Structure of zinc-borate low-melting glasses derived from IR spectroscopy data

The IR spectra of glasses of the ZnO—SrO—B₂O₃ system with constant additions of PbO, Al₂O₃, and Li₂O (20 mol. % in sum) were studied. It is established that on replacement of B₂O₃ by ZnO, the structure of the glasses is characterized by the presence of groupings with the bridge bonds B^III— O—B^III, B^III—O—B^IV, B^IV—O—B^IV and end groups B^III— O⁻; ZnO practically exerts no influence on the coordination transition [BO₃] → [BO₄]. At a high content of ZnO, zinc ions are present in both a six-and a four-coordinated state. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 778–781, November–December, 2005.     

Investigation of the electronic structure of the SbSeBr cluster

The energy levels of valence bands (VB) in SbSeBr crystals were calculated for investigation of the photoelectron emission spectra of A⁵B⁶C⁷ — type crystals. The molecular model of this crystal was used for calculation of VB by the Density Functional Theory (DFT) and Unrestricted Hartree — Fock (UHF) methods. The molecular cluster consisting of 20 molecules of SbSeBr was used for calculations of averaged total density of states including atom vibrations. The spectra of averaged total density of states from VB in the SbSeBr cluster were compared with experimental photoelectron emission spectra from VB of A⁵B⁶C⁷ — type crystals. The results of comparison clarify that the atomic vibrations are one of possible reasons for the smoother appearance of the experimental X-ray photoelectron spectrum (XPS).      

Kinetics of the formation of gas bubbles during polarization of copper and graphite electrodes in electrolytic aqueous solutions

The adsorption and kinetic processes of the formation of gas bubbles passivating the surface during polarization of copper and graphite electrodes in 1% aqueous solution of sulfuric acid have been investigated. Three stages of the process related to the recharging of the double electric layer—adsorption accumulation of the gas escaping from the surface, the critical nucleation of the gas bubbles, and their subsequent growth—have been revealed, distinguished, and quantitatively estimated. It has been shown that potential leveling at the steady-state value specified by the Tafel equation is unambiguously associated with achievement of the limiting surface area screened by the gas bubbles for each particular current density. The surface diffusion constants D _H = (1.5–4.4) × 10⁻⁴ and (0.1–3.8) × 10⁻⁵ cm²/s of hydrogen on copper and graphite, respectively, and D _O = (1.8–4.5) × 10⁻⁷ cm²/s of oxygen on graphite during the motion toward the drain (the gas bubbles) have been calculated.     

Progress in the growth and research of crystals for wide-gap semiconducting materials

The feasibility of a sublimation sandwich method for controlled growth of single crystals and epitaxial layers of different SiC and GaN polytypes is demonstrated. The controlled production of pure (n _i <10¹⁶ cm⁻³) and heavily-doped crystals and epitaxial layers of these materials has made it possible to study their semiconducting parameters in detail and to identify the nature of a number of the most important impurity centers. It is shown for the example of SiC that the typically high chemical-binding energy of atoms in these compounds is the reason for the formation of stable metastable compounds, among them associations and clusters that include intrinsic defects which have a significant effect on the properties of the material. Clusters formed on the surface can serve as seeds for different polytypes during crystal growth. Fiz. Tverd. Tela (St. Petersburg) 41, 822–825 (May 1999)     

Conditions of formation of stable and metastable structures in thin films of AIIBVI compounds on amorphous substrates

On the basis of extensive experimental results relating to actual A^IIB^VI compounds (excluding compounds with mercury) an analysis is made of the general laws of formation of stable and metastable modifications in thin films of these compounds. A connection is shown between the relation of the ionic and covalent components of the bond, the type of crystal structure and the excess of the corresponding component in films of A^IIB^VI compounds, which makes it possible to control the type of crystal structure in thin layers by precipitating them in conditions of deliberate disruption of the ratio of the components in the crystallization zone.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 11, pp. 85–91, November, 1972.     

