bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic material having a flight of magnetization steps caused by temperature variation
Authors: Takahashi Naoko , Shimotomai Satoshi , Ido Hideaki
Published: May 2005

A flight of magnetization steps for increasing temperature has been made by making use of the compounds Mn2−xCrxSb, which are well known to undergo a first-order antiferromagnetic-to-ferrimagnetic phase transition for increasing temperature with the transition temperature increasing as x increases. A linear slope of magnetization for increasing temperature is also obtained in a wide temperature region of 60K for a compound Mn2−xCrxSb with a heterogeneous Cr distribution in the crystal. Temperature hysteresis at the first-order transition temperature is observed for three samples with different Cr compositions. A brief discussion about some potential applications of the magnetic characteristics is given on the basis of the experimental data.
Magneto-optical properties of Co∕Ge(100) with ultrathin Ag buffer layers
Authors: Su C. W. , Tsay J. S. , Hwang C. H. , Yao Y. D.
Published: May 2005

Magnetic properties of Co films (<2nm) with Ag buffer layers (<0.7nm) grown on Ge(100) at room temperature and 200K were studied by surface magneto-optical Kerr effect. Without the buffer, the films reveal in-plane magnetic anisotropy even Co and Ge forms nonmagnetic interfacial alloys. The hysteresis due to intercalation of Ag can be detected at thinner Co thicknesses. The buffer can effectively cutoff the intermixing of Co and Ge. As the thickness of Ag is reduced, out-of-plane magnetic anisotropy due to the interface interactions between Co∕Ag and Co∕Ge was discovered and was only at 200K.
Metamagnetic steps in Eu-based manganite compounds
Authors: Rana D. S. , Kuberkar D. G. , Stone M. B. , Schiffer P. , Malik S. K.
Published: May 2005

Magnetization and specific heat measurements have been carried out on the ABO3 type half-doped (La0.3Eu0.2)(Ca0.3Sr0.2)MnO3 (LE-50) and electron-doped (La0.2Eu0.25)(Ca0.3Sr0.25)MnO3 (LE-55) compounds. These compounds exhibit a metamagnetic transition at low temperatures. In LE-50, the dominant feature of the metamagnetic transition manifests itself at T≲5K. The transition is sharp and steplike at 2K with a critical field (HC) of 3.3kOe and a transition width of 250Oe becoming broad with a width of ∼10kOe at 3K. Similarly, in the LE-55 sample, the steplike behavior occurs at T≲3K with HC=72kOe, whereas at T>3K, the transition is much broader. A feature consistent with the metamagnetic transition appears in the specific heat data in these compounds. Such metamagnetic steps have been observed prior to now only in Pr-based manganites. Observation of the same in Eu-based manganites suggests that these steps have no influence of any property intrinsic to Pr ions.
Dipole-exchange spin waves in rectangular antiferromagnetic nanowires
Authors: Nouri M. , Cottam M. G.
Published: May 2005

A microscopic theory is developed for the dipole-exchange spin waves in long antiferromagnetic nanowires, either singly or in a rectangular array of size M×N, where M and N are arbitrary integers (typically chosen to be less than about 20). The model includes exchange interactions (both intrawire and interwire), the long-range dipole–dipole interactions, single-ion anisotropy (which stabilizes the antiferromagnetic ordering) and a longitudinal applied magnetic field. The spectrum of discrete coupled spin waves is calculated using a method similar to that employed recently to find the dipole-exchange spin waves in ultrathin antiferromagnetic films, and the spin-flop phase transition in some of these cases is also studied. It is found that the spin-wave frequencies are shifted as a result of the dipolar terms and that the transition field to the spin-flop phase is typically reduced in magnitude compared to its value in the exchange limit. The results are illustrated with numerical examples.
Microstructure characterization of ball milled Sm(CoCuFeZr) 2:17 powders
Authors: Zhang Y. , Zeng Q. , Hadjipanayis G. C.
Published: May 2005

