Evolution of the concentration of point defects at the tip of a crack

The evolution of the distribution of interstitial impurity atoms in the plastic zone around the tip of a tension crack is analyzed. The transport of point defects is determined by: 1) the hydrostatic component of the elastic stress at the crack tip, created by the superposition of the elastic fields of the crack and dislocations; 2) the elastic field of moving dislocations (“sweeping out” of interstitial impurity atoms); 3) the dislocation-driven transport of point defects present in the dislocation cores. The contributions of each mechanism of transport of point defects to the crack tip are calculated over the entire time from the start of loading of a sample containing a crack until an equilibrium distribution of plastic deformation is established after the cessation of loading. Numerical calculations are carried out for interstitial hydrogen atoms dissolved in an α-Fe crystal. Fiz. Tverd. Tela (St. Petersburg) 39, 1580–1585 (September 1997)     

On the dominant role of two-photon relaxation in photonic crystals in external fields

It is shown that resonant interaction of a coherent wave with impurity atoms leads to filling of levels of an impurity atom that lie in the gap in the photon density of states and do not belong to resonant transitions, while the interaction of impurity atoms with a nonresonant coherent wave results in effective deactivation of the indicated levels. The main mechanism determining the pumping and decay of an impurity level in a gap are two-photon radiative relaxation processes previously investigated by the present author (Zh. éksp. Teor. Fiz. 102, 1126 (1992) [Sov. Phys. JETP 75, 611 (1992)]). Pis’ma Zh. éksp. Teor. Fiz. 70, No. 7, 434–438 (10 October 1999)     

Evolution of vacancy defects in the surface layers of a metal irradiated with a pulsed electron beam

The distribution of vacancy defects in the surface layers of α-Fe after irradiation with a high-current pulsed electron beam is studied experimentally by unique nuclear-physical methods — low-energy positron annihilation, Rutherford backscattering (RBS), and proton-induced x-ray emission (PIXE). Regions with low local density, which are sources of crater formation on the surface of the irradiated sample, are observed by scanning a proton microbeam. Positron lifetime measurements reveal that as the electron beam power increases, nonequilibrium vacancies tend to be captured by carbon impurity atoms. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 618–622 (25 April 1997)     

Influence of the concentration of Ca impurity on the magnetic threshold of the magnetoplastic effect in NaCl crystals

It is found experimentally that the threshold magnetic field B _c for the magnetoplastic effect, i.e., the field at which the depinning of dislocations from paramagnetic impurities in an external magnetic field begins to be observed, increases with increasing concentration C of Ca impurity in NaCl crystals in the range C=(0.5–100) ppm. It is shown that the dependence B _c(C) exhibits a distinct tendency toward saturation. The physical interpretation of the observed dependence rests on the notion that as the impurity concentration C increases, the average size of the impurity complexes increases and, accordingly, the local atomic configuration around the impurity atoms changes according to a definite pattern. In particular, the average number of cation vacancies among the nearest neighbors increases from 1 to 6 as the number N of Ca atoms in the complex increases, and this trend, in turn, should cause the thermal vibration amplitude of the Ca atoms to increase. In other words, the phenomenon in question appears to be physically analogous in its microscopic mechanisms to the previously observed increase of B _c with increasing temperature. The proposed interpretation is further supported by good correlation of the experimental dependence B _c(C) with the calculated function . Fiz. Tverd. Tela (St. Petersburg) 40, 81–84 (January 1998)     

Off-centering of Pb and Sn impurities in GeTe induced by strong local stress

Noncentrality of large impurity atoms—Pb and Sn atoms—substituting for Ge atoms in a GeTe lattice has been discovered by means of EXAFS investigations in Ge_0.9Pb_0.1Te and Ge_0.85Sn_0.15Te samples. The transition of impurity atoms into a noncentral position under conditions of a strong local stress is explained by the participation of an unshared electron pair from the impurity atoms in the formation of the chemical bond. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 600–603 (25 April 1996)     

Influence of a weak pulsed magnetic field on grain-boundary relaxation in aluminum

It is discovered that the treatment of technical-grade polycrystalline aluminum in a weak pulsed magnetic field during heating leads to displacement of the grain-boundary maximum of the low-frequency internal friction toward lower temperatures. This phenomenon is associated with a decrease in the degree of interaction of the grain boundaries with impurity atoms. Fiz. Tverd. Tela (St. Petersburg) 41, 1985–1987 (November 1999)     

