bluuebunny/crossref_metadata_2025 huggingface dataset:
Topology of the quantum control landscape for observables
Authors: Hsieh Michael , Wu Rebing , Rabitz Herschel
Published: March 2009

A broad class of quantum control problems entails optimizing the expectation value of an observable operator through tailored unitary propagation of the system density matrix. Such optimization processes can be viewed as a directed search over a quantum control landscape. The attainment of the global extrema of this landscape is the goal of quantum control. Local optima will generally exist, and their enumeration is shown to scale factorially with the system’s effective Hilbert space dimension. A Hessian analysis reveals that these local optima have saddlepoint topology and cannot behave as suboptimal extrema traps. The implications of the landscape topology for practical quantum control efforts are discussed, including in the context of nonideal operating conditions.
Magnetoelastic coupling in Fe100−xGex single crystals with 4<x<18
Authors: Petculescu G. , LeBlanc J. B. , Wun-Fogle M. , Restorff J. B. , Yuhasz W. M. , Lograsso T. A. , Clark A. E.
Published: March 2009

In this paper we examine the elastic (c′ and c44) and magnetostrictive (λ100 and λ111) behaviors of Fe100−xGex for 4<x<18, quantities used further to find the fundamental magnetoelastic coupling constants b1 and b2 at room temperature. The x dependence of b1 and b2 for Fe100−xGex is contrasted to those of Fe100−xGax and Fe100−xAlx. While the rhombohedral shear elastic constant c44 is almost insensitive to the type and amount of solute, the tetragonal shear constant c′ shows a pronounced and rapid softening with increasing x for all three alloys but with different decreasing slopes. Similarly, while the rhombohedral magnetostriction λ111 behavior is analogous for all three alloy systems, showing a sign change from negative to positive at the onset of chemical order, the tetragonal magnetostriction λ100 behavior differs. For the Ga and Al alloys, λ100 maintains positive values over the entire x range, both curves showing large peak values, whereas λ100 of Fe100−xGex exhibits a moderate positive peak followed by a negative dip, both of comparable magnitude. Finally the tetragonal coupling constant −b1 of Fe–Ge shows a marked, sharp decrease as chemical order occurs at x∼12 at. % Ge. The decline continues until the ordered D03 phase is fully established at x∼18 at. % Ge. The peak value of |b1| for Fe–Ge is approximately half of those for Fe–Ga and Fe–Al. This smaller value of |b1|, obtained for the higher electron concentration Ge alloy, is consistent with predictions based on band structure calculations. The rhombohedral coupling constant −b2 shows a consistent sign change at the occurrence of chemical ordering in both Fe–Ga and Fe–Ge.
Depinning field of a periodic domain wall array in vicinal nanowires
Authors: Dantas Ana L. , Rebouças G. O. G. , Carriço A. S.
Published: March 2009

We report a theoretical investigation of the magnetic states and depinning field of a periodic array of head-to-head domain walls of flat Fe rectangular nanowires, exchange coupled with a vicinal two-sublattice uniaxial antiferromagnetic substrate. We show that for strong interface exchange energy, domain walls are pinned at interface steps perpendicular to the antiferromagnetic easy axis, separating terraces with opposite interface exchange field. The array sequence, which alternates head-to-head and tail-to-tail domain walls, may form a structure with alternate chirality or with the same chirality. The domain wall dipolar field affects the chirality sequence, which is tunable by the geometrical constraints and the strength of the interface exchange field. The depinning field of 10 nm thick, 1 μm long wires, with widths of 100 and 200 nm, is of the order of the interface field strength, and the depinning process involves domain wall motion and the transversal displacement of a periodic array of vortices.
Magnetic properties, phase evolution, and structure of melt spun SmCo7−xNbx (x=0–0.6) ribbons
Authors: Guo Z. H. , Chang H. W. , Chang C. W. , Hsieh C. C. , Sun A. C. , Chang W. C. , Pan W. , Li W.
Published: March 2009

The phase evolution, microstructure, and magnetic properties of melt spun SmCo7−xNbx (x=0–0.6) ribbons have been investigated using powder x-ray diffraction, transmission electron microscopy, and vibrating sample magnetometer, respectively. SmCo7−xNbx ribbons could crystallize in TbCu7-type structure only for low Nb substitution of x=0–0.3 at high wheel speed of 30–40 m/s. According to the structure refinement, the doping element Nb prefers to occupy the 2e site. The intrinsic coercivity increases dramatically from 1.9 kOe for SmCo7 ribbon to 10.2 kOe for SmCo6.8Nb0.2 ribbon at wheel speed of 40 m/s. The mechanism of the coercivity enhancement has been discussed. The optimal magnetic properties of σr=54.4 emu/g and Hic=10.2 kOe were obtained in SmCo6.8Nb0.2 ribbon.
Magnetic anisotropy and the cone state in Co/Pt multilayer films
Authors: Stillrich Holger , Menk Christian , Frömter Robert , Oepen Hans Peter
Published: March 2009

The magnetic anisotropy of Co/Pt multilayers is investigated. The perpendicular uniaxial anisotropy is discussed in second order approximation under a variation in Co and Pt layer thicknesses. The evolution of anisotropy constants is shown in the phase diagram of first and second order anisotropy constants. A thickness driven spin reorientation via the canted phase is observed for a single Co layer as well as for Co/Pt multilayer films.
Nonlinear-dynamical-system approach to microwave-assisted magnetization dynamics (invited)
Authors: Bertotti G. , Mayergoyz I. D. , Serpico C. , d’Aquino M. , Bonin R.
Published: March 2009

