bluuebunny/crossref_metadata_2025_split huggingface dataset:
Energy band structure of K2SeO4 single crystals
Authors: Kityk I. V. , Andrievskii B. V. 
Published: July 2007

AbstractThe energy band structure and electronic properties of K2SeO4 single crystals (space group D) are presented. The ab initio LCAO method is used. The corresponding calculational technique is described briefly. Anaytical expressions for the overlapping matrix elements are obtained. Results are reported for the valence and conduction band energies and densities of states. In terms of these, a consistent explanation of the experimental measurements on K2SeO4 single crystals is obtained.
Bound hole states in direct‐gap semiconductors with screw dislocations
Authors: Razumova M. A. , Khotyaintsev V. N. 
Published: July 2007

AbstractThe dispersion law of holes bound at a screw dislocation in cubic semiconductors is calculated on the basis of deformation potential theory taking into account the threefold degeneracy of the valence band maximum in a perfect crystal. The lowest dislocation band (the corresponding azimuthal quantum number is m = 0) is slightly affected by the anisotropy of the effective mass. Only the change of the effective mass is observed. The states with azimuthal number m ≠ 0 are considerably more affected by the anisotropy of the effective mass. A mixing of longitudinal and transverse motions (with respect to the dislocation line direction) takes place and terms linear in k‖ appear in the dispersion law. The optical transitions associated with these states are intense for a polarization of light transverse with respect to the dislocation line and may cause circular dichroism in the absorption. The long‐wave edge of the dislocation absorption on the whole is formed by transitions from the deepest band m = 0 and is polarized along the Burgers vector b.
The low‐temperature dependence of coercivity in NdFeB magnets
Authors: Hu Jifan , Kou X. C. , Kronmüller H. 
Published: July 2007

AbstractThe influence of the spin reorientation phenomenon on the nucleation field of NdFeB magnets is analyzed based on the theory of micromagnetism. Results indicate that the temperature dependence of coercivity in the whole temperature range can be well described by the equation Hc = (2K1/Ms) × αKαψ – NeffMs when taking into account the effect of the spin reorientation in the low‐temperature range.
Anisotropy of the ZnO single crystal reflectivity in the region of residual rays
Authors: Venger E. F. , Melnichuk A. V. , Melnichuk L. Yu. , Pasechnik Yu. A. 
Published: July 2007

AbstractThe consistent parameters of hexagonal zinc oxide single crystals are obtained in the frame of the one‐oscillator model by analyzing the dispersion spectra of IR light reflection. The anisotropy coefficients of the electron effective masses and the plasmon damping Coefficients are measured. The calculated values of electron mobility and conductivity for zinc oxide are in good agreement with the reference data available.
The Influence of Permanent Magnetic and Alternative Electric Fields on the Dislocation Dynamics in Ionic Crystals
Authors: Golovin Yu. I. , Morgunov R. B. , Tyutyunnik A. V. 
Published: July 2007

AbstractThe dislocation dynamics are investigated in NaCl single crystals treated by permanent magnetic (B = 1 T) or alternating electric (E = 200 kV/m, f = 50 Hz) fields. It is shown that the field treatment before mechanical loading causes long‐lived (≈1 h at T = 300 K) excited states of both local obstacles and edge dislocations. It leads to an increase of dislocation displacements up to two times and the number of mobile dislocations up to 1.5 times.
Pressure Dependence of Electron and Positron Band Structures in Elemental Semiconductors
Authors: Alkhafaji S. T. , Amrane N. , Bouarissa N. , Badi N. , Soudini B. , Sehil M. , Aourag H. 
Published: July 2007

AbstractOn the basis of the local pseudopotential method and the independent particle approximation, the electron and positron band structures of Si, Ge, C, and α‐Sn with respect to the variation of pressure are discussed. The positron band structure in a periodic lattice is similar to its electron counterpart. Some physical properties of these semiconductors such as the homopolar gap and the bulk modulus are calculated from the positron band structure.
An Analytical Model for Screened Coulomb Interaction in a C60 Cluster
Authors: Cappellini G. , Bechstedt F. , Bosin A. , Casula F. 
Published: July 2007

