bluuebunny/crossref_metadata_2025 huggingface dataset:
Ligand Hyperfine Interaction of Ti2+ in CdS and CdSe on Sites with Trigonal Symmetry C3v
Authors: Böttcher R. , Dziesiaty J.
Published: July 2007

An EPR investigation is made of the hyperfine interactions with ligand nuclei (SHFS) of the Ti2+ centre in single crystals of hexagonal CdS and CdSe on sites with trigonal point symmetry C3v. From the spectra experimental evidence is given for the magnetic nonequivalence of the twelve next‐nearest ligands (cadmium) in wurtzite‐type crystals. The SHFS‐constants for the first three cadmium lattice shells for wurtzite are calculated using MO‐theory. From the SHFS‐constants the probability is estimated of finding the delocalized 3d‐electrons of the Ti2+ ion in the 5s‐orbitals of the cadmium ligands.
Diffusional Relaxation of Crystal Lattice Defects in the Fields of Local Lattice Disturbances
Authors: Estrin Yu. Z. , Khachaturyan A. G.
Published: July 2007

Diffusional relaxation of mobile defects in crystals containing local disturbances is considered. It is shown that the distribution density of the inverse relaxation times in such a system calculated as the imaginary part of the trace of the diffusion equation Green's function may involve resonance maxima. Their positions are determined by the interaction potential of the mobile defect and the immobile local perturbation center. The relevant internal friction peaks when detected provide a possibility of investigating this potential. The distribution density of the inverse relaxation times is calculated for a number of particular systems which model the diffusional relaxation of point defects in the local fields of a point disturbance and of a plane defect and the kink relaxation in the presence of a disturbance. It is shown that several internal friction peaks can be associated with a single relaxation mechanism.
Stress‐Dependence of the Raman Frequencies and Elastic Constants of Cubic Semiconductors
Authors: Bell M. I.
Published: July 2007

A model proposed by Keating to describe the second‐ and third‐order elastic constants of diamond‐structure crystals is used to calculate the parameters (p, q, r) which characterise the stress‐induced shifts and splittings of k = 0 optical phonon frequency. The results for Si and Ge are in good agreement with experiment. If the electrostatic contributions to the elastic constants and Raman frequencies are neglected, the model may be applied to zinc‐blende‐type crystals, but the results obtained in this way for GaAs and InSb are less satisfactory.
Lattice—Ion Interactions in the Ligand Field Theory of Trigonally Distorted Octahedral V3+ Complexes
Authors: Bhattacharyya B. D.
Published: July 2007

The theoretical expressions for the magnetic susceptibility and g‐values of trigonally distorted octahedral V3+ ion have been developed on the basis of molecular orbital method of Stevens [19] and Bose et al. [20]. Results are discussed considering the less strong vibronic coupling (dynamic Jahn‐Teller effect) in which an orbital triplet interacts with the vibrational E‐modes of the ligands. Comparison is made with experimental results on magnetic susceptibility, g‐values, and optical absorption with a unique set of values of parameters in the theoretical expressions at all temperatures. The Jahn‐Teller coupling parameter appears to be appreciable in some of the complexes.
Study of Molecular Motions in Liquid Crystalline MBBA by Cold Neutron Scattering
Authors: Dimic V. , Barbič L. , Bling R.
Published: July 2007

Self‐diffusion in the nematic and isotropic liquid phase of 4‐n‐methoxybenzylidene‐4′‐n‐butylaniline (MBBA) has been studied by incoherent quasielastic neutron scattering. A comparison of NMR and neutron data shows a large difference in the values of the diffusion coefficients, D, indicating that rotational as well as translational diffusion contribute to the self‐diffusion coefficient observed by the neutrons. These measurements are contrasted with those of PAA where translational diffusion is dominant.
The Influence of the Non‐Locality of Shaw's Model Potential on the Band Structure of Be
Authors: Taut M.
Published: July 2007

The influence of the non‐locality of Shaw's model potential on the band structure of Be is investigated. A method is given for the treatment of the energy dependence of a linear energy dependent potential. The energy dependence of Shaw's model potential causes an increase of the band width. The density of states and the Fermi surface using the non‐local model potential agree well with experiment whereas the local approximation is not appropriate.
Non‐Local Screened Exchange in Copper
Authors: Stöhr H. , Bross H.
Published: July 2007

This paper reports an investigation of solid copper using a nonlocal screened exchange. It is shown that the energy bands strongly depend on the screening constant. Its optimum value obtained by a fit to several experiments yields an excellent agreement with measured energy differences of the valence bands as well as reasonable results for the higher core levels.
Coincidence Site Lattices
Authors: Fortes M. A.
Published: July 2007

The general conditions for the occurrence of a coincidence site lattice, i.e., for partial coincidence of lattice points in two identical crystal lattices, are derived and a method is outlined for determining the degree of coincidence. The question of atomic coincidence in two actual crystals is discussed, leading to the concept of mixed coincidence (coincidence of non‐equivalent atomic sites).
Atomare Fehlstellen mit niedriger Symmetrie in inhomogenen Spannungsfeldern
Authors: Hornung W.
Published: July 2007

