Negative thermal expansion in crystals with the zincblende structure: an EXAFS study of CdTe

The extended x-ray absorption fine structure (EXAFS) has been measured at both the K edges of cadmium and tellurium in CdTe, from liquid helium to room temperature, in order to investigate the local thermodynamic behaviour. The temperature dependences of the structural parameters obtained from the separate analysis of the two edges are perfectly consistent. The positive contribution to the thermal expansion due to the bond stretching and the negative contribution due to the tension effects are disentangled and quantified in terms of the bond thermal expansion and the perpendicular mean square relative displacement. The comparison with previous EXAFS results for Ge and CuCl shows that relevant correlations can be established between a number of local parameters measured by means of EXAFS and the properties of the lattice negative thermal expansion of tetrahedrally bonded semiconductors. The effective force constants derived from the EXAFS are compared with the force constants of a valence force field model.     

Photoluminescence properties of GaAs nanowire ensembles with zincblende and wurtzite crystal structure

Self‐standing III–V nanowires (NWs) are promising building blocks for future optoelectronic devices such as LEDs, lasers, photodetectors and solar cells. In this work, we present the results of low temperature photoluminescence (PL) characterization of GaAs NWs grown by Au‐assisted molecular beam epitaxy (MBE), coupled with the transmission electron microscopy (TEM) structural analysis. PL spectra contain exci‐ ton peaks from zincblende (ZB) and wurtzite (WZ) crystal structures of GaAs. The peaks are influenced by the quantum confinement effects. PL bands corresponding to the exciton emission from ZB and WZ crystal phases are identified, relating to the PL peaks at 1.519 eV and 1.478 eV, respectively. The obtained red shift of 41 meV for WZ GaAs should persist in thin NWs as well as in bulk materials. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Adiabatic and isothermal elastic constants for highly anharmonic crystals

The mass operator of the phonon Green function for a highly anharmonic crystal in the ladder approximation is derived. In order to calculate the mass operator in the collision-dominated regime it is necessary to solve the Boltzmann-like equations. These solutions allow us to find the thermal conductivity coefficient and to prove that the thermodynamic relation between adiabatic and isothermal elastic constants is satisfied.     

Elastic moduli inheritance and the weakest link in bulk metallic glasses

We show that a variety of bulk metallic glasses (BMGs) inherit their Young's modulus and shear modulus from the solvent components. This is attributed to preferential straining of locally solvent-rich configurations among tightly bonded atomic clusters, which constitute the weakest link in an amorphous structure. This aspect of inhomogeneous deformation, also revealed by our in situ neutron diffraction studies of an elastically deformed BMG, suggests a rubberlike viscoelastic behavior due to a hierarchy of atomic bonds in BMGs.     

The study of the pressure derivative of the bulk moduli in ionic solids

The expression for the pressure derivative of the Szigeti effective bulk modulus (dB¹/dp)for the ionic solids has been derived. The formulated relation has been utilized to evaluate the values of dB¹/dp in case of some alkali halides, which are found to be much different from their normal values obtained by mechanical methods.     

Dependence of lattice constants and bulk moduli on pseudopotential properties

Using a nearly-free-electron model and model pseudopotentials, an analytical relation between the Wigner-Seitz radius for a metal, r_ws, and the core radius of the model pseudopotential, r_c is derived. In addition, a semi-empirical formula for the bulk moduli is obtained.     

Photoluminescence of bulk Eu-doped GaN crystals

A study of the variation of photoluminescence spectra of bulk Eu-doped GaN samples revealed that the dopant can reside in the crystal in various charge states depending on the total defect concentration in the starting semiconductor host matrix. In crystals with the lowest concentration of shallow-level defects, the ion can exist only in one charge state, Eu³⁺. At higher concentrations of such defects, Eu can be observed in two charge states, Eu²⁺ and Eu³⁺. A rare-earth impurity was found to act as a getter of defects in the starting GaN matrix.     

On the scatter ranges for the elastic moduli of random aggregates of general anisotropic crystals

A randomly inhomogeneous material may have macroscopic properties (elasticity, conductivity) scattered over some uncertainty intervals, despite the idealistic uniqueness assumption of homogenization theory. Based on minimum energy principles and certain statistical isotropy-symmetry hypotheses, our partly third-order bounds on the effective properties of random polycrystals are expected to estimate those scatter ranges. Explicit expressions are given and calculated for the elastic moduli of the random aggregates of some known monoclinic and triclinic crystals, which yield results in agreement with those calculated for higher-symmetry crystals: the moduli are determinable within an accuracy of two or three significant digits in most cases. It is shown, however, that with some real-world exotic crystals the bounds may fall far apart, and further theoretical and experimental studies on them deserve attention.     

