bluuebunny/crossref_metadata_2025 huggingface dataset:
Towards a beyond 1 GHz solid-state nuclear magnetic resonance: External lock operation in an external current mode for a 500 MHz nuclear magnetic resonance
Authors: Takahashi Masato , Ebisawa Yusuke , Tennmei Konosuke , Yanagisawa Yoshinori , Hosono Masami , Takasugi Kenji , Hase Takashi , Miyazaki Takayoshi , Fujito Teruaki , Nakagome Hideki , Kiyoshi Tsukasa , Yamazaki Toshio , Maeda Hideaki
Published: October 2012

Achieving a higher magnetic field is important for solid-state nuclear magnetic resonance (NMR). But a conventional low temperature superconducting (LTS) magnet cannot exceed 1 GHz (23.5 T) due to the critical magnetic field. Thus, we started a project to replace the Nb3Sn innermost coil of an existing 920 MHz NMR (21.6 T) with a Bi-2223 high temperature superconducting (HTS) innermost coil. Unfortunately, the HTS magnet cannot be operated in persistent current mode; an external dc power supply is required to operate the NMR magnet, causing magnetic field fluctuations. These fluctuations can be stabilized by a field-frequency lock system based on an external NMR detection coil. We demonstrate here such a field-frequency lock system in a 500 MHz LTS NMR magnet operated in an external current mode. The system uses a 7Li sample in a microcoil as external NMR detection system. The required field compensation is calculated from the frequency of the FID as measured with a frequency counter. The system detects the FID signal, determining the FID frequency, and calculates the required compensation coil current to stabilize the sample magnetic field. The magnetic field was stabilized at 0.05 ppm/3 h for magnetic field fluctuations of around 10 ppm. This method is especially effective for a magnet with large magnetic field fluctuations. The magnetic field of the compensation coil is relatively inhomogeneous in these cases and the inhomogeneity of the compensation coil can be taken into account.
Photocatalytic treatment of industrial wastewater containing chromium as a model pollutant-effect on process parameters and kinetically studies
Authors: Chaudhary Rubina , Thakur Rajendra Singh
Published: October 2012

This study has focused on treatment of the metal ions, which have infinite lifetimes and build up their concentrations in food chains to toxic levels. Many researches have been performed about use of photocatalysis processes in treatment of industrial wastewater. The additional advantage is coming from sensitivity of the Cr(VI) photo reduction to the surface catalytic effects or to the presence of an additional electron donor and acceptors; then, a synergistic effect was observed in two- or multi-component systems. The efficiency of total chromium reduction by photo catalytic treatment under UV-light in different degradation systems, UV, TiO2/UV, H2O2/UV, TiO2/citric acid/UV, and TiO2/H2O2/UV, was studied and compared. Even when the TiO2/citric acid/H2O2/UV reaction happened to be the most efficient process, both homogeneous and heterogeneous photocatalysis can be considered as suitable methods to reach the complete mineralization of the solutions. Maximum treatment efficiency of total chromium reduction with increasing citric acid concentration from 20 to 80 mg/l was reduced to 62%, 76.7%, 77.91%, and 100%, and with increasing H2O2 concentration from 20 to 100 ml/l was reduced to 40.57%, 41.84%, 50.77%, 61.03%, and 100%, which is more efficient than the H2O2/UV process. Finally, the combination of TiO2/citric acid/H2O2/UV result was observed more efficient than the TiO2/citric acid and TiO2/H2O2/UV processes in the solutions.
Ultrasound-induced release of micropallets with cells
Authors: Guo Sijia , Wang Yuli , Allbritton Nancy , Jiang Xiaoning
Published: October 2012

Separation of selected adherent live cells attached on an array of microelements, termed micropallets, from a mixed population is an important process in biomedical research. We demonstrated that adherent cells can be safely, selectively, and rapidly released from the glass substrate together with micropallets using an ultrasound wave. A 3.3-MHz ultrasound transducer was used to release micropallets (500 μm × 500 μm × 300 μm) with attached HeLa cells, and a cell viability of 92% was obtained after ultrasound release. The ultrasound-induced release process was recorded by a high-speed camera, revealing a proximate velocity of ∼0.5 m/s.
Two-dimensional direct numerical simulation evaluation of the flame-surface density model for flames developing from an ignition kernel in lean methane/air mixtures under engine conditions
Authors: Reddy Harinath , Abraham John
Published: October 2012

Flame surface density (FSD) models are often used in premixed turbulent combustion modeling to provide closure in large eddy simulations (LES) and Reynolds-averaged simulations. In the present study, data obtained from direct numerical simulation, with reduced chemistry, of flames developing from ignition kernels in lean methane-air mixtures is used to study the accuracy of the FSD modeling terms for LES applications. This study is conducted at elevated temperature and pressure conditions relevant to lean-burn natural gas engines. The closure models for four terms in the FSD transport equation are studied. It is observed that the propagation term as resolved by the LES grid adequately models the actual propagation term without additional sub-grid scale modeling. On the other hand, the closure of the curvature term requires a sub-grid scale model. Two sub-grid scale curvature models are studied and it is seen that there are differences between the actual and the modeled curvature terms during the early evolution of the ignition kernel. The agreement improves as the developing flame approaches a statistically steady-state. The modeling of the sub-grid convection term is analyzed and differences between the actual and the modeled terms are observed to grow during the transient evolution of the ignition kernel. A sub-grid scale model for the closure of the strain rate term shows disagreement at both early and late stages of kernel growth. In general, the sensitivity of the strain rate and curvature models to the filter width is found to be greater than for convection and propagation.
Mean velocity and temperature profiles in a sheared diabatic turbulent boundary layer
Authors: Li Dan , Katul Gabriel G. , Bou-Zeid Elie
Published: October 2012

