bluuebunny/crossref_metadata_2025 huggingface dataset:
Field theoretic simulations of polymer nanocomposites
Authors: Koski Jason , Chao Huikuan , Riggleman Robert A.
Published: January 2014

Polymer field theory has emerged as a powerful tool for describing the equilibrium phase behavior of complex polymer formulations, particularly when one is interested in the thermodynamics of dense polymer melts and solutions where the polymer chains can be accurately described using Gaussian models. However, there are many systems of interest where polymer field theory cannot be applied in such a straightforward manner, such as polymer nanocomposites. Current approaches for incorporating nanoparticles have been restricted to the mean-field level and often require approximations where it is unclear how to improve their accuracy. In this paper, we present a unified framework that enables the description of polymer nanocomposites using a field theoretic approach. This method enables straightforward simulations of the fully fluctuating field theory for polymer formulations containing spherical or anisotropic nanoparticles. We demonstrate our approach captures the correlations between particle positions, present results for spherical and cylindrical nanoparticles, and we explore the effect of the numerical parameters on the performance of our approach.
Analytic structure of the multichannel Jost matrix for potentials with Coulombic tails
Authors: Rakityansky S. A. , Elander N.
Published: January 2014

A quantum system is considered that can move in N two-body channels with the potentials that may include the Coulomb interaction. For this system, the Jost matrix is constructed in such a way that all its dependencies on the channel momenta and Sommerfeld parameters are factorized in the form of explicit analytic expressions. It is shown that the two remaining unknown matrices are single-valued analytic functions of the energy and therefore can be expanded in the Taylor series near an arbitrary point within the domain of their analyticity. It is derived a system of first-order differential equations whose solutions determine the expansion coefficients of these series. Alternatively, the unknown expansion coefficients can be used as fitting parameters for parametrizing experimental data similarly to the effective-range expansion. Such a parametrization has the advantage of preserving proper analytic structure of the Jost matrix and can be done not only near the threshold energies, but around any collision or even complex energy. As soon as the parameters are obtained, the Jost matrix (and therefore the S-matrix) is known analytically on all sheets of the Riemann surface, and thus enables one to locate possible resonances.
Generalization of Lieb's variational principle to Bogoliubov–Hartree–Fock theory
Authors: Bach Volker , Breteaux Sébastien , Knörr Hans Konrad , Menge Edmund
Published: January 2014

In its original formulation, Lieb's variational principle holds for fermion systems with purely repulsive pair interactions. As a generalization we prove for both fermion and boson systems with semi-bounded Hamiltonian that the infimum of the energy over quasifree states coincides with the infimum over pure quasifree states. In particular, the Hamiltonian is not assumed to preserve the number of particles. To shed light on the relation between our result and the usual formulation of Lieb's variational principle in terms of one-particle density matrices, we also include a characterization of pure quasifree states by means of their generalized one-particle density matrices.
Geometric momentum for a particle constrained on a curved hypersurface
Authors: Liu Q. H.
Published: January 2014

The canonical quantization is a procedure for quantizing a classical theory while preserving the formal algebraic structure among observables in the classical theory to the extent possible. For a system without constraint, we have the so-called fundamental commutation relations (CRs) among positions and momenta, whose algebraic relations are the same as those given by the Poisson brackets in classical mechanics. For the constrained motion on a curved hypersurface, we need more fundamental CRs otherwise neither momentum nor kinetic energy can be properly quantized, and we propose an enlarged canonical quantization scheme with introduction of the second category of fundamental CRs between Hamiltonian and positions, and those between Hamiltonian and momenta, whereas the original ones are categorized into the first. As an N − 1 (N ⩾ 2) dimensional hypersurface is embedded in an N dimensional Euclidean space, we obtain the proper momentum that depends on the mean curvature. For the spherical surface, a long-standing problem in the form of the geometric potential is resolved in a lucid and unambiguous manner, which turns out to be identical to that given by the so-called confining potential technique. In addition, a new dynamical group SO(N, 1) symmetry for the motion on the sphere is demonstrated.
Revisiting the influence of chain length on the α- and β-relaxations in oligomeric glass formers
Authors: Ngai K. L.
Published: January 2014

Dielectric relaxation measurements of a series of oligo(propylene glycol) dimethyl ethers, CH3–O–[CH2–CH(CH3)–O]N–CH3, including samples with the number of PG units N = 1, 2, 3, 7, 17, 34, and 69, were made by Mattsson et al. [Phys. Rev. Lett. 94, 165701 (2005)] at ambient pressure. The objective of the study was to relate the change of properties of the glass transition dynamics to the number of monomer units N in the chain. Not examined in the previous publication is how the change of the width of the frequency dispersion of the α-relaxation with N is related to the observed change in the α-β bifurcation characterized by the ratio, τα(Tg)/τβ(Tg). In this paper, the frequency dispersion of the dimer, trimer, and heptamer are fitted by the Fourier transform of the Kohlrausch stretched exponential function, ϕ(t) = exp[−(t/τα)1−n]. Determined from experimental data, both τα(Tg)/τβ(Tg) and n increase with N. More interestingly, we find τα(Tg)/τβ(Tg) has approximately the same value as [τα(Tg)/tc]n with tc = 2 ps, in accordance with the prediction of the Coupling Model of approximate relation between τα and τβ given by τβ ≈ (tc)n(τα)1−n. Considered also are previously unpublished dielectric loss spectra of the heptamer taken at different combinations of T and P with τα(T,P) fixed by Roland et al. [Phys. Rev. B 77, 012201 (2008)]. The dielectric loss data show not only the α-loss peaks superpose but also the high frequency flank including the barely resolved JG β-relaxation superposes approximately. This is again consistent with the approximate relation between τα and τβ from the Coupling Model because n is unchanged on varying P and T with τα(T,P) kept constant, and tc is a constant. The additional advance made herein has the benefit of enhancing the impact of the earlier experimental studies of the oligo(propylene glycol) dimethyl ethers on current understanding of the dynamics of glass transition.
Variation and decomposition of the partial molar volume of small gas molecules in different organic solvents derived from molecular dynamics simulations
Authors: Klähn Marco , Martin Alistair , Cheong Daniel W. , Garland Marc V.
Published: January 2014

