bluuebunny/crossref_metadata_2025 huggingface dataset:
Neutron emission Doppler-shift measurements in deuterium–tritium plasmas
Authors: Frenje J. , Ballabio L. , Conroy S. , Ericsson G. , Gorini G. , Källne J. , Prandoni, P. , Tardocchi M. , Traneus E.
Published: July 2002

Spectrometry of the neutron emission is discussed as a probe of the fuel ion velocity distribution in tokamak plasmas where the instrumental energy calibration allows determination of Doppler shifts on an absolute scale or relative to the emission of thermal plasma conditions. Such Doppler shifts have been measured for plasmas of different heating methods at the Joint European Torus (JET) using a neutron spectrometer of the magnetic proton recoil (MPR) type. The principles of the MPR spectrometer as a rotation diagnostic are discussed together with the results obtained for the JET plasmas. The development potential of neutron spectrometry as a rotation diagnostic at the accuracy level of ±1 km/s for toroidal and poloidal rotation is projected, especially, for use on burning plasma tokamaks.
Activation studies of low-dose Si implants in gallium nitride
Authors: Eiting C. J. , Grudowski P. A. , Dupuis R. D. , Hsia H. , Tang Z. , Becher D. , Kuo H. , Stillman G. E. , Feng M.
Published: July 2002

The implantation of Si ions into undoped high-resistivity GaN films is of interest for the realization of high-performance digital and monolithic microwave integrated circuits. We report the effect of postimplant annealing conditions on the electrical, optical, and surface morphology of Si ion-implanted GaN films. We demonstrate high activation efficiencies for low-dose Si implants into unintentionally doped GaN/sapphire heteroepitaxial films. The Si ions were implanted through an epitaxial AlN cap layer at 100 keV and a dose ∼5×1014 cm−2. Samples were subsequently annealed in an open-tube furnace for various times and temperatures. The postanneal electrical activation is correlated with the surface morphology of the film after annealing. The samples annealed at 1150 °C in N2 for 5 min. exhibited a smooth surface morphology and a sheet electron concentration ns∼6.8×1013 cm−2.
An investigation of adatom bonding at metals by the electron localization function
Authors: Bonini N. , Trioni M. I. , Brivio G. P.
Published: July 2002

In this article, within an ab initio density functional framework, we study the charge rearrangement in chemisorption of an isolated atom on a simple metal surface described by jellium. We focus our attention on the electron localization function (ELF) and illustrate how this function can complement the total charge density contour plots for the understanding of chemisorption. The ELF of Li, Cl, Si, and Fe adatoms are discussed. In all such cases the ELF shows itself as a wealthier, more immediate and visually informative pattern of the charge rearrangement. By varying the atom–surface distance of Fe, the formation of the chemisorption bond and the role played by the majority and the minority spin populations are discussed.
Million frames per second infrared imaging system
Authors: Zehnder Alan T. , Guduru Pradeep R. , Rosakis Ares J. , Ravichandran G.
Published: July 2002

An infrared imaging system has been developed for measuring the temperature increase during the dynamic deformation of materials. The system consists of an 8×8 HgCdTe focal plane array, each with its own preamplifier. Outputs from the 64 detector/preamplifiers are digitized using a row-parallel scheme. In this approach, all 64 signals are simultaneously acquired and held using a bank of track and hold amplifiers. An array of eight 8:1 multiplexers then routes the signals to eight 10 MHz digitizers, acquiring data from each row of detectors in parallel. The maximum rate is one million frames per second. A fully reflective lens system was developed, consisting of two Schwarszchild objectives operating at infinite conjugation ratio. The ratio of the focal lengths of the objectives determines the lens magnification. The system has been used to image the distribution of temperature rise near the tip of a notch in a high strength steel sample (C-300) subjected to impact loading by a drop weight testing machine. The results show temperature rises at the crack tip up to around 70 K. Localization of temperature, and hence, of deformation into “U” shaped zones emanating from the notch tip is clearly seen, as is the onset of crack propagation.
Evidence of a phase transition induced in zirconia by high energy heavy ions
Authors: Benyagoub A. , Levesque F. , Couvreur F. , Gibert-Mougel C. , Dufour C. , Paumier E.
Published: July 2002

Monoclinic zirconia samples were irradiated with 300 MeV Ge and Ni ions at increasing ion fluences. Their structural evolution was monitored in situ by x-ray diffraction and ex situ by Raman spectroscopy. No amorphization of zirconia was observed in both cases. However, a transition from the monoclinic to the tetragonal phase was found in the case of Ge ion irradiation. On the contrary, no such effect was detected upon Ni ion irradiation. A comparison of these experiments indicates that the electronic energy loss released by swift heavy ion irradiation needs to be quite in excess of ∼12 keV nm−1 in order to induce a monoclinic to a tetragonal phase transition in pure (i.e., unstabilized) zirconia.
Significant enhancement of photoconductivity in truly two-component and chemically hybridized CdS-poly(N-vinylcarbazole) nanocomposites
Authors: Yang C. L. , Wang J. N. , Ge W. K. , Wang S. H. , Cheng J. X. , Li X. Y. , Yan Y. J. , Yang S. H.
Published: July 2002

