bluuebunny/crossref_metadata_2025 huggingface dataset:
Generalized Locator‐Coherent‐Potential Approach to Ferromag‐netic Systems with Random Local Magnetic Fields and Moments
Authors: Pfeiffer H. , Haubenreisser W.
Published: July 2007

The locator formulation of the coherent‐potential approximation (CPA) is generalized and applied to a Hamiltonian with diagonal and off‐diagonal disorder where the latter comprises additional terms of the type a and ajak. The random variables can take an arbitrary number of values. The off‐diagonal elements are assumed to be separable. The resulting CPA‐equations are represented by relations between 2 × 2‐matrices. The formalism is applied in particular to the problem of inhomogeneous broadening of ferromagnetic resonance lines in polycrystalline materials. Taking into account the usual simplifications, a transcendental equation for the reduced, complex frequency shift is derived. The used approach comprises the results previously obtained by other methods.
Properties of Randomly Diluted Heisenberg Antiferrornagnets with Spin 1/2
Authors: Hattori K.
Published: July 2007

The properties of Heisenberg antiferromagnets with spin ½ randomly diluted with non‐magnetic atoms are theoretically investigated. It is shown that the critical concentration of magnetic atoms pc for which the Néel point becomes 0 K is determined from the relation pc[1 − (1 − pc)z] = l/(z − 1), where z is the number of nearest neighbours. The variations of the Néel point, spin expectation value, and specific heat above the Néel point with concentration of magnetic atoms are also obtained. The Schottky‐type maximum of the specific heat due to the residual short range order above the Néel point is found near the critical concentration of magnetic atoms.
Non‐Central Forces and Stability of the ANB8–N‐Tetrahedral Compounds. I. Bond Charges and the Valence‐Force‐Field Model
Authors: Rusakov A. P. , Anfimov M. V.
Published: July 2007

Stability aspects of the ionic‐covalent crystals are investigated in terms of the bond charge model. A definition of the covalent bond charge is proposed for the A‐NB8–N semiconductors with zinc‐blende structure. The possibility is discussed to use the bond charge as an order parameter to describe the transition from zinblende to NaCl type structures. The trends for the parameter change of the Keating‐Martin valence‐force‐field model with bond charge variations are investigated.
Dispersion of Phonons in Transition Metal Oxides (MnO)
Authors: Goyal R. P. , Sarkar K. K. , Goyal S. C.
Published: July 2007

The theory of the extended three body force shell model (ETSM) is applied to study the dispersion of phonons in transition metal oxides (MnO). The calculated values show a good agreement with the experimental values. The distinct behaviour of alkaline earth oxides and transition metal oxides is also discussed.
Anisotropic g‐Factor of the Conduction Electrons in Bi2Te3
Authors: Köhler H.
Published: July 2007

Shubnikov‐de Haas investigations were performed on n‐type Bi2Te3 in the concentration range from 2 × 1017 up to 2 × 1019 cm−3, at temperatures between 1.6 and 4.2 K. Magnetic fields up to 15 T induction were supplied by a superconducting coil (10 T) and a Bitter‐type magnet (15 T). In contrast to a commonly almost isotropic ratio of spin‐splitting to cyclotron‐splitting in semiconductors, this property shows an anisotropy ratio of more than 20 in n‐Bi2Te3. With knowledge of the electronic mass parameters of the lowest conduction band the angular variation and energy dependence of the corresponding g‐factors were obtained, where the magnitude and sign (g > 0) can he determined. The orbital part of the g‐factor is considerably large along two of the principal axes of the Fermi surface ellipsoids, but completely vanishes along the third principal axis. Hence the g‐factor essen‐rially describes an elliptic disc, the longest principal axis (g′2) tilted by an angle ϑ0 = 33.5° relative to the bisectrix axis within the mirror plane, while the second principal axis (g′1) is parallel to the binary axis. Perpendicular to this plane g′3 ≈ 2 is about an order of magnitude smaller. Values of gmc/(2mo) ≷ 1 may only be explained in a model of more than two interacting energy bands, where additional valence and conduction bands have to be tegarded.
Indirect Interactions and Nuclear Relaxation in TINO3
Authors: Villa M. , Avogadro A.
Published: July 2007

Measurements of the nuclear relaxations times T2T1and Tiq are performed on Tl203 and Tl205 in TINO3 from room temperature up to the melting point (210 °C). The indirect nuclear interactions and their effects on T2, T1 and T1q are discussed. From the experimental data, the values of the T1‐T1 indirect coupling constants and the values of the correlation times for Tl+ diffusion are deduced. Evidence for spin‐lattice relaxation processes driven by lattice vibrations is also shown.
Cyclotron Masses in Semiconducting Bi1−xSbx Alloys
Authors: Oelgart G. , Herrmann R.
Published: July 2007

Cyclotron resonance measurements are carried out on a great number of semiconductive Bi1–x‐Sbx; alloys in the range of 0.07 < x ≦ 0.17. The measuring frequencies are 35 and 70 GHz, and the measurements are carried out at helium temperatures. For the smallest cyclotron mass, a linear relationship to the antimony concentration is found. The more rapid increase of the great cyclotron masses and the associated increase of the elongation of the iso‐energy surfaces with increasing antimony concentration presents the greatest difficulty as compared to the known dispersion relations of the electron bands.
Long‐Wavelength Optical Phonons in Mixed Crystals. II. The persistence of common gaps
Authors: Jahne E.
Published: July 2007

