bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of Luminescence Anisotropy of YAlO3:Nd3+ Single Crystals
Authors: Aminov L. K. , Kaminskii A. A. , Chertanov M. I.
Published: July 2007

Based on the microscopic model of activator centres in crystals theoretical calculations of the anisotropic intensity of luminescent radiation of Nd3+ ions in the YAlO3 single crystals are carried out. The parameters of the effective electric‐dipole moment operator of Nd3+ ions describing the probabilities of the transitions in 4F3/2 → 4I9/2‐15/2 channels are obtained and the interference of two basic electric‐dipole transition mechanisms are taken into account. In one of these mechanisms electric‐dipole transitions are allowed due to the mixing of opposite parity states to the ground 4fN‐configuration of the lanthanide ions by the odd terms of the crystal field, in the another one the f‐f transitions are induced by variable components of the even crystal field modulated by the radiation field. The obtained anisotropy of the luminescent inter‐manifold branching ratios and calculated lifetime of the 4F3/2 state of Nd3+ ions well agree with experimental situation. Using as an example the results obtained in this work, the question of the applicability of the present model for the analysis of the spectral properties of anisotropic crystals doped with Ln3+ ions, is discussed.
Electrical Resistivity and Thermoelectric Power of Spinodal Decomposing Crystals
Authors: Schubert G. , Löser W.
Published: July 2007

The electrical resistivity and the thermoelectric power during spinodal decomposition in binary alloys are calculated, especially submicroscopic fluctuations are taken into account. The thermoelectric power shows a monotonous dependence on time whereas a maximum behaviour is obtained for the electrical resistivity. Both results agree with experimental observations.
The Mass Action Law in Two‐Component Fermi Systems Revisited Excitons and Electron‐Hole Pairs
Authors: Zimmermann R. , Stolz H.
Published: July 2007

Taking up the quantum‐statistical derivation of a mass action law for two‐component Fermi systems with Coulomb interaction given previously an improved density formula for either species is presented. It expresses the densities as a sum of a free quasi‐particle part and a correlated contribution resulting from bound and scattering states. Avoiding the resolvent‐technique for the solution of the T‐matrix equation the correlated density is directly expressed in terms of on‐shell quantities, eventually the scattering phase shifts. The ionisation equilibrium of excitons, electrons, and holes is quantitatively analyzed by model calculations for a cut‐off Coulomb potential. The possibility of Bose‐Einstein condensation of scattering states is inspected.
Colour Centres and Luminescence in KCl:Zn Crystals
Authors: Bhuniya R. C. , Rao S. E.
Published: July 2007

TL and MTL glow curves, TL spectra, X‐ray luminescence, and F bleaching characteristics of KCl:Zn crystals are studied. They are found to be very much different from those of crystals doped with divalent cations of larger ionic radius. Zn++ ion introduces an absorption band at 272 nm. Z1 centres are produced during irradiation but are destroyed upon F bleaching. The V band at 225 nm is very sensitive to F bleaching. The TL glow peak at 90 °C is the only predominant glow peak and it increases with X‐ray dosage. F bleaching reduces this peak and induces a strong one at 125 °C which is a weak shoulder in the unbleached crystal. Only one strong intrinsic emission appears at 425 nm in TL spectra, X‐ray luminescence, and MTL. Other intrinsic emissions are either very weak or are absent. These results are discussed in the light of electron trapping by the impurity and the I‐V complexes of nn and nnn separations it introduces in the crystal.
Kinetics of Surface Segregation in Binary Alloys
Authors: Wille L. T. , Vennik J.
Published: July 2007

A new theory for the kinetics of surface segregation in binary alloys is proposed, which takes into account the concentration in the subsurface layers. This more realistic segregation profile is derived within the regular solution pair‐bonding model. The surface composition as a function of time is determined analytically and it is in qualitative agreement with the experimentally observed behaviour.
Lattice Vibrations of V2O5. Calculation of Normal Vibrations in a Urey‐Bradley Force Field
Authors: Clauws P. , Broeckx J. , Vennik J.
Published: July 2007

A normal coordinate analysis of the vibration spectrum of crystalline V2O5 is carried out in the assumption of a Urey‐Bradley force field. The calculated frequencies are adjusted to 29 experimental infrared and Raman frequencies by an automatic force constant refinement program. Analysis of the potential energy distribution and the atomic displacements allows the classification of the modes into nine types of oxygen vibrations and three types of chain modes. A discussion is given of effective charges and infrared intensities. An assignment of the infrared spectrum of polycrystalline V2O5 is added.
Resonant Scattering of Phonons in ZnS:Fe2+ and CdTe:Fe2+
Authors: Ram P. N. , Bijalwan R. D. , Semalty P. D. , Israel M.
Published: July 2007

The lattice thermal conductivity of ZnS :Fe2+ and CdTe: Fe2+ is analysed on the basis of Callaway's formalism with an emphasis on resonant scattering of phonons from the magnetic levels of the Fe2+ ions. The resonant scattering of E‐mode phonons corresponding to the T1 → T3 transition and T2‐mode phonons corresponding to the T4 → T3 transition are found to be important. An expression of resonance phonon scattering rate due to Klein was used which incorporates temperature dependent widths as essentially the only adjustable parameters. The estimated widths in ZnS :Fe2+ are seen to be low while they are reasonable in CdTe: Fe2+. The calculated results are in reasonable agreement with the experimental data.
Temperature and Field Dependence of Exciton‐Magnon Absorption of NaMnCl3
Authors: Popov E. A. , Kotlyarskii M. M. , Edelman I. S.
Published: July 2007

