bluuebunny/crossref_metadata_2025 huggingface dataset:
Luminescence of Pb2+ and energy transfer from Pb2+ to rare earth ions in silicate oxyapatites
Authors: Lin Jun , Su Qiang
Published: July 2007

The Pb2+ luminescence in a series of silicate oxyapatites Me2(Y, Gd)8(SiO4)6O2, Me4Y6(SiO4)6O(Me = Mg, Ca, Sr) is reported and discussed in relation to the crystal structure. The maximum wavelengths of the excitation (1S03P1) and emission (3P1−1S0) bands of Pb2+ are independent of the Me:Y ratio (2:8 or 4:6), but they have lower energies in MgY‐oxyapatites than in CaY‐ and SrY‐oxyapatites. The Stokes shift of Pb2+ luminescence amounts to 11100 to 11400 cm−1, which does not depend strongly on the host composition. There exists a mutual energy transfer between Pb2+ and Gd2+ in Sr2Gd8(SiO4)6O2. At last, the dependence of the energy transfer efficiency of Pb2+−Sm3+, Tb3+, Dy3+ in Sr2(La, Gd)8(SiO4)6O2 and Ca2(Y, Gd)8(SiO4)6O2 on their doping concentrations was studied in more detail.
Spectroscopic characteristics of color centers produced in a LiF crystal surface layer by microwave discharge
Authors: Ter‐mikirtychev V. V. , Tsuboi T. , Konyzhev M. E. , Danilov V. P.
Published: July 2007

We have studied the absorption and luminescence spectra of color centers in a LiF crystal surface thin layer which are produced by low energy electrons from a secondary electron emission microwave discharge. The intensityratio of the absorption band due to the F2 color center to the band due to the F center was observed to be much higher than the ratio of these color centers produced in the whole bulk crystal. It was found that the F−2 center is produced in a thin layer through a selective two‐step photoionization of F2 color center by irradiation with the 514.5 nm Ar+ laser line. A mechanism of colorcenter formation in the thin layer is discussed.
Effective cubic nonlinear response of a coated granular composite medium
Authors: Liu Xin‐yu , Li Zhen‐ya
Published: July 2007

An approximate expansion method is developed to study the bulk effective response of a nonlinear dielectric granular composite. The system considered here is composed of some grains with the same structure and properties (linear cores and cubic nonlinear concentric shells) randomly distributed in a linear host. An approximate method is invoked for deducing the effective nonlinear susceptibility of such a system. As we introduce a structural parameter λ representing the thickness of the coating shell, we discuss the influence of this structural parameter on the bulk effective nonlinear response in the whole system. Through adjusting both the dielectric properties of each component and the structural parameter, we can obtain the enhancement of the effective nonlinear response of the system. Our solutions are valid and useful to deal with a nonlinear dielectric system with a small volume fraction of nonlinear grains.
Calculation of tensor constants for quasicrystal aggregates
Authors: Divinski S. V. , Dnieprenko V. N. , Zheng Wen‐Chen
Published: July 2007

An approach for calculating the tensor constants of single quasicrystals by the data on textured aggregates of structure elements has been proposed. The distribution of the structure elements on orientation has been approximated by a model function that describes well a broad spectrum of possible textures ranging from a single orientation to the fully isotropic distribution. The anisotropy of two‐rank and four‐rank tensor properties has been analysed for decagonal quasicrystals. The melt‐spun textured decagonal quasicrystals have been found to reveal an appreciable anisotropy of physical properties even in the ribbon plane.
A comparative study of birch and kumar equations of state under high pressure NaCl as an example
Authors: Kumar M. , Bedi S. S.
Published: July 2007

A comparative study of the Birch and Kumar equations of state is presented to demonstrate their validity for a wide range of pressure, viz. from atmospheric pressure up to the structural transition pressure. It is found that these equations of state are equally good in the low pressure range anddeviations occur under the effect of high pressures. The comparison demonstrates that the results obtained by the Kumar equation of state are in better agreement with the experimental data as compared with the Birch equation of state. The application of Kumar's equation of state is also discussed at high temperatures, viz. from room temperature up to the melting temperature. The good agreement between theory and experiment demonstrates the validity of the Kumar equation of state recently proposed for the entire range of pressure and temperature.
Effects of confined longitudinal optical phonons on the binding energy of the impurity in a quantum well
Authors: Moukhliss S. , Sayouri S. , Fliyou M.
Published: July 2007

The effects of the confined LO phonon‐electron interaction on the binding energy of shallow donors confined in a GaAsGa1−xAlxAs quantum well are investigated. The barrier potential height is taken into account. Using the variational method of Lee‐Low‐Pines, we find that the effect of this interaction is negligible for very narrow quantum wells. For thick quantum wells, the polaronic contribution is quite large.
Two‐point conductivity tensor of nonmultilayer magnetic systems
Authors: Sheng L. , Gu R. Y. , Xing D. Y. , Dong Jin‐ming
Published: July 2007

The transport properties of nonmultilayer magnetic systems are described in terms of a semiclassical Boltzmann equation. Interfaces are modeled asthin mixed interlayers so that both bulk impurity scattering and interface scattering are included in a position‐dependent relaxation time. Theresulting expression for the two‐point conductivity tensor is in accordance with that derived from the real‐space Kubo formula, indicating a close connection between the semiclassical method and the quantum approach.
Electronic transmission coefficient for solid‐state structures with spatially variable anisotropic electronic effective mass
Authors: Kupka M.
Published: July 2007

