bluuebunny/crossref_metadata_2025 huggingface dataset:
Electroreflectance Spectra of AlxIn1−xSb Alloys
Authors: Isomura S. , Prat F. G. D. , Woolley J. C.
Published: July 2007

The preparation of homogeneous samples of AlxIn1–xSb alloys from a stoichiometric melt is considered. A slow horizontal Bridgman method with ingots encapsulated in liquid B2O3 has been used to provide samples suitable for optical reflectance measurements and details of the variation of composition with position along the ingot are given. Electroreflectance measurements, using the electrolyte technique, have been made on samples cut from cross sectional slices, and the variation of the transition energy with composition has been determined for the E0, E0 + Δ0, E1 and E1 + Δ1 peaks. The experimental values of E0, Δ0 and Δ1 as a function of x are compared with the previously proposed theoretical equations based on the concept of interband mixing due to disorder of the lattice, and the significance of some of the energy parameters in these equations is discussed.
Electron–Lattice Interaction in Gallium Selenide
Authors: Schmid Ph. , Voitchovsky J. P.
Published: July 2007

In the layered semiconductor GaSe, the interaction of holes with non‐polar, fully symmetric, optical phonons governs both the Hall mobility of the carriers and the temperature shift of the direct energy gap. The results are in agreement with theory and indicate that the interaction of charge carriers with acoustic phonons is rather weak.
F‐Centre Theory in Multivalley Cubic Crystals
Authors: Khazan L. S. , Pekar S. I. , Sheka V. I.
Published: July 2007

The interaction between an electron bound to a Coulomb potential and the LO lattice vibrations in a cubic multivalley ionic crystal is considered for not too weak coupling. The vibrational degrees of freedom in the partition function of the system are eliminated by Feynman's ordered‐operator technique. The complicated operator thus obtained depends only on the electron variables and is approximated by a suitable operator containing some available parameters. The latter provides to calculate the partition function of the F‐centre by means of a variational principle at arbitrary temperatures and thus to obtain the ground state energy of the F‐centre as a function of coupling parameters and of the anisotropy of electron band masses.
The Interaction of an Optical Polaron with Zero Vibrations of the Lattice in High Electric Fields
Authors: Shmelev G. M. , Sibirskii V. A.
Published: July 2007

The mobility of polarons interacting with zero acoustic vibrations of the lattice in high electric fields is evaluated. The polaron is described by the Feynmann‐Kadanoff model. The distribution function of such carriers is obtained as a solution of the Boltzmann equation, which is found to be never normalized. The effect of the escape of polarons in high electric fields leads, in principle, to the appearance of regions with negative differential conductivity in the voltage–current curve.
The Magnetic Properties and Indirect Exchange in Gd(Ni, Cu)2
Authors: Poldy C. A. , Kirchmayr H. R.
Published: July 2007

The magnetic properties of compounds in the Gd(Ni, Cu)2 pseudobinary system are described and the variation of the ordering temperatures with composition is accounted for in terms of the Rudermann‐Kittel‐Kasuya‐Yosida theory. In excellent agreement with experiment the model predicts a transition from the antiferromagnetic state of GdCu2 to the ferromagnetic one upon replacing 13% of Cu by Ni, where it is assumed that the 3d‐states of Ni are filled. Support for this assumption is obtained by considering the Curie temperature variation of the Ni‐rich end of this system in conjunction with the Gd(Co, Ni)2 system.
Energy Level Structure of Biexcitons and Related Optical Transitions
Authors: Bassani F. , Forney J. J. , Quattropani A.
Published: July 2007

The states of the biexciton molecule are related to the band structure of the crystal and effects due to spin–orbit interaction and to electron–hole exchange are taken into account. The cases of CuCl and CdS are explicitly studied and the possible bound states are given. Optical luminescence decay processes and two‐photon excitation processes are discussed and related selection rules are given. In the case of CdS all decay processes are shown to produce polarized photons. Experimental results are analyzed and are compared with theoretical calculations of the biexciton ground state energy.
Trigonal Warping of the Valence Band Fermi Surfaces of Tellurium
Authors: Köhler H.
Published: July 2007

