bluuebunny/crossref_metadata_2025 huggingface dataset:
Existence results for nonlinear periodic boundary-value problems
Authors: Graef John R. , Kong Lingju
Published: February 2009

We study a class of second-order nonlinear differential equations on a finite interval with periodic boundary conditions. The nonlinearity in the equations can take negative values and may be unbounded from below. Criteria are established for the existence of non-trivial solutions, positive solutions and negative solutions of the problems under consideration. Applications of our results to related eigenvalue problems are also discussed. Examples are included to illustrate some of the results. Our analysis relies mainly on topological degree theory.
Chemical reactions in reactive powder metal mixtures during shock compression
Authors: Reding D. J. , Hanagud S.
Published: February 2009

A heterogeneous chemical reaction model is proposed and used to describe shock-induced chemical reactions that occur in reactive granular mixtures during shock compression. The proposed heterogeneous model is intended for application in mesoscale simulations at locations where reactant particles are in contact. Previous studies have employed homogeneous reaction rate models with Arrhenius type kinetics, in which the material transport mechanism is not spatially dependent. In contrast, the spatially heterogeneous model explicitly describes material transport at the interface between the reactants. A transport mechanism permits reactants to flow through the product that is formed between the reactant grains during reaction. Diffusion mechanisms alone are too slow to describe shock-induced chemical reactions. Therefore, the stress contribution is included in the activation energy to affect both the diffusion rate and the surface reaction kinetics. The model is demonstrated for the 2Al+Fe2O3 system, in which the pressure and temperature initial conditions are obtained from a mesoscale simulation. Calculated temperatures are compared with previously reported experimentally measured shock temperatures, showing the capability of the proposed model to describe this shock-induced chemical reaction.
Enhanced magneto-Coulomb effect in asymmetric ferromagnetic single electron transistors
Authors: Jalil M. B. A. , Tan S. G. , Ma M. J.
Published: February 2009

We investigate the magneto-Coulomb (MC) effect in a ferromagnetic single electron tunneling transistor (FM-SETT), with asymmetric junction resistances and FM electrodes. The MC effect enables the conductance of the FM-SETT by an applied magnetic B-field in addition to the usual gate-bias modulation. Under optimal biasing of the asymmetric FM-SETT near the sawtooth edge of its gate oscillation, the sensitivity γB=dI∕dB can be enhanced by a factor α, where α=R1∕R2 denotes the junction resistance asymmetry. The enhanced B-field modulation is, however, susceptible to thermal smearing effects. Finally, an asymmetry in the magnetic properties of the FM leads results in a complex magnetoconductance response, with distinct conductance states.
Spinwave propagation in lossless cylindrical magnonic waveguides
Authors: Xi Haiwen , Wang Xiaobin , Zheng Yuankai , Ryan Pat J.
Published: February 2009

Spinwave propagation in clad cylindrical magnonic waveguides is investigated under linear approximation. With the assumption of no magnetic damping, characteristic equation to determine the bound spinwave modes has been obtained based on the structural and magnetic properties of the waveguides. The study is then applied to homogenous magnetic nanowires with no cladding. Spinwave characteristics and properties, such as the dispersion relationship and group velocity, can be described analytically.
Magnetic field dependence of rectification radio frequency current flowing through a single layered ferromagnetic wire
Authors: Yamaguchi A. , Motoi K. , Miyajima H. , Nakatani Y.
Published: February 2009

The spin and electric charge of electrons in nanoscale artificial magnetic systems exhibit peculiar behaviors especially in a radio-frequency (rf) region via spin-wave excitations. One of the interesting effects is the rectification of the rf current in nanomagnets. We present a detailed experimental magnetic field dependence of the rectifying spectrum induced by a rf current flowing through the ferromagnetic wire and discuss in terms of the quantized spin-wave modes excited by the microwave injection.
Verification of percolating nature of ferromagnetic-paramagnetic transition in bulk manganite La0.75Ca0.25MnO3 with current-pulse conductivity study
Authors: Belevtsev B. I. , Krasovitsky V. B.
Published: February 2009

A study of nonlinear effects in conductivity of a bulk sample of La0.75Ca0.25MnO3 grown by the floating-zone technique is presented. It was found in previous studies that paramagnetic-ferromagnetic (PM-FM) transition in this compound is of first order and has been suggested to have a percolating character. The aim of this study has been to reveal such effects. For this purpose, the conductivity in the PM-FM transition region has been studied by applying single ramp millisecond pulses (“sawtooth” shape) of current that linearly increase with time. Driving current pulse J(t) and its response voltage U(t) were recorded for different temperatures and magnitudes of current pulses. It was found that U(t) behavior in the temperature range of the PM-FM transition has features that must be attributed to a mixed phase state of the manganite sample near the ferromagnetic-paramagnetic transition and percolation character of this transition.
Exotic vortex effect on the alternating order around impurities in two-dimensional antiferromagnets
Authors: Cuccoli Alessandro , Vaia Ruggero
Published: February 2009

