bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic moment investigations of epitaxial magnetite thin films
Authors: Kallmayer M. , Hild K. , Elmers H. J. , Arora S. K. , Wu Han-Chun , Sofin R. G. S. , Shvets I. V.
Published: March 2008

In this report, we provide an x-ray magnetic circular dichroism (XMCD) study for 100nm thick epitaxial magnetite (Fe3O4) films on MgO (001) and Al2O3 (0001) substrates. For XMCD, we recorded the surface sensitive total electron yield and the bulk sensitive transmission spectra. From the analysis of the XMCD data, we find an increased Fe spin moment (10% larger) at the surface of the film on MgO (100) with respect to the corresponding bulk value of the film. Surface and bulk spin moments of the film on Al2O3(0001) are almost equal. For both films, the bulk orbital to spin moments ratio increases from zero at 70K to 0.03–0.04 at 300K. For Fe3O4∕MgO (001), the surface orbital to spin moment ratio behaves similarly to the bulk value, while the orbital to spin moments ratio is increased at the Fe3O4∕Al2O3 (0001) surface to 0.06. The observed differences between films grown on MgO and Al2O3 are explained within the framework of differences in mismatch strain experienced by the films.
The effects of electronic structure and charged state on thermodynamic properties: An ab initio molecular dynamics investigations on neutral and charged clusters of Na39, Na40, and Na41
Authors: Ghazi Seyed Mohammad , Lee Mal-Soon , Kanhere D. G.
Published: March 2008

In this paper we explore the effects of the electronic structure, the charge state, and the nature of energy distribution of isomers on the thermodynamic properties of sodium clusters. The focus of the work is to isolate the effects of these ingredients on thermodynamic behavior by choosing specific clusters. Toward this end we investigate Na39−, Na40, and Na41+, which are the electronic closed shell systems which differ in number of atoms and charge state. We also examine Na39, Na39+, Na40+, and Na41 clusters having different charges of these clusters. Our density functional molecular dynamics simulations show that all electronic shell-closing clusters have similar melting temperature of ≈310K. Remarkably, it is observed that an addition of even one electron to Na39 increases the melting temperature by about 40K and makes the specific heat curve sharper. All the cationic clusters show broadened specific heat curves.
Classical and quantum Coulomb crystals
Authors: Bonitz M. , Ludwig P. , Baumgartner H. , Henning C. , Filinov A. , Block D. , Arp O. , Piel A. , Käding S. , Ivanov Y. , Melzer A. , Fehske H. , Filinov V.
Published: March 2008

Strong correlation effects in classical and quantum plasmas are discussed. In particular, Coulomb (Wigner) crystallization phenomena are reviewed focusing on one-component non-neutral plasmas in traps and on macroscopic two-component neutral plasmas. The conditions for crystal formation in terms of critical values of the coupling parameters and the distance fluctuations and the phase diagram of Coulomb crystals are discussed.
New contributions to the photophysical model for all-trans-polyenesfrom ttbP4, a nonphotolabile octatetraene
Authors: Catalán Javier , Hopf Henning , Martus Mainrad
Published: March 2008

All-trans-octatetraene 3,10-di(tert-butyl)-2,2,11,11-tetramethyl-3,5,7,9-dodecatetraene emits fluorescence in three different regions of the visible spectrum. Thus, it produces an extremely weak emission in the gas phase that can hardly be detected in the condensed phase; such an emission exhibits a negligible Stokes shift with respect to the 1Ag→1Bu absorption transition and can, in principle, be assigned to the 1Bu→1Ag emission for the compound. A second, structureless fluorescence emission, centered in the region of 525nm, is observed in the gas phase and at somewhat higher wavelengths in the condensed phase [viz., 570nm in 2-methylbutane (2MB) and 550nm in squalane (SQ)]. While detectable, this emission increases significantly, with no change in spectral position, as the solution temperature is lowered; also, it is abruptly replaced by a new, strongly blueshifted emission at ∼490nm in 2MB and 455mm in SQ when the viscosity of the medium exceeds a given level. The fact that the two fluorescence emissions considerably depart from the expected behavior for a 1Bu→1Ag emission in an all-trans-polyene, and that one disappears while the other simultaneously appears as the medium becomes more rigid, suggests that the two emissions are produced by two different molecular structures and that the rigidity of the medium switches their production from the originally excited all-trans 1Bu form. The observed spectral behavior is consistent with a recently proposed model [J. Catalan, Chem. Phys. 335, 69 (2007)] in which the 1Bu excited state of octatetraene can give two distinct molecular conformers as a result of twisting about different C–C single bonds.
Attenuation characteristics of near-field electromagnetic noise through microstrip line for magnetic absorbers with different levels of conductivity
Authors: Kim Sang Woo , Lee Jung Hwan , Kim Yoon Bae , Lee Kyung Sup
Published: March 2008