The method for calculating the electronic transmission coefficient for elastic electronic transport through a layered structure with spatially variable anisotropic effective mass and one‐electron potential energy is proposed. In this method, the transmission coefficient is evaluated by means of a certain matrix quantity assigned to any region of the sample. Thismatrix is determined by functional shapes of the reciprocal effective mass tensor and a one‐particle potential energy in a given region as wellas by the energy and group velocity of an incident electron. As an example, the transmission coefficients are calculated for a triple‐barrier structure as well as for a structure obtained by replacing the inner potential barrier by an anisotropic conductive layer.
X, l, and γ lines in low temperature photoluminescence spectra ofal0.47Ga0.53As0.035Sb0.965 alloys
Authors: Tronc P. , Mani H. , Maciejko R. , Reid B. , Leroux M. , Lazzari J. L. , Segura‐fouillant C.
Published: July 2007

In Al0.47Ga0.53As0.035Sb0.965 layers nearly lattice‐matched to GaSb(100) substrates, the X, L, and Γ optical transitions have energies close to each other. By varying the temperature of the samples and the power of the laser beam, one can separatethe contribution of every transition in the photoluminescence spectra and measure the different transition energies versus temperature. The experiments provide very accurate measurements of the difference between the above transition energies and, together with X‐ray double diffraction measurements, show the existence of fluctuations of composition within the alloy.
Two‐photon absorption and magnetoabsorption of P excitons in β‐ZnP2
Authors: Engbring J. , Fröhlich D. , Schepe R. , Spitzer S. , Arimoto O. , Nakamura K.
Published: July 2007

Electronic transitions in biaxial β‐ZnP2 are studied by two‐photon absorption. The anisotropy of the crystal leads to a large energy splitting of the P excitons. The energy shift of the P excitons and Landau transitions are observed in a magnetic field up to 6T. We present a theory which describes the anisotropic P excitons in a magnetic field. The analysis of the experimental data allows the determination of the anisotropic dielectric constants and effective masses of the valence and conduction bands.
Raman scattering in 2‐Amino‐5‐Nitropyridine‐L‐(+)‐Tartrate single crystals
Authors: Oussaïd M. , Kemiche M. , Becker P. , Carabatos‐nédelec C. , Watanabe O.
Published: July 2007

Single crystals of 2‐amino‐5‐nitropyridine‐L‐(+)‐tartrate (ANPT) have been studied by Raman scattering and infrared absorption at room temperature and at 10 K. The crystal ofmonoclinic symmetry space group P21 (C22) undergoes no detectable structural phase transition between these two temperatures. The room temperature spectra were compared with those of parent compounds in order to propose a tentative assignment of the observed vibrational modes. Particular attention was paid to the hydrogen bond modes.
BaTiO3 crystal structure phase transition caused by the Jahn‐Teller effect and the model of like‐hydrogen molecule‐ion (H)
Authors: Zhang Duan‐Ming , Yu Bo‐Ming , Li Yuan , Zhang Xin‐Yu , Lai Jian‐Jun , Li Hua
Published: July 2007

By utilizing the Jahn‐Teller effect, we discuss the structural phase transition of BaTiO3 crystals. By taking into account the simple model of the like‐hydrogen molecule‐ion (H) and the effective point charge, we compute the structural paraelectrical‐ferroelectrical phase transition temperature Tc of BaTiO3. The theoretical value obtained is in good agreement with the experimental results.
Dielectric function and plasmon dispersion relation of a quasi‐two‐dimensional electron gas
Authors: Khanh Nguyen Quoc
Published: July 2007

In this paper we generalize the method suggested by Lee and Spector to calculate the dielectric function of a quasi‐two‐dimensional electron gas without strict restriction on the wave vector q, the conducting layer thickness d, and the dielectric constants of the conducting and surrounding media ϵ, ϵ1, ϵ2. In the case of a thin layer, the plasmon dispersion equation obtained by means of the dielectric function is solved exactly to get the dispersion relation of the collective excitations. We compare our exact solution to the approximated result of Aharonian et al. and find that the latter is valid only for very small values of qd.
Quantized photoemission from rectangular quantum wires of narrow‐gap semiconductors
Authors: Das N. R. , Ghosh K. K. , Ghoshal D.
Published: July 2007

In the present work a theoretical study has been made on the photoemission from rectangular quasi‐one‐dimensional structures of narrow‐gap semiconductors. The effect of reduced dimensionality on the photoemission is seen to be quite prominent. The computations are performed for a temperature of 4.2 K taking strongly degenerate n‐InSb as an example. It is observed from the study that the photoemission exhibits a stair‐case function of incident photon energy and an oscillatory function of the transverse dimensions.
Critical temperatures of the diluted spin‐1/2 Ising system with Gaussian type exchange integrals
Authors: Zheng S. M.
Published: July 2007

The site‐ and bond‐disordered spin‐1/2 Ising ferromagnetic system is studied in which the exchange integral between nearest‐neighbour sites is expressed by the Gaussian probability distribution function. The general expression of the magnetization is derived. The critical temperatures of the system are calculated as functions of concentration and disorder parameter, using the improved correlated effective field theory. For comparison, the corresponding results for the Handrich‐Kaneyoshi type probability distribution function of the exchange integral are also given. It is shown that the Gaussian probability function of the exchange integrals gives more reasonable results.
ESR study of high‐pressure phases of C60 fullerene
Authors: Kozlov M. E. , Zakhidov A. A. , Yakushi K. , Tokumoto M.
Published: July 2007