Methods of nonlinear-dynamical-system theory are applied to the study of magnetization dynamics under the action of microwave magnetic fields. In the case of a system with uniaxial anisotropy subject to a circularly-polarized microwave field, the conditions are derived under which the magnetization reversal field is substantially reduced by the application of the microwave field. The dependence of magnetization switching on microwave-field-pulse duration is discussed and analytical expressions are derived for the minimum pulse duration leading to switching.
High Bs nanocrystalline alloys with high amorphous-forming ability
Authors: Urata A. , Matsumoto H. , Sato S. , Makino A.
Published: March 2009

The soft magnetic property and the structure of the new type nanocrystalline Fe–(P,B,Si,Nb,Cu) alloy with high Fe content about 80 at. % have been investigated. In this paper Fe77.9P3B14Nb5Cu0.1 alloy has a supercooled liquid region (ΔTx) of 33 K defined by the difference between the glass transition temperature (Tg) and crystallization temperature (Tx), which would be relating to the high glass-forming ability leading a thickness of 140 μm melt-spun ribbon with a glassy phase. The Fe80.9P2B10Si2Nb5Cu0.1 alloy with the higher Fe content can be melt spun into the thick ribbon with thickness of 65 μm due to rather large amorphous-forming ability. In addition, a homogeneous nanostructure composed of α-Fe grain with around 15 nm in diameter was realized after crystallization, and the nanostructured alloy exhibits the high Bs of 1.55 T and the low coercivity (Hc) of 4.6 A/m. The Bs and the glass or amorphous-forming ability of the alloys are superior to those for typical nanocrystalline. Therefore, the miniaturization and the realization of higher efficiency of power supply circuits in electric devices are able to be expected using these nanocrystalline Fe–(P,B,Si,Nb,Cu) alloys.
Development of a new magnetic circuit for slim microspeakers
Authors: Lee Chang-Min , Kwon Joong-Hak , Hwang Gun-Yong , Hwang Sang-Moon
Published: March 2009

In the minimultimedia market, functional diversity, high performance, and design are given top priority. Lightweight mobile phones of unique design especially attract customer interest and are usually considered to be highly competitive items in the marketplace. Therefore, mobile phones need wider and thinner liquid crystal displays and smaller microspeakers with high acoustic performance. There are two main directions for development of slim microspeakers. One is reducing the thickness, and the other is reducing the width. In the case of reducing the width, the total magnet volume is maintained through extending the length of the magnet circuit [S. M. Hwang et al., IEEE Trans. Magn. 39, 2003 (2003)]. In contrast, reducing the thickness results in a reduction in total magnet volume unless width is increased, which causes many difficulties in the manufacturing process. In this paper, we introduce a new magnetic circuit for slim microspeakers. This new magnetic circuit, which excludes the top plate, makes it possible to manufacture slimmer microspeakers more cost-effectively without any loss of acoustic performance.
75% inverse magnetoresistance at room temperature in Fe4N/MgO/CoFeB magnetic tunnel junctions fabricated on Cu underlayer
Authors: Komasaki Yosuke , Tsunoda Masakiyo , Isogami Shinji , Takahashi Migaku
Published: March 2009

Pseudo-single-crystal Cu underlayer (UL) with thermal tolerance was obtained on bare Si wafer by employing a diffusion-blocking layer. Fe4N layer fabricated on the Cu UL has an epitaxial relationship and a large grain diameter. Magnetic tunnel junctions in a stacking sequence of Fe4N/MgO/CoFeB exhibited an inverse tunnel magnetoresistance (TMR) effect at room temperature. The largest magnitude of the TMR ratio, −75.1%, was obtained at bias voltage Vb=−250 mV, where electrons flow from CoFeB to Fe4N. The large inverse TMR ratio is due to the improvement of the crystallinity of the Fe4N film because of the small lattice misfit between Fe4N and Cu.
Magnetism of Cr-doped diamond-like carbon
Authors: Santana J. A. Colón , Skomski R. , Singh V. , Palshin V. , Petukhov A. , Losovyj Ya. B. , Sokolov A. , Dowben P. A. , Ketsman I.
Published: March 2009

Chromium-doped hydrogenated diamond-like carbon (Cr-DLC) and chromium carbide hydrogenated DLC alloys were synthesized by plasma-assisted vapor deposition and investigated by x-ray absorption near edge structure spectroscopy, extended x-ray absorption fine structure, and superconducting quantum interference device (SQUID) magnetometry. Structural and magnetic properties of the doped and alloy materials were investigated as a function of the Cr concentration (0.1–20 at. %). Toward the upper end of the concentration range, Cr precipitates in the form of chromium carbide (Cr3C2) nanoclusters. For low Cr concentrations, the systems are ferromagnetic at very low temperatures, whereas the chromium carbide clusters appear to be antiferromagnetic with uncompensated spins at the surface. Cr-DLC films and alloys with various Cr concentrations are used to make heterojunctions on silicon, and the produced diodes are investigated by I-V measurements. The heterojunctions exhibit negative magnetoresistance that saturates at less than 500 Oe and may be suitable for spin-electronics applications.
Pseudo-spin-valve with L10 (111)-oriented FePt fixed layer
Authors: Zha C. L. , Bonetti S. , Persson J. , Zhou Yan , Åkerman Johan
Published: March 2009

