bluuebunny/crossref_metadata_2025 huggingface dataset:
Microscopic theory of the dephasing of coherent optical phonons due to anharmonic decay in polar semiconductors
Authors: Wenschuh U. , Schäfer W. , Zimmermann R.
Published: July 2007

The dephasing of coherent longitudinal optical phonons due to anharmonic decay is investigated from a microscopic point of view. Time‐resolved coherent anti‐Stokes Raman signals are calculated for GaP on the basis of a full phonon dispersion calculation and the relevant decay channels are identified and compared. Anharmonic renormalization effects in the phonon spectrum and their influence on the dephasing signals are investigated. Non‐Markovian dynamics of dephasing are calculated for GaP and result in a 30% faster dephasing signal than the corresponding Markovian dynamics.
Build‐up and decoherence of optical transients in disordered semiconductors
Authors: Velický B. , Kalvová A.
Published: July 2007

For a model two‐band semiconductor with static disorder, suddenly illuminated by intense light, the averaged non‐equilibrium Green function can be obtained within the coherent potential approximation. Explicit solutions found previously are analyzed on numerical examples of the transient photoexcited distribution. The results are used to test the semigroup property of the propagators and the analogous factorization of the particle correlation function known as the generalized Kadanoff‐Baym ansatz. Both properties hold much better if quasiparticle states are used instead of the bare particles. The quasiparticle modification of the ansatz is motivated formally and verified on numerical data.
Homogeneous linewidths of excitons in high and low band gap Ga0.52In0.48P
Authors: Grossmann P. , Feldmann J. , Göbel E. O. , Thomas P. , Arent D. J. , Bertness K. A. , Olson J. M.
Published: July 2007

Photoluminescence and picosecond four‐wave‐mixing experiments are reported on Ga0.52In0.48P grown by organometallic vapor phase epitaxy on GaAs substrates. The spectral behavior of the homogeneous linewidth in the range of the inhomogeneously broadened band gap excitonic resonance is found to be different for a high band gap as compared to a low band gap structure. Whereas the former shows the normal alloy behavior, the behavior of the low band gap sample supports the assumption that its structure consists of ordered domains with varying degrees of order. This means in particular that the main origin of the inhomogeneous broadening is different for the high and low band gap cases.
Dynamics of charge separation in type‐II hetero‐nipi‐structures after interband excitation
Authors: Hauenstein H. M. , Moritz N. , Seilmeier A.
Published: July 2007

A subpicosecond light pulse creates carriers in the GaAs quantum wells of a type‐II GaAs/Al0.3Ga0.7As hetero‐nipi‐structure. Relaxation is monitored via transient absorption changes which reflect band‐filling effects and a decrease of the internal electric field. Thermionic emission of electrons to n‐doped AlGaAs layers is observed which proceeds within several hundred picoseconds. On a time scale reaching from several hundred microseconds to seconds nonexponential interband carrier recombination is found which strongly depends on the density of excited carriers. The time‐resolved data are discussed taking into account the complex band structure changes during the relaxation process in self‐consistent Thomas Fermi approximation.
Vibrational properties of thin films of ultraheavily doped semiconductors
Authors: Alvarado F. L. Castillo , Urbaniak‐Kucharczyk A.
Published: July 2007

The pseudospin model for the G1−xPx semiconductor alloy is considered in the coherent potential approximation. The phonon density of states in ultraheavily doped semiconductor thin films is investigated as a function of impurity concentration and the position of a monatomic layer inside the film. The obtained results show the dependence of the local density of states on the surface conditions.
Influence of rings on the electronic structure of lattices with topological defects
Authors: Lazo E. , Glass B.
Published: July 2007

The influence of rings on the electronic structure of open and closed lattices, like the Bethe and Husimi lattices, respectively, is studied. In both lattices topological defects are introduced changing, in alternating form, the number of nearest neighbors on each site. In the Bethe lattice with or without topological defects the local density of states has the symmetry of a central ring, but topological defects generate a gap in the electronic structure and some bound states in between. In the Husimi model formed with rings of two different numbers of sides, it is found that the electronic bands are symmetrical, if and only if, combinations of rings with even number of sides, independent of the presence of topological defects, are considered. In addition, gaps appear when alternating topological defects are introduced.
The Comparative Studies of Low‐Temperature Nearest‐Neighbour Distances in Alkali Hydride Crystals
Authors: Tyutyunnik V. I.
Published: July 2007

The numerical analysis demonstrates that at low temperatures the change in lattice parameter between the alkali hydride salts is related only to the difference between the radii of the “alkali atoms” which have the same sizes as those in the alkali metals with body‐centred cubic structure. There is no isotope effect on the size of particles constituting an alkali hydride crystal.
Brillouin Zone Centre Lattice Dynamical Studies of Nd2CuO4, Pr2CuO4, and Gd2CuO4
Authors: Amrithraj Mona , Devarajan V.
Published: July 2007

Brillouin zone centre lattice dynamical studies for the tetragonal phase of the electron superconductors Nd2CuO4, Pr2CuO4, and Gd2CuO4 are reported for three potential models viz., short‐range valence force model, rigid ion model, and a polarizable ion model. The models are developed in three stages with the number of parameters increasing at every stage. The calculated TO‐LO frequencies and high‐frequency dielectric tensor values are compared with recent experimental values reported in the literature and the agreement is found to be very good. The eigenvectors for some of the modes are depicted.
The g‐Tensor Theory Based on the Single‐Determinant‐Basis Crystal‐Field Method
Authors: Zhou Kang‐Wei , Yang Jian‐Hua
Published: July 2007

