bluuebunny/crossref_metadata_2025 huggingface dataset:
A reduction theorem for time dependent Schrödinger operator with finite differentiable unbounded perturbation
Authors: Yuan Xiaoping , Zhang Kangkang
Published: May 2013

In this paper, we consider the time dependent Schrödinger operator with a temporal quasi-periodic perturbation which is some function of class Cℓ for any ℓ ⩾ 100(3n + 2τ + 1), by using the KAM technique, we establish a reduction theorem for the above operator and prove that the invariant tori of the nearly-integrable Hamiltonian system with unbounded small perturbation are linearly stable both for \documentclass[12pt]{minimal}\begin{document}$\tilde{d}<d-1$\end{document}d̃<d−1 and the limiting case \documentclass[12pt]{minimal}\begin{document}$\tilde{d}=d-1$\end{document}d̃=d−1.
Intermolecular interactions and the thermodynamic properties of supercritical fluids
Authors: Yigzawe Tesfaye M. , Sadus Richard J.
Published: May 2013

The role of different contributions to intermolecular interactions on the thermodynamic properties of supercritical fluids is investigated. Molecular dynamics simulation results are reported for the energy, pressure, thermal pressure coefficient, thermal expansion coefficient, isothermal and adiabatic compressibilities, isobaric and isochoric heat capacities, Joule-Thomson coefficient, and speed of sound of fluids interacting via both the Lennard-Jones and Weeks-Chandler-Andersen potentials. These properties were obtained for a wide range of temperatures, pressures, and densities. For each thermodynamic property, an excess value is determined to distinguish between attraction and repulsion. It is found that the contributions of intermolecular interactions have varying effects depending on the thermodynamic property. The maxima exhibited by the isochoric and isobaric heat capacities, isothermal compressibilities, and thermal expansion coefficient are attributed to interactions in the Lennard-Jones well. Repulsion is required to obtain physically realistic speeds of sound and both repulsion and attraction are necessary to observe a Joule-Thomson inversion curve. Significantly, both maxima and minima are observed for the isobaric and isochoric heat capacities of the supercritical Lennard-Jones fluid. It is postulated that the loci of these maxima and minima converge to a common point via the same power law relationship as the phase coexistence curve with an exponent of β = 0.32. This provides an explanation for the terminal isobaric heat capacity maximum in supercritical fluids.
A high-impedance attenuator for measurement of high-voltage nanosecond-range pulses
Authors: Yu Binxiong , Liu Jinliang , Zhang Tianyang , Hong Zhiqiang
Published: May 2013

A novel kind of high-impedance cable attenuator for measurement of high-voltage ns-range pulses is investigated in this paper. The input and output ports of the proposed attenuator were both high-impedance ports, and good pulse response characteristics of the proposed attenuator were obtained with pulse response time less than 1 ns. According to the requirement of measurement, two attenuators with lengths at 14 m and 0.7 m were developed with response time of 1 ns and 20 ns, and the attenuation coefficient of 96 and 33.5, respectively. The attenuator with the length of 14 m was used as a secondary-stage attenuator of a capacitive divider to measure the high-voltage pulses at several hundred ns range. The waveform was improved by the proposed attenuator in contrast to the result only measured by the same capacitive divider and a long cable line directly. The 0.7 m attenuator was also used as a secondary-stage attenuator of a standard resistant divider for an accurate measurement of high-voltage pulses at 100 ns range. The proposed cable attenuator can be used to substitute the traditional secondary-stage attenuators for the measurement of high-voltage pulses.
Voltage-driven perpendicular magnetic domain switching in multiferroic nanoislands
Authors: Hu Jia-Mian , Yang T. N. , Chen L. Q. , Nan C. W.
Published: May 2013

We show that, using phase-field simulations, large voltage-driven perpendicular magnetic domain switching can be realized in magnetic-ferroelectric nanoislands with relieved substrate constraint, which is difficult in continuous multiferroic layered thin films due to significant substrate clamping. The as-grown magnetic and ferroelectric domain structures in the heterostructured nanoislands can be tailored by engineering their respective geometric sizes and/or the underlying substrate strain. Influences of the lateral size of the island on the dynamic voltage-driven magnetic domain switching are addressed, whereby an optimum lateral size is identified for illustration. Thus, such three-dimensional multiferroic nanoislands should provide great flexibilities for designing novel high-density spintronic/microelectronic devices with purely voltage-driven means.
Hard x-ray tomographic studies of the destruction of an energetic electron ring
Authors: Wang Y. , Gekelman W. , Pribyl P.
Published: May 2013

A tomography system was designed and built at the Large Plasma Device to measure the spatial distribution of hard x-ray (100 KeV-3 MeV) emissivity. The x-rays were generated when a hot electron ring was significantly disrupted by a shear Alfvén wave. The plasma is pulsed at 1 Hz (1 shot/s). A lead shielded scintillator detector with an acceptance angle defined by a lead pinhole is mounted on a rotary gimbal and used to detect the x-rays. The system measures one chord per plasma shot using only one detector. A data plane usually consists of several hundred chords. A novel Dot by Dot Reconstruction (DDR) method is introduced to calculate the emissivity profile from the line integrated data. In the experiments, there are often physical obstructions, which make measurements at certain angles impossible. The DDR method works well even in this situation. The method was tested with simulated data, and was found to be more effective than previously published methods for the specific geometry of this experiment. The reconstructed x-ray emissivity from experimental data by this method is shown.
A new ab initio based global HOOH(13A″) potential energy surface for the O(3P) + H2O(X1A1) ↔ OH(X2Π) + OH(X2Π) reaction
Authors: Li Jun , Guo Hua
Published: May 2013