Chemically hybridized CdS-poly(N-vinylcarbazole) (PVK) nanocomposites with different molar ratios of CdS to PVK were prepared. A variety of techniques including optical absorption, photoluminescence (PL), photocurrent spectroscopy, and time-resolved PL were employed to investigate the properties of CdS–PVK nanocomposites. A significant enhancement of photoconductivity was observed in CdS–PVK nanocomposites as compared to pure PVK and CdS/PVK nanoblends. We interpret this result as the improved interface quality between CdS and PVK in CdS–PVK nanocomposites, which facilitates fast interfacial carrier transfer and thus increases the charge generation efficiency.
Subgrid scale variance and dissipation of a scalar field in large eddy simulations
Authors: Jiménez C. , Ducros F. , Cuenot B. , Bédat B.
Published: July 2002

Subgrid scale (SGS) variance of a scalar field in large eddy simulations is only properly defined in relation to a probability density function. This solves a reported problem in the variance definition [Cook and Riley, Phys. Fluids 6, 2868 (1994); Cook, Riley, and Kosály, Combust. Flame 109, 332 (1997)] and allows to write a simple evolution equation for the scalar variance. This equation shows that a recently proposed model for scalar dissipation in terms of the large-scale gradients [Pierce and Moin, Phys. Fluids 10, 3041 (1998)] implies dissipation and production canceling out, preventing variance decay and complete mixing at SGS level. An alternative simple model for dissipation in terms of a SGS mixing characteristic time is proposed and tested here.
Hexagonal nanotubes of ZnS by chemical conversion of monocrystalline ZnO columns
Authors: Dloczik L. , Engelhardt R. , Ernst K. , Fiechter S. , Sieber I. , Könenkamp R.
Published: July 2002

Monocrystalline ZnO columns grown in electrodeposition were converted to ZnS using ion exchange reactions in H2S or S vapor. At ∼400 °C the reaction with H2S only affects a thin layer of 10–30 nm thickness at the surface of the ZnO crystallites, and ZnS-coated ZnO columns are produced. Exploiting the large difference in etch resistance between ZnS and ZnO, the ZnO core of the columns can be removed, and a tubular structure of ZnS can be prepared. Typical dimensions of the ZnS tubes are a length of 1–3 μm, a diameter of 100–300 nm, and a wall thickness of 10–30 nm. The ZnS tubes have the same distribution, alignment, and surface morphology as the original ZnO columns. The reaction in S vapor is suitable to produce solid ZnS columns.
Spatial effects on ideality factor of amorphous silicon pin diodes
Authors: Kroon M. A. , van Swaaij R. A. C. M. M.
Published: July 2002

We analyze dark current-voltage and short-circuit current versus open-circuit voltage measurements of hydrogenated amorphous silicon pin and nip diodes. The ideality factor of ∼1.4 is independent of the thickness of the intrinsic layer, indicating that recombination occurs in the p/i interface region rather than in the bulk. These results can be simulated accurately when the defect-pool model is used. Lateral amorphous silicon diodes with a uniform defect density throughout the intrinsic region have ideality factors, which do depend on the size of the intrinsic region and exceed the theoretical value of 2. We present a model to explain the ideality factor and to verify the results with numerical simulations. Values smaller than 2 are caused by unequal shifts of the quasi-Fermi levels for electrons and holes; values larger than 2 arise when the recombination is spread out over a wide region. In that case the distance between the quasi-Fermi levels is smaller than the applied voltage.
Electron-impact ionization of NH3 and ND3
Authors: Rejoub R. , Lindsay B. G. , Stebbings R. F.
Published: July 2002

Absolute partial and total cross sections for electron-impact ionization of NH3 and ND3 are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ion species are collected with equal efficiency irrespective of their initial kinetic energies. Data are presented for the production of (NH3++NH2++NH++N+), H+, H2+, and NH32+ from NH3 and for production of ND3+, ND2+, ND+, N+, D+, D2+, and ND32+ from ND3 and for the total cross sections which are obtained as the sum of these partial cross sections. Data are also presented for formation of (ND+,D+) and (N+,D+) ion pairs. The overall uncertainty in the absolute cross sections for most of the singly charged ions is ±6–8% while that for doubly charged ions is ±20%. It is observed that the isotopic composition of the ammonia target has no discernible effect upon the cross sections and that the partial cross sections for many of the lighter ions are much larger than had been previously reported.
Synthesis of GaN quantum dots by ion implantation in dielectrics
Authors: Borsella E. , Garcia M. A. , Mattei G. , Maurizio C. , Mazzoldi P. , Cattaruzza E. , Gonella F. , Battaglin G. , Quaranta A. , D’Acapito F.
Published: July 2002

GaN nanocrystals (in the wurtzite phase) were synthesized by sequential implantation of Ga and N ions into either crystalline (quartz, sapphire) or amorphous (silica) dielectrics, followed by annealing of the samples in flowing NH3 gas at 900 °C. GaN was formed by reaction of implanted Ga with NH3 combustion products and/or via conversion of Ga oxide/oxynitrides. A blueshift of the near-band-edge photoluminescence (quantum-confinement effect) was observed for GaN nanocrystals with size ⩽2–3 nm, present in all the substrates.
Radio-frequency-driven radial current and plasma rotation in a tokamak
Authors: Chan V. S. , Chiu S. C. , Omelchenko Y. A.
Published: July 2002