In the atmospheric surface layer, modifications to the logarithmic mean velocity and air temperature profiles induced by thermal stratification or convection are accounted for via stability correction functions ϕm and ϕh, respectively, that vary with the stability parameter ς. These two stability correction functions are presumed to be universal in shape and independent of the surface characteristics. To date, there is no phenomenological theory that explains all the scaling laws in ϕh with ς, how ϕh relates to ϕm, and why ϕh ⩽ ϕm is consistently reported. To develop such a theory, the recently proposed links between the mean velocity profile and the Kolmogorov spectrum of turbulence, which were previously modified to account for the effects of buoyancy, are generalized here to include the mean air temperature profile. The resulting theory explains the observed scaling laws in ϕm and ϕh reported in many field and numerical experiments, predicts their behaviors across a wide range of atmospheric stability conditions, and elucidates why heat is transported more efficiently than momentum in certain stability regimes. In particular, it is shown that the enhancement in heat transport under unstable conditions is linked to a “scale-resonance” between turnover eddies and excursions in the instantaneous air temperature profiles. Excluding this scale-resonance results in the conventional Reynolds analogy with ϕm = ϕh across all stability conditions.
Levinson's theorem for graphs II
Authors: Childs Andrew M. , Gosset David
Published: October 2012

We prove Levinson's theorem for scattering on an (m+n)-vertex graph with n semi-infinite paths each attached to a different vertex, generalizing a previous result for the case n = 1. This theorem counts the number of bound states in terms of the winding of the determinant of the S-matrix. We also provide a proof that the bound states and incoming scattering states of the Hamiltonian together form a complete basis for the Hilbert space, generalizing another result for the case n = 1.
Casimir eigenvalues for universal Lie algebra
Authors: Mkrtchyan R. L. , Sergeev A. N. , Veselov A. P.
Published: October 2012

For two different natural definitions of Casimir operators for simple Lie algebras we show that their eigenvalues in the adjoint representation can be expressed polynomially in the universal Vogel's parameters α, β, γ and give explicit formulae for the generating functions of these eigenvalues.
A design for a compact time-of-flight mass spectrometer
Authors: Manard Manuel J.
Published: October 2012

The design of a prototype, compact time-of-flight (TOF) mass spectrometer is described. The system primarily consists of an ion acceleration/focusing/steering assembly (AFSA), an 8 cm field-free region, a 4 cm dual-stage reflectron, and a miniature microchannel plate detector. Consequently, the resulting flight length of the system is 12 cm. The system has been designed with the capability to sample directly from atmosphere at ambient pressures. This is accomplished through the use of an electrodynamic ion funnel, housed in an intermediate-vacuum chamber that is coupled to the inlet of the TOF chamber. TOF spectra were obtained using noble gases (Ar, Kr, and Xe) as test chemicals. These measured flight times were used to probe the performance of the instrument. A temporal resolution (tflight/Δt) of approximately 125, acquired using 129Xe+, has been measured for the system.
Capacitance-voltage profiling on polar III-nitride heterostructures
Authors: Hurni Christophe A. , Kroemer Herbert , Mishra Umesh K. , Speck James S.
Published: October 2012

It has been long recognized that the capacitance-voltage (C-V) profiling technique only yields an apparent carrier concentration n̂, which, in general, differs from the majority carrier concentration. However, it is still possible to extract the conduction band offset ΔEc and interface charge σi from n̂, provided the doping profile and the exact hetero-interface position are known. We will show that in the case of σi∼1012cm−2, which is typical for the polar III-nitrides, an interface misplacement as small as 1 nm leads to a large error in the extracted ΔEc, making the technique challenging to use, even with the help of a computer-assisted reconstruction. We used one-sided p+-n junctions to experimentally investigate σi and ΔEc for Ga-face n-N GaN/AlGaN heterostructures through C-V profiling. The extracted σi matched the polarization charge reported by others.
Thermoreflectance imaging of percolation effects and dynamic resistance in indium tin oxide nanoparticle layers
Authors: Chavez R. , Angst S. , Maize K. , Gondorf A. , Schierning G. , Wolf D. E. , Lorke A. , Shakouri A.
Published: October 2012

Thin films of indium tin oxide nanoparticles are studied using charge-coupled device thoermoreflectance. High resolution sub-micron thermal images confirm that percolation in current conduction induces strongly inhomogeneous heat loads on the thin film. We experimentally show that the inhomogeneous current densities induce thousands of “micro-hotspots” that can be 20% hotter than the average Joule heating in the thin film layer and show comparable behavior in a resistor network. In addition to the percolation induced “micro-hotspots,” we report major hotspots, with non-Joule behavior, whose temperature response is greater than I2. We demonstrate that a temperature dependent resistor can account for an effective exponent larger than 2. Finally, it is shown that while ambient molecules modify the thin film conductivity by at least 20%, current conduction and percolation effects remain largely unchanged, but such chemical reactions can be nonetheless detected with thermoreflectance.
Temperature dependence of electron magnetic resonance spectra of iron oxide nanoparticles mineralized in Listeria innocua protein cages
Authors: Usselman Robert J. , Russek Stephen E. , Klem Michael T. , Allen Mark A. , Douglas Trevor , Young Mark , Idzerda Yves U. , Singel David J.
Published: October 2012

