bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic Properties of the Molten Tl‐AgTlS2 System
Authors: Nakajima T. , Uemura O. , Satow T. , Yagihashi T.
Published: July 2007

The electrical conductivity, the magnetic susceptibility, and the thermoelectric power of liquid Tlx(AgTlS2)1‐x alloys are measured as a function of composition in the temperature range from the melting point to 900 °C. The behaviour of these electronic properties is qualitatively the same as the previous result of liquid Tlx(AgTlTe2)1‐x alloys except for the differences mentioned below. A main difference is that the value of conductivity minimum at x ≈ 2/3 for the sulfide system is considerably smaller than that for the telluride system. Corresponding to the conductivity result, the thermoelectric power becomes as large as 250 μ V/K around this composition. Secondly, for x ≈ 2/3, the conductivity is almost independent of composition in the sulfide alloys, whereas the conductivity of the telluride alloys depends strongly on composition. Furthermore, the anomalous change of electronic properties around x ≈ 2/3 becomes less rapid in sulfide alloys compared with the case of telluride alloys. Discussions on the electron localization for liquid Tlx(AgTlS2)1‐x are made in terms of current electron transport theories for liquid semiconductors using the present data.
Spin Glass of the Site‐Random Ising Model with First and Second Neighbor Interactions
Authors: Katsura S. , Konishi K.
Published: July 2007

The site‐random Ising mixture of atoms A and B, of which the first and second neighbor interaction energies are J1AA. J1BB, J1AB J2AA J2BB, J2AB, is considered. The second neighbor interactions are treated along the line of Hüller. For the mixtures where J1AA > 0 (ferromagnetic) and J1BB < 0 (antiferromagnetic), the Curie, Néel, and spin glass transition temperatures are calculated and the phase diagrams are obtained. The severe situation for the appearance of the spin glass state in the site‐random Ising model with only first neighbor interactions (possible region of J1AA, J1BB, and J1AB for the spin glass is very narrow) is much improved by the introduction of the proper choice of the second neighbor interactions which enhances the frustration.
Effect of γ‐Irradiation on Eu2+ Ions in KCl Crystals
Authors: Opyrchał H. , Nierzewski K. D. , Macalik B.
Published: July 2007

Optical absorption and emission, EPR, ITC, and thermoluminescence spectra of RT γ‐irradiated KCl:Eu2+ crystals are examined. ITC and EPR measurements of heavily irradiated samples point to the lack of any I.V. Eu2+ dipoles. Examination of optical properties permits to exclude the presence of Eu+ and Eu3+ ions. On the other hand, the examination of thermal annealing effects in heavily irradiated samples reveals the lack of any correlation between the recovery of original europium dipoles and the decay of colour centres. The results obtained can be understood by assuming radiation field induced aggregation of europium dipoles.
Two Types of Tetrahedral Sites in the Mixed {Ca2Na} [Mg2] (V3)O12 Garnet
Authors: Wójcik M. , Le Gall H. , Desvignes J. M.
Published: July 2007

The untypical quadrupolarly split NMR spectrum, from 51V in the tetrahedral sites (d), of mixed {Ca2Na} [Mg2] (V3)O12 single crystal is investigated. The EFG distribution in the (d) sites, associated with the random distribution of the Ca2+ and Na+ ions over the nearest neighbour dodecahedral sites in garnet, is calculated. On ground of the theoretical results, the observed NMR lines are indentified as arising from the two types of (d) centres: calcium‐type and sodium‐type, characterized by a different EFG tensor.
Optical Investigation of Ag+ Centers in Cesium Halides
Authors: Pfalzgraf B. , Roder O. , Spaeth J.‐M.
Published: July 2007

In Ag+ doped CsCl and CsBr single crystals several absorption bands are found in the far UV region which strongly depend on crystal growth conditions. By means of a special thermal treatment two different monomer Ag+ centers could be produced reproducibly. From optical absorption, fluorescence emission, and radiative lifetimes as well as optical bleaching experiments it is concluded that the optical properties of these centers are due to bound excitons.
Calculation of Binding Energies of Small Vacancy Clusters in B.C.C. Transition Metals
Authors: Masuda K.
Published: July 2007

The binding energies Ebin of vacancy‐type lattice defects in b.c.c. transition metals are calculated using a tight‐binding type electronic theory and compared with available experimental results. The present calculation takes into account the Born‐Mayer repulsive energies and uses a total energy minimization procedure. It is shown that atomic relaxation around the lattice defects (small vacancy clusters) plays a dominant role in determining the binding energies Ebin: in general, Ebin depends strongly on the configuration of vacancies. Detailed information on the local d‐electron density of states (near the defects) is also given.
Temperature Dependence of the Electrical Conductivity of the Periodic Anderson Model
Authors: Elk K.
Published: July 2007

The temperature dependence of the electrical conductivity σ of the periodic Anderson model is investigated using a modified version of the model of Chen, Weisz, and Sher, describing the electron‐phonon scattering by a temperature dependent stochastic potential. The correlation term of the Anderson model is decoupled in the framework of the alloy analogy approximation taking into account strong electron‐electron interaction. Then the electrical conductivity can be obtained using the Kubo‐Greenwood formula. By numerical methods the density of states and the temperature dependence of σ are calculated. For σ(T) a pronounced minimum is obtained at low temperatures T, whereas at higher temperatures σ increases with T.
Lattice Mechanics of Molecular Solid Sodium Nitrate
Authors: Singh R. K. , Chaplot S. L.
Published: July 2007

