bluuebunny/crossref_metadata_2025_split huggingface dataset:
Pressure‐induced phase transitions and anharmonicity study of alkaline‐earth fluorides
Authors: Kourouklis G. A. , Anastassakis E. 
Published: July 2007

AbstractRoom temperature measurements of Raman scattering under hydrostatic pressure in a diamondanvil cell have shown that the cubic‐to‐orthorhombic (β → α) phase transition in CaF2 and BaF2 occurs at (8.5 ± 0.2) and (2.3 ± 0.2) GPa, respectively. The pressure dependence of the observed phonons in the α‐phase as well as their symmetry assignment based on a comparison to other fluorite‐structure crystals is presented. Anharmonic processes are shown to be small but not negligible and of the same nature in all alkaline‐earth fluorides. It is established that the β → α and α → β transition pressures show a linear dependence on the inverse volume of the unit cell for the alkaline‐earth fluorides CaF2, SrF2, and BaF2.
One‐dimensional dirac equation approach to the effective mass concept in diatomic crystals
Authors: Detcheva V. , Metodieva M. , Khristov T. , Kandilarov B. D. 
Published: July 2007

AbstractA relativistic generalization of the effective‐mass concept is worked out on the basis of simplest one‐dimensional model of diatomic crystals and in the framework of the Dirac equation approach. The numerical analysis performed leads to reasonable values for both, the energy gaps and the effective masses of the carriers. The relativistic corrections thus obtained bring on a firm basis the justification of simpler calculations in the framework of the Schrödinger equation. The need of more powerful approaches beyond the Dirac equation is outlined, especially for crystals of very heavy atoms (Z > 68).
Approximate solution for a two‐dimensional anisotropic ferromagnetic spin lattice with Bloch Wall and Bloch Line. II. Numeric Computation
Authors: Knoop U. , Heber G. 
Published: July 2007

AbstractFor a two‐dimensional classical spin lattice with local anisotropy or exchange anisotropy the nonlinear difference equations, which are derived from the Heisenberg Hamiltonian, are numerically solved. The spin configuration of a spin lattice containing a Bloch wall and a Bloch line is computed.
Low‐temperature IR and Raman scattering spectra of TlInS2 layered crystal
Authors: Allakhverdiev K. R. , Babaev S. S. , Tagiev M. M. , Shirinov M. M. 
Published: July 2007

AbstractAn investigation is made of the polarized far IR and Raman scattering spectra of layered single crystals of ternary compound TlInS2 in the 6 to 300 K temperature range including the series of the phase transition. New unknown low‐frequency Raman and IR active phonons are observed. It is shown that the interpretation of the vibrational properties of TlInS2 crystal are in good agreement with the C space group. A low‐frequency Raman active soft mode is observed and analysed in the ferroelectric phase of this crystal. The pecularities in changing of the spectra of optical phonons, connected with a transition of the crystal from the para to ferroelectric state, from modulating structure, and quadrupling along C‐axis are observed and analysed.
X‐Ray Emission Spectroscopy of the Electronic States in Pb1−xHgxS Thin Films
Authors: Acharya B. S. , Singh S. K. , Nayak B. B. 
Published: July 2007

AbstractX‐ray emission specta in metals and alloys have been studied extensively. However, work on semiconductor alloy thin films is rather scarce. Here the sulphur Kα emission in Pb1−xHgxS thin films prepared by electroless deposition method is investigated. Films in the thickness range 0.2 to 0.3 μm are grown by this method. X‐ray emission spectra of SKα band in Pb1−xHgxS reveal the presence of three‐band structure at about 2, 5 to 7, and 17 to 18 eV from the Fermi level EF. The asymmetric index (x) is found to be higher than that of the pure SKα band. Alloying the film with Hg increases the α in the beginning and then decreases and finally increases it for HgS. It is observed that the density of states (DOS) at EF decreases for alloy thin films. From the shift in the Kα peak energy the charge associated with the S ion in these films is calculated. Besides, the bonding parameter (λ) is calculated from the charge and it is observed that for PbS, the bonding is homopolar and decreases with the increase in Hg concentration reaching a minimum nearer to neutral bonding value and thereafter it increases showing the bonding in HgS to be homopolar again. The satellite peaks are explained on the basis of hybridization of s– and p–d orbitals.
Bloch Oscillations of an Electron in a Crystal under the Action of an Electric Field
Authors: Nazareno H. N. , Gallardo J. C. 
Published: July 2007

