bluuebunny/crossref_metadata_2025_split huggingface dataset:
The electronic structure and phase diagrams of binary alloys with complex lattices: II. MgCd System
Authors: Zhorovkov M. F. , Fuks D. L. , Panin V. E. 
Published: July 2007

AbstractA general theory given in a previous paper which allows to determine theoretically the structures of ordered phases in binary alloys with complex lattice is used for an investigation of the system MgCd. The equilibrium value of the lattice parameter a in the disordered solution is calculated as a function of concentration. For a definite model potential the energetically favoured superstructures are obtained and compared with those observed experimentally.
Magnon relaxation in an XY chain
Authors: Kashcheev V. N. 
Published: July 2007

AbstractIn first‐order perturbation theory it is shown with respect to the Hamiltonian of the magnon–magnon interaction that for a linear ferromagnetic XY chain with large spins the frequency renormalization is proportional to the energy of noninteracting magnons. In higher order with respect to the same interaction the momentum and temperature dependence of the magnon damping is obtained and weakly damped magnons are shown to exist, provided their wavelength is much shorter than the correlation length.
Uniaxial stress effect on the 4A2 → 4T2 zero‐phonon transition of Cr3+ in α‐Al2O3
Authors: Louat R. , Lacroix R. , Duval E. , Champagnon B. 
Published: July 2007

AbstractUniaxial stress induced effects on the zero‐phonon transition 4A2 → 4T2 of Cr3+ in α‐Al2O3 are measured for two directions of the applied pressure making an angle of π/3 with the C3 axis, one lying in the (C2, C3) plane and the other in a plane perpendicular to the latter. These measurements, as well as data obtained previously, determine the subsequent ratio |b′|/|a′| of the interaction constants between the doping ion and the strains which transform as the basis functions of the E representation of the C3v group, without knowing the elastic compliances. On the other hand, these experiments reveal the existence of a large elastic compliance S15*. The experimental results are interpreted by the existence of a Jahn‐Teller effect with vibrational modes which are a mixing of tetragonal and trigonal modes.
Secondary electron emission from some organic crystals
Authors: Bubnov L. Ya. , Frankevich E. L. 
Published: July 2007

AbstractThe coefficients of secondary electron emission (SEE) in copper phtthalocyanine and free phthalocyanine have been determined. They are found to be close to the values determined earlier for the SEE coefficients in aromatic crystals. The relationship found between the SEE coefficients and the energy of first singlet excitations of these crystals is explained on the basis of assumptions advanced earlier on the rectilinear nature of motion of secondary electrons to the surface. A comparison with the alkali‐halogenide crystals is made to verify this assumption. Results on the maximum mean free path of electrons with an energy in the range of 0.6 to 2.2 keV for tetracene and pyrene are reported. They are consistent with the data published for inorganic substances. The experimental curve of the elastic reflection coefficent (k) versus the energy of primary electrons (Ep) for free phthalocyanine exhibits a steep drop when Ep > 200 eV. Such a behaviour k(Ep) is attributed to the scattering of electrons at the carbon atom, and is regarded as one of the causes for the stable position of the maximum of the SEE coefficient in organic crystals for Ep ≈︁ 200 eV.
Compression‐induced magnetic anisotropy of long‐range‐ordered Fe3Al alloy
Authors: Takahashi S. 
Published: July 2007

AbstractTorque measurement and electron microscopie observations of dilocations are made in Fe‐25 at% Al single crystals subjected to different heat treatments and compression. The compression‐induced magnetic anisotropy is explained in terms of dislocation density and its distribution accordeng to a previously proposed model. The expermental results show a better agreement with the calculated results when such an effect is taken tnto acfcount that the dislocation which crosses the thermally induced APB(2) produces the new APB(2) over the slip plane.
Thermodynamical properties of Frenkel excitons
Authors: Marinković M. M. 
Published: July 2007

AbstractAnalysis is presented of the thermodynamical properties of Frenkel excitons. The low temperature behaviour of the parameter of “orderness”, analogue to that usually used in the theory of magnetism, is thoroughly discussed. The reasons are explained why the same methods, usually used in the theory of magnetism, lead here to results of a somewhat lower degree of approximation. For the analysis of the high temperature behaviour the RPA method is applied. In both cases the reduction of the Hamiltonian of the system of excitons to a quasi‐particle conserving one, secures a correct basis for the succeeding treatment.
Nonlinear concentration dependence of the low‐temperature thermal resistivity in metals containing impurity atoms
Authors: Kagan Yu. , Zhernov A. P. , Pashaev Kh. M. 
Published: July 2007

