bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of High Pressure on the Electrical Conductivity of TlInX2 (X = Se, Te) Layered Semiconductors
Authors: Rabinal M. K. , Titus S. S. K. , Asokan S. , Gopal E. S. R. , Godzaev M. O. , Mamedov N. T.
Published: July 2007

The dc electrical conductivity of TlInX2 (X = Se, Te) single crystals, parallel and perpendicular to the (001) c‐axis is studied under high quasi‐hydrostatic pressure up to 7.0 GPa, at room temperature. Conductivity measurements parallel to the c‐axis are carried out at high pressures and down to liquid nitrogen temperatures. These materials show continuous metallization under pressure. Both compounds have almost the same pressure coefficient of the electrical activation energy parallel to the c‐axis, d(ΔE∥)/dP = −2.9 × 10−10 eV/Pa, which results from the narrowing of the band gap under pressure. The results are discussed in the light of the band structure of these compounds.
Superconductors with a Superlattice of Voids
Authors: Rudko V. N. , Shevtsova O. N.
Published: July 2007

The problem of the interaction between the vortex lattice in superconductors with the void superlattice is investigated. It is shown that the effect of the void superlattice is strong enough and under conditions that can be easily realized, changes the symmetry of the latter. Then the minimum of free energy is reached for the vortex lattice whose unit cell before deformation in the vicinity of voids has the form of a parallelogram with, in general, unequal sides.
Magnetoresistance in NiFebCo/Cu/Co/Cu Multilayers
Authors: Miyazaki T. , Oikawa M. , Ishio S.
Published: July 2007

Multilayer films of glass/[3 nm NiFeCo/dCuCu/3 nm Co]10, dCu = 0.5 to 24 nm are prepared by an electron beam evaporation method under a pressure of ≈ 1.3 × 10−4 Pa. The magnetoresistance for these multilayers is investigated in the temperature range between 4.2 and 300 K. The temperature dependence of magnetoresistance is discussed based on the model of thermal excitation of magnons and also the two‐current model.
Eletron‐Spin Dynamics in the Two‐Dimensional Compound [NH3(CH2)4NH3]MnCl4
Authors: Niang A. , Ablart G. , Pescia J. , Khechoubi E. M. , Chanh N. B. , Duplessix R.
Published: July 2007

The two‐dimensional (2D) system [NH3(CH2)4NH3]MnCl4 is investigated by means of electron spin resonance (ESR) at X‐band over the 40 to 300 K temperature range. The linewidth, the g‐factor, the spin‐lattice relaxation time T1, and their thermal and angular dependences are measured. The data are characteristic of a 2D compound. The angular dependence ΔH(θ) indicates, taking also into account the presence of a half‐field line, the diffusive character of the magnetization at long time. The observed thermal dependences are consistent with an antiferromagnetic order at 40 K. The experimental data are interpreted using the Bloembergen and Wang three‐reservoir model since a strong exchange interaction is present.
Magnetic Properties of the Amorphous Transverse Spin‐1 Ising Model
Authors: Benyoussef A. , Amraoui Y. El , Saber M.
Published: July 2007

The amorphization of the spin‐1 transverse Ising model on a square lattice is studied within the effective field theory. The total longitudinal and transverse magnetizations, and quadrupolar moments are obtained. A number of interesting phenomena is found in these quantities, due to the fluctuation of exchange interactions and the applied transverse field.
Time Dependence of Inelastic Mössbauer Scattering with Stepwise Phase Modulation
Authors: Haas M. , Realo E. , Jõgi J.
Published: July 2007

The theory of time dependence of inelastic Mössbauer scattering in the case of stepwise phase modulation of recoilless γ‐radiation is developed. In thin scatterer approximation the theory describes the conversion electron yield for different center shift and phase change values. Inhomogeneous broadening of resonant lines and a realistic law of source motion is taken into account. Experimentally the stepwise phase modulation of the 14.4 keV resonant radiation is performed by an x‐cut quartz piezotransducer to which a 57Co:Pd γ‐source is cemented. By using a resonance detector with thin 57FeAl‐alloy scatterers to detect conversion electrons, an acceptable time resolution (< 30 ns) is achieved. A good agreement between theory and experiments is obtained.
Resonance Trapping of Positrons in Solids
Authors: Dryzek J.
Published: July 2007