The g‐tensor theory of the single‐determinant‐basis (SDB) crystal‐field method which has been published recently is developed. With such an SDB method a unified calculation of both the optical spectrum and EPR parameters of any d5 complex can be carried out in general. It is shown that the g‐value (being isotropic) of a cubic d5 complex decreases with decreasing average orbital reduction factor N for given Dq but increases with decreasing |Dq| for given N and that g is exactly a function of (Dq)2 for |Dq| < 1000 cm−1 and that g also decreases with increasing |ζ| for given B, C, and Dq. The application of the SDB method to Mn2+ ions in CaF2 and KZnF3 shows that not only the calculated optical spectra and the zero‐field splittings but also the g‐values are in good agreement with the findings. The calculated g‐values are, respectively, 2.00203 and 2.00194, while the findings are 2.0010 ± 0.0005 and 2.0021. Since it diagonalizes the electrostatic interaction, the crystal‐field, and the spin‐orbit coupling energies simultaneously, the SDB method can be extended to cases of arbitrary crystal‐field intensity, i.e., to arbitrary transition series.
Electron States of a Vacancy in the Core of the 90° Partial Dislocation in Silicon
Authors: Marklund S. , Wang Yong‐Liang
Published: July 2007

An LCAO scheme (linear combination of atomic orbitals) taking into account ten atomic orbitals (s‐, p‐, and d‐type) is used to calculate the electronic structure of a vacancy present in the core of the reconstructed 90° partial dislocation in silicon. The levels in the band gap are extracted using Lanczos' algorithm and a continued fraction representation of the local density of states. The three‐fold degenerate state of the ideal vacancy is split into three levels with energies 0.26, 1.1, and 1.9 eV measured from the valence band edge.
Electric and Photoelectric Properties of High Porosity Silicon
Authors: Kǔoka J. , Fejfar A. , Pelant I.
Published: July 2007

The basic problems of porous silicon (PS) transport, like long time constants, ageing, and the influence of contacts are discussed. The results of the steady‐state and transient photoconductivity on the self‐supporting p‐ and n‐type PS are presented and the resulting conclusions, related to the microstructure isotropy, trapping, and time‐dependent electric field redistribution are summarized. On the basis of the dark I–U characteristics and their temperature dependencies the different volume and surface transport mechanisms are discussed. The compatibility of the models of explanation of the strong PS photoluminescence with our transport results is discussed.
Thermally Induced Modifications in the Porous Silicon Properties
Authors: Halimaoui A. , Campidelli Y. , Larre A. , Bensahel D.
Published: July 2007

Various porous silicon (PS) properties are significantly modified by thermal annealing. For example, when a freshly prepared sample is annealed at 450°C, there are a significant reduction of the photoluminescence (PL) intensity, a change in the optical constants (refractive index and absorption coefficient), a sharp decrease in the material strain, hydrogen desorption, and change in the microstructure. The modification of all the parameters listed above (strain, PL, absorption coefficient, microstructure etc.) occurred for annealing temperatures between 350 and 450°C, and that is the same temperature range at which hydrogen is desorbed from the PS surface. The relationship between these parameters and the results of this comparison are discussed.
Core Shell Spectroscopy of Rapidly Oxidised Porous Silicon. Linking the Chemical and Electronic Structure
Authors: Bancert U. , Gardelis S. , Hamilton B. , Pettifer R.
Published: July 2007

High resolution scanning transmission electron microscopy (STEM) and synchrotron X‐ray excitation of luminescence (XEOL) is used to probe the chemical nature of the luminescence mechanisms of porous and rapidly oxidised porous silicon. It is concluded that for fresh porous silicon, Si‐Si bonded material is involved in the luminescence. For oxidised material this is probably not the case, the chemical environment being SiO2.
From Porous Silicon to Clustered Matrices: A Comparison of Local Structures
Authors: Bayliss S. C. , Zhang Q. , Hutt D. A.
Published: July 2007

Studies have been made of the local atomic structure ofclustered and porous silicon nanostructures, with the object of obtaining more information on the environments which exist in highly PL‐efficient material. In some structures the Si environments can be interpreted in terms of a small crystalline core surrounded by configurationally disordered material. Finally, although the details of the individual mechanisms responsible for the emission appear to be different for clustered and porous material, the overall behaviour can be linked t o the sizes and dimensionality of the units involved.
Transport Properties of Siloxene
Authors: Rosenbauer M. , Höpner A. , Dettlaff‐Weglikowska U. , Stutzmann M.
Published: July 2007

The transport properties of siloxene (as‐grown and annealed) are investigated. The dark conductivity is thermally activated with an activation energy of about 1 eV for as‐prepared siloxene, in accordance with the insulating nature of this material and its energy gap of about 2.6 eV. The conductivity is very sensitive to the presence of ambient gases. The spectral and temperature dependence of photo‐ conductivity is also discussed.
Anodized Amorphous Silicon: Present Status
Authors: Bustarret E. , Ligeon M. , Rosenbauer M.
Published: July 2007

