bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoelectron spectroscopy and theoretical study of M(IO3)2− (M = H, Li, Na, K): Structural evolution, optical isomers, and hyperhalogen behavior
Authors: Hou Gao-Lei , Wu Miao Miao , Wen Hui , Sun Qiang , Wang Xue-Bin , Zheng Wei-Jun
Published: August 2013

H(IO3)2− and M(IO3)2− (M = Li, Na, K) anions were successfully produced via electrospray ionization of their corresponding bulk salt solutions, and were characterized by combining negative ion photoelectron spectroscopy and quantum chemical calculations. The experimental vertical detachment energies (VDEs) of M(IO3)2− (M = H, Li, Na, K) are 6.25, 6.57, 6.60, and 6.51 eV, respectively, and they are much higher than that of IO3− (4.77 eV). The theoretical calculations show that each of these anions has two energetically degenerate optical isomers. It is found that the structure of H(IO3)2− can be written as IO3−(HIO3), in which the H atom is tightly bound to one of the IO3− groups and forms an iodic acid (HIO3) molecule; while the structures of M(IO3)2− can be written as (IO3−)M+(IO3−), in which the alkali metal atoms interact with the two IO3− groups almost equally and bridge the two IO3− groups via two O atoms of each IO3− with the two MOOI planes nearly perpendicular to each other. In addition, the high VDEs of M(IO3)2− (M = Li, Na, K) can be explained by the hyperhalogen behavior of their neutral counterparts.
Flow structure on a rotating wing: Effect of steady incident flow
Authors: Bross M. , Ozen C. A. , Rockwell D.
Published: August 2013

The flow structure along a rotating wing in steady incident flow is compared to the structure on a rotating wing in quiescent fluid, in order to clarify the effect of advance ratio J (ratio of free-stream velocity to tip velocity of wing). Stereoscopic particle image velocimetry leads to patterns of vorticity, velocity, and Q-criterion (constant values of the second invariant of the velocity gradient tensor), as well as streamlines, which allow identification of critical points of the flow. The effective angle of attack is held constant over the range of J, and the wing rotates from rest to a large angle that corresponds to attainment of the asymptotic state of the flow structure. Prior to the onset of motion, the wing is at high angle of attack and the steady incident flow yields a fully stalled state along the wing. After the onset of rotation, the stalled region quickly gives rise to a stable leading edge vortex. Throughout the rotation maneuver, the development of the flow structure in the leading edge region is relatively insensitive to the value of J. In the trailing-edge region, however, the structure of the shed vorticity layer is strongly dependent on the value of J. Further insight into the effects of J is provided by three-dimensional patterns of spanwise-oriented vorticity, spanwise velocity, and Q-criterion.
Inclusion of persistence length-based secondary structure in replica field theoretic models of heteropolymer freezing
Authors: Weber Jeffrey K. , Pande Vijay S.
Published: August 2013

The protein folding problem has long represented a “holy grail” in statistical physics due to its physical complexity and its relevance to many human diseases. While past theoretical work has yielded apt descriptions of protein folding landscapes, recent large-scale simulations have provided insights into protein folding that were impractical to obtain from early theories. In particular, the role that non-native contacts play in protein folding, and their relation to the existence of misfolded, β-sheet rich trap states on folding landscapes, has emerged as a topic of interest in the field. In this paper, we present a modified model of heteropolymer freezing that includes explicit secondary structural characteristics which allow observations of “intramolecular amyloid” states to be probed from a theoretical perspective. We introduce a variable persistence length-based energy penalty to a model Hamiltonian, and we illustrate how this modification alters the phase transitions present in the theory. We find, in particular, that inclusion of this variable persistence length increases both generic freezing and folding temperatures in the model, allowing both folding and glass transitions to occur in a more highly optimized fashion. We go on to discuss how these changes might relate to protein evolution, misfolding, and the emergence of intramolecular amyloid states.
Numerical simulation of vortex-induced vibration of two circular cylinders of different diameters at low Reynolds number
Authors: Zhao Ming , Yan Guirong
Published: August 2013

Two-degree-of-freedom Vortex-Induced Vibration (VIV) of two rigidly coupled circular cylinders of different diameters at a low Reynolds number of 250 is investigated numerically. While the diameter ratio and the mass ratio are kept constant, the study is focused on the effect of the position angle of the small cylinder on the lock-in regime of the VIV. Simulations are carried out for position angles α of the small cylinder ranging from 0° to 180° with an interval of 22.5° and the reduced velocities ranging from 1 to 15 with an increment of 1. In order to find the effect of the gap between the two cylinders on the vibration, two gap-to-diameter ratios (0 and 0.2) are considered. It is found that compared with a single cylinder case, the lock-in regime of the reduced velocity is widened significantly when the position angle of the small cylinder is α = 0°, 22.5°, 90°, or 112.5°. Pulsed beating phenomenon characterized by regular vibration with occasional high-amplitude disturbances at regular or irregular intervals is observed at G = 0 and α = 90°. At α = 135°, more than one lock-in regimes are observed in the computed range of reduced velocity for both gaps (G = 0 and 0.2). Setting a small gap (gap-to-diameter ratio of 0.2) between the two cylinders mitigates the vibration by narrowing the lock-in regime and reducing the vibration amplitude.
Motion of a helical vortex filament in superfluid 4He under the extrinsic form of the local induction approximation
Authors: Van Gorder Robert A.
Published: August 2013