Plasma rotational shear is potentially important for controlling the formation and positioning of internal transport barriers that could stabilize tokamak microturbulence and improve plasma confinement. A new physical mechanism capable of inducing plasma rotation and rotational shear via the ion cyclotron resonance frequency (ICRF) heating of minority ion species in a tokamak has been proposed [Perkins, White, Bonoli, and Chan, Phys. Plasmas 8, 2181 (2001)]. The present work evaluates the validity of this mechanism under the realistic condition when fast ions are continuously heated and slowed down in a driven system. Ion dynamics are calculated with a Monte Carlo code in which wave-induced diffusion in velocity space is accounted for by a quasilinear operator. The code follows the drift trajectories of test particles in a tokamak geometry under the influence of given rf fields and collisions with the background plasma. When the heating geometry is such that no net toroidal wave momentum is injected, the rotational characteristics described in Perkins et al. are reproduced. A physical picture emerges which ascribes the directionality of the rotation as a consequence of finite drift orbit width. The stochastic nature of the wave-induced diffusion can result in a net toroidal torque on the bulk plasma. A number of predicted features can be experimentally tested as a validation of finite orbit drift physics and radio-frequency-induced orbit diffusion.
Rapid sub-diffraction-limit laser micro/nanoprocessing in a threshold material system
Authors: Tanaka Tomokazu , Sun Hong-Bo , Kawata Satoshi
Published: July 2002

We report on sub-diffraction-limit (SDL) micro/nanofabrication via two-photon-absorption (TPA) photopolymerization. SDL spatial resolution was found possible in a threshold system, where materials responded to light excitation with a pronounced threshold behavior. The diffraction limit in this case became just a measure of focal spot size, but did not put any actual restraint to voxel dimensions. Experimentally, lateral spatial resolution down to 120 nm was realized by using high numerical-aperture optics. In addition, we proposed a profile scanning method, by which the fabrication efficiency was significantly increased. The TPA processing time, in the example given here, was reduced by 90%.
The vibrational properties of xenon hydrate: An inelastic incoherent neutron scattering study
Authors: Gutt Christian , Baumert Julian , Press Werner , Tse John S. , Janssen Stefan
Published: July 2002

We report results from a high-resolution, incoherent inelastic neutron scattering (IINS) study of xenon hydrate. This study extends previous work in which the existence of a strong coupling between localized guest vibrations and the lattice modes was shown1 for the first time [Tse et al., Europhys. Lett., 54, 354 (2001)]. This guest–host coupling might be responsible for the glass-like temperature dependence of the thermal conductivity of the crystalline gas hydrates. Our experiment focused on the low-frequency phonon density of states of the ice-like water lattice of xenon hydrate. We found two broad maxima in the density of states (DOS) at energy transfers of 7.3 and 10.3 meV. The first peak is assigned to the transverse acoustic (TA) phonons near the zone boundary and the second to the fold-back of the TA modes towards the zone center. The guest–host coupling could be confirmed by finding three distinct low energy peaks in the DOS at energy transfers of 2.05, 2.87, and 3.94 meV. In addition, another broad inelastic scattering component extending from 0 to 1.5 meV has been found, which may also be important for the low-temperature behavior of the thermal conductivity. The line positions of the coupled modes shift to higher frequencies with increasing temperature, pointing towards the importance of the repulsive part of the host–guest interaction which is responsible for the stability of gas hydrate structures.
Series and parallel connection of multiterminal quantum Hall-effect devices
Authors: Delahaye François
Published: July 2002

The circuit properties of networks which include multiterminal quantum Hall-effect (QHE) devices are discussed. It is shown that QHE devices can be placed in a series or in parallel using multiple links to give equivalent four-terminal quantized resistances which are in practice, to a high degree of accuracy, independent of contact resistances to the two-dimensional electron gas in the samples and of series resistances in the links. The same technique of multiple links can also be used to incorporate QHE devices in resistance or impedance bridges, resulting in a balance condition which is practically unaffected by contact or series resistances. These properties are established using calculations based on equivalent circuits of QHE devices. Metrological applications include the obtainment of accurate reference standards of resistance with values which are multiples or submultiples of individual quantized Hall resistances (QHRs) and, using a resistance bridge, the precise comparison of QHRs. An experimental verification is reported, demonstrating that the effective equivalent resistance of two QHE devices connected in parallel does not differ from nominal by more than a few parts in 109.
Two-dimensional numerical optimization study of the rear contact geometry of high-efficiency silicon solar cells
Authors: Aberle Armin G. , Heiser Gernot , Green Martin A.
Published: July 2002

Under one-sun illumination, the highest energy conversion efficiencies of silicon solar cells are presently obtained with bifacially contacted n+p cells, where contact to the p-type substrate is made via small openings in the rear passivating oxide. In this work, a state-of-the-art 2-dimensional (2D) semiconductor device simulator is applied to these devices in order to investigate the effects arising from the rear metallization scheme. The impact of various cell parameters [i.e., substrate resistivity, rear surface recombination model (flatband or surface band bending conditions), positive oxide charge density, capture cross section ratio] on the cell’s current-voltage (I-V) characteristics and the optimum rear contact spacing is investigated. The highly nonideal I-V curves of rear point-contacted high-efficiency silicon solar cells made at the University of New South Wales (UNSW) are modeled with a high degree of accuracy. This is achieved by properly accounting for the complex recombination behavior at the rear oxidized surface. The 2D simulations show that the nonideal I-V curves result from the unequal capture cross sections of electrons and holes at the rear Si-SiO2 interface and the surface band bending induced by positive oxide charges and metal/silicon work function differences. For the UNSW cells, optimum one-sun efficiency is obtained on 2 Ω cm substrates and rear contact spacings of 0.2–0.3 mm. The 2D simulations presented in this work clearly confirm the experimental findings and reveal the physical mechanisms which favor this particular contact design.
The effect of thermal annealing on the Ni/Au contact of p-type GaN
Authors: Sheu J. K. , Su Y. K. , Chi G. C. , Chen W. C. , Chen C. Y. , Huang C. N. , Hong J. M. , Yu Y. C. , Wang C. W. , Lin E. K.
Published: July 2002

