Optical absorption of hexagonal boron nitride involving nitrogen vacancies and their complexes

The pseudopotential and supercell methods are used to study the optical absorption of graphite-like hexagonal boron nitride involving deep levels of nitrogen vacancies and their clusters. The impurity-induced absorption is shown to be mainly related to electron transitions between the states that are antisymmetric with respect to the horizontal plane. Therefore, this absorption is highly anisotropic and is maximum for light waves polarized normally to the hexagonal axis. The optical absorption and photoconductivity spectra before and after neutron irradiation and thermal treatment are interpreted, and the activation energies for the thermoluminescence and conductivity of nitrogen-depleted boron nitride before and after fast-neutron irradiation and vacuum annealing are found.     

Effect of bismuth nanolayers on the oriented growth of fullerene C<Subscript>60</Subscript> on an amorphous substrate

The effect of a bismuth sublayer with an effective thickness of 0.5 to 4 nm on the structure of C₆₀ fullerene films grown on amorphous substrates (silicon covered with a natural oxide layer; glass) using the quasi-closed-volume method is studied. An x-ray diffraction study of fullerene films showed that the intensity ratio between the (220) and (111) peaks depends nonmonotonically on the sublayer thickness. In the bismuth sublayer thickness range 0.5–2.0 nm, fullerene films are found to exhibit a growth texture with the 〈110〉 axis; the average crystallite size was ～20 µm. The quality of the texture can be improved by varying the fullerene growth temperature.     

Application of ultrafine-dispersed diamonds in electroplating

The results obtained upon the electrochemical deposition of composite coatings containing ultrafine-dispersed diamonds produced by detonation synthesis are generalized. The main advantages of such coatings over the well-known composite electrochemical coatings are revealed.     

The mechanism of ionic conductivity in stabilized cubic zirconia

The electron-density functional method (in the gradient approximation) and the pseudopotential method are used to study the mechanism of ionic conductivity in the cubic phase of zirconia stabilized with magnesium or yttrium. The oxygen-ion migration in the stabilized zirconia is shown to be a two-stage process, which consists in the formation of active oxygen vacancies and in oxygen-ion jumps from one active vacancy to another. The total activation energy of these processes is calculated to be 1.0–1.5 eV, which agrees with experimental data.     

High-efficiency erbium ion luminescence in silicon nanocrystal systems

The photoluminescence spectra and kinetics of both erbium-doped and undoped multilayer structures of quasi-ordered silicon nanocrystals in a silicon dioxide matrix were studied. It was shown that the optical excitation energy of silicon nanocrystals 2–3 nm in size can be practically completely transferred to Er³⁺ ions in the oxide surrounding the nanocrystals, with its subsequent radiation at 1.5 μm. Possible reasons for the high excitation efficiency of the Er³⁺ ions are discussed, and the conclusion is drawn that the F?rster mechanism is dominant in the energy transfer processes occurring in these structures. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 1, 2004, pp. 105–109. Original Russian Text Copyright ? 2004 by Kashkarov, Kamenev, Lisachenko, Shalygina, Timoshenko, Schmidt, Heitmann, Zacharias.     

Modeling of the structure and electronic structure of condensed phases of small fullerenes C<Subscript>28</Subscript> and Zn@C<Subscript>28</Subscript>

A comparative analysis of the stability factors and electronic structure of two possible crystalline forms of small fullerene C₂₈ and endohedral fullerene Zn@C₂₈ with diamond and lonsdaleite structures is performed using a cluster model. Atoms of elements that, when placed inside C₂₈ cages, have no significant effect on the stability of free small-fullerene molecules are shown to be able to dramatically change the electronic properties and reactivity of the C₂₈ skeleton and to be favorable for forming small-fullerene crystalline modifications, which are covalent crystals. In contrast, if the presence of foreign atoms inside C₂₈ cages stabilizes the isolated nanoparticles, then molecular crystals (such as C₆₀ fullerites) are formed due to weak van der Waals forces.     

Special points for calculating integrals over the primitive cell of a periodic system

A method is proposed for constructing appropriate sets of special points in the Brillouin zone in which the extended unit cell method is used with a subsequent shift from the center of the reduced Brillouin zone. The proposed method offers several advantages over the commonly used Monkhorst-Pack method. The difference in the construction of sets of special points for the direct and reciprocal lattices is discussed for crystals belonging to a nonsymmorphic space group. The cases of a planar square lattice and an fcc lattice are considered to illustrate general results.     

Characteristics of elementary acts in the kinetics of metal fracture

Experimental studies of the fracture kinetics of polycrystalline metals have led to the problem of the barrier and activation volume of elementary fracture acts. A model is proposed where the field binding one atom to its environment in a metal is represented by equivalent bonds directed along three orthogonal axes. These bonds are described using the Morse potential, whose parameters are found from the values of the Young’s modulus and the linear thermal expansion coefficient for metals. The validity of the model is checked by comparing the results obtained with metal sublimation data. The values of the barrier and activation volume of elementary fracture acts are determined for 15 polycrystalline metals. The levels of local overstresses are estimated. The theoretical breaking strengths of the metals are calculated.     

Calculation of the vortex pinning energy in a long periodically modulated Josephson contact

An analytical method for evaluating the Josephson and magnetic energies of a vortex, as well as the vortex pinning energy and its components, in a long periodically modulated Josephson contact is suggested. The method allows one to take into account the variation of the vortex shape with the position of the vortex. The results obtained with this method are much closer to those of exact computer analysis compared with results of the conventional techniques. The discrepancy between the exact shape of the vortex and a function obtained by solving a differential equation approximating the exact difference equation is studied.     

Green luminescence in diffusion-doped layers of zinc selenide

Diffusion-doped layers with a green emission band dominating in the room-temperature luminescence spectrum are obtained by annealing single-crystal zinc selenide substrates in tellurium and zinc vapors.