bluuebunny/crossref_metadata_2025 huggingface dataset:
Stoichiometric, nonstoichiometric, and locally nonstoichiometric SrTiO3 films grown by molecular beam epitaxy
Authors: Fisher P. , Du H. , Skowronski M. , Salvador P. A. , Maksimov O. , Weng X.
Published: January 2008

SrTiO 3 films were grown by reactive molecular beam epitaxy to have varying degrees of both global and local cationic nonstoichiometries (with stoichiometry defined as a 1:1 ratio of Sr:Ti). Slight global excesses of Sr and Ti resulted in two-fold reconstructions in the reflection high-energy electron diffraction patterns along the [110] and [100] azimuths, respectively. Larger global nonstoichiometries (2:1 and 1:2 ratios) were also accommodated into the film’s crystalline structure and affected the long-range crystalline order as observed in the x-ray diffraction patterns, both of which were related to the parent perovskite pattern. Local nonstoichiometries were introduced by depositing multiple monolayers (MLs) (from 2 to 33) of SrO and TiO2 in an alternating fashion, while maintaining the global SrTiO3 stoichiometry. These layered structures of SrO and TiO2 blocks inter-reacted during growth to form highly crystalline epitaxial SrTiO3. Films grown in this manner with blocks thicker than 8 MLs were fully relaxed and, when the block thicknesses ranged between 8 and 10 MLs, the full widths at half maxima of 2θ peaks were narrower than the standard SrTiO3 films having blocks 1 ML thick.
High-field properties of carrier transport in bulk wurtzite GaN: A Monte Carlo perspective
Authors: Chen Shiyu , Wang Gang
Published: January 2008

The transport properties of both electron and hole in bulk wurtzite phase GaN in the high electric field domain are presented by using an ensemble Monte Carlo (EMC) method. In our EMC simulation, the impact ionization process, which is seldom studied due to the lack of experimental data, is included. The impact ionization is treated as an additional scattering mechanism, and the impact ionization rate is described by the Keldysh formula, with the parameters in the formula determined by fitting the simulation results to the numerical calculation results. Such a treatment makes it convenient to simulate the impact ionization initiated by either an electron or hole with the EMC method compared to the previous study of carrier transport properties in GaN up to the high field. Steady-state properties of carriers under an applied electric field up to 1 MV/cm are presented and analyzed. Particularly, the impact ionization process here is further studied and detailed discussions are also given. It is found that the impact ionization coefficients of both the electron and hole upon applied electric field can be described by two simple experiential equations. Moreover, for the first time, to the best of our knowledge, we obtain the ratio of the electron impact ionization coefficient to the hole impact ionization coefficient in wurtzite GaN and find out that it can be smaller than that in InP, which means wurtzite GaN may have good gain noise behaviors according to the present noise theories.
Terahertz mobility measurements on poly-3-hexylthiophene films: Device comparison, molecular weight, and film processing effects
Authors: Esenturk Okan , Melinger Joseph S. , Heilweil Edwin J.
Published: January 2008

We directly compare relative carrier mobilities in semiconducting organic polymer films measured using noncontact optical pump terahertz (THz) probe spectroscopy to those reported in electrical device studies. Relative transient signal amplitude measurements of photoinjected carrier mobility as a function of poly-3-hexylthiophene (P3HT) molecular weight correlate directly with electrical device test values, indicating that the THz method shows promise as a rapid material screening approach. We also present measurements on P3HT conducting films as a function of structural regularity, dispersing solvent, addition of C60 electron traps, sample temperature, and other growth parameters relevant to device manufacture.
High perpendicular coercive field of (100)-oriented CoFe2O4 thin films on Si (100) with MgO buffer layer
Authors: Ning M. , Li J. , Ong C. K. , Wang S. J.
Published: January 2008

CoFe 2 O 4 thin films with large perpendicular magnetic anisotropy were obtained by pulsed laser deposition on Si(100) substrates with MgO buffer layers. Transmission electron microscopy study reveals the columnar structure of these CoFe2O4 films and confirms their (100) texture. Magnetic properties of these films have been investigated in the function of substrate temperature and film thickness. A perpendicular coercivity as high as 7.8 kOe has been achieved in the CoFe2O4 film deposited at 700 °C, with a thickness of 50 nm and a grain size of 30 nm. The high coercivity mechanism is possibly associated with the magnetocrystalline anisotropy, the strain anisotropy, the shape anisotropy due to the columnar structure, and also the appropriate grain size approaching the single-domain critical value.
Performance of the university of saskatchewan compact torus injector with curved acceleration electrodes
Authors: Liu D. , Xiao C. , Hirose A.
Published: January 2008

The first vertical compact torus (CT) injection experiment has been performed in the Saskatchewan Torus Modified (STOR-M) tokamak [Nucl. Fusion 46, 104 (2006)]. To increase the kinetic energy density of the injected CTs for deeper penetration, the University of Saskatchewan Compact Torus Injector (USCTI) was further modified by attaching a 90° curved inner electrode coaxial with the outer electrode. The modification extended the original CT acceleration section from 60to114cm. Effects of the curved acceleration electrodes on the velocity and magnetic field of the CT are reported in this paper. It has been found that the CTs, injected horizontally, were deflected to vertical direction and CT velocity measured at the curved acceleration section increased to 180km∕s, representing a 40% increase compared with the case without the curved inner electrode in a previous experiment. At a higher acceleration bank voltage of 16kV, this velocity increased to about 270km∕s. In addition, amplification of the CT magnetic field in the curved acceleration section has also been observed.
Comment on “Derivation of paleoclassical key hypothesis” [Phys. Plasmas 14, 040701 (2007)]
Authors: Thyagaraja A. , Roach C. M. , Hazeltine R. D.
Published: January 2008