Very recently, Shivamoggi [“Vortex motion in superfluid 4He: Reformulation in the extrinsic vortex-filament coordinate space,” Phys. Rev. B 84, 012506 (2011)]10.1103/PhysRevB.84.012506 studied the extrinsic form of the local induction approximation (LIA) for the motion of a Kelvin wave on a vortex filament in superfluid 4He, and obtained some results in a cubic approximation. Presently, we study the motion of helical vortex filaments in superfluid 4He under the exact fully nonlinear LIA considered in potential form by Van Gorder [“Fully nonlinear local induction equation describing the motion of a vortex filament in superfluid 4He,” J. Fluid Mech. 707, 585 (2012)]10.1017/jfm.2012.308 and obtained from the Biot-Savart law through the equations of Hall and Vinen [“The rotation of liquid helium II. I. Experiments on the propagation of second sound in uniformly rotating helium II,” Proc. R. Soc. London, Ser. A 238, 204 (1956)]10.1098/rspa.1956.0214 including superfluid friction terms. Nonlinear dispersion relations governing the helical Kelvin wave on such a vortex filament are derived in exact form, from which we may exactly calculate the phase and group velocity of the Kelvin wave. With this, we classify the motion of a helical Kelvin wave on a vortex filament under the LIA. The dispersion relations and results, which follow are exact in nature, in contrast to most results in the literature, which are usually numerical approximations. As such, our results accurately capture the qualitative behavior of the Kelvin waves under the LIA. Extensions to other frameworks are discussed.
On the spatial distribution of small heavy particles in homogeneous shear turbulence
Authors: Nicolai C. , Jacob B. , Piva R.
Published: August 2013

We report on a novel experiment aimed at investigating the effects induced by a large-scale velocity gradient on the turbulent transport of small heavy particles. To this purpose, a homogeneous shear flow at Reλ = 540 and shear parameter S* = 4.5 is set-up and laden with glass spheres whose size d is comparable with the Kolmogorov lengthscale η of the flow (d/η ≈ 1). The particle Stokes number is approximately 0.3. The analysis of the instantaneous particle fields by means of Voronoï diagrams confirms the occurrence of intense turbulent clustering at small scales, as observed in homogeneous isotropic flows. It also indicates that the anisotropy of the velocity fluctuations induces a preferential orientation of the particle clusters. In order to characterize the fine-scale features of the dispersed phase, spatial correlations of the particle field are employed in conjunction with statistical tools recently developed for anisotropic turbulence. The scale-by-scale analysis of the particle field clarifies that isotropy of the particle distribution is tendentially recovered at small separations, even though the signatures of the mean shear persist down to smaller scales as compared to the fluid velocity field.
Molecular dynamics of polymer crystallization revisited: Crystallization from the melt and the glass in longer polyethylene
Authors: Yamamoto Takashi
Published: August 2013

Molecular mechanisms of the steady-state growth of the chain folded lamella and the cold crystallization across the glass transition temperature Tg are investigated by molecular dynamics simulation for a system of long polyethylene (PE)-like polymers made of 512 united atoms C512. The present paper aims to reconsider results of our previous simulations for short PE-like polymers C100 by carrying out very long simulations up to 1 μs for more realistic systems of much longer chains, thereby to establish the firm molecular image of chain-folded crystallization and clarify the specific molecular process of cold crystallization. We observe that the chain-folded lamella shows fast thickening-growth keeping marked tapered growth front. Despite the fast growth in much longer chains, the fold-surface is found to be predominantly of adjacent-reentry. Detailed inspections of the molecular pathway give an insightful image that can explain the apparently contradicting results. In addition, the fold-structure with specific spatial heterogeneity is found to give rise to heterogeneous mobility within the crystalline region. On the other hand, investigations of the cold crystallization during slow heating of the glassy film across Tg is found to give a granular texture made of small crystallites. The crystallites are found to nucleate preferentially near the free surfaces having lower Tg, and to be dominantly edge-on showing a definite tendency to orient their chain axes parallel to the free surface.
High sensitive quasi freestanding epitaxial graphene gas sensor on 6H-SiC
Authors: Iezhokin I. , Offermans P. , Brongersma S. H. , Giesbers A. J. M. , Flipse C. F. J.
Published: August 2013

We have measured the electrical response to NO2, N2, NH3, and CO for epitaxial graphene and quasi freestanding epitaxial graphene on 6H-SiC substrates. Quasi freestanding epitaxial graphene shows a 6 fold increase in NO2 sensitivity compared to epitaxial graphene. Both samples show a sensitivity better than the experimentally limited 1 ppb. The strong increase in sensitivity of quasi freestanding epitaxial graphene can be explained by a Fermi-energy close to the Dirac point, leading to a strongly surface doping dependent sample resistance. Both sensors show a negligible sensitivity to N2, NH3, and CO.
Transparent metal electrodes from ordered nanosphere arrays
Authors: Morfa Anthony J. , Akinoglu Eser M. , Subbiah Jegadesan , Giersig Michael , Mulvaney Paul
Published: August 2013

