bluuebunny/crossref_metadata_2025_split huggingface dataset:
Anisotropic chemical shift of nuclear magnetic resonance in single crystals of tellurium and selenium
Authors: Koma A. 
Published: July 2007

AbstractThe anisotropies of the chemical shifts in tellurium and selenium are analysed theoretically. It is shown that the second order paramagnetic chemical shift gives the main contribution to the anisotropies. The anisotropy of the chemical shift is calculated assuming molecular wave functions for the valence electrons derived on the basis of Reitz's band calculation and good agreement is obtained with the observed anisotropy for tellurium. It is found in the analysis that there exists about 2% mixing of the bonding and the antibonding orbitals in the upper valence band states of tellurium. More elaborate calculations, however, seem to be necessary to explain the anisotropic chemical shift in selenium.
Interionic potentials in liquid metals including liquid noble and transition metals
Authors: Waseda Y. , Suzuki K. 
Published: July 2007

AbstractThe Born‐Green equation has been analyzed numerically to derive the effective interionic pair potentials from the structural data observed by X‐ray or neutron diffraction experiments for liquid metals (Mg, Zn, Hg, Al, Ga, In, Tl, Sn, Pb, Sb, Bi), noble metals (Cu, Ag, Au), and transition metals (Fe, Ni) using a linearized simultaneous equation method. In all cases the potentials were found to be insensitive to temperature and to have long‐ranged oscillations. But the damping behaviour of the oscillations for liquid Sb and Bi remarkably deviates from that of the usual metals such as liquid Na. For most metals, the wavelengths of oscillations observed in this work are in agreement with those expected from the electron theory of metals. The viscosity coefficient and surface tension were calculated. Adequate agreement with experiments was found.
Study of F* centre bleaching in SrCl2 doped with an alkaline cation
Authors: Rzepka E. , Baltog I. , Lefrant S. , Yuste M. , Taurel L. 
Published: July 2007

AbstractF* centre bleaching in SrCl2:Na+ at LNT shows that the centres haveC3v symmetry. In the excited state, the probabilities of annihilation of an F* centre by a positive hole centre and of F* electron trapping by a vacancy associated with an alkaline centre are equal. In the case of F* centres in SrCl2:Rb+, F* centre bleaching leads to the formation of associations which can be attributed to perturbed F2 centres. By bleaching, these associations can be dissociated into F* centres or transformed into another types of association according to the wavelength of the bleaching light.
Field at the Nucleus and EPR Line Intensities for Ions in Crystals
Authors: Mialhe P. , Erbeia A. 
Published: July 2007

AbstractThe determination of the intensity of hyperfine lines in an EPR spectrum of ions in a crystal field with axial symmetry is discussed. Expressions are obtained for the magnetic field seen by the nucleus. It is taken as nuclear quantization axis and then the angular variations of line intensities are computed in a first order perturbation approximation and found in good agreement with experimental results on Mn2+–Al203 EPR spectra. Those results are compared with the descriptions given by the method of Bir and by calculations involving the identification of electronic and nuclear quantization axes.
Internal Strain in Zincblende and Wurtzite Crystals
Authors: Lawaetz P. 
Published: July 2007

AbstractA new method of calculating the bond‐bending parameter from experimental elastic properties is presented. The values obtained are used to analyze the piezoelectric constant, and a simple relationship between dipole and quadrupole contributions is found. The results are also applied to the equilibrium wurtzite structure for which the built‐in internal strain is predicted for a number of relevant crystals.
Strain Derivatives of the Static and High‐Frequency Dielectric Constants of Rubidium and Cesium Halides
Authors: Srinivasan R. , Srinivasan K. 
Published: July 2007

AbstractThe strain derivatives of the static and high‐frequency dielectric constants of rubidium and cesium halides have been calculated using the two ion polarizable shell model and the theoretical expressions of Srinivasan. The details of the model used and the relevant theoretical expressions are given. The relative magnitudes of the different elements of the strain derivative tensor is clearly seen to depend on the arrangement of the nearest neighbours. In the rubidum halides the calculated pressure derivatives of the static dielectric constant are in reasonable agreement with experiment while in cesium halides the calculated values are larger than the experimental ones. The strain optical constants calculated for the rubidium and cesium halides are in reasonable agreement with available experimental data. It is pointed out that the large contribution from overlap forces to the anisotropy in the polarizability arising from strain is fairly well represented by this simple shell model.
Self‐consistent Calculation of the Verwey Ordering in Magnetite
Authors: Ihle D. , Lorenz B. 
Published: July 2007