AbstractIt is shown that the specific interference of elastic scattering of electrons by impurity atoms and inelastic scattering of electrons by phonons, taking into account a real angular and energy dependence of the electron distribution function, results in a strongly nonlinear concentration dependence of the electron part of the thermal resistivity of a metal. The curve showing the temperature dependence of the inelastic part of the impurity thermal resistivity has a peak whose value depends on the concentration c of impurities. Simultaneously the coefficient B in the usual low‐temperature representation of the thermal resistivity, BT2, is also found to depend on c. A comparison with the experimental data is made.
Exciton reflection spectra of Fe‐doped CdTe single crystals
Authors: Gavaleshko N. P. , Kurik M. V. , Skitsko I. F. , Ivanchuk R. D. 
Published: July 2007

AbstractThe reflection spectra of CdTe crystals doped with Fe atoms are measured in the exciton region. The dependences are obtained of the exciton line position, and its halfwidth and intensity on impurity concentration. The position of the exciton line shifts linearly to shorter wavelengths with increasing impurity concentration. At 4.2 °K with increasing impurity concentration broadening and decrease in intensity of the line is found. At 77 °K the exciton line halfwidth shows no dependence on the doping level. For concentrations between 2 and 4 × 1019 cm−3 exciton „flash up”︁ is observed (exciton line narrowing in the spectrum), which is explained by the effect of an internal effective magnetic field due to exchange interaction between the impurity ions.
Free carrier absorption in semiconductors with non‐parabolic and ellipsoidal energy band structures
Authors: Das A. K. , Nag B. R. 
Published: July 2007

AbstractFormulas are derived for the calculation of free carrier absorption in semiconductors with ellipsoidal constant energy surfaces, including the effects of non‐parabolicity, for all important scattering mechanisms. It is shown that simple expressions for the absorption coefficient follow from a mathematically exact formulation only in case of acoustic, nonpolar optical, and inter‐valley scattering. Simple, approximate expressions are also suggested for the evaluation of the absorption coefficient in the case of polar optical phonon and impurity scattering.
Optical properties of CaWO4 and CaMoO4 crystals in the 4 to 25 eV region
Authors: Grasser R. , Pitt E. , Scharmann A. , Zimmerer G. 
Published: July 2007

AbstractThe reflection spectra of CaWO4 and CaMoO4 single crystals were measured in the region between 4 and 25 eV. The measurements were performed using the electron synchrotron DESY as a light source. Both crystals show very similar spectral features. A relatively sharp, well separated low energy peak is accompanied on the high energy side by two groups of maxima, each group containing three broad main reflection bands which strongly overlap. Besides this, a less pronounced structure is observed over the whole spectral region. The experimental results are discussed in terms of semiempirical MO calculations of the anionic transition metal complexes.
Effect of sample shape and temperature on the ESR linewidth above the curie point of a ferromagnet
Authors: Kötzler J. , Scheithe W. 
Published: July 2007

AbstractFrom room temperature down to temperatures near the Curie point (TC = 2.21 K) the ESR linewidths of GdCl3 ellipsoids narrow according to the simple law ΔH(T) = ΔH(∞) × [1 – (θ(0) – CNz)/T], where Nz denotes the demagnetization factor parallel to the applied static field. The result is explained by linearized modified Bloch equations, which describe the time development of the magnetization M(t) under the influence of a molecular field and of relaxation forces driving M(t) to a state of equilibrium determined by the instantaneous molecular field.
Changes of the excitonic optical properties of CdS introduced by an electron beam
Authors: Ratsch U. 
Published: July 2007

AbstractReflection spectra and luminescence in the exciton region of CdS at 80 K under the influence of a low energy electron beam (2 to 3 keV) have been investigated. Long‐term changes of the spectral lineshape have been observed. It is found that the electron beam produces a negative surface charge, the electric field of which dissociates the excitons in a surface layer. The reflected intensity of light is determined by interference of waves reflected at the boundaries of the exciton‐free layer. The lineshape of the reflection spectra depends on the thickness of this layer.
On the optical absorption and the magnetic circular dichroism of the transparent ferromagnet K2CuF4
Authors: Laiho R. 
Published: July 2007