Zur Beschreibung der Bewegung atomarer Fehlstellen mit niedrigerer als Gittersymmetrie wird eine allgemeine Methode unter Verwendung von Ratengleichungen angegeben und an speziellen Beispielen durchgeführt. Dabei werden Umorientierungs‐ und Wanderungsprozesse gleichzeitig berücksichtigt. Der Fall inhomogener Spannungsfelder wird durchgeeignete Transformationen auf den Fall homogener Felder zurückgeführt. Das zugehörige hochdimensionale Gleichungssystem wird nach räumlicher Fourier‐Transformation durcheinen Relaxationsansatz gelöst. Bei inhomogenen Feldern tritt eine bisher unbekannte Relaxationsfunktion auf, deren Zeitkonstante mit den von diesen Feldern induzierten Wanderungsschritten verknüpft ist. Die zugehörige Aktivierungsenergie unterscheidet sich von der im Snoek‐Effekt gemessenen, falls die Aktivierungsenergie der lokalen Umorientierung kleiner ist als die der Wanderung.
Dielectric Function of a Model Semiconductor
Authors: Rashenov V. K. , Foigel M. G. , Alarashi R. A.
Published: July 2007

Penn's model of an isotropic semiconductor is used to calculate ε1(q), the real part of the static dielectric function. With the aid of the optical f‐sum rule the expression for the interband transition matrix element is obtained which is different from that of Penn. The calculations reveal close correspondence of ε1(q) with that of other authors obtained from the realistic model of the band structure. The dielectic functions of Si, Ge, Gap, GaAs, and ZnSe crystals are given.
Relaxation and Nuclear Dynamic Polarization in the Case of Inhomogeneous EPR Broadening
Authors: Buishvili L. L. , Zviadadze M. D. , Fokina N.
Published: July 2007

The time of nuclear relaxation and the amplification coefficient of polarization in diluted paramagnetic crystals with inhomogeneous broadening of EPR and with allowance for nuclear spin diffusion are calculated in various limiting cases.
The Influence of Interatomic Correlation on the Instability of the Paramagnetic State in an Orbitally Nondegenerate Narrow Band
Authors: Schneider J. , Heiner E. , Haubenreisser W.
Published: July 2007

The role of the interatomic correlation on the paramagnetic instability against the ferromagnetic state in an orbitally nondegenerate narrow band is discussed in the frame of an extended Hubbard Hamiltonian. It is concluded that the intra‐atomic Hund's‐rule coupling in orbitally degenerate bands seems to be more important to stabilize a ferromagnetic state than the interatomic correlations in an orbitally nondegenerate band.
Temperature Dependence of the Knight Shift in Na in the Nearly‐Free Electron Approximation
Authors: Islam M. M. , Sarker A. Q.
Published: July 2007

In the nearly‐free electron approximation the temperature dependences of the Knight shift in liquid and solid sodium are determined using the experimentally measured structure factors and the temperature‐dependent pseudo potential of Islam and Ball. The calculated values are in excellent agreement with experiment.
Perturbation Study of the Uniaxial Stress Effect on the EPR Spectra of Γ8 Ions
Authors: Passeggi M. C. G.
Published: July 2007

Perturbation theory has been used to derive the shift of the energy levels of a Γ8 system under the effect of a uniaxial stress applied along the [110] axis. The results are correct up to second order in the perturbation parameter f/g (f/g is the ratio between cubic and linear terms in spin variables of the unstressed spin‐Hamiltonian), and up to second order in the stress. The formalism could also be extended to include higher order terms. The results are valid for all values of the angular variable provided that − 1/6 < f/g < 1/9. Otherwise for certain angles, some expressions become invalid.
Surface Phonons in Spectral Oscillations in the Photoconductivity on Silicon (111) Surfaces
Authors: Ibach H. , Mönch W.
Published: July 2007

At clean cleaved silicon (111) surfaces an “oscillatory” photo conductance with excitation of holes out of surface states has been reported [1]. The period of the oscillations is (58 ± 3) meV. A surface phonon (hωs = (56 ± 1) meV) has been found on the same type of surfaces in low energy electron spectroscopy [7]. It is shown that the oscillations in the photoconductivity are caused by the interaction of photoexcited hot holes with the surface phonon.
Antiferromagnetism in Amorphous Carbon Films
Authors: Macfarlane J. M. , McLintock I. S. , Orr J. C.
Published: July 2007

Measurements of the ESR absorption in amorphous carbon films over the temperature range 77 to 300 K provide evidence of an antiferromagnetic transition at 100 K. This transition temperature is not affected by prior annealing of the films in vacuo at temperatures up to 550 K. The ESR line width is independent of temperature between 77 and 300 K. The relationship of these results to current theories on the structure of carbon films and related materials is discussed.
Perturbation Theory for a Spin System
Authors: Trimper S.
Published: July 2007

The spin 1/2 Heisenberg ferromagnet is treated by perturbation theory. To avoid the difficulties connected with the unusual commutation relations of spin operators the Dyson‐Maleev transformation is applied. The corresponding Bose system is described by the Bose‐Green function. To calculate the thermodynamic behaviour of the spin system a relation has been found between spin and Bose‐Green function. The low temperature expansion of the magnetization is calculated. No agreement is found using Dyson's standard result.
Electric Field Effect on Two‐Photon Transitions near Thresholds in Solids I. Three‐Band Model
Authors: Yang E.
Published: July 2007

