On the electronic spectrum of low-dimensional structures with the symmetry of borders

The features of the electronic spectrum in low-dimensional (quasi-one-dimensional) structures with symmetry of borders have been considered. It has been shown that, in the spinless case, the conical massless (Dirac) form of the spectrum (ɛ(k) ∝ ± |k|) appears in groups with glide planes. This spectrum is a consequence of forced degeneracy, which appears due to the joint action of the lattice symmetry elements and time reversal invariance. For the spinor representations, the conical shape of the spectrum appears in all groups.     

Observation of regular defects formed on the surface of PbTe thin films grown by molecular beam epitaxy

Regular shape defects on the surface of PbTe thin films grown by molecular beam epitaxy (MBE) were studied by scanning electron microscope (SEM). Two types of regular shape defects were observed on Te-rich PbTe films grown at substrate temperature T ≥ 235 °C with a beam flux ratio of Te to PbTe (R_f) to be 0.5 and at 280 °C with a R_f ≥ 0.4, which include cuboids and triangular pyramids. The formation mechanism of the observed regular shape defects is interpreted as following: They are the outcome of fast growth rate along {1 0 0} crystal planes that have the lowest surface energy and the enclosure of the {1 0 0} crystal planes. The formation of the regular shape defects in the growth of PbTe needs appropriate substrate temperature and Te-rich ambience. However, when R_f is decreased low enough to make the films slightly Pb-rich, triangular pits that originate from the insufficient glide of the threading dislocations along the main 〈1 1 0〉 {1 0 0} glide system of PbTe in Cottrell atmosphere, will be the main feature on the film surface.     

Dynamics of nanodefects on a loaded gold surface

The dynamics of defects with linear dimensions from ≈1 to ≈100 nm on a Au surface under load have been studied by means of tunnelling microscopy. It is found that the origin, growth, and resorption of the defects is caused by displacements of bands of material from 5 to 50 nm wide, parallel to the {111} slip planes. The defects can be separated into two groups: nonsteady-state defects, whose lifetime does not exceed 15 min, while the depth is ⩽20 nm, and quasi-steady-state defects, with a lifetime three orders of magnitude greater than the first. It is assumed that the nonsteady-state defects are formed when the ensemble of dislocations is being reconstructed, while the quasi-steady-state defects are formed at the instant of formation of dislocation substructures during the creep of the loaded metal. Fiz. Tverd. Tela (St. Petersburg) 40, 2180–2183 (December 1998)     

Thermal lensing study and athermal directions in flashlamp-pumped Nd:KGd(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> laser crystal

Thermal lensing has been studied by a probe beam technique in flashlamp-pumped Nd:KGdW laser rods with rod axes positioned in the N _p–N _g plane and making different angles with the N _g optical indicatrix axis. It was found that the orientation of principal meridional planes of the astigmatic thermal lens in Nd:KGdW corresponds to the directions with maximum and minimum values of thermal-expansion coefficient in the plane perpendicular to the rod axis. The athermal Nd:KGdW laser crystal orientation defined as orientation with weak positive near spherical thermal lens has been determined to make an angle of 30^○–40^○ with the N _g axis for light polarization E∥N _m.     

Magnetically induced spin-rotation transition in iron-garnet films with canted-phase anisotropy

Epitaxial iron-garnet films exhibiting canted-phase and easy-plane anisotropies, with crystallographic (111) surface orientation are investigated by magnetooptical and inductive frequency methods. It is shown that four types of domain structure exist in the films. When the magnetization of the films is reversed by a static magnetic field oriented at various angles to the normal to the plane, anomalous behavior of the magnetic susceptibility is observed in the interval of magnetic-field orientation angles from 0.3° to 1.5°. Fiz. Tverd. Tela (St. Petersburg) 39, 1415–1420 (August 1997)     

Low-temperature motion of dislocations as a possible mechanism of formation of one-dimensional electronic structures in semiconductor crystals

