bluuebunny/crossref_metadata_2025 huggingface dataset:
Deformation Potentials of the Energy Gaps in Trigonal Selenium
Authors: Lingelbach W. , Weiser G.
Published: July 2007

The 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

The 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

Paramagnetic 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

The 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

Phase 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

The 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

The 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

The 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

ESR 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

An 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

The 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

Within 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

Untersucht 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

The 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

The 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

A 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

A 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

Selection 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

A 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

Es 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

The 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

Small 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

The 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

Transverse 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

An 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

Measurements 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

The 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.
A Possible Diffusion Mechanism in B.C.C. Metals
Authors: Neumann G. , Tölle V.
Published: July 2007

Morse potential calculations reveal that the anomalous diffusion behaviour in vanadium and chromium possibly can be explained by single vacancy migration via next nearest neighbour places.
The Luminescence of ZnO under High One‐ and Two‐Quantum Excitation
Authors: Klingshirn C.
Published: July 2007

The 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

The 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

The 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.
On Anisotropic Hopping Conduction in Thin Amorphous Germanium Films
Authors: Thomas P.
Published: July 2007

The hopping conductivity is calculated for cases where the number of hops on a conducting path between the electrodes is small. This model is applied to transverse conduction in thin films. Using an isotropic distributionof localized states the conductivity shows an apparent anisotropy, which decreases with film thickness. Reasonable agreement with existing experimental data on transverse hopping conduction in thin amorphous Ge films is obtained. It is concluded that the experimentally observed thickness dependence of the anisotropy of the conductivity is not necessarily related to structural inhomogenieties. By fitting the model to the experimental data a reasonable value is obtained for the orbital size of the localized states withoutmaking any assumptions about the density of states.
X‐Ray‐Induced Vacuum Ultraviolet Luminescence from Neon Crystals
Authors: Schuberth E. , Creuzburg M.
Published: July 2007

In the X‐ray‐induced emission spectrum of solid neon excitonic lines are found at 744.2, 741.8, 738, and 733 Å, and broad peaks at 770 and 890 Å of unrelaxed Ne*2 molecules which arebelieved to originate only from exciton relaxation. We also observed a peculiarly strong temperature dependence of the emission intensity which can becaused by changes in the crystal structure that influence the exciton formation.
Unsymmetrized self‐consistent field approximation for a crystal. I. Linear chain
Authors: Zubov V. I.
Published: July 2007

In the unsymmetrized self‐consistent field approximation the second‐order correction to the Helmholtz free energy is derived by statistical perturbation theory. This correction takes into account the effect of the anharn micity of odd orders on the Helmholtz free energy and on the thermodynamic properties of a crystal. The case of the linear chain is studied in more detail. Special attention is paid to the case of strong anharmonicity.
Structure of phonon sidebands of the ν1 vibration absorption band of the NCO− impurity ion in KI:KCl and KBr:KCl mixed single crystals
Authors: Kondilenko I. I. , Tsyashchenko Yu. P. , Zaporozhets V. M. , Pinkevich I. P.
Published: July 2007

The change of the structure due to local vibrations of the perturbed environment is studied at 10°K in the phonon sideband (PS) of the ν1 vibration absorption band of the impurity ion NCO− in mixed KI:KCl and KBr:KCl lattices. It is found that at increased concentration of Cl− ions beginning from 1 wt% the intensity of a high‐frequency structure is sharply decreased and the number and half‐width of the gap‐local‐vibration bands increase; one fraction of these shifts to the high‐frequency side, the other to the long‐wave one. These effects are explained by the different locations of Cl− near NCO− in KI and KBr matrix lattices and, consequently, changes different in sign of the perturbed force constants of the axial interaction of the ν1 stretching vibration with lattice phonons.
Spin–lattice relaxation of Tl2+ ions in K2SO4 single crystals
Authors: Aminov L. K. , Kurkin I. N. , Silkin N. I. , Shlenkin V. I.
Published: July 2007

Spin‐lattice relaxation times of Tl2+ ions in potassium sulphate single crystals were measured in the temperature range 1.5 to 25°K at a frequency of about 9 GHz by electron spinecho method. The relaxation is due to direct and Raman processes: T1−1 = 1.0 T + 3.3 × 10−5T7. Deviations from the T ∼ T7 dependence at high temperatures allow to estimate the Debye tempeature of K2SO4 to θ = 120°K. The anomalous temperature dependence of the Raman processes is discussed.
The effect of monovalent impurities on KCl low‐temperature specific heat
Authors: Augst G. R. , Kvavadze K. A.
Published: July 2007

