bluuebunny/crossref_metadata_2025 huggingface dataset:
A Weighted Linear Chain Approximation for the Ising Model Free Energy
Authors: Ferreira J. R. Faleiro , Silva N. P. , Franco B. J. O.
Published: July 2007

3D Ising model thermodynamic functions are evaluated through a variational approach in which the partition function is taken as product of different direction linear chain partition functions.
Magneto‐Optical Phenomena in Semiconductor Superlattices
Authors: Shon Nguyen Hong
Published: July 2007

The Faraday and birefringence effects due to infrared radiation in semiconductor superlattices in quantizing magnetic field are investigated. It is shown that for a non‐degenerate electron gas in the one‐mini‐band approximation the frequency behaviour of magneto‐optical coefficients exhibits finite jumps at ω = nωc ± 2Δ ω is the frequency of electromagnetic radiation, ωc the cyclotron frequency, and Δ the half band‐width). The natural birefringence and dichroism in superlattices are also calculated.
On a Simplified Analysis of the Carrier Contribution to Elastic Constants of Ternary and Quaternary Compounds
Authors: Ghatak K. P. , Mondal M.
Published: July 2007

An attempt is made to study the carrier contribution to the elastic constants of degenerate ternary and quaternary compounds on the basis of a newly derived electron dispersion law taking into account the presence of band tails. It is found, taking n‐Hg1−xCdxTe and In1−xGaxAsyP1−y lattices matched to InP as examples, that the said contribution increases with increasing carrier concentration in different manners. The band tail diminishes the numerical values of the carrier contribution to the second and third order elastic constants for both the compounds. An experimental method is suggested of determining the electronic contribution to the elastic constants in degenerate materials having arbitrary carrier dispersion laws. In addition, the corresponding well‐known results for non‐degenerate wide‐gap materials in the absence of band tails are also obtained from the generalized expressions under certain limiting conditions.
FIR Absorption in Monoclinic ZnAs2
Authors: Weszka J. , Mazurak Z. , Pishchikov D. I.
Published: July 2007

FIR absorption in powdered ZnAs2 crystals is studied. The features within the frequency area 194 to 270 cm−1 are attributed to As helix modes. Low‐frequency features are attributed to modes involving important contributions of Zn atoms. There are some indications that inter‐As helix correlation splitting exists.
Anharmonic Effects in KDP near the Ferroelectric Phase Transition
Authors: Serra K. C. , Melo F. E. A. , Filho F. Mendes , Moreira J. E. , Lemos V.
Published: July 2007

The temperature dependence of some Raman modes of KDP, below the ferroelectric phase transition, is investigated. Two lattice modes display an exponential dependence with T which is explained by third and fourth order anharmonic effects.
Characterization of GaSb Epitaxial Layers on GaSb and GaAs Substrates by Infrared Reflectivity
Authors: Mezerreg A. , Llinares C.
Published: July 2007

Infrared reflectivity measurements on p‐GaSb epitaxial layers on n‐GaSb substrate and n‐GaSb epitaxial layers on semi‐insulating (SI) GaAs are made to determine the mobility and density of free carriers in the bulk and the layer simultaneously by fitting a theoretical model to experimental data. The results obtained are generally in good agreement with those given by Hall measurements.
Electronic Structure of La2O3 and LaF3 Crystals
Authors: Evarestov R. A. , Leko A. V. , Murin I. V. , Petrov A. V. , Veryazov V. A.
Published: July 2007

The band structure and local properties of La2O3 and LaF3 crystals are investigated within the framework of CNDO approximation.
Semiempirical Crystal Field Calculations for fN‐Systems. II. Crystal Field Effects in Hexahalogenide PaX, UX, and UCl Complexes and in UCl4 Crystal
Authors: Mironov V. S. , Rosov S. P.
Published: July 2007

On the basis of a many‐electron semiempirical method developed in the previous paper a program for practical crystal field energy calculations for fN‐systems is written and applied to octahedral PaX, UX, and UCl complexes, and the UCl4 crystal. The results obtained are in very good agreement with the experimental data on crystal field levels in these compounds. The vibronic coupling constants are also calculated for the series of PaX and UX complexes in the frame of this method.
Mechanism of Temperature‐Induced Transitions between Stripe and Undulation Domain Structures
Authors: Zablotskii V. A. , Mamalui Yu. A. , Lamonova K. V.
Published: July 2007

The temperature‐induced instability of thin magnetic film stripe domain structures (SDS) is studied. Dislocations present in the SDS are shown to be nuclei for the formation of an undulation structure, the domain wall bend period being determined by the dislocation strength. The range of thermal stability of the stripe domain structure is calculated.
Structural Studies of Liquid Germanium Using Lebowitz's Solution of Hard‐Sphere Mixtures
Authors: Rao R. V. Gopala , Debnath D.
Published: July 2007

Experimental investigations on liquid germanium show a small subsidiary shoulder in the main peak of the structure factor S(k) near the melting point. Through Lebowitz's solution of hard sphere mixtures the structure factors are treated theoretically investigated. Experiments and molecular dynamic studies conclusively point out that the liquid structure arises from the interplay of two characteristic distances and these are figuratively treated as different diameters. The present theoretical results are in good agreement with those obtained by other methods in the whole range of momentum space. The isothermal compressibilities are calculated by using Kirkwood‐Buff's equation.
Phonon‐Limited Electron Mobility in a Polar Semiconductor Quantum Well
Authors: León H. , León F. , Comas F.
Published: July 2007