AbstractThe optical and infrared absorption as well as the magnetic circular dichroism (mcd) of the transparent ferromagnet K2CuF4 were measured in the paramagnetic and ferromagnetic regions. The absorption bands at 7900, 9500, and at 12300 cm−1 were assigned to the crystal field transitions 2A1g → 2B1g, 2Eg, 2B2g of the Cu2+ ions in the tetragonally distorted octahedral symmetry. A pronounced vibrational structure, with a zero‐phonon line at 8775 cm−1, was identified on the low energy side of the 9500 cm−1 band and compared with the infrared absorption bands between 600 and 1100 cm−1. The first moment change of the MCD band corresponding to the absorption around 9500 cm−1 was explained by the non zero orbital angular momentum in the 2Eg state. The mcd results obtained near the ferromagnetic Curie temperature wer compared with the magnetic properties of the crystal.
Structural stability and trends in band structures of covalent‐ionic compounds
Authors: Altshuler A. M. , Kh. Vekilov Yu. , Umarov G. R. 
Published: July 2007

AbstractThe relationship between the conditions of the structural transition and the particularities of the band structure is discussed. The derivatives of the forbidden gap with respect to temperature and pressure as well as the PT diagrams for a number of single crystals, and the phase diagrams for the SiGe system are calculated, using the analytical relationships obtained for the electron energies in high‐symmetry points of the Jones zone for compounds having structures either of diamond or of ZnS. Effects of collective excitations are also discussed.
Diffusion of triplet excitons in amorphous tetracene
Authors: Hofberger W. , Bässler H. 
Published: July 2007

AbstractAnalysis of the excitation spectrum of extrinsic photoconduction in evaporated amorphous tetracene films in terms of exciton diffusion theory yields a diffusion length ld = (1000 ± 250) Å for triplet excitons. From pulsed photoconductivity measurements the lifetime of free excitons is found to be (0.6 ± 0.1) μs. The triplet exciton diffusion coefficient is D = (1.6 ± 0.8) × 10−4 cm2 s−1.
Ferromagnetic resonance in CdCr2Se4 with Fe addition
Authors: Gurevich A. G. , Karpovich V. I. , Rubalskaya E. V. , Bairamov A. I. , Lapovok B. L. , Emiryan L. M. 
Published: July 2007

AbstractThe temperature and angle dependences of the resonance field and resonance linewidth have been measured for ferromagnetic CdCr2Se4 crystals with Fe addition. An attempt is made to interpret the observations on the basis of the Fe2+ energy level model proposed by Hoekstra et al. and generalized for the case of finite minimum level splitting. The contributions of relaxation mechanisms to the linewidth and resonance field are discussed and the parameters of the model are roughly estimated.
Theory of arbitrary coupling F‐centres in multivalley cubic crystals
Authors: Khazan L. S. , Sheka V. I. , Mozdor E. V. 
Published: July 2007

AbstractA theory of a large radius polaron bound to a positive Coulomb centre in a multivalley ionic cubic crystal is given for arbitrary values of both the electron–phonon coupling and the Coulomb binding. By means of a direct variational method with highly flexible trial functions, general expressions for the ground state and the first excited state energies are obtained depending on the coupling parameters and on the anisotropy of electron band masses. Under some simplifying assumptions the numerical values of these energies and of the oscillator strengths of optic transitions between the ground and excited states are given.
Nonpolar Scattering of Electrons by Optical Phonons in Small‐Gap Semiconductors
Authors: Bogusławski P. 
Published: July 2007

