bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectral properties and source regions of dayside electron acceleration events
Authors: Newell P. T. , Wing S. , Meng C.‐I.
Published: November 2005

Eleven years of DMSP particle data (1984–1994) were used to investigate electron acceleration events on the dayside. Specifically, the characteristic energy (spectral flux peak), total energy flux, and probability of observing acceleration events were studied as a function of the type of dayside precipitation region (cusp, mantle, etc.). Most dayside electron acceleration events are latitudinally smaller than the events found premidnight, so it is usually possible to identify the type of precipitation which surrounds dayside events. All dayside regions contain embedded electron acceleration events, with such instances most common in the dayside boundary plasma sheet (BPS) and least common in the mantle and polar rain. Electron acceleration events in the cusp are not particularly rare, but the acceleration involved is quite mild. This is consistent with findings that large nightside potentials do not form in regions with a large ambient electron population. On the basis of the fraction of spectra which are accelerated (number of accelerated spectra divided by the total number of spectra measured), the dayside regions can be ordered as follows, with the acceleration probability in parenthesis: mantle prenoon (2.4%), mantle postnoon (3.0%), cusp (4.2%), open low‐latitude boundary layer (LLBL) (5.0%), closed LLBL prenoon (6.0%), closed LLBL postnoon (7.2%), BPS prenoon (8.2%), and finally BPS postnoon (8.6%). The total precipitating energy flux from electron acceleration has the same order; thus more dayside auroral luminosity is associated with the BPS than with any other region. When these regions are ordered by the spectral flux peak in acceleration events, the order changes, as follows: cusp (201 eV), open LLBL (234 eV), closed LLBL prenoon (390 eV), mantle prenoon (414 eV), closed LLBL postnoon (459 eV), mantle prenoon (463 eV), BPS prenoon (572 eV), and BPS postnoon (725 eV). The afternoon hot spot is found to have a relatively symmetric partner, a morningside “warm spot,” centered near 0830 MLT. Both the afternoon hot spot and the morning warm spot span the convection reversal boundary, since they include both BPS and LLBL precipitation, with the former region showing the most intense activity (both the highest number of events and the largest electron acceleration energies).
Global estimates of water‐vapor‐weighted mean temperature of the atmosphere for GPS applications
Authors: Wang Junhong , Zhang Liangying , Dai Aiguo
Published: November 2005

Water‐vapor‐weighted atmospheric mean temperature, Tm, is a key parameter in the retrieval of atmospheric precipitable water (PW) from ground‐based Global Positioning System (GPS) measurements of zenith path delay (ZPD), as the accuracy of the GPS‐derived PW is proportional to the accuracy of Tm. We compare and analyze global estimates of Tm from three different data sets from 1997 to 2002: the European Centre for Medium‐Range Weather Forecasts (ECMWF) 40‐year reanalysis (ERA‐40), the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis, and the newly released Integrated Global Radiosonde Archive (IGRA) data set. Temperature and humidity profiles from both the ERA‐40 and NCEP/NCAR reanalyses produce reasonable Tm estimates compared with those from the IGRA soundings. The ERA‐40, however, is a better option for global Tm estimation because of its better performance and its higher spatial resolution. Tm is found to increase from below 255 K in polar regions to 295–300 K in the tropics, with small longitudinal variations. Tm has an annual range of ∼2–4 K in the tropics and 20–35 K over much of Eurasia and northern North America. The day‐to‐day Tm variations are 1–3 K over most low latitudes and 4–7 K (2–4 K) in winter (summer) Northern Hemispheric land areas. Diurnal variations of Tm are generally small, with mean‐to‐peak amplitudes less than 0.5 K over most oceans and 0.5–1.5 K over most land areas and a local time of maximum around 16–20 LST. The commonly used Tm‐Ts relationship from Bevis et al. (1992) is evaluated using the ERA‐40 data. Tm derived from this relationship (referred to as Tmb) has a cold bias in the tropics and subtropics (−1 ∼ −6 K, largest in marine stratiform cloud regions) and a warm bias in the middle and high latitudes (2–5 K, largest over mountain regions). The random error in Tmb is much smaller than the bias. A serious problem in Tmb is its erroneous large diurnal cycle owing to diurnally invariant Tm‐Ts relationship and large Ts diurnal variations, which could result in a spurious diurnal cycle in GPS‐derived PW and cause 1–2% day‐night biases in GPS‐based PW.
Finite element tree crown hydrodynamics model (FETCH) using porous media flow within branching elements: A new representation of tree hydrodynamics
Authors: Bohrer Gil , Mourad Hashem , Laursen Tod A. , Drewry Darren , Avissar Roni , Poggi Davide , Oren Ram , Katul Gabriel G.
Published: November 2005

Estimating transpiration and water flow in trees remains a major challenge for quantifying water exchange between the biosphere and the atmosphere. We develop a finite element tree crown hydrodynamics (FETCH) model that uses porous media equations for water flow in an explicit three‐dimensional branching fractal tree‐crown system. It also incorporates a first‐order canopy‐air turbulence closure model to generate the external forcing of the system. We use FETCH to conduct sensitivity analysis of transpirational dynamics to changes in canopy structure via two scaling parameters for branch thickness and conductance. We compare our results with the equivalent parameters of the commonly used resistor and resistor‐capacitor representations of tree hydraulics. We show that the apparent temporal and vertical variability in these parameters strongly depends on structure. We suggest that following empirical calibration and validation, FETCH could be used as a platform for calibrating the “scaling laws” between tree structure and hydrodynamics and for surface parameterization in meteorological and hydrological models.
Large induced magnetic anisotropy in manganese spinel ferrite films
Authors: Zuo Xu , Yang Aria , Yoon Soack-Dae , Christodoulides Joseph A. , Harris Vincent G. , Vittoria Carmine
Published: November 2005

