bluuebunny/crossref_metadata_2025 huggingface dataset:
Minimum action paths for spin-torque assisted thermally induced magnetization reversal
Authors: Chaves-O’Flynn Gabriel D. , Stein Daniel L. , Kent Andrew D. , Vanden-Eijnden Eric
Published: April 2011

We calculate the most probable reaction paths for thermally induced magnetization reversal of a nanomagnet under the influence of spin transfer torque. The presence of the spin transfer torque implies that the standard reaction rate theory of Kramers cannot be used since the dynamics no longer shows detailed balance and so the magnetization reversals are nonequilibrium transitions. Thin film nanomagnets with a biaxial anisotropy, a shape anisotropy that leads to in-plane magnetization with a preferred axis in the plane, are considered. The reaction pathways and rates are computed using geometrical Minimum Action Method. Our results indicate that the transition state has an out-of-plane magnetization component, in contrast to the case without an applied spin transfer torque.
Phonon anomalies near the magnetic phase transitions in BiFeO3 thin films with rhombohedral R3c symmetry
Authors: Singh Manoj K. , Katiyar Ram S.
Published: April 2011

Raman spectra of epitaxal BiFeO3 thin films, grown on (111) SrTiO3 substrates, have been studied in temperature range 300–800 K. All four prominent A1-symmetry (81, 138, 170, and 214 cm−1) Raman modes show evolution with the temperature near the Néel temperature (TN), which is interpreted due to spin lattice coupling and it could be also linked with the structural instability originated by octahedral tilting near TN as reported by A. Palewicz et al., Acta Cryst B 63, 537 (2007). As another evidence of spin–phonon coupling, softening of the 214 cm−1 stretching mode is observed near and below the magnetic ordering temperature.
Study on corrosion behaviors of sintered Nd–Fe–B magnets in different environmental conditions
Authors: Li J. J. , Li A. H. , Zhu M. G. , Pan W. , Li W.
Published: April 2011

Nd–Fe–B magnets have outstanding magnetic properties, but their corrosion resistance is poor because the rare-earth-rich phases in them are easily oxidized. In this article, we report an investigation of the corrosion behaviors of sintered Nd–Fe–B magnets with varied compositions in different corrosion conditions. The weight losses of the magnets after corrosion testing were measured after brushing off the corrosion products. The magnetic flux losses of the magnets were measured using a fluxmeter. A scanning electron microscope equipped with an energy dispersive x-ray analysis system was employed to observe the corrosion morphology. It was found that the humid-heat resistance of the magnets was obviously improved by partially substituting Dy for Nd and adding minor Co. The corrosion products and morphologies of Nd–Fe–B magnets for the autoclave test were different from those for the constant humid-heat test. The corrosion rates of the magnets for the former were much slower than for the latter; this is probably because the high-pressure steam led to an oxygen-deficient atmosphere, and the liquid film on the surface of the magnet specimens hindered the diffusion of oxygen into the bulk for the autoclave test.
Effect of external magnetic field on magnetic properties of Co–Pt nanotubes and nanowires
Authors: Chen J. Y. , Liu H. R. , Ahmad N. , Li Y. L. , Chen Z. Y. , Zhou W. P. , Han X. F.
Published: April 2011

Well-ordered Co90Pt10 nanotubes and nanowires have been fabricated into anodic aluminum oxide (AAO) templates by using a dc electrodeposition method. The effects of an external magnetic field on the magnetic properties of Co90Pt10 nanotubes and nanowires are investigated. The results show that the external magnetic field during electrodeposition has a significant effect on Co90Pt10 nanowires, as well as nanotubes. The external magnetic field induces good texture and large grain size in Co90Pt10 nanowires, causing a reduction of coercivity. An obvious uniaxial anisotropy is induced in Co90Pt10 nanotubes, causing the crossover of the easy axis along the directions of an external magnetic field.
Ferromagnetic resonance behavior of spark plasma sintered Ni–Zn ferrite nanoparticles produced by a chemical route
Authors: Valenzuela R. , Beji Z. , Herbst F. , Ammar S.
Published: April 2011

Ferrite nanoparticles of composition Zn0.5Ni0.5Fe2O4 were prepared by forced hydrolysis in a polyol (polyol process) from the corresponding iron, nickel, and zinc acetates. Synthesis conditions allowed for obtaining polycrystalline epitaxial clusters of about 22 nm in size with an average crystal size of about 5 nm. These powders were subsequently consolidated by spark plasma sintering (SPS) technique under a pressure of 80 MPa and temperatures in the 350 to 500 °C range for short periods (5 to 10 min). Densities reached 92% to 94% of the theoretical density. Particle size remained smaller than 61 nm, even for the highest temperature. Ferromagnetic resonance experiments at 77 K showed broadened resonance lines. In contrast with powdered nanoparticles, these high-density bodies can be used in many high-frequency applications.
Effects of Co addition on magneto-transport properties of magnetic tunnel junction consisting of CoFeB or CoFeSiB free layer
Authors: Noh Su Jung , Chun Byong Sun , Kim Young Keun , Wang Tianxing
Published: April 2011