The partial molar volumes, $\bar V_i$V¯i, of the gas solutes H2, CO, and CO2, solvated in acetone, methanol, heptane, and diethylether are determined computationally in the limit of infinite dilution and standard conditions. Solutions are described with molecular dynamics simulations in combination with the OPLS-aa force field for solvents and customized force field for solutes. $\bar V_i$V¯i is determined with the direct method, while the composition of $\bar V_i$V¯i is studied with Kirkwood-Buff integrals (KBIs). Subsequently, the amount of unoccupied space and size of pre-formed cavities in pure solvents is determined. Additionally, the shape of individual solvent cages is analyzed. Calculated $\bar V_i$V¯i deviate only 3.4 cm3 mol−1 (7.1%) from experimental literature values. Experimental $\bar V_i$V¯i variations across solutions are reproduced qualitatively and also quantitatively in most cases. The KBI analysis identifies differences in solute induced solvent reorganization in the immediate vicinity of H2 (<0.7 nm) and solvent reorganization up to the third solvation shell of CO and CO2 (<1.6 nm) as the origin of $\bar V_i$V¯i variations. In all solutions, larger $\bar V_i$V¯i are found in solvents that exhibit weak internal interactions, low cohesive energy density and large compressibility. Weak internal interactions facilitate solvent displacement by thermal solute movement, which enhances the size of solvent cages and thus $\bar V_i$V¯i. Additionally, attractive electrostatic interactions of CO2 and the solvents, which do not depend on internal solvent interactions only, partially reversed the $\bar V_i$V¯i trends observed in H2 and CO solutions where electrostatic interactions with the solvents are absent. More empty space and larger pre-formed cavities are found in solvents with weak internal interactions, however, no evidence is found that solutes in any considered solvent are accommodated in pre-formed cavities. Individual solvent cages are found to be elongated in the negative direction of solute movement. This wake behind the moving solute is more pronounced in case of mobile H2 and in solvents with weaker internal interactions. However, deviations from a spherical solvent cage shape do not influence solute-solvent radial distribution functions after averaging over all solvent cage orientations and hence do not change $\bar V_i$V¯i. Overall, the applied methodology reproduces $\bar V_i$V¯i and its variations reliably and the used $\bar V_i$V¯i decompositions identify the underlying reasons behind observed $\bar V_i$V¯i variations.
Interaction of plasmas in laser ion source with double laser system
Authors: Fuwa Y. , Ikeda S. , Kumaki M. , Sekine M. , Cinquegrani D. , Romanelli M. , Kanesue T. , Okamura M. , Iwashita Y.
Published: January 2014

Multiple laser shots could be used to elongate an ion beam pulse width or to intensify beam current from laser ion sources. In order to confirm the feasibility of the multiple shot scheme, we investigated the properties of plasmas produced by double laser shots. We found that when the interval of the laser shots is shorter than 10 μs, the ion current profile had a prominent peak, which is not observed in single laser experiments. The height of this peak was up to five times larger than that of single laser experiment.
Optical and electrical properties of porous silicon impregnated with Congo Red dye
Authors: Chouket Ahmed , Cherif Boutheina , Ben Salah Nasr , Khirouni Kamel
Published: January 2014

Incorporation of molecules into porous silicon (PS) matrix is of particular interest for potential utilization in hybrid organic-semiconductor devices. In this study, the incorporation of Congo Red molecules inside luminescent PS layers was investigated. The resulting structures have been characterized by Fourier Transformer Infrared (FTIR) and photoluminescence (PL) techniques. Based on these characterizations, the infiltration of dye molecules into the porous matrix has been proved. The recuperated PL signal was more important than that of porous silicon alone. A nonradiative excitation transfer due to dipolar interactions was evidenced from the effect of the CR concentration on the PL emission which was also confirmed by FTIR spectroscopy. It was found that the formed composite displays an efficient and stable PL. Preliminary characterizations of the electrical properties of the resulting nanocomposite structure have been also performed. The current–voltage (I–V) characteristics of CR-PS/p-Si were measured at the room temperature (300 K). To study the effect of illumination on CR-PS/p-Si composites, the measurement has been performed in dark and under illumination, at room temperature. The diode characteristics including zero-bias barrier height (ΦB0), ideality factor (n), and series resistance (Rs) were calculated at room temperature in dark and under illumination. The obtained results have shown that these characteristics are largely affected by illumination. The incorporation of dye molecules in porous silicon matrix enhanced photovoltaic properties of resulting structures.
Effective interface state effects in hydrogenated amorphous-crystalline silicon heterostructures using ultraviolet laser photocarrier radiometry
Authors: Melnikov A. , Mandelis A. , Halliop B. , Kherani N. P.
Published: January 2014