It is well known that a magnetic field applied onto a Heisenberg antiferromagnet acts as an effective easy-plane anisotropy leading, in the case of two dimensions, to a Berezinskii–Kosterlitz–Thouless transition at a field-dependent temperature. If a nonmagnetic impurity is introduced in the lattice, an island of finite staggered magnetization develops, for which a recently proposed effective model predicts a universal shape that is almost independent of temperature and whose amplitude decays exponentially with the distance from the impurity. This picture was confirmed for the two-dimensional spin-12 case by quantum Monte Carlo simulations, whose outcomes, however, display also a counterintuitive effect: as the temperature approaches the critical value, the field-induced staggered magnetization surprisingly increases its range, which reaches a maximum before definitely dropping to zero in the disordered phase. In this paper an explanation of such a strange behavior is given as the result of vortex excitation.
Cooperative near-infrared quantum cutting in Tb3+, Yb3+ codoped polyborates La0.99−xYbxBaB9O16:Tb0.01
Authors: Wang Yuhua , Xie Lechun , Zhang Huijuan
Published: February 2009

The experimental identification of cooperative energy transfer from Tb3+ to Yb3+ has been presented by the photoluminescence properties of La0.99−xYbxBaB9O16:Tb0.01 and the mean decay lifetime curves of the D54 emission of Tb3+. Near-infrared quantum cutting involving the F25/2→F27/2 emission of Yb3+ (about 960–1040 nm) is achieved upon the excitation of D54 energy level of Tb3+ at 481 nm. Yb3+ concentration dependent quantum efficiency is calculated and the maximum efficiency approaches 152.5%. Because the energy of Yb3+ transition (about 960–1040 nm) is matched well with the band gap of crystalline Si, the phosphors could be applied potentially in silicon-based solar cells.
Magnetization reversal in exchange biased antidot arrays
Authors: Tripathy D. , Adeyeye A. O.
Published: February 2009

A systematic study of the exchange bias effects in arrays of Cu (10nm)∕NiFe (30nm)∕IrMn (tIrMn)∕Cu (2nm) nanoscale antidot arrays prepared by deep ultraviolet lithography is presented. The magnetic properties of the antidot arrays are compared to that of a continuous film of identical composition. We observed that the antidots demonstrate higher coercivity and exchange bias fields. This enhancement in the exchange bias field, and the relative evolution of exchange bias field with the IrMn layer thickness tIrMn for both the antidot array and the continuous film, is attributed to the physical limitations imposed on the IrMn domain size by the reduced lateral dimensions of the antidots. Magnetoresistance measurements further corroborate the results obtained from the hysteresis loops.
Effect of polymer processing on spin magnetoresistance in organic structures
Authors: Dhandapani D. , Rao A. , Morley N. A. , Das A. , Grell M. , Gibbs M. R. J.
Published: February 2009

Results from studies of spin dependent transport in the hole transporting organic semiconductor regioregular poly 3-hexylthiophene (RR-P3HT) are reported. The organic spin valves were grown on glass, with the structure Fe50Co50∕RR-P3HT∕Ni81Fe19. RR-P3HT was cast from five different solvents with different boiling points; 1,2,4 trichlorobenzene, toluene, xylene, chloroform, and chlorobenzene. Magnetoresistance measurements were carried out at room temperature. Spin dependent transport was only observed when toluene, xylene, and annealed 1,2,4 trichlorobenzene were used as solvents. Results are interpreted on the basis of differences in the RR-P3HT morphology induced by different solvents under fixed processing conditions.
Investigation of slanted and V-shaped domain walls in MnAs films
Authors: Engel-Herbert R. , Hesjedal T.
Published: February 2009

The magnetic domain structure of MnAs stripes exhibits a number of domain transitions that are inclined with respect to the system’s easy axis. Among them are laterally confined slanted and V-shaped domain walls, as well as extended zigzag shaped transitions that are running along the stripe axis. The nature of these unusual domain transitions was investigated with the help of micromagnetic simulations and compared with experimental magnetic force microscopy (MFM) images. All types of inclined walls result from the underlying three-dimensional domain structure of the wires. In the domain transition region, the underlying flux closure pattern gets asymmetrically distorted in the cross-sectional wire plane as the system changes its domain types. The results of the simulations are in excellent qualitative agreement with the stray field patterns observed with MFM.
Interparticle interactions in coupled Au–Fe3O4 nanoparticles
Authors: Frey N. A. , Phan M. H. , Srikanth H. , Srinath S. , Wang C. , Sun S.
Published: February 2009

Complex ac susceptibility measurements are reported on composite Au–Fe3O4 nanoparticles of two different configurations—the so-called “dumbbell” and “flower” configurations. The frequency-dependent blocking temperature was fitted to two separate models in an attempt to understand the relaxation and the role of interactions present in the nanoparticle arrays. While the Néel–Arrhenius model failed to accurately describe the blocking behavior of both types of particles, the Vogel–Fulcher model was shown to fit the dumbbell particles indicating the importance of weak interparticle interactions in this system. The flower nanoparticles, however, failed to yield physical fit parameters for both models, indicating that the interactions present in these particles are not solely dipolar but likely associated with competing intraparticle interactions. Radio-frequency transverse susceptibility measurements also confirm these features.
Influence of the silicon nanocrystal size on the 1.54 μm luminescence of Er-doped SiO/SiO2 multilayers
Authors: Rinnert H. , Adeola G. Wora , Vergnat M.
Published: February 2009

