bluuebunny/crossref_metadata_2025 huggingface dataset:
Quasifinite representations of the Lie superalgebra of quantum pseudodifferential operators
Authors: Boyallian Carina , Meinardi Vanesa
Published: February 2008

In this paper we extend general results obtained by Kac and Liberati [Lett. Math. Phys. 53, 11 (2000)] for quasifinite highest weight representations of Z-graded Lie algebras to 12Z-graded Lie superalgebras, and we apply these to classify the irreducible quasifinite highest weight modules of the Lie superalgebra of quantum pseudodifferential operators.
Implementation of a nonequilibrium Green’s function method to calculate spin-transfer torque
Authors: Heiliger Christian , Czerner Michael , Yavorsky Bogdan Yu. , Mertig Ingrid , Stiles Mark D.
Published: February 2008

We present an implementation of the steady state Keldysh approach in a Green’s function multiple scattering scheme to calculate the nonequilibrium spin density. This density is used to obtain the spin-transfer torque in junctions showing the magnetoresistance effect. We use our implementation to study the spin-transfer torque in metallic Co∕Cu∕Co junctions.
Ferromagnetic resonance and switching dynamics of perpendicular recording heads
Authors: Xing Xinzhi
Published: February 2008

Studies on high data rate perpendicular heads are presented in this work. Ferromagnetic resonance (FMR) was observed for these heads using an impedance measurement technique [X. Xing et al., IEEE Trans. Magn. 43, 2181 (2007)]. From the measured FMR frequency, the effective in plane stiffness of the yoke can be determined. An external field was also applied parallel to the easy axis of the yoke to manipulate the effective in plane stiffness. As external field increases, a higher FMR frequency is observed and the head frequency response improves due to the increased stiffness of the write head. The result indicates that the FMR of the yoke becomes a more important limiting factor of the head switching dynamics.
Magnetic and transport properties of superconductor∕ferromagnet bilayer microbridges
Authors: Matsuda Ken-ichi , Akimoto Yosuke , Uemura Tetsuya , Yamamoto Masafumi
Published: February 2008

We investigated magnetic and transport properties of variable-thickness bridges (VTBs) having a Nb∕Pd0.85Ni0.15 bilayer microbridges. It was found that the superconducting critical current Ic of the VTBs monotonically decreased with increasing PdNi thickness dF, while nonmonotonic behavior of the superconducting transition temperature Tc was observed as a function of dF in large Nb∕PdNi bilayer films.
Study on the magnetization reversal process in a magnetic nanowire and a magnetic dot observed by magnetic field sweeping magnetic force microscopy measurements (invited)
Authors: Endo Yasushi , Fujimoto Hideki , Kumano Shinya , Matsumura Yusuke , Sasaki Isao , Kawamura Yoshio , Yamamoto Masahiko , Nakatani Ryoichi
Published: February 2008

We have studied the details of the magnetization reversal process in Ni–Fe nanowires and dots using magnetic field sweeping (MFS)-magnetic force microscopy (MFM). All the points within the nanowire and the dot show important changes in phase (changes in stray field) including a hysteresis loop, a decrease and an increase in phase, as the magnetic field is varied. From these results, it is demonstrated that domain wall motion dominates the magnetization reversal process of a 10-nm-thick Ni–Fe nanowire with widths between 100 and 1000nm. It is also demonstrated that the nucleation, the movement, and the annihilation of the vortex core can be directly observed in the magnetization reversal process of a 40-nm-thick Ni–Fe circular dot with diameters between 200 and 800nm. Furthermore, it is found that, in the magnetization reversal process of a 10-nm-thick Ni–Fe elliptical dot with several major axial distances between adjacent dots, the magnetization between the adjacent dots magnetostatically couples as the major axial distance decreases.
Valence state and spatial distribution of Fe ions in ferromagnetic Ba(Fe1−xZrx)O3−δ single-crystal films on SrTiO3 substrates
Authors: Matsui T. , Sato R. , Tsuda H.
Published: February 2008

Ba ( Fe 1 − x Zr x ) O 3 − δ single-crystalline films on (001) single-crystal SrTiO3 substrate were successfully synthesized. The films exhibited dielectric and ferromagneticlike characteristics even at room temperature. The saturation magnetization of the samples increased with increasing the Zr substitution amount. Especially for the x=0.8 and 0.9 samples, significant enhancement of the ferromagnetic ordering was observed. X-ray photoelectron spectroscopic analysis revealed that the samples with strong ferromagnetic ordering contained a relatively large amount of Fe4+ ions rather than Fe3+ ions. In addition to this, we revealed the presence of the very small regions having the Fe-rich composition of BFZO within the BFZO thin film. The localized tetravalent Fe ions at these regions causing ferromagnetic superexchange coupling are speculated to be responsible for the observed enhanced magnetization of the x=0.8 and 0.9 samples.
Novel hybrid magnetic overcoats: A prospective solution for low magnetic spacing
Authors: Poh W. C. , Piramanayagam S. N. , Liew T.
Published: February 2008