The formalism developed in Part I in order to describe the long‐wavelength optical phonons in mixed crystals as a system of two coupled modes is now applied to the two mixed systems In1–xGaxP and GaAs1–xPx as representative ones for one‐ and two‐mode behaviour, respectively. Calculated mode frequencies, eigenvectors, and infrared oscillator strengths are presented and their behaviour is discussed in dependence on composition in both mixed systems. Contrary to earlier suggestions we state that in a traditionally named one‐mode system apart from the lattice‐like mode in addition an impurity‐like mode occurs which is always an in‐band mode. A two‐mode system is characterized by a gap between the two sets of optical phonons arising from a gap between the optical phonon bands of the end‐member crystals. A new criterion is given for one‐ or two‐mode behaviour in terms of an overlapping or the existence of a gap between the optical phonon densities of states of the end‐member crystals. It is suggested from several items that the persistence of common gaps of the end‐member crystals is a determining factor in the lattice dynamics of mixed systems.
Wavelength Modulation Spectra and Electronic Band Structure of SnS2 and SnSe2
Authors: Camassel J. , Schlüter M. , Kohn S. , Voitchovsky J. P. , Shen Y. R. , Cohen M. L.
Published: July 2007

The first detailed reflectivity and differential reflectivity measurements performed on the layer compounds SnS2 and SnSe2 for energies ranging from 2 to 6 eV are presented. The results are discussed on the basis of existing band structure calculations. The main structures in the optical response of SnS2 and SnSe2 are found to result from excitations from the top most anion p‐type valence states (crystal field split pz and pxy type levels) into the lowest conduction band (anion and cation s‐ and p‐type levels). The location of these structures in k‐space is found to be very close to symmetry determined “special k‐points” for SnS2.
Electron–Hole Droplet Formation Kinetics in Pure Germanium
Authors: Staehli J. L.
Published: July 2007

The Pokrovskii rate equations for electron–hole droplets (EHD) are generalized by adding a term accounting for statistical droplet nucleation in a supersaturated homogeneous gas of free excitons (FE). The surface tension, which is dominant in this new term, is also used to correct the evaporation coefficient. Examples of numerical solutions show that the size and density of the EHD created are strongly affected by how fast the FE density reaches the threshold at which embryo nucleation starts. Luminescence experiments using pulsed volume excitation are reported, and are in qualitative agreement with the theoretical predictions; e.g. a maximum in FE density occurring during EHD nucleation and a delay in the build‐up of the EHD phase are observed.
Luminescence of Highly Excited ZnTe Crystals
Authors: Opanowicz A. , Marinova K. , Liebing H. , Ruppel W.
Published: July 2007

The luminescent emission of undoped and P‐doped ZnTe single crystals is measured at 1.8 and 77 K under one‐photon excitation by a Cd‐He laser and a ruby laser. At 1.8 K, at low excitation intensities, radiative recombination through bound excitons and impurities predominates. At increasing intensity, inelastic exciton‐exciton collisions become the most important recombination process. At the highest excitation, ionisation of excitons occurs accompanied by band‐to‐band recombination. At 77 K, all bound excitons except those bound to donor–acceptor centres are destroyed. At low excitation intensities, radiation from the decay of free excitons, band‐to‐band transitions, and impurity centres are observed. At high intensities, radiation from inelastic exciton‐electron collisions is predominant.
Landau Emission in n‐InSb
Authors: Bauer G. , Overhamm M. , Grosse P. , Mütller W. , Gornik E. , Pötzl H. W.
Published: July 2007

In n‐InSb (n < 2 × 1013 cm−3) hot electron cyclotron‐resonance emission (Landau emission), absorption, and reflection are investigated at T = 4.2 K. Electric fields up to 20 V/cm and magnetic fields up to 5 T are applied to the samples. From a determination of the optical constants under hot electron conditions, the emission intensity profiles are deduced and the following transitions are identified: 1+ ↔ 0+, 1− ↔ 0− Landau level and 110 ↔ 000 impurity transitions. The population of the various states is not only influenced by the electric and magnetic field but also by the background radiation conditions as evidenced in the variation of the resistivity, the absorption and emission.
Positron Annihilation in Liquid Copper–Bismuth and Copper–Tin Alloys
Authors: Itoh F. , Kai K. , Kuroha M. , Takeuchi S.
Published: July 2007

Angular correlation curves by the positron annihilation are measured on copper–bismuth and copper–tin alloys in the liquid state. A large positron affinity for bismuth and tin in these alloys is found in the concentration dependence of the angular correlation, which is ascribed to the atomic arrangement in the liquid alloys.
Raman and Vibronic Spectra of a New Oxygen Molecule Ion in CaF2
Authors: Bill H. , von der Osten W.
Published: July 2007

Spectra are reported of an unidentified oxygen molecule center of monoclinic symmetry found in natural crystals of CaF2. Two alternative models consisting of a perturbed O and a O molecule ion substituting for two fluorine neighbors are discussed. In contrast to the well‐known unperturbed O center in CaF2, this new center has a visible absorption band. This band exhibits vibronic structure corresponding to an excited‐state vibrational frequency of (900 ± 10) cm−1 and coupling constants S ≈ 6 for this vibration and S ≧ 6 for the lattice phonons. This structure is enhanced in stress‐induced dichroism spectra. Resonance Raman scattering excited in this band shows the ground‐state vibrational fundamental at (1038 ± 1) cm−1 and the overtone at (2076 ± 2) cm−1. The polarization of these features is consistent with the 〈100〉 orientation of the center. The Lorentzian line‐shape, the width, and the relative intensity of the two‐ and one‐phonon scattering peaks are consistent with recent theories of the resonant Raman effect of impurity centers.
Fading Memory in the Theory of Electronic Excitation Transfer
Authors: Sokolov F. F. , Hizhnyakov V. V.
Published: July 2007

