bluuebunny/crossref_metadata_2025 huggingface dataset:
Polariton Fusion in CuCl
Authors: Fröhlich D. , Mohler E. , Uihlein Ch.
Published: July 2007

Selection rules and frequency dependence for fusion of two polaritons to a longitudinal exciton or a transverse polariton are discussed for crystals of point symmetry Td. Within certain approximations, the transition probability depends only on the polarization of the two polariton beams and on the K‐vector of the final state. The theoretical predictions are found to be in good agreement with two‐photon absorption measurements for the first exciton in CuCl.
Band Structure and Pseudopotential Form Factors for AlAs
Authors: Hess E. , Topol I. , Schulze K.‐R. , Neumann H. , Unger K.
Published: July 2007

A combination of the pseudopotential method with the k p‐method was used to, calculate the complete band structure of AIAs. The pseudopotential form factors were determined by fitting to the known optical transition energies. Effective masaes were calculated for the principally important conduction band minima and the valence band maximum and compared with experimental data so far as possible.
EPR Optical Detection of F Centre Pairs in Alkali Halides. II. Spin‐Lattice Relaxation of the F Centre in Its Relaxed Excited State
Authors: Ruedin Y. , Schnegg P.‐A. , Jaccard C. , Aegerter M. A.
Published: July 2007

Like the EPR line of the fundamental state of the F centre in most of the alkali halide crystals, the line attributed to the relaxed excited state detected optically for F centre pairs is also Gaussian in shape. That is explained by an inhomogeneous broadening due to hyperfine interactions with the surrounding nuclei. The measurements of these resonances as a function of the microwave power allow to determine the saturation parameter and the broadening of the elementary line (homogeneous). On the other hand the measurement of the resonance signal as a function of temperature, the other parameters remaining constant, shows that it stays constant for T < 10 °K and decreases for increasing temperature. This is attributed to the spin‐lattice relaxation time T1 of the defect in its relaxed excited state. The relaxation mechanism is characterized by an Orbach process.
Single‐Site Approximation in Alloys with Short‐Range Order
Authors: Streitwolf H.‐W. , Mertsching J.
Published: July 2007

A self‐consistent single‐site approximation in alloys with short‐range order is discussed and applied to a one‐dimensional tight‐binding model. This approximation yields some broadening or narrowing of the electron density of states due to short‐range order, but no specific cluster effects, of course.
Piezoemission Study of CdS:Te
Authors: Goede O. , Fink F.
Published: July 2007

The emission band of the isoelectronic impurity Te in CdS is investigated with static uniaxial stress, σ, applied to the crystal. It is suggested that the bandshape is due to strong electron‐LA‐phonon interaction, the corresponding coupling constant and the effective phonon energy are determined. The measured peak shifts and stress‐induced polarization of the emission band for σ ‖ c and the band splitting for σ ⟂ c are interpreted on the basis of first order perturbation theory. The symmetry group of the defect and the irreducible representations of the electronic states involved in the optical transition are obtained and confirm the bound exciton model proposed in [5].
On the Effects of Applied Electrostatic Fields and Hydrostatic Pressures upon F‐Centre ENDOR Spectral
Authors: Wolbarst A. B.
Published: July 2007

It is suggested that the effects of bulk lattice polarization are not necessarily ignorable in the analysis of F‐centre electric‐field‐effect electron‐nuclear double resonance data, as was thought previously. Estimates of those contributions to contact hyperfine interaction shifts which are attributable to electric field induced lattice polarization are found to be comparable in magnitude to the corresponding values extracted from high pressure ENDOR measurements.
Birefringence of CdS under Uniaxial Stress
Authors: Babonas G. , Reza A. , Šileika A.
Published: July 2007

The dependence of intrinsic birefringence of CdS on uneasily stress has been investigated at room temperature in the spectral region 0.1 to 2.3 eV. The measurements were performed for stresses parallel and perpendicular to the c‐axis of CdS single crystals up to 2500 and 300 kp/cm2, respectively. The experimental results are explained by stress‐induced changes of the parameters of the high energy peak (E1) and exciton lines (A, B, and C).
Optical Properties of Cu‐Complex Centres in KCl Single Crystals
Authors: Kishishita H.
Published: July 2007