We show that perforated metal electrode arrays, fabricated using nanosphere lithography, provide a viable alternative to conductive metal oxides as transparent electrode materials. The inter-aperture spacing is tuned by varying etching times in an oxygen plasma, and the effect of inter-aperture “wire” thickness on the optical and electronic properties of perforated silver films is shown. Optical transmission is limited by reflection and surface plasmons, and for these results do not exceed 73%. Electrical sheet resistance is shown to be as low as 3 Ω ◻−1 for thermally evaporated silver films. The performance of organic photovoltaic devices comprised of a P3HT:PCBM bulk heterojunction deposited onto perforated metal arrays is shown to be limited by optical transmission, and a simple model is presented to overcome these limitations.
Direct observation of self-assembled chain-like water structures in a nanoscopic water meniscus
Authors: Kim Byung I. , Boehm Ryan D. , Bonander Jeremy R.
Published: August 2013

Sawtooth-like oscillatory forces generated by water molecules confined between two oxidized silicon surfaces were observed using a cantilever-based optical interfacial force microscope when the two surfaces approached each other in ambient environments. The humidity-dependent oscillatory amplitude and periodicity were 3-12 nN and 3-4 water diameters, respectively. Half of each period was matched with a freely jointed chain model, possibly suggesting that the confined water behaved like a bundle of water chains. The analysis also indicated that water molecules self-assembled to form chain-like structures in a nanoscopic meniscus between two hydrophilic surfaces in air. From the friction force data measured simultaneously, the viscosity of the chain-like water was estimated to be between 108 and 1010 times greater than that of bulk water. The suggested chain-like structure resolves many unexplained properties of confined water at the nanometer scale, thus dramatically improving the understanding of a variety of water systems in nature.
A rotating directional probe for the measurements of fast ion losses and plasma rotation at Tokamak Experiment for Technology Oriented Research
Authors: Rack M. , Liang Y. , Jaegers H. , Aßmann J. , Satheeswaran G. , Xu Y. , Pearson J. , Yang Y. , Denner P. , Zeng L.
Published: August 2013

This work discusses a new directional probe designed for measurements of fast ion losses and the plasma rotation with a high angular resolution in magnetically confined plasmas. Directional and especially Mach probes are commonly used diagnostics for plasma flow measurements, and their applicability for the fast ion losses detection has been demonstrated. A limitation of static Mach probes is their low angular resolution. At the Tokamak Experiment for Technology Oriented Research, the angular resolution is strongly restricted by the finite number of available measurement channels. In a dynamic plasma, where instabilities can lead to local changes of the field line pitch-angle, plasma flow, or fast ion losses, a low angular resolution makes a precise data analysis difficult and reduces the quality of the measured data. The new probe design, the rotating directional probe, combines the features of early directional probes and Mach probes. It consists of two radially aligned arrays of nine Langmuir probe pins with each array facing opposite directions. During the measurement the probe head rotates along its axis to measure the ion saturation current from all directions. As a result, the rotating directional probe simultaneously provides an angular dependent plasma flow and fast ion losses measurement at different radial positions. Based on the angular dependent data, a precise determination of the current density is made. In addition, the simultaneous measurement of the ion saturation current at different radial positions allows for resolving radially varying field line pitch-angles and identifying the radial dynamic of processes like fast ion losses.
Design, development, and demonstration of a fully LabVIEW controlled in situ electrochemical Fourier transform infrared setup combined with a wall-jet electrode to investigate the electrochemical interface of nanoparticulate electrocatalysts under reaction conditions
Authors: Nesselberger Markus , Ashton Sean J. , Wiberg Gustav K. H. , Arenz Matthias
Published: August 2013

We present a detailed description of the construction of an in situ electrochemical ATR-FTIR setup combined with a wall-jet electrode to investigate the electrocatalytic properties of nanoparticulate catalysts in situ under controlled mass transport conditions. The presented setup allows the electrochemical interface to be probed in combination with the simultaneous determination of reaction rates. At the same time, the high level of automation allows it to be used as a standard tool in electrocatalysis research. The performance of the setup was demonstrated by probing the oxygen reduction reaction on a platinum black catalyst in sulfuric electrolyte.
Gene delivery in conjunction with gold nanoparticle and tumor treating electric field
Authors: Tiwari Pawan K. , Soo Lee Yeon
Published: August 2013

The advances in electrotherapy to treat the diseased biological cell instigate its extension in gene therapy through the delivery of gene into the nucleus. The objective of this study is to investigate the application of moderate intensity alternating electric field, also known as tumor treating electric field on a carrier system consisting of a charged gene complex conjugated to the surface of a gold nanoparticle. The gene delivery mechanism relies on the magnitude and direction of the induced electric field inside the cytoplasm in presence of carrier system. The induced electric field strength is significant in breaking the gene complex-gold nanoparticle bonding, and exerting an electric force pushing the charged gene into the nucleus. The electric force orientation is dependent on the aspect ratio (AR) of the gold nanoparticle and a relationship between them is studied via Maxwell two-dimensional (2D) finite element simulation analyzer. The development of charge density on the surface of carrier system and the required electric field strength to break the bonding are investigated utilizing the Gouy-Chapman-Grahame-Stern (GCGS) theoretical model. A carrier system having the aspect ratio of the gold nanoparticle in the range 1 < AR ≤ 5 and AR = 1 are substantial delivering cationic and anionic genes into the nucleus, respectively.
Transport behavior and electronic structure of phase pure VO2 thin films grown on c-plane sapphire under different O2 partial pressure
Authors: Kittiwatanakul Salinporn , Laverock Jude , Newby Dave , Smith Kevin E. , Wolf Stuart A. , Lu Jiwei
Published: August 2013