The lattice mechanics of sodium nitrate with calcite structure is investigated by means of a rigid molecular ion model of the molecular solids employing the external mode formalism. The phonon dispersion curves calculated are in good agreement with those recently measured from the technique of neutron inelastic scattering. The values of the eigenvectors and the cohesive energy obtained for sodium nitrate also appear to be reasonably correct. The possibilities of further improvement and extension of the model are also discussed.
High Energy Distribution Function in an Electric Field and Electron Impact Ionization in AIIIBV Semiconductors
Authors: Dmitriev A. P. , Mikhailova M. P. , Yassievich I. N.
Published: July 2007

A calculation method of the exponential dependence of the high‐energy distribution function in an electric field E is proposed for polar semiconductors. It is shown that in many‐valley semiconductors with complicated conduction band structure a strong anisotropy of the electron impact ionization coefficient α(E) will be obtained. In the case of E ∥ (100) or E ∥ (110) a ballistic transport in the Γ‐valley can occur and the dependence of In α vs.(E01/E) holds. If E ∥ (111) the ballistic transport in the Γ‐valley is impossible and the high‐energy distribution function is always formed due to electron scattering from X‐valleys. Then the dependence of In α vs.(E02/E)2 will be realized for a wide range of fields. The characteristic fields E01 and E02 are connected with the intervalley coupling constants for Γ—X and X—X scattering, respectively. Impact ionization processes are considered in detail for InSb, GaAs, and InP. The results are compared with accessible experimental data.
Phonon Scattering by Paraelectrical Centers in Crystals II. Lowest Order Approximation
Authors: Akhmadullin I. Sh , Saburova R. V.
Published: July 2007

On the basis of formalism developed in a previous work, a theoretical study of the dynamic of paraelectric centers interacting both, with each other and with lattice vibrations is presented. Spectrum and damping of collective excitations are calculated. Frequency and center concentration dependences of phonon lifetime are analysed. An application of the results obtained from the analysis of thermal conductivity is discussed. A new thermal conductivity mechanism is proposed.
Electron States Associated With Partial Dislocations in Diamond
Authors: Persson A.
Published: July 2007

Electron states associated with 30° and 90° partial dislocations in diamond are examined by the use of Slater‐Koster parameters evaluated form first principles. The partial dislocations give half‐filled bands in the lower half of the band gap. A possible reconstruction of the 90°‐partial is also considered but found to be unlikely. Including non‐spherical parts of the crystal potential in the calculation of the matrix elements for the atoms with a dangling bond did not appreciably alter the result for the 90° partial.
Temperature Dependence of the Linewidth of First‐Order Raman Spectrum for CdF2 Crystals
Authors: Sato T. , Tateyama S.
Published: July 2007

The first‐order Raman spectrum of CdF2 crystal is measured at temperatures from 4.2 to 469 K. From these experiments, the values of the linewidth at these temperatures are obtained. On the other hand, the linewidth is calculated quantitatively theoretically under consideration of only the cubic anharmonic term of the Coulomb and repulsive forces of the crystal potential. A comparison of the experimental and theoretical results shows good agreement.
Statistical Mechanics of the Random Anisotropic Planar Heisenberg Model. Discussion on the Spin Glass
Authors: Katsura S. , Shirakura T.
Published: July 2007

The classical (S = ∞) and the quantum (S = 1/2) planar model with random anisotropic exchange energy, of which the Hamiltonian is given by —βℋ = σKij(Sjx + ηSiySjy) + σ(CixSix + CiySiy), are discussed by the method of the pair approximation. The high‐temperature uniform and spin glass susceptibilities are obtained. The magnetic phase diagrams of the short ranged binary mixture (±J model), the short ranged mixture with the width ΔJ, and the infinitely long ranged Gaussian mixture are obtained from the divergences of the susceptibilities. In the classical case the spin glass transition is realized in the whole range of 0 ≦ η ≦ 1 while in the quantum case the spin glass is not realized in the range of η > ηc (in particular in the isotropic case η = 1).
Theory of Interionic Forces in Alkali Halide Crystals
Authors: Narayanan R. S. , Shankar J. , Gupta H. C. , Tripathi B. B.
Published: July 2007

The widely used Born‐Mayer potential in the classical theory of ionic crystals is replaced by a quantum mechanical analytical potential form following the recent tight binding theory developed by Harrison. It is found that the crystalline data on interionic distance, separation energies, isothermal bulk modulus, and its pressure derivative for all the alkali halides can be fitted to a very good extent by taking the Madelung energy and Harrison's overlap potential form. It is emphasised that Harrison's overlap potential can be a useful tool for studying the properties of ionic crystals within the framework of conventional approach based on Madelung's energies.
Temperature and Pressure Dependence of the Electric Field Gradient at the Br Sites in PbBr2 and SnBr2
Authors: Bastow T. J.
Published: July 2007

Detailed measurements are made of the 79,81Br NQR frequencies in PbBr2 and SnBr2 at temperatures between 77 and 350 K. Pressure coefficients at up to 250 MPa are measured for these frequencies at room temperature. The assumption that the NQR frequency shift with temperature is a linear function of the mean square atomic displacement, which in turn is described by the quasi‐harmonic theory, is shown to provide a satisfactory account of the observed power law (Tn) temperature dependences.
Calculation of Excess Free Energy of Mixing and Analysis of Phonon Spectra With Clustering Parameter in III—V Pseudobinary Alloys
Authors: Yamazaki S. , Kishi M. , Katoda T.
Published: July 2007