The microstructure evolution and coercivity development in ball milled Sm(Co,Cu,Fe,Zr)z powders have been investigated by TEM, SEM, HRTEM, and VSM. The fully heat-treated Sm(Cobal,Fe0.1,Cu0.08,Zr0.03)7.5 and Sm(Cobal,Fe0.1,Cu0.08,Zr0.03)8.3 alloys were ball milled to powders with different ball milling rates and times. At a low ball milling rate, both the particle size and coercivity decrease slowly and monotonically with increasing milling time. Even after 48h milling, both powders with an average particle size of about 2μm still retain about 40%–50% of their initial coercivity. Both the cellular and lamellar structures are still observed in the two powders even after 48h milling, although crystal lattice distortion and defects are introduced into the cellular and lamellar structures in both powders. At a high milling rate, both the particle size and the coercivity drop sharply with milling time for both powders, and even 2min milling leads to the loss of 50% of the initial coercivity; the mean particle size was measured to be 5μm, whereas the initial cellular structure was significantly destroyed and highly strained. Further ball milling leads to sample oxidation and the formation of amorphous phase.
Spin reorientation transition in ultrathin Co film on InP(2×4) reconstructed surface
Authors: Park Yong-Sung , Jeong Jong-Ryul , Shin Sung-Chul
Published: May 2005

We have investigated magnetic properties of monolayer (ML)-thickness Co film deposited on InP(2×4) reconstructed surface using in situ surface magneto-optical Kerr effects (SMOKE) measurement system. InP(2×4) reconstructed surface, obtained by several cycles of sputtering-and-annealing process, was confirmed by reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) measurements. Co film grown on InP(2×4) reconstructed surface shows three distinguishable thickness regions which have different magnetic properties, depending on Co film thickness. In the Co film thickness region smaller than 7 ML, no SMOKE signal was detected. In the thickness region between 8 ML and 15 ML, both longitudinal and polar Kerr hysteresis loops were observed. In the film thickness larger than 16 ML, only longitudinal SMOKE signal without polar signal was detected.
Growth and characterization of sputtered BSTO∕BaM multilayers
Authors: Srinath S. , Frey N. A. , Heindl R. , Srikanth H. , Coffey K. R. , Dudney N. J.
Published: May 2005

Multilayers of Ba0.5Sr0.5TiO3 (BSTO) and BaFe12O19 (BaM), with tunable permeability and permittivity are attractive systems for radio frequency and microwave applications. We have grown multilayers of BSTO and BaM using magnetron sputtering on Al2O3 substrates. Film growth conditions such as sputtering parameters were optimized to obtain high quality multilayers. X-ray diffraction established that both BSTO and BaM were formed and cross-sectional SEM studies showed sharp interfaces between BSTO and BaM layers. Magnetization showed a large coercivity (∼2000Oe) consistent with the hexaferrite component. The hysteresis loops also revealed the distinct influence of magnetocrystalline and shape anisotropies at different temperatures.
The mechanism of Ag top layer on the coercivity enhancement of FePt thin films
Authors: Zhao Z. L. , Ding J. , Yi J. B. , Chen J. S. , Zeng J. H. , Wang J. P.
Published: May 2005

The magnetic properties of the FePt thin films with a Ag top layer prepared by magnetron sputtering have been studied. With 4nm Ag layer deposited right after the deposition of FePt layer, the ordering temperature of the L10 FePt phase decreased to 350°C or below. X-ray photoelectron spectroscopy results indicate that Ag has diffused into the FePt layer when Ag was deposited at 350°C, while Ag remains on the top of the FePt when Ag was deposited at room temperature. The Ag top layer deposited at 350°C was also found to protect the FePt layer from oxidation.
A
-site dependent percolative thermopower and Griffiths phase in Pr(0.7−x)HoxSr0.3MnO3 (x=0.0, 0.04, 0.08, and 0.1)
Authors: Rama N. , Opel M. , Sankaranarayanan V. , Gross R. , Ogale S. B. , Venkatesan T. , Rao M. S. Ramachandra
Published: May 2005

We report our analysis of thermopower S(T) of Pr(0.7−x)HoxSr0.3MnO3 (x=0, 0.04, 0.08, and 0.1) compounds within a percolative framework and correlate the results with the existence of a Griffiths phase. The Griffiths temperature, at which ferromagnetic metallic clusters nucleate above TC, is evaluated. We further show that the Griffiths phase extends over a higher temperature range above TC with increase in the variance of ionic radii of the A site.
Magnetic properties of SmTbFe17−xGax [⩽x⩽8]
Authors: Ingersoll J. C. , Markandeyulu G. , Murty V. S. , Rama Rao K. V. S.
Published: May 2005