Electron magnetic resonance (EMR) spectroscopy was used to determine the magnetic properties of maghemite (γ-Fe2O3) nanoparticles formed within size-constraining Listeria innocua (LDps)-(DNA-binding protein from starved cells) protein cages that have an inner diameter of 5 nm. Variable-temperature X-band EMR spectra exhibited broad asymmetric resonances with a superimposed narrow peak at a gyromagnetic factor of g ≈ 2. The resonance structure, which depends on both superparamagnetic fluctuations and inhomogeneous broadening, changes dramatically as a function of temperature, and the overall linewidth becomes narrower with increasing temperature. Here, we compare two different models to simulate temperature-dependent lineshape trends. The temperature dependence for both models is derived from a Langevin behavior of the linewidth resulting from “anisotropy melting.” The first uses either a truncated log-normal distribution of particle sizes or a bi-modal distribution and then a Landau-Liftshitz lineshape to describe the nanoparticle resonances. The essential feature of this model is that small particles have narrow linewidths and account for the g ≈ 2 feature with a constant resonance field, whereas larger particles have broad linewidths and undergo a shift in resonance field. The second model assumes uniform particles with a diameter around 4 nm and a random distribution of uniaxial anisotropy axes. This model uses a more precise calculation of the linewidth due to superparamagnetic fluctuations and a random distribution of anisotropies. Sharp features in the spectrum near g ≈ 2 are qualitatively predicted at high temperatures. Both models can account for many features of the observed spectra, although each has deficiencies. The first model leads to a nonphysical increase in magnetic moment as the temperature is increased if a log normal distribution of particles sizes is used. Introducing a bi-modal distribution of particle sizes resolves the unphysical increase in moment with temperature. The second model predicts low-temperature spectra that differ significantly from the observed spectra. The anisotropy energy density K1, determined by fitting the temperature-dependent linewidths, was ∼50 kJ/m3, which is considerably larger than that of bulk maghemite. The work presented here indicates that the magnetic properties of these size-constrained nanoparticles and more generally metal oxide nanoparticles with diameters d < 5 nm are complex and that currently existing models are not sufficient for determining their magnetic resonance signatures.
First-principles calculations of rovibrational energies, dipole transition intensities and partition function for ethylene using MULTIMODE
Authors: Carter Stuart , Sharma Amit R. , Bowman Joel M.
Published: October 2012

Large-scale, rovibrational variational calculations are performed for ethylene, using the potential energy surface published by Avila and Carrington [J. Chem. Phys. 135, 064101 (2011)10.1063/1.3617249]. Energies for J = 0 are in very good agreement with their benchmark results. Corresponding energies for J = 1 and J = 2 are also given. Calculations with a slightly reduced basis permit energies to J = 40, allowing a reliable determination of the partition function at 296 K. Using a new ab initio dipole moment surface, reported here, the infrared spectra of five dipole-allowed fundamentals are calculated. Both the partition function and infrared spectra are shown to be in excellent agreement with those in the experimental HITRAN database, with the exception of one band, which we believe is partially mis-assigned in HITRAN.
Specification, construction, and exact reduction of state transition system models of biochemical processes
Authors: Bugenhagen Scott M. , Beard Daniel A.
Published: October 2012

Biochemical reaction systems may be viewed as discrete event processes characterized by a number of states and state transitions. These systems may be modeled as state transition systems with transitions representing individual reaction events. Since they often involve a large number of interactions, it can be difficult to construct such a model for a system, and since the resulting state-level model can involve a huge number of states, model analysis can be difficult or impossible. Here, we describe methods for the high-level specification of a system using hypergraphs, for the automated generation of a state-level model from a high-level model, and for the exact reduction of a state-level model using information from the high-level model. Exact reduction is achieved through the automated application to the high-level model of the symmetry reduction technique and reduction by decomposition by independent subsystems, allowing potentially significant reductions without the need to generate a full model. The application of the method to biochemical reaction systems is illustrated by models describing a hypothetical ion-channel at several levels of complexity. The method allows for the reduction of the otherwise intractable example models to a manageable size.
Bound states at threshold resulting from Coulomb repulsion
Authors: Gridnev Dmitry K.
Published: October 2012

The eigenvalue absorption for a many-particle Hamiltonian depending on a parameter is analyzed in the framework of non-relativistic quantum mechanics. The long-range part of pair potentials is assumed to be pure Coulomb and no restriction on the particle statistics is imposed. It is proved that if the lowest dissociation threshold corresponds to the decay into two likewise non-zero charged clusters then the bound state, which approaches the threshold, does not spread and eventually becomes the bound state at threshold. The obtained results have applications in atomic and nuclear physics. In particular, we prove that an atomic ion with the critical charge Zcr and Ne electrons has a bound state at threshold given that Zcr ∈ (Ne − 2, Ne − 1), whereby the electrons are treated as fermions and the mass of the nucleus is finite.
On the wave breaking phenomena for the generalized periodic two-component Dullin-Gottwald-Holm system
Authors: Chen Caixia , Yan Yonghua
Published: October 2012