Recently we proposed a spin torque oscillator where the fixed layer has its magnetization easy-axis tilted with respect to the film plane to simultaneously achieve zero-field operation and high output power [Y. Zhou, C. L. Zha, S. Bonetti, J. Persson, and J. Åkerman, Appl. Phys. Lett. 92, 262508 (2008)]. Here we take the first step toward the realization of this device and fabricate successfully a pseudo-spin-valve using an L10 (111)-oriented FePt fixed layer with tilted magnetocrystalline anisotropy. A total magnetoresistance (MR) of 0.86% is experimentally observed for the standard pseudo-spin-valve of the L10 FePt/Cu/NiFe structure in applied fields up to 1.5 T. While part of the MR may originate from the FePt film alone, the dominating part of MR correlates with switching of the NiFe free layer.
Correlation between exchange bias field and domain size of ferromagnetic layer in Mn–Ir/Co–Fe bilayers
Authors: Takahashi Hirokazu , Tsunoda Masakiyo , Fukumoto Keiki , Nakamura Tetsuya , Arai Kuniaki , Kinoshita Toyohiko , Takahashi Migaku
Published: March 2009

In order to clarify the correlation between the exchange bias strength and magnetic domain structure in ferromagnetic/antiferromagnetic (AF) bilayers, ferromagnetic domain structure was observed at Co-L3 edge for polycrystalline Mn–Ir/Co–Fe bilayers by using the x-ray magnetic circular dichroism photoelectron emission microscopy technique. From the observation for four different samples prepared with the respective thermal annealing procedures, a positive correlation was found between the strength of exchange bias field and the ferromagnetic domain size. Within the framework of single spin ensemble model, it was indicated that the larger size of AF domain, which means small distribution of fixing direction of AF spins, can induce the larger exchange bias field.
Magnetic ordering in the spinel compound Li[Mn2−xLix]O4(x=,0.04)
Authors: Gaddy John , Lamsal Jagat , Petrovic Marcus , Montfrooij Wouter , Schmets Alexander , Vojta Thomas
Published: March 2009

The two B-site ions Mn3+ and Mn4+ in the stoichiometric spinel structure LiMn2O4 form a complex, columnar ordered pattern below the charge-ordering transition at room temperature. On further cooling to below 66 K, the system develops long-range antiferromagnetic order. In contrast, whereas lithium-substituted Li[Mn2−xLix]O4 also undergoes a charge-ordering transition around room temperature, it only displays frozen in short-range magnetic order below ∼25–30 K. We investigate to what extent the columnar charge-order pattern observed in LiMn2O4 can account for the measured magnetic ordering patterns in both the pure and Li-substituted (x=0.04) compounds. We conclude that eightfold rings of Mn4+ ions form the main magnetic unit in both compounds (x=0,0.04), and that clusters formed out of these rings act as superspins in the doped compound.
Magnetic properties of the thulium layered compound Tm2AlB116: An AlB2-type analogue
Authors: Mori T. , Shishido T. , Nakajima K. , Kiefer K. , Siemensmeyer K.
Published: March 2009

Tm 2 Al B 11 6 with the Y2ReB6-type crystal structure was synthesized, and the magnetic properties, specific heat, and magnetic entropy were investigated. Recent developments in rare earth aluminoboride compounds with two-dimensional boron layers have attracted interest due to the heavy fermion behavior superconductivity discovered in YbAlB4 and the multiple anomalies manifesting below the Néel temperatures in TmAlB4 attributed to be due to intrinsic building defects. Tm2AlB116 has a strong structural analogy to the AlB2-type structure similar to the REAlB4 compounds. Multiple antiferromagnetic transitions were found in Tm2AlB116 at TN1=3.5 K and TN2=2.8 K at zero field. Interestingly, the magnetic field dependence of the two transitions differed sharply. A low field of 4 kOe suppresses the low temperature transition, whereas TN1 initially reduces with applied field but then exhibits a surprising robustness (i.e., steep phase boundary) versus the magnetic field above 20 kOe with the peak temperature hardly shifting up to 70 kOe. This indicates that a high field phase exists with extreme stability. The complex behavior is indicated to be influenced by different factors compared to related systems TmB4 and TmAlB4, e.g., crystal field effects or surprisingly strong magnetic interactions above TN1 indicated by the specific heat behavior.
Synthesis of surface enhanced Raman scattering active magnetic nanoparticles for cell labeling and sorting
Authors: Charan Shobhit , Kuo Chiung Wen , Kuo Yu-Wei , Singh Narendra , Drake Philip , Lin Yuh-Jiuan , Tay Lilin , Chen Peilin
Published: March 2009

Here we report the synthesis of the magnetic nanocomposite nanoparticles with Fe3O4 core and silver shell for cell imaging and separation. When the magnetic nanoparticles are decorated with surface enhanced Raman scattering (SERS) active molecules, they can be used for cell separation with unique optical signature. In this experiment, commercially available superparamagnetic nanoparticles (fluidMAG) with 50 nm diameter were used as the core. The shell layer was produced by the reduction of the silver salts. As a result of the reduction, nanocomposite magnetic nanoparticles with 60 nm diameter were obtained. To create unique SERS patterns for multiplexing, the surfaces of the nanoparticles were further modified with chloro-, bromo-, or fluorobenzenethiol. When these nanoparticles were incubated with 3T3 cells, it was found that the nanoparticles were located around the nucleus in the cytoplasm.
Rapidly solidified Sm–Co–V nanocomposite permanent magnets
Authors: Valiveti R. S. K. , Ingmire A. , Shield J. E.
Published: March 2009

Alloys around the Sm–Co eutectic composition provide an opportunity to form two-phase nanocomposite permanent magnets consisting of nanoscale Co fibers embedded in Sm2Co17.While ternary alloying elements may refine the scale of the structure, they may also disrupt the eutectic growth and lead to the formation of primary Co. Thus, microstructural selection maps were constructed for conventionally solidified Sm–Co–V alloys. It was found that V additions enlarged the primary Sm2Co17-forming region and, at (Sm0.09Co0.91)97V3, resulted in a eutectic structure. Upon rapid solidification, this alloy was determined to have a coercivity of 5 kOe with a high remanent ratio. However, the V addition reduced the magnetization, which limited the energy product to 4.3 MG Oe. The rapidly solidified structure consisted of primary SmCo7 dendrites along with an intergranular Co region, suggesting that eutectic structure formation is skewed by underlying metastable phase relationships.
Size effects in ordered arrays of magnetic nanotubes: Pick your reversal mode
Authors: Bachmann Julien , Escrig Juan , Pitzschel Kristina , Moreno Josep M. Montero , Jing Jing , Görlitz Detlef , Altbir Dora , Nielsch Kornelius
Published: March 2009