An accurate global potential energy surface is developed for the title reaction by fitting more than 36 000 of ab initio points at the CCSD(T)/AVTZ level using the permutation invariant polynomial method. The canonical rate constants for both the forward and reverse directions of the title reaction are determined on the new potential energy surface and the agreement with experiment is satisfactory. In addition, the dynamics of the forward reaction is investigated with the quasi-classical trajectory method. It is found that this direct abstraction reaction has a backward bias in its product angular distribution, consistent with a direct rebound mechanism. The OH product newly formed by the reaction exhibits a bimodal rotational state distribution, due apparently to secondary collisions with the slowly recoiling spectator OH product.
Electrical performance of monolayer MoS2 field-effect transistors prepared by chemical vapor deposition
Authors: Amani Matin , Chin Matthew L. , Birdwell A. Glen , O’Regan Terrance P. , Najmaei Sina , Liu Zheng , Ajayan Pulickel M. , Lou Jun , Dubey Madan
Published: May 2013

Molybdenum disulfide (MoS2) field effect transistors (FET) were fabricated on atomically smooth large-area single layers grown by chemical vapor deposition. The layer qualities and physical properties were characterized using high-resolution Raman and photoluminescence spectroscopy, scanning electron microscopy, and atomic force microscopy. Electronic performance of the FET devices was measured using field effect mobility measurements as a function of temperature. The back-gated devices had mobilities of 6.0 cm2/V s at 300 K without a high-κ dielectric overcoat and increased to 16.1 cm2/V s with a high-κ dielectric overcoat. In addition the devices show on/off ratios ranging from 105 to 109.
Defect segregation at grain boundary and its impact on photovoltaic performance of CuInSe2
Authors: Yin Wan-Jian , Wu Yelong , Noufi Rommel , Al-Jassim Mowafak , Yan Yanfa
Published: May 2013

Defect segregations at grain boundaries (GBs) of polycrystalline thin-film absorber are crucial to solar cell performance. The Σ3(114) GBs in CuInSe2 (CIS) have been studied through first-principle calculations. We reveal that the intrinsic CIS GBs produce deep gap states, which act as nonradiative recombination centers. However, the segregation of CuIn and OSe at GBs can clean the gap states and lead to electrically benign behavior. Our results suggest that the defect segregation at GBs could be an important feature for high efficiency CIS-based photovoltaic solar cells and it provides a general guidance for engineering GBs in other chalcogenide polycrystalline devices.
Response to “Comment on ‘Unified explanation of the anomalous dynamic properties of highly asymmetric polymer blends’ ” [J. Chem. Phys. 138, 197101 (2013)]
Authors: Ngai K. L. , Capaccioli S.
Published: May 2013

The Comment of Colmenero asserts no change in Fs(Q,t) of the poly(ethylene oxide) (PEO) chains in blends with poly(methyl methacrylate) on crossing times of about 1–2 ns in data obtained by neutron scattering experiments and simulations. The assertion is opposite to that reported in the original papers where the neutron data and simulations were published. To make this point clear, we cite the data and the very statements made in the original papers concluding that indeed in the time interval from 60 ps to 1–2 ns the dynamics of PEO chain follows approximately the Rouse model, but becomes slower and departs from the Rouse model in the dependencies on time, momentum transfer, and temperature at longer times past tc = 1–2 ns. It is noteworthy that similar crossover of chain dynamics in entangled homopolymers at the ns time scale was found by neutron scattering.
Theoretical and experimental study on two-stage-imaging microscopy using ellipsometric contrast for real-time visualization of molecularly thin films
Authors: Kajihara Y. , Fukuzawa K. , Itoh S. , Watanabe R. , Zhang H.
Published: May 2013

A two-stage-imaging ellipsometric-contrast microscope (TIEM) has been developed to measure the distribution of film thickness over a wide area of molecularly thin liquid films with a high lateral resolution, wide field of view, high thickness resolution, and high-speed. Moreover, this ellipsometric microscope enables us to achieve simultaneous measurements with other measurement apparatuses. We present the principle used to parallelize an object image to an imaging sensor and to reduce the incident angle entering the imaging sensor. In addition, we discuss the characteristic shape deformation of the object image due to oblique observation. The performance of the actual setup for TIEM was experimentally studied. A lateral resolution of about 1 μm was obtained by measuring the modulation transfer function of the TIEM. We also found that the shape deformation approximately agreed with that from theory. Furthermore, for molecularly thin films, we confirmed linearity between the film thickness and the light intensity measured with TIEM, which enables us to quantify the thickness of the films. TIEM can open up a new field of real-time imaging of thin films such as visualization of a liquid lubricant film under shear.
Turbulence and transport suppression scaling with flow shear on the Large Plasma Device
Authors: Schaffner D. A. , Carter T. A. , Rossi G. D. , Guice D. S. , Maggs J. E. , Vincena S. , Friedman B.
Published: May 2013