The oxygen pressure dependence of magnetic anisotropy in pulse laser deposited manganese ferrite (MnFe2O4) films was investigated. Magnetic anisotropy fields (Ha) are shown to exceed 5kOe when films were processed at oxygen pressures below 5mTorr. Further, it is shown that the magnetically preferred direction of Ha can be aligned either along the film plane (pO2<8mTorr) or perpendicular to it (pO2>8mTorr). The ability to induce large perpendicular magnetic anisotropy in spinel ferrites allows for new applications (i.e., phase shifters, filters, isolators, and circulators) near or above X-band frequencies to be considered.
Spatially resolved in situ measurements of the transport of organic molecules in a polycrystalline nanoporous membrane
Authors: Oh Weontae , Nair Sankar
Published: November 2005

We report spatially resolved, quantitative, in situ, nondestructive measurements of the transport of organic molecules through a polycrystalline, anisotropic, nanoporous molecular sieve membrane, with micron-scale resolution. A method based on photoacoustic spectroscopy experiments during permeation through a nanoporous membrane, combined with a physical model of photoacoustic signal generation from a heterogeneous membrane, allows extraction of concentration profiles in situ. In particular, we demonstrate the steady-state concentration profiling of the organic molecules p-xylene and n-hexane during their transport through a nanoporous zeolite silicalite membrane. The implications for elucidating structure-property relationships in membrane materials for separations, catalytic, or nanotechnology applications are discussed.
Basal plane dislocation-free epitaxy of silicon carbide
Authors: Zhang Z. , Sudarshan T. S.
Published: November 2005

Molten KOH etching was implemented on SiC substrates before growing epilayers on them. It was found that the creation of basal plane dislocation (BPD) etch pits on the substrates can greatly enhance the conversion of BPDs to threading edge dislocations during epitaxy, and thus low BPD density and BPD-free SiC epilayers are obtained by this method. The reason why BPD etch pits can promote the earlier conversion is discussed. The SiC epilayer growth by this method is very promising in overcoming forward voltage drop degradation of SiC PiN diodes.
Photonic-crystal-fiber pigtail device integrated with lens-duct optics for terahertz radiation coupling
Authors: Diwa Gilbert , Quema Alex , Estacio Elmer , Pobre Romeric , Murakami Hidetoshi , Ono Shingo , Sarukura Nobuhiko
Published: November 2005

An integrated optics called terahertz (THz) pigtail, which is comprised of an emitter, an optically transparent launching media, and a waveguide, is devised and fabricated. The InAs emitter under a 1T magnetic field is coupled to the launching media using silicone grease, an index matching liquid. The launching media, a lens duct made from a polymer based on poly 4-methyl pentene-1 (commonly known as TPX), is designed based on the concept of guiding THz radiation into Teflon photonic crystal fiber (PCF) waveguide by means of total internal reflection. It is found that the constructed THz lens duct is able to channel and couple the THz radiation into the PCF waveguide with a loss of <1dB. The results here show that the idea of using the THz pigtail can be a potential means of effectively directing THz radiation.
Enhanced feedback in organic photonic-crystal lasers
Authors: Harbers Rik , Strasser Patric , Caimi Daniele , Mahrt Rainer F. , Moll Nikolaj , Offrein Bert Jan , Erni Daniel , Bächtold Werner , Scherf Ullrich
Published: November 2005

The mode coupling of organic lasers is greatly enhanced by a photonic crystal that consists of a thin layer of titanium dioxide (TiO2) with a rectangular lattice of holes. The use of TiO2 increases the index contrast in the photonic crystal as well as the confinement in the waveguide, which results in larger feedback given to the lasing modes. This in turn leads to lower thresholds and much smaller devices. Vertically emitting laser devices have been fabricated according to optimized parameters, and the spectral features measured are in excellent agreement with simulations. The devices feature a three to five times lower threshold than devices whose feedback structure is etched directly into the fused silica substrate.
Tunable optical fiber filters with magnetic fluids
Authors: Liao Weijun , Chen Xianfeng , Chen Yuping , Pu Shengli , Xia Yuxing , Li Qu
Published: November 2005

This letter presents a tunable optical fiber filter based on a long-period grating (LPG) by applying a magnetic field on a magnetic nanoparticle fluid. The movement of the magnetic fluid jacket surrounding the fiber cladding controlled by an applied magnetic field results in a variation of the external index between that of the magnetic fluid and air. As a result, the attenuation band of the LPG is shifted. The observed center wavelength shift of the attenuation band is as large as 7 nm, which can cover eight dense wavelength division multiplexing channels (100 GHz channel spacing). Repeatable transport of a magnetic fluid at a switching rate of 1 Hz is demonstrated.
Nilpotent representations of classical quantum groups at roots of unity
Authors: Abe Yuuki , Nakashima Toshiki
Published: November 2005

