Torsion method for measuring piezooptic coefficients

We develop and describe analytically a torsion method for measuring piezooptic coefficients associated with shear stresses. It is shown that the method enables to increase significantly the accuracy of determination of piezooptic coefficients. The method and the appropriate apparatus are verified experimentally on the example of LiNbO₃ crystals. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Piezo-optic properties of incommensurately modulated K<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> crystals

The effect that uniaxial pressures along the principal crystallophysical directions have on the dispersion and temperature dependences of the birefringence in incommensurate K₂ZnCl₄ crystals has been investigated in a wide temperature range. This parameter has been established to be fairly sensitive to uniaxial pressures. The spectral and temperature dependences of combined piezo-optic coefficients are analyzed. Significant anomalies of these coefficients are found at the incommensurate-commensurate phase transition. They are due to the change in the induced birefringence that results from spontaneous polarization and to the significant effect of uniaxial stress on the soliton structure of the crystal. The contributions that the electrooptic effect, the elasto-optic effect, and the order parameter make to the spontaneous changes in the piezooptic coefficients are separated.     

Piezo-optic properties of TGS crystals doped with <Emphasis Type="Italic">D</Emphasis>-serine

The effect of uniaxial mechanical pressure along the main crystallophysical axes on the principal values of birefringence Δn _i in TGS crystals doped with 5% D-serine is investigated. It is ascertained that the values of Δn _i are rather sensitive to the action of uniaxial stresses. The temperature shift coefficients ?T _c /?σ_m are determined for the phase-transition points. The temperature and spectral dependences of the combined piezo-optic constants π _im ⁰ are calculated. The contribution of the secondary electro-optic effect to the changes in the birefringence and the piezo-optic constants of doped crystals is determined. It is found that the piezooptic constants π ₂₃ ⁰ and π ₁₂ ⁰ have the same values, which indicates a decrease in the anisotropy of the optical indicatrix of doped crystals under the action of uniaxial stress. The optical and deformation contributions to the relaxation effect of piezo-optical birefringence in doped TGS crystals are found.     

Heterogeneous nucleation (and adhesion) of lysozyme crystals

The heterogeneous nucleation of hen-egg-white lysozyme (HEWL) crystals has been investigated using a double-(thermal)-pulse technique, thus detaching nucleation from the growth stage. n(t) dependencies of the nucleus number, n, vs. time, t, were plotted for surfaces rendered hydrophobic (by means of hexamethyl-disilazane) and for bare glass surfaces. Preferred crystallite orientation supplied additional information. The discussion is based on the classical Stranski–Kaischew theory of crystal nucleation. With a purposely developed new technique the adhesion energies have been measured for HEWL crystals, grown on different supports.     

Surface and bulk magnetic properties of amorphous and nanocrystalline Ni-substituted FINEMET samples

The effect of partial substitution of Fe by Ni on the bulk and surface magnetic properties in as-quenched and nanocrystalline Fe_73.5?xNi_xCu₁Nb₃Si_13.5 (x = 0, 8, 10 and 20) alloys has been investigated. A study of the variation of the surface magnetic properties with oxidation in these alloys is also presented. The surface hysteresis loops were obtained by the transverse magneto-optical Kerr effect. The experimental results show that the as-quenched and annealed samples with lower surface coercive field values are those with lower Fe/Si ratio in the sample surfaces. No significant changes between the values of surface H_c for the oxidized and non-oxidized annealed samples with the same Ni content have been observed.     

Characterization of defects generated by di-and trivalent cations in the potassium-dihydrophosphate structure and their influence on growth kinetics and face morphology

Different types of defect sites generated by the impurities of divalent (M²⁺) and trivalent (M³⁺) metals in the structure of potassium dihydrophosphate KH₂PO₄ (KDP) were revealed by crystal-chemical analysis and computer simulation. These sites cause different deformations of the crystal matrix by generating different local strains, which enhance the inhibiting effect of impurity atoms adsorbed on the surface. This fact accounts for the different influence of di-and trivalent cations on the growth kinetics and face morphology of KDP crystals. The effect of the M³⁺ ions is associated primarily with their adsorption on the face surfaces, whereas the influence of the M²⁺ ions results from their insertion into the surface layer of the crystal.     

Épitaxie en phase liquide des composés III–V sur substrat InP

The surface preparation and characterization of InP substrates have been studied. It is shown that a 1–3% bromine-methanol etching solution gives a smooth surface if 10 μm or more has been removed. Auger spectrometry detected some contamination of carbon and oxygen by the atmosphere during substrate preparation. The concentration of carbon impurities is less than in the case of GaAs. Liquid phase epitaxy of InP and Ga_xIn_1?xAs_1?yP_y was achieved by a slider technique. Equilibrium cooling, supercooling and step-cooling processes have been tried. It is demonstrated that misorientation from a (111) plane gives a “scale” structure, the orientation of which is exactly (111). Smooth LPE surfaces have been obtained with the supercooling and the step-cooling processes.     