Continuous control over azimuthal flow and shear in the edge of the Large Plasma Device (LAPD) [W. Gekelman et al., Rev. Sci. Instr. 62, 2875 (1991)] has been achieved using a biasable limiter. This flow control has allowed a careful study of the effect of flow shear on pressure-gradient-driven turbulence and particle transport in LAPD. The combination of externally controllable shear in a turbulent plasma along with the detailed spatial diagnostic capabilities on LAPD makes the experiment a useful testbed for validation of shear suppression models. Motivated by these models, power-law fits are made to the density and radial velocity fluctuation amplitudes, particle flux, density-potential crossphase, and radial correlation length. The data show a break in the trend of these quantities when the shearing rate (γs=∂Vθ/∂r) is comparable to the turbulent decorrelation rate (1/τac). No one model captures the trends in the all turbulent quantities for all values of the shearing rate, but some models successfully match the trend in either the weak (γsτac<1) or strong (γsτac>1) shear limits.
Functionalization of carbon nanotubes with –CHn, –NHn fragments, –COOH and –OH groups
Authors: Milowska Karolina Z. , Majewski Jacek A.
Published: May 2013

We present results of extensive theoretical studies concerning stability, morphology, and band structure of single wall carbon nanotubes (CNTs) covalently functionalized by –CHn (for n = 2,3,4), –NHn (for n = 1,2,3,4), –COOH, and –OH groups. These studies are based on ab initio calculations in the framework of the density functional theory. For functionalized systems, we determine the dependence of the binding energies on the concentration of the adsorbed molecules, critical densities of adsorbed molecules, global and local changes in the morphology, and electronic structure paying particular attention to the functionalization induced changes of the band gaps. These studies reveal physical mechanisms that determine stability and electronic structure of functionalized systems and also provide valuable theoretical predictions relevant for application. In particular, we observe that functionalization of CNTs causes generally their elongation and locally sp2 to sp3 rehybridization in the neighborhood of chemisorbed molecules. For adsorbants making particularly strong covalent bonds with the CNTs, such as the –CH2 fragments, we observe formation of the characteristic pentagon/heptagon (5/7) defects. In systems functionalized with the –CH2, –NH4, and –OH groups, we determine critical density of molecules that could be covalently bound to the lateral surface of CNTs. Our studies show that functionalization of CNTs can be utilized for band gap engineering. Functionalization of CNTs can also lead to changes in their metallic/semiconductor character. In semiconducting CNTs, functionalizing molecules such as –CH3, –NH2, –OH, –COOH, and both –OH and –COOH, introduce “impurity” bands in the band gap of pristine CNTs. In the case of –CH3, –NH2 molecules, the induced band gaps are typically smaller than in the pure CNT and depend strongly on the concentration of adsorbants. However, functionalization of semiconducting CNTs with hydroxyl groups leads to the metallization of CNTs. On the other hand, the functionalization of semi-metallic (9,0) CNT with –CH2 molecules causes the increase of the band gap and induces semi-metall to semiconductor transition.
Mesoscopic analysis of leakage current suppression in ZrO2/Al2O3/ZrO2 nano-laminates
Authors: Martin Dominik , Grube Matthias , Weinreich Wenke , Müller Johannes , Weber Walter M. , Schröder Uwe , Riechert Henning , Mikolajick Thomas
Published: May 2013

Metal-Insulator-Metal capacitors, with ZrO2/Al2O3/ZrO2 (ZAZ)-nanolaminate thin-films as a dielectric layer, exhibit reduced leakage currents compared to corresponding capacitors based on pure ZrO2 while maintaining a sufficiently high dielectric constant for the DRAM application. This work is a comparative study demonstrating how the incorporation of a small amount of Al is responsible for the suppression of crystallization during deposition. Extensive electrical characterization leads to the identification of a defect band which conductive atomic force microscopy shows to be formed along crystallite grain boundaries, extending through the entire ZrO2-film. The incorporation of a sub-layer of Al2O3 prevents these grain boundaries resulting in an effective reduction of leakage currents, despite the film being in the nanocrystalline phase, necessary for it to exhibit the required high dielectric constant. A transport model based on phonon assisted trap to trap tunneling is proposed.
Computational analysis of hot-spot formation by quasi-steady deformation waves in porous explosive
Authors: Gilbert John , Chakravarthy Sunada , Gonthier Keith A.
Published: May 2013

The impact and shock sensitivity of porous (granular) high-explosives is related to the formation of small mass regions of elevated temperature within the material called hot-spots by dissipative mechanisms such as plastic and friction work. Because of their small size, hot-spots are difficult to experimentally interrogate, particularly for high volumetric strain rates (ϵ̇V>10,000 s−1). In this study, simulations are performed for large ensembles of deformable particles (≈4000 particles) using a combined finite and discrete element technique to characterize statistical distributions of hot-spot intensity, geometry, and spatial proximity within and behind quasi-steady, piston supported uniaxial waves in granular HMX (C4H8N8O8). Emphasis is placed on examining how the material's initial particle packing density, characterized by its effective solid volume fraction ϕ¯s,0, affects hot-spot statistics for pressure dominated waves corresponding to piston speeds within the range 300≤Up≤500 m/s. Predictions indicate that hot-spot intensity is only marginally affected by initial porosity (1−ϕ¯s,0) for all piston speeds, whereas hot-spot size, number density, volume fraction, and volume specific surface area appreciably increase with porosity and exponentially increase with piston speed. Minor variations in particle shape are predicted to be largely inconsequential. Joint distributions of hot-spot intensity and size are combined with thermal explosion data to identify and examine critical hot-spots that quickly react behind waves. These results indicate that the observed increase in sensitivity with initial porosity for sustained loading is likely due to an increase in hot-spot size and number rather than intensity.
Flexoelectric effect on the electroelastic responses of bending piezoelectric nanobeams
Authors: Yan Z. , Jiang L. Y.
Published: May 2013

