bluuebunny/crossref_metadata_2025_split huggingface dataset:
Aggregation Processes in the NaCl:Eu2+, Mn2+ System and Energy Transfer Mechanisms
Authors: Cywiński R. , Fava R. , Manfredi M. , Mugeński E. 
Published: July 2007

AbstractEnergy transfer processes are studied in NaCl:Eu2+, Mn2+ samples, along the annealing treatments that are performed at RT and at 100 °C; the Eu ions and/or Eu aggregates act as donors and Mn ions and/or Mn aggregates act as acceptors. For the samples annealed at RT, a single pair transfer mechanism is evidenced at the early annealing stages, while at late annealing stages a fast energy migration in the donor system followed by a phonon assisted transfer is observed. For the annealing at 100 °C at late aggregation stages the most probable transfer process seems to be connected to a slow energy diffusion in the donor system followed by the transfer to the acceptors. The Mn2+ emission band, that is detectable as a consequence of the EuMn transfer shows a complex behaviour.
Kinetic Theory of the Hydrogen Diffusion in Metals
Authors: Fujita S. 
Published: July 2007

AbstractThe diffusion coefficient D for hydrogen in a metal normally obeys the Arrhenius law: D = D0 exp (– Ea/kBT), where Ea is the activation energy, which represents the barrier potential for the most favorable interstitial channel. The pre‐exponential factor D0 can be related to the mean straight path l in the crystal by Do = (1/3) gvl, where v is the average migration speed and g a statistical weight of order one. The mean straight path l for a pure crystal can be very long, since the migrating atoms cannot readily be scattered or stopped by the (phonon) motion of the regular lattice. This physical picture, reasonable but not widely recognized, is supported by experimental evidence and theoretical considerations. In particular, the distinct isotope effects for hydrogen diffusion in f.c.c. and b.c.c. metals are explained in terms of quantum mechanical zero‐point energies associated with the crystal channel potentials. A few experimental tests for the theory are suggested.
Influence of Many‐Body Effects on the Cyclotron Resonance Mass of Two‐Dimensional Polarons with Application to GaAsAl1−xGaxAs Heterostructures
Authors: Wu Xiaoguang , Peeters F. M. , Devreese J. T. 
Published: July 2007

AbstractThe influence of many‐body effects on the cyclotron resonance mass of two‐dimensional polarons is investigated. Firstly, the occupation effect is considered, which arises from the Fermi‐Dirac statistics. In the calculation the Landau levels are considered to be perfectly sharp. It is found that (1) the occupation effect reduces the polaron effect, i.e. the cyclotron resonance mass renormalization due to the polaron effect is reduced over the whole magnetic field region, (2) for a fixed electron density the cyclotron resonance mass shows an oscillating behaviour as a function of the magnetic field. The polaron cyclotron resonance mass attains minimal values when the filling factor passes through integer values. Secondly, the effect of static screening on the polaron cyclotron resonance mass is examined. The electron screening is found to reduce further the cyclotron resonance mass renormalization. The effect of the finite width of the two‐dimensional electron layer is included into the present calculations by considering the lowest subband. After taking into account the nonparabolicity of the electron energy band, the calculated cyclotron resonance mass is compared to the recent experimental data for GaAsAl1−xGaxAs heterostructures. Good agreement is found in the high magnetic field region, i.e. for magnetic fields H > 12 T.
A Tight‐Binding Description of Isovalent Impurity Clusters. Application to Te Impurities in II‐VI Compound Semiconductors
Authors: Hanke M. , Hennig D. , Kaschte A. 
Published: July 2007

AbstractFor centres of several Te substitutionals in nearest‐neighbour anion position in the host crystals ZnS, ZnSe, CdS, and CdSe a tight‐binding description is given. The matrix of the Hamiltonian in the sp3‐representation includes all interactions up to the second‐nearest neighbours and is based upon a charge self‐consistent parametrization. All Te centres suitable in a tetrahedrally coordinated five‐atom cluster are considered. Using the Koster‐Slater scattering theory the off‐diagonal matrix elements of the impurity potential have to be included. Thereby the results of the calculation are in a good agreement with experimental data for the energy levels of complex Te impurities in II‐VI semiconductors.
Tunnelling in k‐Space. The Case of Time‐Varying Electric Field
Authors: Dargys A. 
Published: July 2007