Within the framework of a model with the Hamiltonian of the electron–phonon interaction linear in the phonon operators, a memory function (MF) has been found which represents a kernel of the integral operator in a generalized master equation describing the migration of the electronic excitations in the crystal. At finite temperatures the microscopic elements of MF obtained fade in time. This is due to the presence of phonon‐assisted transitions which are competing with the zero‐phonon one. Therefore, the coarse‐grained MF has two characteristic fading times.
Influence of a Magnetic Field on the Susceptibility of a Heisenberg Ferromagnet near the Critical Point
Authors: Fechner B. J.
Published: July 2007

Starting from the high temperature series expansion of the free energy, a magnetic equation of state is derived for the paramagnetic region valid for any crystal lattice and any spin value and solvable in various approximations. The equation is used in the Gaussian approximation to study the temperature and field dependence of the magnetic susceptibility near and above the Curie point. The results are compared with the experimental data for nickel. It is deduced that at the Curie temperature and in zero external field, the exchange coupling constant J = 132.6k‐ and the average atomic moment n0 = 0.65′B.
Nucleation Phenomena in Electron‐Hole Drop Formation in Ge and Si. II. Application to Observable Phenomena
Authors: Westervelt R. M.
Published: July 2007

The expressions derived in Part I for electron‐hole drop (EHD) nonequilibrium nucleation rates and dynamic equilibrium concentration are used to calculate the following results for Ge samples under continuous excitation: 1. very slow metastable time dependence of the EHD concentration, 2. EHD signal versus generation rate G for different histories of excitation, 3. mathematically well defined equilibrium and non‐equilibrium EHD thresholds, 4. procedure for accurately determining the EHD surface tension from threshold data at a given temperature, 5. modified EHD‐free exciton phase diagram including the effeets of metastability and finite lifetime, 6. EHD radius versus temperature for different historics of excitation. Results for the modified phase diagram of Si are also presented.
Photon Echo in CaWO4:Nd3+ and LiAl5O8: Cr3+
Authors: Samartsev V. V. , Usmanov R. G. , Khadiev I. Kh. , Kustov E. F. , Baranov M. N.
Published: July 2007

The conditions and peculiarities of the photon echo forming in monocrystals of LiAl5O8:Cr3+ and CaWO4: Nd3+ are experimentally investigated. The photon echo phenomenon for LiAl5O8:Cr3+ crystals is firstly discovered. A value for the transversal nonreversible relaxation time T2 of about 37 ns is found from the experimental dependence of the echo intensity on the time between the pulses. The dependence of the photon echo intensity on a stationary magnetic field is also investigated. When observing the photon echo in CaWO4:Nd3+, the resonance conditions are achieved with the help of a dye laser. A theory of the optical coherent response is developed for both the cases of Δt > T and of Δt < T (Δt pulse duration, T time of the transverse reversible relaxation). Satisfactory agreement of the experimental and theoretical results is found.
Optical and Electrooptical Properties of the Interfaces of Media
Authors: Timashev S. F. , Krykin M. A.
Published: July 2007

In terms of classical Maxwell electrodynamics using the nonlocal matter equation the reflection coefficient of a plane electromagnetic wave at the interface in presence of an arbitrary transition layer is obtained. Instead of traditional parameters of the layer (thickness, index of refraction) linear response functions to the electromagnetic perturbance are introduced.
Phonon Dispersion in V3Si
Authors: Achar B. N. N. , Barsch G. R.
Published: July 2007

The phonon frequencies of V3Si along the principal symmetry directions are calculated for a two band model from the microscopic dielectric screening approach. The d‐electrons are approximated by a δ2 band without interchain coupling. For the electron‐ion interaction a local Heine‐Abarenkov pseudopotential is used, with parameters very similar to those given before for V and Si. The results agree semiquantitatively with inelastic neutron scattering data and exhibit the observed softening of the TA mode in [110] and of the LA mode in [100]. In addition, LA mode softening along [110] and [111] is predicted. The Γ12 optic mode shows a small decrease with decreasing temperature.
Temperature Dependence of Nuclear Quadrupole Resonance in Layer‐Type Crystals
Authors: Lyfar D. L. , Goncharuk V. E. , Ryabchenko S. M.
Published: July 2007

The temperature dependences of the NQR frequencies of halides in layer‐type crystals of CdBr2, Cdl2(4H), Pbl2(2H, 4H)are studied within 10 to 300 K temperature range. The NQR frequency grows weakly with temperature in CdI2, and CdBr2, whereas it decreases in PbI2. For PbI2 the frequency depends linearly on the temperature beginning at T≧20K. The difference of the NQR frequencies of nonequivalent nuclei in (4H) crystals corresponds to the calculated differences of the ion contributions to the gradient, while the sum of the calculated ion and covalent contributions does not account for the observed values of the NQR frequencies. This discrepancy is suggested to arise from the strong polarization of the covalent bonds by the internal electric field in the layer crystals. The growth of the NQR frequency with increasing temperature in CdI2 and CdBr2 is shown to be due to the fact that in these crystals the ionic and covalent contributions to the gradient are of opposite sign. In PbI2 the temperature dependence of the NQR frequency is associated with the peculiarities of the phonon spectrum of the layer crystals.
Excitons and Electron‐Hole Plasma. A Ground State Calculation
Authors: Zimmermann R.
Published: July 2007