A novel hybrid magnetic overcoat (Hy-MOC), consisting of a bilayer, suitable for ultrahigh density recording, has been proposed and investigated. The top layer is a thin layer of carbon (about 1nm) employed mainly for the purpose of lubrication affinity. The bottom layer is a thin CoCrPt alloy layer with silicon oxide/nitride grain boundary (about 4nm), which helps as an overcoat to prevent corrosion and also in reducing the magnetic spacing. Media prepared with such an overcoat showed a better recording performace and thermal stability than the media with a-C overcoat. The microstructure of the media with carbon overcoats and the magnetic overcoats obtained by transmission electron microscopy were comparable. Improved corrosion protection properties were also observed. These results indicate that Hy-MOC provides a possible solution for ultrathin overcoat system necessary for ultrahigh density media.
Thickness dependence of magnetic domain pinning energy in GaMnAs ferromagnetic semiconductor films
Authors: Yea Sun-Young , Chung Sun-Jae , Son Hyunji , Lee Sanghoon , Liu X. , Furdyna J. K.
Published: February 2008

Magnetotransport measurements were performed on GaMnAs films with four different thicknesses. The process of magnetization reorientation was investigated by planar Hall effect, from which the magnetization switching fields were determined for each film thickness. Domain pinning energies were then obtained for the films by fitting the angular dependence of the switching fields using the model developed by Cowburn et al. [J. Appl. Phys. 78, 7210 (1995)]. The results show a systematic increase of domain pinning energies as the thickness of the film decreases. Such dependence of domain pinning can be understood in terms of the carrier dependence of magnetic anisotropy in GaMnAs films on the concentration of carriers.
Partially reversible changes in magnetic properties of CrO2 nanoparticles through electrochemical cycling
Authors: Sivakumar Vikram , Yabuuchi Naoaki , Ross C. A. , Shao-Horn Yang
Published: February 2008

The changes in saturation moment and crystal structure of elongated CrO2 nanoparticles upon galvanostatic electrochemical Li insertion have been investigated via vibrating sample magnetometry and Θ-2Θ x-ray diffraction. Li+ insertion reduces Cr4+ to Cr3+ and dramatically lowers the saturation magnetization Ms, initially by 5μB per Li ion. The changes in Ms are similar in magnitude to prior results on 550 nm long CrO2 particles. Increasing the rate of cycling resulted in smaller amounts of Li being inserted, and therefore smaller changes in Ms. The rutile structure is preserved on Li insertion, though the peaks become broader and shoulders appear at high Li contents.
Microwave generation in MgO magnetic tunnel junctions due to spin transfer effects (invited)
Authors: Nazarov Alexey V. , Nikolaev Konstantin , Gao Zheng , Cho Haeseok , Song Dion
Published: February 2008

Low resistance-area product and high spin polarization lead to current-driven precession of the magnetization in CoFeB∕MgO∕CoFeB magnetic tunnel junctions due to spin-transfer torque (STT) effects. Current-driven precession of the magnetization leads to pronounced narrow peaks in the frequency range of 4–7GHz with a full width at half maximum as low as 21MHz. The peak amplitudes have a threshold dependence on the dc bias current. Experimental results show that the STT-driven microwave generation can also occur in MgO-based junctions at maximum resistance state but at opposite current polarity, which corresponds to precession of the magnetization of the reference layer (RL) electrode. This conclusion is supported by the peak frequency dependence on magnetic field. The maximum generated power was 35nW at a peak frequency of about 6GHz. The estimated maximum angle change of the RL in-plane magnetization rotation is 19° and corresponds to a large angle precession.
The temperature structure of solar coronal plasmas
Authors: Feldman Uri , Landi Enrico
Published: February 2008

In the early 1940s it was at last accepted that the temperature of the solar corona is at least 1MK and varies considerably from region to region throughout the solar activity cycle. It was recognized that during solar minimum periods the electron temperatures of plasmas in polar regions do not exceed 1MK, but during solar maximum periods the plasma temperatures of highly active regions could be as high as 3MK. Nevertheless, until recently the consensus among the solar physics community was that coronal temperatures vary among the different regions in a continuous manner. In the present paper we review the evidence showing that solar coronal plasmas (Te>0.7MK) are isothermal and their temperature can have only a small set of fixed values.
Stochastic magnetic field driven charge transport and zonal flow during magnetic reconnection
Authors: Ding W. X. , Brower D. L. , Craig D. , Chapman B. E. , Ennis D. , Fiksel G. , Gangadhara S. , Den Hartog D. J. , Mirnov V. V. , Prager S. C. , Sarff J. S. , Svidzinski V. , Terry P. W. , Yates T.
Published: February 2008

Magnetic fluctuation-induced charge transport, resulting from particle transport that is not intrinsically ambipolar, has been measured in the high-temperature interior of a reversed-field pinch plasma. It is found that global resistive tearing modes and their nonlinear interactions lead to significant charge transport, equivalent to the perpendicular Maxwell stress, in the vicinity of the resonant surface for the dominant core resonant mode during magnetic reconnection. Finite charge transport can result in a zonal flow associated with locally strong radial electric field and electric field shear. In the presence of stochastic magnetic field, radial electric field is expected to be balanced by radial electron pressure gradient. Direct measurement of local density gradient is consistent with the formation of radial electric field and the zonal flow.
Plasma shield for in-air beam processes
Authors: Hershcovitch Ady
Published: February 2008