Attenuation characteristics of near-field electromagnetic noise through microstrip line attached with electromagnetic thick films with different levels of electrical conductivity for frequencies in the range of 0.1–6GHz were investigated with numerical simulation via the three dimensional finite-element high-frequency structure simulator (HFSS) code. The electromagnetic absorption of the semiconducting magnetic or nonmagnetic films highly increased up to 80% at above 1GHz, while the absorption of the nonmagnetic conductor or the insulating films was significantly diminished. The remarkable enhancement in the loss factor of the semiconducting magnetic or nonmagnetic films was attributed to the increase in loss factor of the eddy-current loss. Even though the reflection of the semiconducting magnetic films at above 58S∕m was significantly lowered by the magnetic loss effect compared to that of the semiconducting nonmagnetic films, the reflection below 1GHz was still high enough to incur electromagnetic interferences. It was confirmed from the numerical results of magnetic field that the radiation noise was highly generated with increasing conductivity at above 58S∕m because of the rereflection of the radiated noise from the surface of the magnetic films with high level of conductivity. As a result, the near-field electromagnetic noise was effectively reduced by controlling conductivity to the semiconducting level of below 5.8S∕m for the magnetic films with high permeability in the measured frequency.
Forbidden patterns in financial time series
Authors: Zanin Massimiliano
Published: March 2008

The existence of forbidden patterns, i.e., certain missing sequences in a given time series, is a recently proposed instrument of potential application in the study of time series. Forbidden patterns are related to the permutation entropy, which has the basic properties of classic chaos indicators, such as Lyapunov exponent or Kolmogorov entropy, thus allowing to separate deterministic (usually chaotic) from random series; however, it requires fewer values of the series to be calculated, and it is suitable for using with small datasets. In this paper, the appearance of forbidden patterns is studied in different economical indicators such as stock indices (Dow Jones Industrial Average and Nasdaq Composite), NYSE stocks (IBM and Boeing), and others (ten year Bond interest rate), to find evidence of deterministic behavior in their evolutions. Moreover, the rate of appearance of the forbidden patterns is calculated, and some considerations about the underlying dynamics are suggested.
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equation of state of platinum to 80GPa and 1900K from internal resistive heating/x-ray diffraction measurements
Authors: Zha Chang-Sheng , Mibe Kenji , Bassett William A. , Tschauner Oliver , Mao Ho-Kwang , Hemley Russell J.
Published: March 2008

The P-V-T equation of state (EOS) of Pt has been determined to 80GPa and 1900K by in situ x-ray diffraction of a mixture of Pt and MgO using a modified internal resistive heating technique with a diamond anvil cell. The third-order Birch–Murnaghan EOS of Pt at room temperature can be fitted with K0=273.5±2.0GPa, K0′=4.70±0.06, with V0=60.38Å3. High temperature data have been treated with both thermodynamic and Mie–Grüneisen-Debye methods for the thermal EOS inversion. The results are self-consistent and in excellent agreement with those obtained by the multianvil apparatus where the data overlap. MgO is taken as the standard because its thermal EOS is well established and based on a wealth of experimental and theoretical data, and because the EOS at room temperature has been determined by a primary method that is completely independent of any assumptions or measurements by other methods. Improvements to previous internal resistive heating methods were made by using a Re gasket that replaces the original gasket composed of diamond and MgO powder. We have thereby extended the P-T range to nearly 80GPa and 1900K. Use of this method in combination with synchrotron radiation has advantages in the study of EOS, phase diagrams, and materials synthesis for a variety of problems in physics, chemistry, geosciences, and material sciences.
Charge creation, trapping, and long phosphorescence in Sr2MgSi2O7:Eu2+, RE3+
Authors: Setlur A. A. , Srivastava A. M. , Pham H. L. , Hannah M. E. , Happek U.
Published: March 2008

This report describes experiments that elucidate the phosphorescence mechanism in Sr2MgSi2O7:Eu2+, RE3+ phosphors. The first step of phosphorescence, the storage of carriers in traps, is traced to thermally assisted photoionization of Eu2+ ions, supported by thermal quenching studies that give the thermal ionization threshold of Eu2+. The trapping of these delocalized electrons has been detected using the spectroscopic signature of Sm2+ ions created by electron capture by Sm3+ in samples doped with Eu2+ and Sm3+ ions. The final steps for phosphorescence, liberation of trapped carriers and recombination with the luminescence center, have also been studied by monitoring the Eu2+ luminescence induced by optical excitation into the Sm2+ 4f6→4f55d1 absorption band. These results show that the phosphorescence in this system starts with the promotion of electrons into the conduction band and subsequent reduction of RE3+ ions, not from hole formation and RE3+ oxidation, as has been reported for similar systems. Finally, the formation of Sm2+ after Eu2+ photoionization also allows for initial spectroscopic measurements of Sm2+ in Sr2MgSi2O7.
Anodic plasma in Hall thrusters
Authors: Keidar Michael
Published: March 2008

In this paper plasma dynamics and ionization of propellant gas are modeled within the anode holes used for gas injection of a Hall thruster. Under conditions of anode coating with dielectric material, the discharge current should close within these holes, which results in ionization and formation of plasma jets emanating from the openings. The model shows that gas ionization inside the anode holes is very significant. For instance, the electron density increases by two orders of magnitude under certain conditions. The potential drop in the anode region which includes the electrostatic sheath inside the hole and potential drop along the hole might be positive or negative, depending on the anode hole radius.
Reduced-order simulation of large accelerator structures
Authors: Cooke S. J.
Published: March 2008

