bluuebunny/crossref_metadata_2025_split huggingface dataset:
High‐ Tc YBaCuO Superconductor Plasma and Collision Frequency Calculated from the Very Far Infrared Transmission Spectra
Authors: Hadni A. , Gerbaux X. , Villegier J. C. 
Published: July 2007

AbstractTransmission spectroscopy in the very far IR (10 to 50 cm−1) is shown to be efficient to get dc conductivity, plasma frequency, and collision frequency at different temperatures in the normal state (e.g. 210 and 125 K), and the superconducting state (e.g. 79 to 65 and 8 K), for an YBa2Cu3O7—δ (YBaCuO) thin film. For a given sample a plasma frequency vp = 6500 ± 1000 cm−1 independent of temperature, a collision frequency depending linearly on temperature, vc = 0.69T (in cm−1) and a negligible transmission τ at v = 10 cm−1 for T = 8 K (τ < 2 × 10−4) are found, showing that the amount of YBa2Cu3O6 insulating impurity is very low. A two‐fluid model is used to explain the spectra at T = 65 and 80 K, with a concentration Ns of superconductive electrons given by a T4 law, Ns = No[1 — (T/Tc)4], which seems to be acceptable as a first approximation where the collision frequency for normal electrons is assumed to be saturated around 40 cm−1 at T < 65 K, to fit the data. Such a collision frequency is corresponding to a mean free path l = 212 Å if we take a Fermi velocity vF = 200 km s−1 and l = vF/2πcvc.
Weak Exchange Effects in EPR Anisotropy of Dichlorobis(1‐Phenyl‐3,5‐Dimethylpyrazole) Copper(II) Single Crystals
Authors: Goslar J. , Hilczer W. , Hoffmann S. K. 
Published: July 2007

AbstractThe EPR spectrum displays a merging effect between two lines from inequivalent Cu(II) sites which produces a partially resolved and partially averaged angular dependence of the resonance fields. Decoupling of the lines, assuming spin exchange, gives an exchange integral |J| = 0.00386 cm−1 which is isotropic in the crystal. A detailed procedure for a decoupling of the two angular dependences of the individual lines in a monoclinic crystal is described. It leads to the following molecular g‐factors of the individual complex: gz = 2.230, gy = 2.050, and gx = 2.036. Temperature affects only the gz‐value and the linewidth value in perpendicular orientations. Both gz and ΔBpp decrease with temperature, and gz behaviour is attributed to a fluxional behaviour of the square‐planar CuCl2N2 complex. The |J|‐value is temperature independent and suggests ferromagnetic coupling between inequivalent Cu(II) sites connected by the exchange pathway ‐Cu‐pz… Cl‐Cu‐Cl… pz‐Cu‐pz‐ in the crystal unit cell.
Surface Spin Waves in a Semi‐Infinite Magnetic Semiconductor with a Site‐Diluted Free Surface in the Narrow‐Band Limit
Authors: Shen Ming‐Rong , Shen Wen‐Zhong , Li Zhen‐Ya 
Published: July 2007

AbstractThe s—d (or s—f) Heisenberg interaction model is used to investigate the surface spin waves (SSW) in semi‐infinite magnetic semiconductors with a site‐diluted free surface in the narrow‐band limit. The dispersion relations of surface excitations are obtained by use of Green's function theory. Results in some particular cases are discussed.
Magnetic—Nonmagnetic Ground State of Mixed‐Valence and Heavy‐Fermion Systems
Authors: Panwar S. , Singh I. 
Published: July 2007

AbstractIn many experimental findings it has been observed that some heavy‐fermion (HF) and mixed‐valence (MV) systems order magnetically at low temperature while many others do not down to the lowest temperature. In this paper, the magnetic properties of HF and MV systems using the Anderson lattice model, including an intersite exchange interaction are studied. It is seen that the ratio of exchange to hybridization dictates the absence or presence of magnetic ordering. The magnetic phase diagram and the variation of susceptibility with exchange and temperature is studied.
The Existence of a Gap Between the Para‐ and Ferromagnetic Phases in a Group of Pr Intermetallics Observed Through a Doublet‐Triplet Model
Authors: Caldas A. , Von Ranke P. J. , Palermo L. 
Published: July 2007

