bluuebunny/crossref_metadata_2025 huggingface dataset:
Quinine carbamate chiral stationary phases: Systematic optimization of steric selector‐selectand binding increments and enantioselectivity by quantitative structure‐enantioselectivity relationship studies
Authors: Lämmerhofer Michael , Franco Pilar , Lindner Wolfgang
Published: June 2006

A series of quinine carbamate‐based chiral stationary phases (CSPs) differing solely in the carbamate residue have been devised and a congeneric set of N‐3,5‐dinitrobenzoyl (DNB) amino acids (AAs) was separated into enantiomers on these CSPs by HPLC using a buffered hydro‐organic mobile phase. Some details on retention and chiral recognition mechanisms have been investigated by application of quantitative structure‐property relationship (QSPR) studies using the linear free energy relationship methodology, i. e., the extrathermodynamic approach. Retention factors of the high affinity enantiomer (log k2) and enantioselectivities (log α) were correlated with Taft's steric parameter as structural descriptor for the variability in the carbamate and AA residues, and statistically significant QSPR models could be obtained. They confirmed that the variance in the dependent variable (log k2, log α) is mainly associated with the steric bulkiness of the selectors' carbamate and of the AA residues. The retention factor of the second eluted enantiomers and the enantioselectivity first increased with steric bulkiness of the carbamate and AA residues, which may be explained by stronger dispersive interactions. After reaching an optimum, the dependent variable, however, declined with further increase of the steric bulkiness of the substituents, probably because of steric hindrance. The variability of the retention factors of the first eluted enantiomer could not be explained by steric descriptors. Instead, it has become obvious that the retention arises mainly from interactions of the DNB‐AA and the quinine carbamate backbone, as it turned out to be more or less constant and solely to a minute amount modulated by the carbamate residue and the AA side chain. The QSPR models were fully in agreement with an earlier postulated chromatographically and spectroscopically derived hypothetical selector‐selectand binding model.
Glutamate release inhibition ineffective in Levodopa‐induced motor complications
Authors: Bara‐Jimenez William , Dimitrova Tzvetelina D. , Sherzai Abdullah , Aksu Murat , Chase Thomas N.
Published: June 2006

Reported benefits of various glutamatergic receptor antagonists in Parkinson's disease (PD) prompted an evaluation of the antidyskinetic effect of a putative glutamate release inhibitor in 15 moderately advanced patients. In a 3‐week, double‐blind, proof‐of‐concept study, riluzole (200 mg/day) failed to alter parkinsonian or levodopa‐induced motor complication severity. Opposing effects of a generalized inhibition of glutamate‐mediated synaptic transmission may limit the usefulness of this approach to treat PD. © 2006 Movement Disorder Society
Helical chirality in hexamethylene triperoxide diamine
Authors: Guo Chunlei , Persons John , Harbison Gerard S.
Published: June 2006

The primary explosive hexamethylenetriperoxide diamine has previously been found to exist in the solid state as a racemic mixture of helically chiral, threefold symmetric enantiomers; another enantiomeric pair of low‐energy conformers has been predicted, but has never been observed. We show by solution 2D NMR at 14 T, in achiral solution and by addition of chiral shift reagents, that all four optically isomeric conformers coexist at slow equilibrium on the NMR timescale at room temperature, and can be observed. Calculations of the 1H and 13C NMR chemical shifts using gauge‐including atomic orbital methods are in excellent agreement with experiment; thermochemical calculation of the free energies in solution are in somewhat worse agreement, but correctly predict the relative stability of the conformers. Analysis of the effects of chiral shift reagents on the NMR spectra suggests that discrimination between chiral isomers is primarily around the molecular equator, around which the enantiomeric gauche OO linkages are arrayed. Copyright © 2006 John Wiley & Sons, Ltd.
1H and 13C chemical shifts for some 6,6a‐dihydrochromeno(3,4‐b)chromene derivatives
Authors: Mebale A‐J. Abogo , Maggiani A. , Brun P.
Published: June 2006

The 1H and 13C NMR resonances of five 6,6a‐dihydrochromeno(3,4‐b)chromene derivatives were assigned completely with certainty using a concerted application of one‐ and two‐dimensional experiments (DEPT, gs‐COSY, gs‐HMQC and gs‐HMBC). Copyright © 2006 John Wiley & Sons, Ltd.
Faster nerve regeneration after sciatic nerve injury in mice over‐expressing basic fibroblast growth factor
Authors: Jungnickel Julia , Haase Kristina , Konitzer Jens , Timmer Marco , Grothe Claudia
Published: June 2006