For calculating the energy per pair ℰo in a highly excited semiconductor a Green's function theory is presented which covers the whole density range from the exciton limit to the electron‐hole plasma at high densities. Extensive use is made of the analogy to the excitonic phase theory which is extended here to include correlation and dynamical screening. In a model with equal‐mass parabolic bands a rather shallow minimum of E, somewhat below the exciton energy is found at the density rs = 3.
Phase Transitions in Perovskite Crystals
Authors: Darlington C. N. W.
Published: July 2007

Atomic displacements are analysed that occur at phase transitions in perovskites caused by the condensation of zone boundary modes. It is shown that for phases in which modes with wave vectors at the M‐point, ½ (a* + (a* + b*), and the R‐point, ½(a* + b* + c*), have condensed, static displacements associated with a normal mode with wave vector ½ + c* of the X‐point are also present. Such induced displacements are to be expected from Landau's theory of continuous phase transitions when terms that couple the parameters X, R, and M me included in the free energy expression. Such an expression is applied to (Na, K)NbO3, and the results derived from it are compared with the experiment.
Stress Dependence of the (El, El + Δ1), Transition in Germanium
Authors: Kühn W.
Published: July 2007

The influence of uniaxial stress on the optical δE1, E1 + Δ1), transition of Ge is reinvestigated theoretically and experimentally. Existing theoretical approaches are extended by explicitly considering the stress dependence of spin‐orbit interaction. For comparison of the results obtained the stress dependence of the (E1, E1, + Δ1), structure is measured by means of electroreflection, where special care is given to the potential stability at the electrolytesemiconductor interface. It can be shown that the combined influence of stress‐induced valence band coupling and modification of the spin‐orbit splitting allows to explain the measured stress dependences.
Heat Conductivity Measurement at V3Si in the Temperature Range 4.2 to 80 K
Authors: Hegenbarth E. , Schmidt B.
Published: July 2007

The heat conductivity of a V3Si‐crystal is measured in the temperature range from 4.2 to 80 K by a stationary method. The analysis of the measuring results shows, that the thermal conductivity of V3Si above the transition temperature can be described by the scattering of electrons on defects and phonons. From the behaviour of the thermal conductivity below the transition temperature the energy gap of V3Si can be determined.
Comparison of the Large Unit Cell and Small Periodic Cluster Approaches to the Quasimolecular Calculations of the Band Spectra of Crystals
Authors: Evarestov R. A. , Petrashen M. I. , Ledovskaya E. M.
Published: July 2007

Two recently developed quasimolecular approaches to the calculation of the band spectra of crystals are compared: the “Large Unit Cell” model and the model of “Small Periodic Cluster”. It appears that the first model has an essential advantage allowing to confine by relatively small quasimoleocules in cases where the second model appears to be impracticable. As examples KCl and layered hexagonal BN crystals are considered.
Dielectric Permittivity of Semiconductors with Narrow Forbidden Gap in the Long‐Wave Limit
Authors: Sheka D. I. , Korol A. N.
Published: July 2007

In the random phase approximation the dielectric permittivity is evaluated for a semiconductor with a band structure described by Kane's energy band scheme (spin‐orbit interaction is neglected and the effect of higher bands is included in a very simplified form). It is demonstrated that the fact that several hole bands have been taken into account, when the operator of the kinetic energy ceased to be the Dirac one plays the principal role in the calculation of the static dielectric permittivity σ(q → 0). The analytical dependence of σ(q → 0) on the parameters of the semiconductor, width of the forbidden band Eg, effective mass, and the lattice constant, is obtained. The spatial dispersion of the dielectric permittivity for zero‐gap semiconductors is considered in the long‐wave limit.
Surface and Size Effects in the Specific Heat of Linear Chains
Authors: Ma S. K. S. , Alldredge G. P. , De Wette F. W.
Published: July 2007

The size and surface effects in the specific heat of the monatomic linear chain are investigated for a small to intermediate number of atoms and for temperatures from O K up to θ0, the zero‐Debye temperature. It is shown that the surface‐excess heat capacity, δC, can reliably be extracted for chains of as few as ten atoms, although size effects are still quite appreciable for 50 to 100 atoms. The low‐temperature limiting value of 1/2kB for δC is reached for T ⪅ 0.07θ0. The freezing out of the lowest non‐zero frequency in δC becomes appreciable only for T ⪅ 0.05θ0, in the ten‐atom chain and, more generally, for T ⪅ 1/2θ0/N in the N‐atom chain.
Theory of Time‐Dependent Mössbauer Emission Spectra under Relaxation Conditions
Authors: Afanasev A. M. , Gorobchenko V. D.
Published: July 2007

General formulas are given for the profile of the Mössbaner emission spectra measured within a finite time interval following the preceding decay, under conditions when the initial electronic state of the Mössbauer atom is in non‐equilibrium. The results obtained are valid for an arbitrary case in which the hyperfine interactions change during the relaxation process, as well as for arbitrary ratios between the lifetime of the Mössbauer nucleus, the relaxation time, and a characteristic hyperfine frequency. The case is discussed in detail when the relaxation process is accompanied by a change of the isomer shift. It is shown that, even for rather short relaxation times, the relaxation process reveals itself through a noticeable asymmetry of the emission line.
Study of the Velocity and Damping of Acoustic Waves Obtained by Brillouin Scattering in the Paraelastic Phase in GMO
Authors: Luspin Y. , Hauret G.
Published: July 2007

