Structural, electronic and optical properties of high pressure stable phases of ZnTe

We have performed full potential linear augmented plane wave calculations to investigate the pressure induced phase transition in ZnTe. Total energies of three phases (zinc-blende, cinnabar and Cmcm) are calculated using density functional theory formalism under generalized gradient approximation and Engel-Vosko generalized gradient approximation for the exchange correlation potential approximation. The pressure stability corresponding to zinc-blende, cinnabar and Cmcm phases of ZnTe are computed. We find that cinnabar phase could be formed as a metastable phase by releasing pressure from the high pressure Cmcm phase. The obtained structural, electronic and optical results are compared with previous calculations and available experimental data. Overall good agreement is found.     

Fe的结构与物性及其压力效应的第一性原理计算

采用基于密度泛函理论的平面波赝势方法，计算了Fe的几种不同晶体结构的总能量曲线，对HCP结构下晶体结构参数c/a随压强的变化关系做计算分析. 能量计算精度取为0.01 eV/atom. 计算得出： 1) 零温下Fe从bcc到hcp结构的相变压强约为15 GPa，与实验结果相一致； 2) 压强的升高会导致Fe的磁矩减小，最终破坏Fe的磁性； 3) 压强升高引起hcp晶体结构参数c/a缓慢增大，而在地核压强(135—360 GPa)范围内，c/a取常量约1.59能够满足计算精度的要求. 关键词： 第一性原理计算 压力效应 Fe的结构与物性     

三能级原子与少周期脉冲串作用中的相干布居捕获

研究了少周期脉冲串作用下三能级原子中的布居转移和相干布居捕获现象.在非旋波近似的情况下求解了密度矩阵方程.研究结果表明在等时间间隔的锁相脉冲作用下,系统能级的布居逐步转移并积累,系统基态相干也逐步积累.在满足脉冲重复频率为基态能级频差的整数分之一倍时,三能级系统和频率梳中两梳齿频率成分作用形成相干布居捕获现象,原子暗态布居值达到最大,介质对脉冲透明.在适当选取少周期脉冲参量的情况下,在0.5个ns的时间内三能级系统相干性演化到最大后到达稳态,相干布居捕获发生.与脉宽为100个fs的多周期脉冲相比,少周期脉冲串在介质中建立相干布居捕获的时间缩短两个数量级.由于频率梳中与三能级系统发生作用的梳频成份有相同的频移,相干布居捕获的条件双光子共振仍然满足.因而,当两基态能级频率差较大时,如果选取少周期脉冲载波频率为系统能级1至2和1至3的传输频率之和的一半ω=(ω₁+ω₂)/2,室温下原子热运动的引起的多普勒频移并不会破坏相干布居捕获.     

第一性原理研究硫化镉高压相变及其电子结构与弹性性质

采用第一性原理研究了CdS的六方纤锌矿(WZ), 立方闪锌矿(ZB) 和岩盐矿(RS)相在高压条件下的相稳定性、 相变点、电子结构以及弹性性能.WZ相与RS 相可以在相应的压强范围内稳定存在, 而ZB相不能稳定存在.压强大于2.18 GPa时, WZ相向RS相发生金属化相变.WZ相中S原子电负性大于Cd, 且电负性差值小于1.7, CdS的WZ相为共价晶体.高压作用下, S原子半径被强烈压缩, 有效核电荷增加, 对层外电子吸引能力提高, 电负性急剧增大, 导致S与Cd的电负性差值大于1.7, CdS的RS相以离子晶体存在. WZ相的C₄₄随压强增加呈下降趋势, 导致WZ相力学不稳定, 并向RS相转变.当压强大于2.18 GPa时, RS相C₁₁, C₁₂随压强增加而增大, 并且C₄₄保持稳定, 说明RS相具有良好的高压稳定性与力学性能. 关键词： 第一性原理 相变 电子结构 弹性性质     

X-ray Diffraction Study of Cd 0.8 Zn 0.2 Te Under High Pressure and Temperature

Energy dispersive X-ray diffraction using synchrotron radiation has been used to study phase transformations of Cd 0.8 Zn 0.2 Te under high pressure and temperature. We confirm the presence of a cinnabar phase between the zinc-blende and rock-salt phases. But contrary to the results in CdTe, this intermediate phase is found to be stable only on pressure decrease and in a narrower pressure and temperature range. Single-phase cinnabar patterns are obtained only at 300 and 373 K. At 673 K and above, even on pressure decrease, no evidence of the cinnabar phase is found. In this temperature range, a phase segregation phenomenon is observed in the zinc-blende phase during the zinc-blende transition in both upstroke and downstroke, and the retrieved sample at ambient conditions presents a double pattern corresponding to two different Zn contents.     

