bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantitative Models for Efficient Cloning of Different Vectors with Various Clone sites
Authors: Zhang Gang , Tandon Anurag
Published: March 2012

We developed an efficient "Combinatorial Strategy” for cloning different vectors with various clone sites. 1) Using originally existed clone sites from circular vectors to prepare the inserts, and if no appropriate sites are available, performing SDM to create compatible sites, to achieve maximal correct digestion of the inserts. 2) Different vectors were digested with various restriction endonucleases, and then dephosphorylated with CIP. 3) Top10 competent cells were used for transformation to increase the transformant colonies. Our results showed that, when blunt sites, or a Xba I site was adopted for ligation, the percentages of positive clones were about 50%. Whereas, when different sites, including one blunt and another Pst I sites, Not I and Xho I sites, the percentages of positive clones were nearly 100%. Using this strategy, most vectors could be successfully cloned through “one ligation, one transformation, three to five minipreps."
Facile synthesis and magnetic property of iron oxide/MCM-41 mesoporous silica nanospheres for targeted drug delivery
Authors: Yu Le-Le , Bi Hong
Published: March 2012

Iron oxide/MCM-41 hybrid nanospheres (designated as MMSN) with a large surface area of 1334 m2/g and a uniform diameter of 85 nm have been synthesized via a facile sol-gel route. Transmission electron microscopy (TEM) imagery shows many ultra-small iron oxide nanoparticles evenly distributed inside the mesoporous silica nanospheres. High-resolution TEM image with a corresponding electron diffraction spectrum and Fourier transform infrared spectra confirm the formation of iron oxide nanoparticles while the ordered mesoporous silica structure is maintained. The MMSN present a ferromagnetic property that ensures them a fast response to an applied magnetic field. Moreover, they are proven to be beneficial for loading an anticancer drug—doxorubicin hydrochloride (DOX), because a considerable loading content of 6.0% and a high entrapment efficiency of 90.5% have been achieved. Most notably, these DOX-loaded MMSN display not only a pH-variable but also a magnetic field-controllable drug release behavior. Further, after 48 h co-incubation, the MMSN did not show any significant cytotoxicity against human HepG2 cells even at a high concentration of 250 μg/mL. These results have demonstrated that the prepared MMSN may be potential drug carriers for both magnetic targeting and stimuli-responsive controlled release.
Self-assembly of Fe nanocluster arrays on templated surfaces
Authors: Lübben Olaf , Krasnikov Sergey A. , Preobrajenski Alexei B. , Murphy Barry E. , Bozhko Sergey I. , Arora Sunil K. , Shvets Igor V.
Published: March 2012

The growth of Fe nanoclusters on the Ge(001) and MoO2/Mo(110) surfaces has been studied using low-temperature scanning tunneling microscopy (STM) and X-ray magnetic circular dichroism (XMCD). STM results indicate that at low coverage Fe atoms self-assemble on both surfaces into well-separated nanoclusters, which nucleate at equivalent surface sites. Their size, shape, and the observed spatial separation are dictated by the substrate and depend on preparation conditions. Annealing the Fe nanoclusters on Ge(001) at 420 K leads to the formation of linear nanocluster arrays, which follow the Ge dimer rows of the substrate, due to cluster mobility at such temperature. In turn, linear Fe nanocluster arrays are formed on the MoO2/Mo(110) surface at room temperature at a surface coverage greater than 0.5 monolayer. This is due to the more pronounced row pattern of the MoO2/Mo(110) surface compared to Ge(001). These nanocluster arrays follow the direction of the oxide rows of the strained MoO2/Mo(110) surface. The Fe nanoclusters formed on both surfaces show a superparamagnetic behavior as measured by XMCD.
Manipulation of magnetization reversal of Ni81Fe19 nanoellipse arrays by tuning the shape anisotropy and the magnetostatic interactions
Authors: Wang Y. , Shi W. H. , Wei H. X. , Atkinson D. , Zhang B. S. , Han X. F.
Published: March 2012

Two series of highly ordered two-dimensional arrays of Ni81Fe19 nanoellipses were nanofabaricated with different aspect ratios, R, and element separations, S, to investigate the influence of the self-demagnetization and the magnetostatic interaction upon the magnetization reversal. For nanostructures with low shape anisotropy, an additional magnetic easy axis was induced orthogonal to the shape-induced easy axis by reducing the separations along both axes. For the structures with larger shape anisotropy, the switching field distribution/coercivity (SFD/Hc) was reduced, and for the array with the smallest separations (20 nm and 35 nm along the long and short axes, respectively), coherent rotation of the whole array occurred. The magnitude of both the shape anisotropy and a configurational anisotropy induced by the magnetostatic interactions have been estimated. These results provide some useful information for the design of potential magnetic nanodot logic and for high-density magnetic random access memory.
Effects of Cu doping on the magnetism of CeO2 nanoparticles
Authors: Seehra M. S. , Suri S. , Singh V.
Published: March 2012