The results of the few groups which have attempted to etch chemically or electrochemically amorphous silicon films prepared in various ways are reviewed briefly with a focus on the photoluminescence of such layers. The particular case of plasma‐deposited boron‐doped hydrogenated amorphous silicon subjected to an anodization in HF solutions followed by an electro‐oxidation in water is elucidated. Many similaritics between the properties of these materials and those of their crystalline counterparts are emphasized, but one significant difference between the resonantly excited PL spectra of the two porous materials is pointed out.
Si/CaF2 Superlattices. A Direct Gap Structure Due to Interface State Coupling
Authors: Ossicini Stefano , Fasolino A. , Bernardini F.
Published: July 2007

One promising approach for the development of silicon‐based light‐emitting devices is the epitaxial growth of Si nanostructures. In this context, the lattice matched system CaF2/Si/CaF2 as prototype of a well controlled and ordered Si‐based system with known microscopic structure is proposed. For this system, a new mechanism is found leading to a direct band gap based on the coupling of interface states in very thin Si layers.
Exciton Energies as a Function of Electric Field in Quantum Wells Finite Potential Barrier Case
Authors: Feng Yuan‐Ping , Spector H. N.
Published: July 2007

A previous variational calculation of the exciton binding energy as a function of electric field in a semiconducting quantum well structure using a 3D hydrogenic trial wave function is extended to the case where the confining potential barrier is finite in height. The results are compared with those obtained for the same case using a 2D hydrogenic trial wave function. The results yield larger exciton binding energies for wide wells and strong electric fields. It is found that the crossover between 2D and 3D behavior occurs at a critical well width which is of the order of the exciton Bohr radius in zero field and which decreases with increasing electric field. It is also found that decreasing the height of the barrier decreases the binding energy of the exciton, as expected.
Effective Pair Interactions between Localised Defects Inside a CuO2 Plane in the Superconducting Copper Oxides
Authors: Germain Ph. , Labbé J.
Published: July 2007

The effective pair interaction potentials between localised defects inside a CuO2 plane are deduced from our calculated susceptibility of the clectron gas. The very peculiar shape of the Fermi line, which is perfectly square for a half‐filled band, leads to a strongly anisotropic interaction, changing from the expected two‐dimensional behaviour along the [1, 0] or [0, 1,] direction to a purely one‐dimensional one along the [1,1] or [1, 1] direction. Thus, the impurities in the CuO2 planes of the new superconductors are not independent since the range of their interactions can be much larger than in a usual two‐dimensional system. This could lead to superstructure effects. These results, applied to magnetic impurities, highly complicate the discussion of the frustration in a possible spin‐glass phase, since the interaction between two localised spins strongly depends on the considered direction.
Study of Molecular Motions in 2‐Adamantanone by Nuclear Magnetic Resonance
Authors: Decressain R. , Amoureux J. P. , Cochon E.
Published: July 2007

The temperature dependence of proton and carbon spin‐lattice relaxation rates in 2‐adamantanone is measured. The present experiments are analyzed in terms of relaxation resulting from intramolecular reorientation modulation of the spin‐spin interaction. In the plastic phase the molecular reorientations involve a superposition of uniaxial rotation and tumbling reorientation of the dipolar axis. The calculation of the corresponding autocorrelation function is made by using a Frenkel model. The resulting expression for T1 leads to a determination of the plastic phase residence times in good agreement with values previously deduced from quasi‐elastic neutron scattering and dielectric relaxation. In the low‐temperature phase the motions are completely frozen and T1 is rather constant.
Cluster Expansion for a Sublattice System with Unequal Coordination Numbers
Authors: Mohri T. , Oates W. A.
Published: July 2007

A cluster expansion of the internal energy is carried out for a phase comprised of sublattices which have different coordination numbers, and the equation for the mixing energy is derived. The resultant expression of the mixing energy is equivalent to that derived using the higher‐order approximation of the Bond Energy Model. It is shown that the effective pair interaction energy of the cluster expansion is exactly the site exchangc energy of the Bond Energy Model. The cluster expansion method is suitable for use in higher‐order approximation calculations of the free energy.
Study of the Electronic Polarizability of Metal Clusters
Authors: Utreras‐Díaz C. A. , García E. Martín , Troncoso M. A. Rojas
Published: July 2007

A study of alkaline metal clusters, both neutral and ionized is presented as a function of the number of atoms in the cluster N in the Thomas‐Fermi‐Dirac‐Weizsäcker approximation for the spherical jellium model. Numerical solutions are found for the electronic density using the program COLSYS and the surface energy and the ionization energy of the clusters are studied as functions of N for N up to 10000 atoms. To calculate the polarizability of the cluster, a one‐parameter trial density is proposed, based upon the numerical solution for neutral clusters, to approximate the distorted electronic density under a constant electric field. An alternative approach is also considered and described, but it is not implemented numerically in this work.
Polaronic Hamiltonian and Polar Optical Vibrations in Multilayer Structures
Authors: Klimin S. N. , Pokatilov E. P. , Fomin V. M.
Published: July 2007

A theory of polar optical vibrations in multilayer semiconductor structures is developed taking into account phonon dispersion. A method of diagonalization of the equations of motion for inertial polarization vectors in the finite basis is used, that is founded on a finite number of degrees of freedom of the system. The Hamiltonian of the electron‐phonon interaction is deduced. The dispersion law and the spatial dependence of the obtained eigenmodes are in good agreement with experimental data and microscopic models. It is shown that the electron scattering rate in a magnetic field depends on the chosen model of polar optical vibrations.
A Study of XANES Associated with the L Absorption Discontinuities of Some Rare Earths in Their Oxides and Cuprates
Authors: Moghe N. V. , Sapre V. B. , Nigam A. N. , Mande C.
Published: July 2007

