bluuebunny/crossref_metadata_2025_split huggingface dataset:
Anomalous Field Dependence of the Hole Mobility in a Molecular Doped Polymer
Authors: Novo M. , van der Auweraer M. , de Schryver F. C. , Borsenberger P. , Bässler H. 
Published: July 2007

AbstractPhotocurrent transients are determined in polycarbonate doped with 17 wt% of p‐p‐EFTP (see structure below) as a function of the electric field strength and the temperature. The hole mobilities are obtained both by a graphical method and by computational analysis based on an analytical expression for the transient photocurrent which has been recently proposed by Scott et al. The comparison of the results obtained with the two methods shows that, although the absolute values of the mobilities differ by a factor of 1.5, the field and temperature dependencies of the hole mobility are similar. The system studied shows an anomalous field dependence of the mobility which at low fields increases with increasing field to a maximum above which it decreases with increasing field. Such a behaviour is analysed in the framework of two different theoretical models: one based on the Marcus theory of electron‐transfer processes and Bässler's model which takes into account the fluctuations in hopping sites due to the simultancous presence of diagonal and off‐diagonal disorder. While the first model does not provide a satisfactory description of the experimental data, the latter cannot explain qualitatively the observed field dependence over a wide range of applied fields. The predicted saturation of the drift velocity of the holes at high fields does not agree with the observed temperature dependence of the mobility high fields. This disagreement with the conventional models is probably due to the large dimensions of the molecules allowing a change in the free energy for nearest‐neighbour hops which is large compared to the polaron binding energy or the width of the distribution of the density of states.
Magnetic Properties of New Layered Heisenberg Antiferromagnets
Authors: Du A. , Wei G. Z. 
Published: July 2007

AbstractThe magnetic properties of layered Heisenberg antiferromagnets with antiferromagnetic interaction between spins within a layer and ferromagnetic interaction between layers are studied by using a spin‐wave theory and double time‐temperature spin Green's function theory. At low temperatures a characteristic temperature is defined reflecting the layered features of the system and the low‐temperature region is divided into two new ones, giving asymptotic expressions of sublattice magnetization and parallel suseeptibility in different temperature region, including the low‐temperature region. the vicinity of the Néel temperature, and the high‐temperature region. Also given is the Néel temperature in an asymptotic form when the interlayer coupling is weak. It is shown that the characteristies of dimensional cross‐over for physical quantities exists at low temperatures. The interlayer coupling is essential to keep long‐range ordering and the properties of this king of antiferromagnets are very similar to those of normal antiferromagnets.
Stability Studies of the AlLiCu Icosahedral Quasicrystal by Means of the Embedded‐Atom Method
Authors: Nie Xiliang , Wang Renhui , Ye Yiying 
Published: July 2007

AbstractTotal energies of the T2‐Al6Li3 Cu icosahedral quasicrystal and its crystalline approximant are calculated by means of the embedded‐atom method and are compared with the fictitious AlLiCu solid solutions and mixtures. For such a calculation, the real atomic structure of the icosahedral T2‐Al6Li3 Cu phase is obtained by the projection method and special cubic spline knots for the embedding function FLi(ϱ) are selected to overcome the difficulty that the local electron density in Al‐Li‐Cu alloys is much larger than that in the pure Li crystal. The results show that both quasicrystalline T2‐Al6Li3 Cu and crystalline R‐Al5Li3 Cu are stable phases compared with the fictitious corresponding solid solutions and mixtures of pure Al, Li, and Cu crystals. These results are consistent with the experiments within the theoretical error.
Empirical Tight‐Binding Total Electronic Energy Calculation for SinH2m (n = 1 to 6, m = 1 to 3) Clusters
Authors: Katircioḡlu S. , Erkoç S. 
Published: July 2007

AbstractThe stable structures of SinH2m (n = 1 to 6, m = 1 to 3) clusters are investigated by the total electronic energy calculation using an empirical tight‐binding (ETB) method. It seems that bridged H‐bond models are also possible for small sinHm clusters.
Long‐Wavelength Limit of the Structure Factor of Liquid Metals from the Charged Hard‐Sphere Reference System
Authors: Akinlade O. 
Published: July 2007

