bluuebunny/crossref_metadata_2025 huggingface dataset:
Coupling with the Jahn‐Teller mode for triplet states of MF6 (M = Mn2+, Cr3+) complexes: Dependence on the M‐F distance and influence on the stokes shift
Authors: Barriuso M. T. , Aramburu J. A. , Moreno M.
Published: July 2007

The microscopic origin of the VE coupling constant with the Jahn‐Teller mode Eg for orbital triplet states corresponding to several excited states of MnF4+6 and the 4T2g state of CrF3−6 is explored by molecular orbital (MO) calculations. VE is shown to be determined by the splittings δe and δt induced in the antibonding e and t levels by a Qθ(∼3z2− r2) distortion and so VE < 0 for the 4T2g state of CrF36 leading to a compressed octahedron as equilibrium geometry. VE values have been derived from self‐consistent charge extended Hückel and MSXα calculations performed at different Qθ values. The results for the 4T1g(G), 4T2g(G), 4T2g(D), and 4T1g(P) states of MnF46 are reasonably close to the experimental figures. For the first excited state 4T1g(G) the value VE ∼ 60 cm−1/pm found for a metal‐ligand distance R equal to 213 pm leads to a Huang‐Rhys factor SE = 1.5 also close to experimental findings. On passing from this case to the 4T2g state of CrF36−[VE] increases by a factor of about two but SE =S 1.2 in agreement with experimental data for Rb2KGaF6: Cr3+. The latter figure implies a Stokes shift E0S(E) due to the Eg mode equal to 1200 cm−1 which is about 50% of the total Stokes shift. As a salient feature it is shown that for both MnF4−6 and CrF3−6 complexes VE strongly depends upon R, as it also happens to the VA coupling constant with the symmetric A1g mode. This reasonably explains the increase of the Stokes shift upon increasing R recently observed for Mn2+−doped fluoroperovskites. Although pure crystalfield (CF) theory gives rise to VE values much smaller than the experimental ones it is shown that, the relation VA/VE = \documentclass{article}\pagestyle{empty}\begin{document}$\documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt 2 $\end{document}$\end{document} derivedforthe first excited state of MnF4−6 and CrF3−6 is no far from the results obtained through MO calculations.
Estimation of the local compressibilities for GD3Ga5O12 : Cr3+ and Gd3Sc2Ga3O12 : Cr3+ crystals from their high pressure spectroscopies
Authors: Zheng Wen‐Chen
Published: July 2007

According to high pressure spectroscopies, in particular, the nearly linear red‐shift of the R‐line at pressures above ≈ 7 GPa for Gd3Ga5O12 : Cr3+ and Gd3Sc2Ga3O12 : Cr3+ crystals, the local linear compressibilities of (CrO6)9− groups in both systems at P ≈ 7 GPa have been determined by using the bond length dependence of the covalency reduction factor obtained from the pressure‐induced spectral shifts of ruby (Al2O3 : Cr3+). Then, the local compressibilities at normal pressure are estimated on the basis of the pressure dependence of compressibility obtained for other garnet crystals. The results are obviously different from the compressibilities of the host crystals. The reasons of this difference are discussed.
Persistent photoconductivity and properties of the Two‐Dimensional electron Gas in modulation‐doped GaInAs/AlInAs heterostructures after illumination
Authors: Saffian B. , Kraak W. , Oelze B. , Künzel H. , Böttcher J.
Published: July 2007

The properties of the two‐dimensional electron gas (2DEG) in MBE‐grown low carrier density Ga0.47In0.53As/Al0.48In0.52As heterostructures after illumination at low temperatures have been investigated. A strong persistent photoconductivity anddrastic changes of low temperature electronic properties were observed. Thepronounced changes of the quantum oscillations of the magnetoresistivity (Shubnikov‐de Haas effect) and of the quantum Hall effect clearly reveal an important increase of both electron concentration and carrier mobility in the two‐dimensional interface layer. The variations of the electronic properties are attributed to photoexcitation of electron‐hole pairs in the GaInAs layer.
A possible new approach of superconductivity
Authors: Rezlescu N. , Gh. Buzea C. , Buzea C. , Agop M. , Ignat M.
Published: July 2007

A new superconducting mechanism is postulated by substituting the exchange interaction between the pairs by an interaction on the “subquantic medium”. Equating the potential of a spin‐polaronlike distribution to the “subquantic potential”, a nonlinear equation for the density of states is obtained. Solving this equation using the WKBJ method, a splitting of the density of states is noticed under particular conditions, the result is understood as the formation of pairs.
Mean‐field insight into the plasmon mechanism of the pairing arrangement in the one‐dimensional electron system
Authors: Yanetka I.
Published: July 2007

The mean‐field temperature of the insulating transition is obtained for the one‐dimensional electron system with the interparticle interaction that can be expressed with the help of the dielectric response function. To illustrate the results a detailed calculation of the transitiontemperature is presented for the one‐dimensional system with the tight‐binding particle spectrum and with the effective interparticle interaction of Coulomb type that is evaluated in the plasmon‐pole approximation.
Numerical solution to the general one‐dimensional diffusion equation in semiconductor heterostructures
Authors: Harrison P.
Published: July 2007

