bluuebunny/crossref_metadata_2025_split huggingface dataset:
Atomic hydrogen at anion sites in LiFH− and NaH− crystals
Authors: Egranov A. V. , Chernyago B. P. 
Published: July 2007

AbstractAnion hydrogen atom centres are studied by optical and ESR echniques in LiFH− and NaFH− crystals. The ESR spectra of these centres are observed after X‐ray irradiation of the crystals at 77K. The optical absorption spectra of H centres consists of two bands at 2.4 and 3.5 eV for LiFH− and at 2.95 and 3.8eV for NaFH−,which are due to the electron charge transfer from a flurine ion to the l s state of the hydrogen atom.
Charge ordering of asymmetrical clusters in mixed‐valence molecular crystals: Mössbauer spectra
Authors: Klokishner S. I. , Gheikhman B. D. 
Published: July 2007

AbstractA crystal consisting of asymmetrical d‐d mixed‐valence (MV) clusters is considered. It is supposed that the cluster distortion results from the difference of crystal fields acting on centers a and b as well as from other factors providing the difference of single‐electron wave functions of centers a and b. The model accepted takes into account Heisenberg‐type and double exchange along with cluster asymmetry and intercluster dipole‐dipole interaction. Conditions of charge‐ordered phase stabilization are elucidated. Ferro‐and antiferroelectric states of crystals comprising asymmetrical MV dimers are investigated. The phase transitions in such crystals take place gradually. Computer simulation of Mössbauer spectra is carried out. The experiments of Mössbauer spectra of a series of biferrocenium derivatives performed by the Hendrickson group are qualitatively interpreted.
Effect of interface energy anisotropy on thermal stability and transformation of lamellar Structures: II. Transformation of lamellae
Authors: Mazurski M. I. , Salishchev G. A. 
Published: July 2007

AbstractThermodynamic regularities of the transformation of a lamellar shape into a spherical one is analyzed on the basis of the comparison of interface energies of spheres and disks. It is revealed that the motive force of spheroidization in two‐phase systems with a high level of anisotropy occurs during high‐temperature deformation due to developing recrystallization processes. In systems with a low level of interface energy anisotropy the motive force considerably increases due to recrystallization. The causes of a specific behaviour of systems consisting of phases with different types of interatomic bonds are noted.
Resonant tunneling and confining phenomena in symmetrical rectangular triple‐barrier structures with C‐type double‐wells
Authors: Yamamoto H. , Tsuji K. , Taniguchi K. 
Published: July 2007

AbstractResonant tunneling is studied theoretically in symmetrical rectangular triple‐barrier structures composed of triple barriers and C‐type double‐wells which are coupled through a central thin barrier. Analytical expressions for the transmission coefficient and the resonance conditions are derived for the first time. Wave functions of an electron at the resonance level are examined and the confining phenomena are studied.
A modified Hückel calculation on the structures and ionization potentials of Ag clusters
Authors: Zhao Jijun , Chen Xiaoshuang , Wang Guanghou 
Published: July 2007

AbstractThe geometrical structures and ionization potentials of silver clusters are studied by a modified Hückel theory including the variable Coulomb integral resulting from different coordinate environments. The stable atomic configurations of the Agn clusters are obtained by comparing the energy of different structural isomers. The clusters with magic number atoms are found to adopt the near spherical host‐like structure with higher symmetry, corresponding to the complete electronic shell. Based on those favorable structures the size dependent ionization potentials of Agn are calculated up to 68 atoms by using this new developed modified Hückel theory. The theoretical values are in quite agreement with the experimental results as well as ab initio configuration interaction (CI) calculations. A transition from “electronic order” to “atomic order” around n = 40 is proposed.
The elastic energy loss related to the lattice instabilities above room temperature in Bi1.7Pb0.3Sr2Ca2Cu30y
Authors: Chen S. O. , Shen H. M. , Huang Y. N. , Xu J. , Wang X. M. , Wang Y. N. 
Published: July 2007

AbstractThe tensile test, the torsional pendulum test, and X‐ray analysis are used to investigate the lattice instabilities of Bi1.7Pb0.3Sr2Ca2Cu3Oy ceramics from room temperature to 250 °C. The results show that there are three energy loss peaks near 35, 95, and 155 °C, which display of strain amplitude dependence and frequency independence. In the vicinity of these peak temperatures step‐like changes of lattice parameters associated with lattice instabilities are observed.
Comparison of Surface‐Enhanced Raman and Hyper‐Raman Spectra of the Triphenylmethane Dyes Crystal Violet and Malachite Green
Authors: Schneider S. , Brehm G. , Freunscht P. 
Published: July 2007