Effect of pre-existing baryon inhomogeneities on the dynamics of quark-hadron transition

Baryon number inhomogeneities may be generated during the epoch when the baryon asymmetry of the universe is produced, e.g. at the electroweak phase transition. These lumps will have a lower temperature than the background. Also the value ofT _c will be different in these regions. Since a first-order quark-hadron (Q–H) transition is susceptible to small changes in temperature, we investigate the effect of the presence of such baryonic lumps on the dynamics of the Q–H transition. We find that the phase transition is delayed in these lumps for significant overdensities. Consequently, we argue that baryon concentration in these regions grows by the end of the transition. We mention some models which may give rise to such high baryon overdensities before the Q–H transition.     

Thermodynamic properties of noble metal clusters:molecular dynamics simulation

The thermodynamics properties of noble metal clusters Au_N, Ag_N, Cu_N, and Pt_N (N = 80, 106, 140, 180, 216, 256, 312, 360, 408, 500, 628, 736, and 864) are simulated by micro-canonical molecular dynamics simulation technique. The potential energy and heat capacities change with temperature are obtained. The results reveal that the phase transition temperature of big noble metal clusters (N ⩾ 312 for Au, 180 for Ag and Cu, and 360 for Pt) increases linearly with the atom number slowly and approaches gently to bulk crystals. This phenomenon indicates that clusters are intermediate between single atoms and molecules and bulk crystals. But for the small noble clusters, the phase transition temperature changes irregularly with the atom number due to surface effect. All noble metal clusters have negative heat capacity around the solid-liquid phase transition temperature, and hysteresis in the melting/freezing circle is derived in noble metal clusters.     

Electronic,elastic and dynamical properties of MgSe under pressure: rocksalt and iron silicide phase

The electronic, elastic and dynamical properties of MgSe in the rocksalt (B1) and iron silicide (B28) phase and the effects of pressure on these properties are investigated using first-principles method. The calculated electronic band structure indicates that the B1 phase of MgSe presents an indirect band-gap feature and the band gaps initially increase with pressure and subsequently decrease upon compression. Remarkably, an indirect-to-direct band-gap transition has been observed at the phase transition pressure. The elastic constants, bulk modulus, shear modulus, Young’s modulus, elastic anisotropy and B/G ratio of MgSe in the B1 and B28 phase at high pressure have also been investigated. The bulk modulus, shear modulus and Young’s modulus all increase monotonously with the increasing of pressure for the B1 and B28 phase of MgSe. The calculated phonon frequencies of the B1 phase at zero pressure agree well with available theoretical results. And the transverse acoustic phonon TA(X) mode of this phase completely softening to zero at 82 GPa. The phonon curves of the B28 phase under pressure have also been successfully investigated.     

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.     

The kinetics of formation and the parameters of radiation defect clusters in silicon

An approximate analytic solution of the system of equations describing the kinetics of formation and the parameters of radiation defect clusters in crystals is obtained. A model corresponding to the real parameters of Si is assumed as a basis for calculation and it is shown that two types of secondary radiation defect clusters may be realized corresponding to the congealing and spreading of the initial vacancy clusters. The cluster type depends on the incident energy of the particle which creates the effects and on the physical parameters of the crystal under irradiation. For a congealing cluster most of the initial vacancies react within the original volume of the damage cascade and for a spreading cluster the final concentration of divacancies and A-centres within the original cascade volume is much less than the initial vacancy concentration, i.e. most of the vacancies form divacancies and A-centres dispersed throughout the crystal volume.

The definition of the concept “threshold energy of cluster formation” as the minimum primarily displaced atom energy for the creation of a “congealing” vacancy cluster is proposed.

It is shown that A-centres form a belt surrounding a central divacancy cluster.     

