Interrelation between the soliton lattice and electric polarization in RMn<Subscript>2</Subscript>O<Subscript>5</Subscript> oxides

Magnetic transitions from the paramagnetic state to an incommensurate magnetic structure and then to an ordered phase with long-range antiferromagnetic order in RMn₂O₅ (R is a rare-earth ion) oxides are analyzed. It is shown that a transition from the paramagnetic to the incommensurate phase is associated with exchange as well as relativistic interactions and can be described, apart from the basic magnetic order parameter, by an associated order parameter (viz., electric polarization along the y axis of the crystal). As a result of such a transition, the emergence of electric polarization in the crystal is not accompanied by a change in crystal symmetry.     

Adaptive interferometers with no external field using reflection gratings in CdTe:Ge at 1550 nm

We demonstrate experimentally the operation of an adaptive two-wave mixing interferometer with no field applied to a photorefractive crystal in which the quadrature condition is introduced by an external π/2 phase step. To achieve high signal to noise ratio the counter-propagating geometry for grating recording is used with a CdTe:Ge crystal, ensuring effective recording of reflection holograms. It is demonstrated that the interferometer with a photorefractive semiconductor crystal provides good sensitivity without the adverse effects that are normally associated with an applied field.     

Surface size effects under plastic deformation of microcrystals and nanocrystals

The size effects associated with the crystal surface as an effective sink for moving dislocations in a thin crystal and as a barrier for these dislocations in the presence of a high-strength film or a special hardened layer on the surface, which favor the accumulation of dislocations in the crystal, have been considered theoretically in terms of the kinetic equation for the density of dislocations concentrated in the crystal in the critical lengths of single-ended (unipolar) dislocation sources. The theoretical results have been illustrated by the experimental data available in the literature for microcrystals and nanocrystals of copper and aluminum. It has been found in accordance with these data that the dependence of the yield stress ??_2% of the crystal on the crystal transverse size D has the form ??_2% ?? D ^?0.75 when there is a free crystal surface for the escape ofthe dislocations and ??_2% ?? D ^?0.5 when there is a high-strength layer on the lateral surface of the crystal..     

Photodielectric effect in TlInS<Subscript>2</Subscript> activated by the La impurity

The influence of light on the low-frequency dielectric properties of the TlInS₂ layered crystal is investigated for the first time. It is established that the illumination of the crystal during measurements leads to a substantial change in the behavior of the temperature dependence of the permittivity ɛ of the TlInS₂ compound doped with the lanthanum impurity in the range of the existence of the incommensurate phase. The temperature dependences of the permittivity ε of the TlInS₂ compound doped with the lanthanum impurity after preliminary cooling of the crystal in constant electric fields with different strengths are studied for the first time. The inference is made that the experimentally observed photodielectric effect is associated with the localization of charge carriers at defect levels in the band gap of the crystal with the formation of local polarized states.     

In-gap spin excitations and finite triplet lifetimes in the dilute singlet ground state system SrCu(2-x)Mgx(BO3)2

High resolution neutron scattering measurements on a single crystal of SrCu(2-x)Mgx(BO3)2 with x approximately 0.05 reveal the presence of new spin excitations within the gap of this quasi-two-dimensional, singlet ground state system. The application of a magnetic field induces Zeeman-split states associated with S=1/2 unpaired spins which are antiferromagnetically correlated with the bulk singlet. Substantial broadening of both the one- and two-triplet excitations in the doped single crystal is observed, as compared with pure SrCu2(BO3)2. Theoretical calculations using a variational algorithm and a single quenched magnetic vacancy on an infinite lattice are shown to qualitatively account for these effects.     

Anisotropic acoustooptic modulator of nonpolarized light based on diffraction by a slow acoustic wave in a paratellurite crystal

The problem of anisotropic Bragg diffraction of nonpolarized light by a slow acoustic wave in a TeO₂ crystal is solved. Two independent acoustic waves are excited in the crystal. Nonpolarized light splits in the crystal into two orthogonally polarized eigenmodes, either diffracting by its associated acoustic beam. Conditions under which the angles of incidence and diffraction are the same for both diffraction processes are found. Depending on the acoustic frequency, the diffracted light at the exit from the crystal may be represented either by a single nonpolarized beam or by two orthogonally polarized beams with different directions and orthogonal polarizations. This may provide a high diffraction efficiency (up to 100%) for nonpolarized light in a TeO₂ crystal. Theoretical calculations are supported by experiments. Modulators capable of controlling a high-power laser operating at a wavelength of 1.06 μm are fabricated.     