The paleoclassical hypothesis, derived in Callen [Phys. Plasmas 14, 040701 (2007)], proposes that electron guiding centers experience additional diffusion which is absent from neoclassical theory. This is claimed to be associated with the diffusion of poloidal magnetic flux, and to be most significant in cold resistive plasmas. In this comment we explain why the paleoclassical hypothesis contradicts electrodynamics.
The ferroelectric phase transition in lithium tantalate single crystals: A composition-dependence study
Authors: Bhaumik Indranil , Ganesamoorthy S. , Bhatt R. , Karnal A. K. , Wadhawan V. K. , Gupta P. K. , Kitamura K. , Takekawa S. , Nakamura M.
Published: January 2008

The dielectric behavior of lithium tantalate (LT) single crystals has been investigated in the vicinity of the ferroelectric phase transition for nonstoichiometric and near-stoichiometric compositions. Fitting of the experimental data to a modified Curie-Weiss equation indicates the occurrence of a mildly relaxor-ferroelectric-like phase transition in congruent LT single crystals (with the Curie-Weiss exponent γ having a value 1.2, rather than the value unity for a regular ferroelectric). As the Li concentration of these crystals approaches the stoichiometric value, the phase transition gets sharper, indicative of an increasing amount of long-range ordering. The bulk concentration of cation vacancies has also been estimated.
Optimizing the magnetoinductive lens: Improvement, limits, and possible applications
Authors: Freire Manuel J. , Marques Ricardo
Published: January 2008

In this contribution, the basic concepts underlying the physics of magnetoinductive (MI) lenses are developed and the main practical limitations to their performances are analyzed. Strategies to overcome such limitations are proposed. Possible applications of MI lenses in magnetic resonance imaging are discussed.
Microstructure and magnetotransport properties of Cu doped Fe3O4 films
Authors: Tripathy D. , Adeyeye A. O. , Boothroyd C. B. , Shannigrahi S.
Published: January 2008

A systematic and study on the microstructure and magnetotransport properties of Cu doped Fe3O4 films prepared by cosputtering is presented. Structural investigations reveal that the Cu doped films are polycrystalline in nature with a preferred growth direction and retain the cubic spinel structure of Fe3O4. Another characteristic feature of the Cu doped films is the transport mechanism being dominated by spin dependent tunneling across grain boundaries. The magnetic properties of the doped films are markedly sensitive to the doping concentration, with both saturation magnetization and coercivity increasing with increasing Cu doping concentration. In-plane magnetoresistance (MR) measurements show a reduction in MR ratio as the Cu doping concentration is increased.
Effects of ferroelectricity and magnetism on electron and spin transport in Fe∕BaTiO3∕Fe multiferroic tunnel junctions
Authors: Velev Julian P. , Duan Chun-Gang , Belashchenko Kirill D. , Jaswal Sitaram S. , Tsymbal Evgeny Y.
Published: January 2008

First principles electronic structure and transport calculations are used to demonstrate the impact of the electric polarization on electron and spin transport in Fe∕BaTiO3∕Fe multiferroic tunnel junctions (MFTJs). We find that the polarization of BaTiO3 reduces the tunneling conductance, as compared to a nonpolarized barrier, due to the change in the electronic structure driven by ferroelectric displacements, similar to that found previously for Pt∕BaTiO3∕Pt. For the MFTJ, however, this effect has different magnitudes for majority- and minority-spin channels and for parallel and antiparallel orientations of the magnetization of the electrodes. As a result, we find a substantial drop in the spin polarization of the tunneling current in the parallel configuration and an inversion of the magnetoresistance as polarization of the barrier is turned on.
Pressure tuning of the magnetic transition in Gd5(Si0.375Ge0.625)4 giant magnetocaloric effect material
Authors: Tseng Y. C. , Haskel D. , Lang J. C. , Mudryk Ya. , Pecharsky V. K. , Gschneidner K. A.
Published: January 2008

The effect of hydrostatic pressure on the ferromagnetic ordering transition of the monoclinic Gd5(Si0.375Ge0.625)4 giant magnetocaloric effect compound was investigated using x-ray magnetic circular dichroism measurements in a diamond anvil cell. The Curie temperature TC increases linearly with applied pressure up to ∼7.2GPa, at which point a discontinuity in dTC∕dP occurs. This discontinuity, which appears when TC reaches ∼277K, is also observed when the unit cell volume is reduced by Si doping and is associated with the volume-driven monoclinic (M) to orthorhombic [O(I)] structural transition.
Half-metallicity at ferromagnetic∕antiferromagnetic interfaces in zincblende transition-metal chalcogenides: A full-potential linearized augmented plane-wave study within LDA+U
Authors: Nakamura Kohji , Akiyama Toru , Ito Tomonori , Freeman A. J.
Published: January 2008

