bluuebunny/crossref_metadata_2025_split huggingface dataset:
A Density‐Matrix Functional Approach to Bound Multiexciton Complexes
Authors: Wünsche H.‐J. , Henneberger K. 
Published: July 2007

AbstractA general Hamiltonian is introduced describing multiexciton complexes bound to impurities in semiconductors. Band couplings by the impurity potential as, e.g., the valley—orbit or intervalence band couplings in cubic many‐valley semiconductors are involved and require a generalization of the conventional density‐functional formalism (DFF). Therefore, instead of the usual densities, the reduced one‐particle density matrices of the electrons and holes are introduced as basic quantities. Within the resulting density‐matrix functional formalism (DMFF) the inhomogeneous multiparticle systems are also described by a system of selfconsistent one‐particle equations. Some general properties of these equations are discussed, especially the consequences concerning the shell structure of bound multiexciton complexes. In case of uncoupled bands they are shown to transform into the conventional Kohn‐Sham equations.
Orthonormal Functions for Fermi Surfaces A Test of Convergence for Nb and Cu
Authors: Khan F. S. , Allen P. B. 
Published: July 2007

AbstractThe electronic mass enhancement λk on the Fermi surfaces of Cu and Nb is expanded in various basis sets. In particular, orthogonal polynomials (‚Fermi surface harmonics’︁) are constructed in the velocity vector components (FSH(v)), in the wave vector components (FSH(k)) and the cubic harmonics (CH). The results indicate that the set FSH(k) gives better convergence than the sets FSH(v) or CH.
The Most One‐Dimensional CTC Belonging to the TTF‐TCNQ Series The Trans DEDMTTF‐TCNQ Compound
Authors: Keryer G. , Delhaes P. , Amiell J. , Flandrois S. , Tissier B. 
Published: July 2007

AbstractAn experimental study of the most one‐dimensional compound of TTF‐TCNQ series, the transdiethyl‐dimethyl TTF‐TCNQ complex, is presented. In parallel with the X‐ray crystallographic structure different physical properties are investigated: magnetic susceptibility, electron paramagnetic resonance, specific heat, and dc electrical conductivity. In spite of the cation size the trans‐isomer only gives a TCNQ charge transfer complex which crystallizes with regular segregated stacks of acceptors and donors and thus is conducting. Because of the pronounced electronic one‐dimensionality, the disorder effect appears predominently at very low temperatures: the magnetism and specific heat temperature dependences are explained in terms of Bulaevskii's model used before for TCNQ anion‐radical salts only.
Ion‐Size Effects on the Energy Levels of F, FA, and FH Centers in Cesium Halides
Authors: Sivasankar V. S. , Sankar Chitra , Whippey P. W. 
Published: July 2007

AbstractThe energy levels of F centers in cesium halides are calculated variationally using the point‐ion lattice approximation with different sets of trial wavefunctions for the ground and the excited states. The effect of the finite size of the lattice ions on these energy levels is then evaluated self‐consistently within the framework of a local model pseudopotential approach. This method is then used to obtain the changes in the ion‐size effects induced by a substitutional monovalent cation impurity in a nearest neighbour position — the FA center — or a substitutional monovalent anion impurity in the second neighbor position — the FH center. In each case the ion‐size effects are calculated for all the trial functions considered. The calculated transition energies are compared with the observed positions of the corresponding optical absorption bands. It is concluded that the results of optical measurements on all the three centers can be accounted for, at least qualitatively, on the basis of a reasonably simple theory.
Effective Charges in Displacive Ferroelectrics
Authors: Gervais F. 
Published: July 2007

AbstractThe temperature dependence of the effective charges in ferroelectrics BaTiO3, KNbO3, and LiTaO3 is investigated in the vicinity of the ferroelectric—paraelectric phase transition. The results are understood in terms of a displacive‐type mechanism rather than order—disorder.
Hyperfine Interaction and Magnetic Phase Transition of BaVS3 Observed by Inelastic Spin‐Flip Scattering of Neutrons
Authors: Heidemann A. , Takano M. 
Published: July 2007

AbstractHigh‐resolution inelastic neutron scattering experiments are performed to study the magnetic behaviour of stoichiometric BaVS3 at low temperatures. A magnetic phase transition is detected at Tc = (31 ± 1) K. The magnetic hyperfine field at a fraction of the vanadium nuclei has a value of (75 ± 10) kOe at T = 10 K. The results are compared with existing data of the magnetic susceptibility.
Effect of Lattice Distortion on Electron—Impurity Scattering Phase Shifts in the Distorted‐Wave Born Approximation
Authors: Sorbello R. S. 
Published: July 2007