Low‐field drift mobility for transport parallel to the interfaces in a polar semiconductor quantum well is calculated following an iterative procedure. Calculations are carried out in the size‐quantum‐limit approximation for a degenerate electron gas. Screening of the electron–phonon interaction via acoustic deformation potential, piezoelectric of the accoustic branch, and longitudinal optical mode couplings is taken into account by means of a dielectric function deduced in an earlier work. Confinement of LO vibrations is also taken into account by means of an appropriate Hamiltonian successfully employed in previous works. The role of the electron–LO‐phonon interaction is accurately established and the conclusion is drawn that this coupling becomes considerable at intermediate and very important at sufficiently high temperatures for quantum wells of the GaAs/AlGaAs type. Comparison with experiment gives an acceptable agreement between calculated and measured mobility values.
Study on Momentum Density of Electrons and Fermi Surface in Magnesium by Positron Annihilation
Authors: Nakashima H. , Kubota T. , Kondo H. , Tanigawa S.
Published: July 2007

The three dimensional electron–positron momentum density in magnesium is reconstructed from measurements of two dimensional angular correlation of positron annihilation radiations (2D‐ACAR) followed by the image reconstruction technique based on a direct Fourier transformation. From these data, informations are obtained about the Fermi surface topology, the high momentum components (HMC), the enhancement effect, and autocorrelation function.
Application of the Linear Response Theory to the Perfect Diamagnetism in Superconductors
Authors: Tokita M. , Zenmyo K. , Haga E.
Published: July 2007

Taking account of the Meissner effect (B = 0 for the superconducting state), the vector potential A is defined not by B = rot A but by H = rot A to calculate the linear magnetic field response of a bulk superconductor. The magnetic interaction in the Bohm‐Pines theory is consistently introduced into the Hamiltonian of the system. On the basis of both the usual linear response theory and the BCS‐Bogolyubov theory, the static magnetic susceptibility χ = −1/4π, i.e. perfect diamagnetism, is obtained for the bulk superconductor at zero temperature.
Electron Irradiation Damage in Pure and Impurity‐Doped RbMgF3 Crystals
Authors: Sardar D. K.
Published: July 2007

The radiation damage processes in pure and impurity‐doped RbMgF3 crystals are studied by identifying and monitoring the radiation induced defects. Interstitials and Vk centers are observed at low temperatures. One of the most important characteristics of RbMgF3 is that the F centers form in this host without their aggregation even at room temperature. Annealing of these samples at higher temperature after room temperature irradiation however forms F‐center aggregates such as F2‐centers and F3‐centers and also impurity‐F center pairs. In the case of impurities such as Er3+ or Mn2+ which substitutionally replace Mg2+ ions in the RbMgF3 lattice, there is no effect on the early stage production of F centers. It is observed that the statistical productions of the F aggregates and Mn2+‐F pairs occur during room temperature irradiation. On the other hand, the larger Eu2+ or Cu2+ ions substitute for Rb+ ions in this host and significant enhancement of the early stage formation of F centers at room temperature is observed. The experimental observations are compared with the proposed model on radiation defect production.
Theory of Dipole–Dipole Energy Transition of Electron Excitation in a Three‐Layer System
Authors: Pokutnyi S. I. , Efremov N. A.
Published: July 2007

A theory of dipole‐dipole energy transition of electron excitation for a three‐layer system is framed: the Förster formula is generalized for the energy transfer probability.
Method of Functional Derivatives in the Theory of Point Defects in Crystals: I. General Theory. Representations of the Self‐Energy
Authors: Kantorovich L. N.
Published: July 2007

A novel method is suggested for the calculation of point defect electronic structure in arbitrary crystals using a non‐orthogonal localized atomic orbital basis set. The method is based on the concept of a cluster Green's function and uses a special truncating procedure for the Green's function chain, such that only the defective region of the crystal is considered precisely, while the rest of the crystal as well as its interaction with the defective region are considered in Hartree‐Fock approximation. It is shown, that the correct incorporation of an electronic correlation in the defective region is connected with a relevant choice of a cluster self‐energy operator. Decompositions of this operator into a series either over screened or unscreened interaction are investigated. An approximation analogous to the simplest Hedin's GW‐approximation is considered. Qualitatively this approach is equivalent to that which uses the generalized Anderson Hamiltonian.
Features Peculiar to Transport Phenomena in Cobalt‐Doped HgSe Crystals
Authors: Tsidilkovskii I. M. , Lerinman N. K. , Sabirzyanova L. D. , Paranchich S. Yu. , Paranchich Yu. S.
Published: July 2007

On Hg1−xCoxSe (5 ⪅ 10−5 x ⪅ 1 × 10−2) crystals the Hall coefficient, resistivity, and Shubnikovde Haas oscillations of transverse magnetoresistance are measured. Experimental findings permit the conclusion that in mercury selenide, cobalt atoms form a resonance donor level. However, by contrast with mercury selenide crystals doped with iron and chromium, the cobalt impurity does not lead to the known electron mobility “anomaly” due to the ordering of the system of charged impurities. Shubnikov‐de Haas oscillations exhibit peculiarities arising from a substantial exchange interaction of conduction band electrons with 3d electrons of cobalt ions.
The Conductivity Anisotropy in Uniaxially Stressed p‐InSb
Authors: Germanenko A. V. , Minkov G. M. , Rut O. E.
Published: July 2007

