bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Theory of a Heisenberg Alloy
Authors: Kassan‐Ogly F. A. , Medvedev M. V. 
Published: July 2007

AbstractMagnetic states of a disordered binary Heisenberg magnet, which is characterized (in the nearest neighbour approximation) by the exchange integrals: IAA > 0; IAB > 0; IBB < 0, are investigated. In the “mean” field approximation it is shown that in such an alloy below the transition temperature into the paramagnetic state ferromagnetic, antiferromagnetic, and noncollinear magnetic structures exist and that non‐trivial sequence of temperature phase transitions from one to another of these states may take place. Rather peculiar behaviour of spontaneous magnetization is obtained.
Theory of Resonance Dechannelling
Authors: Kumakhov M. A. , Wedell R. 
Published: July 2007

AbstractA theory of non‐linear phenomena leading to the resonance ion dechannelling is developed. These phenomena arise when ions pass from the conditions of axial channelling to those of planar one and also when ions channelled in an axial channel have low energies. In the first case, the wavelength of a particle trajectory in the plane normal to the axial channel becomes comparable with the distances between atomic rows forming an atomic plane. In the second case, the wavelength is comparable with the distance between two neighbouring atoms in the atomic row. In both the cases a resonance arises which is due to the fact, that the ion oscillation period in a channel becomes comparable with the lattice potential period. The general nonlinear equations obtained in a number of cases become the Mathieu equation describing a parametric resonance. The theoretical results are compared with those of computer calculations and with experiment.
Powder Reflection Measurements of Some Divalent Lead Halides
Authors: Eijkelenkamp A. J. H. 
Published: July 2007

AbstractPowder reflection spectra are recorded of α‐ and β‐PbF2, PbCl2, PbBr2, PbFCl, and PbFBr. The measurements are compared with available absorption and specular reflectance spectra. The characteristic exciton of the lead halides manifests itself as a maximum in the powder reflection spectra owing to a prevailing specular component of reflectance. An additional low‐energy absorption appearing in the powder spectra of PbF2 after a thermal treatment, is ascribed to incorporation of O2− impurity. The low‐energy absorption visible in the powder reflection spectra of PbFCl and PbFBr is attributed to either (hydr.) oxide impurity or nonequimolar composition regarding F− and Cl− or Br−. α‐ and β‐PbF2 are photochemically active like the other lead halides.
Low‐Energy Gap Semiconductor Mobilities in Presence of Spatial Fluctuations of Carrier Density
Authors: Barjon D. , Raymond A. , Robert J. L. 
Published: July 2007

AbstractIt is shown that the ionized impurity mobility is reduced by a factor close to [1 +(Δn/n0)]−l when spatial fluctuations of the carrier density are present in the crystal. This result is valid for degenerated and non degenerated semiconductors. It is also shown, that acoustical or optical scattering modes are not affected by these fluctuations.
Electronic Density of States of Amorphous CuGaS2 and ZnGeAs2
Authors: Braun W. , Cardona M. 
Published: July 2007

AbstractMeasurements of the energy distribution curves of photoemitted electrons are made for amorphous ZnGeAs2, and CuGaS2. The energies of the exciting photons are 21.2 (HeI), 40.8 (HeII), 16.8 (NeI), and 26.9 eV (NeII). The amorphous thin films are prepared by sputtering from polycrystalline material onto aluminium substrates. The photoemission curves of the valence band of ZnGeAs2, show a two peak structure. The upper portions of the valence band (0 to 6 eV) are composed primarily of p‐orbitals of the As and Ge atoms. A doublet structure is observed and attributed to the different strength of the Zn‐As and Ge‐As bonds. The lower portion of the photoemission spectrum is dominated by a strong feature derived from the Zn 3d‐level. The photoemission spectra of CuGaS2, show a strong dependence upon excitation energy which is due to the varying admixture of 3d electrons of the Cu with the p‐valence electrons of Ga and S. The spectra are decomposed into approximate p and d densities of valence states by using experimental atomic cross sections for the corresponding partial excitation probabilities.
An Operator Method for Optical Lineshape Calculations in Non‐Adiabatic Electron‐Phonon Systems
Authors: Sigmund E. , Wagner M. 
Published: July 2007

AbstractA method has been developed for optical band shape calculations in non‐adiabatic electron‐phonon systems. The basis of this treatment is the calculation of ordered exponential operators by successive partial integrations, which can be summed up in a closed form. The regions of validity are discussed. An application to a doubly degenerate Jahn‐Teller system (E‐e type) is presented. A comparison to experimental results is given.
Susceptibility and Superconductivity of Compounds with A‐15 Structure
Authors: Alekseevskii N. E. , Krasnoperov E. P. , Mydlarz T. 
Published: July 2007

