bluuebunny/crossref_metadata_2025_split huggingface dataset:
A Theory of a Diluted Ferromagnet
Authors: Kaneyoshi T. , Fittipaldi I. P. , Beyer H. 
Published: July 2007

AbstractA new‐type effective field theory with correlations in a diluted ferromagnet is discussed. The critical percolation concentration is obtained analytically. The transition temperatures are given as a universal function of effective number of nearest‐neighbours. The specific heat and internal energy are also obtained.
Band Structures of Zirconium Hydrides
Authors: Darby M. I. , Read M. N. , Taylor K. N. R. 
Published: July 2007

AbstractThe band structures of zirconium hydride compounds are calculated using the APW method. By comparing the bands for a hypothetical f.c.c. compound ZrH0 with f.c.c. ZrH2 (corresponding to the Δ‐phase), it is shown that the hydrogen atoms give rise to a new ‘bonding’ band below the Fermi level. The band structures of two interstitial hydrogen h.c.p. compounds ZrH0.5 and ZrH (α‐phase) are also calculated. For ZrH0.5 with two Zr atoms and one H atom in the primitive unit cell no new bands are formed. For ZrH with two Zr atoms and two H atoms in the primitive unit cell a new band is formed just below the Fermi level but the band structure is similar to that of ZrH0.5 and a‐Zr with the Fermi level raised. Estimates of the total energies show that the band structure contributions favour formation of f.c.c. ZrH2 (corresponding to the Δ‐phase) over an interstitial α‐phase, as is observed experimentally.
An Exactly Soluble Model Antiferromagnet
Authors: Bowers R. G. 
Published: July 2007

AbstractAn exact calculation of the complete equilibrium thermodynamics of a model antiferromagnet is presented. Attention is focussed on the Néel point and associated critical phenomena and many standard critical exponents are calculated. The model has infinite‐range interactions and classical exponent values result. The importance, for theory, of staggered magnetizations and magnetic fields is emphasized. Experimental results frequently refer to total magnetizations and uniform fields. The interrelation between these two descriptions is investigated within the context of the present model.
Pulsed Line Narrowing in Nitrogen‐14 NQR
Authors: Ya. Osokin D. 
Published: July 2007

AbstractOn the basis of the average Hamiltonian theory the possibility of the pulsed narrowing of nitrogen 14 NQR lines in powders is studied theoretically using the fictitious spin‐(l/2) operator formalism. The modified Carr‐Purcell pulse sequence t (τ–t–2τ–t–τ)n is shown to be more acceptable for this purpose. The average Hamiltonians of inhomogeneous broadening and heteronuclear dipole‐dipole interactions as well as the secular part of the Hamiltonian of the interaction of a spin system with torsional oscillations of molecules are proved to commute with the initial density matrix prepared by the first (preparatory) pulse t The experimental curves of the spin echo envelopes in the pulse sequence are given. The decay time constants obtained experimentally at 77 K range between 0.5 s in methylamine and 10 s in sodium nitrite and agree with the spin–lattice relaxation times in these compounds.
Modelling of the Radial Distribution Function of the Hard‐Sphere Liquid in a Quasi‐Crystalline Model
Authors: Medvedev N. N. , Naberukhin Yu. I. 
Published: July 2007

AbstractThe radial distribution function of the hard‐sphere system, which is obtained by the exact solution of the Percus‐Yevick equation, is described within the framework of the quasi‐crystalline model of liquids (crystal lattice with lattice sites smeared out by the Gaussian law). The b.c.c. lattice is found to allow to receive a practically ideal description of all oscillations of the RDF (with an exception of several first peaks) over a range of interparticle distances up to the 16 sphere diameters. Other lattices give a significantly worse description. It is proposed that the quasi‐crystalline model reflects the correlations in the particle positions connected with regularities of the random close packing.
Hyper‐Raman Scattering from Infrared Active Modes in Calcite Crystals
Authors: Polivanov Yu. N. , Sayakhov R. Sh. 
Published: July 2007

AbstractThe hyper‐Raman spectrum of calcite is measured with Nd–YAG laser excitation and single channel detection. The lines corresponding to the normal Raman inactive transverse and longitudinal optic phonons are observed. The hyper‐Raman cross‐section of these lines is measured.
X‐Ray Thermal Diffuse Scattering Measurements of the [110] Phonon Dispersion Curves of Indium Arsenide
Authors: Orlova N. S. 
Published: July 2007

