bluuebunny/crossref_metadata_2025 huggingface dataset:
Propagation Beats of Quadrupole Polaritons in Cu2O
Authors: Fröhlich D. , Kulik A. , Uebbing B. , Langer V. , Stolz H. , von der Osten W.
Published: July 2007

Time‐resolved measurements of transmission in resonance with the 1 S quadrupole polariton in Cu2O reveal a long‐lasting signal which is modulated by pronounced oscillations with time‐dependent period. These oscillations are interpreted as “propagation beats”, which are caused by superposition of excited states of both polariton branches which propagate coherently through the crystal. At higher excitation intensities the decay becomes faster and the beats disappear. Magneto‐quantum beats are also influenced significantly by propagation effects in transmission measurements.
Coherent Polariton Dynamics in Quantum Wells
Authors: Rappen T. , Mohs G. , Wegener M.
Published: July 2007

It is demonstrated that the exciton–photon coupling, or equivalently the polariton nature of the exciton, may lead to qualitative changes in the lineshape of time‐resolved four‐wave mixing. For small oscillator strength and hence small longitudinal–transverse splitting this coupling acts very similar to the recently discovered coherent exciton–exciton interaction, i.e. as an exponential rise of the time‐integrated signal for negative time delay. Here the exciton–photon coupling can be interpreted as a mediated exciton–exciton interaction. For larger coupling a distinct nonperiodic modulation evolves that reflects the oscillation between the two branches of the polariton dispersion. Spectrally and time‐resolved four‐wave mixing is shown to give direct access to the dynamics on the two branches. Corresponding femtosecond experiments on two‐dimensional magneto‐excitons are compared with numerical calculations.
Interband Quantum Kinetics with LO‐Phonon Scattering in a Laser‐Pulse‐Excited Semiconductor I. Theory
Authors: Haug H.
Published: July 2007

Nonequilibrium Green's function theory is employed to derive with the use the generalized Kadanoff‐Baym ansatz quantum kinetic equations for the interband polarization and the densities of electrons and holes. These equations are valid in the regime of ultrafast spectroscopy, where that usual Boltzmann kinetics with its individual energy‐conserving collisions no longer holds. The memory kernel of the retarded quantum kinetic equations is determined by retarded and advanced Green's functions. It is stressed and explicitly shown by numerical solutions in an adjacent paper that these Green's functions have to be approximated consistently with the coupling to phonons.
Electronic Coherence Effects in Absorption Saturation Experiments
Authors: Scholz R. , Stahl A.
Published: July 2007

In pump‐probe experiments with femtosecond pulses, a coherent peak is found in the absorption saturation for short pulse delays. This coherent peak is analysed within a density matrix model, including a realistic multi‐valence band structure and the optical matrix elements to leading order. Dissipative processes are treated in a relaxation time approximation. The parameters of the model are determined by comparison to experimental results.
Optical Excitations near the Fundamental Gap of Solid C60
Authors: Feldmann J. , Fischer R. , Göbel E. O. , Schmitt‐Rink S.
Published: July 2007

Optical studies of solid C60 employing time‐integrated as well as time‐resolved photoluminescence spectroscopy are reported. At low excitation intensity, a richly structured luminescence spectrum is observed. Increasing the excitation intensity leads to pronounced carrier accumulation and to the emission of intense and spectrally broad luminescence. It is concluded that multiple‐step absorption processes lead to carrier recombination originating from higher conduction or lower valence band states.
Tunneling through Single AlGaAs Barriers
Authors: Heberle A. P. , Rühle W. W. , Köhler K.
Published: July 2007

Tunneling transfer through Al0.35Ga0.65As barriers is studied in asymmetric double quantum well structures by time‐resolved photoluminescence measurements in the pico‐ and femtosecond regime. A large variety of electron and hole resonances is detected when electric fields of both signs are externally applied. The ground state resonance shifts, when the electrons tunnel in the reverse direction, revealing the importance of excitonic effects. Longitudinal optical phonon assisted tunneling plays a minor role for narrow quantum wells in comparison to impurity or interface roughness assisted transfer. Resonant electron tunneling times depend exponentially on the integrated square root of the tunneling barrier height and are an order of magnitude faster than resonant hole tunneling times. The n = 2 to n = 1 electronic intersubband scattering time in a 10 nm quantum well is determined to be 550 fs measuring the transfer time through a thin barrier.
Quantum‐Confined Electron–Hole Droplets
Authors: Kalt H. , Nötzel R. , Ploog K. , Giessen H.
Published: July 2007

