bluuebunny/crossref_metadata_2025 huggingface dataset:
High-mobility heterostructures based on InAs and InSb: A Monte Carlo study
Authors: Rodilla H. , González T. , Pardo D. , Mateos J.
Published: June 2009

In this work, by means of Monte Carlo simulations, two different narrow band gap semiconductors, InAs and InSb, and their associated heterostructures, AlSb/InAs and AlInSb/InSb, have been studied. The parameters for the bulk simulations have been optimized in order to correctly reproduce the experimental mobility values. For the correct simulation of the heterostructures, roughness scattering has been included in the model, and its strength has been adjusted to achieve a good agreement with the experimentally measured mobility.
Gap probabilities in non-Hermitian random matrix theory
Authors: Akemann G. , Phillips M. J. , Shifrin L.
Published: June 2009

We compute the gap probability that a circle of radius r around the origin contains exactly k complex eigenvalues. Four different ensembles of random matrices are considered: the Ginibre ensembles and their chiral complex counterparts, with both complex (β=2) or quaternion real (β=4) matrix elements. For general non-Gaussian weights we give a Fredholm determinant or Pfaffian representation respectively, depending on the non-Hermiticity parameter. At maximal non-Hermiticity, that is, for rotationally invariant weights, the product of Fredholm eigenvalues for β=4 follows from the β=2 case by skipping every second factor, in contrast to the known relation for Hermitian ensembles. On additionally choosing Gaussian weights we give new explicit expressions for the Fredholm eigenvalues in the chiral case, in terms of Bessel-K and incomplete Bessel-I functions. This compares with known results for the Ginibre ensembles in terms of incomplete exponentials. Furthermore, we present an asymptotic expansion of the logarithm of the gap probability for large argument r at large N in all four ensembles, up to and including the third order linear term. We can provide strict upper and lower bounds and present numerical evidence for the conjectured values of the linear term, depending on the number of exact zero eigenvalues in the chiral ensembles. For the Ginibre ensemble at β=2, exact results were previously derived by Forrester [Phys. Lett. A 169, 21 (1992)].
Effects of compressional waves on the response of quartz crystal microbalance in contact with silicone oil droplets
Authors: Zhuang Han , Lim Siak Piang , Lee Heow Pueh
Published: June 2009

Droplet quartz crystal microbalance has been demonstrated to be a promising tool for accessing material properties of fluids as well as the diverse solid-fluid interface phenomena. However, a microliter droplet localized on the surface of the electrodes of finite lateral size may cause a nonuniform distribution of the plane velocity, which may lead to surface normal fluid flow and generate the compressional waves above the crystal surface. In the present article, we report systematical investigation on both resonance frequency and dissipation measurements with reference to the small droplets of silicone oils spreading on the surface of the quartz crystal microbalance. Significant cyclical variations in the resonant frequency and resistance of the crystal have been observed as the characteristic sizes of the silicone oil droplets are close to specific values known to favor compressional wave generation. The experimental results have been compared with the theoretical values predicted by the finite element computation associated with a simple hydrodynamic model. Good agreement between theory and experiment has been obtained. The finding indicates that the small droplets on the crystal surface can act as resonant cavities for the compressional wave generation and that the greatest propensity to exhibit periodical resonance behavior in the frequency and dissipation measurements is at droplet height of λc/2 above the crystal surface.
Dipolar energy states in clusters of perpendicular magnetic nanoislands
Authors: Mengotti E. , Heyderman L. J. , Bisig A. , Fraile Rodríguez A. , Le Guyader L. , Nolting F. , Braun H. B.
Published: June 2009

We investigated the energy states in compact clusters of ferromagnetic islands with perpendicular anisotropy arranged on a triangular lattice. Due to their finite nature, we were able to determine the energies of all possible cluster states using dipolar energy calculations. We employed photoemission electron microscopy to observe the magnetic states in arrays of clusters of monodomain Co/Pt multilayer islands and following demagnetization, we observed a shift in the energy distribution to lower energies as the dipolar coupling increased. These multistate island clusters not only provide model arrangements of frustrated Ising-type nanomagnets but are also interesting for data storage applications.
On the spectral analysis of quantum electrodynamics with spatial cutoffs. I.
Authors: Takaesu Toshimitsu
Published: June 2009

In this paper, we consider the spectrum of a model in quantum electrodynamics with a spatial cutoff. It is proven that (1) the Hamiltonian is self-adjoint; (2) under the infrared regularity condition, the Hamiltonian has a unique ground state for sufficiently small values of coupling constants. The spectral scattering theory is studied as well and it is shown that asymptotic fields exist and the spectral gap is closed.
Total current fluctuations in the asymmetric simple exclusion process
Authors: Tracy Craig A. , Widom Harold
Published: June 2009

A limit theorem for the total current in the asymmetric simple exclusion process (ASEP) with step initial condition is proven. This extends the result of Johansson on TASEP to ASEP.
Effects of collisions on electronic-resonance-enhanced coherent anti-Stokes Raman scattering of nitric oxide
Authors: Patnaik Anil K. , Roy Sukesh , Gord James R. , Lucht Robert P. , Settersten Thomas B.
Published: June 2009