AbstractThe dynamical localization problem of an electron in a ID crystal under the action of a uniform de electric field E is solved. Starting with a determined state at t = 0, it is solved for the Wannier amplitudes (propagator) and shown that they oscillate in time with the Bloch frequency. Moreover, it is shown the behaviour of the optical absorption coefficient as a function of the photon frequency ω which shows jumps whenever the photon energies equal multiples of eEa.
Fermi Level Density of States in AgAu Alloys. Nuclear Magnetic Spin‐Lattice Relaxation Rate and Low‐Temperature Specific Heat
Authors: Kokko K. , Ojala E. , Mansikka K. 
Published: July 2007

AbstractThe electronic structure of AgAu alloys is studied using LMTO self‐consistent band structure calculations. Both, the total and partial density of states as function of Au‐content at the Fermi level are presented. Some results of band structure calculations are analysed using a simple cluster model, which reveals that the behaviour of the partial density of states at the Fermi level can be explained by the s–d hybridisation strength and the position of the top of the d‐band. The results obtained for specific heat and spin–lattice relaxation are in general agreement with experimental observations. Furthermore, the calculations predict that the nuclear magnetic spin–lattice relaxation time of Au would decrease with increasing Ag‐content.
Ideal Two‐Dimensional Electron Gas in a Magnetic Field and at Non‐Zero Temperatures. An Alternative Approach
Authors: Wang L. , O'Connell R. F. 
Published: July 2007

AbstractA new exact expression is derived for the free energy of an ideal two‐dimensional electron gas (2DEG) in a uniform magnetic field and at low, but finite, temperatures. The approach eliminates a 2D‐peculiarity that neither the weak nor the strong magnetic field limit can be easily taken in the result derived from the standard Sondheimer‐Wilson treatment. In the strong magnetic field limit, the result reduces to an existing one, for which a misunderstanding needs to be clarified. In the weak field limit, it agrees with a result obtainable through Euler's summation formula. The conditions are discussed under which the nondegenerate situation may occur.
EPR Linewidth in Pb1−xMnxTe
Authors: Korczak S. Z. , Korczak W. , Subotowicz M. , Wasiewicz H. 
Published: July 2007

AbstractEPR in semimagnetic semiconductors Pb1−xMnxTe with x ≦ 0.017 is studied over the temperature range 3.5 to 300 K. The variations of linewidth with Mn content and temperature are analyzed. The analysis of the experimental results is based on the assumption of the presence in the system of both a Korringa‐type relaxation and a bottleneck effect. An approximate value of the exchange constant between localized magnetic ions and carriers, J = (0.08 ± 0.03) eV, is deduced.
Fundamental Solution of the Problem on the Relief Shape of a Kink on a Dislocation in an External Force Field Equal to the Barrier Reaction
Authors: Polyakov M. E. 
Published: July 2007

AbstractA critical analysis is made of the double kink model of dislocations and its disadvantages are revealed. A modified model of single dislocation kink free of the disadvantages of the previous model is proposed. In the new model, the form of the single kink relief is analytically found from the variational problem of the effect of external stress on the dislocation for the case of Peierls sinusoidal barrier. The single kink is shown to be subjected to pseudobreakage. The pseudobreakage area is characterized by an imaginary solution. Analytical dependences are obtained of the single kink and pseudobreak relaxation areas on the barrier height, and the dislocation shift by the external force of the Peierls relief valley.
A Nonlocal Continuum Approach to the Dynamics of Crystals. I. S.C. and F.C.C. Lattices
Authors: Idiodi J. O. A. 
Published: July 2007

AbstractWith a view towards helping to bridge the gap between discrete and continuum models of crystalline elastic solids, a nonlocal continuum theory which describes exactly a class of lattice models that have been widely used for s.c. and f.c.c. lattices, is presented. Particular emphasis is placed on the treatment of the boundary conditions that arise in the presence of a surface defect.
Vibrational Spectra and Phase Transitions in Layered Semiconducting Ferroelectrics with TlGaSe2 Structure. I. Temperature Dependences of Parameters of the Raman Spectrum Lines of TlGaSe2
Authors: Durnev Yu. I. , Kulbuzhev B. S. , Malsagov A. U. , Rabkin L. M. , Torgashev V. I. , Yuzyk Yu. I. 
Published: July 2007