AbstractThe magnetic susceptibilities of the Nb3Al1−xGex, and Nb3Ga compounds are measured in the temperature range 20 to 300 K. It is shown that in the compounds investigated the temperature dependence of the susceptibility is considerably less than in other high‐temper‐ ature superconductors with A‐15 structure. It is difficult to explain the high value of Tc and the small temperature coefficient of the susceptibility by the narrowness of the conduction band. It is assumed that the high coupling constant in these superconductors is determined by the properties of the electron‐phonon interaction arising as a consequence of the nearly one‐dimensional movement of the electrons.
Transport Phenomena and Eigenoscillations of Hot Electrons in Semiconductors with Periodic Structure
Authors: Dykman I. M. , Tomchuk P. M. 
Published: July 2007

AbstractSemiconductors with periodic impurity distribution are studied in electric fields heating the carriers. Both, elastic and inelastic scattering of carriers by lattice vibrations are con‐sidered. The calculations are carried out in a general form without concrete definition of the energy loss mechanism and without introduction of energy relaxation time. An expression for the additional current due to the sample in homogeneity valid over a wide range of fields is obtained. The sign of this current is opposite to the sign of the current in a homogeneous sample. Its absolute value firstly grows with increasing field and then drops up to zero. It is shown that the semiconductor inhomogeneity noticeably influences the spectrum of the eigenoscillations. The dependence of the frequency of these oscillations ω = ωr + iγ on the field strength and the wave vector is determined. The dependence of ω on the wave vector is resonant in nature. The frequency of the longitudinal waves is found to depend not only on the value, but also on the direction of the wave vector.
Formal Results on the Susceptibility of Dilute Alloys. II. Mean‐Field Approach and Applications
Authors: Bisch P. M. , Gomes A. A. 
Published: July 2007

AbstractA mean field approach is adopted and a matrix formulation is used to obtain formal results for the susceptibility of a two band system (α, β). This general result includes the intra‐atomic Coulomb repulsions, one‐electron impurity potentials, and the change in the Coulomb interaction and inter‐band exchange at impurity site. The complete evaluation of the partial susceptibilities involves the calculation of inverse matrices, which is difficult to perform in the general case. Expressions for the self‐polarization hyperfine field, the Knight‐shift, and ESR g‐shift are obtained for transition metal like systems (s‐d bands), using previous results obtained in I. It is shown that these quantities may be evaluated in terms of phase‐shift, involving only the density of states and the parameters of the problem.
Biexciton Luminescence with Interserial Transition of the Formed Exciton
Authors: Khadzhi P. I. , Petrashku K. G. , Moskalenko S. A. 
Published: July 2007

AbstractThe process of biexciton radiative recombination is studied theoretically with the interserial transition of the exciton in semiconductors with direct allowed interband transitions. Analytical expressions for the spectral and temperature dependences of the rates of different radiative recombination channels are obtained along with the formation of the exciton in the ground, as well as in the excited states of the discrete or continuous spectrum. Depending on the band structures of the exciton and biexciton the emission band for radiative recombination with the formation of exciton possesses the boundary (threshold) frequency and the long‐wavelength (short‐wavelength) tail. Numerical calculations given for CdS indicate the giant probability of the treated process.
Zero and First Sound in NaF and RbI
Authors: Loidl A. , Jex H. , Daubert J. , Müllner M. 
Published: July 2007

AbstractZero and first sound as well as the transition region are investigated theoretically in RbI and NaF. The temperature dependence of the first sound, zero sound, and isothermal elastic constants is calculated on the basis of a breathing shell model for the harmonic lattice dynamics and anharmonic parameters from microscopic Grüneisen constants. The theoretical results are compared with ultrasonic data and inelastic neutron scattering results. Own recent neutron experiments in RbI are concerned with the zero sound region at room temperature while the NaF experiments cover also parts of the transition region between first and zero sound at various temperatures between 295 and 700 K. It is shown that the dispersion of sound is satisfactorily described by use of an averaged thermal phonon lifetime. In NaF for each temperature first sound elastic constants and averaged phonon lifetimes are deduced from an analysis of the experimental data.
Electron Mobility of GaxIn1−xSb at Room Temperature
Authors: Barjon D. , Raymond A. , Joullie A. , Robert J. L. 
Published: July 2007