Considered herein is the generalized two-component periodic Dullin-Gottwald-Holm system, which can be derived from the Euler equation with nonzero constant vorticity in shallow water waves moving over a linear shear flow. The precise blow-up scenarios of strong solutions and several results of blow-up solutions with certain initial profiles are described in detail. The exact blow-up rates are also determined. Finally, a sufficient condition for global solutions is established.
Impact of universal mobility law on polycrystalline organic thin-film transistors
Authors: Raja Munira , Donaghy David , Myers Robert , Eccleston Bill
Published: October 2012

We have developed novel analytical models for polycrystalline organic thin-film transistor (OTFT) by employing new concepts on the charge carrier injection to polysilicon thin-films. The models, also incorporate the effect of contact resistance associated with the poor ohmic nature of the contacts. The drain current equations of the OTFT, both in the quasi-diffusion and quasi-drift regimes, predict temperature dependencies on essential material and device parameters. Interestingly, under the drift regime, the polycrystalline OTFT model reveals similar power dependencies on the applied voltages, to those of purely disordered model developed by utilizing the universal mobility law (UML). Such similarities are not thought to be coincidental since the effect of gate voltage on surface potential is influenced by the Fermi level pinning in the grain boundary. Nonetheless, the best fits on the data of 6,13-bis(tri-isopropylsilylethynyl) OTFTs are attained with the proposed polycrystalline rather than the disordered model, particularly at low gate voltages where the diffusive component is dominant. Moreover, in order to understand the effect of grain boundaries, we devise a relationship for the dependency of the effective mobility on carrier concentration, assuming a crystalline region to be in direct contact with a disordered region. Interestingly, we find a similar dependency as the UML in purely disordered materials, which further signifies the conduction to be limited by the grain boundaries. Subsequently, an analytical model for the variation of the effective mobility with gate voltage is established. Such models are vital in assisting the development of more accurate designs of the novel organic circuits.
Frequency effect on streaming phenomenon induced by Rayleigh surface acoustic wave in microdroplets
Authors: Alghane M. , Fu Y. Q. , Chen B. X. , Li Y. , Desmulliez M. P. Y. , Walton A. J.
Published: October 2012

Acoustic streaming of ink particles inside a water microdroplet generated by a surface acoustic wave (SAW) has been studied numerically using a finite volume numerical method and these results have been verified using experimental measurements. Effects of SAW excitation frequency, droplet volume, and radio-frequency (RF) power are investigated, and it has been shown that SAW excitation frequency influences the SAW attenuation length, lSAW, and hence the acoustic energy absorbed by liquid. It has also been observed that an increase of excitation frequency generally enhances the SAW streaming behavior. However, when the frequency exceeds a critical value that depends on the RF power applied to the SAW device, weaker acoustic streaming is observed resulting in less effective acoustic mixing inside the droplet. This critical value is characterised by a dimensionless ratio of droplet radius to SAW attenuation length, i.e., Rd/lSAW. With a mean value of Rd/lSAW ≈ 1, a fast and efficient mixing can be induced, even at the lowest RF power of 0.05 mW studied in this paper. On the other hand, for the Rd/lSAW ratios much larger than ∼1, significant decreases in streaming velocities were observed, resulting in a transition from regular (strong) to irregular (weak) mixing/flow. This is attributed to an increased absorption rate of acoustic wave energy that leaks into the liquid, resulting in a reduction of the acoustic energy radiated away from the SAW interaction region towards the droplet free surface. It has been demonstrated in this study that a fast and efficient mixing process with a smaller RF power could be achieved if the ratio of Rd/lSAW ≤ 1 in the SAW-droplet based microfluidics.
Direct structural characterisation of line gratings with grazing incidence small-angle x-ray scattering
Authors: Wernecke Jan , Scholze Frank , Krumrey Michael
Published: October 2012

Grazing incidence small-angle x-ray scattering (GISAXS) and x-ray reflectometry (XRR) have been used to investigate structural parameters, especially period length, line width, groove width, and line height, of grating test structures in the sub-micron range. The gratings are e-beam written structures on a quartz substrate with a fixed period length, but different line and groove widths, covered by a layer of ruthenium. A Ru layer thickness of 9.4 nm has been determined with XRR. GISAXS was performed in two orientations, with an incident beam alignment perpendicular and parallel to the grating lines. The scattering patterns in parallel orientation have been analysed without numerical simulation by Fourier transformation. The obtained results for line and groove width are in good agreement with nominal values. The analysis method has been validated by analysing simulated scattering data. A superposition of scattering intensities measured for different azimuthal rotation angles close to parallel alignment was used to determine the line height of a grating of 27.3 nm, which is also close to the nominal value. The Fourier analysis procedure opens up the possibility of traceable structure determination with GISAXS in the nanometre range.
Detection of virus-like nanoparticles via scattering using a chip-scale optical biosensor
Authors: Yan Rongjin , Scott Lynn N. , Kingry Luke C. , Yi Zhangjing , Erickson Tim , Slayden Richard A. , Dandy David S. , Lear Kevin L.
Published: October 2012

