bluuebunny/crossref_metadata_2025 huggingface dataset:
Modified Wilkinson power divider using coupled microstrip lines and shunt open‐ended stubs
Authors: Ahmed U.T. , Abbosh A.M.
Published: May 2015

The design of a modified Wilkinson power divider for wideband applications is presented. Instead of using a T‐junction at the input port of the traditional design, the proposed divider uses broadside‐coupled microstrip to slotline configuration to extend the impedance matching bandwidth at the input port. Moreover, a coupled structure with two isolation resistors and two open‐ended stubs is used to enhance isolation and matching of the output ports across a wide band. The device has an overall dimension of 40 × 70 mm using the substrate Rogers RO4003 (dielectric constant = 3.38 and thickness = 0.508 mm). The simulated and experimental results show more than 100% fractional bandwidth assuming 16 dB of isolation and 10 dB of return loss as the reference. The device has equal power division between the two output ports with <0.2 dB amplitude imbalance and <2° phase imbalance across the band 1–3 GHz.
Modelling of OpenFlow‐based software‐defined networks: the multiple node case
Authors: Mahmood Kashif , Chilwan Ameen , Østerbø Olav , Jarschel Michael
Published: May 2015

OpenFlow (OF) is one of the most widely used protocols for controller‐to‐switch communication in a software defined network (SDN). Performance analysis of OF‐based SDN using analytical models is both highly desirable and challenging. There already exists a very elegant analytical model based on M/M/1 queues to estimate the packet sojourn time and probability of lost packets for the case in which a controller is responsible for only a single node in the data plane. However the literature falls short when it comes to the multiple node case, i.e. when there is more than one node in the data plane. In this work we propose a model to address this challenge by approximating the data plane as an open Jackson network with the controller also modeled as an M/M/1 queue. The model is then used to evaluate the system in the light of some of the metrics, such as; how much time a packet spends on average in an OF‐based network and how much data we can pump into the network given the average delay requirements. Finally the PDF and the CDF of the time spent by the packet in an OF‐based SDN for a given path is derived.
Optimised phase disposition pulse‐width modulation strategy for hybrid‐clamped multilevel inverters using switching state sequences
Authors: Ma Mingyao , He Xiangning , Cao Wenping , Song Xueguan , Ji Bing
Published: May 2015

This study describes an optimised modulation strategy based on switching state sequences for the hybrid‐clamped multilevel converter. Two key control variables defined as ‘phase shift angle’ and ‘switching state change’ for a five‐level hybrid‐clamped inverter are proposed to improve all switches’ operation, and by changing their values, different control methods can be obtained for modulation optimisation purposes. Two example methods can solve the voltage imbalance problem of the dc‐link capacitors and furthermore avoid two switches’ simultaneous switching transitions and improve the inverter's performance as compared with the traditional phase disposition pulse‐width modulation strategy. A 6 kW prototype inverter is developed and a range of simulation and experiments are carried out for validation. It is found that simulation and experimental results are in a good agreement and the proposed modulation strategy is verified in terms of low‐order harmonic reduction.
Field evaluation of a photonics‐based radar system in a maritime environment compared to a reference commercial sensor
Authors: Laghezza Francesco , Scotti Filippo , Serafino Giovanni , Banchi Luca , Malaspina Vincenzo , Ghelfi Paolo , Bogoni Antonella
Published: May 2015

This study shows the experimental validation of the first photonics‐based radar system demonstrator in a real maritime environment. The radar system demonstrator exploits photonic technologies for both the generation and the detection of the RF radar signals, and it has been previously proved allowing increased performance and unprecedented potential flexibility. Here, the photonics‐based radar is compared with a commercial system for maritime applications provided by ‘GEM elettronica’. The analysis has checked the performance of the photonics‐based system in terms of transmitted signal integrity and receiver pre‐ and post‐detection capability, by means of ad‐hoc laboratory tests and an on‐field direct comparison, run into the operative scenario of the port of San Benedetto del Tronto (Italy), by observing non‐cooperative targets. The reported results show that the photonics‐based radar system, although at a demonstrator stage, proves comparable performance with the commercial radar used as reference. Beyond the actual implementation of the proposed system, the outcomes of this comparison confirm that the photonics‐based approach can lead new advances in radar architecture, by the development of a completely frequency and waveform agility, application‐transparent photonic transceiver.
Revisiting the blocking force test on ferroelectric ceramics using high energy x-ray diffraction
Authors: Daniel L. , Hall D. A. , Koruza J. , Webber K. G. , King A. , Withers P. J.
Published: May 2015