Polarization of quadrupolar nuclei

A method of obtaining high polarization and pure spin states of impurity nuclei with a moderately strong quadrupole interaction in solid diamagnetic hosts whose nuclei have spin 1/2, a large g factor (like ¹H and ¹⁹F), and a high degree of polarization is proposed. The method employs cross-relaxation transitions of the impurity nuclei with the host spins (with adiabatic variation of the external magnetic field) and simple radio-frequency pulses that invert the host nuclei or give rise to two-spin resonance of the host and impurity nuclei. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 7, 539–543 (10 October 1998)     

Kinetics of impurity segregation at grain boundaries in polycrystals. II. Concentrated solution

The isothermal impurity segregation from a finite-size grain into an intergrain boundary region or at an external free surface is investigated when the impurity concentration in the boundary region is not small. Simple algebraic equations are obtained that describe the impurity concentration for the case of several competing and interacting impurities. The process of segregation of two impurities is discussed in detail, and it is shown that the concentration of one of them can have a maximum as a function of time. Fiz. Tverd. Tela (St. Petersburg) 40, 251–253 (February 1998)     

Far-IR radiation of hot holes in germanium for mutually perpendicular directions of uniaxial pressure and electric field

The advantages of crossed directions of an electric field and uniaxial pressure over a parallel configuration for obtaining lasing (λ≅100 μm) in p-type germanium are substantiated. The results of the first experiments investigating far-IR emission for mutually perpendicular directions of the field and pressure are reported. The pressure dependences of the radiation intensity in various electric fields are explained by the different hole occupation of the states of an impurity center which are split by uniaxial pressure. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 4, 257–261 (25 August 1999)     

Effect of a random crystalline field on the Curie temperature of an anisotropic crystalline ferromagnet

We examine how substitution of nonmagnetic atoms of one species by another that leads to the emergence of a random crystalline field affects the Curie temperature T _C of an anisotropic crystalline ferromagnet. We study the case of low concentrations, in which individual substitutional impurities create isolated clusters of perturbed magnetic ions with additional easy-or hard-magnetization axes. Finally, we analyze the various sign relations among the parameters D and d of regular and impurity second-order anisotropies with Kramers (J=1) and non-Kramers (J=3/2) angular momenta, and show that usually the effects of a random crystalline field lower the Curie temperature as the concentration of the nonmagnetic impurity atoms increases. Zh. éksp. Teor. Fiz. 111, 127–143 (January 1997)     

The effect of hydrogen atoms on the mobility of edge dislocations

This paper discusses the process by which a thermodynamically unstable impurity atmosphere consisting of hydrogen atoms displaces an edge dislocation. Relationships are obtained for the displacement force in the approximation of dilute solid solutions. The results of theoretical analysis are used to explain the hydrogen embrittlement of nonhydride-forming metals. Fiz. Tverd. Tela (St. Petersburg) 41, 451–453 (March 1999)     

Diffusion properties of plastically deformed silicon crystals

The effect of dislocations generated by electroplastic strain on the electric-field-driven transport of impurity atoms of indium in single crystals of P-silicon is investigated experimentally. It is shown that when electrodiffusion of indium and strain are induced simultaneously, the impurity ions are preferentially dragged towards the anode. Fiz. Tverd. Tela (St. Petersburg) 41, 1028–1029 (June 1999)     

Kinetics of the diffusion of an impurity atom on a square lattice

An analytical study of the migration of an embedded impurity atom over a solid surface under the influence of the diffusion of vacancies is presented. The case of small surface coverages of both vacancies ϑ _v and impurity atoms ϑ _i , with ϑ _i ≪ ϑ _v ≪ 1, is considered. It is shown that the realization of multiple collisions of a single impurity atom with vacancies imparts a Brownian character to its motion. At long times, the dependence of the mean square displacement on the time differs little from the linear, whereas the spatial density distribution is close to the Gaussian, features that makes it possible to introduce a diffusion coefficient. For the latter, an analytical expression is derived, which differs from the product of the diffusion coefficient of vacancies and their relative concentration only by a numerical factor η. The dependence of the diffusion coefficient of an impurity atom on the ratio of the frequency of its jumps to the frequency of jumps of vacancies is analyzed. In the kinetic mode, at ω ≪ 1, the diffusion coefficient of impurity atoms depends linearly on ω, whereas at ω ≫ 1, a saturation is observed; i.e., the dependence on the frequency of jumps of the impurity atom disappears. Nevertheless, the value of η remains less than unity, and no total entrainment of impurity atoms with vacancies occurs.     