Crack path instabilities in DCDC experiments in the low speed regime

We studied the low speed fracture regime (10⁻⁴-10⁻⁹ m s⁻¹) in different glassy materials (soda-lime glass, glass ceramics) with variable but controlled length scale of heterogeneity. The chosen mechanical system enabled us to work in pure mode I (tensile) and at a fixed load on double cleavage drilled compression specimen. The internal residual stresses of studied samples were carefully relaxed by appropriate thermal treatment. By means of optical and atomic force microscopy techniques fracture surfaces have been examined. We have shown for the first time that the crack front line underwent an out-of-plane oscillating behavior as a result of a reproducible sequence of instabilities. The wavelength of such a phenomenon is in the micrometer range and its amplitude in the nanometer range. These features were observed for different glassy materials providing that a typical length scale characterizing internal heterogeneities was lower than a threshold limit estimated to few nanometers. This effect is the first clear experimental evidence of crack path instabilities in the low speed regime in a uniaxial loading experiment. This phenomenon has been interpreted by referring to the stability criterion for a straight crack propagation as presented by Adda-Bedia et al. [Phys. Rev. Lett. 76 (1996) 1497].     

Photo-induced phenomena in GeO2 glass

Photo-induced changes in GeO₂ glass have been studied in terms of its optical and structural properties. Band-gap excitation gives rise to modifications in mid-gap optical absorption and photo-luminescence spectra. It also produces electron-spin signals at g ≃ 1.995 and 2.008. Raman-scattering spectrum becomes sharper, X-ray diffraction patterns show a disappearance of a crystalline peak, and the illuminated surfaces expand. These observations can be accounted for by assuming bond transformations from rutile-like to defective quartz-like structures and/or from small to large rings. These photo-induced changes in GeO₂ are discussed in comparison with those in SiO₂ and GeS₂.     

Model for the amorphous roughening transition in amorphous semiconductor deposition

The evolution of surface roughness on hydrogenated amorphous silicon (a-Si:H) films prepared by plasma-enhanced chemical vapor deposition exhibits initial smoothening due to nuclei coalescence, a region of surface stability, and finally a prominent roughening transition (designated a → a) at a critical thickness. The thickness at which the a → a transition occurs, as measured by real time spectroscopic ellipsometry (SE), is found to correlate closely with the electronic properties of the film. Thus, the transition has been incorporated into deposition phase diagrams that have been applied successfully to optimize solar cell performance and stability. A simple continuum model for the evolution of the 1D surface profile using an initial condition designed to be consistent with the nucleation characteristics measured by real time SE is sufficient for insights into the correlation between the roughness evolution and film properties. Good agreement between the experimental results and model calculations support the concept that optimum electronic properties of the films are associated with weakly reactive surfaces and long lifetime radicals that lead to adsorbed precursors with large surface diffusion lengths.     

Application of the optical method for determining of the RMS roughness of porous glass surfaces

The application of an optical method for characterizing surface roughness is presented. This method was used for an examination of porous glass surfaces. The expressions relating the root mean square rms (σ) of a surface to its specular reflectance at normal incidence are used for σ ? λ, (λ - wavelength). For light of sufficiently long wavelength the decrease in the measured specular reflectance due to the surface roughness depends only on the root mean square (rms) height of the surface irregularities. On the basis of reflectance spectra, one can determine σ for the porous glass surfaces after technological processes. The measured reflectance spectra were compared with calculated ones for which the scattered component of light was taken into account. The parameters rms determined from the optical method are comparable to those obtained from atomic force microscopy examinations.     

The influence on a stability analysis involving a prototype solidification problem of including an interfacial surface entropy effect in the heat balance relationship at the interface

A reexamination of the conservation of heat boundary condition usually imposed at the solid-liquid interface during controlled solidification by means of a careful application, to energy conservation at that interface, of a general balance equation from continuum mechanics for surfaces of discontinuity yields a new conservation of heat condition containing an extra term involving interfacial curvature and surface entropy. A standard linear stability analysis by a normal mode technique of the uniform growth of a planar interface of a pure solid into a thermally undercooled liquid shows that the consideration of this extra term has a destabilizing effect on a model which does not include it for this prototype priblem if surface entropy is negative and a stabilizing influence if it is positive.     

Influence of the VPE parameters on the epitaxial film properties for the (Pb1−xSnx)1+yTe1−y system

Epitaxial films have been produced for 0 ? x ? 1 by steps of 0.1. The growth rate, crystalline quality, carrier concentration and composition have been measured as a function of source, substrate and Te₂ source temperature. The growth rate is adjustable between 0.1 and 36 μm h^-1 and the films have smooth and shiny surfaces. The crystalline quality, conduction type and carrier concentration is mainly determined by the Te₂ pressure. It is possible to choose the type and the carrier concentration in the stable region for a source composition x ? 0.2. The films are always p-type if x ? 0.3. The composition of the films is varied by the source and by the substrate temperature. It is possible to vary the film composition compared to the source composition in the range of ±20% by varying the substrate temperature.     