CuI晶体及其缺陷态电子结构的模拟

利用相对论密度泛函理论和嵌入分子团簇方法,模拟计算了具有闪锌矿结构的γ态CuI晶体及其缺陷态的电子结构。结果显示晶体的本征能级结构:价带顶主要由I5p和Cu3d轨道杂化组成,导带底由Cu4s轨道组成,禁带宽度为3.1eV,该结果与实验相符。在不同缺陷态的计算中,四面体间隙铜缺陷相对其他间隙缺陷更易于在晶体中形成,其中Cu3d→4s跃迁能量为3.2eV,推测与CuI晶体发光密切相关。     

Structural phase transition study of NiCl22H2O by EPR

A single EPR line of isotropic and Lorentzian form was observed in a crystal of NiCl₂2H₂O between 113 and 300 K. During a cooling run a sudden line broadening was observed at 200 K associated with a known crystallographic phase transition. On heating, the corresponding transition temperature was 220 K. The EPR spectra are interpreted in terms of a modified Anderson and Weiss model. The crystal field parameter is determined as D = 0.81 K and D = 1.07 K above and below the transition temperature, respectively.     

Relaxor properties and the mechanism of conduction in TlInS<Subscript>2</Subscript> crystals exposed to gamma irradiation

The density of states at the energy levels associated with radiation-induced defects, the localization length of a defect center, and the hopping distance of charge carriers are determined in a TlInS₂ crystal. It is demonstrated that, by varying the dose of gamma irradiation, it is possible to control the dielectric properties of ferroelectrics and to attain a stable relaxor state. In the temperature range of existence of this state, charge carriers execute tunneling from electron levels in the band gap through potential barriers created by an incommensurate superstructure of the TlInS₂ crystal.     

Magnetic state and properties of the Fe0.5TiSe2 intercalation compound

The Fe_0.5TiSe₂ compound with a monoclinic crystal structure has been prepared by intercalation of Fe atoms between Se-Ti-Se sandwiches in the layered structure of TiSe₂. The crystal and magnetic structures, electrical resistivity, and magnetization of the Fe_0.5TiSe₂ compound have been investigated. According to the neutron diffraction data, the Fe_0.5TiSe₂ compound has a tilted antiferromagnetic structure at temperatures below the Néel temperature of 135 K, in which the magnetic moments of Fe atoms are antiferromagnetically ordered inside layers and located at an angle of approximately 74.4° with respect to the layer plane. The magnetic moment of Fe atoms is equal to (2.98 ± 0.05)μ_B. The antiferromagnetic ordering is accompanied by anisotropic spontaneous magnetostrictive distortions of the crystal lattice, which is associated with the spin-orbit interaction and the effect of the crystal field.     

十二重准晶中的方向缺陷

采用多重散射的方法研究了十二重准晶中的缺陷模,发现在十二重准晶中存在方向缺陷,且当缺陷组合围绕对称中心旋转时,在保持原有缺陷模的基础上,出现了新的缺陷模. 关键词： 准晶 缺陷 带隙     

Evolution of the formation and growth of a cavity in pentagonal crystals of electrolytic origin

The experimentally observed formation and evolution of a cavity in a pentagonal crystal grown to a particular size during electrocrystallization of copper are justified theoretically. A channel of relaxation of elastic stresses associated with the formation of a disclination-type defect is revealed, and the subsequent transformation of a pentagonal crystal with a cavity into a single crystal is described. The growing pentagonal crystal is considered an open thermodynamic system in which the formation of a cavity is adequately described within the linear approximation of the thermodynamics of nonequilibrium processes.     

X-ray photoelectron spectroscopy and Raman microscopy of a ferroan platinum crystal from the Kondyor Massif,Russian Far East

X-ray Photoelectron Spectroscopy was used to study a ferroan platinum crystal from the Kondyor Massif, Russian Far East. Prior to the X-ray Photoelectron Spectroscopic analyses, the nature of the crystal was confirmed by X-ray diffraction. The survey scan showed mainly the presence of Pt and Fe, with smaller amounts of O and Si. The high resolutions spectra of the Pt 4f and Fe 2p showed 18.3 atom% Fe in the crystal, which puts the composition on the lower boundary for ferroan platinum and confirms earlier analyses using other methods such as Scanning Electron Microscopy-Energy Dispersive X-ray analysis/microprobe. The binding energy of the Pt 4f5/2 was 74.0?eV and Pt 4f7/2 70.5?eV, while the Fe 2p3/2 for metallic Fe was observed at 707.2?eV. The Fe 2p3/2 for metallic Fe was significantly sharper than that of Fe 2p3/2 at 710.7?eV associated with surface material. The Raman spectrum was dominated by the Pt–Pt stretching mode at 253?cm^?1. Changed orientation resulted in the observation of two bands at 127 and 139?cm^?1, interpreted as being due to stretching modes of two Pt–Pt bonds with the third bond to Fe and Pt fixed. The presence of Ca-Fe-Al-Mg-Si-O on the surface was probably associated with the presence of a clinopyroxene. These minerals can be expected since the crystal came originally from a clinopyroxenite-dunite matrix. The spectra showed a variety of interferences, e.g. Al 2p with Pt 4f, Mg 2p with Fe 3p, and Ca 2p1/2 with Mg Auger, making exact determinations of the ratios of these elements difficult.     

