用子动力学理论研究介观体系的输运——电导响应函数的计算

从子动力学理论出发,计算了介观体系的输运公式,得到了包含高阶修正的Kubo公式,这对于任意外场的作用下的介观体系具有重要的意义。     

Anisotropy of graphite optical conductivity

The graphite conductivity is evaluated for frequencies between 0.1 eV, the energy of the order of the electronhole overlap, and 1.5 eV, the electron nearest hopping energy. The in-plane conductivity per single atomic sheet is close to the universal graphene conductivity e ²/4ħ and, however, contains a singularity conditioned by peculiarities of the electron dispersion. The conductivity is less in the c direction by the factor of the order of 0.01 governed by electron hopping in this direction.     

Hall conductivity of a spin-triplet superconductor

We calculate the Hall conductivity for a spin-triplet superconductor, using a generalized pairing symmetry dependent on an arbitrary phase phi. A promising candidate for such an order parameter is Sr2RuO4, whose superconducting order parameter symmetry is still subject to investigation. The value of this phase can be determined through Kerr rotation and dc Hall conductivity measurements. Our calculations impose significant constraints on phi.     

Electrical Conductivity of Non-ideal Plasmas and the Ion Distribution Function

The theory of electrical conductivity of electron-ion-systems is developed for a density region which reaches from the region of non-ideal plasmas up to the region of liquid metals. The conductivity is expressed by quantum mechanical correlation functions. Different forms of the electron-ion pseudopotentials are considered. The ion distribution function is derived using the mean spherical approximation (MSA) theory or the nonlinear Debye-theory. Higher order scattering effects are treated by introducing scattering phase shifts for the statically screened electron-ion potential. The numerical results for the conductivity show a SPITZER-like behaviour in the low-density non-degenerate limit where higher order scattering is important, and a ZIMAN-like behaviour in the strongly degenerate high-density limit where the ion distribution functions and the form of the electron-ion pseudopotential become more important.     

Electrical conductivity for Lloyd's disorder model in one dimension

We consider the electrical dc-conductivity for Lloyd's disorder model in one dimension. First we calculate the conductivity within the CPA and the MCPA theory of electronic transport. For all values of the disorder parameter γ, which is the fluctuation width of random site energies, the MCPA leads to a substantial reduction of the conductivity as compared with the CPA-treatment. Secondly we investigate the case of large disorder, γ → ∞, by an asymptotic expansion. It is found that the conductivity vanishes order by order in an expansion in γ^?1 which strongly suggests that it vanishes identically for all γ.     

Theoretical study of the lattice thermal conductivity in Ge framework semiconductors

The lattice thermal conductivity of Ge clathrates is investigated by evaluating the linear response theory heat current correlation functions using molecular dynamics. Clathrate crystals with and without guest atoms in their fullerane cages are studied. In comparison with that of diamond-phase Ge, the clathrate conductivity is reduced by approximately 1 order of magnitude due to the open framework itself. The addition of an encapsulated (rattling) Sr guest atom produces a further order of magnitude reduction in the conductivity, making it comparable to that of amorphous Ge. Our results are consistent with experiments, and have impact on the search for improved thermoelectric materials.     

Dense quark matter conductivity in ultra-intense magnetic field

Heavy-ion collisions generate a huge magnetic field of the order of 10¹⁸ G for a duration of about 0.2 fm/c. This time may become an order of magnitude longer if the electrical conductivity of quark matter is large. We calculate the conductivity in the regime of high density and show that contrary to naive expectations it only weakly depends on the magnetic field.     

Dynamical Conductivity of the Dilute Lorentz Gas with Spherically Symmetric Scatterers

The dynamical conductivity of the Lorentz gas with spherically symmetric potentials is studied to lowest order in the density of scatterers. The frequency-dependent friction coefficient is calculated from the Fourier transform of the force–force time-correlation function determined by the dynamics of a single scattering process. The corresponding dynamical conductivity varies with frequency on the scale of the inverse collision time. As an example, the conductivity is calculated for a scattering potential of the Maxwell type.     

QED Electrical Conductivity from the 3PI Effective Action

We study the electrical conductivity in QED using the resummed 3PI effective action to 3-loop order. The resulting expression for the conductivity is explicitly gauge invariant, and the integral equations that resum the pinching and collinear contributions are produced naturally by the formalism. All leading order terms are included, without the need for any kind of power counting arguments.     

Electrical conductivity of a fully ionized plasma at high frequencies

When the frequency of the electric field is of the order of, or larger than, the electron plasma frequency, kinetic equations are no longer valid. The Klimontovich equation linearized in the applied electric field is solved systematically by means of an expansion into powers of the square root of the plasma parameter. A general expression up to second order is obtained for the conductivity σ(ω, k). There exists a large overlap with the results of an earlier paper on the conductivity at low frequencies. This overlap is used to determine a cut-off parameter. The expression for the conductivity serves as a basis for the calculation of the influence of collisions on the dispersion relation for plasma waves. It appears that for small wavenumbers the influence of electron-ion collisions is dominant compared to electron-electron collisions.     

