bluuebunny/crossref_metadata_2025_split huggingface dataset:
First‐harmonic electromodulation in uniaxial crystals
Authors: Suisky D. , Boyn R. , Enderlein R. , Biering B. 
Published: July 2007

AbstractA discussion is given of an electromodulation mechanism which occurs in uniaxial crystals for electric fields perpendicular to the optical axis. This mechanism is based on the electro‐optical Pockels effect, which produces a rotation of the polarization directions of the ordinary and extraordinary light waves propagating in the crystal. In the presence of polarization‐dependent zero‐field transmission of the material, this leads to a change of transmission which is linear in the field. It is shown that this mechanism makes a significant contribution to the F ⟂ c electromodulation spectra of transition metal centres in wurtizite‐type CdS obtained recently, and that on this basis it is possible to understand the spectral shapes of the signals found in this geometry. An analogous mechanism should contribute to the electroreflection of uniaxial crystals.
Long‐range order in nearly one‐ and nearly two‐dimensional Heisenberg ferromagnets. An upper bound for the magnetization
Authors: Treplin W. , Jelitto R. J. 
Published: July 2007

AbstractA modified Heisenberg model of ferromagnetism is considered with variable dimension, which is suitable for a description of substances with nearly linear and nearly planar magnetic coupling. By application of Mermin and Wagner's method an exact upper bound is derived for the spontaneous magnetization as a function of the lattice dimension. This upper bound breaks down in a qualitatively different way for the transition to one and to two dimensions. For the latter case a residual three‐dimensional admixture to the interaction suffices to produce a practically three‐dimensional behaviour.
A new type of F3‐centre in additively coloured CaF2
Authors: Alcalá R. , Orera V. M. , Beamonte J. 
Published: July 2007

AbstractTwo new absorption bands at 450 and 320 nm appear in additively coloured CaF2 crystals by bleaching in the α‐band (375 nm) at temperatures ≈ 160 K. These two bands have dichroic behaviour, from which is found, that they are due to two different optical transitions in the same centre, which have their electric dipoles perpendicular, and parallel to the 〈110〉 directions, for the 450 nm band; and along the 〈100〉 directions, for the 320 nm band. Thermal reorientation measurements show that the dichroism induced by light with E ∥ [110], disappears at 210 K, while the dichroism induced by light with E ∥ [100] does not disappear until 270 K. This 270 K disorientation is accompanied by a transformation to the linear F3‐centre, which appears with some dichroism. A model for the centre responsible for these two bands, consisting of three F‐centres forming a right‐angled triangle in a (100) plane, gives good agreement with the experimental results. This is a new type of F3‐centre in the fluorite structure, in contrast with the linear F3‐centre found by bleaching in the α‐band at RT.
Static and dynamic critical behaviour of uniaxial ferroelectrics and the phase transition in TGS
Authors: Nattermann Th. 
Published: July 2007

AbstractUsing the Larkin‐Khmelnitskii theory for static critical behaviour it is shown how recent renormalization group calculations on static and dynamic phenomena in uniaxial ferroelectrics can be generalized to the whole critical region. The cross‐over between classical and asymptotic behaviour is described in terms of two additional non‐universal parameters b and t0 which are defined by microscopic quantities. The critical temperature dependence of the elastic and electrostrictive constants, the dynamic susceptibility, and the ultrasonic attenuation are treated as a ramification of the Larkin‐Khmelnitskii theory. The correlation parameter δ and the kinetic coefficient T exhibit no anomalous behaviour close to Tc. The ultrasonic attenuation above Tc is shown to be dominated by energy fluctuation. Contradictions in previous interpretations of the data of ferroelectric TGS are removed. At zero temperature Te0 = 4 × 10−7 s is found for the energy relaxation time and ξ0 = 3.7 and ξ∥o = 3.9 Å for the transverse and longitudinal correlation length, respectively. From b = 0.036 and t0 = 0.04 the size of the critical region of TGS is estimated to t = (T – Tc)/Tc ≈︁ 1.6 × 10−2 to 1.8 × 10−3 depending on the quantity which is considered, but the uncertainties in determing t0 are large.
On the microscopic mechanism of the ferroelectric phase transition of TSCC: Mn2+
Authors: Franck S. , Kühnel A. , Wendt S. 
Published: July 2007