The velocity anomalies and damping of acoustic waves in the paraelastic phase of GMO crystals are calculated on the basis of an interaction Hamiltonian which is proportional to the strain and to the square of the order parameter. Within the assumption of a three dimensional soft mode spectrum, good agreement is obtained with the experimental results.
Band Structure of Al and the Laves Phase CaAl2
Authors: Radwan A. M. , Taut M.
Published: July 2007

The band structures of Al and the intermetallic compound CaAl2 are calculated using the full nonlocal, but energy independent Shaw model potential. The energy dependence of the potential is then treated in first order. The effect of a simulated charge transfer in CaAl2 on the band structure is studied. The results are compared with experimental soft X‐ray spectra.
Crystal Field Induced Spin Rotation Observed by Magnetization Study of Polycrystalline NdCo2
Authors: Dublon G. , Kroupp M. , Dariel M. P. , Atzmony U.
Published: July 2007

Magnetization measurements of polycrystalline NdCo2 are reported as a function of temperature (4.2 to 200 K) and magnetic field (0.2 to 26 koe). Anomalies in the temperature dependence of the magnetization are attributed to a [110] to [100] spin rotation around 40 K. Single‐ion crystal field calculations which reproduce the experimental results are presented. These yield a (2.3 ± 0.1) μB Nd3+ moment, 25 < A4 < 40 K α, and −0.088 < A4/A6 < −0.068 α in NdCo2.
Electronic Structure of Aluminium Oxide as Determined by X‐Ray Photoemission
Authors: Balzarotti A. , Bianconi A.
Published: July 2007

The electronic structure of the valence band of crystalline (α and γ) and amorphous Al2 O3 is measured by X‐ray photoemission. No significant changes are found in the different crystalline forms. A broad band observed at about 4 eV below the valence band is associated with the 2s states of the oxygen atoms, in agreement with band calculations. An electronic structure scheme of the aluminium oxide is constructed from photoemission, v. u. v. and X‐ray optical data.
Anisotropic Indirect Absorption Edge in GeSe
Authors: Vlachos S. V. , Lambros A. P. , Thanailakis A. , Economou N. A.
Published: July 2007

The absorption and reflection coefficients of GeSe single crystals are measured at various temperatures in the spectral range 0.8 to 2.2 μm using linearly polarised light. The main absorption mechanism is due to indirect interband electronic transitions with participation of two different phonons. Indirect energy gap values of 1.22 and 1.18 eV are obtained for the two directions of polarisation ϵ ∥ a and ϵ ∥ b, respectively. The corresponding phonon energies at room temperature are found to be 0.023 and 0.089 eV for ϵ ∥ a and 0.013 and 0.043 eV for ϵ ∥ b. The indirect gaps vary linearly with temperature, the temperature coefficient being −8.68 × 10−4 and −5.05 × 10−4 eV K−1 for the two directions of light polarisation, respectively. A donor‐like localised level within the energy gap of GeSe, lying 0.55 eV above the valence band irrespective of direction, is found from the tail of the room temperature absorption curves. Since a similar level has been observed previously in amorphous GeSe it is assumed to be an intrinsic property, introduced by Se vacancies.
Modified Perturbation Method for Diamond‐Type Crystals
Authors: Soma T.
Published: July 2007

The modified perturbation method, which reproduces the observed data of phonon curves and covers the uncertainties due to higher order perturbation, is proposed for diamond‐type crystals. Calculated values of cohesive energies, compressibilities, and band gaps for Si, Ge, and α‐Sn are in good agreement with the experimental data.
Surface Phonon Polaritons on Absorbing Crystals
Authors: Borstel G. , Schuller E. , Falge H. J.
Published: July 2007

The dispersion of surface phonon polaritons is calculated taking into account the experimental set‐up and complex dielectric functions. Under these conditions the results show the dispersion of surface waves in as well as outside of the frequency range of reststrahlenbands. Furthermore the dispersion branches exhibit backbending. The calculated curves are compared with reflection measurements at α‐quartz and discussed in terms of virtual modes. The connection between frequency and angular scans is shown by three‐dimensional representations of the reflectivity as a function of the angle of incidence and frequency.
Directional Dispersion of an Exciton Polariton in a Pentamethinium Cyanine Dye
Authors: Hesse H. J. , Fuhs W. , Weiser G. , Von Szentpaly L.
Published: July 2007

Reflectance spectra of γ‐cyclopropyl‐bis(1, 3, 3‐ trimethyl‐indolenine‐2‐yl)pentamethiniumtetrafluoroborate are measured on different crystal faces. The complex dielectric constant, derived from the reflectivity data, reveals that the broad and highly polarized reflectance bands (R ≈︁ 90%) in the visible spectral range are caused by a narrow but strong molecular excitation (ϵ2 > 300). The resonance energy is different for the various crystal faces and depends on the angle between the wave vector of light, k, and the transition moment of the molecular excitation. The large shift of the resonance energy from 1.73 eV on a {110} face to 2.61 eV on a {100} face is described quantitatively as directional dispersion of an extraordinary polariton.
Bose‐Einstein condensation of free excitons in AgBr‐type crystals at high excitation
Authors: Lelyakov A. V.
Published: July 2007

The shape of the excitonic luminescence band is studied for degenerate excitons of great radius at T ≠ 0 K in crystals with indirect bandgap. A calculation is made of the transition probability from a macroscopically occupied exciton state with wave vector k correeponding to the boundary of the Brillouin zone, under photon and phonon emission. The luminescence band consists of a δ‐type peak and a broader, and less intense, short‐wavelength satellite. The theory is compared with experiments for AgBr crystals.
Quantum theory of microwave conductivity
Authors: Arora V. K. , Qureshi E.
Published: July 2007