Simulating electromagnetic waves inside finite periodic or almost periodic three-dimensional structures is important to research in linear particle acceleration, high power microwave generation, and photonic band gap structures. While eigenmodes of periodic structures can be determined from analysis of a single unit cell, based on Floquet theory, the general case of aperiodic structures, with defects or nonuniform properties, typically requires 3D electromagnetic simulation of the entire structure. When the structure is large and high accuracy is necessary this can require high-performance computing techniques to obtain even a few eigenmodes [Z. Li et al., Nucl. Instrum. Methods Phys. Res., Sect. A 558, 168 (2006)]. To confront this problem, we describe an efficient, field-based algorithm that can accurately determine the complete eigenmode spectrum for extended aperiodic structures, up to some chosen frequency limit. The new method combines domain decomposition with a nontraditional, dual eigenmode representation of the fields local to each cell of the structure. Two related boundary value eigenproblems are solved numerically in each cell, with (a) electrically shielded, and (b) magnetically shielded interfaces, to determine a combined set of basis fields. By using the dual solutions in our field representation we accurately represent both the electric and magnetic surface currents that mediate coupling at the interfaces between adjacent cells. The solution is uniformly convergent, so that typically only a few modes are used in each cell. We present results from 2D and 3D simulations that demonstrate the speed and low computational needs of the algorithm.
Turbulent scaling
Authors: Lundgren Thomas S.
Published: March 2008

This article is a review and extension of several papers which presented a model for inertial range intermittency and anomalous scaling of velocity difference structure functions. The method of matched asymptotic expansions is used with the Navier–Stokes equation to derive a basic law for the instantaneous velocity difference between neighboring points in a turbulent flow. This is a power law with an undetermined exponent which must be specified statistically by making a physical hypothesis. (The simplest hypothesis, mean dissipation independent of Reynolds number makes the exponent be 1/3, the Kolmogorov 1941 result.) The resulting general theory is used to calculate structure functions, anomalous exponents and the probability density function of velocity differences and is extended to include the Reynolds number dependence of the instantaneous dissipation at a point and a theory for passive scalars. (A parameter in the instantaneous dissipation is adjusted to make the mean dissipation approximately independent of Reynolds number.)
Investigations of high mobility single crystal chemical vapor deposition diamond for radiotherapy photon beam monitoring
Authors: Tromson D. , Descamps C. , Tranchant N. , Bergonzo P. , Nesladek M. , Isambert A.
Published: March 2008

The intrinsic properties of diamond make this material theoretically very suitable for applications in medical physics. Until now ionization chambers have been fabricated from natural stones and are commercialized by PTW, but their fairly high costs and long delivery times have often limited their use in hospital. The properties of commercialized intrinsic polycrystalline diamond were investigated in the past by many groups. The results were not completely satisfactory due to the nature of the polycrystalline material itself. In contrast, the recent progresses in the growth of high mobility single crystal synthetic diamonds prepared by chemical vapor deposition (CVD) technique offer new alternatives. In the framework of the MAESTRO project (Methods and Advanced Treatments and Simulations for Radio Oncology), the CEA-LIST is studying the potentialities of synthetic diamond for new techniques of irradiation such as intensity modulated radiation therapy. In this paper, we present the growth and characteristics of single crystal diamond prepared at CEA-LIST in the framework of the NoRHDia project (Novel Radiation Hard CVD Diamond Detector for Hadrons Physics), as well as the investigations of high mobility single crystal CVD diamond for radiotherapy photon beam monitoring: dosimetric analysis performed with the single crystal diamond detector in terms of stability and repeatability of the response signal, signal to noise ratio, response speed, linearity of the signal versus the absorbed dose, and dose rate. The measurements performed with photon beams using radiotherapy facilities demonstrate that single crystal CVD diamond is a good alternative for air ionization chambers for beam quality control.
Influence of metal organic chemical vapor deposition growth parameters on the luminescent properties of ZnO thin films deposited on glass substrates
Authors: Roro K. T. , Dangbegnon J. K. , Sivaraya S. , Leitch A. W. R. , Botha J. R.
Published: March 2008

Highly c-axis oriented zinc oxide (ZnO) thin films with a wurtzite structure have been grown on glass substrates by metal organic chemical vapor deposition. The influence of growth parameters on the luminescent properties of the ZnO layers is investigated. It is shown that the growth temperature and VI/II ratio strongly influence the luminescent properties of ZnO thin films. For the films grown at low temperatures (250–325°C) a broad violet emission band at about 3.1eV has been observed. As the growth temperature increases, ultraviolet emission dominates the spectra and deep level emission is suppressed. It is suggested that the violet emission depends on grain size and hence the width of the free-carrier depletion region at the particle surface; the narrower the depletion width compared to the grain size, the higher the intensity. The result suggests that the mechanism responsible for the violet emission is recombination of delocalized electrons close to the conduction band with holes trapped in neutral zinc vacancy (VZn0) centers. For films grown under different VI/II ratios, the spectra are increasingly dominated by deep level emission as the VI/II ratio is increased. This broad emission is resolved into three bands at ∼2.0, ∼2.3, and ∼2.5eV. Based on the growth conditions, possible origins are proposed and discussed.
Settling of an isolated spherical particle in a yield stress shear thinning fluid
Authors: Putz A. M. V. , Burghelea T. I. , Frigaard I. A. , Martinez D. M.
Published: March 2008

