高压下ZnO的结构、弹性性质和吸收光谱的第一性原理研究

 运用基于密度泛函理论（DFT）的平面波赝势方法（PWP）,结合局域密度近似（LDA）以及广义梯度近似（GGA）,系统地研究了ZnO的纤锌矿结构（B4结构）,NaCl结构（B1结构）和CsCl结构（B2结构）在不同压强下的几何结构、弹性性质和吸收光谱。详细研究了ZnO发生的两次相变（B4→B1及B1→B2相变）,得到了不同近似下的相变压强,以及两次相变过程中其弹性常数随压强的变化,并分析了这种变化与相变的关系。发现在高压作用下,ZnO的吸收光谱发生蓝移。通过计算结果和实验结果的比较可以看出,LDA近似下的计算结果更加符合实验结果。     

Pressure dependence of elastic properties of wurtzite ZnO crystal

We present the pressure dependence of elastic properties of the wurtzite phase of ZnO undergoing wurtzite to the rocksalt phase transition. A simple Landau theory is developed to describe the structural phase transition between wurtzite to rocksalt phases observed in ZnO. We have defined the necessary order parameter of the wurtzite to the rocksalt phase transition. We present a detailed analysis of the pressure dependence of the elastic and shear constants of the wurtzite phase of ZnO. The theoretical predictions are in the line with experimental results.     

Random walk of dislocations following a high-velocity impact

The permanent distortion of an elastic material due to a shock wave generated by a high-velocity impact is modeled by a random walk of dislocations. The dislocation movement is inhibited by a spatial and energetic distribution of activation barriers. The dislocations also experience a radially outward stress bias from the point of impact. The experimentally observed scaling of the total integrated momentum as well as the scaling with time of the penetration distance and strength of the shock wave are obtained in this model.Presented at the Symposium on Random Walks, Gaithersburg, MD, June 1982.Research supported by Defense Advanced Research Projects Agency.     

Study of the wurtzite zinc-blende mixed-structured GaAs nanocrystals grown on Si (111) substrates

The structure and growth mechanism of GaAs nanocrystals grown on Si (111) substrates by using the molecular beam epitaxy method have been studied using transmission electron microscopy. The isolated nanocrystals had hexangular shapes, with aspect ratio ～1 and high symmetry. The crystal structure of the GaAs nanocrystals contains a mixture of a stable state of zinc-blende and a metastable state of wurtzite. A number of thin wurtzite layers parallel to the Si (111) plane are introduced into the zinc-blende GaAs nanocrystals as stacking faults. Formation of partial dislocations near the GaAs/Si interface and the small difference in the Gibbs free energy between the zinc-blende and wurtzite structures could cause formation of wurtzite as stacking faults in the zinc-blende structure     

Ultrasonic waves in crystals with charged dislocations

A system of equations for charged dislocations, where the quadratic nonlinear terms are taken into account, is derived using the variational principle. This system describes the propagation of ultrasonic (US) waves in crystals with charged dislocations. From the linearized system of equations a linear dispersion equation is derived. Formulas for the phase linear velocity of the wave and the absorption coefficient are obtained, which show essential influence of charged dislocations and electrical properties of media on the mentioned quantities. For a nonlinear US wave an equation for the amplitude of the first harmonic is derived and, as a consequence, expressions are obtained for the nonlinear velocity of the US wave, for the attenuation of the first harmonic's amplitude, and for phase variation.     

Structural stabilities, electronic, elastic and optical properties of SrTe under pressure: A first-principles study

The structural, electronic, elastic and optical properties as well as phase transition under pressure of SrTe have been systematically investigated by first-principles pesudopotential calculations. Five possible phases of SrTe have been considered. Our results show that SrTe undergoes a phase transition from NaCl-type (B1) to CsCl-type (B2) structure at 10.9 GPa with a volume collapse of 9.43%, and no further transition is found. We find that SrTe prefer h-MgO instead of wurtzite (B4) structure for its metastable phase because that the ionic compound prefers a high coordination. The elastic moduli, energy band structures, real and imaginary parts of the dielectric functions have been calculated for all considered phases, and we find that a smaller energy gap yields a larger high-frequency dielectric constant. Our calculated results are discussed and compared with the available experimental and theoretical data.     