Amorphous ferromagnetic CoFeB and CoFeSiB layers with various Co concentrations were employed as free layers of magnetic tunnel junctions (MTJs), and their magnetization switching performances were compared. Both analytical measurements and micromagnetic modeling efforts were carried out to understand the dependence of magnetization switching field (Hsw) on Co concentration. In overall, the CoFeSiB free layered MTJs showed a lower Hsw compared to that of the CoFeB ones. This is due to the fact that CoFeSiB possesses a lower saturation magnetization than CoFeB and, moreover, its magnetization switching process shows coherent switching.
Magnetization reversal mechanism of a sintered Nd-Fe-B magnet with Dy segregation
Authors: Suzuki Hiroyuki , Satsu Yuichi , Kohashi Teruo , Motai Kumi , Komuro Matahiro
Published: April 2011

The magnetization reversal mechanism of a sintered Nd-Fe-B magnet with Dy segregation near grain boundaries (GB) was studied using spin polarized scanning electron spectroscopy and magnetization measurements. More fine magnetic domains near GB were found in the magnet with Dy segregation than in the magnet without treatment. Magnetic modifications near the GB had no effect on the magnetization development. The domain wall motion of a quasihard magnetic component in the magnet with Dy segregation was suppressed in comparison with those in the magnet without treatment; this could also be confirmed in the different behaviors of asymmetry components toward a magnetic field direction between both magnets. From analyzing the probability of rotating magnetization near the GB, the magnetization reversal of the magnet with Dy segregation was more difficult to produce than those of the magnet without treatment.
Effects of applied pressure in ZnV2 O4 and evidences for a dimerized structure
Authors: Piñeiro A. , Botana A. S. , Pardo V. , Botana J. , Pereiro M. , Baldomir D. , Arias J. E.
Published: April 2011

The series of V spinels [A2+] V2 O4 (A = Cd, Mn, Zn, Mg) provides an opportunity to tune the V-V distance continuously, in the frustrated pyrochlore lattice of the spinel. This system has been shown to approach the metallic state when V-V distance is reduced. The proximity to the transition leads to a dimerized structure in ZnV2 O4 caused by lattice instabilities. A different manner to tune the V − V distance of this structure is to fix the A2+ cation (in our case, Zn) and apply pressure. We have analyzed the evolution of the electronic structure of the system in the dimerized state. Such structure prevents the system to present a metallic phase at moderate pressures. We have also calculated the transport properties in a semiclassical approach based on Boltzmann transport theory. Our results support the validity of this structural distortion by providing a nice fit with experimental measurements.
Magnetic vector aftereffect model
Authors: Torre E. Della , Cardelli E.
Published: April 2011

In addition to the decay of magnetization with time that occurs in scalar processes, in vector processes the magnetization must also rotate toward the easy axis. In this model, each hysteron is characterized by a critical surface. The energy of the hysteron is proportional to the size of the difference between the applied field and the particular point on the critical surface. An energy landscape is obtained by plotting the size of this difference as a function of magnetization angle. The equilibrium state is one in which the hysterons lie in the potential wells with a density given by Maxwell-Boltzmann statistics at room temperature. The logarithm of the time constant of the approach to equilibrium for a particular hysteron is given by the ratio of the barrier energy to the product of the absolute temperature and Boltzmann’s constant. This barrier energy is obtained by the difference of the nearest maximum to the well depth. The overall behavior is the sum of this effect for all the hysterons.
Coercivity enhancement in Nd-Fe-B sintered permanent magnet doped with Pr nanoparticles
Authors: Sun H. , Liu W. Q. , Zhang X. R. , Yue M. , Zhang D. T. , Zhang J. X.
Published: April 2011

Nd-Fe-B permanent magnets doped with a small amount of praseodymium (Pr) nanoparticles were prepared by conventional sintering method. Effects of the Pr nanoparticles doping amount and the sintering temperature on the magnetic properties of the magnets were studied. As the doping amount increases, the coercivity of the magnets rises gradually, while the remanence and the maximum energy product of the magnets rise first, peak at 1.0 wt. % of Pr nanoparticles doping, then drop. The optimal sintering temperature was determined as 1040 °C. Under optimal doping amount and sintering temperature, the Nd-Fe-B magnet bears good magnetic properties of Br of 1.386 T, Hci of 1170 kA/m and (BH)max of 371 kJ/m3, which are better than the Pr-free magnet’s magnetic properties of Br of 1.358 T, Hci of 1097 kA/m and (BH)max of 354 kJ/m3, suggesting the doping of Pr nanoparticles is a promising way to enhance the magnetic properties of sintered Nd-Fe-B magnets.
Structural and magnetic properties of La0.7Sr0.3MnO3 thin films integrated onto Si(100) substrates with SrTiO3 as buffer layer
Authors: Belmeguenai M. , Mercone S. , Adamo C. , Moch P. , Schlom D. G. , Monod P.
Published: April 2011

La0.7Sr0.3MnO3 (LSMO) thin films with a thickness d of 10, 20, 60, and 100 nm were grown on 20-nm-thick SrTiO3-buffered (100) silicon substrates by a reactive molecular beam epitaxy. For all samples, x-ray diffraction (XRD) revealed an excellent epitaxy with in-plane cubic [100] and [010] axes of LSMO. The XRD measured values of the out-of-plane lattice parameter suggest that the strain state does not vary significantly from sample to sample. A super conducting quantum interference device reveals that the room temperature magnetization at saturation increases with d and nearly reaches the bulk value for d = 100 nm; the Curie temperature ranges in the 320–350 K interval, to compare to 360 K in the bulk. Ferromagnetic resonance (FMR) in cavity (at 9.5 GHz) and microstrip FMR used to investigate the dynamic magnetic properties, revealed a fourfold anisotropy showing its easy axes along the [110] and [11¯0] directions. In the thickest samples (d > 20 nm), the FMR spectra present two distinct resonant modes. This splitting is presumably due to the simultaneous presence of two different magnetic phases.
Magnetic properties of Pr-Fe-Ti-B nanocomposite magnets produced by spark plasma sintering method
Authors: Saito Tetsuji , Ishikawa Hiroyuki , Nishio-Hamane Daisuke
Published: April 2011