Basic fibroblast growth factor (FGF‐2) is expressed in the peripheral nervous system and is up‐regulated after nerve lesion. It has been demonstrated that administration of FGF‐2 protects neurons from injury‐induced cell death and promotes axonal regrowth. Using transgenic mice over‐expressing FGF‐2 (TgFGF‐2), we addressed the importance of endogenously generated FGF‐2 on sensory neuron loss and sciatic nerve regeneration. After sciatic nerve transection, wild‐type and transgenic mice showed the same degree of cell death in L5 spinal ganglia. Also, the number of chromatolytic, eccentric, and pyknotic sensory neurons was not changed under elevated levels of FGF‐2. Morphometric evaluation of intact nerves from TgFGF‐2 mice revealed no difference in number and size of myelinated fibers compared to wild‐type mice. One week after crush injury, the number of regenerated axons was doubled and the myelin thickness was significantly smaller in transgenic mice. After 2 and 4 weeks, morphometric analysis and functional tests revealed no differences in recovery of sensory and motor nerve fibers. To study the role of FGF‐2 over‐expression on Schwann cell proliferation during the early regeneration process, we used BrdU‐labeling to mark dividing cells. In transgenic mice, the number of proliferating cells was significantly increased distal to the crush site compared to wild‐types. We propose that endogenously synthesized FGF‐2 influences early peripheral nerve regeneration by regulating Schwann cell proliferation, axonal regrowth, and remyelination. © 2006 Wiley Periodicals, Inc. J Neurobiol, 2006
Histological identification of bone fragments in archaeology: telling humans apart from horses and cattle
Authors: Cuijpers A. G. F. M.
Published: June 2006

In archaeology, it is not always possible to identify bone fragments. A novel approach was chosen to assess the potential of histology as an identification tool in those instances in which macroscopical study (e.g. by means of bone surface texture) has failed. This study concentrated on the diaphyses of long bones in three species of comparable size which are relevant to archaeology. Late juvenile and adult human diaphyseal bone structure was compared with the bone structure of horses and cattle. Since bone structure can differ even within a single bone, the restrictions in terms of bone category, bone part and species were deemed necessary for the development of a useful identification method for archaeological bone. The reference series comprised long bones from several individuals to broaden the insight into variations in diaphyseal bone structure within a single species.

A general difference in the primary bone types was found between humans and the two large mammals. Human bone displayed lamellar bone types, whereas horses and cattle showed fibrous bone types. The only exceptions were separated growth layers. A difference in the secondary bone structure was also observed. A large number of connecting, primary (Volkmann's) canals, giving the secondary bone a reticular aspect, were common in horses and cattle. They were not, however, present in the human bones studied.

To confirm the validity and applicability of these differences, a blind test was conducted on 15 diaphyseal fragments of identified long bones from archaeological sites. The results were very promising. Although four fragments could not be identified due to severe degradation, all the others were correctly attributed. Copyright © 2006 John Wiley & Sons, Ltd.
Synthesis and aggregation of amine‐cored polyamidoamine dendrons synthesised without invoking a protection/deprotection strategy
Authors: King Amy SH , Martin Ian K , Twyman Lance J
Published: June 2006

The synthesis of a series of amine‐cored N,N‐dimethyl‐terminated polyamidoamine dendrons with the potential to be modified at their focal point is reported. The use of an aniline core enables the target molecules to be synthesised without resorting to the use of a time‐consuming and expensive protection/deprotection strategy. After synthesising these molecules significant frothing in aqueous solution at millimolar concentrations was noticed, a property associated with aggregation. The critical micelle concentrations were therefore measured and found to occur at relatively low dilutions (in the range 10−4–10−5 mol L−1). Copyright © 2006 Society of Chemical Industry
Optical Pyrometry of Fireballs of Metalized Explosives
Authors: Goroshin Samuel , Frost David L. , Levine Jeffrey , Yoshinaka Akio , Zhang Fan
Published: June 2006

Fast‐response optical diagnostics (a time‐integrated spectrometer and two separate fast‐response three‐color pyrometers) are used to record the transient visible radiation emitted by a fireball produced when a condensed explosive is detonated. Measurement of the radiant intensity, in several narrow wavelength bands, is used to estimate the temperature of the condensed products within the fireball. For kg‐scale conventional oxygen‐deficient homogeneous TNT and nitromethane explosive charges, the radiant intensity reaches a maximum typically after tens of milliseconds, but the measured fireball temperature remains largely constant for more than 100 ms, at a value of about 2,000 K, consistent with predictions using equilibrium thermodynamics codes. When combustible metal particles (aluminum, magnesium or zirconium) are added to the explosive, reaction of the particles enhances the radiant energy and the fireball temperature is increased. In this case the fireball temperatures are lower than equilibrium predictions, but are consistent with measurements of particle temperature in single particle ignition experiments.
Wear‐out and breakdown of thermally grown Ta2O5 insulating films on plasma oxynitrided Si substrates
Authors: Novkovski N. , Atanassova E.
Published: June 2006

Wear‐out and breakdown at constant current stress (J = 10 mA/cm2) were studied for Ta2O5 insulating layers (30 nm thick) thermally grown over plasma oxynitrided (5 to 15 s at room temperature) Si substrates. For injected charges Qinj lower than 100 C/cm2 hard breakdown occurs, while for Qinj higher than 100 C/cm2 it appears that only progressive breakdowns are possible. An optimum of the dielectric properties was found for oxynitridation times as long as 10 s, simultaneously for the hard breakdown charge (33 C/cm2 on a capacitors with an area of 2.5 × 10–3 cm2), the equivalent thickness (8.24 nm) and the wear‐out (variation of the gate voltage smaller than 50 mV, during the stress up to 100 C/cm2). The observed optimum was attributed to a defect free relaxed interfacial structure containing approximately 1 × 1014 cm–2 nitrogen atoms in the first atomic monolayer adjacent to the Si substrate, as one with a significant improvement of the SiOxNy interfacial layer relative permittivity (5.5) without considerable reduction of the tunneling barrier heights. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Influence of an external mechanical strain on the character of the magneto‐electric effect in epitaxial films of yttrium iron garnet
Authors: Koronovskyy V. E.
Published: June 2006