Basing on the approach to the calculations of the positron trapping rate presented by Shirai and Takamura [1], an attempt is made to explain the prevacancy effects observed in positron annihilation measurements in real metals. The calculations are done for the point defect and for the dislocation, taking into account the thermal distribution of the positron energy and the thermal expansion of the crystal lattice. In spite of the thermal distribution of energy of thermalized positrons the resonance trapping of positrons can be responsible for prevacancy effects in metals and can help solve some problems in the study of positron annihilation.
Uniaxial Stress Dependence of the Ionic Conductivity of Some Alkali Halides
Authors: Barth M. , Mohler E.
Published: July 2007

A sensitive method to measure ionic conductivity changes of uniaxially stressed crystals is described. The stress effect in the extrinsic regime of the conductivity of NaCl and LiF single crystals is used to determine the elastic dipole tensor of a migrating cation vacancy. The elastic dipole tensor is transformed to the double force tensor which gives insight into the jump path the ion takes and into the elastic displacements in the immediate neighbourhood of the jumping ion.
ESR Study of Deuterated Analogue of (BEDT‐TTF)4Hg3I8
Authors: Yudanova E. I. , Lyubovskaya R. N. , Hoffmann S. K. , Sekretarczyk G. , Graja A.
Published: July 2007

Electron spin resonance g‐factor, linewidth, and spin susceptibility of single crystals of the fully deuterated bis(ethylenedithio)tetrathiafulvalene (D8‐(BEDT‐TTF)) based conductor with octaiodine‐ mercurate anion (Hg3I8)2‐ are found to show pronounced temperature dependences between 4 and 300 K. A first‐order phase transition of the type semiconductor‐semiconductor is observed at 258 K. Discussion of the ESR parameters and the nature of the phase transition is given.
Hamiltonian Dynamics of the Landau Model of Ferroelectric Liquid Crystals. I
Authors: Mundim K. C. , Pitanga P. , Filho A. Ribeiro
Published: July 2007

A Hamiltonian treatment of the Landau theory of phase transitions is presented. This formulation is based on the analogy between the order parameter space and the ordinary coordinate space of a particle moving in two dimensions. As an application, ferroelectric smectic liquid crystals are considered.
Lattice Dynamics, Thermal Expansion, and Third‐Order Elastic Constants of Seminoble Metals
Authors: Barrera G. D. , Batana A.
Published: July 2007

General expressions to calculate the first, second‐, and third‐order coupling parameters considering a general interatomic model that includes both two‐ and three‐body uncoupled forces are given. These formulae are applied to deduce the analytic expressions of the pressure, the second‐(SOEC), and third‐order (TOEC) elastic constants and the dynamical matrix and its strain derivative for the case of metals with face‐centred cubic (f.c.c.) structure and considering two‐body interactions up to third neighbours and common nearest‐neighbour three‐body force ones. A scheme of parametrization is proposed and the four independent parameters of the model are estimated. Within this framework the lattice contribution to the dispersion curves, the TOEC's, and the Grüneisen function for Ni, Pd, and Pt are calculated and compared with the values obtained from experimental data. Also the microscopic information the model gives, particularly the importance of three‐body forces and the fulfilment of the Grüneisen rule is analyzed.
Bandgap Fluctuations in Compensated a‐Si : H (P, B) Films
Authors: Rath J. K. , Fuhs W. , Mell H.
Published: July 2007