AbstractIn ferroelectric TSCC doped by Mn2+ ions Windsch et al. observed a large rotation of some EPR lines, the limiting rotation angle being 30°. Using Newman's superposition model, the EPR fine structure tensor Dij is calculated via the field gradient tensor Vij in a point charge model. It turns out that rather small (some 10−9 cm) shifts together with a small change in the effective charge of the oxygen ions at the sites of the distorted octahedron around the manganese ions are sufficient to explain the large rotation of the EPR lines in a good agreement with the experimental results.
Temperature‐independent intersystem crossing in anthracene crystals
Authors: Michel‐Beyerle M. E. , Haberkorn R. 
Published: July 2007

AbstractThe temperature dependence of the delayed fluorescence of anthracene crystals observed under the condition of inhomogeneous excitation can be explained by the influence of reabsorption on the exciton distribution. It does not reflect a thermally activated intersystem crossing rate.
Release of stored energy of X‐irradiated KCl and CsBr
Authors: Schrey P. , Balzer R. , Peisl H. 
Published: July 2007

AbstractThe energy release of KCl and CsBr single crystals is measured after X‐irradiation at 10 K. Release maxima are found at 22, 28, and 36 K (KCl), and at 24 K (CsBr). The energy release is correlated with the annealing of irradiation‐induced Frenkel pairs by comparison with the results of optical absorption measurements. Below 50 K the release of stored energy is mainly due to the recombination of α + I Frenkel pairs. A value of 5 eV per recombination is found as an upper limit for the α + I Frenkel pair in KCl.
Dynamical screening of excitons by free carriers
Authors: Haug H. , Thoai D. B. Tran 
Published: July 2007

AbstractThe Bethe‐Salpeter equation for the electron‐hole polarization function is treated in the screened ladder approximation. The dynamically screened Coulomb potential is taken into account in the plasmon pole approximation. An iterative procedure leads to an effective Bethe‐Salpeter equation in which the screening is described by a virtual exchange of plasmons between the electron and hole. The resulting exciton binding energy is considerably larger than that obtained by the statically screened Coulomb potential approximation. Results of the numerical calculation of the binding energy as a function of the free carrier density are given. The calculated binding energy vanishes only asymptotically in the limit of high free‐carrier densities.
Electronic elementary excitation in the far UV‐spectral region core level exciton and core hole polarization
Authors: Enderlein R. , Seifert B. , Bechstedt F. 
Published: July 2007

AbstractAn electronic elementary excitation of a crystal is considered which might be of interest for the interpretation of the structures of far UV reflectance and photoemission spectra associated with transitions from core levels into conduction bands. The primarily generated strongly localized core hole polarizes the electronic subsystem. The polarization is accompanied by the generation of a secondary less strongly localized hole to which the conduction band electron is bound. The excitation energy is lowered by the polarization energy of the core hole as compared to the excitation energy of an exciton. The static polarization energy and polarization charge density are calculated for a core hole of the Ga atom by means of the Thomas‐Fermi approximation. Polarization energies of several eV are obtained which decrease nearly inverse proportionally with hole radius. The influence of the polarization energy on UV reflectance and photoemission spectra is discussed. It is concluded that the core hole polarization energy contributes to the difference between band distances obtained by reflectance and photoemission measurements, supposed the dispersion of the polarization energy is important.
Nonlocal theory of elastic interaction between point defects
Authors: Gairola B. K. D. 
Published: July 2007

AbstractA weakly nonlocal version of the nonlocal continuum theory is used to calculate the interaction energy between a pair of identical defects in a face‐centred cubic lattice when the defect spacing is large. The lattice is assumed to be the same that was considered by Hardy and Bullough, namely, an idealized harmonic f.c.c. lattice held together by nearest‐neighbour forces which are such that the crystal is elastically isotropic in the long‐wavelength limit. An analytic expression is obtained for the interaction energy which is orientation dependent and varies as the inverse fifth power of defect separation. This expression for the interaction energy is in complete agreement with the result of Hardy and Bullough which they obtained by using Kanzaki's method of lattice statics. In this way it is demonstrated that the nonlocal continuum theory can reproduce the results of the lattice theory.
The optical spectra of rare‐earth cobalticyanides
Authors: Bonnet M. C. , Antic‐Fidancev E. , Porcher P. , Lemaitre‐Blaise M. , Caro P. 
Published: July 2007