A method of solving numerically the one‐dimensional diffusion equation for arbitrary profiles and arbitrary functional dependencies of the diffusion coefficient on the position, diffusant concentration and the time, is described. The technique is applied to a variety of diffusion problems in semiconductor quantum wells to illustrate its power and versatility. In particular solutions are shown for diffusion of graded interfaces, a concentration dependent diffusion coefficient, and the effect of a depth and time dependent diffusion coefficient on a superlattice, as occurs in ion implantation and the subsequent annealing out of the resulting radiation damage. The depth dependence of the intermixing due to ion implantation and subsequent rapid thermal anneal, of a GaAs/Ga1−xAlxAs multiple‐quantum‐well structure, is deduced from the broadening of the low temperature photoluminescence emission.
Study of the localization lengths in non‐periodic GaAs/Al0.3Ga0.7As superlattices
Authors: Usher Marilyn , Ranganathan Radha
Published: July 2007

Electronic states and the wavefunction localization lengths for non‐periodic GaAs/Al0.3Ga0.7As superlattices are calculated and also studied experimentally using photoluminescence. Non‐periodicity causes the first bound state to shift down in energy. A relation between the shift in energy from the periodic case and the wavefunction localization length is derived and used to obtain the localization length experimentally from photoluminescence measurements. The localization length thus obtained is compared with the results from a direct calculation of the wavefunction.
Exchange coupling between two magnetic layers in a quasi‐one‐dimensional potential well
Authors: Wang Zhongcheng , Wang Shaofeng , Hou Weihong
Published: July 2007

Exchange coupling between two magnetic monolayers mediated by electrons confined in a quasi‐one‐dimensional potential well is investigated. By using a Green's‐function method and approximate expansion, an analytic expression is derived for the weak coupling within the contact interaction approximation. As a main result of the quantum size effect, the number of occupied levels and the Fermi energy are found to be periodic functions of the sample thickness. If the anisotropy in the effective electron mass is taken into consideration, the period is given by \documentclass{article}\pagestyle{empty}\begin{document}$ T = \sqrt[3]{{{{\pi \mu _\parallel } \mathord{\left/ {\vphantom {{\pi \mu _\parallel } {\left({6n_0 \mu _ \bot } \right)}}} \right. \kern-\nulldelimiterspace} {\left({6n_0 \mu _ \bot } \right)}}}} $\end{document}, where μ| and μ⊥ are the effective electron mass in the surface and growth direction, respectively, and n0 is the electron density for one spin. Because of the quantum size effect, the exchange coupling cannot be considered as a simple oscillatory function of the distance between the two magnetic layers. It depends on the distance between the two magnetic layers as well as on the distance from the magnetic layer to the sample surface. For some distance from the magnetic layer to the sample surface, the exchange coupling with the distance between two magnetic layers shows a similar analytic behaviour as the RKKY interaction does. However, for some other distance, there may be just a strength oscillation of the ferromagnetic coupling.
Effective mass of a polaron in a quantum wire under a strong magnetic field
Authors: Lu Tianquan , Zheng Yisong , Zhang Chengxiang , Wu Xuhong
Published: July 2007

Using the Lee‐Low‐Pines variational method the effective mass of a polaron in a rectangular quantum wire subject to a strong uniform external magnetic field along the wire length is studied. Taking the GaAs quantum wire as an example numerical results are given. The results show that the effective mass of the polaron is dominated mainly by the smaller dimension of the quantum wire cross section and the external magnetic field has an interesting influence on it.
1/N expansions for the U = ∞ Hubbard model: Comparison between X‐operator and slave‐boson approach
Authors: Gehlhoff L.
Published: July 2007

We consider the large‐spin‐degeneracy (N) version of the U = ∞ one‐band Hubbard model and present a systematic 1/N expansion method which is based on Hubbard's X‐operator formulation and a Baym‐Kadanoff expansion. We compare this alternative X‐operator approach with the customary time‐continuum slave‐boson (SB) functional‐integral treatment, which has recently been criticized. In particular, we give a detailed derivation of the density Green's function within the new approach and compare the leading‐order results with those as obtained with the commonly used SB treatment, finding total agreement.
Theoretical study of resonance phenomena in n‐fold non‐periodic barrier structures
Authors: Zhao X. D. , Yamamoto H. , Taniguchi K.
Published: July 2007

Resonant tunneling is studied systematically for finite non‐periodic barrier structures with arbitrary potential profiles, which are considered as a network connecting alternately the generalized‐barrier unit with the phase delay unit in series. Analytical expressions of the transmission coefficient and the resonance condition for these structures are derived by taking into account the mass difference between well and barrier layers. It is found that a quasi‐periodic mode may exist in non‐periodic barrier structures. An optimum group of resonant peaks composed of single unity transmission and (n − 2) near‐unity resonant peaks will occur when both the unity resonance condition and the quasi‐periodic mode condition hold simulteneously.
High Pressure Magnetophotoluminescence in Diluted Magnetic Semiconductors Cd1−xMnxSe and Cd1−xCoxSe
Authors: Kuroda N. , Matsuda Y.
Published: July 2007

This paper reviews experimental studies on the magnetophotoluminescence in diluted magnetic semiconductors Cd1−xMnxSe with x = 0.01, 0.05, 0.10, and 0.25 and Cd1−xCoxSe with x = 0.012 at 1.4 and 4.2 K under hydrostatic pressures up to 2 GPa. The studies focus attention to the pressure dependence of the magnetic dichroism of the 1 s exciton state under magnetic fields of 0 to 27 T. Pressure strengthens the exciton‐Mn(Co) and Mn(Co)‐Mn(Co) exchange interactions. An analysis of these results in terms of the kinetic exchange theory shows that the on‐site Coulomb energy U and the chargetransfer energy Δ are reduced markedly by pressure.
Effect of Pressure on the Refractive Index and the Band Gap of Cesium Halides
Authors: Johannsen P. G. , Reiss G. , Bohle U. , Holzapfel W. B.
Published: July 2007