The excess free energy of mixing, ΔGex, is calculated and the phonon spectra for III—V pseudobinary alloys A1–xBxC are analyzed on the basis of a cluster model. ΔGex is a function of the clustering parameter β which represents the deviation from the random atomic distribution. The value of β giving the minimum excess free energy, βs, takes the maximum value at x = 0.5 and it increases with increase in the relative difference of lattice constants, \documentclass{article}\pagestyle{empty}\begin{document}$ \Delta a\sqrt a $\end{document}. The value of β giving the best agreement between the measured phonon frequencies and the calculated ones, βph, takes the maximum value also at about x = 0.5 in systems exhibiting the phonon spectra of partly‐two mode type. The value of βs at x = 0.5 and βph over the entire composition range is zero for two‐mode systems, such as (AlGa)—V.
Monte‐Carlo Calculation of Density of States and Localization in Alloys Including Short‐Range Order
Authors: Brouers F. , Franz J.
Published: July 2007

The alloy density of states calculated with a Monte‐Carlo technique is compared to that calculated with a small cluster approximation. It is shown that the use of small clusters imbedded in an effective medium can give rise to spurious structure in the density of states while the two methods give the same results in the limit of large clusters. The conditions are determined for localization in such systems and some conjectures concerning the nature of localized states observed in real liquid alloys are made.
The Semiconductor—Metal Transition in Liquid Tellurium
Authors: Rokhorenko V. Yaw. , Sokolovskii B. I. , Alekseev V. A. , Basin A. S. , Stankus S. V. , Sklyarchuk V. M.
Published: July 2007

The conductivity and thermoelectric power of liquid tellurium up to 1750 K and its density up to 1500 K are measured. Furthermore, all the parameters are measured at 50 K supercooling. A σ(T) maximum in the region of 1100 K is obtained. The thermoelectric power being positive does not vary in the wide temperature range. The ϱ(T) dependence displays a maximum at the normal crystallization point. The results obtained are interpreted by the approximation of metal character of the conductivity and the double‐structural near order of liquid tellurium.
Self‐Trapping of Polaritons
Authors: Hizhnyakov V. V. , Sherman A. V.
Published: July 2007

The self‐trapping of the exciton component of the polariton is considered in the adiabatic approximation. The possibility of the coexistence of self‐trapped (STP) and coherent (CP) polaritons is demonstrated. Such coexistence is possible even for a large effective mass of the exciton component, if the exciton—photon coupling is sufficiently strong, as it is in the case of core polaritons. The self‐trapping barrier of polaritons depends essentially on the frequency of the exciting source, the character of relaxation processes, and some other factors. The shape of the adiabatic surfaces obtained allows one to suggest an explanation of a number of peculiarities observed in the optical spectra of NaI crystals in which the coexistence of STP and CP has been established previously.
The Initial and Final States for UPS Optical Transitions in Cd0.2Hg0.8Te Crystal
Authors: Orlowski B. A.
Published: July 2007

An UPS experiment is performed to investigate the density of states distribution in the valence and conduction bands of a Cd0.2Hg0.8Te crystal. The crystal is cleaved in UHV (p = 5 × 10−11 Torr) and the EDCs are taken at 300 and 77 K and for UV photon energy hv values very close to each other in the range 8.7 eV ≦ hv ≦ 11.9 eV. A comparison of the measured positions of the EDC peaks with the calculated band structure leads to the conclusion that they originate from the peaks of the density of states in the valence and conduction bands at the L and X points and the σ axis of the Brillouin zone.
Kinks in a Quasi‐One‐Dimensional System with a Nearly 1/3‐Filled Band
Authors: Mertsching J.
Published: July 2007

The formation of kinks in a system with commensurability three is studied in a continuum approximation. The width, the amplitude modulation, the spectrum, and the formation energy of the kinks is approximately calculated within the mean‐field approximation. With increasing ratio of bandwidth to gapwidth, the kinks become broader and less amplitude modulated. The number of localized states in the gap increases, and the energy of formation decreases.
Neutron Diffraction and Magneto‐Optical Study of the Effect of Chloride Doping. On the Incommensurate Magnetic Phase of NiBr2
Authors: Day P. , Turner K. , Visser D. , Wood T. E.
Published: July 2007

The magnetic structures of crystals of NiBr2 doped with 2, 5, and 10 mol % NiCl2 are investigated by neutron diffraction and optical spectroscopy, with special reference to the incommensurate helical phase. Behaviour of the magnetic satellite reflections around (003/2) shows that the propagation vector of the helical phase is reduced from 0.15 nm−1 in NiBr2 to 0.12 nm−1 in the crystal doped with 5% NiCl2. The commensurate‐incommensurate transition temperature Tic is similarly reduced, from 22.5 to 17 K. The propagation direction is unaffected by the dopant, remaining along [110]. Although no resolved magnetic satellite reflections are found in the 10% Cl− ‐doped crystal, a difference plot between 7 and 10 K suggests that the incommensurate phase exists below 10 K. The Néel temperatures of the samples show no significant variation with dopant concentration. The intensity of the “cold” exciton‐magnon combination band between 6050 and 6100 Å, which is associated with the incommensurate phase is used to determine Tic and its behaviour in a magnetic field is used to plot magnetic phase diagrams in the Hab‐T plane. No cold band is observed in the 10% Cl− ‐doped crystal. A hot band. resulting from the ferromagnetic component of the intraplanar coupling, is also studied.
Relativistic Theory of Electron and Positron Diffraction at High and Low Energies
Authors: Gevers R. , David M.
Published: July 2007

