bluuebunny/crossref_metadata_2025 huggingface dataset:
Calculation of electron effective masses in AlAs
Authors: Guo Q. , Ong C. K. , Poon H. C. , Feng Y. P.
Published: July 2007

The ab initio pseudopotential method based on the local density approximation is employed to calculate the band structure and the electron effective masses in AlAs. The expected camel's back structure at the X point of the conduction band is not found in the present calculation, in agreement with previous ab initio local density approximation calculation. The transverse and longitudinal electron effective masses are found to be mt = 0.268m0 and m1 = 1.14m0, respectively. The calculated effective masses are in reasonable agreement with the majority of the previously determined values.
The origin of the “high temperature” oscillations of kinetic coefficients of bismuth in the magnetic field
Authors: Bogod Yu. A. , Libinson A.
Published: July 2007

The electrical conductivity of a Bi monocrystal in high magnetic field (ultraquantum limit) was measured. The special quantum oscillations of the kinetic coefficients in the magnetic field (high temperature oscillations ‐ HTO) discovered in 1973 have been considered. The analysis of the experimental data verified one of two alternative models of HTO. The chosen model relates the HTO with the interband electron‐hole transitions near band limits.
Scintillation in cerium‐activated gadolinium‐based crystals
Authors: Kondratiev D. M. , Korzhik M. V. , Fyodorov A. A. , Pavlenko V. B.
Published: July 2007

Scintillation in cerium‐doped gadolinium‐based crystals results from the energy transfer of excited host matrix Gd3+ ions (6I7/2,11/2; 6P7/2,5/2) to the Ce3+ ions {5d1(2T2)} which radiatively relax to the ground state.
Correlation between ionicity and nonlinear refractive index in alkali halides
Authors: Górski P. , Kin M. , Kucharczyk W.
Published: July 2007

A relationship between the generalized Miller δ1111(ω; ω, ω, ω) coefficient and the Phillips‐van Vechten ionicity in alkali halides is presented. The correlation is employed to estimate the nonlinear refractive index of these crystals.
Semiconductors under Uniaxial Strain
Authors: Cardona M.
Published: July 2007

Uniaxial strain, having a larger number of irreducible components, yields in principle more information than its hydrostatic counterpart. It has, however, the drawback that its magnitude is limited by sample fracture. We discuss the methods of application of uniaxial stress to solid samples for the purpose of performing spectroscopic measurements. We then present some of the highlights of such measurements, concerning phonons and electronic states in semiconductors.
Electronic Structures of Semiconductors under Pressure
Authors: Christensen N. E. , Wenneker I. , Svane A. , Fanciulli M.
Published: July 2007

The dielectric constants of semiconductors under pressure are derived from ab initio calculations of the complex dielectric functions ε(ω). The pressure coefficient of the refractive index is negative for most tetrahedrally bonded compounds and trends are explained within a simple model. Gap deformation potentials as well as “absolute” band edge deformation potentials are derived, and the pressure dependence of the electronic structure of superlattices is discussed. Examples from the group III‐V, in particular nitrides, and II‐VI (ZnS, ZnS/ZnSe) compounds illustrate these investigations. Also, less conventional semiconductors, β‐FeSi2 and ε‐FeSi, are examined. Strong electron correlation effects limit the validity of LDA theory in these cases. It is suggested that experimental pressure studies can shed new light on the properties of Kondo insulators.
Pressure Dependence of the Electronic Band Gap in 6HSic
Authors: Engelbrecht F. , Zeman J. , Wellenhofer G. , Peppermüller C. , Helbig R. , Martinez G. , Rössler U.
Published: July 2007

Photoluminescence experiments on 6H‐SiC doped with nitrogen have been performed at low temperature (T = (29 ± 2) K) under hydrostatic pressure up to 5 GPa. The pressure coefficients of the S0, R0, P0, S02, and R02 emission lines related to the neutral nitrogen donor bound excitons were determined. The pressure coefficient of the indirect gap of 6H‐Sic deduced from the P0 line turns out to be +2.0 meV/GPa. Nonrelativistic band structure calculations within the density‐functional theory based on the local density approximation are used to calculate the pressure coefficients of the indirect band gaps of the 6H, 4H, and 3C SiC polytypes. The comparison with available experimental data shows good agreement with the theoretical results.
Lattice Dynamics of α‐Boron from ab‐initio Calculation and Raman Scattering under High Pressure
Authors: Vast Nathalie , Baroni S. , Zerah G. , Besson J. M. , Polian A. , Grimsditch M. , Chervin J. C.
Published: July 2007

The Raman spectrum of icosahedral α‐boron presents a very sharp peak at 525cm−1 that was consistently rejected from the lattice modes but is still unexplained. New Raman scattering experiments under pressure are compared with ab‐initio lattice dynamics calculations. The very good agreement of the mode frequencies and their pressure coefficients yields unambiguous assignment of all observed features, including the 525 cm−1 line which is a highly harmonic librational mode of the icosahedron. This mode is also identified in the Raman spectrum of other icosahedral boron‐rich solids.
Vibrational Properties of InSe under Pressure: Experiment and Theory
Authors: Ulrich C. , Mroginski M. A. , Goñi A. R. , Cantarero A. , Schwarz U. , Muñoz V. , Syassen K.
Published: July 2007

