bluuebunny/crossref_metadata_2025_split huggingface dataset:
Herzberg‐Teller Interaction and Vibronic Coupling in Molecular Crystals IV. Interaction of Two Exciton States via a Totally Symmetric Vibration
Authors: Zgierski M. Z. 
Published: July 2007

AbstractThe three‐level model of a molecular crystal with two exciton bands coupled by a totally symmetric vibration according to the Herzberg‐Teller interaction mechanism is discussed. The absorption spectrum to a lower, narrow exciton band is investigated. The formula for distribution of intensity among different vibronic lines is obtained and discussed. It is shown that this distribution depends not only on the Herzberg‐Teller coupling constant, the distortion parameter in the lower band, and the ratio of the intensities of the two coupled bands, but it is also very sensitive to the difference between distortion parameters in the two bands, as well as to the angle between the transition dipole moments in the molecule. The one‐phonon region of the spectrum is discussed with respect to the existence of vibronic exciton and dissociated exciton—phonon states. General conditions for the existence of different numbers of vibronic excitons in this region of a spectrum are obtained. The changes introduced by the Herzberg‐Teller coupling in the one‐phonon region of the spectrum are illustrated by an example. The general formulas obtained reduce for zero distortion parameters to the formulas describing the coupling of two exciton states by a non‐totally symmetric coordinate, and for zero Herzberg‐Teller coupling constant they reduce to the formulas describing the simple vibronic coupling in a molecular crystal.
Propriétés thermodynamiques et optiques des oxydes alcalino‐terreux
Authors: Galtier M. , Montaner A. 
Published: July 2007

AbstractUn modèle dynamique décrivant les forces à long rayon d'action sous forme d'interactions dipôles‐dipôles, et les forces à court rayon d'action à l'aide d'intéractions à trois corps, est appliqué aux oxydes alcalino‐terreux de structure NaCl. Les résultats expérimentaux existants (chaleurs spécifiques, absorption infrarouge, diffusion Raman et diffraction des neutrons) sont ensuite comparés aux prévisions théoriques déduites des densités d'état à un et deux phonons. L'accord assez satisfaisant permet de conclure sur la validité du modèle et d'interpréter les spectres optiques.
Deformation Potentials of the Energy Gaps in Trigonal Selenium
Authors: Lingelbach W. , Weiser G. 
Published: July 2007

AbstractThe stress induced shift of the indirect gaps and of some direct transitions at different points of the Brillouin zone has been measured from the dependence of electroreflectance and electroabsorption spectra on uniaxial stress parallel and perpendicular c. Macroscopic and microscopic (generalized) deformation potentials have been derived for the corresponding energy gaps. The results are compared with the electron charge distribution of valence band and conduction band states. The spectra show evidence for a splitting of the conduction band at H by stress perpendicular c.
Theory of Rb87 Nuclear‐Spin Relaxation in RbMnF3 above the Néel Temperature
Authors: Tucker J. W. 
Published: July 2007

AbstractThe temperature dependence of the Rb87 nuclear‐spin relaxation in RbMnF3 is calculated in the paramagnetic phase. The relaxation rate is expressed as a product of the wavevector‐dependent susceptibility and the wavevector‐frequency‐dependent relaxation‐shape function for which a semi‐phenomenological representation, based on the method of moments, is used. The temperature dependence, which is contained in the susceptibility and the moments of the relaxation‐shape function, is estimated on the basis of a spherical model. The results are compared with the recent experimental measurements of Borsa and Bosco.
EPR of BaCl2:Eu2+ and BaCl2:Gd3+
Authors: Wever H. , Den Hartog H. W. 
Published: July 2007

AbstractParamagnetic resonance spectra of Eu2+ and Gd3+ ions at Ba2+ sites in BaCl2 were measured at room temperature and liquid nitrogen temperature. There was little difference between the spectra of the same ion taken at the different temperatures. The important parameters in the spin Hamiltonian are: Eu2+(300 °K): b=24 × 10−4 cm−1, b = −450 × × 10−4cm−1; Eu2+ (77 °K): b = 33 × 10−4cm−1, b = −467 × 10−4cm−1; Gd3+(77 °K): b = 84 × 10−4cm−1, b = −461 × 10−4cm−1; g =1.992. The corresponding theoretical values are: Eu2+: b = 104 × 10−4cm−1, b = −357 × 10−4cm−1; Gd3+: b =99 × × 10−4cm−1, b = −334 × 10−4cm−1; g = 1.993.
Spectroscopic Studies of Tl+ Centres in KF
Authors: Mayer U. , Schmid D. , Seidel H. 
Published: July 2007