A novel concept/apparatus, the Plasma Shield, is introduced in this paper. The purpose of the Plasma Shield is designed to shield a target object chemically and thermally by engulfing an area subjected to beam treatment with inert plasma. The shield consists of a vortex-stabilized arc that is employed to shield beams and workpiece area of interaction from an atmospheric or liquid environment. A vortex-stabilized arc is established between a beam generating device (laser, ion or electron gun) and a target object. The arc, which is composed of a pure noble gas, engulfs the interaction region and shields it from any surrounding liquids like water or reactive gases. The vortex is composed of a sacrificial gas or liquid that swirls around and stabilizes the arc. The successful Plasma Shield was experimentally established and very high-quality electron beam welding with partial plasma shielding was performed. The principle of the operation and experimental results are discussed in the paper.
Influence of interactions on magnetization behavior of arrays of nanostructures with uniaxial anisotropy
Authors: Webb J. L. , Atkinson D.
Published: February 2008

A semianalytical model is presented for representing the magnetization switching in arrays of nanostructures with uniaxial anisotropy that includes both the dipolar interactions between structures in the array and a distribution of switching fields for the structures within the arrays. The net magnetic field experienced by each individual structure is the sum of the externally applied field and the local field from the other structures. Here the local field was calculated by subdividing each nanostructure into discrete magnetic moments. The field dependence of each structure within the array was represented by assigned switching behavior. The simplest behavior assumed a structure was wholly reversed at a given axial field (square switching), while more complex behavior was also implemented using both a Stoner–Wohlfarth representation of switching and switching behavior that included a zero magnetization state to represent a vortex state. A distribution of switching fields within an array was incorporated by discretizing a normal distribution to assign switching fields to each structure. The variation from noninteracting switching to interacting was investigated for permalloy (Ni–Fe) arrays of 1.2×0.6 μm, 0.6×0.3 μm, and 0.2×0.1 μm ellipses. The model is compared with magneto-optic Kerr effect measurements of lithographically defined arrays and shown to give good qualitative and reasonable quantitative agreement with the measurements. The effect of interactions upon the Stoner–Wohlfarth astroid is also shown.
Nucleation mechanism of gallium-assisted molecular beam epitaxy growth of gallium arsenide nanowires
Authors: Fontcuberta i Morral A. , Colombo C. , Abstreiter G. , Arbiol J. , Morante J. R.
Published: February 2008

Molecular beam epitaxy Ga-assisted synthesis of GaAs nanowires is demonstrated. The nucleation and growth are seen to be related to the presence of a SiO2 layer previously deposited on the GaAs wafer. The interaction of the reactive gallium with the SiO2 pinholes induces the formation of nanocraters, found to be the key for the nucleation of the nanowires. With SiO2 thicknesses up to 30nm, nanocraters reach the underlying substrate, resulting into a preferential growth orientation of the nanowires. Possibly related to the formation of nanocraters, we observe an incubation period of 258s before the nanowires growth is initiated.
Phenomenological model for the simulation of hysteresis loops in NiFe∕Cu multilayered nanowires
Authors: Chiriac H. , Óvári T. A. , Pascariu Petronela
Published: February 2008

Results on a new phenomenological model for the simulation of out-of-plane and in-plane hysteresis loops of magnetic∕nonmagnetic multilayered nanowire arrays are reported. Calculated loops are compared with experimental ones obtained using a vibrating sample magnetometer. The measured intrinsic coercivity of the array and the dipolar field created by the array are employed as input parameters in the proposed model. The shapes of the simulated loops are similar to those of the experimentally measured ones and confirm the dominant role of dipolar interactions in dense nanowire arrays. The proposed model offers a convenient tool for studying the correlation between the hysteretic behavior and the geometry of nanowire arrays.
Structural and optical investigations of Nd-doped fluorozirconate-based glass ceramics for enhanced upconverted fluorescence
Authors: Ahrens Bernd , Eisenschmidt Christian , Johnson Jacqueline A. , Miclea Paul T. , Schweizer Stefan
Published: February 2008

We found enhanced upconverted fluorescence in Nd3+-doped fluorozirconate glasses which were additionally doped with chlorine ions. Upon annealing between 240 and 290°C, hexagonal phase BaCl2 nanocrystals between 20 and 180nm in diameter were formed in the glass. During the thermal processing, some of the Nd3+ ions enter the nanocrystals leading to additional splitting of the infrared fluorescence spectra and to increased upconverted fluorescence intensities. The optimum value was found for the 270°C sample where the upconverted fluorescence intensity is approximately a factor of 60 stronger than the one found for the as-made glass.
Ultrasmall iron nanoparticles: Effect of size reduction on anisotropy and magnetization
Authors: Lacroix L.-M. , Lachaize S. , Falqui A. , Blon T. , Carrey J. , Respaud M. , Dumestre F. , Amiens C. , Margeat O. , Chaudret B. , Lecante P. , Snoeck E.
Published: February 2008

Stable iron nanoparticles have been synthesised by the decomposition of {Fe(N[Si(CH3)3]2)2}2 under dihydrogen pressure. Those conditions lead to a system of monodisperse and metallic nanoparticles which diameter is less than 2nm and stabilized by HN[Si(CH3)3]2. The magnetization is found to be MS=1.92μB∕at., i.e., 10% lower than the bulk value. The Mössbauer spectrum is fitted by two contributions of metallic iron. The magnetic anisotropy energy constant increases up to 5.2×105J∕m3, i.e., ten times the bulk one.
Thermal fluctuation effects on spin torque induced switching: Mean and variations
Authors: Wang Xiaobin , Zheng Yuankai , Xi Haiwen , Dimitrov Dimitar
Published: February 2008

