bluuebunny/crossref_metadata_2025_split huggingface dataset:
Comparison of the Large Unit Cell and Small Periodic Cluster Approaches to the Quasimolecular Calculations of the Band Spectra of Crystals
Authors: Evarestov R. A. , Petrashen M. I. , Ledovskaya E. M. 
Published: July 2007

AbstractTwo recently developed quasimolecular approaches to the calculation of the band spectra of crystals are compared: the “Large Unit Cell” model and the model of “Small Periodic Cluster”. It appears that the first model has an essential advantage allowing to confine by relatively small quasimoleocules in cases where the second model appears to be impracticable. As examples KCl and layered hexagonal BN crystals are considered.
Dielectric Permittivity of Semiconductors with Narrow Forbidden Gap in the Long‐Wave Limit
Authors: Sheka D. I. , Korol A. N. 
Published: July 2007

AbstractIn the random phase approximation the dielectric permittivity is evaluated for a semiconductor with a band structure described by Kane's energy band scheme (spin‐orbit interaction is neglected and the effect of higher bands is included in a very simplified form). It is demonstrated that the fact that several hole bands have been taken into account, when the operator of the kinetic energy ceased to be the Dirac one plays the principal role in the calculation of the static dielectric permittivity σ(q → 0). The analytical dependence of σ(q → 0) on the parameters of the semiconductor, width of the forbidden band Eg, effective mass, and the lattice constant, is obtained. The spatial dispersion of the dielectric permittivity for zero‐gap semiconductors is considered in the long‐wave limit.
Surface and Size Effects in the Specific Heat of Linear Chains
Authors: Ma S. K. S. , Alldredge G. P. , De Wette F. W. 
Published: July 2007

AbstractThe size and surface effects in the specific heat of the monatomic linear chain are investigated for a small to intermediate number of atoms and for temperatures from O K up to θ0, the zero‐Debye temperature. It is shown that the surface‐excess heat capacity, δC, can reliably be extracted for chains of as few as ten atoms, although size effects are still quite appreciable for 50 to 100 atoms. The low‐temperature limiting value of 1/2kB for δC is reached for T ⪅ 0.07θ0. The freezing out of the lowest non‐zero frequency in δC becomes appreciable only for T ⪅ 0.05θ0, in the ten‐atom chain and, more generally, for T ⪅ 1/2θ0/N in the N‐atom chain.
Theory of Time‐Dependent Mössbauer Emission Spectra under Relaxation Conditions
Authors: Afanasev A. M. , Gorobchenko V. D. 
Published: July 2007

AbstractGeneral formulas are given for the profile of the Mössbaner emission spectra measured within a finite time interval following the preceding decay, under conditions when the initial electronic state of the Mössbauer atom is in non‐equilibrium. The results obtained are valid for an arbitrary case in which the hyperfine interactions change during the relaxation process, as well as for arbitrary ratios between the lifetime of the Mössbauer nucleus, the relaxation time, and a characteristic hyperfine frequency. The case is discussed in detail when the relaxation process is accompanied by a change of the isomer shift. It is shown that, even for rather short relaxation times, the relaxation process reveals itself through a noticeable asymmetry of the emission line.
Study of the Velocity and Damping of Acoustic Waves Obtained by Brillouin Scattering in the Paraelastic Phase in GMO
Authors: Luspin Y. , Hauret G. 
Published: July 2007

AbstractThe velocity anomalies and damping of acoustic waves in the paraelastic phase of GMO crystals are calculated on the basis of an interaction Hamiltonian which is proportional to the strain and to the square of the order parameter. Within the assumption of a three dimensional soft mode spectrum, good agreement is obtained with the experimental results.
Band Structure of Al and the Laves Phase CaAl2
Authors: Radwan A. M. , Taut M. 
Published: July 2007

AbstractThe band structures of Al and the intermetallic compound CaAl2 are calculated using the full nonlocal, but energy independent Shaw model potential. The energy dependence of the potential is then treated in first order. The effect of a simulated charge transfer in CaAl2 on the band structure is studied. The results are compared with experimental soft X‐ray spectra.
Crystal Field Induced Spin Rotation Observed by Magnetization Study of Polycrystalline NdCo2
Authors: Dublon G. , Kroupp M. , Dariel M. P. , Atzmony U. 
Published: July 2007

