Retardation of dislocations by the Cherenkov mechanism in crystals with a soft mode

The retardation mechanism of dislocations in crystals with a soft mode on account of Cherenkov emission of soft optical phonons is considered in the present work. The conditions in which this retardation mechanism appears are examined for various orientations of the wave vector q_c characterizing structural phase transition. It is shown that the threshold velocity of the retardation mechanism in the case where q_c¦V (V is the velocity of motion of the dislocations) decreases on approaching the phase-transition temperature T_c in proportion to ¦t_c–T_c¦. The dependence of the dynamic retardation coefficient B on the velocity V and the closeness to the point of structural phase transition is obtained.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 59–64, December, 1988.     

Infrared induced visible emission from porous silicon: the mechanism of anodic oxidation

The visible luminescence caused by anodic oxidation of p-type porous silicon has been studied. It is shown that similar luminescence can be observed in n-type material by illumination with near-infrared light. Addition of a suitable reducing agent to the electrolyte solution can both suppress the oxidation of the porous layer and quench its luminescence. These results confirm a previously suggested mechanism, in which the capture of a valence band hole in a surface bond of the porous semiconductor gives rise to a surface state intermediate capable of thermally injecting an electron into the conduction band.     

On the instability of parallel weakly interacting dislocations in soft-mode crystals

The instability of the rectilinear form of parallel weakly interacting dislocations, manifesting itself in their spontaneous wave-like bending, has been investigated. It is shown that softening of a crystal leads to the enhancement of this instability.     

Inelastic incoherent scattering of neutrons by dislocations in crystals

A possibility of determination of the magnitude of the Peierls stress τ_p from the spectrum of neutrons, inelastically scattered by dislocations, is considered. The dislocation dynamic has been described by the flexible string model. It is shown, that in the case of multiphonon scattering one can find τ_p by measuring the temperature dependence of the half-width of the Gaussian curve (8). It is possible to determine τ_p also through the measurement of the width of a gap, which is present in the energetical distribution of neutrons, scattered by means of one-phonon exchange.     

Photonic crystals for the visible spectrum by holographic lithography

We describe a general and flexible technique for the fabrication of three-dimensional photonic crystals that is particularly well adapted to the production of structures with the sub-micron periodicity required for applications in the visible optical spectrum. Three-dimensional microstructure is generated by using a four-beam laser interference pattern to expose a thick layer of photoresist. Exposed areas are rendered insoluble; unexposed areas are dissolved away leaving a three-dimensional photonic crystal formed of cross-linked polymer with air-filled voids. The polymeric structure may be used as a template for the production of photonic crystals with higher refractive index contrast. Photonic crystals made of polymer and of TiO₂ have been characterized by scanning electron microscopy and by optical diffraction measurements.     

Influence of electrical pulses on the distance traveled by dislocations in NaCl single crystals

A study was made of the influence of rectangular electrical pulses, applied either before or simultaneously with mechanical loads, on the distance traveled by edge and screw dislocations. A considerable increase in this distance was observed. The effect was a function of the type of dislocation, impurity composition of crystals, and electric field orientation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 10–14, October, 1980.     

Formation of antisite defects by gliding dislocations in sphalerite-structure crystals

Conditions are discussed under which an interstitial atom absorbed in the core of a gliding 60° dislocation belonging to the glide set can be transformed into an antisite defect. The mechanism considered may be responsible for an increase in the concentration of As_Ga defects observed in GaAs single crystals after their plastic deformation.     

On the nature and mechanism of surface polygonization of NaCl and KCl single crystals produced by electron bombardment and illumination

Abstract

The polygonization phenomenon observed on alkali halide surfaces, irradiated with electrons and subsequently illuminated, was studied by optical and electron microscopy. For the explanation of the nature and mechanism of this process, we have investigated the influence of light (containing the F?centre absorption band) on the development and annihilation in time of some polygonization lines, as well as the correlation of the polygonization patterns with dislocation etch pits and subgrain boundaries revealed by a selective chemical etching.

Preceded by an optical bleaching effect which locally induces surface potential changes, the polygonization process is explained taking into account the mobility of unpinned charged dislocations and the surface atom migration. The disappearance of some polygonization lines may be produced by a vacancy-interstitial annihilation process.