A local evanescent array coupled biosensor is used to detect spherical polystyrene nanoparticles with diameters of 40 nm and 200 nm, whose sizes and refractive index are similar to virus particles. The sensitivity is ∼1%/particle for 200 nm particles and 0.04%/particle for 40 nm particles. Mie scattering in an evanescent field theory is used to model the scattered light intensity for both sizes of nanoparticles.
Inertial squirmer
Authors: Wang S. , Ardekani A.
Published: October 2012

Although the propulsion of micro-organisms has been extensively studied in the literature, current studies have mainly focused on their propulsion in the absence of inertia. Here in this paper, we quantify the effects of convective inertial forces in the limit of small, but nonzero, Reynolds number regime. We analytically quantify the role of inertia on swimming speed, energy expenditure, and flow signature of an archetypal swimming model “squirmer”. Our results suggest that pushers, generating thrust behind their body, have a competitive advantage in swimming due to higher motility in the inertial regime. In contrast, those organisms that generate thrust in front of their body, pullers, have more efficient foraging in the inertial regime compared to their counterparts in the Stokes regime. Inertia enhances the swimming speed of a pusher swimmer and hinders it for a puller, potentially affecting a broad range of abundant millimeter- to centimeter-size organisms living in oceans and lakes.
Driving photochemistry by clustering: The ICl-Xe case
Authors: Glodic Pavle , Kartakoullis Andreas , Fárník Michal , Samartzis Peter C. , Kitsopoulos Theofanis N.
Published: October 2012

We present slice imaging data demonstrating the influence of clustering on the photodissociation dynamics of a diatomic molecule: iodine monochloride (ICl) was dissociated at 235 nm in He and Xe seed gasses, probing both Cl and I photofragment energy and angular distributions. We observe that the kinetic energy releases of both Cl and I fragments change from He to Xe seeding. For Cl fragments, the seeding in Xe increases the kinetic energy release of some Cl fragments with a narrow kinetic energy distribution, and leads to some fragments with rather broad statistical distribution falling off exponentially from near-zero energies up to about 2.5 eV. Iodine fragment distribution changes even more dramatically from He to Xe seeding: sharp features essentially disappear and a broad distribution arises reaching to about 2.5 eV. Both these observations are rationalized by a simple qualitative cluster model assuming ICl dissociation inside larger xenon clusters and “on surface” of smaller Xe species.
Nucleation of colloids and macromolecules in a finite volume
Authors: Lutsko James F.
Published: October 2012

A recently formulated description of homogeneous nucleation for Brownian particles in the over-damped limit based on fluctuating hydrodynamics is used to determine the nucleation pathway, characterized as the most likely path (MLP), for the nucleation of a dense-concentration droplet of globular protein from a dilute solution in a small, finite container. The calculations are performed by directly discretizing the equations for the MLP and it is found that they confirm previous results obtained for infinite systems: the process of homogeneous nucleation begins with a long-wavelength, spatially-extended concentration fluctuation that condenses to form the pre-critical cluster. This is followed by a classical growth processes. The calculations show that the post-critical growth involves the formation of a depletion zone around the cluster whereas no such depletion is observed in the pre-critical cluster. The approach therefore captures dynamical effects not found in classical density functional theory studies while consistently describing the formation of the pre-critical cluster.
On the Chan-Das gauge for the calculation of molecular magnetizabilities
Authors: Lazzeretti Paolo
Published: October 2012

It is proven that the common origin gauge proposed by Chan and Das for the calculation of molecular magnetizabilities within the conventional approach using gaugeless basis sets, i.e., the center of electronic charge, does not minimize the van Vleck paramagnetic contribution to the diagonal tensor components, unless the Arrighini-Maestro-Moccia sum rules for gauge invariance, and charge conservation are satisfied. Additional quantum mechanical constraints for extremum values of formally diamagnetic and paramagnetic contributions have also been obtained for magnetizabilities calculated via procedures allowing for a continuous translation of the origin of the current density.
Surface electrostatic potential transformation of nanodiamond induced by graphitization
Authors: Xu Qian , Yang Tao , Li Sheng-Tao , Zhao Xiang
Published: October 2012

The surface electrostatic potential of raw nanodiamonds is implied to be altered permanently during in the spontaneously occurred graphitization process by recent reports. With all-electron ab initio density functional theory methods, the intrinsic effect of graphitization on the electrostatic potential of nanodiamonds is investigated. It is exposed that while the graphitization process goes on, the dangling bonds on the (111) surface transfer into the inner side and subsequently the surface potential changes from negative to positive. Our results may be of great help in understanding the various electrostatic properties of nanodiamonds.
Disorder-induced dynamics in a pair of coupled heterogeneous phase oscillator networks
Authors: Laing Carlo R.
Published: October 2012