Elastic stiffness coefficients (c11, C33, and C13) for freshly excised and formalin-fixed myocardium from ultrasonic velocity measurements

The goal of this study was to measure elastic stiffness coefficients of freshly excised and subsequently formalin-fixed myocardial tissue. Our approach was to measure the angle-dependent phase velocities associated with the propagation of a longitudinal ultrasonic wave (3-8 MHz) in ovine myocardium using phase spectroscopy techniques and to interpret the results in the context of orthotropic and transversely isotropic models describing the elastic properties of myocardium. The phase velocity results together with density measurements were used to obtain the elastic stiffness coefficients c11, c33, and c13 for both symmetries. The results for the elastic stiffness coefficients c11, c33, and c13 are the same for both symmetries. Measurements for freshly excised myocardium and the same tissue after a period of formalin fixation were compared to examine the impact of fixation on the elastic stiffness coefficients.     

Wave and ray spatial dynamics of the light field in the generation,evolution, and annihilation of phase dislocations

The spatial dynamics of the light field during the generation, evolution, and annihilation of screw phase dislocations in Gaussian optical beams is analyzed. The regularities of transformation of an aberration wave front of a beam into a singular front are exposed. The structure of the field of directions of the energy-flux density vector is analyzed on the basis of a solution of a parabolic wave equation for the complex amplitude of a monochromatic light field with the construction of streamlines and investigation of the full set of critical points in the corresponding dynamic system. It is established that the energy redistribution in the beam and appearance of dislocations occur in accordance with the transformation of singularities: local focusing of a streamline in the vicinity of a node is a precursor of a generated pair of screw dislocations, and the streamline takes on a helix shape near an unstable focus.     

First principles study of the structural,electronic, mechanical and superconducting properties of WX (X=C,N)

The structural, electronic, mechanical and superconducting properties of tungsten carbide (WC) and tungsten nitride (WN) are investigated using first principles calculations based on density functional theory (DFT). The computed ground state properties, such as equilibrium lattice constant and cell volume, are in good agreement with the available experimental data. A pressure induced structural phase transition is observed in both tungsten carbide and nitride, from a tungsten carbide phase (WC) to a zinc blende phase (ZB), and from a zinc blende phase (ZB) to a wurtzite phase (WZ). The electronic structure reveals that these materials are metallic at ambient conditions. The calculated elastic constants obey the Born-Huang criteria, suggesting that they are mechanically stable at normal and high pressure. Also, the superconducting transition temperature is estimated for the WC and WN in stable structures at atmospheric pressure.     

Structural,electronic and mechanical properties of alkaline earth metal oxides MO (M=Be,Mg, Ca,Sr, Ba)

The structural, electronic and mechanical properties of alkaline earth metal oxides MO (M=Be, Mg, Ca, Sr, Ba) in the cubic (B1, B2 and B3) phases and in the wurtzite (B4) phase are investigated using density functional theory calculations as implemented in VASP code. The lattice constants, cohesive energy, bulk modulus, band structures and the density of states are computed. The calculated lattice parameters are in good agreement with the experimental and the other available theoretical results. Electronic structure reveals that all the five alkaline earth metal oxides exhibit semiconducting behavior at zero pressure. The estimated band gaps for the stable wurtzite phase of BeO is 7.2 eV and for the stable cubic NaCl phases of MgO, CaO, SrO and BaO are 4.436 eV, 4.166 eV, 4.013 eV, and 2.274 eV respectively. A pressure induced structural phase transition occurs from wurtzite (B4) to NaCl (B1) phase in BeO at 112.1 GPa and from NaCl (B1) to CsCl (B2) phase in MgO at 514.9 GPa, in CaO at 61.3 GPa, in SrO at 42 GPa and in BaO at 14.5 GPa. The elastic constants are computed at zero and elevated pressures for the B4 and B1 phases for BeO and for the B1 and B2 phases in the case of the other oxides in order to investigate their mechanical stability, anisotropy and hardness. The sound velocities and the Debye temperatures are calculated for all the oxides using the computed elastic constants.     