N‐type compensated films of a‐Si : H (P, B) and a‐Si : H (Li, B) are prepared by the glow discharge decompusition of SiH4. Doping is achieved from the gas phase by addition of PH3 and B2H6 or after film deposition by in‐diffusion of Li. The statistical shift of the Fermi level inferred from the temperature dependence of the dark conductivity is consistent with a pronounced minimum in the density of states distribution in the upper half of the mobility gap. This minimum apparently shifts towards midgap when the boron content of the films is increased. The optical absorption indicates a decrease of the bandgap and an increase of the Urbach‐tail parameter Et, with increasing boron content. Both the statistical shift and the changes in the optical absorption are explained in terms of an apparent broadening of the bandtails due to boron‐induced fluctuations of the bandgap.
Nonlinear Waves of Rayleigh Type
Authors: Adamashvili G. T. , Gapishvili N. G. , Ogiashvili Z. D.
Published: July 2007

The theory of self‐induced transparency for waves of Rayleigh type is applied to a system consisting of a solid layer on a solid surface which contains paramagnetic impurities. Explicit expressions for a soliton and breather are given. It is shown that the parameters of the nonlinear waves of Rayleigh type depend on the transverse structure of the field and are formed at any thickness of the film.
Internal Friction Related to Carriers in High‐ Tc Superconductors
Authors: Huang Y. N. , Wang Y. N. , Chen S. O. , Zhou H. L. , Zhang Q. M. , Shen H. M. , Yang Z. , Zhao Z. X. , Chao G. H.
Published: July 2007

For both BiSrCaCuO and TlBaCaCuO samples, internal frictions (Q−l) in the kHz range reveal a plateau (Qp−1) above Tc and drop rapidly below Tc with the turning points at Tc. This kind of anomaly cannot be observed for nonsuperconducting samples. Moreover, it is discovered that Qp−1 is nearly proportional to the Hall carrier density for Y(Pr)BaCuO with various Pr content. These results show that the drop of Q−1 below Tc is closely related to superconducting condensation. The mechanism of Qp−1 may be attributed to the carrier jump induced by the stress‐induced reorientation of the polaron‐like distortions around the carriers. Further, taking into account the smearing of the superconducting gap structure resulting from the recombination of quasi‐particles and modifying the BCS relative jump rate the calculated results of internal friction below Tc are in fairly good agreement with experimental data. The superconducting gap Δ and the damping rate Γ for both BiSrCaCuO and TlBaCaCuO have also been obtained, they are in accordance with those got by tunneling spectrum and NMR methods, etc.
Study of Crossover Behaviour in Layered Antiferrormagnets
Authors: Singh Ravi P.
Published: July 2007

The 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

The 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

The 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

A 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

A 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

A 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

On 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

The 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

A 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

Analysis 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

A 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

We 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

Contributions 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

The 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

An 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

The 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

The 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

An 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

The 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

It 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

The 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

A 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

The 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

The 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

Current‐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

The 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

The 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

The 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

A 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

The 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.
Theoretical Evaluation of the Dynamic Properties of Amorphous Selenium
Authors: Rao R. V. Gopala , Venkatesh R.
Published: July 2007

The phonon frequencies of amorphous Se are calculated according to Bhatia‐Singh's method assuming the electronic contribution to the bulk modulus is zero. These longitudinal phonon frequencies are used to evaluate the dynamic structure factor in the viscoelastic approximation. It is found that the dynamic structure factor of Se exhibits characteristics similar to that of Al and Rb. The current correlation function JL(k, ω) and S(k, ω) are calculated for six typical momentum vectors. ωmax values are obtained from the computed JL(k, ω) correlation functions. The ωmax(k) versus k plots give a maximum midway between zero and the second peak position of S(k). S(k, ω) versus ω at fixed values of k follows the predictions made by Egelstaff. At large k, the S(k, ω) gives a Lorentzian single‐peaked curve resembling that of a perfect gas. It is found that the dynamic structure factor of amorphous Se resembles that of liquid metals like Al and Rb.
The Effect of Anharmonicities on the Absorption Spectrum of a Diatomic Linear Chain within a Central Force Model
Authors: Held Th. , Pfeiffer I.
Published: July 2007