The XANES region of the LII and LIII absorption discontinuities of some rare earths in their oxides and cuprates is studied. The main absorption peak and the peak adjacent to it are attributed to the transitions of electrons from the inner L levels to the outer levels t2g and eg, which are formed due to the crystalline field in the materials. It is observed that the energy difference between these two peaks varies linearly with the atomic number Z for the oxides as well as the cuprates.
Photoreflectance Study of a Fibonacci Superlattice
Authors: Pánek P. , Munzar D. , Humlíček J. , Lukeš F. , Ploog K.
Published: July 2007

Photoreflectance spectra of a GaAs/GaAlAs Fibonacci superlattice in the temperature range from 20 to 300 K are reported. A new method for the modulation beam shift in sweeping photoreflectance is presented. The experimental results are compared with the theoretical spectra calculated with the help of the envelope function approximation, using the first‐derivative model for the photoreflectance lineshapes. The model predictions are in good agreement with spectra measured at low temperatures. The temperature evolution of the spectra is also discussed.
Rigidity Percolation and Chemical Ordering in Ge‐In‐Se Glasses
Authors: Saffarini G. , Schlieper A.
Published: July 2007

The dependence of the mean atomic volume Va of chalcogenide glasses belonging to the GexIn6Se94−x and GexIn12Se88−x families of the Ge‐In‐Se system, on the average coordination number Z is investigated. The Va‐Z plots for both families display maxima at Z = 2.75 and 2.83 corresponding to the tie‐line composition in In6 and In12 families, respectively. The Va‐Z dependence for the In6 family exhibits a minimum at Z = 2.54 and a broad local maximum at Z = 2.67, and that of the In12 family shows a marked change in slope at Z = 2.50. These results are discussed in the framework of the chemically ordered covalent network model, Tanaka's model of topological transition, and the Phillips‐Thorpe model of rigidity percolation.
Langevin‐Type Charge Carrier Recombination in a Disordered Hopping System
Authors: Albrecht U. , Bässler H.
Published: July 2007

A Monte Carlo simulation study is performed to determine the ratio γ/μ of the rate coefficient for bimolecular charge recombination and the charge carrier mobility in a hopping system with built‐in energetic disorder. The result indicates that γ/μ is independent of disorder but increases at large electric fields at which the charge carrier velocity saturates.
Electronic Pairing Mechanism Due to Band Modification with Increasing Pair Number
Authors: Mizia J.
Published: July 2007

It is shown that a shift of an electron band with electron occupation number n, which is changing during the transition to the superconducting state, can lower the total energy of the system. In fact it will bring a negative contribution to the pairing potential, which is proportional to the product of the electron band shift with occupation number and the charge transfer during the transition to the superconducting state. The shift of the electron band comes from the change of stresses and the change of correlation effects in the CuO2 plane with n, that in turn is caused by the changing oxygen concentration. This model explains the phenomenological success of Hirsch's model, which gives no explanation how the band shift in energy can give rise to superconductivity.
Spin‐Wave Theory on Quasi‐One‐Dimensional Heisenberg Antiferromagnets
Authors: Du A. , Wei G. Z.
Published: July 2007

The low‐temperature of quasi‐one‐dimensional Heisenberg antiferromagnets with ferromagnetic coupling between nerest‐neighbor spins within a chain and antiferromagnetic couplng between nearest‐neighbor spins withina chain and antiferromagnetic coupling between chains are studied by the spin‐wave theory. The effects of interchain coupoling on the ground state energy, sublattice magnetization, and perpendicular susceptibility at zero temperture are calculated numerically. At low temperatures, asympototic expressions of sublattice magnetization, internal energy, magnetic specific heat, parallel and perpendicular susceptibilities are given as functions of temperature and interchain coupling strength within tow sub‐temerature regions. It is found that if interchain coupling is weak, with increasing temperature the properties of the system change from threedimensional behavior (Δm ∝ T2, ΔE ∝T4, Cm ∝ T3, X∥ ∝ T5/2, ΔX⊥ ∝ T2) To quasi‐one‐dimensional bahavior (Δm ∝ T, ΔE ∝ T3/2, Cm ∝ T1/2, X∥ ∝ T3/2, ΔX⊥ ∝ T).
Spectroscopic Study of Cr3+ Impurities Embedded into Crystallites of Mullite Glass Ceramics
Authors: Männik J. , Sildos I.
Published: July 2007

Fluorescence spectra of Cr3+ impurities in a mullite glass ceramic are investigated by using time‐resolved fluorescence‐line‐narrowing spectroscopy. The dependence of fluorescence spectra on the excitation wavelength inside the inhomogeneously broadened R‐like line of Cr3+ ions embedded in mullite crystallites is studied. In these spectra a large inhomogeneous broadening, strong non‐correlation between different electron transitions, and non‐exponential decay patterns of fluorescence in spite of resonant narrow‐band laser excitation are revealed. The influence of the compositional and mesoscopic structure of the mullite glass ceramic on the spectra of the impurities is analyzed and an attempt is made to separate the contributions of these two factors.
Shift of Optical Resonant Reflectance Extrema Due to Damping and Spatial Dispersion
Authors: Grigorchuk N. I.
Published: July 2007