Magnetic properties of (Sm0.5Tb0.5)2Fe17−xGax [mentioned subsequently as SmTbFe17−xGax] have been investigated. All the compounds have formed in rhombohedral Th2Zn17 structure with traces of α-Fe. The saturation magnetization decreases with increasing x and is attributed to the modification of the density of states of 3d band due to the low lying 3p band of Ga. The increase of Curie temperature up to x=3 and subsequent decrease is explained on the basis of band narrowing. The easy direction of magnetization is in the ab plane up to x=5 and changes along the c axis for x=7.
Measurement of field-dependence elastic modulus of iron–gallium alloy using tensile test
Authors: Yoo Jin-Hyeong , Flatau Alison B.
Published: May 2005

An experimental approach is used to identify Galfenol material properties under dc magnetic bias fields. Dog-bone-shaped specimens of single crystal Fe100−xGax, where 18.6⩽x⩽33.2, underwent tensile testing along two crystallographic axis orientations, [110] and [100]. Young’s modulus and Poisson’s ratio sensitivity to magnetic fields and stoichiometry are investigated. Data are presented that demonstrate the dependence of these properties on applied magnetic-field levels and provide a substantial assessment of the trends in material properties for performance of alloys of different stoichiometries under varied operating conditions.
The effect of Cu-doping on the magnetic and transport properties of La0.7Sr0.3MnO3
Authors: Kim M. S. , Yang J. B. , Parris P. E. , Cai Q. , Zhou X. D. , James W. J. , Yelon W. B. , Buddhikot D. , Malik S. K.
Published: May 2005

The effects of Cu-doping on the structural, magnetic, and transport properties of La0.7Sr0.3Mn1−xCuxO3(0⩽x⩽0.20) have been studied using neutron diffraction, magnetization, and magnetoresistance (MR) measurements. All samples show the rhombohedral structure with the R3¯c space-group from 10 K to room temperature (RT). Neutron diffraction data suggest that some of the Cu ions have a Cu3+ state in these compounds. The substitution of Mn by Cu affects the Mn–O bond length and Mn–O–Mn bond angle resulting from the minimization of the distortion of the MnO6 octahedron. Resistivity measurements show that a metal to insulator transition occurs for the x⩾0.15 samples. The x=0.15 sample shows the highest MR(≈80%), which might result from the co-existence of Cu3+–Cu2+ and the dilution effect of Cu-doping on the double exchange interaction.
Low-temperature ordering of FePt by formation of silicides in underlayers
Authors: Lai Chih-Huang , Chiang C. C. , Yang C. H.
Published: May 2005

A low-temperature ordering of FePt was achieved by introducing dynamic stress. The ordering temperature of FePt was reduced to 300°C by using a Cu underlayer on the HF-cleaned Si (001) substrate. An in-plane coercivity as high as 6900Oe can be obtained after post-annealing at 300°C. The formation of copper silicide, Cu3Si, during post-annealing induces a dynamic stress on FePt films, which greatly reduces the ordering temperature. Pt silicides also help to reduce the ordering temperature. The low-temperature ordering of FePt can be realized with Si∕Cu underlayers on glass substrates.
Optimization of DIII-D advanced tokamak discharges with respect to the β limit
Authors: Ferron J. R. , Casper T. A. , Doyle E. J. , Garofalo A. M. , Gohil P. , Greenfield C. M. , Hyatt A. W. , Jayakumar R. J. , Kessel C. , Kim J. Y. , Luce T. C. , Makowski M. A. , Menard J. , Murakami M. , Petty C. C. , Politzer P. A. , Taylor T. S. , Wade M. R.
Published: May 2005