AbstractA detailed study of the long‐wavelength limit S(0) of the structure factor of alkali metals and some polyvalent metals at their freezing points using the charged hard‐sphere reference system is made. In order to investigate the role of refinements in the description of the electron‐ion interaction, use is made of both the Ashcroft empty‐core model (ECM) potential and a non‐local pseudopotential which incorporates high‐order perturbation corrections. The results obtained from the random‐phase approximation (RPA) are compared with those using the mean‐density approximation (MDA). Our calculations indicate that to obtain an accurate description of S(0) within the present framework, a good method of dealing with the electron‐ion pseudopotential is essential. It is also observed that the MDA correction in most cases does not lead to a remarkable improvement on those obtained from the RPA.
Static Lattice Defects in a Quasi‐Continuum
Authors: Vörös G. , Kovács I. 
Published: July 2007

AbstractA new method for the description of the properties of lattice defects in a quasi‐continuum with long‐range elastic interactions is presented. The method is based on the application of the Fourier transformation for the solution of the equation of classical elasticity and takes into account the fact that the wavelength of the elastic waves in a real material cannot be arbitrarily small. The method is applied for the description of the properties of a dilatation centre and a screw dislocation.
On the Hot Phonon Spot Dynamics in Thin Films
Authors: Agaev F. F. , Guseinov N. M. , Rustamov K. A. 
Published: July 2007

AbstractThe hot phonon spot (HPS) formation and development in thin semiconductor films as a result of photoexcited charge‐carrier cooling is discussed. The film is assumed to be thin enough so that the thickness d follows d/v ≪ τ1, (ω), where v is a mean phonon group speed and τ1,(ω) a spontaneous decay time of nonequilibrium longitudinal phonons. Phonon scattering on defects is neglected. The light pumping parameter values that lead to the HPS formation are obtained. The equations that describe the dynamics of spot development are also discussed.
Deformation Potential Approach to the Estimation of the Peierls Phase Transition Temperature
Authors: Zhang Hongqun , Huang Yuanhe , Liu Ruozhuang 
Published: July 2007

AbstractBy means of the deformation potential theory, an expression for the electron‐phonon coupling constant g is derived, thereby a formula for the calculation of the Peierls phase transition temperature is obtained. As an illustration, this formula with the data calculated by a semiempirical crystal orbital method is used to compute the Peierls phase transition temperature of TTF‐TCNQ.
Pressure Dependent Donor Ionization Energies in a Quantum Well
Authors: Benedictal A. , Sukumar B. , Navaneethakrishnan K. 
Published: July 2007

AbstractDonor ionization energies in a GaAs quantum well of the GaAs/Ga1−xAlxAs superlattice are obtained for different hydrostatic pressures up to 40 × 108 Pa. It is found that the ionization energy increases with pressure. While the conduction band nonparabolicity increases, the ionization energy generally decreases (the maximum change by 18% occurring at zero pressure), for high pressures this effect is negligible. The effect of replacing the static dielectric constant by a suitable screening function has the effect of enhancing the ionization energy by about 2%, the maximum contribution arising at high pressures.
Diamagnetic and Dielectric Susceptibilities of AlSb and GaSb
Authors: Sahu T. , Nayak S. K. , Acharya R. N. 
Published: July 2007

AbstractThe magnetic susceptibility (χ) and dielectric constant (ϵ0) of AlSb and GaSb are calculated by adopting a linear combination of hybrids (LCH) method developed by us. We have constructed basis functions for the valence and conduction bands and used them in the general expression for χ of intrinsic semiconductors to derive an expression for χ of these A NB8‐N semiconductors. Our LCH basis states are also used in the Kramers‐Kronig relation at ω = 0 to obtain an expression for ϵ0. The Hall‐Weaire‐Thorpe approximation is used to calculate the matrix elements between different hybrids and made an average energy gap ansatz (Eg) for energy gap denominator occuring in the expression for χ and ϵ0. Our expressions for χ and ϵ0, which are obtained in terms of matrix elements between intrasite hybrids and between adjacent hybrids forming a bond, are free from any scaling parameter, whereas in the earlier theories arbitrary scaling parameters have been used to fit with the experimental results. Our results agree well with the available experimental results.
Fine Structure of the Triplet Relaxed Excited State of Ga+ Centres in Alkali Chlorides Induced by the Off‐Centre Position of the Excited Ga+ Ion
Authors: Kalder K. , Korrovits V. , Nagirnyi V. , Zazubovich S. 
Published: July 2007

