bluuebunny/crossref_metadata_2025_split huggingface dataset:
Long‐Wavelength Optical Phonons of AgBrxCl1−x Mixed Crystals
Authors: Bootz B. , von der Osten W. , Uhle N. 
Published: July 2007

AbstractInfrared reflectivity and second‐order Raman scattering in the mixed crystal system AgBrxCl1−x, were studied at 77 K. The TO(Γ) and LO(Γ) phonon frequencies obtained by means of a Kramers‐Kronig analysis shift continuously between their energetic positions in the pure components indicating one‐mode behaviour of the system. This result is discussed in view of a recent model that describes the optical modes of a mixed crystal. The LO(Γ) phonon frequencies taken from the Raman spectra are found to be slightly lower. A resonant enhancement of the 2L0 scattering process that occurs for z = 0.42 with shorter‐wavelength excitation is used to determine the longitudinal optical phonon frequency.
A New Radiochemical Method for Determining the Oscillator Strength of F Centres in Alkali Halides
Authors: Bosi L. , Lazzarini A. L. Fantola , Lazzarini E. 
Published: July 2007

AbstractA new radiochemical method for determining the concentration of F centres in X‐ or γ‐ray coloured crystals of alkali halides is described. The method is based on the fact that crystals containing F centres upon dissolution in water give rise to the formation of an equal amount of hydrated electrons. Hydrated electrons are able to reduce [Co(NH3)6]3+complex ions to Co2+; thus, by dissolving crystals containing F centres in a water solution of [Co(NH3)6]3+ions, the amount of formed Co2+ions is related to the concentration of F centres in the crystals. The method was applied to the determination of F centre concentration in NaCl, KC1, and KBr crystals coloured by50Co γ‐ray irradiation. The use of these concentrations in the Smakula absorption formula enables to calculate the optical oscillator strength / of F centres. The / values obtained are as follows: ƒNaCl = 0.64 ± 0.02; ƒKCl = 0.68 ± 0.02; /KBr = 0.72 ± 0.02. These results are compared with those already reported in literature.
Modifications of KN Theory for Weak Resonance and Dislocation Scattering of Phonons
Authors: Singh M. P. , Verma G. S. 
Published: July 2007

AbstractThe resonance scattering of phonons in Co++ doped CdS is investigated both in the presence and absence of magnetic fields on the basis of modified expressions of Kozakov and Nagaev, which include weak resonance scattering of phonons for the off‐resonance situations T<TR, where TR is the resonance temperature. It is possible to establish that the major contribution to resistivity due to resonance scattering of phonons comes from the virtual transition between the magnetic field split states | + 1/2 〉 and | −1/2〉. Similar modifications of KN expressions for the weak dislocation scattering as well as the weak resonance scattering for ω>ωr lead to plausible explanations for the unexplained depressions in the phonon conductivity of CaF2 in the temperature range 2 to 4 °K.
Inelastic Scattering of Slow Electrons from a Heisenfoerg Ferromagnet
Authors: Harriague S. , Majlis N. , Makler S. A. , Olmedo A. M. 
Published: July 2007

AbstractThe magnetic inelastic scattering of low‐energy electrons by the (001) surface of a b.c.c. Heisenberg ferromagnet is considered. As a first step, the spectrum of elementary magnetic excitations is obtained, revealing the existence of optical and acoustic surface magnons, as well as a continuum of bulk magnons. The differential cross‐section for inelastic scattering shows the contribution of those types of excitations; localized niagnons give a delta function contribution, while the contribution due to the bulk states is qualitatively different depending on the value of the ratio of the exchange integrals for surface and bulk spins, thereby providing a method for investigating the exchange interactions at the surface.
Computer Modelling of the Order–Disorder Transformation in Cu3Au and Ni3Fe
Authors: Golosov N. S. , Dudka B. V. 
Published: July 2007

AbstractThe problem of the type definition of phase transition is discussed on the example of order–disorder transformation modelling by the Monte‐Carlo method. The discrepancy of the transition temperature of stationary state distributions which were obtained from high‐and low‐temperature chains defines the phase transition type. Necessity of metastable phase overcooling or overheating at the first‐order transition results in the dependence of the critical state of the substance on that one preceeding the transition in the critical one.
Luminescence and Excitation Spectra of Exciton Emission in GaAs
Authors: Heim U. , Hiesinger P. 
Published: July 2007