In this study, the Ni/Au layers prepared by electron beam evaporation and thermal alloying were used to form Ohmic contacts on p-type GaN films. Before thermal alloying, the current–voltage (I–V) characteristic of Ni/Au contact on p-type GaN film shows non-Ohmic behavior. As the alloying temperature increases to 700 °C, the I–V curve shows a characteristic of Ohmic contact. The Schottky barrier height reduction may be attributed to the presence of Ga–Ni and Ga–Au compounds, such as Ga4Ni3, Ga3Ni2, GaAu, and GaAu2, at the metal-semiconductor interface. The diffusing behavior of both Ni and Au have been studied by using Auger electron spectroscopy and Rutherford backscattering spectrometry. In addition, x-ray diffraction measurements indicate that the Ni3N and Ga4Ni3 compounds were formed at the metal-semiconductor interface.
Itinerant electron metamagnetic transition in La(FexSi1−x)13 intermetallic compounds
Authors: Fujita A. , Akamatsu Y. , Fukamichi K.
Published: July 2002

A first-order transition above the Curie temperature for ferromagnetic La(FexSi1−x)13 (x=0.86 and 0.88) compounds has been confirmed by applying a magnetic field. The magnetic state changes from the paramagnetic to the ferromagnetic state and the transition field increases with temperature, indicating an itinerant electron metamagnetic (IEM) transition. The IEM transition is broad in x=0.86 and becomes clearer in x=0.88, which takes a negative slope of the Arrott plot. The volume change just above the Curie temperature for x=0.88 is huge, about 1.5%, which is caused by a large magnetic moment induced by the IEM transition.
Annealing of the deep recombination center in GaInP/AlGaInP quantum wells grown by solid-source molecular beam epitaxy
Authors: Dekker J. , Tukiainen A. , Xiang N. , Orsila S. , Saarinen M. , Toivonen M. , Pessa M. , Tkachenko N. , Lemmetyinen H.
Published: July 2002

Time resolved photoluminescence and deep level transient spectroscopy have been used to monitor the effect of rapid thermal annealing on bulk GaInP and GaInP/AlGaInP quantum wells grown by solid source molecular beam epitaxy similar to those used in 650 nm range lasers. Following rapid thermal annealing at temperatures up to 875 °C, reductions in the concentration of several deep level traps are observed. Correlation of these data with photoluminescent intensity and lifetime measurements indicate that the defect labeled N3, 0.83 eV below the conduction band, is the dominant recombination center. The combination of these two transient spectroscopy measurement techniques is therefore not only able to measure the change in deep level concentration, but also to correlate this change with improved carrier lifetimes and, ultimately, reduced threshold current densities in quantum well lasers. There is also evidence to suggest that this same defect, possibly a phosphorous vacancy or a related complex, plays an important role in other GaInP based devices.
Identification and stability determinations for the gaseous titanium oxide molecules Ti2O3 and Ti2O4
Authors: Balducci G. , Gigli G. , Guido M.
Published: July 2002

The gaseous titanium oxides molecules Ti2O3 and Ti2O4 were observed for the first time by high temperature mass spectrometry in the vapors over a cobalt–titanium–oxygen system. From the study of three all-gas reaction equilibria, the stabilities of these molecules have been derived: D○0,at(Ti2O3) =2312±56 kJ mol−1 and D○0,at (Ti2O4)=3017±47 kJ mol−1. The appearance potentials were measured to be 8.3±0.5 and 10.5±0.5 eV for Ti2O+3 and Ti2O+4, respectively.
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o calculation of argon–argon potential
Authors: McLean A. D. , Liu B. , Barker J. A.
Published: July 2002

We present a high-quality ab initio calculation of the potential energy of interaction of two argon atoms using the method of interacting correlated fragments (ICF). The resulting potential has well depth within about 15% of the experimental value and is expected to be relatively accurate in the repulsive region where it is reasonably close to the most recent potential from high-energy scattering data and in reasonable agreement with experimental gas viscosities. Comparison is made with several experimental and partly experimental potentials and also with high-pressure solid state data using several assumptions concerning the nature of many-body interactions in the solid.
A two-dimensional Fourier transform electron-spin resonance (ESR) study of nuclear modulation and spin relaxation in irradiated malonic acid
Authors: Lee Sanghyuk , Patyal Baldev R. , Freed Jack H.
Published: July 2002

Nuclear modulation in electron-spin-echo spectroscopy is conventionally studied by one-dimensional electron-spin-echo envelope modulation (1D-ESEEM). Two-dimensional Fourier transform electron-spin resonance (2D-FTESR) studies of nuclear modulation have the promise of enhancing the spectral resolution and clarifying the key details of the relaxation processes. We present a 2D-FTESR study on single proton nuclear modulation from γ-irradiated malonic acid single crystals to test the validity of the Gamliel–Freed theory and to assess the value of the new methods. The two pulse spin-echo correlation spectroscopy (SECSY) spectra as a function of orientation of the single crystal show very good agreement with the Gamliel–Freed theory extended to the general case of nonaxially symmetric hyperfine interaction. It is very simply affected by spin relaxation, such that relative intensities are essentially unaffected. Thus SECSY-ESR can most reliably be utilized for studying nuclear modulation. Stimulated SECSY provides the simplest nuclear modulation patterns, which, however, do exhibit the suppression effect well known in three-pulse ESEEM studies. Two-dimensional electron–electron double resonance (2D-ELDOR) provides nuclear modulation patterns similar to that of SECSY-ESR, so the suppression effect is absent. Both three-pulse methods exhibit complex relaxation behavior which can affect relative intensities. This is a feature characteristic of three-pulse ESEEM, but is not well understood.