We visualize the flow induced by an isolated non-Brownian spherical particle settling in a shear thinning yield stress fluid using particle image velocimetry. With Re<1, we show a breaking of the fore-aft symmetry and relate this to the rheological properties of the fluid. We find that the shape of the yield surface approximates that of an ovoid spheroid with its major axis approximately five times greater than the radius of the particle. The disagreement of our experimental findings with previous numerical simulations is discussed.
Impact of germanium related defects on electrical performance of hafnium oxide
Authors: Sun Qing-Qing , Shi Yu , Dong Lin , Liu Han , Ding Shi-Jin , Zhang David Wei
Published: March 2008

Germanium is known to diffuse into high-k dielectrics during germanium-based metal oxide semiconductor transistor process, which causes the degradation of gate dielectrics. In order to explore the origins of the degradation, we performed first-principles calculation based on density functional theory to study germanium related defects in HfO2. According to calculation, Ge at oxygen vacancy is responsible for the experimentally observed negative charge, gate leakage, and hysteresis enlargement while interstitial Ge in HfO2 is only responsible for the increased gate leakage. The results indicate that the passivation of oxygen vacancy is extremely important during germanium-based metal oxide semiconductor transistor processing.
Nonlinear saturation of collisionless trapped electron mode turbulence: Zonal flows and zonal density
Authors: Lang Jianying , Parker Scott E. , Chen Yang
Published: March 2008

Gyrokinetic δf particle simulation is used to investigate the nonlinear saturation mechanisms in collisionless trapped electron mode (CTEM) turbulence. It is found that the importance of zonal flow is parameter-sensitive and is well characterized by the shearing rate formula. The effect of zonal flow is empirically found to be sensitive to temperature ratio, magnetic shear, and electron temperature gradient. For parameters where zonal flow is found to be unimportant, zonal density (purely radial density perturbations) is generated and expected to be the dominant saturation mechanism. A toroidal mode-coupling theory is presented that agrees with simulation in the initial nonlinear saturation phase. The mode-coupling theory predicts the nonlinear generation of the zonal density and the feedback and saturation of the linearly most unstable mode. Inverse energy cascade is also observed in CTEM turbulence simulations and is reported here.
Recursive inverse factorization
Authors: Rubensson Emanuel H. , Bock Nicolas , Holmström Erik , Niklasson Anders M. N.
Published: March 2008

A recursive algorithm for the inverse factorization S−1=ZZ* of Hermitian positive definite matrices S is proposed. The inverse factorization is based on iterative refinement [A.M.N. Niklasson, Phys. Rev. B 70, 193102 (2004)] combined with a recursive decomposition of S. As the computational kernel is matrix-matrix multiplication, the algorithm can be parallelized and the computational effort increases linearly with system size for systems with sufficiently sparse matrices. Recent advances in network theory are used to find appropriate recursive decompositions. We show that optimization of the so-called network modularity results in an improved partitioning compared to other approaches. In particular, when the recursive inverse factorization is applied to overlap matrices of irregularly structured three-dimensional molecules.
Analytical solution for optimal squeezing of wave packet of a trapped quantum particle
Authors: Grigorenko Ilya
Published: March 2008

Optimal control problem with a goal to squeeze wave packet of a trapped quantum particle is considered and solved analytically using adiabatic approximation. The analytical solution that drives the particle into a highly localized final state is presented for a case of an infinite well trapping potential. The presented solution may be applied to increase the resolution of atom lithography.
Properties of asymmetric magnetic reconnection
Authors: Birn J. , Borovsky J. E. , Hesse M.
Published: March 2008

Properties of magnetic reconnection are investigated in two-dimensional, resistive magnetohydrodynamic (MHD) simulations of current sheets separating plasmas with different magnetic field strengths and densities. Specific emphasis is on the influence of the external parameters on the reconnection rate. The effect of the dissipation in the resistive MHD model is separated from this influence by evaluating resistivity dependence together with the dependence on the background parameters. Two scenarios are considered, which may be distinguished as driven and nondriven reconnection. In either scenario, the maximum reconnection rate (electric field) is found to depend on appropriate hybrid expressions based on a magnetic field strength and an Alfvén speed derived from the characteristic values in the two inflow regions. The scaling compares favorably with an analytic formula derived recently by Cassak and Shay [Phys. Plasmas 14, 102114 (2007)] applied to the regime of fast reconnection. An investigation of the energy flow and conversion in the vicinity of the reconnection site revealed a significant role of enthalpy flux generation, in addition to the expected conversion of Poynting flux to kinetic energy flux. This enthalpy flux generation results from Ohmic heating as well as adiabatic, that is, compressional heating. The latter is found more important when the magnetic field strengths in the two inflow regions are comparable in magnitude.
Experimental studies of zonal flow and field in compact helical system plasma
Authors: Fujisawa A. , Itoh K. , Shimizu A. , Nakano H. , Ohshima S. , Iguchi H. , Matsuoka K. , Okamura S. , Minami T. , Yoshimura Y. , Nagaoka K. , Ida K. , Toi K. , Takahashi C. , Kojima M. , Nishimura S. , Isobe M. , Suzuki C. , Akiyama T. , Ido T. , Nagashima Y. , Itoh S.-I. , Diamond P. H.
Published: March 2008