Amorphous melt-spun ribbons of Pr8Fe86–xTixB6 (x = 0–10) alloys were successfully consolidated into bulk materials by the spark plasma sintering method. X-ray diffraction and transmission electron microscopic studies revealed that the resultant bulk materials consisted of nanosized grains of the α-Fe and Pr2Fe14B phases. The coercivity of the Pr-Fe-Ti-B bulk nanocomposite magnets increased with increasing Ti content. The Pr8Fe80Ti6B6 bulk nanocomposite magnet exhibited a high remanence of 10.2 kG with a coercivity of 4.9 kOe, while the Pr8Fe76Ti10B6 bulk magnet exhibited a remanence of 6.5 kG with a high coercivity of 8.2 kOe.
Angular arrangements of triangular fins for controlling the magnetization processes in permalloy rings
Authors: Lin Szu-Yu , Lin Ya-Hui , Ger Tzong-Rong , Huang Hao-Ting , Wei Zung-Hang
Published: April 2011

The influences of triangular fins’ positions on controlling the magnetization processes are investigated. It is observed experimentally that when the included angle between the triangular fins and the field direction is 15° or 30°, the magnetization reversals are repeatable in the sweep-up and sweep-down processes, and the magnetization processes are controllable. In contrast, when the included angle between the triangular fins and the field direction is 45° or 60°, the magnetization reversals are unrepeatable and random in the sweep-up and sweep-down processes. Magnetization reversal can only be controlled when positions of the triangular fins are close to the original nucleation regions of the rings. When triangular fins are away from the original nucleation regions, two kinds of magnetization reversals randomly occur without being controlled by the artificial nucleation sites, and the triangular fins thus lose their control ability.
Onset of morphological instability in two binary liquid layers
Authors: McFadden G. B. , Coriell S. R. , Lott P. A.
Published: April 2011

We consider the linear stability of a horizontal liquid bilayer subject to vertical heating. The two layers consist of a binary liquid that has undergone a phase transition, resulting in a horizontal interphase boundary between two phases with different compositions. We perform linear stability calculations to determine the critical values for the applied temperature difference across the system that is necessary to produce instability using both numerical computations and small-wavenumber approximations. We focus on an instability primarily due to the phase change, which can occur in the absence of buoyancy and surface-tension-driven convection. We find both direct and oscillatory modes of instability, either of which can persist to small wavenumbers that allow approximate analytical descriptions. The interaction of flow with a deforming phase boundary plays a critical role in the instability, and the results are compared to morphological stability results that can be obtained in the absence of flow.
Cyclotron maser emission: Stars, planets, and laboratory
Authors: Vorgul I. , Kellett B. J. , Cairns R. A. , Bingham R. , Ronald K. , Speirs D. C. , McConville S. L. , Gillespie K. M. , Phelps A. D. R.
Published: April 2011

This paper is a review of results by the group over the past decade on auroral kilometric radiation and similar cyclotron emissions from stars and planets. These emissions are often attributed to a horseshoe or crescent shaped momentum distribution of energetic electrons moving into the convergent magnetic field which exists around polar regions of dipole-type stars and planets. We have established a laboratory-based facility that has verified many of the details of our original theoretical description and agrees well with numerical simulations. The experiment has demonstrated that the horseshoe distribution does indeed produce cyclotron emission at a frequency just below the local cyclotron frequency, with polarization close to X-mode and propagating nearly perpendicularly to the beam motion. We discuss recent developments in the theory and simulation of the instability including addressing a radiation escape problem and the effect of competing instabilities, relating these to the laboratory, space, and astrophysical observations.
Mitigation of nonthermal plasma production to measure the pulsed magnetic field threshold for the thermal formation of plasma from thick aluminum surfaces
Authors: Awe T. J. , Bauer B. S. , Fuelling S. , Siemon R. E.
Published: April 2011

Untangling the roles of different plasma formation mechanisms in high-current electrical discharges requires controlling each mechanism and isolating its effects. Recent experiments use this method to find that thermal formation of plasma occurs from 6061-alloy Al conductors by ohmic heating when the surface field reaches a magnetic field threshold of 2.2 MG (independently of ∂B/∂t ranging from 30 to 80 MG/μs) [T. J. Awe et al., Phys. Rev. Lett. 104, 035001 (2010)]. The experiments pulse 1.0 MA peak current on the surface of thick (∼1.0 mm diameter) aluminum rods in 100 ns. Strong electric fields are necessarily introduced, and precursor plasma may form (e.g., from arcing electrical contacts). To ensure plasma formation is predominantly thermal, a systematic study of multiple combinations of high-current electrodes and central-rod profiles was completed. Data from multiple diagnostics show that the time and location of plasma formation strongly correlate with the hardware combination used. An extensive dataset (greater than 100 shots) enabled numerous hypotheses to be tested regarding plasma formation mechanisms. For hardware with compressed, large-diameter current joints, plasma formation is a predominantly thermal process driven by ohmic heating.
Nanodiamond-gated silicon ion-sensitive field effect transistor
Authors: Ahmad Rezal K. , Parada Ana Carolina , Jackman Richard B.
Published: April 2011