Trigonal warping of the valence band Fermi surfaces of tellurium, which was observed earlier by Shubnikov‐de Haas measurements for B ⊥ c (c trigonal axis), was investigated in detail from the oscillatory part of the magnetoresistance using arrangements for the magnetic induction B not normal to the trigonal axis of the crystals. Keeping the angle υ = ∡ (B, c) at 70° while rotating the samples around their c‐axis, a higher degree of reliability was achieved, where only fields up to 10 Tesla (100 kG) magnetic induction were necessary, and the observation of the effect was possible at hole concentrations of less than 1 × 1018 cm−3. Besides, a decision between the main influence of inversion or mirror symmetry of the trigonal warping on Shubnikov‐de Haas effect could be made for the first time. Numerical calculations were performed to fit the results using current ε(k) models of the valence band of tellurium.
Electric‐Field‐Induced Optical Activity and Circular Dichroism of Cr‐Doped KAl(SO4)2 · 12H2O
Authors: Weber H. J. , Haussühl S.
Published: July 2007

The effects of electrical fields on the optical activity and circular dichroism in chromium doped KAl(SO4)2 · 12 H2O crystals have been measured. Both effects are described by a third rank axial tensor, and therefore depend linearly on the electric field strength. The measured values agree with Vegard's rule. In the vicinity of the absorption band at 567 nm, both effects can be interpreted on the basis of a simple dispersion model.
Frequency‐Dependent Relaxation Time of Electrons in PbSe from Infrared Measurements
Authors: Mycielski A. , Aziza A. , Mycielski J. , Balkanski M.
Published: July 2007

Infrared magneto‐reflectivity data for n‐PbSe are analysed and interpreted using a frequency‐dependent momentum relaxation time of free electrons. A peculiar variation of the relaxation time with frequency suggests a dissipation mechanism consisting in short‐wavelength plasmon generation.
Dynamical Observables in Cubic Crystals from the Standing LCAO Wave Functions
Authors: Olszewski S. , Wierzbicki A.
Published: July 2007

Average electron velocity, acceleration, and angular momentum in cubic crystals have been calculated with the aid of standing LCAO wave functions. The calculations are compared with those based on the itinerant LCAO wave functions of Bloch.
Precipitates in Annealed and Quenched NaCl: Cd++ Crystals
Authors: Spengler W. , Kaiser R.
Published: July 2007

Raman scattering, X‐ray, microprobe, and electron microscopy experiments show that precipitates of the crystallographic phase 6NaCl · CdCl2 are present in NaCl crystals doped with CdCl2 in the concentration range from 0.008 to 2.2 mol%. The size of the precipitates increases with increasing Cd concentration. If the NaCl: Cd crystals are quenched in a few seconds from 550 to 20 °C the main Raman peak is shifted to higher frequencies. This shift of the Raman peak is interpreted as being caused by the smaller size of the precipitates in quenched crystals.
Piezoemission Study of Free‐Exciton Exchange Interaction in CdS
Authors: Goede O. , Winter D. , Hasse E.
Published: July 2007

The free‐exciton exchange interaction in CdS is investigated by piezoemission measurements with static uniaxial stress σ ⊥ c and photon wave vector Kph ∥ c. The exchange‐induced splitting of the A(T5)‐exciton emission band and the stress dependence of the optical transition probabilities for the two components with polarizations E ⊥ σand E ∥ σ are measured at 77 and 27 K. The results are quantitatively discussed in the framework of the quasi‐cubic model for wurtzite‐type II–VI semiconductors and the exciton exchange parameter is determined in agreement with the zero‐field data of free exoitons in CdS. It is shown that the exchange parameter obtained by Langeretal. [l] from piezoreflection measurements is too large by about a factor of two because no corrections were made for the stress‐induced change of the transition probabilities increasing the experimentally observed splitting of the reflection minima.
Kinetics of Stress Relaxation in Normal and Superconducting States
Authors: Dotsenko V. I. , Landau A. I. , Pustovalov V. V.
Published: July 2007

