Section images of dislocations normal to the surface of a 6H-SiC single crystal

Section topographs of edge and screw dislocations with an axis along [0001] in 6H-SiC are taken and interpreted, and the image formation is explained for this case. The contrast induced by various arrangements of dislocations within the Borrmann triangle is experimentally studied. The sign of the Burgers vector of an edge or screw dislocation normal to the crystal surface is shown to be unambiguously determined from the section-topograph image of this dislocation. The sign of the Burgers vector of a screw dislocation can also be determined from its image taken with Lang projection topography. The contribution of a long-range strain field to the section images of edge and screw dislocations normal to the crystal surface is revealed. The experimental contrasts recorded using section topography and Borrmann-effect-based topography are compared.     

On the mechanism of the electric field effect on dislocation mobility in alkali-halide crystals

The dependence is investigated of the velocity of motion of edge and screw dislocations on the applied shear stress in annealed and quenched NaCl and KC1 crystals with various impurity content under the action of an electric field of up to 50 kV/cm. It is shown that in impure crystals the action of the field is determined by its effect on damping centers, while in pure crystals, by its effect on charged dislocations. The interaction force of an edge dislocation with impurity-vacancy dipoles of various orientation is calculated. The charge density and sign of a dislocation is estimated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 16–21, May, 1975.     

Functional derivative of Tc with α2(ω)F(ω) for an anisotropic superconductor

The functional derivative δT_c/δα²(ω)F(ω) of the critical temperature (T_c) with the electron-phonon spectral density (α²(ω)F(ω)) gives information on the effectiveness of various phonon modes in enhancing T_c. For an anisotropic superconductor, it is found that δT_c/δα²(ω)F(ω) goes negative at some small but finite phonon energy. This contrasts with the isotropic case for which it is well known that the functional derivative is positive everywhere. Thus, very low energy phonons reduce T_c in an anisotropic superconductor which is similar to the known effects of static impurities that wash out anisotropy and hence reduce T_c.     

高质量三维胶体光子晶体中慢光速与超光速的实验研究

利用飞秒脉冲自相关技术研究了高质量三维胶体光子晶体中的慢光速和超光速效应.实验中使用的胶体光子晶体是采用压力控制的绝热沉积技术（PCIHVD）制备的、由聚苯乙烯小球组成的人工蛋白石结构.由于其中的缺陷和位错密度很低,它们具有很高的通带透过率和陡峭的能带边缘.测量了从通带到带边直至带隙中央群速度的变化,在只有20层左右小球的样品中观察到低至0.43c的慢光速以及高至1.34c的超光速现象.此外,利用时域有限差分（FDTD）方法对短脉冲在三维光子晶体中的群速度进行了数值模拟,并且和 关键词： 三维胶体光子晶体 慢光速 超光速 脉冲自相关     

Electric stimulation of magnetoplasticity and magnetic hardening in crystals

The effect of electric field E on the magnetoplastic effect (MPE) has been investigated in NaCl crystals with different impurities, which provide either the plasticization of the samples in the magnetic field (positive MPE) or their magnetic hardening (negative MPE). The mobility of individual dislocations under the joint action of the magnetic and electric fields and the mechanical load on the crystals has been studied. The sharp electric stimulation of the MPE of both signs has been revealed, i.e., an increase or a decrease in the mean free path of dislocations that is roughly proportional to exp(±E/E ₀) at E ? E ₀ ～ 1–10 kV/m. In particular, in the negative-MPE NaCl(Pb) crystals, the accompanying electric field enhances the magnetic suppression of plasticity. The results are attributed to the electrically induced transformation of the additional part of the pinning impurity ions Me⁺⁺ to the magnetically active state of Me⁺ on the dislocations. The subsequent magnetic transformation of the structure of these pinning centers should lead to a sharper variation of the dislocation pinning force (either an increase or a decrease, depending on the MPE sign).     

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.     