Diamond has excellent surface chemical properties for sensing applications but active device fabrication technology is immature compared to silicon. A method for attaching nanodiamonds to commercially available silicon ion-sensitive field effect transistors is demonstrated, leading to a nanodiamond-silicon hybrid sensor. In response to pH changes, the hybrid device showed a similar response to the Si sensor, at 55 mV/pH, near the Nernst limit. Complete immersion in liquid containing different molarities of explosive precursors such NaNO2 and NaClO3, and exposure to 2,4 dinitrotoluene vapor, an analog for the explosive trinitrotoluene, gave rise to sensitive detection, while the Si ion sensitive field effect transistor device remained insensitive.
A solution processable fluorene-benzothiadiazole small molecule for n-type organic field-effect transistors
Authors: Mutkins Karyn , Gui Ke , Aljada Muhsen , Schwenn Paul E. , Namdas Ebinazar B. , Burn Paul L. , Meredith Paul
Published: April 2011

We report an n-type organic semiconductor [2-({7-(9,9-di-n-propyl-9H-fluoren-2-yl}benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile (herein referred to as K12) for use in organic field-effect transistors (OFETs). K12 can be processed by spin-coating from solution or by vacuum deposition, organizing into highly orientated microcrystalline structures at modest (75 °C) annealing temperatures. OFETs with n-octyltrichlorosilane or hexamethyldisilazane monolayers, or poly(propylene-co-1-butene) (PPCB) modified dielectric surfaces were prepared. The mobility, ON/OFF ratio, threshold voltage, and current hysteresis were found to be dependent on the thermal history of the film and surface onto which it was deposited. The highest OFET mobility achieved was 2.4×10−3 cm2/V s, for spin-coated films with a PPCB modified silicon dioxide dielectric.
Molecular dynamics simulation of thermal energy transport in polydimethylsiloxane
Authors: Luo Tengfei , Esfarjani Keivan , Shiomi Junichiro , Henry Asegun , Chen Gang
Published: April 2011

Heat transfer across thermal interface materials is a critical issue for microelectronics thermal management. Polydimethylsiloxane (PDMS), one of the most important components of thermal interface materials presents a large barrier for heat flow due to its low thermal conductivity. In this paper, we use molecular dynamics simulations to identify the upper limit of the PDMS thermal conductivity by studying thermal transport in single PDMS chains with different lengths. We found that even individual molecular chains had low thermal conductivities (κ ∼ 7 W/mK), which is attributed to the chain segment disordering. Studies on double chain and crystalline structures reveal that the structure influences thermal transport due to inter-chain phonon scatterings and suppression of acoustic phonon modes. We also simulated amorphous bulk PDMS to identify the lower bound of PDMS thermal conductivity and found the low thermal conductivity (κ ∼ 0.2 W/mK) is mainly due to the inefficient transport mechanism through extended vibration modes.
A systematic study of neutral and charged 3d-metal trioxides and tetraoxides
Authors: Pradhan Kalpataru , Gutsev Gennady L. , Weatherford Charles A. , Jena Purusottam
Published: April 2011

Using density functional theory with generalized gradient approximation, we have performed a systematic study of the structure and properties of neutral and charged trioxides (MO3) and tetraoxides (MO4) of the 3d-metal atoms. The results of our calculations revealed a number of interesting features when moving along the 3d-metal series. (1) Geometrical configurations of the lowest total energy states of neutral and charged trioxides and tetraoxides are composed of oxo and/or peroxo groups, except for CuO3− and ZnO3− which possess a superoxo group, CuO4+ and ZnO4+ which possess two superoxo groups, and CuO3+, ZnO3+, and ZnO4− which possess an ozonide group. While peroxo groups are found in the early and late transition metals, all oxygen atoms bind chemically to the metal atom in the middle of the series. (2) Attachment or detachment of an electron to/from an oxide often leads to a change in the geometry. In some cases, two dissociatively attached oxygen atoms combine and form a peroxo group or a peroxo group transforms into a superoxo group and vice versa. (3) The adiabatic electron affinity of as many as two trioxides (VO3 and CoO3) and four tetraoxides (TiO4, CrO4, MnO4, and FeO4) are larger than the electron affinity of halogen atoms. All these oxides are hence superhalogens although only VO3 and MnO4 satisfy the general superhalogen formula.
Short-time pressure response during the start-up of a constant-rate production of a high pressure gas well
Authors: Jin Yan , Chen Kang Ping , Chen Mian , Grapsas Nicholas , Zhang Fu Xiang
Published: April 2011

The start-up flow of a constant-rate production of a high pressure gas well is studied, with emphasis on the effect of gas acceleration. Gas acceleration is important in the near wellbore region for a high pressure gas in a high permeability formation. It is shown that when gas acceleration is important, the system of governing equations for the porous media flow becomes hyperbolic. In response to an impulsively imposed mass flow-rate, a steep pressure front is created at the wellbore and it propagates into the formation. This steep pressure front is trailed by large amplitude pressure waves. As they travel away from the wellbore, the pressure front becomes less steep and the amplitude of the trailing waves decreases due to viscous damping. It is found that the pressure drawdown experiences a rapid increase followed by an oscillatory behavior in short times before approaching the classical logarithmic rise regime. The pressure gradient at the wellbore wall can grow to a large magnitude in the early times, which can cause a tensile failure of the rock materials near the wellbore.
Solution reaction space Hamiltonian based on an electrostatic potential representation of solvent dynamics
Authors: Aono Shinji , Yamamoto Takeshi , Kato Shigeki
Published: April 2011

