Nuclear acoustic resonance in tantalum with low concentrations of hydrogen impurities

Nuclear acoustic resonance (NAR) has been observed in tantalum single crystals with low concentrations of hydrogen (7.8 and 16 ppm) between 4.2 K and room temperature. The temperature dependence of the Δm=1 transition is reported for the first time. For the Δm=2 transition the temperature dependence of the linewidth is analysed in terms of the precipitation of a hydride phase in the Ta-H system. The differences in linewidth between virgin and pre-cooled runs can be interpreted by the effect of dislocations generated during the precipitation.     

Theory of the electret state in crystals with hydrogen bonds

The mechanism of proton migration over hydrogen bonds is considered. On this basis, a system of kinetic equations describing electret charge formation is written and solved in the linear approximation. The relaxation of heterocharge and homocharge is investigated, and a formula determining the electret polarization is obtained. E. A. Buketov Karagandinsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 77–82, June, 1998.     

Theory of the Gorsky effect for low interstitial concentrations

The formalism of the preceding paper is applied to work out the theory of the Gorsky effect, or anelastic relaxation due to the long-range diffusion of interstitials in a host lattice, for non-interacting (low-concentration) interstitials (e.g., H in Nb). It is shown how linear response theory (LRT) provides a number of advantages that simplify the solution of the problem and permit the handling of complications due to specimen geometry and stress inhomogeneity. The multiple-relaxation time creep function of Alefeldet al is first re-derived. Next, the dynamic responseand the short-time behaviour of the creep function are deduced exactly, and theω ^−1/2 fall-off of the internal friction at high frequencies is exhibited. Finally, it is pointed out that the true asymptotic behaviour of the dynamic response must be found by going beyond the diffusion equation model. A two-state random walk analysis is used to predict a cross-over to a trueω ⁻¹ asymptotic behaviour, and the physical reasons for this phenomenon are elucidated.     

Plateau in the temperature dependence of the thermal conductivity of solid hydrogen containing heavy isotopic neon impurities at ultimately low concentrations

The thermal conductivity of solid hydrogen with 0.0001–0.0002 at. % Ne in the form of equilibrium samples grown at a low rate after remelting is investigated in the temperature range 1.5–10.0 K. It is demonstrated that the temperature dependence of the thermal conductivity for the samples containing neon at concentrations considerably lower than the limiting solubility of the heavy impurity exhibits a symmetric plateau. This behavior of the temperature dependence of the thermal conductivity differs qualitatively from the previously observed resonance minimum in the temperature dependence of the thermal conductivity for desublimated samples. A relaxation model is proposed for explaining the observed effect. According to this model, the plateau is explained by the formation of linear impurity structures that are located along dislocation lines and considerably enhance phonon scattering by dislocation cores. The density of linear impurity structures is estimated. The influence of these structures on the thermal conductivity is compared with the corresponding effect of uniformly distributed individual neon atoms in solid hydrogen.     

Infrared spectra of hydrogen impurities in BaTiO3 crystals

The IR spectra in the 2.8 μm region of the OH^? impurities in thick single domain crystals have been measured for different polarization geometries and various temperatures. Simple calculations of the Coulomb interactions between hydrogen and lattice ions show that these interactions are much more important than the hydrogen bonding ones for determining the positions of the hydrogen ions. The appearance of only one strong non-dichroic OH^? stretching line is due to an almost equal population of the proton axial sites. The side bands are interpreted as combinations of the stretching mode with a low-frequency translational mode. The large broadening of the main line just above T_c supports the model which attributes a considerable order—disorder character to the ferroelectric phase transition.     

The influence of hydrogen impurities on the atomic and electronic structures of carbyne crystals

An ab initio DFT study of atomic and electronic structure of carbyne crystals was carried out. The influence of hydrogen impurities on carbyne structure was investigated. Calculations with atomic relaxations showed that carbon chains in the carbyne crystal structure are bow-like curved; free-energy calculations showed that the most probable lengths of those chains are four and six atoms, which is in a good agreement with experiments. Carbyne-crystal electronic-structure analysis showed that there is a small gap of 0.09 eV near the Fermi level in four-atomic carbyne, while there is no such gap in six-atomic carbyne. In studying of the hydrogen impurity influence on the atomic and electronic structure of carbyne crystals, hydrogen atoms were embedded in two directions: across and along carbon chains in the crystal. As a result we found that the crystal structure is not distorted in the case of hydrogen embedded across the chains, while the type of bonding between carbon atoms in carbon chains in the carbyne crystal structure depended on the impurity concentration. The crystal structure was distorted when hydrogen was embedded along the chains. The concentration of impurities influences the conductivity of a carbyne crystal.     