Exciton transitions in photoluminescence spectra of AIAs/GaAs superlattices

Conclusions The studies performed here have shown that the superlattice samples studied exhibit photoluminescence spectra which agree with the Kronig-Penney model, although in calculating the energies of the radiative transitions it is necessary to take into account the binding energy of the excitons,E _B. Due to the exciton—phonon interaction, the 1HH peak breaks up on the long wavelength side into a Poisson distribution. The energy of the LO phonon so determined is 34 meV. Bands due toD ⁰-A ⁰ andD ⁰X transitions, caused by remaining low-level impurities in the GaAs crystals are also observed. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 61, Nos. 3–4, pp. 241–245, September–October, 1994.     

Rotational analysis of the 0–9, 0–10 and 0–11 bands of theA 2II u -X 2II g band system of (16O18O)+

High-resolution spectra of the 0–9, 0–10 and 0–11 bands of theA ²II _u —X ²II _g system of (¹⁶O¹⁸O)⁺ ion have been studied for their rotational structure. This study enables a direct determination of the Λ-doubling parameters of theA ²II _u andX ²II _g states. The model of ‘pure precession’ explains, though not entirely, the Λ-doubling of theX ²II _a state as arising out of its interaction with theB ² Σ _g ⁻ state. The Λ-doubling in theA ²II _u state was found insignificant.     

A Comparison of GaN Epilayers with Multiple Buffer Layers and with a Single Buffer Layer Grown on Si（111） Studied by HRXRD and RBS/Channeling

Two hexagonal GaN epilayers （samples A and B） with multiple buffer layers and single buffer layer are grown on Si （111） by metal-organic vapour phase epitaxy （MOVPE）. From the results of Rutherford backscattering （RBS）/channeling and high resolution x-ray diffraction （HRXRD）, we obtain the lattice constant （a and c） of two GaN epilayers （aA = 0.3190 nm, cA = 0.5184 nm and aB = 0.3192 nm, CB = 0.5179 nm）, the crystal quality of two GaN epilayers （ ХminA=4.87%, ХminB =7.35% along 〈1-↑213〉 axis） and the tetragonal distortion eT of the two samples along depth （sample A is nearly fully relaxed, sample B is not relaxed enough）. Comparing the results with the two samples, it is indicated that sample A with multiple buffer layers have better crystal quality than sample B with a single buffer layer, and it is a good way to grow GaN epilayer on Si （111） substrates using multiple buffer layers to improve crystal quality and to reduce lattice mismatch.     

Ion conduction and space-charge polarization processes in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> crystals

The electrical properties of a lithium heptagermanate (Li₂Ge₇O₁₅) crystal have been studied in DC and AC measuring fields at temperatures from 500 to 700 K. In a DC field, a substantial decrease of electrical conductivity σ with time has been detected. On the basis of kinetic dependences σ(t), estimates of the charge carrier diffusion coefficient D have been obtained. In the frequency range 10¹–10⁵ Hz, the spectra of complex impedance ρ*(f) have been measured. The analysis of diagrams in the complex plane (ρ″–ρ′) has been performed within the equivalent circuit approach. It has been shown that, in the considered temperature and frequency intervals, the electrical properties of Li₂Ge₇O₁₅ crystals have been determined by the hopping conduction of interstitial lithium ions A _Li and accumulation of charge carriers near the blocking Pt electrodes.     

Carbon phosphide (B2 Σ + — A2 Π i) band system: RKR Franck-Condon factors andr-centroids

Franck-Condon factors andr-centroids based on Rydberg-Klein-Rees potential have been computed for the CP (B ² Σ ⁺ —A ² Π _i) band system. These results are compared with the Morse values and discussed.     

Theoretical description of fluctuations in free-standing smectic-A films including spatial non-uniformity of their elastic characteristics

A theoretical study is reported of fluctuations in smectic layer displacements and of the correlations between them in free-standing smectic-A films formed of liquid-crystal compounds with smectic-A-isotropic liquid (Sm-A-I) and Sm-A-nematic (Sm-A-N) bulk phase transitions. The study took into account the dependence of the elastic constants for bending, K, and tension (compression), B, of smectic layers on distance to the free film surfaces. The calculations are compared with the results obtained within Hołyst’s model for spatially uniform, free-standing smectic-A films. It has been established that, below the temperature at which smectic order in the bulk of a liquid crystal disappears, taking into account the profiles of the elastic moduli K and B does not produce noticeable differences from this model. However, at temperatures considerably above the Sm-A-I and Sm-A-N phase transitions, their inclusion results in considerable discrepancies from the predictions of Hołyst’s model. Fiz. Tverd. Tela (St. Petersburg) 41, 1882–1889 (October 1999)     