AbstractThe present work constitutes a part of a continuing series of studies dealing with models, in which only the two lowest levels of the crystal field splitting scheme of the Pr3+ ion in Pr intermetallics are retained. Taking into account the magnetic state equations derived from a model, in which the ground and first excited levels are doublet and triplet, respectively, the existence of a gap between the para‐ and ferromagnetic phases is shown in the space of J0 (effective exchange parameter) versus Δ (crystal field parameter). This study is applied to the available experimental data of several Pr intermetallics, in cubic symmetry, and the results are in line with the previsions of the model.
Light‐Induced Absorption (Amplification) of an Intensive Sound Wave in a Semiconductor Under a Quantizing Magnetic Field
Authors: Yu. Safronov E. , Sinyavskii E. P. 
Published: July 2007

AbstractThe light‐induced absorption (amplification) of an intensive high‐frequency sound wave due to interband transitions of the electrons in a quantizing magnetic field is studied. With the use of the density matrix method the energy dissipation of the sound wave per unit time is calculated and the non‐linear constant γ (I) of the sound absorption is defined. The pecularities of γ (I) which appear with the change of the sound‐wave intensity (I) are discussed.
Anomalies of Spin–Lattice Relaxation and NMR Spectra and Phase Transition in Fluorinated Graphite Intercalation Compounds
Authors: Panich A. M. 
Published: July 2007

AbstractThe anomalies of NMR spectra and spin–lattice relaxation time T1 in a fluorinated graphite layer intercalated with acetone (stage 1) suggest a phase transition and the existence of an “intermediate” phase in the range of fast molecular diffusion. In the high temperature phase T1 and the activation energy Ea do not depend on the molecular content x. Meanwhile, the concentration dependence of Ea and T1 in the “intermediate” phase suggests the essential role of intermolecular interactions leading to cooperative effects. An increase of Ea with decreasing x is interpreted as a consequence of the existence of a molecular sublattice which is incommensurate with the fluorographite lattice. The nature of the phase transition is discussed.
A General Study of Tunnelling through Multibarrier Systems
Authors: Roy C. L. , Khan Arif 
Published: July 2007

AbstractA study of the tunnelling through an arbitrary number N of rectangular barrier‐type potentials is reported. For this purpose, first the general form of the transmission coefficient (TC) for the system just mentioned is derived and therefrom the general form of conditions for resonant tunnelling is obtained. Our study indicates, among others, 1. the role of the Chebyshev polynomials of second kind in regard to TC, 2. the domains of energies for resonant tunnelling, and 3. the validity of one condition for resonant tunnelling through any even number of barriers.
Two‐Dimensional Quantum Antifferomagnet in a Strong Magnetic Field. The Case of Spin 1/2
Authors: Gluzman S. 
Published: July 2007

AbstractThe problem of the two‐dimensional quantum antiferromagnet of spin 1/2 on a square lattice in a strong transverse magnetic field close to saturation is studied in terms of the equivalent Bose gas problem. The existence of a soft sound‐like mode and of a phase transition from the state with specific canted magnetic structure to the disordered ferromagnetic state is demonstrated. The expressions for thermodynamical and magnetic characteristics are derived.
Interband and Gap State Related Transitions in β‐Rhombohedral Boron
Authors: Werheit H. , Laux M. , Kuhlmann U. 
Published: July 2007

AbstractThe anisotropy of the electronic interband transitions of β‐rhombohedral boron is determined by optical absorption measurements. The band gaps (indirect allowed transitions) are 1.32(1) and 1.50(1) eV for E ∥ c, respectively, 1.29(1) and 1.46(1) eV for E ⟂ c (extrapolated to T = 0 K). At low temperatures these gaps decrease, while the thermal equilibrium develops. The temperature dependence of the gap is attributed to the thermal disorder caused by phonons of 121.9 (E ⟂ c) and 94.1 meV üE ⟂ i.c) representing the most prominent intraicosahedral vibrations. Six intrinsic trapping levels for electrons in distances between 0.17 and 1.15 eV from the conduction band edge are determined, the closer ones in agreement with different photo effects. The trapping levels are attributed to distortion states of the icosahedra due to a dynamical Jahn‐Teller effect. This conception allows the interpretation of other physical properties like internal friction, discontinuities of thermal expansion, and electrical conductivity, of the temperature dependence of the ESR linewidth, and of the density of paramagnetic centers (spin density and magnetic susceptibility).
Zero‐Field Splitting and the Optical Fine Structure in Fe3+ :Al2O3 Crystals
Authors: Yu Wan‐Lun , Wang Juen‐Zhong 
Published: July 2007