The pressure dependence of the phonon modes in the layered semiconductor γ‐InSe has been investigated experimentally and theoretically for pressures up to 11 GPa. The mode Grüneisen parameters of all Raman‐active zone‐center phonons have been determined by Raman scattering under pressure. In addition, features corresponding to second and third‐order scattering processes are apparent in the Raman spectra under resonance conditions, from which information about zone‐edge modes can be obtained. For the assignment of the observed Raman features to vibrational modes we have calculated the phonon dispersion curves using a rigid‐ion model including couplings to first‐nearest neighbors and long‐range Coulomb interaction. At about 7 GPa the sample turns from transparent to opaque and a new Raman mode appears in the spectra at around 165 cm−1. This is evidence of a pressure‐induced structural instability of γ‐InSe, which is optically detected but not by X‐ray diffraction.
Pressure Dependence of Band Gaps and Deep Centers in Chalcopyrite Semiconductors
Authors: Choi In‐Hwan , Yu P. Y.
Published: July 2007

We have determined the pressure coefficients of fundamental energy gap and deep centers in two chalcopyrite semiconductors AgGaS2and CuGaS2from their absorption and photoluminescence spectra. The pressure coefficient of a deep center in AgGaS2is found to belargerthan that of the band gap. A deep center with similar behavior has been reported already in AgGaSe2. We discuss the effect of p‐d hybridization on these coefficients.
Galvanomagnetic Measurements of the 2D Electron‐Hole Gas in GaSb/InAs/GaSb Quantum Wells under Pressure up to 2.5 GPa
Authors: Dizhur E. M. , Itskevich E. S. , Kashirskaya L. M. , Voronovsky A. N. , Malik T. A. , Stradling R. A. , Yuen W. T.
Published: July 2007

Galvanomagnetic measurements at 4.2 K in fields up to 7 T under hydrostatic pressures up to 2.5 GPa are presented for a series of GaSb/InAs/GaSb quantum wells with different cap‐to‐well ratios and interface type. The pressure derivatives of electron and hole concentrations have been obtained from Shubnikov‐de Haas (ShdH) oscillations and from fitting the low field data to the classical two‐carrier model. Above the pressureinduced metal‐insulator transition, a strong negative magnetoresistance occurs which is attributed to localisation associated with the presence of GaSb surface donors.
Spin Splitting in a 2D Hole System under Uniaxial Compression
Authors: Hansen O. P. , Kraak W. , Minina N. , Olsen J. S. , Saffian B. , Savin A. M.
Published: July 2007

Magnetotransport experiments under in‐plane [110] uniaxial compression up to 2.4 kbar of a (001) p‐GaAs/Al0.5Ga0.5As single heterostructure have been performed. A pronounced redistribution of carriers in the spin‐split lowest heavy hole subband in favour of the less populated and lighter mass one is established but the total carrier concentration reveals only a 5% decrease. The difference between the effective masses in spin subbands becomes less. The spin subband splitting decreases by a factor of 2.5 under maximum compression.
Hydrostatic Pressure Dependence of the Threshold Current in 1.5 μm Strained Quantum Well Lasers
Authors: Adams A. R. , Silver M. , O'Reilly E. P. , Gonul B. , Phillips A. F. , Sweeney S. J. , Thijs P. J. A.
Published: July 2007

Results are presented which show that the radiative current in 1.5 μm quantum well lasers obeys simple theory, but that the total current through the device at room temperature is dominated by Auger recombination. The measured temperature and pressure dependencies of the threshold current show that it is phonon‐assisted Auger recombination that is operative. This is less sensitive than band‐to‐band Auger processes to the exact form of the band structure and explains why inbuilt strain has little effect on the temperature sensitivity of the threshold current and why there is little change in threshold current as one goes from 1.5 to 1.3 μm devices.
New Structural Systematics in the II–VI, III–V, and Group‐IV Semiconductors at High Pressure
Authors: McMahon M. I. , Nelmes R. J.
Published: July 2007

A systematic study of the II‐VI, II‐V, and group‐IV semiconductors using angle‐dispersive diffraction techniques is revealing new structures and transitions that significantly modify the accepted structural systematics of these materials. Recent new results from Ge, ZnSe, InSb, and GaAs extend the range of some aspects already indicated by our previous work, and new intermediate phases have been found in InSb, and in HgSe and HgTe.
Pretransitional Behavior in Zincblende HgTe under High Pressure and Temperature
Authors: Besson J. M. , Grima P. , Gauthier M. , Itié J. P. , Mézouar M. , Häusermann D. , Hanfland M.
Published: July 2007

The equation of state of mercury telluride, HgTe, has been measured under pressure in its zincblende and cinnabar phases up to 800K, by X‐ray diffraction under synchrotron radiation in a large volume Paris‐Edinburgh cell. The thermal expansion coefficient decreases to zero around 1.1 GPa and then becomes negative up to the first‐order phase transition to the high‐pressure cinnabar phase, around 1.4 GPa. This behavior is related to the increase with temperature of the nonlinearity in the pressure dependence of the bulk modulus. It indicates an overall softening of the phonon branches of zincblende HgTe prior to the phase transition which characterizes a second‐order pretransitional behavior.
High Pressure Structural Study of I–III–VI2 Chalcopyrites
Authors: Mori Y. , Takarabe K. , Iwamoto S. , Minomura S. , Niwa E. , Masumoto K.
Published: July 2007