Magnetic properties of 5.5 nm CuxCe1−xO2 nanoparticles (NPs) with change in Cu doping (x = 0, 2.5, 5.0, 7.5, and 10%) are investigated to examine the controversial issue of room temperature ferromagnetism (FM) in this system. The data of magnetization M versus magnetic field H fit the equation: M = Mf + χpH with Mf (χp) being the FM (paramagnetic) part. The observed FM saturation magnetization MS = 8.5 × 10−5 emu/g for x = 0 is suggested to be due to Fe impurities detected in the samples at 20 ppm level. However, in the doped samples, both χp and MS increase nearly linearly with x with the FM magnetic moment μS ≈ 3 × 10−4μB/Cu atom. This magnitude of μS, although considerably smaller than μS ≈ 1 μB/Cu atom reported in thin films of CuxCe1−xO2, is argued to be an intrinsic effect due to Cu doping. Electron magnetic resonance studies show lines due to Cu2+ ions in the doped samples.
Size effect of Fe nanoparticles on the high-frequency dynamics of highly dense self-organized assemblies
Authors: Kura Hiroaki , Ogawa Tomoyuki , Tate Ryu , Hata Kotaro , Takahashi Migaku
Published: March 2012

Molded Fe nanoparticle (NP) assemblies 4 mm × 8 mm × 0.3 mmt in size were fabricated by a uniaxial press from Fe NPs 3 nm to 22 nm in diameter, and their high-frequency dynamics and static properties were investigated. The freezing temperature of the magnetic moment of the assemblies increased from 25 K to over 400 K with an increasing Fe NP diameter, and for particles larger than 8.1 nm, the Fe NP assemblies showed ferromagnetic behavior even at room temperature. For particles smaller than 8.1 nm, on the contrary, the assemblies showed superparamagnetism at room temperature. From the complex magnetic susceptibility spectra of the assemblies normalized by the volume of the Fe NPs, χ′Fe, and χ″Fe, the minimum ferromagnetic resonance frequency, fr (106 MHz), and maximum χ′Fe at 1 MHz (123) were obtained at 8.1 nm. The fr of the Fe NP assembly increased because of an enhanced magnetostatic interaction field between NPs larger than 8.1 nm. For NPs smaller than 8.1 nm, in the superparamagnetic phase, the thermal magnetic field increasingly dominated the effective magnetic field with a decreasing NP diameter. This indicates the existence of a critical diameter for the thermal fluctuation of the moment, which competes with the magnetostatic interactions between the Fe nanoparticles.
Time-resolved measurements of field-driven domain wall motion in a submicron strip with perpendicular magnetic anisotropy
Authors: Emori Satoru , Bono David C. , Beach Geoffrey S. D.
Published: March 2012

The motion of domain walls (DWs), driven by magnetic field and spanning almost eight orders of magnitude in velocity, has been studied in a submicron-wide strip of Co/Pt with perpendicular magnetic anisotropy. A scanning magneto-optic Kerr effect (MOKE) system is used to conduct time-resolved measurements of DW dynamics in both the thermally activated and viscous flow regimes. MOKE signal transients in the thermally activated regime reveal distributions of stochastic DW propagation events. Transients in the viscous flow regime show deterministic DW motion with velocities ∼20 m/s. The transition between the two dynamic regimes is observed as the relationship between the DW velocity and the driving magnetic field changes from exponential to linear.
Structure and magnetic properties of Pr10(Fe,Co,Ni)84B6 nanocomposite alloys
Authors: Daniil M. , Minter L. , Willard M. A.
Published: March 2012

Nanocomposite ribbons with composition Pr10(Fe1-2xCoxNix)84B6 (x = 0.0, 0.05, 0.10, and 0.15) were prepared by melt-spinning. All melt-spun ribbons are composed of a fine mixture of 2:14:1 and bcc-(Fe,Co,Ni) phases with an additional soft phase (Pr2Fe23B3-based) for x = 0.10 and 0.15. The room temperature coercivity decreases from 7.4 to 2 kOe with Co and Ni substitutions likely due to the decrease of the magnetocrystalline anisotropy of the 2:14:1 phase and the increase of the amount of the soft phases. Saturation magnetization increases slightly while remanence and (BH)max show a slight decrease for x up to 0.10 and then they drop quickly. The intergranular exchange coupling is strong for x ≤ 0.10 but it is reduced significantly for x = 0.15 due to the substantial increase of the volume and size of the soft phases. Thermomagnetic measurements showed a significant increase of the Curie temperature of the 2:14:1 phase from 290 °C for x = 0 to 485 °C for x = 0.15. High temperature hysteresis loop measurements showed improved temperature dependence of remanence and coercivity for x = 0.05.
Spin transfer switching of current-perpendicular-to-plane giant magnetoresistance with various Gd-Fe free-layer compositions
Authors: Aoshima K. , Hashimoto Y. , Funabashi N. , Machida K. , Kuga K. , Kikuchi H. , Shimidzu N. , Ishibashi T.
Published: March 2012

We have investigated the spin transfer switching (STS) properties of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices with various compositions of Gd-Fe free-layers for magneto-optical light modulator applications, whereby iron (Fe) concentrations are varied from 72.5 to 80.3 at. %. Switching current densities decreased dramatically with increases in Fe concentration within the Gd-Fe compound. The switching current density (Jc0) for a device with composition Gd Fe80.3 at.% was 20.5 MA/cm2, which is more than eight times smaller than that for devices with composition Gd Fe72.5 at. %. This reduction in switching current can be attributed to a decrease in the effective perpendicular anisotropy in Fe-richer Gd-Fe compounds.
Single-core fluxgate gradiometer with simultaneous gradient and homogeneous feedback operation
Authors: Janosek Michal , Ripka Pavel , Ludwig Frank , Schilling Meinhard
Published: March 2012