The influence of the silicon nanocrystal (Si-nc) size on the erbium luminescence at 1.54 μm was studied in Er-doped silicon oxide thin films containing Si-nc. Er-doped and undoped SiO/SiO2 multilayers were prepared to control the Si-nc size. The results showed that the Er luminescence was due to an energy transfer process between Si-nc and Er ions. The proportion of Si-nc which couples with Er was dependent on the Si-nc size and the energy transfer process is maximal for a size equal to 4 nm. This preferentially coupling effect is interpreted by the existence of an optimum overlap between the Si-nc emission spectrum and the I49/2 multiplet of Er ions.
Missing row and surface relaxation induced ferromagnetic phase stabilization of Fe(x)Pt(1−x) (110) surface alloy: First-principles calculation
Authors: Kim Miyoung , Kim Hanchul
Published: February 2009

The structural properties and magnetic phase stability of Fe(x)Pt(1−x) alloys in L12 crystal structure in bulk as well as thin film on Pt (110) substrate are studied by means of the highly precise full-potential linearized augmented plane-wave method within generalized gradient approximation. The antiferromagnetic (AFM) phase is found to be preferred over the ferromagnetic (FM) phase for FePt3 bulk alloy in agreement with experiment while FexPt(1−x), where x=0.25 and 0.5 with a film thickness smaller than 0.5nm, favor the FM phase. The total energy calculation assuming pseudomorphic strain reveals that the AFM preference for bulk is maintained in wide range of tetragonal distortion up to the value of 22% reached by the surface relaxation of thin film, implying that the magnetic configuration change from AFM to FM at the thin film surface is originated in rather complex surface effects beyond the structural relaxation. The FM preference of thin film is predicted to fast suppress as the film thickness increases and finally the AFM preference is recovered for the film thickness of five atomic layers.
Multihit two-dimensional charged-particle imaging system with real-time image processing at 1000 frames/s
Authors: Horio Takuya , Suzuki Toshinori
Published: February 2009

A high-speed imaging system developed for two-dimensional counting of charged particles is presented. Microchannel plates coupled with a phosphor screen of a short emission lifetime (<1 μs) are used to visualize the two-dimensional positions of charged-particle impacts, and the image on the phosphor screen is captured with a 1 kHz complementary metal oxide semiconductor (CMOS) image sensor (512×512 pixels). A multistage image intensifier consisting of the first and second generation devices was used to compensate for the low sensitivity of CMOS. The centers of gravity (COG) of individual light spots in each image frame are calculated in real time by a field programmable gate array circuit. The performance of this system is tested by time-resolved photoelectron imaging (TR-PEI) of NO using (1+1′) resonance enhanced multiphoton ionization via the A Σ2+ state with a femtosecond laser operated at 1 kHz. The new system enabled COG detection for more than ten particles in each frame at 1 kHz and achieved an extremely high degree of accuracy in the measurement of photoelectron angular distributions in TR-PEI.
Effects of thermal annealing on the emission properties of type-II InAs/GaAsSb quantum dots
Authors: Liao Yu-An , Hsu Wei-Ting , Chiu Pei-Chin , Chyi Jen-Inn , Chang Wen-Hao
Published: February 2009

We report the effects of thermal annealing on the emission properties of type-II InAs quantum dots (QDs) covered by a thin GaAs1−xSbx layer. Apart from large blueshifts and a pronounced narrowing of the QD emission peak, the annealing induced alloy intermixing also leads to enhanced radiative recombination rates and reduced localized states in the GaAsSb layer. Evidences of the evolution from type-II to type-I band alignments are obtained from time-resolved and power-dependent photoluminescence measurements. We demonstrate that postgrowth thermal annealing can be used to tailor the band alignment, the wave function overlaps, and hence the recombination dynamics in the InAs/GaAsSb type-II QDs.
Effects of crystalline and elastic anisotropies on coercivity of longitudinally oriented CoCrPt thin films grown on CrW underlayer
Authors: Liu Xi , Li Zhenghua , Shi Wenkui , Li Songtian , Wei Fulin , Wei Dan , Liu Xiaoxi
Published: February 2009

The longitudinal CoCrPt thin films have been prepared on CrW underlayer at room temperature. The dependence of CoCrPt’s structure and magnetic properties on the Pt content was investigated. The addition of platinum element in the CoCrPt magnetic layer increases the lattice constant and longitudinal coercivity of the CoCrPt thin films. The coercivity mechanism of the CoCrPt thin film is analyzed by using the micromagnetic models, with careful discussions of the uniaxial crystalline anisotropy and the magnetostriction. It is found that the magnetostriction effects play an important role on the coercivity mechanisms when the Pt content is more than 15 at. % Pt concentrate.
Rectifying characteristics and transport behavior of the La0.9Hf0.1MnO3/Nb-doped SrTiO3 heterojunction
Authors: Wang L. , Gao J.
Published: February 2009

La 0.9 Hf 0.1 MnO 3 (LHMO) films were grown on (001) SrTiO3 (LHMO/STO) and (001) 0.7% Nd-doped STO (LHMO/STON) substrates under the same conditions by using pulsed laser deposition technique. The x-ray diffraction and rocking curve measurements demonstrated high epitaxy and good crystallinity obtained in our films. The curves of resistance versus temperature indicated that the as-grown LHMO film exhibits a paramagnetic-ferromagnetic transition at Curie temperature TC. The LHMO/STON heterojunctions exhibited asymmetric current-voltage characteristics similar to those of traditional semiconductor junctions and a typical temperature-dependent rectifying property in a wide temperature range from 20 to 300 K. The diffusion voltage (Vd) of the junction decreases almost linearly with the decrease in temperature. At a negative bias voltage, the dependence of the junction resistance (V/I) on temperature showed an insulator-metal transition in the vicinity of TC, as the voltage decreased from −2 to −7 V. The results were discussed by considering the band diagram of LHMO/STON junction and the tunneling current through the junction.
Characteristics of metal-ferroelectric-insulator-semiconductor diodes composed of Pt electrodes and epitaxial Sr0.8Bi2.2Ta2O9(001)/SrTiO3(100)/Si(100) structures
Authors: Lu Xubing , Ishiwara Hiroshi , Gu Xing , Lubyshev Dmitri , Fastenau Joel , Pelzel Rodney
Published: February 2009