Properly specializing the parameters in “Schnizer modules,” for types A,B,C, and D, we get its unique primitive vector. Then we show that the module generated by the primitive vector is an irreducible highest weight module of finite dimensional classical quantum groups at roots of unity.
Discrete and continuous cosine transform generalized to Lie groups SU(3) and G(2)
Authors: Patera J. , Zaratsyan A.
Published: November 2005

In the paper we complete the development and description of the four variants of two-dimensional generalization of the cosine transform started in [Patera and Zaratsyan, J. Math Phys. 46, 053514 (2005)]. Each variant is based on a compact semisimple Lie group G of rank 2. Here, the groups are SU(3) and G(2). The cosines are generalized as the corresponding C-functions of the Lie group. A C-function is the contribution to an irreducible character from one orbit of the appropriate Weyl group. An explicit description is provided for expansions of functions given on the fundamental region F of the two compact simple Lie groups into series of C-functions. The fundamental region F is an equilateral triangle for SU(3) and half of such a triangle for G(2). Expansion coefficients are calculated using orthogonality of C-functions on F. Discrete expansions are set up on a grid FM⊂F. The grid is defined group theoretically for all positive integers M. It consists of points in F that represent conjugacy classes of elements of the finite maximal Abelian subgroup of G generated by its elements of order M. The C-functions are orthogonal on such a grid; hence, coefficients of discrete expansions are calculated independently of the continuous expansions. Processing digital data, sampled on triangular lattices, is the motivating application here.
Smoothness of wave functions in thermal equilibrium
Authors: Tumulka Roderich , Zanghì Nino
Published: November 2005

We consider the thermal equilibrium distribution at inverse temperature β, or canonical ensemble, of the wave function Ψ of a quantum system. Since L2 spaces contain more nondifferentiable than differentiable functions, and since the thermal equilibrium distribution is very spread out, one might expect that Ψ has probability zero to be differentiable. However, we show that for relevant Hamiltonians the contrary is the case: with probability 1, Ψ is infinitely often differentiable and even analytic. We also show that with probability 1, Ψ lies in the domain of the Hamiltonian.
Molecular-dynamics simulation of model polymer nanocomposite rheology and comparison with experiment
Authors: Kairn T. , Daivis P. J. , Ivanov I. , Bhattacharya S. N.
Published: November 2005

The shear-rate dependence of viscosity is studied for model polymer melts containing various concentrations of spherical filler particles by molecular-dynamics simulations, and the results are compared with the experimental results for calcium-carbonate-filled polypropylene. Although there are some significant differences in scale between the simulated model polymer composite and the system used in the experiments, some important qualitative similarities in shear behavior are observed. The trends in the steady-state shear viscosities of the simulated polymer-filler system agree with those seen in the experimental results; shear viscosities, zero-shear viscosities, and the rate of shear thinning are all seen to increase with filler content in both the experimental and simulated systems. We observe a significant difference between the filler volume fraction dependence of the zero-shear viscosity of the simulated system and that of the experimental system that can be attributed to a large difference in the ratio of the filler particle radius to the radius of gyration of the polymer molecules. In the simulated system, the filler particles are so small that they only have a weak effect on the viscosity of the composite at low filler volume fraction, but in the experimental system, the viscosity of the composite increases rapidly with increasing filler volume fraction. Our results indicate that there exists a value of the ratio of the filler particle radius to the polymer radius of gyration such that the zero-shear-rate viscosity of the composite becomes approximately independent of the filler particle volume fraction.
Nonadiabatic quantum-classical reaction rates with quantum equilibrium structure
Authors: Kim Hyojoon , Kapral Raymond
Published: November 2005

Time correlation function expressions for quantum reaction-rate coefficients are computed in a quantum-classical limit. This form for the correlation function retains the full quantum equilibrium structure of the system in the spectral density function but approximates the time evolution of the operator by quantum-classical Liouville dynamics. Approximate analytical expressions for the spectral density function, which incorporate quantum effects in the many-body environment and reaction coordinate, are derived. The results of numerical simulations of the reaction rate are presented for a reaction model in which a two-level system is coupled to a bistable oscillator which is, in turn, coupled to a bath of harmonic oscillators. The nonadiabatic quantum-classical dynamics is simulated in terms of an ensemble of surface-hopping trajectories and the effects of the quantum equilibrium structure on the reaction rate are discussed.
Direct calculation of solid-liquid coexistence points of a binary mixture by thermodynamic integration
Authors: Apte Pankaj A. , Kusaka Isamu
Published: November 2005

We present a new thermodynamic integration method that directly connects the liquid and the solid phases of a binary mixture by a reversible path. The states along the path are simulated in the isothermal-isobaric semigrand canonical ensemble, in which temperature, pressure, the total number of particles, and the fugacity fractions of the components are held fixed. The thermodynamic integration yields the chemical-potential difference between the two phases for one of the components and this information is then used to locate the solid-liquid coexistence points. The melting temperatures predicted by our method agree well with those predicted by the Gibbs-Duhem integration for a truncated and shifted Lennard-Jones system with a cutoff radius of 2.5σ.
Stark slowing asymmetric rotors: Weak-field-seeking states and nonadiabatic transitions
Authors: Schwettmann Arne , Franklin Jack , Overstreet K. Richard , Shaffer James P.
Published: November 2005