Ordered arrays of magnetic nanotubes are prepared by combining a porous template (anodic alumina) with a self-limiting gas-solid chemical reaction (atomic layer deposition). The geometric parameters can thus be tuned accurately (tube length of 1–50 μm, diameter of 20–150 nm, and wall thickness of 1–40 nm), which enables one to systematically study how confinement and anisotropy effects affect the magnetic properties. In particular, the wall thickness of such ordered Fe3O4 nanotubes has a nonmonotonic influence on their coercive field. Theoretical models reproduce the size effects that are experimentally observed and interpret them as originating from a crossover between two distinct modes of magnetization reversal.
Magnetic properties of heavy fermion system Ce1−xGdxCoSi3
Authors: Hong J. B. , Oh H. J. , Kwon Y. S.
Published: March 2009

We have investigated magnetic properties of the heavy fermion system Ce1−xGdxCoSi3(0≤x≤1) via specific heat and electrical resistivity measurements. The specific heat shows antiferromagnetism at 9 K for GdCoSi3(x=1.0). With increasing Ce concentration, TN linearly decreases and is suppressed at x=0.1. An anomalous hump in specific heat was found below the ordering temperature for x≥0.4. For x=0.1, non-Fermi-liquid behavior was found in electrical resistivity and −ln T divergence in C/T below 3 K. Our results indicate that x=0.1 is a quantum critical point.
Thermally stable high effective work function TaCN thin films for metal gate electrode applications
Authors: Adelmann C. , Meersschaut J. , Ragnarsson L.-Å. , Conard T. , Franquet A. , Sengoku N. , Okuno Y. , Favia P. , Bender H. , Zhao C. , O’Sullivan B. J. , Rothschild A. , Schram T. , Kittl J. A. , Van Elshocht S. , De Gendt S. , Lehnen P. , Boissière O. , Lohe C.
Published: March 2009

TaCN layers were deposited using metal-organic chemical-vapor deposition for applications as metal gate electrodes in p-type metal-oxide-semiconductor (pMOS) devices. The films were formed by thermal decomposition of tertiary-amylimido-tris(dimethylamido)tantalum (TAIMATA®) between 400 and 600 °C. The composition was dependent on the growth temperature with increasing C and decreasing N content at higher temperature. Films grown below 500 °C were nearly amorphous and became weakly polycrystalline with a cubic structure at higher growth temperature. The layer density was ∼8.1 g/cm3, about half of the TaCN bulk density. Grazing-incidence x-ray diffraction and transmission electron microscopy showed that the films consist of small polycrystalline grains in an amorphous matrix. The resistivity was found to decrease with increasing growth temperature. Lowest resistivity values were around 1 mΩ cm for films grown at 600 °C. The films formed an ∼4 nm thick insulating surface oxide, which leads to a thickness dependence of the film resistivity. Oxygen was also found to diffuse slowly into the bulk metal, which leads to a resistivity aging effect. The effective work function of the TaCN films was found to be 4.8 eV on HfSiO4 and HfSiON and to shift weakly by high thermal budget annealing toward the Si valence band, reaching 4.9 eV on HfSiO4.
Mesoscale simulation of polymer reaction equilibrium: Combining dissipative particle dynamics with reaction ensemble Monte Carlo. II. Supramolecular diblock copolymers
Authors: Lísal Martin , Brennan John K. , Smith William R.
Published: March 2009

We present an alternative formulation of the reaction ensemble dissipative particle dynamics (RxDPD) method [M. Lísal, J. K. Brennan, and W. R. Smith, J. Chem. Phys. 125, 16490 (2006)], a mesoscale simulation technique for studying polymer systems in reaction equilibrium. The RxDPD method combines elements of dissipative particle dynamics (DPD) and reaction ensemble Monte Carlo (RxMC), and is primarily targeted for the prediction of the system composition, thermodynamic properties, and phase behavior of reaction equilibrium polymer systems. The alternative formulation of the RxDPD method is demonstrated by considering a supramolecular diblock copolymer (SDC) melt in which two homopolymers, An and Bm, can reversibly bond at terminal binding sites to form a diblock copolymer, AnBm. We consider the effect of the terminal binding sites and the chemical incompatibility between A- and B-segments on the phase behavior. Both effects are found to strongly influence the resulting phase behavior. Due to the reversible nature of the binding, the SDC melt can be treated as the reaction equilibrium system An+Bm⇌AnBm. To simulate the An+Bm⇌AnBm melt, the system contains, in addition to full An, Bm, and AnBm polymers, two fractional polymers: one fractional polymer either fAn or fBm, and one fractional polymer fAnBm, which have fractional particles at the ends of the polymer chains. These fractional particles are coupled to the system via a coupling parameter. The time evolution of the system is governed by the DPD equations of motion, accompanied by random changes in the coupling parameter. Random changes in the coupling parameter mimic forward and reverse reaction steps as in the RxMC approach, and they are accepted with a probability derived from the expanded ensemble grand canonical partition function. Unlike the original RxDPD method that considers coupling of entire fractional polymers to the system, the expanded ensemble framework allows a stepwise coupling, thus greatly increasing the efficiency of the RxDPD approach. The RxDPD technique rigorously satisfies thermodynamic equilibrium, but not the hydrodynamic behavior. However, the approximate treatment of the hydrodynamics can be minimized by simulating a large number of particles.
Effect of residual stress on the ferroelectric property of (Pb0.90La0.10)Ti0.975O3 thin films
Authors: Wu Jiagang , Xiao Dingquan , Zhu Jianguo
Published: March 2009