The expressions for the two‐photon absorption coefficient of solids in the presence of a uniform electric field are derived in a closed‐form. The result, can be expressed in terms of Airy functions where the effective mass approximation and a three‐band model are assumed for a solid. The finite‐field of the imaginary part of the dielectric function due to two‐photon absorption reduces to the convolution of the zero‐field dielectric function with Airy function. As in the one‐photon case, this calculation predicts different spectral dependences for the absorption coefficient at different critical points in the band structure of solids. It is also shown that the peak heights of the spectra depend on the position of the intermediate state.
Pressure Effect on the Fermi Surface of Molybdenum, Tungsten, and Rhenium
Authors: Svechkarev I. V. , Pluzhnikov V. B.
Published: July 2007

Employing the de Haas‐van Alphen effect, the behaviour of small cross‐sections of the Permi surface under pressure has been studied on Mo, W, and Re. The results are discussed in terms of a simple empirical model for spectrum changes under pressure. The model also involves the deformation of d‐band and its shift relative t o the s·p band. The possibility is shown of obtaining quantitative information on the behaviour under pressure of the fundamental parameters of the transition metal energy spectra on the basis of studying small cross‐sections of the Fermi surface.
A Discussion and Generalization of Bethe's Second‐Order Approximation
Authors: Serneels R. , Gevers R.
Published: July 2007

Using the Howie and Whelan system a perturbation technique is presented, based on the Löwdin partioning technique, to take into account the effect of weak beams in contrast calculations. This method is a generalization of the well known Bethe second order correction method. The usual Bethe method is found in the limit of very small weak beam interaction. Some explicit situations are discussed which show the limitations of the usual Bethe method for practical calculations.
Investigation of Electron Penetration and X‐Ray Production in Solid Targets
Authors: Matsukawa T. , Murata K. , Shimizu R.
Published: July 2007

The Monte Carlo method based on a single scattering model is applied to investigate the behaviour of electrons in pure and mixed specimens using the Electron Probe Microanalyser (Epma) and the Scanning Electron Microscope (SEM). Fairly good agreement is obtained with experiments on electron backscattering, penetration, and X‐ray production. The effect of the mean ionization potential on the final results is also discussed.
A Theory of Magnetic After‐Effect for the Irreversible Magnetization Process in Rayleigh Region
Authors: Tahara Y. , Sugeno T.
Published: July 2007

A theory of magnetic after‐effect is reconstructed by using a new model in which the reversible and irreversible magnetizations are separately taken into account. Rayleigh's law is employed for the expression of the reversible and irreversible magnetizations. Influence of magnetic after‐effect on the reversible magnetization process is described as a usual disaccommodation phenomenon. Influence on the irreversible magnetization process is described as a change of the critical magnetic field at which a domain well starts its irreversible displacement. Initial magnetization curves are derived from these processes for the stabilized 180° and 90° walls. The results are compared with those of an experiment on the Fe‐N system. The theoretical results for 90° walls can explain the experiment very well. They also explain the non‐linear concentration dependence of the stabilization field obtained by experiment, whose origin is shown to consist in the method for obtaining the stabilization field.
Transverse Magnetoresistance of n‐Type Ge in Quantizing Magnetic Fields
Authors: Babich V. M. , Baranskii P. I. , Korolyuk S. L. , Samoilovich A. G. , Chelova T. N.
Published: July 2007

The anisotropy and the magnetic field dependences of ϱ⟂H/ϱ0 were investigated on n‐Ge (ne ≈ 1.59 × 1013 × m−3 cm−3) in the range of dominant phonon scattering (at 77 °K) in quantizing magnetic fields (up to 250 kOe). The developed theory is in quantitative accordance with the experimental data. It is shown that: the difference of magnetic field dependences ϱ⟂/H/ϱ0 = f(H) for H ‖ [001] or [011] (both at j ‖ [100] and at j ‖ [110]) is caused by the effect of intervalley redistribution of current carriers in quantizing H; the crystal imperfections (type of Herring statistically distributed inhomogenities) do not reveal itself noticeably at the quantization.
Electrical Resistivity and Thermoelectric Power of Liquid Alkali Metals
Authors: Rao P. V. S.
Published: July 2007

The self‐consistent treatment of electron correlations at, metallic densities, recently given by Vashishta and Singwi, is applied to pseudopotential calculations of electrical resistivity and thermoelectric power of alkali metals. For the liquid structure factor, recent accurate experimental data of Greenfield et al., have been used in case of Na and K, and the hard sphere structure factors in case of Rb and Cs. The pseudopotential used is a local potential of Ashcroft type, with its parameters fixed to give good agreement with phonon dispersion data. Energy dependence of the pseudopotential form factor is deduced from thermoelectric power data. Calculations have also been done with Shaw's non‐local pseudopotential for the case of Na and K. For Na, results of calculations of resistivity using three other different screening functions of Hubbard type are also presented.
Impurity Induced Raman Spectra of Annealed and Quenched NaCl: Ca Crystals
Authors: Kaiser R. , Spencler W. , Möller W.
Published: July 2007

Raman spectra have been measured of NaCl: Ca crystals quenched or cooled slowly from high temperatures. The spectra show significant changes in structure and intensity as a function of heat treatment. The spectra of the quenched crystals are compared with the projected perturbed densities of states. Possible models are discussed in order to explain the observations.
Colour Centres in SrCI2
Authors: den Hartog H. W. , Mollema P. , Schaafsma A.
Published: July 2007