Stark deceleration is one of the few methods that can be used to slow polyatomic molecules. We present calculations of Stark shift energies, a quantitative analysis of nonadiabatic transition probabilities, and orientational distribution functions applicable to typical Stark slowing conditions for the two small asymmetric rotors nitromethane and acetaldehyde. We show that asymmetric polyatomic molecules are good candidates for Stark slowing.
Ergodic property of Markovian semigroups on standard forms of von Neumann algebras
Authors: Park Yong Moon
Published: November 2005

We give sufficient conditions for ergodicity of the Markovian semigroups associated to Dirichlet forms on standard forms of von Neumann algebras constructed by the method proposed by Park. We apply our result to show that the diffusion type Markovian semigroups for quantum spin systems are ergodic in the region of high temperatures where the uniqueness of the KMS state holds.
On-diagonal singularities of the Green functions for Schrödinger operators
Authors: Brüning Jochen , Geyler Vladimir , Pankrashkin Konstantin
Published: November 2005

We investigate the behavior of the Green functions of Schrödinger operators near the diagonal. The only nontrivial cases, where the on-diagonal singularities are nonzero and do not depend on the spectral parameter, are two and three dimensions. In the case of two dimensions we show that the singularity is independent of both the scalar and the gauge potentials. In dimension three, we obtain conditions for preserving the singularity under perturbations by nonregular potentials. Some examples illustrating dependence of the singularity on general scalar and gauge potentials are presented.
An efficient approach for self-consistent-field energy and energy second derivatives in the atomic-orbital basis
Authors: Liang WanZhen , Zhao Yi , Head-Gordon Martin
Published: November 2005

Based on self-consistent-field (SCF) perturbation theory, we recast the SCF and the coupled-perturbed SCF (CPSCF) equations for time-independent molecular properties into the atomic-orbital basis. The density matrix and the perturbed density matrix are obtained iteratively by solving linear equations. Only matrix multiplications and additions are required, and this approach can exploit sparse matrix multiplications and thereby offer the possibility of evaluating second-order properties in computational effort that scales linearly with system size. Convergence properties are similar to conventional molecular-orbital-based CPSCF procedures, in terms of the number of derivative Fock matrices that must be constructed. We also carefully address the issue of the numerical accuracy of the calculated second derivatives of the energy, in order to specify the minimum precision necessary in the CPSCF procedure. It is found that much looser tolerances for the perturbed density matrices are adequate when using an expression for the second derivatives that is correct through second order in the CPSCF error.
Performance of rf-assisted magnetohydrodynamics power generator
Authors: Murakami Tomoyuki , Okuno Yoshihiro , Yamasaki Hiroyuki
Published: November 2005

The present paper describes the performance of a radio-frequency (rf) electromagnetic-field-assisted magnetohydrodynamics (MHD) electrical power generator and compares it to a multiple-load MHD generator. rf heating which is superimposed on self-excited Joule heating preionizes cesium-seeded helium gas independent of the in situ loading conditions, whereby Hall potential profile is improved and electron temperature is increased. Furthermore, the dynamic stabilization effect cancels ionization instability and homogenizes plasma structure, which is the most important superiority of the rf-power assistance over a conventional plasma actuation by loading-rate control. The power-generating performance is significantly improved with the aid of the rf power under wide seeding and loading conditions, where the electrical load-matching characteristic is slightly changed.
Large-D expansion from variational perturbation theory
Authors: Brandt Sebastian F. , Pelster Axel
Published: November 2005

We derive recursively the perturbation series for the ground-state energy of the D-dimensional anharmonic oscillator and resum it using variational perturbation theory (VPT). From the exponentially fast converging approximants, we extract the coefficients of the large-D expansion to higher orders. The calculation effort is much smaller than in the standard field-theoretic approach based on the Hubbard-Stratonovich transformation.
Density and chain conformation profiles of square-well chains confined in a slit by density-functional theory
Authors: Ye Zhencheng , Cai Jun , Liu Honglai , Hu Ying
Published: November 2005

Density and chain conformation profiles of square-well chains between two parallel walls were studied by using density-functional theory. The free energy of square-well chains is separated into two contributions: the hard-sphere repulsion and the attraction. The Heaviside function is used as the weighting function for both of the two parts. The equation of state of Hu et al. is used to calculate the excess free energy of the repulsive part. The equation of state of statistical associating fluid theory for chain molecules with attractive potentials of variable range [A. Gil-Villegas et al. J. Chem. Phys. 106, 4168 (1997)] is used to calculate the excess free energy of the attractive part. Because the wall is inaccessible to a mass center of a longer chain, there exists a sharp fall in the distribution of end-to-end distance near the wall as the chain length increases. When the average density of the system is not too low, the prediction of this work is in good agreement with computer simulation results for the density profiles and the chain conformation over a wide range of chain length, temperature, and attraction strength of the walls. However, when the average density and the temperature are very low, the prediction deviates to a certain degree from the computer simulation results for molecules with long chain length. A more accurate functional approximation is needed.
On the structure of the FeF3 molecule
Authors: Hargittai Magdolna
Published: November 2005

