bluuebunny/crossref_metadata_2025 huggingface dataset:
Ab initio analytical infrared intensities for periodic systems through a coupled perturbed Hartree-Fock/Kohn-Sham method
Authors: Maschio Lorenzo , Kirtman Bernard , Orlando Roberto , Rèrat Michel
Published: November 2012

A fully analytical method for calculating Born charges and, hence, infrared intensities of periodic systems, is formulated and implemented in the CRYSTAL program, which uses a local Gaussian type basis set. Our efficient formalism combines integral gradients with first-order coupled perturbed Hartree–Fock/Kohn Sham electronic response to an electric field. It avoids numerical differentiation with respect to wave vectors, as in some Berry phase approaches, and with respect to atomic coordinates. No perturbation equations for the atomic displacements need to be solved. Several tests are carried out to verify numerical stability, consistency in one, two, and three dimensions, and applicability to large unit cells. Future extensions to piezoelectricity and Raman intensities are noted.
Effect of fluctuations on the onset of density-driven convection in porous media
Authors: Bestehorn Michael , Firoozabadi Abbas
Published: November 2012

We study the dissolution of CO2 in saline aquifers. The long diffusion times can be accelerated by orders of magnitude from mass transfer that origins from convection. Convection occurs at a critical time via a phase transition from the horizontally homogeneous diffusion state. To start the instability, perturbations that break the horizontal translation symmetry are necessary. We start with the basic equations and the boundary conditions, examine the linearized equations around the diffusive time and z-dependent base state and compare different definitions of the critical time found in the literature. Taking a simple model we show the role of fluctuations for delayed instabilities if the control parameter is slowly swept through the bifurcation point. Apart from the critical time we use a “visible” time where convection is manifested in the vertical CO2 transport. We specify the perturbations with respect to their strength and length scale, and compute the critical times for various cases by numerical integration of the basic equations in two spatial dimensions. Fluctuating concentration at the upper boundary, fluctuating porosity as well fluctuating permeability are studied in detail. For the permeability fluctuation, the compressibility of the fluid becomes important and the velocity field cannot be derived from a stream function. Our work also includes non-isothermal conditions with a prescribed vertical geothermal gradient and space dependent thermal conductivity. Temperature fields for different standard configurations are computed numerically and serve as starting condition for density-driven convection. Based on our work, we conclude that the visible time is much larger than the critical time. The visible time is a strong function of strength and length scale of the perturbations.
Dynamics of droplet coalescence in response to increasing hydrophobicity
Authors: Graham Percival J. , Farhangi Mehran M. , Dolatabadi Ali
Published: November 2012

Coalescence of a falling droplet with a sessile droplet on solid surface with various wettabilities is investigated by a combined experimental and numerical study. In the experiments, the droplet diameter, the impact velocity, and the distance between the impacting droplets were controlled. The evolution of surface shape during the coalescence of two droplets on various surfaces is captured using high speed imaging and compared with numerical results. A two-phase volume of fluid method is used to determine the dynamics of droplet coalescence, shape evaluation, and contact line movement. The spreading length of two coalescing droplets along their original centers is also predicted by the model and compared well with the experimental results. The effect of different parameters such as impact velocity, center to center distance, droplet size, and surface wettability on maximum spreading length are studied and compared to the experimental results. Finally, correlations are developed for predicting the maximum spreading length using both experimental and numerical results.
Breakdown of nucleation theory for crystals with strongly anisotropic interactions between molecules
Authors: Cabriolu Raffaela , Kashchiev Dimo , Auer Stefan
Published: November 2012

We study the nucleation of model two-dimensional crystals in order to gain insight into the effect of anisotropic interactions between molecules on the stationary nucleation rate J. With the aid of kinetic Monte Carlo simulations, we determine J as a function of the supersaturation s. It turns out that with increasing degree of interaction anisotropy the dependence of ln J on s becomes step-like, with jumps at certain s values. We show that this J(s) dependence cannot be described by the classical and atomistic nucleation theories. A formula that predicts the identified J(s) behavior is yet to be derived and verified, and the present study provides the necessary data and understanding for doing that.
A note on turbulent spots over a rough bed in wave boundary layers
Authors: Carstensen S. , Sumer B. M. , Fredsøe J.
Published: November 2012

This study is a continuation of the investigation of turbulent spots in wave boundary layers over a smooth wall reported by Carstensen et al. [J. Fluid Mech. 646, 169–206 (2010)10.1017/S0022112009992825]. The present paper summarises the results of an experimental investigation of turbulent spots in wave boundary layers over a rough wall. Two kinds of experiments were made: flow visualization tests and velocity profile measurements. The experiments show that turbulent spots as observed in the smooth-wall wave boundary layers are present irrespective of wall roughness conditions. As in the smooth-wall case the turbulent spots over the rough wall appear locally in time and space and grow in size as time progresses.
Eccentricity effects of microhole arrays on drag reduction efficiency of microchannels with a hydrophobic wall
Authors: Kashaninejad Navid , Nguyen Nam-Trung , Chan Weng Kong
Published: November 2012