A six-level model is developed and used to study the effects of collisional energy transfer and dephasing on electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) in nitric oxide. The model includes the three levels that are coherently coupled by the three applied lasers as well as three additional bath levels that enable inclusion of the effects of electronic quenching and rotational energy transfer. The density-matrix equations that describe the evolution of the relevant populations and coherences are presented. The parametric dependencies of the ERE-CARS signal on collisional energy transfer and dephasing processes are described in terms of both a steady-state analytical solution and the numerical solutions to the governing equations. In the weak-field limit, the ERE-CARS signal scales inversely with the square of the dephasing rates for the electronic and Raman coherences. In accord with published experimental observations [Roy et al., Appl. Phys. Lett. 89, 104105 (2006)], the ERE-CARS signal is shown to be insensitive to the collisional quenching rate. Parametric dependencies on quenching, rotational energy transfer, and pure electronic dephasing are presented, demonstrating reduced collisional dependence for saturating laser fields.
Stress generated modifications of epitaxial ferroelectric SrTiO3 films on sapphire
Authors: Hollmann E. , Schubert J. , Kutzner R. , Wördenweber R.
Published: June 2009

The effect of lattice-mismatch induced stress upon the crystallographic structure, strain, strain relaxation, and the generation of different types of defects in heteroepitaxially grown SrTiO3 films on CeO2 buffered sapphire is examined. Depending on the thickness of the SrTiO3 layer, characteristic changes in the structural perfection of the layers, their crystallographic orientation with respect to the substrate system, and their strain is observed. For thin films misfit dislocations partially compensate the stress in the SrTiO3 layer, whereas cracks develop in thicker SrTiO3 films. The cracks are orientated along two predominant crystallographic orientations of the sapphire. The structural modifications and the formation of misfit defects and cracks are explained in a model based on lattice misfit induced stress, on the one hand, and energy considerations taking into account the stress release due to crack formation and the energy necessary for the formation of new surfaces at the crack, on the other hand. The impact of lattice misfit is discussed in two steps, i.e., intrinsic and thermal induced misfits during heteroepitaxial film growth at a given temperature and the subsequent cooling of the sample, respectively. The comparison of the theoretical predictions and the experimental observations demonstrate that intrinsic mismatch and thermal mismatch have to be considered in order to explain strain dependent effects in complex heteroepitaxial layer systems such as induced ferroelectricity of SrTiO3 on sapphire.
Influence of structural changes on diffusion in liquid germanium
Authors: Chathoth S. M. , Damaschke B. , Unruh T. , Samwer K.
Published: June 2009

Liquid germanium exhibits a change in the bonding character from being more covalent to more metallic while heating. We used quasielastic neutron scattering to measure the absolute value of self-diffusion coefficients in this liquid. Compared to other monoatomic liquids, such as liquid Ni or Ti, the self-diffusivity is an order faster near the melting temperature and shows a non-Arrhenius-like behavior. Above 1325 K, the activation energy for self-diffusion is low and obeys Stokes–Einstein relation. Even though the packing density of liquid germanium is less than that of simple metallic melts such as Pb or Sn, the temperature dependence of self-diffusivity does not exhibit D∝Tn(n≃2) form, which is observed for uncorrelated binary collisions of hard-spheres.
Flume experiments on intermittency and zero-crossing properties of canopy turbulence
Authors: Poggi Davide , Katul Gabriel
Published: June 2009

How the presence of a canopy alters the clustering and the fine scale intermittency exponents and any possible connections between them remains a vexing research problem in canopy turbulence. To begin progress on this problem, detailed flume experiments in which the longitudinal and vertical velocity time series were acquired using laser Doppler anemometry within and above a uniform canopy composed of densely arrayed rods. The time series analysis made use of the telegraphic approximation (TA) and phase-randomization (PR) methods. The TA preserved the so-called zero-crossing properties in the original turbulent velocity time series but eliminated amplitude variations, while the PR generated surrogate data that preserved the spectral scaling laws in the velocity series but randomized the acceleration statistics. Based on these experiments, it was shown that the variations in the dissipation intermittency exponents were well described by the Taylor microscale Reynolds number (Reλ) within and above the canopy. In terms of clustering, quantified here using the variance in zero-crossing density across scales, two scaling regimes emerged. For spatial scales much larger than the canopy height hc, representing the canonical scale of the vortices dominating the flow, no significant clustering was detected. For spatial scales much smaller than hc, significant clustering was discernable and follows an extensive scaling law inside the canopy. Moreover, the canopy signatures on the clustering scaling laws were weak. When repeating these clustering measures on the PR data, the results were indistinguishable from the original series. Hence, clustering exponents derived from variances in zero-crossing density across scales primarily depended on the velocity correlation function and not on the distributional properties of the acceleration. In terms of the connection between dissipation intermittency and clustering exponents, there was no significant relationship. While the former varied significantly with Reλ, the latter showed only minor variations within and above the canopy sublayer.
Use of Millikan–Lauritsen plots, rather than Fowler–Nordheim plots, to analyze field emission current-voltage data
Authors: Forbes Richard G.
Published: June 2009