AbstractExperimental values are given of electron mobility at 300K in GaxIn1−xSb compounds (0 ≦ × ≦ 0.48). The experimental values of the mobility are compared to those computed in the case of mixed scattering by acoustic, polar optical phonons and ionized impurities. The fluctuations of carrier density which only affect ionized impurity scattering are also taken into account.
Pseudopotential Calculation of the Electronic Valence Charge Density and the Optical Dielectric Constant of NaCl and MgO
Authors: Nagel S. , Maschke K. , Baldereschi A. 
Published: July 2007

AbstractThe pseudopotential method is used to calculate the electronic charge distribution and the optical dielectric constant for NaCl and MgO. The results indicate that a local pseudopotential gives a sufficiently good description of the highly ionic properties of these materials.
Cyclotron Resonance with a Slightly Anomalous Skin Effect
Authors: Kuka G. , Braune W. , Herrmann R. 
Published: July 2007

AbstractMicrowave absorption lines for bismuth and bismuth alloys (with antimony and tin) are investigated on the basis of a conductivity model for Fermi surfaces showing a slight deviation from an elliptic shape. A good fit of cyclotron wave dispersion and agreement of the experimentally determined behaviour of microwave absorption with theory are found.
Transmission and Reflectivity of Highly Excited Semiconductors Due to the Optical Conversion of Excitons into Excitonic Molecules
Authors: Wünsche H.‐J. , Henneberger F. 
Published: July 2007

AbstractThe dielectric function for the optical conversion of excitons into excitonic molecules is derived, using Boltzmann distributions for both the systems. In a second step calculations of the optical constants as well as the corresponding transmission and reflectivity spectra are presented. Appart from some other characteristic features, a region of strong reflection occurs at high exciton densities.
Far‐Infrared Magneto‐Reflection in HgTe
Authors: Suzuki K. , Miura N. , Uchida S. , Tanaka S. 
Published: July 2007

AbstractThe magneto‐reflection spectra in HgTe at 119 μm are investigated over a temperature range from 10 to 77 K in magnetic fields up to 160 kG. Firstly, the magnetic field dependence of the interband Γ8 → Γ8 dielectric function is calculated and its contribution to the intraband Γ8 → Γ8 magneto‐reflection spectra is studied. Secondly, an anisotropy parameter, γ2 → γ2 to be 0.5 ± 0.4 from the measurement of the interband Γ8 → Γ8 magnetoreflection spectra for magnetic field directions parallel to the crystal axes 〈100〉 and 〈111〉 at about 10 K.
Spectrum of Magnetic Excitations of a Ferromagnet with Weakly Bound Antiferromagnetic Interstitial Impurities
Authors: Botvinko M. N. , Ivanov M. A. 
Published: July 2007

AbstractThe spectrum of impurity excitations of a Heisenberg cubic ferromagnet with randomly distributed weakly‐bound antiferromagnetic interstitials is investigated. It is shown that if the impurity concentration c is less than some characteristic concentration c0, the density of impurity states has an exponentially narrow peak at the localized‐level frequency ω0. If, however, c ≫ c0, an impurity band appears between the frequencies ω0 and ω1 = ω0, (1 ‐ c(S1/S)). Impurity excitations in this band are characterized with wave vectork with wave vector k(k ⪅ c1/3a−1), their inverse lifetime being small compared with the band width c(S′/S) ω0. Inelastic scattering of neutrons as well as light absorption are studied in the resonant region near ω0 When c > c0, the spectrum of light absorption due to impurities involves two strongly asymmetrical peaks (not one, as in the case of c < c0). The main peak width (that at ω1)is of order (c/co)2 c(S′/S) ω0 and falls with growth of the impurity concentration.
The electronic structure of trapped hole centres in SnO2
Authors: Zwingel D. 
Published: July 2007

AbstractIn SnO2 single crystals, doped with Al and Ga, the ESR spectra of “trapped holes” are observed after UV excitation. A discussion of the hyperfine and superhyperfine parameters of the spectra shows that, due to the different electron configurations of the impurity ions, the hole is localized at different oxygen anions next to the impurity. Prominent temperature effects of the spectra show that exchange processes between different anion sites have to be taken into account, as well as the coupling to phonon modes of the lattice in order to explain the high‐temperature ESR spectra.
Influence of the crystal field on bent configuration of VK centres in AMF3 compounds
Authors: Rousseau J. J. , Fayet J. C. 
Published: July 2007