No identification was found between the polygonization patterns and the subgrain boundaries.     

On the surface analysis of small metal crystals

Small metal crystals as catalyst particles have often a surface which is composed not only of plane but also of curved faces. Using the example of the electron micrograph of a Pt particle an analysis is described which enables a determination of the orientation, the shape and the faces of a crystal. The analysis shows that the particle has a [320] orientation. The surface consists to ~ 37% of {111} faces, to ~ 28% of {100} faces and to ~ 35% of curved area. The curved region is mainly composed of {011}, {113}, {012} and {133} faces. The curved surface has the structure of a frozen state of thermal roughening. Such frozen structures have special adsorption sites in low concentration which in some cases may be important for the catalytic reaction.     

On the redistribution of dislocations in silicon single crystals in the vicinity of stress concentrators

The mechanism of relaxation of stresses through the multiplication and motion of dislocations in the vicinity of a stress concentrator (scribe) in silicon single crystals is investigated. The energy-related and dynamic characteristics of the observed processes are determined for different angles of orientation of stress concentrators with respect to the [110] direction.     

On surface layers on BaTiO3 single crystals

The possibility of explaining the existence of a surface layer on BaTiO₃ crystals by the presence of Schottky defects in the crystal is investigated. The magnitude of the potential difference between the surface and the interior of the crystal and the thickness of the surface layer, which at temperatures of over 400°C agrees with the measured values, are calculated for the cubic phase from a simplified ionic model.

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BaTiO₃

iO₃ . , 400° .

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In conclusion the author would like to thank Dr. M. Trlifaj and V. Janovec, candidates of physics and mathematics, for valuable remarks.     

Dynamics of kinks on dislocations in SiGe single crystals

The method of intermittent pulse loading is used for obtaining the dependences of the mean free path of individual dislocations in SiGe single crystals with various concentration of Ge (0–5.5 at. %) on the duration of loading pulses and time intervals between them. It is found that these dependences change qualitatively upon an increase in the Ge concentration. It is shown that the motion of dislocations in SiGe crystals under small shear stresses is characterized by a nonlinear drift of kinks and the formation of superkinks. A theory of the motion of dislocations under the action of intermittent pulse loading under the conditions of heterogeneous kink dynamics is developed. Extended quasi-one-dimensional defects repeating the shape of a part of a segment of a moving dislocation are discovered in SiGe crystals containing 0.96 at. % Ge. The mechanism of formation of such defects as the result of the shedding of a part of the impurity atmosphere by a dislocation segment during overcoming of a local obstacle is proposed.     

Visualization of surface acoustic wave scattering by dislocations

Stroboscopic X-ray topography at the synchrotron beam line was used to visualize the propagation of a 580 MHz surface acoustic waves (SAW) in LiNbO3 crystals. For this purpose, the X-ray bursts coming from the synchrotron storage ring with periodicity of 5.68 MHz were synchronized with the SAW frequency in a phase-locked mode. This method allowed us to "stop" the SAW in time and to observe the X-ray diffraction contrast caused by the dynamic deformation field of SAW. The X-ray topographic images showed well-resolved individual acoustic wave fronts of 6 microm SAW as well as their distortions due to SAW scattering by linear dislocations. Some of the images revealed an exceptional contrast of the concentric rings about the dislocation line, which is caused by coherent interaction of the secondary elastic waves. This contrast is similar to the Fresnel zones in optics, and this conclusion is confirmed by direct summation of secondary waves emitted by local elements of a vibrating dislocation string.     