AbstractThe effect of lattice distortion on electron—impurity scattering is estimated using the distorted‐wave Born approximation for phase shifts. For weak impurity scattering the phase shift for the scattering complex is the sum of the phase shifts from the point impurity and the lattice strain field. Model calculations show that there may be important corrections to this picture when the magnitudes of the impurity phase shifts are about π/10 or larger. These corrections arise from backscattering of electrons by the distortion field surrounding the impurity. It is concluded that one may incur appreciable error in calculating pseudopotentials or impurity phase shifts in the presence of lattice distortion if one does not include the lattice backscattering effect discussed in this paper.
Recombination Kinetics Involving Strongly Localized States Radiative Lifetime of a Chain with Light Emitting Ends
Authors: Esser B. , Gärtner B. 
Published: July 2007

AbstractThe problem of the motion of an excited electron (hole) over randomly distributed localized states to its recombination centre is investigated. The one‐dimensional case where the radiative recombination centres are placed at the ends of a chain of localized states is considered. With some simplifying assumptions the problem is shown to be equivalent to that of a Markov chain with absorbing states. The lifetime of the excited state is expressed by the transition probabilities along the chain. The general expression for the lifetime is applied to the special case of an ensemble of chains where the fluctuations of the transition probabilities are mainly due to the fluctuations of the distances between neighbouring states. For Gaussian distributed distances the dependence of the lifetime on the number of states in a chain is explicitly calculated.
Influence of Partial Long‐Range and Short‐Range Orders on the Electrical Resistivity of Substitutional Alloys
Authors: Christoph V. , Richter J. , Schiller W. 
Published: July 2007

AbstractStarting from a resistivity formula the electrical resistivity of substitutional alloys with partial long and short‐range orders is calculated. Restrictions for the distance dependence of the Cowley short‐range order parameters are derived. In the weak scattering limit the resistivity contributions caused by precipitations and AB ordering effects are discussed.
Comments on the Moss Formula
Authors: Gupta V. P. , Ravindra N. M. 
Published: July 2007

AbstractAn analysis is given of the well‐known Moss formula connecting the energy gap to the refractive index of semiconductors. Based on Penn's model for the dielectric function of semiconductors and the treatment of Wemple, the refractive index is related to the energy gap. For small gaps, the relation is shown to reduce to the earlier predicted linear form.
Paramagnetic Spin Relaxation Due to Spin–Spin Interaction Assisted by Spin–Phonon Interaction or Vice Versa (I)
Authors: Srivastava K. K. P. , Singh H. C. 
Published: July 2007

AbstractThe electronic spin–spin relaxation is induced by a dipolar magnetic coupling between the spin states of a pair of paramagnetic ions, whereas the interaction between spin states and lattice vibrations is responsible for the spin–lattice relaxation. Two‐phonon Raman and Orbach spin–lattice relaxation processes are well known where the spin relaxation between a ground doublet proceeds indirectly via transitions to and from an excited state at energy δ with the absorption and emission of two phonons having an energy difference equal to the energy separation between the two states of the ground doublet. In this paper a new type of “mixed spin relaxation process” is considered in which a spin–spin transition is needed to produce effectively an orbit–lattice (i.e. spin–phonon) transition and vice versa. In other words the spin–spin and spin–phonon interactions are simultaneously needed to produce a relaxation transition for this mixed process. A phenomenological approach indicates that the rate of this “mixed spin relaxation process” is proportional to T4[exp (δ/KT) + 1]−l (sin k Rjl/kRjl) and this process should proceed very efficiently at low temperatures when the phonon wavelengths are larger than the separation Rjl between the paramagnetic ions.
Correction to the Breakdown of the Long‐Wavelength Approximation and Characteristic Temperatures in Impurity Ion Paramagnetic Resonance
Authors: Singh A. , Shrivastava K. N. 
Published: July 2007

AbstractIn the temperature dependence of the zero field splitting of PbWO4:Mn2+, the long‐wavelength approximation breaks down near Tb ≈︁ 150 K. A correction factor is obtained using Bessel functions which explains the observation. In the temperature dependence of the zero field splitting of V2+ in NaCl, KCl, and RbCl a minimum is found and a zero at characteristic temperatures, Tc2 and T02, which are related by T02 = √2Tc2. This theoretical relation between the two temper‐atures is varified experimentally.
Piezoreflectance Study of E1 and E2 Transitions in Germanium from 107 to 326 K
Authors: Musilová J. 
Published: July 2007