We grew highly textured phase pure VO2 thin films on c-plane Al2O3 substrates with different oxygen partial pressure. X-ray absorption and photoemission spectroscopy confirm the identical valence state of vanadium ions despite the different oxygen pressure during the deposition. As the O2 flow rate increases, the [010] lattice parameter for monoclinic VO2 was reduced and coincidently distinctive changes in the metal-semiconductor transition (MST) and transport behaviors were observed despite the identical valence state of vanadium in these samples. We discuss the effect of the oxygen partial pressure on the monoclinic structure and electronic structure of VO2, and consequently the MST.
Concentration tuned bandgap and corresponding nonlinear refractive index dispersion in Ga-Ge-Se nanocolloids
Authors: Sebastian Indu , Mathew S. , Nampoori V. P. N. , Radhakrishnan P. , Thomas Sheenu
Published: August 2013

Solution-processing of chalcogenide glass materials has attracted attention for the fabrication of photonic devices. We have carried out linear and nonlinear optical studies on Ga9Ge27Se64 nanocolloidal solutions with varying concentrations and witnessed the opportunity for engineering the optical properties of the material by simply changing the concentration of solute in the solvent. An enhancement of bandgap, deterioration, and blue shift in the band tail states with decrease in concentration were observed from the absorption spectra. Open and closed aperture Z-scan studies reveal tunability of nonlinear absorption coefficient (β) and nonlinear refractive index (n2). As the bandgap approaches the one photon absorption edge, the material shows very large negative nonlinear refraction and also “two step photon absorption” due to the presence of intermediate tail states. The radial phase shift (Δφ0) induced after travelling through the nanocolloidal solution with respect to bandgap is plotted and we predict that the variation of Δφ0 is governed by a quadratic polynomial of Eg.
Communication: Favorable dimensionality scaling of rectangular collocation with adaptable basis functions up to 7 dimensions
Authors: Manzhos Sergei , Chan Matthew , Carrington Tucker
Published: August 2013

We show that by using a rectangular collocation method with a small basis of parameterized functions, it is possible to compute a vibrational spectrum by solving the Schrödinger equation in 7D from a small number of ab initio calculations without a potential surface. The method is ideal for spectra of molecules adsorbed on a surface. In this paper, it is applied to calculate experimentally relevant energy levels of acetic acid adsorbed on the (101) surface of anatase TiO2. In this case, to obtain levels of experimental accuracy, increasing the number of dimensions from 4 to 7 increases the number of required potential points from about 1000 to about 10 000 and the number of basis functions from 126 to 792: the scaling is very attractive.
Note: Matching index technique for avoiding higher order mode resonance in accelerators: INDUS-2 accelerator as a case study
Authors: Jain V. , Bhandarkar U. V. , Joshi S. C. , Krishnagopal S.
Published: August 2013

Resonance between circulating beam frequencies and RF cavity Higher Order Modes (HOMs) of accelerators can lead to coupled-bunch instabilities. Shifting these HOMs to avoid the resonance is a topic of active interest. A study has been carried out for the accelerating cavities of the INDUS-2. For quantitative measure of deciding which modes have to be moved and by how much, we introduce a new index called the matching index (IM), as a measure of how close a HOM is to the nearest beam mode. Depending on the value of IM, the operating scenarios are classified as safe and unsafe.
Calculation of the “absolute” free energy of a β-hairpin in an all-atom force field
Authors: Klenin Konstantin V. , Wenzel Wolfgang
Published: August 2013

We propose a new approach to calculate the conformational free energy of a macromolecule in a compact stable state in implicit solvent. The free energy is evaluated with respect to an artificial reference system without internal interactions that is confined to a small well-defined multidimensional volume of a regular shape occupying approximately the same part of the conformational space as the macromolecule of interest. We present a practical implementation of our method, successfully apply it to a β-hairpin in all-atom representation, verify the results with direct parallel tempering simulations, and discuss the possibilities of further improvements.
Quantum well electron scattering rates through longitudinal optic-phonon dynamical screened interaction: An analytic approach
Authors: Vallone Marco E.
Published: August 2013

A quantum model is developed to obtain electron capture and hot electron intraband relaxation times in a quantum well, for electron-longitudinal optic (LO) phonon scattering. In particular, we have investigated the effect of carrier density and electron energy, obtaining semi-analytic expressions as function of carrier density, a topic often neglected in literature, despite its fundamental interest in semiconductor physics. We demonstrated that the usual approximation of constant scattering time in modeling applications is often not adequate, because these parameters vary considerably with the injected or photogenerated carrier density. Furthermore we show that the scattering through the emission of pure LO-phonons is not a good approximation when the population increases, whereas the interplay between LO-phonon and collective plasma modes must be considered. We obtained novel semi-analytic expressions in the single plasmon pole dynamical form of the random phase approximation, without making use of the more usual static limit of it.
Spectroscopic imaging of self-organization in high power impulse magnetron sputtering plasmas
Authors: Andersson Joakim , Ni Pavel , Anders André
Published: August 2013

Excitation and ionization conditions in traveling ionization zones of high power impulse magnetron sputtering plasmas were investigated using fast camera imaging through interference filters. The images, taken in end-on and side-on views using light of selected gas and target atom and ion spectral lines, suggest that ionization zones are regions of enhanced densities of electrons, and excited atoms and ions. Excited atoms and ions of the target material (Al) are strongly concentrated near the target surface. Images from the highest excitation energies exhibit the most localized regions, suggesting localized Ohmic heating consistent with double layer formation.
Dependence of spin torque diode voltage on applied field direction
Authors: Taniguchi Tomohiro , Imamura Hiroshi
Published: August 2013