AbstractUsing the Cullen‐Callen model of magnetite in the Hartree approximation the Verwey order parameter m(T)is calculated self‐consistently for U/|h| > 2.78 (U Coulomb energy, h overlap integral). Verwey ordering and semiconducting behaviour is obtained for 0 ≦ T < Tg and gapless Verwey ordering and semimetallic behaviour for Tg ≦ T < T0 At T0 a second‐order phase transition to the disordered metallic phase takes place.
A Non‐Linear Canonical Transformation for the Dynamical Jahn‐Teller Problem in Cubic Symmetry (Optical Resonance Effect)
Authors: Grevsmühl U. , Wagner M. 
Published: July 2007

AbstractOne of the two fundamental dynamical Jahn‐Teller problems in point‐symmetry surrounding, the T–t problem, is handled by means of a non‐linear canonical transformation. The latter is established by analogy to the doubly degenerate problem, where the corresponding tranformation is shown to be quasi‐exact. Combinatorial as well as series expansion methods are presented. Pronounced resonance effects are found for the zero, one, and higher phonon lines. This reflects the fact that the effective splitting of the degeneracy in the high energy system may be of the same order of magnitude as the elementary excitation in the low energy one.
Charge Conversion of Chromium and Vanadium Ions in CdF2
Authors: Hauschild B. , Höhne M. , Ulrici W. 
Published: July 2007

AbstractThe optical absorption spectra of CdF2:Cr crystals show that the charge conversion Cr3+ ⟷ Cr2+ takes place completely and reversibly by heating in Cd and halogen vapour, respectively. The correlated investigation of optical and EPR spectra of CdF2:V crystals suggest that the optical spectrum after heating in halogen vapour is caused by small precipitated particles of VF3 whereas the EPR spectrum is due to V3+ on Cd sites. Thus the charge conversion V3+ ⟷ V2+ can be investigated only with the EPR spectra showing that the conversion V3+ → V2+ depends on the total concentration of trivalent ions on Cd sites.
Reduced Lattice Specific Heat Due to Light Vanadium Impurities in Gold
Authors: Tiwari M. D. , Agrawal B. K. 
Published: July 2007

AbstractThe reduction in lattice specific heat due to light vanadium impurity atoms in gold crystals observed earlier by Star et al. is explained on the basis of a low concentration Green's function theory. A localized perturbation model which includes mass change at the impurity site as well as changes in the nearest neighbour force constants has been employed performing the calculations. Reasonable agreement with the experimental data in two (Au‐0.33 at% V and Au‐1 at% V) of the three dilute alloys is obtained. The mass defect contribution is only 20% of the changed specific heat and the remaining contribution is due to force constant changes. The main contributions to the changed specific heat is caused by even parity A1 g‐, E2 g‐, F2 g‐, and F1 g‐symmetry modes, a feature which is not usually observed in other defect‐induced physical properties.
Stacking Sequence Energies from the Asymptotic Potential Approximation
Authors: Harrison E. A. 
Published: July 2007

AbstractThe way is investigated in which the asymptotic potential derived from pseudopotential theory can be used, in whole or in part, to determine the energies of stacking sequence errors in the f.c.c. and h.c.p. structures. It is shown that, in general, the asymptotic potential alone is inadequate, though it can be used for all but a small number of the energy terms, and for these a full potential must be used. For each valency, the long‐range interactions can be incorporated in a sum which has universal application. Since the evaluation of this sum represents almost all of the work needed to obtain the energy of a stacking fault, the sum has been calculated for valencies 1 to 4, and the most significant terms tabulated. These can be used to obtain the relative energy of any pair of close‐packed stacking sequences for any new potential developed to describe an element of one of these valencies.
Influence of Spatial Dispersion on the Properties of the High‐Frequency Waves in Uniaxial Polar Semiconductors
Authors: Gurevich L. E. , Tarkhanian R. G. 
Published: July 2007