On the integrated density of states for crystals with randomly distributed impurities

In the present paper, we discuss spectral properties of a periodic Schrödinger operator which is perturbed by randomly distributed impurities; such operators occur as simple models for crystals (or semi-conductors) with impurities. While the spectrum itself is independent of the concentrationp of impurities, for 0<p<1, we focus our attention on the limiting behavior of the integrated density of states _p of the random Schrödinger operator, inside a spectral gap of the periodic operator, asp0. Denoting byU ₀ the set of eigenvalues (in the gap) of the reference problem having precisely one impurity (located at the origin, say), we show that the integrated density of states concentrates around the points ofU ₀, in the sense that _p (U ) is of orderp, for any fixed -neighborhoodU ofU ₀, while _p (K)C·p ², for any compact subsetK of the gap which does not intersectU .Research partially supported by Deutsche Forschungsgemeinschaft     

New approximation for the interaction potential of light channeled particles with crystals

A new approximation for the continuous interaction potentials of atomic planes and strings with channeled electrons and positrons in crystals is proposed. A model in which atomic electron distributions within a certain atomic subshell are assumed to be exponentially decaying is used. Contrary to the Thomas-Fermi (TF) and Thomas-Fermi-Dirac (TFD) models this approach takes into account the shell structure of atoms in reasonably good agreement with the Hartree-Fock method (HF). Simple analytical expressions for the continuous plane (string) potentials are given. Calculations of several planar potentials for positrons channeled in silicon and tungsten are presented. Results are compared with widely known Lindhard and Molière approximations. Some estimates of the Kumakhov spontaneous radiation intensity under positron planar channeling conditions in the classical approximation are also given.     

Ability of the As=0 group to form a hydrogen bond

Journal of Applied Spectroscopy -     

Theory of multiphonon absorption in the wings of internal vibrational modes of impurities in ionic crystals

Green's function methods are applied to obtain the multiphonon absorption coefficient in crystals containing impurities with internal vibrational modes. The results imply that the shape of the wing absorption from such modes is determined by the host lattice density of states, but with a fall-off rate determined by the range of the lattice-impurity interaction potential.     

Continuum model for the periodic incorporation of aliovalent impurities in ionic crystals

A continuum approach based on the Poisson-Boltzmann equation for the self-consistent electrostatic potential is used to describe the periodic spatial distributions of aliovalent impurities in ionic crystals. Some thermodynamic properties of the model are discussed and compared with experimental data on the Suzuki phase in alkali halides.     

Holograms in CaF2 crystals with high content of alkaline impurities

Similarities and dissimilarities of holograms recorded in fluorite crystals with color centers that are pure or contain relatively high concentrations of an alkaline impurity (sodium) are considered. It is shown that, in these two cases, the mechanism of recording is the same, but formation of the impurity-vacancy complexes in heavily doped crystals causes certain peculiarities in the types of color centers in the initial crystals and those with holograms and determines specific features of spatial redistribution of centers in the process of recording. These features reduce the diffraction efficiency of holograms in heavily doped crystals compared to pure and weakly activated crystals.     

Free energy of a strong-coupling superconductor with magnetic impurities in Shiba-Rusinov approximation

We evaluate numerically the free energy of a strong-coupling superconductor with paramagnetic impurities in the model of Shiba and Rusinov. Our work is based on a recent formula for the free energy given by Yamamoto and Nagi. This formula is based on the same fundamental expression as used by Schossmann and Schachinger but without their simplfying approximations. Our results are numerically quite different from the previous ones. It is still concluded, however, that the thermodynamic critical field is very sensitive to the model chosen to describe the scattering.     

Influence of impurities on the photoluminescence of modified InP crystals

It is shown that a new emission band at 1.35 eV (77 K) is observed in the photoluminescence spectra of laser-modified InP crystals, irrespective of the type of impurity or its concentration in the initial crystal. It is established that the emergence of this band is a common property of the lattice defect state of the modified semiconductors. Zh. Tekh. Fiz. 69, 110–111 (April 1999)     

Single-particle model for the frequency dependence of weak infrared absorption in crystals and molecules at T = 0 K

We obtain a simple expression for the infrared absorption coefficient α within the single-particle model at zero temperature. For potentials admitting harmonic approximation, α is found to decrease nearly exponentially with increasing frequency for small anharmonicity, with departures for larger anharmonicity. Potentials which do not admit harmonic approximation display a frequency dependence of the power-law variety. The present results in the quantum limit are similar to those obtained in the classical limit by Mills and Maradudin.