With increasing computational possibilities the question of critical comparison of computed results with experimental data gains importance. Differences in the physical meaning of parameters determined by different techniques and the uncertainties of both experimental and computed parameters need to be scrutinized in such comparisons. In view of a recent computational study [V. G. Solomonik, J. F. Stanton, and J. E. Boggs, J. Chem. Phys.122, 094322 (2005)] of the FeF3 molecule, its earlier electron-diffraction data were reanalyzed and compared with the theoretical results. Based on this reanalysis we can rule out the suggested source of the slight discrepancy between the computed and measured bond lengths.
Composition, structure, and electrical characteristics of HfO2 gate dielectrics grown using the remote- and direct-plasma atomic layer deposition methods
Authors: Kim Jinwoo , Kim Seokhoon , Kang Hyunseok , Choi Jihoon , Jeon Hyeongtag , Cho Mannho , Chung Kwunbum , Back Sungkwun , Yoo Kyungdong , Bae Choelhwyi
Published: November 2005

Hafnium oxide thin films were deposited using both the remote-plasma atomic layer deposition (RPALD) and direct-plasma atomic layer deposition (DPALD) methods. Metal-oxide semiconductor (MOS) capacitors and transistors were fabricated with HfO2 gate dielectric to examine their electrical characteristics. The as-deposited RPALD HfO2 layer exhibited an amorphous structure, while the DPALD HfO2 layer exhibited a polycrystalline structure. Medium-energy ion scattering measurement data indicate that the interfacial layer consisted of interfacial SiO2−x and silicate layers. This suggests that the change in stoichiometry with depth could be related to the energetic plasma beam used in the plasma ALD process, resulting in damage to the Si surface and an interaction between Hf and SiO2−x. The as-deposited RPALD HfO2 films had better interfacial layer characteristics, such as an effective fixed oxide charge density (Qf,eff) and interfacial roughness than the DPALD HfO2 films did. A MOS capacitor fabricated using the RPALD method exhibited an equivalent oxide thickness (EOT) of 1.8nm with a Qf,eff=−4.2×1011q∕cm2 (where q is the elementary charge, 1.6022×10−19C), whereas a MOS capacitor fabricated using the DPALD method had an EOT=2.0nm and a Qf,eff=−1.2×1013q∕cm2. At a power=0.6MV∕cm, the RPALD n-type metal-oxide semiconductor field-effect transistor (nMOSFET) showed μeff=168cm2∕Vs, which was 50% greater than the value of the DPALD nMOSFET (μeff=111cm2∕Vs). In the region where Vg-Vt=2.0V, the RPALD MOSFET drain current was about 30% higher than the DPALD MOSFET drain current. These improvements are believed to be due to the lower effective fixed charge density, and they minimize problems arising from plasma charging damage.
Manipulating ultrashort intense laser pulses by plasma Bragg gratings
Authors: Wu Hui-Chun , Sheng Zheng-Ming , Zhang Qiu-Ju , Cang Yu , Zhang Jie
Published: November 2005

Propagation of ultrashort intense laser pulses in a plasma Bragg grating induced by two intersecting laser pulses is studied. Such a plasma grating exhibits an ultrawide photonic band gap, near which the strong dispersion appears. It is found that the grating dispersion dominates the dispersion of the background plasma by several orders of magnitude. Particle-in-cell (PIC) simulations show the light speed reduction, pulse stretching, chirped-pulse compression, and fast compression of Bragg grating solitons in the plasma grating. The nonlinear coupled-mode theory agrees well with the PIC results. Since the plasma grating can support an ultrahigh damage threshold, it can be a novel photonic device to manipulate extremely intense femtosecond laser pulses.
Blueshifted Raman scattering and its correlation with the [110] growth direction in gallium oxide nanowires
Authors: Rao R. , Rao A. M. , Xu B. , Dong J. , Sharma S. , Sunkara M. K.
Published: November 2005

The Raman spectrum of gallium oxide (β-Ga2O3) nanowires with [001] growth direction is identical to that of the bulk Ga2O3 [Y. C. Choi et al. Adv. Mater. 12, 746 (2000)] while that of β-Ga2O3 nanowires with [401¯] growth direction is redshifted by 4–23cm−1 [Y. H. Gao et al. Appl. Phys. Lett. 81, 2267 (2002)]. Here we report the Raman and Fourier transform infrared spectra of β-Ga2O3 nanowires with [110] growth direction which is blueshifted relative to the bulk spectra by ∼10–40cm−1. Based on a first principles calculation of the strain dependence of Raman mode frequencies in bulk β-Ga2O3, we correlate the observed frequency shifts to growth-direction-induced internal strains in the nanowires.
Polarization-modulated magnetic soft-x-ray transmission microscopy
Authors: Kang Bo-Sun , Kim Dong-Hyun , Anderson Erik , Fischer Peter , Cho Gyuseong
Published: November 2005

An adjustable aperture element has been integrated into the full-field soft-x-ray microscope at the Advanced Light Source to select either the right or left elliptically polarized x rays emitted at an inclined angle from a bending magnet. Magnetic contrast recorded at the Fe L3 edge in a 59-nm-thin Gd25Fe75 layer can be modulated and scales with the degree of circular polarization in agreement with theoretical calculations. Nonmagnetic background contributions can be reduced and magnetic contrast is enhanced by comparing two images taken with opposite circular polarization. The fast modulation speed of this technique allows for lock-in recording schemes in high-resolution magnetic soft-x-ray microscopy.
Complete spin polarization of electrons in semiconductor layers and quantum dots
Authors: Osipov V. V. , Petukhov A. G. , Smelyanskiy V. N.
Published: November 2005

