bluuebunny/crossref_metadata_2025 huggingface dataset:
Lattice Mode Anharmonicity in Sodium Nitrate
Authors: Brehat F. , Wyncke B. , Hadni A.
Published: July 2007

The anharmonicity of the infrared active lattice modes of sodium nitrate is studied by measurements of the temperature dependence of the reflectivity. The infrared reflectivity is recorded from 10 to 600 cm−1, in the temperature range 23 to 300 °C, and analysed with the four‐parameter semi‐quantum model. The comparison between the results of the quantum theory and the observed dependence of damping shows that cubic anharmonicity drives the lattice dynamics of sodium nitrate in the low‐temperature phase. Damping divergence is observed for several modes in the vicinity of the phase transition temperature.
Emission Spectra of O and N Atoms and the Structure of Solid N2/Ne Mixtures
Authors: Sayer R. J. , Prince R. H. , Duley W. W.
Published: July 2007

The low temperature emission spectra of both the α(N2D−4S) and β(O1S−1D) transitions of N and O atoms trapped in solid nitrogen—neon mixtures are investigated using low energy electron excitation at a variety of N2 concentrations. It is determined that N2/Ne solids, formed fromed from gas phase deposition, solidify in one of three characteristic types of binary solids over the entire concentration region examined. It is shown that information on the crystal structure of these mixtures may be obtained from spectral data, extending information obtained from electron diffraction studies.
The Mechanism of Participation of Non‐Magnetic B‐Site Cations in the Superexchange in Spinels
Authors: Mayer I. , Angelov S.
Published: July 2007

The superexchange between magnetic cations at the A‐sites of spinels with non‐magnetic B‐site cations (e.g. CoAl2O4, Co3O4, CoRh2O4 etc.) is analysed by using a four‐electron, four‐orbital model. The problem is treated in the framework of the CI‐method up to the fourth‐order of the perturbation theory. In the case of Co(III) and Rh(III) in the low‐spin state as B‐site cations four contributions are found: two negative and two positive. One of the ferromagnetic terms is not connected with Hund's rule but has the same origin (delocalization) as the antiferromagnetic ones. The resulting superexchange is always antiferromagnetic. When B is Al3+ there is only one small antiferromagnetic term.
EPR Investigations on the Dynamic Behaviour of Hole Centres in CaWO4
Authors: Cord B. , Hofstaetter A. , Scharmann A.
Published: July 2007

By X‐irradiation holes are created in the p‐orbitals of oxygen ligands of the WO2–4 complexes of CaWO4. These holes are stably trapped at temperatures below 150 K and are characterized by a hyperfine interaction with the W183 nuclei of two neighboured WO4 tetrahedra. When raising the temperature from 4.2 to 77 K this interaction changes from an inequivalent to an equivalent one. This thermally activated change is accompanied by a reorientation of the g‐tensor and a maximum in the EPR linewidth.
Micromagnetic Theory of Phase Transitions in Inhomogeneous Ferromagnets IV. Dynamics of the Spin System
Authors: Fähnle M.
Published: July 2007

A time‐dependent molecular field theory (Landau‐Ginzburg theory) with correlations in space and time is proposed to calculate the dynamical properties of a spin system near the ferromagnetic phase transition. This theory moreover is a mean field theory, but it takes into account the effect of the thermal and spatial spin fluctuations implicitly when calculating the molecular field. The critical slowing down of the relaxation of the magnetization near the transition temperature, Tc, is enhanced in comparison to the result of the conventional molecular field theory as an effect of the finite lifetime of the spin clusters, even in spatially homogeneous materials. A further enhancement of the critical slowing down is obtained for the case of amorphous ferromagnets with spatial fluctuations of the exchange integral due to the finite size of the spin clusters for temperatures T > Tc. The relation between this dynamic theory and the static spatially correlated molecular field theory recently proposed for the calculation of the static paramagnetic susceptibility for amorphous ferromagnets is discussed in full detail.
Orbit–Lattice Interaction Hamiltonian for Iron‐Group Ions in Non‐Cubic Sites
Authors: Barker G. C. , Tucker J. W.
Published: July 2007

The orbit–lattice Hamiltonian for 3d ions in tetragonal and trigonal XY4 and XY6 complexes with D2d, C3v, D4h, and D3d symmetry is derived. A point‐ion model is adopted and the Hamiltonian is expressed in terms of operator equivalents in a form that exhibits clearly the symmetry of each component. The results are applied to a calculation of the phonon‐induced transitions in the ground state of V3+ in Al2O3. The ratio of the strengths of the ΔMs′ = 2 and the ΔMs′ = 1 transitions can depend strongly on whether the correct symmetry of the magnetic ion is accounted for – in particular it is shown that the commonly invoked 'cubic approximation' for the vibrational modes can lead to substantial errors.
A Quantitative Estimation of the Admixture in the Ground State Wave Function of Cu2+ in Low‐Symmetry Crystal Fields
Authors: Bhaskar Rao T. , Narayana M.
Published: July 2007

A method of calculating the ground state wave function for Cu2+ in rhombic crystal fields using the spin‐Hamiltonian parameters is discussed. The expressions for spin‐Hamiltonian parameters are deduced when the ground state contains comparable contributions from |x2 – y2 and |3z2 – r2 including the effect of covalency. The effectiveness of these expressions is illustrated by estimating the wave function for Cu2+ in four pseudo‐Jahn‐Teller systems, with varying degrees of admixture in the ground state.
Temperature Dependence of Second‐Order Spin‐Lattice Coefficients for Eu2+ in MeF2 Single Crystals
Authors: Kuriata J. , Guskos N. , Rewaj T.
Published: July 2007