From Dirac's equation with a periodical electrostatic potential one finds for both, electrons and positrons, a linear system of first order differential equations for the coefficients in a spinor plane‐wave expansion of the total wavefunction. This system has the same simple structure as the well‐known system of Howie and Whelan, but it is valid without any restriction. It promises to be a good starting point to discuss the several typical diffraction regimes (e.g. the Laue case, the Bragg case, diffraction at very high and very low energies,…) from a general point of view.
Exchange‐Conductivity Broadening of SSWR Line in Metallic Thin Films
Authors: Jirsa M.
Published: July 2007

The exchange‐conductivity contribution to the linewidth of the spin‐wave resonance modes in a metallic film is derived analytically for arbitrary values of the spin‐pinning parameter β in the case of parallel as well as perpendicular static magnetisation, under the assumption of weak interaction between the spin‐ and the electromagnetic solution of the secular equation. Analytical results are compared with those obtained numerically for the full model. Very good agreement between the results of both models and a simple functional dependence of the analytical formulae on β gives a possibility to determine the surface anisotropy constant and other film parameters.
Study of CuK Absorption Edge in Cu(I) Compounds
Authors: Rao B. Jagannatha , Chetal A. R.
Published: July 2007

The shapes of the copper K edge in the compounds CuCl, CuBr, CuI, Cu2O, and chalcopyrite mineral are studied using a Cauchois type X‐ray spectrograph. The electronic transitions 1s → 4s and 1s → 4p are observed in the edges. The transitions are explained on the basis of (Z + 1) analogy. It is also shown that the 4s‐4p orbital separation of an ion in a compound depends on the ionicity of the compound.
The nature of silver impurity atoms in antimony telluride
Authors: Horák J. , Lošťák P. , Šiška L. , Stordeur M.
Published: July 2007

Undoped Sb2Te3 and Ag‐doped Sb2Te3 crystals are grown by a modified Bridgman method. Electrical conductivity, Seebeck coefficient, Hall constant, and reflectivity in the region of the plasma resonance frequency are measured. A quantitative analysis of the silver content in the prepared Sb2Te3 crystals is made by atomic absorption spectrometry. For the undoped Sb2Te3 crystals the free carrier concentration and the effective mass in the direction perpendicular to the c‐axis, m⟂, are calculated, using a simplified model. Using m⟂ determined and the value of the plasma resonance frequency the free carrier concentration in the Sb2Te3(Ag) crystals is calculated. According to the results obtained the Ag atoms incorporated into Sb2Te3 crystals have the character of acceptors and the addition of one Ag atom corresponds to the increase in free carrier concentration by two holes. Therefore it is assumed that silver atoms substitute antimony atoms forming thus point defects of Ag″sb with two negative charges. The defects formed by Ag dopants in Sb2Te3 are compared with those in Bi2Se3(Ag) and Bi2Te3(Ag) crystals.
Extended linear chain approximation to the anisotropic ferromagnetic ising model
Authors: Faleiro Ferreira J. R. , Silva N. P.
Published: July 2007

Approximate critical temperatures of anisotropic ferromagnetic Ising models are calculated through a recently proposed extended variational method in statistical mechanics. Using a linear chain Ising Hamiltonian in an effective field as trial Hamiltonian, quite good results are obtained.
Crystalline electric field effects in Pr(La, Y) Al3
Authors: Alekseev P. A. , Sadikov I. P. , Shitikov Yu. L. , Markova I. A. , Chistyakov O. D. , Savitskii E. M. , Kjems J.
Published: July 2007

The results of inelastic neutron scattering experiments on Prx(La, Y)1–xAl3 are presented. CEF Hamiltonian is obtained and the concentration dependence of CEF‐parameters is analyzed. Possible reasons of transition widening between the CEF levels in ternary alloys are considered. Experimental evidence of magnetic excitation dispersion is found and the exchange interaction constant is determined.
Lattice dynamics of InP under hydrostatic pressures
Authors: Patel C. , Sherman W. F. , Wilkinson G. R.
Published: July 2007

The lattice dynamics of InP under variable hydrostatic pressures is investigated on the basis of an “eleven‐parameter” rigid‐ion model. The one‐phonon and two‐phonon density of states functions are computed for various pressures and these results are compared with the available second‐order Raman spectra. The mode Grüneisen parameters are calculated at two arbitrary pressures. The effect of high pressure on the phonon dispersion curves is shown to lead to ‘softening’ in the transverse acoustic modes and eventually to a phase transformation of the compound. Interesting effects in the [1, 1, 0] and [1, 1, 1] directions are shown by the transverse acoustic mode gammas with increasing pressure and suggests the domination of short‐range forces in these two directions.
Infrared optical properties of cerium dioxide
Authors: Mochizuki S.
Published: July 2007