The liquid phase of quantum‐confined electron–hole plasma is observed in a type‐II GaAs/AlAs short‐period quantum‐well wire superlattice grown on a high‐index (311) GaAs substrate surface. This superlattice system is the first low‐dimensional structure to support the condensation of electron–hole droplets as is evidenced by the characteristic properties of photoluminescence spectra.
Luminescence Spectra and Lattice Dynamics of NaNO2 in the Ferroelectric Phase
Authors: Janssen W. B. J. M. , Van Avoird A. Der , Köhler J. , Schmid D.
Published: July 2007

In order to understand the results of recent luminescence experiments harmonic lattice dynamics calculations on the ferroelectric phase of NaNO2 are performed. Starting from an atom‐atom potential and including the polarization of the nitrite ions by means of the shell model the phonon and vibron dispersion curves and densities of states are calculated. The calculated phonon dispersion curves are in good agreement with the inelastic neutron scattering data. The phonon density of states is compared with the phonon side band measured by luminescence. Above 15 K the measured phonon side band resembies the density of states, but at lower temperatures this resemblance is lost because of the non‐uniforn distribution of the excited states over the Brillouin zone. The density of states of the v2‐vibron band is compared with the lineshape of the v2 line in luminescence spectra. It is shown that this lineshape can be explained by inclusion of the dispersion of the singlet exciton.
Positron Annihilation Studies in GaAs
Authors: Panda B. K. , Padhi H. C. , Viswanathan B.
Published: July 2007

Angular correlation of positron annihilation radiation (ACPAR) along different crystallographic directions of GaAs are measured using the long‐slit geometry. The shapes of the profiles indicate in addition to the ideal sp3 orbitals contribution, the contribution of σ bonding along [111] bonding direction and an admixture of σ and π* bondings for the [100] and [110] directions. The ACPAR anisotropies in this case are somewhat less compared to that of Ge which is caused because of the presence of the ionicity in compound semiconductors. Although the zero points of the autocorrelation functions for ACPAR data are shifted from the actual lattice positions, their periodicity remains same as the actual lattice spacing.
On the Possibility of Space‐Charge Wave Amplification in Multielectrode MIS Microstructure
Authors: Tralle I. E. , Filonov A. B.
Published: July 2007

The beam instability in a metal–insulator–semiconductor microstructure with periodical field electrodes is considered. The instability appears as a consequence of the inversion of the electron distribution function in gallium arsenide in a strong electric field. The amplification of a space‐charge wave in such microstructures can be basis for working out of microwave amplifiers.
Tunneling current through a resonant structure
Authors: Lu X. J. , Horing N. J. M.
Published: July 2007

Tunneling transport through a resonant double‐barrier structure is examined, with attention focused on the suppression of transmission current due to Pauli exclusion arising from population growth of the resonant state. A quasi‐analytic theory is developed to self‐consistently determine the population of the resonant state and its exclusion suppression of tunneling. Furthermore, the role of resonant state population in inducing an electrostatic increase of the resonant level energy, which tends to enhace tunneling, is considered. Taking all these features in joint consideration, a quasi‐analytic description of intrinsic bistability in the regime of negative differential conductance is achieved, exhibiting hysteresis in the I‐U characteristic.
Electron‐molecular vibration coupling in the cation‐radical salt of a giant analogue of TTF with ClO anion
Authors: Graja A. , Yartsev V. M. , Garrigou‐Lagrange C. , Sallé M. , Gorgues A.
Published: July 2007

Spectral studies of an ion‐radical salt DClO4 of a giant analogue (D) of TTF are performed in which the dihydro‐TTF core is bearing the two conjugated 1,4‐dithiafulvalen‐6‐yl side arms. Both IR absorption and electronic spectra of the donor molecule are measured; Raman scattering is also accomplished. Our main attention is paid to the electronmolecular vibration coupling in the DClO4 salt. It is shown that the idea of treating the donor molecules as an aggregate of two parts excited by the intramolecular charge transfer is an acceptable first approximation of the EMV coupling in the giant π‐donor. The EMV coupling constants are calculated and compared with those determined for the parent TTF molecule.
Inversion effect of the electric field in the quantum well in the presence of a single diamagnetic impurity
Authors: Monozon B. S. , Hüther P. , Wies P. , Gerlach H. , Schmidt M.
Published: July 2007