The temperature dependence of the second‐order spin‐lattice coefficients is investigated for BaF2:Eu2+ single crystals. The dependence is interpreted in terms of the simple Einstein model for phonon interactions. An analysis of the phenomenological parameters derived for BaF2:Eu2+ together with those published earlier for other systems indicates the fact that good consistency of the experimentally determined parameters with the implications of the Newman‐Urban superposition model is obtained if the rigid‐lattice values are taken into account.
Pressure dependence of superconductivity in non‐transition metals
Authors: Jain S. C. , Kachhava C. M.
Published: July 2007

An expression for the volume dependence of electron‐phonon coupling strength is derived by considering the corresponding variation of Fermi momentum explicitly. Numerical data for the critical volumes deduced from the formulation compare fairly well with the existing theories. The calculated values of logarithmic volume derivative of effective interaction strength are in good accord with experiments. The volume dependence of superconducting transition temperature evaluated for three representative cases of weak, medium, and strong coupling superconductors leads to the conclusion that the theory reproduces the experimental data satisfactorily.
Density of states in amorphous Si:H
Authors: Overhof H. , Beyer W.
Published: July 2007

The density of states in the pseudogap of doped and undoped glow‐discharge prepared amorphous silicon is determined from transport data. Thermopower and conductivity are combined to calculate the position of the Fermi level as a function of temperature and doping rate. An effective density of states model is constructed that can account for these data. The true density of states is then determined under the assumption of a simple model for the correlation energy. It is shown that this model can account in detail for the doping dependence of the spin densities of various spin resonance lines recently reported.
Recombination kinetics of electrons with α‐ centers in colored alkali halides
Authors: Hoffmann H. J.
Published: July 2007

The recombination kinetics of conduction electrons with halogen‐ion vacancies (α‐centers) in additively colored alkali halides is calculated and discussed for the high‐temperature range, in which concurring F′‐centers decay thermally. The steady state concentrations of conduction electrons, α‐, and F′‐centers as well as the time constants are explicitly given under the assumption that the charge carriers recombine with α‐centers predominantly via the excited state F* of the vacancies. These results are compared with experimental data and former theoretical calculations for the case that recombination directly into the ground state is predominant. The results on the effective recombination coefficient can also be applied to other defects in photoconductors and semiconductors if the charge carriers recombine via an excited state.
Disorder‐induced broadening for free direct excitons in II–VI and III–V semiconducting mixed crystals
Authors: Hennig D. , Strehlow R.
Published: July 2007

A theoretical model is presented which is a modified version of an earlier description of indirect excitons in semiconductor mixed crystals. The influence of short‐range disorder on the broadening of free direct exciton lines from the scattering of the exciton by this disorder potential is studied. This leads for II–VI mixed crystals to a remarkable contribution to the observed disorder‐induced broadening of the exciton lines while for the III–V mixed crystals this contribution is negligibly small.
Electronic band structure of pseudodirect chalcopyrite semiconductors: II. ZnGeP2 and ZnSiAs2
Authors: Heinrich A. , Cordts W. , Monecke J.
Published: July 2007

The band structures of the chalcopyrite semiconductors ZnGeP2 and ZnSiA2 are calculated using the empirical pseudopotential method as applied successfully already to ZnSiP2 and CdSiP2. The band structures are in good agreement with experimental data. The influences of all three tetragonal parts of the crystal potentials (antisymmetric cation potentials, tetragonal compressions, and distortions of the anion sublattices) on the band structures and on optical transition energies are investigated in detail.
The CNDO/BW study of lithium clusters. I. Two‐, three‐, and four‐atomic clusters
Authors: Skála L.
Published: July 2007

The CNDO/BW method is used for a detailed study of Li2, Li3, and Li4 clusters. The equilibrium distances between atoms, binding energies, ionization potentials, dipole moments, charges at atoms, and bond orders are investigated for 14 different geometries of the clusters. The most stable geometrical forms are found and compared with available theoretical and experimental results.
Excitation spectra of disordered systems. II. Two‐and three‐dimensional disordered systems
Authors: Sinai J. J. , Wu Shi‐Yu , Chen Y. C.
Published: July 2007

The approximate theory developed earlier for calculating the ensemble average of the Green's function for disordered systems is applied to the dynamics of a disordered square lattice and a disordered cubic lattice. Good agreement with numerical calculations is demonstrated.
Low‐temperature renormalization group study of the random spin systems
Authors: Li Mai Xuan , Duc Nguyen Manh
Published: July 2007

The second‐order phase transition of a weakly disordered system in 2 + ϵ dimensions is investigated by means of the low‐temperature renormalization group technique. It is shown that there are two fixed points: the Migdal‐Polyakov and the unphysical ones. As the Migdal‐Polyakov fixed point is always stable it is concluded that the disordered and the ordered systems have the same critical exponents. These results agree with the Harris hypothesis, which predicts that phase transitions with positive value of α do not occur in the presence of quenched disorder. The effects of the long‐range exchange interaction on the critical behaviour of the quenched system in σ + ϵ dimensions are also discussed. The criteria for the concentration of nonmagnetic impurities in the systems under consideration are established.
The model potential for charged centres in semiconductors
Authors: Olejníková B. , Hrivnák Ľ. , Kedro M.
Published: July 2007