The conductivity anisotropy induced by uniaxial stress in p‐InSb with acceptor concentration NA ≈︁ (1014 to 2 × 1016) cm−3 is investigated in the temperature range T = 2 to 70 K under uniaxial stress up to 6 × 108 Pa. In pure materials pressure and temperature dependences of the conductivity anisotropy are explained by ordinary models in the hopping regime and regime of free hole conductivity. In doped semiconductors these dependences cannot be explained without taking into account the peculiarities of the metal‐dielectric transition induced by uniaxial stress in p‐type narrow‐gap semiconductors with degenerate valence band ⌜8.
Spectrum of Interface Coupling‐Affected Spin‐Wave Modes in Ferromagnetic Bilayer Films. The Emergence of Interface Mode
Authors: Puszkarski H.
Published: July 2007

A microscopic theory of spin‐wave modes in exchange‐coupled bilayer films is presented. Using the interface inhomogeneity model (an effective interface exchange coupling parameter is introduced) as well as the Heisenberg model in the spin (exchange and Zeeman) Hamiltonian, an energy dispersion relation and spin‐wave mode functions for the exchange‐coupled bilayer ferromagnetic films are derived. The theory is valid for Bravais lattices with arbitrary type of film surface/interface orientation at perpendicular (to the film surface) configuration of the static external field and arbitrary ferro/antiferro‐magnetic value of the interface exchange coupling. The semi‐classical picture of the bilayer spin‐wave modes is elaborated and the influence of the interface coupling parameter on the shape of the modes is discussed. Then, a detailed discussion of the conditions for the occurrence of interface‐localized spin‐wave modes is given. The analysis of the interface mode properties is based on the exact solution of the eigenvalue problem of a bilayer film achieved by the interface rescaling approach. Subsequently, a microscopic theory of the ferromagnetic resonance effect in the exchange‐coupled bilayer films is developed. It is shown that when intrinsic surface anisotropies are absent the pattern of the resonance spectrum is determined solely by the nature of the interface exchange‐coupling integral JAB. When this integral couples ferromagnetically (JAB > 0) the sublayer spins at the interface, the resonance spectrum consists of only one resonance line which is insensitive to the strength of the coupling. On the contrary, when coupling is antiferromagnetic (JAB < 0), the resonance spectrum is multipeak in nature with the first two (high‐field side) lines exhibiting pronounced intensities. When antiferromagnetic coupling is weak, the intensity pattern of these two lines is “regular” (i.e. l1 > l2); however, for stronger antiferromagnetic coupling, an inversion of these intensities occurs leading to an “inverted” intensity pattern (l1 < l2) in accordance with certain experimental results. Moreover it is shown that inversion of the intensities (caused by negative JAB) is due to the fact that the highest‐field line arises by excitation of the mode which is localized on the bilayer interface. It is also shown that this mode represents a bilayer complex vibration formed by the in‐phase combination of the sublayer vibrations, and this result corrects the general belief of experimenters that the mode under consideration is due to out‐of‐phase combination of sublayer vibrations.
Electron Paramagnetic Resonance Studies of Cu2+ Ions Doped in Tl2Zn(SO4)2 · 6 H2O Single Crystals
Authors: Krishna R. M. , Rao J. L. , Bhaskar V. V. , Lakshman S. V. J.
Published: July 2007

The results of EPR and optical absorption studies of Cu2+ ions doped in TZSH single crystals in the temperature range 420 to 120 K at X‐band frequency are reported. The ground state wave function for the Cu2+ ion in TZSH is evaluated. The principal g and A parameters are fitted to orthorhombic symmetry.
Surface Magnetizations of a Semi‐Infinite Ferrimagnetic System with a Compensation Point in the Bulk
Authors: Kaneyoshi T.
Published: July 2007

Thermal variations of magnetizations at or near a surface are investigated by using a semi‐infinite mixed spin ferrimagnetic system within the framework of the effective‐field theory and the three‐layer approximation. Depending on the value of single‐ion anisotropy in the bulk or at the surface, the system can exhibit the compensation point in the bulk magnetization or magnetization near the surface. The effects of compensation point in the bulk on magnetizations at or near the surface are in particular examined. A number of interesting phenomena are found, such as a fact that the compensation temperature in the vicinity of the surface may be different from that in the bulk.
Surface Magnetic Properties of an Amorphous Semi‐Infinite Ferromagnet with Random Fields
Authors: Sarmento E. F. , Saber M. , Kaneyoshi T.
Published: July 2007

Surface magnetic properties of an amorphous semi‐infinite spin 1/2 Ising ferromagnet with random fields at surface and bulk are investigated by the use of the effective‐field theory with correlations. A number of characteristic behaviours is found for the surface magnetic properties. In particular, the effects of competition between the random field and the structural fluctuation parameter δ(the reentrant phenomenon in the bulk) on magnetizations at and near the surface are clarified. The surface crystallization of an amorphous ferromagnet may be important in the sense that a surface transition temperature higher than that ofthe bulk can be obtained from the process of an appropriate amorphous semi‐infinite ferromagnet with a random field in the bulk.
On the Bergman Spectral Function of a Two‐Component Hetero‐System with Interchange Symmetry
Authors: Gerlach E.
Published: July 2007

It is shown that for materials with interchange symmetry the Bergman spectral function g(u) obeys the symmetry relation u g(u) = (1 ‐ u). g (l ‐ u), from which a number of interesting conditions for g(u) can be derived.
Short‐Lived Frenkel Defects in α‐Al2O3
Authors: Aluker E. D. , Gavrilov V. V. , Chernov S. A.
Published: July 2007