The refractive indexes of CsCl, CsBr, and CsI show a strong nonlinear density dependence. This behaviour can be related to the closure of the band gap by means of a constant‐joint‐density‐of‐states model for the dielectric function. The results are discussed with respect to the dielectric properties of other AB compounds.
Shubnikov‐de Haas Oscillations in Fe‐ and Co‐Doped HgSe under Pressure
Authors: Neifeld E. A. , Demchuk K. M. , Harus G. I. , Boubnova A. E. , Domanskaya L. I. , Shtrapenin G. L. , Yu. Paranchich S.
Published: July 2007

We have investigated the effect of pressure on Shubnikov‐de Haas oscillations in the gapless semiconductor HgSe doped with Fe and Co. For HgSe(Fe) an increase of pressure results in a decrease of the electron concentration in accordance with the Kane model for constant Fermi energy. In contrast, in HgSe(Co) the electron concentration is independent of pressure, i.e. the Fermi level is not pinned.
Competition of Deep and Shallow Impurities in Wide‐Gap II‐VI Semiconductors under Pressure
Authors: Weinstein B. A. , Ritter T. M. , Strachan D. , Li M. , Luo H. , Tamargo M. , Park R.
Published: July 2007

The effects of applied pressure on deep defects, shallow ↔ deep competition, and related doping problems in II‐VI semiconductors are reviewed. Photoluminescence studies to 100 kbar (at 7 K) on high purity ZnSe, and on several ZnSe samples doped with 1016 to 1019 cm−3 Cl, Ga, N, P, and As reveal striking differences between the pressure‐shifts of broad‐band features involving acceptor and/or donor deep states, and excitonic lines due to shallow band‐edge states. The binding energies of observed deep acceptor states tend to decrease with pressure, suggesting that compression acts to destabilize these states against competing shallow hole levels. A deep donor level assigned to an excited state of donor‐vacancyzn complexes emerges from the electron continuum in n‐type Ga‐ and Cl‐doped samples at ≈︁25 to 28 kbar. This previously unknown state should not affect doping in ZnSe1−xTex alloys since it arises from existing compensation centers.
Investigation of Coexistence of Weakly Relaxed Neutral Deep Donor (D°) and DX− States in AlxGa1−xAs:Se Using ICTS under Pressure
Authors: Takarabe K. , Hirano Y. , Minomura S. , Matsuda K. , Ohnishi H. , Fujita K. , Watanabe T.
Published: July 2007

The coexistence of two deep donors in Al2Ga1−xAs:Se with and without a capture barrier is reported. The difference of their nature is found in the dependence of the electron emission process on filling pulse time, temperature, and pressure.
DLTS Investigations of the Distorted Configuration of the EL2 Defect Stabilized under High Hydrostatic Pressure in GaAs1−xPx
Authors: Przybytek J. , Baj M. , Slupiński T. , Li Ming‐Fu
Published: July 2007

DLTS measurements of GaAs1−xPx, samples with phosphorus content χ ≈︁ 0.1 to 0.35 have been performed under hydrostatic pressure. When pressure was applied to samples with χ ≈︁ 0.1 and 0.2, we observed the emergence of the DLTS peak at ≈︁90 K. For χ ≈︁ 0.2 the amplitude of this peak increased by three orders of magnitude. We can unambiguously attribute this phenomenon to a spontaneous rearrangement of the EL2 defect to its distorted configuration, enabled by the free electron capture and not by illumination. For samples with χ ≈︁ 0.35 the same peak has been observed without pressure, i.e. in this case the distorted configuration of the defect is stable even at ambient pressure. For samples with χ ≈︁ 0.1 and 0.2, under pressure and below T ≈︁ 80 K, illumination and/or applying a long, positive, filling pulse, caused a long‐time kinetics of the DLTS signal. This process is explained as a slow redistribution of the EL2 defect between both distorted and nondistorted configurations.
Photoionization of the Si‐DX Center in AIGaAs: The Effects of Pressure and Local Configuration
Authors: Piotrzkowski R. , Litwin‐Staszewska E. , Przybytek J.
Published: July 2007

We have studied the optical ionization rate of Si‐DX centers in Al0.15Ga0.85As at 77 K by measuring the time variations of the density of photoexcited free electrons. The experiments were performed under hydrostatic pressure and without pressure, using the pressure to fill the DX centers with electrons. In the main experiment we determined the variation of the occupancies of individual alloy‐split DX levels along the photoionization transient. As a result, the distinct photoionization rates for all four alloy‐split configurations DX0 to DX3were found. This proves that optical ionization energy of Si‐DX center in the alloys is configuration dependent. A blue shift of the photoionization spectra under pressure was found. For the spectra measured without pressure, the dependence of photoionization threshold on the initial filling was found. Such behavior is the consequence of alloy‐splitting of the optical ionization energy.
Metal‐Insulator Transition in GaN Crystals
Authors: Perlin P. , Knap W. , Camassel J. , Polian A. , Chervin J. C. , Suski T. , Grzegory I. , Porowski S.
Published: July 2007