High pressure powder X‐ray diffraction studies are reported for the chalcopyrite phases of AgGaX2 (X = S, Se, and Te). Structural parameters have been determined by the Ftietveld method. While the c/a ratio decreases with increasing pressure, the anion displacement parameter u is constant within experimental error for all samples investigated.
Structural Studies of CuInS2 and CuInSe2 under High Pressure
Authors: Tinoco T. , Polian A. , Gómez D. , Itié J. P.
Published: July 2007

CuInS2 and CuInSe2 chalcopyrites have been studied up to 29 GPa in a diamond anvil cell by energy dispersive X‐ray diffraction (EDX) using synchrotron radiation. Phase transitions have been observed at 9.5 and 7.6 GPa and the high‐pressure phases have been indexed as cubic. The pressure dependence of the volume per formula unit has been fitted with a first‐order Murnaghan equation of state. The bulk moduli are 75 and 72 GPa for the chalcopyrite phases and 123 and 114 GPa for the cubic phases of CuInS2 and CuInSe2, respectively. The volume reduction at the transition pressure is roughly 10%. The recovered phases are quite different from the initial ones. For CuInSe2 it is of the zincblende type, for CuInS2 it is an amorphous phase. Thus, the pressure‐induced phase changes are not reversible.
High Pressure Phases of AlSb from ab‐initio Theory
Authors: Rodríguez‐Hernández P. , Muãoz A. , Mujica A.
Published: July 2007

We present the results of an ab‐initio study of the ground state properties and the high pressure phase diagram of AlSb. We have analyzed the structural parameters and the total energy of AlSb in the rocksalt, cesium chloride, ß‐tin, zincblende, and nickel arsenide‐type structures, performing first principles self‐consistent local density calculations with nonlocal pseudopotentials. From the energy versus volume diagram we obtain that at 4.7 GPa AlSb transforms to the NiAs‐type structure, like other III‐V AIX compounds. This is in disagreement with a recent experimental study that reports a transition to a ß‐tin structure. We also predict a second transition to the CsCl‐type structure at 39.5 GPa.
ZnSb and GaSb Bulk Amorphous Semiconductors: Transport Properties
Authors: Antonov V. E. , Barkalov O. I. , Kolyubakin A. I. , Ponyatovsky E. G.
Published: July 2007

Temperature dependencies of the conductivity and thermopower of bulk amorphous semiconducting alloys Zn41Sb59 and Ga100−xSbx with 47.5 < x < 55 were measured at 80 to 370 K and 120 to 370 K, respectively. The samples were prepared by solid state amorphization of the quenched high pressure phases occurring on heating at ambient pressure. The electrical properties of nonstoichiometric a‐Zn41Sb59 are well described by the conventional Mott‐Davis model. Those of both stoichiometric a‐GaSb and nonstoichiometric a‐Ga100−xSbx, appear to be more unusual and require a modification of the model.
Pressure‐Induced Amorphization of LiInGe
Authors: Bräninger S. , Schwarz U. , Syassen K. , Kniep R.
Published: July 2007

We have investigated the structural stability of the ternary Zintl phase LiInGe up to 35 GPa by means of angle dispersive X‐ray powder diffraction. At ambient conditions LiInGe crystallizes in a cubic facecentred structure consisting of a polytetrahedral arrangement of In and Ge atoms. The compound undergoes a reversible phase transition to an X‐ray amorphous high pressure modification between 20 and 30 GPa. We have determined the equation of state for the low pressure phase and the result is discussed with respect to the related binary NaTl‐and zincblendetype compounds. In order to investigate the electronic properties of LiInGe we have performed LMTO band structure calculations.
Raman Spectra of InAs1−xSbx Alloys and InAs0.58Sb0.42/InSb Strained Layer Superlattice under High Pressure
Authors: Nagata K. , Webb S. J. , Stradling R. A.
Published: July 2007

Raman scattering has been measured in InAs1−xSbx alloys (0 ⩽ × ⩽ 1) and an InAs0.58Sb0.42/InSb strained layer superlattice under hydrostatic pressures up to the phase transition. The pressure coefficient of the Raman frequency (δω/δp) of the InAs‐like LO mode in InAs1−xSbx alloys increases with increasing composition ×, while that of the InSb‐like LO mode is almost constant above × = 0.5. The phase transition pressure of InAs1−xSbx alloys decreases almost linearly with composition. On the other hand, both component layers of an InAs0.58Sb0.42/InSb superlattice change phase simultaneously at a pressure which is the mean of the phase transition pressures for bulk InAs0.58Sb0.42 and InSb.
Optical Properties of Pressure‐Polymerized C60
Authors: Venkateswaran U. D. , Sanzi D. , Krishnappa J. , Marques L. , Hodeau J.‐L. , Núñez‐Regueiro M. , Rao A. M. , Eklund P. C.
Published: July 2007

We report Raman, infrared (IR), and photoluminescence (PL) studies of several polymeric phases of solid C60 synthesized under high pressure and temperature conditions. The structure and dimensionality of bonding in the polymer depend on the conditions of synthesis. Relative to pristine C60, the Raman and IR spectra of these polymers exhibit numerous lines which are shifted to low frequencies and are also broadened. These changes are consistent with the loss of the original icosahedral C60 symmetry and an increase in the strength of intermolecular bonding due to polymerization. PL features are also broadened significantly and are redshifted compared to those of pristine C60 which is consistent with the usual trend that polymerization decreases the HOMO‐LUMO gap. The pressureinduced shift of the PL bands in the rhombohedral fullerene polymer is about five times smaller than that reported for solid C60.
The Effect of Pressure on the Concentration of Thermal Donors in Czochralski Grown Silicon
Authors: Misiuk A. , Jung W.
Published: July 2007