AbstractThe luminescence of a RbCl:Ga crystal is investigated by the methods of time‐resolved polarization spectroscopy in the temperature range 0.09 to 400 K. Manifestations of an off‐centre displacement of an excited Ga+ ion and its tunnelling motion are found in the decay kinetics of the triplet AT emission of the Ga+ centre. No such manifestations are observed for the Ax emission. A comparison of luminescence characteristics and peculiarities of the triplet relaxed excited state (RES) structure of the Ga+ centre in RbCl:Ga crystal with those of NaCl:Ga and KCl:Ga crystals is performed. The fine structure of the tetragonal Jahn‐Teller minima of the triplet RES of Ga+ centres in RbCl:Ga is found to be analogous to that of KCl:Ga: the metastable state is split due to tunnelling transitions between various equivalent off‐centre positions of the excited Ga+ ion, but in the emitting state the spin splitting caused by the off‐centre displacement of Ga+ ion considerably dominates over the tunnel splitting of this state. Basing on the analysis of the ODMR spectra as well as of the circular polarization of Ga+ triplet luminescence and its decay kinetics, the assumption is made that the tunnel splitting energy of the triplet metastable state of Ga+ centres is less than 10−5 eV.
Electronic and Magnetic Structure of B.C.C. FeMn Alloys
Authors: Elzain M. E. , Yousif A. A. 
Published: July 2007

AbstractThe first principle discrete variational method in the local density approximation is employed to calculate the electronic and magnetic properties of 15‐atom clusters representing b.c.c. FeMn alloys. The occupation numbers and local densities of states are found to be unaffected by alloying in the case of paramagnetic clusters. When spin polarization is included the local magnetic moments at Fe sites are found to depend on the number of neighbouring Mn atoms and the FeMn coupling. The local moment components Eg and T2g at a site show different orientations. This is explained in terms of coupling T2g moments to nearest neighbours and of Eg moments to next‐nearest neighbours. The average magnetic moment and hyperfine field were calculated and compared with experimental data.
Correlated Effective‐Field Treatment of Spin‐1 Ising Models; Susceptibility
Authors: Sarmento E. F. , Kaneyoshi T. 
Published: July 2007

AbstractThe initial susceptibility of the Blume‐Emery‐Griffiths model is investigated by the use of the new correlated‐effective‐field theory. The general expression for evaluating the paramagnetic susceptibility is obtained. In particular, the present formulation is illustrated in a honeycomb lattice by examining the susceptibility (or the inverse susceptibility) numerically. We find many characteristic phenomena in the system with z = 3, where z is the coordination number.
Static and Dynamic Properties of Vacancies in H.C.P. Metals Many‐Body versus Pair Potentials
Authors: Fernández J. R. , Monti A. M. 
Published: July 2007

AbstractMany‐body interatomic potentials based on the embedded atom method are used to study the static properties of small vacancy clusters and dynamic properties of monovacancies in Mg, α‐Ti, and α‐Zr. In agreement with pair potential predictions, quantitative differences due to the elastic material properties characterize the static calculations. Diffusion constants for the monovacancy mechanism described in terms of the Arrhenius law are found to be strongly dependent on the potential barrier at distances below a, the basal lattice parameter.
The Electronic Structure and the Chemical Bonding in NiO and La2NiO4 Crystals. A Comparison with CuO and La2CuO4
Authors: Choisnet J. , Evarestov R. A. , Tupitsin I. I. , Veryazov V. A. 
Published: July 2007

AbstractModelling from quantum chemical data of the chemical bonding in NiO and La2NiO4 crystals comparatively to CuO and La2CuO4, is proposed. The increase of the ionicity degree of the Ni(II)–O bonding in comparison with Cu(II)‐O bonding is observed. The covalency is low and equally distributed in the equatorial plane and apical direction in the NiO6 octahedra in La2NiO4. As a supplementary data, which is common to La2CuO4 and La2NiO4, the covalency of the apical La(III)–O bonding plays an important role in increasing the overall covalent character of the apical oxygen and triggering a further elongation of MO6 octahedra.
Pressure Dependent Hydrogen Diffusion in the Intermetallic Compound Fe0.5 Ti0.5
Authors: Alfred Raj S. Cecil , Lawrence N. , Sebastian R. , Haridasan T. M. 
Published: July 2007