The energy shifts of light reflectance extrema in the region of exciton resonances are studied taking into account the transitional layers, spatial dispersion (SD), and damping. The values and directions of the shifts of the extrema are determined depending on the round‐trip phase delay in the transitional layer. It is found that in the phase interval nπ ≦ θ < nπ + π/4 (n = 0, 1, …) the long‐wave shift of the maximum is dominant while in the interval nπ + π/4 < θ ≦ nπ + π/2 mainly a short‐wave shift of the minimum occurs. The comparative roles of SD and damping are discussed. The equivalence of the mechanisms of SD and of damping effects on the frequency shift is established; the first one playing a more important role. Theoretical results agree with the available experimental data. Corresponding estimates for CdS, GaAs, and LiH crystals are made.
XPS Studies on Ce/Al2O3 and on CoRh/Nb2O5 Catalysts
Authors: Appel L. G. , Frydman A. , Perez C. A. C. , Eon J. G. , Castner D. G. , Campbell Ch. T. , Schmal Schmal M.
Published: July 2007

The surface properties of supported catalysts are studied. XPS data are used to identify the different surface species but, alone, it is not sufficient to provide enough information to describe the correct form and distribution of these species and phases on the surface. However, XPS supported by an accurate mathematical formalism to model catalysts surface structure, may become a powerful tool to help understanding surface properties. First, a new mathematical model for supported bimetallic Co‐Rh/Nb2O5 catalysts is proposed based on quantitative analysis of XPS intensities assisted only by TPR data. XPS peak intensities are compared to the values predicted from different structural models of the oxide particles. Secondly, the Ce/Al2O3 system generated from different precursors of cerium salts and contents is examined using different techniques. XPS results show the effect of the precursor on the Ce/Al ratio while TEM the morphology. In addition, the EPR data show the Ce3+ species distributed in regions underneath the topmost surface layer. Then, an improved model is proposed based on quantitative analysis of XPS intensities, XRD, and TEM results.
Analysis of hydrogen and paramagnetic defects in aSi: H aerosol particles. Resulting from thermal decomposition of silane
Authors: Onischuk A. A. , Strunin V. P. , Ushakova M. A. , Samoilova R. I. , Panfilov V. N.
Published: July 2007

Hydrogen and paramagnetic centers contained in aSi:H aerosol particles, formed during thermal decomposition of silane are studied. The methods of IR spectroscopy, effusion of hydrogen, and ESR spectroscopy are used. For low temperatures of aerosol formation (T ≦ 833 K) paramagnetic centers with g ∼ 2.0051 are recorded in aerosol particles. For high temperatures (T > 880 K) paramagnetic centers with g ∼ 2.0055 are recorded. The obtained experimental results testify that the existence of paramagnetic centers with g ∼ 2.0051 is related to the presence of polysilane chains. The existence of paramagnetic centers with g ∼ 2.0055 is related to the presence of monohydride groups with the frequency of stretching vibrations near 2100cm−1 located on the surface of interconnected microvoids and microchannels. The quantitative relations between the fractions of hydrogen present in various structural groups and IR absorptions in the corresponding regions of the IR spectra are obtained.
Screening effect on polarizabilities of shallow donors and acceptors in finite‐barrier quantum wells
Authors: Ilaiwi K. F.
Published: July 2007

Polarizabilities of shallow donors and acceptors in finite‐barrier GaAs/Ga1−xAlxAs quantum wells are calculated using Hasse's variational method within the effective mass approximation. The effect of spatially dependent screening on polarizabilities is taken into account with an r‐dependent dielectric response. It is shown that spatially dependent screening effects can be more important for acceptors than for donors in GaAs/Ga1−xAlxAs quantum wells. The effects of electric and magnetic fields on binding energy are also studied.
Studies of the CuO Bond in (Pb, Cd)‐1212 and (Pb, Cd)‐1201 cuprates by X‐ray photoelectron spectroscopy
Authors: Yu W. J. , Mao Z. Q. , Tian M. L. , Wang Y. , Zhou G. E. , Zhang Y. H.
Published: July 2007

It is shown that in (Pb0.5Cd0.5)Sr2(Y0.5Ca0.5)Cu2O7 and (Pb0.5Cd0.5)SrLaCuO5 cuprates, where Cu2+ ions are in pyramidal and octahedral geometry with five and six neighboring oxygens, respectively, the Cu2p3/2(2p53d10L) XPS core line is broader than that in Nd2CuO4 where Cu2+ ions are in a square planar geometry with four coordination oxygen ions, and that for (Pb0.5Cd0.5)SrLaCuO5 is the broadest. To explain these changes in the XPS Cu2p3/2 main line, two different charge transfer mechanisms from the O2p to the Cu3d orbitals are considered. For the square planar geometry (CuO4) only the O2px,y → Cu3d charge transfer is allowed, whereas for pyramidal and octahedral geometries an additional apical O2pz → Cu3d transition could take place, and in octahedral geometry the latter transition is more significant. The valence band spectra of the cuprates also confirm this proposition.
Spin waves of anisotropic ferrimagnets with nearest neighbor and next‐nearest neighbor interactions
Authors: Xue D. S. , Gao M. Z. , Yang J. B. , Kong Y. , Li F. S.
Published: July 2007

An anisotropic Heisenberg model with nearest neighbor and next‐nearest neighbor interactions is introduced to study the spin wave spectra of two‐sublattice ferrimagnets. The matrix Green function is applied to deal with the NaCl‐ and CsCl‐type ferrimagnets. The results show that the spin wave spectra separate into two branches, and the numerical calculation at finite temperature indicates that the absolute value of the spin wave energy increases with increasing extension and strength of the exchange interactions. It is found that the influences of the next‐nearest neighbor and anisotropic interactions are quite different. Simultaneously, the influences of the structure and the temperature are discussed.
The effect of anisotropy on magneto‐optics of type‐II semiconductor superlattices
Authors: Lau W. , Singh M.
Published: July 2007