Electronic structures and half-metallicity at ferromagnetic∕antiferromagnetic (AFM) interfaces in zincblende transition-metal chalcogenides, CrSe∕MnSe and CrTe∕MnTe, are investigated by means of the first principles full-potential linearized augmented plane-wave method within the LDA+U, and the effect of correlation in the 3d states on the half-metallic interfaces is discussed. The uncompensated AFM interface with the antiparallel alignment of the Cr and Mn moments at the interfaces shows an excellent half-metallicity, where the correlation effect tends to manifest the half-metallic interfaces. This indicates that these interfaces offer a key ingredient as promising exchange bias candidates in having interfaces with 100% spin polarization at the Fermi level.
Electrochemical synthesis and properties of CoO2, the x= phase of the AxCoO2 systems (A=Li,Na)
Authors: Motohashi T. , Ono T. , Katsumata Y. , Kanno R. , Karppinen M. , Yamauchi H.
Published: January 2008

Single-phase bulk samples of the “exotic” CoO2, the x=0 phase of the AxCoO2 systems (A=Li,Na), were successfully synthesized through electrochemical deintercalation of Li from pristine LiCoO2 samples. The samples of pure CoO2 were found to be essentially oxygen stoichiometric and possess a hexagonal structure consisting of stacked triangular-lattice CoO2 layers only. The magnetism of CoO2 is featured with a temperature-independent susceptibility of the magnitude of 10−3emu∕molOe, being essentially identical to that of a Li-doped phase, Li0.12CoO2. It is most likely that the CoO2 phase is a Pauli-paramagnetic metal with itinerant electrons.
Structure and magnetic properties of bulk isotropic and anisotropic Nd2Fe14B∕α-Fe nanocomposite permanent magnets with different α-Fe contents
Authors: Yue M. , Niu P. L. , Li Y. L. , Zhang D. T. , Liu W. Q. , Zhang J. X. , Chen C. H. , Liu S. , Lee D. , Higgins A.
Published: January 2008

Chemical coating, hot compaction, and hot deformation techniques have been applied to prepare bulk isotropic and anisotropic Nd2Fe14B∕α-Fe nanocomposite magnets. The effect of α-Fe content on the structure and magnetic properties of the magnets were studied. For the isotropic magnets, the remanence (Br) increases as the α-Fe content increases, while the coercive force (Hci) drops simultaneously. For the anisotropic magnets, the Br rises first, peaking at 2vol% of α-Fe content, then falls as the α-Fe content increases, and Hci drops significantly for all the α-Fe containing anisotropic magnets. Crystal structure analysis shows that only the magnets with no more than 2vol% α-Fe exhibit strong c-axis crystal texture of Nd2Fe14B phase after deformation. Microstructure observation also shows that there are many Nd2Fe14B equiaxial grains even after hot deformation in the magnets with α-Fe more than 2vol%.
Magnetoresistance in nanocontacts formed in NiFe thin films
Authors: Ohsawa Yuichi
Published: January 2008

Magnetoresistance (MR) and current-driven conductance change measurements in NiFe nanocontacts in ion-beam-sputtered thin films at the initial growth stage were performed. A MR ratio of as large as about 140% with about 1nm2 contact whose conductance change was the same as that of current-driven measurements was observed. Furthermore, as it aligned so well in the empirical fit of the contact size dependence to MR ratio based on the previous reports, it was deduced that domain wall plays an important role with respect to the large MR.
Two-phonon coupling to the antiferromagnetic phase transition in multiferroic BiFeO3
Authors: Ramirez Mariola O. , Krishnamurthi M. , Denev S. , Kumar A. , Yang Seung-Yeul , Chu Ying-Hao , Saiz Eduardo , Seidel Jan , Pyatakov A. P , Bush A. , Viehland D. , Orenstein J. , Ramesh R. , Gopalan Venkatraman
Published: January 2008

A prominent band centered at ∼1000–1300cm−1 and associated with resonant enhancement of two-phonon Raman scattering is reported in multiferroic BiFeO3 thin films and single crystals. A strong anomaly in this band occurs at the antiferromagnetic Neel temperature, TN∼375°C. This band is composed of three peaks, assigned to 2A4, 2E8, and 2E9 Raman modes. While all three peaks were found to be sensitive to the antiferromagnetic phase transition, the 2E8 mode, in particular, nearly disappears at TN on heating, indicating a strong spin-two-phonon coupling in BiFeO3.
Enhancement of photoluminescence efficiency in binary quantum dot arrays in hybrid organic/inorganic materials
Authors: Kohary K. , Burlakov V. M. , Pettifor D. G. , Gibson G. A. , Sheng X. , Yang C. C.
Published: January 2008

The photoluminescence (PL) quantum efficiency of dense semiconductor colloidal quantum dot (QD) arrays is significantly reduced by the quenching of the optical excitons via defect-induced nonradiative decay channels. This luminescence quenching is facilitated by the rapid migration of excitons between neighboring QDs via resonant Förster transfer processes. We propose to mitigate this quenching by using nonradiative “spacer” quantum dots (SQDs) to separate radiative primary quantum dots (PQDs). We have identified the maximum and minimum values of the PL quantum efficiency (PLQE) for different compositions of spacer-primary QD arrays. Using the kinetic Monte Carlo technique, we have found that for a given composition, the PLQE is highest for randomly distributed spacer and primary QDs, decreasing dramatically when clusters of SQDs and PQDs are formed. We have modeled cluster formation of QDs in binary QD arrays and determined the resultant PL properties. By comparing our simulation results with those we obtained experimentally, we have estimated the magnitude of the attractive pairwise van der Waals interaction between the QDs which is the driving force for QD cluster formation. We have also explored possible strategies to achieve the highest PLQE of the PQDs in hybrid organic/inorganic materials for photoelectronics and we found that lowering the mixing time of the binary QD mixture can improve the PLQE.
Improved conductivity and mechanism of carrier transport in zinc oxide with embedded silver layer
Authors: Han H. , Theodore N. D. , Alford T. L.
Published: January 2008

