature dependence of the thermal conductivity of solid hydrogen containing heavy isotopic neon impurities at ultimately low concentrations

The thermal conductivity of solid hydrogen with 0.0001–0.0002 at. % Ne in the form of equilibrium samples grown at a low rate after remelting is investigated in the temperature range 1.5–10.0 K. It is demonstrated that the temperature dependence of the thermal conductivity for the samples containing neon at concentrations considerably lower than the limiting solubility of the heavy impurity exhibits a symmetric plateau. This behavior of the temperature dependence of the thermal conductivity differs qualitatively from the previously observed resonance minimum in the temperature dependence of the thermal conductivity for desublimated samples. A relaxation model is proposed for explaining the observed effect. According to this model, the plateau is explained by the formation of linear impurity structures that are located along dislocation lines and considerably enhance phonon scattering by dislocation cores. The density of linear impurity structures is estimated. The influence of these structures on the thermal conductivity is compared with the corresponding effect of uniformly distributed individual neon atoms in solid hydrogen.     

Low-temperature heat conductivity of ZnSe:Ni crystals: An unusually strong resonant change caused by phonon scattering from the anharmonic modes induced by charged impurities

An anomalously strong resonancelike change with a sharp minimum at a temperature of 15±1 K was observed for the heat conductivity of ZnSe:Ni crystals for the first time. The heat conductivity of one of the samples decreased by more than 200 times, as compared to its maximal value in pure ZnSe. A new phonon scattering mechanism including the A processes is suggested to interpret the unusual temperature dependence of heat conductivity. These are the umklapp processes accompanying the phonon scattering from the anharmonic modes induced by charged nickel impurities. As a result, the efficiency of A processes may be very high and comparable at 15 K to the efficiency of the U processes in ZnSe at a temperature of approximately 1000 K.