The infrared spectra of bulk and thin‐film CeO2 are measured at various conditions by reflection, transmission, and thermal‐emission methods. The reststrahlen reflectivity spectra of bulk CeO2 are analyzed by means of a two‐oscillator dispersion model, and the dielectric properties and the lattice vibrational properties of CeO2 are discussed. The virtual mode (polariton) spectra of the thin‐film CeO2 are analyzed by means of the virtual mode theory of the anharmonic ionic crystal slab.
Point‐ion‐lattice calculations on F+ centers in lithium niobate and lithium tantalate
Authors: Ho Fat Duen
Published: July 2007

Wave functions for the ground states and the first excited states of the F+ centers in lithium niobate and lithium tantalate are calculated by using the point‐ion‐lattice model exclusive of the effects of polarizations in the first approximation. The Schrödinger equation is solved variationally. The transition energies are obtained by this very simple treatment, which are compared with the available optical data. This calculation yields the results which agree within some 5% and 23% of the experimental values for LiNbO3 and LiTaO3, respectively.
Impurity–host to host–host coupling force constant ratio for 57Fe in rare gas solids
Authors: Paul Prem , Puri S. P.
Published: July 2007

Lattice dynamical studies of the Mössbauer recoilless fraction and SOD for 57Fe embedded in Ar, Kr, and Xe, are reported by McNab and coworkers, using the rare gas matrix isolation technique. According to them, the f‐values for the various hosts are nearly the same. Making use of the experimentally determined phonon frequency distribution functions for these hosts, an attempt is made to calculate the f‐values in the framework of the Mannheim model. The anharmonicity correction is also applied for temperatures beyond 5 K. It is found that the values of the ratio of the impurity—host to the host—host coupling‐force constants in these lattices change in a way that the f‐values and SOD remain the same, thereby indicating their independence of the host. In accordance with the expectation, no localized modes are found due to the weak binding of the impurity to the host.
A simple model approach to localized‐itinerant magnetism application to rare‐earth intermetallics
Authors: Iannarella L. , Guimarães A. P. , Da Silva X. A.
Published: July 2007

The combined role of intraband and electron‐localized moment exchange in determining the magnetic behaviour of a system composed of itinerant electrons and localized magnetic moments is investigated. Having in mind rare‐earth–transition metal and rare‐earth–normal metal intermetallic compounds, the critical temperature versus de Gennes factors and the temperature dependence of the magnetizations and susceptibilities of the two magnetic species are studied. Results are obtained for two cases, a) a delta‐like band (narrow band limit), and b) a parabolic density of states, using the molecular field approximation both in the treatment of intraband interaction (Stoner‐like description) and electron‐localized spin exchange. Some comments are made on the parallel between computed and measured magnetic quantities in the systems RAl2, RFe2 and R(Fe1–xAlx)2 (0 < x < 1 and R  heavy rare‐earth).
The quantum yield spectra of electron emission of solids in XUV region
Authors: Elango M. , Kikas A. , Nǒmmiste E. , Pruulmann J. , Saar A.
Published: July 2007

An analysis of the physical information containing in the quantum yield spectra of photoelectron emission of solids in the XUV region is presented. It is shown that these spectra may be obtained at least in three meaningful counting regimes: current, pulse, and n‐electron regimes. A proper treatment of the spectra gives quantitative information about secondary electron escape length, escape probability for an electron at surface, and the number of secondary electrons created by one XUV photon. The experiments with alkali chlorides in the 100 to 250 eV photon energy region give the values of 300 to 800 Å for the escape length and 11 to 17 eV for the energy need to create one low‐energy electron.
Experiment investigation on the complex dielectric function of CdS in the exciton and in the plasma limit
Authors: Kreissl A. , Bohnert K. , Lyssenko V. G. , Klingshirn C.
Published: July 2007

The real and imaginary parts of the complex dielectric function of CdS are experimentally investigated for low temperatures and for various excitation conditions. Strong changes of the dielectric function are observed at high excitation in the spectral region below the band edge. Emphasis is put on the variations of the real part at the transition from a low density exciton phase to a high density plasma phase. It is shown, that the Kramers‐Kronig relations are also valid for highly excited semiconductors. The experimental results are compared with recent theoretical predictions.
Theory of auger spectra from disordered alloys the effect of electron correlations in filled narrow bands
Authors: Drchal V. , Kudrnovský J.
Published: July 2007

The theory of the intraatomic core‐valence‐valence Auger spectra from a substitutionally disordered alloy with an initially filled narrow band is formulated within the coherent‐potential approximation in the space of the two‐hole states. The constituent atoms are characterized by their atomic levels Δ and the contact pair interactions U between two electrons (Hubbard model). The exact solution to the problem is found for a particular case with randomness in the pair interactions only. Results of numerical analysis for a simple model based on the scaling‐factor approximation are given to illustrate the basic features of the theory.
Influence of clustering on the spin wave stiffness constant in NiCu alloys
Authors: Jezierski A.
Published: July 2007

The spin wave stiffness constant in NiCu alloys is calculated in the RPA and cluster coherent potential approximation. It is shown that the stiffness constant D decreases when the concentration of Cu increases.
Vibrational relaxation in the excited electronic state
Authors: Hizhnyakov V. V.
Published: July 2007