We have investigated the effect of annealing at temperatures up to 1230 K under hydrostatic argon pressure up to 1.1 GPa on thermal donor formation and on interstitial oxygen precipitation in Czechralski grown silicon. Depending on annealing temperature and applied pressure, the stressen‐hanced formation of thermal donors, enhanced interstitial oxygen loss from the Si‐O solid solution, and generation of oxygen‐related defects were observed.
The Structure of the Ordered Phase in Rocksalt Type Titanium Carbide, Carbidenitride, and Carbidehydride
Authors: Em V. T. , Yu. Tashmetov M.
Published: July 2007

It is shown that on the basis of the disordered δ‐phase (space group Fm3m) two ordered equilibrium phases with cubic (δ′‐phase, space group Fd3m) and trigonal (δ″‐phase, space group R3m or P3121) unit cells can be formed in titanium carbide TiC0.62 and carbidenitrides TiCxNy (x+y ≈︁ 0.63), whereas only a cubic δ′‐phase has been observed in TiC0.62Hy. With increasing temperature the low temperature trigonal δ″‐phase transforms at about 1053 K to the cubic δ′‐phase which experiences disordering. The formation of the cubic δ′‐phase at temperatures lower than 1053 K is explained by the difference between the order of δ → δ and δ → δ″ phase transitions.
Two‐Phonon Magneto‐Raman Scattering in Quantum Wells: Fröhlich Interaction
Authors: Cros A. , Cantarero A. , Belitsky V. I.
Published: July 2007

We have developed a theoretical model of two‐phonon resonant magneto‐Raman scattering in a semiconductor quantum well (QW). Fröhlich electron—phonon interaction has been considered and the corresponding selection rules are derived for Faraday geometry and backscattering configuration. The resonant profiles are analyzed as a function of magnetic field and laser energy. To simplify the discussion a three‐band model with parabolic masses has been used as a first approach, studying later the role of heavy‐hole light‐hole admixture in the scattering process. It is shown that, due to mixing effects, Fröhlich interaction contributes to the two‐phonon Raman spectra in the parallel (z̄(σ±, σ±)z) and also in the usually forbidden crossed (z̄(σ±, σ∓) z) polarizations. The model has been used to calculate the two‐phonon magneto‐Raman spectra of a 35 Å, GaAs/AlAs QW as an example.
Scattering of Excitons by Free Carriers in Semiconducting Quantum Well Structures: Finite Potential Well Model
Authors: Koh Tong San , Feng Yuan Ping , Spector Harold N.
Published: July 2007

We present a theoretical calculation of the elastic and ionization scattering cross sections of excitons due to their scattering by free carriers, in semiconducting quantum well structures of finite width, using the finite confining potential well model. Our results reveal interesting features that differ significantly from the predictions of the infinite confining potential well model. We find that both the elastic and ionization cross sections initially decrease with decreasing well width, but then in‐ crease below a critical well width where the exciton begins to revert to a quasi‐3D behavior. We compare the present results with those obtained using previous models to account for these new effects.
Decorated vacancy clusters in Si and thin C films grown on Si studied by depth profiling positron annihilation spectroscopies
Authors: Brusa R. S.
Published: July 2007

The combined use of different depth profiling positron annihilation spectroscopies give insight on distribution, type and decoration of open‐volume defects. Applications regarding defects produced in modified silicon and at the interface between deposited thin films and the silicon substrate are presented. The attention is focused on selected systems and situations potentially related to technological developments: a) identification of decorated vacancy clusters in silicon implanted by light‐ion (He, He+H) and evolution of the clusters with the thermal treatments; b) release of compressive stress through vacancy‐like defects formation at the interface during the growth of thin (10‐200 nm thick) carbon films. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
PALS study of epoxy matrices: self‐assembly of block copolymers and its capability for nanostructuring thermosetting systems
Authors: Ramos Jose Angel , Serrano Elena , Tercjak Agniezska , Salgueiro Walter , Goyanes Silvia , Mondragon Inaki
Published: July 2007

The free volume in epoxy systems based on diglycidyl ether of bisphenol‐A (DGEBA) fully cured with aminic hardeners having different chemical structures has been studied by means of positron annihilation lifetimes spectroscopy. The results are compared with those obtained from the analysis of the macroscopic specific volume changes by using pressure‐volume‐temperature (PVT) experimental technique. An excellent correlation between the volumes measured at macro and nanoscales was found for the epoxy systems fully cured with aminic hardeners. On the other hand, a systematic study on the dependence of the volumes at nano‐scale in epoxy systems cured with two selected aminic hardeners at different pre‐cure temperatures revealed that the pre‐cure temperature, as well as, the structure of the hardeners governs the packing of the molecular chains of the epoxy network and its influence in the volume and number density of the nanoholes. In the second part of this work, a systematic study of the effect of several epoxidation degrees of butadiene block in a commercial polystyrene‐polybutadiene star block copolymer (SB) has been carried out to control nano‐ordering in thermosetting epoxy systems. For this purpose, blends of DGEBA cured with 4,4′‐methylenebis(3‐chloro‐2,6‐diethylaniline) (MCDEA) containing 30 wt% neat SB and several epoxidized SB star block copolymers have been synthesized. At low epoxidation degrees, interactions with the epoxy matrix are not sufficiently favorable and macroscopic phase separation takes place, leading to a phase‐inverted morphology where the block copolymer may become the matrix. For high epoxidation degrees, however, nanostructured thermosetting systems, being the epoxy‐rich phase the matrix of the blends, have been obtained. It has been proved that, in blends containing SepB61 and SepB76 star block copolymers, the copolymer self‐assembles into a well‐defined hexagonally ordered structure, where cylinders are formed by PS arranged in an epoxy matrix containing both epoxidized and non‐epoxidized butadiene units. SAXS experiments show that there are not strong differences in the long spacing between the different systems because the repeated distance between PS cylinders for all the blends is around 41‐42 nm. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Investigation of nanopores in nanofiltration membranes using slow positron beam techniques
Authors: De Baerdemaeker J. , Boussu K. , Djourelov N. , Van der Bruggen B. , Dauwe C. , Weber M. , Lynn K. G.
Published: July 2007