In the late 1920s, two forms of data plot were used to analyze the current-voltage (i-V) data obtained in experiments on cold field electron emission (CFE). Millikan–Lauritsen (ML) plots have the form [ln{i} versus 1/V]; Fowler–Nordheim (FN) plots have the form [ln{i/V2} versus 1/V]. In both cases common logarithms may be used instead. For historical reasons, it has become customary to use FN plots; but recent mathematical developments in CFE theory made these historical reasons less valid than they formerly were. ML plots are in fact easier to understand and use, and are more flexible when it is wanted to make corrections for all physical sources of voltage dependence in the data, or to estimate uncertainties in derived parameter values when the precise forms of voltage dependences are not known. This paper summarizes historical and recent background and argues that ML plots should now replace FN plots as the basic method of analyzing CFE i-V data (or related data involving current densities and/or fields). A formula for the interpretation of ML plot slopes is presented and discussed, and examples are given of its use.
GaN metal-oxide-semiconductor field-effect transistor inversion channel mobility modeling
Authors: Pérez-Tomás A. , Placidi M. , Perpiñà X. , Constant A. , Godignon P. , Jordà X. , Brosselard P. , Millán J.
Published: June 2009

Lateral n-channel enhancement-mode GaN metal-oxide-semiconductor (MOS) field-effect transistors and lateral capacitors have been fabricated on a p-type epi-GaN substrate semiconductor and electrically characterized at different temperatures. A clear positive behavior of the inversion channel mobility with temperature has been obtained. A physics-based model on the inversion charge and charge trapped in interface states characteristics has been used to investigate the temperature dependence of the inversion MOS channel mobility. The field-effect mobility increase with temperature is due to an increase in the inversion charge and a reduction in the trapped charge for a given voltage gate. Then, for larger gate bias and/or higher temperatures, surface roughness effects become relevant. The good fitting of the model with the experimental data leads us to consider that the high density of charged acceptor interface traps together with a large interface roughness modulates the channel mobility due to scattering of free carriers in the inversion layer. A closed form expression for the experimental inversion MOS channel mobility is proposed.
Photo-oxidation effects of light-emitting porous Si
Authors: Tamura Tomoyuki , Adachi Sadao
Published: June 2009

The effects of light illumination on porous silicon (PSi) properties have been studied using photoluminescence (PL), PL excitation (PLE), and x-ray photoelectron spectroscopy (XPS) measurements. The PL spectrum evolution in PSi sample under light illumination at various wavelengths indicates that the photo-oxidation occurs and causes a decrease in its intensity with increasing illumination time t. The decrease in the PL intensity IPL can be written as logarithmic expression, namely, the Elovich equation IPL∝−α ln t, where α is the quenching rate of the PL intensity associated with the native oxide growth. The α value is dependent on the illuminated photon energy Epo in a manner α=0.050Epo. Each PL spectrum can be deconvoluted into four Gaussian peaks. The higher the PL peak energy, the larger its photo-oxidation-induced blueshift. This fact and XPS results support that the light emission in a porous sample is due to the quantum-size effect, i.e., relaxation of the momentum conservation at and above the indirect absorption edge (supra-indirect-gap emission). The PLE spectra suggest that the surface hydrogen termination should influence the highly excited carrier dynamics in nanocrystalline PSi materials.
Nonlinear light scattering from clusters and single particles
Authors: Dadap Jerry I. , de Aguiar Hilton B. , Roke Sylvie
Published: June 2009

We present sum-frequency-scattering experiments on colloidal dispersions with various concentrations and in different scattering geometries. At small scattering angles, large fluctuations are observed in the intensity of the scattered sum-frequency photons. By considering the angular dependence of the signal, the particle concentration dependence, and the surface vibrational spectra of the particle, we have determined that the fluctuations are caused by scattering from clusters of particles. We further demonstrate that dynamic nonlinear light scattering may be used to measure the size of the correlated particle clusters.
Rotational dynamics and coupling of methyl group rotations in methyl fluoride studied by high resolution inelastic neutron scattering
Authors: Kirstein O. , Prager M. , Schneider G. J.
Published: June 2009

Methyl group rotations in methyl fluoride were studied using the high flux backscattering spectrometer SPHERES at FRM-II. The asymmetry and width of the low temperature tunneling peak was used to determine if coupled rotations between neighboring methyl fluoride molecules exist. The temperature dependent broadening of the tunneling peak was used to determine the first librational transition and compared to the temperature dependent shift of the position of the tunneling peak. The results obtained by using inelastic neutron scattering confirm previous models that assume rotational coupling. This is the first neutron backscattering experiment with sub-μeV resolution at energy transfers up to 31 μeV.
Facile biosynthesis of phosphate capped gold nanoparticles by a bacterial isolate Stenotrophomonas maltophilia
Authors: Nangia Yogesh , Wangoo Nishima , Sharma Saurabh , Wu Jin-Song , Dravid Vinayak , Shekhawat G. S. , Raman Suri C.
Published: June 2009

We report intracellular biosynthesis of gold nanoparticles (GNPs) by a strain Stenotrophomonas maltophilia (AuRed02) isolated from the soil samples of Singhbhum gold mines, India. An aqueous solution of gold chloride was reduced to metallic gold in a suspension of disrupted cell mass of AuRed02, which progressively turns into cherry red within 8 h of incubation at 25 °C. The optical spectrum showed the plasmon resonance at 530 nm and analysis by transmission electron microscopy and dynamic light scattering confirmed the formation of around 40 nm GNPs. Zeta potential and Fourier transform infrared measurements confirmed GNPs are capped by negatively charged phosphate groups of NADP.
Tunable resonant frequencies for determining Young’s moduli of nanowires
Authors: Zeng Dujuan , Wei Xianlong , Liu Jefferson Z. , Chen Qing , Li Xide , Zheng Quanshui
Published: June 2009