Epitaxial films of yttrium iron garnet (YIG) exhibit electric‐field‐dependent magnetic properties that have been experimentally measured as a function of the applied magnetic field and as a function of strain. The laser polarimeter technique was used for researching the magneto‐electric manifestation – the electromagneto‐optical (EMO) effect. Magnetic‐field dependencies, the acquired EMO effect, testify to an influence of an external mechanical strain not only on the value of the EMO effect, but also on the character of the EMO manifestation in YIG films. These magneto‐electric properties are discussed on the basis of the model of lowering of symmetry in the crystal due to nonuniform deformations of the YIG film. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Prediction of intragranular strains in metallic polycrystals with a two‐level homogenisation approach: Influence of dislocation microstructure on the mechanical behaviour
Authors: Gloaguen D. , François M.
Published: June 2006

A two‐level homogenisation approach is applied to the micro‐mechanical modelling of the elasto‐plasticity of polycrystalline materials during various strain‐path changes. The model is tested by simulating the development of intragranular strains during different complex loads. Mechanical tests measurements are used as a reference in order to validate the model. The anisotropy of plastic deformation in relation to the evolution of the dislocation structure is analysed. The results demonstrate the relevance of this approach for FCC polycrystals. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Effect of Ba on ferroelectric and piezoelectric properties of the PLZT (1.2/55/45) system
Authors: Koduri Ramam , Hermosilla Luis Salinas
Published: June 2006

Ferroelectric Pb0.982–zLa0.012Δ0.006Baz(Zr0.55Ti0.45)O3 (PLZT; z = 0–6 mol%) ceramic compositions were fabricated by a mixed‐oxide method. X‐ray diffraction studies of sintered ceramics indicate the coexistence of rhombohedral and tetragonal phases in the 5 mol% Ba‐doped PLZT composition. Grain growth is inhibited with increasing Ba concentration in the PLZT system as evidenced by scanning electron micrographs. Dielectric constant and dielectric loss as a function of temperature suggest that ϵrt and tan δ are increased up to compositions containing 4 and 5 mol% Ba, respectively. The dielectric maximum (ϵTc) decreased to 4 mol% Ba and gradually increased to 6 mol% Ba, whereas, with increasing Ba concentration in the PLZT system, the Curie temperature (TC) decreased from the composition with no Ba to that with 6 mol% Ba. Relaxor behavior as a function of temperature and frequency was studied over the composition series. As the Ba concentration increased, the coercive field (Ec) showed a gradual enhancement. Remanent (Pr) and spontaneous (Ps) polarizations decreased to the 5 mol% Ba composition. Piezoelectric charge coefficient (d33), piezoelectric planar coupling coefficient (kp) and piezoelectric thickness coupling coefficient (kt) increased to the 4 mol% Ba composition, which are potential compositions for sensor and actuator applications. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Single‐crystal nanorings of β‐gallium oxide synthesized using a microwave plasma
Authors: Zhu F. , Yang Z. X. , Zhou W. M. , Zhang Y. F.
Published: June 2006

In this study, a new method for synthesizing single‐crystal ring‐like β‐Ga2O3 nanobelts (here named nanorings), of only 0.2–1 µm in diameter, is presented. A microwave plasma method involving liquid‐phase gallium containing H2O in an Ar atmosphere, using silicon as the substrate, is used for the first time. The morphology and structure of the nanorings were analysed using scanning electron microscopy and X‐ray diffraction. The morphology of β‐Ga2O3 nanobelts was also characterized using transmission electron microscopy (TEM) and high‐resolution TEM in order to propose possible growth mechanisms of the β‐Ga2O3 nanorings. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
p‐type transparent conducting oxides
Authors: Sheng Su , Fang Guojia , Li Chun , Xu Sheng , Zhao Xingzhong
Published: June 2006

The recent advance of p‐type transparent conductive oxide thin films is reviewed. The focus is on p‐type transparent oxide semiconductors CuAlO2, CuGaO2, CuInO2, SrCu2O2, and LaCuOCh (Ch = chalcogen). These materials and related device applications are then shown as examples. Room temperature operation of current injection emission from ultraviolet light‐emitting diodes based on p‐SCO/n‐ZnO p–n junctions has been demonstrated. This changed with the discovery of p‐type transparent conducting oxides, thereby opening up the possibility for all‐oxide transparent electronics. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Influence of strain in the reduction of the internal electric field in GaN/AlN quantum dots grown on a‐plane 6H‐SiC
Authors: Cros A. , Budagosky J. A. , García‐Cristóbal A. , Garro N. , Cantarero A. , Founta S. , Mariette H. , Daudin B.
Published: June 2006

The strain state of stacks of GaN/AlN quantum dots (QDs) grown on (0001) and (11$ \bar 2 $0) 6H‐SiC has been investigated by means of Raman spectroscopy. Depending on the orientation of the wurtzite axis with respect to the growth direction it is found that the piezoelectric contribution to the electrostatic potential may either reinforce that arising from the spontaneous polarization or oppose it. The experimental results are compared with a theoretical model for the strain and polarization field in QDs of both orientations that allows the calculation of the electrostatic potential in the QDs. Both the experimental results and the theoretical model indicate that the internal electric field and electrostatic potential are strongly reduced in the QDs grown on (11$ \bar 2 $0) 6H‐SiC as compared to those grown along the wurtzite c‐axis. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Degenerate four‐wave mixing spectroscopy of GaN films on various substrates
Authors: Ishiguro T. , Toda Y. , Adachi S. , Mukai T. , Hoshino K. , Arakawa Y.
Published: June 2006