Face‐centered cubic and rhombohedral phases of pressure‐temperature treated C60 were investigated by ESR spectroscopy in the 5 to 550 K temperature range. It was found that the ESR spectra of the former differ only slightly from those of the starting C60 powder. The intensity of the ESR signal from this phase reduces upon heating the specimens above 480 K. In contrast, the rhombohedral phase shows an intense narrow ESR line which obeys the Curie law and does not change much upon heating. The origin of paramagnetic centers in the high‐pressure phases of C60 and their relationship to the structure of the substances are discussed.
Magnetic phase transition of ultrathin Fe layers in Fe/Au multilayers
Authors: Yamazaki H.
Published: July 2007

The magnetic phase transition of Fe layers is investigated in Fe(2 Å)/Au(d) multilayers with d = 5 to 200 Å by means of magnetization measurements. For d < 10Å, the Curie temperature Tc is above 300 K. With increasing d, an overall decrease of Tc is observed. Reference samples including a single Fe(2 Å) layer show Tc ≈ 80 K, which is lower than Tc ≈ 150 K for 200 Å ≧ d > 100 Å. The interlayer coupling between Fe layers mediated by Au layers remains even for long distances exceeding 100 Å. It is expected that even a weak interlayer coupling suppresses the spin fluctuation in two‐dimensional magnets and shifts Tc to a higher temperature.
Spin‐one Ising model with biquadratic exchange interaction within functional integration method
Authors: Vatamaniuk O. , Rudavskii Yu.
Published: July 2007

A spin‐one Ising model with biquadratic exchange interaction is investigated within a functional integration approach. The phase diagram is analyzed by means of Landau's free energy expansion. The dependence of the features of the system on the ratio of bilinear to biquadratic exchange interaction constants is discussed. In the random phase approximation, expressions for phase transition temperatures and the tricritical point for the ferromagnetic transition are obtained.
Evaluation of the effective second harmonic generation coefficient of monomethyl urea single crystals
Authors: Romani S. , Razzetti C. , Zha M. , Paorici C.
Published: July 2007

The effective second harmonic generation coefficient of monomethyl urea has been determined relatively to urea by phase matched interaction. Both Type I and Type II phase matching have been achieved by propagating the pump beam at small angles to natural faces of solution grown crystals. The conversion efficiency for the doubling of Nd:YAG fundamental is similar in urea and methylurea.
Quantitative analysis of dislocations in relation with the martensitic transformation in Cu‐Zn‐Al alloys
Authors: Rodriguez P. L. , Condo A. M. , Lovey F. C.
Published: July 2007

A quantitative evaluation of the dislocation energies in relation with the β → 18R martensitic transformation in Cu‐Zn‐Al alloys has been performed. Dislocations produced by macroscopic deformation of the parent or martensitic phase have been considered. The crystallographic changes of the dislocations upon transformation have been established and the anisotropic theory of dislocations has been used to calculate their energy when embedded in β or martensite. Good agreement was found between the calculated separation of the partials and the corresponding line orientations for the minimum energy and the experimental results from transmission electron microscopy observations.
Temperature and mass dependence of anharmonic defect dynamics
Authors: Salamatov E. I.
Published: July 2007

Within the framework of an approach with allowance for fluctuations of the mean energy of a particle interacting with a thermostat, the partial density of vibrational states (PDVS) of a crystal lattice defect located in a two‐well potential has been investigated. The results obtained are in good agreement with those of a numerical study of the corresponding Langevin equation. It has been shown that the mass dependence of the anharmonic defect PDVS is quite complicated and is determined by temperature.
Intense laser field effect on confined hydrogenic impurities in quantum semiconductors
Authors: Fanyao Q. , Fonseca A. L. A. , Nunes O. A. C.
Published: July 2007

The influence of an intense high‐frequency laser field on the binding energy of a shallow neutral donor impurity in a quantum semiconductor structure such as GaAs/AlGaAs is reported. By making use of a nonperturbative theory and the variational approach it was found that for an on‐centre hydrogenic impurity in the given quantum well, the binding energy for different well widths increases, reaches a maximum, and then decreases quite rapidly with increasing laser field amplitude of the quiver motion of the electron.
Variational approach for phonon renormalization effects in photoexcited quantum wires and quantum wells
Authors: Güven K. , Tanatar B.
Published: July 2007

We investigate the effects of screening on polaronic corrections to the effective band edge in photoexcited quasi‐one‐dimensional GaAs quantum wires and two‐dimensional quantum wells. We develop a variational method to calculate the polaron energy of a two‐component plasma (electrons and holes) coupled to LO‐phonons. Screening effects are incorporated within a dynamical scheme. We find that the screening effects and finite well width considerably reduce the polaron energy as the plasma density increases. Many‐body corrections beyond the random‐phase approximation are also considered.
Plasmapolaron selfenergy and effective mass in uniaxial polar crystals
Authors: Capone G. , Cataudella V. , Ninno D. , Iadonisi G.
Published: July 2007

We study the polaron effects in a uniaxial polar semiconductor with electrons in the conduction band. We describe the system as an external charge interacting with three boson fields: two optical longitudinal phonons and a plasmon. The combined effects of these three fields lead to a quasiparticle which we call plasmapolaron. The selfenergy and the anisotropic effective mass of the plasmapolaron as a function of the anisotropy parameters and the electron density are calculated in the intermediate regime for the electron‐phonon and electron‐plasmon coupling. We find that, when the crystal has high polarizability along the c‐axis and a large polaron radius in the a‐b plane, the screening effects due to the plasmons on the electron‐phonon interaction are small, so that the latter is present even at relatively high electron density. In these conditions the induced charges are localized mainly on the a‐b plane. We also calculate the selfenergy of the plasmapolaron in the strong coupling limit and compare it with that calculated in the intermediate regime. We find that, increasing the electron density, the selfenergy in the intermediate regime becomes always lower than that in the strong limit. A possible connection of our results with the high‐Tc superconductors is discussed.
The ground state energy of a bound polaron in the presence of a magnetic field
Authors: Zorkani I. , Belhissi R. , Kartheuser E.
Published: July 2007