Absorption and emission spectra of KCl single crystals doped with various amounts of copper have been investigated a t temperatures from 10 to 300 °K. In highly doped specimens, four new absorption bands appear in wavelength regions longer than the 260 nm absorption band which has been attributed to the lowest transition within a single Cu+ centre. These new absorption bands are found at 288 nm (B‐band), 324 nm (C‐band), 373 nm (D1‐band), and 382 nm (D2‐band) a t 77 °K. The B‐band is assigned as a transition in paired Cu centres. The D1‐ and D2‐bands are attributed t o exciton absorptions in CuCl clusters formed in KCI crystals. As these clusters are small in size, the optical properties of excitons in them are slightly different from those in bulk CuCl crystals: (1) the energy positions of exciton absorption peaks are 20 meV higher than those of bulk CuCl excitons, (2) the absorption lines show relatively large width and asymmetric shapes, (3) the Stokes shifts are large, and (4) there are no bound excitons. These results are discussed as the characterirtic feature of excitons due to the small size of a CuCl micro‐crystal.
Phenomenological Description of Aggregates of Exchange‐Bonded Paramagnetic Ions in Crystalline Environment. I. Effective Hamiltonian Formalism
Authors: Kurzyński M.
Published: July 2007

A general effective Hamiltonian formalism is proposed and applied to the phenomenological description of energetic structure of aggregates consisting of several exchange‐bonded paramagnetic ions in a crystalline environment. The low‐lying energetic multiplets of such a system, resulting from isotropic exchange (superexchange) interaction, split under the further interactions and, for a variety of ions, can be described in terms of effective spin Hamiltonians. Such a description is highly useful from the electron paramagnetic resonance point of view, in which the transitions inside multiplets are observed. The microscopic model for the phenomenologic constants of these Hamiltonians is given in the second order of perturbation treatment, and the case of ions with orbitally nondegenerate ground crystal term is discussed in some detail.
Propagation of Electromagnetic Waves in Semiconductor Plasmas under Crossed Steady Electric and Magnetic Fields
Authors: Brauer M.
Published: July 2007

The propagation of low frequency electromagnetic waves in single component infinite semiconductor plasmas under the combined action of dc crossed electric and magnetic fields, E0, B0, is investigated using the kinetic method. Using a modified form of the well‐known relaxation‐time approximation given by Bhatnagar for the collision term in the Boltzmann equation the dielectric tensor is calculated for propagation vectors q lying in the plane formed by E0 and B0. It is shown that only the refined relaxation‐time assumption can lead to consistent results concerning the conservation of particles. Finally the dispersion relation resulting from the dielectric tensor is discussed for q ‖ B0 and q ‖ E0.
Intra‐ and Interatomic Electron Correlation in an Orbitally Nondegenerate Narrow Band
Authors: Heiner E. , Schneider J. , Haubenreisser W.
Published: July 2007

The influence of interatomic interaction between the electrons on the occurrence of ferro‐magnetism in a nondegenerate narrow band is investigated. Using the Green's function technique it is shown that the unperturbed band splits due to the interatomic interaction into spin‐polarized sub‐bands. For a simple cubic tight‐binding band the ground state energy for the paramagnetic and ferromagnetic configurations are calculated and compared. It is found that in the strongly correlated limit and for sufficiently large number of electrons per atom the ferromagnetic state is stable.
Electrical and Optical Studies of Semiconducting CdF2:In Crystals
Authors: Kunze I. , Ulrici W.
Published: July 2007