Epitaxial metal-ferroelectric-insulator-semiconductor diodes were fabricated by depositing a chemical-solution-decomposed Sr0.8Bi2.2Ta2O9 (SBT) film on an SrTiO3-coated Si(100) wafer. X-ray diffraction analysis revealed that the SBT film was composed mostly of c-axis-oriented grains. In Pt/SBT(300 nm)/SrTiO3(23 nm)/Si diodes, a memory window as wide as 1.1 V was obtained for a voltage sweep of ±7 V in capacitance-voltage measurement. The capacitance change in per decade increase in the retention time was approximately 10% up to 24 h. The origin of the ferroelectricity in a c-axis-oriented SBT film is discussed.
Nonequilibrium work relations for systems subject to mechanical and thermal changes
Authors: Chelli Riccardo
Published: February 2009

Generalized forms of the Crooks fluctuation theorem are derived for non-Hamiltonian and Hamiltonian systems subject to both mechanical and thermal changes. Almost identical derivations are provided for the two cases under rather general assumptions. The basic one is that the probability distribution is a stationary solution of the Liouville equation for fixed values of mechanical control parameters applied to collective variables of the system and for fixed temperature. Generalized expressions for several nonequilibrium work relations derivable from the Crooks fluctuation theorem, such as the Jarzynski equality, path-ensemble averages for systems driven far from equilibrium, Bennett acceptance ratio, and two work-based potential of mean force estimators, are also derived. Although this list is not complete, the extension to other related work theorems is straightforward. The application of the methodology is illustrated for two representative cases, namely, for systems evolving with isochoric-isokinetic and isothermal-isobaric equations of motion.
A new method for determining absorption cross sections out of initially excited vibrational states
Authors: Golan Amir , Mayorkas Nitzan , Rosenwaks Salman , Bar Ilana
Published: February 2009

A first experimental demonstration, combining the methods of vibrationally mediated photodissociation (VMP) and ionization-loss stimulated Raman spectroscopy (ILSRS) for measuring cross sections for dissociation of vibrationally excited levels is reported. The action spectrum obtained in the VMP of methylamine exhibits enhancement of the H photofragment yield as a result of initial vibrational excitation and the ILSRS monitors the fraction of molecules being excited. The partial cross sections for H production out of the sampled vibrational states and the extent of mode selectivity were thus determined.
Higher-order explicitly correlated coupled-cluster methods
Authors: Shiozaki Toru , Kamiya Muneaki , Hirata So , Valeev Edward F.
Published: February 2009

Efficient computer codes for the explicitly correlated coupled-cluster (CC-R12 or F12) methods with up to triple (CCSDT-R12) and quadruple excitations (CCSDTQ-R12), which take account of the spin, Abelian point-group, and index-permutation symmetries and are based on complete diagrammatic equations, have been implemented with the aid of the computerized symbolic algebra SMITH. Together with the explicitly correlated coupled-cluster singles and doubles (CCSD-R12) method reported earlier [T. Shiozaki et al., J. Chem. Phys. 129, 071101 (2008)], they form a hierarchy of systematic approximations (CCSD-R12<CCSDT-R12<CCSDTQ-R12) that converge very rapidly toward the exact solutions of the polyatomic Schrödinger equations with respect to both the highest excitation rank and basis-set size. Using the Slater-type function exp(−γr12) as a correlation function, a CC-R12 method can provide the aug-cc-pV5Z-quality results of the conventional CC method of the same excitation rank using only the aug-cc-pVTZ basis set. Combining these CC-R12 methods with the grid-based, numerical Hartree–Fock equation solver [T. Shiozaki and S. Hirata, Phys. Rev. A 76, 040503(R) (2007)], the solutions (eigenvalues) of the Schrödinger equations of neon, boron hydride, hydrogen fluoride, and water at their equilibrium geometries have been obtained as −128.9377±0.0004, −25.2892±0.0002, −100.459±0.001, and −76.437±0.003 Eh, respectively, without resorting to complete-basis-set extrapolations. These absolute total energies or the corresponding correlation energies agree within the quoted uncertainty with the accurate, nonrelativistic, Born–Oppenheimer values derived experimentally and/or computationally.
Free and bound exciton fine structures in AlN epilayers grown by low-pressure metalorganic vapor phase epitaxy
Authors: Onuma T. , Shibata T. , Kosaka K. , Asai K. , Sumiya S. , Tanaka M. , Sota T. , Uedono A. , Chichibu S. F.
Published: February 2009

Exciton fine structures were observed in partially polarized optical reflectance and cathodoluminescence (CL) spectra of AlN epilayers grown by low-pressure metalorganic vapor phase epitaxy on (0001) Al2O3 substrates. A few free and four bound exciton lines were clearly resolved in the low-temperature CL spectra of the lowest threading dislocation density (∼1×108 cm−2) AlN film. From the energy difference between the ground-state and the first excited states, the hydrogenic A-exciton binding energy in the present compressively strained (Δa/a≈−1.68%) AlN was estimated to be approximately 51 meV.
Microstructure–critical current density model for MgB2 wires and tapes
Authors: Birajdar B. , Eibl O.
Published: February 2009