A theoretical calculation for the ground state energy of a bound magneto‐polaron as a function of the magnetic field is presented. The theory is based on a variational approach using the trial function , representing the envelope wave function of the ground state already proposed by Devreese et al. in absence of a magnetic field. This function is adequate for all electron‐phonon coupling α and γ0 values, where èM0 is the ratio between the LO phonon energy and the Coulomb one. For the electronic part of the trial wave function we have adopted a new linear combination function which covers all magnetic field domains. Analytical results are obtained in the weak coupling limit.
Far‐Infrared Stimulated Emission in p‐Ge under High Uniaxial Pressure
Authors: Altukhov I. V. , Chirkova E. G. , Kagan M. S. , Korolev K. A. , Sinis V. P. , Yassievich I. N.
Published: July 2007

Stimulated emission of far‐infrared radiation from uniaxially compressed p‐Ge in strong electric fields is shown to be due to population inversion of strain‐split acceptor levels. A peak corresponding to the optical transitions between split‐off and ground acceptor states is found in the spectrum of stimulated emission. A strong frequency tuning by stress due to the pressure dependence of the energy splitting of these states is obtained. The inversion takes place as the split‐off acceptor state is in the valence band continuum.
Time‐Resolved Photoluminescence Study of Ordered Ga0.5In0.5P under High Pressure
Authors: Kobayashi T. , Minaki M. , Takashima K. , Uchida K. , Nakahara J.
Published: July 2007

We have measured time‐resolved photoluminescence (TR‐PL) spectra in ordered and disordered Ga0.5In0.5P alloys at 12 K and at pressures up to 3 GPa to investigate the characteristics of the main broad PL spectrum at ≈︁1.91 eV in nominally ordered alloys. At normal pressure, unlike the disordered alloy, the PL‐decay profiles for the ordered alloys change remarkably for small changes in detection energy. This indicates the presence of adjacent overlapping emissions of different nature. Together with the prolonged nonexponential decay curves, a gradual red‐shift of the low‐energy peak with delay time is observed for the first time, indicating spatially indirect recombinations. With increasing pressure, the constituent emissions shift toward higher energies at rates slightly smaller than that for the disordered alloy. At 3 GPa, the PL decay appears to be insensitive to the detection energy, and a faster decay is observed.
Effects of Conduction Band Structure and Dimensionality of the Electron Gas on Transport Properties of InSe under Pressure
Authors: Errandonea D. , Segura A. , Royo J. F. Sánchez , Muñoz V. , Ulrich C. , Grima P. , Chevy A.
Published: July 2007

We report Hall effect and resistivity measurements in InSe under pressure. The electron concentration strongly decreases under pressure in samples exhibiting 3D transport behaviour. This is explained by the existence of an excited minimum in the conduction band moving to lower energies under pressure. The related impurity level traps electrons as it reaches the band gap and approaches the Fermi level. In samples exhibiting 2D behaviour the electron concentration remains constant. This behaviour, together with the pressure dependence of the Hall mobility, is consistent with a previous model which considers high mobility 3D electrons and low mobility 2D electrons to contribute to charge transport along the layers.
Coexistence of DX and A1 States in Highly Doped GaAs:Ge, Te and GaAs:Si, Te
Authors: Wasik D. , Baj M. , Przybytek J. , Słupiński T. , Kudyk K.
Published: July 2007

We studied highly localized states of germanium and silicon impurities in GaAs. Hall effect and resistivity measurements as a function of hydrostatic pressure up to 1.5 GPa and temperature ranging from 4.2 up to 300 K were performed on highly doped LEC GaAs:Ge, Te and LEC GaAs:Si, Te. For Ge impurities our data confirm the two‐electron character of GeDX states and evidence that the GeA1 state does not reveal any metastable features down to liquid helium temperature. In the case of Si impurities we analyzed Hall concentration and Hall mobility dependencies on pressure and illumination in order to investigate the coexistence of DX and A1 states.
Photoluminescence Study of Rare‐Earth Doped Semiconductors under Pressure
Authors: Takarabe K.
Published: July 2007

High pressure photoluminescence (PL) studies are reported for InP:Yb and GaAs:Er, O. In InP:Yb, the thermally quenched intra‐4f luminescence is recovered by applying pressure near room temperature. Simultaneously the band‐edge related emission decreases. This result is interpreted qualitatively by a thermal equilibrium between the photocreated electron‐hole pair of the host InP and the excited state of the 4f shell. In MOCVD grown GaAs:Er, O, the sharp luminescence from the intra‐4f transition of Er is observed at 1.538 μm at atmospheric pressure. This is the main emission associated with the ErGa‐2O center which has C2v symmetry. For increasing pressure the PL intensity of the Er intra‐4f emission increases and also some new PL lines appear at low temperatures. These pressure‐induced new lines are assigned to other Er centers with different atomic configurations.
Pressure and Time‐Resolved Photoluminescence Studies of Mg‐Doped and Undoped GaN
Authors: Teisseyre H. , Kozankiewicz B. , Leszczynski M. , Grzegory I. , Suski T. , Bockowski M. , Porowski S. , Pakula K. , Mensz P. M. , Bhat I. B.
Published: July 2007