Choice of optimal crystal-orientation for terahertz transceiver with zincblende crystal

This paper presents a set of equations describing the terahertz generation and electro-optic detection based on optical rectification in zincblende crystals. The dependence of terahertz emission efficiency on the polarization of incident beam and crystal-orientation is discussed. For the experimental setup with a transceiver which transmits and detects terahertz radiation in the same crystal, we have demonstrated the optimal combination of both parameters above to optimize the working efficiency. Equations supplied in this paper are valid for zincblende crystals with arbitrary crystal-orientation and every possible polarization of an incident beam, which are of great significance for the optimization of a system.     

Ⅳ族FCC晶体基态结构和电子性质第一性原理研究

采用基于密度泛函理论的第一性原理方法,在广义梯度近似下,计算了Ⅳ族元素晶体的面心立方结构和电子性质.结果表明:Ⅳ族元素晶体的面心立方结构均可存在,面心立方结构Ge晶体的结合能最大,结构最稳定;面心立方结构C、Si、Ge和Sn的晶格常数分别为0.3509nm、0.4322nm、0.4225nm、0.4903nm,不随原子序数的增加而单调增加,是由面心立方锗晶体比面心立方硅晶体中电子云交叠小,产生的排斥较弱所导致的;面心立方结构C晶体是间接能隙为6.5e V的宽禁带半导体,面心立方结构Si晶体的导带和价带存在较小的交叠而呈现出半金属性,面心立方结构Ge和Sn的电子结构相似均表现为金属性,Ⅳ族元素面心立方结构晶体的电学性质由宽禁带半导体向金属转变.     

Melting and isothermal bulk modulus of the rocksalt phase of ZnO with molecular dynamics simulation

The Buckingham potential has been employed to simulate the melting of the rocksalt phase of ZnO over a wide pressure–temperature range and isothermal bulk modulus with shell model molecular dynamics method. The melting curve from molecular dynamics simulations is in good agreement with the results obtained from Lindemann melting equation in the pressure below 7 GPa. The calculated null compression bulk modulus is compared with the available experimental data and other theoretical results. At extended temperature ranges, the isothermal bulk modulus has also been predicted up to 3000 K.     

体材料结晶能力的理论预测

引入评价晶体材料缺陷的残缺度及描述晶面表面势场的结晶势两个概念,针对Ni,Cu,Al,Ar单质在不同温度条件下的再结晶过程进行了大量的分子动力学模拟,发现残缺度随结晶势的增大而单调地减小,即结晶势越大则形成单晶体能力越强.由于结晶势能够唯一地确定结晶能力并且其计算简单,因此可从理论上快速方便地预测材料形成单晶体的能力. 关键词： 单晶 结晶 分子动力学 材料设计     

Effects of pressure and temperature on the isothermal bulk modulus of CaO

Molecular dynamics (MD) simulations have been performed to investigate the effects of pressure and temperature on the isothermal bulk modulus of CaO using pair-wise interactions that include polarization effects through the shell model (SM). The dependence of isothermal bulk modulus B_T of CaO on the compression ratio V/V₀ and pressure P have been obtained from MD runs at T=300 K, and compared with the available experimental data and other theoretical results. A good agreement between theory and experiment is obtained. Meanwhile, B_T dependence on temperature T at zero pressure is investigated. At extended pressure and temperature ranges, SM-MD method has also been carried out for predicting the P-V-T equation of state and isothermal bulk modulus at different temperatures along the isotherms 0, 1000, 2000, 3000, and 4000 K, and at different pressures along the isobars 5, 15, 30, 40, and 50 GPa for CaO, respectively.     

Pressure derivatives of isothermal bulk modulus of rare gas solids

In the present study we have derived the expression for the bulk modulus using the Hildebrand approximation and the lattice potential energy for rare gas solids (RGS). The derived relation is further used to derive the expression for first and second order pressure derivatives of the bulk modulus for RGS. The derived relations are also used to compute the values of these constants in the case of Ne, Ar, Kr and Xe which are compared with the available values.