Raman spectra of Cd1-xMnxTe mixed crystals

Raman spectra of the mixed crystals Cd_1-xMn_xTe have been studied over the composition range 0.1 ≤ × ≤ 0.7. They show the two-mode behavior corresponding to the zone center “CdTe-like” and “MnTe-like” LO-TO modes. In addition, a first-order Raman spectrum is observed which reflects the one-phonon density of states. It is attributed to the presence of both translational and spin-disorder in these mixed crystals. We observe an intense feature of A₁ symmetry, whose peculiar lineshape suggests the occurrence of a Fano type interference between a discrete mode and an overlapping continuum of states.     

Generation of phase nuclei by solitons of the new “undulator” type in martensitic phase transitions of crystalline materials

The microdynamics of large-amplitude nonlinear lattice vibrations of plutonium and uranium materials has been investigated at high reactor temperatures in the ranges of martensitic phase transitions. Topologically new large-amplitude solitons of the “undulator” type have been revealed. Transverse and longitudinal “undulator” solitons in crystals with hexagonal and cubic symmetry, depending on the direction of motion, have different kinematic and amplitude characteristics, which differ from the characteristics of the previously known solitons. The transverse “undulator” solitons, like electrons in undulators, are observed with periodic atomic displacements orthogonal to the direction of soliton propagation. The longitudinal “undulator” solitons with displacements of atoms in the direction of soliton propagation are characterized by periodic delays with two-step velocities on the trajectory in a certain analogy with two-period engineering undulator devices. It has been shown that, at high energies, such “undulator” solitons of two types generate nuclei of a new phase in early stages of structural phase transitions.     

Density of states determination of mixed semiconductors by neutron diffraction

The phonon density of states of mixed crystals has been determined by neutron diffraction in powders. In the “two mode type” mixed crystal ZnS_xSe_1-x, the great features of the host lattice density of states, are not modified up to an “impurity” concentration of 25%. On the other hand, the presence of Te in ZnSe_xTe_1-x with x = 0.25, give rise to a “one-mode type” mixed crystal, the density of states of which is an average of that of the two components.These results justify that the eigenfrequencies of a two modes type mixed crystal can be calculated using the unperturbed host lattice density of states (additivity of Green's function).     

Raman scattering from crystalline and amorphous (GaSb)1-xGex semiconducting films

The first reported Raman scattering experiments have been performed on single crystal and amorphous films of the metastable alloy (GaSb)_1-xGe_x with compositions across the pseudobinary phase diagram. In crystalline films, the optical phonons exhibit a “one-two” type mode behavior. Broadening and softening of the Raman peaks with increasing alloy concentrations are attributed to a relaxation of q-vector selection rules due to substitutional disorder on both the cation and anion sublattices. Additionally, disorder induced scattering from the zone-edge acoustic phonons was observed. In amorphous alloy films, the reduced Raman spectra were compared to the one-phonon densities of states of the end-member crystals. The resulting apparent lack of polarization dependence indicated that the amorphous films were of the random network type.     

Theory of the influence of optical activity on Raman effect in crystals

The theory of the “banding” of Raman lines observed in optically active crystals has been fully worked for the different classes of Raman lines. In the case of cubic crystals the Raman lines belonging toE andF classes can exhibit “normal” “reverse” or any intermediate type of banding depending on the crystal orientation, and further the perfection of the banding can vary forE lines. The crystal axes and the sign of optical activity of a cubic crystal can be deduced from Raman effect studies.     

Theoretical calculations of the properties of point defects in solids

In accordance with the terms of reference of this series of journals, we have set out in this review to give as far as is possible a critical overview of the methods currently available for the calculation of the various properties of point defects in, first, ionic crystals and, then, in metals. We have restricted ourselves to these two classes of solids since, at present, such calculations as have been made for other materials are relatively few in number and, in our opinion, relatively unreliable. In attempting to write a critical review of this area we have found it impossible to restrict our discussion entirely to relatively recent papers. For example, when we are discussing ionic crystals, it is essential that we review the Mott-Littleton method, and the original paper by these authors appeared in 1938. The real upsurge in interest in the type of calculation we are reviewing largely parallels the development of modern electronic computers. Thus, such calculations as have been made recently can be regarded as computer “experiments.” In a recent short review, Gilman' has justifiably pointed out that there comes a point at whch, from a “cost effectiveness” point of view, it becomes more economical to spend money doing real experiments on real crystals instead of computer experiments on model crystals. This is a valid point and should be borne in mind when assessing the relative merits of the types of defect calculations we discuss. In fact, there is a real danger in writing a “critical” review of this field in that one may convince the reader that the whole subject is an intellectual dead end, and we have tried to bear this in mind in our subsequent discussion.     