Quantum chemical solvation models usually rely on the equilibrium solvation condition and is thus not immediately applicable to the study of nonequilibrium solvation dynamics, particularly those associated with chemical reactions. Here we address this problem by considering an effective Hamiltonian for solution-phase reactions based on an electrostatic potential (ESP) representation of solvent dynamics. In this approach a general ESP field of solvent is employed as collective solvent coordinate, and an effective Hamiltonian is constructed by treating both solute geometry and solvent ESP as dynamical variables. A harmonic bath is then attached onto the ESP variables in order to account for the stochastic nature of solvent dynamics. As an illustration we apply the above method to the proton transfer of a substituted phenol–amine complex in a polar solvent. The effective Hamiltonian is constructed by means of the reference interaction site model self-consistent field method (i.e., a type of quantum chemical solvation model), and a mixed quantum/classical simulation is performed in the space of solute geometry and solvent ESP. The results suggest that important dynamical features of proton transfer in solution can be captured by the present approach, including spontaneous fluctuations of solvent ESP that drives the proton from reactant to product potential wells.
Infrared absorption of methanol clusters (CH3OH)n with n = 2−6 recorded with a time-of-flight mass spectrometer using infrared depletion and vacuum-ultraviolet ionization
Authors: Han Hui-Ling , Camacho Cristopher , Witek Henryk A. , Lee Yuan-Pern
Published: April 2011

We investigated IR spectra in the CH- and OH-stretching regions of size-selected methanol clusters, (CH3OH)n with n = 2−6, in a pulsed supersonic jet by using the IR–VUV (vacuum-ultraviolet) ionization technique. VUV emission at 118 nm served as the source of ionization in a time-of-flight mass spectrometer. The tunable IR laser emission served as a source of predissociation or excitation before ionization. The variations of intensity of protonated methanol cluster ions (CH3OH)nH+ and CH3OH+ and (CH3OH)2+ were monitored as the IR laser light was tuned across the range 2650−3750 cm−1. Careful processing of these action spectra based on photoionization efficiencies and the production and loss of each cluster due to photodissociation yielded IR spectra of the size-selected clusters. Spectra of methanol clusters in the OH region have been extensively investigated; our results are consistent with previous reports, except that the band near 3675 cm−1 is identified as being associated with the proton acceptor of (CH3OH)2. Spectra in the CH region are new. In the region 2800−3050 cm−1, bands near 2845, 2956, and 3007 cm−1 for CH3OH split into 2823, 2849, 2934, 2955, 2984, and 3006 cm−1 for (CH3OH)2 that correspond to proton donor and proton acceptor, indicating that the methanol dimer has a preferred open-chain structure. In contrast, for (CH3OH)3, the splitting diminishes and the bands near 2837, 2954, and 2987 cm−1 become narrower, indicating a preferred cyclic structure. Anharmonic vibrational wavenumbers predicted for the methanol open-chain dimer and the cyclic trimer with the B3LYP/VPT2/ANO1 level of theory are consistent with experimental results. For the tetramer and pentamer, the spectral pattern similar to that of the trimer but with greater widths was observed, indicating that the most stable structures are also cyclic.
A self-consistent global model of solenoidal-type inductively coupled plasma discharges including the effects of radio-frequency bias power
Authors: Kwon D. C. , Chang W. S. , Park M. , You D. H. , Song M. Y. , You S. J. , Im Y. H. , Yoon J.-S.
Published: April 2011

We developed a self-consistent global simulator of solenoidal-type inductively coupled plasma discharges and observed the effect of the radio-frequency (rf) bias power on the plasma density and the electron temperature. We numerically solved a set of spatially averaged fluid equations for charged particles, neutrals, and radicals. Absorbed power by electrons is determined by using an analytic electron heating model including the anomalous skin effect. To analyze the effects of rf bias power on the plasma properties, our model also combines the electron heating and global transport modules with an rf sheath module in a self-consistent manner. The simulation results are compared with numerical results by using the commercial software package cfd-ace + (ESI group) and experimental measurements by using a wave cutoff probe and a single Langmuir probe.
Near field and cavity effects on coupling efficiency of one-dimensional metal grating for terahertz quantum well photodetectors
Authors: Zhang R. , Guo X. G. , Cao J. C. , Liu H. C.
Published: April 2011

The Modal Method is employed to simulate the coupling between the terahertz wave and one-dimensional (1D) transmissive metal grating on the top of terahertz quantum well photodetectors (THzQWPs). Electrical field patterns and behaviors of 1D grating at different frequencies and device thicknesses are systematically studied. The results show that, the coupling efficiency is not only determined by the grating parameters, multiple reflections in the device and the subsequent multiple diffractions at the grating also play an important role. Different diffracted modes interact with each other, and near field effect caused by the evanescent waves are essential in evaluating the coupling efficiency especially when the frequencies are below the cutoff of the gratings. The optimization conditions of the performance of 1D metal grating coupled THzQWPs are also discussed.
Transient photocurrent measurements of PCDTBT:PC70BM and PCPDTBT:PC70BM Solar Cells: Evidence for charge trapping in efficient polymer/fullerene blends
Authors: Li Zhe , McNeill Christopher R.
Published: April 2011