An enhancement of the specific heat of KCl crystals doped with Ag+ and Na+ impurities in the temperature range 5 to 30 K is observed. A comparison of the experimental results with the theory shows that in the case of Ag+ the force constants (FC) are weakened by four times. The established quasi‐local frequency (QLF) is close to that found in experiments of infrared absorption. The anomalous increase of the low‐temperature specific heat of the KCl crystals with light substitutional Na+ impurities are ascribed to the vibration of isolated ions with FC weakened 2.5 times. The results of the investigation of KCl crystals with monovalent impurities show that the curve shape of the specific‐heat excess differs in accordance with FC strengthening or weakening.
Binding energy of wannier excitons interacting with LO phonons in anisotropic polar crystals
Authors: Fock H. , Kramer B. , Büttner H.
Published: July 2007

The binding energy of an exciton is calculated taking into account its interaction with optical phonons as well as the anisotropy of the band masses and the dielectric constants. For not too large anisotropies one might approximate the exciton–phonon interaction by an isotropic effective potential with averaged material constants. In II–VI compounds the energies calculated with this isotropic potential agree quite well with the experimental results.
The peculiar behaviour of transport coefficients in the presence of inelastic scattering by optical phonons
Authors: Kravchenko A. F. , Kubálková S. , Morozov B. V. , Polovinkin V. G. , Skok E. M.
Published: July 2007

A method is developed of calculating galvanomagnetic and thermomagnetic coefficients taking into account successively the inelastic scattering on the polarisation potential of the optical phonons together with the elastic scattering mechanisms. In the approximation of weak magnetic fields, the solution of the Boltzmann transport equation is expanded into a power series of the magnetic field by means of perturbation theory. Numerical computations for n‐GaAs with impurity concentrations of 1012 to 1020 cm−3 in the temperature range of 4.2 to 600 °K are made. The results are given as nomograms which are convenient for experimental data processing. The peculiarities of the behaviour of transport coefficients depending on the impurity concentration are analysed.
Spin‐one heisenberg antiferromagnet with biquadratic exchange
Authors: Micnas R.
Published: July 2007

By a Green's function formalism, an analysis of the collective excitation spectrum and thermodynamical properties of a two‐sublattice uniaxial spin‐one Heisenberg magnet with biquadratic exchange is proposed. The case of antiferromagnetic and quadrupolar ordering is analysed in detail. The results are compared to those of the molecular field theory.
ESR spectra reflecting the motion of ligands in NH4Cl:Cu2+
Authors: Watanabe K. , Abe H.
Published: July 2007

The ESR spectra of doped impurity (centre I in the system of NH4Cl:Cu2+) which reflect the motions of ligands were examined. The frequency (4 to 33 GHz) and temperature (4.2 to 400 K) dependence of the Cu2+ spectrum can be explained at low temperatures by the motion of Cl− ions and at around room temperature, including partial fading‐out of its hfs, by that of reorienting NH4+ tetrahedra for which the activation energy is deduced as 2570 K.
ESR of the 6S‐ions in a trigonal field: Fe3+ in α‐LiIO3
Authors: Martirosyan V. O. , Meilman M. L. , Marov I. N. , Zhukov V. V. , Sevinyan K. T.
Published: July 2007

The ESR spectra of Fe3+ impurity ions localized in positions of trigonal symmetry in α‐LiIO3 single crystals are investigated in the X‐ and Q‐bands at room temperature. The parameters of spin‐Hamiltonian are: g33 = g‖ = 2.0014; g11 = g22 = g⊥ = 2.0080; b = 7.831 GHz; b = −0.133 GHz; b = −0.101 GHz. In X‐band the temperature dependence of the spectra in the interval (−180 to +180°C) is studied. Possible models of active centres in α‐LiIO3 crystals are discussed. The results show that a preferable substitution scheme in the investigated crystals is Fe2+ → Li+. A close connection is obtained between the fine structure parameters of the ESR spectra of iron group ions in crystals of lithium iodate, niobate, and tantalate.
Quantum statistics of an electron–hole plasma
Authors: Kraeft W. D. , Kilimann K. , Kremp D.
Published: July 2007