This paper experimentally investigates the effects of microhole eccentricity on the slip lengths of Stokes flow in microchannels with the bottom wall made of microhole arrays. The wettability of such microhole structures fabricated by the replica molding of polydimethylsiloxane is first analyzed measuring both static and dynamic contact angles. Subsequently, the drag reduction performance of the microchannels with such hydrophobic microhole surfaces is evaluated. The results indicate that the impact of microhole eccentricity on drag reduction performance correlates well with the contact angle hysteresis rather than with the static contact angle. Furthermore, microhole arrays with large normalized width and zero eccentricity show the minimum contact angle hysteresis of 18.7°. In these microchannels, the maximum percentage increase in the relative velocity is 39% corresponding to a slip length of 2.49 μm. For the same normalized width, increasing the normalized eccentricity to 2.6 increases the contact angle hysteresis to 36.5° that eventually reduces the percentage increase in relative velocity and slip length down to 16% and 0.91 μm, respectively. The obtained results are in qualitative agreement with the existing theoretical and numerical models. These findings provide additional insights in the design and fabrication of efficient micropatterned channels for reducing the flow resistance, and leave open questions for theoreticians to further investigate in this field.
Ultra-thin perfect absorber employing a tunable phase change material
Authors: Kats Mikhail A. , Sharma Deepika , Lin Jiao , Genevet Patrice , Blanchard Romain , Yang Zheng , Qazilbash M. Mumtaz , Basov D. N. , Ramanathan Shriram , Capasso Federico
Published: November 2012

We show that perfect absorption can be achieved in a system comprising a single lossy dielectric layer of thickness much smaller than the incident wavelength on an opaque substrate by utilizing the nontrivial phase shifts at interfaces between lossy media. This design is implemented with an ultra-thin (∼λ/65) vanadium dioxide (VO2) layer on sapphire, temperature tuned in the vicinity of the VO2 insulator-to-metal phase transition, leading to 99.75% absorption at λ = 11.6 μm. The structural simplicity and large tuning range (from ∼80% to 0.25% in reflectivity) are promising for thermal emitters, modulators, and bolometers.
Decaying vortex and wave turbulence in rotating shallow water model, as follows from high-resolution direct numerical simulations
Authors: Lahaye Noé , Zeitlin Vladimir
Published: November 2012

We report results of direct numerical simulations of decaying turbulence in an inviscid rotating shallow water model. We use a new-generation high-resolution well-balanced shock-capturing finite-volume scheme with several types of initializations: “classical” ones with random velocity and/or height fields, or an initialization with randomly oriented coherent vortex dipoles. Together with “full” turbulence simulations we also perform pure wave-turbulence ones, starting from an initial random wave field of small amplitude with zero potential vorticity anomaly and a given initial spectrum. Statistical properties of the rotating shallow water turbulence, as well as the development of coherent structures and their interactions are studied in detail. For all “full” turbulence simulations we find a tendency to form coherent structures with clear cyclone-anticyclone asymmetry and very steep energy spectra, with exponents close to −6. We also observe a decorrelation of the vortex and wave fields in time, even at significant Rossby numbers. However, we do not observe a universal power law in the evolution of coherent vortices, predicted by the “universal decay” theory for the 2D turbulence. A clear sensitivity to the initial conditions is thus established. For wave-turbulence simulations we observe a tendency to form very steep spectra different from the predictions of the so-called weak turbulence, and of both the turbulence of cusped nonlinear waves and the shock turbulence.
Computational study and modeling of turbine spacing effects in infinite aligned wind farms
Authors: Yang Xiaolei , Kang Seokkoo , Sotiropoulos Fotis
Published: November 2012

We study the turbine spacing effects in infinite, aligned wind-turbine arrays using large-eddy simulation (LES) with the wind turbine rotors parameterized as actuator disks. A series of simulations is carried out to systematically investigate the different effects of streamwise and spanwise turbine spacings on the array power output and turbulence intensities. We show that for the same turbine density, increasing the streamwise spacing is more beneficial than increasing the spanwise spacing. Larger streamwise turbine spacing increases the power extraction and lowers the turbulence intensity at each turbine more efficiently than when the spanwise turbine spacing is increased. The reason for the different effects of streamwise and spanwise turbine spacings on wind farm performance is that the wake recovery of wind turbines in infinite arrays depends on the area influenced by the wind-turbine wakes, rather than the land area occupied by each turbine. Based on this idea, an improved effective roughness height model is proposed, which can account for the different effects of streamwise and spanwise turbine spacings in infinite aligned wind farms. The predictive capabilities of the new model are demonstrated via extensive comparisons with results obtained from the LES and previously proposed roughness height models.
Modeling plasmonics: A Huygens subgridding scheme for Lorentz media
Authors: Hu Zixuan , Ratner Mark A. , Seideman Tamar
Published: November 2012

Huygens subgridding for the grid-based solution of the Maxwell equations is a new and promising technique that enables accurate computation of mixed systems, by efficiently reducing the computational cost for simulating structures where increased spatial resolution is required in part of space. The Huygens subgridding approach has previously been derived and tested for perfect electric conductors and Debye media. This work introduces a Huygens subgridding method that is applicable to Lorentz media, thus opening a range of new applications in the field of plasmonics.
Measurement of nematic liquid crystal splay elastic constants by disclination line motion in high pretilt, negative dielectric anisotropy pi-cells
Authors: Walton Harry G.
Published: November 2012