The optimum condition of an applied field direction to maximize spin torque diode voltage was theoretically derived for a magnetic tunnel junction with a perpendicularly magnetized free layer and an in-plane magnetized pinned layer. We found that the diode voltage for a relatively small applied field is maximized when the projection of the applied field to the film-plane is parallel or anti-parallel to the magnetization of the pinned layer. However, by increasing the applied field magnitude, the optimum applied field direction shifts from the parallel or anti-parallel direction. These analytical predictions were confirmed by numerical simulations.
Fabrication and magnetic properties of La-X (X = Co, Ni, and Fe) nanotube arrays prepared by electrodeposition methods
Authors: Chen J. Y. , Shi D. W. , Ahmad N. , Liu D. P. , Zhou W. P. , Han X. F.
Published: August 2013

Well-ordered La-X (X = Co, Ni and Fe) nanotubes, with the average diameter of ∼200 nm, wall thicknesses of ∼40 nm, have been fabricated into anodized aluminum oxide template by potentiostatic electrodeposition method. Various composition of La-X nanotubes were obtained by tuning the applied deposition potential. Magnetization measurements reveal that doped La could enhance the coercivity (Hc) of La-X nanotubes and their easy axis is perpendicular to the nanotube axis. There is a transition from the curling to transverse mode with increase of angle. Temperature dependent magnetization indicates the existence of superparamagnetic nanoparticles and that the surface effect results in the increase of saturation magnetization (Ms) at low temperature. Abnormal behavior of temperature dependent Hc may result from thermal excitation, magnetoelastic anisotropy, as well as oxide layer of nanotube inner surface induced coupling. These one-dimensional rare-earth transition metal nanostructures could have potential applications in novel spintronics device, ultra-small magnetic media, drug delivery, or other nanodevice.
Local carrier dynamics around the sub-surface basal-plane stacking faults of GaN studied by spatio-time-resolved cathodoluminescence using a front-excitation-type photoelectron-gun
Authors: Furusawa K. , Ishikawa Y. , Tashiro M. , Hazu K. , Nagao S. , Ikeda H. , Fujito K. , Chichibu S. F.
Published: August 2013

Local carrier dynamics around the sub-surface basal-plane stacking faults (BSFs) accidentally formed in a low dislocation density c-plane GaN were studied by the spatio-time-resolved cathodoluminescence measurement. A high photoelectron (PE) emission efficiency of the front-excitation-type PE-gun enabled to investigate sub-surface defect structures with low acceleration voltages. As a result, the presence of an energy transfer channel of excitons from neutral donor bound states to I1-type BSF bound states was confirmed. Careful comparisons of cathodoluminescence intensity mapping images taken at 3.305 eV and those corresponding to I1-BSFs indicated the presence of prismatic-plane stacking faults connecting the BSFs into a bundle.
Nanocrystallization in spark plasma sintered Fe48Cr15Mo14Y2C15B6 bulk amorphous alloy
Authors: Singh Ashish , Katakam Shravana , Ilavsky Jan , Dahotre Narendra B. , Harimkar Sandip P.
Published: August 2013

Spark plasma sintering (SPS) is evolving as an attractive process for the processing of multi-component Fe-based bulk amorphous alloys and their in-situ nanocomposites with controlled primary nanocrystallization. Extended Q-range small angle neutron scattering (EQ-SANS) analysis, complemented by x-ray diffraction and transmission electron microscopy, was performed to characterize nanocrystallization behavior of SPS sintered Fe-based bulk amorphous alloys. The SANS experiments show significant scattering for the samples sintered in the supercooled region indicating local structural/compositional changes associated with the profuse nucleation of nanoclusters (∼4 nm). For the samples spark plasma sintered near and above crystallization temperature (>653 °C), the SANS data show the formation of interference maximum indicating the formation and growth of (Fe,Cr)23C6 crystallites. The SANS data also indicate the evolution of bimodal crystallite distribution at higher sintering temperatures (above Tx1). The growth of primary nanocrystallites results in impingement of concentration gradient fields (soft impingement effect), leading to non-random nucleation of crystallites near the primary crystallization.
Effect of conductive atomic force microscope tip loading force on tip-sample interface electronic characteristics: Unipolar to bipolar resistive switching transition
Authors: Singh Bharti , Varandani Deepak , Mehta B. R.
Published: August 2013

This study shows that tip loading force in conductive atomic force microscope (CAFM) is an important parameter determining electronic properties of tip-sample interface. At higher tip loading force, electrochemical breakdown of moisture layer and redox reactions cause changeover from unipolar to bipolar switching in Pt tip-Cu2O junction. Change in forming voltage due to tip force (and thus contact area) is opposite to that observed in micro range due to electric field enhancement effect. Results of the present study are important in studying filamentary conduction, nanometric mapping of charge carriers at the interface of complex oxide heterostructures, and other studies by CAFM.
Magnetic-field assisted performance of InGaAs/GaAsSb terahertz quantum cascade lasers
Authors: Maëro Simon , de Vaulchier Louis-Anne , Guldner Yves , Deutsch Christoph , Krall Michael , Zederbauer Tobias , Strasser Gottfried , Unterrainer Karl
Published: August 2013