MgB 2 wires and tapes were prepared by the powder in tube method using different processing technologies and thoroughly characterized for their superconducting properties. Either prereacted MgB2 (ex situ) or a mixture of Mg+2B (in situ) was used as the precursor powder. In some wires the precursor powder was mixed with SiC. The critical current density (Jc) of these wires was found to differ by orders of magnitude, the highest Jc being 104 A cm−2 at 10.5 T and 4.2 K. The microstructure of these wires was investigated using quantitative electron microscopy and spectroscopy methods [B. Birajdar, N. Peranio, and O. Eibl, Supercond. Sci. Technol. 21, 073001 (2008)]: combined scanning electron microscopy, electron probe microanalysis, and transmission electron microscopy analysis with artifact-free sample preparation, elemental mapping, and advanced chemical quantification. Wires with prereacted MgB2 (ex situ) show oxygen-poor MgB2 colonies (a colony is a dense arrangement of several MgB2 grains) embedded in a porous oxygen-rich matrix introducing structural granularity. Wires with elemental precursors (in situ) are generally more dense but show inhibited MgB2 phase formation with significantly higher fraction of B-rich secondary phases in comparison to the ex situ wires. SiC in the in situ wires results in the formation of Mg2Si secondary phases. In situ and mechanically alloyed samples show smaller (20–100 nm) MgB2 grains, the grain size being slightly larger than the coherence length. All samples show Mg oxide. SiC added samples annealed beyond 950 °C yield formation of Si oxide compounds, whereas Mg2Si is found for annealing temperatures of less than 650 °C. The critical current is limited due to the anisotropy but also due to structural granularity. A microstructure–critical current density model is given to explain the large, orders of magnitude, differences in the Jc of MgB2 wires and tapes. The model contains the following microstructure parameters: (1) MgB2 grain size, (2) colony size, (3) volume fraction of B-rich secondary phases, and (4) oxygen mole fraction. The logarithmic critical current densities as a function of magnetic field were parametrized and the decay field and the critical current density at zero field (Jc0) was quantitatively correlated with the parameters of the microstructure. The MgB2 grain size is negatively correlated with the decay field and the three other microstructure parameters show correlation with Jc0. Sample preparation influencing the microstructure parameters is discussed. A detailed analysis is given to correlate the microstructural data with respect to fundamental parameters of a flux-line pinning model established for anisotropic superconductors.
Phase identification of self-forming Cu–Mn based diffusion barriers on p-SiOC:H and SiO2 dielectrics using x-ray absorption fine structure
Authors: Ablett J. M. , Woicik J. C. , Tőkei Zs. , List S. , Dimasi E.
Published: February 2009

X-ray absorption fine structure spectroscopy has been used to study the chemical and structural properties of self-forming diffusion barrier layers from Cu-8 at. % Mn alloy films on porous low-k and thermally grown SiO2 dielectrics. For the porous low-k/Cu(Mn) system, we provide evidence that the interface is composed of MnSiO3 and MnO with near complete Mn segregation from the alloy film; however, we find that the self-forming process does not go to full completion on thermally grown SiO2 substrates.
Electron momentum spectroscopy study of Jahn–Teller effect in cyclopropane
Authors: Li Zhongjun , Chen Xiangjun , Shan Xu , Liu Tao , Xu Kezun
Published: February 2009

The binding energy spectra of cyclopropane in energy range of 9–19 eV have been measured by the high resolution (e, 2e) spectrometer with 0.5 eV energy resolution. The individual electron momentum distributions for the outer valence orbitals including the two Jahn–Teller splitting components for the highest occupied molecular orbital 3e′ have been obtained. Compared with the results of high level quantum chemistry calculations, the observed different intensities of the experimental momentum profiles for the two Jahn–Teller splitting components have been phenomenologically explained, showing the influences caused by the change in molecular geometry at the instant of ionization from doubly degenerate 3e′ orbital on the electron momentum distributions. In addition, the controversial ordering of the two outer valence orbitals 3a1′ and 1a2″ has also been assigned unambiguously.
Analytical potential energy surfaces for N3 low-lying doublet states
Authors: Wang Zhi , Kerkines Ioannis S. K. , Morokuma Keiji , Zhang Peng
Published: February 2009

Adiabatic potential energy surfaces (PESs) for five low lying doublet states (three A2′ states and two A2″ states) of N3 are constructed based on 1504 single point calculations at the MRCISD(Q) level using aug-cc-pVTZ basis set. A new strategy is adopted to obtain the final PESs by combining global fits of individual adiabatic PESs and local simultaneous fits of two adiabatic PESs in several conical intersection regions with switching functions. These global fits employ basis functions that satisfy permutational invariance with respect to like nuclei and have rms errors around 2–3 kcal/mol. The special local two-state fits are performed at the cyclic, bent, and linear N3 conical intersection regions to take account of intrinsic square root behavior of the potentials and to improve the quality of fitting. Stationary points as well as minima on the concial intersections and seams of crossing are located on these PESs and compared with ab initio results. The agreement is satisfactory in most cases. In addition to the construction of adiabatic PESs, diabatization is performed for the 1 A2′ and 2 A2′ states around their conical intersection at the N3 bent region. These two diabatic PESs and the diabatic coupling potential have been constructed and reported.
Field emission and lifetime of microcavity plasma
Authors: Kim G. J. , Iza F. , Lee J. K.
Published: February 2009