By means of high‐pressure Raman spectroscopy of coupled LO phonon‐plasmon modes, a qualitatively new model of the metal‐insulator transition observed in bulk GaN crystals at pressure around 20 GPa is proposed. This transition was interpreted recently as caused by the emergence of the localized donor state into the energy gap of GaN. Our new experimental evidence suggests that two types of donors can supply electrons to the bulk crystal: the first of a localized character and the second of a shallow character with contributions to the electron concentration of 5 × 1019 and 3 × 1018 cm−3, respectively. The localized state can be associated with the nitrogen vacancy, while the shallow state can be tentatively attributed to impurities such as oxygen. A number of interesting phenomena take place beyond the metal‐insulator transition in bulk GaN and an attempt to explain them will be made in the present work.
Investigation of Dopant Segregation in GaAsAlAs Short‐Period Superlattices by Hall Effect under High Pressure
Authors: Robert J. L. , Bosc F. , Sicart J.
Published: July 2007

Hydrostatic pressure is a powerful tool in the electrical characterization of large lattice relaxation states (e.g. DX states in III‐V semiconductors). We have applied this method on short‐period superlattices to evidence the dopant segregation resulting from planar Si doping performed during molecular beam epitaxy. The characterization is based on the dependence of the electrical activation energy both on the dopant location in the microstructure and the applied pressure.
Pressure‐Tuned Resonance Raman Scattering in InAs/GaSb Superlattices
Authors: Lyapin S. G. , Lomsadze A. V. , Trojan I. A. , Klipstein P. C. , Mason N. J. , Walker P. J.
Published: July 2007

We have performed a pressuretuned resonance Raman scattering study in short‐period GaSb/InAs SLs in the region of E1 and E1 + Δ1 in GaSb. Resonance profiles for polarized Raman scattering by both LO and elastic interface modes reveal two electronic transitions with two different types of Fröhlich‐interaction‐induced scattering. The LO resonance at the E1 gap displays strong interference and we attribute it to intrinsic intrasubband Fröhlich scattering, which is dipole allowed due to the difference in the degree of localization of the electron and hole wave functions. A new resonance enhancement peak, not present in the bulk, has been observed in between the E1 and E1 + Δ1 gaps of GaSb. This resonance appears to be dominated by extrinsic Fröhlich scattering.
Electronic and Structural Properties of BeSe, BeTe, and BeS: Comparison between ab‐initio Theory and Experiments
Authors: Muñoz A. , Rodríguez‐Hernández P. , Mujica A.
Published: July 2007

We have used first principles total energy pseudopotential techniques to calculate the ground state properties and the relative stabilities of the high pressure phases of the beryllium chalcogenides BeSe, BeTe, and BeS. We study the energy‐volume (E–V) relations of the zincblende, NaCI, CsCI, NiAs, and βSn structures of these compounds. The components of the stress tensor were also calculated and used in the structural relaxation of the tetragonal and hexagonal phases. From the E–V curves of the structures considered and the Gibbs construction we find that these compounds transform under pressure from the zincblende structure (B3) to the NiAs structure (B8), and no fürther transitions are found. Our calculated values of the structural parameters and the B3 → B8 transition pressure for BeSe and BeTe agree well with recent experimental results. Very little is known experimentally for BeSe, and our results for this compound have a predictive character.
Second‐Order Pretransitional Effects in the High Pressure Phase Transition of Indium Nitride
Authors: Besson J. M. , Bellaiche L. , Kunc K.
Published: July 2007

Indium nitride, which has the hexagonal wurtzite structure, transforms to an NaCl lattice between 12 and 16 GPa. Prior to this transition nonlinearities occur in the wurtzite phase, particularly steep decrease of the c/a axial ratio. The equation of state and nonlinear behavior of c, a, and c/a of InN under pressure have been calculated by using the first‐principles total‐energy pseudopotential method. The calculations fit with experiment up to the phase transition pressure (15 GPa), which validates their accuracy. Pursuing the calculations in the wurtzite structure above the actual transition pressure the nonlinearities appear to be caused by a virtual second‐order isostructural phase transition between two wurtzite structures with unequal internal positional parameters. This second‐order process is interrupted by the first‐order reconstructive phase transition to the high pressure structure and remains observable only as its precursor.
High Pressure Effects on Optical Properties of CdS
Authors: Kobayashi M. , Iwata H. , Horiguchi T. , Endo S.
Published: July 2007

CdS undergoes a reversible phase transition from the wurtzite to the rock‐salt structure at 2.6 GPa. The LO Raman mode of samples retrieved from the high pressure phase shows a remarkable shift towards lower frequency relative to the LO mode of a single crystal. This shift is explained by the formation of nanocrystals and its interpretation in terms of the spatial correlation model yields nanocrystal diameters ranging from 6.7 to 10 nm. The diameter depends on the highest pressure applied. When pressure is applied again to a sample retrieved from a high pressure cycle, the phase transition pressure has been found to increase compared with that of a single crystal sample. These results lead to an explanation for the question why the wurtzite and rock‐salt structures coexist beyond the onset of the phase transition in CdS.
Raman Frequencies and Angular Dispersion of Polar Modes in Aluminum Nitride and Gallium Nitride
Authors: Filippidis L. , Siegle H. , Hoffmann A. , Thomsen C. , Karch K. , Bechstedt F.
Published: July 2007

We performed angular‐resolved Raman‐scattering experiments on AlN and GaN both grown in the wurtzite structure. Our measurements reveal that the transverse polar modes in AlN and GaN show a pronounced angular dispersion. We varied the angle between the phonon‐propagation direction and the optical axis between 41° and 90° and observed a frequency shift of the A1(TO) mode in the range of 28 cm−1 in AlN and of 15 cm−1 in GaN. The experimental values agree well with our theoretical calculations which were carried out using the DFT‐LDA plane‐wave pseudopotential method.
Morphology and Luminescence of p‐Doped Porous Silicon
Authors: zur Mühlen E. , Chang Da , Rogaschewski S. , Niehus H.
Published: July 2007