We demonstrate that nonequilibrium electrons in thin nonmagnetic semiconductor layers or quantum dots can be fully spin polarized by means of simultaneous electrical spin injection and extraction. The complete spin polarization is achieved if the thin layers or quantum dots are placed between two ferromagnetic metal contacts with moderate spin injection coefficients and antiparallel magnetizations. The sign of the spin polarization is determined by the direction of the current. Applications of this effect in spintronics and quantum information processing are discussed.
Roles of Au and Ag nanoparticles in efficiency enhancement of poly(3-octylthiophene)/C60 bulk heterojunction photovoltaic devices
Authors: Kim Kyungkon , Carroll David L.
Published: November 2005

We obtained improved poly(3-octylthiophene) (P3OT)/C60 bulk heterojunction photovoltaic devices by doping with stable and highly electrically conductive Au and Ag nanoparticles. Doped devices showed 50–70% improved efficiency, with the Ag nanoparticles exhibiting the greatest increase in efficiency. We suggest that a dominate mechanism for the efficiency enhancement of doped photovoltaic device is the improved electrical conductivity through the introduction of “dopant” levels within the band gap of the P3OT.
Statistical properties of an ensemble of vortices interacting with a turbulent field
Authors: Spineanu F. , Vlad M.
Published: November 2005

An analytical formalism is developed with the purpose to determine the statistical properties of a system consisting of an ensemble of vortices with random position in plane interacting with a turbulent field. The generating functional is calculated by path-integral methods. The function space is the statistical ensemble composed of two parts, the first one representing the vortices influenced by the turbulence and the second one the turbulent field scattered by the randomly placed vortices.
Size effect on the static and dynamic magnetic properties of W-type barium ferrite composites: From microparticles to nanoparticles
Authors: Li Z. W. , Lin G. Q. , Chen Linfeng , Wu Y. P. , Ong C. K.
Published: November 2005

The static, dynamic, and attenuation properties of BaCoZnFe16O27 barium ferrites and their composites have been studied using microparticles and nanoparticles. The results showed that BaCoZnFe16O27 microparticles have larger saturation magnetization Ms and smaller coercivity Hc than do nanoparticles. The composites with microparticles have significantly larger real and imaginary permeabilities μ0′ and μmax″ due to their multidomain structures, but low resonance frequency fR, as compared with the composites with nanoparticles. The predicted dependence of the reflection loss on frequency showed that the composites with microparticles have much better attenuation for use as electromagnetic (EM) materials with low reflectivity and broad bandwidth at microwave frequency.
Drift wavs in helically symmetric stellarators
Authors: Rafiq T. , Hegna C. C.
Published: November 2005

The local linear stability of electron drift waves and ion temperature gradient modes (ITG) is investigated in a quasihelically symmetric (QHS) stellarator and a conventional asymmetric (Mirror) stellarator. The geometric details of the different equilibria are emphasized. Eigenvalue equations for the models are derived using the ballooning mode formalism and solved numerically using a standard shooting technique in a fully three-dimensional stellarator configuration. While the eigenfunctions have a similar shape in both magnetic geometries, they are slightly more localized along the field line in the QHS case. The most unstable electron drift modes are strongly localized at the symmetry points (where stellarator symmetry is present) and in the regions where normal curvature is unfavorable and magnitude of the local magnetic shear and magnetic field is minimum. The presence of a large positive local magnetic shear in the bad curvature region is found to be destabilizing. Electron drift modes are found to be more affected by the normal curvature than by the geodesic curvature. The threshold of stability of the ITG modes in terms of ηi is found to be 2∕3 in this fluid model consistent with the smallest threshold for toroidal geometry with adiabatic electrons. Optimization to favorable drift wave stability has small field line curvature, short connection lengths, the proper combination of geodesic curvature and local magnetic shear, large values of local magnetic shear, and the compression of flux surfaces in the unfavorable curvature region.
Spectroscopic properties of Er3+ transitions in SrWO4 crystal
Authors: Jia Guohua , Tu Chaoyang , Li Jianfu , Lu Xiuai , You Zhenyu , Zhu Zhaojie , Wu Baichang
Published: November 2005

Er 3 + : Sr W O 4 crystal of good optical quality was grown by Czochralski technique method. Polarized absorption and emission spectra of Er3+ ions in SrWO4 crystal have been investigated. The Judd-Ofelt (JO) theory was applied to the polarized absorption spectra of Er3+:SrWO4 to obtain the three JO parameters: Ω2=7.41×10−20cm2, Ω4=0.25×10−20cm2, and Ω6=1.71×10−20cm2. The radiative probabilities, radiative lifetimes, and branch ratios of Er3+:SrWO4 were calculated. The stimulated emission cross sections of the potential laser transitions at 1535nm were determined using the reciprocity method and the Fuchtbauer-Ladenberg formula, and the potential laser gain versus wavelength for the I13∕24→I15∕24 transition has been estimated and discussed.
Ba
3
B
P
3
O
12
:
Eu
2
+
—A potential scintillation material
Authors: Duan Cheng-Jun , Wu Xue-Yan , Li Wei-Feng , Chen Hao-Hong , Feng Xi-Qi , Zhao Jing-Tai
Published: November 2005