The experimental studies on zonal flows and turbulence have been carried out in Compact Helical System [K. Matsuoka, S. Kubo, M. Hosokawa et al., in Plasma Physics and Controlled Nuclear Fusion Research, Proc. 12th Int. Conf., Nice, 1988 (International Atomic Energy Agency, Vienna, 1989, Vol. 2, p. 411] using twin heavy ion beam probes. The paper presents the experimental observations of stationary zonal flow, nonlinear couplings between zonal flow and turbulence, and the role of zonal flow in the improved confinement, together with the recent discovery of zonal magnetic field. The presented experimental results strongly support the new paradigm that the plasma transport should be considered as a system of drift wave and zonal flows, and provides the first direct evidence for turbulence dynamo that the structured magnetic field can be really generated by turbulence.
Structural precursor to freezing: An integral equation study
Authors: Brader Joseph M.
Published: March 2008

Recent simulation studies have drawn attention to the shoulder which forms in the second peak of the radial distribution function of hard spheres at densities close to freezing and which is associated with local crystalline ordering in the dense fluid. We address this structural precursor to freezing using an inhomogeneous integral equation theory capable of describing local packing constraints to a high level of accuracy. The addition of a short-range attractive interaction leads to a well known broadening of the fluid-solid coexistence region as a function of attraction strength. The appearance of a shoulder in our calculated radial distribution functions is found to be consistent with the broadened coexistence region for a simple model potential, thus demonstrating that the shoulder is not exclusively a high density packing effect.
Investigation of plasma immersion ion implantation of nickel-titanium rod by multiple-grid particle-in-cell simulation
Authors: Kwok Dixon T. K. , Wu Shuilin , Liu Xiangmei , Fu Ricky K. Y. , Chu Paul K.
Published: March 2008

A multiple-grid-particle-in-cell numerical method has been developed. This method uses grids of different cell sizes and details are needed in only one part of the simulation region and not others. Hence, there are fewer nodes in the simulation thereby reduced computational time without sacrificing details. In the multiple-grid system, a phenomenon is identified to arise at the interface between two grids and a half-cell weighting method is utilized to solve the weighting issue at the boundary. It is shown that the expression of the change of momentum has no weighting function. This method is employed to numerically simulate the plasma immersion ion implantation process into a nickel titanium rod measuring 50mm long and 4.8mm in diameter used in orthopaedic surgery. To conduct more uniform implantation, the NiTi rod is elevated on the sample stage by a metal rod. The nitrogen implantation fluences and depth profiles are simulated and compared to experimental values determined by x-ray photoelectron spectroscopy.
Structural disorder in the cubic Ce0.5Zr0.5O2 catalyst: A possible factor of the high catalytic activity
Authors: Wakita Takahiro , Yashima Masatomo
Published: March 2008

Nuclear-density distribution of cubic Ce0.5Zr0.5O2 compound has been studied at 1832K by the maximum-entropy method (MEM) and MEM-based pattern fitting combined with the Rietveld method using in situ neutron powder diffraction data. The oxygen ions show a large positional disorder and shift to the ⟨111⟩ directions. Possible bulk diffusion paths of the oxygen ions can be seen along the ⟨100⟩ and ⟨110⟩ directions. The spatial distribution of oxygen ions in Ce0.5Zr0.5O2 is greater than that of CeO2, which is consistent with the higher bulk diffusivity of oxygen ions in Ce0.5Zr0.5O2. It is suggested that the greater disorder of Ce0.5Zr0.5O2 is a factor of its higher catalytic activity.
Stability of trapped charges in sapphires and alumina ceramics: Evaluation by secondary electron emission
Authors: Zarbout K. , Si Ahmed A. , Moya G. , Bernardini J. , Goeuriot D. , Kallel A.
Published: March 2008

The stability of trapped charges in sapphires and alumina ceramics is characterized via an experimental parameter expressing the variation of the secondary electron emission yield between two electron injections performed in a scanning electron microscope. Two types of sapphires and polycrystalline alumina, which differ mainly by their impurity content, are investigated in the temperature range 300–663K. The stable trapping behavior in sapphires is attributed to trapping in different defects, whose nature depends on the purity level. In alumina ceramics, the ability to trap charges in a stable way is stronger in samples of high impurity content. In the low impurity samples, stable trapping is promoted when the grain diameter decreases, whereas the reverse is observed in high impurity materials. These behaviors can stem from a gettering effect occurring during sintering. The strong dependence of the variation of the secondary electron emission yield on the grain diameter and impurities enables a scaling of the stable trapping ability of alumina materials.
Subwavelength modulation of surface relief and refractive index in preirradiated liquid-crystalline polymer films
Authors: Yu Haifeng , Shishido Atsushi , Ikeda Tomiki
Published: March 2008

We present a simple method to record subwavelength gratings (SWGs) in an azobenzene (AZ)-containing liquid-crystalline polymer by preirradiation. Since the cooperative motions of mesogens are eliminated in a cis-AZ-rich isotropic phase by the pretreatment, the subwavelength modulation of surface relief and refractive index (RI) was obtained. The surface relief of less than 10nm and the RI modulation were detected by atomic force microscopy. A large phase retardation and formed birefringence were obtained in the recorded SWGs in which a possible schematic illustration of AZ was proposed.
Buckling of ZnO nanowires under uniaxial compression
Authors: Riaz M. , Nur O. , Willander M. , Klason P.
Published: March 2008