Microplasmas with cylindrical hollow cathode have been studied by means of two-dimensional particle-in-cell/Monte-Carlo collision (PIC/MCC) simulations. For a given input power, the onset of field emission from the cathode surface caused by the strong electric field generated in these discharges leads to a reduction of the discharge voltage and an increase in plasma density. The plasma density profile can be strongly influenced by localized enhancements of the electric field, which in turn will affect the erosion profile of the cathode. The cathode erosion profile is predicted in this work by combining the ion kinetic information obtained from the PIC/MCC simulation with the sputtering yield computed using SRIM [J. F. Ziegler, J. P. Biersack, and M. D. Ziegler, SRIM: The Stopping and Range of Ions in Matter (Lulu, Chester, 2008)]. The entrance of the cathode and the center region are the areas most susceptible to ion-induced damage. The lifetime of the device, however, can be extended by operating the device at high pressure and by reducing the operating voltage by means of field emission and/or additional electron emitting processes from the cathode.
A proper-orthogonal-decomposition–based model for the wall layer of a turbulent channel flow
Authors: Podvin Bérengère
Published: February 2009

In this paper we investigate a proper orthogonal decomposition (POD)-based model for the wall layer of a turbulent channel flow. The model is based on the original derivation of Aubry et al. [J. Fluid Mech. 192, 115 (1988)] and includes 220 independent modes with 11 modes in each horizontal direction and one wall-normal mode. We use direct numerical simulation to evaluate and calibrate the closure assumptions. The nature of the energy transfer to unresolved scales is examined in detail. The POD modes integrated from the model are compared to those directly computed from the simulation. The model is shown to be sensitive to initial conditions. Special attention is given to the zero streamwise subspace, which is characterized by chaotic solutions loosely related to a cluster of traveling waves. The statistics of the model are shown to be in relatively good agreement with the simulation, with a limited dependence on the calibration procedure.
A single source femtosecond-millisecond broadband spectrometer
Authors: Carroll E. C. , Hill M. P. , Madsen D. , Malley K. R. , Larsen D. S.
Published: February 2009

Time-resolved measurement of population dynamics extending over femtosecond to millisecond time scales typically requires a combination of transient absorption techniques involving different laser systems and detection schemes. The spectrometer design presented here facilitates transient absorption measurements over 12 decades with a single ultrafast laser system by picking pump and probe pulses independently from the laser oscillator pulse train. Unamplified pulses seed a photonic crystal fiber to a supercontinuum probe source for spectrally resolved measurements. The utility of the system is demonstrated by measuring triplet state dynamics following photoexcitation of vitamin B6 in aqueous solution.
Ferroelectric properties of Pb(Zr,Ti)O3 thin films until 40 GHz
Authors: Defaÿ E. , Lacrevaz T. , Vo T. T. , Sbrugnera V. , Bermond C. , Aïd M. , Fléchet B.
Published: February 2009

The radio frequency characterization of Cu/TiN/Pb(Zr,Ti)O3 stack on glass is performed using coplanar transmission lines. A dielectric relaxation is evidenced around 10 GHz by the correlated decrease in the dielectric constant K together with the dielectric losses increase versus frequency. This phenomenon is attributed to domain wall relaxation. The ferroelectric nature of Pb(Zr,Ti)O3 (PZT) thin films is observed until 40 GHz with a hysteresis curve of K versus dc bias. The high K value (K∼1200) combined with a high tunability (∼35%) and moderate losses (∼1%) suggest that PZT films could be well suited for tunable devices for frequencies lower than 5 GHz.
Minipressure sensor using AlGaN/GaN high electron mobility transistors
Authors: Hung S. C. , Chou B. H. , Chang C. Y. , Lo C. F. , Chen K. H. , Wang Y. L. , Pearton S. J. , Dabiran Amir , Chow P. P. , Chi G. C. , Ren F.
Published: February 2009

AlGaN/GaN high electron mobility transistors (HEMTs) with a polarized polyvinylidene difluoride (PVDF) film coated on the gate area exhibited significant changes in channel conductance upon exposure to different ambient pressures. The PVDF thin film was deposited on the gate region with an ink-jet plotter. Next, the PDVF film was polarized with an electrode located 2 mm above the PVDF film at a bias voltage of 10 kV and 70 °C. Variations in ambient pressure induced changes in the charge in the polarized PVDF, leading to a change in surface charges on the gate region of the HEMT. Changes in the gate charge were amplified through the modulation of the drain current in the HEMT. By reversing the polarity of the polarized PVDF film, the drain current dependence on the pressure could be reversed. Our results indicate that HEMTs have potential for use as pressure sensors.
Miniature high efficiency transducers for use in ultrasonic flow meters
Authors: Joshi S. G. , Zaitsev B. D. , Kuznetsova I. E.
Published: February 2009

The development of a miniature high efficiency transducer for use in ultrasonic flow meters is described. The transducer is based on conversion of energy between surface acoustic waves (SAWs) and bulk acoustic wave (BAWs). It consists of a thin plate of a suitable piezoelectric material whose SAW velocity is greater than velocity of BAW in the fluid. Under these conditions a surface wave traveling on the substrate radiates a bulk acoustic wave in the fluid. We show that with proper design efficient conversion of energy from SAW to BAW and vice versa can be obtained. For example, using lead zirconate titanate as the piezoelectric material and water as the fluid, total conversion loss (SAW to BAW and back from BAW to SAW) of less than 3 dB has been obtained.
Nanolayers on nanochannels for hydrogen purification
Authors: Checchetto R. , Patel N. , Miotello A. , Brusa R. S.
Published: February 2009