The model potential of an electron U(r) = −e2/4πϵ0ϵsr [1 + (ϵs − 1) exp (‐βr)] is, appliedtodeep charged centres in semiconductors instead of the commonly used combination of Coulombic and δ‐function potentials. The ground‐state energy of such a centre, photoionization cross‐section, and ionization energy lowering due to an applied electric field are calculated for various values of the parameters β and the values m* and ϵs corresponding to GaAs.
Nonlinear parametric interaction in a piezosemiconducting medium
Authors: Sinha D. K. , Gupta M.
Published: July 2007

An attempt is made to investigate the nonlinear parametric interaction of acoustic waves in n‐type piezoelectric semiconductors, the nonlinearity arising from the nonlinear transport property of electrons. The equations of mechanical motion and of electricity supplemented by basic equations of piezoelectric media are used to solve the problem. In the present analysis the gain of energy of the signal wave is theoretically investigated for different amplitudes of the pump wave, assuming the interaction to be of degenerate type and the drift velocity of electrons exactly equal to the sound wave.
Planar channeling of relativistic protons in bent crystal
Authors: Taratin A. M. , Vorobiev S. A.
Published: July 2007

A calculation of the dechanneling half‐length x1/2 for relativistic protons in a bent crystal is carried out with account of the valence electron density. The energetic dependence of particle channeling in curved channels is discussed and the influence of the beam divergence on the trapping of relativistic protons in channeling and their dechanneling in bent crystals are considered.
Photosensitive EPR spectra of Pb‐doped ZnS single crystals
Authors: Dziesiaty J. , Backs D. , Mantwitz D.
Published: July 2007

Photosensitive EPR investigations of lead‐doped ZnS single crystals have been made. The results of excitation and quenching measurements are discussed in terms of the A‐centre model, assuming a Zn vacancy associated with a neutral Pb2+ impurity on next cation site. A simple level model, involving the energetic positions of the photosensitive impurities in the band gap of ZnS, is proposed to explain the EPR excitation and quenching spectra.
Aspects of electrical conductivity in a moving base approach
Authors: Wagner M.
Published: July 2007

Three types of unitary transformations (Fröhlich, adiabatic base, modified Bloch‐Whitfield) are contrasted. The adiabatic base approach improves the Fröhlich philosophy: the effective electron‐electron interaction turns out to be purely attractive, and it also is symmetric in the involved electronic wave vectors. The rise of superconductivity proves to be a purely nonadiabatic effect. Electrical resistivity is split into adiabatic and nonadiabatic contributions and depends both on the vibrational and electronic occupation. Moreover it turns out that one never can get rid of diagonal electron‐phonon interactions. These establish selfconsistent interrelations between lattice dynamics and electronic occupation, which most simply may be handled by the Bogolyubov inequality. The modified Bloch‐Whitfield transformation seems to offer a possibility to calculate Tc and electrical conductivity with a somewhat diminished knowledge about the atomic potentials. Perspectives for detailed calculations are given.
Binding energies and chemical shifts of least bound core electron excitations in cubic ANB8−N semiconductors
Authors: Bechstedt F. , Enderleln R. , Wischnewski R.
Published: July 2007

Core electron binding energies EB with respect to the vacuum level and their chemical shifts are calculated for the least bound core levels of cations and anions of cubic ANB8−N semiconductors. Starting from the HF‐binding energy of the free atom absolute values of EB are obtained by adding core level shifts and relaxation energies. Core level shifts are calculated by means of an electro‐static model with ionic and bond charges according to Phillips' bond charge model. For the calculation of relaxation energies the linear dielectric theory of electronic polarization is applied. Valence and core electrons, and diagonal and non‐diagonal screening are taken into account. The theoretical results for chemical shifts of binding energies are compared with experimental values from XPS‐measurements corrected by work function data. Good agreement is obtained in all cases within the error limit of about one eV. Chemical and atomic trends of core level shifts, relaxation energies, and binding energies are discussed in terms of changes of atomic and solid state parameters. Chemical shifts and relaxation energies are predicted for various ternary ANB8−N compounds.
Kinetics of Defect Accumulation and Recombination: II. Diffusion‐Controlled Defect Annihilation
Authors: Kotomin E. , Kuzovkov V.
Published: July 2007

In the present part of the paper the kinetic equations, derived in its first part and taking into account the spatial correlations of both, the similar and dissimilar defects, are applied to both the theory of diffusion‐controlled defect annihilation, A + B → 0, and energy transfer, A + B → A. It is shown that only taking into account the correlation of the similar defects, one can describe the transition from bimolecular to monomolecular kinetics if the correlation within initially created Frenkel pairs varies from a random distribution to a very strong one. Even for the case of initially uncorrelated pairs of dissimilar defects an annihilation stimulates with time the creation of the similar defect correlation. In turn, it results in the deviation from the wellknown constant of quasi‐steady state recombination, k0 = 4πDRa. The present theory is compared with analogous theories by Waite and Antonov‐Romanovskii.
A High‐Order Magnetoelastie Hamiltonian for the Conduction Electron Contribution in Cubic Rare‐Earth Metals
Authors: Del Moral A.
Published: July 2007

The effect of the conduction electrons on the magnetoelastic interaction due to the crystal field in cubic rare‐earth metals is treated within a model similar to the so called deformable ion one. The resultant effective Hamiltonian has the proper dependence on the irreducible strains and angular momentum operators as the usual second‐order Hamiltonian, but it is now modulated by an operator proportional to the unperturbed cubic crystal field Hamiltonian. This Hamiltonian is particularized for a symmetry quantization axis. Expressions for the magnetostrictive equilibrium strains are provided to show their peculiar temperature dependence.
Theory of the Nuclear Magnetic Resonance Lineshape in Magnetically Diluted Crystals
Authors: Aliev M. N.
Published: July 2007