We have carried out time‐integrated and spectrally‐resolved four‐wave mixing (FWM) measurements to study the effects of built‐in strain within GaN films on their exciton structures and on their coherent dynamics. Precise values for exciton energies, linewidths, and oscillation periods of quantum beats were presented. Remarkable phase shifts of the quantum beats were observed in the GaN films on a‐sapphire, suggesting the exciton–exciton interaction caused by the reduction of the crystal symmetry. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Study of InAsN quantum dots on GaAs substrates by molecular beam epitaxy
Authors: Ueta A. , Akahane K. , Gozu S. , Yamamoto N. , Ohtani N.
Published: June 2006

InAsN QDs on (001) GaAs substrates were studied. Each InAsN QD was surrounded by four {011} facets and had a pyramidal shape. However, InAs QDs had a half sphere shape. The InAsN QDs had size distributions with two maxima. This might be explained by the existence of very stable {011} facets and curved surfaces between the {011} facets. The curved surfaces act as the incorporation sites to grow the QD larger. This means that large InAsN QDs will be suppress the further growth, because of the smaller occupations of the curved surfaces to the total surface area of the InAsN QD. Excess In atoms supplied on the {001} plane will accumulate strains and form new small QDs by the Stranski–Krastanow (S–K) growth mode. This turns out two maxima of the size distribution. These results will open the possibility of obtaining the size control and small distributions of the InAsN QDs. We investigated the emission properties of these InAsN QDs. An emission peak of nearly 1300 nm from the InAsN QDs with a plasma power of 350 W was observed. InAsN QDs are an attractive material for achieving emissions in the fiber‐optic communication wavelengths. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Raman study of N bonding in AlGaAs/InGaAsN multiquantum wells
Authors: Lazić S. , Calleja J. M. , Hey R. , Ploog K.
Published: June 2006

We report resonant Raman scattering measurements on InGaAsN/AlGaAs multiquantum wells grown by plasma‐assisted molecular beam epitaxy. Sharp vibration modes have been observed at 323, 402, 454 and 501 cm–1. They are resonant in the energy range 1.81–1.87 eV at decreasing energies for increasing N concentration. The resonance energies reveal the influence of the N concentration on the barrier gap at the multiquantum well interfaces. The peaks are interpreted in terms of local vibrations involving GaN and AlN units and pairs. They seem to form mostly at the quantum well interfaces due to preferential N bonding to Al. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The performance of AlGaN solar blind UV photodetectors: responsivity and decay time
Authors: Cherkashinin G. , Lebedev V. , Wagner R. , Cimalla I. , Ambacher O.
Published: June 2006

The responsivity and the decay time of AlGaN solar blind UV‐detectors have been studied. The photodetectors have shown a good spectral responsivity in a narrow spectral range (220 < λ < 300 nm) and a short time response with the best estimated characteristic time constant of τ ∼ 30 ms measured at room temperature. Possible mechanisms responsible for the persistent photoconductivity (PPC) effect in AlxGa1–xN (x = 0.51) are analyzed. A shape of the spectral response as a function of the applied voltage is analyzed in the frameworks of the space‐charge limited current model. It has been shown that the main source of PPC is traps above the Fermi level. PPC occurs when the density of free carriers equals the density of the traps. The model attributing PPC to the spatial separation of the photoexcited electrons and holes by the macroscopic potential barrier is not supported by our photocurrent studies. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Doping of GaN with Fe and Mg for spintronics applications
Authors: Bonanni Alberta , Simbrunner Clemens , Wegscheider Matthias , Przybylinska Hanka , Wolos Agnieszka , Sitter Helmut , Jantsch Wolfgang
Published: June 2006

Metal‐organic chemical vapour deposition of GaN:Fe and (Ga, Fe)N:Mg has been carried out in order to optimize the growth process and to investigate the effectiveness of Fe ions and Mg acceptors incorporation. All samples have been investigated via high‐resolution X‐ray diffraction, secondary‐ion mass spectroscopy, electron paramagnetic resonance (EPR) and magnetization measurements. Co‐doping of Fe and δ‐Mg has been found to promote the acceptors incorporation and EPR data, supported by superconducting quantum interference device magnetometry experiments, reveal the presence of Fe ions in two charge states, namely the Fe3+ responsible for Curie paramagnetism and the Fe2+ tentatively correlated to van Vleck paramagnetism. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Strong light–matter coupling in GaN microcavities grown on silicon(111) at room temperature
Authors: Sellers I. R. , Semond F. , Leroux M. , Massies J. , Henneghien A‐L. , Disseix P. , Leymarie J. , Vasson A.
Published: June 2006