AbstractLuminescence, excitation, and photoconductivity spectra of epitaxial n‐ and p‐type GaAs at 1.6 K are measured with high resolution, using very low intensity of excitation. Under this condition, it is found that the (D°, X) bound exciton emission is split into two components, and that further structure is observable between the (D°, X) and the luminescence band, recently attributed to lower branch polariton emission. In particular, two very sharp lines are seen ≈︁ 0.5 meV above the (D°, X) emission. These lines are observed in all our samples, and their intensities seem to be related to the strength of the (D°, X) line. Analysis of the excitation spectra lead to the identification of three emission lines. These are the n = 2 free exciton transition, the two‐electron transition of the (D°, X) complex with the donor left in then = 3 excited state, and most probably the n = 2 donor‐to‐valence band transition.
The Critical Voltage Effect in Transmission Electron Microscopy. I. Eigenvalue Degeneracy in the Three‐Beam Case
Authors: Gevers R. , Serneels R. , David M. 
Published: July 2007

AbstractGiven an orientation where only one reflection h is nearly in exact Bragg condition, the others being far from the Ewald sphere, the various physical processes are discussed which may lead to an anomalously low intensity of the reflection h. It is found that exact eigenvalue degeneracy is neither a sufficient nor a necessary condition for an anomalous intensity reduction. Absorption effects being neglected, a general three beam case 0, h, g is discussed. It is shown that a reflection being far from the Ewald sphere may induce a weak intensity for a reflection ft close to the Ewald sphere. The necessary conditions are discussed. The usual linear second order critical voltage effect follows as a particular case. Then it is shown that for the linear three beam case, absorption effects taken into account, particular diffraction conditions exist, i.e. voltage and orientation conditions, for which two complex eigenvalues are equal.
The Linear Dielectric Response of a Semiconductor: A New Analytic Form for the Dielectric Function
Authors: Schulze K. R. , Unger K. 
Published: July 2007

AbstractA simple analytic form for the linear dielectric function ϵ(q) of a semiconductor is constructed, which accounts correctly for the limiting cases q = 0 and asymptotic large q values. For ϵ(0) → ∞ a Weizsäcker type of screening is obtained. Numerical comparison with dielectric functions calculated on the basis of exact band structures shows very good agreement. Analytical expressions for the r‐dependent dielectric function, for the filter function, and for the convolution function are also given and their implications are discussed.
Influence of Uniaxial Pressure on Hot‐Electron Conductivity in n‐Si at Low Temperatures
Authors: Asche M. , Russu E. 
Published: July 2007

AbstractInvestigations of current‐voltage characteristics of n‐Si along 〈100〉 direction at low temperatures when nde should occur are repeated in detail. In order to verify the inter‐valley carrier transfer as origin of current saturation and accompanying oscillations, the valleys are split energetically by uniaxial pressures parallel and perpendicular to the 〈100〉 direction.
On the Determination of the Phonon Density of States by Means of Coherent Inelastic Scattering of Neutrons on Polycrystalline Samples
Authors: Eschrig H. , van Loyen L. , Ziesche P. 
Published: July 2007

AbstractThe possibility, the accuracy, and the necessary experimental conditions for a direct determination of the phonon density of states by means of coherent inelastic neutron scattering on polycrystalline samples in the case of non‐cubic material is tested by model calculations of both the phonon density of states and the spectrum of scattered neutrons corresponding to measurements carried out by Eremeev et al. on Mg. It is found that apart from the low energy range the necessary experimental conditions can easily be fulfilled and that for correcting the experimental results a method earlier proposed by Gorbachev et al. is appropriate.
Electron–Hole Liquid above the Critical Point
Authors: Martin R. W. 
Published: July 2007

AbstractAt sufficiently high pumping levels, carrier densities of more than 2 × 1017 cm−3 can be produced in doped Ge or small crystals of intrinsic Ge. As a result, the crystal can be entirely filled with a degenerate electron–hole plasma. With increasing temperature this system does not undergo a phase transition to a low density free exciton gas, even at temperatures above the critical point.
Lattice Dynamics of Some Transition Metals by an Elastic Force Model
Authors: Goel C. M. , Pandey B. P. , Dayal B. 
Published: July 2007