Flexoelectricity, referring to a spontaneous electric polarization induced by strain gradient in dielectrics, presents a strong size dependency at the nanoscale. In the current work, the influence of the flexoelectric effect on the mechanical and electrical properties of bending piezoelectric nanobeams with different boundary conditions is investigated. Based on the extended linear piezoelectricity theory and the Euler beam model, analytical solutions of the electroelastic fields in the piezoelectric nanobeams subjected to both electrical and mechanical loads are obtained with the inclusion of the flexoelectric effect. Simulation results show that the flexoelectric effect on the elastic behavior of bending beams is sensitive to the beam boundary conditions and the applied electrical load. In addition, for a cantilever piezoelectric nanobeam, an axial relaxation strain is induced from the piezoelectric and flexoelectric effects, while these effects induce a resultant axial force in both the clamped-clamped and simply supported piezoelectric nanobeams. Results also indicate that the flexoelectric effect plays a significant role in the contact stiffness and electric polarization of piezoelectric beams when their thickness is at the nanoscale. It is found that the flexoelectric effect on the electroelastic responses of piezoelectric nanobeams is more pronounced for the beams with smaller thickness. These results are useful for understanding the fundamental mechanical and physical properties of bending piezoelectric nanobeams.
Electronic structure and quantum dynamics of photoinitiated dissociation of O2 on rutile TiO2 nanocluster
Authors: Dholabhai Pratik P. , Yu Hua-Gen
Published: May 2013

The adsorption and photoinitiated dissociation of molecular oxygen on reduced rutile TiO2 nanocluster have been studied using a hybrid density functional theory (DFT)/time-dependent DFT approach and a time-dependent wavepacket dynamics method. Results show that the most favorable state for O2 at the bridging row O-vacancy site of TiO2 is O22− with an orientation parallel to the surface. We find that its dissociation in the electronic ground state involves a spin forbidden intersystem crossing, and therefore has a large barrier along the reaction pathway. However, time-dependent wavepacket calculations reveal that the photoinitiated O2 dissociation on TiO2 is very fast via a direct mechanism on the excited states. The lifetime of excited O2 molecules is predicted to be about 266 fs. Non-adiabatic effects among the singlet electronic states are found to play an important role in the O2 dissociation whereas the spin-orbit effect is negligible. In addition, adsorption of two O2 molecules at an O-vacancy site shows that the second O2 molecule can stabilize the system by about 0.22 eV.
Dedicated vertical wind tunnel for the study of sedimentation of non-spherical particles
Authors: Bagheri G. H. , Bonadonna C. , Manzella I. , Pontelandolfo P. , Haas P.
Published: May 2013

A dedicated 4-m-high vertical wind tunnel has been designed and constructed at the University of Geneva in collaboration with the Groupe de compétence en mécanique des fluides et procédés énergétiques. With its diverging test section, the tunnel is designed to study the aero-dynamical behavior of non-spherical particles with terminal velocities between 5 and 27 ms−1. A particle tracking velocimetry (PTV) code is developed to calculate drag coefficient of particles in standard conditions based on the real projected area of the particles. Results of our wind tunnel and PTV code are validated by comparing drag coefficient of smooth spherical particles and cylindrical particles to existing literature. Experiments are repeatable with average relative standard deviation of 1.7%. Our preliminary experiments on the effect of particle to fluid density ratio on drag coefficient of cylindrical particles show that the drag coefficient of freely suspended particles in air is lower than those measured in water or in horizontal wind tunnels. It is found that increasing aspect ratio of cylindrical particles reduces their secondary motions and they tend to be suspended with their maximum area normal to the airflow. The use of the vertical wind tunnel in combination with the PTV code provides a reliable and precise instrument for measuring drag coefficient of freely moving particles of various shapes. Our ultimate goal is the study of sedimentation and aggregation of volcanic particles (density between 500 and 2700 kgm−3) but the wind tunnel can be used in a wide range of applications.
On the optical and morphological properties of microstructured Black Silicon obtained by cryogenic-enhanced plasma reactive ion etching
Authors: Nguyen K. N. , Basset P. , Marty F. , Leprince-Wang Y. , Bourouina T.
Published: May 2013

Motivated by the need for obtaining low reflectivity silicon surfaces, we report on (sub-) micro-texturing of silicon using a high throughput fabrication process involving SF6/O2 reactive ion etching at cryogenic temperatures, leading to Black Silicon (BS). The corresponding high aspect ratio conical spikes of the microstructured surface give rise to multiple reflections and hence, enhanced absorption under electromagnetic radiation. Aiming a better understanding of this mechanism, we performed a systematic study by varying several plasma process parameters: O2/SF6 gas flow rate ratio, silicon temperature, bias voltage, and etching time. We determined the process window which leads to BS formation and we studied the influence of the process parameters on the surface morphology of the obtained BS samples, through analysis of scanning electron microscopy images. The measured optical reflectance of BS is in the order of 1% in the visible and near infrared ranges (400–950 nm). We noticed that the lowest reflectance is obtained close to the threshold parameters of BS formation. Absorptance spectral response of BS is measured from 1.3 to 17 μm, and we observed a great enhancement of absorptance up to about 75% compared to flat silicon. We also obtained through these experiments, a clear evidence of a correlation between the excellent optical properties and the aspect ratio of the BS conical microstructures in the measured wavelength ranges.
The role of surface states in modification of carrier transport in silicon nanowires
Authors: Rasool Kamran , Rafiq M. A. , Ahmad Mushtaq , Imran Zahid , Hasan M. M.
Published: May 2013