Thermal properties of continuously spun carbon nanotube fibres

As indicated by theory and experimental measurements individual carbon nanotubes (CNTs) have very high values of thermal conductivity. One of the challenges is to achieve high thermal conductivity in macroscopic assemblies of CNTs such as fibres, films and composites, paving the way to a wide range of applications. CNT fibres have tremendous potential in succeeding as the future materials for a variety of applications when properties at the nanoscale are translated to their macroscopic assemblies. In this paper we report the measurements of thermal conductivity of continuously spun CNT fibres and its dependence on temperature. Thermal conductivity measurements were performed using in-house built temperature sensing microscope probe. Specific thermal conductivity of CNT fibres showed an order of magnitude advantage over the traditional materials used for heat dissipation.     

Anisotropic impurity scattering in electrically quantized inversion layers

The Boltzmann Equation is solved for a two dimensional gas of charge carriers in order to calculate the conductivity of field-effect transistors. The charge carriers are assumed to be scattered by point charges. The scattering process is treated in Born approximation. The resulting conductivity tensor is in apparent disagreement with experimental results.     

光谱法研究闪电通道的导电特性

依据无狭缝光栅摄谱仪在青海获得的云对地闪电回击光谱信息,结合空气等离子体传输理论,用四种不同方法计算了同一闪电放电通道的电导率。结果表明：各种方法所得闪电核心通道的电导率数量级均为104 S·m-1;且同一通道内的电导率随通道高度的增加有减小的趋势;通道内电子与一次、二次电离离子的碰撞以及它们各自的碰撞对通道电导率的贡献不可忽略;用碰撞积分的方法计算闪电核心通道的电导率结果更为合理。在通道电导率的基础上估算了回击通道的放电电流,与辐射峰值电场实验资料所得的相应峰值电流相比,其结果在合理的范围内, 并进一步探讨了温度与电流放电特性的相关性, 为研究闪电放电电流提供了一条可行的途径。     

碳纳米管热传导的分子动力学模拟研究

采用改进的经验键序作用势描述碳原子间的相互作用,应用分子动力学方法和Green-Kubo函数计算了碳纳米管的热导率.在模拟中,使用了重叠计算的方法来计算热流相关函数,大大减少了模拟步数.计算结果表明,碳纳米管的热导率以原子间作用力相互做功所引起的热流形式为主;热导率的值随着直径的增加而减小;在室温下,热导率的值随着温度的增加而增加,达到室温后逐渐收敛于定值.计算的单壁碳纳米管热导率在1000W/mK至4000W/mK之间,计算结果与实验结果基本符合. 关键词： 分子动力学 碳纳米管 热导率     

二元无规混合系统的有效介质理论

本文从二元无规混合物的两种不同拓扑结构出发,在Maxwell-Garnett理论的基础上,提出一种自洽的方法以改进类平均场的Bruggeman有效介质理论。导出了二元无规混合物有效电导率的普适式,研究了一些实际材料的电导率和热导率问题,理论计算结果与实验数据符合甚好。 关键词：     

A Study on Fractional Order Theory in Thermoelastic Half-Space under Thermal Loading

In this study, the effect of fractional order derivative on a two-dimensional problem due to thermal shock with weak, normal and strong conductivity is established. The governing equations are taken in the context of Green and Naghdi of type III model (GNIII model) under fractional order derivative. Based on the Laplace and exponential Fourier transformations with eigenvalues approach, the analytical solutions has been obtained. For weak, normal and strong conductivity, the numerical computations for copper-like medium have been done and the results are shown numerically. The graphical results indicate that the effect of fractional order parameter has a major role on all physical quantities involved in the problem.     

Strong electric field effect on weak localization

The influence of an electric field on weak localization is studied by a recently proposed generalized quantum Langevin equation approach to the conductivity problem. A general formula for the memory function of the non-interacting electron gas, in the presence of high order impurity scattering and an arbitrary electric field, is derived. In the low field case, a scale and frequency dependent conductivity is obtained, which reduces to the well known scale dependent conductivity in the static limit. In the high field case, the conductivity is field dependent through the drift velocity. It is shown that the presence of strong electric fields tends to delocalize the one and two dimensional electron systems if one adopts the electron heating model. However, if the electron gas does not heat up, the conductivity will be field independent.     

固态金属中声子热传递的分子动力学模拟研究

固态金属中的热传递是声子和自由电子共同作用的结果。自由电子引起的热导率可以通过电导率,利用Wiedemann-Franz定律得到,声子引起的热导率目前仍然不能进行实验测量,只能借助其他方法来研究。本文采用非平衡分子动力学(NEMD)方法,用镶嵌原子方法(EAM)势能模型,模拟计算了不同厚度(1.760-10.56nm)金属镍薄膜中由于声子-声子作用引起的热导率。然后根据纳米厚度金属薄膜的热导率借助关联式推到宏观尺度下由于声子-声子作用引起的热导率。结果表明,对于纳米厚度金属薄膜,由于声子-声子作用引起的热导率比块体金属镍的热导率小一个数量级;薄膜厚度越小,声子-声子作用引起的热导率越小;对于块体金属镍,由于声子-声子作用引起的热导率约占其总热导率的33.0%左右。     

Mode-mode coupling approach to critical fluctuations in superconductors aboveT c

A set of kinetic equations for a superconductor aboveT _c is derived using Kawasaki's formulation of mode-mode coupling theory. From these equations the conductivity is calculated in various temperature ranges. Far away from the transition the Aslamasov-Larkin conductivity is rederived and modifications of this result are obtained in all physical dimensions. These modifications become important if the critical region is approached. The conductivity inside the critical region is investigated assuming that static scaling for the equal time correlation function of the order parameter is valid.