The effects of an embedded silver layer on the electrical and optical properties of zinc oxide (ZnO)/silver (Ag)/zinc oxide (ZnO) layered composite structures on polymer substrates have been investigated. We have engineered transparent conducting oxide structures with greatly improved conductivity. Optical and electrical properties are correlated with Ag thickness. Film thicknesses were determined using Rutherford backscattering spectrometry. Hall effect, four-point probe, and UV-Vis spectrophotometer analyses were used to characterize electrical and optical properties. The results show that carrier concentration, mobility, and conductivity increase with Ag thickness. Increasing Ag thickness from 8to14nm enhances sheet resistance and resistivity by six orders of magnitude. The optical transmittance of the composite structure decreases when compared to a single ZnO layer of comparable thickness. However, a composite with 12nm of Ag provides conductivity and transmittance values that are acceptable for optoelectronic devices. We describe of the influence of Ag thickness on electrical and optical properties of the ZnO∕Ag∕ZnO composite and propose a conduction mechanism for this system.
Electronic states of chemically treated SiC surfaces
Authors: Nie Shu , Feenstra R. M. , Ke Y. , Devaty R. P. , Choyke W. J.
Published: January 2008

Electronic states at chemically treated SiC surfaces have been studied by scanning tunneling spectroscopy. Charge accumulation on the surface is deduced through a voltage shift observed in the spectra. More charge is observed on electropolished surfaces as compared to untreated (as-received) surfaces. This difference is interpreted in terms of the electropolished SiC surfaces being more insulating than as-received ones, such that on the former the transport of charge is limited and surface charges cannot come into equilibrium with the bulk semiconductor. Observations of tunneling spectra on SiC prepared by various amounts of hydrogen etching are used to support this interpretation.
Tuning the pore wall morphology of mesoporous silicon from branchy to smooth, tubular by chemical treatment
Authors: Kumar Pushpendra , Hofmann Tommy , Knorr Klaus , Huber Patrick , Scheib P. , Lemmens P.
Published: January 2008

The effect of chemical treatment on physical and chemical properties, i.e., pore diameter, porosity, specific surface area, and chemical bonding of electrochemically formed mesoporous silicon were investigated by using of nitrogen sorption isotherm, scanning electron microscopy, and Fourier transform infrared spectroscopy. The adsorption isotherms measurements show the general behavior found for the porous materials, but at the same time, they exhibit clear differences following different chemical treatments of porous layer. It was clearly observed from Fourier transform infrared spectroscopy that the chemical environment of porous silicon wall changes significantly after chemical treatment. In scanning electron microscopy images, we see that the rough dendritic structure of the pore walls is modified to smooth tubular pore wall structure on chemical treatment. The changes in nanocrystalline porous silicon were also clearly observed by an asymmetric broadening and shift of the optical silicon phonons in Raman spectra. Furthermore, changes are observed in the multiphonon regime due to surface assisted multiphonon processes, which are enhanced in highly porous silicon. The chemically modified porous silicon samples suggest possibilities of use as a porous matrix for fundamental study and technological application.
Comprehensive model of damage accumulation in silicon
Authors: Mok K. R. C. , Benistant F. , Jaraiz M. , Rubio J. E. , Castrillo P. , Pinacho R. , Srinivasan M. P.
Published: January 2008

Ion implantation induced damage accumulation is crucial to the simulation of silicon processing. We present a physically based damage accumulation model, implemented in a nonlattice atomistic kinetic Monte Carlo simulator, that can simulate a diverse range of interesting experimental observations. The model is able to reproduce the ion-mass dependent silicon amorphous-crystalline transition temperature of a range of ions from C to Xe, the amorphous layer thickness for a range of amorphizing implants, the superlinear increase in damage accumulation with dose, and the two-layered damage distribution observed along the path of a high-energy ion. In addition, this model is able to distinguish between dynamic annealing and post-cryogenic implantation annealing, whereby dynamic annealing is more effective in removing damage than post-cryogenic implantation annealing at the same temperature.
Circular one-dimensional position-sensitive time-of-flight microchannel plate detector using resistive anode for space plasma measurements
Authors: Yokota Shoichiro , Saito Yoshifumi
Published: January 2008

We have developed circular one-dimensional position-sensitive time-of-flight microchannel plate (MCP) detector for space plasma measurements. The MCP detector is equipped with a 354.375° resistive anode and a grid anode for obtaining position and timing signals simultaneously from one start event. The resistive anode provides high-resolution and continuous position sensing with two electronic channels alone. The grid anode generates start signals with pulse width of 1ns for time-of-flight measurements. The resistive anode is formed on an alumina substrate which is installed behind the output side of the MCP plates. The grid anode is installed between the MCP plates and the resistive anode for the simultaneous detection of position and start signals. Stop signals are obtained from stop events by a center anode formed on the alumina substrate behind the MCP plates. On the basis of the test experiments, we have evaluated that the MCP detector functionally deals with the position and timing signals.
Single-mode octagonal photonic crystal fibers for the middle infrared
Authors: Millo Arnon , Lobachinsky Lilya , Katzir Abraham
Published: January 2008