Pressure and time‐resolved photoluminescence (PL) experiments were performed on undoped and Mgdoped GaN homoepitaxial layers. The pressure coefficients of the following PL lines were determined: (i) exciton bound to a neutral donor (3.472 eV), (ii) exciton bound to a neutral acceptor (3.463 eV), (iii) line centered at about 3.27 eV and its two LO (92 meV) phonon replicas. In the time‐resolved measurements, we observed that the latter PL line corresponds to a slow process (delay time is in the millisecond range). This observation strongly supports the conclusion that donor‐acceptor pairs are responsible for this radiative transition. Identification of the radiative centers and a new explanation of the radiative mechanisms (including optically active E12 phonons) are proposed.
An Investigation of the Formation Mechanisms of Electric Field Domains in GaAs/AlAs Superlattices
Authors: Sun Baoquan , Jiang Desheng , Liu Zhenxing , Zhang Yaohui , Liu Wei
Published: July 2007

We have studied the vertical transport and formation mechanisms of electric field domains in doped weakly‐coupled GaAs/AlAs superlattices. Under hydrostatic pressure two kinds of sequential resonant tunneling are observed within the pressure range from 0 to 4.5 kbar. A transition from Γ‐Γ to Γ‐X sequential resonant tunneling occurs at Pt ≈︁ 1.6 kbar. For P < Pt, the high electric field domain is formed by the Γ‐Γ process, while for P > Pt it is preferentially formed by the Γ‐X process.
Structural and Optical Properties of HgO at High Pressure
Authors: Zhous T. , Schwarz U. , Hanfland M. , Liu Z. X. , Syassen K. , Cardona M.
Published: July 2007

We have investigated high‐pressure properties of HgO by optical spectroscopies and powder X‐ray diffraction. At pressures up to 14 GPa we observe a pronounced softening of two oxygen‐related Raman modes of orthorhombic HgO which is interpreted as a signature of increasing coupling between ‐OHgO‐ chains. The lattice parameters reveal a strongly anisotropic compressibility in this pressure range. At a pressure of 14 GPa, discontinuous changes of Raman active modes indicate a structural phase transition, in agreement with changes in the X‐ray diffraction diagrams. The high‐pressure phase is assigned as tetragonal with the mercury partial structure being close to face centered cubic. An increase in near‐infrared oscillator strength, which starts at 28 GPa as seen in optical reflectivity, is attributed to an insulator‐metal transition resulting from a monotonic increase of valence and conduction band overlap.
Stability of Diamond at Megabar Pressures
Authors: Scandolo S. , Chiarotti G. L. , Tosatti E.
Published: July 2007

The stability of diamond at megabar pressures (1 Mbar = 100 GPa) is studied by means of ab‐initio simulations. Under hydrostatical pressure, diamond is found to be locally stable (i.e. a local enthalpy minimum) up to a pressure as high as 3000 GPa, where it is predicted to collapse into a sixfold coordinated metallic phase never considered so far. On the other hand, if a considerable uniaxial component is added to the stress tensor, as in diamond‐anvil‐cell experiments, a completely different scenario applies whereby much lower pressures are required to bring diamond to mechanical instability. Qualitative considerations on the maximum pressure achievable in diamond anvil cells are presented.
High Pressure Stability in III–V and II‐VI Binary Compounds and the Cmcm Phase A Theoretical Study
Authors: Mujica A. , Needs R. J. , Muñoz A.
Published: July 2007

We present the results of a theoretical study based on the density functional theory of the structural stability of several III‐V and II‐VI binary compounds (GaAs, Gap, InAs, InP, ZnSe, and ZnTe, among others). The recent developments in X‐ray diffraction techniques have led to the identification of several new phases and to a major change in the accepted trends for these families of compounds. A remarkable feature of the emerging new picture is the existence of an orthorhombic phase which is stable in many of the binary compounds studied so far, a distortion of the rocksalt structure which has been named Cmcm after its space group. Our theoretical study focuses on this novel and rather general phase. We have investigated the instability of the rocksalt cell with respect to the distortion leading to Cmcm. The results of our ab initio study are in line with recent experimental studies of these compounds.
Structural Stability of the Group‐VIb Elements under High Pressure
Authors: Nishikawa A. , Niizeki K. , Shindo K. , Ohno K.
Published: July 2007

We have performed band structure calculations of a high‐pressure phase of the group‐VIb elements within the local density‐functional formalism and the norm‐conserving pseudopotential method to investigate the structural stability. This paper mainly deals with the β‐Po type rhombohedral phase of sulphur, and we discuss the effect of the rhombohedral distortion on the electronic structure.
Nonequilibrium Phase Transformations in Diamond and Zincblende Semiconductors under High Pressure
Authors: Lyapin A. G. , Brazhkin V. V. , Popova S. V. , Sapelkin A. V.
Published: July 2007

The problem of nonequilibrium phase transformations is reviewed for compression of diamond, zincblende, and amorphous tetrahedral semiconductors and for decompression of their high‐pressure phases. The importance of lattice or network dynamical properties for the nature of nonequilibrium transformations is shown. Using low‐temperature quenching of high‐pressure phases, we obtained new Crystalline phases for Ge and Ge‐Gash solid solutions, which have X‐ray patterns very similar to corresponding data for rhombohedral R8 silicon. We established that a number of anomalous features, like elastic softness, pressure‐induced geometrical distortion, and strong bulk modulus softening (δB/δP > 0), distinguish the pressure behavior of the amorphous tetrahedral network in a‐GaSb from its crystalline counterpart.
Spectroscopic Study of KTiOAsO4 Single Crystals Doped with In, Sc, Fe
Authors: Mashkovtsev R. I. , Isaenko L. I.
Published: July 2007