The effects of parallel magnetic and electric fields on a shallow donor state in thin semiconductor layers (quantum wells) are investigated. The magnetic field is strong in comparison with the Coulomb field of the electron. It is shown that in such systems the inversion effect appears, i.e. the eigenstates and energies of the impurity states change with the inversion of the direction of the electric field. Using the weak impurity potential approximation analytical expressions for the inversion shift of the energy levels are obtained. The results for the shift of the ground level for GaAs well are obtained in closed form. By measurement of the inversion energy shift it will be possible to determine the position of the impurity centre relative to the boundary of the layer.
Rhombic Fe3+ centers in KTaO3
Authors: Pechenyi A. P. , Glinchuk M. D. , Antimirova T. V. , Kleeman W.
Published: July 2007

The EPR spectrum of rhombic Fe3+ paramagnetic centers in KTaO3 both nominally pure and with Li and Nb impurities is investigated. The absolute values and signs of the parameters of the spin‐Hamiltonian describing the observed EPR spectrum are determined on the basis of the angular and temperature dependences of the mentioned spectrum. The model of a rhombic iron center involving the Fe3+ ion substituted for a Ta5+ ion with a nearby interstitial bivalent positive ion in [110] direction for charge compensation is suggested.
Magnetopolaron in a rectangular quantum wire under a strong magnetic field
Authors: Lü Tian‐Quan , Gu Shi‐Wei
Published: July 2007

Considering the interaction of an electron with the confined LO‐phonons, the ground state energy shift of a magnetopolaron in a rectangular quantum wire is calculated under a strong magnetic field using the effective mass approximation and the variational method. It is shown that when the external magnetic field keeps constant, the absolute value of the ground state energy shift of a magnetopolaron in a rectangular quantum wire as a function of the cross section of the wire increases monotonically with the increase of the size in one direction remaining constant in the other direction and increases very slowly when the size of the wire is very large. At constant cross section of the quantum wire the absolute value of the ground state energy shift increases approximately linarly with the increase of the external magnetic field.
Soliton excitations in the classical one‐dimensional antiferromagnet in a staggered field
Authors: Cassino F. S. L. , Pires A. S. T.
Published: July 2007

Soliton excitations in an easy‐plane classical one‐dimensional antiferromagnet in the presence of a staggered magnetic field transverse to the chain direction are studied. The out‐of‐plane soliton solution is found to have an instability similar to the one found before by Magyari‐Thomas‐Kumar for the easy‐plane ferromagnetic chain in an external magnetic field. Also studied are the soliton stability and the phase shift of spin waves in the presence of the soliton.
Hole Distribution in YBCO Superconductor Ionic Model Studies
Authors: Gong X. G. , Pan Wei , Zheng Q. Q.
Published: July 2007

The total energy is approximated in an ionic model. By applying the obtained total energy formula to the YBa2Cu4O8 superconductor, the possible hole distributions are investigated. The results show that the hole is predominantly on the apical oxygen, only about 0.06 holes per CuO2 plane, which is in agreement with photoemission experiments indicating that the hole has oxygen character.
EPR Studies of Restricted Motion of NH3+ Radicals in X‐Irradiated ADA and Glassy RADA
Authors: Waplak S. , Trybula Z. , Drumheller J. E. , Schmidt V. H.
Published: July 2007

The electron paramagnetic resonance (EPR) spectra of NH3+ radicals in NH4H2AsO4 (ADA)and mixed Rb1−x(NH4)xH2AsO4 (RADA) (x = 0.35) in the temperature region 4.2 to 300 K are measured. It is shown that in ADA at room temperature (RT) NH3+ molecules undergo fast rotation around the principal axes of the crystal field tensor, and make rapid jumps between them. These motions provide nearly isotropic averaging of the spectra. Below TN = 216 K rapid rotation only around the a‐ and b‐axes is present. In contrast to ADA, in RADA the NH3+ groups rotate rapidly only around the a‐ and b‐axes even at RT and inversion tunneling of nitrogen is suggested. The hyperfine structure of the ammonia proton is isotropically averaged down to 115 K in ADA and down to 60 K in RADA.
Optical Absorption of the Fd Center in KCl: Ca2+
Authors: Cárdenas‐García D. , López‐Téllez E. R. , Ruíz‐Mejía C.
Published: July 2007

The optical absorption of the Fd center in KCl: Ca2+ is studied by using point‐ion and ion‐size correction models. For the calculations the Gourary and Adrian wave function types I, II, III and Gaussian functions are used. The principal axes are obtained for several positions of the impurity‐vacancy dipoles, and the corresponding ground and excited states of the Fd center are calculated. Good agreement with the experimental values is found for the Gourary and Adrian wave functions of type I.
Absorption, Emission, and Thermoluminescence of Copper Doped Alkali Sulphate Phosphors
Authors: Kishan Kumar V. S. , Acharyulu B. S. V. S. R. , Sastry S. B. S.
Published: July 2007