The lineshape of the nuclear magnetic resonance in diamagnetic crystals with paramagnetic impurities is calculated with the help of the Green's function method for the case when both, the nuclear and electron spins are ½. The linewidth and the line shift at slow and rapid fluctuations of the local field produced by the paramagnetic impurities on the nuclei of the diamagnetic atoms magnetic are calculated.
Enhancement of Polaron Effect in Thin Dielectric Films
Authors: Sherman A. V.
Published: July 2007

The binding energy and effective mass of a strong‐coupling large polaron in a polar crystal film are calculated. The film with a thickness less than the polaron radius and a high‐frequency dielectric constant larger than unity is assumed to be surrounded by vacuum. It is shown that the absolute value of the binding energy and the effective mass of such a polaron may considerably exceed the corresponding values for an infinite crystal.
Effect of Anharmonicity on the Second‐Order Doppler Shift for a Vacancy‐Trapped Mossbauer Impurity Atom in Metallic Solid Solution
Authors: Acharyya P. K. , Tewari A. K. , Roy S. K. , Bhattacharya D. L.
Published: July 2007

The anharmonic contributions to the second‐order Doppler shift are estimated theoretically for a simple lattice model containing impurities and vacancies at the lattice points by considering the double‐time Green's function technique in the high temperature limit. The cubic anharmonic terms in the expansion of the potential energy function of the proposed model are found to produce a significant change in the time dependence of the correlation functions and result in the appearance of additional terms proportional to the temperature in the expression for the energy shift. The estimated result thus shows that the effects of anharmonicity on the second‐order Doppler shift become important at high temperatures and it becomes possible to estimate the changes in the third‐order force constants of the defect lattice.
Thermally Stimulated Depolarization of NO−3, NO−2, and PO−2 Centres
Authors: Nierste G. , Fischer F.
Published: July 2007

A TSD peak is found between 18 and 33 K in single crystals of sodium and potassium halides doped with NO−2 and NO−3 centres. The peak belongs to NO−2 in sodium halides and to NO−3 in potassium halides. The discrimination is possible by applying pressed pellets as normally used in IR absorption work. This new technique allows TSD measurements below 200 K. The electric dipole moment p of NO−3 centres is of a pure displacement type. In KCI:NO−3 the experimental value of p is 0.52 e Å. It reasonably agrees with that obtained from a model which explains the off‐centre position of the NO−3 mainly by the repulsive potential of the surrounding lattice ions. In case of PO−2 centres TSD peaks are found in potassium halides and in RbBr between 29 and 39 K.
Effect of Solid Dilution and Temperature on the Crystal Field Spectra of Chromium Ions in Single Crystals of Chrome Alum
Authors: Kesavamoorthy R. , Krishnaswamy R.
Published: July 2007

The optical absorption spectra of mixed single crystals of chrome alum and potash alum are studied for different concentrations at room temperature and for one concentration at different temperatures between 100 and 310 K. The cubic crystal field splitting parameter Dq, and the interelectronic repulsion parameter B are obtained using the observed values of the peak positions. It is found that Dq, B, and half width at half maxima Γ of the spectral bands associated with the paramagnetic chromium ion in chrome alum do not change on replacing varying amounts of chromium ions with the diamagnetic aluminium ions. The temperature dependence of Γ is explained on the basis of the vibronic transition and that of Dq on the basis of the results of crystal field theory. A simple model is proposed to explain the temperature dependence of B.
Electronic Properties of Gallium Chalcogenides (Gas, GaSe, and GaTe)
Authors: Gupta V. P. , Gupta A. , Srivastava V. K.
Published: July 2007

Phillips' and Van Vechten's theory (PVV) is applied to gallium chalcogenides (GaS, GaSe, and GaTe), which are layered semiconductors. The derived values of Eh and Ep are found inagreement with the values obtained from Phillips' model, Penn's model, and also with the reflectivity data. Polarizability is investigated using Chemla's relation and the investigated values are shown to be in accordance with the values from the Clausius‐Mossotti relation. Phillips' ionicity (fi) is evaluated and is used to determine the heat of formation (ΔH) for these materials. The values of ΔH are found in good agreement with the experimental data.
EPR Observation of an Au‐Fe Complex in Silicon: I. Experimental Data
Authors: Kleinhenz R. L. , Lee Y. H. , Corbett J. W. , Sieverts E. G. , Muller S. H. , Ammerlaan C. A. J.
Published: July 2007

After quenching of Au‐doped FZ silicon an anisotropic axially symmetric EPR spectrum is observed. The spectrum exhibits a hyperfine interaction with 197Au nuclei (I = ½). In samples which were intentionally doped with isotopically enriched 57Fe (I = ½) an additional hyperfine interaction with this nucleus is observed. Due to the large nuclear electric quadrupole moment of 107Au the nuclear quadrupole interaction strongly influences the EPR spectrum. From this effect a lower limit for the quadrupole interaction can be derived by a computer diagonalization of the spin Hamiltonian. From the observed hyperfine interactions and the known properties of Fe and Au in silicon it is concluded that this spectrum originates from a [AusFei] complex.
Auger Recombination with Deep Impurities in Indirect Band Gap Semiconductors
Authors: Haug A.
Published: July 2007