A novel configuration of a single-core gradiometer, utilizing both homogeneous and gradient feedback operation, is presented. The fluxgate gradiometer comprises of a standard pick-up/feedback coil and an additional gradient pickup/feedback coil with two separate electronic blocks. The 40-mm-long gradient coil is concentric and coaxial with the homogeneous pickup/feedback coil: the gradient coil assembly was slipped over an already existing race-track fluxgate sensor. The gradient coil works as a pick-up coil and it also generates the compensating field which well approximates a first-order gradient field with zero spatial-mean value. Together with the compensating field from the homogeneous feedback coil, it is thus always possible to measure in two independent feedback loops the homogeneous and gradient field components. The 1/f gradient noise is 4 nT/m/√Hz @ 1 Hz, and it can be further improved by separating the gradient feedback and compensating coil.
Epitaxial Fe/MgO/Ge spin-photodiodes for integrated detection of light helicity at room temperature
Authors: Rinaldi C. , Cantoni M. , Petti D. , Bertacco R.
Published: March 2012

We report on spin-photodiodes based on fully epitaxial Fe/MgO/Ge(001) heterostructures for the room temperature integrated detection of light helicity at a wavelength of 1300 nm. The degree of circular polarization of light is related to the spin direction of photo-excited carriers in Ge that are filtered by the MgO barrier, with probabilities differing for the two spin channels. Spin-detection experiments are performed by illuminating spin-photodiodes using left or right circularly polarized light under the application of a magnetic field parallel to the light direction and measuring the current crossing the device. We found that the maximum percentage variation of the photocurrent due to full reversal of the light helicity is on the order of 5.9% at room temperature. This variation can be attributed in part to the magnetic circular dichroism of Fe (0.8%) but mainly to the spin filtering of photo-generated carriers across the MgO barrier.
Phase transformation of alumina-coated FePt nanoparticles
Authors: Johnston-Peck Aaron C. , Tracy Joseph B.
Published: March 2012

Monolayers of FePt nanoparticles (NPs) were coated with 10 nm-thick layers of Al2O3 by pulsed laser deposition (PLD). The Al2O3 coating prevents diffusion and stabilizes the NP monolayer against sintering. Annealing converts the FePt NPs from an alloy phase (A1) into intermetallic phases. Atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) shows that Pt-rich A1 Fe29Pt71 NPs convert into the FePt3 L12 intermetallic phase upon annealing and develop faceted morphologies. HAADF-STEM also reveals the presence of structural defects and Pt-rich shells. These results, in conjunction with prior studies of Al2O3 deposition by atomic layer deposition (ALD), demonstrate that coating arrays of FePt NPs with thin Al2O3 films by PLD or ALD imparts thermal stability and provides comparable results.
Effect of different compositions of CoFeB spin polarizer on magnetoresistance and switching property of Co/Pd multilayers with perpendicular magnetic anisotropy
Authors: Tahmasebi T. , Piramanayagam S. N. , Sbiaa R. , Tan H. K. , Chong T. C.
Published: March 2012

The effect of magnetization of the spin-polarizing layer (SPL) on the magnetoresistance (MR) is investigated by using different compositions of CoxFe80−xB20 (x = 60, 40, and 20) as spin polarizers in Co/Pd multilayers-based pseudo-spin-valves with perpendicular magnetic anisotropy (PMA). In the case of as-deposited films, the giant magnetoresistance (GMR) was found to decrease with increase of CoFeB thickness from 5 Å to 10 Å. However, after post-annealing at 250 °C, 10 Å samples showed a peak of GMR in all compositions. Further increase in annealing temperature causes a reduction in GMR, which was found to be larger in samples with a lower saturation magnetization (Ms). As a function of SPL thickness, the GMR decreases in the case of as-deposited samples. However, post-annealed samples show a larger MR in case of thicker SPL. In the case of thick CoFeB SPL, the magnetization of the soft layer appears to be canted out of the plane. Among the three compositions investigated, the Co60Fe20B20 polarizer shows a stronger PMA due to its lower Ms, leading to the weaker demagnetization.
Structure and magnetic properties of Co-W clusters produced by inert gas condensation
Authors: Golkar Farhad , Kramer M. J. , Zhang Y. , McCallum R. W. , Skomski R. , Sellmyer D. J. , Shield J. E.
Published: March 2012

In this article, inert-gas condensation was used to synthesize Co-W clusters. The formation, structure, and magnetic properties of the clusters were investigated. Sub-10-nm clusters were obtained, and the structures and average sizes were strongly dependent on sputtering power. At low sputtering powers, the clusters were predominantly amorphous, while, at high sputtering power, the clusters were crystalline. X ray diffraction and transmission electron microscopy revealed clusters with hcp structure at high sputtering power. The magnetic properties were dependent on the sputtering power and temperature, with the highest coercivity of 810 Oe at 10 K for high sputtering power.
Electromagnetic characteristics of eccentric figure-eight coils for transcranial magnetic stimulation: A numerical study
Authors: Kato Takuya , Sekino Masaki , Matsuzaki Taiga , Nishikawa Atsushi , Saitoh Youichi , Ohsaki Hiroyuki
Published: March 2012

