Structural,electronic, optical,elastic and thermal properties of ZnXAs<Subscript>2</Subscript> (X = Si and Ge) chalcopyrite semiconductors

We report first principles calculations of solid state properties of ZnSiAs₂ and ZnGeAs₂ chalcopyrite semiconductors. The structural properties are calculated using a Full Potential Linearized Augmented Plane Wave method (FP-LAPW) of the Density Functional Theory (DFT). A Generalized Gradient Approximation (GGA) scheme proposed by Wu and Cohen (WC) has been chosen to calculate electronic and optical properties. Optical features such as dielectric functions, refractive indices, extinction coefficient, optical reflectivity, absorption coefficients and optical conductivities were calculated for photon energies up to 30 eV. The elastic constants at equilibrium in tetragonal structure are also determined. Temperature effect on the volume, thermal expansion, heat capacity, Debye temperature, entropy, Grüneisen parameter and bulk modulus were calculated employing the quasi-harmonic Debye model at different temperatures and pressures and the silent results were interpreted. Finally using semi-empirical relation, we determined the hardness of the materials which attributed to different covalent bonding strengths.     

A new application of Riccati equation to some nonlinear evolution equations

By means of symbolic computation, a new application of Riccati equation is presented to obtain novel exact solutions of some nonlinear evolution equations, such as nonlinear Klein-Gordon equation, generalized Pochhammer-Chree equation and nonlinear Schrödinger equation. Comparing with the existing tanh methods and the proposed modifications, we obtain the exact solutions in the form as a non-integer power polynomial of tanh (or tan) functions by using this method, and the availability of symbolic computation is demonstrated.     

First-Principles Calculations of Elastic and Thermal Properties of Lanthanum Hexaboride

The plane-wave pseudopotentiai method using the generaiized gradient approximation within the framework of density functional theory is applied to anaylse the bulk modulus, thermal expansion coefficient and heat capacity of LaB6. The quasi-harmonic Debye model, using a set of total energy versus volume obtained with the plane-wave pseudopotential method, is applied to the study of the thermal properties and vibrationai effects. We analyse the bulk modulus of LaB6 up to 1500 K. The elastic properties calculations show that our system is mechanically stable. For the heat capacity and the thermal expansion, significant differences in properties are observed above 300K. The calculated zero pressure bulk modulus is in good agreement with the experimental data. Moreover, the Debye temperatures are determined from the non-equilibrium Gibbs functions and compared to available data.     

Theoretical investigations of structural, electronic and thermal properties of Ti2AlX(X=C,N)

We have carried out a first-principles total-energy calculations of the structural and the electronic properties for the series of H-phases compounds Ti₂AlC and Ti₂AlN. We have applied the full-potential linearized augmented plane waves (FP-LAPW) method based on the density functional theory (DFT) using the local-density approximation (LDA) and/or the generalized gradient approximation (GGA). The quasi-harmonic Debye model, using a set of total energy versus volume calculations obtained with the FP-LAPW method which is applied to study the thermal and vibrational effects. Temperature and pressure effects on the structural parameters, thermal expansions, heat capacities and Debye temperatures are determined from the non-equilibrium Gibbs functions.     

Structural, elastic and thermal properties of the compound LaNi4.5Sn0.5

The equilibrium lattice constants, cell volumes, densities of states and electron density distributions of LaNi_4.5Sn_0.5 crystal are evaluated by the density functional theory using the plane wave pseudopotential (PW-PP) method. The quasi-harmonic Debye model, using a set of total energy versus cell volume obtained from the PW-PP method, is applied to the study of thermal and vibrational effects. We have analyzed the bulk modulus of LaNi_4.5Sn_0.5 as a function of temperature up to 1000 K. The thermodynamic properties such as thermal expansion coefficients and heat capacities are also predicted using the quasi-harmonic Debye model. Significant differences in properties are observed at high temperatures and pressures. Moreover, the Debye temperatures are determined from the non-equilibrium Gibbs functions. The calculated results are in excellent agreement with the available experimental data, and compared favorably with other theoretical results.     