Incommensurate phase in the layered hexagonal crystal Cs3Bi2I9

An incommensurate phase is observed in a Cs₃Bi₂I₉ crystal, which belongs to the family of layered A₃B₂X₉ structures. Data from ¹²⁷I NQR and x-ray powder diffraction patterns show that the second-order normal phase—incommensurate phase transition at T _i=220 K is accompanied by a P6₃/mmc↔P2₁/m symmetry change and takes place at the center of the Brillouin zone. The nature of the instability leading to the appearance of this little-studied form of incommensurability is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1105–1108 (June 1997)     

The effect of oxygen on the excition spectra of A2B6 compounds

The exciton cathode luminescence and reflection spectra are studied for pure ZnSe with deviation from stoichiometry within a bilateral homogeneity region and a sharp change in the oxygen concentration from 3·10¹⁸ to 2·10²⁰ cm⁻³. It is confirmed that the exciton b and is not shifted and, consequently, that the forbidden band does not change with the oxygen concentration. This effect is interpreted on the basis of the interaction of oxygen with intrinsic point defects. It is found that dissolution, of oxygen enhances the excitonphonon interaction, which results in additional absorption within the A_ex-1LO or A_ex-2LO spectral region. It is shown that this effect is also characteristic of other compounds A₂B₆. Moscow Institute of Power Engineering, 14, Krasnokazarmennaya St., 111250, Moscow B-250, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 3, pp. 394–399, May–June, 1998.     

Low-stability pre-transitional states,orderdisorder phase transitions,and <Emphasis Type="Italic">B</Emphasis>2–<Emphasis Type="Italic">A</Emphasis>1 structure transformations in the Cu – 40 at.% Pd alloys

Low-stability states and peculiarities of structural changes in the region of phase transitions are investigated using the Cu–Pd ~40 at.% Pd alloys as an example. To this end, the crystal lattice and long-range atomic order parameters and the Debye-Waller factors were determined in situ directly in the temperature range of the B2–A1 phases and the order – disorder phase transition using X-ray structural analysis. Based on the data obtained, peculiarities of the B2 and A1 phases in the CuPd alloys were analyzed near the structure-phase transitions and the behavior pattern of atomic vibrations in the crystal lattice was discussed. It is shown that in the CuPd alloys with ~40 at.% Pd, low-stability states are observed, which exhibit a number of anomalous phenomena (such as anisotropy of atomic displacements, splitting, heterophase fluctuations, nonlinearity of the lattice parameter and long-range order parameter dependences, etc.), which precede transformation of the alloy.     

Nuclear orientation study of160Tb impurity in gadolinium single crystal

The low temperature nuclear orientation of¹⁶⁰Tb in gadolinium single crystal has been studied in the temperature range 5–60 mK and in the external magnetic field B_ext ≤9 T, applied both in the a- and c-axis directions. The obtained results indicate the noncollinearity between the Tb magnetic moments and the magnetization of the gadolinium matrix.     

Effect of neutron irradiation of colorless sapphire (leucosapphire) single crystals on their absorption and luminescence spectra

Features of the thermoradiative changes in the optical absorption and luminescence spectra of leucosapphire (colorless sapphire) crystals irradiated by neutron fluences within the range 5·10¹⁵ to 5·10¹⁹ cm⁻² have been studied in the visible region. The stepwise character of these processes was established as well as the basic steps involved in bleaching the induced color of the wafers as a result of isochronal annealing. Some anomalies have been observed in the temperature dependence of the optical density of the 460–620 nm bands, the activation energies for the color centers as well as the color center concentrations have been calculated, and the nature of the radiation-induced centers has been analyzed. An analytical expression is proposed to describe the accumulation kinetics for the centers during irradiation of the crystals by reactor neutrons. It is concluded that there is an interconnection between, for example, the 460 nm color centers and the 540 nm luminescence centers, and that there is a common mechanism for the process of radiation-induced defect formation, initially responsible for their formation in the crystal. The possible effect of reabsorption of radiation on the behavior of the Y(Φ) curve in the irradiated material is discussed. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 2, pp. 247–251, March–April, 2007.     