AbstractInvestigation of the intensity of X‐ray diffuse scattering by lattice vibrations in indium arsenide along the second‐order symmetry axis is performed and the phonon frequencies of all modes in this direction are determined. An analysis of elliptic modes polarized in the (110) plane is given.
A Two‐Particle Model for the Ground State Energy of the Polaron
Authors: Bader G. , Lépine Y. 
Published: July 2007

AbstractThe dielectric polaron is treated using the Gross unitary transform and the Matz‐Green's function techniques. A two‐particle Feynman‐like model is introduced to compare these methods with the best Feynman path integral formulation. It is found that the introduction of a second particle in the model problem does not improve on the usual result for a one‐particle model. However, a comparison of these different approaches leads to the conclusion that no vertex contributions are contained in the Feynman approximation. It is also concluded that it is the very special treatment of the kinetic energy of the electron in Feynman's theory that makes his results lower in energy.
Interband Electronic Raman Scattering of Polarized Light in CuCl
Authors: An Nguyen Ba , Van Hieu Nguyen , Thang Nguyen Toan , Viet Nguyen Ai 
Published: July 2007

AbstractThe cross‐sections are calculated for the Raman scattering processes in CuCl with polarized photons in the initial and final states and with the transition of the electron from the highest valence band Γ7 to the conduction band Γ6. The exciton effect is taken into account.
Long‐Wavelength Phonons in the Different Phases of KNbO3
Authors: Fontana M. D. , Kugel G. E. , Metrat G. , Carabatos C. 
Published: July 2007

AbstractRaman scattering measurements are made in the tetragonal phase of KNbO3. The results are in good agreement with the previous corresponding neutron data, and can be assigned to symmetry species. Comparison of these results to those available in the other phases enables to observe the effect of the phase transformations on the LO–TO splitting and on the degeneracy lifting of each initial cubic mode. It is shown that the lowest frequency F1u cubic mode is the most affected one by the different cell distortions; at each transition, it splits into “soft” and “stiffened” components; it is the so‐called “ferroelectric” mode which drives the successive phase changes. On the other hand, it is remarkable that the phonons derived from the F2u “silent” mode are nearly degenerate, even in the ferroelectric phases; it is also the case for the phonons issued from the second F1u cubic mode. Finally, it is emphasized that the phase transitions in KNbO3 can be described by a displacive‐type mechanism.
Laser Induced Fluorescence and Raman Studies with KI:NO
Authors: Jodl H. J. , Bolduan F. 
Published: July 2007

AbstractRaman, absorption, and fluorescence spectra associated with NO substitutional impurity in KI as a function of different parameters are reported. This model substance is chosen of special interest to concentrate on the following topics like laser induced fluorescence (3B1 → 1A1), like the structure of zero‐phonon line and sideband in absorption and fluorescence spectra, like Raman scattering of intramolecular vibrations, and like Fermi resonance of matrix isolated species. The data are discussed in the light of various anharmonic effects, of different coupling models, and of special spectroscopic aspects.
Studies on Anharmonic Damping Mechanisms of High‐Frequency Phonons in Crystals Containing Complex Ions
Authors: Tsyashchenko Yu. P. , Krasnyanskii G. E. 
Published: July 2007

AbstractPolarized infrared reflection spectra of CaWO4, KReO4, K2SO4, K2CrO4 single crystals are investigated in the region of 600 to 900 cm−1 within the temperature range of 170 to 625 K. The temperature dependences of the damping constants of the high‐frequency phonons v3(Eu), v3(Au) in CaWO4 and KReO4, v3(B1u) in K2CrO4, v4(B1u) in K2SO4 are obtained by means of the dispersion analysis. The observed temperature behaviour of the damping constants is mainly explained by the anharmonic mechanisms of double‐quantum phonon scattering on other high‐frequency phonons involving low‐frequency ones, as it is demonstrated by a satisfactory agreement between experimental and theoretical curves.
Reflectivity Spectra and Band Structure of the ZnTe–CdTe System
Authors: Sobolev V. V. , Maksimova O. G. , Kroitoru S. G. 
Published: July 2007