Determination of characteristic surface vibration temperatures by molecular beam scattering: Application to specular scattering in the HLiF (001) system

A theory of the determination of characteristic surface vibration temperatures (so-called “surface Debye temperatures”) by molecular beam scattering is outlined. The basis of the theory is the reduction in the intensities of the elastically-scattered beams which is due to the thermal motions of the surface atoms (the “Debye-Waller effect”). A combination of the theory of this effect with the diffraction theory of Cabrera, Celli, Goodman and Manson [Surface Sci. 19 (1970) 67] and of Goodman and Tan [J. Chem. Phys. 59 (1973) 1805] allows calculation of the elastic intensities as functions of the experimental parameters [in particular, as functions of the ratio of (a) the surface temperature and (b) the characteristic surface vibration temperature]. The theory is applied to some experimental data of Hoinkes, Nahr and Wilsch [Surface Sci. 33 (1972) 516] on specular scattering in the HLiF (001) system. It is concluded that a great deal of information about the experimental system, apart from the characteristic surface vibration temperature, is contained in such experimental data.     

On EXAFS Debye‐Waller factor and recent advances

The effects of structural and vibrational disorder on the EXAFS signals are parameterized in terms of the Debye Waller (DW) factor. Here the vibrational contribution is addressed, which for most systems can be singled out by studying the temperature dependence of the EXAFS DW factor, which corresponds to a good accuracy to the parallel mean square relative displacement (MSRD) around the inter‐atomic equilibrium distance. By comparing the first‐shell EXAFS thermal expansion with the crystallographic thermal expansion one can evaluate the perpendicular MSRD. The results of recent measurements on copper and on several tetrahedral semiconductors are here critically compared and some properties of the MSRDs are discussed, such as the dependence of correlation, force constants and vibrational anisotropy on crystal structure and bond ionicity as well as the relative merits of the correlated Debye and Einstein models. The anharmonic contribution to the parallel MSRD of CdTe has been evaluated and a quasi‐harmonic analysis has been attempted, leading to an estimation of the bond Grüneisen parameter.     

Pseudo-potential investigations of structural, elastic and thermal properties of tungsten disilicide

The plane-wave pseudopotential method using the generalized gradient approximation within the density functional theory is used to investigate the structure and bulk modulus of WSi₂. The quasi-harmonic Debye model, using a set of total energy versus cell volume obtained with the plane-wave pseudopotential method, is applied to the study of the elastic properties and vibrational effects. We have analysed the bulk modulus of WSi₂ up to 1600~K. The major trend shows that the WSi₂ crystal becomes more compressible when the temperature rises and the increase of compressibility leads to the decrease of Debye temperature. The predicted temperature and pressure effects on the thermal expansion, heat capacity and Debye temperatures are determined from the non-equilibrium Gibbs functions and compared with the data available.     

The effect of macroscopic polarization on intrinsic and extrinsic thermal conductivities of AlN

The effect of macroscopic polarization on thermal conductivity of bulk wurtzite AlN has been theoretically investigated. Our results show that macroscopic polarization modifies the phonon group velocity, Debye frequency and Debye temperature of the AlN. Using revised phonon velocity and Debye temperature, various phonon scattering rates and combined scattering rate are calculated as functions of the phonon frequency at room temperature. The intrinsic and extrinsic thermal conductivities of AlN have been estimated using these modified parameters. The theoretical analysis shows that up to a certain temperature the polarization effect acts as negative effect and reduces the intrinsic and extrinsic thermal conductivities. However, after this temperature both thermal conductivities are significantly enhanced. High phonon velocity and Debye temperature are the reason of this enhancement which happens due to the polarization effect. The revised thermal conductivities at room temperature are found to be increased by more than 20% in AlN due to macroscopic polarization phenomenon. The method we have developed can be taken into account during the simulation of heat transport in optoelectronic nitride devices to minimize the self heating processes.     

Temperature dependence of the Debye screening length in heavily doped semiconductors having Gaussian band-tails

In the present work, a generalized expression is derived for the Debye screening length of the carriers in heavily doped semiconductors having Gaussian band-tails. The temperature dependence of the screening length is also computed for such semiconductors, taking n-type GaAs as an example.On leave of absence fromthe Department of Physics, B. N. College, University of Patna, India.     