Ultraviolet photocarrier radiometry (UV-PCR) was used for the characterization of thin-film (nanolayer) intrinsic hydrogenated amorphous silicon (i-a-Si:H) on c-Si. The small absorption depth (approximately 10 nm at 355 nm laser excitation) leads to strong influence of the nanolayer parameters on the propagation and recombination of the photocarrier density wave (CDW) within the layer and the substrate. A theoretical PCR model including the presence of effective interface carrier traps was developed and used to evaluate the transport parameters of the substrate c-Si as well as those of the i-a-Si:H nanolayer. Unlike conventional optoelectronic characterization methods such as photoconductance, photovoltage, and photoluminescence, UV-PCR can be applied to more complete quantitative characterization of a-Si:H/c-Si heterojunction solar cells, including transport properties and defect structures. The quantitative results elucidate the strong effect of a front-surface passivating nanolayer on the transport properties of the entire structure as the result of effective a-Si:H/c-Si interface trap neutralization through occupation. A further dramatic improvement of those properties with the addition of a back-surface passivating nanolayer is observed and interpreted as the result of the interaction of the increased excess bulk CDW with, and more complete occupation and neutralization of, effective front interface traps.
Kinks, loops, and protein folding, with protein A as an example
Authors: Krokhotin Andrey , Liwo Adam , Maisuradze Gia G. , Niemi Antti J. , Scheraga Harold A.
Published: January 2014

The dynamics and energetics of formation of loops in the 46-residue N-terminal fragment of the B-domain of staphylococcal protein A has been studied. Numerical simulations have been performed using coarse-grained molecular dynamics with the united-residue (UNRES) force field. The results have been analyzed in terms of a kink (heteroclinic standing wave solution) of a generalized discrete nonlinear Schrödinger (DNLS) equation. In the case of proteins, the DNLS equation arises from a Cα-trace-based energy function. Three individual kink profiles were identified in the experimental three-α-helix structure of protein A, in the range of the Glu16-Asn29, Leu20-Asn29, and Gln33-Asn44 residues, respectively; these correspond to two loops in the native structure. UNRES simulations were started from the full right-handed α-helix to obtain a clear picture of kink formation, which would otherwise be blurred by helix formation. All three kinks emerged during coarse-grained simulations. It was found that the formation of each is accompanied by a local free energy increase; this is expressed as the change of UNRES energy which has the physical sense of the potential of mean force of a polypeptide chain. The increase is about 7 kcal/mol. This value can thus be considered as the free energy barrier to kink formation in full α-helical segments of polypeptide chains. During the simulations, the kinks emerge, disappear, propagate, and annihilate each other many times. It was found that the formation of a kink is initiated by an abrupt change in the orientation of a pair of consecutive side chains in the loop region. This resembles the formation of a Bloch wall along a spin chain, where the Cα backbone corresponds to the chain, and the amino acid side chains are interpreted as the spin variables. This observation suggests that nearest-neighbor side chain–side chain interactions are responsible for initiation of loop formation. It was also found that the individual kinks are reflected as clear peaks in the principal modes of the analyzed trajectory of protein A, the shapes of which resemble the directional derivatives of the kinks along the chain. These observations suggest that the kinks of the DNLS equation determine the functionally important motions of proteins.
Energy and exergy analysis of gas production from biomass intermittent gasification
Authors: Chen Zhao-Sheng , Wang Li-Qun
Published: January 2014

In this paper, thermochemical biomass intermittent gasification was performed in a fluidized-bed reactor with pure steam as gasification agent. A single fluidized bed two-stage gasification process, also known as intermittent gasification process, which involves the time segregated hybridization of coal-combustion and steam biomass-gasification stages in the same vessel, was employed. This research aims to explore the influences of gasification temperature (T) and steam-biomass ratio (S/B) on composition, energy and exergy distribution, as well as energy and exergy conversion efficiencies of the product gas (biomass-gas). Over the ranges of the test conditions used, the total concentration of H2 and CO in the biomass-gas varies between 57.9% and 80.8%, and the exergy and energy efficiencies of the biomass-gas are in the ranges of 54.61%–35.52% and 82.91%–60.78%, respectively. The results show that the chemical exergy values of the biomass-gas are 13.50 to 17.71 times as the corresponding physical exergy values, whereas the chemical energy values are 6.91 to 9.45 times as the corresponding physical energy values. The total energy values of the biomass-gas are higher than the corresponding exergy values, this result in higher energy efficiencies of the biomass-gas. Higher temperature contributes to higher CO and H2 content, physical and chemical energy values, physical and chemical exergy values, as well as energy and exergy efficiencies of the biomass-gas. The steam to biomass ratio is optimal in the intermediate levels for maximal energy and exergy efficiencies of the biomass-gas.
Temperature dependent band gap in PbX (X = S, Se, Te)
Authors: Gibbs Zachary M. , Kim Hyoungchul , Wang Heng , White Robert L. , Drymiotis Fivos , Kaviany Massoud , Jeffrey Snyder G.
Published: January 2014

