bluuebunny/crossref_metadata_2025_split huggingface dataset:
Study of Crossover Behaviour in Layered Antiferrormagnets
Authors: Singh Ravi P. 
Published: July 2007

AbstractThe dimensional crossover from 3D to quasi‐2D character in magnetic properties of layered antiferromagnets such as La2CuO4 (2‐1‐4) is studied. The theoretical framework is based on an anisotropic Heisenberg model. Random phase approximation (RPA) is used to obtain a self‐consistent expression for the magnetization. It is shown that under specific approximations on this theory, free‐spin‐wave (FSW) results can be obtained. With the help of this self‐consistent theory it is shown that the crossover occurs as the value of dimensional anisotropy parameter (Jc/Jab) changes from about 10−3. Numerical results for the sublattice magnetization are obtained depicting this crossover and compared with free‐spin‐wave results. The results obtained from this self‐consistent calculations are in excellent agreement with the experimental results obtained for 2‐1‐4 materials.
Optical Properties of Semiconductors (Ge, Si, GaAs, InSb) in the Region of Fundamental Absorption
Authors: Kudykina T. A. 
Published: July 2007

AbstractThe law of light refraction and analogues of Fresnel's formulas obtained strictly from the Maxwell boundary conditions, are used to calculate the optical parameters of semiconductors. The expressions following from the reflectivity or ellipsometry, for indexes of refraction n and absorption x are presented. The calculations based on our formulas and experimental data on reflectivity at normal incident light for Ge, Si, GaAs, and InSb in the region of band‐to‐band transitions are given. The difficulties which took place earlier in optics of highly absorbing media disappear in our approach: 1. the angle of refraction is a real value; 2. the real part of the dielectric function ε1 ≧ 0, because χ ≦ n; 3. the index of refraction n > 1.
H ‐Centres in LiFH−, Mg2+
Authors: Egranov A. V. 
Published: July 2007

AbstractThe ESR spectrum of an unperturbed hydrogen atom on a cation site is observed after vacuumultraviolet irradiation of LiF‐H−, Mg2+ crystals at room temperature. A production mechanism involving optical destruction of magnesium‐vacancy dipoles and U‐centres, release of the cation vacancies from the radiation‐induced Mg‐related centres, and subsequent capture of hydrogen atoms by the free cation vacancies, is proposed.
Thermodynamic Approximations in High‐pressure and High‐Temperature Physics of Solids
Authors: Shanker J. , Kumar M. 
Published: July 2007

AbstractA number of useful thermodynamic approximations of wide applicability for describing high‐pressure and high‐temperature behaviour of solids are discussed with particular emphasis on the origin and equivalence of such approximations. Calculations of various thermodynamic quantities for NaCl crystals are performed and compared with available experimental data to demonstrate the usefulness of the approximations considered.
Model of Interacting Atomic Chains and Its Application to the Description of the Crowdion in an Anisotropic Crystal
Authors: Landau A. I. , Kovalev A. S. , Kondratyuk A. D. 
Published: July 2007

AbstractA new generalized Frenkel‐Kontorova (discrete sine‐Gordon) model is proposed which treats a crystal as a set of one‐dimensional atomic chains interacting with each other. Here, unlike earlier works, the model does not include the fixed field generated by an external rigid substrate. The two‐dimensional version of the model is considered, but the three‐dimensional generalization can be easily developed. On the basis of the present interacting atomic chain model a computer simulation of the crowdion (or the anticrowdion) in an anisotropic crystal is made. The calculated results completely agree with analytical results of the elasticity theory.
A New Approach to the Determination of the Fe(3+)/Fe(2+) Ratio and Recoil‐Free Factors in Two Garnets by Mössbauer Spectroscopy
Authors: Karfunkel U. , Pollak H. 
Published: July 2007

AbstractA new approach to the analysis of Mössbauer spectra for garnets with mixed valence iron is made. Values for the recoil‐free fraction ratios are extracted and a more acceptable approximation procedure is obtained for the derivation of the Fe(3+)/Fe(2+) ratio.
Enhanced Diffusion of Impurities and Defects in Crystals in Conditions of Ultrasonic and Radiative Excitation of the Crystal Lattice
Authors: Pavlovich V. N. 
Published: July 2007