The behaviour of the optical absorption bands of NaMnCl3, is experimentally studied in the region of the 6Alg → 4Alg, 4Eg(4G) transition of the Mn2+ ion in the temperature range 1.8 to 20 K and in magnetic fields up to 20 T. Exciton and its “hot” magnon sidebands are observed in the spectrum. The anomalous temperature dependence of the exciton‐magnon bands intensity is explained by participation of magnons with wave vector k ± o in absorption. The low energy exciton‐mag‐ non band is supposed to be due to simultaneous creation of an exciton and annihilation of three thermally created magnons.
Electronic Structure of the Cubic Laves Phase of γ‐CeAl2
Authors: Lopez‐Aguilar P. , Costa‐Quintana J.
Published: July 2007

The electronic structure of the cubic Laves phase of γ‐CeAl2, is performed by means of a selfconsistent first‐principles pseudopotential model which includes the Coulomb correlation (Cc) effects within the band Hamiltonian. The electronic structure without the pseudopotential is also calculated and both results are compared. The wave functions are determined of the electronic states with 4f character and their couplings with other extended states. The main features of the electronic structure of this Ce alloy consist in the existence of peaks corresponding to the different 4f configurations and the complex location of the occupied 4f bands (≈ 2.7 eV below the Fermi level). All these characteristics are quantitatively determined and are in good agreement with previous theoretical and experimental results.
Influence of Damping on the Energy Loss Straggling of Low Velocity Protons in an Electron Gas
Authors: Gras‐Marti A. , Anderson V. E. , Ritchie R. H. , Ashley J. C.
Published: July 2007

When damping is included in an electron gas, an increase in the straggling parameter of low energy ions is predicted. The magnitude of this effect is estimated. The calculations provide a step forward towards a better understanding of experimental data on low energy straggling, which appears to be consistently higher than existing calculations based on the (undamped) free electron gas model. More experiments for low velocities would be desirable in order to check the theoretical predictions.
Non‐Equivalence Effects in Cluster Self ‐Consistent Calculations of Ionic Solids
Authors: Matos Maria , Maffeo B.
Published: July 2007

A simple model is used to analyze actual cluster calculations and an interpretation of the existence of central levels well separated from band levels is proposed for INDO and Xα methods. The analysis shows that the convergence to the infinite solid through the use of a point charge boundary condition cannot in general eliminate this symmetry non‐equivalence effect, which is related to the approximate treatment of two‐electron interactions inherent to each kind of cluster method.
The Structure of the Magnetic Centre in Cs3CoCl5
Authors: Lukin S. N. , Teslya O. P.
Published: July 2007

The influence of mechanical deformation of the tetragonal crystal Cs3CoCl5, on the EPR spectrum of the Co2+ ion tetrahedral centres is studied. The contributions of two main mechanisms of forming of ground state zero‐field splitting (modifications of volume and “shape” of the magnetic complex) are separated. The calculated angle of the distortion of tetrahedral nearest environment of the paramagnetic ion (γ – γ0 = −0.8°) is in good agreement with the X‐ray analysis data. The correlation of sign of the tetrahedral magnetic centre distortion with the sign of the zero‐field parameter D is considered.
Thermoelectric Power Due to Small Polaron Hopping Motion in Disordered Systems
Authors: Triberis G. P.
Published: July 2007

A theory is developed of the thermoelectric power associated with electronic phonon‐assisted hopping motion between energetically inequivalent sites with different electron–lattice interaction parameters. The general results are applied to the high‐temperature small polaron hopping case. Comparison is made with other theories.
Determination of the photoemission generation depth with use of experiments on the dynamic scattering of X‐Rays
Authors: Kruglov M. V. , Solomin I. K. , Lunev A. V.
Published: July 2007

The angular dependences of photoemission are measured in Bragg diffraction of X‐rays on germanium and silicon crystals covered with amorphous layers of various thickness. The function K(z) – the integral of electron escape probability versus generation depth – is obtained from the experimental data. The escape depth is found to be substantially lower than the estimates often used whereas K(z) is satisfactorily described by Liljequist's theory. In the photoeffect technique, the depth of the explored layer depends on the primary‐to‐secondary electron recording efficiency ratio. The capabilities of two methods of deep scanning – by changing the initial energy of electrons by a variation of the radiation wavelength and by means of the escape energy selection of electrons – are compared.
On SCCF theory of magnetic properties of 1‐D binary alloys (magnon spectrum, magnetization)
Authors: Šlechta J.
Published: July 2007

A detailed SCCF calculation of the magnon densities of states of 1‐D binary alloys, both with statistical short range order and purely random ones, is presented and the results are compared with the electronic densities of states of those alloys. Also the demonstration of the higher spatial correlations (HSC) in the shapes of the magnon densities of states is discussed. It is shown that for some constitutions of 1‐D binary alloys the long range magnetic order is likely to exist and results for the temperature dependence of the magnetization of those binary alloys is also given.
Critical exponents of the magnetostriction in amorphous alloys
Authors: Vázquez M. , Hernando A. , Kronmüller H.
Published: July 2007