Negative dielectric anisotropy nematic liquid crystals confined to high pretilt pi-cells undergo a discontinuous, disclination mediated transition between a splayed and a twisted director configuration. A critical voltage, Us, exists at which the two states are in equilibrium and hence the movement of disclination lines ceases. We show that measurement of Us provides a simple and convenient means to determine the splay elastic constant, K11, of the liquid crystal to an accuracy of 10% or better.
New method to assess the loss parameters of the photovoltaic modules
Authors: Bencherif M. , Chermitti A.
Published: November 2012

The determination of an effective method able to estimate the parameters of a photovoltaic panel is essential for the development and the performance of analysis of such equipment. In this paper, we present a new simple and effective method in order to extract the parameters of the photovoltaic (PV) modules by using the standard diode model. This model requires the five parameters be known, the photocurrent Iph, the reverse saturation current Is, the resistance Rs, the shunt resistance Rsh, and the curve fitting parameter A (the ideality factor) or the thermal voltage a. In order to find the five parameters, we formulated an equation binding only the two loss parameters Rs and Rsh. In order to solve this equation and to determine the shunt resistance and the thermal voltage, we expose the computation models of the series resistance Rs, which give excellent approximations. While the other parameters Iph and Is depend exclusively on the parameters Rs, Rsh, a, and the short circuit current Isc, and the open circuit voltage Voc. This method is validated experimentally by three different flat plat PV modules of various technologies (monocrystalline, polycrystalline, and thin film panels) and manufacturers.

This article is being retracted effective 7 August 2014.
Pulsed laser ablation plasmas generated in CO2 under high-pressure conditions up to supercritical fluid
Authors: Kato Toru , Stauss Sven , Kato Satoshi , Urabe Keiichiro , Baba Motoyoshi , Suemoto Tohru , Terashima Kazuo
Published: November 2012

Pulsed laser ablation of solids in supercritical media has a large potential for nanomaterials fabrication. We investigated plasmas generated by pulsed laser ablation of Ni targets in CO2 at pressures ranging from 0.1 to 16 MPa at 304.5 K. Plasma species were characterized by optical emission spectroscopy, and the evolution of cavitation bubbles and shockwaves were observed by time-resolved shadowgraph imaging. Ni and O atomic emissions decreased with increasing gas pressure; however, near the critical point the intensities reached local maxima, probably due to the enhancement of the plasma excitation and effective quenching resulting from the large density fluctuation.
Fabrication of quasi-periodic surface microcavities by selective etching of self-organized superalloys for high-temperature photonics
Authors: Shimizu Makoto , Konno Kiyotaka , Iguchi Fumitada , Yugami Hiroo
Published: November 2012

Large-area fabrication of periodic microstructures on refractory metals surface is a key technology supporting the practical application of spectrally controlled thermal radiation using surface microcavities. This report describes large-area fabrication of two-dimensional submicron quasi-periodic microcavities using self-organization on a nickel-based superalloy. The surface microcavities on a bulk metal are obtained by heat treatment and simple chemical etching. The emission peak attributed to the confined modes inside cavities can be tuned by controlling the microcavity size from 0.27 to 0.53 μm. Emittance enhancement and thermal stability are also confirmed at 973 K.
Stochastic generation scheduling considering wind power generators
Authors: Niknam Taher , Reza Massrur Hamid , Bahmani Firouzi Bahman
Published: November 2012

This paper presents a model for the computation of costs of the generation of the electrical power in a power system. It is based on the stochastic optimization of mid-term generation scheduling wind integrated power system with respect to the uncertainty on both wind generation and load forecast. The uncertainty of the two above mentioned elements is modeled by scenario-based method. In the previous works, the impact of the generation of wind power on the generation scheduling (GS) problem using stochastic methods has been studied on daily basis. However, this study assesses the effect of the generation of wind power on the GS problem on a yearly manner. The problem of GS is solved under the various constraints of the power system. These constraints embrace the thermal and wind units' constraints, power balance, available wind power, and reserve requirements. A population-based method called teaching-learning-based-optimization (TLBO) is proposed to solve the stochastic generation scheduling problem. TLBO is a robust and effective search algorithm. The most salient advantage of this algorithm is that it does not require the tuning of any kind of controlling parameters. The efficiency of the proposed formulation is studied by employing the three test cases. Numerical results are presented for these case studies.
Effects of weak ties on epidemic predictability on community networks
Authors: Shu Panpan , Tang Ming , Gong Kai , Liu Ying
Published: November 2012

Weak ties play a significant role in the structures and the dynamics of community networks. Based on the contact process, we study numerically how weak ties influence the predictability of epidemic dynamics. We first investigate the effects of the degree of bridge nodes on the variabilities of both the arrival time and the prevalence of disease, and find out that the bridge node with a small degree can enhance the predictability of epidemic spreading. Once weak ties are settled, the variability of the prevalence will display a complete opposite trend to that of the arrival time, as the distance from the initial seed to the bridge node or the degree of the initial seed increases. More specifically, the further distance and the larger degree of the initial seed can induce the better predictability of the arrival time and the worse predictability of the prevalence. Moreover, we discuss the effects of the number of weak ties on the epidemic variability. As the community strength becomes very strong, which is caused by the decrease of the number of weak ties, the epidemic variability will change dramatically. Compared with the case of the hub seed and the random seed, the bridge seed can result in the worst predictability of the arrival time and the best predictability of the prevalence.
The suppressed negative bias illumination-induced instability in In-Ga-Zn–O thin film transistors with fringe field structure
Authors: Chen Yu-Chun , Chang Ting-Chang , Li Hung-Wei , Hsieh Tien-Yu , Chen Te-Chih , Wu Chang-Pei , Chou Cheng-Hsu , Chung Wang-Cheng , Chang Jung-Fang , Tai Ya-Hsiang
Published: November 2012