The blocking force test is a standard test to characterise the properties of piezoelectric actuators. The aim of this study is to understand the various contributions to the macroscopic behaviour observed during this experiment that involves the intrinsic piezoelectric effect, ferroelectric domain switching, and internal stress development. For this purpose, a high energy diffraction experiment is performed in-situ during a blocking force test on a tetragonal lead zirconate titanate (PZT) ceramic (Pb0.98Ba0.01(Zr0.51Ti0.49)0.98Nb0.02O3). It is shown that the usual macroscopic linear interpretation of the test can also be performed at the single crystal scale, allowing the identification of local apparent piezoelectric and elastic properties. It is also shown that despite this apparent linearity, the blocking force test involves significant non-linear behaviour mostly due to domain switching under electric field and stress. Although affecting a limited volume fraction of the material, domain switching is responsible for a large part of the macroscopic strain and explains the high level of inter- and intra-granular stresses observed during the course of the experiment. The study shows that if apparent piezoelectric and elastic properties can be identified for PZT single crystals from blocking stress curves, they may be very different from the actual properties of polycrystalline materials due to the multiplicity of the physical mechanisms involved. These apparent properties can be used for macroscopic modelling purposes but should be considered with caution if a local analysis is aimed at.
Ferromagnetic behavior and exchange bias effect in akaganeite nanorods
Authors: Tadic Marin , Milosevic Irena , Kralj Slavko , Saboungi Marie-Louise , Motte Laurence
Published: May 2015

We report ferromagnetic-like properties and exchange bias effect in akaganeite (β-FeOOH) nanorods. They exhibit a Néel temperature TN = 259 K and ferromagnetic-like hysteresis behavior both below and above TN. An exchange bias effect is observed below TN and represents an interesting behavior for akaganeite nanorods. These results are explained on the basis of a core-shell structure in which the core has bulk akaganeite magnetic properties (i.e., antiferromagnetic ordering) while the shell exhibits a disordered spin state. Thus, the nanorods show ferromagnetic properties and an exchange bias effect at the same time, increasing their potential for use in practical applications.
Low-k material damage during photoresist ashing process
Authors: Lee Woohyun , Kim Hyuk , Park Wanjae , Kim Wan-Soo , Kim Donghyun , Kim Ji-Won , Cheong Hee-Woon , Whang Ki-Woong
Published: May 2015

The change of –OH and –CH3 component ratios in Fourier transform-infrared analysis of low-k materials during photoresist (PR) ashing processes were compared to assess the differences in the damages to low-k materials in a reactive ion etch (RIE) chamber and a magnetized-inductively coupled plasma (M-ICP) chamber. In M-ICP, the PR ashing rate was 28.1% higher than that of RIE, but the low-k material damage in M-ICP decreased when typical ashing conditions were used in each machine. The dependences of low-k material damage and PR ashing rate on the pressure, source power, and bias power in the M-ICP chamber were studied. We measured the ion energy distributions using an ion energy analyzer from which the flux could be also obtained. We found that the PR ashing rate increased as the ion flux increased, while the low-k material damage also increased as the ion flux and the incident ion energy increased. However, as the pressure decreased, the ion flux increased dramatically and the ion energy decreased. As a result, the PR ashing rate could be high and the low-k material damage low.
Binding kinetics of lock and key colloids
Authors: Colón-Meléndez Laura , Beltran-Villegas Daniel J. , van Anders Greg , Liu Jun , Spellings Matthew , Sacanna Stefano , Pine David J. , Glotzer Sharon C. , Larson Ronald G. , Solomon Michael J.
Published: May 2015

Using confocal microscopy and first passage time analysis, we measure and predict the rates of formation and breakage of polymer-depletion-induced bonds between lock-and-key colloidal particles and find that an indirect route to bond formation is accessed at a rate comparable to that of the direct formation of these bonds. In the indirect route, the pocket of the lock particle is accessed by nonspecific bonding of the key particle with the lock surface, followed by surface diffusion leading to specific binding in the pocket of the lock. The surprisingly high rate of indirect binding is facilitated by its high entropy relative to that of the pocket. Rate constants for forward and reverse transitions among free, nonspecific, and specific bonds are reported, compared to theoretical values, and used to determine the free energy difference between the nonspecific and specific binding states.
A rivulet of a power-law fluid with constant contact angle draining down a slowly varying substrate
Authors: Al Mukahal F. H. H. , Duffy B. R. , Wilson S. K.
Published: May 2015

Locally unidirectional steady gravity-driven flow of a thin rivulet of a power-law fluid with prescribed volume flux down a locally planar substrate is considered. First, the solution for unidirectional flow of a uniform rivulet down a planar substrate is obtained, and then it is used to obtain the solution for a slowly varying rivulet with prescribed constant (nonzero) contact angle down a slowly varying substrate, specifically flow in the azimuthal direction around the outside of a large horizontal circular cylinder. The solution is shown to depend strongly on the value of the power-law index of the fluid. For example, a rivulet of strongly shear-thinning fluid “self-channels” its flow down a narrow central channel between two “levées” of slowly moving fluid that form at its sides, and in the central channel there is a “plug-like” flow except in a boundary layer near the substrate. On the other hand, in a rivulet of a strongly shear-thickening fluid the velocity profile is linear except in a boundary layer near the free surface. Another notable qualitative departure from Newtonian behaviour is that, whereas the mass of a rivulet of a Newtonian or a shear-thinning fluid is theoretically infinite, the mass of a rivulet of a shear-thickening fluid is finite.
Optical conductivity of topological insulator thin films
Authors: Li L. L. , Xu W. , Peeters F. M.
Published: May 2015