The magnetostriction of some amorphous alloys is investigated in the temperature range of the magnetic phase transition. Measurements are made with the help of a method described previously, and which is based on the observation of the macroscopical torsional strain under a longitudinal electrical current. The magnetostriction near the ferromagnetic phase transition is found to obey a (Tc – T)χ‐law with critical exponents depending on the type of alloy and values varying in the range of χ = 1.1 to 1.4.
Monte‐Carlo studies of energy straggling of electrons in solids
Authors: Kotera M. , Murata K. , Nagami K.
Published: July 2007

An energy straggling distribution is incorporated in the Monte‐Carlo simulation of keV‐electron scattering in solids. This distribution is obtained by modifying the original Blunck and Leisegang distribution based on the Landau theory. The average energy loss of the distribution is adjusted to fit the stopping power of the Rao Sahib and Wittry equation (the modified Bethe equation). Since an electron trajectory is simulated by a stepwise fashion, the energy straggling of an electron even in a bulk specimen can be calculated. By comparing with an experimental energy distribution of transmitted electrons from a thin film, the validity of the present model is discussed. The electron path length distribution in a bulk specimen due to the energy straggling is also discussed.
Plasmon bands in X‐ray spectra of polycrystalline beryllium
Authors: Papademetriou D. K. , Katsanos D. , Doukas A. G.
Published: July 2007

The plasmon bands are observed in the X‐ray spectra of polycrystalline Be, using CuKα1, SaL2 radiation for k > kc. The spectra show four peaks at the long wavelength region of Raman bands, assigned to the 3spxpy‐3spz splitting of the 3ps hybridized band.
The band structure of Co3Ga and Co2MnGa and the electrical conductivity of related alloys
Authors: Fletcher G. C.
Published: July 2007

Band structure calculations are carried out for hypothetical nonmagnetic alloys Co2Ga and Co2MnGa and for the ferromagnetic alloy Co2MnGa, all with the L21 structure. The results are used to investigate the electrical conductivity of (Co, Ga) and Co(Ga, TM) alloys (where TM is a transition metal), particularly as this involves hopping conduction. Explanation of the latter by the presence of antistructure Co or TM atoms on the Ga subslattice seems to be confirmed although they do not affect normal conduction appreciably. Mn assists hopping conduction less than antistructure Co since it provides fewer good hopping states, albeit better ones.
Electronic properties of iron chalcogenides FeS, FeS2, FeSe2, and FeTe2
Authors: Singh O. P. , Gupta V. P.
Published: July 2007

Lavin's modifications of Phillips and Van Vechten (PVV) theory are discussed and hence ionic gap (Ec), homopolar gap (Eh), and ionicity (fi) are investigated for iron chalcogenides (FeS, FeS2, FeSe2, and FeTe2). A correlation between evaluated values of ionicity and Mossbauer isomeric shift is also discussed.
Solitary excitations in molecular chains in the presence of vibronic states
Authors: Pushkaeov Kh. I. , Peimatabova M. T.
Published: July 2007

Solitary excitations due to the combination of an electronic excitation with intramolecular (both totally symmetric and nontotally symmetric) vibrations are shown to take place in molecular chains. Two cases are considered: a) when the molecules of the chain are fixed at their equilibrium positions and b) when the chain molecules are oscillating. In the second case solitary bound states of the chain deformation and the exciton‐intramolecular excitation can exist and propagate along the chain. The parameters of the solitons and the energy of the system are calculated.
Bond charge approximation for valence electron density in elemental semiconductors
Authors: Bashenov V. K. , Gorbachev V. E. , Marvakov D. I.
Published: July 2007

The spatial valence electron distribution in silicon and diamond is calculated in adiabatic bond charge approximation at zero temperature when bond charges have Gaussian shape and their tensor character is taken into account. Agreement between theory and experiment is achieved. For this purpose Xia's ionic pseudopotentials and Schulze‐Unger's dielectric function are used. By two additional parameters AB and Z′B the spatial extent is described of the bond charge and local‐field corrections, respectively. The parameter Z′B accounts for the ratio between the Coulomb and exchange correlation interactions of the valence electrons. Its silicon and diamond values have different signs.
Optical properties of EuF3 and GdF3 and paramagnetic susceptibility of EuF3
Authors: Jayasankar C. K. , Antic‐Fidancev E. , Blaise M. Lemaitre , Porcher P.
Published: July 2007

The optical properties of EuF3 and GdF3 are reinvestigated. Crystal field parameters derived from the data by using the D3h → C2v descending symmetry procedure for EuF3 are (in cm−1): B = 465, B = 375, B = −232, B = −96, B = −114, B = −490, B = 83, B = −518, B = −162. The temperature dependent paramagnetic susceptibility of EuF3, is calculated by using the simulated wave functions. The calculated and experimental paramagnetic susceptibilities are in very good agreement.
15N‐NMR investigations of dynamical processes near the ferroelectric phase transition of TSCC
Authors: Engelke F. , Michel D. , Windsch W. , Petersson J.
Published: July 2007

15N‐NMR measurements are performed to investigate the proton dynamics in the hydrogen bonds of type NH … Cl near the ferroelectric phase transition (Tc = 130 K) of tris‐sarcosine calcium chloride (TSCC), enriched with 15N nuclei. In the ferroelectric phase the second moment and the shape of the 15N‐NMR lines change significantly with decreasing temperature. The experimental data can be explained on the basis of a quantum, mechanical torsion oscillator model for the NH2 group. This interpretation allows the definition of a local order parameter from the NMR spectra which changes with the temperature at T < Tc similarly as the spontaneous polarization.
Multiple scattering approach to the XANES theory of alkali halide crystals. II. Band structure approach and cluster approximations
Authors: Datsyuk V. N. , Gegusin I. I. , Vedrinskii R. V.
Published: July 2007