AbstractThe absorption and emission spectra of KF single crystals containing Tl+ ions at low concentrations are studied in the UV and vacuum UV spectral range between 12 and 300 K. Four absorption bands (A, B, C, and D), which are characteristic for the TL+ impurity centre were observed, but only two emission bands could be detected. These results are compared to the results on the hitherto investigated TL+ centres.
Phase Transitions in a Rigorously Solvable Model
Authors: Georgiev G. , Milev N. , Gocheva S. 
Published: July 2007

AbstractPhase transitions in dependence on temperature, T, and applied magnetic field are determined in a one‐dimensional spin X‐Y‐model with partial account of next‐nearest neighbour interaction (characterized by the parameter α) at final N and N → ∞, where N is the spin number. The analytic behaviour of the fundamental thermodynamic characteristics of the system is investigated at high and low temperatures in different regions of the (α, H) plane. The same calculations are made at N → ∞ for the X‐Y‐Model, containing partial X‐Y interaction between all of the spins of the system. A dependence between the order of the type‐II phase transitions and the multiplicity of the zeroes of the dispersion function is found for the rigorously solvable model considered. Numerical data are given for the magnetic susceptibility and the specific heat of the system at arbitrary values of T.
Theory of Energy Migration in Doped Molecular Crystals
Authors: Golubov S. I. , Konobeev Yu. V. 
Published: July 2007

AbstractThe kinetics of diffusion‐controlled sensitized luminescence is analyzed in molecular crystals in which a small concentration of acceptor molecules is embedded. It is shown that if the probability of the excitation energy transfer is determined by the dipole‐dipole interaction between donor and acceptor molecules the host luminescence decay law n(t) changes from Förster's law (n(t) ∼ exp (−α \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt t $\end{document}))to an exponential one (n(t) ∼ exp (−βt)) with the electronic excitation diffusion length increasing from 0 to l ≧ 5R0, where R0, is the effective radius of the energy transfer by resonance. In addition the host luminescence decay law n(t) is found with account for the electronic excitation migration over donor molecules in molecular crystal doped by donor and acceptor impurities of small concentrations. It is shown that for such a system the function n(t) cannot be derived from any appropriate diffusion equation.
Piezoelectric Scattering in Semiconductors under Longitudinal Quantizing Magnetic Field
Authors: Basu P. K. 
Published: July 2007

AbstractThe possibility of defining a relaxation time for piezoelectric scattering in the presence of a longitudinal quantizing magnetic field is examined. The mobility calculated from the relaxation time, existing only at the extreme quantum limit, is compared with the acoustic scattering mobility and also with the piezoelectric mobility calculated with the displaced Maxwellian distribution.
Galvanomagnetic Effects in Alloys in Quantizing Magnetic Fields
Authors: Středa P. , Smrčka L. 
Published: July 2007

AbstractThe one‐electron version of the Kubo formula is used to derive the components of the conductivity tensor for a simple model of a binary alloy characterized by the parabolic dispersion law and the zero range of potentials of scatterers. The configurational averaging is performed using the coherent potential approximation and therefore the final formulae apply for a broad range of alloy parameters. They are shown to yield proper high field and low field limits and to agree, for appropriate values of parameters, with the quasi‐classical approximation.
ESR Investigation of Localized States in Amorphous Boron
Authors: Gewinner G. , Kubler L. , Koulmann J. J. , Jaéglé A. 
Published: July 2007

AbstractESR signals of amorphous boron samples are observed between 1.5 and 800 °K. The samples are found to yield a large signal at g = 2.0028±2 × 10−4 corresponding to a spin density as high as 5 × 1019 spins/cm3. This ESR line is attributed to a true bulk effect and the study of line width versus temperature gives direct evidence for the localized nature of the involved electronic states. Diffusion of carriers in these states is observed and the hopping frequency versus temperature determined. The contribution of these singly occupied states to the electrical conductivity is estimated to σ ≈ 10−4 (ω cm)−1 at 300 °K. Spin‐lattice relaxation times are measured below 77 °K and a consistent model for line width and line shape proposed. The intrinsic or extrinsic nature of the observed line is also discussed.
On the Mechanism of Radiative Annihilation of Excitonic Molecules in CdS Single Crystals at Different Excitation Intensities
Authors: Voigt J. , Mir F. , Kehrberg G. 
Published: July 2007