We present a detailed theoretical study on the optoelectronic properties of topological insulator thin film (TITFs). The k·p approach is employed to calculate the energy spectra and wave functions for both the bulk and surface states in the TITF. With these obtained results, the optical conductivities induced by different electronic transitions among the bulk and surface states are evaluated using the energy-balance equation derived from the Boltzmann equation. We find that for Bi2Se3-based TITFs, three characteristic regimes for the optical absorption can be observed. (i) In the low radiation frequency regime (photon energy ℏω<200 meV), the free-carrier absorption takes place due to intraband electronic transitions. An optical absorption window can be observed. (ii) In the intermediate radiation frequency regime (200<ℏω<300 meV), the optical absorption is induced mainly by interband electronic transitions from surface states in the valance band to surface states in the conduction band and an universal value σ0=e2/(8ℏ) for the optical conductivity can be obtained. (iii) In the high radiation frequency regime (ℏω>300 meV), the optical absorption can be achieved via interband electronic transitions from bulk and surface states in the valance band to bulk and surface states in the conduction band. A strong absorption peak can be observed. These interesting findings indicate that optical measurements can be applied to identify the energy regimes of bulk and surface states in the TITF.
Semi-analytical solution to the frequency-dependent Boltzmann transport equation for cross-plane heat conduction in thin films
Authors: Hua Chengyun , Minnich Austin J.
Published: May 2015

Cross-plane heat transport in thin films with thicknesses comparable to the phonon mean free paths is of both fundamental and practical interest for applications such as light-emitting diodes and quantum well lasers. However, physical insight is difficult to obtain for the cross-plane geometry due to the challenge of solving the Boltzmann equation in a finite domain. Here, we present a semi-analytical series expansion method to solve the transient, frequency-dependent Boltzmann transport equation that is valid from the diffusive to ballistic transport regimes and rigorously includes the frequency-dependence of phonon properties. Further, our method is more than three orders of magnitude faster than prior numerical methods and provides a simple analytical expression for the thermal conductivity as a function of film thickness. Our result enables a straightforward physical understanding of cross-plane heat conduction in thin films.
Relativistic Eulerian Vlasov simulations of the amplification of seed pulses by Brillouin backscattering in plasmas
Authors: Shoucri M. , Matte J.-P. , Vidal F.
Published: May 2015

We apply an Eulerian Vlasov code to study the amplification by Brillouin scattering of a short seed laser pulse by a long pump laser pulse in an underdense plasma. The stimulated Brillouin backscattering interaction is the coupling of the pump and seed electromagnetic waves propagating in opposite directions, and the ion plasma wave. The code solves the one-dimensional relativistic Vlasov-Maxwell set of equations. Large amplitude ion waves are generated. In the simulations we present, the density plateau of the plasma is ne=0.3 nc (nc is the critical density), which excludes spurious stimulated Raman scattering amplification (which can occur only if ne<nc/4). We also varied the duration and/or amplitude of the short input seed pulse to study how these influence its subsequent behaviour. An initially broad pulse grows more rapidly than an initially narrow pulse. Furthermore, for an initially broader seed pulse, towards the end of the simulation, it is seen to become narrower and to gradually detach from the trailing signal. On the contrary, initially very narrow seed pulses are seen to broaden. The absence of noise in the Vlasov simulations allows to simulate long plasma amplifier lengths, and to follow the evolution of the system with a fully kinetic description and with an accurate representation of the phase-space structures of distribution function.
Electron attachment to the phthalide molecule
Authors: Asfandiarov N. L. , Pshenichnyuk S. A. , Vorob’ev A. S. , Nafikova E. P. , Lachinov A. N. , Kraikin V. A. , Modelli A.
Published: May 2015

Phthalide, the simplest chain of conductive polymer thin film, was investigated by means of Electron Transmission Spectroscopy, Negative Ion Mass Spectrometry, and density functional theory quantum chemistry. It has been found that formation of gas-phase long-lived molecular anions of phthalide around 0.7 eV takes place through cleavage of a C–O bond of the pentacyclic ring of the parent molecular anion to give a vibrationally excited (electronically more stable) open-ring molecular anion. The energy of the transition state for ring opening of the parent negative ion is calculated to be 0.65 eV above the neutral ground state of the molecule. The energy (2.64 eV) evaluated for the corresponding transition state in the neutral molecule is much higher, so that the process of electron detachment from the anion must lead to a neutral molecule with its initial pentacyclic structure. The average lifetime of the molecular negative ions formed at an electron energy of 0.75 eV and 80 °C is measured to be about 100 μs. The known switching effect of thin phthalide films could stem from the presence of a similar open/closed transition state also in the polymer.
Probing structural features of self-assembled violanthrone-79 using two dimensional infrared spectroscopy
Authors: Cyran Jenée D. , Krummel Amber T.
Published: May 2015

Two-dimensional infrared (2D IR) spectroscopy was used to characterize the structure of a self-assembled polycyclic aromatic hydrocarbon (PAH), violanthrone-79. A local mode basis was constructed using spectroscopic and computational results of anthrone and monomer violanthrone-79. The vibrational modes in the spectral region 1550–1700 cm−1, carbonyl stretching and in-plane ring breathing, are used as vibrational probes. The local mode basis and an electrostatic coupling model were applied to three nanoaggregate structures: parallel, antiparallel, and a chiral configuration produced by a 28° rotation from parallel. Angular disorder within each nanoaggregate configuration was also explored. This investigation is a first approach to probe self-assembled PAHs with 2D IR spectroscopy. The experimental and calculated 2D IR spectra align best when the violanthrone-79 molecules are in an anti-parallel configuration within the nanoaggregate.
Magnetoelectric effect in layered ferrite/PZT composites. Study of the demagnetizing effect on the magnetoelectric behavior
Authors: Loyau V. , Morin V. , Chaplier G. , LoBue M. , Mazaleyrat F.
Published: May 2015