Experimental results are reported obtained on additively and substractively coloured SrCI2 crystals. By means of combined optical and ESR experiments the position of the F‐band could be located. It was found that 02‐ impurities play an important role in the additive and substractive colouration process. The presence of O2‐ ions very strongly enhances the coagulation rate of electron excess centres formed during additive colouration. Models for radiation damage processes in O2‐ doped SrCI2 crystals are discussed.
Phenomenological Description of Aggregates of Exchange‐Bonded Paramagnetic Ions in Crystalline Environment. II. Microscopical Model for a Rectangular Aggregate of Four Paramagnetic Ions with Spin One Half
Authors: Kurzyński M.
Published: July 2007

The general theory of Part I is applied to an aggregate consisting of four exchange‐bonded paramagnetic ions with spin one half forming a planar rectangle. As a zero‐order approximation, eigenvalues and eigenstates of the isotropic Heisenberg Hamiltonian inside the sixteen dimensional suhspace of the low‐energetical states of the system are found. The isotropic interactions form one energetical quintet, three triplets, and two singlets; the further splitting of which are caused by anisotropic interactions and can he described in terms of spin Hamiltonian with effective spins S = 2, 1, and 0, respectively. The magnetic dipole–dipole interactions and also simple anisotropic exchange interactions are considered as & perturbation, and the microscopical model for the phenomenological parameters of the spin Hamiltonians is given in second order. Finally, the effective g‐tensors are discussed.
M Centers in Potassium Iodide
Authors: Gudat A. E. , Neubert T. J.
Published: July 2007

Absorption spectra of preferentially aligned M centers in Kl were measured with polarized light at 20.5, 77.4, and 243 °K. The band maxima, halfwidths, and relative intensities of the (M1, M4, M2, and M2 bands and the peak position of other anisotropies (M3, M5, M6, M7, M3, M5, M7) were determined from dichroic difference spectra. The results have been compared with M‐center data for other alkali halides and the origins of the various anisotropies have been considered.
High‐Field Antiferromagnetic Resonance in α‐Fe203
Authors: Morrison B. R. , Morrish A. H. , Troup G. J.
Published: July 2007

Millimetre microwaves and pulsed magnetic fields have been used to investigate the magnetic resonances in a‐Fe2O3. Above the field and temperature induced phase boundary the coupling of the two weak‐ferromagnetic resonance modes has been observed. The coupling equation has been derived taking into account second anisotropy, basal plane anisotropy and magnetostriction. Below the phase boundary the antiferromagnetic resonance modes have been investigated and the resonance equations derived for the field in the basal plane. The observation of the phase boundary using a conventional resonance spectrometer has been satisfactorily interpreted in terms of the large changes in susceptibility which a numerical analysis shows often occur at or near this boundary. A small quantitative difference has been shown to exist between phenomenological theory and experiment at high magnetic fields.
Interband Magnetoreflection of ZnO
Authors: Hümmer K.
Published: July 2007

In magnetic fields up to 50 kOe and for photon energies greater than the band gaps for light polarized with E ⊥ c and E ‖ c oscillatory changes of reflectivity are observed on ZnO‐crystals at helium temperatures. The anisotropic reduced masses and hole masses and the energy gaps are deduced from the shift of the reflection maxima in dependence on the magnetic field (H ‖ c, H ⊥ c). The influence of the Coulomb interaction of holes and electrons and the electron‐phonon interaction on the Landau levels are discussed. The results are compared with the valence band structure of wurtzite type crystals and the band parameters of ZnO previously reported.
The M+ Center in CaF2
Authors: Collins W. C.
Published: July 2007

S‐irradiation of additively colored CaF2 at low temperature produces two new absorption bands at 545 and 360 nm at the expense of the M band at 519 nm. These same bands can also be produced by ultraviolet bleaching at low temperature of additively colored CaF2 doped with electron trapping impurities. In addition, excitation into either band gives rise to a luminescence band peaking at 645 nm, and both absorption bands can be reversibly bleached with visible light. We propose that these bands arise from the M+ center.
Broad Neutron Groups in Inelastic Neutron Scattering and Anomalous Anisotropy of the Energy of the Dispersion Branches
Authors: Comès R.
Published: July 2007

It is suggested that the broad neutron groups observed in inelastic neutron scattering and in relation with structural phase transitions can be explained by the finite triple axis spectrometer resolution combined with an important energy anisotropy of the lower dispersion branches. The damped or overdamped mode concept should consequently only be used with great care in such circumstances.
Anisotropic Light Absorption in the Crystalline RbMnF3 in the Frequency Region of the 6A1g(6S) → 4E1g(4G) Transition in the Magnetic Ion Mn2+
Authors: Petrov E. G. , Kharkyanen V. N.
Published: July 2007

A theory of one‐particle collective excitations of the cubic antiferrodielectric RbMnF3 in an external magneticfield is developed for the doubly degenerate 4E1g(4G) state of the magnetic ion Mn2+. The crystal anisotropy is taken into account. It is shown that the change in the equilibrium spin configuration in a magnetic field (which breaks the magnetic symmetry of a crystal) leads to the spin‐orbit splitting of the degenerate 4 Elg(4G) level and to the further splitting of the obtained levels into the Davydov components. The formulas for the above splittings and for the intensities of optical transitions are obtainedfor different directions of a magnetic field. The results of theoretical calculations are used to explain experimental data.
Thermodynamic Correlations in Crystalline Ferroelectric Semiconductor Solid Solutions Involving Consideration of Pressure
Authors: Yurkevich V. E. , Rolov B. N.
Published: July 2007

