铁冲击相变的晶向效应

采用基于火炮加载的三样品精细波剖面对比测量,研究了晶向效应对铁弹-塑性转变及体心立方结构(bcc,α相)至六角密排结构(hcp,ε相)相变特性的影响.观测到单晶铁异常的弹-塑性转变行为,这与基于位错密度描述的黏塑性本构模型计算结果相符,对应的Hugoniot弹性极限δ_(HEL)均大于6 GPa,且具有晶向相关性,即δ(111)/(HEL)δ(110)/(HEL)δ(100)/(HEL);系统获取了相变起始压力P_(PT)晶向相关性的实验数据,[100],[110]和[111]晶向的PPT实测值分别为13.89±0.57 GPa,14.53±0.53 GPa,16.05±0.67 GPa,其变化规律与非平衡分子动力学计算结果相符.上述结果揭示出冲击压缩下单晶铁存在塑性与相变微观机理的强耦合,为完善用于冲击实验描述的相场动力学模型提供了重要的实验支撑.     

Linear polarization of radiation from ZnCdSe/ZnSe quantum wells induced by the anisotropic profile of interfaces

The low-temperature spectra of the exciton luminescence of the ZnCdSe/ZnSe quantum well grown in the [100] direction have been analyzed. It has been found that the observed radiation is linearly polarized in the well plane along either the [011] axis or the orthogonal $[0\bar 11]$ axis depending on the energy of exciting light. Polarization along the [011] axis is associated with the anisotropy of the geometric profiles of interfaces and corresponds to the polarization of excitons localized in the terraces elongated along the [011] axis, which are regions corresponding to an increase in the well thickness by a depth of one or several monolayers. The terraces appear due to the existence of growth steps on the interfaces. Anisotropy in the distribution of growth steps, i.e., a longer length of the steps along the [011] axis, is also responsible for the appearance of polarization along the $[0\bar 11]$ axis. Such a polarization appears due to the found specific channel of anisotropic exciton absorption. As a result of such absorption, free excitons with large wave vectors are excited. The excitation of excitons by light is indirect, because it involves additional processes of the elastic scattering of excitons on the interface growth steps. Prevailing scattering on an ensemble of longer growth steps (along the [011] axis) leads to the alignment of the wave vectors of excited excitons. A difference between the absorption probabilities for the polarizations of excitation along the [011] and $[0\bar 11]$ axes is caused by a difference between the probabilities of scattering on steps for excitons whose dipole moments and, correspondingly, the axes of the P-type wave functions for the holes involved in the process are oriented along and across the direction of the alignment of wave vectors of excitons.     

Experimental determination of third-order elastic constants of diamond

To determine the nonlinear elastic response of diamond, single crystals were shock compressed along the [100], [110], and [111] orientations to 120 GPa peak elastic stresses. Particle velocity histories and elastic wave velocities were measured by using laser interferometry. The measured elastic wave profiles were used, in combination with published acoustic measurements, to determine the complete set of third-order elastic constants. These constants represent the first experimental determination, and several differ significantly from those calculated by using theoretical models.     

Some physical solutions of Yang’s equations forSU(2) gauge fields, Charap’s equations for pion dynamics and their combination

Some previously obtained physical solutions [1–3] of Yang’s equations forSU(2) gauge fields [4], Charap’s equations for pion dynamics [5,6] and their combination as proposed by Chakraborty and Chanda [1] have been presented. They represent different physical characteristics, e.g. spreading wave with solitary profile which tends to zero as time tends to infinity, spreading wave packets, solitary wave with oscillatory profile, localised wave with solitary profile which becomes plane wave periodically, and, wave packets which are oscillatory in nature.     

Modulated structure of the superconducting phases in Pb-modified Bi[sbnd]Sr[sbnd]Ca[sbnd]Cu[sbnd]O

Abstract

High resolution electron microscopy has shown that a modulated structure occurs in the Bi[sbnd]Sr[sbnd]Ca[sbnd]Cu[sbnd]O system. Electron diffraction and x-ray diffraction observations show that this modulation produces extra reflections. Diffraction patterns of the Pb-free material have superstructure reflections with wave vectors [0, 1/4.7, 1] and [0, ?1/4.7, 1]. These two waves vectors can generate all of the superstructure relections associated with each fundamental reflection. When PbO is added to the BSCCO system a new wave vector [0. 1/8.5, 0] is introduced. Observations show that within a single grain the two wave vectors coexist for a Pb and Bi ratio of about 0.25. This produces a non-uniform modulated structure which may be due to a superposition of the two wave vectors. The intensity of the [0, 1/4.7, 1] wave vector is substantially reduced in the Pb-modified material. The b' component of both wave vectors in incommensurate with the lattice period. The same superstructures occurs in both the low-T_c phase and the high-T_c phase.     