AbstractIntervalence transitions between light and heavy mass subbands due to strong time‐varying electric fields are analyzed within the effective mass framework in two limiting cases, when the spin‐orbit interaction is weak and strong. Explicit formulae for the resonance lineshape, lineshift, and frequency of quantum mechanical Rabi oscillations are presented for one‐ and two‐photon absorption. The relationship between the detuning and the amplitude of Rabi oscillations is found to have exactly the same form, as in two‐level atomic systems. In case of p‐type germanium the Rabi oscillation period becomes comparable to the optical phonon scattering time at an intensity of the CO2‐laser of 5 MW/cm2. The conditions for the observation of Rabi oscillations are especially favourable at small energies of light quanta, when optical phonon emission due to energy conservation is forbidden. By this reason in p‐type semiconductors at strong time‐varying electric fields the non‐linear quantum effects similar to those found in two‐level atomic systems e.g. photon echos, self‐induced transparency etc., are expected to be observable as well.
Soliton Mechanism of Superconductivity in Organic Quasi‐One‐Dimensional Crystals
Authors: Brizhik L. S. , Davydov A. S. 
Published: July 2007

AbstractThe soliton model is used to develop a hypothesis on superconductivity in organic quasi‐onedimensional crystals formed by stacks of plane molecules. The pairing of solitons and the dynamics of motion of many solitons are investigated.
On Continuum Mechanics and Special Relativity
Authors: Günther H. 
Published: July 2007

AbstractIt is shown that the quasi elastic model of Maxwell's equations according to MacCullagh‐Sommerfeld can be defined by a non‐covariant gauge for the four potential Aμ. The quasi elastic displacement vector s then is described by the vector potential A. Lorentz invariance of the mechanical model depends on the gauge invariance of the physically measurable quantities. A theoretical relation to relativistic kink equation can be seen.
Metallic Glasses and Chiral Nematic Liquid Crystals
Authors: Trebin H.‐R. , Longa L. , Salzgeber B. 
Published: July 2007

AbstractGeometrical frustration exists in both metallic glasses and chiral nematic liquid crystals. Applying results from the Landau‐Ginzburg theory of liquid crystals and from polytope models for the amorphous state, a frustrated free‐energy density for metallic glasses is derived. It is expressed in terms of an order parameter that characterizes the bond orientation of neighbouring atoms.
A Comparison of Electronic Properties of Clusters and Bulk of Copper and Palladium. Bandwidth, Density of States, Deformation Density
Authors: Fritsche H.‐G. 
Published: July 2007

AbstractF.c.c. clusters Cu14 and Pd14 are investigated by use of the scattered‐wave‐Xα method. Electronic properties of valence electrons (bandwidth, density of states, deformation density) are compared with that ones of the bulk in order to study their dependence on the periodicity of lattice.
On the Theory of Photoemission from Adsorbate‐Induced Surface States
Authors: Beckmann A. 
Published: July 2007

AbstractOn the basis of a tight‐binding model, the photoemission from adsorbate‐covered transition metal surfaces is investigated. The photoemission current from the bulk states and the adsorbate‐induced surface states is deduced from both poles of the excitation function. The conditions for the existence of surface states for a given overlayer structure are given and compared with the case of a 1 × 1 structure. The connection between the peak widths and the various lifetimes is elucidated.
Diffusion Mechanism in F.C.C. Metals
Authors: Mundy J. N. 
Published: July 2007

AbstractCurvature in the Arrhenius plot of self‐diffusion in metals is commonly interpreted as resulting from a monovacancy jump process at low temperatures with an increasing contribution from a divacancy jump process at higher temperatures. Recently it has been suggested that the Arrhenius curvature in b.c.c. metals could be attributed to monovacancies with temperature‐dependent activation enthalpies. The grounds why the same model could not be applied to f.c.c. metals are examined with particular emphasis on the diffusion and defect properties of aluminum.
Stress Induced Anisotropic Diffusion of Intrinsic Point Defects towards Dislocations in H.C.P. Crystals
Authors: y‐De Grande N. Smetniansk , Savino E. J. , Tomé C. N. 
Published: July 2007