AbstractOn the basis of the quantum theory of diffusion the influence of crystal lattice excitations (ultrasonic, nuclear irradiation) on the diffusion of impurity atoms is analysed. The effects considered in the paper are determined by two factors: the change of population of the impurity oscillator levels due to interaction with nonequilibrium excitations of the crystal lattice, and the changé of the probability of diffusion transitions on the given level. The effects are qualitatively different for heavy impurity atoms whose diffusion is enhanced in nonequilibrium conditions and for light impurity atoms for which the effect of additional localization is possible.
Calculations of Energy Bands and Optical Properties of Solid‐State Superlattice Materials
Authors: Yee J. H. , Khanka G. H. , White W. T. , Orvis W. J. 
Published: July 2007

AbstractThe calculational results of the electronic and optical properties of some III‐V semiconductor compounds obtained from the pseudopotential energy band method are compared with results obtained from other energy band calculational techniques and with experiments are compared. The calculation of real and imaginary parts of the optical refractive index of the GaAs–AlAs superlattice shows good agreement with experimental data. The calculations of the energy gap as a function of the number of atomic layers were carried out for GaAs–AlAs and GaSb‐InAs superlattices. The result obtained from these calculations shows that it is about 10 to 15% below the existing experimental data. However, if the band gap of experimental data is correctly normalized to the correct room temperature value, our calculations show excellent agreement with experimental data. Comparison of the results obtained from our calculations concerning the charge density and the charge confinement are carried out with the results obtained from other energy band calculation techniques.
Model of Charge Transfer Induced by EL2→EL2* Transformation in Semi‐Insulating GaAs
Authors: Kiliulis R. , Kažukauskas V. 
Published: July 2007

AbstractA numerical simulation of EL2 level transition to its metastable state in semi‐insulating (si) GaAs is performed solving the system of equations describing the charge transfer between EL2 and two other levels, namely a donor and an acceptor. The metastable EL2* state is involved as electrically and optically inactive. The time dependence of the changes of photoconductivity, Hall mobility, and also of electron population in EL2, donor, and acceptor levels is obtained for a different initial compensation ratio of si GaAs samples. It is shown that the photoquenching process consisting of up to three exponential parts is ruled by the state changes of one EL2 level and is very sensitive to the defect concentrations involved.
Analysis of Thermodynamic Properties for High‐Temperature Superconducting Oxides
Authors: Kushwah S. S. , Shanker J. 
Published: July 2007

AbstractAnalysis of thermodynamic properties such as specific heat, Debye temperature, Einstein temperature, thermal expansion coefficient, bulk modulus, and Grüneisen parameter is performed for rare‐earth‐based, TI‐based, and Bi‐based superconducting copper oxides. Values of thermodynamic parameters are calculated and reported. The relationship between the Debye temperature and the superconducting transition temperature is used to estimate the values of Tc, using the interaction parameters from Ginzburg.
The Lowest Approximation for Inelastic Acoustic Phonon Scattering Consideration in a Monte Carlo Procedure
Authors: Kancleris Ž , Willander M. 
Published: July 2007

AbstractA simple algorithm to account for acoustic scattering inelasticity in the Monte Carlo procedure is suggested. The temperature range is evaluated at which the proposed method is suitable. Good agreement of the computed results using the proposed algorithm with an analytical solution of the kinetic equation and the data obtained by other techniques is established.
The Thermalization Process of Photoexcited Electrons and Holes in the Second Kinetic Stage of Relaxation
Authors: Maia Ana E. P. , Filho A. P. C. Pires , Sampaio A. J. C. , Freire V. N. 
Published: July 2007

AbstractWe analyze the process of dissipation of energy in excess of equilibrium in photoexcited carriers (electrons and holes) during their second kinetic stage of relaxation (SKS). Quantum kinetic equations obtained from Zubarev's nonequilibrium statistical operator are used, and different initial effective temperatures for electrons and holes are taken. It is shown that holes relax their excess energy faster than electrons for all levels of photoexcitation, and that the mutual thermalization of electron and hole sub‐systems depends on the degree of photoexcitation. The lapse of time for electrons and holes to attain mutual equilibrium is estimated to be of the order of 6ps for a density of photoinjected carriers in GaAs of the order of n = 2.0 × 1018 cm − 3, when the initial excess of kinetic energy per pairis taken as 2.4 eV.
Properties of a Collective Excitation in Quasi‐One‐Dimensional Antiferromagnets
Authors: Pang Xiao‐Feng 
Published: July 2007