This study investigates the suppressed negative gate bias illumination stress (NBIS) -induced instability of via-type amorphous indium-gallium-zinc-oxide (a-IGZO) thin film transistors (TFTs) with fringe field (FF) structures. The less negative threshold voltage shifts of devices after NBIS are showed when device has larger FF structures. This finding is attributed to more dispersive distribution of photo-generated holes in the width direction of a-IGZO during NBIS, which reduce the hole trapping phenomenon in the front channel interface. The a-IGZO TFT with FF structure is expected to be an effective method to increase the electrical reliability of devices after NBIS.
Modulation of external electric field on surface states of topological insulator Bi2Se3 thin films
Authors: Liu Genhua , Zhou Guanghui , Chen Yong-Hai
Published: November 2012

We study theoretically surface band structure and topological phase transition for Bi2Se3 thin films under an external electric field. It is demonstrated that an electric field vertical to the film surface can close or reopen a gap for surface states. We also study the spin-dependent transport property through an interface junction between regions without and with electric field on the film. Interestingly, the opacity and transparency of spin-down Dirac fermions through the junction can be modulated by changing the incident angle and the electric field strength. On the contrary, the spin-up Dirac fermions always penetrate through the junction.
Idealized numerical simulation of breaking water wave propagating over a viscous mud layer
Authors: Hu Yi , Guo Xin , Lu Xinhua , Liu Yi , Dalrymple Robert A. , Shen Lian
Published: November 2012

Direct numerical simulation and large-eddy simulation are developed to investigate water waves propagating over viscous fluid mud at the bottom, with a focus on the study of wave breaking case. In the simulations, the water surface and the water–mud interface are captured with a coupled level-set and volume-of-fluid method. For non-breaking water waves of finite amplitude, it is found that the overall wave decay rate is in agreement with the existing linear theory. For breaking water waves, detailed description of the instantaneous flow field is obtained from the simulation. The time history of the total mechanical energy in water and mud shows that during the early stage of the wave breaking, the energy decays slowly; then, the energy decays rapidly; and finally, the decay rate of energy becomes small again. Statistics of the total mechanical energy indicates that the mud layer reduces the wave breaking intensity and shortens the breaking duration significantly. The effect of mud on the energy dissipation also induces a large amount of energy left in the system after the wave breaking. To obtain a better understanding of the underlying mechanism, energy transport in water and mud is analyzed in detail. A study is then performed on the viscous dissipation and the energy transfer at the water–mud interface. It is found that during the wave breaking, the majority of energy is lost at the water surface as well as through the viscous dissipation in mud. The energy and viscous dissipation in mud and the energy transfer at the water–mud interface are strongly affected by the wave breaking at the water surface.
Raman spectroscopic calibrations of phonon deformation potentials in wurtzitic AlN
Authors: Zhu Wenliang , Leto Andrea , Hashimoto Ken-ya , Pezzotti Giuseppe
Published: November 2012

A controlled biaxial stress field generated in a ball-on-ring configuration was applied on a thin hexagonal (0001)AlN film grown on (001)Si substrate, and the phonon deformation potential (PDP) constants of the A1 (TO), E2 (high), and A1 (LO) Raman modes were retrieved from the respective spectral shifts. Measurements were done: (i) at given fixed locations upon increasing the externally applied load; and (ii) along spectral line scans performed at a fixed load. The reliability of the retrieved PDP values, in comparison with values previously reported in the literature, was examined by a further evaluation of the residual stress field stored in the AlN/Si system associated with the tip of a crack propagated from the corner of a pyramidal indentation. The PDP assessments were confirmed to be reliable owing to a close correspondence among stress intensity factors independently retrieved from different Raman bands.
Compositional dependence of the absorption edge and dark currents in Ge1−x−ySixSny/Ge(100) photodetectors grown via ultra-low-temperature epitaxy of Ge4H10, Si4H10, and SnD4
Authors: Beeler R. T. , Xu Chi , Smith D. J. , Grzybowski G. , Menéndez J. , Kouvetakis J.
Published: November 2012

Lattice-matched Ge1−x−ySixSny (x ≤ 0.2, y ≤ 0.05) alloys were deposited defect-free on Ge(001) substrates via low-temperature (330–290 °C) reactions of Ge4H10, Si4H10 and SnD4 hydrides, and used to fabricate pin photodetectors. The growth is carried out under gas-source molecular beam epitaxy conditions in a specially designed single-wafer reactor. Optical responsivity measurements reveal absorption edges between 0.88 eV and 0.98 eV, which are used to determine the compositional dependence of the direct band gap. A study of the I-V characteristics of the diodes shows that the dark current is very weakly correlated with the number of Si-Sn bonds in the alloy.
Nonlinear output properties of cantilever driving low frequency piezoelectric energy harvester
Authors: Xu Chundong , Ren Bo , Liang Zhu , Chen Jianwei , Zhang Haiwu , Yue Qingwen , Xu Qing , Zhao Xiangyong , Luo Haosu
Published: November 2012