AbstractThe reflectivity spectra of ZnTe:Cs, ZnTe:Ga2O3, and ZnxCd1−xTe crystals (x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.7, 0.8, 1.0) in the range 1 to 12.5 eV at 293 K and in the range 1 to 5.5 eV at 90 K are investigated. The experimental results obtained prove the hypothesis about a great detailed similarity of ZnxCd1−xTe band structures in a wide energy range and the existence of a straightline dependences of interband transition energies on molar composition. A strong effect of fresh cleavage and impurities on the reflectivity spectra is detected.
Anisotropic Ferromagnet with Two Spins per Site
Authors: Piasecki R. 
Published: July 2007

AbstractGreen's function diagrammatic technique is used to investigate a model ferromagnet in the case, when the effective spin at a site is compounded of two anisotropically interacting electron spins. The results are: the transverse magnetic excitations (Δm = ± 1) and the longitudinal ones (Δm = 0) are obtained in the random phase approximation, magnetization and critical temperature are calculated to the first order in 1/z (the reciprocal of the effective number of ions interacting with a given ion), and exchange interaction at the same site gives the contrary effects in changes of the magnitudes mentioned above in dependence on the exchange‐isotropy parameter I and the exchange‐anisotropy parameter D.
Semiempirical Calculations of the Impurity Level Positions with Respect to the Perfect Crystal Bands
Authors: Ermoshkin A. N. , Kotomin E. A. , Evarestov R. A. 
Published: July 2007

AbstractThe semiempirical calculation of the electronic structure of a perfect KCl crystal carried out within the framework of the large unit cell model is compared with those performed using the cluster model. The positions of the ground state one‐electron levels of the impurity ions Tl+ and Tl2+ within the gap are also calculated using molecular clusters of different sizes. The procedure which takes into account the crystal lattice polarization produced by a charged impurity is proposed for the calculation of Tl2+ levels. The electronic density distributions and the impurity level positions with respect to the KCl bands are compared for both the defects.
Extended Deformable Shell Model Calculation of the Lattice Statics and Dynamics of LiH and LiD Crystals
Authors: Ghosh S. , Basu A. N. , Sengupta S. 
Published: July 2007

AbstractAn attempt is made for a unified study of the lattice statics and dynamics of the LiH–LiD crystals. So far it has not been possible to obtain a unified description of the different properties of this crystal on the basis of a single model and with a single set of model parameters. All the previous calculations suffer from this inadequacy and their results show that there are certain problems regarding the stability of the static lattice structure, the cohesive energy, and a consistent description of the dielectric properties and the dispersion of phonons for this crystal. The present calculation based on the extended deformable shell model removes the earlier difficulties and provides a good overall description of the lattice mechanical properties of this crystal with a single set of parameters. Another salient feature of the present investigation is to indicate the effect of the quadrupolar distortion of the charge cloud, in particular, of the hydrogen ion which has been neglected in all the earlier investigations of the lattice mechanics of these crystals. The calculation clearly points out that without including this effect it is not possible to obtain a coherent description of the lattice mechanics in all its totality. Some qualitative justification for this effect being significant for the crystals under consideration is also presented.
Infrared Absorption Studies on Crystalline α‐P4S3
Authors: Chattopadhyay T. , Von Schnering H. G. 
Published: July 2007

AbstractInfrared absorption spectra of crystalline α‐P4S3 are obtained at 80 and 275 K. Absorption features due to low frequency external modes are observed and are identified for the first time. Absorption due to internal molecular modes is compared with the data reported in previous works. α‐P4S3 belongs to the centrosymmetric space group Pmnb(D2h16) and therefore none of the infrared active modes are Raman active. The present work is complementary to a recent Raman study on α‐P4S3.
The Change of Electronic Structure and Optical Properties in FeCo at the Order‐Disorder Transition
Authors: Kulkova S. E. , Egorushkin V. E. , Anokhina I. N. , Fadin V. P. 
Published: July 2007

AbstractThe change of the electronic structure and the optical properties of the metallic alloy FeCo is studied at the order–disorder transition. Energy‐band calculations for ordered and disordered alloys FeCo are performed by the KKR and the model Hamiltonian methods. In the model of direct and indirect transitions the optical conductivity is calculated. The calculations are in satisfactory agreement with the experimental data.
On the Dynamics of the Plasma Phase Transition in Highly Excited Direct Gap Semiconductors
Authors: Koch S. W. 
Published: July 2007