A new mechanism is proposed for the formation of one-dimensional electronic structures in semiconductor crystals. The mechanism is based on controllable low-temperature glide of dislocations. Moving dislocations generate associations of intrinsic point defects in the form of one-dimensional chains, and the decay of the associations is impeded by low temperature. Experimental results and numerical estimates are presented for cadmium sulfide crystals. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 10, 639–644 (25 November 1997)     

Numerical calculation of the rate of strain of interstitial solid solutions under irradiation. I. Model of radiation creep

A model of radiation creep of interstitial solid solutions is developed on the basis of the combined motion of dislocations, including their glide and climb past obstacles. The obstacles considered are forest dislocations and pileups of radiation-induced point defects. A computational formula for the rate of strain is derived which describes creep at high stresses, when the gliding dislocations overcome some of the barriers by force, and a method is described for determining the average distance traversed by a dislocation in the glide plane under the influence of the stress until it is stopped by barriers. The results are compared with those of other authors. It is shown that the formula obtained for the rate of strain goes over in particular cases to those given by the previously known SIPA, Gittus-Mansur, and glide-climb models of radiation creep. Zh. Tekh. Fiz. 69, 64–71 (January 1999)     

Anomalous orientation of ferroelectric liquid crystal films in an electric field

An anomalous orientation of ferroelectric liquid crystals in an electric field is observed. The tilt planes of the molecules are oriented in a direction parallel to the electric field. The anomalous orientation is observed in freely suspended films above the smectic-C*-smectic-A bulk phase transition temperature. The effect is explained by the appearance of giant flexoelectric polarization, 10⁴ times greater than that in bulk liquid-crystal samples, in the film. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 6, 414–418 (25 September 1997)     

Angular dependences of the peak effect in Yba2Cu3Ox single crystals

Angular dependences of the magnetization hysteresis loops have been studied at T=77 K on YBCO single crystals exhibiting the peak effect. The peak effect is shown to be related to the pinning of longitudinal vortices along the c axis at twin boundary-type ordered defects. The behavior of the peak effect at intermediate angles is explained by anisotropic magnetic field penetration into quasi-two-dimensional superconductors. In thin crystals with a dilute ordered-defect structure this can result in an enhanced peak effect due to formation of a vortex kink structure and to “internal” pinning of transverse vortex segments at Cu-O sheets. Fiz. Tverd. Tela (St. Petersburg) 39, 425–431 (March 1997)     

Influence of the Flux Peaking Effect on the DD Reaction Yield Upon D + Ion Channelling in Deuterated Palladium

The interaction between a deuterium ion (D+) beam and deuterium-enriched Pd and Ti targets are studied using the HELIS ion accelerator (Lebedev Physical Institute). The neutron yield is determined from the DD reaction in the deuterated Pd target whose surface is irradiated with a D+-ion beam with an energy of 20 keV. The neutron flux is measured in the D+-ion-beam direction as a function of the angle β of target rotation relative to the beam axis using a multichannel detector based on 3He counters. Significant anisotropy (orientation effect) of the neutron yield is observed; it is two times higher at β = 0° than at β = ±30°. The orientation effect can be associated with channeling and the so-called flux-peaking effect. It is studied by computer simulations using the BCM-2.0 code. The enhanced density of the D+ flux between the (200) planes of a Pd crystal (where implanted D is located) at zero angle of incidence with respect to these planes makes it possible to qualitatively explain the observed orientation effect. No effect is observed in a homogeneous target of deuterated titanium.     