Cantilever driving low frequency piezoelectric energy harvester (CANDLE) has been found as a promising structure for vibration energy harvesting. This paper presents the nonlinear output properties of the CANDLE to optimize the performance of the device. Simulation results of the finite element method illustrate that nonlinear contacts between the cymbal transducers and the cantilever beam are main reasons of the nonlinear output. However, high excitation acceleration of the nonlinear leap point limits the application of the device. Based on the simulation results and theory analysis, the excitation acceleration is reduced to 30 m/s2 by increasing the proof mass.
Investigation of non-stationary self-focusing of intense laser pulse in cold quantum plasma using ramp density profile
Authors: Habibi M. , Ghamari F.
Published: November 2012

The authors have investigated the non-stationary self-focusing of Gaussian laser pulse in cold quantum plasma. In case of high dense plasma, the nonlinearity in the dielectric constant is mainly due to relativistic high intense interactions and quantum effects. In this paper, we have introduced a ramp density profile for plasma and presented graphically the behavior of spot size oscillations of pulse at rear and front portions of the pulse. It is observed that the ramp density profile and quantum effects play a vital role in stronger and better focusing at the rear of the pulse than at the front in cold quantum plasmas.
Fiber-optic rotation of micro-scale structures enabled microfluidic actuation and self-scanning two-photon excitation
Authors: Black Bryan J. , Luo Dijun , Mohanty Samarendra K.
Published: November 2012

Here, we report non-restricted, controlled fiber optic rotation of micro-motor, in counter-propagating fiber-optic beams having transverse-offset, for actuation of microfluidic flow. Ray-optics based simulations of the torque (and angular velocity) were conducted for different fiber transverse-offsets in order to determine optimal geometry for effective actuation. Further, self-scanning two-photon excitation of the fiber-optically rotated microscopic object is achieved by use of an ultrafast laser beam in one of the fiber arm.
The roles of metallic rectangular-grating and planar anodes in the photocarrier generation and transport of organic solar cells
Authors: Sha Wei E. I. , Choy Wallace C. H. , Cho Chew Weng
Published: November 2012

A multiphysics study carries out on organic solar cells (OSCs) by solving Maxwell's and semiconductor equations simultaneously. By introducing a metallic rectangular-grating as the anode, surface plasmons are excited resulting in nonuniform exciton generation. Meanwhile, the internal E-field of plasmonic OSCs is modified with the modulated anode boundary. The plasmonic OSC improves 13% of short-circuit current but reduces 7% of fill factor (FF) compared to the standard one with a planar anode. The uneven photocarrier generation and transport by the grating anode are physical origins of the dropped FF. This work provides fundamental multiphysics modeling and understanding for plasmonic OSCs.
Field dependency of magnetoelectric coupling in multilayered nanocomposite arrays: Possible contribution from surface spins
Authors: Lu Xiaoli , Dong Sining , Li Xiaoguang , Alexe Marin , Hesse Dietrich , Hao Yue
Published: November 2012

Highly ordered BaTiO3/CoFe2O4 (BTO/CFO) epitaxial heterostructures were grown via modified stencil-assisted pulsed laser deposition. With proper particle size and good crystal quality, the magnetoelectric (ME) coupling effect of the as-prepared nanodots can be probed in a low field. Unexpected coupling relaxation was found after BTO phase transition onset in low field (50 Oe) but was suppressed in high field (50 kOe). This field dependency of ME coupling is proposed to originate in different contributions from bulk and disordered surface spins. The results may provide a way to enhance ME coupling at nanoscale for the design of low-field operated multifunctional ME devices.
A combined first principle calculations and experimental study on the spin-polarized band structure of Co-doped PbPdO2
Authors: Chen S. W. , Huang S. C. , Guo G. Y. , Chiang S. , Lee J. M. , Chen S. A. , Haw S. C. , Lu K. T. , Chen J. M.
Published: November 2012

With x-ray spectroscopy and first-principles calculations, we expose the electronic structure, near the Fermi level, of Co-doped PbPdO2 composed of O 2p-Pd 4d hybridized states with an additional contribution of a spin-polarized Co 3d state at either a greater or smaller energy. The spin-polarized Co 3d states interacting with O 2p-Pd 4d hybridized states cause spin splitting at the band edge. Fascinating physical properties such as high-temperature ferromagnetism thus arise in Co-doped PbPdO2. Results will help in the design of materials with desired electronic structures and the control of spin polarization with chemical doping.
Defect induced mobility enhancement: Gadolinium oxide (100) on Si(100)
Authors: Sitaputra W. , Tsu R.
Published: November 2012

Growth of predominantly single crystal (100)-oriented gadolinium oxide (Gd2O3) on a p-type Si(100) and growth of a polycrystal with a predominant Gd2O3(100) crystallite on a n-type Si(100) was performed using molecular beam epitaxy. Despite a poorer crystal structure than Gd2O3(110), an enhancement in carrier mobility can be found only from the Gd2O3(100)/n-type Si(100) interface. The mobility of 1715-1780 cm2/V · s was observed at room temperature, for carrier concentration >1020 cm−3. This accumulation of the electrons and the mobility enhancement may arise from the two-dimensional confinement due to charge transfer across the interface similar to transfer doping.
Ultra-thin silicon-on-insulator strip waveguides and mode couplers
Authors: Gould Michael , Pomerene Andrew , Hill Craig , Ocheltree Stewart , Zhang Yi , Baehr-Jones Tom , Hochberg Michael
Published: November 2012