The effects of impurity concentration and hydrostatic pressure on the behaviour of the physical properties of crystals are considered in the vicinity of the phase transition point. First order as well as second order phase transitions are studied. A number of useful correlations between the physical characteristics of a crystal is obtained.
Field Effect on the Excitonic Luminescence in CsI
Authors: Grigorev V. A. , Pollini I.
Published: July 2007

The electric field ionization of the self‐trapped exciton is studied and the results are interpreted in terms of the Schottky effect theory. Though the possibility of internal radiationless transitions is not excluded, the idea of thermal ionization of the self‐trapped Exciton finds experimental support.
Hydrogenic Impurity Bands in an External Magnetic Field
Authors: Lukes T. , Nrx B. , Rogers B. , Suprapto B.
Published: July 2007

A method used previously to investigate the influence of charged donors on the density of states when an impurity band is formed is extended to the case when a magnetic field is present. The density of states is calculated for various magnetic fields for concentrations typical of InSb at the metal–insulator transition. For sufficiently low concentrations splitting of the baud can take place as in the absence of a magnetic field.
Diamagnetism and Superexchange Electron Interaction in Cs2Sb (IV) Cl6, Semiconducting Crystals
Authors: Ionova G. V. , Ionov S. P. , Makarov E. F. , Aleksandrov A. Yu.
Published: July 2007

Crystals of Cs2Sb SbCl6Sb are diamagnetic and have a cubic structure of the antifluorite type. These facts may be explained either in terms of statistical disorder between Sb(III) and Sb(V) or in terms of electron exchange between SbCl and SbCl. A calculation is presented of the exchange interaction between the nearest SbCl complexes. The exchange integral £ is taken in form £ = (Es − Et)/2 where Es and Et are energies of singlet and triplet states. It is established that CI atoms mainly contribute to the exchange interaction. Sign and value of the exchange integral correspond to an antiferromagnetically ordered state in Cs2SbCl6 at room temperature.
Cation Diffusion in Single Crystal MgO up to 2500 °C
Authors: Harding B. C.
Published: July 2007

The diffusion of Ca2+ and Ge4+ in single crystal MgO has been studied over the temperature range 1850 to 2400 °C using a sectioning technique. The activation energy for the diffusion of Ca2+ (3.2 eV) is similar to the values obtained for other divalent cations at the same temperatures, but the value for Ge4+ (4.0 eV) is significantly greater. For ions of a given valence, it is concluded that the activation energy for diffusion in MgO is independent of the “hard‐sphere” ionic radius of the diffusing ion, while the pre‐exponential factor, Do, appears to depend on its mass and electronic polarisability. If results for aliovalent ion diffusion are interpreted in terms of the association theory developed for the alkali halides, an empirical relation between the charge of the diffusing ion and its enthalpy of migration emerges.
Effective Charges and Lattice Dynamics of Zineblende Structure Crystals I. Effective Charges
Authors: Mani K. K. , Singh R. P.
Published: July 2007

The cohesive energies of the zineblende and wurtzite structure are expressed as a sum of covalent and ionic parts. It is shown that the Lippincott potential function is a good representation of the covalent contribution and that effective charges may be obtained from the experimental bulk moduli and the Lippincott function. These charges are compared with other estimates. Heats of atomisation for a series of II–VI and III–V compounds are correlated.
NMR and Magnetic Susceptibility of Cr2Al11
Authors: Coldea M. , Crisan M. , Niculescu V. , Pop I.
Published: July 2007

The temperature dependence of the magnetic susceptibility and the Knight shift are measured. The magnetic susceptibility has a minimum around a temperature of To≈ 400 K and for the Knight shift a quadratic dependence of the temperature is found. The NMR and magnetic susceptibility measurements are correlated and the results are discussed in terms of localized Anderson magnetic moments and spin fluctuations. The expression found for the magnetic susceptibility from a theoretical model is in good agreement with experimental results.
Polarization of Singlet–Triplet Transitions in Perylene‐Type Molecular Crystals
Authors: Witkowski A. , Zgierski M. Z.
Published: July 2007

The polarizations of singlet–triplet transitions in molecular crystals with four molecules in the unit cell and group symmetry C52h are analyzed. It is shown that only transitions to factor group symmetry forbidden triplet states possess intensity and are no longer polarized perpendicular to any crystal axis.
Polarized luminescence and the mössbauer effect of tin impurity centres in NaCl crystals
Authors: Realo E. , Zazubovich S.
Published: July 2007

An investigation is made of various types of impurity centres in NaCl;Sn and KCl:Sn crystals using the Mössbauer effect and polarized luminescence. Two types of Sn2+ centres are found in NaCl: Sn. The centres of type I have polarized emission at 2.78 eV due to electronic transitions allowed along the C4 axis of the crystal. A small quadrupole splitting (<0.4 mm/s) of the Mössbauer absorption line at +4.32 mm/s is observed for these centres. The centres of type II show the polarized emission at 2.3 eV due to electronic transitions allowed along the C2 axis of the crystal. For these centres a considerable value of quadrupole splitting Δ = 0.97 mm/s of the Mössbauer lines with the isomer shift + 3.75 mm/s is found. Only the centres of the type I are found in KCl: Sn crystals. It is supposed, that the investigated centres in NaCl:Sn represent two types of associates of Sn2+ ions with cation vacancies vc: associates with C4V symmetry (the type I centres) and associates with C2v symmetry (the type II centres) in the ground electronic state.
Influence of acceptor concentration on the hall effect in tellurium
Authors: Bastard G. , Hau Nguyen Hy , Rigaux C. , Vieren J. P.
Published: July 2007