An investigation is described of the kinetics of stress relaxation and postrelaxational effects in normal (N) and superconducting (S) states of polycrystalline 99.9995% pure lead at 4.2 °K. The work‐hardening coeificient θ(r) during stress relaxation has been found to be much larger than that θ(r) during dynamic deformation. It has also been revealed that θ(r)depends on the state of the sample (N or S state) where θ>θ.
The Pseudopotential Method and the Problem of Ordering in Alloys. II. Multi‐Ion Interaction
Authors: Krasko G. L. , Makhnovetskii A. B.
Published: July 2007

The problem of multi‐ion interaction in ordered alloys is considered. Khachaturyan's theory of ordering is modified to take into account this interaction. A pseudopotential approach is developed and a procedure suggested to calculate the “main” terms of third and fourth order perturbation theory including the effects of the Fermi‐surface distortion. Expressions are found for the thermodynamic characteristics of alloys in terms of pseudo‐potentials of ion‐components.
Observation of Magnetostriction in Cu Ferrite Single Crystals
Authors: Arai K. I. , Tsuya N.
Published: July 2007

The magnetostriction constants λ100 and λ111 of Cu ferrite single crystals quenched from 820, 900, and 950 °C were measured by a capacitance method in a temperature range from liquid N2 to room temperatures. For the crystal quenched from 900 °C, the magnetostriction constants λ100 and λ111 at room temperature are −87.3 × 10−6 and 9.8 × 10−6, respectively. The absolute value of the constant λ100 has a maximum (about 200 × 106) near 155 K, and the maximum value decreases with the increase of the quenching temperature, while the value of the constant λ111 decreases monotonically with the increase of the temperature.
Optical Phonons in ZnGeP2 and CdGeP2
Authors: Bettini M. , Miller A.
Published: July 2007

The phonon spectra of ZnGeP2 and CdGeP2, determined by Raman scattering on single crystals at room and liquid nitrogen temperature, are presented. The close coincidence of the zone center phonon energies of ZnGeP2 with the corresponding τ, X, and W modes of its isoelectronic analogue, GaP, bears out the expected similarity of lattice vibrations in ternary chalcopyrite compounds and binary zincblende materials. Differences between ZnGeP, and CdGeP2 can be attributed to mass and force constant differences when Zn is substituted by the heavier Cd atom.
The Dynamics of Molecular Crystals under Pressure
Authors: Pawley G. S. , Mika K.
Published: July 2007

A general procedure for the calculation of the lattice modes in molecular crystals under pressure is given. Only a few modifications are necessary to the general lattice dynamical computer program. Calculations have been performed and the results are presented for naphthalene, anthracene, and orthorhombic sulphur. Although the basic program uses a sum over atom‐atom interactions to derive intermolecular interactions, this is not a necessary assumption in the procedure for the application of pressure. For naphthalene and anthracene the calculated increase in phonon frequency per kilobar of pressure is roughly 0.045 and 0.12 THz for modes with frequency 2 and 6 THz, respectively. Not all modes behave in this manner, and so the calculated pressure variation could prove very useful in mode assignment. The effect of uniaxial pressure on orthorhombic sulphur was also investigated, showing a markedly different frequency variation for some modes but not for others. Thus uniaxial pressure, if experimentally possible, could prove further the success of mode assignment. At 23 kbar uniaxial pressure the orthorhombic structure of sulphur became unstable, resulting in a monoclinic form in the space group C2/c. It is of interest that the simple model for intermolecular interaction can predict a soft mode phase transition, though whether this occurs in practice remains to be seen.
Impurity Effect on the Metal–Semiconductor Transition of NiS
Authors: Ohtani T. , Kosuge K. , Kachi S.
Published: July 2007