AbstractThe ZFS parameters D, F, and a of Fe3+ in Al2O3 are evaluated by taking the spin‐ orbit interaction as the perturbation to the crystal‐field and the electrostatic potentials. Calculations are carried out up to fifth order and show a good convergence. Meanwhile, the optical fine structure is analyzed by diagonalizing the complete energy matrices. The obtained results are in good agreement with the experimental data reported in the literature.
Effective Single‐Band Hamiltonian for the Three‐Dimensional Hubbard Model
Authors: Rodríguez‐núntez J. J. , Medina R. , Silva P. 
Published: July 2007

AbstractThe four‐band Hubbard model is mapped onto an effective single‐band model in the Hilbert sub‐space of one‐ and two‐particle wave functions. The parameters of the effective interaction depend on the single‐particle energies. The Hartree‐Fock (H‐F) approximation is used. The 3D isotropic effective interaction produces superconductivity for different values of the physical parameters. The applicability of this Hamiltonian to physical systems is discussed. This may restrict the magnitude of the on‐site Coulomb repulsion.
The Warm Electron Coefficient in n‐InSb in the Presence of a Quantizing Magnetic Field at Low Temperatures
Authors: Santra K. , Sarkar C. K. 
Published: July 2007

AbstractThe warm electron coefficient (β) in n‐InSb in the presence of a quantizing magnetic field at low temperature is investigated theoretically to interpret the experimental results in n‐InSb at TL = 4.2K. The theoretical model used for the calculation of β considers the displaced Maxwellian distribution for electrons and other complexities such as the band nonparabolicity, the non‐equipartition of phonons, the free‐carrier screening, and the Landau‐level broadening. The theoretical results of β obtained by the above model are found to agree qualitatively with the experimental results taking the standard material parameters of n‐InSb. To fit the theoretical values of β with the experimental values, the acoustic deformation potential constant (E1) is found to be 13 eV which agrees with other observations.
One‐ and Two‐Spin Cluster Approximations for the Site‐Diluted Spin‐1 Ising Model
Authors: Bobák A. , Mockovčiak S. , Sivuľka J. 
Published: July 2007

AbstractOne‐ and two‐spin cluster approximations employing the differential operator technique are applied to study the second‐order phase boundaries for a quenched site‐diluted spin‐1 Ising model. The critical concentration below which the transition temperature reduces to zero is determined for the honeycomb, square, and simple cubic lattices. The results are compared to those of the site‐diluted spin‐1/2 Ising model.
The Dilute Random‐Field Ising Model by the Finite Cluster Approximation
Authors: El Atri A. , Saber M. 
Published: July 2007

AbstractUsing the finite cluster approximation, the phase diagram of site‐diluted three‐dimensional simple cubic Ising models with a random field is determined. The phase diagram exhibits only second‐order transition or second‐order and first‐order transition lines, according to an appropriate probability distribution of these random variables.
Evaluation of the Grüneisen Constant of Molten GaAs
Authors: Rao R. V. Gopala , Debnath D. 
Published: July 2007

AbstractThe effective potential function of molten GaAs is derived from experimental neutron diffraction data. From the second and third derivatives of the derived potential function the Grüneisen constant is calculated. As an interconsistent check the first derivative of the potential function is utilized to calculate the pressure of the fluid. The energy of vibration is computed from Debye's equation. The calculated pressure, experimental volume, and Debye vibrational energy are then used to evaluate the Grüneisen constants. The results calculated by these two methods are found to be similar.
Theoretical Description of the Crowdion in an Anisotropic Crystal Based on the Frenkel‐Kontorova Model Including and Elastic Three‐Dimensional Medium
Authors: Kovalev A. S. , Kondratyuk A. D. , Kosevich A. M. , Landau A. I. 
Published: July 2007

AbstractThe well‐known Frenkel‐Kontorova theoretical model is generalized by including the elastic properties of the three‐dimensional medium surrounding the discrete atomic chain considered. The generalized sinuse‐Gordon equation (SGE) and the integro‐differential equation describing the crowdion in a highly anisotropic crystal are described. The numerical solution of these equations corroborates all the analytical estimates.
Multiphoton Polariton Spectroscopy on ZnO
Authors: Fiebig M. , Fröhlich D. , Pahlke‐Lerch Ch. 
Published: July 2007