Optical absorption and electrical conductivity of In‐doped CdF2 crystals have been measured at various temperatures. The results confirm the assumption of Trautweiler et al. [12] that the In3+ centers offer two localization possibilities t o the electrons introduced by annealing the crystals in Cd vapour: an atomic ground state (In2+) and a hydrogenic ground state (In3+ :e−). From an analysis of the results 0.055 eV is obtained for the energy distance between the two ground states and approximate values for their positions below the bottom of the conduction band. The latter depend on the degree of conversion. At temperatures below 50 K the occupation ratio between the two ground states can be strongly altered permanently in favour of the In3+ : e− donor states by irradiation with visible or UV light producing a photo‐induced conductivity and absorption. From measurements of the dependence of the photo‐induced conductivity below 50 K on temperature, irradiation time, and intensity, some properties of the In3+:e− donors are deduced.
On the Derivation and the Properties of an Effective Hamiltonian for a Polaron in an External Magnetic Field
Authors: Röseler J. , Henneberger K. , Fischbeck H. J.
Published: July 2007

The lowest eigenvalues of Frohlich's Hamiltonian with a magnetic field are approximately determined by solving the eigenvalue problem of an effective Hamiltonian for a polaron in a magnetic field. The effective Hamiltonian can be diagonalized without approximations, and the eigenvalue problem is reduced to the determination of the energy‐momentum relation of a free polaron. In the present paper it is shown in which manner the effective Hamiltonian can be derived from Frohlich's Hamiltonian with a magnetic field. The approximations being necessary for this derivation are justified on the condition ωc/ω0 = |e| · |B|/m c ω0 ≪ 1 in the case of weak and intermediate coupling. Furthermore, the properties of the eigen‐value spectrum belonging to the effective Hamiltonian are discussed.
Infrared Reflectivity and Raman Scattering of Mg2SiO4 Single Crystal
Authors: Servoin J. L. , Piriou B.
Published: July 2007

The lattice vibrations of Mg2SiO4 forsterite single crystal have been studied using infrared and Raman techniques. The site symmetry method allows to study the degeneracy lifting of the silicate ion vibrational levels and to assign the modes of vibration (45 infrared active modes and 36 Raman active modes), as internal or external. The analysis of the reflectivity data between 150 and 10000 cm−l by classical dispersion theory allows to find the oscillator parameter values. Most of the infrared lattice bands could be identified. All the predicted modes have been observed in the Raman spectra and the vl1 v2, and v4 lines have been identified. For some modes, specially in the low frequency range, the difference between internal and external modes is not obvious, taking into account the SiO4 tetrahedra deformation.
Theory of Electron Galvanomagnetics in Crystals: Hall Effect in Semiconductors and Semimetals
Authors: Rode D. L.
Published: July 2007

Using the contraction mapping principle, we show how iterated solutions of the classical Boltzmann equation with Fermi statistics are obtained. The mathematical formalism allows manipulation of the Boltzmann equation separately from considerations of particular electron scattering mechanisms and proves existence, uniqueness, and convergence properties of the solutions. This technique potentially applies to a broad range of electronic transport problems, but in particular, Hall effects and magnetoresistance are considered in group [V, III‐V, and II‐VI semiconductors and semimetals. These results show the corrections between Hall mobility, magnetoresistance mobility, and drift mobility, e.g. Hall mobility differs from drift mobility by less than 30% for all the crystals considered. In addition, measurements of the Hall factor (rH) in GaAs from 0.1 to 90 kG are compared with theory and agreement is found to 4%.
Optical Study of Ni2+ and Co2+ Ions in Garnets with Only Octahedral Magnetic Sublattice
Authors: Sokolov V. I. , Szymczak H. , Wardzynski W.
Published: July 2007

The optical absorption spectra of two garnets, NaCa2Ni2V2O12 and NaCa2Co2V3O12 have been measured at various temperatures between 4.2 °K and room temperature. The data have been fitted to a crystal field theory which includes spin‐orbit coupling and the trigonal field of the S6 site. The parameters of the trigonal field for a best fit for both crystals were v = 2100 cm−1 and v′ = −500 cm−1. A discussion of the results refered to the magnetic properties of garnets is given.
Experimental Study of a New Mg Centre in NaCl and KCl
Authors: Voszka R. , Watterich A.
Published: July 2007

In OH‐free NaCl(Mg) and KCl(Mg) electron centres are produced by electrolytical coloration and “decoloration”. It is most probable that the centre is a Mg atom in a vacancy pair. The oscillator strength of its absorption band is determined.
Magnetic Ordering in NdS
Authors: Golosovskii I. V. , Plakhty V. P.
Published: July 2007