A new shoulder in luminescence spectra due to the triplet excitons in AgBr

Magneto-optical measurements of pure AgBr have been extended to cover the luminescence study. A new and intrinsic shoulder in luminescence spectra supposed to be due to “the indirect allowed triplet excitons” has been found, for the first time, for zone refined AgBr crystals at high magnetic fields up to 50 kOe.     

Vibrational spectra and ab initio computations of sarcosinium oxalate monohydrate

Single crystals of sarcosinium oxalate monohydrate (SOM) are grown by the slow-evaporation technique at ambient temperature, and vibrational spectroscopic analysis is carried out using NIR-FT Raman, FT-IR, and SERS spectra. The normal mode frequencies and corresponding vibrational analysis of SOM are examined theoretically using the Gaussian’98 set of quantum chemical codes. The two bands present in the SOM ν C=O region, clearly observed in the Raman spectrum, are assigned to “free” and “bonded” carbonyl groups with the hydrogen atom. Vibrational analysis indicates the presence of C-H—O hydrogen bonding interaction producing a blueshift of the C-H stretching frequency.     

含油岩心显微红外光谱成像方法的研究

虽然用红外光谱显微探针方法研究岩心中碳质物已有很多工作,但是通常对诸如岩心等样品物性的光谱学微探针实验研究多是局限于对经过复杂处理分离出的微小样品或样品中个别位点所得的结果,缺乏对复杂样品各组分(或基团)的空间分布及其相互关系的研究。近年来新发展起来的光谱成像分析系统将光谱技术与成像技术有机地结合起来融为一体,可在光谱和空间两个方面对目标样品进行分析和识别并已得到广泛应用,但是用红外显微成像光谱研究岩心的工作则少有报道。文章报道了应用透射和衰减全反射(ATR)两种方式对含油岩心进行了显微红外光谱成像(Mapping)的研究。从灵敏度、信噪比、光谱分辨率和空间分辨率以及工作效率等方面对两种方式所得实验结果进行了比较和评估。     

Vibrational EELS studies of CO chemisorption on clean and carbided (111), (100) and (110) nickel surfaces

Room temperature adsorption of CO on bare and carbided (111), (100) and (110) nickel surfaces has been studied by vibrational electron energy loss spectroscopy (EELS) and thermal desorption. On the clean (100) and (110) surfaces two configurations of CO adsorbed species, namely “terminal” and bridge bonded CO, are observed simultaneously. On Ni(111), only two-fold sites are involved. The presence of superficial carbon lowers markedly the bond strength of CO on Ni(111)C and Ni(110)C surfaces, while no adsorption has been detected on the Ni(100)C surface. Moreover, on the carbided Ni(110)C surface, the adsorption mode for adsorbed CO is changed with respect to the clean surface; only “terminal” CO is then observed.     

ESR of Fe3+ -Rhombic,Fe2+ -Vo and Fe1+ -Vo complexes in SrTiO3

Three new Fe-defect complexes have been identified by ESR in Fe doped crystals of SrTiO₃. These are an Fe³⁺ center with rhombic fine structure in the “as-grown” crystals, and the vacancy associated complexes Fe²⁺ -V_o and Fe¹⁺ -V_o in “reduced” crystals. Photo-excited charge transfer effects are observed for these centers.     

Visualization of magnetic flux penetration and trapping in superconducting Bi2Sr2CaCu2Ox single crystals

Magnetic flux profiles in superconducting Bi₂Sr₂CaCu₂O_x single crystals have been observed using a magnetic garnet film as a magnetic field sensor. Measurements have been performed for the “zero-field-cooled” and “field-cooled” regimes as a function of temperature and magnetic field. An analysis of the observed patterns has revealed some peculiarities in the flux trapping effects. In particular, a nonuniform magnetic structure has been directly observed in the remanent state after “field-cooled” processes.     

Results of irradiating aluminum and homogeneous alloy YMn2 by 23 MeV γ-quanta in a molecular deuterium atmosphere at 2 kbar pressure

Specimens of a number of metal were placed successively along the length in a deuterium high-pressure chamber of the “finger type” (DHPC-FT). The specimens were: two aluminum rods, a copper rod, two YMn₂ alloy specimens, and stainless steel. The molecular deuterium pressure in the DHPC-FT chamber was 2 kbar. The specimens were irradiated by braking γ-quanta with boundary energy 23 MeV. After irradiation, all specimens were investigated on scanning electron microscopes (SEM) with electron probe X-ray microelement analysis (XMA). Considerable changes in the structure of the surfaces and elemental composition of the measured aluminum, destruction of the homogeneous YMn₂ alloy specimen, and the “formation of monocrystalline specimens” of the YMn₂ type and structures resembling manganese-based “crystals” were observed. A phenomenological explanation of the observed phenomena and effects based on nuclear reactions is proposed with consideration of certain new approaches, which are examined.     