AbstractAn investigation is made of the influence of spatial dispersion on the propagation of electromagnetic waves in uniaxial polar semiconductors. It is shown that spatial dispersion eliminates the longitudinal—transversal wave resonances at the frequencies of the coupled phonon‐plasmon oscillations and leads to the possibility of propagating an additional wave. Expressions for refractive index and extinction coefficient are given in the limiting cases of weak and strong spatial dispersion. The transmitting regions, the cut‐off frequencies, and the resonances of the waves are studied.
Valence‐Band Properties of Bi2Te3 from Galvanomagnetic Measurements
Authors: Sologub V. V. , Goletskaya A. D. , Lang I. G. , Pavlov S. T. 
Published: July 2007

AbstractGalvanomagnetic effects in Bi2Te3 are investigated in magnetic fields up to 38 kOe at 1.6 and 4.2 °K on six single‐crystal samples with hole concentrations between 1.4 × 1018 and 2.4 × 1019 cm−3. A proper arbitrary‐field theory is developed for highly degenerated p‐ or n‐type Bi2Te3 on the basis of a six‐ellipsoid Drabble‐Wolfe model (DM). In accordance with Shubnikov‐de Haas oscillations, the experimental data in the concentration range over 3 to 4 × 1018 cm−3 exhibit considerable deviations from a simple DW behaviour. Using an extension of the galvanomagnetic theory to the case of two six‐ellipsoidal subbands, a good fit with the data is obtained, which do not fit to the DW description. However, a test two‐band estimate in terms of the calculated concentrations and density of states masses results in a too narrow energy gap between the subbands in the 7.1 × 1018 cm−3 sample (an order of magnitude lower than the chemical potential). The upper subband should populated at hole concentrations of the order 1017 cm−3, in contradiction to the threshold concentration of 3 to 4 × 1018 cm−3, which has been obtained in the high‐field analysis of the whole series of samples investigated.
On Magnetization of Impurities in Dilute Alloys
Authors: Rudra P. 
Published: July 2007

AbstractA linearized thermodynamic Green's functional analysis has been made to obtain the magnetization of impurity ions in s‐band host metals. Fermi contact type antiferromagnetic coupling between the impurity spin and the conduction electron spin leads to a sudden discontinuous jump in the impurity magnetization as a function of temperature. The impurity magnetization also shows extremum properties for a parameter χ less than 1.2. In the case of ferromagnetic coupling the impurity magnetization is a monotonous function of temperature.
Low Temperature Specific Heat of Ordered and Disordered FeCo
Authors: Kuentzler R. 
Published: July 2007

AbstractMeasurements of the specific heat of configurationally ordered and disordered FeCo between 1.2 and 4.2 °K are presented. The coefficients of the electronic and harmonic phonon contributions decreased on ordering (by 16% and 20%, respectively). The effects on the electronic term are mainly associated with a change in the density of states.
Structure Studies by Electron Diffraction on Amorphous Ge Films
Authors: Gandais M. , Theye M. L. , Fisson S. , Boissonade J. 
Published: July 2007

AbstractElectron diffraction experiments performed in an electron microscope on amorphous Ge films, which allowed to determine the elastically scattered intensity for s = 2 sin θ/λ between 0.1 and 3 Å−1, are described. The method of elimination of the inelastically scattered intensity, as well as the correction for experimental errors using the properties of the statistical distribution function, are exposed in detail. The interference functions and the statistical and radial distribution functions obtained for different samples are compared to those obtained by various authors from electron or X‐ray diffraction.
Misalignment of Flux Lines in a Type II Superconductor Studied by Neutron Small‐Angle Diffraction
Authors: Lippmann G. , Schelten J. , Hendricks R. W. , Schmatz W. 
Published: July 2007

AbstractThe mutual misalignment of flux lines has been measured by neutron small‐angle diffraction with the incident beam parallel to the flux lines. The resolution function of the apparatus for this new geometry has been evaluated and various physical contributions to the broadening of the observed diffraction peaks are discussed. The resulting misalignment angles were found to be largest (≈ 4°) at low flux densities, and decreased to a constant value (8′ ± 5′) at flux densities larger than 0.5 Hc2.
A Variation in Applying the Born Model to Ultrasonic Data for NaCl Structures
Authors: Fisher E. S. 
Published: July 2007