Electron paramagnetic resonance and optical absorption in the visible and infrared regions have been used to study flux‐grown KTiOAsO4 crystals doped with In, Sc, and Fe. An unstable broad absorption band and paramagnetic hole centres in KTA: In or Sc after γ‐irradiation are observed. The analysis of EPR data leads to a model wherein the holes are trapped on bridging oxygen ions between two titanium ions. The optical absorption band at 2.7 eV is ascribed to the light‐induced transfer of holes from one oxygen ion to another. Infrared absorption bands at 3584 and 3594 cm−1 are observed in all crystals and possible positions of the OH group in the structure are discussed.
On the Theory of Huang X‐Ray Scattering Caused by Point Defects in Hexagonal Crystals
Authors: Michelitsch Th. , Wunderlin A.
Published: July 2007

The theory of Huang scattering of point defects in hexagonal crystals is treated. The concept of “elementary defects” which is based on group theoretical methods is introduced. From this point of view, point defects in hexagonal crystals can be understood as unique superpositions of three types of incoherent elementary scattering defects. These elementary defects reflect characteristical defect symmetries. This concept yields a simple analytical expression for the Huang intensity in the case of arbitrary scattering vectors. The derived intensity function may be a useful tool for the investiga‐ tion of defect structures in h.c.p. metals by Huang scattering measurements.
Half‐Space Jellium and Its Natural Occupation Numbers: Generalized Neutrality Sum Rule and Correlation Entropy
Authors: Ziesche P. , Gersdorf P.
Published: July 2007

A conceptional analysis of half‐space jellium in terms of its oneparticle density matrix (diagonalized by natural orbitals and natural occupation numbers as eigenfunctions and eigenvalues) is presented. This allows to apply the concept of correlation entropy as a measure of correlation strength. Exact expressions for the electron density, the kinetic energy density, and the correlation entropy density are given. A generalized Sugiyama‐Langreth neutrality sum rule for the phases which describe the bulk asymptotics of the natural orbitals is derived. Kinetic surface energy and correlation surface entropy depend on these phases and on the bulk momentum distribution of the uniform electron gas.
X‐Ray Spectra and Electronic Structure of the Compounds in the ZrCoSi System
Authors: Shcherba I. D. , Antonov V. M. , Gorelenko Yu. K. , Plotnikov M. O. , Margolych I. I. , Stadnyk Yu. V. , Yatsyk B. M.
Published: July 2007

X‐ray spectral studies were used to establish the electronic structure of the ternary compounds from the ZrCoSi system. X‐ray emission spectra of the components of ZrCo2Si2, ZrCoSi, ZrCo5Si3, and Zr6Co16Si7 compounds were presented on a unique energy scale. The partial and total densities of electronic states were calculated by the half‐relativistic LMTO method for the ZrCo2Si2 compound. Agreement between calculated and experimental X‐ray emission bands was observed for the components of the ZrCo2Si2 compound.
A Nonlocal Pseudopotential Calculation of the Thermopower of Liquid Metals
Authors: Akinlade O.
Published: July 2007

An accurate pseudopotential has been used to calculate the thermopower of some simple liquid metals using a formula that incorporates the nonlocality of the potential. Firstly, we investigate the role of the structure factor in thermopower calculations and find it very significant and secondly we infer that refinements in the pseudopotential theory and thus the expression for thermopower incorporated into our calculations does not have a remarkable effect on computed thermopower values when compared with experiment.
D‐Wave Superconductivity Induced Renormalization of Phonons in Strongly Interacting Fermion Systems
Authors: Mierzejewski M. , Zieliński J. , Entel P.
Published: July 2007

We have calculated changes in the phonon self‐energy originating from d‐wave superconductivity in the two‐dimensional Hubbard model (U → ∞ limit). Strong local correlations have been incorporated within the mean‐field approximation for auxiliary boson fields. Due to the presence of correlated quasiparticles at the Fermi surface, changes in phonon‐energy shifts and linewidths strongly depend on the concentration of holes. In particular, pronounced effects have been derived for low frequency q = 0 phonons at low doping, where superconductivity starts to set in.
Band‐Edge Photoluminescence at Room Temperature from ZnGeP2 and AgGaSe2
Authors: Petcu M. C. , Giles N. C. , Schunemann P. G. , Pollak T. M.
Published: July 2007

Photoluminescence near the band edge has been observed at room temperature from bulk single crystals of ZnGeP2 and AgGaSe2. Both of these materials are ternary chalcopyrite semiconductors, but they have quite different edgerelated emission behavior. The emission from ZnGeP2 is observed to peak near 2.35eV and is dominated by transitions from the upper Γ7 conduction band to the upper Γ6 valence band. The lack of emission at 1.99eV from the lowest Γ6 conduction band in ZnGeP2 indicates a Γ6 conduction‐band‐energy minimum at the high‐symmetry point T (T1 + T2 band), rather than at Γ(k = 0). In contrast, AgGaSe2 is a direct‐band‐gap semiconductor and exhibits k = 0 radiative recombination at 1.793 eV. Based on the power dependence of the PL emis‐ sion intensity, this 1.793 eV band is shown to be excitonic in nature, thus giving a room‐temperature band‐gap energy near 1.814 eV for AgGaSe2.
Optical Reflectivity and Transmissivity of Misfit Layer Compounds (MS)nTS2 (T  Ta, Nb; M  Pb, Sm, Tb; 1.08 < n < 1.23)
Authors: Rüscher C. H.
Published: July 2007