Impurity effects on the metal‐semiconductor transition of hexagonal N P have been investigated by magnetic susceptibility, electrical resistivity, and thermoelectric power measurements. The transition temperature Tt is raised by addition of Ti, V, and Fe, and lowered by the addition of Co and Cu. Cr ions do not seem to be dissolved in NiS lattice. The sample containing 2.0% Cr shows, however, a negative Seebeck coefficients below Tt, which implies electron conduction, in contrast with pure NiS. Conduction electrons in this phase seem to be produced from sulfur vacancies. The transition mechanism is explained by the electronic phase diagram based on the Mott‐Hubbard model.
The fluorescence properties of crystalline anthracenes and their dependence on the crystal structures
Authors: Cohen M. D. , Ludmer Z. , Yakhot V.
Published: July 2007

The fluorescence properties of a number of crystalline anthracenes are described. It is found that in excimer‐emitting crystals the energy of the emission is shifted by change in temperature much more strongly in infinite‐stack structures than in structures based on pair‐wise packing. A theoretical treatment is given of the general problem of the excimer emission from a crystal, and is applied to the anthracenes. This theory shows that the sensitivity to temperature for high temperatures is determined by the ratio of the frequencies of intra‐pair vibrations in ground and excited states of the particular material. An interpretation of the difference in this ratio for stack and pair structures is given, and some predictions based on this interpretation are given and tested. It is confirmed that structures in which molecular overlap is small give rise to monomer emission.
Elastic interaction of point defects in isotropic and anisotropic cubic media
Authors: Schaefer H. E. , Kronmüller H.
Published: July 2007

Using the Green's tensor function as the solution of the elastic equations for isotropic and anisotropic cubic media the elastic interaction energy is calculated for pairs of defects which have isotropic, tetragonal, or orthorhombic symmetry. Special attention is given to the interaction of a one‐dimensionally migrating interstitial, in crowdion configuration, with other defects. The differences of the interaction in elastically isotropic and anisotropic materials, the influence of defect anisotropy on the interaction potential, and the relation of these calculations to experimental results are discussed.
Binding energies and wave functions of wannier excitons in uniaxial crystals — a modified perturbation approach. I. theory
Authors: Gerlach B. , Pollmann J.
Published: July 2007

The effective mass Hamiltonian for excitons in uniaxial crystals is discussed by means of a modified perturbation approach. This approximation scheme gives upper limits for the exact energies; the validity of the approach is tested by deriving lower limits for the exact energies. The spacing of the two limits is shown to be satisfactorily small in a wide range of the anisotropy parameter. This parameter free approach leads to energies, which show good agreement with the results of involved variational calculations. In addition analytical expressions are obtained for the wave functions, which are easy to handle and can be used in more complex situations e.g. to include the description of external field influences.
Optical absorption spectrum of CoIII in LaCoO3
Authors: Marx R. , Happ H.
Published: July 2007

The optical spectrum of Co111 in LaCoO3 has been investigated between 6500 and 34000 cm−1. The observed crystal field splittings are explained by a crystal potential of cubic and axial symmetry. The cubic and axial ligand field parameters and the Raccah parameters describing the electron interaction are determined and their consequences with respect to the magnetic properties of LaCoO3 are discussed. A critical summary of the existing theories concerning the absorption spectra of 3d electrons of transition metals in cubic and axial fields will be given.
An analysis of models for the ionic conductivity of silver chloride
Authors: Coerish J. , Jacobs P. W. M.
Published: July 2007