We report the use of high magnetomechanical coupling ferrites in magnetoelectric (ME) layered composites. Bilayer samples combining (Ni0.973 Co0.027)1−xZnxFe2O4 ferrites (x = 0–0.5) synthesized by non conventional reactive Spark Plasma Sintering and commercial lead zirconate titanate (PZT) were characterized in term of ME voltage coefficients measured at sub-resonant frequency. Strong ME effects are obtained and we show that an annealing at 1000 °C and a quenching in air improve the piezomagnetic behavior of Zn-rich compositions. A theoretical model that predicts the ME behavior was developed, focusing our work on the demagnetizing effects in the transversal mode as well as the longitudinal mode. The model shows that: (i) high ME coefficients are obtained when ferrites with high magnetomechanical coupling are used in bilayer ME composites, (ii) the ME behavior in transversal and longitudinal modes is quite similar, and differences in the shapes of the ME curves are mainly due the demagnetizing effects, (iii) in the transversal mode, the magnetic field penetration depends on the ferrite layer thickness and the ME coefficient is affected accordingly. The two later points are confirmed by measurements on ME samples and calculations. Performances of the ME composites made with high magnetomechanical coupling ferrites are compared to those obtained using Terfenol-D materials in the same conditions of size, shape, and volume ratio. It appears that a ferrite with an optimized composition has performances comparable to those obtained with Terfenol-D material. Nevertheless, the fabrication processes of ferrites are quite simpler. Finally, a ferrite/PZT based ME composite was used as a current sensor.
Simultaneous observation of nascent plasma and bubble induced by laser ablation in water with various pulse durations
Authors: Tamura Ayaka , Matsumoto Ayumu , Fukami Kazuhiro , Nishi Naoya , Sakka Tetsuo
Published: May 2015

We investigate the effects of pulse duration on the dynamics of the nascent plasma and bubble induced by laser ablation in water. To examine the relationship between the nascent plasma and the bubble without disturbed by shot-to-shot fluctuation, we observe the images of the plasma and the bubble simultaneously by using two intensified charge coupled device detectors. We successfully observe the images of the plasma and bubble during the pulsed-irradiation, when the bubble size is as small as 20 μm. The light-emitting region of the plasma during the laser irradiation seems to exceed the bubble boundary in the case of the short-pulse (30-ns pulse) irradiation, while the size of the plasma is significantly smaller than that of the bubble in the case of the long-pulse (100-ns pulse) irradiation. The results suggest that the extent of the plasma quenching in the initial stage significantly depends on the pulse duration. Also, we investigate how the plasma-bubble relationship in the very early stage affects the shape of the atomic spectral lines observed at the later delay time of 600 ns. The present work gives important information to obtain high quality spectra in the application of underwater laser-induced breakdown spectroscopy, as well as to clarify the mechanism of liquid-phase laser ablation.
Note: Inverted heat pulse method to measure heat capacity during cooling: A counterpart of conventional quasi-adiabatic heat pulse method
Authors: Bag Pallab , Singh Vikram , Rawat R.
Published: May 2015

A simple method to extract known amount of heat from a sample within a given time interval has been proposed. Using this method, which we call inverted heat pulse (IHP) method, absolute values of heat capacity (CP) during cooling can be measured in a manner similar to conventional quasi-adiabatic heat pulse method of measuring CP during warming. Absolute accuracy of the measured CP using IHP method is found to be better than 2% in the temperature range 100–300 K. Applicability of this method to a broad and hysteretic first order transition is tested by measuring CP of Rh doped FePt sample, which shows a antiferromagnetic to ferromagnetic first order transition with a transition width of ∼35 K and hysteresis of ∼6 K. The peak value of the measured CP using IHP during cooling as well as entropy change calculated from measured data is found to be in good agreement with that measured during warming using conventional quasi-adiabatic heat pulse method.
Thin film cryogenic thermometers defined with optical lithography for thermomagnetic measurements on films
Authors: Nelson J. , Goldman A. M.
Published: May 2015

Resistance thermometers are common secondary thermometers in cryogenic applications. Bulk RuO2 thermometers are used in dilution refrigerators because of their low magnetoresistances in addition to their temperature sensitivity. Thermoelectric and thermomagnetic measurements require multiple thermometers to measure temperature differences. Here, we present a method to fabricate thin film RuO2 thermometers directly onto an experimental substrate. This enhances thermal contact between thermometers and films whose thermoelectric or thermomagnetic properties may be measured. Commercial thermometers have higher temperature sensitivities than the thermometers presented in this study, but commercial thermometers must be carefully heat sunk to the cryostat or sample to be useful. Thin film thermometers can be patterned with ultraviolet (UV) lithography. This allows both the size of the thermometer and its distance from the sample, when also patterned with UV lithography, to be on the order of micrometers. A universal calibration curve for these thin film thermometers has not been produced. The efficacy of these thermometers has been demonstrated through measurements of the Nernst effect in Nb. In this study, the thin film thermometers were calibrated using the cryostat thermometers.
Effects of the d-donor level of vanadium on the properties of Zn1−xVxO films
Authors: García-Hemme E. , Yu K. M. , Wahnon P. , González-Díaz G. , Walukiewicz W.
Published: May 2015