AbstractRaman spectra of TlGaSe2 single crystals are obtained in the temperature range 85< T < 300 K, which includes the phase transition points. Fitting of the spectra, using the model of additive damped oscillators and a relaxator, yields quantitative information on parameters of the lines. No critical rise of the relaxation time is detected near phase transitions. On the other hand, the strength of the relaxator experiences an approximately twofold increase. An anomalous behavior of parameters of the lines is observed near phase transitions, which is thought to be associated with ordering processes. The lowest‐frequency lines in the 15 cm−1 region are interpreted as the lines which correspond to rigid‐layer modes.
Strain Effects on the Band Structure of Si/Si1–xGex (001) Superlattices
Authors: Rücker H. , Enderlein R. , Bechstedt F. 
Published: July 2007

AbstractThe electronic structure of strained layer Si/Si1–xGex superlattices is calculated by means of the empirical tight‐binding method. The influence of strain on band structure is treated by applying interatomic interaction parameters depending on bond lengths and bond angles. Strain effects on band discontinuities are studied. A strain induced type I‐type II transition in Si/Si0.5Ge0.5 superlattices on substrates with different lattice constants is obtained. Threshold energies and oscillator strengths of optical band‐to‐band transitions are reported for short period SinGen superlattices grown on Si1‐yGey in dependence on n and y.
On the Correct Use of Perturbation Theory for the Calculation of One‐Particle Green Functions
Authors: Monecke J. 
Published: July 2007

AbstractPerturbation theory conventionally is applied either to one‐particle Green functions (or to equivalent quantities) directly or to their inverses, i.e. to the self‐energies. Both methods, however lead to wrong, in the first case even to negative and divergent, densities of states in the corresponding order. Here the system of equations for the Green functions is solved exactly in the standard form G = N/D and perturbation theory is applied to both, the nominator N and the denominator D separately. It is shown that this method is free of divergencies and results in the correct density of states in each order. This new perturbation theory is especially suited for the calculation of electron and phonon spectra of superlattices.
Effect of Laser Irradiation on Low‐Temperature Photoconductivity and Photoluminescence Spectra of Gallium Selenide
Authors: Mozol P. E. , Skubenko N. A. , Skubenko P. A. , Gnatenko Yu. P. , Salkov E. A. , Kovalyuk Z. D. 
Published: July 2007

AbstractFeatures of photoluminescence (PL) and photoconductivity (PC) spectra, taken at 4.2 to 60 K, of GaSe crystals of stoichiometric composition and grown with an excess and with a lack of Ga atoms are analyzed. It is shown that the presence of excessive Se atoms in the matrix of GaSe crystals results in the formation of centres where triplet excitons are bound. Excessive Ga atoms formed in a crystal both, in the process of its growth and under the effect of laser irradiation give rise to regions with another polytype composition (γ‐modification). The presence of such (γ‐polytype) regions is shown to produce packing defects whre localization of excited electron states (n > 1) occurs. An effect of PC signal increase with increasing exposure dose is found, which is attributed to the formation of acceptor centres.
Diffusion Studies in CdTe and HgTe
Authors: Madhavan Y. , Ramachandran K. , Haridasan T. M. 
Published: July 2007

AbstractLattice dynamical calculations of the jump frequencies for various isotopes of cation and anion in both, CdTe and HgTe, are worked out following the general theory of diffusion for metals by Achar, assuming an active role for vacancy. The isotope effect, which is a measure of the diffusion rate in both the cases, is also deduced. The results indicate that the diffusion in these systems is governed prominently by a single vacancy mechanism as predicted from experiments.
Transmission Coefficient and Resonance Condition in Rectangular n‐Fold Barrier Structures
Authors: Yamamoto H. , Kanie Y. , Taniguchi K. 
Published: July 2007

AbstractResonant tunneling is studied theoretically in rectangular n‐fold barrier structures. Analytical expressions of the transmission coefficient and the resonance condition are derived for the first time. It is believed that the expressions are useful for evaluating the energy variations of the transmission coefficient and estimating the resonance energy, and for resonant tunneling device fabrications.
Conductivity and the Metal–Dielectric Transition in Compensated n‐ZnSe
Authors: Kasiyan V. A. , Nedeoglo D. D. , Simashkevich A. V. , Timchenko I. N. 
Published: July 2007

AbstractIn the temperature range from 1.6 to 300 K the dcresistivity of n‐ZnSe in the region of the metal–dielectric transition induced by compensation is investigated. Over the dielectric part of the transition the power dependence ϱ(T) with an exponent smaller than zero in the critical region is replaced at sufficiently low temperatures by a dependence of the form ϱ(T) = ϱ0 exp (T0/T)1/2. The observable exponential dependence of ϱ on T is treated in the frames of a model of hopping conductivity with a variable range hopping over the states of the Coulomb gap near by the Fermi level. Such parameters as the screening radius Rs, which determines the size of large‐scale fluctuations, the radius of localization of the electron wave function a, the dielectric constant x, and the coefficient g0 which determines the density of the localized states near the Fermi level are calculated. It is established that g0, χ, and a increase with the decrease of K = NA/ND, that is with the approach to the metal–dielectric transition. It is assumed that this is connected with the decrease of the width of the Coulomb gap and its possible disappearance in vicinity of the transition.
LCAO Recursion Approach for the Bound Electron States at the 90°‐Partial Dislocation in Silicon
Authors: Wang Yong‐Liang , Teichler H. 
Published: July 2007