GaN结构相变、电子结构和光学性质

运用第一性原理平面波赝势和广义梯度近似方法,对纤锌矿结构和氯化钠结构GaN的状态方程及其在高压下的相变进行计算研究,分析相变点附近的电子态密度、能带结构和光学性质的变化机制.通过状态方程和焓相等原理得到GaN从纤锌矿到氯化钠结构的相变压强分别为43.9 Gpa和46.0 Gpa;在相变的过程中,GaN由典型的直接带隙半导体转变为间接带隙半导体材料;氯化钠结构GaN相比于纤锌矿结构,介电函数主峰值增强,本征吸收边明显往高能方向移动,氯化钠结构GaN在低能区域的光学性质差于纤锌矿结构.     

Structural, electronic and vibrational properties of InN under high pressure

The structural, electronic and vibrational properties of InN under pressures up to 20 GPa have been investigated using the pseudo-potential plane wave method (PP-PW). The generalized-gradient approximation (GGA) in the frame of density functional theory (DFT) approach has been adopted. It is found that the transition from wurtzite (B4) to rocksalt (B1) phase occurs at a pressure of approximately 12.7 GPa. In addition, a change from a direct to an indirect band gap is observed. The mechanism of these changes is discussed. The phonon frequencies and densities of states (DOS) are derived using the linear response approach and density functional perturbation theory (DFPT). The properties of phonons are described by the harmonic approximation method. Our results show that phonons play an important role in the mechanism of phase transition and in the instability of B4 (wurtzite) just before the pressure of transition. At zero pressure our data agree well with recently reported experimental results.     

A computational approach towards modelling dislocation transmission across phase boundaries

To study the nanoscopic interaction between edge dislocations and a phase boundary within a two-phase microstructure the effect of the phase contrast on the internal stress field due to the dislocations needs to be taken into account. For this purpose a 2D semi-discrete model is proposed in this paper. It consists of two distinct phases, each with its specific material properties, separated by a fully coherent and non-damaging phase boundary. Each phase is modelled as a continuum enriched with a Peierls–Nabarro (PN) dislocation region, confining dislocation motion to a discrete plane, the glide plane. In this paper, a single glide plane perpendicular to and continuous across the phase boundary is considered. Along the glide plane bulk induced shear tractions are balanced by glide plane shear tractions based on the classical PN model. The model's ability to capture dislocation obstruction at phase boundaries, dislocation pile-ups and dislocation transmission is studied. Results show that the phase contrast in material properties (e.g. elastic stiffness, glide plane properties) alone creates a barrier to the motion of dislocations from a soft to a hard phase. The proposed model accounts for the interplay between dislocations, external boundaries and phase boundary and thus represents a suitable tool for studying edge dislocation–phase boundary interaction in two-phase microstructures.     

圆管结构中周向导波非线性效应的模式展开分析

在二阶微扰近似条件下, 采用导波模式展开分析方法研究了圆管结构中周向导波的非线性效应. 伴随基频周向导波传播所发生的二次谐波, 可视为由一系列二倍频周向导波模式叠加而成. 从动量定理出发, 结合柱坐标系下非线性应力张量及其散度的数学表达式, 针对圆管中某一基频周向导波模式, 推导出相应的二倍频应力张量及二倍频彻体驱动力的数学表达式, 建立了确定二倍频周向导波模式展开系数的控制方程, 得到了伴随基频周向导波传播所发生的二次谐波声场的形式解. 理论分析和数值计算表明, 当构成二次谐波声场的某一二倍频周向导波模式与基频周向导波的相速度匹配时, 该二倍频周向导波模式的位移振幅表现出随传播周向角积累增长的性质; 当两者的相速度失配时, 二倍频周向导波的振幅随传播周向角表现出“拍”效应.     