We report the effect of d-levels of vanadium atoms on the electronic band structure of ZnO. Polycrystalline layers of Zn1−xVxO with 0 ≤ x ≤ 0.08 were synthesized using magnetron sputtering technique. Electrical measurements show that electron concentration increases with vanadium up to x = 0.04 and then decreases and films become insulating for x > 0.06. Optical characterization reveals that the absorption edge shifts to higher energy, while the photoluminescence (PL) peak shows a shift to lower energy with increasing vanadium content. This unusual optical behavior can be explained by an anticrossing interaction between the vanadium d-levels and the conduction band (CB) of ZnO. The interaction results in an upward shift of unoccupied CB (E+) and the downward shift of the fully occupied E− band derived from the vanadium d-levels. The composition dependence of optical absorption edge (E+) and PL peak (E−) can be fitted using the Band Anticrossing model with the vanadium d-level located at 0.13 eV below CB of ZnO and a coupling constant of 0.65 eV.
First principles-based multiparadigm, multiscale strategy for simulating complex materials processes with applications to amorphous SiC films
Authors: Naserifar Saber , Goddard William A. , Tsotsis Theodore T. , Sahimi Muhammad
Published: May 2015

Progress has recently been made in developing reactive force fields to describe chemical reactions in systems too large for quantum mechanical (QM) methods. In particular, ReaxFF, a force field with parameters that are obtained solely from fitting QM reaction data, has been used to predict structures and properties of many materials. Important applications require, however, determination of the final structures produced by such complex processes as chemical vapor deposition, atomic layer deposition, and formation of ceramic films by pyrolysis of polymers. This requires the force field to properly describe the formation of other products of the process, in addition to yielding the final structure of the material. We describe a strategy for accomplishing this and present an example of its use for forming amorphous SiC films that have a wide variety of applications. Extensive reactive molecular dynamics (MD) simulations have been carried out to simulate the pyrolysis of hydridopolycarbosilane. The reaction products all agree with the experimental data. After removing the reaction products, the system is cooled down to room temperature at which it produces amorphous SiC film, for which the computed radial distribution function, x-ray diffraction pattern, and the equation of state describing the three main SiC polytypes agree with the data and with the QM calculations. Extensive MD simulations have also been carried out to compute other structural properties, as well the effective diffusivities of light gases in the amorphous SiC film.
Magnetic properties of the surface layer and its magnetic interaction with the interior of Nd-Fe-B sintered magnets
Authors: Kobayashi Kurima , Nakamura Michi-hide , Urushibata Kimiko
Published: May 2015

The magnetization and demagnetization mechanisms in the mechanically polished surface layers (SL) of the c-plane and a-b plane of Nd-Fe-B sintered magnets were investigated. The magnetic interaction between the SL and the interior of the magnet was clarified by using vibrating sample magnetometer measurements of a whole sample and magneto-force microscopy observations of the domain structure of the SL layer. The polishing eliminated the Nd-rich grain boundary phases, which was only about 2 nm thick, from the SL crystal grains in the sintered magnets. The a-b plane polishing caused the independent magnetic reversal of the a-b plane SL, which was about 5.5 μm thick, as in the platy samples. The coercivities (μ0Hc) of the SL were less than 0.3 T. In contrast, the c-plane polishing did not produce independent magnetic reversal of the SL, although the coercivity of bulk samples was clearly decreased by the formation of the c-plane SL. The grains in the SL should form clusters that alter the demagnetizing factors, depending on the shape of cluster as a magnetic unit. The magnetic interaction among the SL crystal grains is expected to play an important role in the coercivity mechanism of Nd-Fe-B sintered magnets.
Magnetic field induced insulator-metal transition in nanocrystalline Pr0.67Ca0.33MnO3 compounds: Evidence of large temperature co-efficient of resistance
Authors: Das Kalipada , Das I.
Published: May 2015

We report the electronic transport, magneto-transport, and magnetic properties of nanocrystalline Pr0.67Ca0.33MnO3 compound. A magnetic field induced insulator—metal transition appears for the external magnetic field higher than 50 kOe. We have obtained large value of the temperature coefficient of resistance (TCR) along with magnetoresistance and field coefficient of resistance (FCR). The value of TCR is 135%/K at 48 K. The calculated magnetoresistance is about −9.8 × 107% for 70 kOe and maximum FCR is about 320%/kOe around 75 K. Due to the application of the external magnetic field, charge ordered state of the compound is destabilized leading to such large values of TCR and FCR. Large values of TCR and FCR along with the large magnetoresistance exhibited by the material is interesting from the application point of view.
Temperature effect on vortex-core reversals in magnetic nanodots
Authors: Kim Bosung , Yoo Myoung-Woo , Lee Jehyun , Kim Sang-Koog
Published: May 2015