The conduction band structures of six alkali halide crystals (LiF, NaF, KF, LiCl, NaCl, KCl) are studied by means of the augmented plane wave method (APW). The local partial densities of states (LPDOS) and X‐ray absorption K‐spectra are calculated. The band structure predictions are compared with the results of cluster model calculations obtained within the framework of a Green's function approach employing multiple scattering technique. The effect of cluster size is examined for the clusters containing various numbers of atoms. It is shown that 57‐atom clusters are sufficiently large to represent the band results in XANES region. The hole effect is exhibited as the intensity redistributions and the shifts of the peak positions, although more serious changes in the case of Li K‐spectrum in LiF are found.
Electron tunneling from an ultrathin quantum well in constant and alternating electric fields
Authors: Kundrotas J. , Dargys A.
Published: July 2007

The change of electron tunneling probability with time after instantaneous application of an electric field is calculated for a one‐dimensional ultrathin quantum well (QW). The electronic wave function bound to such QW is characterized by relatively large exponential tails on both sides of the QW. By solving the time dependent Schrödinger equation exactly, it is demonstrated that the tunneling rate is substantially larger than that predicted by standard approximations. This property may appear useful in constructing new ultrahigh‐speed solid state devices based on isolated QWs.
Amorphous ferromagnets with two magnetic components A BPW approximation
Authors: Áwiderczak K. , Zagórski A. , Król J.
Published: July 2007

The influence of chemical short‐range order (SRO) on the Curie temperature and magnetic susceptibility is examined for a model amorphous ferromagnet consisting of two kinds of magnetic atoms (SA = 1/2 and (SB = 2). A Bethe‐Peierls‐Weiss‐like approximation (BPW) is used for the classical Heisenberg model. A strong dependence of TC on SRO is established.
Band and branch inversion phenomena in GaAs–AlxGa1–xAs superlattices
Authors: de Digs Leyva M. , Sabin J. , Gondae J. López
Published: July 2007

A detailed analysis of the symmetry properties of the Bloch envelope functions in GaAs–AlxGa1–xAs superlattices (SL) for finite values of k⟂ is made. It is pointed out that in the (E, k, k⟂) space, where k is the SL wave vector, the energy spectrum is built up of independent branches. Each one of these branches contains two curves of definite parity placed at the planes k = 0 and k = π/d, where d is the SL period. It is shown that with increasing magnitude of k⟂, an interchange of symmetries between branches (similar to the well known band inversion phenomenon) may occur at the plane k = 0 or k = π/d. Furthermore, it is also shown that a magnetic field may induce interchange of symmetries between contiguous bands.
Effect of chemical and spin disorder on shallow impurity states in semiconductor alloys
Authors: Mycielski J. , Witowski A. M.
Published: July 2007

The effect of chemical and spin disorder in semiconductor alloys, in particular in semimagnetic semiconductors (diluted magnetic semiconductors), on shallow donor states is discussed in detail. Assuming short‐range composition and spin correlations, and the isotropy of spin–spin interaction, the averages of products of the matrix elements of the disorder Hamiltonian are calculated. The theory is applied to the problem of ground‐state spin‐splitting and spin‐flip transitions, as well as to Is → 2s transitions.
Optical absorption of EL20 and EL2+ in semi‐insulating GaAs: Cr
Authors: Ulrici W.
Published: July 2007

The analysis of the optical absorption spectra of semi‐insulating GaAs: Cr measured at T = 78 K before and after illumination with light quenching the absorption due to EL2 centres shows that besides Cr2+, Cr3+, and filled (EL20) deep‐donor centres also empty (EL2+) centres may be present. These EL2+ centres cause an optical absorption with spectral shape in accordance with the photoionization cross section σp. The optical quenching of EL2+ is connected with an increase of [Cr3+] by hole capturing. A procedure is proposed to evaluate [EL20], [EL2+], [Cr2+], and [Cr3+] from the optical absorption spectra.
About the role of surface scattering of electrons in the theory of the anomalous acoustic skin‐effect
Authors: Ivanovski G. J. , Galbova O.
Published: July 2007

The effect of transformation of electromagnetic into weakly attenuated acoustic waves is studied in massive metal samples. The role of the δ‐shaped surface force is estimated which occurs under diffuse reflection of the conduction electrons. The study is performed with an arbitrary coefficient of electron reflection and in absence of a constant magnetic field, when the deformation mechanism of transformation is primary.
Lattice dynamics of solid N2O
Authors: Shibata Y. , Ishi K. , Takahashi S.‐I.
Published: July 2007

The lattice dynamics of the molecular crystal N2O is treated throughout the Brillouin zone within the framework of classical harmonic approximation. By using the Kohin type potential, dispersion curves, density of states, and heat capacity at low temperatures are obtained. The results are in good agreement with available experimental data. It is also shown that the lattice frequencies of the isomorphous crystal CO2 calculated by using the same potential agree well with neutron scattering data.
Electron structure and phase diagrams of low‐temperature rare‐earth magnets
Authors: Mitsek A. I. , Yu. Guslienko K. , Pavlovskii S. V.
Published: July 2007