AbstractAn analytical model for the static screened Coulomb interaction of a semiconductor sphere that accurately reproduces the screening properties of an icosahedral C60 cluster is presented. Compared with the static screened Coulomb interaction determined after polarizability calculations it yields the same value for nearest‐neighbor interaction energy and the same antiscreening effect for distant sites for points located on the nuclear grid of a free molecule. Also the static dipole molecular polarizability turns out to be in good agreement with that calculated after different theoretical methods. When extended to finite frequencies, without introducing other parameters, the present model reproduces the main resonance around 20 eV of the optical absorption spectrum for the gas phase C60 and the giant plasmon resonance around 28 eV observed in C60 fullerites via electron energy loss spectroscopy. These results turn out to be largely insensitive to the details of the model.
Interaction of Curved Vortex Lines with the Surface of a High‐Temperature Superconducting Sphere
Authors: Saif A. G. 
Published: July 2007

AbstractThe uniaxial anisotropic London equation is used to study the effects of the surface of a high‐temperature superconducting sphere on a curved vortex lattice. Using the generalized spherical functions (GSF), the field distributions inside and outside the sample are determined. Moreover, the current density, the forces on the vortex segments, and the Gibbs free energy of the system are calculated in the frame of the GSF. Agreement with previous results is achieved in the isotropic limiting case.
Temperature Dependence of Conductance of Fibonacci Superlattices
Authors: Roy C. L. , Basu Chandan , Domínguez‐Adame F. , Maciá E. 
Published: July 2007

AbstractStudies of the dc conductance of Fibonacci superlattices at finite temperature are carried out. Exact solutions of the Schrödinger equation are used for our treatment of the problem. The results show a strong dependence of electrical conduction on the chemical potential, which acts as a probe of the underlying electronic structure. In particular, it is found that the Fibonacci superlattice shows metallic behaviour when the chemical potential lies within allowed bands and it behaves as a semiconductor when the chemical potential lies within gaps.
Parametric Interaction of Electromagnetic and Acoustic Waves in Cubic Semiconductors with Strain‐Dependent Dielectric Constant
Authors: Artemenko O. L. , Sevruk B. B. 
Published: July 2007

AbstractThe phenomenon of parametric excitation of a transverse hypersonic wave by laser and microwave radiation in cubic semiconductor crystals with strain‐dependent dielectric constant and piezoelectric effect induced by a constant transverse electric field is investigated. The numerical analysis of the exact dispersion equation is performed providing resonance dispersion relations of the acoustic wave gain factor for a wide range of electromagnetic wave frequencies and semiconductor plasma parameters. The boundaries of the absolute instability region and electromagnetic field threshold value are set up, as well as the optimal relationship between the collision frequency and the frequencies of electromagnetic and plasma waves governing acoustic wave generation and amplification.
Electronic and Positronic Structure of Diamond under Normal and High Pressure
Authors: Aourag H. , Abderrahmane S. Ait , Amrane Na. , Amrane N. , Bouarissa N. 
Published: July 2007

AbstractEmpirical nonlocal pseudopotentials of diamond which can describe the electronic energy structure over a wide energy range of more than 20 eV from the bottom of the valence band are determined. The nonlocality of the potential is described by the Gaussian model. The optimized nonlocal pseudopotential reproduces the energy band structure within 5%. The valence electron and positron charge densities in diamond are obtained from wave functions derived from this model at normal and under hydrostatic pressure. It is found that the positron density is maximum in the open interstices and is excluded not only as usual, from the ion cores but also to a considerable degree from the valence bonds.
Luminescence Bands and their Proposed Origins In Highly Porous Silicon
Authors: Canham L. T. 
Published: July 2007

AbstractThe striking photoluminescent properties of porous silicon nanostructures have received intense study since 1990. This literature review focuses on the large number of models proposed to account for efficient light emission. Two distinct luminescence bands in the visible spectral range have received most study. Strong evidence now exists that quantum confined carriers in crystalline Si can yield efficient red to green (‘S band’) luminescence. The blue (‘F band’) emission reported to date in oxidised porous Si is, however, unlikely to arise from quantum confinement effects.
From Minutes to Months: Ageing of Porous Silicon Single Layers and Superlattices
Authors: Theiss W. , Arntzen M. , Hilbrich S. , Wernke M. , Arens‐Fischer R. , Bercer M. G. 
Published: July 2007