We demonstrate an ultra-thin silicon waveguide for wavelengths around 1.55 μm, and mode converters designed for transitions to and from standard 500 nm × 220 nm strip waveguides. The devices were fabricated in a CMOS-compatible process requiring two photolithography and etch steps. The ultra-thin waveguides exhibited losses of 2.01 ± 0.231 dB/cm, exhibited bend radii as small as 30 μm with losses of 0.05 ± 0.005 dB per bend, and exhibited coupling losses of 0.66 ± 0.014 dB to standard strip waveguides.
Quasi-equilibrium electron density along a magnetic field line
Authors: Mao Hann-Shin , Wirz Richard
Published: November 2012

A methodology is developed to determine the density of high-energy electrons along a magnetic field line for a low-β plasma. This method avoids the expense and statistical noise of traditional particle tracking techniques commonly used for high-energy electrons in bombardment plasma generators. By preserving the magnetic mirror and assuming a mixing timescale, typically the elastic collision frequency with neutrals, a quasi-equilibrium electron distribution can be calculated. Following the transient decay, the analysis shows that both the normalized density and the reduction fraction due to collision converge to a single quasi-equilibrium solution.
Intrinsic rotation produced by ion orbit loss and X-loss
Authors: Stacey W. M. , Boedo J. A. , Evans T. E. , Grierson B. A. , Groebner R. J.
Published: November 2012

A practical calculation model for the intrinsic rotation imparted to the edge plasma by the directionally preferential loss of ions on orbits that cross the last closed flux surface is presented and applied to calculate intrinsic rotation in several DIII-D [J. Luxon, Nucl. Fusion 42, 614 (2002)] discharges. The intrinsic rotation produced by ion loss is found to be sensitive to the edge temperature and radial electric field profiles, which has implications for driving intrinsic rotation in future large tokamaks.
Simulation of relativistically colliding laser-generated electron flows
Authors: Yang X. H. , Dieckmann M. E. , Sarri G. , Borghesi M.
Published: November 2012

The plasma dynamics resulting from the simultaneous impact, of two equal, ultra-intense laser pulses, in two spatially separated spots, onto a dense target is studied via particle-in-cell simulations. The simulations show that electrons accelerated to relativistic speeds cross the target and exit at its rear surface. Most energetic electrons are bound to the rear surface by the ambipolar electric field and expand along it. Their current is closed by a return current in the target, and this current configuration generates strong surface magnetic fields. The two electron sheaths collide at the midplane between the laser impact points. The magnetic repulsion between the counter-streaming electron beams separates them along the surface normal direction, before they can thermalize through other beam instabilities. This magnetic repulsion is also the driving mechanism for the beam-Weibel (filamentation) instability, which is thought to be responsible for magnetic field growth close to the internal shocks of gamma-ray burst jets. The relative strength of this repulsion compared to the competing electrostatic interactions, which is evidenced by the simulations, suggests that the filamentation instability can be examined in an experimental setting.
Valid flow combinations for stable sheath in a magnetized multiple ion species plasma
Authors: Sharma Devendra , Kaw Predhiman K.
Published: November 2012

Theoretical study is done of the entry criterion for the plasma flow into the electrostatic boundary layer, or sheath, forming in a magnetized multiple ion species plasma. Finding valid entry velocity combinations in a magnetized set up requires a magnetized equivalent of the generalized Bohm criterion. A magnetized generalized entry criterion is obtained with the scale length distribution in a region of validity for the stable solutions. The analysis finds that the valid entry flow velocity combinations with distinct values of individual ion species can correspond to a unique system phase velocity. Magnetization effects govern the region of validity whose boundaries collapse to the unmagnetized sheath criterion in the limit of normal incidence, independent of the strength of the magnetic field. Considerably smaller entry velocities, in comparison to the unmagnetized system sound velocity, are recovered for the species in appropriate regime of magnetization in the cases of oblique incidences.
Influence of Ni impurities on the thermoelectric properties of Ca-partially filled skutterudites CaxCo4Sb12
Authors: Puyet M. , Lenoir B. , Dauscher A. , Candolfi C. , Hejtmanek J. , Stiewe C. , Müller E.
Published: November 2012

Polycrystalline samples of the skutterudite compounds CaxCo4Sb12 were synthesized and investigated by means of electrical resistivity, thermal conductivity, and thermopower in the 300–800 K temperature range. Additional Hall effect measurements were performed between 2 and 300 K. Both Ni-free and Ni-containing Co powders were used as starting materials to elucidate the impact of Ni impurities at the ppm level on the thermoelectric properties. The presence of minute amounts of Ni atoms in the crystal structure leads to enhanced thermopower values with respect to the Ni-free samples, likely associated with the development of additional electron pockets near the Fermi level. Ni impurities, thus, play a significant role on the transport properties and lead to a spectacular increase in the dimensionless thermoelectric figure of merit ZT of up to 150% at 800 K.
Multi electron species and shielding potentials in plasmas
Authors: Khan Arroj A. , Rasheed A. , Jamil M. , Murtaza G.
Published: November 2012

The phenomenon of Debye shielding is investigated in electron ion plasmas using the approach of two temperature electrons. We get different profiles of potential for different parameters and observe that the potentials fall very slowly than the standard Coulomb and Debye potentials. The importance of work is pointed out in the introduction.
Solution processable carbon nanotube network thin-film transistors operated in electrolytic solutions at various pH
Authors: Haeberle Tobias , Münzer Alexandra M. , Buth Felix , Antonio Garrido Jose , Abdellah Alaa , Fabel Bernhard , Lugli Paolo , Scarpa Giuseppe
Published: November 2012