Highly (001)-, (100)-, (101)-, and (110)-oriented (Pb0.90La0.10)Ti0.975O3 (PLT) thin films were deposited on the LaNiO3(001)/Pt(111)/Ti/SiO2/Si(100), LaNiO3/SiO2/Si(100), PbTiO3/Pt(111)/Ti/SiO2/Si(100), and LaNiO3(110)/Pt(111)/Ti/SiO2/Si(100) substrates by rf magnetron sputtering. The orientation dependence of ferroelectric properties of the PLT films was investigated. The result shows that the (001)-oriented PLT thin films exhibit enhanced ferroelectric properties (2Pr=61.1 μC/cm2 and 2Ec=179 kV/cm). The residual stress of the PLT thin films with different orientations was measured by x-ray diffraction (sin2 Ψ method) for illuminating the related physical mechanisms. The result shows that the enhanced ferroelectric properties of (001)-oriented PLT thin films should be attributed to low residual stress.
Exciton diffusion lengths of organic semiconductor thin films measured by spectrally resolved photoluminescence quenching
Authors: Lunt Richard R. , Giebink Noel C. , Belak Anna A. , Benziger Jay B. , Forrest Stephen R.
Published: March 2009

We demonstrate spectrally resolved photoluminescence quenching as a means to determine the exciton diffusion length of several archetype organic semiconductors used in thin film devices. We show that aggregation and crystal orientation influence the anisotropy of the diffusion length for vacuum-deposited polycrystalline films. The measurement of the singlet diffusion lengths is found to be in agreement with diffusion by Förster transfer, whereas triplet diffusion occurs primarily via Dexter transfer.
Transient drain current of polymer transistors
Authors: Fujieda Ichiro , Street Robert A.
Published: March 2009

We have measured transient drain current in millisecond range for some poly(thiophene) transistors with different channel length. Under fixed gate bias and with source grounded, a voltage pulse is applied periodically to the drain and the current waveform is recorded. The drain current of a 50 μm channel device is almost proportional to the drain bias at 0.2 ms after the drain bias application. As the time progresses, the transient output curve starts to show a sign of saturation at a large drain bias. For a shorter-channel device, this initial resistorlike characteristic is less apparent. Two mechanisms are considered to account for this behavior. First, we analyze contribution from the channel charges that exist in excess of the steady-state distribution. Although the linear dependency on the drain bias is correctly predicted, its time dependence underestimates the observed drain current. This suggests that the number of mobile carriers in the channel is more than those balancing the gate potential during the early stage. Second, we consider relaxation of this excess current. Its decay is well fit by a stretched exponential function. Using an empirical expression for the data at a certain low drain bias, transient drain current at other drain biases is calculated. The resultant output curves more or less reproduce the measurement.
Dielectric relaxation mechanisms of BiMn2O5 ceramics
Authors: Lin Y. Q. , Wu Y. J. , Chen X. M. , Gu S. P. , Tong J. , Guan S.
Published: March 2009

Dielectric properties of multiferroic BiMn2O5 ceramics were evaluated over broad temperature and frequency ranges. Two Debye-type dielectric relaxations were observed at low temperatures (130–250 K) and high temperatures (200–450 K), respectively. The low temperature relaxation with an activation energy of 0.18 eV was attributed to charge carrier hopping process between Mn3+ and Mn4+. The high temperature dielectric relaxation with an activation energy of 0.38 eV, which is similar to the activation energy of conductivity, was associated with oxygen vacancies related defect complex. The dielectric response at high temperatures was significantly suppressed and the dc conductivity increased after oxygen annealing.
Magnetic properties of a molecular-based magnet AFeIIFeIII(C2O4)3 with biaxial crystal field
Authors: Jiang Wei , Wang Wei , Zhang Fan , Ren Wei Jun
Published: March 2009

The molecular-based magnetic materials AFeIIFeIII(C2O4)3 have a honeycomb structure in which FeII(S=2) and FeIII(S=5/2) occupy sites alternately, which is described by mixed spin-2 and spin-5/2 Ising model with ferrimagnetic interlayer coupling. Within the effective-field theory with self-spin correlations, the effects of the biaxial crystal field on the magnetization, initial susceptibility, and internal energy of this molecular-based system have been studied numerically
Edge radial electric field structure and its connections to H-mode confinement in Alcator C-Mod plasmas
Authors: McDermott R. M. , Lipschultz B. , Hughes J. W. , Catto P. J. , Hubbard A. E. , Hutchinson I. H. , Granetz R. S. , Greenwald M. , LaBombard B. , Marr K. , Reinke M. L. , Rice J. E. , Whyte D. ,   Alcator C-Mod Team
Published: March 2009