The theory of Auger recombination involving traps is extended to indirect band gap semiconductors, indeed with special emphasis on deep impurities. Effects of the strong localization and of the lattice relaxation combined with these impurities are taken into account. The results give an explanation of the influence of goldimpurities on the recombination in silicon.
Electronic Structure of the Cd85Hg35 Alloy and Its Temperature Dependence near the Phase Transition
Authors: Surma M. , Maciejewski S. , Kubiak R. , Wolcyrz M.
Published: July 2007

The Cd65Hg35 alloy changes its crystal structure from the ordered phase, at temperatures below 238 K, into the disordered phase at temperatures above 238 K. The temperature involved change in the electronic structure of the alloy within the temperature range 202 to 290 K is analyzed. The band structure of the alloy at 202, 243, and 290 K is calculated using a pseudopotential method and taking into account the temperature lattice parameter changes only. The Debye‐Waller effect is neglected in the calculation because of its inessential influence on the dispersion relation that is discussed. The theoretical analysis of the electronic structure change of the Cd65Hg35 alloy is confirmed by X‐ray data.
From Band to Hopping Diffusion of Excitons and Charge Carriers in Molecular Crystals: I. Principles
Authors: Ĉápek V. , Munn R. W.
Published: July 2007

The diffusion coefficient is deduced for an exciton or charge carrier interacting with phonons in a perfect molecular crystal. The method derived is based on the generalized master equation description of the exciton or carrier kinetics. Correct results are obtained in both the band and the hopping limits, namely a Boltzmann‐like expression with a transport relaxation time where appropriate, and a polaron hopping formula, respectively. An interpolation between these two limits is also obtained.
Hot Luminescence of Self‐Trapped Excitons in Alkali Halide Crystals
Authors: Hizhnyakov V. V. , Plekhanov V. G. , Shepelev V. V. , Zavt G. S.
Published: July 2007

The hot luminescence of self‐trapping excitons is discovered in NaI and KI single crystals. The temperature behaviour of hot emission spectra in the range of 4.2 to 170 K and the corresponding excitation spectra are studied. Basing on the theory of resonant secondary emission developed recently a description of the observed luminescence in terms of the relaxation of a configurational coordinate is given. The calculations of local lattice dynamics of a self‐trapped exciton determining its relaxation law and the shape of the hot emission spectra are presented. A comparison of the theory with the experiment provides a detailed picture of the exciton relaxation into the self‐trapped state.
Effet du champ magnétique sur la luminescence de l'ion Pb2+ dans les luminophores CaO, CaS, CaSe et MgS
Authors: Asano S. , Yamashita N.
Published: July 2007

Emission and excitation spectra of CaO, CaS, CaSe, and MgS phosphors activated with Pb2+ ions are measured at 300, 80, and 4.2 K. For these phosphors two emission bands appear corresponding to the transitions 3T1U, 3A1u → 1A1g. The zero‐phonon line on the 3A1u → 1A1g emission band is markedly enhanced by the external magnetic field. The magnetic field dependence of the intensity of the zero‐phonon line is well explained by the model containing the magnetic electronphonon interaction with the τ1g vibrational mode.
EPR and Magnetic Studies on γ:bis(N‐Methyl Salicylaldimine) Copper(II)
Authors: De D. K. , Ghoshal A. K. , Pal D. , Pal A. K.
Published: July 2007

EPR and magnetic studies on single crystals of the dimeric copper (II) compound γ:bis (N‐methyl salicylaldimine) Cu(II) down to 1.5 K reveal that each Cu(II) dimer is ferromagnetically exchange coupled having negligible interdimer exchange interaction. The magnetic properties are best described with C4v ligand field symmetry around each Cu(II) ion. The principal axes of the susceptibility and the g‐tensor are not strictly coincident at low temperature.
Electronic Structure of Transition Metal Impurities in Zinc Sulphide
Authors: Majewski J. A.
Published: July 2007

The Green's function method is used to calculate the electronic states associated with the following neutral substitutional transition‐metal impurities in zinc sulphide:scandium, titanium, vanadium, manganese, iron, cobalt, nickel. Clusters of 17 atoms in a tetrahedral configuration are used to simulate the bulk and the locally perturbed crystal. The results indicate that the impurities studied yield acceptor energy levels deep in the crystal fundamental band gap. The role played by the metal d states in the formation of the impurity centres is disscussed and compared with available experimental data.
EPR Linewidth Studies of Gd3+‐Doped YbxY1–xCl3 · 6 H2O Single Crystals Yb3+ Spin‐Lattice Relaxation Times
Authors: Misra S. K. , Mikolajczak P.
Published: July 2007

The EPR linewidths of Gd3+ ion doping single crystals of YbxY1–xCl3 · 6 H2O for x = 0.75, 0.50, 0.25 are measured from room temperature down to liquid helium temperature in order to study the spin‐lattice relaxation times of the Yb3+ ion. It is found that for all samples there are three distinct regions in which the plots are linear: T > 180 K, 180 K > T > 30 K, and T < 30 K; the Debye temperatures of these samples are about 180 K and the Yb3+ ions solely populate the lowest lying Kramer's doublet below about 30 K. It is concluded that Gd3+ cannot be used as a probe to measure the spin‐lattice relaxation times of host Yb3+ ions at temperatures below 180 K.
Higher‐Order Test of the Kenkre‐Knox Relation for Excitonic Optical and Transport Properties of Molecular Crystals
Authors: Nedbal L.
Published: July 2007