AbstractOn calculating the admixture of the 2IIg molecular state in the 2∑u ground state by a C2v crystal field, an attempt is made to determine the angle which characterizes the bent configuration of the VK centres in the fluoperovskite lattice.
Spin‐wave measurements of exchange constant A in Ni‐Fe alloy thin films
Authors: Hennemann O. D. , Siegel E. 
Published: July 2007

AbstractThe ferromagnetic exchange constant is measured using spin‐wave resonance in eight nickel‐iron alloys with a composition range from about 55 to 95% Ni. The exchange constant A varies with composition as A = (2.25 ‐ 1.4CNi)erg/cm. The data are in good agreement with the values published by Wilts and Lai.
Magnetic moment distribution in Cu‐Ni alloys
Authors: Sacchetti F. , De Gasperis P. , Menzwger F. 
Published: July 2007

AbstractThe magnetic moment distribution is measured on two f.c.c. Cu‐Ni alloys with composition of 14 and 45 atyo Cu. The results are compared both with those of other similar materials and with the predictions of the coherent potential approximation (CPA). It is found that the composition dependence of the asphericity agrees with the CPA calculations, as it was found also for Co‐Ni and Fe‐Ni alloys. The spherical form factor is found to be contracted in both the Cu‐Ni and Co‐Ni alloys with respect to pure Ni.
On the change of electronic polarisability of an ion in a crystal
Authors: Chakrabarti S. K. , Sarkar S. K. , Sengupta S. 
Published: July 2007

AbstractA theory for the change in polarisabilities of ions in crystal environment from their free ion values is presented. The contribution to this change is shown to come from the pure third order perturbation and second order exchange terms in the perturbation energy expression for the system. These contributions are separately estimated. The main contribution is shown to arise from third order perturbation effect. The inadequacies of the earlier treatments by Ruffa [1] and by Ledovskaya [2] are pointed out. Calculations for the change in polarisability due to third order perturbation effects are carried out for cations only. For anions some difficulties in the application of the perturbation theory are discussed. The results are then compared with the empirical values of Tessmann, Kahn, and Shockley [3] and also with those of Ruffa.
Optical spectra of aggregate centres in KCl:Pb crystals
Authors: Mărculescu L. 
Published: July 2007

AbstractAbsorption and emission spectra of the Pb2+ ion‐cation vacancy complexes are measured. The existence of two emission bands due to aggregate centres and their shift to lower energies are explained using a model of Pb2+ luminescent centres in the framework of Johnson and Williams' thermodynamical theory.
Luminescence of a “Cold” gas of biexcitons in CuCl
Authors: Levy R. , Klingshirn C. , Ostertag E. , Phach Vu Duy , Grun J. B. 
Published: July 2007

AbstractThe emission due to the radiative decay of biexcitons in CuCl is studied at low temperatures. Biexcitons formed of free excitons mainly under LO phonon emission, contribute to the broad emission bands observed. In the case of direct creation of biexcitons by resonant two‐photon absorption, a “cold” biexciton gas is created which is not in thermal equilibrium and gives rise to the narrow emission lines. A Bose‐Einstein condensation of the biexcitons seems to be very unlikely.
Magnetic stability at 0 K of a simple cubic ferromagnetic array of pseudo‐spin S = ½ with anisotropic exchange between nearest neighbours
Authors: Belorizky E. , Casalegno R. , Fries P. 
Published: July 2007

AbstractThe spin wave spectrum is calculated of a pseudo‐spin S = ½ system forming a simple cubic ferromagnetic array, by considering the most general interaction between nearest neighbours allowed by a fourfold order symmetry of the bond ℋ︁ij = J‖SizSjz + J⊥(SixSjz + + Siy,Sjy) for a pair (i, j) oriented along the z‐axis. It is shown that, in zero external magnetic field, a ferromagnetic order is allowed at 0 K only for J‖ and J ⊥ both negative, with the easy direction of magnetization along a 0. [100] axis. The zero point energy of the spin waves and the mean spin deviation in the approximate ferromagnetic ground state are also studied.
Electron spin‐lattice relaxation of Cu2+ ions in planar coordination geometries: Calculation and experimental data of T1 for Cu2+ in Ni(II) bis(diethyl‐dithiocarbamate) crystals
Authors: Aminov L. K. , Kirmse R. , Solovev B. V. 
Published: July 2007