AbstractRare earth cobalticyanides Ln[Co(CN)6] · n H2O have two cristalline forms, an hexagonal one from La to Nd with n = 5 and an orthorhombic one from Ce to Lu with n = 4. The optical absorption spectra of the Nd3+ (4f3) and the fluorescence spectra of the Eu3+ (4f6) in the pure and doped salts are recorded at low temperatures (4.2 or 77 K) in the polycristalline form. The orthorhombic phase exhibits spectra corresponding to a lanthanide in a C2v site under a weak crystal field (Bq2, Bq4 < 500 cm−1). The spectra for Nd3+ and Eu3+ in the hexagonal lanthanum compounds are compatible with a C3h site symmetry. There is evidence of selection rules for the dipole electric transitions in 4f3, and the 4f6 configuration has levels which are Kramers doublets.
Crystal growth and EPR study of Cr+‐doped MgTe single crystals
Authors: Boulanger D. , Martin M.‐S. 
Published: July 2007

AbstractEPR measurements are made for X and Ka bands on Cr+ in MgTe single crystals. Growth of these single crystals is described. The spin‐Hamiltonian parameters for Cr+ are determined at liquid nitrogen temperature as follows: g = 2.0038 ± 0.0005; |A| = (13.4 ± 0.4) × 10−4 cm−1; |a| = (9.2 ± 0.5) × 10−4 cm−1; |D| = (28.1 ± 0.3) × 10−4 cm−1 and |a – F| = (11 ± 0.4) X × 10−4 cm−1. The dependence of D on temperature is studied. For Cr+, Mn2+, and Fe3+: phenomenological relationships for the g‐factor are established for various II–VI compounds with blende or wurtzite structure.
Dynamic nuclear polarization in TCNQ salts
Authors: Alizon J. , Berthet G. , Blanc J. P. , Gallice J. , Robert H. , Fabre J. M. , Giral L. 
Published: July 2007

AbstractTemperature dependence of the dynamic nuclear polarization enhancement AL of the protons in tetrathiafulvalinium‐tetracyanoquinodimethan salt (TTF‐TCNQ) and potassium‐TCNQ salt is reported. In TTF‐TCNQ, AL is temperature‐independent in the range 150 to 300 K; in K‐TCNQ, AL is strongly temperature‐dependent. These results, and the knowledge of the frequency dependence of the relaxation time T1 of the protons allow to determine the cut‐off frequency vC of the ld diffusion mechanism with respect to the nuclear and electronic frequencies vI and vS. vC is larger than 10 GHz in TTF‐TCNQ and vC is of the order of vI in K‐TCNQ. Moreover, because of the large enhancement AL observed in K‐TCNQ, the T1 anisotropy is determined on single crystals.
Calculation of the g‐shift and superhyperfine structure tensor of Mn2+ ion in AIIBVI tetrahedral semiconductors
Authors: Hamera M. 
Published: July 2007

AbstractMultiple scattering method is used to obtain wave functions and energy levels of the 17‐atom cluster MnB4A12. The influence of all other atoms of the crystal is assumed to produce an additional constant potential only. At the Mn site, the correction to the potential is introduced to give most probable values of the cubic field splitting parameter Δ. For these values of Δ, the g‐shift and superhyperfine structure tensor are calculated and compared with experimental data.
Localized magnons around an impurity in a spin chain with next‐nearest neighbour interaction
Authors: Georgiev G. , Gochev I. 
Published: July 2007

AbstractLocalized magnons around a ferromagnetic impurity spin in a linear Heisenberg ferromagnet with next‐nearest neighbour interaction are discussed. It is shown that the number and existence regions of these localized states are essentially dependent on the next‐nearest neighbour coupling intensity.
Electro‐birefringence from paraelectric defects in cubic crystals
Authors: Lüty F. , Diaz‐Gongora A. 
Published: July 2007