Single-mode optical fibers for the middle infrared are needed for many applications such as infrared fiber lasers, spatial filtering, and interferometry. Index guiding photonic crystal fibers offer many advantages over regular step-index fibers such as a wide spectral range and large core area. In this paper, we report the design and fabrication of single-mode octagonal photonic crystal fibers. These fibers were fabricated from silver halide polycrystalline materials which have high transmission in the middle infrared. The fibers showed a single-mode behavior at 10.6μm with low losses and a large mode diameter (110μm).
Variable temperature thin film indentation with a flat punch
Authors: Cross Graham L. W. , O′Connell Barry S. , Pethica John B. , Rowland Harry , King William P.
Published: January 2008

We present modifications to conventional nanoindentation that realize variable temperature, flat punch indentation of ultrathin films. The technique provides generation of large strain, thin film extrusion of precise geometries that idealize the essential flows of nanoimprint lithography, and approximate constant area squeeze flow rheometry performed on thin, macroscopic soft matter samples. Punch radii as small as 185nm have been realized in ten-to-one confinement ratio testing of 36nm thick polymer films controllably squeezed in the melt state to a gap width of a few nanometers. Self-consistent, compressive stress versus strain measurements of a wide variety of mechanical testing conditions are provided by using a single die-sample system with temperatures ranging from 20to125°C and loading rates spanning two decades. Low roughness, well aligned flat punch dies with large contact areas provide precise detection of soft surfaces with standard nanoindenter stiffness sensitivity. Independent heating and thermometry with heaters and thermocouples attached to the die and sample allow introduction of a novel directional heat flux measurement method to ensure isothermal contact conditions. This is a crucial requirement for interpreting the mechanical response in temperature sensitive soft matter systems. Instrumented imprint is a new nanomechanics material testing platform that enables measurements of polymer and soft matter properties during large strains in confined, thin film geometries and extends materials testing capabilities of nanoindentation into low modulus, low strength glassy, and viscoelastic materials.
Rosette screw arms created by ac-domain structures and dislocations in barium titanate during nanoindentation
Authors: Schneider G. A. , Scholz T. , Espinoza-Beltrán F. J.
Published: January 2008

The nanoindentation in out-of-plane domains of a {001} oriented BaTiO3 single crystal was performed with a conical indenter of 0.9μm tip radius. The first pop-in occurs at 5±0.5mN load. The surface deformation was studied with atomic force microscopy and piezoresponse force microscopy. In addition to dislocation structures inside the remanent indentation, a rosette arm pattern is observed. The four identified screw arms are due to created a-domains as well as to at the free surface emerging dislocations of the {110}⟨11¯0⟩ glide system. For parts of the screw arms, a detailed analysis of the domain and dislocation structure is presented.
Infinite-dimensional representations of the rotation group and Dirac monopole problem
Authors: Nesterov Alexander I. , Aceves de la Cruz Fermín
Published: January 2008

Within the context of infinite-dimensional representations of the rotation group, the Dirac monopole problem is studied in detail. Irreducible infinite-dimensional representations, which have been realized in the indefinite metric Hilbert space, are given by linear unbounded operators in infinite-dimensional topological spaces, supplied with a weak topology and associated weak convergence. We argue that an arbitrary magnetic charge is allowed, and the Dirac quantization condition can be replaced by a generalized quantization rule yielding a new quantum number, the so-called topological spin, which is related to the weight of the Dirac string.
Source generation of the Davey-Stewartson equation
Authors: Hu Juan , Wang Hong-Yan , Tam Hon-Wah
Published: January 2008

The “source generation” procedure (SGP) proposed by Hu and Wang [Inverse Probl. 22, 1903 (2006)] provides a new way to systematically generate so-called soliton equations with self-consistent sources. In this paper, we apply this SGP to a Davey-Stewartson (DS) equation based on the Hirota bilinear form, producing a system of equations which is called the DS equation with self-consistent sources (DSESCS). Meanwhile, we obtain the Gramm-type determinant solutions to the DSESCS. Since the DS equation is a (2+1)-dimensional integrable generalization of the nonlinear Schrödinger (NLS) equation, the DSESCS may be viewed as a (2+1)-dimensional integrable generalization of the nonlinear Schrödinger equation with self-consistent sources. These results indicate the commutativity of source generation procedure and (2+1)-dimensional integrable generalizations for the NLS equation.
Relations between convergence rates in Schatten p-norms
Authors: Albini Paolo , Toigo Alessandro , Umanità Veronica
Published: January 2008

In quantum estimation theory and quantum tomography, the quantum state obtained by sampling converges to the “true” unknown density matrix under topologies that are different from the natural notion of distance in the space of quantum states, i.e., the trace class norm. In this paper, we address such problem, finding relations between the rates of convergence in the Schatten p-norms and in the trace class norm.
Nanostructure and magnetic properties of L1 FePt:X films
Authors: George Tom A. , Li Zhen , Yan Minglang , Xu Yingfan , Skomski Ralph , Sellmyer David J.
Published: January 2008

Nonepitaxial FePt:X films (X=Cu,Au,CuAu) with tunable magnetic properties are fabricated and investigated. Emphasis is on controlling and adjusting the magnetic properties of high-density perpendicular recording media through exchange decoupling and anisotropy. The films are initially deposited as multilayers with the structure [FePt∕X]n and have individual thicknesses from about 0.06to1.1nm. To create an (001)-oriented granular L10 structure, the films are then annealed at temperatures of 600°C for 5min and 550°C for 10min. The data indicate that Cu enters the L10 lattice whereas Au segregates at the grain boundaries and reduces the intergranular exchange coupling. For X=CuAu, we obtain coercivities Hc below 10kOe, and slopes α=(dM∕dH)Hc of about 1. For X=Cu, we find a favorable reduction in Curie temperature and Hc.
Leader effects on femtosecond-laser-filament-triggered discharges
Authors: Fujii Takashi , Miki Megumu , Goto Naohiko , Zhidkov Alexei , Fukuchi Tetsuo , Oishi Yuji , Nemoto Koshichi
Published: January 2008