We present experimental results demonstrating strong light–matter coupling at room temperature in bulk GaN microcavities grown on silicon(111). Simple low finesse planar microcavities show a Rabi energy as high as 60 meV at room temperature. We also demonstrate room temperature strong‐coupling from a bulk GaN microcavity with epitaxial distributive Bragg reflectors (DBRs). Furthermore, for this structure at low temperature, the strong coupling of both the A and B excitonic features of GaN with the cavity mode are clearly resolved for the first time in such a microcavity. At room temperature a Rabi energy of 50 meV is observed, and well reproduced using numerical analysis describing the interaction of both the A and B excitonic states with the photonic mode. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Localized donor state above the conduction band minimum in InN revealed by high pressure and temperature transport experiments
Authors: Dmowski L. H. , Dybko K. , Plesiewicz J. , Suski T. , Lu H. , Schaff W. , Kurouchi M. , Nanishi Y. , Konczewicz L. , Cimalla V. , Ambacher O.
Published: June 2006

We have studied the pressure and temperature dependences of Hall electron concentration and mobility in degenerated, not intentionally doped InN samples. The results obtained for a whole set of samples with electron concentrations from 5.4 × 1017 cm–3 to 3.3 × 1018 cm–3 consistently revealed the existence of a localized donor type state resonant with the conduction band. The concentration of this state is of the same order in all studied samples and not exceeds 1018 cm–3. Population of this state determines electrical properties of InN samples with low carrier concentration. This donor state is not the main source of conducting electrons in these not intentionally doped samples and can be entirely populated and hidden in samples with high electron concentration exceeding about 3 × 1018 cm–3. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optical polarization anisotropy in strained A‐plane GaN films on R‐plane sapphire
Authors: Ghosh Sandip , Misra Pranob , Grahn Holger T. , Imer Bilge , Nakamura Shuji , DenBaars Steven P. , Speck James S.
Published: June 2006

We have used polarized photoreflectance (PR) spectroscopy to study [11$ \bar 2 $0]‐oriented A‐plane GaN films on R‐plane sapphire substrates. For this type of films, the c‐axis of GaN lies in the film plane. When the probe beam is polarized perpendicular to the c‐axis, we observe a single feature in the PR spectrum, while two features at higher energies are observed for parallel polarization. Comparing the transition energies with the ones obtained by electronic band‐structure calculations, we identify the observed PR spectral features as the three possible transitions around the fundamental band gap of GaN. However in comparison to unstrained GaN, their polarization properties and energies have been significantly modified by the in‐plane anisotropic strain in the film. Finally, we can also determine the individual in‐plane anisotropic strain components. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Four‐wave mixing measurements of biexcitons in uniaxially‐strained GaN films
Authors: Adachi Satoru , Toda Yasunori , Ishiguro Tetsuro
Published: June 2006

Biexcitons of GaN under uniaxial strain have been investigated by spectrally‐resolved four‐wave mixing (FWM) technique. Two kinds of biexcitons were observed clearly and the binding energies were obtained. Since the uniaxial strain changes the exciton structure, the biexciton binding is modified and biexciton has also fine structure, which was revealed in the FWM spectra that were obtained by rotating the linearly polarized excitation against the direction of uniaxial strain. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Pressure and composition dependence of the electron effective mass in GaAs1–xNx
Authors: Gorczyca I. , Christensen N. E. , Svane A.
Published: June 2006

The electronic band structures of GaAs1–xNx alloys are calculated ab initio using a supercell approach in connection with the full‐potential linear muffin‐tin‐orbital method. Alloying GaAs with small amounts of GaN modifies substantially the conduction bands. The lowest conduction band exhibits a strong nonparabolicity, which is reflected in the dependence of the electron effective mass on the k‐vector. The calculations indicate that for x above 0.01 the alloys have electron effective masses, which are significantly larger than that of pure GaAs. In addition, the effective mass decreases with x. Hydrostatic pressure is found to strongly influence the electron effective masses. The pressure coefficient of the mass at the conduction band minimum varies non‐linearly with x. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Reflectance and photoluminescence studies of InGaN/GaN multiple‐quantum‐well structures embedded in an asymmetric microcavity
Authors: Lin D. Y. , Shiu J. J. , Lin C. F.
Published: June 2006

Using reflectance (R) and photoluminescence (PL) measurements InGaN/GaN multiple‐quantum‐well (MQW) structures embedded in an asymmetric microcavity with different thickness of stacking pairs have been studied. The asymmetric microcavity structures are composed of a cavity sandwitched between the air/semiconductor interface and a mirror using distributed Bragg reflector (DBR). For the DBR with thinner AlN layers the high‐reflectivity stop band locates at higher photon energy. The luminescence efficiency and the spectrum of InGaN/GaN multiple‐quantum‐well structures will be modified by the microcavity. A comparison of PL with R spectra shows that the emission efficiency can be enhanced by matching up the luminescence spectrum coming from the MQW and the high‐reflectivity stop band. From the blue shift of the cavity modes as a function of incident angles the refractive index and cavity length can be determined. By measuring the PL spectra as a function of emission angle, it is found that the PL spectra were predominatly determined by microcavity resonances. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Resonant Raman scattering in InGaN alloys
Authors: Davydov V. Yu. , Klochikhin A. A. , Goncharuk I. N. , Smirnov A. N. , Sakharov A. V. , Skvortsov A. P. , Yagovkina M. A. , Lebedev V. M. , Lu Hai , Schaff William J.
Published: June 2006