AbstractThe Green's function technique is used in the scattering matrix formalism to compute the mean square displacement (MSD) of hydrogen interstitials and the reaction coordinate technique is used to calculate the hydrogen diffusion parameters in the intermetallic compound Fe0.5 Ti0.5 from normal pressure to 2.5 GPa pressure for low concentration of hydrogen. The MSD of the metal atoms surrounding the interstitial are found to be smaller than those of the host crystal in agreement with NbHx. The diffusion of hydrogen into the alloy is found to decrease with increase in pressure as experimentally observed in PdHx.
Pertubative Approach to Spin‐1/2 Heisenberg Antiferromagnet
Authors: Trimper S. 
Published: July 2007

AbstractThe temperature Green's function matrix for a two‐sublatticc antiferromagnet is studied taking into account the complete algebraic properties of spin operators. The non‐diagonal part of Green's functions gives rise to a temperature dependent effective interaction. The staggered magnetization is calculated by summing up additional diagrams originated from the spin commutation relations.
On the Nucleation of Dislocations at a Crystal Surface
Authors: Beltz G. E. , Freund L. B. 
Published: July 2007

AbstractAn exact expression for the elastic energy associated with a semicircular shear dislocation loop emanating from a free surface is derived (within continuum dislocation theory) and compared with an earlier approximation. The energy required to activate a semicircular dislocation loop into its unstable “saddle‐point” configuration is then re‐calculated, based on the modified expression for the self‐energy. It is found that the shear stress necessary to emit the loop, as a function of temperature, is almost 50% less than earlier estimates. The principal drawback to this type of calculation is also discussed, namely, that the critical radius of an incipient dislocation loop is on the order of one atomic spacing, which is too small for a continuum theory to be valid.
Electronic and Chemical Properties of Barium and Indium Clusters
Authors: Onwuagba B. N. 
Published: July 2007

AbstractThe ground state electronic and chemical properties of divalent barium and trivalent indium are investigated in a self‐consistent manner using the spin‐polarized local density approximation in the framework of the density functional theory. A jellium model is adopted in the spirit of Gunnarsson and Lundqvist's exchange and correlation energies and the calculated properties primarily associated with the s‐p orbitals in barium and p orbitals in indium provide deepend insight towards the understanding of the mechanisms to the magic numbers in both clusters.
Magnetic and EPR Studies of Oxygenated and Non‐Oxygenated LaBaSrCu3O6+x Compounds in the Tetragonal Phase
Authors: Guskos N. , Likodimos V. , Calamiotou M. , Kuriata J. , Wabia M. , Paraskevas S. M. , Windsch W. , Metz H. , Koufoudakis A. , Mitros C. , Gamari‐Seale H. , Psyharis V. , Niarchos D. 
Published: July 2007

AbstractOxygenated and non‐oxygenated LaBaSrCu3O6+x ceramics in the tetragonal phase are studied using X‐ray, EPR, and magnetic measurements. A Meissner signal for the oxygenated sample corresponding to almost perfect diamagnetism is measured below 35 K. However, EPR data indicate that at least some parts of the sample become superconducting already below 62 K. Magnetic susceptibility of the non‐oxygenated sample exhibit a significant paramagnetic contribution attributed to Cu(1) ions ferromagneticall coupled to the Cu(2) planes.
Thermal Conductivity of Ice
Authors: Ahmad N. 
Published: July 2007

AbstractThe thermal conductivity of ice, formed at different cooling rates during the solidification process, is measured between 2 and 80 K. It is found that the thermal conductivity depends upon the cooling rate. The density of vibrational states is determined from the heat capacity data available in literature and is used in the analysis of thermal conductivity. The umklapp processes, boundary scattering, and point defects are considered along with dislocations or tunnelling states as scattering mechanisms. It is found that the model based on two‐level tunnelling states provides a better fit to the experimental data than the model based on the presence of dislocations.
EPR Study of Phase Transitions and Modulated Structure in [Mg(H2O)6]GeF6 Crystals
Authors: Hrabański R. 
Published: July 2007

AbstractThe phase transitions in [Mg(H2O)6]GeF6 are investigated with an Mn2+ EPR probe substituted for Mg2+ ions in the lattice. An axial symmetry phase with space group R 3 is observed above 403 K. The ferroelastic phase transition at 318 K is of first order and leads to monoclinic symmetry and to the development of a domain structure. In the intermediate phase the spectra exhibit anomalous changes of the lineshapes resulting from incommensurate modulation of the crystal field fine structure tensor. The results of EPR studies are interpreted both in terms of the plane‐wave and multi‐soliton models.
On a New Type of Dynamic Quantum Wells and Quantum Dots
Authors: Tralle I. E. 
Published: July 2007