Dual structural transition in small nanoparticles of Cu-Au alloy

Cu-Au alloy nanoparticles are known to be widely used in the catalysis of various chemical reactions as it was experimentally defined that in many cases the partial substitution of copper with gold increases catalytic activity. However, providing the reaction capacity of alloy nanoparticles the surface electronic structure strongly depends on their atomic ordering. Therefore, to theoretically determine catalytic properties, one needs to use a most real structural model complying with Cu-Au nanoparticles under various external influences. So, thermal stability limits were studied for the initial L1₂ phase in Cu₃Au nanoalloy clusters up to 8.0 nm and Cu-Au clusters up to 3.0 nm at various degrees of Au atom concentration, with molecular dynamics method using a modified tight-binding TB-SMA potential. Dual structural transition L1₂?→?FCC and further FCC?→?Ih is shown to be possible under the thermal factor in Cu₃Au and Cu-Au clusters with the diameter up to 3.0 nm. The temperature of the structural transition FCC?→?Ih is established to decrease for small particles of Cu-Au alloy under the increase of Au atom concentration. For clusters with this structural transition, the melting point is found to be a linear increasing function of concentration, and for clusters without FCC?→?Ih structural transition, the melting point is a linear decreasing function of Au content. Thus, the article shows that doping Cu nanoclusters with Au atoms allows to control the forming structure as well as the melting point.     

Neutron diffraction study of the pressure-driven structural transition in the HgTe<Subscript>0.85</Subscript>S<Subscript>0.15</Subscript> ternary compound

The structure of the HgTe_0.85S_0.15 ternary mercury compound was studied by neutron diffraction at high pressures of up to 40 kbar. A phase transition from the cubic (sphalerite-type) to the hexagonal (cinnabar-type) structure was established to occur with increasing pressure and to be accompanied by an abrupt change in the unit-cell volume and interatomic distances. The unit cell parameters, the positions of the Hg and Te/S atoms in the hexagonal cinnabar phase, and their pressure dependences were found.     

A model for baryogenesis in superstring unification

The problem of baryogenesis is studied in superstring unification with intermediate gauge symmetry breaking. Baryon number asymmetry may be generated by the decay of the coherent Higgs field which is produced by the phase transition associated with the intermediate gauge symmetry breaking. It will be possible to obtain an appreciable baryon number asymmetry if certain phenomenological conditions are satisfied for the model parameters.     

The electrical transportation and carrier behavior of ZnSe under high pressure

With in situ electrical resistivity and Hall effect measurement, electrical transportation property and charge carrier behavior of ZnSe were investigated under high pressure using a diamond anvil cell (DAC). The electrical resistivity changed discontinuously at 7.7 and 11.9?GPa, corresponding to the phase transitions of ZnSe. In the pressure interval of 7.7–11.9?GPa, the electrical resistivity changed continuously, indicating the existence of the intermediate phase between the zinc blende and rock salt phases. The difference of carrier characteristic between the intermediate and rock salt phases can also suggest the existence of the intermediate phase. For the intermediate phase, the increase in electrical resistivity is from the decrease in mobility. While for the rock salt phase, the increase in charge carrier concentration leads to the decrease in electrical resistivity.     

Phase transitions in the Falicov–Kimball model away from half-filling

The canonical Monte Carlo method is used to study the order-disorder phase transition of the Falicov–Kimball model away from half-filling. It is shown that the transition from various inhomogeneous ground-state phases to the disordered phase can be either direct or indirect. The indirect transition means that the ground-state phase first, at critical temperature τ _c , changes to a different ordered phase and at the temperature, that can be several times higher than τ _c , finally changes to the disordered phase. It is shown that the Falicov–Kimball model, depending on the ground state phase, undergoes first order or second order phase transition or can even undergo both for the same parameters and different temperatures if the transition from the ground-state phase to the disordered phase is indirect.     

Quantum interferometry by using delayed phase-locked ultrashort pulses: implementation of quantum gate operation in molecules

It will be shown that by using two phase-locked delayed ultrashort pulses of different durations one can selectively populate odd or even vibrational levels of an excited state of a molecule. One can control the enhancement or damping in a selective vibrational level by controlling the laser parameters e.g. delay, phase difference, temporal width and carrier frequency of two ultradhort pulses. Conversely by reading the population distribution In vibrational levels of an excited state of a molecule one can study the amplitude and the phase of the wavepacket generated by the interference of two wavepackets excited by two ultrashort pulses i.e. by quantum interferometry. It will be examined whether the information stored in a molecule by quantum computation can be retrieved efficiently by using quantum interferometry.     

