bluuebunny/crossref_metadata_2025_split huggingface dataset:
Electronic Structure and Superconductivity in Solid Solutions Based on In, Pb, and TI
Authors: Sulkowski C. 
Published: July 2007

AbstractThere exist an anomaly in the Tc‐course versus electron concentration Z in indium alloys at the value Z = 2.98 electrons/atom. This anomaly is caused by changed of electronic structure. Results of Tc‐measurements in ternary and quaternary indium solid solutions in the Tc anomaly area are presented. The minimum value of Tc for these alloys occurs at Z = 2.98. The effective Z, denoted Z*, is determined from the alloy composition at the minimum Tc resulting in following numbers: (2.0 ± 0.1) for Mg, (2.6 ± 0.1) for T1, (3.9 ± 0.1) for Pb, (4.0 ± 0.1) for Sn, and (3.25 ± 0.2) electrons/atom for Hg. Unique dependence of Tc on Z* is found for each group of alloys when the effective electron concentrations are calculated from the Z*‐number.
Raman Study of Dynamic Disorder and the Ferroelectric Transition in K4Fe(CN)6.3 H20
Authors: Savatinova I. , Anachkova E. 
Published: July 2007

AbstractPolarized Raman spectra of ferroelectric K4Fe(CN)6 ˙ 3 H2O are measured at various temperatures from 140 K up to above the phase transition (Tc = 249 K). The transition becomes apparent through strong phonon coupling of a band at 62 cm‐1 with a low‐frequency continuum in the Bg/A″ spectrum. A detailed analysis of the temperature dependence of this phenomenon is made by Fano's expressions. Intensity measurements indicate the dominant contribution of the water disorder mechanism to the continuum. An anomalous broadening of the Bg/A″ bands in the 80 to 180 cm‐1 region is observed. In the vicinity of Tc the C ≡ N stretching vibrations reveal some changes which are assigned to a slight distortion of the [Fe(CN)6]4‐complexes.
Dielectric response of saturated semiconductors
Authors: dos Reis F. G. , Cerdeira Hilda A. , Luzzi R. 
Published: July 2007

AbstractThe optical properties of a direct‐gap semiconductor are investigated under conditions of a strong optical pumping by an intense laser source of frequency slightly larger than the energy band gap. By taking into account the Coulomb interaction between electrons and holes we found that the energy gap in the quasi‐particle spectrum diminishes. The dielectric constant of the system is calculated in the generalized Hartree‐Fock approximation. The Raman cross‐section is found to vanish for energies below 2Δ, where Δ is the quasi‐particle energy gap.
A simple model for the calculation of heat of formation for high‐concentration substitutional defects in alkali halide crystals
Authors: Paul S. , Sengupta S. 
Published: July 2007

AbstractAn analytical expression for the heat of formation of substitutional defects in high concentration is developed on the basis of a simple model. In a previous paper the authors developed a simple formula for the heat of formation of substitutional defects with low concentration which convenient formula is utilized for calculating the heat of formation when the defect concentration is high. In this model the defect concentration is developed stepwise and in every step the defect crystal is considered as equivalent to a perfect crystal with a modified lattice parameter determined by the Vegard's law. Values for the heat of formation over the entire composition range are calculated and compared to experimental values for eight different solid solutions. For negative ion substitution, the values are in good agreement with the experimental results. The reason for the slightly larger discrepancy for the positive ion substitution may be ascribed to the effect of polarizability of the negative ions.
A non‐diagonally screened model potential for semiconductors applied to the band structure calculation of Si
Authors: Pasemann L. 
Published: July 2007

AbstractOn the basis of a non‐linear approximation of the electronic charge density in the perturbing potential the screening of the ionic model potential by the valence electrons of a semiconductor is described by a screening matrix and the depletion hole charge. Using the bare model potantial of Shaw the constructed screening model is tested by local and non‐local band structure calculations of Si. The band structure calculated with the non‐local model potential is in good agreement with experimentally established data. The local approximation is not appropriate.
Interband electronic Raman scattering by a three‐band semiconductor in the presence of a resonant electromagnetic field
Authors: van Hieu Nguyen , Ai Viet Nguyen 
Published: July 2007