We consider a pair of coupled heterogeneous phase oscillator networks and investigate their dynamics in the continuum limit as the intrinsic frequencies of the oscillators are made more and more disparate. The Ott/Antonsen Ansatz is used to reduce the system to three ordinary differential equations. We find that most of the interesting dynamics, such as chaotic behaviour, can be understood by analysing a gluing bifurcation of periodic orbits; these orbits can be thought of as “breathing chimeras” in the limit of identical oscillators. We also add Gaussian white noise to the oscillators' dynamics and derive a pair of coupled Fokker-Planck equations describing the dynamics in this case. Comparison with simulations of finite networks of oscillators is used to confirm many of the results.
Scattering of a spinless particle by an asymmetric Hulthén potential within the effective mass formalism
Authors: Aydoğdu Oktay , Arda Altuğ , Sever Ramazan
Published: October 2012

Effective mass Klein-Gordon equation for the asymmetric Hulthén potential is solved in terms of hypergeometric functions. Results are obtained for the scattering and bound states with the position dependent mass and constant mass, as a special case. In both cases, we derive a condition for the existence of transmission resonance (T = 1). We also study how the transmission resonance depends on the particle energy and the shape of the external potential.
Model GW study of the late transition metal monoxides
Authors: Ye Lin-Hui , Asahi Ryoji , Peng Lian-Mao , Freeman Arthur J.
Published: October 2012

The model GW method [F. Gygi and A. Baldereschi, Phys. Rev. Lett. 62, 2160 (1989)]10.1103/PhysRevLett.62.2160 is an efficient simplification to the standard GW approximation which uses model dielectric function to describe the long range Coulomb interactions in semiconductors. In this work, the model GW method is used to calculate the quasiparticle band structures of MnO, FeO, CoO, and NiO. All four late transition metal monoxides are predicted to be insulators. The band gaps, magnetic moments, and quasiparticle spectra are in good agreement with the experiments, except for the satellite structures which are missing in the density of states because the model GW self-energy is static. The high accuracy of model GW is due to the usage of the accurate dielectric constants in the construction of the model dielectric functions which ensures the correct asymptotic behavior of the long range Coulomb interactions. Besides, we find that the transition metal 4s states are irrelevant to the formation of the band gaps, which supports the local approaches and the experimental interpretations of the band gaps by photoemission and electron energy loss spectroscopy, while contradicts the recent calculations by hybrid functionals, exact exchange, and one shot GW approximations.
Dependence of radiative efficiency and deep level defect incorporation on threading dislocation density for InGaN/GaN light emitting diodes
Authors: Armstrong A. , Henry T. A. , Koleske D. D. , Crawford M. H. , Westlake K. R. , Lee S. R.
Published: October 2012

The influence of threading dislocation (TD) density on electroluminescence and deep level defect incorporation in the multi-quantum well regions of InGaN/GaN light emitting diodes (LEDs) was investigated. LED efficiency increased with decreasing TD density. To elucidate the impact of TD density on deep level defect incorporation and resulting radiative efficiency, deep level optical spectroscopy and lighted capacitance voltage measurements were applied to the LEDs. Interestingly, the concentration of all observed deep levels decreased with TD density reduction, but their concentration also varied strongly with depth in the multi-quantum well region. These trends indicate that (1) TDs strongly influence point defect incorporation in InGaN/GaN LEDs and (2) TDs, possibly in conjunction with point defects, are detrimental to LED efficiency.
Generalized interface models for transport phenomena: Unusual scale effects in composite nanomaterials
Authors: Pavanello Fabio , Manca Fabio , Luca Palla Pier , Giordano Stefano
Published: October 2012

The effective transport properties of heterogeneous nanoscale materials and structures are affected by several geometrical and physical factors. Among them, the presence of imperfect interfaces plays a central role being often at the origin of the scale effects. To describe real contacts between different phases, some classical schemes have been introduced in literature, namely the low and the high conducting interface models. Here, we introduce a generalized formalism, which is able to take into account the properties of both previous schemes and, at the same time, it implements more complex behaviors, already observed in recent investigations. We apply our models to the calculation of the effective conductivity in a paradigmatic structure composed of a dispersion of particles. In particular, we describe the conductivity dependence upon the size of the inclusions finding an unusual non-monotone scale effect with a pronounced peak at a given particle size. We introduce some intrinsic length scales governing the universal scaling laws.
Intermodal four-wave mixing in a higher-order-mode fiber
Authors: Cheng Ji , Pedersen Martin E. V. , Charan Kriti , Wang Ke , Xu Chris , Grüner-Nielsen Lars , Jakobsen Dan
Published: October 2012

We demonstrate a high-efficiency intermodal four-wave-mixing process in an all-fiber system, comprising a picosecond fiber laser and a high-order-mode (HOM) fiber. Two pump photons in the LP01 mode of the fiber can generate an anti-Stokes photon in the LP01 mode and a Stokes photon in the LP02 mode. The wavelength dependent mode profiles of the HOM fiber produce significant spatial overlap between the modes involved. The anti-Stokes wave at 941 nm is generated with 20% conversion efficiency with input pulse energy of 20 nJ. The guidance of the anti-Stokes and Stokes waves in the HOM fiber enhances system stability.
Efficiency droop due to electron spill-over and limited hole injection in III-nitride visible light-emitting diodes employing lattice-matched InAlN electron blocking layers
Authors: Choi Suk , Ji Mi-Hee , Kim Jeomoh , Jin Kim Hee , Satter Md. M. , Yoder P. D. , Ryou Jae-Hyun , Dupuis Russell D. , Fischer Alec M. , Ponce Fernando A.
Published: October 2012