The relation between optical and transport properties of the Frenkel excitons was derived by Kenkre and Knox by the second‐order theory in the regime of intramolecular vibrations. In this paper, the validity of this relation in case of a perfect molecular crystal with arbitrary phonon dispersion is tested. Higher orders of the dipole‐dipole interaction are taken into account. For these purposes, new formulas for the optical spectra of molecular crystals are derived, too.
Some Properties of Stochastic Diffusion in Constant Potential Fields
Authors: Das A. K.
Published: July 2007

Some unusual features of Brownian motion in a class of one dimensional constant potential fields are discussed. The Smoluchowski diffusion equation for the problem is derived and solved. An anomalous (t + t2)‐behaviour of 〈x2(t)〉 obtains for a repulsive ‘unphysical’ potential. A similar result for a nonlinear repulsive potential is commented on.
Model of a Single Charged Dislocation in Tetravalent Semiconductors
Authors: Milshtein S. , Senderichin A.
Published: July 2007

The electric field around a single edge dislocation in a semiconductor is analyzed taking into account the screening effect of free electrons. Theoretical calculations are made on the temperature dependent single dislocation electrical barrier (SDEB). SDEB dependence on concentration of impurities is computed. The existence of a very narrow inversion layer around a single dislocation is strongly limited by the temperature and donor concentration. The validity of Read's dislocation model is discussed and the new model is considered.
Exciton Spectra of Orthorhombic Indium Bromide
Authors: Yoshida M. , Ohno N. , Nakamura K. , Nakai Y.
Published: July 2007

Absorption and reflection spectra of InBr single crystal are measured at 2 K with polarized light. Dichroic exciton peaks are observed in the reflection spectrum around 2.33 eV. Binding energy of the direct excition for E ‖ c is estimated as (12 ± 1) meV. Indirect absorption edge is confirmed for E ‖ c by using a wavelength modulation technique. The indirect exciton gap is determined to be 2.129 eV. Absorption components associated with five different phonon energies are resolved, one of which is presumably due to a two‐phonon process. The binding energy of the indirect exciton is estimated to be (6.9 ± 0.4) meV. The indirect gap increases rapidly with increasing temperature.
Spontaneous Radiation of Relativistic Electrons under Planar Channeling and Band Structure of Transverse Energy
Authors: Lasukov V. V. , Pivovarov Yu. L. , Kostareva O. G.
Published: July 2007

The concrete calculation of the band structure and spectral characteristics of the spontaneous electron radiation at channeling in the periodic potential of a system of crystal planes is carried out. The spectrum broadening due to multiple scattering is studied in detail.
EPR Study of the Relaxed Excited State of F‐Centres in Alkali Halides
Authors: Romanov N. G. , Veshchunov Yu. P. , Vetrov V. A. , Baranov P. G.
Published: July 2007

EPR spectra in the relaxed excited state (RES) of F‐centres in isotope‐enriched crystals 41KCl, 39KCl, 85RbCl, and 87RbCl are studied by means of an optical detection technique. The EPR linewidth is established to be due to the hyperfine interactions of the F‐centre electron with the ligand nuclei. It is shown that the EPR data for the RES of F‐centres in alkali halides can be accounted for with a rather localised RES wavefunction. A hydrogenlike 2s wavefunction with the extremum in the region of the first coordination sphere is used as an envelope function. Additional lines in the optically detected spectra of F‐centres are studied.
Acceptor Point Defects in n‐Type α‐Sn
Authors: Myhra S.
Published: July 2007

Electrically active point defects are formed in n‐type α‐Sn as a result of electron irradiation at 5 K. These defects give rise to acceptor resonance states near the conduction band edge. Anomalies in the carrier transport phenomena are examined and are interpreted within the framework of a model which proposes that high‐mobility carriers are transferred to low‐mobility resonance states as a result of defect injection. Therefore, in order of increasing defect concentration, the thermoelectric power will reverse polarity, the electrical resistivity will pass through a maximum and, finally, the Hall coefficient will reverse its polarity.
ESR Study of the Monoclinic—Orthorhombic Phase Transition in Neodymium Pentaphosphates
Authors: Parrot R. , Canny B.
Published: July 2007

Very accurate electron spin resonance measurements are performed on Gd3+ impurities in single‐domain crystals of neodymium pentaphosphates. It is shown that the transition from Cs symmetry above the critical temperature (Tc = 421 K) to C1 symmetry below Tc can be interpreted in terms of a temperature‐dependent three‐dimensional rotation R(α, β, γ) of the principal axes of the tensor D. The Euler's angle β(T) and (α + γ) (T) are given by β(degrees) = 0.415 (Tc − T)1/2 − 0.007 (Tc − T) and (α + γ) (degrees) = 0.11 (Tc − T)1/2. Finally, the temperature‐dependent rotation of the tensor D is compared to the rotation of the quadrupolar tensor (tensor Q) appearing in the multipolar expansion of the internal crystal field.
Field Effect in a‐Si:H Films. Influence of Annealing and Light Exposure
Authors: Weber K. , Grünewald M. , Fuhs W. , Thomas P.
Published: July 2007

The field effect is studied in undoped glow discharge deposited a‐Si:H films. The reproducibility and reliability of the method is examined. The density of states is calculated using a new scheme which computes the charge density without explicit solution of Poisson's equation from the experimental field effect characteristics. The density of states near the gap center is raised by annealing and light exposure from below 1017 to about 1018 cm−3 eV−1.
Generalized Internal Strains
Authors: Anastassakis E. M.
Published: July 2007