AbstractThe influence is investigated of a resonant electromagnetic field on the interband electronic Raman scattering of a three‐band semiconductor with direct band gaps. Explicit expressions for the cross‐sections are given. The form of the Raman spectra depends on the effective masses of electrons and holes.
A low temperature equation of state for monovalent metals
Authors: Goel S. C. , Midha Y. R. , Nanda V. S. 
Published: July 2007

AbstractAn equation of state for metals based upon a simple model, which ignores the effect of free electrons, was given in an earlier paper. An improved equation of state is obtained for monovalent metals by taking account of the electronic contribution in the free energy expression. This result is used to explain the thermal expansivity of copper in the range between 1 and 12 K. The same set of parameters give theoretical specific heat values which are in fairly good agreement with the reported experimental data in the same range of temperature.
A new approach to lattice properties of alkali halide crystals
Authors: Sharma M. N. , Kant A. 
Published: July 2007

AbstractA new form of overlap repulsion as a function of interorbital distance as suggested earlier is employed to calculate some more properties of alkali halide crystals, such as infrared absorption frequency (v0), Grüneisen (γ), and Anderson‐Grüneisen (δ) parameter. The results are compared with the observed values and those calculated by earlier workers. In general the results show a marked improvement, establishing the validity of the proposed model.
The magnetic properties of charge‐ordered crystals
Authors: Lubimov V. S. , Makarov E. F. , Ionov S. P. 
Published: July 2007

AbstractThe energy spectrum of the low‐lying collective triplet excitations is calculated. It is shown that the boundary frequency of such states is lower than that of a singlet collective mode taking into account the interband exchange interaction. Without the exchange both types of excitation have identical spectra. The susceptibility of the charge‐ordered state is calculated and some effects are discussed which are caused by the collective states.
On rate equations in the theory of diffusion‐controlled reactions
Authors: Kalnin Yu. H. , Pirogov F. V. 
Published: July 2007

AbstractA connection between the two equations frequently used in the theory of diffusion‐controlled reactions, the Antonov‐Romanovskii and the Waite rate equations, is established. A new rate equation is proposed, which in the case of not very strong correlations between the reaction partners can substitute the more complicated Waite equation.
Spectroscopic investigation of optically excited doublet states of Ho3+ in magnetically ordered (Ho0.01, Gd0.99) VO4
Authors: Kasten A. , Becker P. J. 
Published: July 2007

AbstractThe splittings of optically excited doublets of Ho3+ in (Ho0.01, Gd0.99)VO4, caused by the antiferromagnetic ordering are studied, and exchange fields in the antiferromagnetic as well as in the saturated paramagnetic state are derived by means of Zeeman effect spectroscopy. The description of the superexchange interactions in terms of effective magnetic fields holds for most doublets, but at least for two states the Ho3+–Gd3+ exchange interaction is of a more complicated form.
Dynamic behaviour of the ising model in a transverse field
Authors: Kühnel A. , Wendt S. , Wesselinowa J. 
Published: July 2007

AbstractThe Ising model in a transverse field is treated by means of Green function technique to calculate the dynamic structure factor above and below Tc. The soft mode is overdamped in the disordered phase and near Tc, while it is nearly undamped at low temperatures. The coupling between the transverse soft mode and the relaxing longitudinal mode produces a central mode in the dynamic structure factor. The temperature dependence of the dynamic structure factor for zero wave vector is calculated numerically.
One‐particle spectrum of correlated electrons near the free‐electron limit
Authors: Monecke J. 
Published: July 2007

AbstractAn approximate solution of the Hubbard model is proposed which is correct in second order in I/Δ. In difference to the energy‐dependent Born approximation it preserves the presumed particle–hole symmetry of the Hamiltonian and is correct in the atomic limit, too. The one‐particle spectrum is calculated neglecting finite lifetime effects.
Singly, doubly, and multiply bottlenecked EPR based on the linear response theory
Authors: Sugawara K. 
Published: July 2007

AbstractBased on the linear response theory, the spin–spin relaxation rate T2−1 of S‐state magnetic impurities in metallic hosts is investigated. It is found that the EPR linewidths of S‐state impurities in metals developed by Hasegawa (singly bottlenecked EPR) and by Hirst et al. (doubly bottlenecked EPR) can be naturally obtained using physical principles contained in the theory of Moriya and Obata which was originally developed for an insulator.
The kinetic exchange interaction in doubly degenerate narrow bands
Authors: Chao K. A. , Spałek J. , Oleś A. M. 
Published: July 2007