Experimental observations and new theoretical calculations are given of the impurity influence on the valence band population in tellurium. Galvanomagnetic measurements were performed at temperatures between 1.6 and 77 K, in magnetic fields up to 60 kG, on samples with acceptor concentrations from 1014 to 2 × 1016 cm–3. The temperature and the magnetic field dependences of the Hall coefficient R1 in low doped samples (NA < 2 × 1015 cm–3) is quantitatively explained by the “two acceptor levels” model proposed by Thanh. Results observed for a higher density of acceptors show the presence of an impurity band above the valence band.
Theory of bloch wall tunnelling
Authors: Egami T.
Published: July 2007

The quantum mechanical tunnelling of a very narrow Bloch wall through the intrinsic barrier is considered. A microscopic theory of the spin wave description of narrow domain walls is developed and subsequently extended to describe the domain wall tunnelling phenomena. The tunnelling probability is presented in two ways, firstly, by the quasi‐particle approach using the conventional W.K.B. approximation and secondly by the method of direct product of state vectors. These two methods are complementary to each other. The results indicate that below the temperature of liquid helium the domain wall diffusion in the rare earth metals and compounds which exhibit high intrinsic coercivity is likely to be governed by the tunnelling process.
A perturbation method for calculating absorption coefficients of degenerate bloch waves in the dynamical theory of electron diffraction
Authors: Sheinin S. S. , Cann C. D.
Published: July 2007

The standard method for calculating absorption coefficients in the dynamical theory of electron diffraction is based on perturbation theory for the non‐degenerate state. The presence of Bloch wave degeneracies, such as those which occur in the critical voltage case or in the presence of certain non‐systematic reflections, requires that a different approach be adopted. A method based on perturbation theory for the doubly degenerate state has therefore been developed. Application of this method to the critical voltage case indicates that the errors obtained in using the standard approach depend on the strength of the systematic reflections in the row involved. In the non‐systematic case, application of this method to the calculation of intensities gives results in agreement with those obtained from diagonalization of the complex matrix.
Investigation of the electron energy spectrum in some phases of CsCl‐type
Authors: Nemoshkalbnko V. V. , Ya. Nagorny V. , Nikolaev L. I. , Mindlina M. A. , Gigolashvili G. P.
Published: July 2007

The results of X‐ray spectral investigations of some phases of the CsCl‐type composed by alloying of a series of transition metals of the first, second, and the third main periods are presented. The K, L, and M emission bands of the atoms constituting the alloys are investigated. Some conclusions about the type of interatomic interaction and the nature of bonding in the considered compounds are presented.
Non‐conservation of quasiparticles and bound states
Authors: Dordević R. , Ivezić T. , Tošić B. S. , Žakula R. B.
Published: July 2007

An analysis is made of bound states in systems in which the number of quasi‐particlea is not conserved. All calculations are performed using the exact Bose representation of the Pauli operators as well as the Dyson‐Maleev representation, and corrections to the bound state energies with respect to Heitler‐London approximation are found. It is also shown that in this case the approximate second quantization method does not give the correct zero‐order spectrum of the free excitations.
Antiferromagnetic Resonance in CuCl2 · 2 H2O in Inclined Magnetic Field near Intermediate State
Authors: Galkin A. A. , Kovner S. N. , Popov V. A.
Published: July 2007

Detailed experimental and theoretical investigations of the resonance properties of biaxial antiferromagnetic CuCl2 · 2 H2O are made in the range of low frequencies v = 0.65 to 5.2 GHz at T = 1.57°K in an increasing external magnetic field H. The theory shows a satisfactory agreement with the experiment.
A Localized Soft Mode Theory for Martensitic Transformations
Authors: Clapp P. C.
Published: July 2007

Anharmonic terms of the elastic free energy of a solid are used to locate a surface of instability or “strain spinodal”. The strains near lattice defects bring some regions close to the strain spinodal causing a rapid change in local elastic properties and resulting in localized “soft mode” centers. These regions are proposed as the primary nucleation sites for martensitic transformations. A number of experiments are proposed to test the physical aspects of the theory and some recent evidence is cited supporting several features of the model.
Piezoelectricity in Zincblende‐ and Wurtzite‐Type Crystals
Authors: Hübner K.
Published: July 2007

A phenomenological model for the piezoelectric effect in zincblende‐ and wurtzite‐type crystals is proposed on the basis of optic and dielectric lattice properties. By introduction of a deformation charge it is shown that the sign of the longitudinal piezoelectric constant is determined by the ionicity of the chemical bond. The piezoelectric constant is calculated with correct sign and found to be in good numerical agreement with experimental data. For a number of I–VII, II–VI, and III–V compounds piezoelectric constants are predicted. With the piezoelectric effect the sign of the dynamic effective charges is determined.
On the Pre‐Exponential Factor in the Rate Equation for Plastic Flow
Authors: Sueek T. , Luton M. J. , Jonas J. J.
Published: July 2007