Repetitive transcranial magnetic stimulation (rTMS) is effective for treatment of several neurological and psychiatric diseases. We proposed an eccentric figure-eight coil, which induces strong eddy currents in the target brain tissue. In this study, numerical analyses were carried out to obtain magnetic field distribution of the eccentric figure-eight coil and eddy current in the brain. The analyses were performed with various coil design parameters, such as the outer and inner diameters and number of turns, to investigate the influence of these parameters on the coil characteristics. Increases in the inner diameter, outer diameter, and number of turns caused increases in the maximum eddy current densities. Coil inductance, working voltage, and heat generation also became higher with the increases in these design parameters. In order to develop a compact stimulator system for use at home, we need to obtain strong eddy current density, keeping the working voltage as low as possible. Our results show that it is effective to enlarge the outer diameter.
Nonlinear structures in a nonextensive electron-positron-ion magnetoplasma
Authors: El-Tantawy S. A. , Tribeche M. , Moslem W. M.
Published: March 2012

In outer space physics and astrophysics, there is a considerable amount of anomalous phenomena that support nonextensive particle distribution. This is because of the relevance of gravitational forces (which are long ranged) as well as to a variety of dynamical nonlinear aspects. Here, we investigate the nonlinear properties and the existence conditions of a plasma system consisting of ion fluid as well as electrons and positrons modelled by nonextensive distributions. The numerical analysis of the evolution equation shows that both subsonic and supersonic electrostatic waves may exist. These nonlinear waves admit positive and negative localized structures. The dependence of the latter on the plasma parameters is investigated numerically.
Nonlinear instabilities in two-dimensional hexagonal dust-lattice formed by paramagnetic particles
Authors: Farokhi B. , Shahmansouri M. , Abdikian A.
Published: March 2012

The propagation of linear and nonlinear transverse off-plane dust lattice waves in 2D hexagonal dusty plasma crystals including the paramagnetic effects (magnetized particles) is investigated theoretically. The linear dispersion characteristics of transverse dust lattice waves are studied, including the dispersion relations, group velocities, and an evolution equation for the modulated amplitude of the first harmonic. The dispersion relations show a negative group velocity of the wave for a weak paramagnetic effect and a positive group velocity for a strong paramagnetic effect. The modulated wave packet is described by a nonlinear Schrodinger equation. The modulational stability of carrier wave depends on the form of the electric field, the magnetic field, and the magnetic permeability of dust particles. We found that the wave is modulationally unstable for long wavelength limit and weak (or zero) paramagnetic effects.
Ion acoustic shock waves in presence of superthermal electrons and interaction of classical positron beam
Authors: Shah Asif , Mahmood S. , Haque Q.
Published: March 2012

The effect of positron beam on the ion-acoustic shock wave is studied. The plasma system under-consideration consists of inertial ions and superthermal electrons and the positron beam with classical streaming velocities. Kinematic viscosity of the ions component is responsible of the dissipation in plasma system. The Burgers equation has been derived by employing the reductive perturbation method. It is noticed that both the amplitude and steepness of the ion-acoustic shock wave accrue, as the spectral index of the superthermal electrons, and concentration of impinging positron beam are enhanced. However, the shock amplitude is found to diminish as the beam streaming velocity and temperature ratio of positrons to electrons are increased in the plasma system. Our findings may be helpful in the understanding of laboratory beam plasma interaction experiments as well as the space and astrophysical plasmas with effects of superthermality and interacting beam.
Diblock copolymer lamellae on sinusoidal and fractal surfaces
Authors: Ranjan Amit , Kulkarni Manish , Karim Alamgir , Sharma Ashutosh
Published: March 2012

A scaling analysis of equilibrium orientation of diblock copolymer molecules on fractal surfaces and a brief comparison with a particular experiment is presented in this paper. This work is motivated by a recent experimental finding that a diblock copolymer film of polystyrene-PMMA, when deposited on a rough substrate, can orient its lamellae from a parallel to a perpendicular configuration depending on the topographical characteristics of the substrate surface. It was found that the RMS height itself is not enough to effect the equilibrium configuration, but the fractal dimension of the surface is also important. In general, the orientation of lamellae is a function of the the power spectral density (PSD) curves of the underlying substrate surface. Assuming the diblock lamellae to behave like an Alexander-deGennes brush, we obtain the free energy expressions for this brush in both parallel and perpendicular orientations in various asymptotic regimes. Comparison of their free energy expressions predicts the equilibrium configuration. By examining the PSD curves and using our scaling results, we are able to qualitatively explain some aspects of the experimental observations regarding the equilibrium orientation of the diblock copolymer lamellae on rough surfaces.
The continuous spectrum of time-harmonic shear layers
Authors: Hack M. J. Philipp , Zaki Tamer A.
Published: March 2012