AbstractA canonical transformation is applied to the doubly‐degenerate‐band Hubbard model with arbitrary n in the strong correlation regime. Projection operators are constructed to project out the splitted subbands. Only the inter‐subband electron hopping is treated as a perturbation which couples different subbands at the second order. In localized picture the coupling is of the Heisenberg type, corresponding to the virtual hopping of electrons and generating the spin waves, the orbital waves, and the coupled spin‐orbital waves. For n = 1 the ground state has ferromagnetic spin and antiferromagnetic orbital ordering. For n = 2 the spins in the ground state have ferromagnetic intra‐site but antiferromagnetic inter‐site ordering. These orderings are due to Hund's rule, the effect of which gets weakened by the intra‐subband electron hopping as n deviates from an integer. This canonical‐transformed effective Hamiltonian is suitable for investigating the phase diagram of the Hubbard model with orbital degeneracy.
Theory of the ESR spectra of Tl2+ ions in K2SO4 single crystals
Authors: Eremin M. V. , Silkin N. I. 
Published: July 2007

AbstractThe electron paramagnetic resonance spectrum of β‐K2SO4: Tl2+ is considered. The cluster model of the paramagnetic centres is used. It is suggested that the spectrum anisotropy is caused by the crystalline electric field which mixes the state 2P of the Tl2+ ion to the ground state 2S. Such an electric field is absent in the point charge model. An additional proof of the important role of the overlap of the Tl2+ and oxygen ion electron orbits is obtained when analysing the hyperfine structure of the ESR spectrum.
On the theory of resonant secondary radiation of excitons weakly interacting with phonons
Authors: Hizhnyakov V. V. , Sherman A. V. 
Published: July 2007

AbstractFormulae are obtained for the partial spectra of the resonant secondary radiation of excitons weakly interacting with optical phonons, separated from the Rayleigh line by two or three phonon frequencies. The cases of the Frenkel exciton as well as that of the Wannier‐Mott one are considered. The obtained shapes of the spectra depend upon the type of the dispersion law of the exciton band and on the type of the dependence of the exciton—phonon interaction constants on the wave vector. A classification of these spectra according to the transformation mechanism from the primary photon into secondary is made. The criteria are obtained identifying the spectra under consideration as scattering, ordinary or hot luminescence.
Comments on the theory of hopping conduction
Authors: Zvyagin I. P. 
Published: July 2007

AbstractSome aspects of the existing quantum‐mechanical theory of hopping conduction are discussed. Special attention is paid to the problems of the definition of current density in the site represetation, to the effects of variation of occupation numbers of the sites and to the approximations used in the quantum‐mechanical derivation of the rate equation. The applicability of the random resistor network model to the problem of hopping conduction is further confirmed.
On the use of generalized Langevin equations in phonon transport problems
Authors: Michel H. , Wagner M. 
Published: July 2007

AbstractA system of phonons and local excitations is considered. The coupling of both subsystems is taken linear in the phonon coordinates. The generalized Langevin equations of the phonon density operator and of the subsequent mixed dipole operators are considered. The elimination of the dipole operators, a decomposition of the averaged second order occupations and specified further assumptions lead to the evolution equation of the phonon distribution function, which contains the local occupation numbers. In full analogy the time evolution of the local occupation numbers can be derived. A Markoff assumption, which is justified for realistic systems, and a restriction to slowly varying phonon densities lead to a rate equation type of a Peierls‐Boltzmann equation, which is coupled to the kinetic equations of the local occupations.
Lattice dynamics of the molecular crystals KCN and NaCN in their disordered cubic phase
Authors: Jex H. , Maetz C. J. 
Published: July 2007

AbstractA model for the lattice dynamics of the disordered cubic high temperature phase of KCN and NaCN is proposed using a shell model for the translationnal motion, while the librational degrees of freedom of the CN− ‐ddumbbells are taken into account in the approximation of rigid molecular ions. The calculted dispersion curves are in adequate agreement with the experimental data from inelastic neutron scattring especially for the TA‐branches, which show an anomalous dispersion.
The phonon induced zero field splitting in vanadium doped alkali chlorides
Authors: Vanhaelst M. , Matthys P. , Boesman E. 
Published: July 2007