A theory of the temporal evolution of the distribution of the configurational coordinate in the excited electronic state of an impurity centre of a crystal, allowing for the process of pulse excitation, is proposed. The lattice vibrations are considered to be harmonic and the quadratic vibronic interaction is taken into account. In this model relaxation is caused by the dephasing of phonons conditioned by their dispersion. Formulas are obtained relating the relaxation characteristics regarded with the dynamical Green's function. The present solution takes exactly into consideration the effect of mixing normal modes at electronic transition and it describes the influence of quantum effects upon the classical turning points of the configurational coordinate. The results of the theory are illustrated with the numerical calculations of the distribution of configurational coordinate and optical spectra.
The structural transition from NaCl to CsCl type in cesium hydride
Authors: Bashkin I. O. , Degtyareva V. F. , Dergachev Yu. M. , Ponyatovskii E. G.
Published: July 2007

The CsH volume is measured in dependence on pressure up to 35 kbar. A structural NaCl to CsCl type transition associated with a volume jump of 8.6% is found at a pressure of about 6.5 kbar. The T—P phase diagram of CsH is plotted in the temperature range 200 to 525 K. The high pressure phase at liquid nitrogen temperature remains metastable down to atmospheric pressure. The lattice parameter of this phase is a = 3.820 Å at 100 K and atmospheric pressure, the lattice parameter of the initial (NaCl type) phase is a = 6.365 Å under the same conditions.
Magnetic and neutron diffraction study of the phase transition in TmFe1−xMnxO3
Authors: Krezhov K. A. , Mikhov M. T. , Kadomtseva A. M. , Krinetskii I. B. , Lukina M. M.
Published: July 2007

Magnetic and neutron diffraction investigations on polycrystalline samples of TmFe1−xMnxO3 compounds (x = 0.1 and x = 0.3) reveal some interesting features of the Mn3+‐ion influence on the thulium orthoferrite magnetic properties. Preliminary X‐ray diffraction data indicate an increase of the unit cell distortion with growing content of Mn3+ ions. A decrease of the Néel temperature with higher Mn3+ amount is observed. For the compound with x = 0.1 a reorientation transition from GxFz to GzFx (Γ4 − Γ2) spin configuration is observed, whereas in the case of the compound with x = 0.3 a phase transition to antiferromagnetic phase (Γ1) is established. The neutron diffraction results point out to the occurrence of a complicated transitions sequence in the last case.
A model of titanium luminescence centers in lithium and sodium fluorides
Authors: Radzhabov E. A.
Published: July 2007

The absorption, emission, excitation spectra, and polarized luminescence of titanium centers in lithium and sodium fluorides are studied. The 2.42 and 3.1 eV emissions in LiF:TiO2 and 2.2 and 2.95 eV emissions in NaF:TiO2:MgF2 are related to the electronic transitions of titanium centers. The low energy emissions at relatively low temperatures have the same azimuthal dependence of the polarisation degree as the emission of oscillators oriented along the C2 crystal axis. The high energy emissions are polarized along the C3 crystal axis. The high temperature stability of the polarization degree shows that the luminescence centers have point defects near the titanium ion. The efficiency of transformation of titanium luminescence centers by X‐irradiation grows above 200 K. This points out that the cation vacancies are part of luminescence centers. A model of titanium luminescence centers in lithium and sodium fluorides is suggested. The center consists of a Ti4+ ion at cation site, an O2− ion at anion site, and two associated cation vacancies.
EPR linewidth studies of Gd3+‐doped LaxCe1−xF3 single crystals
Authors: Korczak W. , Paradowski M. , Subotowicz M.
Published: July 2007

EPR linewidths of Gd3+ ions (0.1%) in LaxCe1−xF3 single crystals at the frequency 9.4 GHz and temperatures of 77 to 297 K are investigated for H ∥ z. The generalized theory of the magnetic resonance by Kubo and Tomita and Mitsuma's mechanism of spin‐lattice modulated narrowing are adapted to explain the temperature dependence of the EPR linewidths of the Gd3+ ions at the Kramers host Ce3+ ions (4f1, 2F5/2) in LaxCe1−xF3 single crystals for x ⩽ 0.2. The spin‐lattice narrowing is not observed for x > 0.2. EPR linewidth of Gd3+ does not depend on the temperature for x > 0.2. Peak‐to‐peak linewidth ΔHpp increases with the decreasing amount of diamagnetic lanthanum ions in LaxCe1−xF3 single crystals for 0.6 ⩽ x ⩽ 1.
A pair potential calculation of vacancy formation and migration energies for aluminium and copper
Authors: Rautioaho R. H.
Published: July 2007

Vacancy formation and migration energies and enthalpies are computed for aluminium and copper using potential functions derived earlier by fitting to phonon frequencies. Also, the local density correction proposed by Finnis and Sachdev is evaluated for the formation energy in both the metals. The calculated formation and migration enthalpies, 0.84 and 0.65 eV for Al and 1.38 and 0.80 eV for Cu, respectively, are close to the corresponding experimental values. The results are considered in terms of third‐order contributions of the total energy. The third‐order contribution to the formation energy is expected to be of the order of the experimental formation enthalpy in the case of Al but considerable smaller in the case of Cu. On the other hand, the two‐body potential approach almost suffices to explain the experimental figures for the migration enthalpy, since the third‐order terms for the initial and saddle‐point configurations in the migration process largerly cancel each other.
Theory of the diffusivity—mobility ratio in cadmium arsenide
Authors: Singh M. , Leotin J. , Wallace P. R.
Published: July 2007