AbstractThe electro‐birefringence effects are discussed and produced in an (electro‐optically inactive) cubic crystal by the presence of electric dipole defects which are alignable by electric fields. Based on the electronic polarizability tensor of the defects and their paraelectric alignment functions, expressions for the field‐induced birefringence (Kerr or Pockels effect) can be derived for all interesting symmetries of dipoles, electric fields, and light‐geometry, and their connections — by Kramers‐Kronig relations — to electro‐dichroism can be given. Various methods for electrobirefringence measurements with dc, ac, dc + ac, and birefringence‐bias techniques producing linear, quadratic, and quartic E‐dependence of the electro‐optical effect are discussed. The derived behavior is illustrated by experimental examples selected from Kerr effect measurements on CN− defects in alkali halides.
Thermal properties and Grüneisen parameters of gold using Toya's first principle approach
Authors: Prasad B. , Srivastava R. S. 
Published: July 2007

AbstractThe phonon frequencies and Grüneisen parameters are calculated for different values of the phonon wave vector in the three principal symmetry directions for gold using Toya's first principle approach. The average value of the Grüneisen parameter γth is also calculated for different temperatures and the results are compared in the light of available experimental and theoretical results.
Large unit cell calculations of the band structure of ionic crystals using the Mulliken‐Ruedenberg approximation
Authors: Evarestov R. A. , Ermoshkin A. N. 
Published: July 2007

AbstractThe Mulliken‐Ruedenberg approximation which has been developed for the molecular calculations is generalized for solids in the framework of the large unit cell approach. In such a model the periodical boundary conditions are taken into account. The calculations of the main features of the band structure for the series of alkali halides are performed using the LUC model in the Mulliken‐Ruedenberg approximation. A comparison is given between the results obtained and both the experimental data and results of band structure calculations using the LCAO basis set. The conclusion is drawn that the application of the Mulliken‐Ruedenberg approximation for the Hamiltonian matrix element estimation allows to obtain quite satisfactory results for ionic crystals without any preliminary calibration of atomic parameters.
Observation of precursory phonons of an F.C.C.‐H.C.P. transition
Authors: Sacchetti F. , Bosi P. , Dupré F. , Frollani G. , Menzinger F. , Spinelli M. C. 
Published: July 2007

AbstractA study of the phonon dispersion relation in a Co0.65Ni0.35 alloy shows some novel features, which may be connected to the transition from the f.c.c. to the h.c.p. phase, which is expected at lower temperatures. In fact, in the study of the [ζζζ] T branch in the f.c.c. phase the presence of extra phonon peaks is observed at intermediate q‐values, though the static f.c.c. phase cannot allow for the presence of a new branch. It is believed that a softened optical branch of the h.c.p. phase which interacts with the acoustic branch of the f.c.c. phase may cause the observed phonon structure.
A computer simulation of point defects and planar defects in nickel
Authors: Miller K. M. , Bristowe P. D. 
Published: July 2007

AbstractThe properties of point defects and planar defects in nickel are investigated using computer simulation techniques. An empirical pair potential consisting of nine cubic splines is derived for nickel. This potential reproduces the experimental vacancy formation and migration energies, the intrinsic stacking fault energy, and the elastic constants. The displacement fields, formation energies, and formation volumes are calculated for the vacancy, the saddle‐point configuration associated with vacancy migration, and six interstitial configurations. In addition, results are presented for the intrinsic and extrinsic stacking faults and the twin boundary. The vacancy results are in agreement with previous calculations but the interstitial formation energies are significantly larger than those calculated previously. The crowdion is found to be the most stable interstitial configuration with a formation energy of 8.77 eV and formation volume of 0.42 atomic volumes. The energies of the twin, intrinsic, and extrinsic stacking faults are found to be 97, 158, and 199 mJm−2, respectively.
On the no‐phonon structure in mixed crystals with indirect band structure
Authors: Strehlow R. 
Published: July 2007

AbstractThe no‐phonon structure occurring in the optical interband absorption spectra of mixed crystals with indirect band structure is investigated. Starting from a simple model for describing Wannier excitons in mixed crystals this structure is shown to be due to disorder‐induced indirect exciton formation. The calculations yield lineshape and composition dependence which are in good agreement with experiment.
To the theory of the tunnel effect in a semiconductor in constant electric and magnetic fields
Authors: Oleinik V. P. , Sinyak V. A. 
Published: July 2007