Acoustoplastic effect and internal friction of aluminum single crystals in various deformation stages

The dependences of the acoustoplastic effect and the internal friction on the oscillatory strain amplitude are measured in various deformation stages of low-purity aluminum single crystals. It is discovered that the acoustoplastic effect is observed not only in the macroscopic plastic region of the stress-strain diagram, but also for microplastic deformation in the “elastic” loading and unloading stages. The sign of the effect reverses during unloading. An increase in the strain rate leads to enhancement of the acoustoplastic effect and the absorption of the energy of ultrasonic vibrations causing this effect with a frequency of about 100 kHz. It is concluded that the acoustoplastic effect observed during both macro-and microplastic deformation is caused by the irreversible high-speed motion of dislocations through the long-range stress field of the other dislocations after breaking through the Cottrell atmospheres. Fiz. Tverd. Tela (St. Petersburg) 39, 1794–1800 (October 1997)     

Low-temperature heat transport in α-HgI2 single crystals

The thermal conductivity of several samples from α-HgI₂ crystals grown by two different methods has been measured from 50 mK to 200 K. The thermal conductivity is found to be intrinsic but anisotropic above 15 K: it is smaller along c-axis than along a-axis, the anisotropy ratio being about 5 between 15 and 200 K. Below 15 K, the thermal conductivity is sample dependent and the calculated Casimir limit is not reached at the lowest temperatures. The results have been interpreted considering phonon scattering by structural defects. A simple quantitative analysis of the curves suggests that phonons are scattered mainly by large clusters of interstitial defects due to the lack of stoichiometry of the crystals; the typical dimensions of these clusters are not smaller than 1 μm perpendicular to c-axis and 0.3 /gmm along c-axis. The presence of plane defects is also detected. Point defect scattering is relatively small and explained by residual metallic impurities and carbon at interstitial sites. The intrinsic anisotropy is briefly discussed.     

Strain-field scattering of fast-transverse acoustic phonons by dislocations in LiF

The mean-free-paths of transverse acoustic phonons generated in surface hot spots and propagating ballistically along [100] in LiF are calculated exactly for the case of scattering by the static strain fields of edge and screw dislocations of Burger's vector $\text{1}\text{2}\text{a [}\text{1}\text{10}\text{]}$. Due to wavevector conservation the screw dislocations are shown not to scatter such phonons at all while the edge dislocations lead to mean-free-paths greater than 12 cm for dislocation densities ?2×10⁸ cm^-2 and frequencies ?5×10¹¹ Hz. This is consistent with the observation by Northrop et al. (1982) of the ballistic propagation of such heat-pulse phonons across deformed crystals of 1 cm dimensions.     

A theoretical study of stress-induced effects on Mössbauer impurities forming dislocations in some b.c.c. and f.c.c. metals

Pramana - The external stress values required to move the dislocations in phase with the Mössbauer impurities, forming edge dislocations in different crystals are evaluated. The vibrating...     

Two-phonon bound states in imperfect crystals

The question of the occurrence of two-phonon bound states in imperfect crystals is investigated. It is shown that the anharmonicity mediated two-phonon bound state which is present in perfect crystals gets modified due to the presence of impurities. Moreover, the possibility of the occurrence of a purely impurity mediated two-phonon bound state is demonstrated. The bound state frequencies are calculated using the simple Einstein oscillator model for the host phonons. The two-phonon density of states for the imperfect crystal thus obtained has peaks at the combination and difference frequencies of two host phonons besides the peaks at the bound state frequencies. For a perfect crystal the theory predicts a single peak at the two-phonon bound state frequency in conformity with experimental observations and other theoretical calculations. Experimental data on the two-phonon infrared absorption and Raman scattering from mixed crystals of GA_1−c Al _c P and Ge_1−c Si _c are analysed to provide evidence in support of impurity-mediated two-phonon bound states. The relevance of the zero frequency (difference spectrum) peak to the central peak observed in structural phase transitions, is conjectured; This work is a part of the thesis to be submitted by one of the authors (SS) in partial fulfilment of the degree of Doctor of Philosophy to Utkal University, Bhubaneswar, India.     