AbstractContributions of mangon‐phonon coupling and magnon‐magnon interactions to the formation of localized solitions in common quasi‐one‐deimensional antiferromagnets are closely studied and discussed by using Makhankov's and Pang's method with double‐sublattice model. The anomalous effect in the (CH3)4 NMnCl3 system is also discussed by using these results.
Optical Absorption Studies in Zn1−xMnxSe
Authors: Martinez L. , Gonzalez L. R. , Giriat W. 
Published: July 2007

AbstractThe optical absorption spectra of Zn1−xMnxSe single crystals are investigated, for x = 0.05, 0.06, 0.15, 0.30, 0.45, and 0.50. The measurements were taken from 28 K to RT in the wavelength range between 390 to 520 nm. Four well‐resolved bands below the fundamental absorption edge are observed corresponding to internal transitions from the ground to the excited state within the Mn ion. An initial decrease of the energy gap with the concentration x for all temperatures is found. A break in the monotonic dependence of the energy gap with temperature is observed at high Mn concentrations and low temperatures. This anomalous behavior is associated with the s–d and p–d exchange interactions occurring in diluted magnetic semiconductors (DMS).
Stimulated Brillouin Scattering of Electromagnetic Waves Propagating Obliquely in a Magnetoactive Centrosymmetric Semiconductor Plasma
Authors: Dubey Swati , Ghosh S. 
Published: July 2007

AbstractAn analytical investigation is made for the stimulated Brillouin scattering (SBS) of an electromagnetic wave propagating obliquely with the externally applied magnetic field, in a centrosymmetric, doped semiconductor plasma. The origin of this nonlinear interaction lies in the third‐order optical susceptibility arising due to the nonlinear current density and electrostriction of the medium. The magnitude of the third‐order nonlinear optical susceptibility determined from our analysis is found to agree with the theoretically and experimentally quoted values. The threshold field for inciting SBS and the corresponding gain constant is also identified. The magnetic field and the obliquity of the pump are found to augment the gain constant. The gain constant is found to increase with the scattering angle and is maximum for the backscattered mode. The transmitted intensity of the scattered mode and Brillouin cell efficiency have also been deduced when the cell length is quite large compared to the pump wavelength. The cell efficiency is found to be maximum when the magnetic field is applied across the propagation direction. The analysis established the possibility of optical phase conjugation in a semiconductor plasma and the corresponding threshold cell length is found to be of the order of nm.
Fe2+ Quadrupole Splitting in Dolomite under Orbit‐Lattice Interaction. A mossbauer study
Authors: Sinha T. P. , Ganguli S. , Bhattacharya M. 
Published: July 2007

AbstractThe temperature dependence of the Mössbauer quadrupole splitting of Fe2+ ions in dolomite is theoretically analysed from 12 to 295 K by invoking orbit‐lattice interaction (dynamic crystal field potential through vibronic coupling). This results in a good agreement between theoretical and experimental data. It is shown that the orbit‐lattice interaction is important for ferrous ions and its effect should be duly included in the interpretation of Mössbauer observables as a function of temperature.
Electron Paramagnetic Resonance of Ce3+ in SrS Powder
Authors: Kreissl J. 
Published: July 2007

AbstractThe EPR spectra of SrS : Ce powders with different contents of Ce are studied. The isotropic signal at g = 1.3085 is assigned to the transition within the Γ7 ground state doublet of Ce3+ placed on a high symmetric octahedral site.
An Improved Correlated Effective‐Field Theory for the S = ½ Pure Ising Model
Authors: Zheng S. M. 
Published: July 2007

AbstractAn improved correlated effective‐field theory is introduced for the S = ½ pure Ising model. The theory is based on an approximate decoupling formula for a spin operator which is more reasonable than that in the correlated effective‐field theory (CEFT) and is applied to some lattices for calculating the Curie temperatures. The results are superior to CEFT.
FH(O2−) Centers in Calcium Fluoride Crystals
Authors: Radzhabov E. 
Published: July 2007