AbstractVibrational spectra of two dyes, malachite green and crystal violet are presented. Raman and hyper‐Raman spectra are recorded as surface‐enhanced (hyper‐) Raman spectra on silver colloids. These data are compared to the respective IR spectra with respect to the point group of the molecules (C2 and C3). Theoretical calculations (PM3 semiempirical) and polarized Raman spectra are used to give an assignment of some vibrational bands. The advantage of SERS (surface‐enhanced Raman scattering) and SEHRS (surface‐enhanced hyper‐Raman scattering) is discussed for the analysis of vibrational spectra.
Vibronic Two‐Photon Intensities in the 1La and 1Lb Transitions of Anthracene
Authors: Hohlneicher G. , Wolf J. 
Published: July 2007

AbstractVibronic intensities for a two‐photon transition from the electronic ground state of anthracene to the first two excited singlet states (1La and 1Lb) are calculated within first‐order Herzberg‐Teller theory. Dushinsky effects are fully included. The results are compared to recent experimental observations.
Level Correlations and Dephasing‐Induced Resonances in Molecular Aggregates
Authors: Chernyak V. , Mukamel S. 
Published: July 2007

AbstractDephasing‐induced resonances in the nonlinear optical response of molecular aggregates are expressed in terms of level correlations. The third‐order optical response is related to the three‐dimensional spectral density function χ(ε′, ε,ε″). Expressions for χ(ε′, ε,ε″) derived using the Frenkel exciton model show the existence of collective (dephasing‐induced) resonances.
On the Phase Transition Sequence in Deuterated Betaine Calcium Chloride Dihydrate under Hydrostatic Pressure
Authors: Chaves M. R. , Almeida A. , Kiat J. M. , Tolédano J. C. , Schneck J. , Glass R. , Schwarz W. , Ribeiro J. L. , Klöpperpieper A. , Albers J. 
Published: July 2007

AbstractAn elastic neutron scattering study of deuterated betaine calcium chloride dihydrate under hydrostatic pressure is presented. It is shown that a previous phenomenological model based on a one‐dimensional order parameter P, and involving a term of the form P2(δP/δz)2 remains adequate to account satisfactorily for the effects of the hydrostatic pressure. From the study of the temperature dependence of the integrated intensity of the satellites it is inferred that the critical exponent β related to the primary order parameter is close to 0.5 for 2.9 × 108, 3.4 × 108, and 4 × 108 Pa.
Energy Relaxation of Hot‐Electrons in a Quantum Well. Confined Optical and Acoustic Phonon Effects
Authors: León‐Avila F. , Rodríguez‐Coppola H. , Comas F. 
Published: July 2007

AbstractEnergy‐loss rates of hot electrons are calculated for a semiconductor quantum well (QW) of the GaAs/AlGaAs prototype. Couplings with acoustic phonons (AP) and with confined polar‐optical phonons (OP) are considered. Hot electrons are assumed in internal equilibrium at a temperature Te (the temperature model). Comparison with previous calculations of other authors shows that OP confinement with the proper dispersion relation is of utmost importance for the correct interpretation of energy‐loss rates in the temperature range of 25 to 100 K. In the lower temperature regime the coupling with AP cannot be neglected. Thus reasonable agreement with experimental data is obtained even within the relatively simple model for the calculation when both mechanisms are taken into account. Other effects (hot phonons, etc.) are not considered and also no fitting parameters of any kind are introduced.
Effect of Forming Pressure on the Critical Temperature in YBa2Cu3O7−δ Superconductors
Authors: Al‐Shibani K. , Al‐Aql A. , Al‐Muezzin A. , Humeida H. , Nakhmedov E. P. , Firat T. , Öztürk O. , Saçli Ö. A. 
Published: July 2007

AbstractThe effect of pressure on the critical temperature Tc is studied for YBa2Cu3O7–δ compounds prepared under various forming pressures. A systematic decrease in lattice parameters is observed with forming pressure. Resistivity measurements show an increase of the onset temperature Tc0 with forming pressure. A value of dTc0/dP ≈ (0.070 ± 0.010) × 10−8 K/Pa is determined. A new mechanism based on the variation of the interlayer tunneling integral t⟂ is used to analyse the experiment results.
Influence of Surface Electric Field on Exciton Photoluminescence of CdS. Self‐Reversal of Spectral Lines
Authors: Rakovich Yu. P. , Yablonskii G. P. , Gladyshchuk A. A. , Smal A. S. 
Published: July 2007