Although it is not generally possible to calculate the pre‐exponential factor ϵ0 directly from experimental results, the assumption that the obstacle‐dislocation interaction has the same temperature dependence as the shear modulus permits ϵ0 to be determined from the available data. An iterative procedure is described which, when convergent, is consistent with the thermodynamic properties of such obstacles. The approach is applied to high temperature yield data for zirconium and the results are compared with various physical models for the pre‐exponential term. The values of the activation parameters and their dependence on stress and temperature are consistent with the interpretation that the rate controlling process in the high temperature yielding of zirconium is the unpinning of the nodes associated with a specific group of segment lengths selected from the total segment length distribution present within the material.
Structural and Electronic Properties of a Clathrate‐Like Model of Amorphous Germanium
Authors: Henderson D.
Published: July 2007

The tetrahedral network of the xenon hydrate structure exhibits bond distortions and five‐atom rings. Hence, this clathrate network is a simple model of amorphous Ge which is presumed to contain these structural elements. The radial distribution function and electronic density of states, using a tight‐binding Hamiltonian, are calculated for this model and are found to have many similarities with those of amorphous Ge.
Linear Magnetic Birefringence in the Antiferromagnetic Iron Group Difluorides
Authors: Jahn I. R.
Published: July 2007

The linear optical birefringence of the iron‐group difluorides (rutile‐type) has been determined between 5 and 700 K. A magnetic contribution to the birefringence occurs during the ordering of the antiferromagnetic compounds MnF2, FeF2, CoF2, and the weak ferromagnetic NiF2; the linear magnetic birefringence (LMB) can be well separated from the temperature dependent natural optical anisotropy. Both the magnetic short‐range as well as the long‐range order contribute to the LMB. The temperature derivative of the LMB follows the course of the magnetic specific heat to a very good degree. It is shown that this proportionality is to be expected if all spin pair correlation functions exhibit the same temperature behaviour. Whether this condition is valid for the lattice distortions is discussed using the existing structural data. Assuming the observed LMB is mainly due to spontaneous lattice distortions, the influence of the fluorine parameter seems to play an important role.
Production of F and F‐Aggregate Centres in CaF2 and SrF2 by Irradiation
Authors: Hayes W. , Lambourn K. F.
Published: July 2007

The efficiency of production of F centres in CaF2 by X‐irradiation has been studied as a function of temperature and compared with the results of an earlier detailed investigation of SrF2 [1]. The efficiency begins to fall gradually at about 20 K and at 100 K has reached a very low value. It does not recover at higher temperatures. It is found that M+, M, and R centres can be produced in CaF2 and SrF2 after heavy irradiation with 1 MeV electrons at 77 K. The measured optical properties of these centres are identical with those found in additively coloured crystals. Mechanisms for the production of F and F‐aggregate centres in irradiated crystals are discussed.
Theory of the Hopping Thermo‐Electric Effects in Amorphous Semiconductors and Glasses
Authors: Čápek V.
Published: July 2007

The lowest‐order single phonon contributions to the Peltier and Seebeck coefficients of amorphous semiconductors and glasses are calculated from first principles. Their temperature dependence and sign are discussed.
Influence of Free Carriers on the Faraday Rotation and Faraday Ellipticity in the Exciton Absorption Region of GaAs
Authors: Zvára M. , Prosser V.
Published: July 2007

Investigations of n‐GaAs samples with different free carrier concentrations, both, below and in the absorption edge region, confirm the dominating influence of exciton transitions in Faraday rotation and Faraday ellipticity spectra. The theoretical curves, taking into account exciton and Landau level contributions in the real hand structure of GaAs, show good qualitative agreement with experimental data.
The Effective Charges of Fe Impurity Atoms in AIIBIVC Crystals from Mössbauer Studies
Authors: Yarmarkin V. K. , Teslenko S. P. , Dubrovskii G. P. , Grekov F. F. , Prochukhan V. D.
Published: July 2007

Results of Fe57 Mössbauer measurements are used for the analysis of the electronic state of Fe impurity atoms in AIIBIVC ternary compounds. The Mössbauer parameters have been analyzed and the effective charges of the Fe impurity atoms are evaluated on the basis of a MO (molecular orbital) cluster formed by the central Fe atom and the atoms of the matrix compound located in the two first coordination spheres in the AIIBIVC crystal. The possibility of using the results obtained for the analysis of the state of Fe impurity atoms in AIIIBV binary compounds is discussed.
Temperature Dependence of the Resistivity in a Kondo System: Influence of the Crystal Field
Authors: de Gennaro S. , Borchi E.
Published: July 2007

Using the s‐f exchange model the influence of the crystal field is examined on the temperature dependence of the resistivity for a Kondo system. The case of a cerium impurity ion in a lattice of cubic symmetry is considered. An exact algebraic expression for the resistivity versus temperature is obtained. Some resistivity versus T curves are reported and discussed.
Modulated Spectroscopy of Cubic Semiconductors. Calculation of Piezomodulation Parameters for Degenerate Semiconductors
Authors: Mathieu H. , Auvergne D. , Camassel J.
Published: July 2007

The effects of a modulating strain on an M0 critical point structure in reflectivity are considered for a study of degenerate semiconductors for which the kinetic energy of excited carriers is greater than the energy threshold modulation. Using the Pikus‐Bir formalism to describe the strain induced change in the valence band of cubic compounds, optical matrix elements and modulation amplitudes for several strain arrangements are calculated. For (001) and (111) strains, it is shown that the strain modulated part of ΔR can be described by an effective modulation ΔẼg which is independent of the kind of carriers (light or heavy holes) involved in the transition.
Single Impurity Effect on the Electron—Hole T‐Matrix in Gadolinium
Authors: Holz A.
Published: July 2007