Porous silicon (PS) layers were created with different etching conditions on boron doped silicon wafers. Morphologies of the PS layers and their surfaces were investigated with atomic force and scanning electron microscopy. Low current densities during anodization led to homogeneous layers, while high current densities resulted in strain induced mechanical instabilities. The strain leads to a peeling off of the layer for mitre or mesoporous silicon. In contrast, in macroporous films strain can be relieved laterally leading to the local formation of channels. Luminescence properties of the samples are investigated and related to the PS morphology.
Dynamical Charge and Force Constant Calculations in c‐BN under Pressure
Authors: Badi N. , Bousetta A. , Bensaoula A. , Aourag H.
Published: July 2007

A combination of an adjusted pseudopotential method and the bond‐orbital model of Harrison was used to derive the polarity and the force constants of cubic boron nitride (c‐BN) under pressure up to 900 kbar. The results show that the hybrid covalent energy dominates at high pressure and the transverse effective charge changes by less than 3.3% between normal condition and 900 kbar. The idea of taking the free‐atom term values and the 1/d2 formulae for the covalent energy is not a good approximation to estimate the variation of the transverse effective charge under pressure. In fact, this approximation overestimates the relative variation to 10.2% in the same range of pressure.
Paramagnetism and Magnetic Relaxation in Melt‐Textured Grown GdBa2Cu3O6+y4)
Authors: Qin M. J. , Jin X. , Yao X. X. , Rong X. S. , Ni Y. M. , Xiao L. , Fu X. K. , Ren H. T.
Published: July 2007

Magnetic measurements have been performed on a melt‐textured grown (MTG) GdBa2Cu3O6+y sample. Because of the paramagnetism of the Gd3+ ions, the magnetization relaxation rate is increased by a factor of 4πχ. The effect of paramagnetism on the U(J) relationship of the sample has also been discussed. At the end, we extraxted the U(J) relationship based on the field sweep rate H, which is in agreement with the one after corrections for paramagnetism.
Temperature Dependence of Nuclear Magnetic Resonance and Transferred Hyperfine Field of 133Cs in a CsMnCl3 Single Crystal
Authors: Lim A. R. , Choh S. H. , Jeong S. Y.
Published: July 2007

The nuclear magnetic resonance of 133Cs in a CsMnCl3 single crystal grown by the Czochralski method has been investigated by employing a Bruker FT NMR spectrometer. The resonance lines of two different Cs(I) and Cs(II) were recorded. The quadrupole coupling constants of Cs(I) and Cs(II) in CsMnCl3 decrease as the temperature increases. The temperature dependence of e2qQ/h is explained with a single torsional frequency of the Cs‐Cl ion by means of the simple Bayer theory. Also, we have investigated the transferred hyperfine interaction due to the transfer of spin density from the Mn2+ ion to the Cs+ ion in the CsMnCl3 single crystal using the magnetic susceptibility and the paramagnetic shift using nuclear magnetic resonance.
Exciton Lineshapes in Weakly Disordered Systems
Authors: Mertsching J.
Published: July 2007

Exciton absorptison lines are calculated for weakly disordered tight‐binding systems, using the coherent‐potential approximation (CPA). The scaled lineshapes depend only on the dimension of the system. The CPA results are compared with known exact results for chains, and numerical results for finite two‐and three‐dimensional systems.
Defect‐related aspects of Mn‐doped ZnS nanorods and morphological changes revealed from positron annihilation spectroscopy
Authors: Nambissan P. M. G. , Biswas Subhajit , Kar Soumitra , Chaudhuri Subhadra
Published: July 2007

Manganese‐doped zinc sulfide (Mnx Zn1‐x S) nanorods were synthesized through solvothermal processes and were characterized by x‐ray diffraction and scanning and transmission electron microscopy. Positron lifetime and Doppler broadening measure‐ments were carried out on powdered samples with different dopant concentrations. A transformation from hexagonal to cubic structure was observed almost concomitantly with the filling of Zn2+ vacancies and/or substitution of Zn2+ ions by Mn2+ ions. The lifetime of positrons annihilating at the surfaces of nanorods decreased with increase in their diameter during increased doping by Mn2+ ions. At concentrations above 20%, pyramid‐like coarse crystals of MnS were found coexisting with the ZnS nanostructures. The interior of the nanorods seemed to be free of vacancy‐type defects unlike in the case of ZnS nanoparticles. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Physics with dense positronium
Authors: Cassidy David B. , Mills Allen P.
Published: July 2007

Our recent experiment on Ps‐Ps spin exchange quenching represents the first time that two Ps atoms have been observed interacting with each other. The implication is that experimentation on various systems containing more than one Ps atom is now possible, including the Ps2 molecule, the Ps Bose‐Einstein condensate, the Ps atom laser and the annihilation gamma ray laser. Here we describe various aspects of such research, including the development of new techniques, work currently underway and some possible future experiments. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Non‐thermal food processing/preservation technologies: a review with packaging implications
Authors: Morris Caroline , Brody Aaron L. , Wicker Louise
Published: July 2007

Non‐thermal food processing/preservation methods interest food and food packaging scientists, manufacturers and consumers because they exert a minimal impact on the nutritional and sensory properties of foods, and extend shelf life by inhibiting or killing microorganisms. They are also considered to be more energy efficient and to preserve better quality attributes than conventional thermally based processes. Non‐thermal processes also meet industry needs by offering value‐added products, new market opportunities and added safety margins.