Stimulated by the work of Chakravarti et al. [1], the theory of the Einstein relation for the ratio of diffusivity, D, to mobility, μ, in cadmium arsenide crystals in a magnetic field at low temperatures is developed. D/μ shows characteristic de Haas‐Shubnikov oscillations which vary considerably with orientation of the crystal due to its anisotropy. In fact, for low enough magnetic fields it is shown that it may oscillate as a function of the angle θ between the c‐axis of the crystal and the field direction. Measurements of D/μ would thus constitute a sensitive test of anisotropy. By measuring the variation both, with respect to field and angle for a single sample, all the band parameters can be determined in principle.
Exchange interaction in superconducting metals
Authors: Krüger E.
Published: July 2007

From the band structures of superconductors it is concluded that the formation of Cooper pairs is connected with exchange interaction existing only in the conduction bands of superconductors. Nevertheless, the pairing is mediated by phonons. This new coupling mechanism, which does not contradict the BCS mechanism, provides an understanding of superconducting properties of materials.
Dynamical susceptibility and electrical resistivity of disordered spin systems
Authors: Goedsche F. , Richter A. , Vojta G.
Published: July 2007

A general relation between the electrical resistivity and the dynamical susceptibility of the scattering system, valid in the weak scattering limit and for arbitrary temperatures, is extended to disordered spin systems including elastic spin‐conserving as well as inelastic spin‐flip scattering. Averaging procedures for the position‐dependent susceptibilities are discussed. As an application, the low‐temperature resistivity R is calculated for the Heisenberg‐Mattis model as a site‐random magnet with ferro‐antiferromagnetic exchange disorder. The superposition of the spin‐dependent elastic scattering contributions to R and the contributions due to inelastic scattering by quasipropagating excitations with linear dispersion and quadratic disorder damping yields a weak resistivity minimum. Comparisons with other theoretical papers are made, and the relevance of the results for the interpretation of experimental data is discussed.
Theoretical compton profiles of magnesium
Authors: Rennert P. , Carl G. , Gläser U.‐H.
Published: July 2007

Starting from a pseudopotential calculation Compton profiles for h.c.p. magnesium single crystals are calculated. They are given for nine directions and for the directionally averaged case. The complete 〈001〉 profile of Mg is compared with an experimental result. The experimental Compton profile is slightly broader than the theoretical one because of the electron‐electron correlation. The anisotropy for I〈001〉–I 〈100〉 and I 〈001〉–I 〈110〉 is found to be smaller than for experimental profiles but larger than for theoretical ones calculated by the APW method.
A localized orbital description of Si
Authors: Downey M. , Smith P. V.
Published: July 2007

In order to describe the change in electronic structure arising from the presence of impurities in a metal or semiconductor it is convenient to have a localized basis representation of the host. To optimize such impurity calculations it is also necessary that this set of localized basis functions be as small as possible. The best representation of the electronic structure of the diamond and zinc‐blende semiconductors in terms of a small set of localized basis functions has been obtained by Chadi. In this method Bloch sums for describing the pseudo‐wave‐functions and energy bands are constructed from Slater orbitals of different symmetry centred on each atomic site. Using a variational approach to determine the decay constants parametrizing these various orbitals Chadi obtains an accurate description of the bandstructures with a simple four state per atom s–p basis set and reliable pseudo‐charge densities with only ten states per atom. These original calculations of Chadi are not, however, carried through to full convergence. Therefore this analysis is repeated for the case of Si and it is found that, while Chadi's results are indeed fortuitous, it is possible to accurately describe both the bandstructure and valence electron charge density of these semiconductors in terms of a minimum set of well localized basis functions.
Calculation of magnetic hyperfine interactions in the rare‐earth orthoferrites with model covalency parameter
Authors: Flyagin M. Ya. , Men A. N.
Published: July 2007

The magnetic hyperfine fields for some orthoferrites are calculated using the model suggested by Boekema, Van Der Woude, and Sawatzky. The s electrons of the oxygen ion are taken into account also. An approximate formula is proposed to estimate the covalency parameters. The unknown parameters of this formula are determined using the experimental data for LaFeO3. The proposed model describes satisfactorily the observed hyperfine fields for orthoferrites. It is found that the contribution of 2s electrons to the hyperfine field is small.
Huang diffuse scattering from self‐interstitials in a trigonal lattice
Authors: Khanna Rita
Published: July 2007

Using the continuum theory of linear elasticity, the Huang diffuse scattering (HDS) is calculated from self‐interstitials in a trigonal lattice. Various defect configurations are represented according to the point group symmetry of the defect site and the contribution to HDS from all possible equivalent orientations of a defect configuration (assumed to be equally populated) is averaged It is possible to clearly distinguish between defects of different long‐range symmetry by measuring HDS about appropriately chosen reflections and directions.
Optical properties of inhomogeneous colloidal centres in alkali halides
Authors: Ekmanis Yu. A. , Radchenko I. S.
Published: July 2007

Calculations are made of optical absorption spectra for the following inhomogeneous colloidal centers of sodium in NaCl: for metallic particles of sodium; particles with oxidized shell; particles with a dielectric unrelaxed shell, as well as the shell in the form of a transitional layer with a low concentration of free electrons acting in plasma oscillations, etc. The experimental observations of the position of the maximum of the absorption band enable the identification of a number of the models studied in irradiated as well as in additively coloured NaCl crystals.
Non‐radiative transitions: Fundamental difficulties of the adiabatic base approach
Authors: Wagner M.
Published: July 2007