Instability and buckling characterization of vertical well aligned single crystal of ZnO nanowires grown on SiC substrate was done quantitatively by nanoindentation technique. The critical load was found to be 477μN and the corresponding buckling energy was 3.46×10−11J. Based on the Euler model for long nanowire and Johnson model which is an extension of the Euler model for intermediate nanowire, the modulus of elasticity of single wire was calculated. Also, the critical buckling stress and strain were determined for the as grown single wire of ZnO. We found how the modulus of elasticity is dependent on the slenderness ratio.
Design of a double-pass shear mode acousto-optic modulator
Authors: Chang Chih-Hao , Heilmann R. K. , Schattenburg M. L. , Glenn P.
Published: March 2008

The design of a compact double-pass shear mode acousto-optic modulator for high power operation in the UV is presented. Using a spherical mirror retroreflector in the second pass to correct for beam deflection, changes in beam position and angle during frequency tuning can be eliminated. The use of the shear mode acousto-optic interaction offers several key advantages, but the strain-induced birefringence creates significant levels of spectral leakage in the system. These effects and the polarization of the beams are analyzed to minimize leakage. Using a heterodyne interferometry scheme, two double-pass acousto-optic modulators with offset acoustic frequency inputs are set up to measure the leakage intensity. The designed double-pass shear mode acousto-optic modulator was tested with λ=351.1nm and demonstrated a peak double-pass efficiency of 42.6% and a bandwidth of 28MHz. Over this bandwidth the spectral leakage was reduced to ∼0.01%–0.04% relative intensity.
Recent progress of the improved magnetically insulated transmission line oscillator
Authors: Fan Yu-Wei , Zhong Hui-Huang , Li Zhi-Qiang , Shu Ting , Zhang Jian-De , Liu Jin-Liang , Yang Jian-Hua , Zhang Jun , Yuan Cheng-Wei , Luo Ling
Published: March 2008

The improved magnetically insulated transmission line oscillator (MILO) is a gigawatt-class L-band high power microwave tube driven by a 550kV, 57kA, 50ns electron beam. It has allowed us to generate 2.4GW pulse of 22ns duration. The recent progress of the improved MILO is presented in this paper. First, a field shaper cathode is introduced into the improved MILO to avoid the cathode flares in the triple point region. The experimental results show that the cathode flares are avoided, so the lifetime of the velvet cathode is longer than that of the taper cathode. Furthermore, the shot-to-shot reproducibility is better than that of the taper cathode. Second, In order to prolong the pulse duration and increase the radiated microwave power, a self-built 600kV, 10Ω, 80ns pulser: SPARK-03 is employed to drive the improved MILO. Simulation and experimental investigation are performed. In simulation, when the improved MILO is driven by a 600kV, 57kA electron beam, high-power microwave is generated with output power of 4.15GW, frequency of 1.76GHz, and relevant power conversion efficiency of 12.0%. In experiments, when the diode voltage is 550kV and current is 54kA, the measured results are that the radiated microwave power is above 3.1GW, the pulse duration is above 40ns, the microwave frequency is about 1.755GHz, and the power conversion efficiency is about 10.4%.
Fast, high bit number pattern generator for electron and ion beam lithographies
Authors: Pennelli Giovanni
Published: March 2008

High resolution electron and ion beam lithographies, fundamental tools for nanofabrication and nanotechnologies, require fast and high precision (high bit number) pattern generators. In the present work a solution for increasing the bit number, and preserving the speed of the system, is presented. A prototype with effective 18bit resolution and with a write speed as fast as 10MHz has been successfully tested: details of the adopted hardware solution are presented and described. This solution is very general and can be used in all those applications that require the generation of control voltages with an high bit number (high precision) at high speed, such as, for example, the scanning probe microscopy and nanomanipulation. Software solutions for increasing the data transfer efficiency are also presented; the aim of the adopted solutions is to preserve the flexibility and adaptability of the pattern generator to different writing strategies.
Exchange bias and fourfold magnetic anisotropy in Permalloy thin film on epitaxial hematite antiferromagnet
Authors: Bali R. , Stelmashenko N. A. , Blamire M. G.
Published: March 2008

Fourfold anisotropy was observed in NiFe films deposited on epitaxial antiferromagnet α-Fe2O3. The anisotropy was preserved despite off-axis magnetic field annealing (MFA) below the Néel temperature (TN). The cumulative effect of this fourfold anisotropy and exchange coupling due to MFA determined the exchange bias (Hex) distribution. Physically, this indicated the existence of two types of antiferromagnet domains—those that were frozen in below TN and contributed to the fourfold anisotropy and those that coupled with the annealing magnetic field and contributed to the conventional Hex distribution.
Spin current diode based on an electron waveguide with spin-orbit interaction
Authors: Zhai Feng , Chang Kai , Xu H. Q.
Published: March 2008

We propose a spin current diode which can work even in a small applied bias condition (the linear-response regime). The prototypal device consists of a hornlike electron waveguide with Rashba spin-orbit interaction, which is connected to two leads with different widths. It is demonstrated that when electrons are incident from the narrow lead, the generated spin conductance fluctuates around a constant value in a wide range of incident energy. When the transport direction is reversed, the spin conductance is suppressed strongly. Such a remarkable difference arises from spin-flipped transitions caused by the spin-orbit interaction.
Electron spin resonance of antiferromagnetism in Nd0.43Sr0.57MnO3 and Nd0.35Sr0.65MnO3
Authors: Geetha Kumary T. , Lin J. G.
Published: March 2008