AbstractElectron transition probabilities and high temperature relaxation time for nonpolar optical‐phonon scattering in small‐gap and symmetry‐induced zero‐gap semiconductors are calculated. The band structures of these materials are described by Kane's three‐level model. Matrix elements are expressed by means of two deformation‐potential constants. The contribution of transverse phonon branches to the nonpolar optical relaxation time is higher than that of the longitudinal branch. Spin‐flip and spin‐conserving processes are of similar importance. The role of the nonpolar mode is comparable to polar optical‐phonon scattering at high electron concentrations in both InSb‐like and HgTe‐like semiconductors, being of particular importance in homopolar α‐Sn. The present theory is applicable to the valence band of III–V compounds, overcoming simplifications used by other authors.
Study of Crystal Field Splitting of Pr3+ in PrF3 by Inelastic Neutron Scattering
Authors: Feldmann K. , Hennig K. , Kaun L. P. , Lippold B. , Matthies S. , Matz W. , Savenko B. N. , Welsch D. 
Published: July 2007

AbstractThe electrical crystal field (ECF) levels of the Pr3+ ion in PrF3 are studied by means of inelastic neutron scattering. Spectra are obtained at sample temperatures of 20, 80, and 300 K. From the theoretical analysis basing on an ECF parameter set for the isostructural Pr: LaF3 a complete scheme of the 3H4 multiplet of Pr3+ in PrF3 is proposed.
On the Nature of the Charge Carriers in Magnetite
Authors: Buchenau U. 
Published: July 2007

AbstractThe nature of the charge carriers in magnetite is discussed both from theoretical arguments and experimental evidence. Estimates are given for the bandwidth, the polaron localization energy, and the strength of electron‐electron interactions. The low mobility, the ferromagnetic value of the exchange constant JBB and NMR results in the low temperature phase are explained qualitatively in terms of electrons localized in polaronic Fe‐Fe bonds.
Exact Treatment of the ESR Lineshape of Triplet Excitons in Molecular Crystals with Two Molecules per Unit Cell
Authors: Reineker P. 
Published: July 2007

AbstractThe motion of triplet excitons in molecular crystals with two inequivalent molecules per unit cell is treated as a hopping process, using the stochastic part of the Hamiltonian of the Haken‐Strobl model. The dynamics of the spin is described by the usual Hamiltonian, taking into account the fine structure interaction. Using a correlation function formalism, the line shape is calculated as a function of the frequency at fixed magnetic field and as a function of the magnetic field at fixed frequency. The relation between the eigenvalues of the correlation functions and the lineshape is discussed, and it is shown that the lineshape may be described approximately by four and two eigenvalues, if the hopping rate of the exciton is small and large, respectively.
Fermi Resonance in Raman Spectra of KI:NO
Authors: Haldre T. J. , Rebane L. A. , Liapzev A. V. , Kiselev A. A. 
Published: July 2007

AbstractThe structure of the v1 fundamental line having two intense and two weak components is studied in the Raman spectrum of the NO impurity ion in the KI crystal. The two intense components are explained assuming that they are caused by Fermi resonance of the v1 vibration and v3 + v combination tone involving the v3 internal vibration and the libration of NO The corresponding anharmonicity constant and the relative infrared and Raman intensities of the perturbed vibrations are estimated.
Dynamical Theory of Ferroelectrics with Asymmetrical Hydrogen Bonds. Application to Rochelle Salt‐Type Crystals
Authors: Konsin P. 
Published: July 2007

AbstractA dynamical theory of ferroelectric phase transitions in crystals with asymmetric hydrogen bonds and several proton sublattices is developed. Owing to the presence of the proton‐phonon interaction and the tunnelling of protons, the phase transitions are of displacive type and are induced by the soft phonon‐proton mode. The temperature dependences of the spontaneous polarization and the dielectric constants of Rochelle salt (RS) and deuterated Rochelle salt (DRS) are calculated. The parameters of the theory are determined semi‐empirically. Good agreement with the experiment is obtained. The expected dependences of the frequencies of the soft phonon‐proton mode and pseudosoft proton‐phonon modes on temperature in RS are calculated.
ESR Lineshape and Dynamics of Triplet Excitons in Molecular Crystals with Two Molecules per Unit Cell
Authors: Reineker P. 
Published: July 2007