AbstractThe application of the Born model for ionic crystals to the ultrasonic data for C44 of NaCl type crystals gives effective valences and repulsive parameters that are strongly dependent on the cation and are more consistent with the observed cation dependence of dC44/dP than the assumption of integral valences.
Surface Lattice Dynamics of Ionic Crystals by the Green Function Method
Authors: Benedek G. 
Published: July 2007

AbstractIt is shown that the surface lattice vibrations of semiinfinite ionic crystals can be easily studied and calculated by an approximate Green's function method based on a suitably defined effective short‐range perturbation. Breathing shell model calculations for the (001) surface of NaCl are presented and compared with the direct lattice‐dynamical calculation carried out by Chen et al. for a 15‐layer slab. The reliability and the computational advantages of the Green's function technique with respect to the slab method are discussed in connection with the calculation of the surface phonon response functions.
Theory of Singlet—Triplet Exciton Fusion
Authors: Rahman T. S. , Knox R. S. 
Published: July 2007

AbstractFörster's theory of resonant transfer of energy has been used to calculate the probability per unit time of transfer of energy from an excited singlet to a triplet, raising the latter to a higher triplet. The magnitude of the effect is computed in applications to chlorophyll, rhodamine 6G, and anthracene, and in all cases it is found that the corresponding Förster radius R is of the order of 40 to 50 Å. The rate constant kST for singlet—triplet exciton collisions resulting in quenching of the singlet is then calculated. It is found that the theoretical value of R, for the case of crystalline anthracene, which is much larger than that predicted by Babenko et al., leads to better agreement with experiment and is to be preferred. Assuming a joint singlet—triplet diffusion constant of 10−5cm2/s, the rate constant kST for chlorophyll in solution is predicted to be 4.5 × 10−11 cm3/s.
Mean Square Vibrational Amplitudes in PbTe
Authors: Ghezzi C. 
Published: July 2007

AbstractX‐ray diffraction experiments on single crystals and powder specimens are performed in order to obtain the values of the mean‐square vibrational amplitudes of lead and tellurium atoms in PbTe. From both dependences of the integrated intensities on sin θ/λ and on temperature, the values 〈u〉 =(2.33 ± 0.15) × 10−18 cm2 and 〈u = (2.09 ± 0.14) × 10−18 cm2 are derived. The X‐ray Debye temperature is found to be θM = (107 ± 3) °K. The analysis of the experimental data is given in detail. The results are discussed in relation to previous works tending to substantiate the Brooks‐Yu theory on the temperature dependence of the energy band gap.
Laser Emission in CdSe Due to Exciton—Exciton and Exciton—Electron Interaction
Authors: Braun W. , Bille J. , Fischer T. , Huber G. 
Published: July 2007

AbstractSpectra of the spontaneous and stimulated emission of CdSe are presented in the temperature range from liquid helium to room temperature under electron beam excitation. At temperatures below 70 K the laser process results from the exciton—exciton interaction and from the annihilation of a free exciton associated with the emission of one longitudinal optical phonon. At temperatures above 90 K the laser emission is found to shift to lower energies much stronger than the bandgap. This shift can quantitatively be accounted for by the exciton—electron interaction process.
A Model Potential for Alkali Halide Crystals Including a Short Range Three‐Body Interaction
Authors: Sarkar A. K. , Sengupta S. 
Published: July 2007

AbstractThe model potential including a short range three‐body interaction which was used by Sarkar and Sengupta [1,2] to calculate various static properties of some alkali halide crystals has been extended to include overlap interaction between next nearest neighbours. This has been used to calculate the cohesive energy, harmonic elastic constants, phase transition pressure, and transitionvolume for transition from sodium chloride to the cesium chloride structure in the case of potassium and rubidium halides. The results have been compared with experimental values and with the theoretical calculations done on other models. It is found that the inclusion of overlap interaction between next nearest neighbours considerably improves the overall agreement between theoretical and experimental results.
Coexistence of the Itinerant Antiferromagnetism with Localized Magnetic Moments in Cr—Mn Alloys
Authors: Iuşan V. , Pop I. , Chechernikov V. I. 
Published: July 2007

AbstractThe magnetic susceptibility of Cr—Mn alloys with concentrations 0 to 30 wt % Mn have been measured in the temperature range 100 to 950 K. The temperature dependence of the magnetic susceptibility curves show a strong anomaly (maximum) at the Néel temperature for all concentrations. In the antiferromagnetic domain a new anomaly was found which goes to the Néel temperature with increasing the Mn concentration and vanish at about 28 wt % Mn. The coexistence of the itinerant antiferromagnetism with the localized magnetic moments is clearly shown especially for high Mn concentrations.
Antiferromagnetic Properties of NiO and CoO I. Symmetry Properties
Authors: Morgan F. D. , Joshua S. J. 
Published: July 2007