We studied the temperature effect on vortex-core reversals in soft magnetic nanodots by micromagnetic numerical calculations within a framework of the stochastic Landau-Lifshitz-Gilbert scheme. It was determined that vortex-core-switching events at non-zero temperatures occur stochastically, and that the threshold field strength increases with temperature for a given field frequency. The mechanism of core reversals at elevated temperatures is the same as that of vortex-antivortex-pair-mediated core reversals found at the zero temperature. The reversal criterion is also the out-of-plane component of a magnetization dip that should reach −p, which is to say, mz,dip = −p, where p is the original polarization, p = +1 (−1), for the upward (downward) core. By this criterion, the creation of a vortex-antivortex pair accompanies complete vortex-antivortex-annihilation-mediated core reversals, resulting in the maximum excess of the exchange energy density, ΔEexcri ≈ 15.4 ± 0.2 mJ/cm3. This work provides the underlying physics of vortex-core reversals at non-zero temperatures, and potentiates the real application of vortex random access memory operating at elevated temperatures.
Metaporous layer to overcome the thickness constraint for broadband sound absorption
Authors: Yang Jieun , Lee Joong Seok , Kim Yoon Young
Published: May 2015

The sound absorption of a porous layer is affected by its thickness, especially in a low-frequency range. If a hard-backed porous layer contains periodical arrangements of rigid partitions that are coordinated parallel and perpendicular to the direction of incoming sound waves, the lower bound of the effective sound absorption can be lowered much more and the overall absorption performance enhanced. The consequence of rigid partitioning in a porous layer is to make the first thickness resonance mode in the layer appear at much lower frequencies compared to that in the original homogeneous porous layer with the same thickness. Moreover, appropriate partitioning yields multiple thickness resonances with higher absorption peaks through impedance matching. The physics of the partitioned porous layer, or the metaporous layer, is theoretically investigated in this study.
Time dependent Doppler shifts in high-order harmonic generation in intense laser interactions with solid density plasma and frequency chirped pulses
Authors: Welch E. C. , Zhang P. , Dollar F. , He Z.-H. , Krushelnick K. , Thomas A. G. R.
Published: May 2015

High order harmonic generation from solid targets is a compelling route to generating intense attosecond or even zeptosecond pulses. However, the effects of ion motion on the generation of harmonics have only recently started to be considered. Here, we study the effects of ion motion in harmonics production at ultrahigh laser intensities interacting with solid density plasma. Using particle-in-cell simulations, we find that there is an optimum density for harmonic production that depends on laser intensity, which scales linearly with a0 with no ion motion but with a reduced scaling if ion motion is included. We derive a scaling for this optimum density with ion motion and also find that the background ion motion induces Doppler red-shifts in the harmonic structures of the reflected pulse. The temporal structure of the Doppler shifts is correlated to the envelope of the incident laser pulse. We demonstrate that by introducing a frequency chirp in the incident pulse we are able to eliminate these Doppler shifts almost completely.
Enhanced tunneling magnetoresistance and perpendicular magnetic anisotropy in Mo/CoFeB/MgO magnetic tunnel junctions
Authors: Almasi H. , Hickey D. Reifsnyder , Newhouse-Illige T. , Xu M. , Rosales M. R. , Nahar S. , Held J. T. , Mkhoyan K. A. , Wang W. G.
Published: May 2015

Structural, magnetic, and transport studies have been performed on perpendicular magnetic tunnel junctions (pMTJ) with Mo as the buffer and capping layers. After annealing samples at 300 °C and higher, consistently better performance was obtained compared to that of conventional pMTJs with Ta layers. Large tunneling magnetoresistance (TMR) and perpendicular magnetic anisotropy (PMA) values were retained in a wide range of samples with Mo layers after annealing for 2 h at 400 °C, in sharp contrast to the junctions with Ta layers, in which superparamagnetic behavior with nearly vanishing magnetoresistance was observed. As a result of the greatly improved thermal stability, TMR as high as 162% was obtained in junctions containing Mo layers. These results highlight the importance of the heavy-metal layers adjacent to CoFeB electrodes for achieving larger TMR, stronger PMA, and higher thermal stability in pMTJs.
The Kastler-Kalau-Walze type theorem for six-dimensional manifolds with boundary
Authors: Wang Jian , Wang Yong
Published: May 2015

In this paper, we define lower dimensional volumes of spin manifolds with boundary. We compute the lower dimensional volume V ol6(1,3) for 6-dimensional spin manifolds with boundary and derive the gravity on boundary from the noncommutative residue associated with Dirac operators. For 6-dimensional manifolds with boundary, we also get a Kastler-Kalau-Walze type theorem for a general fourth order operator.
Experimental evidence of hot carriers solar cell operation in multi-quantum wells heterostructures
Authors: Rodière Jean , Lombez Laurent , Le Corre Alain , Durand Olivier , Guillemoles Jean-François
Published: May 2015