Compton profiles and nesting of Fermi surfaces for B2-TiNi and B2-TiPd

The band structures of the shape memory alloys B2-TiNi and B2-TiPd are calculated by the full potential linearized augmented plane wave method with the local density approximation. The theoretical Compton profiles for B2-TiNi and B2-TiPd are calculated. In addition, the three-dimensional (3D) occupation number densities obtained by Lock-Crisp-West (LCW) analysis are presented for the first time. These 3D occupation number densities are in good agreement with the Compton scattering experiment for TiNi. Both shape memory alloys are based on martensitic transformation, which is caused by soft phonons. The charge-density wave is created by nesting of Fermi surfaces, which leads to phonon softening. To examine the nesting vectors quantitatively, we calculate the generalized susceptibility χ(q). χ(q) shows peaks at 0.315[110]2π/a and 0.4[111]2π/a for TiNi and at 0.275[110]2π/a and 0.395[111]2π/a for TiPd. Although the nesting vector in the [110] direction agrees with that from the phonon experiment, the nesting vector in the [111] direction differs from that in the experimental results.     

m=±1 and m=±2 mode helicon wave excitation

The characteristics of plasma produced by plane polarized m=±1 and m=±2 mode helicon waves were investigated for the first time. Plane polarized m=±1 and m=±2 mode helicon waves were mainly excited using a Nagoya type III antenna and a quadrupole antenna, respectively. Two-dimensional cross-field measurements of ArII optical emission induced by hot electrons were made to investigate the RF power deposition. The components of the wave magnetic field measured with a magnetic probe were compared with the field profiles computed for the m=±1 and m=±2 modes. Two and four high intensity plasma columns were observed for the m=±1 and m=±2modes, respectively. These columns were located at the regions between the antenna legs. The radial profiles of the wave magnetic field were in good agreement with computations     

Magnetic Excitations in the antiferromagnet CoCl2

Magnetic neutron inelastic scattering has been used to investigate the non degenerate spim waves propagating in the [00.1] direction in CoCl₂. The neutron results are explicable in terms of a model containing isotropic exchange as proposed by Lines [12, 13] with the addition of a small “in plane” anisotropy. At zero wave vector the anisotropy produces an energy gap in the lower magnon branch which is observed by AFMR to be 70 GHz. Since the maximum spin wave frequency at the zone centre and boundary is 0.56 THz and 0.42 THz respectively the calculations were made within the ground doublet of the ⁴T_1g term. The derived exchange parameters are consistent with those obtained from susceptibility and light scattering. Finally there was no direct evidence to suggest the presence of a strong magnon-phonon hybridization in the [00.1] direction of CoCl₂.     

单轴[110]应力硅电子迁移率

基于k·p微扰法研究单轴[110]应力作用下硅的导带结构,获得单轴[110]应力硅的导带底能量及电子有效质量.在此基础上,考虑电子谷间、谷内及电离杂质散射,采用弛豫时间近似计算单轴[110]应力硅沿不同晶向的电子迁移率.结果表明：单轴[110]应力作用下硅的电子迁移率具有明显的各向异性.在[001]、[110]及[110]输运晶向中,张应力作用下电子沿[110]晶向输运时迁移率有较大的增强,由未受应力时的1 450 cm²·Vs^-1提高到2 GPa应力作用下的2 500 cm²·Vs^-1.迁移率增强的主要原因是电子有效质量的减小,而应力作用下硅导带能谷分裂导致的谷间散射几率的减小对电子迁移率的影响并不显著.     