Thermodynamic functions of an electron–hole plasma in an optically excited semiconductor are derived in the nondegenerate region. The existence of excitons and the ionization equilibrium are discussed in the ne–T (electron density–temperature) plane. Above the Mott curve (r0 = 0.84 a0, r0 screening length, a0 Bohr's radius) excitons do not exist. Besides the well known electron–hole droplet phase transition the critical point of which lies above the Mott curve, a region of instability is found below the Mott curve which might correspond to a plasma phase transition (weakly ionized plasma → strongly ionized plasma). Outside of this instability region at the Mott curve a diffuse phase transition (Mott‐transition) exists.
Statistical interpretation of lattice response in amorphous semiconductors
Authors: Mitra S. S. , Tsay Y. F. , Paul D. K. , Bendow B.
Published: July 2007

A statistical approach to the prediction of lattice response in amorphous solids is presented which emphasizes the role of crystalline‐analogue allowed processes. The method is shown to provide a successful interpretation of the principal features of first‐order Raman and IR spectra in tetrahedrally bonded amorphous semiconductors.
Unsymmetrized self‐consistent field approximation for a crystal. II. Three‐dimensional lattice
Authors: Zubov V. I.
Published: July 2007

The corrections to the Helmholtz free energy, equations of state, and thermodynamic properties of strongly anharmonic crystals are considered in the second order of the statistical perturbation theory. As a zeroth‐order approximation the unsymmetrized selfconsistent field method is used. In this case the statistical perturbation theory is applicable up to the melting temperatures. The thermodynamic properties of the solid noble gases are calculated. For Ar the pair potentials of Barker‐Bobetic and of Barker‐Fisher‐Watts were used. For Kr calculations for the Barker‐Bobetic pair potential are made. The threebody forces of Axilrod‐Teller‐Muto are included. For Xe the Lannard‐Jones pair potential is used. The results are compared with Monte Carlo calculations.
N4,5 excitations in metallic cesium. Single‐electron behaviour and collective effects
Authors: Petersen H.
Published: July 2007

The N4,5 excitation spectrum of Cs is measured with yield spectroscopy from the onset of transitions at 77 to 180 eV photon energy. Synchrotron radiation served as a light source. A broad maximum due to delayed transitions to f‐symmetric continuum states is the dominating feature of the spectrum. An indication for existence of localized f‐symmetric final states is found. Above the N4,5 edges at (77.1 ± 0.1) and (79.3 ± 0.1) eV photon energy transitions to p‐symmetric extended metal states are observed. Their qualitative agreement with APW band structure calculations is discussed. EDC's obtained with photon energies 75 and 111 eV are presented. The observed asymmetric shape of the 4d photoemission maxima in the EDC's and the shape of the N5 edge are discussed with regard to the final state interaction theory by Mahan, Nozieres, and de Dominicis.
Millimeter wave and far‐infrared investigation on KDP with asymmetric interferometers
Authors: Gauss K. E. , Happ H. , Rother G.
Published: July 2007

The complex dielectric constant in KDP is determined at ω = 4.62 cm−1 (Δ = 2.16mm) for E ‖ c and E ⊥ c between 100 and 295 K by measurement of the complex reflection coefficient using a Michelson interferometer. At room temperature the complex dielectric function (ω) is determined for E ‖ c and E ⊥ c in the far infrared using an asymmetric Fourier spectrometer. The data for E ‖ c, T > TC are discussed with the model of modemode coupling. The soft mode frequency of this model is found to vary as ω = A(T‐117.7)/(T + 155) with temperature. For E ⊥ c a heavily damped mode is found below 80 cm−1 which might be due to a proton E‐mode.
Phonon calculation in Nb–Ta and Pd–Pt alloys in the coherent potential approximation
Authors: Sacchetti F. , Spinelli M. C.
Published: July 2007

The phonon dispersion relations of Nb–Ta and Pd–Pt alloy systems are calculated in the framework of the coherent potential approximation. Though only a few experimental data are available to be compared with the theory, the satisfactory agreement obtained, indicates the necessity of further experimental and theoretical effort in the study of alloy phonon dispersion relations.
Zone‐centered phonons in AIBIIIS2 chalcopyrites
Authors: Koschel W. H. , Bettini M.
Published: July 2007