A strong resonant behavior of the Raman scattering from LO‐phonons in n‐InGaN alloys at excitation near the interband absorption threshold was observed. An approach has been developed to describe the resonant Raman cross sectional profile in the presence of a Burstein‐Moss shift of the interband optical transitions. It has been shown that a simultaneous study of absorption, photoluminescence, and Raman spectra provides reliable information about the band gap and can be efficient for the alloy characteriza‐tion. Our data show that the band gap composition dependence of InGaN is characterized by the strongly nonlinear behavior with the large bowing parameter of 2.5–2.6 eV. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Electric fields in AlGaN/GaN quantum well structures
Authors: McAleese C. , Costa P. M. F. J. , Graham D. M. , Xiu H. , Barnard J. S. , Kappers M. J. , Dawson P. , Godfrey M. J. , Humphreys C. J.
Published: June 2006

This paper presents a comparative study of the magnitude of the electric fields in AlGaN/GaN quantum well structures, measured using electron holography in a transmission electron microscope and estimated from a comparison of low temperature photoluminescence peak energies with calculated values. The values derived from the two techniques were found to be in reasonable agreement for the structures examined here. A larger field across the GaN well was observed from a single quantum well compared to a 10 period structure with equivalent well thickness, while the presence of an electric field across the barrier of the single quantum well was also detected by electron holography. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Growth and optical characterization of InAsN quantum dots
Authors: Tsurusawa H. , Nishikawa A. , Katayama R. , Onabe K.
Published: June 2006

Self‐assembled InAsN quantum dots (QDs) have been grown by rf‐plasma‐assisted molecular beam epitaxy (RF‐MBE) on GaAs(001) substrates via a 2‐monolayer (ML)‐thick InAs wetting layer. The InAsN QDs are typically 30–70 nm in diameter and 3–7 nm in height. The dot density was 3.6 × 109–2.3 × 1010 cm–2 for the fluxes corresponding to the nominal thicknesses of 3.0–4.0 MLs. As the nominal thickness increases, the dot density increases, while the diameter decreases. The low‐temperature (10 K) photoluminescence (PL) of the QD samples shows emission spectra in the wavelength range of 1.2–1.6 μm apparently caused by the N incorporation (0.8–2.0%) into the QDs. The QD size distribution causes broad emission spectra. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
X‐ray absorption near edge spectroscopy at the Mn K‐edge in highly homogeneous GaMnN diluted magnetic semiconductors
Authors: Sancho‐Juan O. , Cantarero A. , Martínez‐Criado G. , Olguín D. , Garro N. , Cros A. , Salomé M. , Susini J. , Dhar S. , Ploog K.
Published: June 2006

We have studied by X‐ray absorption spectroscopy the local environment of Mn in highly homogeneous Ga1–xMnxN (0.06 < x < 0.14) thin epilayers grown by molecular beam epitaxy on [0001] SiC substrates. The measurements were performed in fluorescence mode around the Ga and Mn K‐edges. In this report, we focus our attention to the X‐ray absorption near edge spectroscopy (XANES) results. The comparison of the XANES spectra corresponding to the Ga and Mn edges indicates that Mn is substitutional to Ga in all samples studied. The XANES spectra measured at the Mn absorption edge shows in the near‐edge region a double peak and a shoulder below the absorption edge and the main absorption peak after the edge, separated around 15 eV above the pre‐edge structure. We have compared the position of the edge with that of MnO (Mn2+) and Mn2O3 (Mn3+). All samples studied present the same Mn oxidation state, 2+. In order to interprete the near‐edge structure, we have performed ab initio calculations with a 2 × 2 × 1 supercell (∼6% Mn) using the full potential linear augmented plane wave method as implemented in the Wien2k code. The calculations show the appearance of Mn anti‐bonding t2g bands, which are responsible for the pre‐edge absorption. The shoulder and main absorption peaks are due to transitions from the valence band 1s‐states of Mn to the p‐contributions of the conduction bands. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Superconductivity of InN with a well defined Fermi surface
Authors: Inushima T. , Kato N. , Maude D. K. , Lu Hai , Schaff W. J. , Tauk R. , Meziani Y. , Ruffenack S. , Briot O. , Knap W. , Gil B. , Miwa H. , Yamamoto A. , Muto D. , Nanishi Y. , Higashiwaki M. , Matsui T.
Published: June 2006

In this report we present our recent investigation on the co‐existence of superconducting and semiconducting properties in InN grown on sapphire (0001) by the use of MBE and MOCVD methods. Magnetotransport measurements were done using InN with a carrier density ne from 4 × 1017 to 4 × 1020 cm–3 in the temperature range from 20 mK to 2 K and under the magnetic field (B) up to 23 T. As a result, even InN samples with ne as low as 4 × 1017 cm–3 showed superconductivity at Tc = 0.12 K. The vortex solid of InN was melted by thermal fluctuation and by the external field, similarly to the observation in layered high Tc superconductors. By the Shubnikov–de Haas oscillation measurements it was found that there were substantial carriers spread in the a–b plane which played an essential role not only for the occurrence of the superconductivity, but also for many other unexpected electronic and optical properties of InN. Based on these results we present a possible mechanism of superconductivity in terms of the interaction between the conduction electrons and the fixed d‐electrons of In atoms spread in the a–b plane. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The dominant shallow 0.225 eV acceptor in GaN
Authors: Monemar B. , Paskov P. P. , Bergman J. P. , Paskova T. , Figge S. , Dennemarck J. , Hommel D.
Published: June 2006