On the generation of vacancies by moving dislocations

New experiments of Molenaar and Aarts, Blewitt and others seem to confirm the view of the author, previously based only on the experiments of Gyulai and Hartly and Stepanow on sodium chloride, that vacant lattice sites, and possibly interstitial atoms, are generated during plastic flow in ductile crystals, particularly in metals. It is pointed out that the average temperatures near a moving dislocation are probably not sufficiently high to evaporate vacant lattice sites or interstitial atoms as a result of thermal effects alone. Instead, one apparently must conclude that the imperfections are generated either by purely geometrical means during the looping of dislocations about appropriate obstacles, as the result of dynamical instability in the motion of a dislocation, possibly near a jog, or in the very high thermal pulses or ‘spikes’ which are generated either in the zone where two dislocations of opposite sign annihilate one another or near impediments where dislocations are strongly curved. It is pointed out that a pair of vacancies is probably stable near room temperature and may diffuse more rapidly than a single vacancy. It is also proposed that vacancies retained during quenching of Al-Cu alloys and those generated by cold-work play an important role in the precipitation process. The origin of work hardening in single crystals is discussed and several alternative interpretations, which involve the impediment of Frank-Read generators either directly or indirectly as a consequence of the generation of vacancies, are presented. The importance of prismatic dislocations formed by condensation of vacancies is restated. The role that vacancies formed by cold-work may play in determining the stored energy and decrease in density and in affecting processes such as creep and the hardening of latent slip planes is also discussed. Finally a few experiments are proposed, typical of those which could prove decisive in isolating the influence of vacancies.     

On the surface energy of crystals of inert gases

The dependences of the specific surface energy (σ) and its isochoric temperature derivative (?σ/?T) _V on the degree of compression (V/V ₀) of the crystal are calculated on the basis of the Mie-Lennard-Jones pair potential of interatomic interaction. The calculations are performed for all face-centered cubic crystals of inert gases (from Ne to Rn) to the degree of compression V/V ₀ = 0.016 along three isotherms: 1K, T _m and 300 K, where T _m is the melting temperature at zero pressure (V/V ₀ = 1). The activation processes such as the creation of vacancies and self-diffusion are taken into account in the calculations. It is shown that the isotherm σ(V/V ₀) reaching its maximum at (V/V ₀)_max sharply decreases upon further compression. The surface energy becomes negative (σ(V/V ₀) _fr =0) at V/V ₀ ≤ (V/V ₀) _fr < (V/V ₀)_max which should stimulate the process of crystal fragmentation, i.e., an increase in the specific (per atom) intercrystallite surface. It is shown that at high temperatures the condition of fragmentation holds in the crystal in the case of uniform tension, but it is already in the region of the liquid phase. The values of σ, (?σ/?T) _V , the vacancy concentration and the fraction of the diffusion atoms are estimated at the points: V/V ₀ = 1, (V/V ₀)_max and (V/V ₀) _fr at 1 K, Tm and 300 K. The size evolution of the surface and activation parameters is studied using neon as an example.     

Effects of size on the dynamics of dislocations in ice single crystals

Single crystals of ice subjected to primary creep in torsion exhibit a softening behavior: the plastic strain rate increases with time. In a cylindrical sample, the size of the radius affects this response. The smaller the radius of the sample becomes while keeping constant the average shear stress across a section, the softer the response. The size-dependent behavior is interpreted by using a field dislocation theory, in terms of the coupled dynamics of excess screw dislocations gliding in basal planes and statistical dislocations developed through cross slip occurring in prismatic planes. The differences in the results caused by sample height effects and variations in the initial dislocation microstructure are discussed.     

单晶铁金属表面污染物的激光烧蚀机理

激光惯性约束核聚变装置中要求光学元件能够承受极高的激光通量,因此对装置内部洁净度有很高的要求.研究表明装置内部的颗粒污染物主要来源于装置内的机械结构件,杂散光作用下机械结构件表面的损伤将产生颗粒污染物.精密金属构件的激光诱导损伤问题是制约高功率激光装置超洁净制造的重要因素.由于机械结构件表面不可避免地存在污染物,因此本文基于传统的分子动力学能量耦合方式,模拟了激光与表面吸附污染物单晶铁的相互作用过程,探讨了铁材料在激光作用下的烧蚀行为,并分析了激光加载方式和激光能量密度对铁材料烧蚀的作用情况,对比研究了材料表面有无污染物对材料烧蚀的影响情况.研究表明：激光作用下铁材料表面原子在污染物原子的剧烈碰撞下呈现出不同的运动状态;激光能量瞬时加载时更容易烧蚀铁材料;当激光能量密度低于0.0064 J/cm²时,将去除铁材料表面的污染物并不会对铁材料产生烧蚀现象,进一步分析表明铁材料表面吸附污染物时更容易被激光烧蚀.研究结果可为提高高功率激光装置的内部洁净度、实现系统超洁净控制提供理论依据.