We report measurements of the turn-on and turn-off photocurrent dynamics as a function of applied voltage for efficient polymer/fullerene bulk heterojunction solar cells composed of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT): [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) and poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT):PC70BM blends. In particular we present evidence for charge trapping that facilitates recombination in these systems. For the PCDTBT:PC70BM system, an initial transient photocurrent peak 5–10 μs after turn-on is observed for operating voltages between 0.5 V and open-circuit. Furthermore, a long photocurrent tail is observed in the decay dynamics of PCDTBT:PC70BM devices with charge still being extracted hundreds of microseconds after turn-off. These features in the PCDTBT:PC70BM device are attributed to trapping and detrapping of charge on the microsecond time scale, with charge trapping facilitating recombination either through trap-assisted recombination or space-charge effects. For the PCPDTBT:PC70BM system, evidence for charge trapping is also observed albeit on a faster time scale. No initial transient photocurrent peak is observed, however the faster PCPDTBT:PC70BM decay dynamics show only a weak voltage dependence consistent with rapid trapping and recombination of charge. For both systems the amount of extracted charge as a function of applied voltage follows a similar form to the measured current-voltage curves providing evidence that photocurrent is hampered by the extraction, and not just the separation, of charge in these systems. The origin of charge trapping and the nature of recombination is discussed, along with the influence of additives on charge transport in the PCPDTBT:PC70BM system.
Nonlinear transport in quantum-cascade lasers: The role of electric-field domain formation for the laser characteristics
Authors: Wienold M. , Schrottke L. , Giehler M. , Hey R. , Grahn H. T.
Published: April 2011

We report on terahertz quantum-cascade lasers exhibiting discontinuities in the continuous-wave (cw) current-voltage and output power characteristics, which are related to the presence of electric-field domains (EFDs). The investigated lasers are based on a GaAs/Al0.25Ga0.75As heterostructure emitting between 4.2 and 4.4 THz and operate in cw mode up to 50 K. The observed effects related to EFDs are similar to those occurring in weakly coupled superlattices and can be described by the same equations, using an effective drift velocity-field relation. A qualitative agreement between experiments and simulations has been found.
Enhancement of (nearly) homogeneous fields in a (effective) zero-index cavity
Authors: Jiang Hai-tao , Wang Zi-li , Sun Yong , Li Yun-hui , Zhang Ye-wen , Li Hong-qiang , Chen Hong
Published: April 2011

Enhanced homogeneous fields are realized in a zero-index medium embedded in Bragg reflectors. When the zero-index medium has Kerr-type nonlinearity, the threshold for optical bistability can be reduced noticeably due to the enhanced uniform fields. Based on a transmission line, we fabricate an effective zero-index medium sandwiched in electric and magnetic walls and experimentally demonstrate the enhanced nearly uniform fields.
Thermal conductivity of deformed carbon nanotubes
Authors: Zhong Wei-Rong , Zhang Mao-Ping , Zheng Dong-Qin , Ai Bao-Quan
Published: April 2011

We investigate the thermal conductivity of four types of deformed carbon nanotubes by using the nonequilibrium molecular dynamics method. It is reported that various deformations have different influences on the thermal properties of carbon nanotubes. For bending carbon nanotubes, the thermal conductivity is independent of the bending angle. However, the thermal conductivity increases lightly with xy-distortion and decreases rapidly with z-distortion. The thermal conductivity does not change with the screw ratio before the breaking of carbon nanotubes, but it decreases sharply after the critical screw ratio.
Hidden role of trace gas impurities in chemical vapor deposition growth of vertically-aligned carbon nanotube arrays
Authors: In Jung Bin , Grigoropoulos Costas P. , Chernov Alexander A. , Noy Aleksandr
Published: April 2011

Carbon nanotubes (CNTs) grow in a seemingly simple catalytic chemical vapor deposition (CVD) process, yet the detailed mechanism of the process has continued to puzzle researchers. We have examined the role of trace amounts of gas impurities on the kinetics of atmospheric pressure CVD growth of CNTs. Our studies, which used an in situ height monitoring system, revealed that even the nominally ultrapure gases contain enough trace amounts of oxygen-containing species to affect the growth drastically. We were able to obtain the “clean” kinetics of the CNT array growth by passing the feed gases through the high performance gas purifiers. Our data show a remarkable decrease in the catalytic lifetime after the removal of the trace oxygen containing impurities. We suggest that the gas purification is an essential step in obtaining reliable nanotube growth data.
Piezoresistive cantilever force-clamp system
Authors: Park Sung-Jin , Petzold Bryan C. , Goodman Miriam B. , Pruitt Beth L.
Published: April 2011