The fast luminescence, the transient absorption spectra, and the recombination kinetics are observed and investigated in α‐Al2O3. Absorption spectra reveal short‐lived bands at 2.75 and 2.15 eV. Their decay times remain the same (30 to 200 ns) in the temperature interval 80 to 293 K. They are found to be almost the same as the lifetime of the fast components of the luminescence band at 3.8 eV. They are expected to be due to interstitial aluminium ions. Experimental results argue for subthreshold creation of short‐lived Frenkel defect pairs in the cation sublattice of α‐Al2O3.
Nucleation and Growth of Adsorbed Layer Self‐Consistent Approach Based on Kolmogoroff‐Avrami Model
Authors: Dubrovskii V. G.
Published: July 2007

The kinetic model for the growth of adsorbed monolayer is developed in order to describe the growth process beginning from the initial stage of nucleation up to the formation of a continuous film. The nucleation of clusters on the uncovered substrate region is studied within the frame of kinetic theory of condensation. Direct coalescence of clusters is described by means of Kolmogoroff‐Avrami model for two‐dimensional crystallization. The basic self‐consistent equation for the dynamics of adatom supersaturation is derived. Numerical solutions for supersaturation, mean substrate coverage, boundary perimeter, and total number of clusters are obtained. Depending on the numerical value of the governing parameter, three known experimental growth modes are determined. In the regime of incomplete condensation the analytical solutions for the basic kinetic and morphological characteristics of the growing layer are obtained, which are in agreement with numerical results.
Kubo Formalism for Resonant Tunneling in Double Barrier Heterostructures
Authors: Montenegro N. Porras , Latgé A. , Anda E. V.
Published: July 2007

The electronic and transport properties of double barrier structures are studied using the Kubo formula for the electrical conductivity and a tight‐binding respresentation for the Hamiltonian. The theory is able to reproduce the experimental I‐U characteristic curves. A self‐consistent Hartree treatment permits to study the charge distribution and its influence on the position of the resonant energy levels and on the tunneling properties as a function of the doping of the sample.
Resonant Photoemission from Gd on Cu(100)
Authors: Scarfe J. A. , Law A. R. , Hughes H. P. , Bland J. A. C. , Roe G. M. , Walker A. P.
Published: July 2007

Synchrotron‐radiation‐excited photoemission is studied from a monolayer film of Gd on Cu(100). Resonant enhancement of the 5s, 5p, and 4f core‐level intensities is observed for photon energies in the region of the Gd 4d → 4f threshold. The intensity ratios of the components of the Gd 5p and 5s multiplet structures are found to be modified at the resonance as a consequence of the different parentage of the photoexcited final states involved; satellite structures are also observed corresponding to final states that are unobtainable off‐resonance.
Method of Functional Derivatives in the Theory of Point Defects in Crystals: II. General Theory. Total Energy, Electronic Density, and Density of States
Authors: Kantorovich L. N.
Published: July 2007

A novel method for the calculation of point defect electronic structure in arbitrary crystals in a non‐orthogonal localized atomic orbital basis set, based on the concept of a cluster Green's function and a cluster self‐energy matrix, has been suggested in the preceding paper. In the present paper the general formulas for the total electronic energy, density of states, and electronic density matrix for a defect‐containing crystal are obtained within an approximation analogous to the famous Hedin's GW‐approximation. Besides relevant properties of the cluster Green's function are carefully derived and discussed.
Nanocrystalline versus Nanoamorphous Materials
Authors: Gonser U. , Fujita F. E.
Published: July 2007

With the advent of any new material it is highly desirable to determine its structure and, in addition, new definations must be formulated. Regarding nanocrystalline and nanoamorphous materials, the typical structure are in both cases on a nano‐dimensional scale.
How to Distinguish Dislocation Drag Processes from Thermal Activation
Authors: Galligan J. M.
Published: July 2007

Experiments are described which demonstrate the influence of viscosity on thermal activation. In addition experiments which delineate the magnetooscillations in the drag coefficient are detailed. The first set of experiments shows that the creep rate, after plastic deformation, exhibits a minimum and a maximum. This is explained by considering the temperature dependence of the electron drag in the superconducting state. The magnetooscillations a allow a distinction to be made between an anharmonic drag mode and a flutter mode; the experiements show that a flutter mode is preferred for temperatures below 5 K.
Driving Force and Mobility for Microstructural Evolutions. The Rate of Grain Rotation across a Grain Boundary
Authors: Martin G.
Published: July 2007

The orientation dependence of the grain‐boundary energy is a driving force for grain rotation across the boundary in a bicrystal. This has been observed experimentally. Since the energy gained during such a rotation must be entirely transformed into heat, and since this process occurs because of the friction of the grain‐boundary dislocations on the lattice, a simple expression of the rate of energy dissipation as a function of the rate of rotation can be established. It is then demonstrated that the rate of rotation of the grains is the product of a “torque” (gain of energy per unit change of misorientation) times a mobility coefficient. The latter is a function of the misorientation angle, a feature which had been overlooked in the classical theory. Close to zero misorientation, where the grain‐boundary energy exhibits a cusp, the mobility decays algebraically with the misorientation angle, while the torque diverges logarithmically only. As a consequence, the rate of rotation decreases with the misorientation, in agreement with experiment. Existing quantitative data are reasonably well fitted by this model.
EBIC Experiments at Dislocations in Germanium
Authors: Hess J. , Schreiber J. , Hildebrandt S. , Labusch R.
Published: July 2007