Measuring the electric-field-induced resonance frequency of nanowires and nanotubes as cantilever beams has become a popular method to derive their elastic modulus. However, this method is sensitive to the experimental uncertainties of the clamp point conditions. In a preceding paper, we found that such uncertainty could lead to 30%–70% errors of the derived elastic modulus and then we proposed a tunable resonance method as a remedy. In this paper, we implement the tunable resonance method in experiment. The fundamental resonant frequencies of a Si nanowire cantilever stimulated in an electrical field are measured with an extra support localized at different positions along the nanowire. Our results show that based on this set of measured frequencies, the uncertainties of the clamp boundary condition can be easily identified and filtered out and thus the deduced Young’s modulus is more reliable.
Control of the charge-ordering-insulating phase in epitaxial La1−xCaxMnO3 (x=0.30–0.45) thin films under the anisotropic strain
Authors: Huang Zhen , Gao Guanyin , Yin Zhizhen , Feng Xianxian , Chen Yunzhong , Zhao Xiaoru , Sun Jirong , Wu Wenbin
Published: June 2009

The control of charge-ordering-insulating (COI) phase in epitaxial La1−xCaxMnO3/NdGaO3(001) (x=0.30–0.45) thin films with essentially the ferromagnetic metal ground state as observed for the bulk counterparts has been realized via the anisotropic strain relaxation. This epitaxial system is special in that there is a negligible average lattice mismatch but a large anisotropic strain in between the film and the substrate. By changing the film thickness, postannealing temperature, along with the doping level for strain relaxation, the COI phase in the films can be tuned to either melt completely under 1 T, producing a huge low-field magnetoresistance (MR) in a wide temperature range (e.g., for the 20 nm film with x=0.33 and annealed at 780 °C, the MR can be over 70% at 0.2 T and 97% at 0.5 T in 10–200 K), or survive under a high magnetic field of 6 T. The results demonstrate the crucial role of anisotropic strain relaxation in inducing the inhomogeneity in manganites films, thus providing a forward understanding of the strain field in manganite physics.
Critical behavior of Zn1−xMnxO doped by nitrogen
Authors: Igamberdiev Kh. T. , Yuldashev Sh. U. , Kang T. W. , Pelenovich V. O. , Shashkov A. G.
Published: June 2009

The thermal diffusivity measurements have been used to study the magnetic phase transition in Zn0.95Mn0.05O doped by nitrogen. The critical parameters of the ferromagnetic-paramagnetic transition have been obtained through the analysis of the inverse of the thermal diffusivity near the Curie temperature. Taking into account that the inverse of the thermal diffusivity has the same critical behavior as the specific heat, the critical exponent α has been determined. The results are consistent with the three-dimensional Heisenberg model for the magnetic phase transition.
Molecular dynamics simulation study of deposition and annealing behaviors of Al atoms on Cu surface
Authors: Kim S.-P. , Lee K.-R. , Chung Y.-C. , Sahashi M. , Kim Y. K.
Published: June 2009

Deposition and annealing behaviors of Al atoms on rough Cu (111) surface were investigated on the atomic scale by three-dimensional classical molecular dynamics simulation. The rough Cu surface was modeled by depositing 5 ML of Cu on Ta (011) substrate. Most Al atoms deposited on the rough Cu surface placed on the atomic steps, preserving the major features of the surface during Al deposition. This behavior was discussed in terms of the smaller barrier of the surface diffusion than Ehrlich–Schwoebel barrier of Al on Cu (111) surface. By annealing at 700 K, significant intermixing between Al and Cu rapidly occurs with decrease in the surface roughness. This behavior reveals that the exchange process of Al with substrate Cu dominates during the initial stage of high temperature annealing.
Silver high-aspect-ratio micro- and nanoimprinting for optical applications
Authors: Buzzi Stefano , Galli Matteo , Agio Mario , Löffler Jörg F.
Published: June 2009

Metal direct nanoimprinting is a fabrication technique based on plastic deformation of ductile metals such as silver and gold pressed into a structured rigid mold. While this process can be exploited to manufacture metallic micro- and nanoparts by removing the mold after processing, it can also be used as a metallization method for producing two-dimensional metallo-dielectric composites. Dense silver-pillar arrays with diameters down to 140 nm, aspect ratios up to 13, and excellent uniformity over large areas were fabricated. The sample quality was confirmed by near-infrared optical reflectances of Si–Ag photonic crystals, which showed strong collective surface plasmon-polariton resonances.
Quantifying thermodynamics of collagen thermal denaturation by second harmonic generation imaging
Authors: Hovhannisyan Vladimir A. , Su Ping-Jung , Lin Sung-Jan , Dong Chen-Yuan
Published: June 2009