Dynamics of laser filaments in strong nonuniform electric fields is studied with high temporal and spatial resolution. Considerable reduction of the breakdown potential is found and is attributed to a filament-induced leader. Two breakdown modes, fast and slow, are found in 0.4MV positive dc-voltage discharges activated by filaments that are induced by 65fs, 170mJ laser pulses. In the fast mode with duration order of a few microseconds, the filament may acquire the electrode potential and temporarily maintain it, becoming a leader. This gives rise to an average electric field over the attachment instability threshold between a leader head and cathode. Ionization waves precede the breakdown with maximal voltage reduction up to 40% for this mode. The slow mode with its duration order of 1ms appears with a considerably smaller voltage reduction when the leader decays before the secondary streamer; the breakdown delay depends on negative and positive ion mobilities in this case.
Thermodynamic analysis of hydrogen sorption reactions in Li–Mg–N–H systems
Authors: Moysés Araújo C. , Scheicher Ralph H. , Ahuja Rajeev
Published: January 2008

We report a comprehensive first-principles study of the thermodynamics of the hydrogen release reaction from xLiH–yMg(NH2)2 mixtures involving the composition ratios (x=2, y=1), (x=8, y=3), and (x=12, y=3), with special emphasis on the effect of the different intermediate steps. For all three mixing ratios of LiH∕Mg(NH2)2 we find that the hydrogen release is initiated by the same reaction with an enthalpy of 46.1kJ∕mol of H2 in excellent agreement with recent experimental results. Additionally, we also investigated the substitution of LiH by MgH2 as reaction partner of Mg(NH2)2 in the fully hydrogenated state.
Effect of phase transition in shock-recovered silicon
Authors: Kishimura Hiroaki , Matsumoto Hitoshi
Published: January 2008

A series of shock-recovery experiments on a single crystal of silicon up to 38 GPa and characterizations of the recovered samples by x-ray diffraction analysis, Raman spectroscopy, and microscopic observations were performed for a better understanding of residual effects after shock loading by using a propellant gun. The x-ray diffraction trace of each sample revealed the absence of additional constituents including metastable phases and high-pressure phases of silicon except for 11 and 38 GPa. At 11 GPa, small amounts of metastable phases of silicon were obtained. The formation of copper silicide (Cu3Si) was confirmed in the sample shocked at 38 GPa. Considering the surface morphology revealed by microscopic observation, a thermochemical reaction through the melting of silicon resulted in the formation of Cu3Si. An additional band and the center frequency deviation of a peak were shown in the Raman spectroscopy results. The results of x-ray diffraction and Raman spectroscopy indicated that crystalline size reduction rather than the formation of metastable phases occurred. Structural deformation rather than the thermal effect caused by a shock-induced temperature rise may be responsible for the disappearance of metastable phases, which were observed in other high-pressure experiments.
A frequency-selective circulator via mode coupling between surface waveguide and resonators
Authors: Liu A. Q. , Khoo E. H. , Cheng T. H. , Li E. P. , Li J.
Published: January 2008

In this letter, a frequency-selective circulator via mode coupling between a circular photonic crystal (CPC) surface waveguide and resonator systems is developed. The curve surface waveguide is formed by reducing the outermost rods radii. It has a transmission efficiency of 95% when the surface concentric distance is reduced by half. The resonator system, which consists of rod(s) are placed at each of the CPC perpendicular axis. Using optimized resonator physical parameters, selected frequencies are dropped at different locations with efficiency of 97%. It is suggested that several circulators can be cascaded to build a large scale frequency-selective multiplexing system.
Steady state comparative-longitudinal heat flow method using specimen of different thicknesses for measuring thermal conductivity of lotus-type porous metals
Authors: Chiba H. , Ogushi T. , Nakajima H. , Torii K. , Tomimura T. , Ono F.
Published: January 2008

Lotus-type porous metal with many straight pores is attractive as a heat sink, because larger heat transfer capacity is obtained, owing to the small diameter of the pores. In order to use lotus-type porous metal effectively as a heat sink, it is important to clarify the effective conductivity and to consider the pore effect on heat conduction in the lotus-type porous metal. Because lotus-type porous metal is an anisotropic material, a steady state comparative-longitudinal heat flow (SCHF) method for thermal conductivity, referring to the ASTM standard, is thought to be better than a nonsteady method such as a laser flash method. This paper investigated the variable of the steady state comparative-longitudinal heat flow method by using specimens of different thicknesses (SCHF-DT) method for measuring the effective thermal conductivity. As a result, the uncertainty of effective thermal conductivity of a specimen was found to be smaller, as the diameter of a rod became larger and the length of a rod became shorter. In addition, it was found by error analysis that a dominant factor in the uncertainty of this method was the contact thermal difference between the rod and specimen.
Determination of the dissolution mechanism of Al2O3 in CaO-Al2O3-SiO2 liquids using a combined experimental-numerical approach
Authors: Verhaeghe F. , Liu J. , Guo M. , Arnout S. , Blanpain B. , Wollants P.
Published: January 2008