The purification of hydrogen rich gases is of great technological importance in the “hydrogen economy” and is achieved by selective membranes made of organic or inorganic materials. In this field, a strong challenge is the synthesis of defect-free ultrathin Pd-based selective membranes. We present a study on the synthesis and performances of a bilayer structure consisting of 100 nm nanoporous silica coated with a 150 nm Pd–Ag layer. An alumina disk having periodic microsieves structure was used as support for the bilayer. The hydrogen transport through this nanocomposite membrane is controlled by the dissociation of molecular hydrogen at the surface of the Pd–Ag functional layer. When operating at 573 K, the membrane exhibits high H2/N2 selectivity (a factor as high as 600–900), high H2 permeance (∼10−6 mol m−2 s−1 Pa−1), and operative stability on long-term operations.
Two-dimensional longitudinal and transverse relaxation time correlation as a low-resolution nuclear magnetic resonance characterization of ancient ceramics
Authors: Casieri Cinzia , Terenzi Camilla , De Luca Francesco
Published: February 2009

Longitudinal and transverse relaxation time correlation (T1-T2) is employed as a nuclear magnetic resonance noninvasive characterization tool for archeological ceramics. This paper is aimed at investigating whether the most relevant firing-induced changes in ceramics, including those involving pore space properties and paramagnetic mineral structures, could be used as markers of the firing process and therefore of ceramics themselves. Ancient ceramics are made up of naturally available clays, often rich in iron impurities, which undergo relevant modifications of pore distribution upon firing. The firing process also assists chemical and physical rearrangement of iron-bearing species, yielding mineral structures with different magnetic properties. That being so, T1-T2 maps are expected to show the interdependence between ceramic structure and firing technology. T1 and T2 distributions are basically proportional to pore-size distribution, but T2, which is also sensitive to magnetic susceptibility effects, may give information on the porous matrix composition as well. Such a methodology has first been employed on laboratory-prepared ceramic samples, with different paramagnetic compositions and controlled firing temperatures, in order to tackle the problem of model-ceramic selection. Then, the T1-T2 correlation approach has been used on medieval ceramic findings in order to get information about their thermal and compositional history. The information obtained by means of two-dimensional correlation maps proves coherent with archeological dating, thus illustrating the capabilities of this method.
Development of a versatile high temperature top seeded solution growth unit for growing cesium lithium borate crystals
Authors: Reddy J N Babu , Elizabeth Suja , Bhat H. L. , Karnal A. K.
Published: February 2009

The design and fabrication of a state of the art high temperature top seeded solution growth (HTTSSG) unit with seed and crucible rotation options is discussed. Crystals from solid solutions of high viscosity could be grown using the setup. The fabricated setup is used to grow high optical quality single crystals of cesium lithium borate with dimensions 50×40×40 mm3 for frequency conversion of ir to UV laser through harmonic generation.
Physical and optoelectronic characterization of reactively sputtered molybdenum-silicon-nitride alloy metal gate electrodes
Authors: Patel P. , Nadesalingam M. , Wallace R. M. , Buchanan D. A.
Published: February 2009

With continued transistor scaling, work function tuning of metal gates has become important for advanced complementary-metal-oxide-silicon applications. The work function tuning of reactively sputtered MoxSiyNz (also referred to as MoSiN) gates has been studied through the incorporation of nitrogen. The nitrogen concentration in the MoSiN films was altered by controlling the gas flow ratio, RN=N2/(N2+Ar), during gate deposition. The sheet resistance (Rs) of blanket MoSiN films, measured using four-point resistance method, was found to increase as the gas flow ratio was varied from 10% to 40%. Current-voltage measurements confirmed excellent electrical stability of MoSiN/SiO2/p-Si gate stack for applied electric fields ranging up to 6 MV/cm. High frequency capacitance-voltage measurements were used to extract the MoSiN work function (Φm) using the relationship between the flatband voltage (VFB) and the oxide thickness (tox). The extracted MoSiN/SiO2 interfacial barrier heights, obtained through the internal photoemission of electrons, were used to corroborate the extracted values of MoSiN work function. The MoSiN work functions (Φm), extracted independently using both techniques, were consistent and were observed to decrease with increasing gas flow ratio [N2/(N2+Ar)]. Secondary ion mass spectrometry depth analysis revealed uniform distribution of nitrogen throughout the bulk MoSiN films, with no piling up at gate-dielectric interface. X-ray photoelectron spectroscopy surface analysis suggested a steady increase in the Mo–N bonds, and therefore the total nitrogen concentration (from ∼20% to 32%), as the gas flow ratio is increased from 10% to 40%. A similar trend was observed in the nitrogen concentration (in percent), measured using Rutherford backscattering spectroscopy, for these gate deposition conditions. These material characterization results demonstrate that the increase in nitrogen concentration in MoSiN films is consistent with the lowering of MoSiN work function. Moreover, the work function of reactively sputtered MoxSiyNz films was found to be variable over ∼0.3 eV by adjusting the nitrogen concentration.
Modeling optical spectra and Van Vleck paramagnetism in Er3+:YAlO3
Authors: Gruber John B. , Chandra Sreerenjini , Sardar Dhiraj K. , Valiev Uygun V. , Juraeva Nafisa I. , Burdick Gary W.
Published: February 2009