The transition ion states are described by mixed many‐electron operators. The hybridization coefficients are calculated by the variational principle. Low‐temperature magnets are considered within the weak‐exchange energy limit. The destruction of the magnetic order is studied at the increase of the single‐ion anisotropy and (Ising's) exchange anisotropic energy. The magnetic phase diagrams (MPD) of the uniaxial and cubic magnets are calculated. The restrictions of the moment value J and anisotropy constant k at which these critical phenomena occur are determined here. The influence of superconductivity (SC) on the MPD is studied. An enhancement of SC is shown in the antiferromagnetic state. The intermediate helix phase is studied between the SC and ferromagnetic states.
Fluctuation‐induced broadening of exciton reflection spectra in AII BVI solid solutions
Authors: Ablyazov N. N. , Akeshkin A. G. , Melekhin V. G. , Suslina L. G. , Fedobov D. L.
Published: July 2007

The validity of different fluctuation models for an explanation of the concentration dependence of exciton reflection line broadening in AIIBVI solid solutions is discussed on the example of ZnxCd1—xS, ZnxCd1—xTe, and ZnSxSe1—x systems. It is shown that the concentration dependence can be explained with the help of models describing the interaction of excitons with largescale fluctuations of solid solution composition. At small concentrations of one component of solid solution the model of localization of the exciton as a whole by a fluctuation potential can be used. In the range of comparable concentrations of components there can be applied models accounting for the interaction of only the hole, or only the electron with the fluctuation potential, depending on whether there is anion or cation substitution.
The influence of uniaxial strain on the band to band recombination in Pb1‐xSnxTe
Authors: Genzow D. , Mocker M. , Normantas E.
Published: July 2007

The theory of radiative and Auger‐recombination in multivalley semiconductors with nondegenerate valleys is developed. The lifetime of excess carriers is enhanced because of repopulation of the energy valleys. The theory is applied to Pb1‐xSnxTe where uniaxial strain in (lll)‐direction removes the star‐degeneracy of the L‐point valleys. Lifetime enhancements of more than two orders of magnitude at 10 K are possible.
Anomalous asymmetries in X‐ray π‐modes near n‐beam interactions
Authors: Juretschke H. J.
Published: July 2007

General conditions are established for the asymmetries in the shoulders of multiple interaction peaks in Renninger scans to be of opposite sense for σ‐ and π‐polarizations. The method is applied to identify all possible anomalous asymmetries in Ge for CuKα radiation. Several specific cases are worked out in detail to contrast scans for the two polarizations, and to compare their contributions to reflections observed for unpolarized incident radiation. Excellent agreement is found between experiment and theory in the neighborhood of the prominent anomalous asymmetry of Ge 222/131. The two polarizations give strikingly different scans, but neither here, nor in the other cases examined, is the anomalous asymmetry of the π‐mode sufficiently strong to change the direction of the asymmetry observed with unpolarized incident radiation.
Phase shift functions in rare‐earth ternary compounds
Authors: Ghatikar M. N.
Published: July 2007

The empirical phase shifts in the extended X‐ray absorption fine structure (EXAFS) are calculated in a series of rare‐earth based ternary compounds. The phase shifts are compared with the experimental phase shifts obtained from the measured EXAFS data in these systems. The dependence of the phase shift function on the atomic number of the back scattering atom is critically examined.
Optical superradiance in pyrene‐doped biphenyl crystals and effect of phonons on its formation
Authors: Naboikin Yu. V. , Andrianov S. N. , Zinoviev P. V. , Malyukin Yu. V. , Rudenko E. N. , Samartsev V. V. , Silaeva N. B. , Sheibut Yu. E.
Published: July 2007

For the first time peculiarities of the formation of Dicke optical superradiance are studied experimentally and theoretically in a pyrene‐doped biphenyl crystal under active phonon effect (the energy level is 1Ag−1B2u; wavelength 373.93 nm; pyrene concentration 0.25 mol%; temperature 1.5 to 4.2 K). The possibilities of this phenomenon to obtain spectroscopic data are demonstrated. The pseudolocalized phonon lifetime τphon and the activation energy ΔE are estimated to ΔE = (7 ± 3) cm−1 and τphon = 2 to 20 ps by the temperature dependence of the superradiance intensity.
Thermodynamic properties and the equation of state of alkali hydride crystals
Authors: Srivastava G. M. S. , Agrawal G. G. , Shanker J.
Published: July 2007

Recently developed interionic potentials are used to evaluate some important thermodynamic quantities and the high‐pressure equation of state of alkali hydride crystals. Values of short range overlap repulsive interactions estimated with the help of overlap integrals along with recent values of van der Waals interactions are used in calculations. Values of the Grüneisen parameter and its volume derivative, the Anderson parameters, volume thermal expansion coefficient, and the Debye temperature are calculated and reported. The high‐pressure equation of state for LiH and NaH are investigated using a detailed picture of the interionic potentials.
Nonhomogeneous generalized master equation and nonhomogeneous second‐order integrodifferential equation for the time‐dependent back‐jump effect
Authors: Kutner R.
Published: July 2007

Equivalence between the nonhomogeneous generalized master equation and the generalized continuous‐time random walk model, including the time dependent correlations over consecutive jumps of a tagged particle diffusing in a concentrated lattice gas, is established by means of an explicit general relation between the waiting‐time distributions and the memory kernels. It is proved also that for the Bravais nontriangular lattice the self‐correlation function obeys with good approximation the nonhomogeneous second‐order integrodifferential equation. A special example of the waiting‐time distributions, and hence of the memory kernels is also presented; application of the latter in the formalism proposed leads to the higher‐order differential equations, already without the integral kernels.
Small vacancy clusters in H.C.P. metals
Authors: Bisio P. H. , Monti A. M.
Published: July 2007