It is shown how the 2D-FTESR methods enable one to obtain the details of the complex spin relaxation, and in the process, obtain very good agreement between experiment and theory. 2D-ELDOR exhibits exchange cross peaks as well as coherence peaks from the nuclear modulation. It is shown how experiments, as a function of mixing time, enable one to separate the effects of the two. It is pointed out that such experiments are in the spirit of 3D spectroscopy. A new observation of the broadening of the 2D-ELDOR main peaks with an increase in mixing time is ascribed to the effects of solid-state dynamical processes that are slow on the ESR time scale and may thus be studied in ‘‘real time’’ in such experiments. The analysis of spin relaxation in this study is enabled by a full Liouville space derivation of the combined effects of nuclear modulation and spin relaxation in 2D-FTESR.
Bridging and looping in multiblock copolymer melts
Authors: Matsen M. W.
Published: July 2002

We apply both self-consistent-field theory (SCFT) and strong-segregation theory (SST) to the lamellar phase of a melt of linear multiblock copolymers. We present a new solution for the SST which now agrees with that obtained for the SCFT. Using both theories, we calculate the distribution of monomers from the center of blocks originating from a single interface, the lamellar spacing, and the fraction of blocks which bridge across lamellae.
Infrared spectroscopy of small size-selected water clusters
Authors: Huisken Friedrich , Kaloudis Michael , Kulcke Axel
Published: July 2002

Infrared molecular beam depletion and fragment spectroscopy has been employed to study the absorption behavior of small water clusters [(H2O)n, n=2,3,4,5]. The spectral region between 3300 and 3800 cm−1 was covered with an injection-seeded optical parametric oscillator. Size-specific information has been obtained by dispersing the cluster beam with a secondary helium beam and measuring the depletion as a function of the scattering angle. Three absorption bands could be assigned to the water dimer (H2O)2, with the bonded OH stretch being localized at 3601 cm−1. For each of the larger water clusters (n=3,4,5), which have cyclic structures, two absorption bands could be identified, one belonging to the free OH stretch and the other being due to the excitation of the OH ring vibration. The measurements on free water clusters were complemented by studies on small water complexes formed on large argon clusters. The positions of the absorption bands observed in these spectra are close to those found for (H2O)n in argon matrices.
Angular dependence of the dynamic displacement of O2 from Pt(111) by atomic oxygen
Authors: Wheeler M. C. , Seets D. C. , Mullins C. B.
Published: July 2002

Impingement of O16-atoms on an O218 covered Pt(111) surface at 77 K induces the evolution of gas-phase O218 and O16O18 in a ratio of ∼4:1 independent of atomic flux or kinetic energy. The total initial probability of desorption of molecular oxygen induced by impingement of atomic oxygen is measured as a function of incident O-atom flux, kinetic energy, and angle. Interestingly, the probability is found to depend on incident angle with values ranging from 0.25±0.02 for a 60° angle of incidence to 0.42±0.02 for normal incidence.
After-effects of the Co57(EC)57Fe nuclear decay in tris(2,2′-bipyridyl)cobalt(II) encapsulated in the supercage of zeolite Y
Authors: Vankó György , Homonnay Zoltán , Nagy Sándor , Vértes Attila , Spiering Hartmut , Gütlich Philipp
Published: July 2002

Fe 57 emission Mössbauer spectroscopy was used to study the after-effects of the Co57(EC)57Fe decay in tris(bipy)cobalt(II) (bipy=2,2′-bipyridyl) encapsulated in the supercage of zeolite Y. The spectra revealed two low-spin species, [57Fe(bipy)3]2+ and [57Fe(bipy)3]3+, as well as a high-spin species of [57Fe(bipy)3]2+. An anomalously high spectral contribution of FeIII was found with an anomalous temperature dependence, i.e., the amount of FeIII diminished with the temperature elevation. A time-delayed Mössbauer experiment was performed to determine the time scale of this phenomenon, which indicated that the processes responsible for the FeIII to FeII transition was completed in the first nanoseconds after the nuclear decay. Another effect of the zeolite encapsulation was the absence of a second high-spin [57Fe(bipy)3]2+ species identified in pristine crystalline compounds as an excited ligand field state of Fe2+. This observation is attributed to the spatial restriction implied by the supercage. The possible fragmentation origin of the first high-spin [57Fe(bipy)3]2+ species, the complex ion with a damaged ligand, is also discussed.
Why clathrates are good thermoelectrics: A theoretical study of Sr8Ga16Ge30
Authors: Blake Nick P. , Mo/llnitz Lone , Kresse Georg , Metiu Horia
Published: July 2002