The high‐temperature conductivity of pure AgCl has been re‐examined with a view to establishing more precisely the mobility parameters for non‐collinear interstitialcy motion of Ag+ interstitials. The possible roles of direct interstitial jumps, Schottky defects, and temperature dependent activation energies has been assessed. Fomulae are given for the activity coefficients of interstitials and vacancies evaluated to O(x2) in an approximation that uses a sphericalized Brillouin zone and inlcudes only the cycle sum contributions. These activity coefficients are compared with those evaluated from the Debye‐Hückel formula and their effect on the defect parameters obtained from conductivity data is assessed.
Band structure of solid o‐H2 with the molecular tight‐binding and OPW method
Authors: Parravicini G. Pastori , Villa I. , Vittori M.
Published: July 2007

The band structure of f.c.c. solid hydrogen is computed using the molecular tight‐binding method for the valence band and the molecular OPW method for the conduction bands. The direct energy gap is found to be 15.1 eV and corresponds to the allowed transition L → L. The pressure dependence of relevant states is obtained and it is shown that the energy gap decreases with pressure. Some features of the far vacuum ultraviolet excitation spectrum are qualitatively interpreted.
An investigation of the structure of Cu‐H centers in ZnO by EPR
Authors: Zwingel D.
Published: July 2007

In ZnO single crystals, doped both with hydrogen and copper, three new EPR spectra are reported which are assigned to different CuH complexes. It is found that an OH group is formed next to a substitutionally incorporated Cu2+ ion (or pairs thereof). The paramagnetic properties of these centers are markedly different from the known isolated ZnO :Cu‐center and may be described rather well by simple crystal field theory of a 3d9 ion.
The dynamical conductivity of electrons in semiconductors assuming scattering by dislocations
Authors: Gerlach E. , Rautenberg M.
Published: July 2007

Using a method, which is based on the equivalence of energy loss and Joule heat, the dynamical conductivity of semiconductors is calculated. Scattering by dislocations is assumed, which are described by a simple monopole and dipole line model. The theory is applied to Te for frequencies 107 to 1014 Hz (10−7 to 1 eV) and for temperatures T = 10, 20, and 30 K.
Antiferromagnetic resonance in NIO:Co2+ and NIO:Fe2+
Authors: Ph. Lau C. R. Becker , Geick R. , Wagner V.
Published: July 2007

The antiferromagnetic resonance (AFMR) in NiO doped with either Fe2+ or Co2+ was studied at liquid helium temperatures in the far infrared. A strong frequency shift of the AFMR line was observed in Co2+‐doped crystals. In the case of the Fe2+‐doped crystals, this shift was much smaller. In addition, a second absorption line was found at larger Co2+ concentrations (5% to 7%), the nature of which has not yet definitely been established. A simple model calculation which primarily employs the single ion anisotropies of the Co2+ and Fe2+ ions was performed for a qualitative interpretation of the experimental results.
Cyclotron resonance of electrons in semimetallic bismuth‐antimony alloys
Authors: Herrmann R. , Brbune W. , Kuka G.
Published: July 2007

The cyclotron resonance of electrons in semimetallic BiSb alloys is investigated within a range of 0 to 4 at % antimony at 35 GHz in three crystallographic planes. A nonlinear dependence of the cyclotron masses on the antimony concentration (x at %) is found. The analysis of spectra indicates limiting‐point resonances being associated with the cyclotron wave propagation.
A theory of DC‐hopping conductivity in a disordered one‐dimensional system accounting for electrical fields of arbitrary strength
Authors: Bottger H. , Bryksin V. V.
Published: July 2007

The current‐voltage characteristics are investigated for the dc‐hopping processes in disordered one‐dimensional systems with nearest‐neighbour hopping only. It is shown that in such systems strong local fields can give rise to a nonlinear current‐voltage dependence at much lower external fields than in a corresponding ordered system. Qualitatively similar results may also be expected for three‐dimensional systems.
Lattice dynamics of rare‐gas solids by Kihara's potential: I. Phonon spectra
Authors: Agrawal T. N. , Gupta R. K. , Gupta G. L.
Published: July 2007