In boundary layers, eigenfunctions which belong to the continuous spectrum of the Orr–Sommerfeld equation have been established as a suitable basis for the expansion of general free-stream vortical disturbances. They are oscillatory in the free stream, and attenuate inside the boundary layer due to shear sheltering. The extent of modal penetration into the shear depends on the disturbance frequency and orientation, with the low-frequency, streamwise elongated modes being the most effective triggers of a high-amplitude streak-like response. The influence of introducing a time-periodic, spanwise mean flow on modal sheltering is investigated. The evaluation of the continuous modes in this case requires a Floquet expansion in the fundamental frequency of the base flow. Appropriate treatment of the free-stream boundedness condition is developed, and quantitative measures of modal sheltering are computed. The time-dependent, spanwise motion is shown to significantly enhance shear sheltering, and to change the optimal orientation of the continuous modes for penetration into the shear. An explanation is provided, in the limit of low frequency of the base flow, using asymptotic analyses.
Effect of substrate strain on critical dimensions of highly lattice mismatched defect-free nanorods
Authors: Sburlan Suzana , Nakano Aiichiro , Dapkus P. Daniel
Published: March 2012

Free-standing semiconductor nanorods grown on lattice-mismatched substrates have become of interest as novel devices, including solar cells. Here, we study the effect of substrate strain on the critical diameter for defect-free growth of free-standing nanorods using continuum elasticity theory. Unlike a thin film, nanorods release strain by relaxing in the lateral direction. We find that substrate relaxation is crucial for defect-free growth of InAs nanorods fabricated on GaAs. The calculation results show that below a critical diameter, the nanorods can grow to infinite length without emitting dislocations. Our findings lend support to the recent experimental efforts to grow defect-free arrays of nanorods providing improved absorption efficiency for next-generation solar cell technology.
Impurity transport due to electromagnetic drift wave turbulence
Authors: Moradi S. , Pusztai I. , Mollén A. , Fülöp T.
Published: March 2012

Finite β effects on impurity transport are studied through local linear gyrokinetic simulations with GYRO [J. Candy and E. Belli, General Atomics Report No. GA-A26818, 2011]; in particular, we investigate the parametric dependences of the impurity peaking factor (zero-flux density gradient) and the onset of the kinetic ballooning modes (KBMs). We find that electromagnetic effects even at low β can have significant impact on the impurity transport. The KBM instability threshold depends on the plasma parameters, particularly strongly on plasma shape. We have shown that magnetic geometry significantly influences the results, and the commonly used s-α model overestimates the KBM growth rates and ITG stabilization at high β. In the β range, where the KBM is the dominant instability the impurity peaking factor is strongly reduced, with very little dependence on β and the impurity charge.
Spatiotemporal temperature and density characterization of high-power atmospheric flashover discharges over inert poly(methyl methacrylate) and energetic pentaerythritol tetranitrate dielectric surfaces
Authors: Tang V. , Grant C. D. , McCarrick J. F. , Zaug J. M. , Glascoe E. A. , Wang H.
Published: March 2012

A flashover arc source that delivered up to 200 mJ on the 100s-of-ns time-scale to the arc and a user-selected dielectric surface was characterized for studying high-explosive kinetics under plasma conditions. The flashover was driven over thin pentaerythritol tetranitrate (PETN) and poly(methyl methacrylate) (PMMA) dielectric films and the resultant plasma was characterized in detail. Time- and space-resolved temperatures and electron densities of the plasma were obtained using atomic emission spectroscopy. The hydrodynamics of the plasma was captured through fast, visible imaging. Fourier transform infrared spectroscopy (FTIR) was used to characterize the films pre- and post-shot for any chemical alterations. Time-resolved infrared spectroscopy (TRIR) provided PETN depletion data during the plasma discharge. For both types of films, temperatures of 1.6–1.7 eV and electron densities of ∼7–8 × 1017/cm3 ∼570 ns after the start of the discharge were observed with temperatures of 0.6–0.7 eV persisting out to 15 μs. At 1.2 μs, spatial characterization showed flat temperature and density profiles of 1.1–1.3 eV and 2–2.8 × 1017/cm3 for PETN and PMMA films, respectively. Images of the plasma showed an expanding hot kernel starting from radii of ∼0.2 mm at ∼50 ns and reaching ∼1.1 mm at ∼600 ns. The thin films ablated or reacted several hundred nm of material in response to the discharge. First TRIR data showing the in situ reaction or depletion of PETN in response to the flashover arc were successfully obtained, and a 2-μs, 1/e decay constant was measured. Preliminary 1 D simulations compared reasonably well with the experimentally determined plasma radii and temperatures. These results complete the first steps to resolving arc-driven PETN reaction pathways and their associated kinetic rates using in situ spectroscopy techniques.
Study of the dynamical features of the austenite-martensite phase transition in the Ni50(Mn, 1%Fe)34In16 alloy using scanning Hall probe imaging
Authors: Chattopadhyay M. K. , Morrison K. , Dupas A. , Sharma V. K. , Sharath Chandra L. S. , Cohen L. F. , Roy S. B.
Published: March 2012