AbstractAC piezorefleetance spectra of pure germanium from 1.8 to 4.6 eV are studied within the temperature range 107 to 326 K using uniaxial stress and polarized light. The interpretation of corresponding spectra of the piezooptic tensor components is based on the critical point concept. The line shapes of the piezooptic tensor are well fitted by the 3DM1 model in the region of E1, E1+ Δ1 transitions. For the E2 structure the hydrostatic component of the piezooptic tensor agrees with the model of two degenerate saddle points, 3DM1 + 3DM2, however, the other two components are not found to correspond to single transitions. Values of transition parameters and their temperature coefficients are obtained. Also the deformation potentials are determined, using both, the experimental and calculated dielectric function data for numerical analysis.
On the Statistical Theory of Vacancies in Strongly Anharmonic Crystals
Authors: Zubov V. I. 
Published: July 2007

AbstractThe Gibbs free energy of the vacancy formationg(T, P) in a strongly anharmonic crystal is obtained, using the quasi‐chemical approximation for multi‐component systems and the improved self‐consistent field method for a crystal. The enthalpyh, entropys, and volumevof the vacancy formation are derived versus the temperature and pressure. The vacancy formation parameters, concentration of vacancies, and their effects on the thermodynamic properties of strongly an‐ harmonic crystals of Ar, Kr, and Xe are calculated along the solid–gas coexistence curves, computed in the preceding paper. The pair potential of Lennard‐Jones and those of Barker et al. are used. The three‐body interactions are included. All potentials givev= (1.02 to 1.04) atomic volume over the whole temperature range. The enthalpy and entropy of the vacancy formation decrease with increasing temperature, with the valuesh/kT= 9.48 to 9.60 ands/k= 3.14 to 3.40 at the triple points. The results are compared with experimental data for the thermodynamic properties and also with the available Monte Carlo computations and experimental estimations for the vacancy formation parameters.
The Lattice Structure Parameters and Configuration of Cu2+ Jahn‐Teller Centres in Tutton Salt Crystals
Authors: Petrashen V. E. , Yablokov Yu. V. , Davidovich R. L. 
Published: July 2007

AbstractEPR of Cu(H2O) pseudo Jahn‐Teller (JT) centres in dia‐ and paramagnetic Tutton salt crystals A2M(SO4)2. 6H2O (I) with A = Rb, Cs, NH4 and M=Zn, Cu and K2M(ZrF6)2. 6H2O (II) crystals in the temperature range 300 to 4.2 K at frequencies 9.5 and 37 GHz is studied. The dynamic JT nature of Cu(H2O) centres in the whole class of Tutton salt crystals is established basing on the results of EPR study and X‐ray data. The influence of the lowering of lattice symmetry on the configuration and properties of centres and crystals is analyzed. The character of the distortion of Cu(H2O) complexes in “copper” crystals I and II is discussed in terms of cooperative JT effect. The main JT parameters of Cu(H2O) complexes for the crystals under consideration are estimated.
Symmetry Analysis of Some Properties of Ni2+ Ions in the Planar Antiferromagnet K2NiF4 and the Canted‐Spin Antiferromagnet NiF2
Authors: Joseua S. J. , Deonarine S. 
Published: July 2007

AbstractUsing standard group theoretical techniques, earlier investigations on the symmetry properties of the Ni2+ ion in the 2D (planar) antiferrornagnet K2NiF4 and the 3D antiferrornagnet NiF2 are extended.
Soft X‐Ray Appearance Potential Spectra of Iron–Cobalt Alloys
Authors: Berndt K. , Engelmann J. , Storbeck F. , Blasek G. 
Published: July 2007

AbstractThe appearance potential spectra of ferromagnetic Fe–Co alloys in the b.c.c. and f.c.c. phases are measured under ultrahigh vacuum conditions. The results are discussed in terms of the electronic states above the Fermi energy of both the constituents Fe and Co and are compared with the APS spectra of the pure metals. The important role of the final state effects in the broadening of the APS spectra is taken into account. The width of the unoccupied local d states is only slightly changed upon alloying.
Thermally Stimulated Depolarization of NCS− Centres in Potassium and Rubidium Halide Crystals
Authors: Wilbrand A. , Fischer F. 
Published: July 2007