AbstractThe structure of luminescence spectra of free excitons in CdS single crystals at T = 77 K is investigated. The dip formation is found in the top part of free A and B exciton resonant peaks of photoluminescence (PL) spectra after heating of the CdS crystals followed by fast cooling down to 77 K. Similar effects are observed when an external electric field is applied. It is shown that the structure of exciton PL spectra is due to self‐absorption of resonance radiation in the near‐surface layer with low exciton concentration. Our calculations demonstrate that the formation of an exciton concentration gradient may be caused by the influence of a surface electric field on the binding of electron‐hole pairs to excitons. It is assumed that the increase of surface electric fields after heat treatment is caused by the thermostimulated adsorption of oxygen, which results in the formation of electron trap levels.
Electronic Structure of Vanadyl Phthalocyanine. Investigations of Thin Film Samples by Electron Energy‐Loss Spectroscopy
Authors: Dahms U. , Mrwa A. 
Published: July 2007

AbstractThe energy‐loss function Im (−ε−1) of a thin polycrystalline vanadyl phthalocyanine film is determined in the range from 1 to 37 eV from energy‐loss experiments with 50 keV electrons. The electronic structure of the film investigated is analyzed by computing the complex dielectric function ε(E), optical constants (n(E) + ik(E), R(E), α(E)), and neff(E) using the Kramers‐Kronig analysis of the energy‐loss function. The spectrum of valence electron excitations is primarily interpreted in terms of π−π* and σ−σ* transitions of individual molecules and in terms of the collective excitation of the valence electron ensembles (σ + π plasmon).
Al3+ Ion Site Symmetry in (NH4)2[AlF5 · H2O] Single Crystals
Authors: Darabont Al. , Borodi Gh. , Nistor S. V. , Velter‐Stefănescu M. 
Published: July 2007

AbstractThe production of pure and Cr3+ doped (NH4)2[AlF5 · H2O] single crystals and the crystal structure of this compound are described. The infrared spectra recorded experimentally at room temperature show – using the results of group theory – that the [AlF5 · H2O]2− complex ion has at most the symmetry of the C4v group. EPR measurements are carried out for a more exact determination of the site symmetry of Al3+ ions using the Cr3+ ion on Al3+ site as “probe ion”. The determined values of spin‐Hamiltonian parameters suggest that the symmetry of the [AlF5 · H2O]2− complex ion has at most the symmetry of the C2v group.
Theory of the AC Hall Effect and Magnetoresistance in Polycrystalline Systems
Authors: Fishchuk I. I. 
Published: July 2007

AbstractThe theory of the ac Hall effect and magnetoresistance for systems which consist of randomly oriented anisotropic crystallites is developed. The effective‐medium theory is used. The large frequency region, where ω < τ−1 (τ is the electron mean free time) is investigated. It is shown that the real parts of the dynamic conductivity, the Hall conductivity, the Hall mobility, the Hall coefficient, the magnetoconductivity, and the magnetoresistance have low‐ and high‐frequency plateaus. The high‐frequency plateau begins in the vicinity of the frequency ω0 ≈ τ, where τM is the Maxwell relaxation time (τM » τ).
Pair‐Breaking and Magnetic Ordering in the Pseudo‐Quaternary Intermetallic Series (Y, Gd)Ni2B2C
Authors: El Massalami M. , Budko S. L. , Giordanengo B. , Fontes M. B. , Mondragon J. C. , Baggio‐Saitovitch E. M. , Sulpice A. 
Published: July 2007

AbstractAlloying influences on the superconducting and magnetic properties of the (GdxY1−x)Ni2B2C series are studied. For x < 0.25, Tc is degraded linearly with x as expected from the theory of Abrikosov‐Gorkov. Magnetization and magnetoresistivity measurements for x = 0.22 show that the magnetic saturation is attained only for fields as high as 80 kOe implying that even for such a low Gd concentration, antiferromagnetic couplings are quite strong. On the other hand, the magnetic dilution in the Heisenberg antiferromagnet GdNi2B2C causes a linear depression of the magnetic ordering temperatures as expected from molecular field theory. The abrupt quench of superconductivity for x > 0.25 and the coexistence of superconductivity with long‐range antiferromagnetic order for 0.1 < x < 0.25 are discussed.
Estimating Critical‐Point Parameters of Modulated Reflectance Spectra
Authors: Hosea T. J. C. 
Published: July 2007