We have performed scanning Hall probe imaging experiments to study the martensite to austenite phase transition in the Ni50(Mn, 1%Fe)34In16 alloy as a function of temperature and magnetic field. We observe that the martensite and austenite phase regions are separated by a distinct interface. The relative growth of phase across the phase transition is associated with the movement of this interface. The movement of the interface becomes arrested at low temperature, which leads to the formation of a “magnetic glass” state in the alloy. The dynamics of the martensite to austenite phase transition in the Ni50(Mn, 1%Fe)34In16 alloy is found to be qualitatively different when the transition is field induced than what it is when the same transition is induced by temperature. While both nucleation and growth of the martensite phase is observed during the austenite to martensite phase transition in the alloy during cooling down, the martensite to austenite phase transition during warming up appears to be growth oriented. In contrast, both nucleation and growth of the product phases are observed during the field induced martensite to austenite phase transition both during increasing and decreasing field experiments. The physical reasons behind these different observations are explored.
Fluid-fluid and fluid-solid transitions in the Kern-Frenkel model from Barker-Henderson thermodynamic perturbation theory
Authors: Gögelein Christoph , Romano Flavio , Sciortino Francesco , Giacometti Achille
Published: March 2012

We study the Kern-Frenkel model for patchy colloids using Barker-Henderson second-order thermodynamic perturbation theory. The model describes a fluid where hard sphere particles are decorated with one patch, so that they interact via a square-well potential if they are sufficiently close one another, and if patches on each particle are properly aligned. Both the gas-liquid and fluid-solid phase coexistences are computed and contrasted against corresponding Monte Carlo simulations results. We find that the perturbation theory describes rather accurately numerical simulations all the way from a fully covered square-well potential down to the Janus limit (half coverage). In the region where numerical data are not available (from Janus to hard-spheres), the method provides estimates of the location of the critical lines that could serve as a guideline for further efficient numerical work at these low coverages. A comparison with other techniques, such as integral equation theory, highlights the important aspect of this methodology in the present context.
Detailed atomistic Monte Carlo simulations of a polymer melt on a solid surface and around a nanoparticle
Authors: Pandey Yogendra Narayan , Doxastakis Manolis
Published: March 2012

The molecular factors that govern interfacial interactions between a polymer melt and a solid surface remain largely unclear despite significant progress made in the last years. Simulations are increasingly employed to elucidate these features, however, equilibration and sampling with models of long macromolecules in such heterogeneous systems present significant challenges. In this study, we couple the application of preferential sampling techniques with connectivity-altering Monte Carlo algorithms to explore the configurational characteristics of a polyethylene melt in proximity to a surface and a highly curved nanoparticle. Designed algorithms allow efficient sampling at all length scales of large systems required to avoid finite-size effects. Using detailed atomistic models for the polymer and realistic structures for a silica surface and a fullerene, we find that at the extreme limit where particles are comparable to the polymer Kuhn segment length, curvature penalizes the formation of long train segments. As a result, an increased number of shorter contacts belonging to different chains are made competing with the anticipated decrease of the bound layer thickness with particle size if polymer adsorbed per unit area remained constant. For very small nanoparticles, formation of new train segments cannot compete with the overall reduction of adsorbance which is present irrespective of the enthalpic interactions; a result that demonstrates the need for an accurate description of polymer rigidity at these length scales.
Semiconductor laser simulations using non-equilibrium Green’s functions
Authors: Miloszewski Jacek M. , Wartak Marek S.
Published: March 2012

We present a novel method to simulate quantum well, edge-emitting semiconductor lasers in non-equilibrium steady-state. Our method is based on non-equilibrium Green’s functions approach which allows for a fully quantum mechanical description of carrier and photon dynamics. This microscopic approach significantly reduces the number of phenomenological parameters needed to simulate laser. General equations for photon Green’s functions and polarizations are derived as well as their coupling to electron Green’s functions through self-energies. Series of approximations are done to photon Green’s function to allow for efficient numerical approach. All equations are written in the non-orthogonal basis suitable for numerical calculations. As an example, the theory is applied to analyze Al0.2Ga0.8As/GaAs quantum well laser with the effective mass Hamiltonian. Major laser characteristics such as material and modal gain, threshold parameters, carrier and current densities were determined.
Thermodynamically consistent closure approximation for hard spheres systems
Authors: Carbajal-Tinoco Mauricio D.
Published: March 2012

We present a new closure relation that is an extension of the Percus-Yevick approximation. In the proposed closure, we introduce an additional term and a mixing coefficient that can be determined by imposing a condition of thermodynamic self-consistency. Moreover, the mixing coefficient is calculated analytically within a linear approximation. In the case of a monodisperse system of hard spheres, we compare the results of our model to well-established thermodynamic expressions and also to the structural properties of fairly known closure approximations. In the second case, and using an equivalent scheme, the new closure relation is extended to the depletion potential between two large hard spheres immersed in a liquid of small hard spheres. In both cases, the results of our model are in good agreement with numerical simulations performed at intermediate concentrations.
Reflection mechanism for generating spin transfer torque without charge current
Authors: Autès G. , Mathon J. , Umerski A.
Published: March 2012

A reflection mechanism for generating spin-transfer torque is proposed. It is due to interference of bias-driven nonequilibrium electrons incident on a switching junction, with the electrons reflected from an insulating barrier inserted in the junction after the switching magnet. It is shown, using the rigorous Keldysh formalism, that this out-of-plane torque T⊥ is proportional to an applied bias and is as large as the torque in a conventional junction generated by a strong charge current. However, the charge current and the in-plane torque T∥ are almost completely suppressed by the insulating barrier. This junction thus offers the highly applicable possibility of bias-induced switching of magnetization without charge current.
Constructal design for pedestrian movement in living spaces: Evacuation configurations
Authors: Lui C. H. , Fong N. K. , Lorente S. , Bejan A. , Chow W. K.
Published: March 2012