Magnetic moment distributions and form factors in ferromagnetic nickel-ruthenium alloys

Using polarised neutrons, the full three-dimensional magnetic structure amplitudes in the Ni_1?c Ru _c single crystals forc = 0·027, 0·033 and 0·046 were measured. Moment density maps in various portions of the Wigner-Seitz cell were obtained. It is seen from these maps that unlike Ni-based alloys with 3d impurities, the introduction of Ru to the Ni matrix produces extensive perturbations in the diffuse moment density, giving rise to a netpositive diffuse moment which tends to increase with Ru concentration. The asphericity of the host moment at first increases and then decreases with increasing Ru content. Another significant outcome of the present study is the evidence for the reversal of the sign of the Ru moment, from negative to positive, obtained by comparing the shape of the spherical site form factors of the three-alloy concentrations with the Ni spherical form factor itself. The sign reversal of the impurity moment is confirmed by the form factor analyses. Strong local environmental effects seem to play a major role in this alloy system.     

Deformation-Induced Destruction of the Long-Range Order in Single Crystals of Ni3Fe Alloy

In the present work, x-ray diffraction and electron microscopy are used to investigate the state of a Ni₃Fe single crystal during plastic deformation. It is demonstrated that the observed destruction of long-range atomic order is accompanied with splitting of superdislocations and generation of single dislocations. The change of the long-range order parameter for single crystals deformed in the [001] direction estimated from the degree of splitting of superdislocations is in good agreement with x-ray measurements of this parameter. The destruction of long-range order for intermediate deformations < 40% is well described by the model of mechanical mixing of the alloy, the main role in which is played by the mechanisms of plane defect generation by shears along slip planes.     

Modeling of Dislocation Generation and Interaction During High-Speed Deformation of Metals

Recent experiments by Kiritani et al. [1] have revealed a surprisingly high rate of vacancy production during high-speed deformation of thin foils of fcc metals. Virtually no dislocations are seen after the deformation. This is interpreted as evidence for a dislocation-free deformation mechanism at very high strain rates. We have used molecular-dynamics simulations to investigate high-speed deformation of copper crystals. Even though no pre-existing dislocation sources are present in the initial system, dislocations are quickly nucleated and a very high dislocation density is reached during the deformation. Due to the high density of dislocations, many inelastic interactions occur between dislocations, resulting in the generation of vacancies. After the deformation, a very high density of vacancies is observed, in agreement with the experimental observations. The processes responsible for the generation of vacancies are investigated. The main process is found to be incomplete annihilation of segments of edge dislocations on adjacent slip planes. The dislocations are also seen to be participating in complicated dislocation reactions, where sessile dislocation segments are constantly formed and destroyed.     

Kinetics of accumulation of vacancy complexes in dislocated silicon under 60Co γ-ray irradiation

Abstract

Radiation defect accumulation in ⁶⁰Co γ-ray-irradiated n-type Si single crystals (ρ=150ωcm) with various densities of dislocations (N_D = 1 × 10⁴ to 1 × 10⁷ cm ^?2) introduced at plastic deformation was studied. The temperature dependences of the Hall coefficient were measured. The probabilities of interaction of vacancies with oxygen, phosphorus atoms, and dislocation line elements were determined. It has been established that with the increase of N_D they can increase at the expense of complication of dislocation structure, decrease during formation of impurity atmosphere near dislocations and compensation of deformation fields, and they do not change if complex formation of vacancies with impurities occurs far from dislocations. Kinetics of A- and E-centre accumulation in the crystals containing dislocations with different impurity atmosphere was described.     