We investigate the electronic properties of solution-gated carbon nanotube (CNT) thin-film transistors, where the active layer consists of a randomly distributed single-walled CNT network of >90% semiconducting nanotubes, deposited from an aqueous solution by spin-coating. The devices are characterized in different electrolytic solutions, where a reference electrode immersed in the liquid is used to apply the gate potential. We observe a gate-potential shift in the transfer characteristic when the pH and/or ionic strength of the electrolytic solution is changed with a pH sensitivity of ≈19 mV/pH. This sensitivity is attributed to a surface charging effect at the CNT/electrolyte interface.
Frequency agile characteristics of a dielectric filled relativistic magnetron with diffraction output
Authors: Li Wei , Zhang Jun , Liu Yong-gui , Yang Han-wu , Shi Di-fu
Published: November 2012

This paper reports the investigations of a dielectric filled relativistic magnetron with diffraction output (MDO) on frequency agile. The mechanism of frequency agile is theoretically analyzed. Particle-in-cell simulations and preliminary experiments prove the analytics. In experiments, under the working conditions, 605 kV and 0.3 T, a microwave with 1.98 GHz, 200 MW is radiated from an A6 type MDO when the 95% Al2O3 ceramics with the total thickness of 0.9 cm are filled. Compared with the microwave of 3.72 GHz, 240 MW obtained without the ceramics filled, the frequency agile from S band to L band is achieved.
Conductance modulation of Si nanowire arrays
Authors: Li Chuanbo , Krali Emiljana , Fobelets Kristel , Cheng Buwen , Wang Qiming
Published: November 2012

The conductance modulation of vertically aligned Si nanowire arrays with ammonia is investigated. Ammonia adsorption on the surface of the nanowires (NWs) in the array greatly increases the electrical conductivity of n-type NWs. This effect can be potentially applied to boost figure of merit for thermoelectric applications by combining the phonon confinement effect of NWs with the electrical conductivity increase. By using 1/f noise measurements, the mechanism of conduction modulation is investigated. The enhancement of the electrical conduction is interpreted in terms of electron trap filling of the native oxide via ammonia adsorption.
Photoacoustic spectrum analysis for microstructure characterization in biological tissue: A feasibility study
Authors: Xu Guan , Dar Irfaan A. , Tao Chao , Liu Xiaojun , Deng Cheri X. , Wang Xueding
Published: November 2012

This study investigates the feasibility of characterizing the microstructures within a biological tissue by analyzing the frequency spectrum of the photoacoustic signal from the tissue. Hypotheses are derived from theoretical analyses on the relationships between the dimensions/concentrations of the photoacoustic sources within the region-of-interest and the linear model fitted to the power spectra of photoacoustic signals. The hypotheses are validated, following the procedures of ultrasound spectrum analysis, by simulations and experiments with phantoms fabricated by embedding the polyethylene microspheres in porcine gelatin, indicating that photoacoustic spectrum analysis could be a potential tool for characterizing microstructures in biological samples.
CdSe quantum dots synthesized by laser ablation in water and their photovoltaic applications
Authors: Horoz Sabit , Lu Liyou , Dai Qilin , Chen Jiajun , Yakami Baichhabi , Pikal J. M. , Wang Wenyong , Tang Jinke
Published: November 2012

CdSe quantum dots (QDs) have been prepared by a facile and clean synthesis method––laser ablation in water. The structural and luminescent properties of the CdSe QDs have been investigated. The CdSe QDs of wurtzite crystal structure have an average particle size of about 5 nm. The QDs can be attached to ZnO nanowires making them ideal for applications in QD-sensitized nanowire solar cells. A uniqueness of the QDs attached to the ZnO nanowires by this laser ablation method is that they do not contain ligands, and the preparation avoids the complicated process of ligand exchange.
Fiber-cavity-based optomechanical device
Authors: Flowers-Jacobs N. E. , Hoch S. W. , Sankey J. C. , Kashkanova A. , Jayich A. M. , Deutsch C. , Reichel J. , Harris J. G. E.
Published: November 2012

We describe an optomechanical device consisting of a fiber-based optical cavity containing a silicon nitride membrane. In comparison with typical free-space cavities, the fiber-cavity's small mode size (10 μm waist, 80 μm length) allows the use of smaller, lighter membranes and increases the cavity-membrane linear coupling to 3 GHz/nm and the quadratic coupling to 20 GHz/nm2. This device is also intrinsically fiber-coupled and uses glass ferrules for passive alignment. These improvements will greatly simplify the use of optomechanical systems, particularly in cryogenic settings. At room temperature, we expect these devices to be able to detect the shot noise of radiation pressure.
Enhanced and persistent photoconductivity in vertical silicon nanowires and ZnS nanoparticles hybrid devices
Authors: Ahmad Mushtaq , Rasool Kamran , Rafiq M. A. , Hasan M. M.
Published: November 2012