AbstractThe spatial structure of FH(O2−) centers in CaF2 is studied by optical dichroism and ESR methods. The dichroism of absorption is induced by light in both FH bands in additively and X‐ray colored samples. The results are correlated with the C2 crystal axes of FH defects. The ESR spectrum of FH centers is rather similar to that of pure F centers. Both dichroism and ESR results prove that an oxygen ion occupies a (110) anion site around the F center.
Correlations Destroy Acoustic Plasmons in Superlattices
Authors: Golden K. I. , Kalman G. 
Published: July 2007

AbstractIt is demonstrated that interlayer correlations bring about important qualitative changes in the collective mode structure of superlattices, as compared to the random‐phase‐approximation predictions. The RPA acoustic plasmon is converted into an optic‐like mode where the k = 0 long‐wavelength frequency is a sensitive function of the strength of the interlayer correlations and of q, the wave number along the direction perpendicular to the layers. The result should motivate Raman measurements on GaAs/AlGaAs samples in parameter domains where the d/a ratio (d is the separation distance between adjacent layers, a the interparticle spacing within a layer) is of order unity and where rs > 2.
The Scattering Matrix Method for n‐Beam Dynamical Calculations of HRTEM Images
Authors: Watanabe K. , Mitsuishi K. , Hashimoto I. 
Published: July 2007

AbstractThe scattering matrix method combined with the layer‐doubling method is applied to n‐beam dynamical calculations at 100 and 300 kV for aluminum, copper, and gold and the validity of the method has been critically examined in comparison with multi‐slice and Bethe's eigenvalue methods. The method is shown to be an efficient dynamical method for high resolution electron microscopy (HRTEM).
Analysis of the Thermal Expansion Coefficient and Its Temperature Dependence for Alkali Halides
Authors: Kumar M. , Upadhyay S. P. 
Published: July 2007

AbstractA thermodynamic analysis of the Anderson parameter is found to yield useful relations for estimating the temperature dependence of interatomic separations r(T) and thermal expansion coefficient α(T). The relations can be used to predict r(T) and α(T) up to the melting temperatures of solids. The results obtained for sixteen alkali halides are compared with the available data reported in the literature. The values of r(T) are in close agreement with the data based on the theory of interionic potentials. The variation of the thermal expansion coefficient is discussed in the light of other investigations.
Relationship between Bulk‐Modulus Temperature Dependence and Thermal Expansivity
Authors: Ledbetter H. 
Published: July 2007

AbstractThe relationship between two anharmonic physical properties is considered: the bulk‐modulus temperature dependence (1/B) (∂B/∂T) and the thermal expansivity (1/V) (∂V/∂T). For the first, Varshni's relationship is used, which derives from an Einstein‐oscillator model of crystals. For the second, a relationship from Slater is used with a similar basis. The ratio of the two properties is found to be γ + 1, where γ denotes the usual Gröneisen parameter, Varshni's adjustable parameter s is interpreted in terms of the Gröneisen parameter, the Einstein temperature, and the atomic volume. Applied to metals, our relationship works well for those with s‐p‐electron bonding, but poorly for those where d‐electrons contribute to bonding.
Phase Transition in Heavily Doped Gallium Arsenide
Authors: Prudnikov V. V. , Prudnikova I. A. , Semikolenova N. A. 
Published: July 2007

AbstractThe results of investigation of GaAs single crystals heavily doped by group VI elements are considered. The doping effects are studied by methods of free carrier infrared absorption, photoluminescence, ultrasonic, and calorimetric methods. For the explanation of the observed peculiarities in doped gallium arsenide the thermodynamic model of phase transitions is proposed using the properties of complex formation in this semiconducting compound.
Investigation of Carrier Tunneling in Short‐Period a‐Si/SiO2 Quantum Structures with a Scanning Tunneling Microscope
Authors: Golonzka O. V. , Smolyaninov I. I. , Vinogradov E. A. , Zayats A. V. , Pudonin F. A. 
Published: July 2007