High-resolution charge-exchange recombination spectroscopic measurements of B5+ ions have enabled the first spatially resolved calculations of the radial electric field (Er) in the Alcator C-Mod pedestal region [E. S. Marmar, Fusion Sci. Technol. 51, 261 (2006)]. These observations offer new challenges for theory and simulation and provide for important comparisons with other devices. Qualitatively, the field structure observed on C-Mod is similar to that on other tokamaks. However, the narrow high-confinement mode (H-mode) Er well widths (5 mm) observed on C-Mod suggest a scaling with machine size, while the observed depths (up to 300 kV/m) are unprecedented. Due to the strong ion-electron thermal coupling in the C-Mod pedestal, it is possible to infer information about the main ion population in this region. The results indicate that in H-mode the main ion pressure gradient is the dominant contributor to the Er well and that the main ions have significant edge flow. C-Mod H-mode data show a clear correlation between deeper Er wells, higher confinement plasmas, and higher electron temperature pedestal heights. However, improved L-mode (I-mode) plasmas exhibit energy confinement equivalent to that observed in similar H-mode discharges, but with significantly shallower Er wells. I-mode plasmas are characterized by H-mode-like energy barriers, but with L-mode-like particle barriers. The decoupling of energy and particle barrier formation makes the I-mode an interesting regime for fusion research and provides for a low collisionality pedestal without edge localized modes.
Weakly relativistic and ponderomotive effects on the density steepening in the interaction of an intense laser pulse with an underdense plasma
Authors: Niknam A. R. , Hashemzadeh M. , Shokri B.
Published: March 2009

The effect of the weakly relativistic ponderomotive force in the interaction of an intense laser pulse with an underdense plasma is studied. This force modifies the electron density distribution. Furthermore, the existence of intense laser pulses in plasma causes the electron relativistic mass to appear and, consequently, the plasma frequency decreases. It is clear that the electron temperature also affects the dielectric permittivity of plasma. By considering the weakly relativistic and ponderomotive effects, the nonlinear dielectric permittivity of plasma is obtained. Taking into account the Maxwell equations and the nonlinear dielectric permittivity, the electric and magnetic field profiles in plasma are investigated. It is shown that the electromagnetic field profiles deviate from sinusoidal structure. Also, the steepening of the electron density profile decreases by increasing the electron temperature and decreasing the laser pulse intensity. The wavelength of oscillations decreases by increasing the energy flux. Finally, it is found that the electric and magnetic field profiles are lengthened by increasing the electron temperature.
On the dissociation of I2 by O2(a 1Δ): Pathways involving the excited species I2(A′ Π32u,A Π31u), I2(X 1∑,υ), and O2(a 1Δ,υ)
Authors: Azyazov V. N. , Pichugin S. Yu. , Heaven Michael C.
Published: March 2009

Kinetic studies were carried out to explore the role of the excited species I2(A′ Π32u,A Π31u), I2(X 1∑,υ), and O2(a 1Δ,υ) in the dissociation of I2 by singlet oxygen. A flow tube apparatus that utilized a chemical singlet oxygen generator was used to measure the I2 dissociation rate in O2(a 1Δ)/I2 mixtures. Vibrationally excited I2(X) is thought to be a significant intermediate in the dissociation process. Excitation probabilities (γυ) for population of the υth I2(X) vibrational level in the reaction I2(X)+I(P21/2)→I2(X,υ>10)+I(P23/2) were estimated based on a comparison of calculated populations with experimentally determined values. Satisfactory agreement with the experimental data [Barnault et al., J. Phys. IV 1, C7–647 (1991)] was achieved for total excitation probabilities partitioned in two ranges, such that Γ25≤υ≤47=∑υ=2547γυ≈0.1 and Γ15≤υ≤24=∑υ=1524γυ≈0.9. A multipathway I2 dissociation model was developed in which the intermediates are I2(A′ Π32u,A Π31u) and I2(X,υ). It was shown that the iodine dissociation process passes predominantly through the I2(A′ Π32u,A Π31u) intermediate. These states are populated by collisions of I2 with vibrationally excited O2(a 1Δ,υ) at the initiation and the chain stages, when the mole fraction of I2 is small (ηI2<1%). For higher I2 concentrations (ηI2≥1%) the excited states are populated in the chain stage by collisions of I2(X,15≤υ≤24) with O2(a 1Δ).
Enhanced multiferroic characteristics in NaNbO3-modified BiFeO3 ceramics
Authors: Ma Yan , Chen Xiang Ming
Published: March 2009

Modification of BiFeO3 multiferroic ceramics were investigated by introducing NaNbO3 to form (Bi1−xNax)(Fe1−xNbx)O3 (x=0.1, 0.3, and 0.5) solid solutions. With increasing NaNbO3 content, the crystalline structure changed and the ferroelectric properties were improved. The significantly reduced leakage resulted in more regular ferroelectric hysteresis loop for all compositions. Dielectric characteristics of (Bi1−xNax)(Fe1−xNbx)O3 (x=0.1, 0.3, and 0.5) ceramics were evaluated. Two dielectric relaxations were observed in the temperature ranges of 550–600 and 650–710 K in (Bi0.9Na0.1)(Fe0.9Nb0.1)O3 ceramics, and the higher-temperature dielectric relaxation was related to the antiferromagnetism transition, which indicated the coupling between the ferroelectric and magnetic orders essential for the multiferroic materials. Moreover, weak ferromagnetism with remnant magnetization (Mr) of 0.14 emu/g and coercive field (Hc) of 6.48 kOe were observed in the sample with x=0.1, which suggested that the present modified ceramics had enhanced multiferroic characteristics.
Microwave absorption properties of the hierarchically branched Ni nanowire composites
Authors: Qiao Liang , Han Xianghua , Gao Bo , Wang Jianbo , Wen Fusheng , Li Fashen
Published: March 2009

Microwave absorber using hierarchically branched Ni nanowires based composite was introduced in this work. The nanostructure constituents greatly suppressed the eddy current and improved the magnetic anisotropy. Their composite with wax matrix showed strong and broadband electromagnetic attenuation around 8.23 GHz where the magnetic mode was excited. The resonance absorption character was discussed by fitting the permeability spectrum using well-known theoretical equations (Landau-Lifshitz-Gilbert equation and Kittel formula).
Magneto-optical study of thermally annealed InAs-InGaAs-GaAs self-assembled quantum dots
Authors: Nabavi E. , Badcock T. J. , Nuytten T. , Liu H. Y. , Hopkinson M. , Moshchalkov V. V. , Mowbray D. J.
Published: March 2009