Thermal fluctuation effects on mean and variation of spin torque induced magnetic element switching are analyzed. Asymptotic forms of the switching time distribution from the stochastic Landau–Lifshitz–Gilbert equation, and numerical solutions of the first and second moments of switching time from the corresponding Fokker–Planck equation, are used to characterize switching time and switching current density for the whole time range, from the second thermal reversal region to the nanosecond dynamic reversal region. It is shown that as time scales become shorter, switching time distributions become narrower, whereas switching current distributions may become broader. This paper provides a physical understanding of these different scaling behaviors.
Magnetic giant magnetoresistance commercial off the shelf for space applications
Authors: Michelena M. D. , Oelschlägel W. , Arruego I. , del Real R. P. , Mateos J. A. D. , Merayo J. M.
Published: February 2008

The increase of complexity and miniaturizing level of Aerospace platforms make use of commercial off the shelf (COTS) components constitute a plausible alternative to the use of military or rad-tolerant components. In this work, giant magnetoresistance commercial sensors are studied to be used as COTS, the next missions to be launched in the framework of the Spanish National Space Program: OPTOS and SEOSAT. This technology of magnetic sensors is interesting due to their high operating range up to 2mT and the high temperature dynamic range from −50 up to 150°C. However, in contrast, it presents high hysteresis and nonlinearity, temperature dependence, and poor repeatability. To improve the hysteretic, nonlinear and nonrepetitive behavior, a method consisting of a combination of reset and biasing has been designed and implemented for the ±75μT linear region centered around 300–375μT biasing field.
Spatial and temporal evolution of a dielectric barrier discharge
Authors: Malik D. A. , Orlov K. E. , Miroshnikov I. V. , Smirnov A. S.
Published: February 2008

Experimental study of spatial and temporal evolution of a dielectric barrier discharge (DBD) during a single current pulse is reported. The discharge light emission has been imaged using a high-speed intensified charge-coupled device camera. The time-resolved light emission data have been combined with the discharge current and applied voltage waveforms measurements. The discharge behavior has been found to be dependent on the external circuit impedance and bandwidth. Being nonstationary in principle, the DBD can exhibit both time evolution and development over the electrode area. For a typical DBD setup with flat parallel electrodes, the discharge breakdown occurs in the electrode’s central part; then, the plasma column expands to the electrode’s peripheries and gradually fills up the entire electrode area. The powering system should be able to provide enough current value during the discharge glow to achieve a simultaneous breakdown over the whole electrode area. In this case, the discharge appears over the entire electrode’s area and the plasma column exhibits temporal evolution only. The measured discharge characteristics for different power supply architectures had shown the evidence of a “normal current density” effect in DBD glow.
Effect of thermal conductivity on the Knudsen layer at ablative surfaces
Authors: Pekker L. , Keidar M. , Cambier J.-L.
Published: February 2008

In this article we develop an analytical model of the Knudsen layer at the ablative wall taking into account the temperature gradient in the bulk gas. The analysis is based on the premise that the temperature gradient in the bulk gas can be taken into account in the velocity distribution function at the outer boundary of the Knudsen layer. The model uses a bimodal velocity distribution function in the Knudsen layer, which preserves the laws of conservation of mass, momentum, and energy and converges to the Chapman–Enskog velocity distribution function at the outer boundary of the layer. The model is applied to polyethylene ablation, for which two cases are considered: (a) the ablation process is due to pure heat conduction to the surface, with no external heating of the ablated surface, and (b) the ablation is due to both the thermal conduction and an external heating of the surface, e.g., vaporization of a metal exposed to laser radiation. The region of validity of the existing models and effect of the temperature gradient on the Knudsen layer properties are calculated.
Low-temperature dielectric properties of Langmuir–Blodgett ferroelectric polymer films
Authors: Meng X. J. , Kliem H. , Lin T. , Chu J. H.
Published: February 2008

Dielectric properties of the copolymer of vinylidene fluoride and trifluoroethylene films derived from the Langmuir–Blodgett technology have been investigated over the temperature range from 300 to 150 K at various frequencies. Below ∼240 K, peaks in the dielectric loss were observed to increase with increasing frequency. Nonlinear dielectric susceptibility and ferroelectric properties have been studied at different temperatures. The results show that the nonlinear dielectric susceptibility undergoes a remarkable change at ∼230 K. The results suggest a β relaxation, which is related to the freezing of the molecular motions in the amorphous phase in the polymer films. The anomaly in the electrical properties at ∼230 K is due to the freezing of the molecular chains motions.
Terahertz photon-assisted tunneling in carbon nanotube quantum dots
Authors: Kawano Y. , Fuse T. , Toyokawa S. , Uchida T. , Ishibashi K.
Published: February 2008

The authors have studied the transport properties of carbon nanotube quantum dots under terahertz (THz) wave irradiation. The experimental data have shown that the satellite currents are generated with the THz irradiation, and that the peak position of the satellite currents varies linearly with the THz photon energy. These results provide experimental evidence for photon-assisted tunneling in the THz region. The present observation provides the interesting possibility of developing a highly sensitive and frequency-tunable THz detector capable of high-temperature operation.
Neoclassical toroidal viscosity and error-field penetration in tokamaks
Authors: Cole A. J. , Hegna C. C. , Callen J. D.
Published: February 2008