Molecular dynamics simulations of phase transition of n-nonadecane under high pressure

The phase transition behavior of n-nonadecane under high pressure was investigated with molecular dynamics (MD) simulations method. A simplified model with amorphous structure and periodic boundary conditions in constant-temperature, constant-pressure ensemble was used in this study. The results showed that the whirling and molecules motion of n-nonadecane chains were restrained by the high pressure. The simulated phase transition temperature of n-nonadecane under high pressure is higher than that under atmospheric pressure. The order parameter of n-nonadecane decreases with the increase in temperature. The simulations reveal that MD is an effective method to understand the phase transition of alkane-based phase change materials on molecular and atomic scale.     

SnSe分子外场下的基态性质和激发态性质

对Sn原子使用SDB-cc-pVTZ基组, Se原子采用6-311++G^**基组, 利用密度泛函中的B3LYP方法研究了电场强度为-0.04–0.04 a.u.的外电场对SnSe基态分子的几何结构、 电荷布居分布、 HOMO能级、 LUMO能级、 能隙、 费米能级、 谐振频率和红外光谱强度的影响. 继而使用含时密度泛函(TD-B3LYP) 方法研究了SnSe分子在外场下的激发特性. 结果表明, 外电场的大小和方向对SnSe分子基态的这些性质有明显影响. 在所加的电场范围内(-0.04 a.u.–0.04 a.u.), 随着正向电场的增大, 核间距先减小后增大, 在F=0.03 a .u.时取得最小值0.2317 nm; 分子电偶极矩μ近似线性地增大; E_L, E_H、 费米能级E_F和能隙E_g均减小. 随着正向电场逐渐增大, 分子总能量和谐振频率均先增大后减小; 红外谱强度则先减小后增大, 在F=0.03 a.u.时, 取得最小值 0.1138 km·mol^-1. 由基态到第1–10个单重激发态的波长均随着正向电场的增大而增大. 激发能均随着正向电场的增大而减小. 电场的引入可改变SnSe分子激发态出现的顺序并使得一些禁止的跃迁变得可能. 关键词： SnSe 外电场 能隙 激发特性     

Electronic and optical properties of high pressure stable phases of ZnS: Comparison of FPLAPW and PW-PP results

A theoretical study of the structural phase transformation of ZnS under high pressure has been performed using first principle plane wave pseudopotential (PW-PP) and full potential linear augmented plane wave method (FPLAPW) calculation in which Zn-3d states are treated as valence states. In both methods, we have used a generalized gradiant approximation for the study of phase transformation and structural parameters. The calculated difference in lattice constants (Δα₀) by PW-PP and FPLAPW methods for zinc-blende, cinnabar and rocksalt structures is equal to 0.003, 0.01 and 0.001 Å respectively. There is a very good agreement between the results of PW-PP and FPLAPW calculations that shows soundness of our choice of pseudopotential. The calculated transition pressure for zinc-blende → rocksalt is in agreement with available measured data. We present calculations of the optical properties for three phases of ZnS. The band gap of different phases of ZnS decreases in order of zinc-blende → cinnabar → rocksalt mainly due to red shift of Zn-s states in the lowest conduction band. Besides, the optical band gap decreases from 2.84 eV (direct) to 0.188 eV (indirect). The shift of calculated complex dielectric function ε₂(ω) for zinc-blende → cinnabar → rocksalt is also discussed in details of optical transition that occurred in different phases.     

Pressure dependences of elastic and lattice dynamic properties of AlAs from ab initio calculations

Ab initio calculations, based on norm-conserving nonlocal pseudopotentials and density functional theory (DFT), are performed to investigate the structural, elastic, dielectric, and vibrational properties of aluminum arsenide AlAs with zinc-blende (B3) structure and nickel arsenide (B8₁) structure under hydrostatic pressure. Firstly, the path for the phase transition from B3 to B8₁ is confirmed by analyzing the energies of different structures, which is in good agreement with previous theoretical results. Secondly, we find that the elastic constants, bulk modulus, static dielectric constants, and the optical phonon frequencies are varying in a nearly linear manner under hydrostatic pressure. What is more, the softening mode of transversal acoustic mode at X point supports the phase transition in AlAs.