Results are presented from comparisons of modeling and experiment in studies to assess the best choices of safety factor q profile, pressure profile, and discharge shape for high β, steady-state, noninductive advanced tokamak operation in the DIII-D device [J. L. Luxon, Nucl. Fusion 42, 614 (2002)]. These studies are motivated by the need for high qminβN to maximize the self-driven bootstrap current while maintaining high toroidal β to increase fusion gain. Modeling shows that increases in the normalized beta βN stable to ideal, low toroidal mode number (n=1,2), instabilities can be obtained through broadening of the pressure profile and use of a symmetric double-null divertor shape. Experimental results are in agreement with this prediction. The general trend is for qminβN to increase with the minimum q value (qmin) although βN decreases as qmin increases. By broadening the pressure profile, βN≈4 is obtained with qmin≈2. Modeling of equilibria with near 100% bootstrap current indicates that operation with βN≈5 should be possible with a sufficiently broad pressure profile.
Theoretical study on the excited states of HCN
Authors: Nayak Malaya K. , Chaudhuri Rajat K. , Krishnamachari S. N. L. G.
Published: May 2005

In the flash-photolysis of oxazole, iso-oxazole, and thiozole a transient band system was observed in the region 2500–3050 Å. This band system was attributed to a meta-stable form of HCN, i.e., either HNC or triplet HCN. Theoretical investigations have been carried out on the ground and excited states of HCN to characterize this and other experimentally observed transitions. The predicted geometries are compared with the experiment and earlier theoretical calculations. The present calculations show that the band system in the region 2500–3050 Å corresponds to the transition 43-A′←13-A′ of HCN.
Cooperative effects in two-dimensional ring-like networks of three-center hydrogen bonding interactions
Authors: Parra Rubén D. , Bulusu Satya , Zeng X. C.
Published: May 2005

Cooperative effects in two-dimensional cyclic networks containing intermolecular three-centered hydrogen bonding interactions of the type H1…A…H2 are investigated by means of ab intio molecular orbital and density functional theory calculations. Ring-like clusters consisting of three and up to nine monomers of the cis–cis isomer of carbonic acid H2CO3 are used as basic models, where each unit acts simultaneously as a double hydrogen-bond donor and double hydrogen-bond acceptor. Cooperative effects based on binding energies are evident for (H2CO3)n, where n goes from 2 to 9. Thus, the ZPVE-corrected dissociation energy per bifurcated hydrogen bond increases from 11.52kcal∕mol in the dimer to 20.42kcal∕mol in the nonamer, i.e., a 77% cooperative enhancement. Cooperative effects are also manifested in such indicators as geometries, and vibrational frequencies and intensities. The natural bond orbital analysis method is used to rationalize the results in terms of the substantial charge delocalization taking place in the cyclic clusters. Cooperativity seems close to reaching an asymptotic limit in the largest ring considered, n=9.
Helium induced pressure broadening and shifting of HCN hyperfine transitions between 1.3 and 20 K
Authors: Ronningen Theodore J. , De Lucia Frank C.
Published: May 2005

We have measured the helium induced pressure broadening and shifting of the distinct hyperfine components of the j=1←0 and j=2←1 transitions of HCN14 at temperatures between 1.3 and 20 K. The HCN molecules were cooled to these temperatures using the collisional cooling technique. As a test of this cooling technique we measured the Doppler contribution to the spectral lines, and these measurements confirm that the molecules are at the same temperature as the walls of the spectroscopic cell. We observed that the hyperfine components of the 2←1 transition have distinct broadening coefficients that differ from one another by as much as 5%. The measured differences are in reasonable agreement with theoretical predictions. We have also performed molecular scattering calculations on three He–HCN potential energy surfaces in order to compare our results with theoretical expectations. At the lowest temperatures these calculations predict broadening coefficients that are considerably larger than the measured coefficients. We have previously found a similar discrepancy for two other molecules at these low temperatures, and we discuss possible experimental and theoretical origins for this persistent discrepancy.
Tests of the homogeneous nucleation theory with molecular-dynamics simulations. I. Lennard-Jones molecules
Authors: Tanaka Kyoko K. , Kawamura Katsuyuki , Tanaka Hidekazu , Nakazawa Kiyoshi
Published: May 2005