A generalized force (e.g. strain, electric or magnetic field, gradients of forces) applied to a crystal can introduce new atomic position parameters known as internal strains. Each internal strain may be looked as a “frozen” q ≈ 0 optical phonon of appropriate symmetry. Explicit matrix forms can be worked out for each internal strain produced by any type of external force and crystal class. This enables one to determine “selection rules” for deciding the type of force and choice of its components which are necessary to produce an internal strain of given symmetry. Related crystal parameters can be determined in any case with X‐ray diffraction techniques.
Size Effect on the Infrared Absorption Spectrum of Clustered Ionic Spheres
Authors: Clippe P. , Ausloos M.
Published: July 2007

The theory of Clippe, Evrard, and Lucas is extended for the infrared absorption spectrum of small aggregates in order to take into account the variation in size (radius) of ionic spheres. The lowenergy part of the spectrum cannot be explained due to size effects only in this crude dipolar coupling approximation, but a more recent theory taking into account retardation effects may explain the origin of the lower‐frequency absorption.
Anisotropy of the Relaxation Time T1 of NH3 Groups in Triglycine Sulphate Crystals
Authors: Ślósarek G. , Idziak S. , Piślewski N. , Stankowski J.
Published: July 2007

The anisotropy of the spin‐lattice relaxation time of NH3 groups in triglycine sulphate crystals is studicd. Measurements on powdered TGS crystalled to the determination of the spin‐lattice relaxation time minimum at 217 and 315 K at v0 = 90 MHz. T1 measurements performed on single crystal samples, at 217 and 302 K, close to the minimum temperatures, permit a selective study of the spin‐lattice relaxation rate, anisotropy of NH3 groups of individual glycine ions. This measurements confirm the conclusion of papers already published, that the low‐temperature minimum is associated with hindered rotation of NH3 group of glycine GI (activation energy Ea = 22.2 kJ/mol) and the high‐temperature minimum is due to hindered rotation of NH3 groups of glycines GII and GIII (Ea = 27.6 kJ/mol). Non‐exponential spin‐lattice relaxation due to the cross correlation of the intramolecular dipole‐dipole interaction is observed. A calculation of angular dependence of T1 is also performed and compared with experimental results.
The CNDO/BW Study of Lithium Clusters III. Different Crystallographic Structures
Authors: Skála L.
Published: July 2007

The CNDO/BW method is used to investigate the electronic properties of Li clusters in dependence on their crystallographic structure. Clusters with the b.c.c., f.c.c., and s.c. structure and linear equidistant chains are considered. The equilibrium distances between atoms, binding energies, and one‐electron energies of investigated clusters are given and discussed in detail.
L3‐Absorption Edge Studies of Rare Earths in Rare Earth–Aluminum Intermetallics
Authors: Prabhawalkar V. , Padalia B. D.
Published: July 2007

The L3 absorption edge region of rare earths in rare earth—aluminum intermetallics, RAl2 is recorded using an X‐ray focussing spectrograph. Chemical shifts are measured and valence states of rare earth ions are determined. The effective charge on the rare earth ion is estimated using Siegbahn's method. The present results are examined in the light of available reports on the RAl2 system nsing other techniques.
On the Density of States of a Broad‐Band Ferromagnetic Semiconductor at T ≧ TC
Authors: Auslender M. I. , Bebenin N. G. , Zolotovitskii A. B. , Kalashnikov V. P.
Published: July 2007

The density of states ϱ(E) of a broad band ferromagnetic semiconductor in the vicinity of the Curie point is calculated by means of a computer solution of Dyson equation without vertex renormalization. Non‐quasipartical behavior of ϱ(E) at E < 0 (convexivity of ϱ(E), dϱ(E)/dE ≠ 0 at the edge) is found.
Strong‐Field Hopping in Disordered Semiconductors A Problem of Directed Percolation
Authors: van der Meer M. , Schuchardt R. , Keiper R.
Published: July 2007

Starting from a density matrix formulation the hopping current induced by phonons and a strong electric field in disordered semiconductors is discussed in dependence on the temperature and the electric field strength. The treatment of the corresponding rate equation makes use of the directed percolation method. Essential informations about the non‐ohmic behaviour can be obtained from the analysis of the vertex star.
Energy Band Calculation and Zero Energy Gap Conditions for Semiconductor Superlattices
Authors: Milanović V. , Tjapkin D.
Published: July 2007

The energy spectrum of the superlattice structure is analysed in detail, assuming that the spatial distributions of the potential energy and the effective mass are rectangular. It is shown that both wells and barriers may appear in the first and second materials, depending on the transverse component value of the wave vector. Two ZEG (zero energy gap) conditions are also derived; the second one being a direct consequence of the spatial dependence of the effective mass. The numerical analysis of results is performed on AlXGa1−xAs‐GaAs superlattice structure. This numerical results confirm theoretical results exposed in the first part of the paper.
Non‐Linear Optical Properties of the Degenerate Band Charge Carrier Gas in the Magnetic Field
Authors: Pokatilov E. P. , Klymin S. N.
Published: July 2007

A theory of non‐linear optical properties of a band charge carrier gas of arbitrary concentration and temperature due to the influence of electromagnetic radiation on the probability of the carrier scattering by lattice vibrations is developed. A general expression for the polarization created by the carrier response in a laser field in the presence of other electromagnetic fields and a constant magnetic field is obtained. The generalized dielectric susceptibility tensor is calculated for various scattering mechanisms and arbitrary laser field intensities. The non‐linearity of the laser radiation absorption coefficient and the dependence of the probe wave absorption coefficient on the laser field intensity are studied.
Second‐ and Third‐Order Processes in the Theory of Spin‐Lattice Relaxation
Authors: Shrivastava K. N.
Published: July 2007