We observe persistent photoconductivity, enhanced detectivity, responsivity, and external quantum efficiency (EQE) in hybrid ZnS nanoparticles (NPs) and vertical silicon nanowire (Si NW) devices. ZnS NPs (diameter ∼30 nm) were prepared by co-precipitation method. Si NWs (length ∼30 μm, diameter ∼30-400 nm) were prepared by electroless chemical etching. Hybrid device shows ∼10, 3, and 10 times enhancement of EQE, detectivity, and responsivity, respectively, as compared with the Si NWs only devices. The enhancement is attributed to presence of low refractive index ZnS around Si NWs causing funneling of photon energy into Si NWs.
Plasma flares in high power impulse magnetron sputtering
Authors: Ni Pavel A. , Hornschuch Christian , Panjan Matjaž , Anders André
Published: November 2012

Self-organized ionization zones and associated plasma flares were recorded with fast cameras in side-on view. Flare velocities were estimated to be about 20 000 m/s suggesting that the local tangential field Eξ is about 2000 V/m based on a concept where flare-causing electrons are initially ejected by Eξ×B drift. At distances of 10 mm and greater from the target, where the electric field is very small, plasma flares are guided by the magnetic field B.
Low-temperature grown graphene films by using molecular beam epitaxy
Authors: Lin Meng-Yu , Guo Wei-Ching , Wu Meng-Hsun , Wang Pro-Yao , Liu Te-Huan , Pao Chun-Wei , Chang Chien-Cheng , Lee Si-Chen , Lin Shih-Yen
Published: November 2012

Complete graphene film is prepared by depositing carbon atoms directly on Cu foils in a molecular beam epitaxy chamber at 300 °C. The Raman spectrum of the film has indicated that high-quality few-layer graphene is obtained. With back-gated transistor architecture, the characteristic current modulation of graphene transistors is observed. Following the similar growth procedure, graphitization is observed at room temperature, which is consistent with the molecular dynamics simulations of graphene growth.
Impact of the surface-near silicon substrate properties on the microstructure of sputter-deposited AlN thin films
Authors: Schneider M. , Bittner A. , Patocka F. , Stöger-Pollach M. , Halwax E. , Schmid U.
Published: November 2012

In micro-/nanomachined devices and systems, aluminum nitride (AlN) thin films are widely used due to their piezoelectric properties. This work evaluates the potential of modifying the interface between the AlN thin film and the silicon (Si) wafer serving as bottom electrode for optimized crystallographic orientation and, hence, improved electrical and piezoelectric properties. The films were analyzed using temperature-dependant leakage current measurements, transmission electron microscopy, and x-ray diffraction. By preconditioning of the Si substrate surface applying sputter etching prior to film deposition, leakage current levels are substantially decreased and an increased (002) orientation of the AlN grains is observed.
Universal scaling of resistivity in bilayer graphene
Authors: Gopinadhan Kalon , Jun Shin Young , Yang Hyunsoo
Published: November 2012

We report the temperature dependent electrical transport properties of gated bilayer graphene devices. We see a clear evidence of insulating behavior due to electron-hole charge puddles. The electrical resistivity increases while the mobility decreases with decreasing temperature, a characteristic due to carrier inhomogeneity in graphene. The theoretical fittings using an empirical formula of single electron tunneling indicate that electrical resistivity follows a universal curve with a scaling parameter. The scaling parameter is determined to be a measure of the fluctuations in the electron-hole puddle distribution.
Increase in interparticle distance of colloidal dipolar chain in nematic liquid crystal by trapping it on splay-bend wall
Authors: Tagashira K. , Asakura K. , Nakazawa G. , Yoshida H. , Ozaki M.
Published: November 2012

We demonstrate an increase in the interparticle distance of a colloidal dipolar chain in a nematic liquid crystal (NLC). Applying an in-plane electric field perpendicular to the rubbing direction induces a splay-bend wall defect in the middle of the electrode gap, which traps a dipolar chain. Above the Freedericksz threshold electric field, the interparticle distance increases with increasing applied electric field, owing to the reorientation of the NLC molecules. The maximum increase is 32% of the particle diameter.
Light emission from InGaAs:Bi/GaAs quantum wells at 1.3 μm
Authors: Ye Hong , Song Yuxin , Gu Yi , Wang Shumin
Published: November 2012

Highly strained InGaAs:Bi quantum wells (QWs) were grown on (001)-oriented GaAs substrates by molecular beam epitaxy (MBE). Photoluminescence (PL) reveals strong improvements in the optical properties evidenced by 10 times enhancement in PL intensity and extended emission wavelength up to 1.29 μm when Bi is introduced to InGaAs/GaAs QWs. The improved optical quality results from the Bi surfactant effect as well as the Bi incorporation. Post growth thermal annealing shows that Bi atoms in InGaAs/GaAs QWs do not show good thermal stability at 650 °C and tend to diffuse out of the QWs resulting in large wavelength blue-shifts.
Terahertz resonances due to edge magnetoplasmons in a wide armchair graphene ribbon with a weak superlattice potential
Authors: Balev O. G. , Ramos A. C. A.
Published: November 2012

A strong modulation of the resonance frequencies and the gaps of edge magnetoplamons (EMPs), in the ribbon at the ν = 2 quantum Hall effect regime with a superlattice and a smooth steplike electrostatic lateral potential, due to a modulation of the interedge distance is shown. We obtain that, in a wide range of the interedge distances, within THz region the EMPs show wide frequency gaps and at peculiar frequencies the response of the system should manifest resonances. For the latter frequencies the group velocities are nullified.