The sample of Eu2+-doped Ba3BP3O12 was prepared by a solid-state reaction at high temperature and was showed to be an excellent scintillator when excited with hard x ray s. The phosphor showed a broad emission band in the wavelength range of 400–650 nm and a comparable light yield with that of Bi4Ge3O12 powders. Its room temperature fluorescence decay curve exhibited a two-exponent shape with decay time of 20.0±1.4ns (98.97%) and 522±9ns (1.03%), respectively. Considering the suitable emission wavelength range, large light yield, short decay time, moderate density, moderately low melting point, and nonhygroscopic property of Ba3BP3O12:Eu2+, one has reason to assume that this compound might find an application as a new scintillation material.
InGaAs quantum wells on wafer-bonded InP∕GaAs substrates
Authors: Hayashi S. , Sandhu R. , Wojtowicz M. , Chen G. , Hicks R. , Goorsky M. S.
Published: November 2005

Wafer bonding and hydrogen implantation exfoliation techniques have been used to fabricate a thin InP template layer on GaAs with intermediate silicon nitride bonding layers. This template layer was used to directly compare subsequent metal organic vapor phase epitaxial growth of InGaAs∕InAlAs quantum-well structures on these wafer-bonded templates to growth on a standard InP substrate. Chemical mechanical polishing of the bonded structure and companion InP substrates was assessed. No effects from the coefficient of thermal mismatch are detected up to the growth temperature, and compositionally equivalent structures are grown on the wafer-bonded InP template and the bare InP substrate. However, after growth dislocation, loops can be identified in the InP template layer due to the ion implantation step. These defects incur a slight mosaic tilt but do not yield any crystalline defects in the epitaxial structure. Low-temperature photoluminescence measurements of the InGaAs grown on the template structure and the InP substrate exhibit near-band-edge luminescence on the same order; this indicates that ion implantation and exfoliation is a viable technique for the integration of III-V materials.
Interband optical-absorption spectra of a finite quantum dot superlattice in a cylindrical nanowire
Authors: Zhang T. Y. , Zhao W. , Cao J. C. , Qasim G.
Published: November 2005

Theoretical calculations of the interband optical-absorption spectra of a finite quantum dot superlattice in a wire (QDSLW) in the presence of a homogeneous dc or terahertz field are presented. The QDSLW has miniband structures of the electron states as general quantum well superlattices do and shows the two features common to one-dimensional structures, viz., strong excitonic absorption below and weak absorption above the band edge. The applied dc field causes a redshift of the location and a decrease in height of the main excitonic peak and leads to an increase in height of the minor peak next to the main peak. In the presence of a terahertz field, the Autler-Townes splitting of the main excitonic peak and the emergence of one-photon and two-photon gain peaks on the spectra are demonstrated.
Partial encapsulation of Pd particles by reduced ceria-zirconia
Authors: Sun H. P. , Pan X. P. , Graham G. W. , Jen H.-W. , McCabe R. W. , Thevuthasan S. , Peden C. H. F.
Published: November 2005

Direct observation of metal-oxide interfaces with atomic resolution can be achieved by cross-sectional high-resolution transmission electron microscopy (HRTEM). Using this approach to study the response of a model, single-crystal thin film automotive exhaust-gas catalyst, Pd particles supported on the (111) ceria-zirconia (CZO) surface, to a redox cycle, we have found two distinct processes for the partial encapsulation of the Pd particles by the reduced CZO surface that depend on their relative crystallographic orientations. In the case of the preferred orientation found for Pd particles on CZO, Pd(111)[110]∕∕CZO(111)[110], a flat and sharp metal/oxide interface was maintained upon reduction, while ceria-zirconia from the adjacent surface tended to accumulate on and around the Pd particle. In rare cases, Pd particles with other orientations tended to sink into the oxide support upon reduction. Possible mechanisms for these encapsulation processes are proposed.
Gap soliton memory in a resonant photonic crystal
Authors: Mel’nikov Igor V. , Aitchison J. Stewart
Published: November 2005

We consider the interaction of two coherent optical pulses in a one-dimensional, resonant photonic crystal which includes a defect produced by a coherent pump. Using numerical methods, the defect is shown to provide a selective reversible trap for a gap soliton mediated by the long-range coupling. This opens up new opportunities for signal transmission control, light localization and a simple one bit optical memory.
Room-temperature diluted magnetic semiconductors p-(Ga,Ni)N
Authors: Huang Rong-Tan , Hsu Chen-Feng , Kai Ji-Jung , Chen Fu-Rong , Chin Tsung-Shune
Published: November 2005

High concentration (5at.%) Ni was incorporated into a chemical vapor deposition-grown GaN film by using a thin protecting Ni layer on top of the GaN film during ion implantation. After etching off the protecting layer, subsequent annealing up to 800°C under flowing N2 resulted in a p-type GaN with apparent ferromagnetic behavior up to ∼320K. In addition, the ferromagnetic behavior became more manifest with increasing annealing temperature that increases hole concentration. No presence of any other second phases nor clusters in the Ni-implanted region was identifiable, at least to the 0.2nm point-to-point resolution of high resolution transmission electron microscopy. This novel indirect implantation process that being easy to implement appears promising for attaining room-temperature diluted magnetic semiconductors which are applicable to magnetotransport, magneto-optical and spintronics devices, among others.
Multilevel charge storage in silicon nanocrystal multilayers
Authors: Lu T. Z. , Alexe M. , Scholz R. , Talelaev V. , Zacharias M.
Published: November 2005