Pinning of spiral fluxons by giant screw dislocations in YBa2Cu3O7 − δ single crystals: Josephson analog of the fishtail effect

By using a highly sensitive homemade AC magnetic susceptibility technique, the magnetic flux penetration has been measured in YBa₂Cu₃O_{7 ? δ} single crystals with giant screw dislocations (having the structure of the Archimedean spirals) exhibiting a = 3 spiral turnings, the pitch b = 18.7 μm and the step height c = 1.2 nm (the last parameter is responsible for creation of extended weak-link structure around the giant defects). The magnetic field applied parallel to the surface enters winding around the weak-link regions of the screw in the form of the so-called spiral Josephson fluxons characterized by the temperature dependent pitch b _f (T). For a given temperature, a stabilization of the fluxon structure occurs when b _f (T) matches b (meaning an optimal pinning by the screw dislocations) and manifests itself as a pronounced low-field peak in the dependence of the susceptibility on magnetic field (applied normally to the surface) in the form resembling the high-field (Abrikosov) fishtail effect.     

Size dependent ultrasonic absorption in superfluid helium

Ultrasonic waves in liquid He II at temperatures below 0.6°K can be absorbed by thermal phonons in 3- or 4-phonon processes. For the case that the mean free path of thermal phonons is sufficiently long 3-phonon processes are generally excluded because energy and momentum cannot be conserved. The conservation requirements cannot be fullfilled because the group velocity of thermal phonons is lower than the ultrasonic velocity due to dispersion. — The purpose of this note is to point out that insmall samples of liquid the uncertainty in phonon energy prevents a violation of the conservation laws for 3-phonon processes. In small volumes of liquid, ultrasonic waves are therefore strongly absorbed in 3-phonon processes, while in larger volumes of liquid this contribution to the absorption is absent. — A similiar size dependent absorption is to be expected for longitudinal waves at low temperatures in ideal crystals.     

Nonmetallic crystals with high thermal conductivity

Nonmetallic crystals transport heat primarily by phonons at room temperature and below. There are only a few nonmetallic crystals which can be classed as high thermal conductivity solids, in the sense of having a thermal conductivity of > 1 W/cmK at 300K. Thermal conductivity measurements on natural and synthetic diamond, cubic BN, BP and AIN confirm that all of them are high thermal conductivity solids. Studies have been made of the effect on the thermal conductivity of nitrogen impurities in diamond, and oxygen impurities in AIN. The nitrogen impurities scatter phonons mostly from the strain field, the oxygen impurities scatter phonons mostly from the mass defects caused by aluminum vacancies. Pure A1N as well as pure SiC, BeO, BP and BeS conduct heat almost as well as does copper at room temperature, while pure natural and synthetic diamonds conduct heat five times better than copper.All of the nonmetallic solids that are known to possess high thermal conductivity have either the diamond-like, boron carbide, or graphite crystal structure. There are twelve different diamond-like crystals, a few boron carbide-type crystals, and two graphite structure crystals that have high thermal conductivity. Analyses of the rock-salt, fluorite, quartz, corundum and other structures show no candidates for this class. The four rules for finding crystals with high thermal conductivity are that the crystal should have (1) low atomic mass, (2) strong bonding, (3) simple crystal structure, and (4) low anharmonicity. The prime example of such a solid is diamond, which has the highest known thermal conductivity at 300K.     

The magnetoelastic interaction of optical phonons in Ce c La1−c F3 single crystals

Splitting and renormalisation of doubly-degenerate optical phonons in Ce _c La_1?c F₃ single crystals (0≦c≦1) has been observed in Raman-scattering experiments. The effects depend on the 4f-electron concentrationc, the magnetic field and temperature. A quantitative interpretation is given using a Jahn-Teller type Hamiltonian with only one phenomenological coupling constant.     

Application of the model of an edge dislocation in the form of a uniformly charged filament for description of light diffraction from alkali halide and covalent crystals

The interaction of an optical electromagnetic wave with a system of parallel charged edge dislocations in alkali halide and covalent crystals is studied. The optical activity of dislocations is associated with the existence of shielding clouds of mobile point defects with a radius of the order of the wavelength of incident light. Expressions relating the amplitude of the diffracted wave to the parameters of crystal defects (the density of point defects and the density and linear charge of dislocations) are obtained. Methods for the experimental estimation of defect parameters from the characteristics of the diffraction pattern are proposed.