A new kind of EBIC contrast is observed at 60°‐dislocations in thin slabs of germanium. Minority carriers are collected near one surface by the electrostatic potential of the dislocation and are carried along its conducting core to a contact on the opposite side. This experiment nicely demonstrates the existence of the dislocation potential as well as the conductivity of the dislocation core. A theoretical model of the effect which includes also enhanced recombination at the dislocation is developed and evaluated in a first‐order approximation. The results reflect important features of the experimental contrast profiles. Further experimental and theoretical work is expected to provide better insight into the mechanisms of capture and recombination at dislocations.
Solute‐Atom Segregation at Symmetrical Twist Boundaries Studied by Monte Carlo Simulation
Authors: Udler D. , Seidman D. N.
Published: July 2007

Detailed Monte Carlo simulations are performed of solute‐atom segregation at (002) twist boundaries in the Au–Pt system at 850 K; the particular single‐phase bicrystal alloys studied are Pt–1 at% Au and Au–1 at% Pt. The emphasis in this paper is on studying the distribution of solute atoms at low‐angle boundaries. For the Pt–1 at% Au alloy the distribution of sites enhanced in the solute species Au is found to form a bipyramid based on the square cells of the orthogonal primary grain boundary screw dislocations. In the case of the Au–1 at% Pt alloy the solute species Pt is found to be depleted and it also forms a similar bipyramidal pattern. The Gibbsian interfacial excesses of Au and Pt are found to be positive and negative, respectively, for the Pt–1 at% Au and Au–1 at% Pt bicrystal alloys. The absolute values of these Gibbsian interfacial excesses both increase with increasing twist angle.
Magnetic Resonance of Conduction Electrons in Dislocated Germanium
Authors: Pilar von Pilchau A. , Alexander H. , Wattenbach M. , Shreter Yu. , Ionov A.
Published: July 2007

Pakulis and Jeffries first observed Zeeman transitions in germanium crystals which they attributed to the grown‐in dislocations. The authors explained the effect as a change of the electrical conductivity of the crystal due to spin dependent scattering of the carriers. The spin flip was ascribed to dislocation dangling bonds. Using plastically deformed germanium we can show that the signal is due to combined resonance of conduction electrons in one valley of the conduction band the energy of which is lowered by the strain field of dislocations. Additionally two more combined resonance lines are identified which also are assigned to the glide geometry. The delocalised states of electrons causing these lines by spin flip are not yet determined.
Urbach Law and Lifshitz Singularity in Spectra of Localized States of Disordered Systems
Authors: Klochikhin A. A. , Ogloblin S. G.
Published: July 2007

The density of localized states within the energy interval ranging from the mobility threshold up to the Lifshitz border is found in an instantaneous approach with exponetial accuracy for the three‐dimensional two‐component random system described by single‐band Hamiltonian taking into account diagonal disorder. It is shown that just below the mobility threshold an energy region is placed where the density of states dependence is obeying the Urbach law. Exact and approximate expressions for the Urbach parameter are obtained. Its dependence on the model parameters and the estimation of the absolute value show that the experimental data can be accounted for many disordered systems at reasonable parameters of the Hamiltonian. Arguments are presented speaking in behalf of the existence of an Urbach law in one. and two‐dimensional disordered systems.
Isomers and Pairing Mechanism in Alkali Metal‐Doped Fullerides
Authors: Kasperczyk J. , Büttner H.
Published: July 2007

A possible pairing mechanism in the alkali metal‐doped fullerides A3C60 is studied. We have calculated the total π‐electron enecrgies of different ions C60n of the Buckminster fullerene and its isomers (with \documentclass{article}\pagestyle{empty}\begin{document}$ n = 0, \mp 1,\, \mp 2, \mp 3, \mp 4, \mp 5, \mp \, {\rm or}\, \mp 6 $\end{document}). It is shown that anions of the isomers are energetically more stable than the ideal Buckminster fullerene. We have calculated the formation energies of electron local pairs on the C60 clusters. For the phascs consisting of a single type isomer such a formation seems not to be possible. For a mixture of two isomers, however, the electron local pair is stable on anions with odd (in average) valencies. This can serve as a purely electronic pairing mechanism for the so‐called local pair model of superconductivity.
Extinction Related Approach to the Neutron Diffraction on Holmium around the 20 K Magnetic Transition
Authors: Pearce A. , Baruchel J. , Kulda J.
Published: July 2007

The 20K phase transition temperature range is investigated in a high quality and purity single crystal of holmium. The sample was characterized through magnetization measurements and investigated by neutron diffraction. The comparison of the neutron diffraction and magnetization results gives clues about the onset of the conical phase, and its relationship to the 1/6 and 2/11 commensurate phases. The severe extinction present in the diffraction results was treated mainly by a primary extinction variant of the RED model. The obtained extinction parameters show, in contrast with results on other magnetic materials, that the perfection of the magnetic lattice can be higher than that of the nuclear lattice. They give, in addition, indications about the way the transition occurs, with no important reduction of the magnetic coherence length.
Upconversion in Er3+‐Doped Fluorite‐Type Crystals Excited at 800 nm
Authors: Jouart J. P. , Oomen E. W. J.
Published: July 2007