A model for field error penetration is developed that includes nonresonant as well as the usual resonant field error effects. The nonresonant components cause a neoclassical toroidal viscous torque that tries to keep the plasma rotating at a rate comparable to the ion diamagnetic frequency. The new theory is used to examine resonant error-field penetration threshold scaling in ohmic tokamak plasmas. Compared to previous theoretical results, the plasma is found to be less susceptible to error-field penetration and locking, by a factor that depends on the nonresonant error-field amplitude.
An optimal, parallel, fully implicit Newton–Krylov solver for three-dimensional viscoresistive magnetohydrodynamics
Authors: Chacón L.
Published: February 2008

The conceptual development and implementation of a scalable nonlinear solver for the time-dependent three-dimensional (3D) compressible resistive magnetohydrodynamics model (MHD) is discussed. The approach is based on Jacobian-free Newton–Krylov technology, preconditioned with multigrid methods for algorithmic scalability. The key to the approach is the reformulation of the hyperbolic MHD system into a parabolic one, which is amenable to multigrid techniques. Such reformulation (parabolization) aims to render the modified system block diagonally dominant (unlike the original MHD system, which is diagonally submissive for implicit time steps Δt larger than the explicit Courant–Friedrichs–Lewy time step). The algorithm has been tested on a variety of 2D and 3D configurations which demonstrate its excellent algorithmic scalability properties. In particular, it is shown that, serially, the CPU time for a given simulation scales linearly with the number of unknowns, and that large CPU gains (∼30) are attainable with respect to an explicit approach even for moderate grids (256×256). In parallel, the algorithm features excellent scalability properties up to thousands of processors (4096) and millions of unknowns (∼1.3×108).
Magnetization reversal by microwave in magnetic tunnel junctions
Authors: Moriyama T. , Cao R. , Xiao John Q. , Lu J. , Wang X. R. , Wen Q. , Zhang H. W.
Published: February 2008

Microwave assisted magnetization switching was investigated using Fe30Co70∕Al2O3∕Ni80Fe20 magnetic tunnel junctions incorporated with a coplanar waveguide. Coercivity reduction (or switching field) of Ni80Fe20 layer was reduced with a microwave excitation. It was found that the coercivity reduction depends on both frequency and power of the microwave. The numerical simulation based on the Landau-Lifshitz-Gilbert equation reproduced the trend of the experimental data. The results indicate that microwave can be an efficient mean to switch the magnetization of a thin film.
Spin transfer switching in TbCoFe∕CoFeB∕MgO∕CoFeB∕TbCoFe magnetic tunnel junctions with perpendicular magnetic anisotropy
Authors: Nakayama Masahiko , Kai Tadashi , Shimomura Naoharu , Amano Minoru , Kitagawa Eiji , Nagase Toshihiko , Yoshikawa Masatoshi , Kishi Tatsuya , Ikegawa Sumio , Yoda Hiroaki
Published: February 2008

Spin transfer (ST) switching in the TbCoFe∕CoFeB∕MgO∕CoFeB∕TbCoFe magnetic tunnel junction (MTJ) was studied. The TbCoFe∕CoFeB free layer with a large coercive field of 1.2kOe and a large thermal stability factor of 107 at room temperature was switched by a 100ns pulse current with a current density of 4.7MA∕cm2. This is the first report of ST switching in a MTJ with perpendicular magnetic anisotropy. The temperature dependence of the coercive field was also investigated to estimate the magnetic anisotropy in the case of rising temperature due to the Joule heating effect. The measured coercive field at 87°C, which was the simulated temperature during the switching pulse current, was about 0.34kOe. The ratio of the switching current density to the coercive field under the switching current in the MTJ with the TbCoFe∕CoFeB free layer is smaller than that in a typical MTJ with an in-plane magnetized CoFeB free layer. This result indicates that a MTJ with perpendicular magnetic anisotropy is advantageous for realizing low switching current magnetoresistive random access memory.
Cracking in hydrogen ion-implanted Si∕Si0.8Ge0.2∕Si heterostructures
Authors: Shao Lin , Wang Y. Q. , Swadener J. G. , Nastasi M. , Thompson Phillip E. , Theodore N. David
Published: February 2008

We demonstrate that a controllable cracking can be realized in Si with a buried strain layer when hydrogen is introduced using traditional H-ion implantation techniques. However, H stimulated cracking is dependent on H projected ranges; cracking occurs along a Si0.8Ge0.2 strain layer only if the H projected range is shallower than the depth of the strained layer. The absence of cracking for H ranges deeper than the strain layer is attributed to ion-irradiation induced strain relaxation, which is confirmed by Rutherford-backscattering-spectrometry channeling angular scans. The study reveals the importance of strain in initializing continuous cracking with extremely low H concentrations.
Direct electrical characterization of embedded ferroelectric lead titanate nanoislands
Authors: Clemens S. , Dippel A.-C. , Schneller T. , Waser R. , Rüdiger A. , Röhrig S.
Published: February 2008

We report on the fabrication and characterization of lead titanate nanoislands on platinized silicon substrates embedded into a low-k dielectric. Our findings with Pt and Au as collective top electrodes are compared to previous results and thin films, and we discuss the coercive field and the remanent polarization with special care devoted to capacitive and leakage contributions of the nonpolar matrix. A direct electrical characterization of sub-100-nm ferroelectric nanoislands becomes feasible if they are measured in parallel, providing that the thin film material parameters of both ferroelectric and spin-on glass are independently determined.
Modeling and simulation for effective permittivity of two-phase disordered composites
Authors: Cheng Yonghong , Chen Xiaolin , Wu Kai , Wu Shengnan , Chen Yu , Meng Yongmeng
Published: February 2008