Low-temperature absorption spectra are reported for Er3+:YAlO3 (YAP) between 1700 and 350 nm. The low-temperature vacuum ultraviolet absorption spectra are also reported between 400 and 190 nm. A total of 134 experimental energy (Stark) levels representing 30 multiplets with energies below 44 000 cm−1 were modeled using a parametrized Hamiltonian defined to operate within the 4f11 electronic configuration of the Er3+ ions substituting for Y3+ in YAP, an orthorhombically distorted perovskite. The Y3+ sites have low symmetry (CS). The crystal-field energy-level parameters were determined through use of a Monte Carlo method in which 14 independent parameters are given random starting values, which are optimized using standard least-squares fitting between calculated and experimental levels. The best solution obtained has a standard deviation of 6.92 cm−1 (rms error of 6.09 cm−1). In the presence of a magnetic field the Er3+ ions occupy two magnetically inequivalent sites. As an independent check of the crystal-field modeling results, crystal-field wave functions for the I415/2 ground-state manifold of Er3+(4f11) were used to calculate the orientation-dependent anisotropic magnetic susceptibility as a function of temperature over the Curie–Weiss region. The calculated susceptibilities along the a-, b-, and c-axes of the crystal are in excellent agreement with experimental values reported as part of the present study. Van Vleck paramagnetism must be included in the calculations in order to achieve agreement. The calculated angle φ (39.6°) associated with the magnetic moment of the Er3+ sublattice along the a- and b-axes is in good agreement with the corresponding angle φ reported for Er3+ in the orthoferrite structures.
Raman scattering study on anisotropic property of wurtzite GaN
Authors: Lin Hung Chiao , Feng Zhe Chuan , Chen Ming Song , Shen Ze Xiang , Ferguson Ian T. , Lu Weijie
Published: February 2009

The phonon anisotropy property of the GaN wurtzite crystal is studied using angular dependent Raman spectroscopy both theoretically and experimentally. The polarized Raman scattering spectra were recorded from the cross section of c-axis oriented GaN films as a function of the angle between the incident laser polarization direction and the film normal direction in three different configurations. The Raman intensity of A1(TO) showed a sinusoidal dependence on the rotating angle, as also did the E1(TO) mode, while the E2 mode has a quite different behavior. The theoretical analysis takes into account the susceptibility contribution and the phase differential of different vibrating elements.
How to make thermodynamic perturbation theory to be suitable for low temperature?
Authors: Zhou Shiqi
Published: February 2009

Low temperature unsuitability is a problem plaguing thermodynamic perturbation theory (TPT) for years. Present investigation indicates that the low temperature predicament can be overcome by employing as reference system a nonhard sphere potential which incorporates one part of the attractive ingredient in a potential function of interest. In combination with a recently proposed TPT [S. Zhou, J. Chem. Phys. 125, 144518 (2006)] based on a λ expansion (λ being coupling parameter), the new perturbation strategy is employed to predict for several model potentials. It is shown that the new perturbation strategy can very accurately predict various thermodynamic properties even if the potential range is extremely short and hence the temperature of interest is very low and current theoretical formalisms seriously deteriorate or critically fail to predict even the existence of the critical point. Extensive comparison with existing liquid state theories and available computer simulation data discloses a superiority of the present TPT to two Ornstein–Zernike-type integral equation theories, i.e., hierarchical reference theory and self-consistent Ornstein–Zernike approximation.
On the existence of Weibel instability in a magnetized plasma. I. Parallel wave propagation
Authors: Lazar M. , Schlickeiser R. , Poedts S.
Published: February 2009

In a uniform static magnetic field (B0) the nonresonant mechanism of Weibel instability is certainly affected, because the free energy stored in the temperature anisotropy (e.g., T⊥>T∥, where ⊥ and ∥ denote directions relative to the background magnetic field) can drive the parallel whistler instability due to the cyclotron resonance with plasma particles. However, it has already been shown that the nonresonant mechanism should exist and drive the whistler instability, at least, for anisotropic distribution functions with no parallel thermal velocity spread. Here the investigation is extended to a magnetized plasma with a bi-Maxwellian temperature anisotropy. New frequency limits are introduced and the necessary physical conditions are provided: The nonresonant whistler instabilities have in general small frequencies, smaller than the growth rate, ωr<ωi, and for a large temperature anisotropy, the nonresonant instabilities are limited to a β⊥=8πn0T⊥∕B02>1 regime. These criteria restricts in general the magnetic field to strengths less than a threshold value proportional to the square root of the temperature anisotropy. In the presence of an ambient magnetic field, the growth rate of the nonresonant instability is reduced and the large wavelengths are stabilized.
Pulse distortion and modulation instability in laser plasma interaction
Authors: Jha Pallavi , Singh Ram Gopal , Upadhyay Ajay K.
Published: February 2009

The present paper deals with the propagation of a short, intense, Gaussian laser pulse in plasma. Using a one dimensional model, a wave equation including finite pulse length and group velocity dispersion is set up and solved to obtain the intensity distribution across the laser pulse. It is shown that the pulse profile becomes asymmetric as it propagates through plasma. Further, the growth rate of modulation instability and range of unstable frequencies across the laser pulse have been derived and graphically analyzed.