A study of the green and red emissioins from different Er3+ sites of CdF2, CaF2, SrF2, and BaF2 for excitation around 800 nm by a Ti sapphire laser is presented. For CaF2, the green emission intensity depends on the square of the excitation power, whereas for CdF2 (codoped with Na+), SrF2, and BaF2 a cubic dependence is observed. Excitation mechanisms are proposed and discussed, with, in particular, a three‐step absorption process for CdF2(Na), SrF2, and BaF2.
Internal Friction Studies on Oxygen–Oxygen Interaction in Niobium. II. Application of Long‐Range Interaction Models
Authors: Haneczok G. , Weller M. , Diehl J.
Published: July 2007

The oxygen Snoek relaxation in niobium is studied under the assumption of a long‐range interaction among interstitial atoms dissolved in b.c.c. metals. Two models describing this interaction in a mean field approximation (MFA), namely (i) the paraelastic and (ii) the random cooperative strain interaction (RCSI) model are discussed in detail. In the RCSI model the MFA treatment is modified by introducing the “Onsager correction”, and a new formula for the internal friction peak of the Snoek effecct is derived. Based on this modified RCSI model, a quantitative analysis of our experimental data from Nb with 0.12 to 0.85 at% O (presented in Part I) is performed by applying nonlinear regression. The model is able to account for all essential experimental results, (i) the shift of the Snoek maximum to higher temperatures with increasing O content, (ii) the broadening of the maximum increasing with O content, and (iii) a constant value of the activation enthalpy (H = 1.15 eV), independent of the oxygen content. The critical temperature characterizing the interaction of oxygen atoms increases from 110 K for 0.12 at% O to 280 K for 0.85 at% O and is, as predicted by the modified RCSI model, proportional to the square root of the oxygen concentration.
Theory of Macroscopic Local Single‐Particle Charge States in Quasi‐Zero‐Dimensional Structures Bulk Local States
Authors: Pokutnyi S. I. , Efremov N. A.
Published: July 2007

A theory of macroscopic local one‐particle charge states in quasi‐zero‐dimensional structures is developed. Localization of charge carriers on a small sphere placed into some medium is considered. Within the framework of electrostatics a general expression is obtained for the potential energy of a charge in the field of image forces on a spherical interface of two media with arbitrary values of permittivities. It is shown that under certain conditions local states of various types appear in the field of electrostatic image forces. A new type of local state of charge carriers inside a small particle, namely bulk states, is predicted.
The Equation for an Electron in a Deformed Lattice in Effective Mass Approximation
Authors: Dubrovskii I. M.
Published: July 2007

A critical analysis is made of the existing derivations of the effective wave equation for electrons in deformed crystals. A new method is proposed which is applicable also in the case of deformations caused by dislocations. The equation is deduced in the effective mass approximation, i.e., the wavelength of the quasiparticles is supposed to be large compared with the characteristic length of deformation variations. The significance of the region near the dislocation line (dislocation core) is analysed. Effects of terms in the Hamiltonian corresponding to an inhomogeneous effective mass and of the dislocation core on the electron energy spectrum are investigated. The method is applied to a crystal containing a screw dislocation.
Ballistic Transport in Crossed Quantum Wires. An Analytical Approach
Authors: Gaididei Yu. B. , Malysheva L. I. , Onipko A. I.
Published: July 2007

An analytic approach to the ballistic electron transport in crossed 2D wires is presented. It is shown that within and near the first transverse mode the two‐mode approximation gives a correct description of the system. Simple formulas for the bound‐state energies and the single‐mode scattering probabilities are derived.
The Effect of Uniaxial Stress on the Spectrum of a Neutron‐Coloured Sapphire Studied by Spectral Hole Burning
Authors: Malkin E. , Aizengendler M. , Dolindo I. , Sildos I.
Published: July 2007

The influence of uniaxial compressive stress on the 755.6 nm line of colour centres in neutron‐irradiated sapphire at LHeT is studied. To improve the sensitivity spectral hole burning was applied. The shift and the splitting of spectral holes under stress were accounted for by the removal of the orientational degeneracy of colour centres. The corresponding piezospectroscopic constants are found and an assumption about the structure of the centres is made.
Determination of the Quadratic Electrostrictive Coefficients of Some Doped KCl Crystals
Authors: Ramesh N. , Srinivasan K.
Published: July 2007

The quadratic electrostrictive coefficients of KCl containing substitutional impurities (KCl:Br−, KCl0.8Br0.2, KCl0.4Br0.6, KCl:Tl+, KCl:Cd2+, and KCl:Mn2+) are determined from the stress dependence of static dielectric constants. Small changes in the capacitance of the samples under the application of uniaxial stress are measured with the aid of a bridge circuit and a lock‐in amplifier. The values of the components of the electrostriction tensor of the doped systems are found to be enhanced by one to two orders of magnitude compared to those of the pure KCl crystal. The increment of the electrostrictive coefficients of KCl on doping is attributed to (i) the phenomenological description of a point defect as an anisotropic elastic dipole and the orientation of such defects; (ii) an additional deformation of the lattice arising from the reorientation of the I–V dipoles in the case of KCl containing divalent impurities.
Femtosecond Coherent Spectroscopy of GaAs Quantum Wells
Authors: Kim Dai‐Sik , Shah Jagdeep , Schäfer W. , Schmitt‐Rink S.
Published: July 2007