A theory of cyclotron resonance (CR) for type‐II (InAs/GaSb) superlattices based on the 8 × 8 k · p matrix Hamiltonian (Bodnar model), which includes the effect of electron‐hole interactions is developed. In this model, the effect of nonparabolicity and anisotropy is included. Using linear response theory, an expression for the cyclotron absorption coefficient is obtained. It is predicted for the first time that due to the anisotropy of the band structure, the CR transitions are sensitive to the orientation of the sample relative to the growth direction. This prediction agrees with the experiments on a qualitative basis. In addition it is predicted that as the strength of the magnetic field increases the square of the matrix elements of the CR transitions also increases. The present theory can also be applied to study the interband magneto‐optical transitions along with the intraband transitions.
On the pressure dependence of the Grüneisen parameter and debye temperature for CsI crystals
Authors: Kulkarni R. G. , Abd‐Elmeguid M. , Bukshpan S. , Milants K. , Verheyden J. , Pattyn H.
Published: July 2007

An analysis of the pressure dependence of the Grüneisen parameter γ and the Debye temperature θ for CsI based on the model developed by Kumari and Dass is presented. From the pressure dependence of the recoilless fraction behaviour γ is deduced which shows that γ varies linearly with pressure as γ(p,T) = γ0 − ηp, where η is found to be pressure independent. θ(p) also displays a linear variation with p, which is consistent with the recoilless fraction behaviour of γ‐ray emission or absorption in the Mössbauer effect for low pressures. Various thermodynamic approximations implied in the analysis are tested and found to be true.
The formation of germanium nanocrystals by thermal annealing of a‐SiOx: H/a‐GeOx: H multilayers
Authors: Freistedt H. , Stolze F. , Zacharias M. , Bläsing J. , Drüsedau Tilo P.
Published: July 2007

Multilayer films consisting of alternating sublayers of hydrogenated amorphous silicon oxide and germanium oxide were prepared by dual magnetron sputtering of silicon and germanium targets in an argon/water vapor atmosphere. The number and thickness of the a‐GeOx: H sublayers ranged from one to eight and from 80 to 7 nm, respectively. The structure of the multilayers and their components were investigated by means of optical transmission and reflection, infrared spectroscopy, wide‐angle X‐ray scattering, and X‐ray reflectometry. Upon annealing, first an ordering of the amorphous germanium oxide network connected with an outdiffusion of oxygen and an increase of the surface roughness are observed. Depending on sublayer thickness, the onset of the formation of Ge nanocrystals is observed around 700 K. Crystallization of the germanium oxide sublayers is nearly completed at 870 K, and the crystallite diameter ranges from 4 to 13 nm depending on sublayer thickness. With crystallization the surface roughness decreased and the structure of the silicon oxide was improved.
Electron structure near abrupt edges in 2DEG under strong magnetic field
Authors: Bartos I. , Rosenstein B.
Published: July 2007

Energies and wave functions of edge states in a two‐dimensional electron gas are evaluated for a finite step potential barrier model. The spectrum, instead of smooth bending of Landau branches in the vicinity of the barrier acquires a step‐like form; unexpected edge plateaus and significant energy gap reduction take place between the neighbouring Landau branches above the barrier tops. The origin of these phenomena is traced. Stability with respect to modifications is established. Manifestation of the qualitatively new features of electron densities of states in abrupt confinements through magnetooptical and nuclear magnetic spin relaxation effects is proposed.
Dynamics of an inhomogeneous anisotropic antiferromagnetic spin chain
Authors: Daniel M. , Amuda R.
Published: July 2007

Nonlinear spin excitations in the two‐sublattice model of a one‐dimensional classical continuum Heisenberg inhomogeneous antiferromagnetic spin chain are investigated. The dynamics of the inhomogeneous chain reduces to that of its homogeneous counterpart when the inhomogeneity assumes a particular form. Apart from the twists, some planar configurations representing the nonlinear dynamics of spins are obtained.
Refined RBS and AES techniques for the analysis of thin films used in photovoltaic devices
Authors: Bohne W. , Fenske F. , Kelling S. , Schöpke A. , Selle B.
Published: July 2007

The analytical potential of refined RBS and AES techniques is demonstrated by two examples: (i) thin FeSi2 absorber layers on Si and (ii) TiNiAg multilayer contact structures. Both systems have prospects as components in photovoltaic cell devices. In the case of RBS, the refinement concerns an improved mass resolution achieved by applying projectile ions of higher masses (15N, 22Ne) at higher energies (up to 25 MeV). For AES the principal component analysis (PCA) formalism is adopted starting with a peak‐to‐peak height normalization of the relevant Auger peaks in the measured spectra. This evaluation procedure, which takes advantage of the chemical information inherent in the Auger line shape, gives depth profiles of the elements with respect to their chemical bonding state.
Vegard's rule and Sb2Te3−xSex crystals
Authors: Karamazov S. , Nesládek P. , Horák J. , Matyas M.
Published: July 2007