The room temperature transmission and reflection behaviour of misfit layer compounds (MLCs) (MS)nTS2 (TNb, Ta; MPb, Sm, Tb) were investigated in the spectral range between 1000 and 20000 cm−1 for incident light normal to the layered structure (k ⊥ (001)). A characteristic near infrared transmission window (TW) is observed between the Drude free carrier threshold (4000 cm−1 < ω0 < 6000 cm−1) and the onset of stronger interband transitions (above 13000 cm−1). Evaluations of the absorption profiles show an interband absorption superimposed on the free carrier absorption in the TW which are related to transitions involving states in the conduction band (or conduction bands). Values of ω0 are found to be slightly smaller for E ∥ b compared to E ∥ a (parallel to the incommensurate direction) polarized spectra for all compounds under study. This effect is interpreted in terms of slightly smaller effective masses for the optical conductivity for the E ∥ a components. The temperature dependence (213 to 423 K) of the free carrier reflectivity for (PbS)1.13TaS2 shows a linear increase of the damping parameter with temperature both for E ∥ a and E ∥ b. This is suggested to be due to ordinary electron‐phonon scattering of the free carriers. Reflection spectra for E ∥ c indicate ω0 values of about 1500 cm−1 for (SmS)1.19TaS2 ((TbS)1.21NbS2) and below 1300 cm−1 for (PbS)1.13TaS2 ((PbS)1.21NbS2).
Defect profiles in semiconductor structures
Authors: Coleman P. G.
Published: July 2007

Variable‐energy positron annihilation spectroscopy (VEPAS) has found applications in structural and electronic analysis of thin films and near‐surface layers, nanoporous materials, ion implantation, silicon photonics, and vacancy engineering. In all this applied work it is essential that VEPAS is treated as a normal member of the arsenal of spectroscopies available to the semiconductor research community. Examples are presented of how useful insights into current problems in semiconductor physics and technology have been gained by combinations of VEPAS and other techniques such as secondary ion mass spectrometry, transmission electron microscopy, electrical characterization, and optical techniques such as photoluminescence and optically‐detected magnetic resonance. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Positron annihilation spectroscopy in the study of quasicrystals and their related materials
Authors: Sato K. , Murakami H. , Kanazawa I. , Kobayashi Y.
Published: July 2007

Recent advances in the study of quasicrystals (QC's) and their related materials (crystalline approximants) by using positron annihilation spectroscopy are reviewed. We describe how a series of positron annihilation spectroscopy has been applied for the studies of aperiodic QC's. Positron lifetime measurements revealed the presence of high concentration of structural vacancies giving rise to saturation trapping for QC's and approximants studied. Chemical environments around the structural vacancies were successfully clarified by coincident Doppler broadening measurements. In addition, the concentrations of structural vacancies were estimated by analyzing the positron diffusion data obtained by a slow positron beam under the saturation trapping condition. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Positron annihilation characteristics in UO2: for lattice and vacancy defects induced by electron irradiation
Authors: Barthe M.‐F. , Labrim H. , Gentils A. , Desgardin P. , Corbel C. , Esnouf S. , Piron J. P.
Published: July 2007

In this work both 22Na based positron lifetime spectroscopy (PALS) and slow positron beam based Doppler annihilation‐ray broadening spectrometry (SPBDB) have been used to characterize respectively the bulk and the first micron under the surface of sintered UO2 disks that have been polished and annealed at high temperature (1700 °C/24 h/ArH2). Results show the presence of negative ions that are tentatively identified to negatively charged oxygen atoms located in interstitial sites. The positron annihilation characteristics in the UO2 lattice have been determined and are equal to SL(UO2) = 0.371(5), WL(UO2) = 0.078(7), τL(UO2) = 169 ± 1 ps. Such disks have been irradiated at room temperature with electrons and α particles at different fluences. After irradiation SPBDB and PALS measurements show the formation of U‐related vacancy defects after a 2.5 MeV electrons irradiation whereas no defects are detected for an irradiation at 1 MeV. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Positron studies of surfaces, structure and electronic properties of nanocrystals
Authors: Eijt S. W. H. , Barbiellini B. , Houtepen A. J. , Vanmaekelbergh D. , Mijnarends P. E. , Bansil A.
Published: July 2007

A brief review is given of recent positron studies of metal and semiconductor nanocrystals. The prospects offered by positron annihilation as a sensitive method to access nanocrystal (NC) properties are described and compared with other experimental methods. The tunability of the electronic structure of nanocrystals underlies their great potential for application in many areas. Owing to their large surface‐to‐volume ratio, the surfaces and interfaces of NCs play a crucial role in determining their properties. Here we focus on positron 2D angular correlation of annihilation radiation (2D‐ACAR) and (two‐detector) Doppler studies for investigating surfaces and electronic properties of CdSe NCs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Future directions of positron annihilation spectroscopy in low‐k dielectric films
Authors: Gidley D. W. , Vallery R. S. , Liu M. , Peng H.‐G.
Published: July 2007