Recent results on femtosecond time‐integrated and time‐resolved four‐wave‐mixing experiments in intrinsic and modulation‐doped GaAs quantum wells are reported. A variety of new phenomena ranging from giant excitonic resonance for intrinsic samples, Fermi liquid behavior in modulation‐doped samples, and interesting temporal evolutions provide new insights into the physical process responsible for the coherent signals.
Coherent Pulse Breakup in Femtosecond Pulse Propagation in Semiconductors
Authors: Peyghambarian N. , Harten P. A. , Knorr A. , Lee S. G. , Jin R. , Brown Colstoun F. De , Wright E. M. , Khitrova G. , Gibbs H. M. , Koch S. W.
Published: July 2007

Coherent pulse breakup is observed for below‐exciton resonance femtosecond excitation in a room‐temperature GaAs/AlGaAs multiple quantum well waveguide using the cross‐correlation technique. This pulse breakup is present only for large area pulses with durations less than the dephasing time of the closest exciton. For pulses longer than the exciton dephasing times, but with the same and even larger areas, the pulse breakup is not observed. The breakup is due to neither self‐induced transparency nor temperoral soliton. Instead, calculations based on the coupled semiconductor Maxwell‐Bloch equations show that coherent self‐phase modulation during propragation drives the system out of the initial adiabatic following regime into oscillatory excitation density, and eventually, pulse shape modulations.
Excitonic Molecules in Nonlinear Optical Response
Authors: Keldysh L. V.
Published: July 2007

A model which allows to treat self‐consistently many‐body exciton effects, including biexciton formation, is proposed for the theoretical description of an excitonic polarization induced by an intense electromagnetic field. In the presence of a pump field the normal spectrum of electromagnetic waves in the vicinity of the nonlinear biexciton resonance is shown to be composed of four polariton branches, where each is the superposition of two photons and two excitons. In the self‐consistent field approximation, the structure of the interconnected, strongly nonlinear exciton and biexciton resonances is studied, including optical Stark shift, multistable behaviour, etc. For a certain range of the exciton interaction parameters at sufficiently strong pump wave the system of excitons and excitonic molecules transforms abruptly to a dense coherent electron–hole “liquid”. Appearance and temporary evolution of an exciton–biexciton system after switching on a pump is also considered.
Model Calculation for the Transient Luminescence Spectra in Optical Excited GaAs Quantum Wells
Authors: Thoai D. B. Tran , Haug H.
Published: July 2007

The kinetics of the carrier distribution functions in an optically excited GaAs quantum well is determined by numerical integration of the coupled Boltzmann equations for electrons and holes. Calculated transient luminescence spectra are compared with corresponding time‐resolved picosecond measurements by Daiminger et al.
Transient Strongly Anisotropic Luminescence from an Optically Excited Semiconductor
Authors: Ivanov A. L. , Haug H.
Published: July 2007

A strongly anisotropic transient luminescence of a semiconductor which has been excited by an intensive short electromagnetic pulse is predicted and described. This phenomenon occurs on a femtosecond time scale and results directly from the unrelaxed total momentum transferred to the dense electron–hole plasma by the initial optical excitation pulse.
Luminescence Measurements of Two‐Dimensional Electrons in the Regime of the Integer and Fractional Quantum Hall Effects
Authors: Pulsford N. J. , Kukushkin I. V. , Hawrylak P. , Ploog K. , Haug R. J. , von Klitzing K. , Timofeev V. B.
Published: July 2007

The radiative recombination of two‐dimensional electrons is studied in GaAs–GaAlAs heterojunctions with holes localised on acceptors in strong magnetic fields. The optical measurements are emphasized as a probe of the properties of the two‐dimensional electron layer. In the case of two subbands occupied, spatial changes in the self‐consistent potential due to a population in the second subband are demonstrated. In the regime of the fractional quantum Hall effect, downward cusps occur in the mean energy of the luminescence at fractional filling factors and values for the quasiparticle energy gaps are derived.
Optical Properties of Barrier‐Modulated InGaAs/GaAs Quantum Wires
Authors: Gré;us Ch. , Orth A. , Daiminger F. , Butov L. V. , Reinecke T. L. , Forchel A.
Published: July 2007

The optical properties of barrier‐modulated InGaAs/GaAs quantum wires with dimensions down to 25 nm are investigated. The wires show a high quantum efficiency that is mainly due to the processing which is free of defects and also due to efficient carrier capture effects. For wire widths below about 50 nm a blue shift of the emission due to lateral quantization, is observed. In high excitation experiments a second subband is observed for narrow wire widths. The experimentally observed wire width dependence of the band edge and of the subband splitting are in agreement with model calculations based on the geometry of our structures.
Excitonic Effects in Neutral Electron–Hole Magnetoplasma in Quantum Wells at Low Temperature
Authors: Kulakovskii V. D. , Butov L. V.
Published: July 2007

High excitation luminescence spectra from a homogeneous electron–hole system are investigated in undoped unstrained InxGa1−xAs/InP quantum wells in magnetic fields up to 12 T. Clear excitonic effects are observed in the optical spectra of neutral, dense magnetoplasmas at low temperatures. These appear as the suppression of the density dependence of transition energies between the uppermost occupied (Je = jh = N) Landau levels in the range of filling factors N < v/2 < N + 1. The effect disappears with increasing temperature. The experimental results are in agreement with theoretical calculations including both the inter‐Landau‐level coupling and screening effects. In a magnetoplasma with an integer filling factor, Landau‐level transitions near the Fermi level are well described in terms of excitons and deexcitons.
Influence of Doping Concentration on Intersubband Relaxation in Modulation‐Doped Quantum Well Structures
Authors: Plödereder U. , Dahinten T. , Seilmeier A. , Weimann G.
Published: July 2007