Recent measurements have shown that the inorganic clathrate Sr8Ga16Ge30 has good thermoelectric properties. This discovery has caused intense experimental activity to synthesize and test other compounds in this class. It has been conjectured that clathrates may be good thermoelectrics if they satisfy several conditions. The Sr atoms, trapped inside the clathrate cages, scatter phonons efficiently, leading to low thermal conductivity. Electric conductivity takes place mostly through the clathrate frame and the conduction electrons are not scattered by Sr vibrations. The compounds, being made of atoms that are semiconductors in the solid state, may have a high Seebeck coefficient. There has been no direct evidence, experimental or theoretical, for this scenario. By performing density functional calculations we show that these ideas are correct. The Sr atoms are weakly bound to the cage and do undergo large-amplitude motion. An analysis of conductivity shows that the largest contribution comes from a band in which the electrons are located on the clathrate frame. Bands originating from the Sr atoms contribute little to conductivity. There is very little charge transfer between the Sr atoms and the frame, and as a result, Sr vibrations are weakly coupled to the conduction electrons. The calculated Seebeck coefficient is in reasonable agreement with the measured one. We find that it is strongly affected by the positions of the Ga atoms in the frame and by doping.
Bilinear phase-plane distribution functions and positivity
Authors: Janssen A. J. E. M.
Published: July 2002

There is a theorem of Wigner that states that phase-plane distribution functions involving the state bilinearly and having correct marginals must take negative values for certain states. The purpose of this paper is to support the statement that these phase-plane distribution functions are for hardly any state everywhere non-negative. In particular, it is shown that for certain generalized Wigner distribution functions there are no smooth states (except the Gaussians for the Wigner distribution function itself) whose distribution function is everywhere non-negative. This class of Wigner-type distribution functions contains the Margenau–Hill distribution. Furthermore, the argument used in the proof of Wigner’s theorem is augmented to show that under mild conditions one can find for any two states f, g with non-negative distribution functions a linear combination h of f and g whose distribution function takes negative values, unless f and g are proportional.
Green’s function for motion in Coulomb-modified separable nonlocal potentials
Authors: Talukdar B. , Laha U. , Sasakawa T.
Published: July 2002

A closed form expression is derived for the outgoing wave radial Green’s function 𝒢(+)l (r,r′) for motion in the Coulomb plus rank one separable nonlocal potential with form factor vl(r)=2−l ×(l!)−1 rle−βlr. Some possible applications of the result are discussed.
Symmetries of hyper-Kähler (or Poisson gauge field) hierarchy
Authors: Takasaki K.
Published: July 2002

Symmetry properties of the space of complex (or formal) hyper-Kähler metrics are studied in the language of hyper-Kähler hierarchies. The construction of finite symmetries is analogous to the theory of Riemann–Hilbert transformations, loop group elements now taking values in a (pseudo-) group of canonical transformations of a simplectic manifold. In spite of their highly nonlinear and involved nature, infinitesimal expressions of these symmetries are shown to have a rather simple form. These infinitesimal transformations are extended to the Plebanski key functions to give rise to a nonlinear realization of a Poisson loop algebra. The Poisson algebra structure turns out to originate in a contact structure behind a set of symplectic structures inherent in the hyper-Kähler hierarchy. Possible relations to membrane theory are briefly discussed.
Some spectral properties of anti-self-adjoint operators on a quaternionic Hilbert space
Authors: Horwitz L. P.
Published: July 2002

Following the spectral methods established by Adler in his treatment of scattering and decay in the quaternionic quantum theory, it is shown that an anti-self-adjoint operator with quaternion defined spectrum in [0,∞) has a symmetrical effective spectrum, from the point of view of functional analysis, under quite general conditions. In the case of an operator with absolutely continuous spectrum in [0,∞), the effective spectrum is absolutely continuous in (−∞,∞), and a canonically conjugate operator exists. If the anti-self-adjoint operator is the generator of motion in time (Hamiltonian), the conjugate operator is a ‘‘time operator.’’ Moreover, according to a theorem of Misra, Prigogine, and Courbage, such a system may admit a Lyapunov operator, and therefore describe irreversible behavior. It is shown directly that no evident contradiction (as is found in the semibounded case in complex quantum mechanics) arises from the definition of a Lyapunov operator in quaternionic quantum mechanics.
Spontaneous symmetry breaking in the SO(3) gauge theory to discrete subgroups
Authors: Etesi Gábor
Published: July 2002

In this paper we give a systematical description of the possible symmetry breakings in the SO(3)-gauge theory and show an algorithmical method to construct SU(2)- or SO(3)-invariant Higgs potentials in an arbitrary irreducible representation using regular graphs. We close our paper with the explicit construction of the Lagrangian of the simplest SO(3)→A4 theory.
Gauge transformations in quantum mechanics and the unification of nonlinear Schrödinger equations
Authors: Doebner H.-D. , Goldin G. A. , Nattermann P.
Published: July 2002

Beginning with ordinary quantum mechanics for spinless particles, together with the hypothesis that all experimental measurements consist of positional measurements at different times, we characterize directly a class of nonlinear quantum theories physically equivalent to linear quantum mechanics through nonlinear gauge transformations. We show that under two physically motivated assumptions, these transformations are uniquely determined: they are exactly the group of time-dependent, nonlinear gauge transformations introduced previously for a family of nonlinear Schrödinger equations. The general equation in this family, including terms considered by Kostin, by Bialynicki-Birula and Mycielski, and by Doebner and Goldin, with time-dependent coefficients, can be obtained from the linear Schrödinger equation through gauge transformation and a subsequent process we call gauge generalization. We thus unify, on fundamental grounds, a rather diverse set of nonlinear time evolutions in quantum mechanics.
Information content for quantum states
Authors: Brody Dorje C. , Hughston Lane P.
Published: July 2002