The feasibility of multilevel charges in layered arranged Si nanocrystals in a metal-oxide-semiconductor structure is investigated. The structures are created with up to three layers of size-controlled Si nanocrystals having a size of around 3.9nm (±0.4nm). Using a suitable write bias, the apparent states of charge storage are evident in the series of capacitance-voltage (C-V) curves. These memory effects are due to the successive charging of a varied number of Si nanocrystal layers in the floating gate. The widths of the memory windows were estimated by a modified charge equation for the multilayered nanocrystal samples, i.e., each memory window is quantified by charging a different number of Si nanocrystal layers. The static current-voltage (I-V) curve can be fitted by Fowler–Nordheim tunneling which represents the dominant tunneling behavior through the Si nanocrystal multilayers separated by thin silicon dioxide. Time retention investigations demonstrate the stability of the programming states. The results demonstrate the feasibility for using such a stack structure in multibit∕cell nonvolatile memories. In addition, due to the fact that more then one Si nanocrystal layer can be arranged and completely charged, the charge density can be easily increased to 3×1012cm−2 as required for device applications.
32 nm digitally tunable laser diode with a 0.58 nm wavelength grid using a vertically integrated Mach-Zehnder interferometer
Authors: Jacke T. , Todt R. , Meyer R. , Amann M.-C.
Published: November 2005

A forward-coupling laser is presented that uses a monolithically integrated Mach-Zehnder interferometer (MZI) inside the laser cavity for discrete wide wavelength tuning. The electronically tunable MZI filter selects one individual mode out of the discrete Fabry-Pérot spectrum. The emission wavelength is thereby controlled by one current only. Internal reflections are a key issue for all forward coupling lasers as they limit the number of accessible modes. Employing an appropriate optical resonator design, the present laser structure overcomes this drawback and demonstrates full and regular wavelength accessibility across the free spectral range of 32 nm.
GaN strain reduction by growth on compliant GaN-rich GaNP
Authors: Tsuda Yuhzoh , Mouri Hirokazu , Yuasa Takayuki , Taneya Mototaka , Hasuike Noriyuki , Harima Hiroshi
Published: November 2005

For investigating the strain reduction in a GaN layer on a sapphire substrate, double heterostructures of the upper GaN layer/GaN-rich GaNP interlayer as a compliant buffer layer/lower GaN layer were grown by metalorganic chemical vapor deposition. It has been confirmed that wafer bending of the double heterostructure reduces on increasing the mole fraction of phosphorus incorporated into GaNP. Further, the results of x-ray diffraction and Raman scattering analysis revealed that strain relaxation occurs abruptly on the GaNP interlayer and that nonuniform distortion of the upper GaN layer scarcely distributes within the thickness.
Model for the easy-axis alignment of chemically synthesized L1 FePt nanoparticles
Authors: Harrell J. W. , Kang S. , Jia Z. , Nikles D. E. , Chantrell R. , Satoh A.
Published: November 2005

Analytical and computational models of the easy-axis orientation process in magnetic nanoparticle systems are investigated and compared with experimental data. The computational model gives good agreement with experiment on the assumption of a reduced anisotropy constant, which is ascribed to the existence of aggregation. It is found that the degree of orientation depends on the parameter KV∕kT, with K the anisotropy constant and V the nanoparticle volume, which governs the coupling of the easy axis to the magnetic moment. A simple and useful analytical expression is derived, from which the primary conditions for good alignment are determined.
Twelve-spin “Schrödinger cat”
Authors: Lee Jae-Seung , Khitrin A. K.
Published: November 2005

Pseudopure “cat” state, a superposition of quantum states with all spins up and all spins down, is experimentally demonstrated for a system of 12 dipolar-coupled nuclear spins of fully C13-labeled benzene molecule oriented in a liquid-crystalline matrix.
Electromagnetic wave transmission through subwavelength metallic meshes sandwiched between split rings
Authors: Hou Bo , Wen He , Leng Yang , Wen Weijia
Published: November 2005

We report the extraordinary enhanced transmission of microwaves through a subwavelength metallic mesh sandwiched between two identical split-ring arrays. Such split-rings/metallic mesh/split-rings structure demonstrates some unique electromagnetic (EM) characteristics. It is found that the transmittances of EM wave are significantly enhanced at some frequencies near the stop bands of the split rings. The theoretical simulation indicates that two different physical mechanisms dominate those transmissions: negative refractive index effect and electromagnetic wave tunneling when EM wave penetrates through negative permittivity media sandwiched between two high permittivity slabs.
A Study of Raman Excitation Profiles for Soluble cis-Polyacetylene
Authors: Maisuradze Gia G.
Published: November 2005

A quantitative analysis of the Raman excitation profiles (REPs) for soluble cis-polyacetylene, cis-(CH)x, in toluene observed by Sassi et al. was undertaken. This analysis took two approaches, namely improved vibronic interaction theory and resonance-Raman scattering theory for solute molecules coupled homogeneously and strongly to a continuum of environmental nuclear modes. For the former theory, a ‘static’ approach, the effect of inhomogeneous environmental broadening of REPs of a solute molecule due to the solvent–molecule interactions was investigated as well as the role of diagonal and non-diagonal terms of the scattering matrix. For the latter theory, a ‘dynamic’ approach which rests on vibrational density matrices including both molecular and solvent modes in the ground and excited electronic states, the solvent effects are regarded as dynamic and homogeneous. The values of the equilibrium coordinate shifts for the Raman-active mode and the homogeneous and inhomogeneous broadening parameters derived from the REPs by both approaches are compared with theoretical results of previous studies of soluble and of solid cis- and trans-(CH)x.