We report a magneto-optical study of InAs-InGaAs-GaAs self-assembled quantum dots (QDs) subjected to post-growth thermal annealing at different temperatures. At low temperatures annealing strongly affects the bimodal distribution of QDs; at higher temperatures a strong blueshift of the emission occurs. Magnetophotoluminescence reveals that the annealing increases the QD size, with a larger effect occurring along the growth axis, and decreases the carrier effective masses. The main contribution to the blueshift is deduced to be an increase in the average Ga composition of the QDs. The inadvertent annealing which occurs during the growth of the upper AlGaAs cladding layer in laser structures is also studied.
Effect of nitrogen-vacancy complex defects on the electronic transport of carbon nanotube
Authors: Wei Jianwei , Hu Huifang , Wang Zhiyong , Zeng Hui , Wei Yan , Jia Jinfeng
Published: March 2009

Effect of nitrogen-vacancy complex defects on the transport properties of single-walled nanotubes are simulated using density functional theory and nonequilibrium Green’s functions. We find that the defect state in carbon nanotubes becomes spatially localized and develops one half-filled impurity band near the Fermi level for either N-vacancy or N3-vacancy defect. The impurity bands are favorable to the electronic transport of the semiconducting nanotube (8, 0) but weaken that of metallic (4, 4). The studied results show that the differential conductance of the nanotubes behaves obvious oscillation characteristic.
Microscopic insights into nucleation in a sulfuric acid–water vapor mixture based on molecular dynamics simulation
Authors: Matsubara Hiroki , Ebisuzaki Toshikazu , Yasuoka Kenji
Published: March 2009

We conducted a molecular dynamics simulation of the binary nucleation in the vapor mixture consisting of water and a small amount of sulfuric acid and investigated the microscopic process in relation to the structure of the hydrate (binary cluster composed of sulfuric acid and water). It was observed that the nucleation rate increased with the concentration of sulfuric acid. It was found that the formation of the hydrate is stable as long as its size is small enough, and the hydrate growth by coagulation played a major part in the enhanced nucleation. The rate of coagulation became larger by the uptake of more sulfuric acid in the hydrate, while it was suppressed with increasing the hydration number. We found that such features came from the structure of the small hydrate which was a multishell structure composed of an inner shell of sulfuric acid and diffused outer shell of water.
Minimizing curl in a multiconnected domain
Authors: Pan Xing-Bin
Published: March 2009

Motivated by problems in the mathematical theory of liquid crystals and of superconductivity, we consider the problem of minimizing the L2 norm of curl of vector fields on a three dimensional multiconnected domain with prescribed tangential trace on the boundary. The existence and estimate of the minimizers are proven.
Plasma structures observed in gas breakdown using a 1.5 MW, 110 GHz pulsed gyrotron
Authors: Hidaka Yoshiteru , Choi E. M. , Mastovsky I. , Shapiro M. A. , Sirigiri J. R. , Temkin R. J. , Edmiston G. F. , Neuber A. A. , Oda Y.
Published: March 2009

Regular two-dimensional plasma filamentary arrays have been observed in gas breakdown experiments using a pulsed 1.5 MW, 110 GHz gyrotron. The gyrotron Gaussian output beam is focused to an intensity of up to 4 MW/cm2. The plasma filaments develop in an array with a spacing of about one quarter wavelength, elongated in the electric field direction. The array was imaged using photodiodes, a slow camera, which captures the entire breakdown event, and a fast camera with a 6 ns window. These diagnostics demonstrate the sequential development of the array propagating back toward the source. Gases studied included air, nitrogen, SF6, and helium at various pressures. A discrete plasma array structure is observed at high pressure, while a diffuse plasma is observed at lower pressure. The propagation speed of the ionization front for air and nitrogen at atmospheric pressure for 3 MW/cm2 was found to be of the order of 10 km/s.
Turbulent and neoclassical impurity transport in tokamak plasmas
Authors: Fülöp T. , Nordman H.
Published: March 2009

Impurity particle transport in tokamaks is studied using an electrostatic fluid model for main ion and impurity temperature gradient (ITG) mode and trapped electron (TE) mode turbulence in the collisionless limit and neoclassical theory. The impurity flux and impurity density peaking factor obtained from a self-consistent treatment of impurity transport are compared and contrasted with the results of the often used trace impurity approximation. Comparisons between trace and self-consistent turbulent impurity transport are performed for ITER-like profiles. It is shown that for small impurity concentrations the trace impurity limit is adequate if the plasma is dominated by ITG turbulence. However, in case of TE mode dominated plasmas the contribution from impurity modes may be significant, and therefore a self-consistent treatment may be needed.
Theoretical prediction on the structural, electronic, and polarization properties of tetragonal Bi2ZnTiO6
Authors: Wang Hai , Huang Haitao , Lu Wei , Chan Helen L. W. , Wang Biao , Woo C. H.
Published: March 2009

We present first-principles investigations on the structural, electronic, and polarization properties of Bi2ZnTiO6 using density-functional theory within the generalized gradient approximation. The theoretical structure we obtained confirms the extra large tetragonality observed by experiment. The materials exhibit a semiconductor behavior with an indirect band gap determined by the occupied O 2p and unoccupied Bi 6p states. There are strong hybridization effects between Bi–O ions, as well as Ti–O and Zn–O ones. The resulting covalent bondings, having a PbTiO3-type two-dimensional character, strengthen each other and favor the coupling between the tetragonal distortion of unit cell and the off-center displacement of A and B-site cations and O anions due to the existence of Zn, and result in the large tetragonality of this compound. Berry-phase calculation gives the polarization as high as 122 μC/cm2.
Photoluminescence and positron annihilation spectroscopy of MeV Si+ ion-irradiated SiyO1−y:Er (y≈1/3) thin films
Authors: Blakie D. E. , Zalloum O. H. Y. , Wojcik J. , Irving E. A. , Knights A. P. , Mascher P. , Simpson P. J.
Published: March 2009