Temperature and magnetic field dependence of the absorption edge and the conduction band width of ferromagnetic semiconductors

We investigate the shift of the absorption edge and the behaviour of the conduction band width of ferromagnetic semiconductors as functions of temperature T and an external field B, respectively. Numerical results are given for EuO and EuS. As a consequence of electron-magnon scattering processes the band width of EuO is enhanced by more than 14% in the temperature region: 40 K…T…80 K. The external field tries to surpress this effect.     

Structural phase transition and elastic properties of ZnSe at high pressure

We have evolved an effective interionic interaction potential to investigate the pressure-induced phase transitions from zinc blende (B3) to rock salt (B1) structure in II-VI [ZnSe] semiconductors. The elastic constants, including the long-range Coulomb and van der Waals (vdW) interactions and the short-range repulsive interaction of up to second-neighbor ions within the Hafemeister and Flygare approach, are deduced. Keeping in mind that both of the ions are polarisable, we employed the Slater-Kirkwood variational method to estimate the vdW coefficients. The estimated value of the phase transition pressure (P _t ) is higher than in the reported data, and the magnitude of the discontinuity in volume at the transition pressure is consistent with that data. The major volume discontinuity in the pressure-volume phase diagram identifies the structural phase transition from zinc blende to rock salt structure.

The variation of second-order elastic constants with pressure resembles that observed in some binary semiconductors. It is inferred that the vdW interaction is effective in obtaining the thermodynamic parameters such as the Debye temperature, the Gruneisen parameter, the thermal expansion coefficient and the compressibility. However, the inconsistency between the thermodynamic parameters as obtained from present model calculations and their experimental values is attributed to the fact that we have derived our expressions by assuming the overlap repulsion to be significant only up to the nearest second-neighbor ions, as well as neglecting thermal effects. It is thus argued that full analysis of the many physical interactions that are essential to binary semiconductors will lead to a consistent explanation of the structural and elastic properties of II–VI semiconductors.     

Temperature and pressure behaviour of narrow-gap semiconductors including galena

The structural high pressure and temperature investigation of narrow-gap semiconductors (lead chalcogenides) has been performed in the present article. A realistic approach for room temperature and high temperature study of narrow-gap semiconductors has been used. It is examined that the present compounds are more stable in NaCl-phase and they transform to CsCl-phase at high pressure. In the present article, the phase transition pressures and volume collapses of lead chalcogenides have been investigated at room and high temperatures. Phase transition pressures have been reported at high temperature range from 0 to 1200 K. Elastic and anharmonic constants have also been reported at room temperature. A structural study of the narrow-gap semiconductors have been carried out using the realistic model including temperature effect. The temperature and pressure behaviour of elastic constants for the present compounds have also been discussed. Furthermore, various mechanical and thermo dynamical properties like modulus of elasticity, Debye temperatures etc. are also presented.     

Temperature-Dependent Debye Temperature and Specific Capacity of Graphene

The shot-range interaction and the atomic anharmonic vibration are both considered,and then the analytic functions of the Debye temperature,the specific capacity and the thermal conductivity of graphene with the temperature are obtained.The influence of anharmonic vibration on these thermal physical properties is also investigated.Some theoretical results are given.If only the harmonic approximation is considered,the Debye temperature of the graphene is unrelated to the temperature.If the anharmonic terms are considered,it increases slowly with the increasing temperature.The molar heat capacity of the graphene increases nonlinearly with the increasing temperature.The mean free path of phonons and the thermal conductivity of the graphene decrease nonlinearly with the increasing temperature.The relative changes of the Debye temperature,the specific heat capacity and the thermal conductivity caused by the anharmonic terms increase with the increasing temperature.The anharmonic effect of atomic vibration becomes more significant under higher temperature.     