PbTe is an important thermoelectric material for power generation applications due its high conversion efficiency and reliability. Its extraordinary thermoelectric performance is attributed to band convergence of the light L and heavy Σ bands. However, the temperature at which these bands converge is disputed. In this letter, we provide direct experimental evidence combined with ab initio calculations that confirm an increasing optical gap up to 673 K and predict a band convergence temperature of 700 K, much higher than previous measurements showing saturation and band convergence at 450 K.
Identical scaling behavior of saturated dynamic hysteresis in rhombohedral lead zirconate titanate bulk ceramics
Authors: Chen Xuefeng , Dong Xianlin , Zhou Zhiyong , Wang Junxia , Cao Fei , Wang Genshui , Zhang Hongling
Published: January 2014

The scaling behaviors of dynamic hysteresis were investigated in serial rhombohedral Nb-doped Pb(Zr1-xTix)O3 (PZT) bulk ceramics as a function of frequency (f) and field amplitude (E0). Three distinct regions were plotted including linear loops, minor loops, and saturated loops with an increase of E0. When the external fields were over ∼1.5 times of coercive field (Ec), the scaling relations of saturated loops for these PZT ceramics could be expressed with an identical form as hysteresis area ⟨A⟩ ∝ f 0.01E00.10, which indicated that similar ferroelectric systems with different compositions could display a uniform scaling law under high-E0 and low-f regions.
Use of Arrott plots to identify Néel temperature (TN) in metamagnetic Ni48Co6Mn26Al20 polycrystalline ribbons
Authors: Singh Rohit , Kumar Srivastava Saurabh , Nigam Arun K. , Khovaylo Vladimir V. , Varga Lajos K. , Chatterjee Ratnamala
Published: January 2014

(Ni48Co6)Mn26Al20 polycrystalline ribbons with B2 structure at room temperature are investigated. Considering the presence of competing magnetic interactions, Arrott-plot analysis gives TN ∼ 170 K. A broad ferromagnetic-paramagnetic transition (TC) is observed at ∼200 K. H-T phase-diagram is used to validate the presence of competing exchange interactions that persist till very close to TC. Based on Néel theory, a cluster model is used to explain the presence of ferromagnetic and antiferromagnetic clusters in the sample. Formation of ferromagnetic clusters can be understood in terms of positive exchange interactions among the Mn atoms that are neighboring Co atoms located at Ni sites.
Mechanical properties of structural amorphous steels: Intrinsic correlations, conflicts, and optimizing strategies
Authors: Liu Z. Q. , Zhang Z. F.
Published: January 2014

Amorphous steels have demonstrated superior properties and great potentials for structural applications since their emergence, yet it still remains unclear about how and why their mechanical properties are correlated with other factors and how to achieve intended properties by designing their compositions. Here, the intrinsic interdependences among the mechanical, thermal, and elastic properties of various amorphous steels are systematically elucidated and a general trade-off relation is exposed between the strength and ductility/toughness. Encouragingly, a breakthrough is achievable that the strength and ductility/toughness can be simultaneously improved by tuning the compositions. The composition dependences of the properties and alloying effects are further analyzed thoroughly and interpreted from the fundamental plastic flow and atomic bonding characters. Most importantly, systematic strategies are outlined for optimizing the mechanical properties of the amorphous steels. The study may help establish the intrinsic correlations among the compositions, atomic structures, and properties of the amorphous steels, and provide useful guidance for their alloy design and property optimization. Thus, it is believed to have implications for the development and applications of the structural amorphous steels.
Chemical-state-resolved depth profiles of Al/Pr0.7Ca0.3MnO3 stacked structures for application in resistive switching devices
Authors: Toyoda S. , Namiki T. , Sakai E. , Nakata K. , Oshima M. , Kumigashira H.
Published: January 2014

We have investigated the interfacial chemical reaction between Al metal electrodes and Pr0.7Ca0.3MnO3 perovskite oxides to shed light on the underlying mechanism of resistive switching phenomenon that can be utilized in random access memory devices. Depth profiles of the Al/Pr0.7Ca0.3MnO3 interface show the formation of Al2O3 layers resulting from redox reactions between Al metal and perovskite oxide. Angle-resolved Mn 2p core-level photoemission spectra, which are independent of the Al metal electrode thickness, show that a Mn-Al alloy is formed at the interface between Al metal and Al2O3 layers during the initial growth of the Al metal electrode. This suggests that the Mn impurity level in the band gap of Al2O3 layers plays an important role in resistive switching characteristics.
The application of the integral equation theory to study the hydrophobic interaction
Authors: Mohorič Tomaž , Urbic Tomaz , Hribar-Lee Barbara
Published: January 2014

The Wertheim's integral equation theory was tested against newly obtained Monte Carlo computer simulations to describe the potential of mean force between two hydrophobic particles. An excellent agreement was obtained between the theoretical and simulation results. Further, the Wertheim's integral equation theory with polymer Percus-Yevick closure qualitatively correctly (with respect to the experimental data) describes the solvation structure under conditions where the simulation results are difficult to obtain with good enough accuracy.
A two-coil mutual inductance technique to study matching effect in disordered NbN thin films
Authors: Kumar Sanjeev , Kumar Chandan , Jesudasan John , Bagwe Vivas , Raychaudhuri Pratap , Bose Sangita
Published: January 2014