Time-lapse second harmonic generation (SHG) microscopy was applied for the extraction of thermodynamic parameters of collagen thermal denaturation. We found that at sufficiently high temperatures, temporal dependence of SHG intensity from the isothermal treatment of chicken dermal collagen was single exponential and can be modeled by the Arrhenius equation. Activation energy and the frequency factor of chicken dermal collagen thermal denaturation were determined using temporal decays of SHG intensity at different temperatures. Our results show that time-lapse, high temperature SHG imaging can be used to quantify kinetic properties of collagen thermal denaturation within a microscopic volume of 1 nl.
Thermal conductivity of bismuth telluride nanowire array-epoxy composite
Authors: Biswas Kalapi G. , Sands Timothy D. , Cola Baratunde A. , Xu Xianfan
Published: June 2009

Electrodeposition of nanowire array in porous anodic alumina (PAA) templates combine the performance benefits offered by crystallographic texture control, lattice thermal conductivity suppression through boundary scattering of phonons, elastic relaxation of misfit strain, and scalablity essential for high efficiency thermoelectric devices. The template material, however, can serve as a thermal shunt thereby reducing the effective thermoelectric performance. Here, we demonstrate a process of minimizing the parasitic thermal conduction by replacing the PAA matrix with SU-8 (κ∼0.2 W/m K). We report a reduction in the performance penalty from 27% for Bi2Te3/PAA to ∼5% for Bi2Te3/SU-8 nanocomposite by thermal conductivity measurements using a photoacoustic technique.
Heterogeneous nucleation on convex spherical substrate surfaces: A rigorous thermodynamic formulation of Fletcher’s classical model and the new perspectives derived
Authors: Qian Ma , Ma Jie
Published: June 2009

Fletcher’s spherical substrate model [J. Chem. Phys. 29, 572 (1958)] is a basic model for understanding the heterogeneous nucleation phenomena in nature. However, a rigorous thermodynamic formulation of the model has been missing due to the significant complexities involved. This has not only left the classical model deficient but also likely obscured its other important features, which would otherwise have helped to better understand and control heterogeneous nucleation on spherical substrates. This work presents a rigorous thermodynamic formulation of Fletcher’s model using a novel analytical approach and discusses the new perspectives derived. In particular, it is shown that the use of an intermediate variable, a selected geometrical angle or pseudocontact angle between the embryo and spherical substrate, revealed extraordinary similarities between the first derivatives of the free energy change with respect to embryo radius for nucleation on spherical and flat substrates. Enlightened by the discovery, it was found that there exists a local maximum in the difference between the equivalent contact angles for nucleation on spherical and flat substrates due to the existence of a local maximum in the difference between the shape factors for nucleation on spherical and flat substrate surfaces. This helps to understand the complexity of the heterogeneous nucleation phenomena in a practical system. Also, it was found that the unfavorable size effect occurs primarily when R<5r∗ (R: radius of substrate and r∗: critical embryo radius) and diminishes rapidly with increasing value of R/r∗ beyond R/r∗=5. This finding provides a baseline for controlling the size effects in heterogeneous nucleation.
Self-learning multiscale simulation for achieving high accuracy and high efficiency simultaneously
Authors: Li Wenfei , Takada Shoji
Published: June 2009

Biomolecular systems are inherently hierarchic and many simulation methods that try to integrate atomistic and coarse-grained (CG) models have been proposed, which are called multiscale simulations. Here, we propose a new multiscale molecular dynamics simulation method which can achieve high accuracy and high sampling efficiency simultaneously without aforehand knowledge on the CG potential and test it for a biomolecular system. In our method, a self-learning strategy is introduced to progressively improve the CG potential by an iterative way. (1) A CG model, coupled with the atomistic model, is used for obtaining CG structural ensemble, (2) which is mapped to the atomistic models. (3) The resulting atomistic ensemble is used for deriving the next-generation CG model. Two tests show that this method can rapidly improve the CG potential and achieve efficient sampling even starting from an unrealistic CG potential. The resulting free energy agreed well with the exact result and the convergence by the method was much faster than that by the replica exchange method. The method is generic and can be applied to many biological as well as nonbiological problems.
Diluted chirality dependence in edge rough graphene nanoribbon field-effect transistors
Authors: Tseng F. , Unluer D. , Holcomb K. , Stan M. R. , Ghosh A. W.
Published: June 2009

We investigate the role of various structural nonidealities on the performance of armchair-edge graphene nanoribbon field effect transistors (GNRFETs). Our results show that edge roughness dilutes the chirality dependence often predicted by theory but absent experimentally. Instead, GNRs are classifiable into wide (semimetallic) versus narrow (semiconducting) strips, defining thereby the building blocks for wide-narrow-wide all-graphene devices and interconnects. Small bandgaps limit drain bias at the expense of band-to-band tunneling in GNRFETs. We outline the relation between device performance metrics and nonidealities such as width modulation, width dislocations and surface step, and nonideality parameters such as roughness amplitude and correlation length.
A methodology for improving laser beam induced current images of dye sensitized solar cells
Authors: Navas Francisco Javier , Alcántara Rodrigo , Fernández-Lorenzo Concha , Martín Joaquín
Published: June 2009