We present a simulation for the effective permittivity of two-phase disordered composites based on a three-dimensional disordered model. In this model, the inclusions can be arranged at random positions in a composite material that is simulated by 10×10×10 cubic cells. Under the condition of the quasistatic approximation, numerical calculations are performed using the finite element method. The dependence of the calculated effective permittivity on the orientation of the disordered model is investigated. In order to get the macroscopical dielectric properties of isotropic composites, an averaging method is adopted because each topological structure of the disordered model is anisotropic. Then, the effect of the permittivity contrast between the inclusions and the matrix phase of composites is studied. The numerical results obtained by the disordered model are also compared with those obtained by the classical mixing rules and the conventional periodic models. The advantage of this presented disordered model is that it can succeed in predicting the effective permittivity in the cases of high permittivity contrast and large volume fractions of inclusions by considering overlapping of inclusions.
High- and low-field dielectric characteristics of dielectrophoretically aligned ceramic/polymer nanocomposites
Authors: Tomer V. , Randall C. A. , Polizos G. , Kostelnick J. , Manias E.
Published: February 2008

Polymer/ceramic composites with controlled spatial distribution of fillers are synthesized, and the corresponding changes in their properties are studied. Using dielectrophoretic assembly, we create anisotropic composites of aligned BaTiO3 particles in silicone elastomer and study their electrical properties as a function of ceramic volume fraction and composite morphology. These structured composites show an increase in the permittivity compared to composites with the same composition and randomly dispersed (0–3) fillers. This study emphasizes the important role of conductivity, permittivity, and, particularly, local cluster distribution in controlling high-field dielectric behavior. Designed anisotropy in dielectric properties can provide unexampled paradigms for the development of high energy density materials and gain important insights into the mechanisms that control dielectric breakdown strengths and nonlinear conduction at high fields in polymer/ceramic composites.
A modified field model of waveguide reflection dielectric resonator for microwave measurements of dielectric properties
Authors: Sheen Jyh
Published: February 2008

A modified electromagnetic field model of a waveguide reflection dielectric resonator is suggested for measurements of dielectric properties of the homogeneous and isotropic medium in the microwave frequencies. Reflection signal is measured for the calculations of dielectric properties. A dielectric rod sample is put inside of a rectangular cavity made by a microwave waveguide. The sample’s dielectric constant and loss tangent are computed from the unloaded quality factor and the resonant frequency of the TE01δ mode as well as the structure dimensions. For first time, this waveguide reflection dielectric resonator is applied on dielectric constant measurement. A modified field model of the waveguide reflection resonator is developed from the Itoh-Rudokas model [IEEE Trans. Microwave Theory Tech. MTT-25, 52 (1977)] of the parallel-plate dielectric resonator. This modification is justified by the dramatic improvement in the accuracy of dielectric constant measurements. The main merit of this field model is that it provides very simple electromagnetic field expressions of this TE01δ field mode. In addition, accuracies of various methods for calculating the power factor and conducting loss, which have never been given before, will be investigated in this article.
A simple method for deducing properties of laminar wall jets in uniform pressure fields, with undetermined far-field conditions
Authors: Candelier F. , Bournot H.
Published: February 2008

A simple method is proposed in order to draw various information concerning boundary layers that develop axisymmetrically on a flat plate, when the outer pressure field is uniform, and when the far-field conditions, perpendicularly to the plate, are unknown. It is shown that if, at an arbitrary radial coordinate r0, one measures both the value of the maximum radial velocity (i.e., Um) and the height (i.e., δ) where the radial velocity reaches its maximum value, one can build a nondimensional number, which we shall call A. This number is an invariant in this study. It also allows us to calculate several parameters, such as, for instance, the local shear rate on the plate or the power laws for the radial velocity decrease and the boundary layer thickness increase, according to the downstream distance, corresponding here to the radial coordinate r.
Short exciton radiative lifetime in submonolayer InGaAs∕GaAs quantum dots
Authors: Xu Zhangcheng , Zhang Yating , Tackeuchi Atsushi , Horikoshi Yoshiji , Hvam Jørn M.
Published: February 2008

The exciton radiative lifetime in submonolayer (SML) InGaAs∕GaAs quantum dots (QDs) grown at 500°C was measured by using time-resolved photoluminescence from 10to260K. The radiative lifetime is around 90ps and is independent of temperature below 50K. The observed short radiative lifetime is a key reason for the high performance of SML QD devices and can be explained by the theory of Andreani et al. [Phys. Rev. B 60, 13276 (1999)] calculating the radiative lifetime of QDs formed at the interface fluctuations of a quantum well, as the SML QDs are 20–30nm in diameter and embedded within the lateral InGaAs QW.
Gate width dependence on backscattering characteristics in the nanoscale strained complementary metal-oxide-semiconductor field-effect transistor
Authors: Liao M. H. , Liu C. W. , Yeh Lingyen , Lee T.-L. , Liang M.-S.
Published: February 2008