The spectra of the zone‐centered phonons in five compounds of the AIBIIIS2 chalcopyrite (ch) family are completed by IR absorption and Raman scattering measurements. The ch extended Keating model is applied to these phonons. The experiments and calculations clarify the assignments of the Γ3‐modes and the low‐energy modes which originate from the transverse acoustic zincblende (zb) phonons. The central force constants between nearest neighbours αBS of the homopolar bonds are very large corresponding to the small bond lengths. On the other hand, the strong heteropolar A–S bonds yield very low αAS‐values. The model parameters, which are determined by optical phonon fitting, also reproduce in a zb approximation the elastic moduli of AgGaS2.
Magnetic interactions in NiSO4
Authors: Scheerlinck D. , Peetroons C. , Speybrouck P. , Hautecler S. , Michel K.
Published: July 2007

The energy distributions of neutrons scattered by NiS04 in its paramagnetic state are measured by means of a time‐of‐flight technique. Restricting the range of the exchange interactions to the first and second nearest‐neighbours, the relation (J1/kB)2 + 2(J2/kB)2 = = (149±7) °K2 is obtained from the second moment of these energy distributions. A second relation between the two exchange parameters is obtained from a computation of the Néel temperature in the Green's function approximation. From the two relations J1/kB = = (8 ± 0.8) °K and J2/kB = (−6.5 ± 0.6) °K are derived. Taking properly into account a temperature‐independent contribution to the susceptibility, no experimental indication is found that there are other significant exchange interactions.
Molecular cluster approach to magnesium and calcium oxide crystals. I. Perfect crystals
Authors: Ermoshkin A. N. , Kotomin E. A. , Evarestov R. A.
Published: July 2007

The model of the molecular cluster (MC) is used to determine the properties of the perfect crystals MgO and CaO (the widths of the valence bands and forbidden gap, the effective charges of ions). The effective charges in the perfect crystal are shown to be ±1.6 in MgO and ±1.8 in CaO, which indicates high degree of covalency effects of the chemical bonding in these crystals. The obtained band widths are in reasonable agreement with the results of the other band structure calculations. The best results are obtained when the MC simulating the perfect crystal is chosen in the form of the LUC, that means that even the indirect consideration of the translational symmetry turns to be more important than that of the point symmetry.
On exciton absorption, band structure, and phase transformation of GaSe under pressure
Authors: Panfilov V. V. , Subbotin S. I. , Vereshchagin L. F. , Ivanov I. I. , Molchanova R. T.
Published: July 2007

The spectra of the absorption coefficient (α) of GaSe at the photon energy range 1.94 to 2.64eV and at pressures between 1 bar and 12 kbar are investigated at 295°K. Data at 1 to 4360 bar confirm that the edge exciton of GaSe is the direct allowed one. The pressure coefficient of the band gap (dEg/dP = −6.2 × 10−6eV bar−1), one of the Rydberg exciton constant (dR/dP = −0.6 × 10−6eV bar−1), and one of the exciton effective mass [(1/μ) (dμ/dP) = −2.4 × 10−5 bar−1] are approximately determined. It is supposed that two interband exciton transitions at 2.001 and 3.25eV give the contribution into the α spectrum above 2.1eV. The assignment of these transitions is discussed in connection with the band structure calculation of Schlüter. At 5280 bar a discontinuous jump of the absorption edge shape is observed for GaSe, with a change of the sign and a value for dEg/dP of +2.0 × 10−6eV bar−1 within the range of 5280 to 11780 bar. This effect is interpreted as the transformation of one GaSe polytype into another, i.e. as a possible phase transformation close to that of second kind.
On the Theory of Mössbauer Emission Relaxation Spectra
Authors: Afanasev A. M. , Gorobchenko V. D.
Published: July 2007

By utilizing a method previously developed, an expression is derived for the shape of the Mössbauer emission spectrum in the presence of relaxation. The final formula coincides with that of Hartmann‐Boutron, but the derivation is much simpler.
Strain‐Associated Band Tailing Effect in Semiconductors
Authors: Leyral P. , Bois D. , Pinard P.
Published: July 2007