We investigate transport properties of polyacrylic acid (PAA) capped n and p-type silicon nanowire (SiNW) arrays. PAA diluted with deionized water at different concentrations was spun directly on vertically grown SiNW arrays prepared by metal assisted electroless chemical etching. PAA provides mechanical support to electrical contacts and acts as a source of interface doping by creating acceptor like states (holes) on SiNWs surface. PAA capping results in increase in current in p-type SiNWs and decrease in current in n-type SiNWs. Schottky emission model fits current voltage (IV) characteristics of p-type SiNWs/PAA device. Ohmic like conduction at lower voltages followed by space charge limited current (SCLC) with and without traps is observed in p-type SiNWs, n-type SiNWs, and n-type SiNWs/PAA devices. Using SCLC model with exponential distribution of traps, the extracted trap density was 7.20 × 1011/cm3 and 6.0 × 1011/cm3 for p-type SiNWs and n-type SiNWs devices, respectively. Our findings also demonstrate that the carrier concentration in SiNWs depends not only on doping concentration but also depends significantly on density of surface states.
Theoretical model of superconducting spintronic SIsFS devices
Authors: Bakurskiy S. V. , Klenov N. V. , Soloviev I. I. , Bol'ginov V. V. , Ryazanov V. V. , Vernik I. V. , Mukhanov O. A. , Kupriyanov M. Yu. , Golubov A. A.
Published: May 2013

Motivated by recent progress in the development of cryogenic memory compatible with single flux quantum (SFQ) circuits, we have performed a theoretical study of magnetic SIsFS Josephson junctions, where “S” is a bulk superconductor, “s” is a thin superconducting film, “F” is a metallic ferromagnet, and “I” is an insulator. We calculate the Josephson current as a function of s and F layers thickness, temperature, and exchange energy of F film. We outline several modes of operation of these junctions and demonstrate their unique ability to have large product of a critical current IC and a normal-state resistance RN in the π state, comparable to that in superconductor–insulator–superconductor tunnel junctions commonly used in SFQ circuits. We develop a model describing switching of the Josephson critical current in these devices by external magnetic field. The results are in good agreement with the experimental data for Nb-Al/AlOx-Nb-Pd0.99Fe0.01-Nb junctions.
Preparation and characterization of Ni(111)/graphene/Y2O3(111) heterostructures
Authors: Dahal Arjun , Coy-Diaz Horacio , Addou Rafik , Lallo James , Sutter Eli , Batzill Matthias
Published: May 2013

Integration of graphene with other materials by direct growth, i.e., not using mechanical transfer procedures, is investigated on the example of metal/graphene/dielectric heterostructures. Such structures may become useful in spintronics applications using graphene as a spin-filter. Here, we systematically discuss the optimization of synthesis procedures for every layer of the heterostructure and characterize the material by imaging and diffraction methods. 300 nm thick contiguous (111) Ni-films are grown by physical vapor deposition on YSZ(111) or Al2O3(0001) substrates. Subsequently, chemical vapor deposition growth of graphene in ultra-high vacuum (UHV) is compared to tube-furnace synthesis. Only under UHV conditions, monolayer graphene in registry with Ni(111) has been obtained. In the tube furnace, mono- and bilayer graphene is obtained at growth temperatures of ∼800 °C, while at 900 °C, non-uniform thick graphene multilayers are formed. Y2O3 films grown by reactive molecular beam epitaxy in UHV covers the graphene/Ni(111) surface uniformly. Annealing to 500 °C results in crystallization of the yttria with a (111) surface orientation.
Effects of high energy x ray and proton irradiation on lead zirconate titanate thin films' dielectric and piezoelectric response
Authors: Bastani Y. , Cortés-Peña A. Y. , Wilson A. D. , Gerardin S. , Bagatin M. , Paccagnella A. , Bassiri-Gharb N.
Published: May 2013

The effects of irradiation by X rays and protons on the dielectric and piezoelectric response of highly (100)-textured polycrystalline Pb(ZrxTi1−x)O3 (PZT) thin films have been studied. Low-field dielectric permittivity, remanent polarization, and piezoelectric d33,f response all degraded with exposure to radiation, for doses higher than 300 krad. At first approximation, the degradation increased at higher radiation doses, and was stronger in samples exposed to X rays, compared to the proton-irradiated ones. Nonlinear and high-field dielectric characterization suggest a radiation-induced reduction of the extrinsic contributions to the response, attributed to increased pinning of the domain walls by the radiation-induced point defects.
Coercivity enhancement of anisotropic die-upset Nd-Fe-B powders by Pr-Cu alloy diffusion
Authors: Wan Fangming , Han Jingzhi , Zhang Yinfeng , Zhang Xiaodong , Yang Jinbo , Yang Yingchang , Hu Boping , Rao Xiaolei , Cai Daoyan , Chen Zhian , Chen Renjie , Yan Aru
Published: May 2013