AbstractThe temporal behaviour of the system of electronic excitations in highly excited direct gap semiconductors is treated within a hydrodynamic model. A functional Fokker‐Planck equation for the probability distribution of the local density of electron–hole (e–h) pairs is established taking into account the diffusion of e–h pairs due to the local chemical potential as well as recombination losses due to the finite e–h pair lifetime and generation of e–h pairs by external laser excitation. Several time dependent moments of the probability distribution functional are calculated together with the structure factor, i.e. the Fourier transform of the two‐point density correlation function. The results show that for e‐h pair densities inside the instability region of the thermodynamic chemical potential no true phase separation into an e–h pair gas phase and an e–h liquid phase takes place.
Electronic States of CdIn2Se4 and ZnIn2Se4: Role of the Cation Pseudovacancies
Authors: Margaritondo G. , Katnani A. D. , Lévy F. 
Published: July 2007

AbstractThe two defect‐zincblende semiconductors of the II–III2–VI4 family, CdIn2Se4 and ZnIn2Se4, contain a regular network of vacant cation zincblende sites with several properties similar to those of true vacancies. Electronic state calculations predict dramatic effects of these pseudovacancies in the upper valence band. These effects are searched for by photoemission spectroscopy with synchrotron radiation. The spectra do reveal the states related to the pseudovacancies. However, they rule out a theoretically predicted pseudovacancy gap in the valence band density of states. These results agree with the recent hypothesis of a partial cancellation of the pseudovacancy effects by lattice relaxation around each pseudovacancy.
Temperature Investigation of Infrared Absorption in DMB–TCNQ Salt
Authors: Graja A. , Świetlik R. , Sekretarczyk G. 
Published: July 2007

AbstractIR spectra of the simple salt TCNQ with dimethylbenzimidazole vs. temperature are studied. The temperature dependence of the absorption coefficient for bands due to activation of the ag mode is explained in terms of changes in energy level population, geometry, and electron interactions.
Low‐Temperature Heat Conduction of γ‐Irradiated MgO Crystals
Authors: Abramishvili M. G. , Altukhov V. I. , Kalabegishvili T. L. , Kvachadze V. G. 
Published: July 2007

AbstractThe effect of γ‐irradiation on heat conduction χ(T) and the spectra of optical and paramagnetic absorption of MgO crystals are studied. For the first time two stages of thermal destruction of optical absorption bands are found and their relation with the process of thermal recovery χ(T) is established. Contributions to the heat conduction due to both, resonance scattering of phonons on paramagnetic ions Cr2+ (generated in the process of irradiation on account of the change of the impurity valence of Cr2+ and Fe2+) and Rayleigh scattering on radiation (including proper) defects of the MgO lattice are analysed. From the comparison of the calculated data of heat conduction χ(T) with pertinent experimental data on Cr2+ ion concentration and effectiveness of Rayleigh and anharmonic phonon scattering in MgO crystals are obtained.
On the Theory of Double γ‐Nuclear Magnetic Resonance
Authors: Dzyublik A. Ya. 
Published: July 2007

AbstractThe amplitude for resonant scattering of γ‐quanta by Mössbauer nucleus in a crystal is calculated when there are NMR transitions between sublevels of the excited nucleus. It is shown that under the influence of the NMR transitions the scattering amplitude splits into some resonant terms. The cross‐sections for absorption and scattering of γ‐quanta are expressed over the amplitude. The calculations agree with the experiment of Heiman et al.
Interstitial and Vacancy Distributions in a Simple Model of Radiation Damage
Authors: Pázsit I. 
Published: July 2007

AbstractThe spatial distribution of vacancies and interstitials in a displacement cascade is investigated in the Kinchin‐Pease model of radiation damage. The one‐dimensional defect equations are solved in an infinite medium and a half space. The numerical results illustrate the different spatial behaviour of the defect types, and the effect of the free boundary can be assessed.
Phonon Frequencies and Dynamics of the OH− Molecule in Sodium Hydroxide
Authors: Giessler A. , Schaack G. , Bleif H. 
Published: July 2007

AbstractRaman and infrared reflection spectra of the fundamental lattice modes of NaOH are observed in a broad temperature range in the low‐temperature orthorhombic and in the monoclinic phase (T ≧ 514 K). The temperature dependent wave numbers and line shapes especially of the (OH) libration and of the (OH) valence mode are determined. The results are discussed using data from X‐ray and neutron scattering experiments and are compared with the conclusions of recent theories on molecular dynamics in crystals.
Electron Paramagnetic Resonance Studies of Ferroelectric Fe3+‐Doped Tris‐Sarcosine Calcium Chloride Monocrystals
Authors: Kröber T. , Wartewig S. , Windsch W. 
Published: July 2007