AbstractThe dynamics of triplet excitons in molecular crystals of the anthracene type is described by a model Hamiltonian consisting of two parts H0 and H1(t). H1(t) is the stochastic part of the Hamiltonian of the Haken‐Strobl model, describing the hopping motion of the excitons in the crystal, and H0 is the spin‐Hamiltonian comprising the Zeeman and the fine structure terms. Starting from this model, an eigenvalue problem for correlation functions describing the lineshape is derived. In the case of rapid exciton motion the eigenvalues relevant for the lineshape are calculated analytically by second order perturbation theory, which allows to describe the positions and the widths of the ESR lines for fixed value of the magnetic high field as a function of the frequency of the alternating magnetic field as well as for fixed frequency as a function of the value of the high field. These general expressions have been specialized to the cases of rotation of the magnetic field around the a‐, b‐, and c′ ‐axes of the crystal.
Raman Scattering in Thallium‐Doped Cesium Halides
Authors: Buchanan M. , Onari S. , Martin T. P. 
Published: July 2007

AbstractImpurity induced first order Raman spectra are presented for thallium doped CsCl and CsBr. The results of a Green's function calculation assuming that only nearest neighbor motion contributes to the Raman scattering are reported and compared with experiment. It is possible to separate the spectral contribution to the Raman scattering from each of the four symmetry coordinates. A reversal is found of the intensity ordering of these contributions when comparing the present results for positive impurities to previous results for negative impurities.
Temperature Dependence of Coupled Optical Phonons and Debye Relaxations in Ferroelectric Order‐Disorder Crystals
Authors: Borstel G. , Andrade P. Da R. , Merten L. 
Published: July 2007

AbstractIn a preceding paper a set of coupled equations describing the effect of a disorder mechanism on the dispersion of optical phonons and polaritons was given. These equations are valid for the paraelectric region of ferroelectric crystals. The set of coupled basic equations is extended to be valid also for the ferroelectric region. The properties of the different wave types as solutions of these equations are very similar to those of the paraelectric region. At the Curie point the coupling leads in general to a hard core frequency for which a closed expression is given. However, in the limiting case of small Debye relaxation times the theory results in a soft‐mode‐like behaviour for the transverse optical phonons. The algebraic and numerical results are discussed in detail.
The Central Peak in SbSI
Authors: Steigmeier E. F. , Auderset H. , Harbeke G. 
Published: July 2007

AbstractNew measurements of light scattering in SbSI as a function of temperature are reported which extend the previous results to lower frequencies by improving the spectral purity of the spectrometer. The light is found to couple to the central peak in SbSI only indirectly via the soft mode. Using this fact as a condition, from a two‐coupled‐oscillator analysis of the soft‐mode/central‐mode system all the oscillator parameters are determined. Within the first 10 deg below the phase transition the central mode turns out to be a strongly overdamped oscillator of width Γω/2γ1 of 0.4 to 4 cm−1. This width compares very well with the value obtained from the hydrodynamic theory of light scattering; using measured thermal conductivity and specific heat data of SbSI one obtains a width of 0.3 to 1 cm−1 for a ΔT of 2 to 10 deg. Thus, the central peak in SbSI has been directly spectrally resolved and it seems justified to assign this feature to light scattering by phonon density fluctuations.
The Two Band Models in the Theory of Disordered Substitutional Alloys
Authors: Kudrnovský J. , Petru̇ J. 
Published: July 2007

AbstractFor description of the electronic structure of a binary substitutional alloy, a two band model with hybridized S and D bands is introduced, which uses the concept of multiplicative off‐diagonal randomness thus representing a matrix generalization of the single band model of Shiba. All matrix elements may be random permitting to interpolate between the band structure of both pure component crystals with different band locations and/or widths, and with different hybridization strength and k‐dependence as well. All previously described two‐band alloy models are special cases of the present scheme. Despite the high flexibility of the model, its numerical implementation is easy, because of the special Hamiltonian structure.
Temperature Dependence of E1 and E1 + Δ1 Maxima in the Fundamental Reflection for Cdx Hg1‐xTe Solid Solutions
Authors: Kisiel A. , Podgórny M. , Rodzik A. , Giriat W. 
Published: July 2007

AbstractThe width of the energy gap at the A‐point of the Brillouin zone was measured in the temperature range from 100 to 315 K for CdxHg1‐xTe solid solutions. Measurements were carried out for Cd content from 0 to 17%. The thermal coefficients of the El and El + Δl transitions were determined for the above mentioned range of composition. Some anomalies in the E1(T) dependence have been found for volume compositions x̄ = 0.125. A possible connection between these anomalies and the inversion of band structure is discussed.