We present a microelectromechanical device-based tool, namely, a force-clamp system that sets or “clamps” the scaled force and can apply designed loading profiles (e.g., constant, sinusoidal) of a desired magnitude. The system implements a piezoresistive cantilever as a force sensor and the built-in capacitive sensor of a piezoelectric actuator as a displacement sensor, such that sample indentation depth can be directly calculated from the force and displacement signals. A programmable real-time controller operating at 100 kHz feedback calculates the driving voltage of the actuator. The system has two distinct modes: a force-clamp mode that controls the force applied to a sample and a displacement-clamp mode that controls the moving distance of the actuator. We demonstrate that the system has a large dynamic range (sub-nN up to tens of μN force and nm up to tens of μm displacement) in both air and water, and excellent dynamic response (fast response time, <2 ms and large bandwidth, 1 Hz up to 1 kHz). In addition, the system has been specifically designed to be integrated with other instruments such as a microscope with patch-clamp electronics. We demonstrate the capabilities of the system by using it to calibrate the stiffness and sensitivity of an electrostatic actuator and to measure the mechanics of a living, freely moving Caenorhabditis elegans nematode.
Polymer reversal rate calculated via locally scaled diffusion map
Authors: Zheng Wenwei , Rohrdanz Mary A. , Maggioni Mauro , Clementi Cecilia
Published: April 2011

A recent study on the dynamics of polymer reversal inside a nanopore by Huang and Makarov [J. Chem. Phys. 128, 114903 (2008)]10.1063/1.2890006 demonstrated that the reaction rate cannot be reproduced by projecting the dynamics onto a single empirical reaction coordinate, a result suggesting the dynamics of this system cannot be correctly described by using a single collective coordinate. To further investigate this possibility we have applied our recently developed multiscale framework, locally scaled diffusion map (LSDMap), to obtain collective reaction coordinates for this system. Using a single diffusion coordinate, we obtain a reversal rate via Kramers expression that is in good agreement with the exact rate obtained from the simulations. Our mathematically rigorous approach accounts for the local heterogeneity of molecular configuration space in constructing a diffusion map, from which collective coordinates emerge. We believe this approach can be applied in general to characterize complex macromolecular dynamics by providing an accurate definition of the collective coordinates associated with processes at different time scales.
Modeling of self-assembled inorganic oxide semiconductor based electric-double-layer thin film transistors
Authors: Dai Mingzhi , Wu Guodong , Yang Yue , Huang Jin , Li Li , Gong Jun , Wan Qing
Published: April 2011

The effects of the channel thickness on the parameters used for subgap density of states (DOS) modeling for self-assembled oxide semiconductor based electric-double-layer (EDL) thin film transistors (TFTs) with high specific gate capacitance (>1 μF/cm2) were investigated. For indium-tin-oxide-based EDL TFTs, the channel current is affected by the channel thickness. The subgap DOS model with different parameters for different channel thicknesses, together with equations based on device physics, can explain such channel thickness dependence. Our study might lead to a better understanding of inorganic semiconductor EDL TFTs for improved device control.
Atomic structure and strain field of threading dislocations in CeO2 thin films on yttria-stabilized ZrO2
Authors: Hojo Hajime , Tochigi Eita , Mizoguchi Teruyasu , Ohta Hiromichi , Shibata Naoya , Feng Bin , Ikuhara Yuichi
Published: April 2011

Threading dislocations in CeO2 thin films grown on yttria-stabilized ZrO2 substrates were investigated by transmission electron microscopy (TEM), high-resolution TEM, and scanning TEM. It is revealed that there are two kinds of threading dislocations with the Burgers vector of b=1/2⟨110⟩: one is pure edge-type and the other is mixed-type. Comparing the strain field of the mixed-type dislocations with that of the Peierls–Nabarro and the Foreman dislocation models, we find that the Foreman model better describes it in CeO2.
Mixed quantum-classical theory for the collisional energy transfer and the rovibrational energy flow: Application to ozone stabilization
Authors: Ivanov Mikhail V. , Babikov Dmitri
Published: April 2011

A mixed quantum-classical approach to the description of collisional energy transfer is proposed in which the vibrational motion of an energized molecule is treated quantum mechanically using wave packets, while the collisional motion of the molecule and quencher and the rotational motion of the molecule are treated using classical trajectories. This accounts rigorously for quantization of vibrational states, zero-point energy, scattering resonances, and permutation symmetry of identical atoms, while advantage is taken of the classical scattering regime. Energy is exchanged between vibrational, rotational, and translational degrees of freedom while the total energy is conserved. Application of this method to stabilization of the van der Waals states in ozone is presented. Examples of mixed quantum-classical trajectories are discussed, including an interesting example of supercollision. When combined with an efficient grid mapping procedure and the reduced dimensionality approximation, the method becomes very affordable computationally.
Plasmon-enhanced photoluminescence from bioconjugated gold nanoparticle and nanodiamond assembly
Authors: Liu Y. L. , Sun K. W.
Published: April 2011

In this letter, we coupled nanodiamonds (NDs) with gold nanoparticles of different sizes using two complementary DNA sequences. After hybridizing the gold nanoparticles on the NDs, we observed the enhancement of the photoluminescence (PL) signals originating from the nitrogen-vacancy (N-V) center of the ND. The enhancement was attributed to the plasmon field created by the gold nanoparticles. The line shape of the enhanced PL spectra was also affected by the sizes of the attached nanoparticles due to their different resonant plasma frequencies. The signal enhancement can be used as an indexing tool for biosensing applications.
Mechanically and optically reliable folding structure with a hyperelastic material for seamless foldable displays
Authors: Kwon Hyuk-Jun , Shim HongShik , Kim Sunkook , Choi Woong , Chun Youngtea , Kee InSeo , Lee SangYoon
Published: April 2011