AbstractMagnetization measurements of polycrystalline NdCo2 are reported as a function of temperature (4.2 to 200 K) and magnetic field (0.2 to 26 koe). Anomalies in the temperature dependence of the magnetization are attributed to a [110] to [100] spin rotation around 40 K. Single‐ion crystal field calculations which reproduce the experimental results are presented. These yield a (2.3 ± 0.1) μB Nd3+ moment, 25 < A4 < 40 K α, and −0.088 < A4/A6 < −0.068 α in NdCo2.
Electronic Structure of Aluminium Oxide as Determined by X‐Ray Photoemission
Authors: Balzarotti A. , Bianconi A. 
Published: July 2007

AbstractThe electronic structure of the valence band of crystalline (α and γ) and amorphous Al2 O3 is measured by X‐ray photoemission. No significant changes are found in the different crystalline forms. A broad band observed at about 4 eV below the valence band is associated with the 2s states of the oxygen atoms, in agreement with band calculations. An electronic structure scheme of the aluminium oxide is constructed from photoemission, v. u. v. and X‐ray optical data.
Anisotropic Indirect Absorption Edge in GeSe
Authors: Vlachos S. V. , Lambros A. P. , Thanailakis A. , Economou N. A. 
Published: July 2007

AbstractThe absorption and reflection coefficients of GeSe single crystals are measured at various temperatures in the spectral range 0.8 to 2.2 μm using linearly polarised light. The main absorption mechanism is due to indirect interband electronic transitions with participation of two different phonons. Indirect energy gap values of 1.22 and 1.18 eV are obtained for the two directions of polarisation ϵ ∥ a and ϵ ∥ b, respectively. The corresponding phonon energies at room temperature are found to be 0.023 and 0.089 eV for ϵ ∥ a and 0.013 and 0.043 eV for ϵ ∥ b. The indirect gaps vary linearly with temperature, the temperature coefficient being −8.68 × 10−4 and −5.05 × 10−4 eV K−1 for the two directions of light polarisation, respectively. A donor‐like localised level within the energy gap of GeSe, lying 0.55 eV above the valence band irrespective of direction, is found from the tail of the room temperature absorption curves. Since a similar level has been observed previously in amorphous GeSe it is assumed to be an intrinsic property, introduced by Se vacancies.
Modified Perturbation Method for Diamond‐Type Crystals
Authors: Soma T. 
Published: July 2007

AbstractThe modified perturbation method, which reproduces the observed data of phonon curves and covers the uncertainties due to higher order perturbation, is proposed for diamond‐type crystals. Calculated values of cohesive energies, compressibilities, and band gaps for Si, Ge, and α‐Sn are in good agreement with the experimental data.
Surface Phonon Polaritons on Absorbing Crystals
Authors: Borstel G. , Schuller E. , Falge H. J. 
Published: July 2007

AbstractThe dispersion of surface phonon polaritons is calculated taking into account the experimental set‐up and complex dielectric functions. Under these conditions the results show the dispersion of surface waves in as well as outside of the frequency range of reststrahlenbands. Furthermore the dispersion branches exhibit backbending. The calculated curves are compared with reflection measurements at α‐quartz and discussed in terms of virtual modes. The connection between frequency and angular scans is shown by three‐dimensional representations of the reflectivity as a function of the angle of incidence and frequency.
Directional Dispersion of an Exciton Polariton in a Pentamethinium Cyanine Dye
Authors: Hesse H. J. , Fuhs W. , Weiser G. , Von Szentpaly L. 
Published: July 2007

AbstractReflectance spectra of γ‐cyclopropyl‐bis(1, 3, 3‐ trimethyl‐indolenine‐2‐yl)pentamethiniumtetrafluoroborate are measured on different crystal faces. The complex dielectric constant, derived from the reflectivity data, reveals that the broad and highly polarized reflectance bands (R ≈︁ 90%) in the visible spectral range are caused by a narrow but strong molecular excitation (ϵ2 > 300). The resonance energy is different for the various crystal faces and depends on the angle between the wave vector of light, k, and the transition moment of the molecular excitation. The large shift of the resonance energy from 1.73 eV on a {110} face to 2.61 eV on a {100} face is described quantitatively as directional dispersion of an extraordinary polariton.
Bose‐Einstein condensation of free excitons in AgBr‐type crystals at high excitation
Authors: Lelyakov A. V. 
Published: July 2007

AbstractThe shape of the excitonic luminescence band is studied for degenerate excitons of great radius at T ≠ 0 K in crystals with indirect bandgap. A calculation is made of the transition probability from a macroscopically occupied exciton state with wave vector k correeponding to the boundary of the Brillouin zone, under photon and phonon emission. The luminescence band consists of a δ‐type peak and a broader, and less intense, short‐wavelength satellite. The theory is compared with experiments for AgBr crystals.
Quantum theory of microwave conductivity
Authors: Arora V. K. , Qureshi E. 
Published: July 2007