Static properties of small vacancy clusters are investigated in an h.c.p. lattice by using computer simulation techniques and a short‐range pair interatomic potential representative of magnesium. The defect relaxation volume and the corresponding formation and binding energies and entropies are calculated for 20 clusters of up to seven vacancies. The most closely‐packed clusters are found to have the lowest energies and unlike the f.c.c. structure, only small defect lattice relaxations are observed in every studied case.
Anisotropy in the Shubnikov‐de Haas effect in Hg1−xMnxSe
Authors: Schleijpen H. M. A. , Blom F. A. P.
Published: July 2007

Anisotropy in the Shubnikov‐de Haas effect in Hgo.0.975Mn0.025 is studied in fields up to 2 T. The angular dependence of the oscillation period is well described by the modified Pidgeon Brown model (PB), using parameters reported in literature. The anisotropy observed in nodes in the SdH oscillation amplitude cannot be explained by the PB model. A (simple) model shows that the spin splitting of the Landau levels is augmented by an extra splitting due to the lack of inversion symmetry in the zinc blende lattice.
Optical activity of semiconductors with chalcopyrite structure
Authors: Marcinkevičius S. , Babonas G. , Korzun B. V. , Tregub I. G.
Published: July 2007

The spectra of the gyration tensor component g11, are measured at 300 K for CdSiP2, CdSnP2, ZnSiP2, ZnGeP2, and CuInS2, using the polarization modulation technique. The dispersion of g11 is analyzed according to the model of the effective oscillators and the band theory of optical activity. A correlation between the magnitude of the optical activity and the distortion of the characteristic building units of the chalcopyrite structure is found.
Recombination radiation and electroreflection of excitons in ZnxCd1−xSe solid solutions
Authors: Mashchenko V. E. , Ziborov A. I.
Published: July 2007

The simultaneous investigation of electroreflection (ER) and secondary radiation (SR) spectra of ZnxCd1−xSe solid solutions under off‐resonance (resonance) laser excitation at 5 to 300 K reveals the fine structure of the fundamental absorption edge which arises from the direct and indirect creation and annihilation of excitons of series Γ6–Γ8 or Γ7–Γ9 und Γ7–Γ7. The shape of the SR exciton line at low temperatures is determined by the exciton trapping processes at random potential fluctuations of composition. This statistic random potential manifests itself in a blue shift of the SR exciton line with the N2‐laser excitation. The behaviour of the SR exciton line at 77 K is strongly governed by the exciton annihilation processes with the simultaneous emission or absorption of LO phonons. A possible effect of extended and self‐trapped exciton states in SR spectra at high temperature is considered.
An efficient technique for the evaluation of lattice sums in crystal orbital (CO) calculations
Authors: Ramírez R. , Böhm M. C.
Published: July 2007

An approximate approach for the determination of lattice sums is presented in two‐ or three‐dimensional solids within the crystal orbital (CO) formalism as derived in the tight‐binding approximation. The solid is divided into two domain types (I and 11) where the interatomic interaction energies are determined in different degrees of sophistication. The matrix elements of the Fock operator are treated exactly within the (inner) spherical domains of type I. An electrostatic model is employed to calculate the coupling terms to atoms localized in the outer domain 11. In this case only the diagonal elements of the mean‐field Hamiltonian are taken into account. Numerical examples to estimate the convergence properties of this approximation are presented for the graphite layer, diamond, two‐ and three‐dimensional BN in the framework of an improved INDO CO approach.
Indirect forbidden fundamental absorption edge in germanium selenide single crystals
Authors: Elkorashy A. M.
Published: July 2007

The absorption coefficient of GeSe is determined from the measured values of transmissivity and reflectivity. The measurements are carried out with plane polarized light with the plane of polarization parallel to the a‐ and c‐axes which lie in the cleavage plane in the temperature range from room temperature to near liquid nitrogen temperature. Near the fundamental absorption edge the results are fitted to an equation corresponding to an indirect forbidden transition with a single type of phonon engaged in the interband absorption process. The optical energy gap at room temperature is found to be (1.075 ± 0.017) and (1.080 ± 0.016) eV for the a‐ and c‐axes, respectively. The dependence of the energy gap on temperature is linear in the temperature range from 96 to 250 K with a negative temperature coefficient dEg/dT equal to −0.43 and −0.49 meV/K for the a‐ and c‐axis, respectively.
Optical absorption lineshape of R′ centres in alkali halides
Authors: Sati R. , Thakur O. P. , Medeiros e Silva J.
Published: July 2007

Using the method of moments an analytical expression for the R′ optical absorption lineshape is obtained, in quantum approach. No use of the Born‐Oppenheimer approximation is made. The electron‐lattice interaction is taken to be linear. The effects of both, E and A1 modes are considered. It is found that the lineshape calculated using moments up to eighth order is convergent and that the presence of the A1 mode accelerates the convergence. The calculated lineshape is more exact than those reported in the earlier works. It is studied under different conditions of temperatures and coupling strengths. The obtained R′ lineshape in KCl and LiF exhibits all the experimentally observed features.
Effects of a thin transition layer on the dispersion relation of surface polaritons on a randomly rough surface
Authors: Pechstedt R. , Leine L.
Published: July 2007