AbstractTwo new methods, based on the 3‐point method and a Kramers‐Kronig transformation, were recently published for estimating the energy, broadening, phase, and amplitude of critical points in modulated reflectance spectra which are describable by Aspnes' third derivative functional form. The use of the techniques is demonstrated by applying them to example experimental spectra, which are photo‐reflectance measurements of stress‐induced splitting of the direct band gap E0 transition in bulk GaAs. The results are compared and contrasted with those obtained from a least‐squares fit of a theoretical model to the experimental spectra.
Ultrafast Carrier Dynamics in Porous Silicon
Authors: Fauchet P. M. 
Published: July 2007

AbstractThe carrier dynamics in porous silicon is investigated by two time‐resolved techniques. The blue photoluminescence of oxidized porous silicon is measured with 100 ps time resolution as a function of the oxidation method, emission wavelength, excitation intensity, and measurement temperature. The blue luminescence has a distinct origin from the well‐studied rcd luminescence and is attributed to defects in the oxide. Femtosecond photoinduced absorption measurements are performed on thin red‐emitting porous silicon films. The wavelength and intensity dependence of the recovery are interpreted in terms of trapping and of Auger recombination at high excitation intensity. It is shown that red‐emitting porous silicon is not a direct‐gap semiconductor.
Light Emission from Silicon Nanostructures Produced by Conventional Lithographic and Reactive Ion Etching Techniques
Authors: Nassiopoulos A. G. , Grigoropoulos S. , Papadimitriou D. , Gocolides E. 
Published: July 2007

AbstractSub‐ten nanometer diameter silicon pillars and silicon walls of the same thickness are fabricated by using conventional optical lithography based on deep‐UV exposure and reactive ion etching using fluorine only containing gases. The produced structures are studied for their luminescence properties. Visible photoluminescence with a peak in the range 580 to 650 nm is observed under Ar laser irradiation.
Internal Friction and Sound Wave Velocities in Pure C60 Fullerite
Authors: Kobelev N. P. , Soifer Ya. M. , Bashkin I. O. , Gurov A. F. , Moravskii A. P. , Rybchenko O. G. 
Published: July 2007

AbstractThe temperature dependence of the attenuation and sound wave velocity of compacted pure C60 fullerite is measured in the temperature interval from 100 to 300 K at a frequency of about 5 MHz. Two internal friction peaks are observed at 250 and 215 K. The conclusion is made that the internal friction peak at 250 K is connected with the structural phase transition and the peak at 215 K is due to the relaxation by reorientation of C60 molecules in the stress field of ultrasonic waves. The magnitudes of the elastic moduli of C60 fullerite are determined by sound velocity measurements. The results are compared with theoretical estimations of the elastic moduli and experimental data of other authors.
EPR of CrO Radicals in High Temperature Phases of (NH4)3H(SeO4)2 Crystal
Authors: Aucustyniak M. A. , Hoffmann S. K. 
Published: July 2007

AbstractEPR X‐band studies of CrO radicals substituting SeO4, groups in (NH4)3H(SeO4)2 crystals are performed in the temperature range 280 to 400 K, where transitions to two superionic phases with protonic conductivity appear at 304 and 330 K. In the room temperature ferroelastic phase four types of CrO radicals with different g‐factors and direction cosines are identified. Three of them (with 40% of the total spectrum intensity) are identified as Halperin‐Varma‐type defects. These defects vanish in superionic phases. The electronic structure of the CrO3‐4 radical is different from that, predicted by the geometry of the host SeO, groups, and is described by the d ground state, characteristic of an elongated tetrahedron of distorted D2d symmetry. Only part of the CrO3‐4 radicals show the dynamics expected for SeO4 groups and follow their behaviour at the phase transitions. The transition to the superionic phase II at 304 K is observed as a rapid EPR line narrowing due to an increase in the crystal lattice dynamics and an increase in the SeO4 reorientation rate. In the phase transition to the superionic phase I (330 K) a shift and broadening of the lines are observed. It allows to suggest that the crystal structure of phase I is dynamically disordered with two equally populated orientations of the SeO4 group.
The Dependence of the Debye Temperature on the Elastic Constants
Authors: Siethoff H. , Ahlborn K. 
Published: July 2007

AbstractSophisticated methods have been put forward in the literature to calculate Debye temperatures from the elastic constants cik. As a simple alternative method for cubic crystals, a relation is often used that correlates Debye temperature and bulk modulus (c11 + 2c12)/3 by a square‐root law. It is shown that, with exception of the alkali metals, such a law is only poorly fulfilled for other cubic elements and compounds. If one takes, however, elastic moduli such as [c44(c11 − c12) (c11 + c12 + 2c44)]1/3 or {c44[(c11 − c12)c44/2]1/2 (c11 − c12 + c44)/3}1/3 instead of the bulk modulus the square‐root law is established for various cubic material classes. This procedure eventually allows to easily evaluate hitherto unknown Debye temperatures from the elastic constants with high precision.
DSC, Dilatometric, Dielectric, and 1H NMR Studies of Phase Transitions and Molecular Motions in [N(C2H5)4]3M2Cl9 (M = Sb, Bi) Crystals
Authors: Zaleski J. , Medycki W. , Piślewski N. , Jakubas R. 
Published: July 2007