The pore distribution of the commercial nanofiltration membranes Desal 5DL, Desal 51HL, N30F, NTR 7450 and NF‐PES‐10 were analyzed by positron annihilation spectroscopy using slow positron beam analysis. From the depth selective 3γ/2γ analysis a dense nanoporous filtration layer could be determined. From the depth selective lifetime analysis the size and distribution of the nanopores could be extracted. The measurements indicate the presence of 2 pore sizes in the samples with a radius of 1.25‐1.55 Å and 3.2‐3.95 Å. These experimental values are compared with the molecular weight cut‐off values of these membranes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Positron and positronium scattering
Authors: Walters H. R. J. , Starrett C.
Published: July 2007

A short overview of coupled‐(pseudo)state methods and results for positron and positronium scattering by atomic targets is presented. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Positron experiments at the new positron beam facility NEPOMUC at FRM II
Authors: Hugenschmidt C. , Brunner T. , Legl S. , Mayer J. , Piochacz C. , Stadlbauer M. , Schreckenbach K.
Published: July 2007

The in‐pile positron source NEPOMUC of the Munich research reactor FRM II delivers a low‐energy positron beam of highest intensity. Different source potentials can be applied in order to vary the primary kinetic energy of the positrons in the range between 15 eV and 1 keV. The maximum yield of slow positrons is up to 4 × 107 and 5 × 108 moderated positrons per second, respectively. New instruments for beam diagnostics have been implemented for the determination of the positron intensity and for positron beam profile measurements. The long‐term stability and the degradation of the platinum moderator as well as the recovery procedure were investigated. In the present arrangement of NEPOMUC's instrumentation the monoenergetic positron beam is magnetically guided to different experiments: a coincident Doppler broadening spectrometer (CDBS) and an analysis chamber for positron induced Auger electron spectroscopy (PAES). In addition, an apparatus for the production of the negatively charged positronium ion was connected to the beam line in order to investigate this bound leptonic system in collaboration with the Max‐Planck Institute for nuclear physics. An overview of the beam performance, the current status of the positron beam facility and the experiments is presented. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Atomic structure of pre‐Guinier–Preston zones in Al alloys
Authors: Staab T. E. M. , Haaks M. , Zamponi C. , Modrow H. , Maier K.
Published: July 2007

We propose X‐ray absorption (XAS) measurements as a novel approach to determine the atomic structure of pre‐Guinier–Preston zones. These nano‐clusters are formed during very early stages of aging AlCu alloys, immediately after solution heat treatment and quenching. X‐ray absorption near‐edge structure (XANES) spectra were taken from technical aluminum alloys at the copper K edge, revealing the local atomic environment of copper. The spectra of – after solution heat treatment – freshly quenched and of artificially aged alloys differ significantly from each other. We compare the measured XANES spectra with those calculated by the FEFF‐8 code. We show the importance of employing realistic, i.e. relaxed, atomic coordinates around the scattering atom type from ab‐initio calculations with SIESTA. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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Effect of grape seed extract on lipid peroxidation, antioxidant activity and peripheral blood lymphocytes in rats exposed to x‐radiation
Authors: Enginar Hüseyin , Cemek Mustafa , Karaca Turan , Unak Perihan
Published: July 2007

The present studies were designed to evaluate supplemental grape seed extract (GSE) and vitamin E supplements on lipid peroxidation, on antioxidant systems and peripheral blood lymphocytes in rats exposed to x‐rays. Three groups of rats were investigated: a control group (CG) received intraperitoneal (i.p.) physiological serum 1 mL/day (n = 10), i.p.; a vitamin E group (VG) received 50 mg/kg/day (n = 10); an i.p. grape seed extract group received 50 mg/kg/day (n = 10). Four weeks later, a 6 Gy radiation dose was given to the rats. Blood samples were taken 24 h later after irradiation and lymphocyte, malondialdehyde (MDA), reduced glutathione (GSH), nitrate, nitrite, reduced ascorbic acid, retinol, β‐carotene and ceruloplasmin concentrations were analysed. The levels of GSH (p < 0.05), retinol (p < 0.001), β‐carotene (p < 0.05) and ceruloplasmin concentration (p < 0.001) in the GSE group were found to be higher than in the control group but the level of MDA (p < 0.001) and nitrite concentration (p < 0.05) in rats supplemented with GSE were found to be lower than in the control group.