AbstractA quantum theory using a density matrix for electrical conductivity with an alternating electric field is described. The relaxation time, and the effeot of the microwave frequency on it, is obtained in a natural way without making any assumptions regarding its existence, as in the solution of the Boltzmann transport equation. Starting from the formal manipulation of the density matrix equation, the theory emphasizes various approximations under which the density matrix can be obtained in a usable form, assuming the electron‐phonon interaction to be a dominant mechanism of scattering in the nonpolar model. This density matrix is then used to find the expectation value of the current operator and hence the miorowave conductivity.
EPR of Cu2+ and radiation centres in anatase (TiO2)
Authors: Grunin V. S. , Davtyan G. D. , Ioffe V. A. , Patrina I. B. 
Published: July 2007

AbstractThe EPR spectra of three different Cu2+ centres and the spectrum of the radiation hole centre in single crystals of anatase are investigated. The constants of the spin‐Hamiltonians for all the spectra and the ground state wave functions are obtained. The conditions of incorporation into the crystal, the site of impurity in the lattice, and the mechanism of charge compensation are discussed.
Microscopic theory of clean type‐II superconductors in the entire field‐temperature plane
Authors: Brandt E. H. 
Published: July 2007

AbstractThe magnetization curves, the local magnetic field, and the energy gap of clean type‐II superconductors are calculated from Gorkov's theory for all possible values of B, T, and χ by a Ritz variation method. The free energy functional used is derived and discussed in two previous papers. The solutions coincide with all results known so far from approximate theories (T ≈ Tc,, B ≈ Hc2, B«Hc2). The first principle method yields, for the first time, magnetization curves with a Metastable (S‐shaped) part, thus explaining the jump Bo of induction observed at Hcl. At not too large values of T and B, the field at the vortex centres increases and the vortex core shrinks both with decreasing B and T. The third form factor, h20, is found to change its sign at B/Hc2≈0.7.
ESR study of paramagnetic centres in X‐irradiated AgCIO3
Authors: Mathai R. , Rao K. Narahari , Kumaraswamy A. , Rajasekharan T. , Rao K. V. S. Rama 
Published: July 2007

AbstractParamagnetic centres trapped in X‐irradiated AgCIO3, single crystals are studied at room temperature using an X‐band ESR spectrometer. The paramagnetic centres produced in these crystals are identified as O, and CIO2. The CIO2, centres are trapped at two magnetically inequivalent sites. The O, centre is also trapped at two inequivalent sites which are not well resolved at X‐band frequencies. The spin‐Hamiltonian parameters of CIO2, are gxx = 2.004 ± 0.001, gyy = 2.010 ± 0.001, gzz = 2.012 ± 0.001, Axx = (73±1) G, Ayy = (‐39 ± 1) G, and Azz = (‐4 ± 1) G.
Spin‐lattice relaxation of the O centre in RbI and RbBr
Authors: Matthys P. , Vanhaelst M. , Boesman E. 
Published: July 2007

AbstractPulse saturation measurements of spin‐lattice relaxation rates of O centres in rubidium bromide and iodide are reported. A theoretical calculation for the direct process in these salts is summarised. Phonon integrals appearing in the theoretical formulas are calculated numerically using an anisotropic continuum model, thus improving agreement with experiment. The significance of an unexpected anisotropy for two principal directions, perpendicular to the molecular axis, is discussed in the RbBr case.
Structure and conductivity of bilinear organic compounds
Authors: Phillips J. C. 
Published: July 2007

AbstractFerroelastically driven order‐disorder transitions of static domains are proposed to explain anomalous conductivities of molecular salts such as TTF‐TCNQ. If L is the length of a molecular chain, the ferroelastic driving energy is of order L2 compared to dynamical fluctuation energies of order L log L.
Microscopic theory of spin‐wave resonance in ferromagnetic films with surface‐region hihomogeneities
Authors: Wachniewski A. 
Published: July 2007

AbstractA new general approach to spin wave resonance in ferromagnetic films with surface‐region inhomogeneities is proposed. This approach is based on Wallace's perturbation method with a zeroth‐order Hamiltonian corresponding to surface inhomogeneity model. Simple formulae for energies and intensities of resonance peaks are given and numerical examples illustrate the application of the method.
Electric field effect on the thallium emission bands in HBr and KI
Authors: Grassano U. M. , Tanga A. , Valli M. 
Published: July 2007