Although matching effects in superconducting anti-dot arrays have been studied extensively through magneto-resistance oscillations, these investigations have been restricted to a very narrow temperature window close to the superconducting transition. Here, we report a “two coil” mutual inductance technique, which allows the study of this phenomenon deep in the superconducting state, through a direct measurement of the magnetic field variation of the shielding response. We demonstrate how this technique can be used to resolve outstanding issues on the origin of matching effects in superconducting thin films with periodic array of holes grown on anodized alumina membranes.
Effects of emission layer doping on the spatial distribution of charge and host recombination rate density in organic light emitting devices: A numerical study
Authors: Li Yanli , Zhou Maoqing , Zheng Tingcai , Yao Bo , Peng Yingquan
Published: January 2014

Based on drift-diffusion theory, a numerical model of the doping of a single energy level trap in the emission layer of an organic light emitting device (OLED) was developed, and the effects of doping of this single energy level trap on the distribution of the charge density, the recombination rate density, and the electric field in single- and double-layer OLEDs were studied numerically. The results show that by doping the n-type (p-type) emission layer with single energy electron (hole) traps, the distribution of the recombination rate density can be tuned and shifted, which is useful for improvement of the device performance by reduced electrode quenching or for realization of desirable special functions, e.g., emission spectrum tuning in multiple dye-doped white OLEDs.
Magnetic properties of two-dimensional nanodots: Ground state and phase transition
Authors: Kasperski Maciej , Puszkarski Henryk , Hoang Danh-Tai , Diep H. T.
Published: January 2014

We study the effect of perpendicular single-ion anisotropy, \documentclass[12pt]{minimal}\begin{document}$-As_{\text{z}}^2$\end{document}−Asz2, on the ground-state structure and finite-temperature properties of a two-dimensional magnetic nanodot in presence of a dipolar interaction of strength D. By a simulated annealing Monte Carlo method, we show that in the ground state a vortex core perpendicular to the nanodot plane emerges already in the range of moderate anisotropy values above a certain threshold level. In the giant-anisotropy regime the vortex structure is superseded by a stripe domain structure with stripes of alternate domains perpendicular to the surface of the sample. We have also observed an intermediate stage between the vortex and stripe structures, with satellite regions of tilted nonzero perpendicular magnetization around the core. At finite temperatures, at small A, we show by Monte Carlo simulations that there is a transition from the the in-plane vortex phase to the disordered phase characterized by a peak in the specific heat and the vanishing vortex order parameter. At stronger A, we observe a discontinuous transition with a large latent heat from the in-plane vortex phase to perpendicular stripe ordering phase before a total disordering at higher temperatures. In the regime of perpendicular stripe domains, namely with giant A, there is no phase transition at finite T: the stripe domains are progressively disordered with increasing T. Finite-size effects are shown and discussed.
Studies on the chemical synthesis and characterization of lead oxide nanoparticles with different organic capping agents
Authors: Arulmozhi K. T. , Mythili N.
Published: January 2014

Lead oxide (PbO) nanoparticles were chemically synthesized using Lead (II) acetate as precursor. The effects of organic capping agents such as Oleic acid, Ethylene Diamine Tetra Acetic acid (EDTA) and Cetryl Tri Methyl Butoxide (CTAB) on the size and morphology of the nanoparticles were studied. Characterization techniques such as X-ray diffraction (XRD), Fourier Transform-Infrared spectroscopy (FT-IR), Photoluminescence (PL) Field Emission Scanning Electron Microscopy (FE-SEM), Energy Dispersive Spectroscopy (EDS) and Transmission Electron Microscopy (TEM) were used to analyse the prepared nanoparticles for their physical, structural and optical properties. The characterization studies reveal that the synthesized PbO nanoparticles had well defined crystalline structure and sizes in the range of 25 nm to 36 nm for capping agents used and 40 nm for pure PbO nanoparticles.
Anharmonic behavior and structural phase transition in Yb2O3
Authors: Pandey Sugandha Dogra , Samanta K. , Singh Jasveer , Sharma Nita Dilawar , Bandyopadhyay A. K.
Published: January 2014

The investigation of structural phase transition and anharmonic behavior of Yb2O3 has been carried out by high-pressure and temperature dependent Raman scattering studies respectively. In situ Raman studies under high pressure were carried out in a diamond anvil cell at room temperature which indicate a structural transition from cubic to hexagonal phase at and above 20.6 GPa. In the decompression cycle, Yb2O3 retained its high pressure phase. We have observed a Stark line in the Raman spectra at 337.5 cm−1 which arises from the electronic transition between 2F5/2 and 2F7/2 multiplates of Yb3+ (4f13) levels. These were followed by temperature dependent Raman studies in the range of 80–440 K, which show an unusual mode hardening with increasing temperature. The hardening of the most dominant mode (Tg + Ag) was analyzed in light of the theory of anharmonic phonon-phonon interaction and thermal expansion of the lattice. Using the mode Grüneisen parameter obtained from high pressure Raman measurements; we have calculated total anharmonicity of the Tg + Ag mode from the temperature dependent Raman data.
Influence of thermal agitation on the electric field induced precessional magnetization reversal with perpendicular easy axis
Authors: Cheng Hongguang , Deng Ning
Published: January 2014