We report on a magnetic-field investigation of a In0.53Ga0.47As/GaAs0.51Sb0.49 terahertz quantum cascade laser. Owing to the suppression of inter-Landau level non-radiative scattering, the device performances are strongly improved at high magnetic field. Working temperature up to 190 K and current threshold of 450 A/cm2 are measured at 11 T, comparable to the state-of-the-art GaAs/AlGaAs terahertz lasers. The nominally symmetric three-well structure presents significantly better performance with negative bias polarisation because of the inverted-interface roughness impact on the laser action.
Band alignment of two-dimensional transition metal dichalcogenides: Application in tunnel field effect transistors
Authors: Gong Cheng , Zhang Hengji , Wang Weihua , Colombo Luigi , Wallace Robert M. , Cho Kyeongjae
Published: August 2013

Tunnel field effect transistors (TFETs) based on vertical stacking of two dimensional materials are of interest for low-power logic devices. The monolayer transition metal dichalcogenides (TMDs) with sizable band gaps show promise in building p-n junctions (couples) for TFET applications. Band alignment information is essential for realizing broken gap junctions with excellent electron tunneling efficiencies. Promising couples composed of monolayer TMDs are suggested to be VIB-MeX2 (Me = W, Mo; X = Te, Se) as the n-type source and IVB-MeX2 (Me = Zr, Hf; X = S, Se) as the p-type drain by density functional theory calculations.
Surface bulk differences in a conventional superconductor, ZrB12
Authors: Thakur Sangeeta , Biswas Deepnarayan , Sahadev Nishaina , Biswas P. K. , Balakrishnan G. , Maiti Kalobaran
Published: August 2013

We studied the electronic structure of a conventional superconductor, ZrB12, using high resolution x-ray photoemission spectroscopy and single crystalline samples. Experimental results with different bulk sensitivity reveal boron deficiency at the surface while the bulk is stoichiometric and different valence states of Zr at the surface relative to those in the bulk. Signature of satellite features is observed in the Zr core level spectra corresponding to the bulk of the material suggesting importance of electron correlation among the conduction electrons in the bulk while the surface appears to be uncorrelated. These results provide an insight on surface-bulk differences, which is important for fabrication of devices based on such superconductors.
Nanocluster production for solar cell applications
Authors: Al Dosari Haila M. , Ayesh Ahmad I.
Published: August 2013

This research focuses on the fabrication and characterization of silver (Ag) and silicon (Si) nanoclusters that might be used for solar cell applications. Silver and silicon nanoclusters have been synthesized by means of dc magnetron sputtering and inert gas condensation inside an ultra-high vacuum compatible system. We have found that nanocluster size distributions can be tuned by various source parameters, such as the sputtering discharge power, flow rate of argon inert gas, and aggregation length. Quadrupole mass filter and transmission electron microscopy were used to evaluate the size distribution of Ag and Si nanoclusters. Ag nanoclusters with average size in the range of 3.6–8.3 nm were synthesized (herein size refers to the nanocluster diameter), whereas Si nanoclusters' average size was controlled to range between 2.9 and 7.4 nm by controlling the source parameters. This work illustrates the ability of controlling the Si and Ag nanoclusters' sizes by proper optimization of the operation conditions. By controlling nanoclusters' sizes, one can alter their surface properties to suit the need to enhance solar cell efficiency. Herein, Ag nanoclusters were deposited on commercial polycrystalline solar cells. Short circuit current (ISC), open circuit voltage (VOC), fill factor, and efficiency (η) were obtained under light source with an intensity of 30 mW/cm2. A 22.7% enhancement in solar cell efficiency could be measured after deposition of Ag nanoclusters, which demonstrates that Ag nanoclusters generated in this work are useful to enhance solar cell efficiency.
Selective dynamic concentration of peptides at poles of cation-selective nanoporous granules
Authors: Chen Hsiao-Ping , Tsai Chia-Chun , Lee Hung-Meng , Wang Shau-Chun , Chang Hsueh-Chia
Published: August 2013

The authors exposed a non-equilibrium dynamic counterion and coion analyte concentration to an AC electric field to selectively concentrate peptides at the poles of a cation-selective granule. The counterion polarization results from the focusing of the electric field show a discontinuous drop in the intra-granule counterion electromigration flux at the pole. The coion concentration polarization is due to the combined external convective and electromigration fluxes toward the pole that neutralize the accumulating counterions. Because the electromigration mobility of the peptide anion analyte depends on the pH, the authors determined a 20 000-fold high concentration factor for a near-neutral pH of 6.0 to 7.7. Because the peptide is protonated at the acidic pole and its absolute charge ranges from −0.3 to −1.9, the concentration factor scales exponentially with the absolute charge, thus allowing extremely selective concentrations of various peptides, which is demonstrated by fluorescein isothiocyanate tagged angiotensin I (pI ∼ 5.8) and Texas red tagged avidin (pI ∼ 10.5). This dynamic concentration effect can substantially enhance the sensitivity of bio-assays.
Giant d33 in (K,Na)(Nb,Sb)O3-(Bi,Na,K, Li)ZrO3 based lead-free piezoelectrics with high Tc
Authors: Cheng Xiaojing , Wu Jiagang , Wang Xiaopeng , Zhang Binyu , Zhu Jianguo , Xiao Dingquan , Wang Xiangjian , Lou Xiaojie
Published: August 2013