高功率微波效应的多层次多物理场协同算法

针对半导体器件、电路、电子系统的高功率微波效应,提出了一套全新的多层次多物理场协同计算方法。该算法基于半导体器件的物理结构模型,联立并求解由电磁场、半导体物理和热力学方程构成的多物理场方程组,实现了器件级高功率微波效应的仿真;通过器件多物理场仿真和电路仿真的协同计算完成电路级效应仿真;最后进行电路效应和电磁环境的协同计算,获取由多个电路、外壳封装、孔缝和线缆等组成的电子系统的高功率微波效应数据。介绍了该算法的原理和流程,以商业PIN二极管为例,计算了该器件及组成限幅器电路的温度效应、正向恢复特性、半封闭腔体内空间微波辐射等效应,通过与实验测试的对比验证了算法的正确性,同时对效应现象给出了物理机理解释。     

Electronic structure, optical and thermodynamic properties of orthorhombic UCoGe under pressure

The electronic structures, optical and thermodynamic properties of orthorhombic UCoGe are investigated using the generalized gradient approximation (GGA) formalism in the framework of the density functional theory (DFT). The obtained lattice parameters, bulk modulus B and its pressure derivative B′ of UCoGe are in agreement with the available experimental data. From the analysis of band structure and density of states coming out from our calculations, we can see that UCoGe in the ground state belongs to a typical metallic alloy. Various optical properties, including the dielectric function and absorption coefficient as functions of the photon energy are calculated. The thermodynamic properties of UCoGe are predicted using the quasi-harmonic Debye model for the first time. The Debye temperature, the Grüneisen parameter, the heat capacity and the thermal expansion coefficient are obtained at high pressures and temperatures.     

Deformation potentials of bulk semiconductors

A theory of deformation potential electron-phonon scattering coefficients is presented for elemental and compound semiconductors. Expressions for the acoustical deformation potential constants at symmetry points are obtained in closed form for direct-gap bulk semiconductors. The deformation potential vs k is predicted for GaAs.     

Anomalous lattice response at the Mott transition in a quasi-2D organic conductor

Discontinuous changes of the lattice parameters at the Mott metal-insulator transition are detected by high-resolution dilatometry on deuterated crystals of the layered organic conductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br. The uniaxial expansivities uncover a striking and unexpected anisotropy, notably a zero effect along the in-plane c axis along which the electronic interactions are relatively strong. A huge thermal expansion anomaly is observed near the end point of the first-order transition line enabling us to explore the critical behavior with very high sensitivity. The analysis yields critical fluctuations with an exponent alpha approximately 0.8+/-0.15 at odds with the novel criticality recently proposed for these materials [Kagawa et al., Nature (London) 436, 534 (2005)]. Our data suggest an intricate role of the lattice degrees of freedom in the Mott transition for the present materials.     

Influence of Electron–Phonon Interaction on the Lattice Thermal Conductivity in Single-Crystal Si

Lattice thermal conductivity can be reduced by introducing point defect, grain boundary, and nanoscale precipitates to scatter phonons of different wave-lengths, etc. Recently, the effect of electron–phonon (EP) interaction on phonon transport has attracted more and more attention, especially in heavily doped semiconductors. Here the effect of EP interaction in n-type P-doped single-crystal Si has been investigated. The lattice thermal conductivity decreases dramatically with increasing P doping. This reduction on lattice thermal conductivity cannot be explained solely considering point defect scattering. Further, the lattice thermal conductivity can be fitted well by introducing EP interaction into the modified Debye–Callaway model, which demonstrates that the EP interaction can play an important role in reducing lattice thermal conductivity of n-type P-doped single-crystal Si.     

Macroscopic polarization and thermal conductivity of GaN

We theoretically investigated the effect of macroscopic polarization (sum of spontaneous and piezoelectric polarization) on the thermal conductivity of wurtzite GaN. Macroscopic polarization contributes to the effective elastic constant of the GaN and thus modifies the phonon group velocity. We used the revised phonon velocity to estimate the Debye frequency and temperature. Different phonon scattering rates were calculated as functions of the phonon frequency. The thermal conductivity of GaN was estimated using revised parameters such as the phonon velocity and phonon relaxation rate. The revised thermal conductivity at room temperature increased from 250 to 279 W m⁻¹ K⁻¹ due to macroscopic polarization. The method we developed can be used for thermal budget calculations for GaN optoelectronic devices.