  This study reviewed non‐thermal processing technologies currently available or developmental for the inactivation of microorganisms and thus microbiological shelf life in foods, and to identify packaging interactions that might result. Processes include ultra‐high pressure, ionizing radiation, pulsed X‐ray, ultrasound, pulsed light and pulsed electric fields, high‐voltage arc discharge, magnetic fields, dense phase carbon dioxide and hurdle technologies. Copyright © 2007 John Wiley & Sons, Ltd.
Ensemble‐based chemical data assimilation. II: Covariance localization
Authors: Constantinescu Emil M. , Sandu Adrian , Chai Tianfeng , Carmichael Gregory R.
Published: July 2007

Data assimilation is the process of integrating observational data and model predictions to obtain an optimal representation of the state of the atmosphere. As more chemical observations in the troposphere are becoming available, chemical data assimilation is expected to play an essential role in air‐quality forecasting, similar to the role it has in numerical weather prediction. Considerable progress has been made recently in the development of variational tools for chemical data assimilation. In this paper we implement, and assess the performance of, a localized ‘perturbed‐observations’ ensemble Kalman filter (LEnKF). We analyse different settings of the ensemble localization, and investigate the joint assimilation of the state, emissions and boundary conditions. Results with a real model and real observations show that LEnKF is a promising approach for chemical data assimilation. The results also point to several issues on which future research is necessary. Copyright © 2007 Royal Meteorological Society
Alignment of resection planes in total knee replacement obtained with the conventional technique, as assessed by a modern computer‐based navigation system
Authors: Belvedere C. , Ensini A. , Leardini A. , Bianchi L. , Catani F. , Giannini S.
Published: July 2007

Background

To improve the anatomy‐based alignment of prosthetic components in total knee replacement, surgical navigation systems have recently been developed, based on anatomical reference frame definitions through landmark digitations and functional calibration. In this study, femoral and tibial resection plane alignments, obtained by conventional tecnique, were measured intraoperatively during total knee replacements by a navigation system to quantify potential errors in conventional bone preparation techniques.

Methods

Femoral and tibial resection plane alignments, obtained by conventional femoral intramedullary and tibial extramedullary cutting guides, were measured intraoperatively in 25 primary total knee replacements by a navigation system. This system enabled the surgeon to calculate, before definitive bone sawing, the final position and orientation of all resection planes.

Results

The measurements revealed unsatisfactory alignments in nearly all anatomical planes. Except for tibial varus/valgus, final plane orientations were considerably different from those targeted by the surgeon via the navigation system, respectively 7° , 8° and 10° apart in varus‐valgus and flexion‐extension at the femur, and in flexion‐extension at the tibia.

Conclusion

Modern computer‐aided surgery in total knee replacement, once relevant precision has been established in all femur and tibia anatomical planes, can in the future limit the current critical component misalignments. Copyright © 2007 John Wiley & Sons, Ltd.
Three‐dimensional surface acquisition systems for the study of facial morphology and their application to maxillofacial surgery
Authors: Kau Chung How , Richmond Stephen , Incrapera Angela , English Jeryl , Xia James Jiong
Published: July 2007

Background

There has been a growing interest in three‐dimensional (3D) surface imaging devices over the last few years.

Methods

This comprehensive review discusses the various emerging technologies in this field of 3D imaging and applies the use of technology to oral and maxillofacial imaging.

Results

The paper shows that there is increased awareness and application of technology to the field.

Conclusion

3D surface acquisition technology is improving at a rapid pace and has a place in oral and maxillofacial surgery. Copyright © 2007 John Wiley & Sons, Ltd.
On J‐Lossless coprime factorizations and H∞ control
Authors: Tsai Mi‐Ching , Postlethwaite Ian
Published: July 2007

This paper presents a relatively simple approach, based on chain scattering descriptions, to the synthesis of internally stabilizing controllers which result in a cost function with H∞ norm strictly less than a prespecified bound. State‐space formulae for a generator of all suboptimal controllers are found by solving two coupled J‐lossless coprime factorizations of a chain scattering matrix description. A mixed‐sensitivity design problem is used to illustrate the solution procedure, and to demonstrate how in certain cases only one algebraic Riccati equation need be solved to get a solution.
Combined ℋ︁∞/LQG control via the optimal projection equations: On minimizing the LQG cost bound
Authors: Mustafa Denis
Published: July 2007

The Optimal Projection equations for combined ℋ︁∞/LQG control are considered. Positive semidefiniteness of the associated Lagrange multiplier is shown to be necessary for the LQG cost bound to be minimal. It follows that all four Optimal Projection equations have a role to play, even in the full‐order case.
H∞‐optimal controller discretization
Authors: Keller J. P. , Anderson B. D. O.
Published: July 2007

A redesign method for discretizing a continuous‐time controller is proposed. The resulting hybrid control system, for example with continuous‐time plant and discrete‐time controller, is stable, and performance including the system's inter‐sampling behaviour can be optimized by approximating some chosen reference transfer function of the continuous‐time control system. In order to obtain a tractable problem, the continuous‐time part of the hybrid system and the reference transfer function are approximated by a discrete‐time system with arbitrary fast sampling. After lifting the resulting periodic system, the approximation problem can be formulated as a standard H∞‐problem which is solved using standard software for H∞‐controller design.
H∞ generalized minimum sensitivity control design
Authors: Casavola Alessandro , Mosca Edoardo
Published: July 2007