The manganites Nd0.43Sr0.57MnO3 and Nd0.35Sr0.65MnO3, with A-type and C-type antiferromagnetic orders, respectively, are prepared and their physical properties are studied by electrical resistivity, ac∕dc susceptibility, and electron spin resonance (ESR) measurements. Results of the temperature dependent ESR linewidth suggest that the spin-spin interaction is the dominant relaxation mechanism in these layered manganites. The g factors are nearly temperature independent in the paramagnetic state, but it varies dramatically near TN, indicating a strong spin-orbit coupling near the transition. Different temperature-dependencies of g factor in these two samples reflect the distinct features in the layer∕chain antiferromagnetism.
High spatial resolution fiber-optic Fizeau interferometric strain sensor based on an in-fiber spherical microcavity
Authors: Li Enbang , Peng Gang-Ding , Ding Xin
Published: March 2008

We present a fiber-optic Fizeau interferometric strain sensor consisting of an in-fiber spherical microcavity of 39μm in diameter. The spherical microcavity was formed by splicing a normal single-mode fiber with a hollow-core photonic crystal fiber. We demonstrate that strain sensing can be realized by using the interference between the light signals reflected by the front and rear surfaces of the sphere. Experiments have shown that the strain sensor has a strain sensitivity of 3.36pm∕με and a temperature sensitivity of 1.35pm∕°C.
Effective-barrier-thickness fluctuation and critical-current spread of grain-boundary Josephson junctions
Authors: Jeng J. T. , Wu C. H.
Published: March 2008

The phenomenological formulas for the critical-current spread of the bicrystal grain-boundary Josephson junctions are proposed. By considering the intrinsically shunted-junction model and the gamma distribution, the fluctuation of effective-barrier thickness from 0.01to0.1nm corresponds to the 1−σ critical-current spread from 14% to 106%. With a fixed scale parameter, the critical-current spread is inversely proportional to the square root of the bridge width. When the bridge width is fixed, the spread is inversely proportional to the square root of the mean critical-current density. These predictions are qualitatively in agreement with the experimental results reported to date.
Narrow-band radiation sensing in the terahertz and microwave bands using the radiation-induced magnetoresistance oscillations
Authors: Mani R. G.
Published: March 2008

Tuned narrow-band sensing of microwave and terahertz radiation can be realized by subjecting an irradiated two-dimensional electron system to both a static and a time varying magnetic field, and detecting at the harmonics of the modulation. A third harmonic sensor is considered here in conjunction with periodic-in-the-inverse-magnetic-field radiation-induced magnetoresistance oscillations.
Growth of M-plane MnAs on GaAs(111)B by molecular beam epitaxy
Authors: Takagaki Y. , Herrmann C. , Jenichen B. , Herfort J. , Brandt O.
Published: March 2008

We describe a growth procedure utilizing molecular beam epitaxy that produces a (11¯00)-oriented MnAs layer on GaAs(111)B despite the incompatible unit mesh symmetry. An amorphous MnAs layer is deposited at a low temperature beyond the critical thickness for coherent growth. When solid-phase epitaxy is initiated by reducing the background As4 pressure, the layer crystalizes in the M-plane orientation with its c axis being along the {112¯} directions of the substrate. The magnetization components associated with the coexisting c-axis orientations are independent of each other, suggesting that the structural domains are much larger in size than the atomic scales.
Dynamic elastic properties and magnetic susceptibility across the austenite-martensite transformation in site-disordered ferromagnetic Ni–Fe–Al alloy
Authors: Mukhopadhyay P. K. , Kaul S. N.
Published: March 2008

ac susceptibility, sound velocity, and internal friction study of the martensitic transformation in melt-quenched (site-disordered) Ni55Fe20Al25 alloy unravels the irreversible thermoelastic changes (thermal hysteresis) that occur in the austenite phase in the premartensitic regime. In the heating cycle, the system retains the “memory” of the initiation and subsequent growth of the martensitic phase (at the expense of the parent austenite phase) that had taken place during the cooling cycle in the austenite-martensite phase coexistence region. These and other findings are discussed in this communication.
Studies of two distinct types of (Ba,Sr)TiO3∕Pt interfaces
Authors: Xia Yidong , Yin Kuibo , Xu Chun , Zhang Yi , Xu Bo , He Weiye , Meng Xiangkang , Yin Jiang , Liu Zhiguo
Published: March 2008

High resolution transmission electron microscopy is used to investigate the interfacial layer at interfaces between (001) (Ba,Sr)TiO3 (BST) films and (111) Pt electrodes. Two distinct types of interfaces are observed. One case is the presence of interfacial layer with distorted structure, whereas another case is the good interfacial match without any interfacial layer. Based on the analyses of crystallographic relationships between BST and Pt, it is proposed that the in-plane lattice structure plays an important role in determining the feature of the interface. The impact of A-O and B-O terminations of BST films on the interfacial layer is also discussed.
Back-illuminated separate absorption and multiplication GaN avalanche photodiodes
Authors: Pau J. L. , Bayram C. , McClintock R. , Razeghi M. , Silversmith D.
Published: March 2008

The performance of back-illuminated avalanche photodiodes with separate absorption and multiplication regions is presented. Devices with an active area of 225μm2 show a maximum multiplication gain of 41 200. The calculation of the noise equivalent power yields a minimum value of 3.3×10−14WHz−1∕2 at a gain of 3000, increasing to 2.0×10−13WHz−1∕2 at a gain of 41 200. The broadening of the response edge has been analyzed as a function of bias.
Theoretical investigation of size and shape effects on the melting temperature and energy bandgap of TiO2 nanostructures
Authors: Guisbiers G. , Van Overschelde O. , Wautelet M.
Published: March 2008