Closed‐form expressions for the Fourier‐transformed anharmonic force constants of a diatomic linear chain within a central force model are obtained. Some differences compared to those expressions obtained by some other authors, especially in Peierls' approximation, are discussed and illustrated by numerical calculations of the linewidth and frequency shift of the “reststrahlen” peak of a KCl crystal up to second order of the perturbation theory.
Composite Anderson‐Newns Model and Density of States Due to Chemisorption
Authors: Guleria R. , Ahluwalia P. K. , Sharma K. C.
Published: July 2007

A systematic variation in the density of states (DOS) of a substrate with respect to the chemisorption of hydrogen on transition metals using the composite Anderson‐Newns model is investigated for different coverages. The substrate is a one‐dimensional periodically arranged chain of atoms. A comparison between the change in DOS using semi‐elliptical weighted DOS and triangular weighted DOS is also made for non‐magnetic and magnetic cases. The coverage dependence shows a discerning behaviour on the formation of bonding and antibonding peaks. As the coverage increases, the bonding and antibonding peaks are found to shift towards higher energies and at the same time the relative height of the peaks increases, that clearly shows, at which coverage more chemisorption is likely to take place.
Power Losses of a High‐Temperature Superconducting Semi‐Infinite Space
Authors: Saif A. G.
Published: July 2007

An exact solution of the anisotropic London equation is obtained for the system of a vibrating wire carrying dc current (a non‐vibrating wire carrying ac current) parallel to a high‐temperature superconducting planar surface. Also, the distribution of the external magnetic field outside the superconductor is exactly determined. The power losses and the reactive power losses on the surface of the sample are completely determined. It is found that these losses along the versor of the anisotropy axis are increased as λ increases, moreover, they are greater than the power losses along the crystal symmetry direction a. In the isotropic case a satisfactory agreement with previous results is achieved.
Pairing in a Two‐Dimensional Two‐Band Very Anisotropic Model in the Mean Field Approximation
Authors: Fazakas A. B. , Pitis R.
Published: July 2007

A two‐dimensional model is proposed: there are two kinds of sites, with one electronic state per site; tunneling takes place only in one direction. The interaction involves only electrons on different sites. The existence of a phase transition involving interband pairing of electrons is discussed in the mean‐field approximation.
Spectral Studies of Isostructural Organic Metals (BEDT‐TTF)3(HSO4)2 and [Ni(dddt)2]3(HSO4)2
Authors: Świetlik R. , Kushch N. D. , Kushch L. A. , Yagubskii E. B.
Published: July 2007

The polarized reflectance spectra of (BEDT‐TTF)3(HSO4)2 and [Ni(dddt)2]3(HSO4)2 crystals are measured in the IR region (660 to 5200 cm−1) at room temperature. Moreover, the powder absorption spectra in the frequency range 400 to 40000 cm−1 are reported. Differences between charge transfer bands of both compounds are found. In [Ni(dddt)2]3(HSO4)2 the strong coupling of electrons with vibrations of C = C ring bonds is observed. The phase transition in (BEDT‐TTF)3(HSO4)2 at the temperature T = 130 K influences the IR powder absorption spectra, especially within the range of C = C vibrations.
Small‐Angle Scattering by Spherical Inhomogeneities of Different Radii and Refractive Indexes
Authors: Pomortsev R. V. , Skryabin Yu. N.
Published: July 2007

The cross‐section σ of small‐angle scattering of neutrons by inhomogeneities of a wide variety of sizes Rmin < R < Rmax is considered when the scattering crossover from diffraction R < Rc to refraction R > Rc takes place within the available range of size distribution. When qRc ≫ 1 the cross‐section averaged by the distribution function P(R) ∝︁ R−3−Δ(0 < Δ < 1) results in the dependence σ(q) ∝︁ q−4 + 4 in the Born approximation, where q is the transfer momentum, and in case qRc ≪ 1 the averaging leads to the dependence σ(q) ∝︁ q−2+Δ. In the case of scattering in condensed matter containing large scale and loose pores, to which a distribution of the scattering potential can be attributed, the averaging in refractive index fluctuations is carried out. The result of such an averaging is demonstrated by means of a homogeneous distribution.
An Identical Quasicrystal‐Structure Model for T2‐ (AI6CuLi3) from Two Different Six‐Dimensional Crystals
Authors: Pan G. Z. , Li F. H.
Published: July 2007