The results indicate that GSE enhanced the antioxidant status and decreased the incidence of free radical‐induced lipid peroxidation in blood samples of rats exposed to x‐radiation. The antioxidant effect of GSE given to animals was more effective than vitamin E administered before whole‐body irradiation in rats. Copyright © 2007 John Wiley & Sons, Ltd.
Necessary and sufficient conditions for the stability of polynomials with linear parameter dependencies
Authors: Abdallah C. , Docampo D. , Jordan R.
Published: July 2007

In this paper we show that Kharitonov's theorem will provide a necessary and sufficient condition for the stability of a particular family of polynomials where the coefficients are real and linearly dependent on a set of uncertain parameters. The main results are obtained by restricting the linear map from the parameter to the coefficient space to contain the Kharitonov polynomials. The results are applied to both continous‐time and discrete‐time polynomials and future extensions are discussed.
Stability radii of linear discrete‐time systems and symplectic pencils
Authors: Hinrichsen D. , Son N. K.
Published: July 2007

In this paper, we introduce and analyse robustness measures for the stability of discrete‐time system x(t + 1) = Ax(t) under parameter perturbations of the form A→A + BDC where B,C are given matrices. In particular we characterize the stability radius of the uncertain system x(t + 1) = (A + BDC)x(t), D an unknown complex perturbation matrix, via an associated symplectic pencil and present an algorithm for the computation of that radius.
A first principles solution to the non‐singular H∞ control problem
Authors: Petersen Ian R. , Anderson Brian D. O. , Jonckheere Edmond A.
Published: July 2007

This paper presents an elementary solution to the non‐singular H∞ control problem. In this control problem, the underlying linear system satisfies a set of assumptions which ensures that the solution can be obtained by solving just two algebraic Riccati equations of the game type. This leads to the central solution to the H∞ control problem. The solution presented in this paper uses only elementary ideas beginning with the Bounded Real Lemma.
Adaptive motion control of robot manipulators: A unified approach based on passivity
Authors: Brogliato Bernard , Landau Ioan‐Doré , Lozano‐Leal Rogelio
Published: July 2007

This paper presents a unified approach to direct adaptive motion control laws for robot manipulators that have been studied during the last few years by several authors. It provides a general approach based on sensivity to demonstrate the global asymptotic stability of adaptive schemes applied to rigid multilinked manipulators. It is shown that most of the schemes fit within this framework, which presents the advantage of being more systematic than other techniques and therefore will enable a unified presentation of the several schemes proposed to date and will increase our understanding of adaptive control of robot manipulators.
Robust control of robotic manipulators without velocity feedback
Authors: Yuan Jing , Stepanenko Yury
Published: July 2007

This study concerns the problem of robust control of robotic manipulators without joint velocity feedback. A robust lead + bias controller is studied. The bias signal is intended to compensate the nonlinear dynamics of the robot. The focus of this study is robustness when the nonlinear compensation is not perfect and the external disturbances are not negligible.

A conservative polynomial bound is introduced to describe the worst feedback effect of the compensation error and the external disturbances. The polynomial bound covers a class of possible bias signals, synthesized according to the available knowledge about the robot dynamics. Based on the polynomial bound, the tracking errors of a lead + bias controller are proved to be uniformly bounded. They can be minimized by a proper design of the bias signal. In the ideal case where the bias signal compensates the robot dynamics perfectly, the tracking errors will converge to zero.
Outer factor ‘absorption’ forH∞control problems
Authors: Flamm David S.
Published: July 2007

We present a constructive method for ‘absorbing’ an irrational outer factor of a plant into the ‘Q‐parameter’ in theH∞optimal weighted sensitivity problem for single‐input/single‐output distributed parameter systems, when the plant has finitely many irrational zeros on the imaginary axis. This problem could not be solved using previous results. We also extend our new results to the mixed sensitivity problem.
Computation of infimalH∞norm over a finite horizon
Authors: Subrahmanyam M. B.
Published: July 2007

In this paper we develop necessary conditions for a minimax problem involving control and exogenous inputs. The problem can be regarded as a finite horizon version of theH∞optimal control problem. We consider problems involving generalized cost functional and non‐zero initial conditions. A criterion for the evaluation of the performance index is given in these cases. Our computational experience shows that the finite horizon performance is useful in computing the infimalH∞norm in the infinite horizon case, as the final time becomes large. Also, expressions are derived for the variation in performance in terms of system parameter variations. These linear expressions are useful in the evaluation of the robustness of the proposed control strategy.
H∞ estimation for uncertain systems
Authors: Fu Minyue , de Souza Carlos E. , Xie Lihua
Published: July 2007

This paper deals with the problem of H∞ estimation for linear systems with a certain type of time‐varying norm‐bounded parameter uncertainty in both the state and output matrices. We address the problem of designing an asymptotically stable estimator that guarantees a prescribed level of H∞ noise attenuation for all admissible parameter uncertainties. Both an interpolation theory approach and a Riccati equation approach are proposed to solve the estimation problem, with each method having its own advantages. The first approach seems more numerically attractive whilst the second one provides a simple structure for the estimator with its solution given in terms of two algebraic Riccati equations and a parameterization of a class of suitable H∞ estimators. The Riccati equation approach also pinpoints the ‘worst‐case’ uncertainty.
Modelling and decentralized control of a multizone crystal growth furnace
Authors: Abraham R. , Lunze J.
Published: July 2007