Sb2Te3−xSex crystals were prepared from 5N purity elements and their crystal parameters a, c and Hall coefficients RH were measured as functions of the Se concentration up to x = 1. Well defined deviations of both crystal parameters from the linear Vegard's law are observed up to x = 0.4. At higher Se concentrations the linear Vegard's law is fulfilled. The main idea explaining this behaviour is the decrease of antisite defects of the type SbTe with increasing Se concentration. This process is complicated and its details are given in the paper.
Heat capacity of niobium and tantalum carbides NbCy and TaCy in disordered and ordered states below 300 K
Authors: Gusev A. I. , Rempel A. A. , Lipatnikov V. N.
Published: July 2007

The low‐temperature heat capacity of nonstoichiometric niobium and tantalum carbides in disordered and ordered states is measured by an adiabatic calorimetry method. It is found that in the temperature interval from 4 to 300 K the investigated ordered carbides have a higher heat capacity than the disordered carbides of the same composition. The effect of ordering on the heat capacity of nonstoichiometric carbides is interpreted in terms of the order parameter functional (OPF) method. Changes in heat capacity due to ordering in niobium and tantalum carbides are calculated. The degree of long‐range order in ordered carbides is estimated by comparing of the calculated and experimental results.
Presurface exciton spectrum at finite temperatures
Authors: Tkach M. V. , Paziuk V. V.
Published: July 2007

The Hamiltonian of the presurface excitons interacting with the bulk and interface phonons in a heterosystem with a plane interface is obtained in the representation of second quantization in all variables. The spectrum parameters (shift and half‐width) of the shape function of the basic band at arbitrary temperatures are calculated with the help of the Green function method. The temperature dependence of the spectrum parameters is studied and the physical mechanism of their formation is established on the example of particular heterosystems.
Effect of magnetic field reversals on the shape of Mössbauer spectra
Authors: Dzyublik A. Ya.
Published: July 2007

The Mössbauer effect in soft magnetic materials subject to an external radiofrequency magnetic field is analyzed. Assuming the magnetic field at the nucleus to have only one component which periodically alternates general formulas for the emission spectrum as well as for absorption and scattering cross‐sections are obtained. They are applied to the step‐like and harmonic field models. Besides, the expressions are derived for the instantaneous cross‐sections being periodical functions of time. Transient effects are regarded, which arise when a constant magnetic field changes its direction to the opposite one.
On the reorientation of surface magnetization in semi‐infinite ferromagnets
Authors: Zhao Ping‐Bo , Li Bo‐Zang , Pu Fu‐Cho
Published: July 2007

By establishing an effective transfer matrix technique, we calculate the problem of the surface magnetization direction canting between perpendicular and in‐plane states by the classical XY model. Our calculations show many new features which differ from the results obtained by other molecular field approximations. A possible mechanism for the slope discontinuity on the surface magnetization is proposed.
Infrared Spectra of Solid Trifluoroiodomethane
Authors: Anderson A. , Ko Y. M. , Smith W.
Published: July 2007

Infrared spectra of solid trifluoroiodomethane, CF3I, at temperatures between 12 and 125 K in both the lattice and internal mode regions are presented and discussed. A reversible phase transition at (115 ± 3) K is confirmed. The upper phase exhibits broad spectral features, characteristic of a disordered or highly anharmonic crystal. At 12 K, the spectra are typical of an ordered harmonic molecular crystal, and are compatible with a centrosymmetric unit cell containing at least four molecules.
Optical properties of (Bi1−xSbx)2Se3 single crystals
Authors: Navrátil J. , Lošťák P. , Drasar Č. , Blachut T.
Published: July 2007

(Bi1−xSbx)2Se3 (x = 0 to 0.2) single crystals prepared from elements of 5N purity by means of a modified Bridgman method were characterized by measurements of infrared reflectance and transmittance. Values of the plasma resonance frequency ϵp, optical relaxation time τ, and high‐frequency permittivity ϵ∞ were determined by fitting the reflectance spectra. It was found that the substitution of Sb atoms for Bi atoms in the Bi2Se3 crystal lattice leads to an increase in the ωp values, whereas in the range of higher Sb content the ωp values decrease. This effect is accounted for by a model of point defects in the crystal lattice of (Bi1−xSbx)2Se3. The dependence of the absorption coefficient K on the energy of incident photons was determined from the transmittance spectra. The position of the short‐wavelength absorption edge is discussed. With regard to the longwavelength absorption edge it was found that the values of α in the relation K ∼ λα lie in the interval 2.2 to 2.5, hence the dominant scattering mechanism of the free charge carriers in (Bi1−xSbx)2Se3 crystals at room temperature is the scattering by acoustic phonons.
Diffusion by vacancy mechanism in Ni, Al, and Ni3Al: Calculation based on many‐body potentials
Authors: Debiaggi S. B. , Decorte P. M. , Monti A. M.
Published: July 2007

The self‐diffusion coefficients for the monovacancy mechanism, described in terms of the Arrhenius law, are calculated for Ni, Al, and the intermetallic compound Ni3Al by computer simulation techniques. Many‐body interatomic potentials, based on the embedded atom model, are used together with the static relaxation method to study the static and dynamic properties of vacancies. The defect formation and migration energies, their corresponding relaxation volumes and vibration entropies, are evaluated. Special interest is devoted to the results obtained for Ni diffusion in Ni3Al. The present calculations predict that simple vacancy‐Ni atom interchange jumps (both of ordered and disordered types) are energetically more favourable than the correlated six‐jump cycles. Even though a difference in energies of δ = 0.35 eV favours migration via ordering jumps with respect to disordering jumps, the entropy factors behave in an opposite way suggesting that both types of jumps contribute to the Ni diffusion. Only for low temperatures (T ≤ 770 K) disordering jumps can be neglected. For this case, a pre‐exponential factor D0 = 0.2 × m2 s−1 is obtained.
Diffusion of vacancies under the order‐disorder transition temperature in CuAu
Authors: Hashimoto I. , Yamagata K. , Oketani Y. , Yamaguchi H.
Published: July 2007