Positronium Annihilation Lifetime Spectroscopy (PALS) has become recognized in the microelectronics industry as one of only several methods capable of quantitatively characterizing engineered nanopores in next‐generation (k < 2.2) interlayer dielectric (ILD) thin films. Successes and shortcomings of PALS to date will be assessed and compared with other methods of porosimetry such as ellipsometric and X‐ray porosimetries (EP and XRP). A major theme in future low‐k research focuses on the ability to integrate porous ILD's into chip fabrication; the vulnerability of porous dielectrics to etching, ashing, and chemical‐mechanical polishing in process integration is delaying the introduction of ultra‐low‐k films. As device size approaches 45 nm the need to probe very small (sub‐nanometer), semi‐isolated pores beneath thin diffusion barriers is even more challenging. Depth‐profiled PALS with its ability to determine a quantitative pore interconnection length and easily resolve 0.3 nm pores beneath diffusion barriers or in trench‐patterned dielectrics should have a bright future in porous ILD research. The ability of PALS (and PAS in general) to deduce evolution and growth of pores with porosity should find broad applicability in the emerging field of high performance materials with strategically engineered nanopores. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Effects of baicalin and wogonin on mucin release from cultured airway epithelial cells
Authors: Heo Ho Jin , Lee Hyun Jae , Kim Young Sik , Kang Sam Sik , Son Kun Ho , Seok Jeong Ho , Seo Un Kyo , Lee Choong Jae
Published: July 2007

Scutellaria baicalensis Georgi has been used for the treatment of diverse chronic inflammatory diseases including respiratory disease in oriental medicine and its major components – baicalin, baicalein and wogonin – were reported to have various biological effects. This study investigated whether baicalin, baicalein and wogonin affect basal and ATP‐induced mucin release from cultured airway epithelial cells. Confluent primary hamster tracheal surface epithelial (HTSE) cells were metabolically radiolabeled using 3H‐glucosamine for 24 h and chased for 30 min in the presence of varying concentrations of each agent to assess the effects on 3H‐mucin release. The results were as follows: (1) Baicalein did not affect both basal and ATP‐induced mucin release significantly. (2) Baicalin and wogonin increased basal mucin release at the highest concentrations (10−3 m). (3) However, baicalin and wogonin significantly inhibited ATP‐induced mucin release. It is concluded that baicalin and wogonin can slightly increase basal mucin release whereas they can inhibit ATP‐induced mucin release, by directly acting on airway mucin‐secreting cells. It is suggested that baicalin and wogonin be further investigated for the possible use as mucoregulators during the treatment of chronic airway diseases. Copyright © 2007 John Wiley & Sons, Ltd.
Construction of switching sequences for reachability realization of switched impulsive control systems
Authors: Ji Zhijian , Feng Gang , Guo Xiaoxia
Published: July 2007

This paper studies the construction of switching sequences for reachability realization of switched impulsive control systems. An approach is proposed to design switching sequences so that the reachable subspace of switched impulsive control systems is expressed in terms of the reachable state sets of the designed switching sequences. For a class of switched impulsive control systems, it is proved that a single switching sequence can be designed with its reachable state set coinciding with the reachable subspace. Periodic switching sequences are also constructed in this paper for the reachability realization problem. The results present a new way to exploit the switching mechanism to achieve the reachability and controllability of switched impulsive control systems. Copyright © 2007 John Wiley & Sons, Ltd.
Best analytic approximation of rational functions of degree 2: Symmetrization of formulae of N. J. Young
Authors: Spain Philip G.
Published: July 2007

Formulae, explicitly symmetric in the roots of the denominator, are derived for the best approximation to a rational function of degree 2 by an analytic function and by a function with at most one pole.
Robust stability—degree assignment and its applications to the control of flexible structures
Authors: Kimura Hidenori , Oike Taro , Miura Atsuyoshi , Akai Kenji , Kida Takashi
Published: July 2007

A new method of robust controller synthesis is introduced which is an extension of robust stabilization to robust stability‐degree assignment. This method guarantees a lower bound of stability degree for all uncertainties within a prescribed magnitude band.

The method is applied to the control of four types of flexible structures, namely, colocated beam, non‐colocated beam, beam with moving base and large scale satellite model. Excellent performances have been obtained experimentally with respect to the response speed and the robustness.
Quadratic robust tracking control — A Polynomial Matrix Approach
Authors: Kim Jeong‐Yeol , Ko Myoung‐Sam
Published: July 2007

A quadratic robust tracking problem is solved using a polynomial matrix approach. Because of the possibly unstable mode of the control sequence we propose a new quadratic cost function of error and control sequences to facilitate the optimization. The optimal controller makes the plant output track the reference sequence robustly and minimizes the proposed quadratic cost function. We also present the parametrized set of suboptimal controllers which yield finite costs so that there exists the central optimal controller in the set when the parameter is set to zero. The usefulness of our proposed cost function is demonstrated by simulation examples. Our design procedure has advantages over two other possible approaches: the LQ state‐space approach and the Wiener–Hopf approach which may be tried to tackle the same problem. Our approach obviates the need to construct a dynamic observer compared to the state‐space approach and it is computationally attractive compared to the Wiener–Hopf approach.
Branch and bound algorithm for computing the minimum stability degree of parameter‐dependent linear systems
Authors: Balakrishnan V. , Boyd S. , Balemi S.
Published: July 2007

We consider linear systems with unspecified parameters that lie between given upper and lower bounds. Except for a few special cases, the computation of many quantities of interest for such systems can be performed only through an exhaustive search in parameter space. We present a general branch and bound algorithm that implements this search in a systematic manner and apply it to computing the minimum stability degree.