We investigated a semiconductor heterostructure based on InGaAsP multi quantum wells (QWs) using optical characterizations and demonstrate its potential to work as a hot carrier cell absorber. By analyzing photoluminescence spectra, the quasi Fermi level splitting Δμ and the carrier temperature are quantitatively measured as a function of the excitation power. Moreover, both thermodynamics values are measured at the QWs and the barrier emission energy. High values of Δμ are found for both transition, and high carrier temperature values in the QWs. Remarkably, the quasi Fermi level splitting measured at the barrier energy exceeds the absorption threshold of the QWs. This indicates a working condition beyond the classical Shockley-Queisser limit.
Electric field and substrate–induced modulation of spin-polarized transport in graphene nanoribbons on A3B5 semiconductors
Authors: Ilyasov Victor V. , Nguyen Chuong V. , Ershov Igor V. , Hieu Nguyen N.
Published: May 2015

In this work, we present the density functional theory calculations of the effect of an oriented electric field on the electronic structure and spin-polarized transport in a one dimensional (1D) zigzag graphene nanoribbon (ZGNR) channel placed on a wide bandgap semiconductor of the A3B5 type. Our calculations show that carrier mobility in the 1D semiconductor channel of the ZGNR/A3B5(0001) type is in the range from 1.7×104 to 30.5×104 cm2/Vs and can be controlled by an electric field. In particular, at the critical value of the positive potential, even though hole mobility in an one-dimensional 8-ZGNR/h-BN semiconductor channel for spin down electron subsystems is equal to zero, hole mobility can be increased to 4.1×105 cm2/Vs for spin up electron subsystems. We found that band gap and carrier mobility in a 1D semiconductor channel of the ZGNR/A3B5(0001) type depend strongly on an external electric field. With these extraordinary properties, ZGNR/A3B5(0001) can become a promising materials for application in nanospintronic devices.
First-principles study on electronic and optical properties of Cu2ZnSiV I4 (VI=S, Se, and Te) quaternary semiconductors
Authors: Zhang Xuebiao , Rao Dewei , Lu Ruifeng , Deng Kaiming , Chen Dongguo
Published: May 2015

The electronic and optical properties of Cu2ZnSiS4, Cu2ZnSiSe4 and Cu2ZnSiTe4 in kesterite and stannite structures are systematically studied using first-principles calculations. Crystal field splitting, optical transitions, p-d bonding, and anti-bonding overlapping are analyzed. The physical and chemical trends in these properties are investigated with respect to the crystal structure and anion atomic number. The optical spectra, such as dielectric function, refractive index, reflectivity and absorption coefficient are explored in a broad range of energy. A good agreement between the calculated results and experimental data is obtained.
Trip Generation of Seniors and the Geography of Walking in Montreal
Authors: Moniruzzaman Md , Páez Antonio , Scott Darren , Morency Catherine
Published: May 2015

With the graying of populations across the world, the travel behavior of seniors has become a topic of growing interest in planning and research. Most attention in the field of transportation has been devoted to motorized travel. However, the use of various modes of transportation, including nonmotorized travel, remains to be fully investigated. In this paper the multimodal trip generation of seniors in Montreal Island is studied. Personal, mobility tools, neighborhood, and accessibility variables are considered in a trivariate ordered probit model of three modes: car, transit, and walking. Geographical analysis of the walking component of the model helps to identify locations within the region where walking is more or less prevalent among older adults.
Modelling Multilevel Variations in Distance Moved between Origins and Destinations in England and Wales
Authors: Thomas Michael , Stillwell John , Gould Myles
Published: May 2015

Using individual records from a large and geographically detailed national research opinion survey, this paper uniquely adopts a multilevel cross-classified statistical framework to demonstrate the relative importance of individual and simultaneous origin/destination contextual (neighbourhood and city region) variations in the distance moved by residential migrants in England and Wales. The model results confirm distinct microlevel variations in the distance moved according to certain key variables such as household income, educational attainment, and housing tenure whilst simultaneously revealing the importance of substantial origin and destination place-based macrogeographic variations. More specifically, the results indicate the extent to which those out-migrants from the main metropolitan cores, together with those in-migrants to the rural/coastal (amenity-rich) destinations, move over considerably longer distances than the average distance of move.
Mobility and the Model: Policy Mobility and the Becoming of Israeli Homeland Security Dominance
Authors: Machold Rhys
Published: May 2015

Israel's dominant position within the homeland security (HLS) and counterterrorism field is now quite widely acknowledged within academic literature and beyond. While existing accounts of Israel's HLS industry have made a number of critical contributions to understanding its rise, they fail to readily address how Israeli HLS approaches have come to be understood as mobile and universal strategies to be applied worldwide. They also negate the extent to which Israel's security ‘solutions’ are (re)invented as they move geographically. I argue that Israeli claims to HLS dominance are realized through their encounters beyond Israel/Palestine in ways that stabilize their practices and technologies as global exemplars. Building on policy mobilities scholarship, I suggest that bound up with the development of an Israeli HLS ‘model’ is the constitution of an audience, which is central to giving these approaches their global appeal and respective political force. In developing these claims I explore the role of geographical imaginations in the constitution of mobilities and argue for the need to move from a framework of discourse to one of performativity.
The Political Ecology of a Highway through Belize's Forested Borderlands
Authors: Wainwright Joel , Jiang Shiguo , Mercer Kristin , Liu Desheng
Published: May 2015