The diffusion coefficient of vacancies in CuAu has been measured from the shrinkage rate of voids during annealing. The observations of voids have been carried out by transmission electron microscopy under the order‐disorder transition temperature in the temperature range between 453 and 653 K. The suppression of the diffusion of vacancies emitted from voids is seen at temperatures above 493 K. The reason is discussed briefly in comparison with the results for Cu3Au in terms of the diffusivity of vacancies around the voids.
X‐ray studies of the low‐temperature phase of K2SO4
Authors: Ahmed S. A.
Published: July 2007

The low‐temperature phase transition of potassium sulphate (K2SO4) has been investigated by X‐ray diffraction measurements on polycrystalline material. From the temperature dependence of the lattice parameters it was confirmed that K2SO4 crystals undergo a structural phase transition of second‐order type at 56 K. Above the transition point (β‐phase) the K2SO4 crystal belongs to the Pnam (D) orthorhombic space group with lattice parameters a = 0.74780 nm, b = 1.00707 nm, and c = 0.57701 nm at 293 K. The crystal symmetry of the low‐temperature phase (below 56 K, γ‐phase) is probably monoclinic with space group P21/n11 (C). The low‐temperature phase transformation is thought to be of displacive type.
Transition from spinodal decomposition to the stage of coalescence in undercooled glasses and solid solutions
Authors: Feldman E. P. , Stefanovich L. I.
Published: July 2007

The relaxation of intermediate decomposed structures of glasses and solid solutions in the stages following the complete spinodal decomposition is considered. The time dependence of the characteristic period of the concentration structure both at the early stages of binodal decomposition and at the stage of coalescence is obtained. The temperature dependences of relaxation time and the characteristic scale of the structure are established for a one‐dimensional model. Estimates of the interface thickness at all stages of concentration separation are presented.
Hopping conductivity in p‐type β‐FeSi2
Authors: Lisunov K. G. , Arushanov E. K. , Kloc Ch. , Malang U. , Bucher E.
Published: July 2007

The results of resistivity measurements on p‐type β‐FeSi2 single crystals down to the liquid helium temperature are presented and analyzed within the framework of different hopping conductivity models. The values of the characteristic and transition temperatures, the density of states, and the width of the Coulomb quasigap as well as the localization radius and the dielectric constant are determined.
Slow components in the photoluminescence and scintillation decays of PbWO4 single crystals
Authors: Nikl M. , Nitsch K. , Polak K. , Mihokova E. , Dafinei I. , Auffray E. , Lecoq P. , Reiche P. , Uecker R. , Pazzi G. P.
Published: July 2007

The decay kinetics of PbWO4 luminescence and scintillation is investigated in a broad time scale 10−9 to 10−3 mainly at room temperature using a selected set of PbWO4 crystals. The light sum released in different time gates is estimated showing rather high content of slow recombination processes related mainly to the green PbWO4 emission component. Correlations are found among the absolute green emission intensity, crystal light yield, and the amplitude of very slow processes in the PbWO4 scintillation decay.
Structure of the a band of TI+ in Na1−xKxI mixed crystals
Authors: Inaba M. , Hashimoto S.
Published: July 2007

Thin Na1−xKxI: TI+ mixed crystals with about 1 μm thickness have been prepared from melt, and the A band of TI+ on the absorption spectra has been observed in the temperature range 11 to 300 K. A distinct splitting of the band is recognized in the composition range of 0.1 < × < 0.6 at low temperatures though it does not occur in NaI: TI+ (x = 0) and KI: TI+ (x = 1). The low‐energy component of the band shifts continuously with x similarly as the peak of the exciton bands. On the basis of the temperature dependence, the structure is attributed to the static perturbation in addition to the dynamical Jahn‐Teller effect.
Optical absorption and photoluminescence of narrow‐Gap Hg1−x−yCdxMnySe single crystals
Authors: Mazur Yu. I. , Tarasov G. G. , Lavorik S. R. , Tomm J. W. , Kissel H. , Hoerstel W.
Published: July 2007

The optical properties of Hg1−x−yCdxMnySe (x = 0.09 to 0.11, y = 0.02 to 0.03) single crystals are investigated by means of infrared transmission and photoluminescence. The photoluminescence is promoted by manganese doping. The presence of Mn significantly improves the structure of mercury selenides. The optical dielectric constant, refractive index, and energy gap are determined as functions of temperature.
Water adsorption on the stepped si(110) surface
Authors: Katircioglu Ş. , Erkoç Ş.
Published: July 2007

We have investigated the possible water adsorption forms on the stepped Si(110) surface. Calculations have been performed by using the empirical tight‐binding method. Two types of adsorption model of water on a single layer stepped Si(110) surface have been considered, one of them is the dissociative type (H, OH) and the other one is the molecular type (H2O). The total electronic energy and TDOS calculations lead to a dissociative type of water adsorption on the stepped Si(110) surface excluding the molecular type as it was found on the flat Si(110) surface.