The adiabatic base approach is extensively used in literature (“Condon” approximation, “non‐Condon” approximations). Two fundamental difficulties are indicated: (a) for large Huang‐Rhys factors it is not the non‐adiabaticity operator which governs the non‐radiative decay but tunneling; (b) for a small transitive coupling parameter Δ very high potential barriers (with divergent area) are built up as a non‐adiabatic consequence of non‐crossing. A possible way out is shown by suggesting a release of the non‐crossing requirement.
Modified Thomas‐Fermi approximation for systems with finite surface barriers
Authors: Bartoš I. , Kolář M. , Paasch G.
Published: July 2007

Incorporation of a boundary condition into the Thomas‐Fermi approximation is generalized for systems with finite surface barriers. The standard and the modified procedures for the determination of the charge density are tested on the image‐type model potential corresponding to the bent bands at surfaces of semimetals or highly doped semiconductors. Substantial improvement of the description of the electron charge density is achieved in the modified Thomas‐Fermi approximation.
Vibronic and electron‐phonon coupling in spectra of substituted polyenes in rigid matrices
Authors: Nikitina A. N. , Osadko I. S. , Vasilieva I. A. , Yanovskaya L. A.
Published: July 2007

It is found that the electronic excited state 11Bu does not influence the intensity distribution in the electronic 11Ag → 21Ag transition though it is partially allowed. It is determined with the help of the electron‐vibrational interaction theory using the joint analysis of fluorescence spectra and fluorescence excitation spectra of three arylpolyenes in n‐paraffin matrices at 4.2 K. Elastic forces, binding the polyene molecules with the environment, are shown to change in the phototransition.
Consistency condition in the energy expression of the pseudopotential theory of metals and unified study of alkali metals
Authors: Sen D. , Sarkar S. K. , Sengupta S. , Roy D.
Published: July 2007

From a careful analysis of the energy expression of the second‐order pseudopotential theory, the reason for the breakdown of the consistency condition is pointed out. The problem is resolved by introducing the concept of local volume strain which introduces some modifications in the dynamical matrix and leads to the equality of static and dynamic elastic constants. A unified study of the alkali metals with the Heine‐Abarenkov model pseudopotential in conjunction with Taylor's dielectric function, and taking proper care of the consistency condition, yields satisfactory results for both the static and dynamic properties of these metals. One interesting feature of the present investigation is that the theory satisfying the consistency condition reproduces the crossing of the longitudinal and transverse branches in the (100) direction for Li, while reproducing the normal behaviour for the dispersion curve of other metals.
Phonon‐assisted decay of excitons in anthracene crystals
Authors: Marisova S. V. , Myasnikov E. N. , Lipovchenko A. N.
Published: July 2007

The reflection spectra, R(ω), are investigated from the (201) face of anthracene single crystals at low temperatures (4.2 to 60 K). Absence of surface effects makes possible to judge from the reflection curve on the spectral dependence of the decay parameter, γ, due to exciton‐phonon interaction. The calculation of γ(ω) is made by use of a nomogram graphically representing the functional relation between the values R, ω, and γ. It is shown that the decay parameter is essentially frequency dependent at low temperatures. On the basis of the calculated γ(ω) the mean value is estimated of the interaction energy of excitons with acoustic and optical phonons in anthracene crystals.
Spin‐phonon interactions in the ising model with a transverse field
Authors: Wesselinowa Julia M.
Published: July 2007

A Green function technique is used to study the effects of spin–phonon interactions in the Ising model with a transverse field. The renormalized energy of the coupled mode, the damping of the spin waves, and the phonon damping are evaluated. The transverse dynamic structure factor exhibits three peaks due to the coupling of the transverse soft‐mode and the acoustic phonon mode. The width of the central peak is proportional to the phonon damping.
Comparison of different approaches to the study of local defects in crystals. I. Theoretical considerations and computational schemes
Authors: Pisani C. , Dovesi R. , Ugliengo P.
Published: July 2007

The standard way of treating local defects in crystals is to correct the Green function of the perfect periodic host system so as to account for the defect. This approach, which is termed here as “perturbed crystal”, is compared with a “perturbed cluster” approach where instead the molecular solution for a cluster of atoms around and including the defect is corrected in order to account for the presence of the infinite surrounding medium. It is first shown that when a unique basis set is used to describe the crystal both, before and after the defect is created, the perturbed crystal approach is essentially simpler. When however the defect is complicated and the use of different basis sets is practically compulsory, then the other approach is more natural and the corresponding computational scheme is more easily implementable. New explicit formulae for the perturbed cluster approach are derived which are more powerful and general than those used in previous works.
Effect of uniaxial long‐range exchange interaction on the properties of surface spin waves in a Heisenberg ferromagnet
Authors: Petrova T. G. , Syrkin E. S.
Published: July 2007

The low‐ and high‐frequency surface states in a single‐magnon spectrum of the Heisenberg ferromagnet with uniaxial exchange interaction of the long‐range type are studied. The energies and damping decrements of surface spin waves and the surface projected spectral densities of magnon states are obtained. It is found that the considered anisotropic interaction results in new formation conditions of the surface spin waves and essentially affects their characteristics.