Data and analysis are presented for the study of efficiency droop in visible III-nitride light-emitting diodes (LEDs) considering the effects of both electron spill-over out of active region and hole injection into the active region. Performance characteristics of blue LEDs with lattice-matched In0.18Al0.82N electron-blocking layers (EBLs) with different thicknesses were measured in order to exclude the effects of strain and doping efficiency of the EBL, and the quantum efficiencies were analyzed taking account of the electron spill-over current and the relative hole concentration. The results suggest that the highest efficiency in LEDs with a 15-nm In0.18Al0.82N EBL is due to relatively lower hole-blocking effect, hence higher hole injection than in LEDs with a 20-nm EBL, while providing a higher potential barrier for reduced electron spill-over than in LEDs with thinner EBLs. This study suggests that the EBL hole-blocking and electron-confinement effects should be considered in order to achieve higher light output power and reduced efficiency droop at high currents.
Hybrid white organic light-emitting diodes with a double light-emitting layer structure for high color-rendering index
Authors: Zhao Fangchao , Sun Ning , Zhang Hongmei , Chen Jiangshan , Ma Dongge
Published: October 2012

High-efficiency hybrid white organic white light-emitting diodes (WOLEDs) with a double light-emitting layer structure for high color-rendering index (CRI) have been fabricated. The double light-emitting layer structure is composed of a red phosphorescent layer and a blue-green layer of a blue light-emitting fluorescent host doped with a green phosphor separated by a bipolar spacer. The simultaneous emission of a blue light from the fluorescent host and a green light from the phosphor was realized by controlling the doping concentration of the green phosphor in the fluorescent host. The resulting hybrid WOLEDs achieved a CRI of 90 and kept rather stable spectral emission with Commission Internationale de L'Eclairage coordinates of (0.42, 0.44) independent of driving voltages. Furthermore, the hybrid WOLEDs also exhibited a high efficiency that the maximum current efficiency, external quantum efficiency and power efficiency reached 29.4 cd/A, 13.8% and 34.2 lm/W, and still remained at 25.4 cd/A, 11.9% and 23.0 lm/W at 1000 cd/m2, respectively. Mechanism of the two emission layers and role of the bipolar spacer were systematically studied and we have attributed the significant improvement in device performances to the effective and stable control of exciton recombination in the emissive regions by the bipolar functionality of the used spacer.
Structure- and composition-dependent electron field emission from nitrogenated carbon nanotips
Authors: Wang B. B. , Ostrikov K. , Gong C. S. , Xie E. Q. , Wang R. Z.
Published: October 2012

The electron field emission (EFE) properties of nitrogenated carbon nanotips (NCNTPs) were studied under high-vacuum conditions. The NCNTPs were prepared in a plasma-assisted hot filament chemical vapor deposition system using CH4 and N2 as the carbon and nitrogen sources, respectively. The work functions of NCNTPs were measured using x-ray photoelectron spectroscopy. The morphological and structural properties of NCNTPs were studied by field emission scanning electron microscopy, micro-Raman spectroscopy, and x-ray photoelectron spectroscopy. The field enhancement factors of NCNTPs were calculated using relevant EFE models based on the Fowler-Nordheim approximation. Analytical characterization and modeling results were used to establish the relations between the EFE properties of NCNTPs and their morphology, structure, and composition. It is shown that the EFE properties of NCNTPs can be enhanced by the reduction of oxygen termination on the surface as well as by increasing the ratio of the NCNTP height to the radius of curvature at its top. These results also suggest that a significant amount of electrons is emitted from other surface areas besides the NCNTP tops, contrary to the common belief. The outcomes of this study advance our knowledge on the electron emission properties of carbon nanomaterials and contribute to the development of the next-generation of advanced applications in the fields of micro- and opto-electronics.
Ultraviolet photovoltaic effect in BiFeO3/Nb-SrTiO3 heterostructure
Authors: Li H. , Jin K. X. , Yang S. H. , Wang J. , He M. , Luo B. C. , Wang J. Y. , Chen C. L. , Wu T.
Published: October 2012

We report on ultraviolet photovoltaic effects in a BiFeO3/Nb-doped SrTiO3 heterostructure prepared by a pulsed laser deposition method. The heterostructure exhibits rectifying behaviors in the temperature range from 80 K to 300 K. The photovoltage of heterostructure is about 0.33 V at T = 80 K when it is illuminated by a KrF excimer laser with a wavelength of 248 nm. The peak photovoltages decrease with increasing the temperature because of the accumulation of photogenerated carriers. Moreover, the peak photovoltages of heterostructure almost linearly increase with an increase of the power density of laser at T = 300 K. The results reveal some properties that may be useful for possible applications in multiferroic photoelectric devices.
Optical characterization of textured SnO2:F layers using spectroscopic ellipsometry
Authors: Akagawa Masataka , Fujiwara Hiroyuki
Published: October 2012