AbstractThe ageing of porous silicon under ambient conditions must be taken into account in the design of optical devices using this material. The time dependence of the optical properties of porous silicon single layers and superlattices is studied in detail using reflectance spectroscopy. In the infrared the Si–H and Si–O vibrational modes are examined which reflect the state of thc internal surfaces. The optical properties in the visible and ultraviolet spectral range are investigated as well. Spectra are recorded on time scales ranging from minutes to months after sample preparation. The observed changes of the optical properties are discussed. It is verified that the stability of porosity superlattice filters in time can be improved significantly by thermal oxidation.
Temperature Dependent Photoluminescence and Optical Absorption of Wide‐Gap Amorphous Silicon Carbon Alloys
Authors: Rüter D. , Rolf S. , Bauhofer W. 
Published: July 2007

AbstractThe photoluminescence (PL) properties of “porous silicon” prepared by spark erosion and of amorphous silicon carbon alloys. amorphous silicon carbon oxygen alloys and hydrogenated amorphous carbon prepared by CVD methods from liquid precursors are measured. Ellicient blue‐green PL is measured as a function of temperature up to 800 K. Optical absorption data in the high energy range (UV) are obtained from transmission measurements. Very low absorption coellicients in the visible spectral range are determined in light‐guiding thin film structures which are grown as a three‐layer sandwich on a silicon substrate.
The Fraunhofer Diffraction by Large Scale Inhomogeneities
Authors: Skryabina S. N. , Skryabin Yu. N. 
Published: July 2007

AbstractThe small‐angle scattering amplitudes by large scale spherical and nonspherical inhomogeneities are considered both in the refraction and diffraction ranges. In particular, the scattering by vortex lines in superconductors is discussed. In all cases the range is determined where the Fraunhofer diffraction part of the amplitude gives the main contribution to the scattering amplitude. This is important since the Fraunhofer diffraction leads to another dependence on the transfer momentum than the refraction contribution in the refraction range.
The Displacement and Stress Fields due to a Sinusoidal Edge Dislocationa
Authors: Anongba P. N. B. 
Published: July 2007

AbstractExpressions for the displacement and stress fields of a sinusoidal edge dislocation in an infinitely extended isotropic medium are given. It is suggested that information on the physical properties of non‐planar cracks propagating along 〈110〉 in intermetallics with cubic structures may be obtained by studying particular cracks which consist of continuous distributions of sinusoidal edge dislocations perpendicular to 〈110〉 with infinitesimal Burgers vectors.
High Pressure Properties of the Alkaline‐Earth Sulphides
Authors: van Camp P. E. , van Doren V. E. , Martins J. L. 
Published: July 2007

AbstractThe alkaline‐earth sulphides MgS, CaS, SrS, and BaS naturally occur in the rocksalt (Bl) structure. Under pressure they undergo a structural phase transformation to the cesiumchloride (B2) structure. Ab‐initio local density nonlocal pseudopotential calculations of the total energy of the first three compounds are performed in the B1, B2, and in the zincblende (B3) structure. It is found that the B1 structure is stable at normal conditions while the B3 structure is unstable under all conditions. The B1 to B2 phase transformation occurs at, respectively, 230.7, 49.8, and 20.9 GPa. Recent experimental estimates of these transition pressures are 44 and 37.1 GPa for CaS. The calculated lattice constants, bulk moduli, and their pressure derivatives agree well with the experimental values.
Power Losses of a High‐Temperature Superconducting Slab in the Presence of Alternating Currents
Authors: Saif A. G. , El‐Sabagh M. A. 
Published: July 2007

AbstractThe power loss and its two constituent terms due to dissipated and reactive energies are developed for a system of a normal conducting wire (two wires) carrying ac current parallel to a high‐temperature type‐II superconducting infinite slab (moving slab) of arbitrary thickness. The reactive term is found to be either capacitively or inductively reactive. When the thickness of the slab tends to infinity it is found that the results will be in good agreement with the previous results. It is found that the loss along the versor of the anisotropy axis increases as λ‖ increases, moreover it is greater than the power loss along the crystal symmetry direction a. In the isotropic case a satisfactory agreement with previous results is achieved.
TL and IRSL Emission Spectra of Detrital Feldspars. New Experimental Data
Authors: Rendell H. M. , Townsend P. D. , Wood R. A. 
Published: July 2007