AbstractThe zero field splitting in V2+ doped NaCl, KCl, and RbCl is temperature dependent and has a shallow minimum near 50 K. Previously reported experiments are compared with theoretical results based on an anisotropic continuum phonon model. The qualitative behaviour of the spin‐Hamiltonian constant D as well as the quantitative agreement of the coefficients, which determine D as a function of temperature, are discussed.
On the crossing of dispersion curves of alkali metals in [ζ00] direction
Authors: Ramamurthy V. , Singh K. K. 
Published: July 2007

AbstractThe experimentally observed dispersion curves of lithium, sodium, and potassium in [ζ00] direction, especially the degeneracy of the zone boundary frequencies and the anomalous crossing in lithium, are reproduced completely by a lattice dynamical which model which takes into account the umklapp processes and the screening associated with the electrons. It is shown that the observed crossing in lithium is associated with the large positive value of the bulk modulus of the electron gas, together with the variation of umklapp contribution to these modes of vibration with the wave vector. The latter leads to the observed degeneracy at the zone boundary point. The role played by umklapp processes in producing effects is discussed.
On the properties of cobalt impurities in zinc selenide crystals II. The optical properties in near‐infrared. 4A2(4F) ↔ 4T2(4F) Transitions
Authors: Radlinski A. P. 
Published: July 2007

AbstractAbsorption and luminescence spectra due to the 4A2(4F) ↔ 4T2(4F) crystal field transitions in ZnSe:Co2+ are presented. It is shown that the structure of the absorption band is caused by the spin‐orbit interaction within the 4T2(4F) state and the coupling to the lattice modes of A1 symmetry. According to a model proposed for the thermal depopulation of the excited 4T2(4F) state, the ionization energy of the Co2+ impurity in ZnSe is estimated to be about 5600 cm−1 (0.69 eV).
Structural phase transitions of antiferroelectric and displacive modulated types caused by electron‐phonon interaction
Authors: Konsin P. 
Published: July 2007

AbstractA generalization of the vibronic theory of ferroelectric phase transitions is made and the theory of antiferroelectric and structural modulated phase transitions, caused by the electron‐phonon interaction in dielectrics and semiconductors, is developed. The criteria for the occurrence of such transitions are obtained. The basic characteristics (spontaneous lattice distortion, dielectric susceptibility, frequencies of soft modes, etc.) are calculated. The dispersion of electronic states, corrections to the mean field approximation for the driving vibronic interaction, and phonon anharmonicity are taken into account. The relation between ferroelectric and antiferroelectric phase transitions is discussed.
Magnetic freeze‐out effect in noncrystalline semiconductors
Authors: Majerníková E. , Barta Š. 
Published: July 2007

AbstractThe density of states of an electron and an exciton gas in a strong magnetic field and in a Gaussian random potential is calculated. The effect of the magnetic freeze‐out of the tail states owing to the Landau shifts, the dependence of the „damping”︁ of the tails on the magnetic field, as well as the magnetic field dependent quantum shift of the deep tails are found. In some cases, the effect can provide means for a statement of the presence of excitons in non‐crystalline solids.
Some aspects of spin relaxation in ferromagnets in a magnetic field
Authors: Kashcheev V. N. 
Published: July 2007

AbstractIn the Vaks‐Larkin‐Pikin spin model the asymptotics of the pair spin correlators are derived in the right neighbourhood of the Curie point and at the non‐zero external magnetic field. Using this asymptotics the critical anomalies of the NMR line are found as well as the phonon damping and renormalization of the sound velocity in the isotropic Heisenberg ferromagnet.
An exact calculation of the average density of states in disordered systems
Authors: Zawisz S. 
Published: July 2007

AbstractAn exact non‐self‐consistent formula for the direct calculations (i.e. without solving the eigenvalue problem for the Hamiltonian) of the density‐of‐states function for a three‐dimensional disordered system is presented. An interpretation is given and an example of using the formula is discussed.
One‐ and two‐phonon states in tetragonal ZnP2 and CdP2 crystals
Authors: Gorban I. S. , Gorinya V. A. , Dashkovskaya R. A. , Lugovoi V. I. , Makovetskaya A. P. , Tichina I. I. 
Published: July 2007