The SnO2:F transparent conductive oxide (TCO) layers with submicron-size textured structures have been characterized using spectroscopic ellipsometry (SE). By applying the Drude model to the SE analysis, the optical carrier concentration, mobility, and sheet resistance of the textured SnO2:F layers have been estimated, in addition to the detailed layered structures. The excellent agreement has been observed between the results obtained from SE and other characterization techniques including scanning electron microscope, atomic force microscope, and Hall measurements, confirming that SE is a reliable measurement technique even for the textured structures. Nevertheless, the electron mobility obtained from SE is lower than that deduced electrically from the Hall measurement, although the carrier concentrations estimated from these techniques are quite similar. From the SE analysis, the optical constants of the textured SnO2:F layers have been extracted, and the free carrier absorption in the SnO2:F layers increases drastically with decreasing optical mobility. The SE technique developed in this study can be applied further to perform the nondestructive mapping characterization of large-area TCO textures incorporated into thin-film solar cell modules.
Atomic-scaled investigation of structure-dependent luminescence in Sialon:Ce phosphors
Authors: Xu Fangfang , Sourty Erwan , Zeng Xionghui , Zhang Linlin , Gan Lin , Mou Xinliang , Shi Wei , Zhu Yingchun , Huang Fuqiang , Zhao Jingtai
Published: October 2012

Here, we present direct correlation of luminescence properties to the atomic structures of Sialon:Ce phosphors. Luminescence of β-Sialon is revealed attributed to the intergrowth of a unit layer of α-Sialon:Ce-type structure on β-grain surfaces. The dense doping at fault sites either on the surface of or inside the α-Sialon crystallites might lead to intensified emission of light depending on the fault morphology. It seems that concentration quenching would not appear for Sialon:Ce phosphors unless high density of dislocations is created upon dense doping. Finally, structural evolution has been discussed aiming to greatly enhance the luminescence of Sialon phosphors.
Defect mediated ferromagnetism in Ni-doped ZnO nanocrystals evidenced by positron annihilation spectroscopy
Authors: Chen Zhi-Yuan , Chen Z. Q. , Zou B. , Zhao X. G. , Tang Z. , Wang S. J.
Published: October 2012

NiO/ZnO nanocomposites with NiO content of 4 at. % and 20 at. % were annealed up to 1200 °C to get Ni doped ZnO nanocrystals. Raman scattering spectra illustrate a broad and strong band at 500-600cm−1 in all nanocomposites after annealing above 700 °C, which suggests incorporation of Ni in the ZnO lattice. However, x-ray diffraction measurements show that NiO phase can be still observed in all nanocomposites after annealing, which indicates that Ni is partially doped into the ZnO structure. Positron annihilation measurements reveal large number of vacancy defects in the interface region of all nanocomposites, and they are gradually recovered with increasing annealing temperature up to 1000 °C. Room temperature ferromagnetism can be observed in the NiO/ZnO nanocomposites, which is stronger in the 20 at. % NiO/ZnO nanocomposites, and the magnetization decreases continuously with increasing annealing temperature. This indicates that the ferromagnetism at low annealing temperatures originates from the NiO nanograins, and they become antiferromanetic after subsequent higher temperature annealing which leads to the weakening of ferromagnetism. After annealing up to 1000 °C, the ferromagnetism in both the two samples becomes nearly invisible. The disappearance of ferromagnetism shows good coincidence with the recovery of vacancy defects in NiO/ZnO nanocomposites. It can be inferred that the ferromagnetism is mediated by vacancy defects which are distributed in the interface region.
Mechanisms of magnetization reversal in stadium-shaped particles
Authors: Castillo-Sepúlveda S. , Altbir D. , Vargas N. M. , Allende S.
Published: October 2012

During the last years, cylindrical structures like dots, wires, and tubes have been intensively investigated. However, stadium-shaped particles have been scarcely investigated because of the complexity associated to the control of its geometry. In this paper, we used a scaled atomistic representation with Monte Carlo simulations to investigate systematically the magnetization reversal process in stadium-shaped magnetic structures as a function of the size of the central volume. Different reversal mechanisms have been identified and its existence has been explained. The reversal process for short, almost circular, stadium structures occurs via the formation of a single vortex, whereas for longer stadium structures the reversal process involves either single vortices or vortex pairs. For the longest structures investigated, the reversal always involves two vortices that generate a central diamond region or a central antivortex, depending on the region of the nucleation of the vortices. Our results provide guidelines for the use of stadium-shaped particles in the control of vortex motion when a magnetic field is applied.
Thermal annealing influence on poly(3-hexyl-thiophene)/phenyl-C61-butyric acid methyl ester-based solar cells with anionic conjugated polyelectrolyte as cathode interface layer
Authors: Shi Ting , Zhu Xiaoguang , Yang Dong , Xie Yanhua , Zhang Jian , Tu Guoli
Published: October 2012

Thermal annealing dependent performance was demonstrated in poly(3-hexyl-thiophene):phenyl-C61-butyric acid methyl ester based organic photovoltaics with anionic conjugated polyelectrolyte (PFEOSO3Na) as the cathode interface layer. The best performance can be achieved when the device based PFEOSO3Na was treated by post-annealing, exhibiting about 20% higher power conversion efficiency than the control device. Atomic force microscopy studies showed that the morphology of interface layers changed under different device treatment processes, leading to various electron extraction efficiencies. Compared with different interface materials, it further demonstrated the best charge extraction efficiency in the device with PFEOSO3Na layer, due to its stronger interface dipole.