A method for representing probabilistic aspects of quantum systems by means of a density function on the space of pure quantum states is introduced. In particular, a maximum entropy argument allows us to obtain a natural density function that only reflects the information provided by the density matrix. This result is applied to derive the Shannon entropy of a quantum state. The information theoretic quantum entropy thereby obtained is shown to have the desired concavity property, and to differ from the conventional von Neumann entropy. This is illustrated explicitly for a two-state system.
Anisotropy of local and hyperfine magnetic fields in Fe57 nuclei in the hexaferrite SrFe12O19
Authors: Kuntsevich S. P.
Published: July 2002

Local magnetic fields and their anisotropy in Fe57 nuclei are measured in the domain boundaries for the sites 4f1 of Fe3+ ions of the hexaferrite SrFe12O19 in the temperature range 4.2–295 K. The anisotropy of the magnetic moment moduli of Fe3+ ions is determined at low temperatures. The anisotropy of the magnetic moment moduli is considered as the physical factor responsible for the hyperfine field anisotropy. The connection between the anisotropy of magnetic moment moduli and the magnetic characteristics of the ferrite is considered in the molecular field approximation.
Nonlinear stability of a laminar flow past a two-dimensional grid
Authors: Coscia Vincenzo
Published: July 2002

Sufficient conditions for nonlinear stability of an exact solution to the Navier–Stokes equations (Kovasznay flow) are given by use of the energy method. These results markedly improve on earlier ones obtained by Lin and Tobak [Phys. Fluids 30, 3388 (1987)].
Effects of organized turbulence structures on the phase distortion in a coherent optical beam propagating through a turbulent shear flow
Authors: Truman C. Randall , Lee Moon Joo
Published: July 2002

Effects of organized turbulence structures on the propagation of an optical beam in a turbulent shear flow have been analyzed. An instantaneous passive-scalar field in a computed homogeneous turbulent shear flow is used to represent index-of-refraction fluctuations, and phase distortion induced in a coherent optical beam by turbulent fluctuations is calculated. The organized vortical structures (‘‘hairpin-shaped’’ eddies) in the turbulent flow give rise to a scalar distribution with elongated regions of intense fluctuation, which have an inclination (about 30°) with respect to the mean flow, similar to that of the characteristic ‘‘hairpin’’ eddies. Two-point correlations of vorticity and scalar fluctuations support a proposed physical model in which the regions of intense scalar fluctuation are produced primarily by hairpin vortices. It is found that the spatial distribution of the phase distortion has a substantial variation with the direction of propagation. A highly localized distribution of intense phase distortions is produced when the optical beam propagates at an angle (45°) close to the inclination of hairpin vortices; at larger angles of propagation the distribution shows an elongated pattern with smaller phase distortions. It is also found that the root-mean-square phase distortion depends significantly on the propagation direction, and the phase distortion can be minimized at an angle of propagation approximately normal to the inclination of hairpin eddies. This study shows how the characteristics of an optical beam propagating through a turbulent shear flow are affected by the geometrical configurations of organized vortical structures.
Laminar flow and heat transfer in Rayleigh–Bénard convection with shear
Authors: Mohamad A. A. , Viskanta R.
Published: July 2002

Three- and two-dimensional laminar flow in a cavity heated from below and cooled from above, with and without lid motion, was investigated numerically. The effects of applied shear at the upper lid on the flow structure and heat transfer in a shallow cavity were studied and results are reported for Ra=3500 and 4000 with Pr=5.84 (water). The lid moves parallel to either the ξ or ζ direction. The numerical predications and flow visualization results showed that the lid motion destroys all types of convective cells due to heating from below for finite size cavities. Two-dimensional heat transfer results compare favorably with those based on a three-dimensional model for Gr/Re2≤1.
Resonance cones in non-Maxwellian plasmas
Authors: Oelerich-Hill G. , Piel A.
Published: July 2002

Resonance cones are studied experimentally in hot drifting plasmas and in various beam–plasma situations. In a drifting plasma the upstream–downstream asymmetry of the main cone is suitable for diagnostics of plasma drifts. The evaluation of electron temperature from the interference pattern is discussed in terms of the low-temperature–low-drift approximation (LTLDA) and by comparing with numerical kinetic theory calculations. In this way the range of applicability of this method is extended above the LTLDA. In a beam–plasma situation, the downstream resonance cone exhibits a new interference pattern, which can be attributed to resonant particle effects. The upstream resonance cone is only slightly affected and is found still applicable for Te and ne diagnostics. As a result of comparison with numerical calculations, in which the actual distribution function is used, the downstream interference pattern is proposed to be useful as a diagnostic method for obtaining the mean beam energy. The waves generated by the beam–plasma interaction are analyzed by digital cross-correlation techniques, and found to propagate obliquely to the magnetic field direction at the resonance cone angle.
Wall-pressure fluctuations in turbulent pipe flow
Authors: Lauchle Gerald C. , Daniels Mark A.
Published: July 2002

Wall-pressure fluctuation measurements are reported for the fully developed turbulent flow of glycerine in a long pipe. Because of the relatively large viscous scales associated with glycerine, it has been possible to perform pressure fluctuation spectral measurements for 0.7≤d+≤1.5, where d+ is the transducer diameter expressed in wall units. The data presented are for d+ values smaller than ever before reported.
Nonlinear stability of a flow with bound eddies
Authors: Lin S. P. , Tobak M.
Published: July 2002