AbstractMeasurements of thermoluminescence (TL) and infrared stimulated luminescence (IRSL) emission spectra are presented for natural samples of potassium‐rich detrital feldspars separated from middle‐late Pleistocene fluvial sediments from central Spain. Although IRSL‐based sediment dating techniques have developed rapidly since late 1980's, the understanding of the underlying physics, and of the relationship between TL and IRSL, remains rudimentary. The natural IRSL spectra for all three detrital feldspar samples show three emission bands at 335,400, and 550 nm, in contrast, the natural TL spectra exhibit not only these emission bands but also an additional one at 290nm. The effects of both preheating and IR stimulation on the TL emission spectra are investigated.
Elastic Interaction between Nanocrystals and a Crystalline Matrix: CuCl in NaCl
Authors: Weber H.‐J. , Lüghausen K. , Haselhoff M. , Siegert H. 
Published: July 2007

AbstractIt is demonstrated that doping of NaCl with CuCl increases the elastic constants of NaCl and modifies their dependence on temperature. The changes are significantly larger than estimated for a statistical distribution of the doping material. The crystallinity of embedded CuCl is considered to be responsible for additional elastic effects. This conclusion is verified by studying different kinds of annealing effects. The elastic interaction between guest and host crystals is interpreted as a relaxation of the NaCl lattice which closes the spatial gap caused by the lattice misfit. This results in a spontaneous strain which modifies the elastic constants due to the contribution of nonlinear terms.
Tunneling through Ultrasmall NIS Junctions in Terms of Andreev Reflection. A Nonlinear Response Approach
Authors: Hädicke A. , Krech W. 
Published: July 2007

AbstractThe Andreev current through an ultrasmall NIS junction is calculated in a systematic way by means of a nonlinear response approach basing on the elementary Hamiltonian of quasiparticle tunneling. The voltage dependence of current and differential conductance as well as the Andreev conductance are derived for low‐ and high‐impedance environments, respectively.
Hole Transport in Di‐p‐Tolyl‐p‐Nitrophenylamine Doped Poly(styrene)
Authors: Borsenberger P. M. , Gruenbaum W. T. , Kaeding J. E. , Magin E. H. 
Published: July 2007

AbstractDi‐p‐tolyl‐p‐nitrophenylamine (DTNA) is a highly polar donor molecule with a dipole moment of 5.78 D. Hole mobilities have been measured in DTNA doped poly(styrene) over a range of fields, temperatures, and DTNA concentrations. The results are described within the framework of a formalism based on disorder, due to Bässler and coworkers. In the disorder formalism, it is assumed that charge propagation occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. A key parameter of the formalism is the energy width of the hopping site manifold. For DTNA doped poly(styrene), the width is approximately 0.15 eV and independent of the intersite distance, or DTNA concentration. The width is described by a model based on dipolar disorder. The model is premised on the assumption that the width is determined by a dipolar component due to the DTNA molecule and a van der Waals component. The interpretation of the experimental results leads to the conclusion that the van der Waals component increases with increasing intersite distance.
Comparative EPR Studies of Dynamics and Exchange Coupling in Ammonium and Potassium Copper (II) Tutton Salts
Authors: Marciniak M. , Hoffmann S. K. , Augustyniak M. A. , Hilczer W. 
Published: July 2007

AbstractSingle crystal EPR studies on (NH4)2Cu(SO4)2 · 6 H2O and K2Cu(SO4)2 · 6 H2O in the temperature range 4.2 to 360 K show that the vibronic g‐factor averaging is completed in the ammonium salt at 340 K but not in the potassium salt. The vibronic effects are discussed in terms of a thermally activated two‐level model which shows that the barrier between two wells of the adiabatic potential surface increases at low temperatures and tends to zero at high temperatures. Exchange coupling effects are different in the ammonium and the potassium salts. The coupling between inequivalent Cu sites appears below 100 K in the ammonium salt and grows up to J = 0.0023 cm−1 at 4.2 K. This coupling is not observed in potassium salt where, moreover, the coupling between equivalent sites is very small allowing the hyperfine structure to be more resolved at low temperature. A giant line broadening and line shift are observed below 30 K in the potassium salt. The above effects are discussed in relation to the possible superexchange pathways and differences in the space distribution in the electron density of the ground state of Cu(II) in both crystals.
Peculiarities of the Piezooptic Effect in Incommensurate Crystals
Authors: Sveleba S. A. , Polovinko I. I. , Bublyk M. I. , Kapustianik V. B. , Krochuk A. S. 
Published: July 2007