AbstractA modified Sharma‐Joshi model has been employed to compute the phonon dispersion relations in three transition metals, namely gold, palladium, and platinum. The results have been computed in three principal symmetry directions and are found to agree very well with recent experimental data.
A study of spin‐phonon interactions in ferromagnetic insulators. II. The phonon spectrum and spin‐lattice relaxation bate
Authors: Jones M. J. , Cottam M. G. 
Published: July 2007

AbstractThe formalism of a previous paper is employed to evaluate the renormalised energy and damping of phonon excitations due to spin–phonon interactions in a Heisenberg ferro‐magnet. The calculations make use of a field theoretical perturbation technique and an expansion in powers of the high density parameter 1/z. The results are applicable over a wide range of temperatures below Tc. Calculations are also presented for the spin‐lattice relaxation rate, which can be closely related to the phonon damping.
Radiative recombination from non‐equilibrium states in cathodo‐excited semiconductors
Authors: Zehe A. , Röpke G. 
Published: July 2007

AbstractApplication of the theory of non‐equilibrium processes is made to the relaxation and recombination of high‐excited electrons in semiconductors. Following the scheme of a hot‐electron theory based on the method of nonequilibrium statistical operator, developed by Zubarev and, independently, by McLennan, expressions for relaxation and recombination times of hot carriers has been established, resulting directly from a microscopial description of the system. As an example a free‐to‐bound transition is discussed (transitions between donor levels and the valence band or between acceptor levels and the conduction band) in (Ga, Al)As. With this en the one hand the concept of hot electrons is shown to be related to the general theoretical description and on the other hand, intensity curves of time‐resolved spectra, i.e., the time‐variation of spectra after an excitation pulse, are given.
Influence of E–E scattering on the phenomenological energy relaxation time in nonpolar semiconductors
Authors: Dienys V. , Kancleris Ž 
Published: July 2007

AbstractThe dependence of the phenomenological energy relaxation time τε on carrier concentration in n‐and p‐type Ge and n‐type Si at 77°K was measured. The measurements were carried out in a warm electron region using the harmonic mixing technique. A considerable decrease of τε with concentration was observed. It is shown that this effect can be explained by the influence of interelectronic scattering on the energy losses caused by the emission of high energy (optical or intervalley) phonons.
Linear magnetoresistance, hall effect, and the geometry of anisotropic quantum scattering
Authors: Hsu W. Y. , Falicov L. M. 
Published: July 2007

AbstractA series of model calculations for the magnetoresistance and Hall effect of free‐electron‐like metals is presented. They involve anisotropic‐quantum‐flucfruation coherent scattering and yield linear longitudinal and transverse magnetoresistances. The models here studied exhibit various anisotropic behaviors for the magnetoresistance slopes. The Hall coefficient is not affected by the coherent scattering and is a constant equal to the free‐electron model in all cases. The results are compatible with existing experimental data for potassium.
Improved model for the stark effect of the F‐center in KCl
Authors: Bonciani M. , Grassano U. M. 
Published: July 2007

AbstractThe results of the Stark effect measured with light polarized parallel to the field, have been reexamined using a more complete theoretical model which takes into account four excited states of the F‐center. The very good agreement between the fitted curve and the experimental data confirms the evaluation of the energy position of the even parity states 2s and 3d already known from previous works.
Shape and structure of phonon sidebands of the ν1 stretching‐vibration absorption band of the NCO− impurity ion in alkali halide crystals
Authors: Tsyashchenko Yu. P. , Pinkevich I. P. , Zaporozhets V. M. 
Published: July 2007