AbstractInterband tunnelling in a semiconductor in constant electric E and magnetic H fields is studied in the framework of the two‐band model. An accurate formula is deduced for the tunnel current on the basis of the quantum transition theory in systems with unstable ground state, which is correct for arbitrary values of magnitude and direction of E and H. Giving an example of the two‐band model, those striking features of the dynamical description of the physical system which are due to the instability of its growth and state concerning the creation of electron–hole pairs are discussed.
Effect of atomic size on the effective polarizability of physisorbed atoms
Authors: Meixner W. C. , Antoniewicz P. R. 
Published: July 2007

AbstractCalculations of the effective polarizability of atoms adsorbed on metal surfaces have usually represented the atom by a polarizable point dipole. A simple model which includes an atomic size parameter but neglects exchange and correlation effects between the atom and the metal is used to investigate the validity of the polarizable point atom approach. A gradient expansion technique within the density functional formalism is used in this calculation. Results indicate that atomic size can greatly reduce the enhancement of the effective polarizability found from the polarizable point atom approximation.
Temperature behaviour of the dielectric constant of tetragonal BaTiO3
Authors: Lima C. A. S. , Scalabrin A. , Miranda L. C. M. , Vargas H. , Porto S. P. S. 
Published: July 2007

AbstractA model is developed which assumes the temperature dependence of the dielectric constant of BaTiO3, along the ferroelectric axis in the tetragonal phase to be essentially determined by a relaxation mechanism involving dipole reorientation controled by tunneling across a double well potential barrier. Model parameters are determined from fits to existing data and it is shown that the fitted curves adjust well to the data within experimental error. The results support the hypothesis that the ferro‐to‐paraelectric transition in BaTiO3 is not governed by phonon assisted processes but must be ascribed to an intrinsic type of disorder.
Configuration interaction and intensity of crystal field spectra of the tetrahedral Ni2+ Ion
Authors: Gomathy V. , Basu C. , Ghosh U. S. 
Published: July 2007

AbstractThe tetrahedral crystal field acting on the F‐state ion Ni2+ in ZnO mixes the odd parity states of excited 3d74p and 3d74f configurations with the even parity states of 3d8 configuration and this mixing has some contribution to the intensity of electric dipole transition between crystal field levels. The contribution of such configuration interaction to the oscillator strength for transitions from the lowest crystal field level T1 to the higher ones is estimated within the framework of crystal field theory using the powerful Racah method and Richardson function for the radial integrals. The results indicate that such contribution to the intensity is quite significant; it accounts for an appreciable amount of intensity observed particularly for one of the transitions T1 → T2. Moreover, this contribution to all the three transitions considered, is found to be much larger than the intensity for the corresponding magnetic dipole transitions.
Low temperature polaronic hopping conduction
Authors: Thomas P. , Wuertz D. 
Published: July 2007

AbstractThe de‐hopping conductivity and thermopower is calculated for temperatures below half the Debye temperature θD. A percolation approach with multiphonon hoprates is used. Two sets of energetically overlapping states near the Fermi level are consdered. The coupling to the phonons is assumed to be larger for the lower lying set of states. The resulting thermopower is negative and slowly varying with temperature, but bends down to higher neegative values at low (≈︁ 0.1θD) temperatures. The conductivity essentially follows a In σ ∼ T−1/4 law. The thermopower is much more stable with respect to changes in the density of states as compared to a model without multiphonon effects.
Improvements of the equation of motion technique for disordered systems
Authors: von Heimendahl L. 
Published: July 2007

AbstractThe equation of motion technique for the calculation of dynamical properties of large disordered systems is reconsidered and two improvements are proposed. Several procedures for the numerical integration of systems of second‐order differential equations are discussed and one particular efficient integration formula is selected. A modified cosine transformation for finite integration intervals is presented, which gives a positive, bell‐shaped function as a representation of the δ‐function instead of sin (ωT)/ωT. This allows an easier discussion of highly structured spectra. The results are demonstrated by a simple example.
Resonant Raman scattering in InP
Authors: Sinyukov M. , Trommer R. , Cardona M. 
Published: July 2007

AbstractMeasurements are made of resonant first‐ and second‐order Raman scattering by phonons in InP for photon energies from 1.75 to 3.05 eV. The behaviour of all observed resonances can be well explained by effects due to the E0 + Δ0 and the E1 band gaps. A comparison of the first‐order deformation potential at E1 with the one of GaAs is made and several electron‐two‐phonon deformation potentials are evaluated.