The grain boundary diffusion treatment using the low melting point Pr-Cu eutectic alloy was applied to enhance the coercivity of anisotropic die-upset Nd-Fe-B powders. It was found that the room temperature coercivity of the powders increases from 13 kOe to 24 kOe. The thickness of the grain boundary layer was increased and the Fe and Co content in the grain boundary phases was decreased. The modified boundary layers may thus increase the domain wall nucleation field, leading to an enhancement in the coercivity. The diffusion of Pr-Cu alloy prevents the loss of continuity of the Nd-rich grain boundary phase and retains the crystal texture in the initial grains.
Note: A compact, rigid, and easy-to-build piezo motor: The intact-tube GeckoDrive
Authors: Wang Qi , Hou Yubin , Lu Qingyou
Published: May 2013

We report an extremely simple, rigid, low machine tolerance, yet high performance piezoelectric motor, in which two rings are coaxially glued at the ends of one intact piezotube, respectively, using the proper gluing method. A central shaft is pushed to press against the inner edges of the rings by a spring strip at a proper axial position and in the gap between the shaft and the inner wall of the piezotube. It is compared with other important forms of three-friction driven motors and shows advantageous structure and unexpectedly excellent performance, hence deserving a new name: the GeckoDrive.
Low frequency driven oscillations of cantilevers in viscous fluids at very low Reynolds number
Authors: Cranch G. A. , Lane J. E. , Miller G. A. , Lou J. W.
Published: May 2013

The motion of submerged cantilevers driven by viscous fluids is experimentally investigated and a previously published theoretical model is verified over a broad range of Reynolds number covering 4×10−3≤Re≤2000 at frequencies up to 1 kHz. Both planar and cylindrical cantilevers are implemented using short length (few centimeters) fiber lasers, which are also used to measure the deflections. The driving forces are analyzed in detail illustrating how the dominant force transitions from a pressure related force to a viscous force depending on the Reynolds number of the fluid flow around the cantilever. Simplified, approximate expressions for the tip displacement of cantilevers oscillating in the highly viscous regime are also presented. These results will enable accurate, a priori, calculation of the motion of driven cantilevers over a range of dimensions, geometries, and fluid properties.
Nonlinear instability in simulations of Large Plasma Device turbulence
Authors: Friedman B. , Carter T. A. , Umansky M. V. , Schaffner D. , Joseph I.
Published: May 2013

Several simulations of turbulence in the Large Plasma Device (LAPD) [Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] are energetically analyzed and compared with each other and with the experiment. The simulations use the same model, but different axial boundary conditions. They employ either periodic, zero-value, zero-derivative, or sheath axial boundaries. The linear stability physics is different between the scenarios because the various boundary conditions allow the drift wave instability to access different axial structures, and the sheath boundary simulation contains a conducting wall mode instability which is just as unstable as the drift waves. Nevertheless, the turbulence in all the simulations is relatively similar because it is primarily driven by a robust nonlinear instability that is the same for all cases. The nonlinear instability preferentially drives k∥=0 potential energy fluctuations, which then three-wave couple to k∥≠0 potential energy fluctuations in order to access the adiabatic response to transfer their energy to kinetic energy fluctuations. The turbulence self-organizes to drive this nonlinear instability, which destroys the linear eigenmode structures, making the linear instabilities ineffective.
Graphene Hall bar with an asymmetric pn-junction
Authors: Milovanović S. P. , Ramezani Masir M. , Peeters F. M.
Published: May 2013

We investigated the magnetic field dependence of the Hall and the bend resistances in the ballistic regime for a single layer graphene Hall bar structure containing a pn-junction. When both regions are n-type the Hall resistance dominates and Hall type of plateaus are formed. These plateaus occur as a consequence of the restriction on the angle imposed by Snell's law allowing only electrons with a certain initial angles to transmit though the potential step. The size of the plateau and its position is determined by the position of the potential interface as well as the value of the applied potential. When the second region is p-type, the bend resistance dominates, which is asymmetric in field due to the presence of snake states. Changing the position of the pn-interface in the Hall bar strongly affects these states and therefore the bend resistance is also changed. Changing the applied potential, we observe that the bend resistance exhibits a peak around the charge-neutrality point (CNP), which is independent of the position of the pn-interface, while the Hall resistance shows a sign reversal when the CNP is crossed, which is in very good agreement with a recent experiment [J. R. Williams and C. M. Marcus, Phys. Rev. Lett. 107, 046602 (2011)].
Bessel-beam-written nanoslit arrays and characterization of their optical response
Authors: Sahin R. , Morova Y. , Simsek E. , Akturk S.
Published: May 2013