The restriction for obtaining an identical quasicrystal by using the projection method from different high‐dimensional crystals is discussed. Two kinds of six‐dimensional crystal‐structure models, (Al, Cu)7Li and (Al, Cu)7Li20, are used to obtain the icosahedral quasicrystalline phase T2‐ (Al6CuLi3). It is demonstrated that an identical idealized icosahedral phase T2‐A16CuLi3 with the same chemical composition, mass density, and atomic arrangement can be obtained by projecting anyone of two 6D crystals onto the physical space. Idealized projection windows are determined on the basis of the rhombic tiling of body‐centered cubic crystals and similarities between the local atomic arrangement of the icosahedral phase T2‐A16CuLi3 and that of its crystalline approximant R‐Al5CuLi3. The chemical formula and the mass density of the idealized T2 phase is given.
Spectrum of an Electron Localized by Electrostatic Image Forces in Thin Semiconductor Layers
Authors: Tkach M. , Holovatsky V. , Voitsekhivska O.
Published: July 2007

The spectrum of an electron confined in a layer with the dielectric constant ϵ, embedded between two media with larger (ϵ> > ϵ) and smaller (ϵ< < ϵ) values of the dielectric constant is calculated. The dependence of the spectrum on the layer thickness and on the ratio between the dielectric constants of the layer and the media is calculated. It is shown that if the crystal thickness increases, the whole spectrum of electrons shifts into the region of negative energies and becomes quasi‐hydrogenic there.
Bound State of Electrons in a One‐Dimensional Chain
Authors: Brizhik L. S. , Eremko A. A.
Published: July 2007

The problem of dynamical interaction of two electrons interacting with acoustic phonons in a one‐dimensional chain is investigated using the variational method in nonadiabatic approximation. It is shown that under certain conditions electrons form a bound delocalized singlet spin state. The energy of this state and the binding energy are calculated.
Thermal Strain Effects in YBa2Cu3O6+δ Superconductors
Authors: Nechiporenko I. N. , Popov V. P. , Sirenko V. A.
Published: July 2007

Results of the dilatometric study of superconductors with different contents of oxygen and normal phase are presented. They suggest a significant influence of the competition between strain and composition effects upon the superconducting order parameter formation and crystallographic ordering, which is revealed in anomalies on the measured temperature depcndencies of thermal strains.
Tricritical Behaviour in a Simple Model of an Itinerant Antiferromagnet
Authors: Chamati H.
Published: July 2007

The thermodynamics of a simple electron—spin system describing a paramagnetic—antiferromagnetic phase transition is considered for an arbitrary value of the coupling constant in the presence of doping controlled by the chemical potential μ. The ground state is studied exactly, and a second‐order phase transition with respect to μ is found. The case of nonzero temperature is examined numerically. The Landau expansion depending on two parameters is analyzed near the critical line and in the vicinity of the tricritical point (μ*, T*) in the (μ, T)‐plane. The phase diagram in the (n, T)‐plane (n is the electron concentration) shows a coexistence phase for 0 < T ≦ T* and the existence of an isolated point at T = 0.
Phase Diagrams of the Dilute Ashkin‐Teller Model
Authors: Benyoussef A. , Loulidi M.
Published: July 2007

The phase diagram of the dilute Ashkin‐Teller model is investigated within a real‐space renormalization scheme. Using two probability distributions which correspond to the two‐spin coupling, K2, dilution and the four‐spin coupling, K4, dilution, a rich variety of phase diagrams are obtained. The percolation phase diagram is obtained at T = 0. It describes a system of two types of links with different concentrations.
Electron Paramagnetic Resonance of an O, Ion Trapped in KI
Authors: Moens P. , Callens F. , van Doorslaer S. , Maes F. , Matthys P.
Published: July 2007