We investigated the influence of thermal agitation on the electric field induced precessional magnetization switching probability with perpendicular easy axis by solving the Fokker-Planck equation numerically with finite difference method. The calculated results show that the thermal agitation during the reversal process crucially influences the switching probability. The switching probability can be achieved is only determined by the thermal stability factor Δ of the free layer, it is independent on the device dimension, which is important for the high density device application. Ultra-low error rate down to the order of 10−9 can be achieved for the device of thermal stability factor Δ of 40. Low damping factor α material should be used for the free layer for high reliability device applications. These results exhibit potential of electric field induced precessional magnetization switching with perpendicular easy axis for ultra-low power, high speed and high density magnetic random access memory (MRAM) applications.
Pressure driven spin crossover and isostructural phase transition in LaFeO3
Authors: Javaid Saqib , Javed Akhtar M. , Ahmad Irfan , Younas Muhammad , Shah Shafqat H. , Ahmad Iftikhar
Published: January 2014

We have studied the behavior of LaFeO3 under pressure (P) using density functional theory (DFT) and atomistic simulations. Ground state structural properties of LaFeO3 are correctly described by atomistic simulations. The effect of high pressure shows that there is an isotropic compression up to 100 GPa. However, DFT calculations show that within pressure range 0 < P < 32.4 GPa, LaFeO3 retains its ground state electronic structure. On the other hand, at P ∼32.4 GPa high to low spin magnetic phase transition is observed, which is accompanied by 6.9% volume collapse of LaFeO3 unit cell, while retaining the ground state orthorhombic crystal structure, i.e., isostructural phase transition. Furthermore, the band gap is closed leading insulator to metal transition. This differing behavior observed by the two techniques can be attributed to the omission of magnetic effects in static simulations. The simultaneous magnetic, electrical, and structural (volume collapse) phase transitions of LaFeO3 under compression as revealed by DFT calculations corroborate experimental findings. From these results, we can elaborate the mechanism of phase transition in LaFeO3: increasing crystal field induces a high spin to low spin transition, which in turn drives the electrical transitions and volume collapse.
Anomalous Hall effect in magnetic disordered alloys: Effects of spin orbital coupling
Authors: Ma L. , He P. , Gao W. B. , Zhou S. M. , Shi Z. , Miao J. , Jiang Y.
Published: January 2014

For disordered ternary Fe0.5(Pd1−xPtx)0.5 alloy films, the anomalous Hall effect obeys the conventional scaling law ρAH=aρxx+bρxx2 with the longitudinal resistivity ρxx and anomalous Hall resistivity ρAH. Contributed by the intrinsic term and the extrinsic side-jump one, the scattering-independent anomalous Hall conductivity b increases with increasing Pt/Pd concentration. In contrast, the skew scattering parameter a is mainly influenced by the residual resistivity. The present results will facilitate the theoretical studies of the anomalous Hall effect in magnetic disordered alloys.
Strain controlled metal-insulator transition in epitaxial NdNiO3 thin films
Authors: Xiang P.-H. , Zhong N. , Duan C.-G. , Tang X. D. , Hu Z. G. , Yang P. X. , Zhu Z. Q. , Chu J. H.
Published: January 2014

We have fabricated epitaxial thin films of NdNiO3 (NNO) on various single crystal substrates. The transport properties of NNO films are very sensitive to substrate-controlled epitaxial strain. As the strain varies from tensile to compressive, the Mott metal-insulator transition of NNO films shifts to low temperatures. Under a larger compressive strain, the film on LaSrAlO4 substrate exhibits a practically metallic transport characteristic. We have found that the conductivities of NNO films at low temperatures follow Mott's variable range hopping mechanism rather than thermal activation model and the epitaxial strain has a strong effect on Mott's parameters of NNO films. These findings demonstrate that the electronic transport of NNO thin films can be tuned by the epitaxial strain for next-generation perovskite-based microelectronic devices.
Self-focusing and stimulated Brillouin back-scattering of a long intense laser pulse in a finite temperature relativistic plasma
Authors: Niknam A. R. , Barzegar S. , Hashemzadeh M.
Published: January 2014

The nonlinear dynamics of electromagnetic waves propagating through a plasma considering the effects of relativistic mass and ponderomotive nonlinearities is investigated. The modified electron density distribution, the dispersion relation, and the spatial profiles of electromagnetic wave amplitude in the plasma are obtained. It is shown that the cut-off frequency decreases, and there is an intensity range in which the ponderomotive self-focusing takes place. In the upper limit of this range, the laser beam is defocused due to the relativistic ponderomotive force. In addition, the stability of electromagnetic waves to stimulated Brillouin scattering is studied, and the backscattered wave resulting from decay of high power electromagnetic beam is resolved in relativistic regime. The study of effects of electron density and temperature on the growth rate of backscattered wave has been shown that by increasing these effects, the growth rate of instability increases.
Intermittent and sustained periodic windows in networked chaotic Rössler oscillators
Authors: He Zhiwei , Sun Yong , Zhan Meng
Published: January 2014

Route to chaos (or periodicity) in dynamical systems is one of fundamental problems. Here, dynamical behaviors of coupled chaotic Rössler oscillators on complex networks are investigated and two different types of periodic windows with the variation of coupling strength are found. Under a moderate coupling, the periodic window is intermittent, and the attractors within the window extremely sensitively depend on the initial conditions, coupling parameter, and topology of the network. Therefore, after adding or removing one edge of network, the periodic attractor can be destroyed and substituted by a chaotic one, or vice versa. In contrast, under an extremely weak coupling, another type of periodic window appears, which insensitively depends on the initial conditions, coupling parameter, and network. It is sustained and unchanged for different types of network structure. It is also found that the phase differences of the oscillators are almost discrete and randomly distributed except that directly linked oscillators more likely have different phases. These dynamical behaviors have also been generally observed in other networked chaotic oscillators.
Low temperature crystal structure and magnetic properties of RAl2
Authors: Pathak Arjun K. , Paudyal D. , Gschneidner K. A. , Pecharsky V. K.
Published: January 2014