Using the laser beam induced current (LBIC) technique for the study of solar cells and photovoltaic devices, it is possible to obtain images representing the different degrees of quantum efficiency observed on the surface of these elements. Dye sensitized solar cells (DSSCs) or photoelectrochemical solar cells, in contrast to those based on solid-solid interfaces, show a slow response to irradiance variations—up to tens of seconds. This is basically due to both viscous matter transport processes and load transfer. This response is inappreciable when the device is functioning continuously but when a LBIC scan is performed, in which the laser moves quickly from one point to another, the slow response produces a memory effect and the signal generated at one given point depends on the conversion efficiency coefficients of the previously excited positions, resulting in diffuse images and a lack of sharpness. This work presents a methodology to correct high-resolution LBIC mappings of DSSCs using an algorithm based on the kinetics of the discharge process of the irradiated zone. The validity of the proposed method has been evaluated by carrying out experiments where the algorithm has been applied to LBIC mappings.
Theoretical and kinetic studies of the reactions of CF2HCFHCF2H and CF3CFHCFH2 with hydroxyl radicals
Authors: Gao Hong , Liu Jing-yao , Sun Chia-chung
Published: June 2009

The hydrogen abstraction reactions of fluoroalkane isomers CF2HCFHCF2H and CF3CFHCFH2 with the OH radicals have been studied theoretically by a dual-level direct dynamics method. Optimized geometries and frequencies of all the stationary points and extra points along the minimum-energy path are obtained at the BB1K/6-31+G(d,p) level of theory, and then the energy profiles are refined at G3(MP2) level of theory. Using the improved canonical variational transition-state theory (ICVT) with the small-curvature tunneling correction (SCT), the rate constants for each channel are calculated over a wide temperature range of 200–1000 K. Our results show that the tunneling correction plays an important role in the rate constant calculation in the low temperature range. The calculated ICVT/SCT rate constants are consistent with available experimental data. Our calculations indicate the contribution of the abstraction from the –CFH– group of isomeric compounds CF2HCFHCF2H and CF3CFHCFH2 to the overall reactions is quite different over the whole temperature range due to the effect of different groups at both sides of –CFH– group. Furthermore, to further reveal the thermodynamic properties, the enthalpies of formation of the two reactants CF2HCFHCF2H, and CF3CFHCFH2, and the product radicals CF2HCFCF2H, CF2HCFHCF2, CF3CFCFH2, and CF3CFHCFH are obtained by using isodesmic reactions.
Direct evidence for mode-specific vibrational energy relaxation from quantum time-dependent perturbation theory. III. The ν4 and ν7 modes of nonplanar nickel porphyrin models
Authors: Zhang Yong , Straub John E.
Published: June 2009

The time scales and pathways of vibrational energy relaxation (VER) of the ν4 and ν7 modes of three nickel porphyrin models, nickel porphine (NiP), nickel protoporphyrin IX (Ni-heme), and nickel octaethylporphyrin (NiOEP), were studied using a non-Markovian time-dependent perturbation theory at the B3LYP/6-31G(d) level. When NiP is calculated with D4h symmetry, it has the planar structure and the same VER properties as ferrous iron porphine (FeP). The porphine cores of both Ni-heme and NiOEP were distorted from a planar geometry, assuming a nonplanar structure, similar to that of the heme structure in cytochrome c. The VER time scales of Ni-heme are found to be similar to those predicted for a planar iron heme, but the derived pathways have distinctly different features. In particular, the strong coupling between the ν7 mode and the overtone of the ∼350 cm−1 γ7 mode, observed for planar porphyrins, is absent in both nonplanar nickel porphyrins. Direct energy exchange between the ν4 and ν7 modes is not observed in NiOEP, but is found to play an essential role in the VER of the ν4 mode in Ni-heme. The Ni-heme isopropionate groups are involved in the dominant VER pathways of both the ν4 and ν7 modes of Ni-heme. However, in contrast with VER pathways derived in planar iron heme, the isopropionate groups are not observed to play an essential role relative to other side chains in spatially directing the vibrational energy flow.
A branch-point approximant for the equation of state of hard spheres
Authors: Santos Andrés , López de Haro Mariano
Published: June 2009

Using the first seven known virial coefficients and forcing it to possess two branch-point singularities, a new equation of state for the hard-sphere fluid is proposed. This equation of state predicts accurate values of the higher virial coefficients, a radius of convergence smaller than the close-packing value, and it is as accurate as the rescaled virial expansion and better than the Padé [3/3] equations of state. Consequences regarding the convergence properties of the virial series and the use of similar equations of state for hard-core fluids in d dimensions are also pointed out.
The three-dimensional wake of a cylinder undergoing a combination of translational and rotational oscillation in a quiescent fluid
Authors: Nazarinia M. , Lo Jacono D. , Thompson M. C. , Sheridan J.
Published: June 2009

Previous two-dimensional numerical studies have shown that a circular cylinder undergoing both oscillatory rotational and translational motions can generate thrust so that it will actually self-propel through a stationary fluid. Although a cylinder undergoing a single oscillation has been thoroughly studied, the combination of the two oscillations has not received much attention until now. The current research reported here extends the numerical study of Blackburn et al. [Phys. Fluids 11, L4 (1999)] both experimentally and numerically, recording detailed vorticity fields in the wake and using these to elucidate the underlying physics, examining the three-dimensional wake development experimentally, and determining the three-dimensional stability of the wake through Floquet stability analysis. Experiments conducted in the laboratory are presented for a given parameter range, confirming the early results from Blackburn et al. [Phys. Fluids 11, L4 (1999)]. In particular, we confirm the thrust generation ability of a circular cylinder undergoing combined oscillatory motions. Importantly, we also find that the wake undergoes three-dimensional transition at low Reynolds numbers (Re≃100) to an instability mode with a wavelength of about two cylinder diameters. The stability analysis indicates that the base flow is also unstable to another mode at slightly higher Reynolds numbers, broadly analogous to the three-dimensional wake transition mode for a circular cylinder, despite the distinct differences in wake/mode topology. The stability of these flows was confirmed by experimental measurements.
Integrated complementary graphene inverter
Authors: Traversi Floriano , Russo Valeria , Sordan Roman
Published: June 2009