AbstractAn investigation is made of the correlation of excitonic molecule emission in CdS single crystals under stationary (high pressure mercury lamp) and pulse laser (400 kW N2 laser) excitation in a very extended range of excitation intensities at 1.8 K. It is shown that the discrepancies existing in the literature up to now due to the appearance of different molecule lines under different excitation conditions can be resolved assuming a model for molecule annihilation which involves both Γ6‐excitons and Γ5‐polaritons.
Surface Energies of Simple Metals (II)
Authors: Hietschold M. , Paasch G. , Ziesche P. 
Published: July 2007

AbstractThe simple model for the electronic structure of surfaces of simple metals given in part I is used to investigate three problems: 1. possibility and physical background for systematization of surface energies, 2. anisotropy and 3. temperature dependence of surface energies, dipole barriers, and relative lattice relaxation. It is shown that the surface energies show a characteristic dependence from the density parameter rs and from the valency Z which seems to be most pronounced for valencies one and two. The model is reliable for closed‐packed surfaces up to a value of the surface packing fraction of 0.7. The static electronic part of surface energy considered here yields only a very small temperature dependence.
Influence of Structure Fluctuations on the Curie Temperature and Susceptibility of Amorphous Ferromagnets
Authors: Schreiber J. , Kobe S. , Handrich K. , Richter J. 
Published: July 2007

AbstractWithin the stochastical lattice model of amorphous Heisenberg and Ising ferromagnets the influence is investigated of fluctuations of exchange integrals on the Curie temperature Tc for different spin values and numbers of nearest neighbours using the constant coupling approximation and a high temperature expansion for the susceptibility χ. TC and χ are always reduced by the fluctuations of the exchange integrals within the Heisenberg model for any spin and within the Ising model for S = 1/2 (for S ≧ 1 the situation is more complicated). The structure fluctuations lead to two opposite effects: 1. weakening of magnetic long range order and 2. increase of magnetic clustering (short range order).
Positronenannihilation in Ionenkristallen I. Zur Annihilation in F‐Zentren
Authors: Brümmer O. , Brauer O. , Andrejtscheff V. 
Published: July 2007

AbstractUntersucht wurde die Halbwertsbreite (FWHM) des von F‐Zentren stammenden Beitrages c(θ) in der Winkelkorrelation von additiv verfärbtem KCl, KBr und KJ. In der Literatur vorhergesagte Änderungen dieser FWHM können nur teilweise bestätigt werden. Die durch F‐Zentren bewirkte Verengung der Winkelkorrelation läßt sich auch durch Messung der Dopplerverbreiterung der Annihilationslinie mit einem Ge(Li)‐Detektor nachweisen.
Polarons in the Degenerate‐Band Case
Authors: Trebin H.‐R. , Rössler U. 
Published: July 2007

AbstractThe Fröhlich polaron model has been applied so far only to the case of electron states with simple parabolic bands. An extension is given of the polaron concept to more complex states as e.g. the valence bands in zincblende‐type crystals. The band structure is described as in the spherical model of Baldereschi and Lipari. The electron‐phonon interaction in the Fröhlich approximation is treated by perturbation theory. Besides a renormalization of the band parameters the electron‐phonon coupling results in a partial mixing of electronic states from different subbands, i.e. of light and heavy holes.
The Green Function of Ideal Crystals and Electronic Properties of Localized Defects in Metals
Authors: Lehmann G. 
Published: July 2007

AbstractThe Green function Gc of an ideal crystal whose potential is treated in muffin‐tin approximation is investigated by means of the multiple scattering formalism. The evaluation of Gc is possible by transforming the equation for Gc corresponding to the transformation used in deriving energy‐independent H‐NFE‐TB schemes for band structure calculations from the KKR equation. Gc is used for the investigation of electronic properties of localized defects representing a perturbation of the muffin‐tin potential at several lattice sites without lattice distortion. The wave function in the neighbourhood of the defect, the Friedel sum rule and the scattering properties of strongly localized defects are studied in detail.
Pseudopotential Method and Thermodynamics of Alloys I. Disordered Solid Solutions
Authors: Fuks D. L. , Zhorovkov M. F. , Panin V. E. 
Published: July 2007

AbstractA pseudopotential method is applied to construct the thermodynamical potential of a binary solid solution. The state equation is solved for a disordered solid solution. The mixing energy is shown to be a complex concentration function depending on the type of the crystalline lattice. The influence of a uniform compression on the solubility of alloy components is investigated, the enthalpy and work function under adiabatic compression are determined. The numerical calculations are carried out for MgZn and BaCd systems.
A Theory of Transport Phenomena in Disordered Systems in Presence of Arbitrary Electrical Fields
Authors: Böttger H. , Beyksin V. V. 
Published: July 2007