Low temperature crystal structure and magnetic properties of RAl2 (R = Pr and Nd) have been studied using temperature dependent powder x-ray diffraction, magnetization, and heat capacity measurements. Unlike PrAl2, NdAl2 retains cubic MgCu2-type structure from room temperature down to 5 K, which is also confirmed from first principles electronic structure calculations. The magnetization measurements show both PrAl2 and NdAl2 order ferromagnetically at TC = 32 K and 77 K, respectively. However, the magnetization measurements show the former is a hard ferromagnet compared to the latter which is a soft ferromagnetic material. The magnetic entropy change obtained from heat capacity measurements at ΔH = 30 kOe for PrAl2 and NdAl2 are 3.15 J mol−1 K−1 and 1.18 J mol−1 K−1, respectively.
Performance optimization of apodized FBG-based temperature sensors in single and quasi-distributed DWDM systems with new and different apodization profiles
Authors: Mohammed Nazmi A. , Ali Taha A. , Aly Moustafa H.
Published: January 2014

In this work, different FBG temperature sensors are designed and evaluated with various apodization profiles. Evaluation is done under a wide range of controlling design parameters like sensor length and refractive index modulation amplitude, targeting a remarkable temperature sensing performance. New judgment techniques are introduced such as apodization window roll-off rate, asymptotic sidelobe (SL) decay level, number of SLs, and average SL level (SLav). Evaluation techniques like reflectivity, Full width at Half Maximum (FWHM), and Sidelobe Suppression Ratio (SLSR) are also used. A “New” apodization function is proposed, which achieves better performance like asymptotic decay of 18.4 dB/nm, high SLSR of 60 dB, high channel isolation of 57.9 dB, and narrow FWHM less than 0.15 nm. For a single accurate temperature sensor measurement in extensive noisy environment, optimum results are obtained by the Nuttall apodization profile and the new apodization function, which have remarkable SLSR. For a quasi-distributed FBG temperature sensor the Barthann and the new apodization profiles obtain optimum results. Barthann achieves a high asymptotic decay of 40 dB/nm, a narrow FWHM (less than 25 GHZ), a very low SLav of −45.3 dB, high isolation of 44.6 dB, and a high SLSR of 35 dB. The new apodization function achieves narrow FWHM of 0.177 nm, very low SL of −60.1, very low SLav of −63.6 dB, and very high SLSR of −57.7 dB. A study is performed on including an unapodized sensor among apodized sensors in a quasi-distributed sensing system. Finally, an isolation examination is performed on all the discussed apodizations and a linear relation between temperature and the Bragg wavelength shift is observed experimentally and matched with the simulated results.
Finite-temperature second-order many-body perturbation and Hartree–Fock theories for one-dimensional solids: An application to Peierls and charge-density-wave transitions in conjugated polymers
Authors: He Xiao , Ryu Shinsei , Hirata So
Published: January 2014

Finite-temperature extensions of ab initio Gaussian-basis-set spin-restricted Hartree–Fock (HF) and second-order many-body perturbation (MP2) theories are implemented for infinitely extended, periodic, one-dimensional solids and applied to the Peierls and charge-density-wave (CDW) transitions in polyyne and all-trans polyacetylene. The HF theory predicts insulating CDW ground states for both systems in their equidistant structures at low temperatures. In the same structures, they turn metallic at high temperatures. Starting from the “dimerized” low-temperature equilibrium structures, the systems need even higher temperatures to undergo a Peierls transition, which is accompanied by geometric as well as electronic distortions from dimerized to non-dimerized forms. The conventional finite-temperature MP2 theory shows a sign of divergence in any phase at any nonzero temperature and is useless. The renormalized finite-temperature MP2 (MP2R) theory is divergent only near metallic electronic structures, but is well behaved elsewhere. MP2R also predicts CDW and Peierls transitions occurring at two different temperatures. The effect of electron correlation is primarily to lower the Peierls transition temperature.
A small-signal generator based on a multi-layer graphene/molybdenum disulfide heterojunction
Authors: Tan Zhen , Tian He , Feng Tingting , Zhao Lianfeng , Xie Dan , Yang Yi , Xiao Lei , Wang Jing , Ren Tian-Ling , Xu Jun
Published: January 2014

In this work, we fabricate a heterojunction small-signal generator (HSSG) based on a graphene-molybdenum disulfide (MoS2) heterojunction. The HSSG is fundamentally different from any analog device developed previously. The HSSG is composed of two quasi-2D heterojunctions and has three terminals named injector (I), recombinator (R), and generator (G). MoS2 serves as I and G, and graphene works as R in the HSSG. The scale coefficient (β = IG/IR) of the HSSG is 1.14 × 10−4 (VIG, IR = 0.2 V) to 1.95 × 10−4 (VIG, IR = 1 V). The current generated from G could be as low as pA scale, which reveals the good performance of the HSSG.
Magnetotransport measurements of current induced effective fields in Ta/CoFeB/MgO
Authors: Zhang Chaoliang , Yamanouchi Michihiko , Sato Hideo , Fukami Shunsuke , Ikeda Shoji , Matsukura Fumihiro , Ohno Hideo
Published: January 2014