A theory of spin‐lattice relaxation of isolated single paramagnetic ions present as impurities in insulating materials is discussed. A new case of the two‐phonon Raman process with a new temperature dependence of relaxation rate is found, T1−1 ∼ T4 exp (‐Δ/kBT). A new third‐order process is also pointed out, which for closely spaced levels leads to a T3 behavior.
Impact Ionisation and Quantum Efficiency in InSb
Authors: Beattie A. R.
Published: July 2007

Transition rates for impact ionisation initiated by a) a conduction electron, b) a light hole, and c) double impact ionisation by a conduction electron are calculated using time‐dependent perturbation theory. The relative yields of secondary carriers produced by these processes, taking into account the relative numbers of primary carriers created and the loss of light holes by various transitions from the heavy hole band, are evaluated. It is shown that while process b) is a major contributor to the quantum yield along with a), it is unlikely that c) has an appreciable effect. It is also suggested that b) may provide a small contribution to electric field induced impact ionisation, particularly in p‐type material.
Cohesive Energy of Ionic Crystals within the Framework of Compressible Ion Theory
Authors: Gupta V. P. , Sipani S. K.
Published: July 2007

Cohesive energy for 24 alkali, silver, and thallous halides and 15 alkaline earth chalcogenides crystallizing in NaCl and CsCl structures are calculated within the framework of compressible ion theory proposed by Narayan and Ramaseshan, splitting the short‐range repulsive interaction into its ionic components. The nearest and next‐nearest neighbour interactions as well as van der Waals interactions are taken into account to evaluate the cohesive energy for these diatomic ionic solids. The results obtained are found to be in close agreement with recent experimental data.
The Paramagnetic Resonance of Nd3+ and Yb3+ in Zircon Structure Silicates
Authors: Ball D.
Published: July 2007

The electron paramagnetic resonance of the rare‐earth ions Nd3+ (4f3) and Yb3+ (4f13) in synthetic single crystals of zircon and hafnon is reported. The spectra, taken to be axial, have highly anisotropic g‐values making it possible to observe the spectra fully only along one axis. For Nd3+ g‖ < 0.5 and for Yb3+ g ⊥ < 0.5 Fully resolved hyperfine structure is observed for Nd3+ in ⊥ direction and for Yb3+ in ⊥‐direction. The spin Hamiltonian parameters are established by fitting the spectra observed for various angles θ between ‖‐ and ⊥ ‐directions. The parameters indicate the resonance takes place between the Zeeman split levels of an isolated doublet, though fitting the g‐values fully would require consideration of level mixing.
Random Walk of a Di‐Vacancy in an F.C.C. Lattice
Authors: Sakagami Y.
Published: July 2007

The diffusion‐process of a di‐vacancy on an f.c.c. lattice is investigated with the aid of the Markovian master equation which describes an evolution of a probability distribution on a diffusion lattice of a di‐vacancy in f.c.c. lattice. The problem is formulated in the Green function formalism. The dispersion relation, the relaxation spectrum, and the spectral function are calculated. An effective master equation is also obtained for an aged system. The usefulness of the Green function is discussed.
Crystal Field in the Laves Phase Compound PrNi2
Authors: Andreeff A. , Frauenheim Th. , Goremychkin E. A. , Griessmann H. , Lippold B. , Matz W. , Chistyakov O. D. , Savitskii E. M.
Published: July 2007

The crystal field splitting of the 3H4 ground state multiplet of Pr3+ in PrNi2 is studied by inelastic neutron scattering (TOF). The crystal field parameters are found to be B = (24.5 ± 1.8) × 10−4 meV and B = – (65.7 ± 6.9) × 10−6 meV. The ground state is the nonmagnetic Γ3 doublet. The derived crystal field level scheme is proved by measuring the specific heat. From magnetic measurements the molecular field constant λ = 67 kOe/μB and from the temperature dependence of the thermoelectric power the direct scattering potential of the conduction electrons V = –0.057 eV and the exchange coupling parameter J11 = –0.029 eV are derived.
Far‐Infrared Reflectivity of Optical Magnons in FeF2 and CoF2
Authors: Häussler K. M. , Lehmeyer A. , Merten L.
Published: July 2007

Antiferromagnetic optical magnons are observed by means of their far‐infrared reflectivity for the first time. The structure of these “magnetic reststrahlenbands” is calculated approximately for normal incidence of the infrared radiation. Spin wave damping can be taken into account by introducing complex eigenfrequencies. The tensor elements of the frequency dependent complex magnetic permeability fulfill a generalized Kurosawa relation.
On the Diffusion Theory of Kink‐Pair Formation on Dislocations
Authors: Mann E.
Published: July 2007

The process of kink‐pair formation on dislocations under the action of an applied shear stress is studied by means of the diffusion theory for the limiting cases of high or low kink mobility. In both cases exact expressions for the mean lifetime or formation time of a kink pair are derived. They may be converted into analytical forms under fairly general assumptions. In the case of low kink mobility a general formula is obtained that is valid for applied shear stresses of arbitrary magnitude and thus connects the results known hitherto only for high and low stresses, respectively. From the expressions for the lifetime the rate of kink‐pair formation is calculated by introducing the free energies of the system. The appendix deals with the exactly solvable diffusion equation in the case of high mobility.