AbstractA theory is developed for the current in a disordered one‐band tight‐binding system holding for arbitrary electrical fields and for electron–phonon interaction of arbitrary strength. The current is expressed by the density matrix for which the equation of motion is derived. In case of localized states, i. e. in case of hopping conductivity, this equation takes the form of a rate equation with memory.
Selection Rules for the Spinel Structure
Authors: Gashimzade F. M. , Rustamov K. A. 
Published: July 2007

AbstractSelection rules are presented for indirect optical, intervalley transitions, and two‐phonon and three‐phonon processes for the points \documentclass{article}\pagestyle{empty}\begin{document}$ \Gamma (000),\,{\rm X}\frac{{2\pi }}{a}(100),\,{\rm L}\frac{\pi }{a}(111),\,{\rm W}\frac{\pi }{a}(120) $\end{document} of the Brillouin zone. The results obtained may be used for any compound of the symmetry group O.
Quasi‐Ionic Model for Image Potentials in Metals
Authors: Waber J. T. , Rao P. V. S. 
Published: July 2007

AbstractA quasi‐ionic model has to deal with the image potential induced in a metal by an ion when the latter is a few interatomic distances away from the surface. At such proximity, the potential is „granular”︁ and the assumptions of the classical treatment are not valid. The proposed model emphasizes the local surface structure and treats metal as the ion cores which are incompletely screened by the conduction electrons. A parameter δ is introduced to take account of the greater shielding of ion‐cores in the second layer. This heuristic model has been used to estimate the splitting of orbital levels in the approaching ion and to determine how the splitting depends on δ. It has also been used to calculate the attraction energy between the ion and the metal substrate. The results are compared with results of other models, classical image, Gomer‐Swanson and Newns. Separation energy of an electron from the metal surface calculated for alkalis using the quasi‐ionic model is shown to have a dependence on lattice parameter similar to that exhibited by the barrier potential.
Die dynamische Suszeptibilität eines van Vleck‐Ions
Authors: Schotte K.‐D. 
Published: July 2007

AbstractEs wird die frequenzabhängige Suszeptibilität eines van Vleck‐Ions berechnet. Es wird die Mori‐Methode benutzt, um eine Resolvente zu berechnen. Aus methodischen Gründen wird die Rechnung auf zwei verschiedene Arten durchgeführt. Die Übereinstimmung der Ergebnisse wird in Störungstheorie zweiter Ordnung im Austauschintegral nachgeprüft. Danach können Schlüsse über die Vorzüge und Grenzen dieser Methoden gezogen werden.
Solid State Broadening in the Auger Spectra of Rare Gases
Authors: Nuttall J. D. , Gallon T. E. 
Published: July 2007

AbstractThe L23MM Auger spectrum of solid argon, the M45NN Auger spectrum of solid krypton, and the M45N45N45 Auger spectrum of solid xenon have been measured at high resolution. The data has been compared with a computer simulation in which solid data is represented by the gas phase data with broadened lines. The solid phase breadths are 3.6, 4.6, and 2.9 eV, respectively. Characteristic energy loss spectra of the three solids are also reported.
Superconducting Nuclei at Inhomogeneities in Type‐II Superconductors above Hc2
Authors: Brandt E. H. 
Published: July 2007

AbstractSmall inhomogeneities in a type‐II superconductor may lead to the nucleation of superconducting regions with diameter ≈︁ ξ (coherence length) at fields above Hc2. Energy, order parameter, and microscopic field of nuclei caused by one‐, two‐, and three‐dimensional model inhomogeneities are calculated from the Ginsburg‐Landau theory. The nuclei may be larger or smaller than the inhomogeneities; their extension parallel to the field is always ≫ ξ. The influence of these nuclei on the flux line lattice below Hc2 and on the volume pinning force is discussed. Three‐dimensional calculations yield a rapid increase of the flux‐line stiffness as H → Hc2.
Lattice Dynamics of Several ANBS–N Compounds Having the Zincblende Structure. I. Deformable‐Bond Approximation
Authors: Kunc K. , Balkanski M. , Nusimovici M. A. 
Published: July 2007