This paper presents a two‐stage approach to the H∞ generalized sensitivity minimization problem (H∞‐GSM) for SISO, continuous‐time plants. Besides some possible advantages in the control law evaluation with respect to alternative polynomial methods, the proposed approach provides a direct link with the solution of an underlying generalized minimum variance (H2‐GMV) problem and allows one to identify the class of the joint H2/H∞‐GMV (equalizing H2‐GMV) optimal controllers.
Adaptive dynamical variable structure feedback regulation for nonlinear uncertain systems
Authors: Sira‐Ramírez Herbertt , Zribi Mohamed
Published: July 2007

In this article an adaptive discontinuous dynamical feedback strategy is presented for asymptotic output stabilization problems defined on nonlinear controlled systems exhibiting linear parametric uncertainty. A dynamical feedback controller, ideally achieving output stabilization via exact linearization, is obtained by means of output differentiation and sliding mode control ideas. The adaptive version of the dynamical variable structure controller is then obtainable via standard, direct, overparametrized adaptive control techniques available for linearizable systems through static state feedback. An illustrative example from the chemical process control area, including simulations, is provided.
New formulations for quantitative feedback theory
Authors: Nordgren R. E. , Nwokah O. D. I. , Franchek M. A.
Published: July 2007

New formulations are presented for quantitative feedback theory that do not require the direct manipulation of templates in the classical sense. This enables the explicit incorporation of high‐frequency unstructured uncertainty into the problem statement, and allows for constructive existence conditions for the solvability of the QFT problem. Necessary and sufficient conditions for robust stability and robust performance are presented for plants with both parametric and high‐frequency unstructured uncertainty, that parallel their corresponding H∞ counterparts. This work extends the recent contribution of Jayasuriya and Zhao to plants with more general non‐minimum phase characteristics such as RHP poles and zeros and time delays.
The H∞ control problem using static output feedback
Authors: Skelton R. E. , Stoustrup J. , Iwasaki T.
Published: July 2007

In this paper we shall consider the H∞ control problem using static output feedback. The approach uses some recent results from linear algebra. The main result shows that the H∞ control problem is solvable by a static output feedback controller if and only if there exists a positive definite matrix satisfying two certain quadratic matrix inequalities. A parametrization of all static output feedback H∞ controllers is given.
Achieving vector relative degree for nonlinear systems with parametric uncertainties
Authors: Schwartz Carla A. , Gibbens Peter W. , Fu Minyue
Published: July 2007

Several algorithms for adaptive control, as well as for static state feedback decoupling, feedback linearization, or inversion of nonlinear multivariable systems require that the systems have full vector relative degree, in order to be applied. In this paper, we provide a parameter‐independent method of achieving full vector relative degree for nonlinear multivariable systems which do not have it. We determine conditions under which a diagonal dynamic precompensation is sufficient to achieve vector relative degree for multivariable nonlinear systems, and describe a simple algorithm which determines such compensation.
A transfer matrix framework approach to the synthesis of H∞ controllers
Authors: Tsai Mi‐Ching , Tsai Chin‐Shiong
Published: July 2007

This paper presents a transfer matrix framework approach to the now well‐known Riccati solution to the nonsingular H∞ control problem. The approach taken is a frequency‐domain one based on the concepts of coprime factorization and J‐lossless transfer matrices that lead to the development of relatively simple method for obtaining the state‐space solutions. Using two dual pairs of coupled coprime factorizations of certain chain scattering‐matrices which describe the problem, we derive an identity, analogous to the Bezout identity for all stabilizing controllers, to generate all proper real‐rational H∞ controllers. We also point out the connections between algebraic Riccati solutions and J‐lossless matrices.
Frequency‐domain criteria of robust stability: Discrete‐time and continuous‐time systems, a unified approach
Authors: Tsypkin Y. Z. , Furuta K.
Published: July 2007

This paper is concerned with the stability and robust stability of linear discrete‐time and continuous‐time systems. The characteristic polynomials of linear systems are transformed into the general polynomials of discrete‐time systems, and similarly to the hodogram of the discrete‐time system, the stability and the robust stability for the general characteristic polynomials with parameter uncertainty are analysed by using the zero exclusion method, where the order of the system is not used specifically a condition for a stability.
Global effects estimation for multidimensional outcomes
Authors: Travison T. G. , Brookmeyer R.
Published: July 2007

Many health studies focus on multifaceted outcomes that are not easily measured with a single variable; examples include studies on quality of life (QOL) and general health. To fully explore such an outcome, researchers typically collect information on multiple endpoints. The resulting measurements constitute multidimensional outcome data. An object of great interest is the overall—or global—effect of a covariate, such as a treatment intervention, on the multidimensional outcome. Quantifying such an effect can be difficult because multiple clinical outcomes are usually measured on different scales; the problem is enhanced by the fact that multiple measurements on a given subject are typically correlated. We present a regression modeling scheme permitting estimation of global treatment effects when multiple continuous endpoints are examined in concert either once or for several times. The global effect is conceptualized as a change in the distribution functions of the outcome variables. It may thus be interpreted as a connection between outcome distribution quantiles for the treatment and control groups. This concept allows the presentation of a global effect as a scalar quantity applicable to all outcomes simultaneously, easing interpretation of results. Model estimation proceeds directly from existing methods for multivariate survival analysis. The assumption that the treatment effect is homogenous across different outcomes is testable. To illustrate the application, we present data analytic results from a motivating example, an analysis of patients' QOL during recovery from lower limb trauma. We also explore the performance properties of global effects estimation through simulation. Copyright © 2007 John Wiley & Sons, Ltd.
Plasmagestützte Vakuumbeschichtungsverfahren — Verfahrensvarianten und Entwicklungstendenzen
Authors: Kienel Gerhard
Published: July 2007