Here we show that the configuration of an inhabited area controls the time required by all the pedestrians to vacate the space. From the minimization of the global evacuation time emerges the optimal configuration of the area. This is a fundamental principle for designing living spaces with efficient evacuation quality, and it is demonstrated here with several simple building blocks that can be used as components of more complex living structures: single walkway, corner, and T-shaped walkway. We show analytically and numerically that the ratio of the widths of the stem and branches of the T-shaped walkway has an optimal value that facilitates the evacuation of all the inhabitants. This result is fundamental, and is the crowd-dynamics equivalent of the Hess-Murray rule for the ratio of diameters in bifurcated ducts with fluid flow.
Dynamics of surface-coupled microcantilevers in force modulation atomic force microscopy – magnetic vs. dither piezo excitation
Authors: Xu Xin , Koslowski Marisol , Raman Arvind
Published: March 2012

Force modulation atomic force microscopy is widely used for mapping the nanoscale mechanical properties of heterogeneous or composite materials using low frequency excitation of a microcantilever scanning the surface. Here we show that the excitation mode – magnetic or dither piezo, has a major influence on the surface-coupled microcantilever dynamics. Not only is the observed material property contrast inverted between these excitation modes but also the frequency response of the surface-coupled cantilever in the magnetic mode is near-ideal with a clear resonance peak and little phase distortion thus enabling quantitative mapping of the local mechanical properties.
Coulomb blockade phenomenon in ultra-thin gold nanowires
Authors: Guerin Hoël , Yoshihira Masanori , Kura Hiroaki , Ogawa Tomoyuki , Sato Tetsuya , Maki Hideyuki
Published: March 2012

Ultra-thin gold nanowires with uniform diameters of 2 nm and lengths of over 100 μm are synthesized via the reduction of gold(III) chloride in an oleylamine matrix. The gold nanowires, dispersed on an oxidized substrate, are top-contacted with metallic electrodes to manufacture back gated transistors. We investigate the transport properties in the fabricated devices as a function of the gate voltage, the bias voltage, and the temperature. The nonlinear current-bias voltage characteristics from 7 K up to 300 K are well described by the Coulomb blockade model in a nearly one-dimensional quantum dot array (which results from the gold nanowires’ thermal fragmentation into a granular material). Our results support a picture in which the electronic transport is governed by sequential tunneling at an applied bias above the global Coulomb blockade threshold, whereas in the Coulomb blockade regime, inelastic cotunneling is dominant up to 70 K, at which point it crosses over to activated behavior. The current dependence on the gate voltage that shows irregular oscillations is well explained by the superimposition of Coulomb oscillation patterns generated by each different dot in the one-dimensional array. We find that the competitive effects of excitation energy and stochastic Coulomb blockade balance the number of current peaks observed.
Charging and coupling of a vertically aligned particle pair in the plasma sheath
Authors: Carstensen Jan , Greiner Franko , Block Dietmar , Schablinski Jan , Miloch Wojciech J. , Piel Alexander
Published: March 2012

The phenomenon of particle chain formation is studied in a two-particle system. A wake of positive ions leads to an alignment of the negatively charged particles parallel to the ion flow. The dynamic response of this dust system to a small external perturbation is evaluated. It is shown that the eigenfrequency of the downstream particle is reduced compared to an isolated particle. This effect can be identified as a decharging of the particle by the focused ion flow in the wake of the upstream particle. Furthermore, a strong asymmetry of the particle interaction parallel to the ion flow is found. This asymmetry may not be attributed entirely to the interaction forces mediated by the ion wake.
The role of relativity and dispersion controlled inter-chain interaction on the band gap of thiophene, selenophene, and tellurophene oligomers
Authors: Chattopadhyaya Mausumi , Sen Sabyasachi , Alam Md. Mehboob , Chakrabarti Swapan
Published: March 2012

Ab initio relativistic density functional theoretical calculations have been carried out on π-conjugated oligomers of increasing length with S, Se, and Te as heteroatoms. The band gap of the corresponding polymers has been obtained by plotting lowest unoccupied molecular orbital (LUMO)-highest occupied molecular orbital (HOMO)gap against the reciprocal of the number of monomer units (1/N) and extrapolating the curve to 1/N = 0. With B3LYP functional, we predict that role of relativistic correction terms is not very significant in the determination of final band gap of thiophene, selenophene, and tellurophene polymer. The origin of this observation is provided through the density of states (DOS) analysis which manifests that DOS contribution across the Fermi level of these polymers is mostly governed by C atoms and as a consequence relativistic correction terms due to heavy heteroatom remain insignificant to the band gap modification. We also inspected the role of inter-chain interaction in determining the net LUMO-HOMO gap of π-stacked double chain oligomers of increasing length. We have found that due to the exciton splitting in the stacked configurations, the LUMO-HOMO gap decreases steadily. Furthermore, we have noticed that dispersion force has important role in the reduction of the LUMO-HOMO gap of the oligomers studied.
Infrared spectroscopy and modeling of co-crystalline CO2·C2H2 aerosol particles. II. The structure and shape of co-crystalline CO2·C2H2 aerosol particles
Authors: Preston Thomas C. , Signorell Ruth
Published: March 2012