In this work, a lead-free piezoelectric system based on (1−x)(K0.48Na0.52)(Nb0.95Sb0.05)O3-xBi0.5(Na0.7K0.2Li0.1)0.5ZrO3 [(1−x)KNNS-xBNKLZ] is developed, and a rhombohedral-tetragonal phase boundary is constructed in this system. The relationship between the phase boundary and the piezoelectric properties of the (1−x)KNNS-xBNKLZ ceramics is illuminated. The coexistence of a tetragonal phase and a rhombohedral phase is identified in the composition range of 0.03 < x < 0.05. For such composition, the ceramics show giant d33 of ∼380 pC/N, high Tc of ∼290 °C, and a good thermal-depolarization behavior (d33 > 300 pC/N) of ∼210 °C. We believe that the (1−x)KNNS-xBNKLZ system is very promising for lead-free piezoelectric applications.
Optical frequency response analysis of light-emitting transistors under different microwave configurations
Authors: Wang Hsiao-Lun , Huang Yin-Jie , Wu Chao-Hsin
Published: August 2013

In this report, the optical frequency responses of InGaP/GaAs heterojunction bipolar light-emitting transistors are measured and analyzed using electrical small-signal equivalent circuit models under common-emitter, common-base, and common-collector configurations. Different optical modulation amplitudes and bandwidths, f3 dB, are obtained due to different input impedances and electrical transfer functions. The optical response of light-emitting transistors is able to be “tuned” by different input ports due to unique three-terminal characteristics along with carrier-photon conversion in the base region. From microwave analyses, identical “intrinsic” optical responses can be determined under three configurations at same DC bias condition.
Magnetic properties and magnetoresistance effect of Pb2FeMoO6
Authors: Yuan Xueping , Xu Mingxiang , Chen Yanbin
Published: August 2013

Magnetic properties and magnetoresistance effect of Pb2FeMoO6, synthesized under high pressure at high temperature, were systematically studied. The sample shows ferromagnetic behavior with Curie temperature about 243 K. The field dependent of paramagnetic susceptibility was discussed by the magnetic inhomogeneous related to Fe/Mo ions disorder. Electrical resistivity of the sample exhibits a semiconductor behavior, which can be well understood by variable-range hopping model. A maximum magnetoresistance effect value about 15% was observed at 20 K and 70 kOe. The pronounced linear magnetoresistance effect is attributed to the suppression of spin-fluctuations by the magnetic field.
Impact of the contact's geometry on the line resistivity of carbon nanotubes bundles for applications as horizontal interconnects
Authors: Chiodarelli N. , Fournier A. , Dijon J.
Published: August 2013

To use carbon nanotubes bundles as realistic interconnects, contacts must be devised so to electrically interface all the tubes encompassing a bundle. Here, we address this challenge by fabricating contacts at both the tip and the side of nanotubes bundles, in a so-called “All-Around” geometry. The measured resistivity outperforms that of identical bundles contacted at the tip only (End-Bonded geometry) proving more efficient current injection. The result is technologically relevant for all applications targeting the ultimate electrical performances of nanotubes bundles as demonstrates that good electron transparency of the contacts alone is not a sufficient condition for lowest line resistance.
Increased negatively charged nitrogen-vacancy centers in fluorinated diamond
Authors: Cui Shanying , Hu Evelyn L.
Published: August 2013

We investigated the effect of fluorine-terminated diamond surface on the charged state of shallow nitrogen vacancy defect centers (NVs). Fluorination is achieved with CF4 plasma, and the surface chemistry is confirmed with x-ray photoemission spectroscopy. Photoluminescence of these ensemble NVs reveals that fluorine-treated surfaces lead to a higher and more stable negatively charged nitrogen vacancy (NV−) population than oxygen-terminated surfaces. NV− population is estimated by the ratio of negative to neutral charged NV zero-phonon lines. Surface chemistry control of NV− density is an important step towards improving the optical and spin properties of NVs for quantum information processing and magnetic sensing.
Submicrosecond-timescale readout of carbon nanotube mechanical motion
Authors: Meerwaldt H. B. , Johnston S. R. , van der Zant H. S. J. , Steele G. A.
Published: August 2013

We report fast readout of the motion of a carbon nanotube mechanical resonator. A close-proximity high electron mobility transistor amplifier is used to increase the bandwidth of the measurement of nanotube displacements from the kHz to the MHz regime. Using an electrical detection scheme with the nanotube acting as a mixer, we detect the amplitude of its mechanical motion at room temperature with an intermediate frequency of 6 MHz and a timeconstant of 780 ns, both up to five orders of magnitude faster than achieved before. The transient response of the mechanical motion indicates a ring-down time faster than our enhanced time resolution, placing an upper bound on the contribution of energy relaxation processes to the room temperature mechanical quality factor.
Enhanced water window x-ray emission from in situ formed carbon clusters irradiated by intense ultra-short laser pulses
Authors: Chakravarty U. , Rao B. S. , Arora V. , Upadhyay A. , Singhal H. , Naik P. A. , Chakera J. A. , Mukherjee C. , Gupta P. D.
Published: August 2013