A quantum theory using a density matrix for electrical conductivity with an alternating electric field is described. The relaxation time, and the effeot of the microwave frequency on it, is obtained in a natural way without making any assumptions regarding its existence, as in the solution of the Boltzmann transport equation. Starting from the formal manipulation of the density matrix equation, the theory emphasizes various approximations under which the density matrix can be obtained in a usable form, assuming the electron‐phonon interaction to be a dominant mechanism of scattering in the nonpolar model. This density matrix is then used to find the expectation value of the current operator and hence the miorowave conductivity.
EPR of Cu2+ and radiation centres in anatase (TiO2)
Authors: Grunin V. S. , Davtyan G. D. , Ioffe V. A. , Patrina I. B.
Published: July 2007

The EPR spectra of three different Cu2+ centres and the spectrum of the radiation hole centre in single crystals of anatase are investigated. The constants of the spin‐Hamiltonians for all the spectra and the ground state wave functions are obtained. The conditions of incorporation into the crystal, the site of impurity in the lattice, and the mechanism of charge compensation are discussed.
Microscopic theory of clean type‐II superconductors in the entire field‐temperature plane
Authors: Brandt E. H.
Published: July 2007

The magnetization curves, the local magnetic field, and the energy gap of clean type‐II superconductors are calculated from Gorkov's theory for all possible values of B, T, and χ by a Ritz variation method. The free energy functional used is derived and discussed in two previous papers. The solutions coincide with all results known so far from approximate theories (T ≈ Tc,, B ≈ Hc2, B«Hc2). The first principle method yields, for the first time, magnetization curves with a Metastable (S‐shaped) part, thus explaining the jump Bo of induction observed at Hcl. At not too large values of T and B, the field at the vortex centres increases and the vortex core shrinks both with decreasing B and T. The third form factor, h20, is found to change its sign at B/Hc2≈0.7.
ESR study of paramagnetic centres in X‐irradiated AgCIO3
Authors: Mathai R. , Rao K. Narahari , Kumaraswamy A. , Rajasekharan T. , Rao K. V. S. Rama
Published: July 2007

Paramagnetic centres trapped in X‐irradiated AgCIO3, single crystals are studied at room temperature using an X‐band ESR spectrometer. The paramagnetic centres produced in these crystals are identified as O, and CIO2. The CIO2, centres are trapped at two magnetically inequivalent sites. The O, centre is also trapped at two inequivalent sites which are not well resolved at X‐band frequencies. The spin‐Hamiltonian parameters of CIO2, are gxx = 2.004 ± 0.001, gyy = 2.010 ± 0.001, gzz = 2.012 ± 0.001, Axx = (73±1) G, Ayy = (‐39 ± 1) G, and Azz = (‐4 ± 1) G.
Spin‐lattice relaxation of the O centre in RbI and RbBr
Authors: Matthys P. , Vanhaelst M. , Boesman E.
Published: July 2007

Pulse saturation measurements of spin‐lattice relaxation rates of O centres in rubidium bromide and iodide are reported. A theoretical calculation for the direct process in these salts is summarised. Phonon integrals appearing in the theoretical formulas are calculated numerically using an anisotropic continuum model, thus improving agreement with experiment. The significance of an unexpected anisotropy for two principal directions, perpendicular to the molecular axis, is discussed in the RbBr case.
Structure and conductivity of bilinear organic compounds
Authors: Phillips J. C.
Published: July 2007

Ferroelastically driven order‐disorder transitions of static domains are proposed to explain anomalous conductivities of molecular salts such as TTF‐TCNQ. If L is the length of a molecular chain, the ferroelastic driving energy is of order L2 compared to dynamical fluctuation energies of order L log L.
Microscopic theory of spin‐wave resonance in ferromagnetic films with surface‐region hihomogeneities
Authors: Wachniewski A.
Published: July 2007

A new general approach to spin wave resonance in ferromagnetic films with surface‐region inhomogeneities is proposed. This approach is based on Wallace's perturbation method with a zeroth‐order Hamiltonian corresponding to surface inhomogeneity model. Simple formulae for energies and intensities of resonance peaks are given and numerical examples illustrate the application of the method.
Electric field effect on the thallium emission bands in HBr and KI
Authors: Grassano U. M. , Tanga A. , Valli M.
Published: July 2007

The effect of an applied electric field on the emission bands of thallium centers is observed in KBr and KI. The results show a field quenching of the luminescence excited in the C and D absorption bands. From the temperature and field dependence of the relative luminescence change is established that the effect is due to a'decrease of the radiative quantum yield due to a field assisted tunneling of the electron from the excited level toward a nearby trap.
Mechanisms of energy transfer between rare earth ions
Authors: Lowther J. E.
Published: July 2007

The effect of phonon interactions on energy transfer probabilities because of multipolar coupling between two rare earth ions is examined. Both acoustic and optical phonons me oonsidered and the role of localized vibrations in a glass host is discussed.
Influence of magnetic impurities on superconducting properties of some compounds with A‐15 structure
Authors: Alekseevskii N. E. , Bazan C. , Mitin A. B. , Mydlarz T. , Krasnoperov E. P. , Rączka B.
Published: July 2007