In order to explain the band tails experimentally observed in undoped semiconductors especially in GaAs, a model for band tails associated with inhomogeneous strain fields has been developed. An analytical law for the optical absorption edge is deduced and compared with experimental results obtained in bulk or epitaxial GaAs samples. Two cases are considered: spherical and cylindrical strain fields. The origin of such strain fields, dislocations or precipitates, is discussed.
Self‐Consistent Local Orbitals for Silver Halide Crystals
Authors: Mickish D. J. , Kunz A. B.
Published: July 2007

Self‐consistent local orbitals are reported for the crystals AgF, AgCl, and AgBr. The formalism of Adams and Gilbert, expanded to first order in overlap, is used. All relativistic effects are neglected. Only small changes in the Ag+ one‐electron wavefunctions are found in going from the ion to the solid‐state local orbitals but large changes are observed in the halide wave functions. Changes in the ion local orbitals and the surrounding environment are traced during the self‐consistency process.
Interband‐Current Theory for Esaki Diodes
Authors: Wünsche H.‐J. , Henneberger K.
Published: July 2007

A new theoretical concept for calculating diode characteristics is presented, using the splitting of the total current into an intraband and interband part. The direct calculation of the interband current inside the junction leads to results, which agree considerably better with experimental data, than the results of the traditional theory. Furthermore, the formalism allows to consider many‐body effects without of conceptual difficulties.
Local‐Mode Absorption of H− and D− in SrCl2:RE and SrF2:RE Single Crystals
Authors: Timans J. W. J. , Den Hartog H. W.
Published: July 2007

IR studies on localized vibrations of hydride ions in SrCI2 and SrF2 are reported. In hydrogenated and deuterated samples doped with trivalent rare earth ions two different axially symmetric complexes giving rise to two doublets are observed. For SrCI2:RE3+, H− the positions of the absorption lines can be predicted with reasonable accuracy. Lattice relaxations appear to play more important role for C4v oscillators in SrF2 than in SrCI2 and thereforeit is more difficult to give a theoretical description of these tetragonal complexes.
α‐Oxygen Doublets II. Experiment
Authors: Gaididei Yu. B. , Loktev V. M. , Prikhotko A. F. , Shanskii L. I.
Published: July 2007

Experimental results of two‐exciton light absorption in α‐O2 at T = 1 K are presented. On the basis of a theory which was previously developed, the interpretation of the doublet structure of the observed absorption is proposed. The effect of resonance interaction on the formation of the vibronic bimolecular bands and the importance of the excited collective states are discussed.
The Formulation of the Formation Energy of a Vacancy as an Alloy Problem
Authors: Tanigawa S. , Doyama M.
Published: July 2007

The formation energy of a vacancy E is studied as an alloy problem. For this purpose, the pseudopotential formalism for binary random substitutional alloys of simple metals is developed and the expression of the total energy of such alloys is given. In the present approach, a vacancy is taken as a solute atom which has the valency of zero and an atomic volume equal to the formation volume of a vacancy. It was found that (a) the expression derived for E is the same as that of Harrison's approach, in which the formation volume was assumed to be zero, (b) E can be properly evaluated both by the constant volume and by the constant pressure cycle, and (c) E does not depend an the formation volume of a vacancy under equilibrium conditions at zero pressure. This seems to arise from the neglect of the lattice relaxation around a vacancy.
EPR Investigation of the Very High Zero‐Field Splitting of Gd3+ in La2O3 Single Crystals
Authors: Vivien D. , Kahn A. , Lejus A. M. , Livage J.
Published: July 2007

Single crystalsof lanthanum oxide doped with Gd3+ are grown by the flame fusion method and studied by ESR. The very high zero‐field parameter B (0.131 cm−l) is of the same order of magnitude asthe energy quantum of X‐band radiation. This gives an abnormal spectrum which has been resolved by computer. These crystals may find interesting applications in solid state masers.
EPR Investigations of a Structural Phase Change in RbCaF3, RbCdF3, and TlCdF3
Authors: Rousseau J. J. , Rousseau M. , Fayet J. C.
Published: July 2007

EPR investigations of the structural phase change which occurs in the compounds RbCaFa: Mn2+, TlCdF3:Fez+, and RbCdF3:Fe3+ were made. For RbCdF3 we have determined the critical index and measured the order parameter using two different probes, namely the Fe3+ ion and the Vk center.
Electron Paramagnetic Resonance in Anisotropic Magnets
Authors: Hùber D. L. , Seehra M. S.
Published: July 2007