We evaluate current-induced effective magnetic fields in perpendicularly magnetized Ta/CoFeB/MgO structures from the external magnetic field angle dependence of the Hall resistance. We confirm the presence of two components of effective fields. The dependence of their magnitudes on Ta thickness implies that both components are related to the spin current in Ta layer generated by the spin Hall effect.
Core-shell nanowire serves as heat cable
Authors: Liu Yue-Yang , Zhou Wu-Xing , Tang Li-Ming , Chen Ke-Qiu
Published: January 2014

To analyze the thermal transport properties in core-shell nanowires, we calculate systematically the distributions of heat flux in InAs/GaAs and GaAs/InAs core-shell nanowires by using nonequilibrium molecular dynamics simulations. The results show that for InAs/GaAs core-shell nanowires, the heat current tends to transport in the shell, while for GaAs/InAs core-shell nanowires the heat current tends to transport through the core. Moreover, a simple equation is presented to describe the relationship of the thermal conductance among the core, the tubular shell, and core-shell nanowire. It is suggested that the core-shell nanowires can be served as heat cable.
Ferri-to-ferro-magnetic and ferro-to-para-magnetic transitions in Ni48Co2Mn35In13Ga2 Heusler alloy
Authors: Ovichi Maryam , Ghahremani Mohammadreza , Della Torre Edward , Bennett Lawrence H. , Johnson Francis , Srivastava Vijay
Published: January 2014

Heusler alloys feature both conventional and inverse magnetocaloric effects near room temperature as they undergo two different transitions. In this paper, new data are presented and analyzed and a new mechanism to explain the complex hysteretic behavior of a Ni48Co2Mn35In13Ga2 Heusler alloy is developed. This mechanism explains isothermal loops near room temperature. The various descriptions and classifications of these transitions, however, is not critical to this analysis.
Helium bubble bursting in tungsten
Authors: Sefta Faiza , Juslin Niklas , Wirth Brian D.
Published: January 2014

Molecular dynamics simulations have been used to systematically study the pressure evolution and bursting behavior of sub-surface helium bubbles and the resulting tungsten surface morphology. This study specifically investigates how bubble shape and size, temperature, tungsten surface orientation, and ligament thickness above the bubble influence bubble stability and surface evolution. The tungsten surface is roughened by a combination of adatom “islands,” craters, and pinholes. The present study provides insight into the mechanisms and conditions leading to various tungsten topology changes, which we believe are the initial stages of surface evolution leading to the formation of nanoscale fuzz.
Coupling of magnetic field and lattice strain and its impact on electronic phase separation in La0.335Pr0.335Ca0.33MnO3/ferroelectric crystal heterostructures
Authors: Zheng M. , Li X. Y. , Yang M. M. , Zhu Q. X. , Wang Y. , Li X. M. , Shi X. , Chan H. L. W. , Li X. G. , Luo H. S. , Zheng R. K.
Published: January 2014

Phase-separated La0.335Pr0.335Ca0.33MnO3 films were epitaxially grown on (001)- and (111)-oriented ferroelectric single-crystal substrates. Upon poling along the [001] or [111] direction, dramatic decrease in resistance, up to 99.98%, and complete melting of the charge-ordered phase were observed, caused by poling-induced strain rather than accumulation of electrostatic charge at interface. Such poling-induced strain effects can be effectively tuned by a magnetic field and mediated by electronic phase separation. In particular, our findings show that the evolution of the strength of electronic phase separation against temperature and magnetic field can be determined by measuring the strain-tunability of resistance [(ΔR/R)strain] under magnetic fields.
Steady-state domain wall motion driven by adiabatic spin-transfer torque with assistance of microwave field
Authors: Wang Xi-guang , Guo Guang-hua , Nie Yao-zhuang , Wang D. , Xia Qing-lin , Tang Wei , Zeng Zhong-ming
Published: January 2014

We have studied the current-induced displacement of a 180° Bloch wall by means of micromagnetic simulation and analytical approach. It is found that the adiabatic spin-transfer torque can sustain a steady-state domain wall (DW) motion in the direction opposite to that of the electron flow without Walker Breakdown when a transverse microwave field is applied. This kind of motion is very sensitive to the microwave frequency and can be resonantly enhanced by exciting the domain wall thickness oscillation mode. A one-dimensional analytical model was established to account for the microwave-assisted wall motion. These findings may be helpful for reducing the critical spin-polarized current density and designing DW-based spintronic devices.
Phase transitions in heated Sr2MgTeO6 double perovskite oxide probed by X-ray diffraction and Raman spectroscopy
Authors: Manoun Bouchaib , Tamraoui Y. , Lazor P. , Yang Wenge
Published: January 2014

Double-perovskite oxide Sr2MgTeO6 has been synthetized, and its crystal structure was probed by the technique of X-ray diffraction at room temperature. The structure is monoclinic, space group I2/m. Temperature-induced phase transitions in this compound were investigated by Raman spectroscopy up to 550 °C. Two low-wavenumber modes corresponding to external lattice vibrations merge at temperature of around 100 °C, indicating a phase transition from the monoclinic (I2/m) to the tetragonal (I4/m) structure. At 300 °C, changes in the slopes of temperature dependencies of external and O–Te–O bending modes are detected and interpreted as a second phase transition from the tetragonal (I4/m) to the cubic (Fm-3m) structure.