The influence of iron group impurities on the critical temperature, critical field, and magnetic susceptibility of some superconducting compounds with A‐15 structure is investigated. Following systems are examined: Nb3A1 with Ni, Cr, and Fe, Nb3Sn with Fe, Co, and Mn, V3Si with Fe. It is pointed out that the impurities do not change the superconducting parameters substantially.
Bulk properties of lithium hydride up to 100 Mbar
Authors: Perrot F.
Published: July 2007

The electronic structure and equation of state of lithium hydride are investigated along the zero‐temperature isotherm up to 100 Mbar. The electronic part of energy and pressure are calculated using the augmented plane‐wave method (including Kohn‐Sham exchange and correlations). The lattice contribution is treated in a semi‐empirical model. Good agreement with experiment is obtained for the equilibrium lattice constant and minimum total energy. The compressibility modulus is about 20% too small. A semi‐metallic transition (2 Mbar) and a, metallic transition (30 Mbar) are predicted.
Band structure calculation of ZnSiP2
Authors: Pasemann L. , Cordts W. , Heinrich A. , Monecke J.
Published: July 2007

The band structure of ZnSiP2 is calculated using the empirical pseudopotential method. The form factors are deduced from those of ZnS, Si, AIP, and Gap. The band structure obtained is in good agreement with recent experimental data. The influence of the distortion of the anion sublattice on the energy levels at r is examined and the optical transitions in the region from 3 to 5 eV are discussed.
Relativistic excitations of a thin graphite film by inelastic electron scattering in theory and experiment
Authors: Wessjohann H. G.
Published: July 2007

Recently Chen and Silcox have published measurements of the inelastic electron scattering by a thin graphite slab. A theory formulated by the author is applied to this experiment. For the calculations the dielectric functions of graphite measured by Zeppenfeld are used. The numerical results and the maxima of the inelastic electron scattering probability are discussed. There is a good agreement between theory and experiment.
Dielectric function of hexagonal metals: II. Beryllium
Authors: Taut M. , Hanee W.
Published: July 2007

The longitudinal dielectric function ϵ(q,ω) of beryllium is calculated using realistic wave functions and band energies. Comparison with experiment is performed in the optical properties (including anisotropy), plasmon dispersion relation, and half width of the plasmon peak. The relaxation times for inclusion of damping are fitted. The experimentally observed peak splitting of the loss function is explained as a band structure effect.
Multiple s‐scattering in a correlated medium in the low‐density limit
Authors: Blaudeck P. , Lenk R.
Published: July 2007

The multiple scattering of waves in a random low‐density medium is investigated with consideration of the scatterers pair correlation. A given scatterer interacts with itself via the surrounding medium whose density is non‐uniform due to the presence of a correlation cloud. It is shown that correlations influence only the attenuation of the wave by incoherent elastic scattering. The real part of the wave number and, consequently, the density of states are not affected.
Localized non‐magnetic impurities in metals: I. General theory
Authors: Mrosan E. , Lehmann G.
Published: July 2007

A general formalism is developed for localized non‐magnetic impurities in metals. In the limit of cubic symmetry and for non‐zero scattering phase shifts for l ≦ 2, it is shown, that the T‐matrix, describing the scattering properties of the impurity, is diagonal and contains all effects coming from multiple scattering and from the anisotropic band structure of the host, given only by four coefficients of the Green function of the pure host. For localized impurities the Boltzmann equation has a degenerate kernel and can be transformed in an inhomogeneous algebraic set of linear equations of finite rank. The impurity potential is calculated in the muffin‐tin approximation. Screening and recharging effects are described by a constant potential shift chosen in such a way that a Friedel sum rule for the scattering phase shifts is fulfilled.
Approximative BCS‐like solution of the eliashberg gap equation
Authors: Nowotny H.
Published: July 2007

An approximative solution of the Eliashberg gap equation is parametrized to give an equation similar to the BCS gap equation. The connection between a characteristic phonon energy and the parameters corresponding to the BCS coupling constant is discussed. These parameters can be determined from the input data of the Eliashberg theory, i.e. the electron‐phonon spectral function α2(ω) F(ω) and the Coulomb pseudopotential μ*. Numerical results are given for Pb and In.
Theory of the photon‐drag effect in semiconductors with elliptical isoenergetic surfaces and its application to tellurium crystals
Authors: Genzow D. , Normantas E.
Published: July 2007

The components σiklm of the photon‐drag tensor are calculated within the framework of the Boltzmann equation for a model band structure with elliptical isoenergetic surfaces. Allowed direct optical transitions between two parabolic sub‐valence‐bands are considered. The theoretical results are applied to the photon‐drag effect in tellurium. Scattering of carriers on acoustic and polar‐optical phonons in the relaxation time approximation is taken into account. Comparison with experimental results is made.
Phonons in the mixed crystal K0.5Rb0.5I
Authors: Aslam J. , Rolandson S. , Beg M. M. , Butt N. M. , Khan Q. H.
Published: July 2007

Phonons propagating in the symmetry directions Δ, ∑, and A in K0.5Rb0.5I are obtained by inelastic scattering of thermal neutrons at 300 K. The acoustic phonons are well defined and no anomalous broadening is observed, but the transverse zone centre phonon, the only observed optic phonon, has a width about twice the instrumental resolution. The phonon frequencies in the mixed crystal are generally below the averaged frequency values of KI and RbI. The softening of modes is more prominent near the zone boundaries. The results are analysed in terms of the shell model and the rigid ion model for ionic crystals, and compared with similar calculations for KI end RbI. The elastic constants obtained from the eleven parameter shell model in units of 1011 dyn/cm2 are C11 = 3.04, C12 = 0.12, and C44 = 0.50.
The parallel and perpendicular Stark effect of the F‐center in RbBr and RbI
Authors: Banaii N. , Jacobs G.
Published: July 2007