AbstractCurrent‐voltage (I‐V) characteristics of short‐period (≦2.0 nm) amorphous Si/SiO2 quantum structures (QWs) are investigated with a scanning tunneling microscope (STM). The tunneling current regime has been realized so that the applied bias is dropped mainly across the tip‐sample contact and the nondistorted energy structure of the QW conduction band is probed by tunneling electrons. Such measurements give the possibility to obtain energy positions of QW subbands without additional assumptions about the voltage drop across the layers. The I‐V curves observed are described taking into account sequential resonant tunneling. In the flow current regime the negative differential resistance caused by alignment of ground and first excited electron states is observed even at room temperature due to the high barrier in a‐Si/SiO2 QWs.
Energy of a Non‐Regular Hexagonal Network of Misfit Dislocations in Anisotropic Elasticity
Authors: Bouzaher A. , Bonnet R. 
Published: July 2007

AbstractThe elastic energy per unit surface associated to a non‐regular hexagonal network of misfit dislocations lying along a planar interface is expressed analytically for any two anisotropic crystals. The present expression generalizes recent results only valid for a regular hexagonal network formed by dislocations of the same kind of Burgers vectors. It takes into account the specific cut radius associated to each dislocation segment. A numerical application is presented for dislocation network observed at the Cu/Fe interface.
Two‐Dimensional Deformation Structure in the Potassium Molybdenum Purple Bronze K0.9Mo6O17
Authors: Colaitis D. 
Published: July 2007

AbstractThe structure and different electronic properties of K0.9Mo6O17 are well known. Up to now, a charge density wave instability is associated with the metal‐metal transition observed at 120 K. It is shown that the distortion can be associated with a two‐dimensional deformation structure. The atoms move, especially the Mo atoms of the layers situated in the middle of the molybdenum‐oxygen slab of composition Mo6O17. The structural rearrangement, originating from a single physical phenomenon, namely the reestablishment of the balance of electrostatic charges when the slab contracts with decreasing temperature, is supported by the observation of the relative intensities of the superlattice reflections in electron diffraction. It is in agreement with the temperature dependence of the conductivity. The relationships between effects and causes of the transition are discussed in the earlier and the new interpretation.
Some Magnetic Properties of Thin Ferromagnetic Films in the Coherent Potential Approximation (S = 1)
Authors: Urbaniak‐Kucharczyk A. 
Published: July 2007

AbstractThe coherent potential approximation method taking into account natural disorder connected with different states of spin components is applied to thin magnetic films with spin S = 1. The distribution of spontaneous magnetization as well as the temperature dependence of the critical temperature are obtained for b.c.c. (100) films. The results obtained using Tyablikov decoupling of higher order Green function are improved in relation to the Green function results obtained with the same decoupling procedure by including the line profiles of elementary excitations.
Direct Creation of Excitonic Molecules by Two‐Photon Absorption in Quantum Wells
Authors: Hassan A. R. 
Published: July 2007

AbstractA theory of the direct creation of excitonic molecules (biexcitons) by direct two‐photon absorption in semiconductor quantum well structures is developed. An analytical expression of the biexciton two‐photon absorption coefficient is given for both photon polarizations. Excitonic‐, interband‐two‐photon, and exciton one‐photon processes in quantum wells (QW) are also investigated. The analytical forms and numerical values of the momentum matrix elements involved in each process are provided. The biexciton matrix element is calculated adopting a six‐parameter variational wave function. A numerical estimation for GaAs QW shows that the biexciton two‐photon absorption (BTPA) process dominates the other processes by different orders of magnitude. This enhancement essentially comes from the resonance effect and the structure of the matrix elements included in each process. Furthermore, the BTPA process in the QW for both polarizations is enhanced over its bulk value by about four and five orders of magnitude. This increase is interpreted as due to the spatial confinement of the QW.
Simulation and Experimental Examination of the Evolution of Orientation Gradients in Single Grains during Rolling of Body Centered Cubic Polycrystals
Authors: Raabe D. 
Published: July 2007

AbstractThe evolution of orientation gradients within single grains during cold rolling of body centered cubic (b.c.c.) polycrystals is investigated by means of simulations and single orientation measurements. In two grains with different initial orientations for which unlike orientation gradients are predicted the local misorientations are measured by means of single orientation measurements in the scanning electron microscope. For this purpose pure iron oligocrystals, i.e. polycrystals consisting of only a few large grains are applied. The simulations as well as the experiments clearly show that during cold rolling of b.c.c. polycrystals the misorientations between adjacent regions within a single grain increase and that fragmentation of initially homogeneous grains can happen. It is revealed that the starting orientation of the undeformed grain has a strong impact on the formation of orientation gradients.