An interpretation of the large 15 μΩcm per at% residual electrical resistivity of Mg in a Gd host is suggested in terms of the formation of bound interband electron—hole states at the Mg site. The theory is developed for a single Mg ion in a Gd host and allows a qualitatively correct interpretation of the experiments. According to an order of magnitude estimate an instability of the paramagnetic ground state in the presence of a single Mg ion seems to be possible when band structure effects on the screening of the Coulomb interaction are taken qualitatively into account.
Excitation Spectra of Luminescence in Cadmium Sulfide
Authors: Planel R. , Bonnot A. , la Guillaume C. Benoità
Published: July 2007

Excitation spectra of the excitonic luminescence in CdS are presented for three types of samples: doped, pure, and surface contaminated samples. The positions and relative magnitudes of peaks appearing in some of these spectra are analyzed and quantitatively explained in terms of indirect creation of free excitons. The different behaviour of the three types of samples investigated allows a more general discussion which leads to important discriminations between direct and indirect optical absorption and between volume and surface radiative recombination.
Friedel's Law in Electron Diffraction as Applied to the Study of Domain Structures in Non‐Centrosymmetrical Crystals
Authors: Serneels R. , Snykers M. , Delavignette P. , Gevers R. , Amelinckx S.
Published: July 2007

It is shown that a strong contrast may arise in dark‐field electron microscope images between two crystal parts related by an inversion operation, as a result of the violation of Friedel's law. Friedel's law is violated only in the diffracted beam and only in cases where a multiple‐beam situation prevails. Moreover the excited reflections must belong to B zone which reveals the non‐centrosymmetrical character of the crystal. The interfaces between two domains related by an inversion operation, which are proposed to call inversion boundaries are imaged by α‐like fringes which nevertheless have properties which differ somewhat from those of pure α‐fringes. These considerations are applied to the domain structure observed in the γ‐phase. In this case the inversion boundaries separate regions which have suffered displacements as well. It is shown that the displacement vector of such boundaries can be given an unambiguous meaning, although strictly speaking the structures in the two parts cannot be derived one from the other by means of a parallel displacement. It is demonstrated in a concrete case that the observed contrast can be accounted for by a four. beam calculation.
Thermally Activated Dislocation Motion past Point Obstacles Including the Effects of Dislocation Shape
Authors: Dimelfi R. J. , Nix W. D.
Published: July 2007

Thermally activated motion of dislocations past point obstacles is studied by treating a special case of radially symmetric obstacles created by considering a position dependent dislocation line energy. Detailed consideration of the shape of the dislocation in the equilibrium and activated states is made by applying the techniques of the calculus of variations. The activation energy is calculated as a function of the applied stress and the shape and height of the obstacle. It is shown that the stress dependence of the activation energy is similar in form to that found for linear obstacles by Dorn and Rajnak. An estimate of the activation area, made by directly integrating the shape of the dislocation, compares favorably with the activation area found by considering the stress dependence of the activation energy. The shape of the dislocation in both the equilibrium and activated states is also calculated. Two athermal stress levels for the point obstacle problem are identified and studied. These involve dislocation bowing between obstacles and athermal cutting of the obstacles, respectively.
Optical Photoionization Spectra of CdSe:Cr
Authors: Jastrzębski L. , Baranowski J. M.
Published: July 2007

It is shown that the absorption tail in CdSe:Cr (d4) is due to photoionization of Cr impurities. The absorption spectrum is well described by a formula proposed by Allen in a wide range of Cr concentration ranging from 5 × 1017 to 5 × 1019 cm−3. The value of the matrix element of the optical transition from Cr (d4) to the conduction band suggests that the transition is from the extended s‐like bonding orbital rather than from 3d orbital. For high Cr concentrations about 1019 cm−3 interaction effects between impurities occur. It is also shown that two charge states: Cr2+ and probably Cr3+ are possible in CdSe.
Simulation of Positron Channeling in Single Crystals
Authors: Kudrin V. V. , Timoshnikov Yu. A. , Vorobev S. A.
Published: July 2007

A simulation method of charged particle scattering in a crystal lattice is described. Angular distributions of protons and positrons leaving the crystal are calculated and the orientational distributions of axial channeling particles are received. The influence of the experimental geometry and the crystal condition on the obtained results is discussed. It is shown that binary collision calculation gives agreement with experimental results.
On the Problem of Cross Relaxation in Systems with Different Axes of Quantization
Authors: Korsava L. G.
Published: July 2007

Specimens with two different types of spins S and I are considered. There is a slight difference between their Zeeman energies and the dependence of the cross‐relaxation probability (1/T12)on the angle between the axes of spin quantization is considered in the general form. It is shown that 1/T12, increases almost by an order of magnitude when the angle between the axes of spin quantization change from 0 to π. In addition, the problem of cross‐relaxation probability is considered for a system of non‐equivalent non‐Kramers S and I ions (doublets). The dependence of 1/T12 on the value of the constant magnetic field and on the parameters Δ of the Hamiltonians of non‐Kramers doublets is obtained. In particular, it is shown that in a simple case 1/T12∼1/H.
On the Theory of Hopping Transport in Disordered Semiconductors
Authors: Zvyagin I. P.
Published: July 2007

The electron hopping in presence of both an electric field and a temperature gradient is considered. Expressions for the transport coefficients are obtained, and the temperature dependence of the hopping thermopower is discussed for different temperature regions.