The parallel and perpendicular Stark‐effects of the F‐center are measured in RbBr and RbI single crystals. The position of the 2s and 3d states are determined before relaxation of the lattice. The changes of the moments of the absorption band by the electric field are calculated and the dipole matrix elements between the unperturbed states mixed by the electric field are evaluated.
Static approximation for the Hubbard model: Magnetic phase transitions
Authors: Mertschlng J.
Published: July 2007

Magnetic phase transitions are studied within the Hubbard model using the static singlesite approximation for zero temperature and the Bragg‐Williams theory. For a half‐filled bend, the antiferromagnetic transition comes out reasonably except for small correlation energies. However, the static approximation does not yield ferromagnetism even for large correlation energies.
The multiplet structure of the electron absorption spectra of naphthalene—ß‐naphthol mixed molecular crystals
Authors: Fedorov V. P. , Kind V. V. , Korshunov A. V.
Published: July 2007

The electron absorption spectra of naphthalene—β‐naphthol mixed molecular crystals are measured in polarized light at 4.2 to 300 K. The fine structure of spectra in the region of naphthalene and β‐naphthol absorption (the lowest singlet—singlet transitions) is shown to result from localized and pseudolocalized electron states in the Davydov splitting region of naphthalene, 31482 to 31638 cm−1. It consists of phonon side bands for electron transitions of β‐naphthol in naphthalene with v = 29960 cm−1 and v = 30100 cm−1. The long‐wave doublet 29697, 29774 cm−1 is connected with the formation of clusters.
Millimeter wave investigation of the complex dielectric constant in DKDP
Authors: Gauss K. E. , Happ H.
Published: July 2007

The temperature dependence of the complex dielectric constant in K(H1−xDx)2PO4 with x = 0.64 and 0.86 is determined at ω = 4.62 cm−1 (λ = 2.16 mm) for E parallel and transverse to the ferroelectric axis c by measurements of the complex amplitude reflection coefficient using a Michelson interferometer. For E ‖ c the temperature dependence is due to a low frequency mode with a characteristic frequency below 6 cm−1. Apart from the vicinity of TC the data can reasonably be described with a Debye model. For E ⟂ c the results indicate a temperature dependent transverse protonic mode between 5 and 10 cm−1. ϵ″ shows a sharp anomalous peak at TC.
Quantum mechanical ab initio calculations of the surface structure of crystalline lithium hydride
Authors: Schulz W. , Gründler W.
Published: July 2007

The structure of the (100) and (110) surface of crystalline lithium hydride is calculated by the FSGO pair‐potential approximation. An outward displacement of the hydride ions and an inward displacement of the lithium ions relative to the ideal surface is determined in the case of the (100) surface. Also an alternating arrangement of the particles of the first and second surface layer is noticed. An inward displacement takes place not only for the lithium ions but also for the hydride ions in the case of the (110) surface. The surface energies and the ratio of the surface energies of the (110) and (100) surface are calculated in good agreement with semi‐empirical values of alkali halide crystals.
Madelung energies of tetragonal and orthorhombic systems
Authors: Soma T. , Umenai T.
Published: July 2007

The Madelung energies for tetragonal and the orthorhombic systems (simple, face‐centered, and body‐centered structures) are calculated. As applications, the stability for In which forms face‐centered tetragonal lattice, and the high‐pressure phase of group IV diamond‐type crystals are investigated.
Parameter calculation from zinc blende zone boundary conditions of CuI
Authors: Haxton M. B.
Published: July 2007

A rigid ion, second neighbor ionic model has been proposed by Banerjee and Varshni which is successful in fitting the dispersion curves of some zinc blende structures. Additional equations are offered derived from zone boundary conditions in the [100] and [111] directions, to aid in the calculation of a dispersion curve fit. These equations are applied to CuI and parameters are chosen which fit the equations and the physical situation better than those previously suggested.
Electric paramagnetic resonance and combined resonance of conductivity electrons in InSb at 24 GHz
Authors: Rusiniak L.
Published: July 2007

Measurements of small samples at 24 GHz with circular electromagnetic wave polarization are performed in the 60 to 100 K temperature range. The wide resonant line on the CRA side is observed. A new interpretation of this line as a combination of lines arising from three different transitions is proposed. The transitions involve both the electric paramagnetic resonance and combined resonance. The effect of amplification for combined resonance transitions during the dynamic temperature variation is also noted.
A double d‐band model of transition metals
Authors: Zieliński J.
Published: July 2007

A double d‐band model of a transition metal is considered and dynamic susceptibilities are calculated. A correspondence is found to the modified Zener model at least in the limiting case. The temperature dependence of the effective intraatomic Coulomb interaction Ieff is however different from that of the modified Zener model. Ieff exhibits a more or less pronounced maximum when plotted versus temperature. Consequences of such a behaviour are briefly discussed.
Multiphonon absorption in molecular crystals
Authors: Broude V. L. , Umarov L. M. , Sheka E. F.
Published: July 2007

The classification of molecular crystal multiphonon absorption is suggested taking into account quantitative difference of the energy spectrum of internal and external phonon modes. The experimental observations are given of phonon side bands following the bands of high frequency internal modes of the naphthalene crystal. The absorption observed is considered as a two‐phonon one caused by internal and external mode interaction. The dependence of the phonon side band shapes on the different geometry of the atom displacement for different internal modes is discussed.