It is found that the ballistic efficiency, channel backscattering ratio, and injection velocity, which are the most important parameters for the ballistic transport, are greatly influenced by the stress characteristic in the channel even on the same gate length device. The narrower gate width device provides the best performance for the n-type field-effect transistor (n-FET) with the same gate length. Thus, the multichannel device is proposed to enhance the n-FET performance in the circuit design. The stress distribution with different device structures were simulated by the three-dimensional finite element mechanical stress simulation, and ballistic efficiency and injection velocity were calculated theoretically based on the stress characteristic. The theoretical calculation and the experimental data indicate the causes of the higher ballistic efficiency and injection velocity in narrower gate width devices to be the strain-induced modulation of the carrier mean-free path and smaller electron effective mass under the biaxial-like stress.
Effects of perpendicular anisotropy on the interlayer coupling in perpendicularly magnetized [Pd∕Co]∕Cu∕[Co∕Pd] spin valves
Authors: Thiyagarajah Naganivetha , Bae Seongtae , Joo Ho Wan , Han Young Chul , Kim Jongryoul
Published: February 2008

The effects of perpendicular anisotropy on the interlayer coupling in perpendicularly magnetized [Pd∕Co]∕Cu∕[Co∕Pd] spin valves have been explored. It was clearly confirmed that the perpendicular anisotropy of soft and hard [Co∕Pd] multilayers plays a crucial role in determining the physical characteristics of perpendicular interlayer coupling in the [Pd∕Co]∕Cu∕[Co∕Pd] spin valves. In addition, theoretical calculations demonstrated that the behavior of experimentally observed perpendicular interlayer coupling dominantly followed a Ruderman–Kittel–Kasuya–Yoshida oscillation modified by including a physical parameter directly relevant to the angle deviation of soft or hard [Co∕Pd] magnetizations from the perpendicular direction in the spin valves rather than a topologically induced interlayer coupling.
The electronic properties of the interface between nickel phthalocyanine and a PEDOT:PSS film
Authors: Petraki F. , Kennou S. , Nespurek S.
Published: February 2008

Thin films of nickel phthalocyanine (NiPc) are deposited on an as-received and on a mildly sputtered PEDOT:PSS film, spin coated on fluorine tin oxide coated glass. The electronic properties of the PEDOT:PSS surface, both as loaded and upon thermal treatment and sputtering, as well as of the interfaces between NiPc and PEDOT:PSS are studied by x-ray and UV photoelectron spectroscopies in order to investigate both the electronic and the chemical properties of the materials. Surface analysis of the PEDOT:PSS films showed that upon sputtering the insulating PSS film is removed leading to lower work function, as well as to an increase of the density of occupied states close to the Fermi level. The investigation of the interfaces between NiPc and PEDOT:PSS revealed charge transfer and a pinning of the Fermi level across the interface. The hole injection barrier was found significantly lower compared with that for the NiPc/Au interface, indicating that the presence of the PEDOT:PSS layer facilitates the carrier injection between the electrode and the organic semiconductor.
Scanning heterodyne laser interferometer for phase-sensitive absolute-amplitude measurements of surface vibrations
Authors: Kokkonen Kimmo , Kaivola Matti
Published: February 2008

We describe a scanning heterodyne interferometer for imaging surface vibrations with a wide frequency range, with current electronics, up to 6GHz. The heterodyne operation facilitates measurement of absolute amplitude and phase of the surface vibration without calibration. Currently, the setup allows detection of vibration amplitudes down to ∼1pm with a lateral resolution of <1μm. The interferometer is designed to accommodate the different sample types, e.g., surface and bulk acoustic wave devices and micromechanical resonators. The absolute-amplitude and phase information allows for a thorough characterization of surface vibrations in such components and provides direct information of the vibration fields not obtainable via electrical measurements.
A representation formula for maps on supermanifolds
Authors: Hélein Frédéric
Published: February 2008

We analyze the notion of morphisms of rings of superfunctions which is the basic concept underlying the definition of supermanifolds as ringed spaces (i.e., following Berezin, Leites, Manin, etc.). We establish a representation formula for all (pull-back) morphisms from the algebra of functions on an ordinary manifolds to the superalgebra of functions on an open subset of a superspace. We then derive two consequences of this result. The first one is that we can integrate the data associated with a morphism in order to get a (nonunique) map defined on an ordinary space (and uniqueness can be achieved by restriction to a scheme). The second one is a simple and intuitive recipe to compute pull-back images of a function on a manifold M by a map from a superspace to M.
Spectral averaging techniques for Jacobi matrices
Authors: del Rio Rafael , Martinez Carmen , Schulz-Baldes Hermann
Published: February 2008

Spectral averaging techniques for one-dimensional discrete Schrödinger operators are revisited and extended. In particular, simultaneous averaging over several parameters is discussed. Special focus is put on proving lower bounds on the density of the averaged spectral measures. These Wegner-type estimates are used to analyze stability properties for the spectral types of Jacobi matrices under local perturbations.
Noncommutative scalar fields from symplectic deformation
Authors: Daoud M. , Hamama A.
Published: February 2008

This paper is concerned with the quantum theory of noncommutative scalar fields in two dimensional space-time. It is shown that the noncommutativity originates from the the deformation of symplectic structures. The quantization is performed and the modes expansions of the fields, in the presence of an electromagnetic background, are derived. The Hamiltonian of the theory is given and the degeneracies lifting, induced by the deformation, is also discussed.
Bosonic quasideterminants and eigenvalue problems of generalized spin-orbit operators
Authors: Ben Geloun Joseph , Hounkonnou M. Norbert
Published: February 2008