This paper examines how a new highway in Belize will change a tropical forest landscape. Since the end of British colonialism (1981), the Maya communities in Belize's Toledo District have struggled with the state for control of the lands they have customarily used to produce a livelihood. We were approached by some Maya leaders, who recognize that the paving of a new highway through their villages could transform Maya agricultural practices and thus land use and cover change (LUCC), and asked to produce an analysis that could assist them in managing their lands. We braid together historical, social, political, economic, and satellite data to answer two questions: (1) How has land cover changed since 1975 in areas with Maya customary versus noncustomary land-use practices? (2) What will be the consequences of paving the new highway through this region for the landscape and the Maya communities? Using a multifaceted LUCC/political ecology analysis we found that forest clearing is greatest where noncustomary farming practices are employed. Noncustomary practices are spatially concentrated along the Belize-Guatemala border; the shift to noncustomary practices resulted from inmigration by war refugees who maintained close ties to Guatemalan markets. The paving of the highway promises to reduce the functional distance to Guatemala's markets, which could change land use in other villages to the detriment of the forests because of the likely diffusion of noncustomary farm practices. Forest change will be shaped by the region's complex political geography.
Markets, Nature, Neoliberalism, and Conservation through Private Protected Areas in Southern Chile
Authors: Holmes George
Published: May 2015

A vibrant literature has emerged in recent years exploring moves towards neoliberal forms of conservation, with a reduced role for the state and an enhanced role for markets and private and civil society actors. Yet there is a need for studies which explore how and why this trend has emerged, and what impact it has on both people and nature. The author provides a detailed examination of private protected areas, which are often associated with neoliberal approaches to conservation, in Chile—a country which has had a long and deep engagement with neoliberalism. It is found that private protected areas demonstrate a broad range of attitudes towards the use of markets in conservation, from enthusiasm to hostility. Yet all have been made possible, indeed incentivised, by Chile's liberalised property markets and individualistic political culture—products of earlier neoliberal reforms within Chile's society and economy. As such, they provide only a limited challenge to the social and environmental consequence of the integration of southern Chile's natural resources into global neoliberal economic chains. The author emphasises the importance of considering how broader neoliberal economic, political, and social reforms have allowed certain forms of conservation to emerge and thrive.
Effects of age on follicular fluid exosomal microRNAs and granulosa cell transforming growth factor-β signalling during follicle development in the mare
Authors: da Silveira Juliano C. , Winger Quinton A. , Bouma Gerrit J. , Carnevale Elaine M.
Published: May 2015

Age-related decline in fertility is a consequence of low oocyte number and/or low oocyte competence resulting in pregnancy failure. Transforming growth factor (TGF)-β signalling is a well-studied pathway involved in follicular development and ovulation. Recently, small non-coding RNAs, namely microRNAs (miRNAs), have been demonstrated to regulate several members of this pathway; miRNAs are secreted inside small cell-secreted vesicles called exosomes. The overall goal of the present study was to determine whether altered exosome miRNA content in follicular fluid from old mares is associated with changes in TGF-β signalling in granulosa cells during follicle development. Follicular fluid was collected at deviation (n = 6), mid-oestrus (n = 6) and preovulation (n = 6) for identification of exosomal miRNAs from young (3–12 years) and old (20–26 years) mares. Analysis of selected TGF-β signalling members revealed significantly increased levels of interleukin 6 (IL6) in granulosa cells from mid-oestrus compared with preovulatory follicles, and collagen alpha-2(I) chain (COL1A2) in granulosa cells from deviation compared with preovulatory follicles in young mares. In addition, granulosa cells from old mares had significantly altered levels of DNA-binding protein inhibitor ID-2 (ID2), signal transducer and activator of transcription 1 (STAT1) and cell division cycle 25A (CDC25A). Finally, changes in exosomal miRNA predicted to target selected TGF-β members were identified.
Effect of summer livestock grazing on plant species richness and composition in the Himalayan rangelands
Authors: Aryal Suman , Cockfield Geoff , Maraseni Tek Narayan
Published: May 2015

Grazing systems in the high Himalayas are changing in response to socioeconomic factors and policy decisions. As well as effects on livelihood of herders, such changes may have significant impacts on plant species richness and composition. The objectives of this study were to explore how plant species richness and composition respond to livestock grazing in the high Himalayan rangelands. The study was conducted in three mountainous Protected Areas of Nepal viz. Sagarmatha (Mt Everest) National Park, Gaurishankar Conservation Area and Khaptad National Park. Species and environmental data were collected along perpendicular transects outwards from goths (semi-permanent stopping and camping points). It was found that the distance from goths generally represents a grazing disturbance gradient. The core areas near goths had low species richness per plot (α-diversity) where nitrophilous and grazing-tolerant species were commonly found. However, the highest species richness, total numbers of species and occurrences of rare species at mid- and farther distances from goths within 800 m suggest that negative effects of summer livestock grazing were small and confined to limited areas near goths. Altitude, soil moisture content, percentage shrub cover and distance from goths were the variables significantly correlated with species composition, although the relative importance of these factors varied across study sites. This research can be used to inform decision-making about seasonal livestock grazing in the Himalayan rangelands, which should incorporate both ecological as well as socioeconomic considerations.