Experimental results obtained from the in situ observation of the dissolution of spherical Al2O3 particles in CaO-Al2O3-SiO2-containing melts at elevated temperatures are analyzed using a lattice Boltzmann dissolution model. Through a comparison of the experimental dissolution curve with analytical predictions and numerical simulations, the rate-limiting step is identified as diffusion control. Estimations of the effective binary diffusion coefficient are obtained, together with an estimate of the activation energy for the diffusion process.
Effect of annealing atmosphere on the recombination activity of copper precipitates formed by rapid thermal process in conventional and nitrogen-doped Czochralski silicon wafers
Authors: Wang Weiyan , Yang Deren , Ma Xiangyang , Zeng Yuheng , Que Duanlin
Published: January 2008

The effect of annealing atmosphere of Ar, N2, or O2 on the recombination activity of copper (Cu) precipitates, formed by the rapid thermal process (RTP), in conventional Czochralski (CZ) silicon and nitrogen-doped CZ (NCZ) silicon wafers have been investigated. It was revealed that the recombination activity of Cu precipitates formed under N2, Ar, and O2 atmospheres decreased in turn. Moreover, the RTP under O2 atmosphere led to a higher critical temperature for Cu precipitation in comparison with the RTP under Ar or N2 atmosphere. Furthermore, it was found that the Cu precipitates formed under the same conditions possessed stronger recombination activity in CZ silicon than in NCZ silicon. The above results have been tentatively explained.
Meniscus model for noncontacting and contacting sphere-on-flat surfaces including elastic-plastic deformation
Authors: Xue Xiaojie , Polycarpou Andreas A.
Published: January 2008

Single asperity adhesive contact is a fundamental problem in physics and mechanics and a significant body of research works have been published on capillary adhesion of noncontacting and contacting spheres and surfaces in general. The existing works, however, do not specifically consider the change of the projected meniscus area due to the solid asperity deformation when contacting with the flat solid surface. In this paper, accounting for the effects of both elastic and plastic deformations, we present an improved meniscus model for a deformable sphere on a rigid flat surface covering a large range of interference values from noncontact to a fully plastic contact. For elastic contact, the real contact area, the deformed sphere profile, and the projected meniscus area were determined by the Hertzian solution. For elastic-plastic contact, a finite element-based spherical solution was used to calculate the real wetted area and meniscus force. The model predicts that due to the spherical deformation, the meniscus force increases with increasing interference. The effects of asperity radius, humidity level, and different liquid environments, as well as the valid range of the model, were also discussed.
Modeling tunable bulk acoustic resonators based on induced piezoelectric effect in BaTiO3 and Ba0.25Sr0.75TiO3 films
Authors: Vendik Irina B. , Turalchuk Pavel A. , Vendik Orest G. , Berge John
Published: January 2008

A model for tunable thin film bulk acoustic resonators (TFBARs) based on ferroelectric films is proposed. The model is based on electromechanical equations taking into account piezoelectric effect and electrostriction effect induced by the dc electric field. The dc field induced shift of the resonant frequency is explained by the high-order nonlinear effects in the ferroelectric material. The main contribution to the tunability of the resonant frequency under dc electric field can be attributed to electrostriction, which is nonlinear with respect to the mechanical deformation. It is shown that the upward or downward shift in the resonant frequency is given by the sign of the nonlinear component of the electrostriction tensor M. The model is verified by comparing the results with the measured microwave input impedance of BaTiO3 and Ba0.25Sr0.75TiO3 based TFBARs. For a positive sign of the nonlinear coefficient of electrostriction M, the model predicts an upward shift of the resonant frequencies (resonance and antiresonance) under dc biasing in case of the TFBAR based on BaTiO3, whereas a negative sign of the nonlinear coefficient of electrostriction M predicts downward shift of the resonant frequencies for TFBAR based on Ba0.25Sr0.75TiO3 films.
An apparatus for simultaneous thermodynamic and optical measurements, with large temperature excursions
Authors: Giannasi A. , Celli M. , Grazzi F. , Ulivi L. , Zoppi M.
Published: January 2008

We report the design and realization of an integrated system for measuring, at the same time, the thermodynamic and spectroscopic features of nanoporous materials interesting for hydrogen storage purposes. The whole investigation cycle, from thermal activation to the actual investigation of uptake and release of hydrogen, is carried out in the same vacuum tight vessel, equipped with an optical window, whose temperature can range between 10 and 750K, up to a maximum pressure of 50bars. The system has been designed to investigate properties of carbon nanotubes but its use can be extended to any kind of nanoporous sample such as, for example, carbon nanofibers, zeolytes, metal organic frameworks, and similar materials.
Enhancement of the light output power of InGaN/GaN light-emitting diodes grown on pyramidal patterned sapphire substrates in the micro- and nanoscale
Authors: Gao Haiyong , Yan Fawang , Zhang Yang , Li Jinmin , Zeng Yiping , Wang Guohong
Published: January 2008

Sapphire substrates were patterned by a chemical wet etching technique in the micro- and nanoscale to enhance the light output power of InGaN/GaN light-emitting diodes (LEDs). InGaN/GaN LEDs on a pyramidal patterned sapphire substrate in the microscale (MPSS) and pyramidal patterned sapphire substrate in the nanoscale (NPSS) were grown by metalorganic chemical vapor deposition. The characteristics of the LEDs fabricated on the MPSS and NPSS prepared by wet etching were studied and the light output powers of the LEDs fabricated on the MPSS and NPSS increased compared with that of the conventional LEDs fabricated on planar sapphire substrates. In comparison with the planar sapphire substrate, an enhancement in output power of about 29% and 48% is achieved with the MPSS and NPSS at an injection current of 20 mA, respectively. This significant enhancement is attributable to the improvement of the epitaxial quality of GaN-based epilayers and the improvement of the light extraction efficiency by patterned sapphire substrates. Additionally, the NPSS is more effective to enhance the light output power than the MPSS.
Dynamo action in an annular array of helical vortices
Authors: Volk R. , Odier P. , Pinton J.-F.
Published: January 2008