The controller of a crystal growth furnace with 15 heating zones must have a decentralized structure. It is shown that each control station must necessarily satisfy a certain robustness property and that the design of the whole decentralized controller can be replaced by the problem of designing the control stations separately from each other as robust centralized controllers. As a basis for this, it is shown that the model can be considerably reduced. Although the zones are strongly coupled it will be seen here that, owing to structural properties of the plant, the furnace can be described by a model which has regard to only three zones. Experimental results with furnace illustrate the modelling and design methods used.
Robust control design of power system stabilizers using multivariable frequency domain techniques
Authors: Chow J. H. , Harris L. P. , Kale M. A. , Othman H. , Sanchez‐Gasca J. J. , Terwilliger G. E.
Published: July 2007

H∞ and structured singular value optimization techniques are used to design robust power system stabilizers (PSS) for a single‐machine and a two‐machine system with varying operating conditions. Realistic uncertainty models to represent the possible operating conditions as perturbations from a nominal operating condition are developed. System experience is used to select weighting functions to provide adequate damping and shape the controller frequency response. Computer simulations show that the PSS designed using the proposed technique provides improved damping compared to a conventional PSS.
Zero cancelling compensators for singular control problems and their application to the inner–outer factorization problem
Authors: Copeland B. R. , Safonov M. G.
Published: July 2007

Many control problems fall into the category of what we call singular control problems, i.e., problems for which known solutions fail owing to the fact that the relevant transfer functions have zeros on the extended imaginary axis, 𝒞0e. An example of this is the inner–outer factorization (IOF) problem for such transfer functions.

In this paper, we show how to design compensators which would allow for a direct solution to these problems through cancellation of the offending 𝒞0e zeros. This is demonstrated for the IOF problem mentioned above. Our interest lies primarily with infinite zeros but we develop formulae for finite 𝒞0e compensators as well. The paper also presents some discussions on the infinite zero structure of linear time‐invariant systems.
Feedback control of nonlinear systems
Authors: Isidori Alberto
Published: July 2007

This paper deals with the design of (memoryless) state‐feedback laws for systems modelled by nonlinear differential equations which are affine in the inputs. The purpose of the design is to obtain a (locally) internally stable closed‐loop system in which the effect of exogenous inputs on a prescribed error (or, more in general, on a penalty variable) is attenuated. Two standard setups are considered: in the first one, the ratio between the energy associated with the penalty variable and that associated with the exogenous input is required to be bounded by a constant 0 < γ this setup includes (to some extent) the standardH∞control problemof linear system theory. In the second one, the penalty variable is required to converge to 0 ast∞; this setup generalizes the so‐calledservomechanism problemof linear system theory.
State‐space solutions to the ℋ︁∞/LTR design problem
Authors: Stoustrup Jakob , Niemann Hans Henrik
Published: July 2007

The LTR design problem using an ℋ︁∞ optimality criterion is presented for two types of recovery errors, the sensitivity recovery error and the input‐output recovery error. For both errors two different approaches are presented. First, following the classical LTR design philosophy, a Luenberger observer based approach is proposed, where the ℋ︁∞ part of the controller is appended to a standard full‐order observer. Second, allowing for general controllers, an ℋ︁∞ state‐space problem is formulated directly from the recovery errors. Both approaches lead to controller orders of at most 2n. In the minimum phase case, though, the order of the controllers can be reduced to n in all cases. The control problems corresponding to the various controller types are given as four different singular ℋ︁∞ state‐space problems, and the solutions are given in terms of the relevant equations and inequalities.
Robust exponential stability of time‐varying linear systems
Authors: Hinrichsen D. , Pritchard A. J.
Published: July 2007

This paper investigates the robustness of time‐varying linear systems under a large class of complex time‐varying perturbations. Previous results8 which were restricted to bounded linear perturbations of output feedback type are generalized to unbounded and nonlinear perturbations of multi‐output feedback type. We establish a lower bound for the stability radius of these systems and show how it may be possible to improve the bound using time‐varying scalar transformations of the state, input and output variables. The results are applied to derive Gershgorin type stability criteria for time‐varying linear systems.
Stabilization of uncertain boundary control systems
Authors: Rebiai S. E. , Zinober A. S. I.
Published: July 2007

We consider a class of linear infinite‐dimensional systems subject to boundary uncertainties, which are linear and time‐invariant. Using a deterministic approach to the design of stabilizing feedback controls, uniform exponential stability of the zero state is achieved. Lyapunov techniques are combined with Datko or Ichikawa theory. Three illustrative examples are presented.
Nonquadratic cost and nonlinear feedback control
Authors: Bernstein Dennis S.
Published: July 2007

Nonlinear controllers offer significant advantages over linear controllers in a variety of circumstances. Hence there has been significant interest in extending linear‐quadratic synthesis to nonlinear‐nonquadratic problems. The purpose of this paper is to review the current status of such efforts and to present, in a simplified and tutorial manner, some of the basic ideas underlying these results. Our approach focuses on the role of the Lyapunov function in guaranteeing stability for autonomous systems on an infinite horizon. Sufficient conditions for optimality are given in a form that corresponds to a steady‐state version of the Hamilton‐Jacobi‐Bellman equation. These results are used to provide a simplified derivation of the nonlinear feedback controller obtained by Bass and Webber (1966)38and to obtain a deterministic variation of the stochastic nonlinear feedback controller developed by Speyer (1976).45.
On approximate linearization of nonlinear control systems
Authors: Guzzella L. , Isidori A.
Published: July 2007