Dünne Schichten mit guten elektrischen, optischen, chemischen und mechanischen Eigenschaften lassen sich nur bei einer ausreichend hohen Mobilität der Teilchen beim Kondensationsprozeß herstellen. Sie ist vom Schmelzpunkt des Beschichtungsmaterials, der Substrattemperatur und der Teilchenenergie beim Auftreffen auf dem Substrat abhängig. Da der Substrattemperatur Grenzen gesetzt sind, kann vor allem bei hochschmelzenden Metallen oder Verbindungen eine Mobilitätserhöhung nur über die Teilchenenergie erfolgen. Teilchenenergien bis zu einigen 10 eV sind erforderlich, um Schichten hoher Dichte zu erzeugen. Unter vergleichbaren Beschichtungsbedingungen sind daher plasmagestützte Prozesse dem klassischen Aufdampfen im Hochvakuum überlegen.

Gleichmäßige Kondensationsbedingungen auch auf großen Substratflächen und die Erzeugung einer möglichst hohen Ladungsträgerkonzentration zur Erhöhung der Kondensationsgeschwindigkeit bei reaktiven Beschichtungen sind vorrangige Ziele der Weiterentwicklung.
Kathodenzerstäubung und ihre Anwendung in der Dünnschichttechnologie
Authors: Teves Heinz
Published: July 2007

Es existieren verschiedene Varianten der Kathodenzerstäubung zur Herstellung von dünnen Schichten im Vakuum. Die oft angewendete Diodenzerstäubung mit Gleichspannung oder Hochfrequenz, mit und ohne Magnetfeldunterstützung, die reaktive Zerstäubung und das Zerstäuben mit Vorspannung am Substrat werden beschrieben. Grundsätzliche Betrachtungen zum Aufbau von Kathodenzerstäubungsanlagen werden an‐ und die Vorteile dieses Beschichtungsverfahrens herausgestellt.
Vakuumgerechte Werkstoffe und Verbindungstechnik
Authors: Fritsch Reinhard
Published: July 2007

In Part 1 (Henning, Materials used in Vacuum Technology), the application possibilities of different materials in vacuum technology were described. Part 2 describes the most important non‐detachable connections of these materials to produce components that can be used in vacuum technology.

The following processes are suitable for this purpose:

Welding

Soldering

Glueing

Fusion

For this reason, these four processes will be discussed in detail here; their advantages and disadvantages described and their worthiness for use in vacuum technology indicated.

It should be noted here that other nondetachable connections such as rivets, point welding, projection seam welding or roll seam welding do not meet the demands of gas tightness made in vacuum technology and cannot, therefore, be used for such purposes.
Zentrale Vakuumsysteme in der Lebensmittelverpackung
Authors: Schmidt Immo
Published: July 2007

At the beginning of the food packaging there weren't special demands about vacuum pumps. When the permeation rates of the foils became lower the requirements of the vacuum systems increased. During the evacuation especially for rapidly spoiling food it is required to have a lower pressure. This necessitates a high pumping speed in the range where the water evaporates. The rotary vane vacuum pumps couldn't cope with the requirements. The importance of the roots blowers in the food packaging was discovered. The complete vacuum system could be installed out of the packaging room. The systems were equipped with large tubes which are used as volume buffers. This allows a fast evacuation of the food. The combination of rotary vane vacuum pumps and roots blowers needs a lower power consumption. Those advantages show that such vacuum systems find an increasing importance in the packaging industry.
Fehler vermeiden beim Betrieb von Vakuumpumpen
Authors: Barfuss Heinz
Published: July 2007

A wide spectrum of todays vacuum applications are asking for custom taylored, hightech products, which of course require for highest uptime and optimal utilization certain maintenance and to follow operational advices. From a prospectus and experience of a components' manufacturer, the following article would like to give you helpful advices for the use, maintenance and proper system integration for the most common vacuumpumps utilized and to realize the lowest cost of ownership.
Zuverlässigkeit von Turbomolekularpumpen
Authors: Ganschow O. , Eberl W. , Koci B. , Lotz H. , Mädler M. , Reuschling H. , Rippl A.
Published: July 2007

Reliability of turbomolecular pumps today results in mean times to failure in excess of 50 000 h for analytical instrumentation. Methods to investigate potential failure mechanisms, design principles to eliminate them and verification methods to validate the resuits are discussed.
Extruderentgasung beim Kunststoffrecycling
Authors: Redmann Mark
Published: July 2007

The increasing efforts to recycled plastics is marked by high quality parameters and requires expensive recycling plants.

Each component of the plant contributes to the required quality of the final product. Therefore the responsibility of each supplier stops not at his scope of supply, he has to pay attention to the bordering components of the plant. This essay should show the problems at the example extruder vacuum system.

Plastic recycling is especially significant for the complex technical problems. The components of this plant have to be robust and universal.

Concerning the vacuum system, it is a compromise for different types of plastics.

The compromise is supported by the technical properties of the vacuum system, which are stable suction, low pressure, lead away of wastes without any problems and a universal unit design.