AbstractThe formalism of a simplified version of the deformation‐dipole model, called deformable‐bond approximation, is presented and the physical meaning of this lattice‐dynamical model is discussed for the case of crystals having the zincblende structure. Various simplifications used in the description of short‐range forces are also summarized.
Galvanomagnetic and Microwave Transport Coefficients of n‐InSb at 77 K
Authors: Nag B. R. , Dutta G. M. 
Published: July 2007

AbstractTransverse magnetoresistance, Hall factor, and microwave complex conductivity of n‐InSb are studied for a lattice temperature of 77 °K using an iteration method to solve the Boltzmann equation. All the relevant scattering mechanisms, band non‐parabolicity, and overlap integrals, and also the effects of electron screening have been included in the analysis. Computed results are presented showing the variation of the coefficients with magnetic field and also for different microwave frequencies. Values of galvanomagnetic coefficients calculated with an acoustic phonon deformation potential of about 20 eV are found to agree well with experiments. The microwave conductivity is also found to agree with experiments to within ten percent.
Reduction Symmetry Method in Crystal Field Theory (II)
Authors: Kustov E. F. 
Published: July 2007

AbstractAn extension is given of the reduction symmetry method for many‐electron configurations of ions in crystals. The subgroup coefficients for the product of the irreducible representations are introduced and the main symmetry properties of this coefficients are received. A few examples for application of this method for the calculation of the matrix elements of themany‐electron tensor operators are given. In addition a formula is given for the intensity of the electric dipole transitions between Stark levels. The Judd‐Ofelt formula is a special case of this formula (when Stark splitting is much less then kT).
Fundamental Reflection of CdxHg1−xTe Crystals in the 1.9 to 3.1 eV Energy Range
Authors: Kisiel A. , Podgórny M. , Rodzik A. , Giriat W. 
Published: July 2007

AbstractMeasurements of positions of the E1 and El Δ1fundamental reflection maxima for Cdx Hg1−x;Te samples within the composition range from x  0 to 0.2 were performed. The relations E1(x) and [E1 + Δ1]J (x) were found in this composition range. The previous results referring to the same substances which had been questioned by otherinvestigators were fully confirmed. An analysis of all the results known until recently is presented.
The Calculation of the Superexchange Interaction Parameters in EuO
Authors: Falkovskaya L. D. , Sapozhnikov V. A. 
Published: July 2007

AbstractThe superexchange interaction parameter for the next‐nearest neighbouring pair of magnetic ions in EuO, connected by the 180° exchange interaction has been calculated according to the method of configuration interaction. The effect of the non‐homogeneous potential created by the crystal ions in the region of the chosen complex “metal—ligand‐metal” is taken into account. Satisfactory agreement with experimental data is obtained.
The Luminescence of ZnO under High One‐ and Two‐Quantum Excitation
Authors: Klingshirn C. 
Published: July 2007

AbstractThe light emission of ZnO under high one‐ and two‐quantumexcitation is investigated in the temperature region from 4 to 300 K. Emission bands are observed which can be attributed to the interactions of free excitons with phonons, free excitons and free electrons, and of bound excitons with phonons and free and bound electrons.
Redetermination of the Dielectric Function of Graphite
Authors: Venghaus H. 
Published: July 2007

AbstractThe dielectric functions ϵ| arid ϵ⟂ of graphite were redetermined by electron energy loss spectroscopy. At 11 eV a strong maximum in Im (ϵ|)is obtained, which is in accordance with earlier electron energy loss experiments, but in disagreement with recent reflectance measurements. A detailed discussion of uncertainties in the energy loss function Im(‐ 1/ϵ|) and in the Kramers‐Kronig analysisconfirms, that the 11 eV‐maximum of Im (ϵ|) is established beyond experimental error. Our ϵ| data are in agreement with earlier electron energy loss and optical results.
Relativistie Augmented Plane Wave Method
Authors: Sarker A. Q. , Islam Taj‐ul 
Published: July 2007

AbstractThe relativistic augmented plane wave method for the determination of electronic energy eigenvalues in crystals is reformulated assuming that boththe upper and lower components of the trial wave functions are continuous at the Slater sphere. A variational expression for the energy, suitable for this kind of wave functions, is given and the matrix elements are evaluated. The resultant secular determinant is shown to be variational when the Dirac energy eigenvalue for the solution of the Dirac radial wave equations, with the atomic muffin‐tin potential within the Slater spheres, is equal to the crystal energy. The theory reduces, in the suitable limits, to that one formulated earlier by Loucks, who assumed that only the upper (large) components of the wave functions are matched leaving the lower (small) components discontinuous at the Slater spheres.