Influence of interatomic forces on the lattice dynamics of YBa2Cu3O7 (molecular dynamics method)

The method of molecular dynamics is used to study the dynamic properties of the high-temperature superconductor YBa₂Cu₃O₇. It is shown in the system La_2−x Sr_xCuO₄ that, due to the presence of strong anharmonism and a substantial difference in the interatomic forces, local “hot” regions arise around the Ba⁴⁺ charge defects in the CuO₂ and CuO planes, where the mean kinetic energy of the vibrations of the oxygen atoms reaches ∼0.5 eV. All other types of defects: oxygen and copper vacancies, substitutional atoms, and charge defects, do not lead to such effects. Fiz. Tverd. Tela (St. Petersburg) 41, 1729–1733 (October 1999)     

Local heating and quasiathermal plastic deformation of crystals at low temperatures

A theoretical discussion is given of the relationship between local heating of a crystal by glide lines and bands and deviations observed in the temperature dependences of the flow stresses and their rate coefficients at low temperatures (<10 K) from the dependences characteristic of thermally activated plastic deformation. The appearance of plateaus in these dependences is currently explained by the onset of quantum-mechanical, athermal mechanisms for overcoming local barriers. In this paper it is shown that softening and the apparent athermicity of low-temperature deformation are caused by heating of sites where the strain is localized. Fiz. Tverd. Tela (St. Petersburg) 39, 2019–2022 (November 1997)     

Investigation of the role of the projectile-target orientation angles on the evaporation residue production

The measured yield of evaporation residues in reactions with massive nuclei have been well reproduced by using the partial fusion and quasifission cross sections obtained in the dinuclear-system model. The influence of the orientation angles of the projectile- and target-nucleus symmetry axes relative to the beam direction on the production of the evaporation residues is investigated for the ⁴⁸Ca + ¹⁵⁴Sm reaction as a function of the beam energy. At the low beam energies only the orientation angles close to αP = 30° (projectile) and αP = 0°–15° (target) can contribute to the formation of evaporation residues. At large beam energies (about E _c.m. = 140–180 MeV) the collisions at all values of orientation angles αP and α _T of reactants can contribute to the evaporation residue cross section which ranges between 10–100 mb, while at E _c.m. > 185 MeV the evaporation residue cross section ranges between 0.1–1 mb because the fission barrier for the compound nucleus decreases by increasing its excitation energy and angular momentum.     

Kinematics of edge dislocations. I. Involutive distributions of local slip planes

The geometry of continuous distributions of dislocations and secondary point defects created by these distributions is considered. Particularly, the dependence of a distribution of dislocations on the existence of secondary point defects is modeled by treating dislocations as those located in a time-dependent Riemannian material space describing, in a continuous limit, the influence of these point defects on metric properties of a crystal structure. The notions of local glide systems and involutive distributions of local slip planes are introduced in order to describe, in terms of differential geometry, some aspects of the kinematics of the motion of edge dislocations. The analysis leads, among others, to the definition of a class of distributions of dislocations with a distinguished involutive distribution of local slip planes and such that a formula of mesoscale character describing the influence of edge dislocations on the mean curvature of glide surfaces is valid.     

Kikuchi-band formation in medium-energy electron-diffraction patterns

To reveal the mechanism of Kikuchi-band formation, the total Si(100) diffraction pattern produced by 2-keV quasi-elastically backscattered electrons is compared to model calculations made in the single-scattering approximation for clusters constructed with different numbers of close-packed (110) planes. The formation of the Kikuchi bands is shown to be governed by two types of electron scattering in a crystal. The dominant contribution to enhanced electron-scattering intensity within a band comes from the forward-focusing effect as the electrons move along the numerous interatomic directions in the (110) planes. The other mechanism responsible for the sharp edge regions in the Kikuchi bands involves electron scattering from the nearest planes. It is proposed to use the specific profile of the Kikuchi bands in estimating the shape and size of light-element crystallites forming during initial stages of island-film growth. Fiz. Tverd. Tela (St. Petersburg) 41, 411–417 (March 1999)     

On the glide of [100] dislocation and the origin of ‘pencil glide’ in Mg2SiO4 olivine