We numerically study the induction mechanisms generated from an array of helical motions distributed along a cylinder. Our flow is a very idealized geometry of the columnar structure that has been proposed for the convective motion inside the Earth’s core. Using an analytically prescribed flow, we apply a recently introduced iterative numerical scheme [M. Bourgoin, P. Odier, J.-F. Pinton, and Y. Richard, Phys. Fluids 16, 2529 (2004)] to solve the induction equation and analyze the flow response to externally applied fields with simple geometries (e.g., azimuthal, radial). Symmetry properties allow us to build selected induction modes whose interactions lead to dynamo mechanisms. Using an induction operator formalism, we show how dipole and quadrupole dynamos can be envisioned from such motions. The method identifies the main induction mechanisms that generate dynamo action in the selected geometry. Here, it emphasizes the competition between α-effect and field expulsion as well as the role of scale separation.
Effects of K content on the dielectric, piezoelectric, and ferroelectric properties of 0.95(KxNa1−x)NbO3−0.05LiSbO3 lead-free ceramics
Authors: Wu Jiagang , Xiao Dingquan , Wang Yuanyu , Zhu Jianguo , Yu Ping
Published: January 2008

The effects of K content on the dielectric, piezoelectric, and ferroelectric properties of 0.95(KxNa1−x)NbO3−0.05LiSbO3 (0.95KxNN−0.05LS) (x=0.25–0.75) lead-free piezoelectric ceramics prepared by conventional solid-state sintering were studied. The experimental results show that the dielectric, piezoelectric, and ferroelectric properties strongly depend on K content in the 0.95KxNN−0.05LS ceramics. The 0.95KxNN−0.05LS (x=0.40) ceramics exhibit enhanced electrical properties (d33≈280 pC/N; kp≈49.4%; Tc∼364 °C; To-t=25 °C; εr≈1463; tan δ≈2.3%; Pr∼30.8 μC/cm2; Ec∼14.0 kV/cm). The enhanced electrical properties of 0.95KxNN−0.05LS (x=0.40) ceramics are attributed to the polymorphic phase transition near room temperature. These results show that 0.95KxNN−0.05LS (x=0.40) ceramic is a promising lead-free piezoelectric material.
Ultrathin films of homeotropically aligned columnar liquid crystals on indium tin oxide electrodes
Authors: Charlet E. , Grelet E. , Brettes P. , Bock H. , Saadaoui H. , Cisse L. , Destruel P. , Gherardi N. , Seguy I.
Published: January 2008

We report the achievement of very thin films (thickness of about 50nm) of thermotropic columnar liquid crystal in homeotropic (columns normal to the interface) orientation on indium tin oxide (ITO) electrodes. The face-on alignment of the discotic compound has been obtained by thermal annealing without any intermediate coating between the mesophase and the ITO substrate. Such a columnar mesophase alignment is thus shown on a substrate of technological interest in open supported thin film reaching the thickness range suitable for organic photovoltaic devices.
Fabrication, characterization, and the rectifying properties of the interfacial structural controlled LaSrMnO–Si heterojunctions
Authors: Wang K. , Han X. D. , Zhang Z. , Li T. , Zhang M. , Yan H.
Published: January 2008

The La0.9Sr0.1MnO3∕Si p-n junctions with optimized rectifying properties were synthesized using radio frequency magnetron sputtering followed by an in situ annealing process at a series of temperatures. The thickness of the SiOx barrier layer d determines the rectifying properties of the heterojunctions by Id3=constant (I is the rectifying current). The dead layer was determined to act as a semiconductor layer in the conduction process. The annealed samples show Mn4+ rich interface which correlated closely to the electrical transportation properties of the heterostructures. Lowering the annealing temperature, the electric sensitivity of the heterojunction can be significantly enhanced by simply tuning the thickness of the barrier layer.
Acceptor levels in GaSe:In crystals investigated by deep-level transient spectroscopy and photoluminescence
Authors: Cui Yunlong , Dupere Ryan , Burger Arnold , Johnstone D. , Mandal Krishna C. , Payne S. A.
Published: January 2008

Deep-acceptor levels associated with indium in indium-doped GaSe crystals have been measured. High-quality Schottky diodes of GaSe:In have been fabricated and characterized using current-voltage, capacitance-voltage, and deep-level transient spectroscopy (DLTS). Four DLTS peaks at 127, 160, 248, and 319K, corresponding to 0.21, 0.22, 0.44, and 0.74eV above the valence band, were well resolved and assigned to be an indium-on-gallium antisite (InGa), a gallium vacancy (VGa), an indium gallium vacancy complex (VGa-In), and a native defect associated with stacking fault or dislocation, respectively. Low-temperature photoluminescence (PL) spectroscopy measure-ments were performed on GaSe and GaSe:In crystals. The ground and the first excited states of the free exciton emissions were identified and the band-gap energies were determined. The results that the peak of exciton bound to acceptor (A0,X) disappeared and the peak of donor-acceptor pair appeared in GaSe crystal after indium doping are consistent with the DLTS acceptor assignments.