Amorphous erbium-doped silicon oxide (SiyO1−y:Er, y≥1/3) thin films are currently under investigation as a luminescent material system for complementary metal-oxide semiconductor compatible light emitters. We have grown films with y≈1/3 and investigated their properties using both positron annihilation and photoluminescence (PL) spectroscopies. Films were characterized “as deposited,” following irradiation with 1 MeV Si+ ions and after isochronal annealing. The PL yield from both Er3+ ions and sensitizing defects is reduced by irradiation, depending strongly on the irradiation fluence and reaching saturation at ∼4×1013 Si+/cm2. Higher implantation fluences result in an open-volume defect structure in the film that persists after annealing. This annealing behavior is similar to that of an unrecoverable quenching effect on Er3+-related PL near 1540 nm, and we suggest that these open-volume defects may cause a decoupling of the Er3+ ions from sensitizing oxide point defects that form as a result of the film deposition process.
Calculations of the thermal conductivities of ionic materials by simulation with polarizable interaction potentials
Authors: Ohtori Norikazu , Salanne Mathieu , Madden Paul A.
Published: March 2009

Expressions for the energy current of a system of charged, polarizable ions in periodic boundary conditions are developed in order to allow the thermal conductivity in such a system to be calculated by computer simulation using the Green–Kubo method. Dipole polarizable potentials for LiCl, NaCl, and KCl are obtained on a first-principles basis by “force matching” to the results of ab initio calculations on suitable condensed-phase ionic configurations. Simulation results for the thermal conductivity, and also other transport coefficients, for the melts are compared with experimental data and with results obtained with other interaction potentials. The agreement with experiment is almost quantitative, especially for NaCl and KCl, indicating that these methodologies, perhaps with more sophisticated forms for the potential, can be used to predict thermal conductivities for melts for which experimental determination is very difficult. It is demonstrated that the polarization effects have an important effect on the energy current and are crucial to a predictive scheme for the thermal conductivity.
Terahertz reflection response measurement using a phonon polariton wave
Authors: Inoue Hayato , Katayama Kenji , Shen Qing , Toyoda Taro , Nelson Keith A.
Published: March 2009

We developed a new technique for the measurement of terahertz reflection responses utilizing a propagating phonon polariton wave. Frequency tunable phonon polariton waves were generated by the recently developed continuously variable spatial frequency transient grating method [K. Katayama, H. Inoue, H. Sugiya, Q. Shen, T. Taro, and K. A. Nelson, Appl. Phys. Lett. 92, 031906 (2008)]. The phonon polariton wave traveled in a ferroelectric crystal in an in-plane direction with an inclined angle of 26°, and the wave reflected at the crystal edge where a sample was positioned. The reflected polariton wave was detected by the same method as that used for the generation of the polariton waves. By comparing the reflection intensities in the presence and absence of the sample, reflectivity of the polariton wave was calculated, and the refractive index and absorption in the terahertz region were obtained.
Polarization behavior in diffuse phase transition of BaxSr1−xTiO3 ceramics
Authors: Teranishi Takashi , Hoshina Takuya , Takeda Hiroaki , Tsurumi Takaaki
Published: March 2009

Ultrawide band dielectric spectroscopy analysis of barium strontium titanate [BaxSr1−xTiO3 (BST)] ceramics at different temperatures disclosed a contribution of the dipole and the ionic polarizations to their diffuse phase transition (DPT). In the BST ceramics, thermal behavior of the ionic polarization governs the DPT. The dielectric maximum temperature (Tm) is in agreement with the maximum temperature of the permittivity determined by the ionic polarization. The maximum of the permittivity determined by the ionic polarization at the Tm is caused from the decrement of the vibration frequency of the soft mode. The dipole polarization gives small contribution to the DPT. The gradual increase in the permittivity determined by the dipole polarization with decreasing temperature could be explained by the increase in the size of the polar nanoregions.
Normal and abnormal grain growth in nanostructured gold film
Authors: Ruffino F. , Grimaldi M. G. , Bongiorno C. , Giannazzo F. , Roccaforte F. , Raineri V. , Spinella C.
Published: March 2009

Thin nanostructured gold films were deposited on SiO2 by the sputtering technique at room temperature. Films of different thicknesses were deposited ranging from 2 to 16 nm. The film morphology as a function of the thickness was analyzed by microscopic techniques such as atomic force microscopy and transmission electron microscopy. These analyses allowed us to clarify the growth mechanism of the gold nanograins forming the film: in a first stage of growth (2–6 nm) normal grain growth proceeds; then (8–16 nm) the grain surface energy anisotropy drives the growth of abnormal large gold grains by annihilation of the normal ones. During the abnormal growth other normal grain continue to growth. The normal grain size distribution is showed to be a monomodal log-normal distribution that evolves toward larger mean grain radius continuously following a scaling law. By determination of the grain growth exponent, the kinetic mechanism responsible of the grain growth is demonstrated to be the gold atomic diffusion on grain boundaries. By fitting the experimental data using established theoretical models, the room-temperature gold grain boundary coefficient diffusion and mobility were derived. The abnormal grain grows, manifest itself as a bimodal grain size distribution: with the log-normal distribution of the normal grain size, a second Gaussian grain size distribution rises, shifting toward lower size increasing the film thickness. The abnormal grain growth continues until all the abnormal grain boundaries meet each other.