Pressure dependence of single crystal elastic constants and anharmonic properties of wurtzite

The single crystal, elastic constants of wurtzite (ZnS) have been measured by means of the ultrasonic pulse superposition technique at 25°C as a function of pressure, and at 1 bar as a function of temperature between 25 and 100°C. Within experimental accuracy the pressure dependence in the range up to 10 kbar was found to be linear for the longitudinal modes and quadratic for the shear modes. The elastic constants and their first pressure derivatives agree approximately with theoretical values for the hcp structure with first nearest neighbor central force interaction. The experimental data are used for the discussion of the Born stability limit in relation to the high pressure transformations of wurtzite into the sphalerite and rock salt structures, for the calculation of the Grüneisen parameter in the anisotropic elastic continuum approximation, and for the calculation of the isothermal equation of state at high pressure. The elastic properties of sphalerite (ZnS) are calculated from the data for wurtzite in the ‘equivalent sphalerite approximation’ of Sullivan and used for discussing the dependence on ionic radii of the pressure coefficients of the elastic constants in sphalerite-type II–VI compounds. The Born stability limit, the Grüneisen parameter and the isothermal equation of state of sphalerite are also calculated and compared with the corresponding quantities for wurtzite.     

Effect of elastic excitations on the surface structure of hadfield steel under friction

The structure of the Hadfield steel (H13) surface layer forming under dry friction is examined. The deformation of the material under the friction surface is studied at a low slip velocity and a low pressure (much smaller than the yields stress of H13 steel). The phase composition and defect substructure on the friction surface are studied using scanning, optical, and diffraction electron microscopy methods. It is shown that a thin highly deformed nanocrystalline layer arises near the friction surface that transforms into a polycrystalline layer containing deformation twins and dislocations. The nanocrystalline structure and the presence of oxides in the surface layer and friction zone indicate a high temperature and high plastic strains responsible for the formation of the layer. It is suggested that the deformation of the material observed far from the surface is due to elastic wave generation at friction.     

Electronic structure and physical properties of ScN in pressure: density-functional theory calculations

Local density functional is investigated by using the full-potential linearized augmented plane wave （FP-LAPW） method for ScN in the hexagonal structure and the rocksalt structure and for hexagonal structures linking a layered hexagonal phase with wurtzite structure along a homogeneous strain transition path. It is found that the wurtzite ScN is unstable and the layered hexagonal phase, labelled as ho, in which atoms are approximately fivefold coordinated, is metastable, and the rocksalt ScN is stable. The electronic structure, the physical properties of the intermediate structures and the energy band structure along the transition are presented. It is found that the band gaps change from 4.0 to 1.0 eV continuously when c/a value varies from 1.68 to 1.26. It is noticeable that the study of ScN provides an opportunity to apply this kind of material （in wurtzite[h]-derived phase）.     

基于弹性波超表面的Lamb波透射调控

根据广义Snell定律,该文提出了一种新型曲梁型弹性波超表面结构。通过该结构对入射波进行相位调制,实现了板中Lamb波(A0模态)的异常透射调控,从而实现了波的负折射、非对称传输、声聚焦等功能。进一步的研究结果显示,若将该类超表面结构贴置在完整的铝板表面,通过引入吸声材料(阻尼),在保证薄板完整性的同时,可以起到显著消除薄板和超表面透射波场叠加的作用。该方法实现了对完整板结构Lamb波透射方向的有效调控,也为弹性波超表面提供了新的研究思路。     

Surface acoustic waves and elastic constants of InN epilayers determined by Brillouin scattering

The surface acoustic wave velocity in InN has been experimentally determined by means of Brillouin scattering experiments on c ‐ and m ‐face epilayers. From simulations based on the Green's function formalism we determine the shear elastic constants c₆₆ and c₄₄ and propose a complete set of elastic constants for wurtzite InN. The analysis of the sagittal and azimuthal dependence of the surface acoustic wave velocity indicates a slightly different elastic behavior of the m ‐face sample that basically affects the c₄₄ elastic constant. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Allowance for the elastic field of a rectilinear dislocation in a wave description of martensite growth

The boundaries of the region that are optimum for the nucleation and growth of martensite crystals (MC) with the habits {557}, {225} in the wave regime are established from an analysis of the elastic fields of rectilinear dislocations in an isotropic medium.