AbstractDifferent nonlinear techniques are applied to investigate excitonic from the upper valence bands (symmetry Γ7, Γ9, Γ7) to the lowest conduction band (symmetry Γ7) in ZnO. Coherent techniques like three‐photon difference frequency generation are used for resonances on the lower polariton branch, whereas incoherent techniques are well suited for the investigation on the upper polariton branch. Paraexcitons are detected without external perturbation. The angular dependence of the longitudinal transverse splitting is investigated by excitation of mixed mode polaritons and analyzed with use of a suitable ansatz for the dielectric tensor in the classical wave equation. The isotropic point at 3.157 eV is studied by non‐resonant second harmonic generation.
Exact Solutions for the Quantum Nonlinear Trimer
Authors: Andersen J. D. , Kenkre V. M. 
Published: July 2007

AbstractExact analytical solutions of the discrete nonlinear Schrödinger equation for three‐site systems are presented by generalizing methods employed previously for the quantum nonlinear dimer. The transition from localized to delocalized states is markedly different from that in the dimer.
Mössbauer Spectra and Quantum Beats of γ‐Rays Backscattered from a Resonance Absorber on an Oscillating Quartz
Authors: Kuhn D. , Jex H. 
Published: July 2007

AbstractThe elastic and inelastic backscattering from a resonance absorber bound to an oscillating quartz crystal is analyzed by means of 57Co Mössbauer γ‐radiation. The experiments are performed in the energy coordinate using the Doppler motion of the source and in the time domain with quantum beat techniques. Examples are presented of modulated backscattering spectra from the energy analysis and quantum beat signals from excitations with frequencies even below the radiation width.
Light‐Induced Dissociation of PH Complexes in Hydrogenated n‐Type Si
Authors: Shlopak N. V. , Ulyashin A. G. , Bumai Yu. A. 
Published: July 2007

AbstractThe influence of illumination by white light on the stability of PH complexes at room temperature is investigated. It is found that the passivated phosphorus in hydrogenated n‐type silicon is reactivated at room temperature by illumination with photon energies exceeding the band gap of silicon. The annealing kinetics of PH complexes under illumination has a rapid initial step and follows a second‐order process for long illumination times. A dissociation model is discussed. It is assumed that annealing under illumination similar to thermal annealing results in the formation of molecular hydrogen species.
Magnetic Models of Non‐Stoichiometric High‐Temperature Superconductors
Authors: Sikakana I. Q. , Lowther J. E. 
Published: July 2007

AbstractValence charge fluctuations on copper in high‐temperature superconductors can only give a spin‐½ system when copper is in the charge state Cu2+. As charge on the copper fluctuates because of lattice stoichiometry in turn related to the concentration of oxygen atoms, so the concentration of spin‐½ atoms varies. This is modelled with the magnetic interaction topology varying from being cubic to rectangular planar. In all cases various magnetic properties especially the magnetic transition temperature and susceptibility are calculated. It is suggested that the observed magnetic behaviour of oxygen deficient high‐temperature superconducting materials may be interpreted within such a magnetic model, with the magnetic interaction between adjacent superconducting layers varying with oxygen stoichiometry.
Intrinsic Emissions in the Cathodoluminescence Spectra of KBr and KCl Crystals
Authors: Yang B. , Townsend P. D. 
Published: July 2007

AbstractCathodoluminescence (CL) measurements made on pure KCl and KBr crystals are reported for the temperature range 40 to 300 K. The F‐band self‐absorption is studied by using different beam energies and different incident angles. The intrinsic emissions such as self‐trapped exciton (STE) emission, perturbed STE emission, and α‐centre emission are identified after the correction for the self‐absorption. The temperature dependence of the intensities of these emission bands are discussed as well.
The Temperature Dependence of the Energy Gap in Cd0.22Hg0.78Te. (I). Optical Measurement
Authors: Hlídek P. , Kučera Z. , Koubele V. , Höschl P. , Zvára M. 
Published: July 2007

AbstractThe energy gap Eg is accurately determined from interband Landau level transitions observed in spectral measurements of photoconductivity at low temperatures in an external magnetic field. The temperature dependence of the optical absorption coefficient is measured on samples with different slopes of the exponential Urbach absorption edge. The results are compared with empirical formulae reported for Eg(x, T). In the temperature region 10 to 300K, the best agreement is found for the rclation published by Seiler et al., but with a modified value of the temperature coefficient.
Green's Function Method in Thermo‐Field Dynamics: Applications to Plasma and Laser Heated Matter
Authors: Ye Zhen 
Published: July 2007