A neutron diffraction study has been made of powdered NdS samples at 4.2, 27, and 293 °K. A noncollinear magnetic structure with two propagation vectors has been revealed. All spins lying in a {110} plane are collinear and arranged antiferromagnetically. From plane to plane the spin direction rotates by a right angle. The possible and really observed spin configurations are discussed within the, framework of the molecular field method.
Effects of Diffusion on Energy Transfer by Resonance in Solids
Authors: Golubov S. I. , Konobeev Yu. V.
Published: July 2007

A theory of sensitized luminescence in solids accounting effects of electronic excitation diffusion on energy transfer between molecules is developed. Using an exact averaging procedure the quantum yield of acceptor luminescence is calculated in the case of small impuritity concentrations.
On the Soft X‐Ray Appearance Potential Spectrum of Graphite and Less Well‐ordered Carbons
Authors: Bradshaw A. M. , Menzel D.
Published: July 2007

The technique of soft X‐ray appearance potential spectroscopy (APS) has been applied to pyrolytic graphite and other carbons. The prominent, sharp satellites in the spectrum are assigned, in agreement with Houston and Park, to a coupling of the CK core hole excitation to strong plasmon emission (first and higher orders of the 7 eV plasmon and first order of the 27 eV plasmon). However, the peak separations of the higher order 7 eV plasmons are found to be not constant. The broad loss peaks are assigned to coupling with interband transitions. Increasing the degree of disorder of the carbon markedly affects the spectrum. The results are discussed in the light of more direct experimental investigations of electronic excitations in graphite, and of recent theoretical considerations of plasmon coupling in APS.
The Mobility of Grown‐in Dislocations in the Intermetallic Compound MgZn2. III. Dependence of Basal Slip on Chemical Composition within the Homogeneity Range and on Temperature
Authors: Kubsch H. , Paufler P. , Schulze G. E. R.
Published: July 2007

The dislocation velocity, , in basal slip shows a maximum within the range of homogeneity of the Laves phase MgZn2. The temperature dependence of can be described by an thermally activated mechanism with an activation enthalpy of (1.7 ± 0.3) eV in agreement with macroscopic deformation experiments.
Localized Modes Due to Impurity Pairs in a Heisenberg Ferromagnet
Authors: Tucker J. W. , Walls J.
Published: July 2007

The energy of localized modes due to a nearest‐neighbour pair defect in a simple cubic Heisenberg ferromagnet is investigated. Particular attention is paid to the p‐symmetry mode oriented along the direction of the inter‐impurity bond since for this mode the energy depends on the impurity‐impurity exchange constant. It is found that the results of a two‐parameter variational calculation, along the lines suggested by Tucker and Kilby, are in very good agreement with those of an exact treatment.
Quantum‐Statistical Theory of Light Propagation in Crystals
Authors: Davydov A. S. , Serikov A. A.
Published: July 2007

The density matrix method is used to investigate the passage of light (the frequency of which corresponds to the exciton excitation) through an optically isotropic crystal, The investigation is made in a wave packet representation which enables one to observe the spatial motion of photons and excitons. The radiationless energy transfer from an electronic excitation to a dissipative subsystem is taken into account.
Diffusion of Divalent Impurities from the Volume to the Surface in NaCl Crystals
Authors: Yacamán M. J.
Published: July 2007

In NaCl: Ca2+, NaCl: Cd2+, and NaCl: Cu2+ crystals heated in air at temperatures of 400 to 450 °C during 6 to 12 h the impurities tend to migrate to the crystal surface. There the impurities react with air and a new phase is produced on the NaCl surface. Both, transmission and scanning electron microscopy is used to study the structure and characteristics of the new phase. It is found that for NaCl: Ca2+ and NaCl: Cd2+ the new phase corresponds to CaO and CdO, respectively. The phenomenon is discussed using the theory of a double electrical layer that arises in the vicinity of the surface in ionic crystals. However this theory can not explain it completely. Diffusion to the surface must be taken into account in every work that involves a thermal treatment of a ionic crystal containing divalent impurities.
Lattice Dynamics of Crystals of the Calcite Structure II. Dispersion Curves and Phonon Densities
Authors: Plihal M.
Published: July 2007