A method for the approximate linearization of nonlinear control systems based on the ‘state‐space exact linearization’ method is presented. An explicit procedure, both for the single‐input and for the multiple‐input case, is given, which is straightforward to implement.
A simplified approach for the design of basically non‐interacting multiple‐input‐multiple‐output systems using qft
Authors: Oldak S. , Horowitz I.
Published: July 2007

Quantitative feedback theory (QFT) has presented techniques for the design of multiple‐input‐multiple‐output (MIMO) linear time invariant (LTI) systems with structured parameter uncertainty in the plant P for the satisfaction of specifications on the closed loop transfer function matrix T = [Tij]. In many practical applications the specifications are of the basically non‐interacting (BNIA) type, i.e. aii(ω) < | Tii(jω) | < bii(ω), | Tij(jω) | < bij(ω), (i ≠ j) and bij(ω) < aii(ω) in a significant range of frequencies. In one QFT technique the design is based on expressing magnified image when the matrix of compensators G = diag(Gii(s)), Li = GiiQii, P−1 = [1/Qij], Dij a disturbance due to plant interaction between the different system channels. It is shown in this paper that when the specifications are BNIA and F = diag(Fii(s)), the effect of the disturbance acting on the main diagonal terms (i.e. Dii) can be neglected. This observation saves some computational burden because satisfaction of specifications on the Tiis becomes a single‐input‐single‐output (SISO) design problem instead of the more elaborated multiple‐input‐single‐output (MISO) design problem which had to be designed originally.

A detailed 2‐input‐2‐output design example is presented illustrating the simpler approach, stressing the importance of considering the correlation between specifications in the design procedure.
Explicit construction of quadratic Lyapunov functions for the small gain, positivity, circle, and popov theorems and their application to robust stability. Part I: Continuous‐time theory
Authors: Haddad Wassim M. , Bernstein Dennis S.
Published: July 2007

The purpose of this paper is to construct Lyapunov functions to prove the key fundamental results of linear system theory, namely, the small gain (bounded real), positivity (positive real), circle, and Popov theorems. For each result a suitable Riccati‐like matrix equation is used to explicitly construct a Lyapunov function that guarantees asymptotic stability of the feedback interconnection of a linear time‐invariant system and a memoryless nonlinearity. Lyapunov functions for the small gain and positivity results are also constructed for the interconnection of two transfer functions. A multivariable version of the circle criterion, which yields the bounded real and positive real results as limiting cases, is also derived. For a multivariable extension of the Popov criterion, a Lure‐Postnikov Lyapunov function involving both a quadratic term and an integral of the nonlinearity, is constructed. Each result is specialized to the case of linear uncertainty for the problem of robust stability. In the case of the Popov criterion, the Lyapunov function is a parameter‐dependent quadratic Lyapunov function.
PD‐control for stablewise tracking with finite reachability time: Linear continuous‐time mimo systems with state‐space description
Authors: Grujić Ljubomir T. , Mounfield William Pratt
Published: July 2007

The key problems addressed and solved in the framework of time‐invariant continuous‐time multiple‐input multiple‐output (MIMO) linear systems are the following: (a) the necessary and sufficient condition on the system under which there exists a control that can force the system to exhibit a requested tracking property without using any information about the mathematical description of the system internal dynamics and external disturbances; (b) the synthesis of an output error algorithm expressing stablewise tracking with finite (pre‐specified) reachability time; (c) the synthesis of a Proportional‐Derivative (PD) control algorithm for stablewise tracking with finite (given) reachability time without utilizing a mathematical model of the system internal dynamics, its state and the unmeasured, unknown time‐varying disturbances. Simulation results illustrate the theory with an unstable MIMO system under different desired stablewise tracking settling times.
A delta transform approach to loop gain‐phase shaping design of robust digital control systems
Authors: Punyko A. J. , Bailey F. N.
Published: July 2007

This paper addresses the existence of loop gain‐phase shaping (LGPS) solutions for the design of robust digital control systems for SISO, minimum‐phase, continuous‐time processes with parametric uncertainty. We develop the frequency response properties of LGPS for discrete‐time systems using the Δ‐transform, a transform method that applies to both continuous‐time and discrete‐time systems. A theorem is presented which demonstrates that for reasonable specifications there always exists a sampling period such that the robust digital control problem has a solution. Finally, we offer a procedure for estimating the maximum feasible sampling period for LGPS solutions to robust digital control problems.
Quantitative feedback theory with applications in welding
Authors: Bentley Anthony E.
Published: July 2007

Two different control systems were developed using quantitative feedback theory for weld quality improvement. The first application to be discussed was designed for the resistance pinch welding process and uses electrode displacement and force as feedback parameters. A correlation between weld quality and electrode displacement is established for constant electrode force. The system is capable of producing repeatable welds of consistent quality, with wide variations in weld parameters. This is the first time feedback control has been successfully applied to pinch welding. The second example presented in this paper was developed for penetration control of the gas‐tungsten‐arc welding process. The feedback signal is obtained by measuring the amount of light emitted from the back side of the weld. Welds of constant penetration have been demonstrated in tests with travel speeds varying from 1.5 to 6 inches per minute and with 200 per cent changes in part thickness.