Electron paramagnetic resonance in anisotropic magnets is analyzed. The resonance field at fixed frequency is shown to be a function of the principal values of the external susceptibility tensor and hence can be temperature dependent. The predictions of the theory are compared with data on the temperature dependence of the resonance field in CrBr3. Excellent agreement is obtained.
Electron Spin Resonance Study of Cr3+ and Fe3+ in Alums
Authors: Fǎrcas S. I. , Darabant A. , Nicula A.
Published: July 2007

The angular dependence of the electron spin resonance spectrum of Cr3+ and Fe3+ ions in diluted ammonium alum has been measured, and the spin Hamiltonian parameters have been derived. In addition to the allowed fine structure lines some small‐intensity lowfield lines have been observed. These lines were identified as forbidden fine structure transitions due to Cr3+ and Fe3+ in alum. A good agreement between the theoretical angular dependence with that experimentally found confirmed the lines and the local symmetry of the ions to be correctly determined.
Polariton Scattering and Formation of Exciton Luminescence Spectrum in Anthracene Crystals
Authors: Brodin M. S. , Dudinskii M. A. , Mariscva S. V. , Myasnikov E. N.
Published: July 2007

The exciton luminescence spectrum of anthracene crystals with different thickness has been studied within the temperature range 30 to 1.6 K. The formation of this spectrum was analyzed from a polmiton viewpoint taking into account exciton–photon mixing. It is shown that inelastic (combination) polariton scattering processes on lattice and intra‐ molecular phonons are the most significant ones in the formation of the exciton lumines‐ cence spectrum.
Photoabsorption Coefficient of Alloys of AI with Transition Metals V, Fe, Ni and with Cu and Pr from 30 to 150 eV Photon Energy
Authors: Hagemann H.‐J. , Gudat W. , Kunz C.
Published: July 2007

The absorption coefficient of VAI3, FeAI, NiAI, NiAl2, CuAI2, PrAl2, and of disordered V–AI(16 at% AI, 28%, 41%) and FeAI (11%) alloys has been measured in the region of the M2, 3 absorption of the transition metals and the Labsorption of AI. The strong changes of the AI spectrum in the region of the 100 eV maximum upon alloying are explained as another evidence of the EXAFS (extended X‐ray absorption fine structure) nature of these structures. The broad, prominent absorption peaks from the 3p excitations in V and Fe and from the 4d excitations in Pr are influenced only little on allyoing and thus appear to be of atomic origin. The fine structure at the onset of the Pr 4d transitions is identical in the metal and the alloy but differs from that of Pr oxide. The only M2, 3 edge which is detectably shifted is that of Ni (up to 2.1 eV), whereas the onset of the AI L2, 3 edge is shifted in all the alloys (up to 1.1 eV). The shifts are interpreted in accordance with X‐ray fluorescence and nuclear resonance measurements as changes of the density of states in the valence band of the alloys.
On the Theory of Photon‐Assisted Zener Tunneling
Authors: Henneberger K.
Published: July 2007

Zener tunneling assisted by one‐photon absorption is studied theoretically. A general expression for the transition rate is obtained and further evaluated for both a two‐band and a three‐band process, where analytical results are presented in terms of Airy functions. Furthermore, an interband current is produced by photon assisted tunneling which is shown to be proportional to the corresponding transition rate. It can be concluded from these results that photon assisted tunneling noticeably contributes to macroscopically observable quantities like electroabsorption and electric current, especially, if two nearly degenerate valence or conduction bands exist. In the latter case an interband current is produced flowing opposite to the field direction.
Photoluminescence Excitation Spectra of Solid Krypton
Authors: Ackermann Ch. , Brodmann R. , Hann U. , Suzuki A. , Zimmerer G.
Published: July 2007

The excitation spectrum of the intrinsic photoluminescence of solid krypton has been investigated between 9.8 and 14eV. Samples with a clean surface yield a quantum efficiency which is within an uncertainty of ≈10% independent of the kind of initially excited states. Surface contamination reduces the steady‐state concentration of excited states leading to quenching of the intrinsic luminescence. In the region of excitonic excitation the energy transfer between excitons and the surface is investigated. Assuming exciton diffusion, a diffusion length L0 = 200 Å is obtained for the n= 1 and n′ = 1 excitons. Under the assumption of long range dipole–dipole interaction a critical radius R0 = 22 Å is deduced.