Phonon dispersion curves of the external and internal modes of trigonal CaCO3 have been calculated for some directions in the Brillouin zone. The one‐phonon density, the two‐phonon density, and the specific heat have also been determined. The results are compared with available experimental data on the specific heat and the two‐phonon absorption spectrum. The frequencies of the external modes have been calculated neglecting the internal degrees of freedom of the complex ions CO−3−, using a shell model which takes the short range interaction of nearest neighbours, the Coulomb interaction, and the polarizability of the complex ions into account. The variation of the frequencies of the internal modes with wave vector y has been determined by considering only the coupling of the transition dipoles of the complex ions. For this purpose the dipole‐dipole interaction formalism normally used at wave vector y = 0 to calculate the Davydov splitting of the internal modes is extended to wave vectors |y| ≠ 0.
Integral and Differential Equations of the Dynamical Theory of Electron Diffraction within a Distribution Formalism
Authors: Lannes A.
Published: July 2007

Using the distributions theory, an integral formulation is developed of electron dynamical theory including boundaries conditions. Two methods are proposed and the results are given in the general case of forward and backward diffusion. A differential formalism is derived in the second, first, and zeroth order. I n the column approximation, the Howie‐Whelan's formulation may be regarded as the convolution inverse of the integral form. It is suggested that this formalism is very useful as a basis for perturbation methods, diffuse scattering, and image formation.
Impurity‐Induced Far Infrared Absorption in CaF2, BaF2, and SrF2 Containing H‐Ions
Authors: Harrington J. A. , Weber R.
Published: July 2007

The far infrared impurity‐induced absorption has been measured for CaF2, BaF2, and SrF2 containing either H−or D−ions. In doped CaF2, only a broad one‐phonon absorption appears. For BaF2 and SrF2, H−and D−ions induce a weak continuous absorption and, in addition, resonance type modes occur at 127.0 and 139.2 cm−1, respectively. The spectra are compared with the calculated density of states curves. It is suggested that the 127 cm−1 line in BaF2 is the low lying T2 resonance mode of the H−ion.
The Kinetics of F–F′‐Transitions in Coloured Alkali Halides
Authors: Hoffmann H. J. , Stöckma F. , Tödheide‐Haupt U.
Published: July 2007

Reaction and rate equations of the kinetics of photoconductivity of the F–F′‐conversion in alkali halide crystals arc discussed and integrated for the experimentally interesting cases.
Derivation of an Expression for the Correlation Factor for Diffusion by Simple Arguments
Authors: Bakker H.
Published: July 2007

Equations for the correlation factors for diffusion via single vacancies and via more complex defects are derived by simple arguments without making use of the random‐walk expression for ƒ.
Anisotropic Chemical Shift of Nuclear Magnetic Resonance in Single Crystals of Tellurium and Selenium. I. Experimental Studies
Authors: Koma A.
Published: July 2007

Nuclear magnetic resonances of 125Te and 77Se have been observed in single crystals of tellurium and selenium, respectively. They consist of three resonance lines, each of which shows a highly anisotropic chemical shift. The anisotropy of the chemical shift has been investigated in detail. Especially in the case of tellurium the difference in the anisotropy has been made clear between the right and the left‐handed crystals. The chemical shift tensors of tellurium and selenium are determined from the experimental results.
Valence Band Structure of CdTe
Authors: Čápek V. , Zimmerman K. , Koňák Č. , Popova M. , Polivka P.
Published: July 2007

The valence band structure near the I‐point in CdTe has been investigated. Experimentally, the absorption curves obtained for p‐type CdTe doped with Te, P, As are presented, discussed, and compared with theory [18]. This comparison yields the ratios of the hole effective masses m/m; = 7.5 and m/m = 5. The theoretical valence band structure near the r‐point is fitted to correspond with the optical absorption measurements.
On the influence of electronic exchange and correlation on pseudopotential calculations
Authors: Hafner J.
Published: July 2007