The dispersion relation of an unretarded surface polariton propagating on a rough surface covered with a thin transition layer is derived. The interfaces are assumed to be completely correlated Numerical calculations of the dispersion relation are performed for an Al substrake with an Al2O3 layer. It is found that the magnitude of the frequency shift in comparison with an analogous smooth system depends on the layer thickness and the correlation length of the roughness. The position of the maximum shift depends on the correlation length only.
Two‐ and Three‐Body Interactions and Phonons in Some F.C.C. Metals
Authors: Singh G. , Rathore R. P. S.
Published: July 2007

Two‐ and three‐body interactions in f.c.c. metals are derived from the general Morse potential. These interactions are employed to develope the dynamical matrix, which involves only three force constants. The dispersion curves for the f.c.c. metals like calcium, strontium, ytterbium, and lanthanum are derived from the elements of the matrix so developed. The predictions show good agreement with the recently measured phonon dispersion in the said metals.
An Analytical Theory of FMR in Bulk Metals (Parallel Configuration). IV. Exchange Dominated Surface Modes
Authors: Fraitová D.
Published: July 2007

The analytical FMR surface impedance formula, calculated in Part III of this paper, is applied to the case of arbitrary large uniaxial surface anisotropy energy Es, the symmetry axis of which points normal to the sample surface. It is shown that the theory for certain values of Es > 0 leads to the excitation of exchange dominated surface spin‐wave modes; the discussion concerns the accuracy of the results and the applicability of the theory to the experiments.
Ideal Magnetization in Sintered Iron Materials
Authors: Posselt M. , Diener G. , Seidel M.
Published: July 2007

A previously developed self‐consistent approach to the ideal magnetization in heterogeneous ferromagnetics is specified to the case of statistical isotropy, spherical grains, and a Gaussian distribution of the stochastic field. The results are applied to calculate the law of approach to saturation of sintered steel. For this purpose sintered steel can be considered as a mixture of a ferromagnetic and a non‐ferromagnetic component. The magnetization behaviour of the ferromagnetic component is obtained from experimental data of such steels which do not contain any non‐ferromagnetic constituents. The volume fraction of the non‐ferromagnetic component is usually small, i.e. in the order of 1 to 10%. Expanding the fundamental equations of the self‐consistent method with respect to this volume fraction a simple explicit expression is obtained for the effective magnetization. The comparison of the calculated curves with the experimental data shows a satisfactory agreement.
Intermediate Valence Model of the Hyperfine Field and Relaxation Rate of Cerium in Gadolinium Metal
Authors: Foglio M. E. , Schlottmanst P.
Published: July 2007

The temperature dependence of the hyperfine field of Ce in Gd, measured by Thiel et al. is interpreted within a model that considers a mixed valent Ce ion in the molecular field of the Gd matrix and the hexagonal crystal field. A simple Brillouin‐Wigner perturbation is employed to obtain the magnetization of the Ce ions and a mode‐mode coupling approach for the relaxation rate.
Nonlinear Effects in the Piezoresistivity of p‐Type Silicon
Authors: Lenkkeri J. T.
Published: July 2007

Piezoresistivity measurements are carried out on heavily doped ion‐implanted p‐type resistors on the surfaces of cantilever beams cut from a (100) silicon wafer. Tensile and compressive stresses, up to 85 MPa along the [110] direction, are applied by bending the beams with a micrometer. Nonlinear effects are clearly seen in the measurements where the resistor contains sections both parallel and perpendicular to the stress. The results are analyzed in terms of the first‐ and second‐order piezoresistance coefficients. Values of the piezoresistance coefficients are calculated using analytical expressions for the strain dependent energy‐wave vector relations of the valence bands and the known deformation potentials. The results of the calculations are compared with the experimental data including some earlier measurements reported in literature. The calculated values of the first‐order coefficients are found to agree fairly well with the experimental data. The calculated second‐order coefficients are able to describe the observed anisotropy in the nonlinear piezoresistivity of the second order in stress.
A BCS Theory ot Superconductivity in Heavy Fermion Systems
Authors: Bose I. , Jana A. R. , Chatterjee S.
Published: July 2007

A superconducting Hamiltonian is considered in which both intraband and interband pairing interactions are included. This Hamiltonian is recast in a form suitable for the study of heavy fermion superconductivity (HFS). The superconducting transition temperature Tc is calculated and some characteristic features of HFS explained. Effects of interband pairing interaction are discussed.
Application of Band Representations of Space Groups in the Theory of Phase Transitions and Point Defects in Crystals
Authors: Evarestov R. A. , Smirnov V. P.
Published: July 2007

It is shown that simple band representations tables can be used to solve the following two problems: 1) to find the irreducible components of the space group representations induced by the irreducible representations of their site symmetry subgroups; 2) to find the irreducible components of the site symmetry group representations subduced by irreducible representations of a space group. The solution of both the problems requires also the induced representation table for the crystallographic point grousp which is given here. The space group D is considered as an example. The simple band representations table for this group is also given. The application of the band representations of space groups in the theory of second‐order phase transitions and in the theory of point defects in crystals is discussed.
Multipole Interaction Effects on Cohesive and Anharmonic Properties of Alkali Cyanide Crystals
Authors: Singh R. K. , Kumar N. , Varshney G. B.
Published: July 2007