Specific Features in the Spectrum of Nonlinear Magnetoelastic Waves Propagating in a (111) Plate with Combined Anisotropy

Nonlinear magnetoelastic waves with stationary profiles propagating in a cubic ferromagnetic with [111] uniaxial induced anisotropy are investigated theoretically. It is demonstrated that a new type of nonlinear waves engendered by the resonant motion of the 60-degree domain wall propagates along the [111] axis without magnetization escape from the plane of spin rotation. Some other cases of nonlinear magnetoelastic wave propagation in the examined magnetic are also studied.     

Investigation of superconductivity effects in single-phase and multiphase Bi2Sr2CaCu2O8 single-crystals at 2-MM wavelengths

Experimental investigations of superconductivity effects in single-phase and multiphase Bi₂Sr₂CaCu₂O₈ single-crystals have been carried out at 142 GHz frequency by means of the standing wave profile method [1]. Josephson harmonic generation has been observed to be responsible for the appearence of additional peaks on the standing wave profile of the open dielectric resonator loaded with a properly orientated multiphase high-T_c superconductor specimen. This leads to the conclusion that most of the Josephson junctions in multiphase crystals are located in certain crystallographic planes. The investigations of temperature dependencies showed that sharp resonant peaks of conductivity observed earlier [2] at 60 GHz could also be observed at 142 GHz.     

Influence of Interfacial Stress on Microstructural Evolution in NiAl Alloys

A phase-field model for the phase transition between austenite and martensite and twinning between two martensitic variants is presented from our previous theory [1] with the main focus on the influence of interfacial stress that is consistent with the sharp interface limit. Each variant-variant transformation can be represented by only one order parameter. Thus, it allows us to get the analytical solution of interface energy and width. Coupled phase-field and elasticity equations are solved for cubic-to-tetragonal phase transformation in NiAl shape memory alloy. The effects of interfacial stress are studied for martensite-martensite interfaces in detail, which was absent in [1]. Additionally, stress and temperature-induced growth of the martensitic phase inside austenite and twining are simulated. Some of the nontrivial experimentally observed microstructures reproduced in the simulations [1] are analyzed in detail. It includes tip splitting and bending, and twins crossing. This theory can be extended for electric, reconstructive, and magnetic phase transformations.

     

基于原位X射线衍射技术的动态晶格响应测量方法研究

获取动态压缩条件下结构演化过程是冲击相变及其动力学机理研究最为关注的基础问题之一.对此,基于激光驱动瞬态X射线衍射技术,通过系列实验的物理状态关联和抽运-探测时序控制,实现了静态与动态晶格衍射信号的同时获取,消除了不同实验的装置结构和样品差异带来的测量误差,建立了一种基于原位X射线衍射技术的动态晶格响应测量方法.利用上述实验方法,成功实现了激光冲击加载下[111]单晶铁晶格压缩过程的原位测量,获取弹性及塑性响应的晶格压缩度与宏观雨贡纽测量结果完全符合,从晶格层面证实了超快激光加载下的高屈服强度(雨贡纽弹性极限值大于6 GPa),以及可能与晶向效应或加载率效应相关的相变迟滞现象(至终态压力23.9 GPa仍为体心立方结构),相关物理机制仍有待进一步研究.上述测量方法的建立为后续开展相变动力学机理研究提供了可行的技术途径和重要的参考价值.     

Observation of charge-density wave domains on the Cr(110) surface by low-temperature scanning tunneling microscopy

We have studied the Cr(110) surface with low-temperature scanning tunneling microscopy at 6 to 145 K. For tunneling voltages below +/-150 mV we observe a surface charge-density wave (CDW) with a wavelength of 42 A and wave fronts aligned with the [001] in-plane direction. The observed wave pattern is identified as the surface projection of the bulk CDW's arising from the spin-density wave ground state of Cr with the Q vector parallel to [100] and [010], respectively. The bulk CDW with Q parallel to the [001] in-plane direction appears, however, to be strongly suppressed at the Cr(110) surface.     

Interface separation in the transonic range caused by a plane stress pulse

This is a continuation of the authors' investigation [1] of the localized separation between two contacting solids that is caused by an incident plane elastic wave of arbitrary form. The interface is taken as frictionless and incapable of transmitting tensile tractions. It is further assumed that the two solids have identical mechanical properties, and that the angle of incidence of the incoming wave is such that the disturbance propagates along the interface with a transonic speed. Moving dislocations are employed in the formulation and the problem is reduced to a singular integral equation with a Cauchy kernel. Specific results are worked out for an incident tensile stress pulse of a parabolic shape. An intriguing aspect of the results is that, in spite of the incident wave being continuous, the elastic fields are singular at the trailing edge of the separation zone.     