ABSTRACT

Olivine with chemical composition (Mg,Fe)₂SiO₄ is a silicate which is supposed to strongly constrain the flow of the Earth’s upper mantle under thermal convection. Its mechanical properties are thus of primary importance. Slip systems in olivine involve two types of dislocations with [100] and [001] Burgers vectors. In this study, we report atomistic modelling of screw dislocations with [100] Burgers vector and their intrinsic properties. We show that the [100] screw dislocation core exhibits several configurations corresponding to spreading in different planes and different relative stabilities. At low pressure, we identify a clear tendency for core spreading in (010). At higher pressure, relevant for the Earth’s mantle, we show that pressure promotes a change in core configuration with spreading into equivalent {021} planes. Based on the systematic investigation of the minimum energy path between the different configurations, we show that the variability of core structures allows complex glide paths which has been described at the macroscopic level as ‘pencil glide’. Our results suggest that the pencil glide is more efficient at high pressure.     

Stripe motion in CuO2 planes of Y1−x Pr x Ba2Cu3O7 as observed from the Cu(2) quadrupole resonance

The NQR spectra of copper in the compounds YBa₂Cu₃O₇, TmBa₂Cu₃O₇, and Y_0.9Pr_0.1Ba₂Cu₃O₇ at temperatures of 4.2–200 K are investigated by a pulsed NQR technique at frequencies of 28–33 MHz. Quantitative analysis of the spectra shows that the shape of the “plane” Cu(2) spectra is well described by using a model of 1D correlations of the charge and spin distribution in the CuO₂ planes (stripe correlations). In the undoped superconductors the charge-spin stripe structure moves fast in the CuO₂ planes, but doping the YBa₂Cu₃O₇ lattice with praseodymium slows this motion down. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 10, 782–787 (25 May 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Properties of misfit dislocations and pseudodislocations not typical for defects of homogeneous crystals

The structural properties of SiGe/Si heterosystems with a stepped interface obtained by the deflection of a surface from the (001) plane by the rotation by several degrees about the [110] axis are studied. Dislocation-free systems and systems containing misf it dislocations (MDs) are investigated. The model of pseudodislocations describing the mutual rotation of (001) crystallographic planes of the film and substrate is proposed. The pseudodislocations are the elastically strained interface steps, and the magnitude of their effective Burgers vector is determined by the step height and lattice’s mismatch parameter. Long-range normal and shear stress fields in the epitaxial film are considered for the systems containing dislocations. The ambiguity of the value of the MDs Burgers vector is discussed. The mechanism for a small angle boundary formation under MDs introduction into the vicinal interface is proposed. The expression is obtained allowing one to calculate the fractions of MDs glide in the 111 planes inclined to the interface at the maximum and minimum angles based on the parameters determined from X-ray diffraction reflection curves.     

Stiffness of reinforced and unreinforced Bi2Te3 single crystals

We investigate the thermodynamics of Ω- and V-shaped bending of Bi₂Te₃ single crystals. To change the shape of the bending from Ω to V, the sample is reinforced with thin steel rods, arranged perpendicular to the cleavage planes. We find that reinforcement increases the ultimate strength of the samples by more than twofold. We show that the transition Ω→V leads to a substantial change in the spectrum of energy dissipation in the straining of the sample. Some aspects of the practical application of the observed effects are considered. Zh. Tekh. Fiz. 67, 50–54 (February 1997)     

Long-lived excited states and photoluminescence excitation spectra in single crystals of fullerene C60

Photoluminescence, optical absorption spectra, and photoluminescence excitation spectra were measured on large (2–3 mm), very pure crystals of fullerene C₆₀ at 5 K. It is shown that the main contribution to the photoluminescence of these crystals is from singlet and triplet excitons captured on crystal defects. The concentration of these defects does not exceed 10¹⁸ cm⁻²³, and the lifetime of triplet excitons on these defects is about 3 ms. It is shown that the symmetry distortion of the C₆₀ molecules at the defects is rather large and causes the oscillator strength of the zero-phonon optical transitions to be comparable to the most intense optical transitions with the participation of intramolecular vibrations. Zh. éksp. Teor. Fiz. 113, 734–746 (February 1998)