The operation of a digital logic inverter consisting of one p- and one n-type graphene transistor integrated on the same sheet of monolayer graphene is demonstrated. Both transistors initially exhibited p-type behavior at low gate voltages, since air contamination shifted their Dirac points from zero to a positive gate voltage. Contaminants in one transistor were removed by electrical annealing, which shifted its Dirac point back and therefore restored n-type behavior. Boolean inversion is obtained by operating the transistors between their Dirac points. The fabricated inverter represents an important step toward the development of digital integrated circuits on graphene.
Photodissociation of (ICN)2 van der Waals dimer using velocity imaging technique
Authors: Zhang Xiao-peng , Lee Wei-Bin , Zhao Dong-feng , Hsiao Ming-Kai , Chen Yu-Ling , Lin King-Chuen
Published: June 2009

Photodissociation of (ICN)2 dimer from 265 to 270 nm are studied using time-of-flight mass spectrometry combined with velocity imaging technique. Both I+ and I2+ ions are found in the mass spectra. The I2+ ions result from (1+1) resonant ionization of the neutral I2 fragment that is produced in the photodissociation of dimer, but not from dissociative ionization of (ICN)2; i.e., (ICN)2++hν→I2++2CN. The dissociation channels of I2+ leading to I+ are all found with parallel character. The total kinetic energy distributions and anisotropy parameters of the I+ channels produced by (ICN)2 are almost the same as those from a neutral I2 sample, thereby confirming that the I2 fragments are obtained in cold state. With the aid of ab initio calculations, a plausible dissociation mechanism is proposed.
Pressure effect on the rotator-II to rotator-I transition of alkanes
Authors: Mukherjee Prabir K.
Published: June 2009

We examine the effect of pressure on the rotator-II (RII) to rotator-I (RI) (RII-RI) transition within Landau phenomenological theory. The influence of pressure on the RII-RI transition is discussed by varying the coupling between the lattice distortion and the area/molecule. The pressure dependence of the area/molecule and the lattice distortion are calculated near the RII-RI transition. The RII-RI transition is found to be first order even at elevated pressure. The Landau coefficients are calculated from the experimental data. The theoretical results are in good agreement with available experimental results.
Enhanced directional lasing by the interference between stable and unstable periodic orbits
Authors: Lin Yuan Yao , Chen Chih-Yao , Chien Wei , Pan Jin-Shan , Lee Tsin-Dong , Lee Ray-Kuang
Published: June 2009

We demonstrate the idea of surface-assisted microstructures to tailor the modes and resulting emission from a dynamical localized vertical cavity surface emitting laser at room temperature. With a slightly mismatch between the surface structure and the native oxide cavity layer, near field intensity images corresponding to the spontaneous emission patterns, whispering-gallery modes (WGMs), dynamical localized modes (DLMs), and unidirectional light emissions are measured and identified in experiments and simulations. Moreover, the coherent superposition of WGM and DLM, i.e., stable and unstable periodic orbits, respectively, is reported to provide an alternative and general method to control lasing characteristics as well as to study quantum chaos in mesoscopic system.
Metal-rich Au-silicide nanoparticles for use in nanotechnology
Authors: Moyen E. , Macé M. , Agnus G. , Fleurence A. , Maroutian T. , Houzé F. , Stupakiewicz A. , Masson L. , Bartenlian B. , Wulfhekel W. , Beauvillain P. , Hanbücken M.
Published: June 2009

We present a route to functionalize chemically and magnetically silicon surfaces by a local passivation, taking advantage of Stranski–Krastanov growth mode of the Au–Si(111) system. Metal-rich Au-silicide nanoparticles, supported on a Si-rich two-dimensional Au-silicide layer, are obtained. Subsequently deposited Co is used to form magnetic nanostructures. The two Au silicides display a different chemical reactivity with Co enabling the fabrication of localized magnetic Co nanodots. These magnetic nanostructures can be aligned along step bunches of a vicinal Si(111) surface. By varying the growth parameters, the particle density can be tuned from 109 to the low 1012 dots/in.2.
Low-noise GaN ultraviolet p-i-n photodiodes on GaN substrates
Authors: Zhang Yun , Shen Shyh-Chiang , Kim Hee Jin , Choi Suk , Ryou Jae-Hyun , Dupuis Russell D. , Narayan Bravishma
Published: June 2009