AbstractElectron movement in a spatially periodic high‐frequency electric field is considered. If the spatial period is small enough and the frequency is sufficiently high, the effect of this field on the charge particle is in a sense equivalent to the effect of a time‐independent effective potential that is a sequence of deep ‘dynamic’ quantum wells where the electrons are confined. The physical model is proposed which could be described in terms of such a Hamiltonian. Also the influence of an external quantizing magnetic field on such quantum well is considered and the existence of a quantum dot in this system is demonstrated.
Interference between Parametric and Coherent Bremsstrahlung Radiation Mechanisms of a Fast Charged Particle in a Crystal
Authors: Kleiner V. L. , Nasonov N. N. , Safronov A. G. 
Published: July 2007

AbstractParametric and coherent Bremsstrahlung radiation of a fast charged particle, moving in a crystal is studied from a single point of view. Such an approach permits to observe the essential change of the radiation structure in the region where CBR and PXR radiation patterns overlap that may be interpreted as an interference of these two radiation mechanisms. The dependence of interference items from charge sign is predicted. The PXR expression admitting a continuous transition from the dynamic regime of radiation to the kinematic one and the parameter of this transition are given.
Analysis of Magnetic Point‐Defect Relaxations in Electron‐Irradiated Magnetite
Authors: Walz F. , Kronmüller H. 
Published: July 2007

AbstractPronounced relaxation peaks occurring in the magnetic after‐effect (MAE) spectra of low‐temperature electron‐irradiated single‐crystalline magnetite (Fe3O4) are numerically analyzed and associated with anisotropic point defect configurations. As basic defects, cation interstitials and octahedral vacancies have been identified to exist, each in two closely related configurations, giving rise to reorientation‐type double peaks at 230 K (interstitials) and 300 K (vacancies). The analysis of these relaxations yields reorientation (migration) enthalpies of 0.69 eV and 0.73 eV for the interstitials and of 0.86 eV and 0.93 eV for the vacancies. The role of radiation damage for the relaxation mechanisms below the Verwey temperature (Tv = 125 K), i.e. tunnelling (4 K < T < 35 K) and hopping (45 K < T < 125 K) of electrons is discussed and a consistent recovery model, comprising all types of radiation‐induced lattice defects, developed.
Infrared Spectra of Solid Chlorodifluoromethane
Authors: Anderson A. , Beardsall A. J. , Fraser J. M. 
Published: July 2007

AbstractInfrared spectra of solid Chlorodifluoromethane (CHClF2 or Freon 22) at temperatures between 12 and 105 K are presented and discussed. The evidence for a reversible solid state phase transition at (55 ± 5) K is described, confirming the results of a previous Raman study. Two metastable phases are also detected. Multiplet structure is observed for all nine internal modes and interpreted in terms of crystal field effects. This confirms that both phases are ordered, with rather large unit cells, and that the transition is of a displacive type. Coincidences with Raman frequencies suggest that both structures are probably non‐centrosymmetric. The lattice spectra are compatible with these conclusions and are tentatively assigned on the basis of inertia and intensity arguments.
Free Carrier Absorption in Quantum Well Structures Due to Confined and Interface Optical Phonons
Authors: Bhat J. S. , Mulimani B. G. , Kubakaddi S. S. 
Published: July 2007

AbstractA theory of free carrier absorption in quantum wells when electrons are scattered by confined LO phonons described by the Huang and Zhu model, Fuchs‐Kliewer slab modes, and Ridley's guided mode models is given. The effect of interface phonon modes on free carrier absorption is also studied. Numerical results for frequency and well width dependence are given for parameters characteristic of the GaAs/AlAs quantum well. A comparison is made with the calculations based on a bulk description of phonons.
On the M‐Centre Luminescence in Alkali Halides. The Case of NaF and LiF
Authors: Bosi L. , Gallo D. 
Published: July 2007

AbstractNew results concerning the M luminescence in NaF and LiF are presented. The ageing of the samples is investigated particularly in connection with MA‐centre formation in NaF : Li. Trends and correlations are proposed between halfwidths, effective modes, and Huang‐Rhys factors in alkali halides.