Structural changes across the glass-transition in a glassy-crystal

The changes in molecular geometry, short-range order and crystal lattice parameter across the calorimetric glass-transition from the orientational-glass state into a plastic crystal, as well as structural changes within the molten state, have been determined from neutron diffraction for the most fragile orientational-glass former known so far, i.e., 1,2-difluoro-1,1,2,2-tetrachloroethane. In addition to its high fragility, the material is of interest due to the existence of two conformers having nonpolar and polar characters, which gives rise to different molecular potential energy surfaces generating an intrinsic source of competing interactions. The radial distributions as well as changes of the intermolecular structure were monitored by the temperature dependence of the lattice parameter. They evidence a remarkably strong interplay between internal and external molecular modes.     

Termination effect in X-ray diffraction studies of disordered systems

The effect of termination on the structural parameters of the radial distribution function of atoms has been analyzed with the use of the structure factor calculated for the model of soft spheres. It is shown that in the range of the experimentally attainable magnitudes of the diffraction vector S _max, the position R ₁ and the area Z ₁ ^sym of the first maximum of the radial distribution function of atoms linearly depends on 1/S _max. The method is suggested for taking into account the effect of termination on the structure parameters.     

The Effective Temperature of Surfaces of Liquid Metals and Some Non-Metals

An estimation of decrease of atomic bond energy by raising temperature for surfaces of liquid metals and some non-metals had been made. The calculation values of the derivative du_sdu_b where u_s and u_b, are bond energy of atoms in surface and bulk of a liquid, are significantly above 1 and in limits from 2 to 10. It was supposed that this results from anomalously intensive thermal expansion of surface layers of liquids – the effect which is well known for crystal surfaces. This expansion was explained by the effective temperature of these surfaces which, thus, was found to be by several times above those for bulk of liquids.     

On the internal nucleation of melting

Crystals of quartz have been superheated by some 450°C and crystals of albite by some 185°C above their respective melting points. In all cases, melting took place by the nucleation of liquid at the external surfaces (and internal boundaries as well in the case of albite). No evidence for the internal nucleation of liquid was found at any superheat for either material.The results of quartz indicate an exceptionallu large barrier to the internal nucleation of liquid. It is suggested that this large nucleation barrier is associated with the strain energy of forming a liquid nucleus within the crystalline phase.It is also indicated that the nucleation of liquid at the external surfaces of crystals at negligible superheats suggests that the free surfaces of liquids do not per se serve as preferred nucleating sites for crystallization — and that the crystal nucleation often observed at external surfaces or internal surfaces is in fact associated with condensed second-phase impurities.     

Study of microindentation cracks in bismuth-doped arsenic selenide glasses

The formation of radial cracks from Vickers indentations on the surfaces of both unirradiated and irradiated samples of (As_1 ? xBi_x)₂Se₃ glasses with x ≤ 0.1 is investigated as a function of indentation load P. It was found that radial cracks around indentations are produced on the surfaces of all samples at loads exceeding a particular applied load P_c^app but this threshold load is higher by a factor of 15 to 30 from the value of P_c calculated from the theoretical dependence of ratio c/d of crack length c measured from the center of indentation impression to indentation impression diagonal d on applied load P. By fitting the experimental P(c^3/2) data in an expression relating c and P according to expanding cavity for elastic/plastic solid and from a theoretical expression relating load-independent hardness H and threshold load P_c for crack formation estimated from the experimental data of the dependence of c/d on P, the value of indentation fracture toughness K_C of As₂Se₃ glasses, irrespective of Bi content in the samples and their irradiation, was found to be about 0.52 MPa?m^1/2.     

Energy criteria for orientation of smectic C* liquid crystals in electrooptic elements

Energy analysis of the types of orientation of smectic C* liquid crystals is performed. Criteria for the type and quality of their orientation in electrooptic elements with surfaces of orientants of different nature are revealed. An energy parameter of the optical quality of planar orientation of a smectic C* liquid crystal is proposed, which determines the contrast ratio of electrooptic elements independent of the type and nature of the smectic C* liquid crystal and the orientant.     

Anisotropy of the electro-optic effect in magnesium-doped LiNbO3 crystals

Specific features of constructing indicatory surfaces of the electro-optic effect (EOE) are described. The generalized case for the symmetry class 3m and all three possible indicatory surfaces under orthogonal experimental conditions, when the directions of light propagation and electric field are orthogonal, is considered. The EOE surfaces are constructed for magnesium-doped lithium niobate crystals. The specific features of anisotropy of these surfaces are discussed and their extreme values and corresponding angular coordinates are found. To construct the indicatory surfaces, all absolute coefficients of the linear electro-optic effect are determined by the interferometric method. The difference in the electro-optic coefficients of pure and magnesiumdoped crystals does not exceed 10%. An advantage of doped crystals is their high durability to high-power laser radiation.