Infrared absorption spectra of co-crystalline CO2·C2H2 aerosol particles were modeled using a combination of two methods. Density functional theory was used to model several bulk CO2·C2H2 co-crystal structures and to calculate their lattice energies and frequency-dependent dielectric tensors. This was necessary as there currently exists no crystallographic or refractive index data on co-crystalline CO2·C2H2 due to its metastability. The discrete dipole approximation was then used to calculate infrared absorption spectra of different model particles using the dielectric tensors calculated using density functional theory. Results from these simulations were compared to the experimental spectrum of co-crystalline CO2·C2H2 aerosol particles. The aerosol particles after the decomposition of the co-crystalline phase were studied in Part I.
Slow packets of electromagnetic waves in magnetized plasma with ferrite grains
Authors: Mesfin Belayneh , Mal’nev V. N.
Published: March 2012

The propagation of narrow Gaussian electromagnetic wave packets in the collisionless magnetized plasma with ferrite grains (MPFG) is studied. In this system, in the frequency range close to the frequency of the electron cyclotron resonance, which coincides with the frequency of the ferromagnetic resonance, the dispersion of the electric permittivity (electron subsystem) and magnetic permeability (ferrite grain subsystem) must be taken into account simultaneously. The refractive index η of MPFG can be negative and with increase in frequency becomes positive passing through zero. The strong dispersion of η and the group velocity vg results in the spreading of the wave packets, which propagate with velocity much smaller than the speed of light. Their phase velocity is negative if η<0 and positive if η>0. For η<0, the group velocity can have a maximum at some frequency. The wave packets centered at this frequency are weakly spreading. The existence of slow waves in the frequency range, where η≤0, may be a general property of left handed media.
Time versus temperature rescaling for coarse grain molecular dynamics simulations
Authors: Accary J. B. , Teboul V.
Published: March 2012

Coarse graining procedures are intended to well reproduce the structure of a material while increasing the simulations efficiency. However, the dynamics usually accelerates with coarse graining and a scaling procedure has to be used for dynamical data calculations. Most often a simple time-scaling coefficient is used for this purpose. However, for low temperature liquids this simple scaling procedure is questionable. Because supercooled liquids in their approach to the glass transition temperature do not follow a simple dynamics. In order to test if this scaling procedure is still pertinent at low temperature, we use molecular dynamics simulations of a coarse grain model of the methylmethacrylate molecule compared to simulations with the All atom model. We compare two different rescaling procedures, a time rescale and a temperature rescale procedure. Using these two procedures we compare the behaviors of the mean square displacements, the incoherent scattering functions, the self and distinct part of the Van Hove correlation functions and the non-Gaussian parameters. Results show that the temperature rescaling procedure reproduces well the All atom dynamical data at low temperatures, while the time rescaling procedure is correct only in the Brownian regime. We also find that the melting and the glass-transition temperatures are relatively well reproduced with the temperature rescaling procedure.
Nonlinear Marangoni waves in a two-layer film in the presence of gravity
Authors: Nepomnyashchy Alexander , Simanovskii Ilya
Published: March 2012

Longwave Marangoni convection in two-layer films under the action of gravity is considered. The analysis is carried out in the lubrication approximation. A linear stability analysis reveals the existence of monotonic and oscillatory instability modes, depending on the way of heating and the value of the Biot number. Numerical simulations are performed in the case of an oscillatory instability, which takes place by heating from above. Periodic boundary conditions are applied on the boundaries of the computational region. A sequence of nonlinear wavy regimes, which develop by the increase of the Galileo number, is studied. That sequence includes three-dimensional and two-dimensional structures. The multistability of wavy patterns with different spatial periods has been revealed.
Combined ellipsometry and refractometry technique for characterisation of liquid crystal based nanocomposites
Authors: Warenghem Marc , Henninot Jean François , Blach Jean François , Buchnev Oleksandr , Kaczmarek Malgosia , Stchakovsky Michel
Published: March 2012

Spectroscopic ellipsometry is a technique especially well suited to measure the effective optical properties of a composite material. However, as the sample is optically thick and anisotropic, this technique loses its accuracy for two reasons: anisotropy means that two parameters have to be determined (ordinary and extraordinary indices) and optically thick means a large order of interference. In that case, several dielectric functions can emerge out of the fitting procedure with a similar mean square error and no criterion to discriminate the right solution. In this paper, we develop a methodology to overcome that drawback. It combines ellipsometry with refractometry. The same sample is used in a total internal reflection (TIR) setup and in a spectroscopic ellipsometer. The number of parameters to be determined by the fitting procedure is reduced in analysing two spectra, the correct final solution is found by using the TIR results both as initial values for the parameters and as check for the final dielectric function. A prefitting routine is developed to enter the right initial values in the fitting procedure and so to approach the right solution. As an example, this methodology is used to analyse the optical properties of BaTiO3 nanoparticles embedded in a nematic liquid crystal. Such a methodology can also be used to analyse experimentally the validity of the mixing laws, since ellipsometry gives the effective dielectric function and thus, can be compared to the dielectric function of the components of the mixture, as it is shown on the example of BaTiO3/nematic composite.