High-speed superplasticity of microcrystalline alloys under conditions of local grain boundary melting

A model is proposed for the high-speed superplasticity of materials under conditions of local grain boundary melting at temperatures close to solidus. It is shown that the local melting of grain boundaries containing segregations of impurity atoms, results in the formation of a structure consisting of liquid-phase regions and solid intergranular bridges which provide cohesion of the grains during the deformation process. The equilibrium concentration, dimensions, and activation energy for the formation of solid bridges are determined as a function of the temperature, initial impurity concentration in the boundary, and the boundary thickness. A mechanism is proposed for grain-boundary slip under conditions of local grain boundary at anomalously high strain rates. Zh. Tekh. Fiz. 68, 38–42 (December 1998)     

Numerical calculations of the radiation-induced strain rate of interstitial solid solutions. II Allowance for the main channels for the influence of impurities on radiationinduced creep

A previously developed numerical method for calculating the radiation-induced creep rate [Yu. S. Pyatiletov and A. D. Lopuga, Tech. Phys. 45 (1999)] is used to study the influence of impurity atmospheres around dislocations and pores, impurity traps, and mobile impurity-vacancy and impurity-interstitial complexes on the radiation-induced strain rate of interstitial alloys. Quantitative data are obtained on the creep rate as a function of impurity concentration, and a physical interpretation allowing for the recombination of interstitial atoms and vacancies directly with one another, on impurity traps, and on mobile complexes is put forward. Zh. Tekh. Fiz. 69, 19–23 (March 1999)     

The metal-insulator transition and the phase transition in metal-ammonia solutions

The ground state of impurity metal (sodium) atoms in liquid ammonia close to the solvated state of the free electrons is considered. It is shown that the critical solubility point lying on the metal side of the metal-insulator transition is determined by the Coulomb interaction between the ions and electrons in the overlapping impurity states, classically accessible spheres of which form an infinite percolation cluster. The percolation conductivity via the impurity states is estimated. The estimate agrees with the experimental data near the critical solubility point. Zh. éksp. Teor. Fiz. 111, 938–948 (March 1997)     

Tunneling ionization of Rydberg molecules

The theory of tunneling ionization of atoms is generalized to ionization of symmetric top molecules, either polar or nonpolar. Low-lying excited states of molecules, for which the ordinary Born-Oppenheimer approximation holds, and high-lying excited states, for which the inverse Born-Oppenheimer approximation holds, are discussed. Ionization in a constant external field is analyzed, as is ionization in an alternating field. It is shown that the orientation of the molecule’s axis along the field does not lead to any appreciable increase in the ionization probability as compared to other orientations. Zh. éksp. Teor. Fiz. 112, 115–127 (July 1997)     

Double ionization of atoms by multiply charged ions and a strong electromagnetic field: Effects of correlation in the continuum

A nonstationary theory of double ionization of two-electron atoms in collisions with multiply charged ions or by an intense electromagnetic field is developed. An approach that permits investigating both problems by a single method is formulated. A two-electron continuum wave function that takes into account the interaction of the electrons with the atomic nucleus and the external ionizer as well as with one another is obtained as a product of Coulomb waves with modified Sommerfeld parameters. The computational results obtained for the double ionization of helium atoms by multiply charged ions are in good quantitative agreement with the existing experimental data. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 1, 23–26 (10 July 1997)     

Spin dynamics of the muon in muonium in normal metals

The question of the charge state of the proton (the positive muon) in metals is of fundamental importance for the theory of metal hydrides. The theory developed here permits determination of the charge state of μ ⁺ in normal metals. The experimental possibilities of the observation of Mu atoms in metals at various strengths of the external magnetic field and various temperatures are analyzed. Zh. éksp. Teor. Fiz. 111, 730–736 (February 1997)     

Chaotic Rabi vacuum oscillations in cavity quantum electrodynamics

It is shown in numerical simulations with two-level atoms moving through a single-mode high-Q cavity that spontaneous emission of a new type — chaotic Rabi vacuum oscillations — arises in the strong atom-field coupling regime. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 11, 801–806 (10 June 1997)