AbstractThe diffusion equations for vacancies and interstitials in the field of straight edge dislocations are numerically solved for a crystal under irradiation and uniaxially stress. The main dislocation systems in hexagonal closed packed metals are considered. The diffusion model takes into account the full lattice and defect symmetry in the equilibrium and migration configurations. A diffusivity tensor for strained h.c.p. metals is deduced, which depends linearly on the strain field. This tensor allows to solve analytically the steady state defect flux towards a hollow cylinder in a homogeneously strained crystal. The resulting vacancy and interstitial absorption (sink strength) are calculated and compared with the ones obtained numerically when the dislocation field is taken into account. It is shown that both approaches result in a similar dependence of the sink strengths on the homogeneous strain as far as a correct capture radius is adopted for the hollow cylinder. The dependence of the results on the boundary conditions and the defect symmetry considered is discussed. For the numerical calculations defect symmetries consistent with those calculated for magnesium are used.
Investigation of the Resonant Vibration Modes of Radiation Induced Interstitial Atoms in Aluminum by Diffuse Inelastic Neutron Scattering
Authors: Urban R. , Ehrhart P. , Schilling W. , Schober H. R. , Lauter H. 
Published: July 2007

AbstractDiffuse inelastic neutron scattering measurements are reported which are used, for the first time, to study the dynamics of radiation‐induced self interstitial atoms. Large single crystalline aluminum samples are electron‐irradiated in a specially built facility at T ≦ 20 K to defect concentrations of up to 500 at ppm and subsequently transported in the cold state to the high flux reactor at Grenoble. The measurements are done at T = 6 K on the triple axis spectrometer IN3 and are repeated after annealing at T = 46 K (above stage I) and at T ≈ 300 K (after complete recovery). The irradiated samples show a significant increase of the inelastic scattering intensity at transferred energies between 0.7 and 2 THz. By comparison with the results of computer calculations the different frequencies of the energy loss spectrum can be attributed to the different vibration modes of interstitial atoms. The frequencies of the most significant modes of the self interstitial atom are determined to be ν = 0.75 THz and ν = 1.35 THz for the translation‐(A2u)‐and libration‐(Eg)‐mode, respectively.
H2 Dissociation in Nickel. A Cluster Model
Authors: Vargas P. , Kronmüller H. , Böhm M. C. 
Published: July 2007

AbstractA cluster formed by 38 Ni atoms and two H atoms is studied by a semiempirical Hartree‐Fock (HF) self‐consistent‐field (SCF) variant based on the INDO (intermediate neglect of differential overlap) approximation. The energy of the system is analyzed as a function of the spatial localization of the two H atoms. The energy minimum in the considered part of the singlet‐type potential surface corresponds to a spatial arrangement where the H atoms occupy nearest‐neighbor octahedral sites in the f.c.c. Ni‐lattice (d(HH) = 0.249 nm). This configuration is ≈ 1 eV below another local minimum which is associated to a “H2” molecule (d(HH) = 0.085 nm) in the host system. The H atoms in the Ni‐lattice can be characterized as overscreened protons.
Muon‐Spin Resonance in Metals. RF‐Field Penetration, Diffusion, and Trapping Effects
Authors: Brandt E. H. , Messer R. 
Published: July 2007

AbstractThe longitudinal spin polarization in a μ+‐spin‐resonance experiment is calculated for positive muons diffusing to traps in a metal foil of finite thickness. General expressions are given for the shape of the resonance lines of freely moving and of trapped muons as a function of foil thickness, rf amplitude, trapping probability, and of the local magnetic fields and relaxation rates Γ1, Γ2 of the free and trapped muons.
Computer Modeling of Relaxation Peaks Involving a Gaussian Distribution of Activation Energies
Authors: Berry B. S. , Pritchet W. C. 
Published: July 2007

AbstractTo extend previous work in the frequency domain, as a function of temperature, the forms of anelastic or dielectric loss peaks are computed directly involving a Gaussian distribution of activation energies. The calculations provide the height, width, asymmetry, and frequency‐shift behavior of the loss peak as a function of a dimensionless parameter defining the width of the distribution. The calculations are used to determine how well the input distribution can be recovered or deconvoluted from a set of peaks, obtained for several different frequencies, by a simple empirical procedure termed a cross‐cut analysis. The procedure is found to work well for peaks whose width is more than twice the Debye width.
The Influence of Magnetic Fields on the Thermal Conductivity of the Cooperative Vibronic System Tm VO4
Authors: Mutscheller W. , Wagner M. 
Published: July 2007