An ozonide ion is detected in single crystals of KI by means of electron paramagnetic resonance. The new data support the monovacancy model for the location of the O ion in the alkali halides proposed recently. Theoretical calculations using self‐consistent field Hartree‐Fock methods are performed in order to determine the principal g‐tensor values for the O molecular ion. The results are in good agreement with the experimental data.
Implantation Profile and Positronium Fraction for Low‐Energy Positrons
Authors: Shrivastava S. B. , Agrawal Kavita
Published: July 2007

The diffusion equation for positrons is solved subjected to two types of implantation profiles, namely, exponential and derivative of Gaussian type. The positronium fraction obtained using the two profiles has been compared with experimental results. It is found that in general, the use of the derivative of the Gaussian‐type profile gives better results. The maximum yield of the positronium as a function of temperature is predicted corresponding to the different orientations of the aluminum surface.
Influence of Hydrostatic Pressure on the Fermi Surface of Vb Transition Metals
Authors: Solanki A. K. , Ahuja R. , Auluck S.
Published: July 2007

Ab initio calculations of the effect of hydrostatic pressure on the Fermi surface of transition metals, vanadium (V), niobium (Nb), and tantalum (Ta) using the linear mufin‐tin orbital method in the atomic‐sphere approximation are presented. The self‐consistent calculations are performed with the use of various exchange‐correlation potentials. The results are compared with available experimental data and theoretical Calculations.
Oscillator Strengths of the Intraband Transitions in a Semiconductor Superlattice in an Electric Field
Authors: Tronc P.
Published: July 2007

The oscillator strengths of the intraband optical transitions in a semiconductor superlattice in an electric field parallel to the growth axis is calculated using a perturbation model with Bloch envelope functions. The applied electric field induces strength asymmetry between the oblique +p and — p transitions of the Wannier‐Stark ladder.
Rotational Hysteresis Work and Rotational Hysteresis Integral of Polycrystalline Permanent Magnets
Authors: Elk K.
Published: July 2007

The possible information on the magnetization processes of the grains available from the rotational hysteresis work and the rotational hysteresis integral of polycrystalline samples is investigated. A combined theoretical model for the switching field of the grains is considered, and numerical computations are carried out for different values of the model parameters. Especially, the influence of different orientations of the rotational plane of the applied field is studied.
EPR Identification of the Trigonal Ti‐Sp Pair in GaP
Authors: Kreissl J. , Ulrici W. , Gehlhoff W.
Published: July 2007

EPR experiments on titanium‐doped n‐ and p‐type GaP are reported. In n‐type GaP:Ti co‐doped with sulphur additional to the known isolated defects an angular dependent spectrum showing trigonal symmetry can be observed. The analysis suggests that the spectrum is caused by a donor‐acceptor nearest‐neighbour pair defect [Ti
‐Sp]. Theparameters of the spin‐Hamiltonian are determined as follows: S = l, g | = g⟂ = 1.95(1), |D| = 0.0670(5) cm−1. The concentration of thepair defect confirms that Coulomb attraction is responsible for its preferential formation. However, no EPR spectrum of a pair can be proved for p‐type GaP: Ti co‐doped with Zn, although the existence of such a pair is also most probable. The lack of the EPR spectrum could have spectroscopical reasons.
Dispersion in the Linear and Second‐Order Bond Polarizabilities of Alkali Halides
Authors: Kucharczyk W.
Published: July 2007

An analysis of dispersion in the linear bond polarizability and in the lattice contribution to the second‐order polarizability of some alkali halides is performed. The linear and nonlinear polarizabilities are evaluated employing the elasto‐optic coefficients p11; p12, and p44. The results obtained show significant wavelength dependence of the linear polarizability and the longitudinal component of the second‐order polarizability. The anisotropy of the bond polarizability is also found to be frequency dependent.
Time Resolved X‐Ray Interference in the Presence of Ultrasonic Excitations
Authors: Follath R. , Jex H.
Published: July 2007

The ultrasonic waves running into an X‐ray interferometer are studied. Their influence is demonstrated in conventional as well as time dependent interference patterns. The time signals correlate the registration time of the X‐ray photons with the phase of the phonons.