We have studied the optical signatures of the Mg acceptor in GaN, using samples that are doped with Mg during MOCVD growth. In order to reduce the defect density in the material and thus achieve narrow linewidths in optical spectra we have used thick HVPE grown GaN layers as templates in the MOCVD growth. The photoluminescence (PL) spectra show two acceptor‐related bound exciton peaks at 3.466 eV and 3.455 eV respectively. In the lower photon energy range the 3.27 eV emission with its LO‐phonon replicas is dominant, riding on a broad background emission peaking at about 3.1 eV. These results, together with previous data in the literature, indicate that there are two acceptors in Mg‐doped GaN, one dominating the optical spectra (the 3.466 eV and the 3.27 eV emissions) and another related to the 3.455 eV and the 3.1 eV emissions. We suggest that the latter is related to the Mg acceptor, while the former is a H‐related complex, not necessarily involving Mg. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Hot carrier energy losses in conducting layers of AlGaN/GaN heterostructures grown on SiC and Al2O3 substrates
Authors: Danilchenko B. A. , Zelensky S. E. , Drok E. , Vitusevich S. A. , Danylyuk S. V. , Klein N. , Lüth H. , Belyaev A. E. , Kochelap V. A.
Published: June 2006

The energy relaxation rate for hot electrons in AlGaN/GaN heterostructures was measured over the temperature range 4.2–300 K. Samples grown on sapphire and 4H‐SiC substrates were studied. The most important result is that the hot carrier energy dissipation differs for samples grown on sapphire and SiC substrates. In the case of sapphire substrate, the dissipation can be described by the emission of optical phonons with an energy of 90 meV and relaxation time of 25 fs. In the case of SiC substrate, both activation energy and relaxation time exceed the values characteristic of the electron‐LO‐phonon dissipation process. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Structural evaluation of GaN/sapphire grown by epitaxial lateral overgrowth by X‐ray microdiffraction
Authors: Drakopoulos M. , Laügt M. , Riemann T. , Beaumont B. , Gibart P.
Published: June 2006

The cover picture from the article [1] depicts the geometry of X‐ray diffraction on a GaN/sapphire structure made by a two‐step epitaxial lateral overgrowth (2S‐ELO) process. Below, the X‐ray rocking curve as a function of the vertical beam position in 2S‐ELO GaN/sapphire is shown. A low intensity line diverging from the main peak position (arrow) may be interpreted as the disappearance of a winged crystal region due to dislocation curvature.

This paper is a presentation from the 6th International Conference on Nitride Semiconductors held in Bremen, Germany, in 2005. Further articles from ICNS‐6 are also being published in phys. stat. sol. (a) 203, No. 7 (2006) and phys. stat. sol. (c) 3, No. 6 (2006).
UPLC/MSE; a new approach for generating molecular fragment information for biomarker structure elucidation
Authors: Plumb Robert S. , Johnson Kelly A. , Rainville Paul , Smith Brian W. , Wilson Ian D. , Castro‐Perez Jose M. , Nicholson Jeremy K.
Published: June 2006

A new approach to obtain fragmentation information in liquid chromatography/mass spectrometry (LC/MS) studies of small molecules in complex mixtures is presented using simultaneous acquisition of exact mass at high and low collision energy, MSE. LC/MS‐TOF and LC/MS/MS‐TOF are powerful tools for the analysis of complex mixtures, especially those for biological fluids allowing the elucidation of elemental composition and fragmentation information. In this example the composition of rat urine was studied using this new approach, allowing the structures of several endogenous components to be confirmed in one analytical run by the simultaneous acquisition of exact mass precursor and fragment ion data. The spectral data obtained using this new approach are comparable to those obtained by conventional LC/MS/MS as exemplified by the identification of endogenous metabolites present in rat urine. Copyright © 2006 John Wiley & Sons, Ltd.
Characterization of nodularin variants in Nodularia spumigena from the Baltic Sea using liquid chromatography/mass spectrometry/mass spectrometry
Authors: Mazur‐Marzec Hanna , Meriluoto Jussi , Pliński Marcin , Szafranek Janusz
Published: June 2006

Nodularin is a potent hepatotoxic cyclic pentapeptide produced by planktonic cyanobacterium Nodularia spumigena. Bloom and culture samples of the cyanobacterium collected and isolated from the Gulf of Gdańsk, southern Baltic Sea, were analyzed. Hybrid quadrupole‐time‐of‐flight liquid chromatography/mass spectrometry/mass spectrometry (TOF‐LC/MS/MS) with ionspray (ISP) and collision‐induced dissociation (CID) were used to characterize nodularin and its analogues. The identification process was based on the comparison of recorded product ion spectra with the previously reported FAB‐MS/CID (high‐energy) mass spectra of the corresponding nodularin variants. Amino acid structures and sequences were derived from the fragmentation pattern of the [M+H]+ ions. Apart from unmodified nodularin with an arginine residue (NOD‐R), three demethylated variants have been found. The sites of demethylation were located on aspartic acid [Asp1]NOD, the Adda residue [DMAdda3]NOD, and dehydrobutyric acid [dhb5]NOD. In two other nodularin variants an additional methyl group is located in the Adda [MeAdda]NOD and Glu [Glu4(OMe)]NOD residues. The linear NOD and the geometrical isomer of NOD‐R, reported earlier in N. spumigena from New Zealand, have also been detected. Two of the total eight nodularin variants characterized in the present study, [dhb5]NOD and [MeAdda]NOD, have not been described earlier. Copyright © 2006 John Wiley & Sons, Ltd.
Flushing time and storage effects on the accuracy and precision of carbon and oxygen isotope ratios of sample using the Gasbench II technique
Authors: Paul Debajyoti , Skrzypek Grzegorz
Published: June 2006