This paper deals with an extension of the applications of the paper by Gelfand and Retakh [Funct. Anal. Appl. 25, 91 (1991)] on quasideterminant (QsD) algebraic method to eigenvalue problems in quantum mechanics. Using relevant identities on the free 1-mode bosonic algebra, we build characteristic QsDs associated with generalized spin-orbit Hamiltonians with a well defined representation which allows us to explicitly and straightforwardly compute analytical expressions of eigenenergies. Specific instances are provided on f-deformed generalized Jaynes–Cummings models and other Hamiltonian classes widely used in condensed matter physics.
Analysis of surface thermal lens signal in optical coatings with top-hat beam excitation
Authors: Li Bincheng , Chen Xiaoxiao , Gong Yuan
Published: February 2008

An explicit theoretical model based on three-dimensional thermal conduction, thermoelasticity, and Fresnel diffraction integral is developed to describe the surface thermal lens (STL) with a continuous-wave (cw) modulated excitation. The surface displacements and STL amplitudes obtained with both top-hat and Gaussian beam excitations are theoretically computed and compared. A STL experiment is performed with the cw modulated top-hat beam excitation to investigate the dependences of the STL amplitude on the experimental parameters, namely, the detection distance, the modulation frequency, the radius of the excitation beam, etc. Good agreements between the experimental and theoretical results are obtained. The results indicate that high sensitivity of the STL technique can be achieved with the top-hat beam excitation and by optimizing the experimental configuration.
Production and detection of reducing and oxidizing radicals in the catalytic decomposition of H2∕O2 mixtures on heated tungsten surfaces
Authors: Umemoto Hironobu , Moridera Masashi
Published: February 2008

H atoms, O atoms, and OH radicals were identified in the catalytic decomposition of H2∕O2 mixtures on heated polycrystalline tungsten surfaces. In order to suppress the oxidization of the tungsten catalyzer surfaces, the H2∕O2 pressure ratio was kept more than 83, while the catalyzer temperature was kept below 2000K. The absolute density of H atoms was determined by a vacuum-ultraviolet laser absorption technique, while one-photon and two-photon laser-induced fluorescence techniques were employed to extend the dynamic range. Since the O-atom density was much smaller, only a vacuum-ultraviolet laser-induced fluorescence technique could be used for the detection. The absolute density could be estimated by comparing the induced fluorescence intensity with that for H atoms. OH radicals could be identified by a laser-induced fluorescence technique in the ultraviolet region. The absolute density was determined by comparing the induced fluorescence intensity with that of Rayleigh scattering caused by Ar. The H-atom density decreased with the increase in the O2 partial pressure stepwisely. The O-atom density increased with the O2 partial pressure monotonously, but the increase was rather slow at low O2 pressures. The OH-radical density showed saturation against the O2 partial pressure. These results can be explained by the change in the coverage conditions of the catalyzer surfaces.
Static and dynamic analysis of a diamagnetic bearing system
Authors: Cansiz Ahmet
Published: February 2008

A unique numerical calculation method was constructed in order to analyze the static and dynamic characteristics of the diamagnetic bearing. An experimental setup consisting of a permanent magnet rotor, levitated with the arrangement of ferrite magnet and bismuth blocks, was used to confirm theoretical calculations. The stability analysis of the bearing relating to the vertical and lateral forces was incorporated with the dynamic model. The interaction force between the rotor magnet and the diamagnetic materials was modeled with the Amperian current approach via the diamagnetic-mirror-image concept. The magnetic potential of the system, which provides the equation of motion, was stated in terms of diamagnetic and gravitational interactions. The equations of motion of the dynamic part of the system were stated as a function of lateral, vertical, and tilt angles. Correlation of the vibrations such as vertical, radial, and tilt was determined with a numerical calculation obtained by the Runge–Kutta method. Translational and rotational tests together with the theoretical calculations showed the equilibrium position of the rotor to be stable statically and dynamically. The permanent magnet was spun up under vacuum conditions and then let to free spin down to analyze its dynamics.
Investigation on damage process of GaAs induced by 1064nm continuous laser
Authors: Qi Haifeng , Wang Qingpu , Zhang Xingyu , Liu Zejin , Liu Zhaojun , Chang Jun , Xia Wei , Jin Guofan
Published: February 2008

A single crystal GaAs wafer with front face mirror polished was irradiated by a 1064nm continuous laser experimentally. The damage process was investigated by real-time surface reflectivity and optical distribution monitoring. No evident melting and oxidation occurrence was found and the components of Ga and As almost kept constant during the whole damaging process. The distinct differences from the damage by 532nm continuous laser were analyzed as well. For the current wavelength, the damage threshold was greatly dependent on the boundary conditions, consequently only an estimated threshold value about 2kW∕cm2 was obtained.
Atomic size mismatch strain induced surface reconstructions
Authors: Bickel Jessica E. , Modine Normand A. , Van der Ven Anton , Mirecki Millunchick Joanna
Published: February 2008

The effects of lattice mismatch strain and atomic size mismatch strain on surface reconstructions are analyzed using density functional theory. These calculations demonstrate the importance of an explicit treatment of alloying when calculating the energies of alloyed surface reconstructions. Lattice mismatch strain has little impact on surface dimer ordering for the α2(2×4) reconstruction of GaAs alloyed with In. However, atomic size mismatch strain induces the surface In atoms to preferentially alternate position, which, in turn, induces an alternating configuration of the surface anion dimers. These results agree well with experimental data for α2(2×4) domains in InGaAs∕GaAs surfaces.