AbstractResults in the dependence of stoichiometry of obtained tetraethylammonium (TEA) chloroantiomonatc and chlorobismuthate salts on the molar ratio of reactants used in the synthesis are presented. Seven tetraethylammonium salts are obtained: (TEA)6M8Cl30, TEAMCl4, (TEA)3M2Cl9 (M = Sb, Bi) and (TEA)2SbCl5. Preliminary X‐ray diffraction studies on (TEA)3M2Cl9 (M = Sb, Bi) show that they are isomorphous, crystallizing at room temperature in monoclinic symmetry. The dilatometric, dielectric, and DSC studies show that (TEA)3Bi2Cl9 undergoes two phase transitions at Tc2 = 144K and at Tc1 = 322K while (TEA)3Sb2Cl9 undergoes three transitions at Tc3, = 185. Tc2 = 219, and Tc1 = 321 K. Both crystals are ferroelastic below the ferro‐paraelastic transition at Tc1. The room temperature domain structure belongs to the Aizu species 6/mmmF2/m. The spin‐lattice relaxation times and the values of the second moments of the 1 NMR line for the (TEA)3Sb2Cl9 crystal are measured in the temperature range of 50 to 350K. Three minima of T1 are observed. Two low‐temperature minima are attributed to CH3 group reorientations, while the high‐temperature one to reorientations of the whole cations. The activation energies for the processes are determined.
X‐Ray Diffraction Study of Thin Porous Silicon Layers
Authors: Lomov A. A. , Bellet D. , Dolino G. 
Published: July 2007

AbstractHigh resolution X‐ray diffraction measurements are performed on porous silicon samples (p+ type, 40% porosity) of various thicknesses (from 0.03 to 20 μm). In the thicker samples narrow Bragg peaks are observed for as‐formed samples, with profiles close to those of dynamical diffraction theory. For thin samples (in the 0.1 μm range) the Bragg peak of the porous layer is not resolved, but appears as a shoulder on the substrate peak. Thickness fringes are observed, from which simulations give information on these thin PS layers, showing that the porous layer surfaces are well defined.
Consistent Analysis of the Tl and Pb Center ESR Parameters
Authors: Nistor S. V. , Schoemaker D. 
Published: July 2007

AbstractThe electron spin resonance parameters of the Tl and Pb dimer centers produced by electron trapping at pairs of neighboring Tl+ or Pb2+ impurity ions in alkali chlorides are reanalyzed. The analysis which uses a consistent set of molecular parameters is based on the molecular modcl previously employed to describe the ESR properties of the homonuclear Vk self‐trapped hole center, which has a complcrnentary electronic configuration. In a rough approximation the analysis is extended to the self‐trapped electron (STEL) center in PbCl2 crystals, which is a bent Pb molecular ion in a low symmetry crystal field.
Computer Simulations of I‐Center Annealing in KCl and KBr Crystals. Theoretical Interpretation of Thermostimulated Experiments
Authors: Eglitis R. I. , Popov A. I. , Kotomin E. A. 
Published: July 2007

AbstractResults of computer simulations of the kinetics of correlated annealing of pairs of close α–I and F–I centers in KCI and KBr crystals, enhanced by I‐center diffusion and Coulomb or elastic attractions, respectively, are presented. Special attention is paid to the conditions under which multi‐stage annealing stages arise as it has been observed experimentally more than once. Our general conclusions are: (i) a weak elastic interaction affects the recombination kinetics and the survival probability even for relatively well‐separated F–I pairs, the more so is true for the case of Coulomb attraction between charged α–I pairs; (ii) the multi‐step (kink) structure arises only for close (typically, up to fourth‐nearest neighbour) defects. To explain the strongly separated stages observed experimentally for α–I (17 to 21 K, 30 to 39 K in KCI and 18 to 22 K and 27 to 30 K in KBr) a very particular initial distribution of defects is required consisting of two groups of close and well‐separated defects. It gives strong support to the results of recent computer simulations that an interstitial ion created due to electron trapping by an H center can be transformed into a crowdion which could be displaced athermally by a long distance from a vacancy (five to seven interionic distances).