Two kinds of the homogeneous nucleation theory exist at the present: the classical nucleation theory and the semiphenomenological model. To test them, we performed molecular-dynamics (MD) simulations of nucleation from vapor to liquid with 5000–20 000 Lennard-Jones-type molecules. Simulations were done for various values of supersaturation ratios (from 2 to 10) and temperatures (from 80to120K). We compared the size distribution of clusters in MD simulations with those in the theoretical models because the number density of critical clusters governs the nucleation rate. We found that the semiphenomenological model achieves excellent agreements in size distributions of the clusters with all MD simulations we done. The classical theory underestimates the number density of the clusters in the temperature range of 80–100K, but overestimates in 100–120K. The semiphenomenological model also predicts well the nucleation rate in MD simulations, while the classical nucleation theory does not. Our results confirmed the validity of the semiphenomenological model for Lennard-Jones-type molecules.
The mechanism of the self-assembly of associating DNA molecules under shear flow: Brownian dynamics simulation
Authors: Yeom Min Sun , Lee Jysoo
Published: May 2005

A shear flow induces the assembly of DNAs with the sticky spots. In order to strictly interpret the mechanism of shear-induced DNA assembly, Brownian dynamics simulations with the bead-spring model were carried out for these molecules at various ranges of the Weissenberg numbers (We). We calculate a formation time and analyze the radial distribution function of end beads and the probability distribution of fractional extension at the formation time to understand the mechanism of shear-induced assembly. At low Weissenberg number the formation time, which is defined as an elapsed time until a multimer forms for the first time, decreases rapidly, reaching a plateau at We=1000. A shear flow changes the radial distribution of end beads, which is almost the same regardless of the Weissenberg number. A shear flow deforms and stretches the molecules and generates different distributions between end beads with a stickly spot. The fractional extension progresses rapidly in shear flow from a Gaussian-like distribution to a uniform distribution. The progress of the distribution of fractional extension increases the possibility of meeting of end beads. In shear flow, the inducement of the assembly mainly results from the progress of the probability distribution of fractional extension. We also calculate properties such as the radius of gyration, stretch, and so on. As the Weissenberg number increases, the radius of gyration at the formation time also increases rapidly, reaching a plateau at We=1000.
Reaction mechanism and kinetics of the NCN+NO reaction: Comparison of theory and experiment
Authors: Huang Chih-Liang , Tseng Shiang Yang , Wang Tzu Yi , Wang Niann S. , Xu Z. F. , Lin M. C.
Published: May 2005

The rate constants for the NCN+NO reaction have been measured by laser photolysis/laser-induced fluorescence technique in the temperature range of 254–353K in the presence of He (40–600Torr) and N2 (30–528Torr) buffer gases. The NCN radical was produced from the photodissociation of NCN3 at 193nm and monitored with a dye laser at 329.01nm. The reaction was found to be strongly positive-pressure dependent with negative-temperature dependence, as was reported previously. The experimental data could be reasonably accounted for by dual-channel Rice–Ramsperger–Kassel–Marcus calculations based on the predicted potential-energy surface using the modified Gaussian-2 method. The reaction is predicted to occur via weak intermediates, cis- and trans-NCNNO, in the A″2 state which crosses with the A′2 state containing more stable cis- and trans-NCNNO isomers. The high barriers for the fragmentation of these isomers and their trapping in the A′2 state by collisional stabilization give rise to the observed positive-pressure dependence and negative-temperature effect. The predicted energy barrier for the fragmentation of the cis-NCNNO (A′2) to CN+N2O also allows us to quantitatively account for the rate constant previously measured for the reverse process CN+N2O→NCN+NO.
Low-noise collision operators for particle-in-cell simulations
Authors: Lewandowski J. L. V.
Published: May 2005

A new method to implement low-noise collision operators in particle-in-cell simulations is presented. The method is based on the fact that relevant collision operators can be included naturally in the Lagrangian formulation that exemplifies the particle-in-cell simulation method. Numerical simulations show that the momentum and energy conservation properties of the simulated plasma associated with the low-noise collision operator are improved as compared with standard collision algorithms based on random numbers.
Photodissociation dynamics of acetoxime in gas phase
Authors: Dhanya Suresh , Upadhyaya Hari P. , Kumar Awadhesh , Naik Prakash D. , Saini Rameshwar D.
Published: May 2005