AbstractA general formula is derived for the direct spin‐lattice relaxation process of paramagnetic ions with S = ½ for arbitrary orientations of an external magnetic field relative to the crystal axes. Using the formula obtained, T1 of Cu2+ ions in square planar coordination geometries is calculated in the point charge approximation. The temperature, concentration, and angular dependence of T1 of Cu2+ ions in Ni(II)bis(diethyl‐dithiocarbamate) single crystals are reported. Including some features of covalency a reasonable agreement between the theoretical and experimental values of T1 is reached.
On the shear modulus of the flux line lattice
Authors: Brandt E. H. 
Published: July 2007

AbstractThe shear modulus c66 of the flux line lattice in type II superconductors can be obtained from a two‐body interaction between the flux lines even a t large inductions B ≈Hc2. The potential is composed of a repulsive and an attractive part and has a range diverging at Hc2. An explicit expression for the Ginsburg‐Landau c66 is given for arbitrary B and x. From Gorkov's microscopic theory c66 is derived for large B and arbitrary temperature. Simple approximate formula are given.
Energy band structure and Ferrni surface of potassium
Authors: Faust A. , Merkel C. , Bross H. 
Published: July 2007

AbstractWave functions and energy eigenvalues for potassium are calculated with the MAPW method using a nonlocal screened exchange potential. From the results obtained for several symmetry directions, the Fermi energy, the Fermi surface, its extremal cross‐sections, different masses, and the contribution of conduction electrons to the Knight shift are determined and related to experimental data.
Description of reactions between defects in solids
Authors: Pletnev M. N. 
Published: July 2007

AbstractA new scheme is suggested for describing the system of reacting particles using the Fock space of distribution functions. This method can be applied to calculate any function of the number of particles in a system, in particular, the moments of any order. Diffusion‐limited reactions and simultaneous generation and annihilation of particles as the result of reaction are considered.
Dynamical properties of crystals of Sr(NO3)2, Ba(NO3)2, and Pb(NO3)2. I. Infrared spectra and structure
Authors: Bon A. M. , Benoit C. , Bernard O. 
Published: July 2007

AbstractThe far infrared reflection spectra of three single crystals of Sr(NO3)2, Ba(NO3)2, and Pb(NO3)2 are measured from 2500 to 10 cm−1 and from 10 to 300 K. The results are discussed on the basis of phonon and polariton theory. A method is found to determine the number of ordinary and extraordinary phonons in the case of general symmetry and a relation is established between the phonon frequencies and the polariton frequencies. These results are applied to discuss the spectra of Sr(NO3)2, Ba(NO3)2, and Pb(NO3)2. Then, among several structures proposed by different authors, the T4 structure is proposed from the results.
Spin—lattice relaxation of several nuclei near the ferroelectric transitions of TGS and TGFBe
Authors: Müller D. , Petersson J. 
Published: July 2007

AbstractSpin—lattice relaxation time, T1, measurements are reported and interpreted of the nuclei H1, C13, and N15 in TGS and F19 in TGFBe. In particular the behaviour of T1 is investigated near the ferroelectric phase transition temperatures of these crystals. The ferroelectric contribution to the C13 relaxation rate in TGS doped selectively in the CH2 group as well as the ferroelectric contribution to the F19 relaxation rate in TGFBe show an anomalous temperature dependence near Tc which can be described by a logarithmic singularity. The ferroelectric contribution to the spin—lattice relaxation rate of the protons in TGS shows a rather complicated temperature and frequency dependence. T1 of N15 doped TGS is nearly constant in a wide temperature range. The presented experimental results are in accordance with the assumption of an order—disorder model for the motions of the molecular groups which cause ferroelectricity in crystals of the TGS group.
Theory of side‐bands in a cooperative Jahn‐Teller material
Authors: Gehring G. A. , Laugsch J. 
Published: July 2007

AbstractThe theory of optical side‐bands is developed for a crystal with a cooperative Jahn‐Teller phase transition containing rare‐earth ions. A tight binding scheme is used to describe the rare‐earth energy levels over the whole Brillouin zone. The possible two‐ion couplings which can give rise to side‐bands are discussed; these include the diagonal and off‐diagonal contributions of the virtual phonon interaction.
Lattice dynamics of rare‐gas solids by Kihara's potential III. Green functions
Authors: Bohra A. K. , Gupta R. K. 
Published: July 2007

AbstractThe phonon Green functions are presented for solid argon, krypton, and xenon. The Green functions are computed by studying the lattice dynamics of these solids, using the Kihara hard core potential. A set of potential parameters fitted to neutron data is used in the computation. The localized mode due to neon in argon is studied as an example of the use of these Green functions in defect problems.