AbstractThe effect of an applied electric field on the emission bands of thallium centers is observed in KBr and KI. The results show a field quenching of the luminescence excited in the C and D absorption bands. From the temperature and field dependence of the relative luminescence change is established that the effect is due to a'decrease of the radiative quantum yield due to a field assisted tunneling of the electron from the excited level toward a nearby trap.
Influence of magnetic impurities on superconducting properties of some compounds with A‐15 structure
Authors: Alekseevskii N. E. , Bazan C. , Mitin A. B. , Mydlarz T. , Krasnoperov E. P. , Rączka B. 
Published: July 2007

AbstractThe influence of iron group impurities on the critical temperature, critical field, and magnetic susceptibility of some superconducting compounds with A‐15 structure is investigated. Following systems are examined: Nb3A1 with Ni, Cr, and Fe, Nb3Sn with Fe, Co, and Mn, V3Si with Fe. It is pointed out that the impurities do not change the superconducting parameters substantially.
Bulk properties of lithium hydride up to 100 Mbar
Authors: Perrot F. 
Published: July 2007

AbstractThe electronic structure and equation of state of lithium hydride are investigated along the zero‐temperature isotherm up to 100 Mbar. The electronic part of energy and pressure are calculated using the augmented plane‐wave method (including Kohn‐Sham exchange and correlations). The lattice contribution is treated in a semi‐empirical model. Good agreement with experiment is obtained for the equilibrium lattice constant and minimum total energy. The compressibility modulus is about 20% too small. A semi‐metallic transition (2 Mbar) and a, metallic transition (30 Mbar) are predicted.
Band structure calculation of ZnSiP2
Authors: Pasemann L. , Cordts W. , Heinrich A. , Monecke J. 
Published: July 2007

AbstractThe band structure of ZnSiP2 is calculated using the empirical pseudopotential method. The form factors are deduced from those of ZnS, Si, AIP, and Gap. The band structure obtained is in good agreement with recent experimental data. The influence of the distortion of the anion sublattice on the energy levels at r is examined and the optical transitions in the region from 3 to 5 eV are discussed.
Relativistic excitations of a thin graphite film by inelastic electron scattering in theory and experiment
Authors: Wessjohann H. G. 
Published: July 2007

AbstractRecently Chen and Silcox have published measurements of the inelastic electron scattering by a thin graphite slab. A theory formulated by the author is applied to this experiment. For the calculations the dielectric functions of graphite measured by Zeppenfeld are used. The numerical results and the maxima of the inelastic electron scattering probability are discussed. There is a good agreement between theory and experiment.
Dielectric function of hexagonal metals: II. Beryllium
Authors: Taut M. , Hanee W. 
Published: July 2007

AbstractThe longitudinal dielectric function ϵ(q,ω) of beryllium is calculated using realistic wave functions and band energies. Comparison with experiment is performed in the optical properties (including anisotropy), plasmon dispersion relation, and half width of the plasmon peak. The relaxation times for inclusion of damping are fitted. The experimentally observed peak splitting of the loss function is explained as a band structure effect.
Multiple s‐scattering in a correlated medium in the low‐density limit
Authors: Blaudeck P. , Lenk R. 
Published: July 2007

AbstractThe multiple scattering of waves in a random low‐density medium is investigated with consideration of the scatterers pair correlation. A given scatterer interacts with itself via the surrounding medium whose density is non‐uniform due to the presence of a correlation cloud. It is shown that correlations influence only the attenuation of the wave by incoherent elastic scattering. The real part of the wave number and, consequently, the density of states are not affected.
Localized non‐magnetic impurities in metals: I. General theory
Authors: Mrosan E. , Lehmann G. 
Published: July 2007

AbstractA general formalism is developed for localized non‐magnetic impurities in metals. In the limit of cubic symmetry and for non‐zero scattering phase shifts for l ≦ 2, it is shown, that the T‐matrix, describing the scattering properties of the impurity, is diagonal and contains all effects coming from multiple scattering and from the anisotropic band structure of the host, given only by four coefficients of the Green function of the pure host. For localized impurities the Boltzmann equation has a degenerate kernel and can be transformed in an inhomogeneous algebraic set of linear equations of finite rank. The impurity potential is calculated in the muffin‐tin approximation. Screening and recharging effects are described by a constant potential shift chosen in such a way that a Friedel sum rule for the scattering phase shifts is fulfilled.
Approximative BCS‐like solution of the eliashberg gap equation
Authors: Nowotny H. 
Published: July 2007