AbstractEarlier investigations of several workers on the symmetry properties and dispersion relations of antiferromagnetic NiO and CoO are extended. Using standard group theoretical methods, crystal field splittings, symmetry properties of excitons, possible symmetries of the spin wave spectra in the paramagnetic phase and the symmetry characters of the zero wave vector two magnon states me studied. The latter are useful for determining the selection rules for two magnon absorption and in fact these selection rules we have derived for CoO.
Photoemission and density of valence states of the II–VI compounds. I. ZnTe, CdSe, CdTe, HgSe, and HgTe
Authors: Shevchik N. J. , Tejeda J. , Cardona M. , Langer D. W. 
Published: July 2007

AbstractThe density of valence states of the II–VI compounds ZnTe, CdSe, CdTe, HgSe, and HgTe have been determined with far uv (h v = 21.2 and 40.8 eV) photoemission (UPS). Although the upper portions of the valence bands are found to have shapes in qualitative agreement with theory, they are consistently 1 to 2 eV wider than predicted. With the high resolution obtainable in uv photoemission, the shapes of the d‐bands of the cation can be resolved to permit an accurate determination of their positions relative to the top of the valence band and their spin‐orbit splittings. The “apparent” spin‐orbit splittings of the uppermost d‐orbitals of Zn and Cd are found to be 50% larger in the metallic form then in the compounds.
Magnetic circular dichroism of the vibrationally induced d → d transition 4A2g(F) → 4T1g(P)on Co2+ ions in CdF2
Authors: Donecker J. , Kluge J. 
Published: July 2007

AbstractMeasurements of the magnetic circular dichroism of the vibrationally induced transition 4A2g(F) → 4T1g(P) in Co2+ doped CdF2 crystals at temperatures from 2.5 to 300 K and at magnetic fields up to 52 kOe are reported. The measured line shape of the magnetic circular dichroism qualitatively corresponds to that calculated by Cho for the Jahn‐Teller effect of the 1A1g(S) → 1T1u(P) transition. Therefore, we give support to the explanation of the structure of this absorption band due to a Jahn‐Teller effect. The method of moments of Henry, Schnatterly, and Slichter is extended to vibrationally induced transitions. Assuming that vibrations belonging to one irreducible representation dominate, a reduced spin‐orbit coupling constant of λ′ = (70 ± 10) cm−1 is found for the 4T1g(P)state.
Charge Carrier Generation by Singlet–Singlet Exciton Interaction in Crystalline Anthracene under Weakly Absorbed Light Illumination
Authors: Schott M. , Berrehar J. 
Published: July 2007

AbstractA study of double quantum photoconductivity processes due to weakly absorbed light in crystalline anthracene is used to precise the conditions where singlet‐singlet, singlet‐photon, or singlet‐triplet processes are observable. It is found that αss = (6 ± 3) × 10−12 cm3 s−1 and the average singlet photoionization cross section for photons between 4200 and 5000 Å is σsΦ ≦ 2 × 10−19cm2. αss is weakly dependent on T above 200 °K, the corresponding activation energy being (0.03 ± 0.03) eV. Results found in the literature are discussed and in some cases conclusions are corrected.
Elastic and optical properties of rutile‐structure fluorides in the rigid‐ion approximation
Authors: Striefler M. E. , Barsch G. R. 
Published: July 2007

AbstractThe elastic constants and the infrared and Raman frequencies of the difluorides XF2 with X = Mg, Mn, Fe, Co, Ni, Zn have been calculated for a rigid ion model with an effective ionic charge and with first and second nearest neighbor central force short range interaction. Explicit expressions for the elastic constants are given with the internal strain contributions expressed in terms of the Raman frequencies. The nine parameters of the model are determined from the three equilibrium conditions and from least squares fits over some or all available experimental elastic and optical data. The model is able to account well for the magnitude of the experimental data and for their almost monotonic decrease with increasing cation radius, but the calculated deviations from the Cauchy relations are smaller than the observed values.