We report a mechanically and optically robust folding structure to realize a foldable active matrix organic-light-emitting-diode (AMOLED) display without a visible crease at the junction. A nonlinear stress analysis, based on a finite element method, provided an optimized design. The folding-unfolding test on the structure exhibited negligible deterioration of the relative brightness at the junction of the individual panels up to 105 cycles at a folding radius of 1 mm, indicating highly reliable mechanical and optical tolerances. These results demonstrate the feasibility of seamless foldable AMOLED displays, with potentially important technical implications on fabricating large size flexible displays.
Spectroscopic observation and structure of CS2 dimer
Authors: Rezaei M. , Norooz Oliaee J. , Moazzen-Ahmadi N. , McKellar A. R. W.
Published: April 2011

Infrared spectra of the CS2 dimer are observed in the region of the CS2 ν3 fundamental band (∼1535 cm−1) using a tunable diode laser spectrometer. The weakly bound complex is formed in a pulsed supersonic slit-jet expansion of a dilute gas mixture of carbon disulfide in helium. Contrary to the planar slipped-parallel geometry previously observed for (CO2)2, (N2O)2, and (OCS)2, the CS2 dimer exhibits a cross-shaped structure with D2d symmetry. Two bands were observed and analyzed: the fundamental (C–S asymmetric stretch) and a combination involving this mode plus an intermolecular vibration. In both cases, the rotational structure corresponds to a perpendicular (ΔK = ±1) band of a symmetric rotor molecule. The intermolecular center of mass separation (C–C distance) is determined to be 3.539(7) Å. Thanks to symmetry, this is the only parameter required to characterize the structure, if the monomer geometry is assumed to remain unchanged in the dimer. From the band centers of the fundamental and combination band an intermolecular frequency of 10.96 cm−1 is obtained, which we assign as the torsional bending mode. This constitutes the first high resolution spectroscopic investigation of CS2 dimer.
Effective potentials between nanoparticles in suspension
Authors: Grest Gary S. , Wang Qifei , Veld Pieter in't , Keffer David J.
Published: April 2011

Results of molecular dynamics simulations are presented for the pair distribution function between nanoparticles in an explicit solvent as a function of nanoparticle diameter and interaction strength between the nanoparticle and solvent. The effect of including the solvent explicitly is demonstrated by comparing the pair distribution function of nanoparticles to that in an implicit solvent. The nanoparticles are modeled as a uniform distribution of Lennard-Jones particles, while the solvent is represented by standard Lennard-Jones particles. The diameter of the nanoparticle is varied from 10 to 25 times that of the solvent for a range of nanoparticle volume fractions. As the strength of the interactions between nanoparticles and the solvent increases, the solvent layer surrounding the nanoparticle is formed which increases the effective radii of the nanoparticles. The pair distribution functions are inverted using the Ornstein–Zernike integral equation to determine an effective pair potential between the nanoparticles mediated by the introduction of an explicit solvent.
Measurement of the electric quadrupole moment of N2O
Authors: Chetty Naven , Couling Vincent W.
Published: April 2011

Measurements of the temperature dependence of the Buckingham effect (electric-field-gradient-induced birefringence, EFGIB) for gaseous nitrous oxide are presented. Measurements span the temperature range 298.5–473.9 K, which allows for separation of the temperature-independent hyperpolarizability term from the temperature-dependent quadrupole term, yielding a quadrupole moment of Θ = (−11.03 ± 0.41) × 10−40 C m2, and a hyperpolarizability term of b = (−0.638 ± 0.063) × 10−60 C3 m4 J−2. For dipolar molecules, the quadrupole moment is origin dependent, and the value reported here is referred to an origin called the effective quadrupole center (EQC). Comparison of this value with the center of mass (CM) quadrupole moment obtained from MBER experiments yields information about the dynamic dipole-quadrupole and dipole-magnetic dipole polarizabilities. The temperature-independent term, previously assumed to contribute negligibly to the EFGIB, is found to contribute some (5.2 ± 0.6)% to the effect at room temperature and clearly needs to be accounted for if the quadrupole moment is to be definitively established.
Mucosal wrinkling in animal antra induced by volumetric growth
Authors: Li Bo , Cao Yan-Ping , Feng Xi-Qiao , Yu Shou-Wen
Published: April 2011

Surface wrinkling of animal mucosas is crucial for the biological functions of some tissues, and the change in their surface patterns is a phenotypic characteristic of certain diseases. Here we develop a biomechanical model to study the relationship between morphogenesis and volumetric growth, either physiological or pathological, of mucosas. Theoretical analysis and numerical simulations are performed to unravel the critical characteristics of mucosal wrinkling in a spherical antrum. It is shown that the thicknesses and elastic moduli of mucosal and submucosal layers dictate the surface buckling morphology. The results hold clinical relevance for such diseases as inflammation and gastritis.
Mechanism for transient migration of xenon in UO2
Authors: Liu X.-Y. , Uberuaga B. P. , Andersson D. A. , Stanek C. R. , Sickafus K. E.
Published: April 2011

In this letter, we report recent work on atomistic modeling of diffusion migration events of the fission gas product xenon in UO2 nuclear fuel. Under nonequilibrium conditions, Xe atoms can occupy the octahedral interstitial site, in contrast to the thermodynamically most stable uranium substitutional site. A transient migration mechanism involving Xe and two oxygen atoms is identified using basin constrained molecular dynamics employing a Buckingham type interatomic potential. This mechanism is then validated using density functional theory calculations using the nudged elastic band method. An overall reduction in the migration barrier of 1.6–2.7 eV is obtained compared to vacancy-mediated diffusion on the uranium sublattice.