Enhanced water window x-ray emission (23–44 Å) from carbon clusters, formed in situ using a pre-pulse, irradiated by intense (I > 1017 W/cm2) ultra-short laser pulse, is demonstrated. An order of magnitude x-ray enhancement over planar graphite target is observed in carbon clusters, formed by a sub-ns pre-pulse, interacting with intense main pulse after a delay. The effect of the delay and the duration of the main pulse is studied for optimizing the x-ray emission in the water window region. This x-ray source has added advantages of being an efficient, high repetition rate, and low debris x-ray source.
Low energy structures of lithium-ion battery materials Li(MnxNixCo1−2x)O2 revealed by first-principles calculations
Authors: Shang ShunLi , Wang Yi , Wang William Y. , Fang Huazhi , Liu Zi-Kui
Published: August 2013

A long-standing issue regarding the low energy structures for the partially disordered cathode materials Li(MnxNixCo1−2x)O2 has been probed by first-principles calculations. It is found that the transitional metals Mn, Ni, and Co in Li(MnxNixCo1−2x)O2 follow the maximum entropy probability distribution (MEPD), instead of the random distribution, according to the distributions of the minimal partial radial distribution functions and the correlation functions. Here, the MEPD is proposed to understand the low energy structures of the partially disordered lithium-ion battery materials.
Tractable Shape Grammars
Authors: Yue Kui , Krishnamurti Ramesh
Published: August 2013

In this paper we explore the theoretical basis for a concept of ‘computation-friendly’ shape grammars, through a formal examination of tractability of the grammar formalism. Although a variety of shape grammar definitions have evolved over time, it is possible to unify these to be backwards compatible. Under this unified definition, a shape grammar can be constructed to simulate any Turing machine from which it follows that: A shape grammar may not halt; its language space can be exponentially large; and in general, its membership problem is unsolvable. Moreover, parametric subshape recognition is shown to be NP. This implies that it is unlikely, in general, to find a polynomial-time algorithm to interpret parametric shape grammars, and that more pragmatic approaches need to be sought. Factors that influence the tractability of shape grammars are identified and discussed.
Alkaline Earth Metal Organoamide Complexes Displaying Close Ae–F(C) (Ae = Ca, Mg) Interactions
Authors: Deacon Glen B. , Junk Peter C. , Kelly Rory P.
Published: August 2013

Protolysis of [Ca{N(SiMe3)2(thf)2] (thf = tetrahydrofuran) with the N,N-dialkyl-N′-2,3,5,6-tetrafluorophenylethane-1,2-diamines, p-HC6F4NH(CH2)2NR2 (LRH; R = Me or Et), yields [Ca(p-HC6F4N(CH2)2NR2)2(thf)2], whilst protolysis of di-n-butylmagnesium by the same amines yields [Mg(p-HC6F4N(CH2)2NR2)2] complexes. X-Ray crystal structures show that the calcium derivatives contain eight-coordinate calcium atoms, with tridentate (N,N′,o-F) coordination of the organoamide ligands and cisoid thf donors. It is noteworthy that the Ca–F(C) bond lengths are shorter than the Ca–NR2 bond lengths. By contrast, magnesium forms centrosymmetric, six-coordinate, homoleptic complexes, in which the Mg–NR2 bond lengths are shorter than the Mg–F(C) bond lengths. 19F{1H} NMR spectra of the R = Me complexes support retention of Ae–F(C) linkages in solution at low temperatures. Heating the complexes for a prolonged period in C6D6 at 80°C showed no evidence of C–F activation reactions.
Synthesis, Structure, NMR Spectroscopy, and Electrochemistry of the Sterically Congested Triarylarsine Dipp3As: EPR Characterization of its Radical Cation
Authors: Taghavikish Mona , Price Brock L. , Roemmele Tracey L. , Boeré René T.
Published: August 2013

The synthesis, NMR spectroscopy, single-crystal X-ray structure, and solution electrochemistry of the new compound [2,6-{CH(CH3)}2C6H3]3 As, abbreviated as Dipp3As, is reported. The molecule, prepared by reaction of AsCl3 with a pre-formed aryl copper reagent, Dipp4Cu4, crystallizes in the hexagonal space group R3 as a racemic twin. The sum of angles around As, ∑∠{CAsC}, is 329.13(3)° in the X-ray structure and 329.17° from an R-B3LYP/6-31G(d,p) hybrid density functional theory calculation. The aromatic rings are quite distorted with both the ipso carbon and especially the As atom significantly out of plane by 0.503(3) Å. The ambient temperature NMR spectrum fits for C3 symmetry implying that inversion is slow on the NMR timescale. Cyclic voltammetry on a glassy carbon electrode in CH2Cl2 with 0.4 M [nBu4N][PF6] over scan rates of 0.05–0.8 V s–1 and temperatures of 22 ± 2°C produced one quasi-reversible process with Em1 = +0.43 V at a scan rate of 0.20 V s–1 and a second irreversible process with a peak potential of +1.45 V (v. Fc+/0). The diffusion coefficient has been measured as 3.3 ± 0.1 × 10–6 cm2 s–1 in CH2Cl2 solution containing 0.4 M [nBu4N][PF6]. Chemical oxidation with AgPF6 in CH2Cl2 in degassed solutions in sealed vessels allowed for recording of characteristic EPR spectra; at 293 K, a (75As) = 26.1 mT and g = 2.021. In frozen solution, an almost isotropic spectrum is obtained (g║ = 2.000 mT and g⊥ = 2.003 mT) and the hyperfine splitting constants are a║ = 47.9 and a⊥ = 19.0 mT, leading to an estimate for the structure being slightly pyramidal with ∑∠{CAsC} ≈ 351°.