Critical anomaly in the electron spin-lattice relaxation at the ferroelectric phase transition of X-irradiated KDA

The spin-lattice relaxation of X-irradiated ferroelectric KDA has been investigated by means of the electron spin-echo method in the range between 2 and 200 K. In the vicinity of the phase transition point an anomalous increase of T₁ has been observed. This effect could not be detected for KDA-KDP mixed crystals with a high concentration of KDP. The anomaly of the spin-lattice relaxation at the phase transition is explained by the increased damping of the “hard” optical mode which governs the relaxation behaviour at this temperature region.     

Electrical resistivity of antiferromagnetic GdP

The electrical resistivity of GdP single crystals has been measured between 4.2 and 300 K as function of the stoichiometry. The metallic type of resistivity exhibits critical scattering of electrons at the Néel temperature. Within the range of stoichiometries investigated, the type of scattering changes between “Ornstein-Zernike” like and “Fisher-Langer” like. The magneto-resistance is negative.     

Modulating lamb wave band gaps using an elastic metamaterial plate

Modulating band gaps (extending the bandwidths or shifting into a lower frequency range) is a challenging task in phononic crystals. In this paper, elastic metamaterial plates composed of a square array of “hard” stubs or “soft” stubs on both sides of a 2D binary locally resonant plate are proposed, and their band structures are studied. The dispersion relationships and the displacement fields of the eigenmodes are calculated using finite element methods. Numerical results show that the band gaps are shifted to lower frequencies and the bandwidths are enlarged compared to classic elastic metamaterial plates. A conceptual “analogousrigid mode” that includes an “out-of-plane analogous-rigid mode” and an “in-plane analogous-rigid mode” is developed to explain these phenomena. The “out-of-plane analogous-rigid mode” mainly adjusts the band gaps into the lower frequency range, and the “in-plane analogous-rigid mode” mainly enlarges the bandwidth. Furthermore, the band gap effects of composite “hard” stubs and “soft” stubs are investigated. The results show that the location of the band gaps can be modulated into a relatively lower frequency and the bandwidth can be extended by the use of different composite stubs. These elastic wave properties in the proposed structure can be used to optimize band gaps and possibly produce low-frequency filters and waveguides.     

Electromagnetic waves in uniaxial crystals with metallized boundaries: Mode conversion,pure reflections,and bulk polaritons

A theory is constructed for the reflection of plane electromagnetic waves in uniaxial crystals with a positive definite permittivity tensor and an arbitrarily oriented metallized boundary. The problem is solved both for general-position orientations corresponding to three-partial reflection and for special conditions allowing two-partial reflections: mode conversions when the incident and reflected waves belong to different sheets of the refraction surface and “pure” reflections when both waves belong to the same sheet. The space of pure reflections is shown to be formed by two types of optical-axis orientations: arbitrary directions in the plane of the crystal surface and in the plane of incidence. The configurations of the conversion surface for optically positive and negative crystals are investigated. A subspace of pure reflections that transform into one-partial bulk polaritons with the energy flux parallel to the surface at grazing incidence has been found. The domain of existence of such bulk eigenmodes is bounded by two “lines” of solutions. These are any directions along the boundary containing the optical axis for ordinary polaritons and the direction along the projection of the optical axis onto the surface at an arbitrary orientation of the axis with respect to the boundary for extraordinary polaritons.     

Temperature-Dependent Raman Study on Vibrational Modes in LiTaO3

In this paper the results of temperature-dependent Raman study on the vibrational modes in LiTaO₃ crystals are presented. It is found that the two low-frequency A₁ modes show significant changes during the temperature changing process. “Modes softening” and “frequency-repulsion” effect was observed and dicussed.     

Ferroelasticity in non-group-subgroup transitions

Aizu's definition of ferroelasticity can be extended to transitions between two crystals whose point groups are not group-subgroup related. In general, two different variant structures then appear on both sides of the transition, each variant structure being determined by the operators “lost” at the transition. If a minimal common supergroup exists, then it is the prototype group. For each variant structure, a spontaneous strain tensor can be defined. Ferroelasticity occurs for both structures, and is termed “double ferroelasticity”, if none of the groups belong to the same symmetry system as the common minimal supergroup. Examples are discussed.