AbstractExperimental data are compared of the shape and structure of the phonon sidebands (PS) of the ν1 intramolecular vibration (IMV) of the impurity ion NCO− in KCl, KBr, and KI matrices obtained at 10 °K with their theoretical calculations. It is shown that in f.c.c. host lattices the shape (distribution of absorption coefficient K(ωk)) of the acoustic branch of PS is in quantitative agreement with the expression magnified image × × 〈sin2 (κ, ι)〉 g (ωκ), where ωk is the frequency displacement from the centre of the IMV absorption band (frequency of lattice phonon), ωm the maximum frequency of the phonon spectrum of the matrix, g(ωk) the density of its phonon states, k the quasimomentum of matrix phonon, and l the vector of the nearest neighbour of the impurity. In the optical branch of PS the agreement is qualitative. Assuming a change of the force constants of the two nearest axial neighbours of the linear impurity conditions are obtained of the appearance of local vibrations (LV) separating into the gap of the phonon spectrum of the matrix crystal and above ωm· The treatment is given of a sharp structure observed at the short‐wave side of PS and in the region of PS corresponding to the frequency gap of the f.c.c. matrices (doublets). Corresponding limiting changes of the force constants in the impurity crystals are evaluated.
A study of structurally disordered single‐element magnetic materials beyond the homogeneous molecular field approximation
Authors: Šlechta J. 
Published: July 2007

AbstractA usual assumption in the theory of amorphous magnetic materials is 〈Si〉 = S. It is shown that in amorphous materials this is a very crude approximation. The magnetic properties of small random clusters of localised spins were studied beyond the above simplification. It is found that in this case the fluctuations of the exchange interaction around its crystalline mean value cause an increase of the Curie temperature. In the light of this conclusion most of the experimental results in this direction should be reinterpreted. It is argued that the magnetic properties inherently depend on higher spatial correlations than pair–pair ones.
Transfer matrix approach to axial hyperchanneling
Authors: Corciovei A. , Apostol M. 
Published: July 2007

AbstractThe mechanism of channeling is reviewed and a discrete model for hyperchanneling is formulated, which holds both for planar and axial channeling. It is assumed that the moving particle in the crystal interacts with the atoms in the planes of the lattice which are perpendicular to its direction of motion. An axial channel consists in a succession of parallel rings of atoms. The interaction between hyperchanneled particle and these atomic rings is described in terms of classical collisions. From the linearized equations of motion in the momentum approximation one obtains that the interaction with the rings can be described by a transfer matrix which relates the mechanical states of the particle before and after a collision with an atomic ring. Then by induction the mechanical state parameters of the particle after the n‐th collision are obtained in terms of the initial conditions. Minimum critical hyperchanneling energies are derived. Expressions are given for the critical angles as well as for the energy losses in the case of the f.e.e. 〈100〉 channel. Numerical evaluations show a good agreement between the theoretical results and the experiment.
Isothermal electrocaloric effect in KCI:OH
Authors: Korrovits V. , Liidja G. , Mihkelsoo V. 
Published: July 2007

AbstractThe entropy of a noninteracting dipole system with the equilibrium dipole moment μ ‖ 〈100〉 in a rigid cubic lattice is obtained for the external electric field E‖ [100] of arbitrary strength, taking into account the 90° tunneling. In thermostating experiments the KCI‐OH monocrystals were illuminated with light pulses of rectangular shape (single 10−2 s shots, absorbed power ranging 3 × 103 erg/s cm3) and the electric field applied to the crystal was simultaneously removed. The electric field pulse shape, needed for maintaining the crystal temperature constant within some millidegrees at 0.75 K, is close to the theoretical one assuming a “working” dipole number NE ≈ 0.1Nopt.
Surface energies of simple metals (I)
Authors: Paasch G. , Hietschold M. 
Published: July 2007

AbstractThe surface energy and the dipole barrier are calculated for eleven simple metals. The potential of the positive ions is replaced by Ashcroft‐model potentials. The decrease of the electron density at the surface and the relaxation of the top lattice plane are described by two parameters. Using the theory of the inhomogeneous electron gas the surface energy is calculated and minimized with respect to the surface parameters. The influence of the inhomogeneity of the electron density on the exchange and correlation energy has to be taken into account in order to obtain reasonable results for both the surface energy and the dipole contribution to the work function.
Calculation of nonradiative multiphonon capture coefficients and ionization rates for neutral centres according to the static coupling scheme: I. Theory
Authors: Pässler R. 
Published: July 2007

AbstractCapture coefficients and ionization rates for nonradiative multiphonon transitions of charge carriers at neutral imperfections in semiconductors are calculated within the framework of the static coupling scheme. The temperature averaging connected with the thermal distribution of free carriers over the band states is performed on the basis of a three‐param‐etrical overlap‐factor formula which enables a reduction of the analytical expressions for the quantities in consideration to simple functions of five imperfection parameters.
Domain structure at ferromagnetic‐antiferromagnetic phase transitions as superheated‐undercooled state of a solid
Authors: Popov V. A. , Polyakov M. L. 
Published: July 2007