Nanoslit arrays are fabricated on thin metal film coated glass slides using femtosecond laser pulses with Bessel beam profiles. The optical properties of the fabricated structures with different periodicities are characterized with transmission spectroscopy. Experimental results reveal the existence of two separate surface plasmon resonance modes occurring at the metal-air and metal-glass interfaces. These two resonance modes cause two minima in the high transmission spectra of the sub-skin depth thick thin films in the visible and near infrared regions. The existence of double surface plasmon resonance modes is verified with additional experiments, theoretical and numerical studies. Due to its relaxed alignment constraints, reduced aberrations, scalability property to shorter wavelengths, and resulting shorter dimensions, nanofabrication with diffraction-free Bessel beams is an easy, cheap, and advantageous alternative to regular lithography techniques to fabricate nanoslit arrays. The shift of the resonance wavelength with a change in the refractive index of the surrounding medium can be exploited for enhanced sensing.
Thermal and optical properties of glass and crystalline phases formed in the binary R2O3-Al2O3 (R = La-Lu and Y) system
Authors: Vijaya Kumar Malahalli , Ishikawa Takehiko , Basavalingu B. , Okada Junpei T. , Watanabe Yuki
Published: May 2013

Rare-earth aluminate (RAlO3, R = La–Lu and Y) glass and crystalline phases were prepared by containerless levitation in an aerodynamic levitation furnace. In the RAlO3 system, La, Nd and Sm aluminum perovskites solidified as glass and Eu–Lu and Y aluminum perovskites solidified as crystalline phases. The glass forming region decreased with decreasing ionic radius of the rare-earth element. Scanning electron microscopy images and x-ray diffraction results revealed the formation of a single RAlO3 phase from the undercooled melt. The glass transition temperature, Tg, and density increased and the molar volume decreased with decreasing rare-earth element ionic radius. The refractive index at 589 nm exceeds 1.85 in each composition and a transparency of approximately 72% was achieved for the LaAlO3 glass.
Electronic transport and Schottky barrier heights of p-type CuAlO2 Schottky diodes
Authors: Lin Yow-Jon , Luo Jie , Hung Hao-Che
Published: May 2013

A CuAlO2 Schottky diode was fabricated and investigated using current density-voltage (J-V) and capacitance-voltage (C-V) methods. It is shown that the barrier height (qϕB) determined from J-V measurements is lower than that determined from C-V measurements and qϕB determined from C-V measurements is close to the Schottky limit. This is due to a combined effect of the image-force lowering and tunneling. Time domain measurements provide evidence of the domination of electron trapping with long-second lifetime in CuAlO2. Carrier capture and emission from charge traps may lead to the increased probability of tunneling, increasing the ideality factor.
Pyroelectric electron emission from nanometer-thick films of PbZrxTi1−xO3
Authors: Fletcher Patrick C. , Mangalam Vengadesh Kumara R. , Martin Lane W. , King William P.
Published: May 2013

We report pyroelectric emission from PbZrxTi1−xO3 (PZT) thin films on nanometer-sharp tips. The epitaxial PZT films are 30 nm thick and grown directly on single-crystal silicon tips. Pyroelectric emission occurs for heating rates of ≥50 °C/min in a 20 V/μm external field. The emission current is a maximum of 240 nA when the heating rate is 100 °C/min and the electric field strength is ≥6.7 V/μm. The emitted charge is ∼7% of that expected for a perfect thin film emitter of epitaxial PbZrxTi1−xO3. We calculate that pyroelectric emission can occur without an applied field if the heating rate exceeds 4.0 × 107 °C/min.
InGaN/GaN light-emitting diode with a polarization tunnel junction
Authors: Zhang Zi-Hui , Tiam Tan Swee , Kyaw Zabu , Ji Yun , Liu Wei , Ju Zhengang , Hasanov Namig , Wei Sun Xiao , Volkan Demir Hilmi
Published: May 2013

We report InGaN/GaN light-emitting diodes (LED) comprising in situ integrated p+-GaN/InGaN/n+-GaN polarization tunnel junctions. Improved current spreading and carrier tunneling probability were obtained in the proposed device architecture, leading to the enhanced optical output power and external quantum efficiency. Compared to the reference InGaN/GaN LEDs using the conventional p+/n+ tunnel junction, these devices having the polarization tunnel junction show a reduced forward bias, which is attributed to the polarization induced electric fields resulting from the in-plane biaxial compressive strain in the thin InGaN layer sandwiched between the p+-GaN and n+-GaN layers.
Spatially-resolved spectroscopic measurements of Ec − 0.57 eV traps in AlGaN/GaN high electron mobility transistors
Authors: Cardwell D. W. , Sasikumar A. , Arehart A. R. , Kaun S. W. , Lu J. , Keller S. , Speck J. S. , Mishra U. K. , Ringel S. A. , Pelz J. P.
Published: May 2013

Simultaneous temperature-dependent measurements of resistance transients (RTs) and spatially resolved surface potential transients were made after bias switching on AlGaN/GaN high electron mobility transistors (HEMTs). We find an Ec − 0.57 eV trap, previously correlated with HEMT degradation, located in the GaN buffer and not in the AlGaN barrier or at the AlGaN surface. The amplitude of the Ec − 0.57 eV trap in RTs depends strongly on the Fe-concentration in the GaN buffer. Filling of this trap occurs only under bias conditions where electric fields penetrate into the GaN buffer.
Tunable 1.6–2 μm near infrared few-cycle pulse generation by filamentation
Authors: Driever S. , Bigourd D. , Fedorov N. , Cornet M. , Arnold M. , Burgy F. , Montant S. , Petit S. , Descamps D. , Cormier E. , Constant E. , Zaïr A.
Published: May 2013