AbstractThe influence of magnetic fields on the thermal conductivity of Tm VO4 is investigated theoretically. A method developed in an earlier paper is used in complete analogy to the case without magnetic field. The results are in a good agreement with the experimental findings.
Phase Transformation in F.C.C. Metals and Their Stability on the Path of Transformation
Authors: Thakur K. P. , Ahmad S. 
Published: July 2007

AbstractCalculations are carried out to locate the stress‐free b.c.c. phase in four f.c.c. metals (Ni, Cu, Al, and Pd). Internal energies corresponding to the unstressed b.c.c. and f.c.c. phases, and the required stresses and energy changes for the f.c.c. → b.c.c. transformation in these crystals are presented. Ranges of G and S stabilities are examined according to the Born stability criterion, by calculating the values of corresponding elastic moduli on the line of deformation connecting the stress‐free b.c.c. and stress‐free f.c.c. phases of the crystal. The Morse potential model is used to carry out the calculations.
Scattering Properties of Non‐Spherical Muffin‐Tin Potentials of Surface Atoms
Authors: Rennert P. , Seifert F. 
Published: July 2007

AbstractThe calculation of the generalized phase shifts and of the partial wave amplitudes is described. An expansion into lattice harmonics is preferred to include the simplifications due to the symmetry in a direct way. The non‐spherical potential components are considered for a Ni atom in a surface layer of an (111) surface. Results are presented for an electron energy of 4 Ryd. Even at this relatively high energy remarkable deviations from the spherical case can be observed. It is discussed in detail, how the scattering amplitude depends both, on the direction of the incoming wave and on the azimuthal angle, that means cases are considered, in which the scattering amplitude in the spherical case has always the same value.
Exchange‐Scattering Effects on Band Energies in Magnetically Mixed Semiconductors
Authors: Ryabchenko S. M. , Semenov Yu. G. , Terletskii O. V. 
Published: July 2007

AbstractChanges of the band energies of a free electron, hole, and exciton due to virtual processes of exchange scattering of these particles on the localized spin moments in magnetically mixed semiconductors with the cubic lattice are considered. The result is presented as an effective spin‐Hamiltonian governing the effects of (1) the fluctuation paramagnetic energy shift, (2) its partial compensation in the antiferromagnetic coupling within a system of magnetic ions, (3) renormalization of the exchange coupling constants in describing the giant spin splitting, (4) the non‐Heisenberg correction to the spin interaction between a hole and magnetic ions, and (5) the indirect electron‐hole exchange interaction for excitons. Numerical estimates of these effects are obtained for Cd1−xMnxTe. The results are compared with published experimental data; to verify one of the theoretical conclusions, the asymmetry of the spin splittings in the exciton spectra, a special experiment is conducted for Cd1−xMnxTe which confirms the theoretical predictions.
Electrical Conduction in Amorphous Cu10As40Se50 with Potential Fluctuations
Authors: Abdalla S. , Dongol M. , Ibrahim M. M. 
Published: July 2007

AbstractHall measurements of amorphous and annealed Cu10As40Se50 samples are performed to evaluate the carrier density and to evaluate the mobility in the temperature range 300 to 580 K. It is found that the free hole density pf, the Fermi level, and the mobility is a function of the annealing processes. The data are interpreted using the potential fluctuation theory. Differential thermal analysis (DTA) is performed. The endothermic changes of DTA have been attributed, in fact, to the shrinkage of the maximum height of the potential hills with temperature; while the exothermic changes have been regarded as a reason of the reduction of the disorder energy with annealing.
Liquid Site Model and Self‐Consistent Potentials in the Theory of Quantum Hall Effect
Authors: Ziep O. , Keiper R. 
Published: July 2007

AbstractTo explain the quantum Hall effect (QHE) as a macroscopic quantum phenomenon with interference character the liquid site model is discussed in more detail. The calculated thermodynamic potential containing the magnetic field energy shows minima near the filling factors v = p/(2n + 1) (p = 1, 2, …; n = 0, 1, 2, …). Electron correlation in strong magnetic fields leads to a local Hall potential satisfying the two‐dimensional Laplace equation. The liquid site model permits to discuss the necessity for an optimum degree of disorder as well as the high precision attainable in QHE measurements.
Magnetic Susceptibility of a Two‐Dimensional Electron Gas in the Strong Magnetic Field Limit and for Non‐Zero Temperatures
Authors: Wang L. , O'Connell R. F. 
Published: July 2007