Spin waves in nickel thin films

Dynamic susceptibility and spin waves in nickel thin films are investigated within the framework of the multiband model. Parameters of the model are chosen in such a way that one-electron ground-state results consistent with ab initio theories are obtained. The dispersion relation for spin waves which propagate in the plane parallel to the surface along the [1 0] and [1 1] directions is calculated. Acoustic surface mode is observed up to the Brillouin zone boundary for wave vector q along the [1 1] direction and localization of the mode in the surface layer gradually increases with increasing q. On the other hand, for q in the [1 0] direction acoustic surface mode appears only for low q values and it disappears in the band of bulk modes for greater values of q. Near the zone boundary only bulk modes are observed. It is suggested that the disappearance of the surface mode can be a result of crossing of spin-wave branches.     

FTIR spectroscopic and quantum-chemical studies on some tribromoindenes and their isomers

Combined experimental and computational studies on molecular structure of newly synthesised transtirans 1,2,3-tribromo-1,2,3-trihydro-¹H-benz[f]indene (TTTBI) were reported. Also, only computational studies were done for cis-trans-1,2,3-tribromo-1,2,3-trihydro-¹H-benz[f]indene (CTTBI) and cis-cis-1,2,3-tribromo-1,2,3-trihydro-¹H-benz[f]indene (CCTBI) in order to understand the vibrational spectra and molecular parameters of them. The geometry optimization and vibrational wave numbers of the title molecules were carried out with the Gaussian98 program package by using Hartree-Fock (HF) and Density Functional Theory (DFT) with B3LYP functional and 6–31G (d) basis set. All calculations were done for the title compounds in their ground states. Especially for CTTBI and CCTBI, which could not be synthesized yet, these kind of pre-calculations take an important role for their synthesis process. Also crystal structural parameters of TTTBI by single-crystal X-ray diffraction method was used as input for computational study of it. Observed and calculated vibrational wave numbers were compared. Because the use of benz[f]indene as a cyclopentadienyl ligand attracted much attention because an annulated benzo ring might increase both the stereocontrol and productivity of catalytic system, TTTBI and other computationally studied and modeled two molecules may play an important role of other types of compounds as a starting structures.     

高纯铝层裂的自由面速度剖面特征分析

 采用任意反射面激光干涉测速（VISAR）系统,对高纯铝（纯度为99.999%）材料开展了层裂实验研究,获得了未完全层裂和完全层裂样品的自由面速度剖面。基于实验测量中观察到的层裂回跳（Pullback）信号的脉冲宽度、速度幅值差异以及回跳信号上升过程中的两次速度斜率变化特征,详细讨论了高纯铝材料从出现损伤到完全断裂的过程中波剖面演化的行为特征。结合“软回收”样品的细观分析,指出这些自由面速度变化特征可能与材料中应力波的能量释放速率以及材料中晶粒的断裂行为密切相关。研究结果可以为延性金属损伤断裂的演化过程提供有意义的认识。     

An improved algorithm for detecting point of impact in anisotropic inhomogeneous plates

Conventional triangulation techniques fail to correctly predict the acoustic source location in anisotropic plates due to the direction dependent nature of the elastic wave speeds. To overcome this problem, Kundu et al. [1] proposed an alternative method for acoustic source prediction based on optimizing an objective function. They defined an objective function that uses the time of flight information of the acoustic waves to the passive transducers attached to the plate and the wave propagation direction (θ) from the source point to the receiving sensors. Some weaknesses of the original algorithm proposed in Ref. [1] were later overcome by developing a modified objective function [2]. A new objective function is introduced here to further simplify the optimization procedure and improve the computational efficiency. A new algorithm for source location is also introduced here to increase the source location accuracy. The performance of the objective function and source location algorithm were experimentally verified on a homogeneous anisotropic plate and a non-homogeneous anisotropic plate with a doubler patch. Results from these experiments indicate that the new objective function and source location algorithm have improved performance when compared with those discussed in Refs. [1] and [2].