Fluorescence studies of CS2 and SO2 at 121.6, 73.6–74.4 and 58.4 nm

The fluorescence spectra of CS₂ and SO₂ have been studied at three incident photon wavelengths of 121.6, 73.6–74.4 and 58.4 nm and relative production cross sections for different product states have been measured. The CS(A ¹Π→X ¹Σ⁺) system between 240 and 290nm has been obtained when CS₂ is photoexcited at 121.6nm whereas CS ₂ ⁺ (B ²Σ _u ⁺ →X ²Π _g ) and CS ₂ ⁺ (A ²Π _u →X ²Π _g ) systems have been produced between 276 and 295 and 437 and 555nm respectively when excited by both the incident photon wavelengths of 73.6–74.4 and 58.4nm. The fluorescence spectra of SO₂ obtained at 121.6 and 73.6–74.4nm include the vibrational bands of SO(A ³Π→X ³Σ⁻) and SO(B ²Π⁻→X ³Σ⁻) systems from 240 to 268 and 268 to 442nm respectively whereas the emission spectrum at 58.4nm, has contributions from the two SO systems and SO⁺(A ²Π→X ²Π) system. In all these emission spectra, the fluorescence bands of different systems have been analyzed and their relative production cross sections have been measured. The results obtained in the present investigations have been compared with a few recent reliable measurements reported in literature.     

Sum-of-Squares Results for Polynomials Related to the Bessis–Moussa–Villani Conjecture

We show that the polynomial S _m,k (A,B), that is the sum of all words in noncommuting variables A and B having length m and exactly k letters equal to B, is not equal to a sum of commutators and Hermitian squares in the algebra R〈X,Y〉, where X ²=A and Y ²=B, for all even values of m and k with 6≤k≤m−10, and also for (m,k)=(12,6). This leaves only the case (m,k)=(16,8) open. This topic is of interest in connection with the Lieb–Seiringer formulation of the Bessis–Moussa–Villani conjecture, which asks whether Tr (S _m,k (A,B))≥0 holds for all positive semidefinite matrices A and B. These results eliminate the possibility of using “descent + sum-of-squares” to prove the BMV conjecture.     

Isotope effect in charge transport of LuB12

The galvanomagnetic properties of single-crystal samples with various isotopic boron compositions have been investigated for the first time for the normal state of superconductor LuB₁₂ (T c ≈ 0.44 K). Precision measurements of the resistivity, Hall coefficient, and magnetic susceptibility have been performed over a wide temperature range of 2–300 K in magnetic fields up to 80 kOe. A change of the charge transport regime in this nonmagnetic compound with metallic conduction is shown to occur near T* ≈ 50−70 K. As a result, a sharp peak with significantly different amplitudes for Lu¹⁰B₁₂ and Lu¹¹B₁₂ is recorded in the temperature dependences of the Hall coefficient R _H(T) near T*. A significant (about 10%) difference (in absolute value) of the Hall coefficients R _H for the Lu¹⁰B₁₂ and Lu¹¹B₁₂ compounds at helium and intermediate temperatures has been found and the patterns of behavior of the dependence R _H(H) for T < T* in an external magnetic field H ≤ 80 kOe for Lu¹⁰B₁₂ and Lu¹¹B₁₂ are shown to differ significantly. Analysis of the Curie-Weiss contribution to the magnetic susceptibility χ(T) leads to the conclusion about the formation of magnetic moments μ_eff ≈ (0.13−0.19)μ_B in each unit cell of the fcc structure of LuB₁₂ compounds with various isotopic compositions. The possibility of the realization of an electronic topological 2.5-order transition near T* and the influence of correlation effects in the 5d-band on the formation of a spin polarization near the rare-earth ions in LuB₁₂ is discussed.