AbstractThe evaluation of bound electron states at extended lattice defects in Si is studied within an LCAO scheme under full inclusion of s‐, p‐, and d‐states and their mutual non‐orthogonality. The levels are determined from a continuous‐fraction recursion treatment. Application to a supercell model of the unreconstructed 90°‐partial dislocation demonstrates that the method works well for basis sets of 2560 complex basis functions. The present results for the bound electron states at the 90°‐partial dislocation in Si appreciably differ from the earlier tight binding estimates due to the here included dangling bond interactions. Two one‐dimensional bands of bound states are found showing large band splitting inside the Brillouin zone and degeneracy of the bands at the zone boundary with an occupation limit between empty and full states 0.12 eV above the valence band edge.
VUV and UV Spectroscopy of Pb2+ and Bi3+ Centres in Alkaline‐Earth Fluorides
Authors: Oboth K. P. , Lohmeier F. J. , Fischer F. 
Published: July 2007

AbstractAbsorption and emission studies on Pb2+ and Bi3+ centres in different alkaline‐earth fluorides are reported. The A absorption is observed in the ultraviolet and the B and C absorption in the vacuum ultraviolet spectral range. The A band in CaF2 exhibits a vibrational structure. At low temperatures the symmetry‐forbidden B′ emission (3Eu, 3T2u → 1A1g) and the spin‐ and symmetryforbidden α emission (3A1u → 1A1g) are observed. In CaF2 and SrF2 the α emission exhibits a distinct vibrational structure. Its zero‐phonon line is due to a strong hyperfine coupling. At higher temperatures the α emission as well as the B′ emission vanish and only the A′ emission is left.
Calculation of Interband Tunneling in Inhomogeneous Fields
Authors: Schenk A. , Stahl M. , Wünsche H.‐J. 
Published: July 2007

AbstractA new approach to the interband tunneling in semiconductor junctions is developed. It generalizes the traditional WKB and EMA theories, which fail in the limits of strong fields and inhomogeneous fields, respectively. The given expression for the energetic tunneling rate remains valid in both limits. In the strong field case it tends to the parabolic band EMA result, whereas for low fields with medium inhomogeneity any band structure model is possible. A Kane model is used to calculate tunneling and total I–U‐characteristics for narrow gap diodes. The breakdown behaviour yields information about the electrically active doping profile.
Electronic Properties of AmBn Complex Crystals
Authors: Kumar V. , Prasad G. M. 
Published: July 2007

AbstractA simple model is proposed which relates the homopolar and heteropolar energy gaps, ionicity, dielectric constant, and other physical and chemical properties of AmBn complex crystals with their plasmon energy. Special care of d‐electrons is taken into account in the case of transition metal compounds. The calculated values are in fair agreement with the values reported by other workers.
Spin‐Correlated Crystal‐Field Analyses of the 4f2(Pr3+) Energy Levels in LiYF4:Pr3+ and LiBiF4:Pr3+
Authors: Jayasankar C. K. , Richardson F. S. 
Published: July 2007

AbstractComparative analyses of the 4f2(Pr3+) energy level structure in LiYF4:Pr3+ and LiBiF4:Pr3+ are reported. These analyses are based on a phenomenological crystal‐field model that includes some (partial) consideration of electron‐correlation effects in the 4f‐electron‐crystal‐field interactions. Parametric fits of calculated‐to‐experimental energy level data are reported for a 48‐level data set for LiYF4:Pr3+, a 38‐level data set for LiBiF4:Pr3+, and a data set that contains 37 levels assigned in common for the two systems. The 4f2 energy level structure is qualitatively similar in the two systems, but the LiYF4:Pr3+ system exhibits slightly stronger crystal‐field interactions. Inclusion of spin‐correlated crystal‐field (SCCF) interactions in the phenomenological energy level analyses produces slightly improved agreement between calculated and empirical results, but it does not resolve major discrepancies between calculated and observed crystal‐field splittings within the 1G4 and 1D2 multiplet manifolds.
Origin of Periodic Fluctuations with Temperature of Ultrasonic Attenuation in zinC, Aluminium, and Iron
Authors: Youssef Moheb H. , Roushdy M. , Gobran N. K. 
Published: July 2007