We report low-noise GaN visible-blind homojunction p-i-n photodiodes. The devices are grown on a freestanding bulk GaN substrate and are fabricated using a “ledged” surface depletion technique to suppress the mesa sidewall leakage. For an 80-μm-diameter photodetector, the dark current density is lower than 40 pA/cm2. A room-temperature noise equivalent power of 4.27×10−17 W Hz−0.5 and a detectivity of 1.66×1014 cm Hz0.5 W−1 are achieved at a reverse bias of 20 V. The noise performance of the reverse-biased GaN p-i-n photodiodes are among the best values reported to date and demonstrate the potential of GaN photodiodes for low-noise high-speed UV detection.
Modeling positive ion current to a planar probe in low-pressure electronegative discharges
Authors: Chung T. H.
Published: June 2009

A fluid model is utilized to describe the plasma-sheath boundary for a negatively biased planar probe immersed in electronegative plasmas. The model equations are solved on the scale of the electron Debye length and calculate the spatial distributions of electric potential, velocity, and density of positive ions in front of the probe. The position of sheath edge, the positive ion velocity at sheath edge (the Bohm velocity), and the positive ion flux collected by the probe are determined and compared with analytic (or scaling) formulas. Effects of control parameters on the Bohm velocity, the sheath thickness, and on the positive ion flux are investigated. A larger thermal motion of negative ions causes the Bohm velocity to increase, the sheath to increase, and the positive ion flux collected by the probe to increase. An increase in collision causes the Bohm velocity to decrease and the sheath to decrease resulting in a decrease in the positive ion flux. An increase in electronegativity causes both the Bohm velocity and the sheath thickness to decrease, resulting in an increase in the positive ion flux. As the value of the non-neutrality parameter q increases, the Bohm velocity and the sheath thickness are found to decrease, and the positive ion flux collected by the probe increases. The behavior of the positive ion flux entering the sheath is discussed as functions of control parameters. A careful comparison of theoretical positive ion flux with the experimental flux can allow us to obtain the electronegativity, the plasma ionization rate (q), and the collision parameter (δ).
Calculation of the electrostatic potential of lipid bilayers from molecular dynamics simulations: Methodological issues
Authors: Gurtovenko Andrey A. , Vattulainen Ilpo
Published: June 2009

The electrostatic properties of lipid membranes are of profound importance as they are directly associated with membrane potential and, consequently, with numerous membrane-mediated biological phenomena. Here we address a number of methodological issues related to the computation of the electrostatic potential from atomic-scale molecular dynamics simulations of lipid bilayers. We discuss two slightly different forms of Poisson equation that are normally used to calculate the membrane potential: (i) a classical form when the potential and the electric field are chosen to be zero on one of the sides of a simulation box and (ii) an alternative form, when the potential is set to be the same on the opposite sides of a simulation box. Both forms differ by a position-dependent correction term, which has been shown to be proportional to the overall dipole moment of a bilayer system (for neutral systems). For symmetric bilayers we demonstrate that both approaches give essentially the same potential profiles, provided that simulations are long enough (a production run of at least 100 ns is required) and that fluctuations of the center of mass of a bilayer are properly accounted for. In contrast, for asymmetric lipid bilayers, the second approach is no longer appropriate due to a nonzero net dipole moment across a simulation box with a single asymmetric bilayer. We demonstrate that in this case the electrostatic potential can adequately be described by the classical form of Poisson equation, provided that it is employed in conjunction with tin-foil boundary conditions, which exactly balance a nonzero surface charge of a periodically replicated multibilayer system. Furthermore, we show that vacuum boundary conditions give qualitatively similar potential profiles for asymmetric lipid bilayers as compared to the conventional periodic boundaries, but accurate determination of the transmembrane potential difference is then hindered due to detachment of some water dipoles from bulk aqueous solution to vacuum.
Nonlocal effects in nonisothermal hydrodynamics from the perspective of beyond-equilibrium thermodynamics
Authors: Hütter Markus , Brader Joseph M.
Published: June 2009

We examine the origins of nonlocality in a nonisothermal hydrodynamic formulation of a one-component fluid of particles that exhibit long-range correlations, e.g., due to a spherically symmetric, long-range interaction potential. In order to furnish the continuum modeling with physical understanding of the microscopic interactions and dynamics, we make use of systematic coarse graining from the microscopic to the continuum level. We thus arrive at a thermodynamically admissible and closed set of evolution equations for the densities of momentum, mass, and internal energy. From the consideration of an illustrative special case, the following main conclusions emerge. There are two different source terms in the momentum balance. The first is a body force, which in special circumstances can be related to the functional derivative of a nonlocal Helmholtz free energy density with respect to the mass density. The second source term is proportional to the temperature gradient, multiplied by the nonlocal entropy density. These two source terms combine into a pressure gradient only in the absence of long-range effects. In the irreversible contributions to the time evolution, the nonlocal contributions arise since the self-correlations of the stress tensor and heat flux, respectively, are nonlocal as a result of the microscopic nonlocal correlations. Finally, we point out specific points that warrant further discussions.
Conceal an entrance by means of superscatterer
Authors: Luo Xudong , Yang Tao , Gu Yongwei , Chen Huanyang , Ma Hongru
Published: June 2009

Based on the fact that superscatterer can look like a scatterer bigger than its geometric size, we propose a design of hidden portal in which an entrance is concealed from the electromagnetic wave detection. Such a superscatterer is realized by coating a negative index material shell on a perfect electrical conductor rectangle cylinder. The results are numerically confirmed by full-wave simulations both in the far field and near field.