The applicability of Kihara's potential function to rare‐gas solids has been investigated by studying their lattice dynamics. The intermolecular potential parameters are determined from the experimental sublimation energy and lattice spacing extrapolated to 0 °K, taking into account the zero point energy. These parameters are used to compute the theoretical phonon spectra for the solidified rare gases neon, argon, krypton, and xenon. The theoretical results are found to compare well with available experimental data.
The electronic structure and phase diagrams of binary alloys with complex lattices: I. The application of static concentration waves and pseudopotential methods for calculating the phase diagrams
Authors: Zhorovkov M. F. , Fuks D. L. , Panin V. E.
Published: July 2007

It is shown that the structures of the ordered phases in binary alloys with complex Ising lattice (made up by the α‐Bravais lattices displaced with respect to each other) can be determined theoretically by means of the static concentration wave (s.c.w.) method and pseudopotential theory. The expression which gives the relationship between the phenomenological parameters of the s.c.w. method and the energy‐wavenumber characteristicof thepseudopotential theory is obtained. The superstructures which may be stable with respect to the formation of an antiphase boundary structure is obtained for the hexagonal close‐packed lattice.
Tight‐binding calculations of the valence bands of diamond and zincblende crystals
Authors: Chadi D. J. , Cohen M. L.
Published: July 2007

Using the tight‐binding method, the valence band structures and densities of states for C, Si, Ge, GaAs, and ZnSe are calculated. Very good agreement is obtained with other calculations when all nearest‐ and one second‐nearest‐neighbor interactions are included. The effects of the various interactions on the density of states are discussed.
Radio‐frequency size effect in bismuth
Authors: Müller H.‐U. , Hess S. , Herrmann R. , Scholz H.
Published: July 2007

The anisotropy of the momentum of the Fermi surface of electrons has been investigated by the radio‐frequency size effect. By variation the polarization of the radio‐frequency field it is possible to determine completely the anisotropy of the momentum in the trigonal and binary planes. The shape of the Fermi surface in the binary plane is shown to be different from the ellipsoidal one and not symmetric to the axis.
Exchange integral determination in Co3O4
Authors: Scheerlinck D. , Hautecler S. , Wegener W.
Published: July 2007

The energy distributions of neutrons scattered by Co3O4 in its paramagnetic state have been measured by means of a time‐of‐flight technique. In this normal spinel with only A‐A interactions, the JAA exchange integral has been derived from the second moment of these energy distributions. The result, JAA/kB = (7.27 ± 0.25) K, has been compared with values deduced from an analysis of the macroscopic magnetic data.
Dispersion relation for surface wannier excitons
Authors: Rimbey P. R.
Published: July 2007

The surface exciton dispersion relation is derived for a spatially dispersive medium including the additional boundary condition Px(0+) = 0. The dispersion is complex for long‐wavelength P‐polarized light in the absence of phenomenological damping suggesting surface excitons exist as critically damped wavepackets.
Effect of the anisotropy of atomic motions on Mössbauer spectra of polycrystals
Authors: Goldanskii V. I. , Karyagin S. V.
Published: July 2007

Examples of anisotropic motions (vibrations, diffusion, eddy flows) are given which lead to the dependence of the shape of Mossbauer spectra on the relative orientation of the system studied (a part of a monocrystal) and the direction of observation of γ;‐rays. The anisotropy of continuous diffusion in polycrystals is found to result in different broadenings of the π and σ components. The jump diffusion is found to induce a virtually equal broadening of the π and σ components. The line shape in this case is non‐Lorentzian. Study of line shapes can give essential information on the diffusion mechanism.
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

A 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

In 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

Uniaxial 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

The 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

Torque 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

Analysis 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

It 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

The 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

Formulas 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

The 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

From 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

Reflection 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

The 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

The 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

Analysis 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

The 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

A 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

Electron 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

The 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

The 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

The 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

The 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

A 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

The 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

Impurity 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

In 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

New 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

For 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

The 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.