Results of optical absorption (OA), fluorescence, and thermoluminescence (TL) studies made on copper doped sulphates of cesium, rubidium, and potassium are reported and discussed. Optical absorption spectra revealed that the impurity enters these lattices in both its mono‐ and divalent states. Fluorescent emission of Cu+ is detected in the three phosphors. Both Cu+ and Cu2+ are found to be acting as electron traps when the samples are exposed to γ‐rays. Absorption due to Cu0 thus formed is detected around 3.45 eV in the irradiated samples. TL glow curves show a three‐peak pattern and are assigned to the thermal mobilization of hole centres formed during irradiation. Unlike in alkali halides, the TL emission spectra do not always contain the characteristic Cu+ emission band.
Heavy Hole—Light Hole Splitting in GaAs/AlAs Superlattices
Authors: Munzar D.
Published: July 2007

Eppenga's six‐band k · p model for the description of the valence band structure of GaAs/AlAs quantum wells is modified for the k⟂ = 0 states in GaAs/AlAs superlattices. The model takes into account the coupling between the light holes and the spin—orbit split‐off holes and yields a very good agreement between experimental and calculated values of the splitting between the first heavy and light hole energy levels, which is not obtained in the simple effective mass approximation.
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

Transmission 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

The 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

The 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

In 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

The 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

The 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

The 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

A 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

The 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

The 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

The 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

The 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

The 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

One‐ 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

Using 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

The 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

The 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

Different 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

Exact 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

The 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

The 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

Valence 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

Cathodoluminescence (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

The 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

This 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

A 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

The 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

Spectroscopic 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

The 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

Photoemission 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.
Exciton States of Lithium Hydride in LiH1−xFx Solid Solutions
Authors: Polienko A. N. , Tabachnik M. E. , Cholakh S. O. , O'Connel‐bronin A. A.
Published: July 2007

Exciton states of LiH1−xFx in the exciton band of lithium hydride are investigated by optical methods. It was found that spectra of the lowest‐energy exciton band of these crystals follow the persistent type of behaviour. The relation of exciton energy to solution composition points to the effect of symmetry changes of the conduction band bottom and average sizes of lithium hydride clusters. The structure of the luminescence spectra is determined by direct, phonon‐forbidden indirect, or disorder‐assisted quasidirect transitions with increasing fluorine content.
On the Efficiency of Different Schemes for the Evaluation of the Density of States and Related Properties in Solids
Authors: Bross H.
Published: July 2007

It is shown that similar to the tetrahedron method the method originally proposed by Gilat and Raubenheimer can be used for all kinds of Bravais lattices to evaluate the density of states and related properties. For a great variety of such properties analytical expressions well suited for numerical computations are given. Based on explicit calculations in the case of Cu it is shown that the improved Gilat‐Raubenheimer method needs by a factor of 2 less CPU time than the tetrahedron method in order to obtain the density of states functions with the same accuracy.
Energy Levels in Quantum Wires with Finite Barrier Potential
Authors: Nag B. R. , Gangopadhyay Samita
Published: July 2007

Formulae are derived for the calculation of the energy eigenvalues in cylindrical and elliptic quantum wires with finite potential barriers. Calculated values are given for quantum wires of the Ga0.47 In0.53As/InP system and experimental results are explained.
On the Photoconductivity in Hydrogenated Amorphous Silicon
Authors: Licea I. , Tomozeiu N.
Published: July 2007

Some experimental data about the photoconductivity of n‐type a‐Si: H samples obtained by glow discharge technics are presented. The photoconductivity and the shape of the lux ampere characteristics presenting two straight lines with variable slopes are explained in terms of electron trapping and recombination kinetics. A comparison between the experimental data and the corresponding expressions which result from the considered model is made.
Magnetic Properties and Structure of (VxAl1−x)2O3 Solid Solutions
Authors: Viktorov V. V.
Published: July 2007

On the basis of the magnetic properties the statistical homogeneity of the solid solutions in the Al2O3‐V2O3 system is investigated. It is shown that the V3+‐ion distribution in the ( VxAll−x)2O3 lattice in the range of vanadium content 0.0096 ≦ x ≦ 0.195 does not obey statistical regularities. Clusters of two and three exchange‐interacting V3 + ions are observed in the structure of the (VxAl1−x)2O3 solid solutions.
Optical Study of Valence Band Splitting in Weakly Strained In1−xGaxAsyP1−y/InP Heterostructures
Authors: Weihofen R. , Weiser G.
Published: July 2007