Intersubband scattering in multiple n‐type modulation‐doped quantum well structures is studied by an infrared bleaching technique at room temperature. The repopulation of the lowest subband of the well is directly observed. In GaAs/Al0.35Ga0.65As quantum wells (thickness 5.9 nm) an increase of the relaxation time from 4 to 7.5 ps is found for increasing doping concentrations from 3.2 × 1011 to 1.4 × 1012 cm−2. Carrier transfer between the wells and barrier states is discussed as a process which determines the relaxation time.
Time‐Resolved Investigation of Recombination and Carrier Capture in Spatially Homogeneous 2D Electron–Hole Plasmas
Authors: Daiminger F. , Neppert B. , Forchel A. , Straka J.
Published: July 2007

The time‐resolved photoluminescence of a homogeneous electron‐hole plasma (EHP) is investigated in a 5 nm strained InGaAs/GaAs quantum well. Good spatial homogeneity is achieved by uniformly exciting mesa structures of about 150 μm width with ps laser pulses. Direct comparison of measurements on mesa and planar structures shows that spatial inhomogeneities in the plasma may strongly influence the quantitative evaluation of the time‐resolved data. The transient spectra of mesa structures clearly show band‐filling effects which last for more than 1ns after the excitation. Nonresonant excitation leads to effective trapping of carriers from the barrier layers.
Temperature Dependence of the Lattice Dynamics of the Layered Semiconductor In2Se3
Authors: Julien C. , Balkanski M.
Published: July 2007

The temperature dependence of the infrared reflectivity of the layerd semiconductor In2Se3 is studied in the range from 10 to 500 cm−1. The behaviour of normal modes is reported from 5 K up to room temperature. Reflectivity spectra are fitted with a four‐parameter dispersion model based on the factorized form of the dielectric function. The temperature dependence of the phonon frequencies and damping is reported. An anomaly in the LO frequency which is observed at 160 K, is related with the α′‐phase transition reported by electron microscopy measurements.
ESR Investigation of Phase Transitions in Some Elpasolite‐Type Fluorides
Authors: Dance J. M. , Grannec J. , Tressaud A. , Moreno M.
Published: July 2007

Structural phase transition temperatures are determined for some elpasolite‐type fluorides Rb2KMF6 (M = Ga, Cr, Fe) through electron spin resonance. The results are compared with low temperature X‐ray diffraction and calorimetric results. For the new elpasolite Rb2KGaF6 a transition temperature of T ≈ 120 K is detected using Cr3+ and Fe3+ as paramagnetic probes. In the case of Rb2KGaF6:Fe3+ the isotropic superhyperfine constant As is well resolved in the powder spectrum. An analysis of experimental As values found for Fe3+ in different fluoride lattices is reported. In the case of paramagnetic elpasolites, the variation of the linewidth of the ESR spectra of Rb2KCrF6 and Rb2KFeF6 with temperature confirms the occurrence of a phase transition at T(Cr) ≈ 150 K and T(Fe) ≈ 170 K. The Cr3+ or Fe3+ doped K2NaGaF6 elpasolite which remains cubic down to very low temperatures is also checked. As expected no change in the ESR spectra occurs in this case.
Field Ionization of Shallow Donors in Germanium
Authors: Žurauskas S. , Dargys A. , Žurauskienė N.
Published: July 2007

Transient currents due to field ionization of shallow donors are investigated in thin germanium samples by transient tunneling spectroscopy. In particular, the electron tunneling time from antimony atoms to the conduction band is measured as a function of electric field strength, lattice temperature, and electric field direction. The results are compared with a simple hydrogenic model. Various factors which may influence the TTS spectra, such as impurity impact ionization, doping inhomogeneity, etc., are discussed. It is concluded that pure electron tunneling from the ground 1 s state of antimony atoms takes place at lattice temperatures smaller than 7 K.
Anomalous Hall Effect in a Nonmagnetic Semiconductor
Authors: Obukhov S. A. , Volkova I. K.
Published: July 2007

The anomalous Hall effect is considered in a system of magnetic impurities on the example of the nonmagnetic semiconductor InSb: Mn. From an analysis of the Hall constant as a function of impurity (Mn) concentration, temperature, and magnetic field strength the magnetic susceptibility of the system and the magnetic ordering temperature,T≈ 1 K, are determined. A physical model is proposed giving a qualitative explanation of the anomalous Hall effect in a system of magnetic impurities. It is argue that the exchange interaction at the boundary for the metal–insulator transition in a system of magnetic impurities damps Coulomb fluctuations.
Anisotropy of the Cuprate Superconductors in the Three‐Dimensional Bigausson Model
Authors: Rajilić; Z. , Mirjanić D.
Published: July 2007

The bigausson model of high‐temperature superconductivity is generalized to three dimensions. The stability of the bigaussons is investigated and the possibility of a bigausson condensate is shown. The anisotropy for the following parameters of cuprate superconductors is calculated: coherence length, energy gap, and critical current density. The obtained results are in agreement with the experimental data for Y1Ba2Cu3O7–δ.
Electromagnetic Response of the η‐Pairing Eigenstate in the Hubbard Model
Authors: Su Gang , Ge Mo‐Lin , Zhao Bao‐Heng
Published: July 2007

Some electromagnetic properties of the η‐pairing eigenstate of the Hubbard model are investigated. The persistent current density Js in the eigenstate is discussed, and the distribution of magnetic field in the studied periodic lattice is analyzed. It is shown that the behaviour of the electromagnetic response is peculiar.