AbstractAn approximative solution of the Eliashberg gap equation is parametrized to give an equation similar to the BCS gap equation. The connection between a characteristic phonon energy and the parameters corresponding to the BCS coupling constant is discussed. These parameters can be determined from the input data of the Eliashberg theory, i.e. the electron‐phonon spectral function α2(ω) F(ω) and the Coulomb pseudopotential μ*. Numerical results are given for Pb and In.
Theory of the photon‐drag effect in semiconductors with elliptical isoenergetic surfaces and its application to tellurium crystals
Authors: Genzow D. , Normantas E. 
Published: July 2007

AbstractThe components σiklm of the photon‐drag tensor are calculated within the framework of the Boltzmann equation for a model band structure with elliptical isoenergetic surfaces. Allowed direct optical transitions between two parabolic sub‐valence‐bands are considered. The theoretical results are applied to the photon‐drag effect in tellurium. Scattering of carriers on acoustic and polar‐optical phonons in the relaxation time approximation is taken into account. Comparison with experimental results is made.
Phonons in the mixed crystal K0.5Rb0.5I
Authors: Aslam J. , Rolandson S. , Beg M. M. , Butt N. M. , Khan Q. H. 
Published: July 2007

AbstractPhonons propagating in the symmetry directions Δ, ∑, and A in K0.5Rb0.5I are obtained by inelastic scattering of thermal neutrons at 300 K. The acoustic phonons are well defined and no anomalous broadening is observed, but the transverse zone centre phonon, the only observed optic phonon, has a width about twice the instrumental resolution. The phonon frequencies in the mixed crystal are generally below the averaged frequency values of KI and RbI. The softening of modes is more prominent near the zone boundaries. The results are analysed in terms of the shell model and the rigid ion model for ionic crystals, and compared with similar calculations for KI end RbI. The elastic constants obtained from the eleven parameter shell model in units of 1011 dyn/cm2 are C11 = 3.04, C12 = 0.12, and C44 = 0.50.
The parallel and perpendicular Stark effect of the F‐center in RbBr and RbI
Authors: Banaii N. , Jacobs G. 
Published: July 2007

AbstractThe parallel and perpendicular Stark‐effects of the F‐center are measured in RbBr and RbI single crystals. The position of the 2s and 3d states are determined before relaxation of the lattice. The changes of the moments of the absorption band by the electric field are calculated and the dipole matrix elements between the unperturbed states mixed by the electric field are evaluated.
Static approximation for the Hubbard model: Magnetic phase transitions
Authors: Mertschlng J. 
Published: July 2007

AbstractMagnetic phase transitions are studied within the Hubbard model using the static singlesite approximation for zero temperature and the Bragg‐Williams theory. For a half‐filled bend, the antiferromagnetic transition comes out reasonably except for small correlation energies. However, the static approximation does not yield ferromagnetism even for large correlation energies.
The multiplet structure of the electron absorption spectra of naphthalene—ß‐naphthol mixed molecular crystals
Authors: Fedorov V. P. , Kind V. V. , Korshunov A. V. 
Published: July 2007

AbstractThe electron absorption spectra of naphthalene—β‐naphthol mixed molecular crystals are measured in polarized light at 4.2 to 300 K. The fine structure of spectra in the region of naphthalene and β‐naphthol absorption (the lowest singlet—singlet transitions) is shown to result from localized and pseudolocalized electron states in the Davydov splitting region of naphthalene, 31482 to 31638 cm−1. It consists of phonon side bands for electron transitions of β‐naphthol in naphthalene with v = 29960 cm−1 and v = 30100 cm−1. The long‐wave doublet 29697, 29774 cm−1 is connected with the formation of clusters.
Millimeter wave investigation of the complex dielectric constant in DKDP
Authors: Gauss K. E. , Happ H. 
Published: July 2007

AbstractThe temperature dependence of the complex dielectric constant in K(H1−xDx)2PO4 with x = 0.64 and 0.86 is determined at ω = 4.62 cm−1 (λ = 2.16 mm) for E parallel and transverse to the ferroelectric axis c by measurements of the complex amplitude reflection coefficient using a Michelson interferometer. For E ‖ c the temperature dependence is due to a low frequency mode with a characteristic frequency below 6 cm−1. Apart from the vicinity of TC the data can reasonably be described with a Debye model. For E ⟂ c the results indicate a temperature dependent transverse protonic mode between 5 and 10 cm−1. ϵ″ shows a sharp anomalous peak at TC.