Local strain effects for isolated pairs of ortho-H2 molecules in para hydrogen

The crystal fields resulting from the local strains associated with a defect consisting of an isolated pair of ortho-H₂ molecules in solid para hydrogen are calculated. The Kanzaki method for the determination of the static properties of lattice defects is used. It is found that the crystal field parameters depend on the pair state |F,±M > and differ considerably for in-plane and out-of-plane pairs. Theories of Luryi, et al. and Harris, et al. for the energy levels of pairs of ortho-H₂ molecules in nearly pure para hydrogen are compared with our results.     

The effect of bulk impurities on localized surface states

The effect of the occurrence of bulk impurities in a crystal on the existence and localization of surface states is examined by investigating a simple one-dimensional model. It is found that the bulk impurity has a small effect on the existence conditions for localized states associated with the surface, but the degree of localization of the corresponding charge density can be altered appreciably by the presence of such impurities. The penetration depth associated with localized surface levels can be much greater that than estimated for a pure crystal. The charge density associated with the electrons which occupy localized surface states may vary in a real crystal over a broader region near the surface than would be deduced from models of pure crystals. This will affect physical quantities, such as surface impedance, which are dependent on this charge distribution.     

强激光条件下5CB液晶中的激光诱导衍射环现象

强激光经5CB液晶传播时会产生非线性自相位调制现象,利用532 nm和1 064 nmCW激光研究了强激光条件下5CB液晶中的激光诱导衍射环现象,并对强激光在5CB液晶中产生自相位调制的现象和机理进行了讨论.当532 nm激光和1 064 nm激光功率密度分别大于10 kW/cm2和300 W/cm2时,接收白屏上有较明显的衍射环现象|当激光分别持续作用数十毫秒和数百毫秒量级时间时,接收屏上的衍射环现象消失.分别利用基尔霍夫衍射积分公式的菲涅耳近似和夫朗和费近似形式对5CB中激光自相位调制和激光诱导衍射环进行了数值模拟,数值结果与实验结果符合的较好.基于热传导理论定性分析了5CB液晶在不同波长和入射条件下的三阶非线性系数.结果表明：在强激光入射条件下,热效应是自相位调制的主要原因,这种三阶非线性系数除了与液晶的吸收系数和作用时间相关,还与激光作用面积甚至激光诱导指向矢转动过程相关.     

高压下MgS的弹性性质、电子结构和光学性质的第一性原理研究

采用基于密度泛函理论(density functional theory)基础上的第一性原理赝势平面波方法, 计算研究了MgS晶体B2构型在不同压强下的几何结构、弹性性质、电子结构和光学性质. 计算结果表明, 在高压作用下, 该结构的导带能级有向高能级移动的趋势, 而价带能级有向低能级移动的趋势. 同时, 对照态密度分布图及高压下能级的移动情况, 分析了MgS B2构型在高压作用下的光学性质, 发现高压作用下, 吸收光谱发生了明显的蓝移.     

Kinetics of photoluminescence decay of KCl-Yb

We have investigated the kinetics of the luminescence decay of a KCl-Yb crystal excited in one of the long-wavelength absorption bands of the Yb²⁺ ion by nitrogen laser radiation with a power density less than 10³ W/cm². We found that, in both luminescence bands, both the one associated with the allowed transition (399 nm) and the one associated with the forbidden transition (430 nm), there are two stages of decay—an initial exponential stage that gives way to a hyperbolic stage. Non-exponential decay due to the optical electron tunneling back to the luminescence center is treated as evidence of delocalization of the electron upon intracenter excitation and is explained by the formation of an excimer-like molecular bond upon photoexcitation of the crystal. Fiz. Tverd. Tela (St. Petersburg) 41, 1763–1765 (October 1999)     

Electron-irradiation-induced radiolytic oxygen generation and microsegregation in silicon dioxide polymorphs

The first direct in situ observations of the production and microsegregation of radiolytic interstitial oxygen resulting from electron beam irradiation of crystal and amorphous oxygen deficient SiO2 polymorphs has been made using cathodoluminescence (CL) microanalysis (spectroscopy and microscopy). Previously unreported near-infrared CL emission is observed at 0.968+/-0.003 eV from crystal alpha-SiO2 (quartz) and at 0.971+/-0.003 eV from amorphous a-SiO2 (fused quartz and silica glasses) at 290 K. The energy and width of the near-infrared CL emission from electron-irradiated alpha-SiO2 polymorphs is consistent with the O2 (1)Delta(g)-->(3)Sigma(-)(g) transition associated with molecular oxygen.     

Crossover from weak to strong Kondo behaviour in CeAl2

High pressure electrical resistance experiments performed on a single crystal of CeAl₂ allow one to determine a characteristic line in the pressure-temperature plane where a continuous passage from weak to strong Kondo behaviour is observed. This line is not associated with any sizeable volume anomaly as shown by X-ray experiments.