Continuous‐flow isotope ratio mass spectrometry interfaced with a Gasbench II is used for automated and faster analyses of δ13C and δ18O in water, carbonate, and air samples that are accurate and highly precise. Prior to online chemistry and measurement using the Gasbench technique, rubber septa‐capped glass vials are routinely flushed to remove air. Due to the small amounts of sample gas required for isotope analyses using current techniques, care should be taken to properly flush these vials to avoid contamination of sample gas with air. Our results indicate that isotopic composition of sample CO2 gas remains constant when 10 mL vials are flushed (rate of 100 mL/min) for ≥600 s, whereas for vials flushed <600 s, the isotopic composition becomes substantially lighter with decreasing time of flushing, which affects the accuracy of analyses. This largely depends on the isotopic composition (and volume) of air that still remains after flushing. This effect is more pronounced on δ18O than on δ13C of sample CO2 gas because there is very little carbon in the air. After 24 h storage in vials with punctured septa, both δ13C and δ18O of CO2 become isotopically heavier compared with first day analyses, suggesting time‐dependent changes in isotopic composition. The magnitude of shift depends on the concentration and the isotopic composition of CO2 in laboratory air as well as on fractionation due to outflow of sample gas or inflow of air via punctured septa. Contamination of sample gas with air can be observed as a secondary peak on chromatograms that precedes sample peaks, and the intensity of these peaks depends on the amount of air. Such peaks are always present with short flushing times. For accuracy and better precision, irrespective of the magnitude of the secondary peaks, the analyses should be discarded if these appear in the chromatograms. Copyright © 2006 John Wiley & Sons, Ltd.
Contributions of common sources of polycyclic aromatic hydrocarbons to soil contamination
Authors: Simon John A. , Sobieraj James A.
Published: June 2006

Asphalt products, particularly sealants, are prepared using petroleum products that contain a com‐plex mixture of aliphatic and aromatic hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs). Clearly, these products are ubiquitous in urban environments, which raises an issue regard‐ing the potential for PAHs to be transported from parking lots to underlying or adjacent soil, surface‐water bodies, or groundwater. Based on a literature review, there are limited studies focus‐ing on this issue; however, the studies that have been published have fascinating conclusions. The literature shows, as expected, that asphalt‐based products contain PAHs. The highest PAH concen‐trations are present in asphalt sealants, particularly those manufactured using coal tar. Furthermore, due to the low solubility and high partition coefficients of PAHs, the potential for PAHs to leach from asphalt surfaces is negligible, which has been confirmed by leachability studies. Thus, there is little risk that PAHs will be present in stormwater runoff or leach into groundwater from asphalt‐paved areas in a dissolved form. However, asphalt pavement and sealants produce particulate matter that can contain concentrations of PAHs in the sub‐percent range (100s to 1,000s mg/kg total PAHs) that is transported in stormwater runoff. Some studies show that this can cause soil and sediment con‐tamination with total PAH concentrations in the range of 1 to 10 mg/kg. From a remediation per‐spective, many site cleanups are conducted to remediate the presence of PAHs to cleanup goals below 1 mg/kg or, in some cases, 0.1 mg/kg or lower. From a total risk perspective, remediating sites to low PAH cleanup goals may be unwarranted in light of the risk of transportable PAHs produced from paved parking surfaces. In other words, is it reasonable to conduct a cleanup to remediate low PAH concentrations and then redevelop the area with asphalt pavement and sealant, which may pose a greater PAH‐related risk? © 2006 Wiley Periodicals, Inc.
Environmental cleanup of the nation's former nuclear weapons sites: Unprecedented public‐private challenges at the largest facilities
Authors: Mayer Henry , Faass Josephine , Greenberg Michael R. , Lewis David
Published: June 2006

In 1994, the U.S. Department of Energy (DOE) initiated a contract reform program intended to strengthen oversight capabilities and encourage the creation of contract and incentive structures, which would effectively facilitate the treatment of onsite contamination and waste. The remedia‐tion and disposal of these legacy wastes is the core of the Department's environmental manage‐ment mission (Government Accountability Office [GAO], 2003). Despite a concerted effort toward achieving the goals of the reform, progress has been slow. Many projects continue to necessitate cost and time extensions above those originally agreed upon. Although the Department insti‐tuted an accelerated cleanup program in 2002, promising to shave some $50 billion and 35 years from its earlier cost and schedule projections, there have been delays in critical project areas that call into question the attainability of the proposed reductions (GAO, 2005). Numerous explana‐tions have been offered as to why achieving these goals has proven so difficult, many of which have concluded that flawed contracting practices are to blame. This article concludes that the root of the problem is much deeper and that the organizational criticisms aimed at DOE are as much a legacy as the waste itself. Although the focus of this article is on large former nuclear weapons sites, these types of contracting and organizational issues are often found at other gov‐ernment and private complex hazardous waste sites. © 2006 Wiley Periodicals, Inc.