In this letter, we report a theoretical investigation concerning the size effect on the melting temperature and energy bandgap of TiO2 nanostructures. Within the thermodynamical approach, we predict a structural phase transition from rutile to anatase for the sizes around 40, 29, and 48nm, respectively, in the cases of spherical nanoparticles, cylindrical nanowires, and nanotubes. For spherical nanoparticles, this means that the more stable phase is anatase for sizes smaller than ∼40nm and rutile for sizes larger than ∼40nm. The energy bandgap of these structures is also estimated.
Photosensitization of nonlinear scattering and photoacoustic emission from single-walled carbon nanotubes
Authors: Lee I.-Yin Sandy , Matsuo Tomomi , Suzuki Honoh
Published: March 2008

Enhancement of laser-induced nonlinear scattering has been observed from an aqueous suspension of single-walled carbon nanotubes (SWCNTs) doped with near-infrared dye chromophores. The underlying mechanism involves cavitation triggered by optical heating and bubble pulsation, which are further enhanced by the dye adsorbed on SWCNT surfaces, resulting in the scattering enhancement and a modification of the accompanying photoacoustic waveform. These photosensitizing effects suggest potential feasibility of dye-doped nanotubes as optically switchable nanoemitters of ultrasound for biomedical applications such as ultrasonic therapy and imaging.
Visualization of heat flows in high-power diode lasers by lock-in thermography
Authors: Ziegler Mathias , Tomm Jens W. , Elsaesser Thomas , Erbert Götz , Bugge Frank , Nakwaski Włodzimierz , Sarzała Robert P.
Published: March 2008

Lock-in thermography is applied to analyze thermal properties of high-power diode lasers. With a temporal resolution of about 100μs for thermal imaging of the entire device, microscopic heat flows occurring on a millisecond time scale and propagating infrared light are distinguished. This allows for a measurement of heat transport in the device on a 100μm length scale and an identification of “hot spots” at the device edges as scattered thermal radiation. Thermal transients are monitored and described quantitatively by finite element modeling. Thermographic images of the laser side identify the front facet as a major heat source.
Photoacoustic measurements to determine acoustic velocities in shocked condensed materials: Application to liquid benzene
Authors: Hemmi N. , Torchinsky D. H. , Zimmerman K. A. , Winey J. M. , Nelson K. A. , Gupta Y. M.
Published: March 2008

Experimental methods were developed to obtain single-shot, photoacoustic measurements in shock compressed thin samples. Impulsive stimulated thermal scattering measurements were used to measure acoustic velocities normal to the shock propagation direction in liquid benzene compressed to nearly 1GPa. Although the resulting pressure-temperature states correspond to the solid region of the benzene phase diagram, shocked benzene remains a liquid at pressures well above the static compression threshold for freezing. Photoacoustic measurements provide a useful approach to examine structural changes in shocked materials.
Near-field optical characterization of surface-plasmon-mediated light emission from electrically biased metal-insulator-semiconductor tunnel junctions
Authors: Seidel J. , Göhler T. , Grafström S. , Eng L. M.
Published: March 2008

We report on near-field optical observations of surface-plasmon-mediated light emission in electrically biased metal-insulator-semiconductor (MIS) tunnel junctions fabricated from Au and p-type Si with a native oxide layer as the tunneling barrier. Our junctions exhibit stable broadband macroscopic light emission in the visible spectrum. Inspection of the optical near-field reveals highly localized hot spots due to local plasmon excitation and scattering, which are investigated with spectral distinction. Such MIS tunneling junctions are compatible with common complementary metal-oxide semiconductor technology and thus open up an interesting route toward the development of novel integrated optoelectronic and plasmonic devices.
Genomic imprinting effects on adult body composition in mice
Authors: Cheverud James M. , Hager Reinmar , Roseman Charles , Fawcett Gloria , Wang Bing , Wolf Jason B.
Published: March 2008

Genomic imprinting results in the differential expression of genes, depending on which allele is inherited from the mother and which from the father. The effects of such differential gene expression are reflected in phenotypic differences between the reciprocal heterozygotes (
Aa
vs.
aA
). Although many imprinted genes have been identified and play a key role in development, little is known about the contribution of imprinting to quantitative variation in trait expression. Here, we examine this problem by mapping imprinting effects on adult body composition traits in the F
3
generation of an intercross between the Large (LG/J) and Small (SM/J) inbred mouse strains. We identified eight pleiotropic imprinted quantitative trait loci (
i
QTL) located throughout the genome. Most
i
QTL are in novel locations that have not previously been associated with imprinting effects, but those on chromosomes 7, 12, and centromeric 18 lie in regions previously identified as containing imprinted genes. Our results show that the effects of genomic imprinting are relatively small, with reciprocal heterozygotes differing by ≈0.25 standard deviation units and the effects at each locus accounting for 1% to 4% of the phenotypic variance. We detected a variety of imprinting patterns, with paternal expression being the most common. These results indicate that genomic imprinting has small, but detectable, effects on the normal variation of complex traits in adults and is likely to be more common than usually thought.