AbstractThis paper, on one hand, is an introduction to thermo‐field dynamics (TFD), on the other hand a derivation of some interesting features of Green's functions in the frame of thermo‐field dynamics. It is proved that in the language of TFD, Green's functions at finite temperature take a very simple form. Through the applications of TFD to many‐body systems it is shown that TFD is a straightforward method in dealing with finite temperature many‐body systems. As an application of Green's function method in TFD, we derive a Kogan‐type formula for the non‐equilibrium dynamic conductivity or laser‐pulse heated condensed matters, where electrons and ions are held at different temperatures.
Effect of dZ2‐Cu and p‐O Orbital Mixing in the High‐Tc Superconducting Materials. A Three‐Dimensional Anisotropic Interaction
Authors: Rodríguez‐Núñez J. J. 
Published: July 2007

AbstractA three‐dimensional (3D) anisotropic interaction is obtained for the oxide high‐Tc superconductors when we take into account the mixing of dZ2‐Cu with the p‐O orbitals. Two hopping matrix elements, tl, on the plane, and t2, along the z‐axis, are used to study anisotropy in the original Hubbard Hamiltonian. The derived interaction contains a local Hubbard term, U, and two non‐local superexchange contributions, J1, and J2. Both J1 and J2 strongly depend on the anisotropy ratio (α ≡ t2/tl). However, for α = 0, J2, → 0, and J1, the superexchange on the plane, coincides with results previously published in 2D. Furthermore, when α = 1, J1 ⟺ J2. The latter result is also known in the literature. The effective Hamiltonian presented here is more physical for studying high‐Tc superconducting materials, since the third dimension is playing an increasing role as it has been pointed out by several authors. It is also found that the hole occupation on the p,‐orbital (〈npz〉), for α ≪ 1, is related to the hole occupation on the px‐ and py‐orbitals, by means of the relationship 〈npz〉 ≈ α2 〈npx〉, which is in qualitative agreement with experimental and theoretical findings that 〈npz〉 is very small. The calculated Tc versus n shows agreement with recent experiments in La2−xSrxCuO4.
Second‐Order Displacive Phase Transition in Ferroelectric Solid Solutions
Authors: Orbukh V. I. , Manaev O. A. 
Published: July 2007

AbstractThe method of the average field is employed to study the second‐order displacive phase transition in ferroelectric solid solutions. Computation of the free energy of the solid solution is made for ferroclectric and paraelectric phases. Temperature dependenccs of thc spontaneous polarization, concentration dependences of the phase transition temperature, Curie constant, and dielectric susceptibi‐ lity as well as the condition for the existence of the phase transition in mixed crystals are obtained.
Time‐Resolved Emission Spectra from Laser‐Induced Plasma in Sodium Chloride
Authors: Yago H. , Furuta K. , Ishikawa K. , Komura H. 
Published: July 2007

AbstractSpectroscopic investigations are carried out on the radiation emitted from laser‐induced plasma on a NaCl crystal surface. It is found that all atomic lines emitted from plasma produced by laser irradiation in air are broadened and some of them are self‐reversed, while the self‐reversal is not observed for plasma induced in vacuum. Self‐reversal occurs for the pressure of ambient gas above 1.3 kPa. It is found from line broadening, mainly caused by resonance and the Stark effect, that the Na atom concentration in the plasma is 5.3 × 1024 m−3. Observed time‐resolved spectra agree with a model that the plasma generated by laser irradiation is composed of a hot core and a low‐temperature periphery, both behaving, respectively, as emission and reabsorption centers and expanding at a speed determined by the ambient gas pressure.
Luminescence Quenching of Strongly Coupled Systems
Authors: Koepke Cz. , Lempicki A. , Wojtowicz A. J. 
Published: July 2007

AbstractThe case of temperature quenching of luminescence of broad‐band emitters is examined. On the basis of published and newly acquired data, it is concluded that the theory of nonradiative transitions based on coupling to a single vibrational mode, whether calculated in the Mott‐Seitz terms or improved by the procedures developed by Struck and Fonger, is totally incapable of accounting for observed radiationless rates. There is no evidence for the operation of selection rules for nonradiative transitions.
Photoemission Calculations Using Kronig‐Penney Model and Spatially Dependent Vector Potential. Application to Pd
Authors: Thapa R. K. 
Published: July 2007

AbstractPhotoemission from the bulk and the surface of Pd is calculated as a function of incident photon energy. The effect of the location of the surface state, with respect to the surface plane on the photocurrent is studied. For the Fermi level photoemission, the initial state wave function |ψ〉 deduced by employing the Kronig‐Penney model is used. The photocurrent is calculated by incorporating the vector potential whose spatial dependence plays a major role during photoemission.