AbstractTSD measurements are carried out on crystals of the chlorides, bromides, and iodides of potassium and rubidium doped with thiocyanate. In all systems a TSD band is found between 24 and 47 K which is attributed to isolated NCS− centres. With the aid of the measured bands the activation enthalpies and fundamental relaxation times for reorientation of the centres as well as their electric dipole moment are determined. The dependence of the activation enthalpies upon the host crystal is compared to that for NCO− centres.
Optical Gain and Induced Absorption in High‐Density Exciton System in CuCl
Authors: Hvam J. M. , Bivas A. 
Published: July 2007

AbstractIn CuCl at 1.7 K optical gain/absorption spectra are recorded in the spectral range of bound exciton and biexciton emission. Energy, effective temperature, and oscillator strength of biexcitons are determined. The observed kinetics indicates that the total pair density in the exciton‐biexciton system is pinned at about 1.6 × 1016 cm−3 for all excitation densities above 10 kW/cm2. At intense excitation, a Fano‐interference is observed between a discrete impurity absorption line and a continuum absorption tail. The latter is followed by a band of positive gain towards lower photon energies.
Random Torsional Oscillation Model for Rotational Molecular Motion. Cold Neutron Scattering Investigation of Cholesteryl Propionate, Nonanoate, and Myristate
Authors: Cvikl B. , Srebotnjak E. , Dušić M. , Dimic V. , Krištof E. 
Published: July 2007

AbstractThe incoherent scattering function for the case of simple translation of the centre of mass and the random torsional oscillations of the rigid planar body along the long molecular axes of an amplitude φ of perfectly ordered molecules in nematic phase, is calculated. The model of a random molecular torsional motion is built on the assumption that each proton in the molecule suffers a large number of equal angular displacements between two perfectly reflecting barriers at φ and −φ. The dynamics of molecular tail is described by uniaxial free rotational diffusion and freely rotating spherical top models, respectively. The predictions of the calculations are tested on measured time of flight spectra, obtained on three members of a homologous series of cholesteric liquid crystals and agreement is obtained.
On the Phase Separation of Carriers in Nitrogen‐Doped GaP
Authors: Thuselt F. 
Published: July 2007

AbstractUnusual phase separation of excited carriers in isoelectronically doped GaP is discussed in terms of two models: (i) a perturbed neutral electron‐hole plasma, and (ii) electrons assumed to be strongly localized at the isoelectronic impurities embedded in a non‐neutral electron‐hole plasma. For low carrier densities where excitons are stable particles due to low screening, the special situation involving nitrogen pairs is taken into account.
On the Theory of Interstitial Alloys in the Coherent Potential Approximation (CPA)
Authors: Urbański M. K. 
Published: July 2007

AbstractThe single‐site CPA self‐consistent condition for the effective Hamiltonian of an interstitial disordered alloy is obtained. It is assumed that the Hamiltonian of the tight‐binding model is composed of the following terms: the Hamiltonian of the pure solvent and two disordered parts, the interstitial atom energy and the solvent‐interstitial interaction. In the model the interstitial atom interaction is neglected in order to use the single‐site approximation. The total density of electron states can be derived explicitly. A simple numerical example of the density‐of‐states calculation is presented.
Study of the Temperature Dependence of the Dielectric Properties of LiTaO3 by Raman Spectroscopy
Authors: Umarov B. S. , Vetelino J. F. , Abdullaev N. S. , Anikiev A. A. 
Published: July 2007

AbstractThe phonon spectra of the A1‐type symmetry vibrations in LiTaO3, are measured by Raman scattering techniques over a wide temperature range which include the phase transition point. These spectra are then analyzed using the Kramers‐Kronig relations to obtain the temperature and frequency dependence of the static dielectric constant, ϵzz, and the infrared (IR) reflection spectra. Good agreement is obtained with results of direct experimental measurement.
Simple Electronic Structure Calculation of Substitutional sp Impurities in α‐Iron
Authors: Demangeat C. , Parlebas J. C. 
Published: July 2007

AbstractTaking into account a Slater‐Koster fit to first‐principles band structures for the host, a simple tight‐binding model is proposed for the electronic structure of sp impurities (Al, Si, P … Sb) substituted in ferromagnetic metals. The aim of the model is to extend calculations to future more complicated problems like chemical binding energies between point defects. The method is applied to dilute σ‐iron based alloys: it is shown that a localized impurity model gives reasonable accord with experimental data on saturation magnetization, every time the dilution law is almost satisfied (FeAl, FeSi, FeP).
Mode Gammas and Thermal Expansion of CaF2
Authors: Govindarajan R. , Haridasan T. M. , Govindarajan J. 
Published: July 2007