AbstractAnalytic expressions are derived for the magnetic susceptibility and the Fermi energy of a non‐interacting two‐dimensional electron gas in the strong field limit and for non‐zero temperatures. The results at zero temperature agree with existing calculations but the results at finite temperature are different from those obtained recently by other authors.
General Principles of Spin‐Echo Formation in Solids
Authors: Whitaker M. A. B. 
Published: July 2007

AbstractThe mechanism of formation of spin‐echoes in solids is discussed, particularly in connection with the height and position of the main echo peak, and possible formation of subsidiary peaks. For values of Γ in the main part of the free‐induction decay, the value of Γ′ for the maximum will be less than τ. The value of τ′max — τ is connected with the slope of 〈Ix〉 (τ, τ). The method of time‐expansion should be regarded as descriptive, not prescriptive. Simple echo responses are analysed, and the results of a calculation for a simple cubic lattice of spins‐½‐ are discussed.
Infrared Investigation of Phase Transitions in CsHSO4
Authors: Döbler U. , Happ H. 
Published: July 2007

AbstractThe IR spectrum of CsHSO4(CHS) single crystals is investigated in reflection between 15 and 1500 cm−1 supplemented by absorption measurements on CHS‐KBr pellets between 2000 and 4000 cm−1, the region of OH stretching vibrations. The measurements extend from the room temperature phase up to the phase of superionic conduction above 412 K. The room temperature spectra are consistent with a structure of space group C(P21) containing HSO groups as molecular units allowing ferroelectric polarization along the crystal b‐axis. Additional dielectric measurements show that below the first known phase transition at about 360 K no further dielectric anomaly exists. The spectra in the superionic phase indicate rapid reorientational motion of SO groups and protons as mobile particles.
Magnetotransport Properties and Subband Structure of the Two‐Dimensional Electron Gas in the Inversion Layer of InSb Bicrystals under Hydrostatic Pressure
Authors: Herrmann R. , Kraak W. , Handschack S. , Schurig Th. , Kusnick D. , Schnackenburg B. 
Published: July 2007

AbstractMagnetotransport properties of the n‐inversion layer formed at the grain boundary in p‐InSb bicrystals are investigated. Hydrostatic pressure up to 103 MPa is used to characterize the properties of the two‐dimensional electron gas in the inversion layer. At atmospheric pressure in addition to previous results four series of quantum oscillations are revealed, indicating that four electric subbands are occupied. From quantum oscillations of the magnetoresistivity ϱxx the characteristic parameters of the electric subbands (subband population nsi, subband energies EF — Ei) and their pressure dependences are established. A strong decrease of the carrier concentration in the inversion layer and of the corresponding subband population is observed when pressure is applied. A simple theoretical model based on the triangular‐well approximation and taking into account the pressure dependence of the energy band structure of pure InSb is used to calculate the energy band diagram of the quantum well and the pressure dependence of the subband parameters.
On the Charge Structure of Polarons in Trans‐Polyacetylene
Authors: Heiner E. , Drechsler S. L. , Malek J. 
Published: July 2007

AbstractThe charge structure of polarons (centered at long bonds) and antipolarons (centered at short bonds) is investigated on rings with N = 4 M sites in the SSH‐model and in the continuum model. In contrast with previous statements the charge distribution has to be described by two smooth penetrating functions. They can be reasonable well obtained within the continuum theory.
Electrical Conductivity Anisotropy in Tin‐Doped n‐Type Indium Selenide
Authors: Pomer F. , Bonet X. , Segura A. , Chevy A. 
Published: July 2007

AbstractElectrical conductivity anisotropy of n‐type indium selenide samples doped with different concentrations of tin is studied between 30 and 300 K. The anisotropy ratio is found to depend exponentially on the inverse temperature, the activation energy E, and the pre‐exponential factor varying with free carrier concentration. This behaviour is explained by the existence of potential wells created by planar aggregates of ionized donor impurities screened by free electrons. A simple model in which the depth of these potential wells is calculated as a function of temperature, charge density in the planar defects, and electron concentration, gives account of the measured activation energies, but does not explain the high value of the anisotropy ratio. The existence of strictly two‐dimensional electrons bound to planar defects not contributing to the electrical conduction across the layers is proposed to explain this result.