AbstractThe influence of uniaxial mechanical stress on the modulated structure which consists of a superstructure sequence is investigated in [N(CH3)4]2MeCl4 (Me=Cu, Fe, Zn) crystals. The peculiarities of the effective piezooptic coefficients π within the incommensurate phase of these crystals are studied by Senarmont's method. The influence of uniaxial mechanical stress disturbs the conditions of modulated structure interaction with defects what leads to the appearance of the widening of metastable states and to anomalies in the physical characteristics. It causes the change of the region of the localization of the incommensurability wave vector k on the commensurate values of higher order.
Study of Excitonic Molecules in Monoclinic Zinc Diphosphide Crystals
Authors: Gorban I. S. , Biliy M. M. , Dmitruk I. M. , Yeshchenko O. A. 
Published: July 2007

AbstractThe photoluminescence (PL) spectra of the monoclinic modification of the semiconductor crystal ZnP2 (β‐ZnP2) are studied under high, intermediate, and low laser excitation intensities at liquid helium temperature. Under high excitation intensities the inverse hydrogenlike M‐series was observed. It is symmetrical with the hydrogen‐like excitonic series. This M‐series is due to the two‐electron transitions from the excitonic molecule (biexciton) state to the ground, excited, and ionized excitonic states. The binding energy of the biexciton in β‐ZnP2 is determined as Δbex = 14.9 meV (0.32 of exciton Rydberg). Under intermediate excitation intensities another G‐band appears in the PL spectrum of β‐ZnP2. This band is interpreted as a result of two‐photon annihilation of free excitonic molecules in the gas phase. A narrow C‐line appears in the PL spectrum under intermediate excitation intensities, too. The analysis of the temperature dependence of the C‐line intensity allows us to assume this line to be the result of two‐photon annihilation of the biexcitons condensed into a dielectric liquid with the binding energy ΔL ≈︁ 0.5 meV.
Electronic Structures of Tetragonal Sixfold‐Coordinated Iron (III) Complexes with High‐Spin—Intermediate‐Spin Transitions
Authors: Kuang Xiao‐Yu , Morgenstern‐Badarau Irène 
Published: July 2007

AbstractA 102 × 102 matrix for describing the d5 configuration ions within the tetragonal ligand‐field is constituted taking into account the electron‐electron repulsion, ligand‐field, spin—orbit coupling, and Zeeman interactions in the weak‐field scheme. Two different high‐spin—intermediate‐spin transitions, which are, respectively, associated with stronger and weaker axial ligand fields of tetragonal sixfold‐coordinated iron(III) complexes, are investigated. The diagrams of the energy level positions, magnetic moments, and effective EPR g‐factors are derived and expressed as functions of the parameter Δ/ζ, Δ being the energy gap between the high‐spin state (6A1) and intermediate‐spin states (4A2 or 4E) and ζ being the spin—orbit coupling coefficient.
Exciton and Electron Traps on Structural Defects in Molecular Crystals with Dipolar Molecules
Authors: Izvekov S. V. , Sugakov V. I. 
Published: July 2007

AbstractThe depths of exciton and electron traps arising due to the presence of structural defects in organic crystals consisting of dipolar molecules are calculated. In the model studied a structural defect appears if a molecule is rotated by 180° around the long axis as compared to the bulk molecules. The contribution of the electrostatic interaction to the depth of the trap state on such a defect is studied in detail. For crystals consisting of molecules which have a large dipole moment in the ground state and adopting large values in the excited state this contribution may be dominant. The calculations are carried out for triplet excitons and electrons in benzophenone crystal.
Exchange Integrals in the Helimagnetic Phase of ZnCr2xAl2−2xS4 (0.85 ≦ x ≦ 1)
Authors: Hamedoun M. , Rachadi A. , Hourmatallah A. , El Allam D. , Benyoussef A. 
Published: July 2007

AbstractThe spinel solid solutions ZnCr2xAl2−2xS4 are studied by neutron diffraction experiments. They show a helimagnetic phase in the concentration range 0.85 ≦ x ≦ 1. The helix angle decreases as x is decreased. The three first exchange integrals and their evolution with x are obtained. The disappearance of the long‐range order is shown to be due to the large decrease of the ferromagnetic in‐plane interaction. The results are extended to obtain the fourth exchange integral.