AbstractThe perturbation approach of Achar and Barsch is employed to work out the mode gammas and the thermal expansion of CaF2. A shell model similar to that of Axe is employed. The pressure dependent dynamical matrix components are computed using the available data on the pressure derivatives of the elastic constants and the mode gammas of the long wavelength optical phonons. The modified moments of the phonon density of states are also computed. These results agree fairly well with the experimental values reported by Baily and Yates and the previously reported work of Ruppin.
Fourier Transform of Deuteron Modulation of Electron Spin Echo Envelope of Silver Atoms in Deuterated Ice and Solid Methanol
Authors: Aruna B. , Ramakrishna Y. V. S. , Narayana P. A. 
Published: July 2007

AbstractThe analysis of electron spin echo modulation in the frequency domain is studied by Fourier transforming the time domain data. Two systems are chosen for this purpose, silver atoms in deuterated ice and specifically deuterated solid methanol (CH3OD). It is shown that some of the artifacts introduced into the Fourier transform spectrum can be reduced by reprocessing the time domain data. The advantage of this type of analysis over the direct simulation of modulation is pointed out.
Effective Mass of Conduction Band Electrons in Hg1−xMnxTe from Shubnikov‐de Haas Oscillations
Authors: Jaczyński M. , Dobrowolski W. 
Published: July 2007

AbstractThe Shubnikov‐de Hass effect (SdH) is investigated in n‐type Hgl−xMnxTe mixed crystals for several compositions in the range 0.02 ≦ x ≦ 0.04. Experiments are carried out on samples with electron concentrations between 2 × 1015 and 8.5 × 1017 cm−3 at temperatures from 2 to 25 K. Prom the temperature dependence of the amplitude of the SdH oscillations the effective masses of electrons are determined. The data are analyzed in terms of a model developed to account for anomalies observed earlier in the SdH effect in this material.
Free Energies of Solid and Liquid Metals. Thermodynamic Calculation of Melting Temperature
Authors: Belan‐Gaiko L. V. , Bogdanov V. I. , Fuks D. L. 
Published: July 2007

AbstractFor the investigation of the free energy of metals and their temperature dependence in solid and liquid phases the pseudopotential method is used. In calculating the free energy in a solid phase the lattice vibration contribution is considered. In definition of the pair potential in liquid metals the volume change of the metal during its melting is taken into account. The structure of liquid metals is calculated with the help of the found potential using the integral equation method. The solutions of different equations for the radial distribution function are compared and the influence of the model form factor and the method of screening in the electron gas on the liquid metal structure is discussed.
Charge and Spin Density Waves in Jellium
Authors: Perdew J. P. , Datta T. 
Published: July 2007

AbstractIs the uniform electron gas unstable against charge and spin density waves (CDW's and SDW's) of infinitesimal amplitude, which break the translational symmetry of the Hamiltonian? Within density functional theory, a CDW of wavevector Q ≈︁ 1.1(2kF) appears at low densities. The local spin density approximation (LSDA) for exchange and correlation places the transition at rs ≈︁ 30, while inclusion of the next term in the gradient expansion suppresses the transition to densities as low as rs ≈︁ 200. The best estimate is that the CDW appears out of the uniform ferromagnetic phase of jellium at rs ≈︁ 70; this is also where Ceperley, in a recent calculation with correlated wave functions, found Wigner crystallization. Although no SDW in jellium is predicted within LSDA, the calculated uniform spin susceptibility diverges at rs ≈︁ 35, close to the highest density (rs ≈︁ 26) at which Ceperley found ferromagnetism.
A Wide Concentration Range Induced Moment System: (TbcLa1−c)Be13
Authors: Clad R. , Bouton J. M. , Herr A. 
Published: July 2007

AbstractMagnetic measurements are reported for polycrystalline compounds (TbcLa1−c)Be13, with 0.02 ≦ ≦ c ≦ 1, in the temperature range 1.6 to 300 K and for fields up to 70 kOe. The observed ordering temperature TN and the downward shift of the inverse susceptibility curve, in the paramagnetic regime (T ≧ TN), scales linearly versus c. The M(H, T) dependences are calculated by diagonalizing the Hamiltonian including crystal field, molecular, and Zeeman terms along the 〈100〉, 〈110〉, and 〈111〉 directions. The best fit to the magnetic data is obtained for the crystal field parameters x = −0.8 and W varying linearly from 0.05 K for c = 1 to 0.13 K for c = 0.16. A transition from Van Vleck paramagnetism to an ordered magnetic structure occurs at low temperature, as the terbium concentration exceeds the threshold value for ordering of about c = 0.06.