The cohesive and anharmonic properties of alkali cyanide crystals are investigated using a three‐body potential consisting of long‐range Coulomb and three‐body interactions, and the short‐range van der Waals attraction and overlap repulsion operative up to the second neighbour ions. The results on cohesive energy, compressibility, molecular force constant, and infrared absorption frequencies are in better agreement with their experimental data than those obtained earlier from a logarithmic potential. An overall systematic trend, followed by the values of third‐and fourth‐order elastic constants and the pressure and temperature derivatives of the second‐order elastic constants, is discussed.
Directional Compton Profiles of CaF2
Authors: Das G. , Padhi H. C. , Aikala O.
Published: July 2007

59.54 keV γ‐ray Compton profiles of CaF2 along the [100] and [110] directions are reported. Both, the absolute profiles and their anisotropy are derived from the LCAO model and compared with experiments. One‐dimensional Fourier transforms are determined from both, experimental and theoretical profiles and compared with each other. Theoretical results are found to be in qualitative agreement with the experiments. It is also shown that momentum distributions are more sensitive to anisotropy in electron distributions than X‐ray structure factors.
Theory of Free‐Carrier Infrared Absorption in GaAs
Authors: Klelnert P. , Giehler M.
Published: July 2007

Free‐carrier infrared absorption spectra in GaAs are calculated for carrier scattering on ionized impurities and phonons due to Coulomb, Fröhlich, and deformation potential interaction. Starting from the Kubo formula and applying the diagram technique the interactions of electrons with electrons, phonons, and ionized impurities are taken into account in the random phase approximation (RPA). The obtained absorption spectra exhibit coupled plasmon‐phonon peaks which collapse for vanishing electron‐LO phonon interaction. By taking into account only plasmon and screening effects of the electron gas the calculated absorption spectra show plasmon bands in the case of carrier‐ionized impurity scattering and both, a LO phonon emission edge and weak plasmon‐LO phonon combination peaks in the case of carrier‐LO phonon scattering. The scattering of electrons by acoustic phonons is not influenced by plasmons. The dependences of various carrier scattering mechanisms on frequency, temperature, and material parameters are determined for n‐GaAs.
Ising Ferromagnet with Biquadratic Exchange Interaction and Uniaxial Anisotropy
Authors: Iwashita T. , Uryǔ N.
Published: July 2007

For Ising ferromagnets with S = 1, S = 3/2, and S = 2, the magnetization 〈Sz〉 and the Curie temperature TC are calculatded with the use of pair mode lapproximation. Assuming the bilinear and biquadratic exchange interactions (J and J′) between nearest‐neighbouring pair of spins and the uniaxial single‐ionic anisotropy D for each spin, the behaviours of 〈Sz〉 and TC as a function of these exchange parameters and the uniaxial anisotropy parameter are investigated for the Ising spin systems with various values of the spin.
Ionized‐Impurity‐Mediated Free‐Carrier Absorption in n‐GaAs Slabs
Authors: Kleinert P.
Published: July 2007

The theory of impurity‐mediated free‐carrier absorption in n‐GaAs bulk material is extended to quasi‐two‐dimensional structures. The surfaces are treated within the classical infinite‐barrier model and the electron—electron interaction within the random‐phase approximation. Screening due to phonons is also considered. Using temperature‐dependent Green's functions and starting from the Kubo formula a general expression is derived for the free‐carrier absorption coefficient including both bulk and surface contributions. In thick n‐GaAs slabs coupled phonon—plasmon bulk excitations give rise to distinct peaks in the absorption spectrum. These structures are increasingly washed out with decreasing thickness. For comparison impurity‐mediated free‐carrier absorption spectra of non‐interacting electrons have also been calculated. At and below the bulk plasmon frequency the absorption is reduced due to screening. The absorption increases on the other hand with decreasing sample thickness.
Photoluminescence Study of the Gallium Defect Spectrum at ≈ 1.049 ev (Ga2) in Irradiated Silicon
Authors: Schall U. , Thonke K. , Sauer R.
Published: July 2007

The Ga2 spectrum with its no‐phonon transitions at 1.0472 eV (triplet) and 1.0501 eV (singlet) is studied as a function of temperature and in magnetic and uniaxial stress fields. Like in the case of Ga3, this singlet—triplet pair exciton emission is accompanied by thermalizing higher energy singlet lines s (+2.9 meV), s* (+3.8 meV), and s** (+4.7 meV). Localized vibrational mode replicas of the triplet/singlet are observed with ħω = 11.0 meV and ħω = 18.8 meV, along with lattice modes and combination lines. Zeeman measurements indicate that the center belongs to the rhombic I (C2v) point group symmetry, and the triplet transition splits as gx = 2.7, gy = 2.1, and gz = 1.6. The effective g‐value of the exciton, geff = 2.13, is consistent with thermal data suggesting the hole to be the tightly bound particle (≈100 meV) whereas the electron is bound by only ≈23 meV. Uniaxial stress measurements can be explained in terms of valley—orbit interactions by the weakly bound “pseudo‐donor” electron. Finally, the gallium related centers Ga 1, Ga2, and Ga3 are compared.
Photon Recycling in Double Heterostructures. I. The Case of Perfect Optical Confinement
Authors: Enders P.
Published: July 2007

The influence of photon recycling onto the minority carrier distribution in double heterostructures is investigated. For perfect optical confinement of the luminescence radiation, the corresponding integro‐differential equation (without drift term) is solved exactly. The enlargement of the radiative lifetime, as measured in photoluminescence experiments, is calculated for various forms of the absorption coefficients, appropriate for band‐to‐band processes in direct‐gap semiconductors.