Valence band splitting resulting from strain ε ≈ 10−3 or less is observed in electroabsorption spectra of quaternary LPE material grown on (001) InP. Although exciton lines are not resolved their contribution leads to narrow, near‐gap features in electroabsorption which provides the necessary spectral resolution. The variation of lineshape with dc bias, temperature, and strain is studied in detail. Small fields and low temperature are required to resolve splitting by a few meV. Even if linewidth broadening prevents resolution of valence band splitting, strain is evident by characteristic distortion of the spectral lineshape, different for compressive and tensile strain. Small splitting is resolved in case of tensile strain but not for compressive strain. This difference is attributed to stronger coupling of the valence bands for compressive strain where calculations show anti‐crossing for dispersion in the layer plane.
Investigation of True Ni2+–Halogen Distances in Alkali Halide Lattices
Authors: Ma Xuanrong , Du Maolu
Published: July 2007

Using the μkα model of Ni2+−6X− clusters the true distances between impurity Ni2+ cation and halogen anion in alkali halide lattices are estimated from the optical spectral data. The results suggest that the true impurity cation‐ligand anion distances are close to the sum of ionic radii of the impurity cation and anion but not the cation‐anion distance of the perfect lattices in the case that the ionic radius of host cation is larger than that of impurity cation.
Reflectivity of GaAs Thin Films
Authors: Bassani F. , Chen Y. , Czajkowski G. , Tredicucci A.
Published: July 2007

An accurate study of the optical properties of GaAs thin layers in the excitonic absorption region, obtained with a real‐space density‐matrix approach adapted to the case of thin films with degenerate valence bands is reported. The computed polarization decreases gradually to zero near the surfaces in a transition layer whose thickness decreases with increasing photon energy. The oscillatory behaviour of the reflectivity is well reproduced and can be interpreted in terms of polaritonic effects by relating the dispersion of the propagating modes to the calculated spectra. The effect of the confining material on the lineshapes and on the intensity of the peaks is also explained.
A Theoretical Study of Si Chemisorption on Ru(001) Surface
Authors: Wei Pan , Gong X. G. , Qi Zheng Qing
Published: July 2007

The embedded cluster model is used to study the electronic structure of Si chemisorption on Ru(001) surface, within the framework of local‐density theory regime. The Kohn‐Sham equation is solved self‐consistently using the discrete variational method. The most possible adsorbed site of Si is obtained by calculating the adhesive energy for two positions. The distributions of Si 3s, Si 3p, and Ru 4d orbitals are clearly identified by our calculations. The interaction between Si adatom and Ru(001) surface and its influence on the d‐d interatomic interaction of Ru atoms are also discussed.
Impurity Effects on the Local Density of States of an Hexagonal Husumi Cactus
Authors: Glass B. , Lazo E.
Published: July 2007

The electronic structure of a hexagonal Husumi cactus with a finite number of substitutional impurities in essentially two different cases is studied: (i) one impure hexagon in a pure Husumi cactus system and (ii) a Husumi cactus formed by infinite repetition of a contaminated hexagon (heterocactus). The tight‐binding Hamiltonian is used considering interaction to nearest neighbors and one orbital per site, with diagonal and off‐diagonal disorder. The Dyson equation describing the electron dynamics is solved using two types of effective transfer matrices. Localized states appear in the electronic gap for the heterocactus formed by infinite repetition of a hexagon with two impurities on sites 1. All genuine heterocactuses generate a gap, but only one has a localized state in between as a function of the normalized self‐energy ϵ.
Power Losses of a Type‐II Superconducting Slab of Arbitrary Thickness
Authors: Saif A. G.
Published: July 2007

The distribution of the internal and external magnetic vector potentials of systems of a type‐II superconducting infinite slab of arbitrary thickness subjected to (i) a vibrating wire (between two vibrating wires) carrying dc currents, (ii) a normal conducting wire (between two wires) carrying ac currents, are obtained. A modified London equation is developed to study these systems in case the slab is moving. The power losses on the surface of the slab and its two constituent terms due to dissipated and reactive energies are calculated. The rcactive term is found to be either capacitivel or inductively reactive. It is found that the power losses are dependent on frequency, London penetration depth, permeability, conductivity, velocity, and the distance between the normal conductors and the surfaces of the superconducting slab. Moreover, the power losses decrease as the distance between the normal conductors and the surfaces of the superconducting slab decreases, and increase as frequency, London penetration depth, permeability, conductivity, velocity increase. These analytical solutions can be used for testing the superconducting slabs in motion and the related problem of the losses of moving superconductors. In the limiting case, a satisfactory agreement with previous results is achieved.