We present results on tunable few-cycle laser pulses generated in the near infrared region obtained by filamentation in a krypton cell combined with group velocity dispersion compensation in fused silica. We obtain a spectral broadening of a factor ∼2–3 over the entire spectral domain studied. The central wavelength is tuned from 1.6 μm to 2 μm via an optical parametric amplification source. In optimum experimental conditions, where the input central wavelength is set to 1.7 μm, 1.8 μm, and 1.9 μm, the incident power to the gas cell exceeded the critical power of Krypton by a factor of ∼4 and the achieved spectral broadening covered ∼300 nm. Using group velocity compensation in bulk fused silica, we obtain near infrared output pulses as short as 2–3 optical cycles with 200 μJ energy per pulse. This near infrared filamentation tunable few-cycle pulse source is an important achievement for strong field physics applications such as attoscience, where wavelength scaling has an important effect.
Revisiting the “In-clustering” question in InGaN through the use of aberration-corrected electron microscopy below the knock-on threshold
Authors: Baloch Kamal H. , Johnston-Peck Aaron C. , Kisslinger Kim , Stach Eric A. , Gradečak Silvija
Published: May 2013

The high intensity of light emitted in InxGa1−xN/GaN heterostructures has been generally attributed to the formation of indium-rich clusters in InxGa1−xN quantum wells (QWs). However, there is significant disagreement about the existence of such clusters in as-grown InxGa1−xN QWs. We employ atomically resolved CS-corrected scanning transmission electron microscopy and electron energy loss spectroscopy at 120 kV—which we demonstrate to be below the knock-on displacement threshold—and show that indium clustering is not present in as-grown In0.22Ga0.78N QWs. This artifact-free, atomically resolved method can be employed for investigating compositional variations in other InxGa1−xN/GaN heterostructures.
Single-peak narrow-bandwidth mid-infrared thermal emitters based on quantum wells and photonic crystals
Authors: Inoue Takuya , De Zoysa Menaka , Asano Takashi , Noda Susumu
Published: May 2013

We experimentally demonstrate single-peak narrow-bandwidth thermal emission with a quality factor (Q factor) of more than 100 at a wavelength of 9.1 μm. The emission is significantly suppressed at all other wavelengths. Our emitter is based on an intersubband transition in a multiple quantum well structure combined with a single high-Q resonant mode in a two-dimensional photonic crystal slab, which allows strong light-matter interaction only at a specific wavelength. Strong thermal emission is exhibited only in a limited angular range (∼20°) from the normal direction. Our results have potential applications in bio- and environmental sensors.
Tropical forest responses to increasing atmospheric CO2: current knowledge and opportunities for future research
Authors: Cernusak Lucas A. , Winter Klaus , Dalling James W. , Holtum Joseph A. M. , Jaramillo Carlos , Körner Christian , Leakey Andrew D. B. , Norby Richard J. , Poulter Benjamin , Turner Benjamin L. , Wright S. Joseph
Published: May 2013

Elevated atmospheric CO2 concentrations (ca) will undoubtedly affect the metabolism of tropical forests worldwide; however, critical aspects of how tropical forests will respond remain largely unknown. Here, we review the current state of knowledge about physiological and ecological responses, with the aim of providing a framework that can help to guide future experimental research. Modelling studies have indicated that elevated ca can potentially stimulate photosynthesis more in the tropics than at higher latitudes, because suppression of photorespiration by elevated ca increases with temperature. However, canopy leaves in tropical forests could also potentially reach a high temperature threshold under elevated ca that will moderate the rise in photosynthesis. Belowground responses, including fine root production, nutrient foraging and soil organic matter processing, will be especially important to the integrated ecosystem response to elevated ca. Water use efficiency will increase as ca rises, potentially impacting upon soil moisture status and nutrient availability. Recruitment may be differentially altered for some functional groups, potentially decreasing ecosystem carbon storage. Whole-forest CO2 enrichment experiments are urgently needed to test predictions of tropical forest functioning under elevated ca. Smaller scale experiments in the understorey and in gaps would also be informative, and could provide stepping stones towards stand-scale manipulations.
Reactive oxygen species regulation and antioxidant defence in halophytes
Authors: Ozgur Rengin , Uzilday Baris , Sekmen Askim Hediye , Turkan Ismail
Published: May 2013

Production of reactive oxygen species (ROS), which are a by-product of normal cell metabolism in living organisms, is an inevitable consequence of aerobic life on Earth, and halophytes are no exception to this rule. The accumulation of ROS is elevated under different stress conditions, including salinity, due to a serious imbalance between their production and elimination. These ROS are highly toxic and, in the absence of protective mechanisms, can cause oxidative damage to lipids, proteins and DNA, leading to alterations in the redox state and further damage to the cell. Besides functioning as toxic by-products of stress metabolism, ROS are also important signal transduction molecules in controlling growth, development and responses to stress. Plants control the concentrations of ROS by an array of enzymatic and non-enzymatic antioxidants. Although a relation between enzymatic and non-enzymatic antioxidant defence mechanisms and tolerance to salt stress has been reported, little information is available on ROS-mediated signalling, perception and specificity in different halophytic species. Hence, in this review, we describe recent advances in ROS homeostasis and signalling in response to salt, and discuss current understanding of ROS involvement in stress sensing, stress signalling and regulation of acclimation responses in halophytes. We also highlight the role of genetic, proteomic and metabolic approaches for the successful study of the complex relationship among antioxidants and their functions in halophytes, which would be critical in increasing salt tolerance in crop plants.