AbstractSecond‐order Raman spectra are studied in tetragonal ZnP2 and CdP2 crystals. The main feature of the spectra is a strong dominance of the A1‐component over the E‐ and B2‐components. Whereas the former is due to overtones, the latter ones are associated with combination tones. Besides, the A1‐component reveals a number of bands associated with a high density of one‐phonon states having a strongly expressed anisotropy: the width of the phonon bands is large (≈︁ 20 cm−1) along the directions of phosphorus chains and is extremely small (about 1 to 2 cm−1) in other directions.
Influence of the position of the interface boundary on the existence of interface states
Authors: Kandilarov B. D. , Tashkova M. G. , Petrova P. C. , Detcheva V. 
Published: July 2007

AbstractNecessary and sufficient conditions for the existence of interface states localized at the interface in two one‐dimensional models of deformed heterojunctions are derived in terms of a scattering theoretical method. The existence conditions thus obtained are compared with the corresponding expression for an ideal undeformed heterojunction. The impact of both symmetrical and asymmetrical deformations on the existence of interface states is discussed.
Electronic band structure of SnSe
Authors: Car R. , Ciucci G. , Quartapelle L. 
Published: July 2007

AbstractThe electronic bands of SnSe are calculated along all high symmetry lines of the Brillouin zone using the local empirical pseudopotential method. Spectralization of the involved large matrices is performed by means of a new approximate folding‐down method which locates the s‐states of valence bands much better than the Löwdin‐Brust perturbative technique. The parameters of the analytical form used to interpolate the pseudopotentials are roughly adjusted to obtain a band structure consistent with optical and photoelectric data. Satisfactory results for the general features are obtained, a better agreement requiring a more cautious tuning of pseudopotential values in the region of small wave vectors.
A pseudopotential study of the electronic properties of noble metals
Authors: Sharma O. P. 
Published: July 2007

AbstractA local model pseudopotential, depending on ionic and core radii and which incorporates the sd hybridization effects phenomenologically, is employed to calculate the phonon frequencies, liquid metal resistivity, binding energy, and the effective ion‐ion interaction of the noble metals Cu, Ag, and Au in conjunction with a dielectric function in the non‐interacting band scheme. Reasonably good agreement is found between the computed and the experimental results.
Magnetic solutions of the Hubbard model
Authors: Schumacher W. , Lindner U. , Jauch R. 
Published: July 2007

AbstractExistence and stability of antiferromagnetic and of ferromagnetic solutions of the Hubbard model are investigated in the framework of the Hartree‐Fock approximation. It is found that the Hubbard model for simple cubic lattice and certain parameters admits stable magnetic solutions in a finite temperature range, even in cases, where for these parameters at T = 0 no magnetic ordering exists (so‐called heat magnetization).
The theory of autolocalized electron states in ferromagnetic semiconductors at arbitrary relation between the band width and the spin–electron interaction
Authors: Egorov B. V. , Krivoglaz M. A. 
Published: July 2007

AbstractThe conduction electron energy is determined in the adiabatic approximation for ferromagnetic semiconductors with large atom spins and an arbitrary relation between the band width ΔE and the spin–electron interaction constant A. The thermodynamical potential of the system is found. The conditions are discussed when it is thermodynamically favourable to form large radius autolocalized electron states (fluctuons) and their characteristics are determined. A consideration is given for the temperature range where the spin‐wave approximation or that one of self‐consistent field are applicable. It is shown that the temperature range of fluctuonic states is the most wide in crystals with large |A|/kTC and with |A| S and ΔE of the same order.
F Centers in Alkaline‐Earth Fluorides. Inadequacy of the Muffin‐Tin Approximation
Authors: Oliveira L. E. , Oliveira P. Murilo , Maffeo B. 
Published: July 2007

AbstractThe SCF‐MSXα method is applied in the study of the electronic structure of F centers in CaF2, SrF2, and BaF2. The predicted optical transition energies are in disagreement with the experimental data. An explanation for the discrepancy is provided showing the inadequacy of the spherical averaging of the potential within the muffin‐tin approximation.