Kovasznay [Proc. Cambridge Philos. Soc. 44, 58 (1948)] obtained an exact solution of the Navier–Stokes equations that describes a flow with periodic bound eddies. The stability of this nonparallel flow with respect to three-dimensional finite amplitude disturbances is analyzed by use of the energy method. A sufficient condition for global stability is obtained. It is shown that the additional nonhomogeneity in shear stress distribution in a nonparallel flow is a destabilizing factor.
Atomic point-contact imaging
Authors: Smith D. P. E. , Binnig G. , Quate C. F.
Published: July 2002

In tunneling microscopy a potential barrier separates a pointed tip from the sample to be investigated. In this letter we show that atomic resolution can be achieved in special cases where the gap spacing has been reduced to the point where the potential barrier may have completely collapsed. In this example the tip may be said to be touching the sample. The forces between the tip and sample are then strongly repulsive and the possibility exists for studying tribology on an atomic scale.
Expression profiling of pancreatic β cells: Glucose regulation of secretory and metabolic pathway genes
Authors: Webb Gene C. , Akbar Murtaza S. , Zhao Chongjian , Steiner Donald F.
Published: July 2002

Pancreatic β cells respond to changes in blood glucose by secreting insulin and increasing insulin synthesis. To identify genes used in these responses, we have carried out expression profiling of β cells exposed to high (25 mM) or low (5.5 mM) glucose by using oligonucleotide microarrays. Functional clustering of genes that averaged a 2.2-fold or greater change revealed large groups of secretory pathway components, enzymes of intermediary metabolism, cell-signaling components, and transcription factors. Many secretory pathway genes were up-regulated in high glucose, including seven members of the endoplasmic reticulum (ER) translocon. In agreement with array analysis, protein levels of translocon components were increased by high glucose. Most dramatically, the α subunit of the signal recognition particle receptor was increased over 20-fold. These data indicate that the translocon and ribosome docking are major regulatory targets of glucose in the β cell. Analysis of genes encoding enzymes of intermediary metabolism indicated that low glucose brought about greater utilization of amino acids as an energy source. This conclusion was supported by observations of increased urea production under low-glucose conditions. The above results demonstrate genome-wide integration of β-cell functions at the level of transcript abundance and validate the efficacy of expression profiling in identifying genes involved in the β-cell glucose response.
A polymorphic dinucleotide repeat in the rat nucleolin gene forms Z-DNA and inhibits promoter activity
Authors: Rothenburg Stefan , Koch-Nolte Friedrich , Rich Alexander , Haag Friedrich
Published: July 2002

Many sequences in eukaryotic genomes have the potential to adopt a left-handed Z-DNA conformation. We used a previously described assay based on the binding of a mAb to Z-DNA to inquire whether Z-DNA is formed in the rat nucleolin (Ncl) gene in metabolically active, permeabilized nuclei. Using real-time PCR to measure Z-DNA formation, the potential Z-DNA sequence element Z1 [(CA)10(CG)8] in the promoter region was found to be enriched 571- to 4,040-fold in different cell lines, whereas Z2 [AC(GC)5CCGT(CG)2] in the first intron was enriched 12- to 34-fold.Nclpromoter activity was 1.5- to 16-fold stronger than that of the simian virus 40 promoter and enhancer. This activity was further increased 36–54% when Z1 was deleted. The inhibitory effect of Z1 onNclpromoter activity was independent of location and orientation. The Ncl Z1 element is identical to the genetic marker D9Arb5. Five allelic variants of Z1 were identified by sequence analysis of genomic DNA from various rats. The two most common alleles differed significantly (up to 27%) in their capacity to inhibitNclpromoter activity. This finding suggests that differences in Z-DNA formation by polymorphic dinucleotide repeats may be one of the factors contributing to genetic variation.
Cloning of the
Arabidopsis WIGGUM
gene identifies a role for farnesylation in meristem development
Authors: Ziegelhoffer Eva C. , Medrano Leonard J. , Meyerowitz Elliot M.
Published: July 2002

Control of cellular proliferation in plant meristems is important for maintaining the correct number and position of developing organs. One of the genes identified in the control of floral and apical meristem size and floral organ number in
Arabidopsis thaliana
is
WIGGUM
. In
wiggum
mutants, one of the most striking phenotypes is an increase in floral organ number, particularly in the sepals and petals, correlating with an increase in the width of young floral meristems. Additional phenotypes include reduced and delayed germination, delayed flowering, maturation, and senescence, decreased internode elongation, shortened roots, aberrant phyllotaxy of flowers, aberrant sepal development, floral buds that open precociously, and occasional apical meristem fasciation. As a first step in determining a molecular function for
WIGGUM
, we used positional cloning to identify the gene. DNA sequencing revealed that
WIGGUM
is identical to
ERA1
(enhanced response to abscisic acid), a previously identified farnesyltransferase β-subunit gene of
Arabidopsis
. This finding provides a link between protein modification by farnesylation and the control of meristem size. Using
in situ
hybridization, we examined the expression of
ERA1
throughout development and found it to be nearly ubiquitous. This extensive expression domain is consistent with the pleiotropic nature of
wiggum
mutants and highlights a broad utility for farnesylation in plant growth and development.