AbstractFerroelectric tris‐sarcosine calcium chloride (TSCC) single crystals are investigated using the EPR method in X‐ and Q‐band in a temperature range from 3.7 to 300 K. The spectra of the Fe3+ ions substituted for Ca2+ ions are well described by a spin Hamiltonian of orthorhombic symmetry with the following parameters at room temperature: g = 2.005 ± 0.002, D = ( −1250 ± 10) × 10−4 cm−1, E = (−70 ± 5) × 10−4 cm−1, F = (30 ± 6) × 10−4 cm−1, a = (81 ± 7) × x 10−4 cm−1. The structural ferroelectric phase transition is evidenced in a splitting of the EPR lines, which is proportional to the spontaneous polarization of the crystal. This proportionality allows to determine the coefficients A, B, and C of the Landau expansion of the free energy G. The results are compared with dielectric measurements and EPR measurements of TSCC:Mn2+.
Theory of Multi‐Phonon Transitions beyond the Condon Approximation
Authors: Teichler H. 
Published: July 2007

AbstractFor light interstitials in deformable host‐materials (“small‐polaron”‐like systems) the theory of multi‐phonon transitions is developed beyond the so‐called Condon approximation, i.e., the effect of phonon fluctuations on the defect transfer‐integrals J is explicitly taken into account. In particular the rate of incoherent defect transitions is evaluated up to second order in J and the temperature dependence of the multi‐phonon processes is discussed in the regime above half of the Debye temperature.
The Temperature Dependence and Dynamical Properties of the Bond Charge in GaAs and InSb
Authors: Pietsch U. , Unger K. 
Published: July 2007

AbstractUsing a bond charge model for the interpretation of X‐ray structure factors the temperature dependence of parameters describing the bond charge (amount, position, and spatial extend) is studied for GaAs and InSb. Four structure factors each at eight temperature values are taken from Bilderback, three factors showing a detectable influence of bond charges. The mean squared displacements of the bond charges are found to be larger than those of the matrix atoms. This is an analogous finding to that in silicon and can be explained by the dynamical change of the bond charge amount for changing bond lengths. The constant of dynamical charge transfer are determined to be 0.185 electrons/Å and 0.165 electrons/Å for GaAs and InSb, respectively.
Self‐Interstitials in Normal Metals
Authors: Augst G. R. 
Published: July 2007

AbstractGeneral expressions describing the formation energy of interstitials of different configurations in simple metals are obtained. The electronic contribution is considered within the framework of pseudopotentials theory and is based on the linear response approximation. To obtain the lattice relaxation energy two terms of the displacement expansion iteration series are calculated. Numerical calculations for aluminium show that the minimum energy corresponds to a 100 dumb‐bell, with the distance of each ion from the vacancy being 0.52a/2. For interstitials of other configurations the disturbance of the lattice is so strong that the iteration series for the relaxation energy is divergent.
Band Structure and Electron–Electron Interaction in Samarium Monosulphide
Authors: Farberovich O. V. 
Published: July 2007

AbstractThe method of augmented plane wave (APW) is used to obtain the band structure of the SmS compound in the semiconductor and metal phases. The noncentral part of the Coulomb electron‐electron interaction is taken into account in the first order perturbation theory. In this case the radial part of the wave APW‐function is taken as a zero approximation function. A multiplet structure of the excited configuration f5d, which provides a good description of the X‐ray photoelectron spectrum and optical spectrum ϵ2(ω), is obtained. The configuration fd is calculated for the interpretation of the optical absorption spectrum of the samarium monosulfide metal phase.
Unsymmetrized Self‐Consistent Field Approximation for Strongly Anharmonic Crystals with Taking into Account the Polarizability of the Ions
Authors: Zubov V. I. , Sh. Soulayman S. 
Published: July 2007

AbstractThe unsymmetrized self‐consistent field method for strongly anharmonic (nonmetallic) crystals is improved by taking into account the ionic polarization caused by the displacement of the outer electron shell relative to the ionic core during the oscillations of the ions. The self‐consistent potential and the Helmholtz free energy of a strongly anharmonic crystal are found using the adiabatic approximation. In the case of the strong anharmonicity of fourth order, the equations of state of a diatomic crystal are obtained. In the zeroth approximation, the inclusion of the ionic polarizability leads to an added renormalization of the force coefficients of second order. The corrections to the Helmholtz free energy and to the equations of state are considered.