AbstractThe ultrasonic attenuation—temperature curves for zinc, aluminium, and iron are found to comprise a periodic fluctuation of the attenuation with temperature as measured at 2, 4, and 8 MHz. This fluctuation is superposed on the well‐known types of ultrasonic damping in metals. The periodicity of such fluctuations is explained in terms of successive thermal levels of atomic vibrations. This approach affords successfully the visualization of the responsible pair of levels which effectively predominates along a certain interval of temperature and furnishes a lossive two‐state model. The successive models are also supported by a previous work on nickel.
On Hydrogen Chemisorption on Simple Metals
Authors: Taranko E. , Gavrilenko G. M. , Taranko R. , Fedyanin V. K. 
Published: July 2007

AbstractA theoretical study of the single hydrogen adatom chemisorption on free electron‐like metal surfaces is performed using an earlier published microscopic approach which includes the possibility of an influence of the adatom orbital occupancy on the hybridization of the adatom and substrate electron wave functions. The matrix elements inherent to the model are calculated from first principles using free‐electron wave functions for describing the substrate metal and treating the IS‐orbital of the adatom as an auxiliary orbital. The effect of the screening charge is considered phenomenologically in terms of an effective on‐adatom Coulomb integral. The self‐consistent Hartree‐Fock calculations show much more preference for hydrogen absorption rather than adsorption and slightly more adatom substrate—surface distance‐dependence of the chemisorption energy compared to results obtained within the local density functional method.
Quantum Calculation of Multiphonon Capture Cross‐Sections in the Model of Tangent Adiabatic Potentials
Authors: Belousov A. V. , Kovarskii V. A. , Chebotar V. N. , Pishkova T. V. 
Published: July 2007

AbstractThe multiphonon capture cross‐section of deep impurities in semiconductors is considered taking into account the tangency of the adiabatic potential. The calculation is based on the quantum description of the one‐coordinate vibrational model. A comparison with the quasiclassical approach is given.
Electronic States of a Hydrogen Atom Near a Hard Wall
Authors: Kovalenko A. F. , Sovyak E. N. , Golovko M. F. 
Published: July 2007

AbstractAn exact solution of the Schrödinger equation for a hydrogenic atom located near a hard wall is found. The discrete electronic spectrum and the set of semi‐bounded hydrogenic wave functions are obtained. The atom dipole moment which arises due to the presence of the wall is calculated.
Comparative Crystal‐Field Analyses of 4fN Energy Levels in LiYF4:Ln3+ Systems
Authors: Jayasankar C. K. , Reid M. F. , Richardson F. S. 
Published: July 2007

AbstractComparative analyses of the 4fN(Ln3+) energy level structures in a series of LiYF4:Ln3+ systems are reported. These analyses focus on empirical energy level data currently available in the literature for the Pr3+, Nd3+, Eu3+, Dy3+, Ho3+, Er3+, and Tm3+ systems, and they are based on the use of a phenomenological Hamiltonian that permits trans‐series comparisons of 4f‐electron/crystal‐field interactions. Spin‐correlated crystal‐field (SCCF) interactions are considered in the phenomenological energy level analyses of the Pr3+, Nd3+, Er3+, and Tm3+ systems. Differences between analyses carried out with and without inclusion of SCCF effects are most apparent for the Er3+ and Tm3+ systems. For these systems, inclusion of SCCF effects leads to significant improvement in agreement between calculated and observed crystal‐field splittings within certain multiplet manifolds. The calculated crystal‐field energy level structures within the 1G4 and 1D2 multiplets of Tm3+ and the lowest‐energy 2H11/2 multiplet of Er3+ exhibit particular sensitivity to the SCCF components of the model Hamiltonian. Comparisons between the present work and previously reported analyses of LiYF4:Ln3+ systems are presented.
Electron Energy Structure and Optical Properties of Microcrystalline Silicon
Authors: Gavrilenko V. I. , Humliček J. , Klyui N. I. , Litovchenko V. G. 
Published: July 2007

AbstractSpectroellipsometry and electroreflectance are used to determine the dielectric functions and the direct optical transition energies in microcrystalline silicon produced by chemical vapor deposition and by recrystallization of amorphous silicon. Changes in the optical gaps depending on the average microcrystallite volume are found. These results are interpreted with a tight binding model which incorporates the virtual crystal approximation.