AbstractA theory is developed for the coherent domain structure at the temperature‐induced phase transition FM⇄AFM. The domain sizes, energy, and thickness of the domain boundaries and their temperature dependences have been calculated. Heat capacity of the FM‐AFM‐domain structure is shown to depend on temperature and achieve very high values. The possibility of a libration phase transition between two FM‐domain structures is discussed.
Ferromagnetism for the hubbard model in the alloy analogy approximation
Authors: Schneider J. , Drchal V. 
Published: July 2007

AbstractThe stability of the strongly ferromagnetic ground state in the Hubbard model for U → ∞ is investigated, using the Hubbard alloy analogy result. In this approximation the saturated ferromagnetic state is not stable for a nearly half‐filled band. In fact, for n < 1, only the paramagnetic solution exists.
Linear electroabsorption due to substitutional deep impurities in wurtzite‐type crystals: 1. Theoretical analysis for A → T absorption bands
Authors: Boyn R. , Gardavský J. 
Published: July 2007

AbstractA theoretical study is made of the first‐order effects of an external electric field on optical transitions between strongly localized electronic states of isolated substitutional impurities in a wurtzite‐type crystal taking into account the near‐tetrahedral symmetry of these centers. The field effect on optical absorption bands due to A → T type transitions is treated on the basis of the method of moments which is extended to include mixing with the A ground level. The field‐induced changes of the zeroth and first moment (corresponding to changes in transition probability and to the Stark splitting, respectively) are calculated as functions of field and polarization direction. The polarization dependence of the zero‐field moments which is due to the axial part of the impurity potential is also discussed. On the basis of a rigid‐shift approximation a brief semi‐phenomenological treatment is given of the field‐induced changes in band shape.
Cluster of four superexchange‐coupled Cu2+ ions in copper‐doped triglycine fluoroberyllate. A theoretical interpretation of the EPR spectrum
Authors: Kurzyńaski M. , Kowalewski L. 
Published: July 2007

AbstractThe unusual EPR spectrum of copper‐doped triglycine fluoroberyllate is shown to originate in large complcx molecules of the hypothetical formula [Cu2+·(NH2CH2COO−)2·H2BeF4]4, built into the distorted host lattice. Superexchange couplings of Cu2+ ions in the cluster through CuFBeFCu paths are discussed. The isotropic superexchange interactions split the 16‐fold degenerate ground crystal state of the four Cu2+ ions into multiplets the further splitting of which, originated by anisotropic interactions, can be described in terms of effective spin Hamiltonians with S = 2 and 1. The observed spectrum is attributed to transitions within the quintet and one of the triplets. By second‐order perturbation procedure, the phenomenological parameters of the effective spin Hamiltonians are expressed in terms of parameters directly describing anisotropic as well as isotropic superexchange interactions in the cluster. The magnitudes of these interactions are thus determined from experimental data.
Effect of piezoelectric potential scattering on the microwave faraday effect in n‐CdS
Authors: Srivastava G. P. , Mathur P. C. , Shyam R. , Kataria N. D. , Chandra I. 
Published: July 2007

AbstractThe theory of free‐carrier Faraday effect in semiconductors previously developed by Donovan and Webster for lattice and impurity scattering is extended to the case of n‐type CdS, taking into account piezoelectric and deformation potential scattering. The high frequency magnetoconductivity tensors are derived considering the detailed structure of the conduction band of CdS. The Faraday rotation measurements in n‐type CdS along the c‐axis are performed at liquid nitrogen temperature and room temperature at 24 GHz. The experimental results are found to agree well with the theory, for β = p/a = 1.9, where a and p being the deformation potential and piezoelectric scattering coefficients at T = 90 °K, respectively.
Theory of diffraction due to substitutional disorder
Authors: Szabó P. 
Published: July 2007

AbstractIt is shown that existing theories of the diffraction due to substitutional disorder are partly wrong and partly do not correspond to the case of a large number of crystallographically equivalent positions in the unit cell. The pertinent diffraction theory is elaborated. Certain striking differences are found between the new results and those of former theories. So it is shown, that some experimental inferences referring to spinels and reported in the literature are unfounded.