The influence of different local statistical exchange and correlation approximations for the core‐core and conduction band‐core interactions on a calculation of binding energies and stable crystal structures within Harrison's first principles pseudopotential formalism is discussed. It is shown that with a convenient choice of the exchange parameters it is possible to calculate the binding energies with high accuracy and to predict the correct crystal structure in each case. These exchange parameters agree well with those yielding the best form factors and effective interionic pair potentials in a previous paper [1].
On positronium‐like state in F‐centres of potassium chloride
Authors: Hautojárvi P. , Nieminen R. , Jauho P.
Published: July 2007

The trapping of positrons in F‐colour centres in KCl is investigated in some model calculations. The wave function of the bound state of the positron and F‐centre is determined in various approximations with and without the electron‐positron correlation, and annihilation parameters are evaluated using this function. It is suggested that the screening of the electron‐positron interaction in the bulk of the crystal has a strong influence on the trapping by promoting the localization of a neutral positronium‐like state in the vacant anion site. In this case agreement with experiments is obtained.
Thermoelectric power of n‐type Bi2Se8 in strong transverse magnetic fields
Authors: Middendorff A. , Köhler H. , Landwehr G.
Published: July 2007

From the saturation values of the magneto‐Seebeck coefficient and Hall constant in strong transverse magnetic fields the density of states and the chemical potential were determined as a function of electron concentration in the conduction band of Bi2Se3 single crystals. The results confirm in an independent manner the non‐parabolic one‐valley model for the lowest conduction band, which was derived from Shubnikov‐de Haas investigations.
On the description of multiple scattering processes in amorphous and crystalline semiconductors
Authors: Maschke K. , Overhof H. , Thomas P.
Published: July 2007

The electronic properties of amorphous semiconductors can be calculated from the configurational averaged Green's function. In this method the atomic structure of the system is incorporated into the scattering correlation function (SCF). To get a tractable form, the SCF is factorized into products of two‐center SCF's. The approximations involved in this decomposition are discussed. A comparison is made to similar approximations common in KKR energy band calculations for crystals (Ewald's summation technique). The connection with scattering by “frozen” hot crystals is demonstrated. For the amorphous case, it is shown that at least one and two‐center scattering contributions to the Green's function can be treated correctly.
On non‐local extensions of the ginsburg‐landau theory for type‐II superconductors valid at all temperatures
Authors: Brandt E. H.
Published: July 2007

The free energy of a superconductor is expressed as a two‐dimensional infinite series which may be interpreted in three different ways: as an expansion with respect to a) the gap parameter, b) the gauge‐invariant gradient, or c) the temperature difference Tc‐ T. By partial summation of the series we obtain a simple non‐local functional containing as special cases the extended Ginsburg‐Landau theory (Neumann‐Tewordt) and the nonlocal electrodynamics (Pippard, Gorkov). For small magnetization it gives the exact gap (Helfand‐Werthamer) and the magnetic field distribution (Delrieu) in the entire temperature range 0≦ T < TcIn the general case the functional is expected to be a good approximation for T > 0.7 Tc By numerical minimization of the functional one may hope to solve fairly complicated problems such as, e.g., the perturbed flux‐line lattice.
Anisotropic chemical shift of nuclear magnetic resonance in single crystals of tellurium and selenium
Authors: Koma A.
Published: July 2007

The 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

The 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

F* 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

The 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

A 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

The 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

Using 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

One 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

The 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

The 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

The 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

An 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

Galvanomagnetic 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

A 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

Measurements 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

Electron 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.
Hori‐Matsuda Method and Relativistic Saxon‐Hutner Theorem
Authors: Subramanian R.
Published: July 2007

A new proof of the Saxon‐Hutner theorem in the relativistic domain is presented, employing the Hori‐Matsuda method.
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

The 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

The 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

It 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

Förster'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

X‐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

Spectra 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

The 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

The 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

Earlier 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

The 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

Measurements 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

A 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.