The dynamics of photodissociation of acetoxime at 193nm, leading to the formation of (CH3)2CN and OH fragments, has been investigated. The nascent OH radicals, which are both rotationally and vibrationally excited, were probed by laser photolysis–laser induced fluorescence technique. OH fragments in both v″=1 and v″=0 vibrational states were detected with a ratio of population in the higher to lower level of 0.07±0.01. The rotational temperatures of v″=0 and 1 levels of OH radicals are 2650±150K and 1290±20K, respectively. More than 30% of the available energy, i.e., 115±21kJmol−1 is partitioned into the relative translational energy of the fragments. The results of excited electronic state and transition state calculations at the configuration interaction with single electronic excitation level suggest that the dissociation takes place with an exit barrier of ∼126kJmol−1 at the triplet state (T2) potential energy surface, formed by internal conversions/intersystem crossing from the initially populated S2 state. Using the calculated transition state geometry and its energy, the observed energy distribution pattern can be reproduced by the hybrid model within experimental uncertainties. The presence of an exit barrier is further supported by the observation of N–OH dissociation upon 248nm excitation, where the relative translational energy of the fragments is found to be ∼96kJmol−1. The photodissociation dynamics of acetoxime is compared with C–OH dissociation in enols and carboxylic acid and N–OH dissociation in nitrous acid. The observed emission (λmax=430nm) and the N–OH dissociation dynamics indicate crossing of the initially populated state to an emissive state of acetoxime, which is different from the dissociative state.
An efficient approach for ab initio energy calculation of biopolymers
Authors: Chen Xihua , Zhang Yingkai , Zhang John Z. H.
Published: May 2005

We present a new method for efficient total-energy calculation of biopolymers using the density-matrix (DM) scheme based on the molecular fractionation with conjugate caps (MFCC) approach. In this MFCC-DM method, a biopolymer such as a protein is partitioned into properly capped fragments whose density matrices are calculated by conventional ab initio methods which are then assembled to construct the full system density matrix. The assembled full density matrix is then employed to calculate the total energy and dipole moment of the protein using Hartree–Fock or density-functional theory methods. Using this MFCC-DM method, the self-consistent-field procedure for solving the full Hamiltonian problem is avoided and an efficient approach for ab initio energy calculation of biopolymers is achieved. Two implementations of the approach are presented in this paper. Systematic numerical studies are carried out on a series of extended polyglycines CH3CO–(GLY)n–NHCH3(n=3–25) and excellent results are obtained.
A photoemission study of Pd ultrathin films on Pt (111)
Authors: Mun Bongjin Simon , Lee Choongman , Stamenkovic Vojislav , Markovic Nenad M. , Ross Philip N.
Published: May 2005

The origin of surface core-level shift (SCLS) of Pd thin films on Pt (111) substrate is investigated. At submonolayer coverage of Pd thin films, the splitting of Pd 3d core-level peaks indicate the contribution of both initial and final states of photoionization processes while no change on valence band (VB) spectra is found. When the coverage of Pd reaches to single monolayer, the final-state relaxation effect on the Pd 3d vanishes and only the initial-state effect, a negative SCLS, is present. Also, the VB spectrum at Pd monolayer films shows a clear band narrowing, that is, the origin of the negative SCLS at monolayer coverage. As the Pd coverage is increased to more than monolayer thickness, the Pd 3d peaks start to show the surface layer contribution from second and third layers and the VB spectra show even narrower bandwidth, possibly due to the formation of surface states and strained effect of Pd adlayers on top of the first pseudomorphic layer.
Dissipative particle dynamics study on the interfaces in incompatible A∕B homopolymer blends and with their block copolymers
Authors: Qian Hu-Jun , Lu Zhong-Yuan , Chen Li-Jun , Li Ze-Sheng , Sun Chia-Chung
Published: May 2005

Dissipative particle dynamics, a simulation technique appropriate at mesoscopic scales, has been applied to investigate the interfaces in immiscible binary A∕B homopolymer blends and in the ternary systems with their block copolymers. For the binary blends, the interfacial tension increases and the interface thickness decreases with increasing Flory–Huggins interaction parameter χ while the homopolymer chain length is fixed. However, when the χ parameter and one of the homopolymer chain length is fixed, increasing another homopolymer chain length will induce only a small increase on interfacial tension and slight decrease on interface thickness. For the ternary blends, adding the A-b-B block copolymer will reduce the interfacial tension. When the mole number of the block copolymer is fixed, longer block chains have higher efficiency on reducing the interfacial tension than the shorter ones. But for the block copolymers with fixed volume fraction, shorter chains will be more efficient than the longer ones on reducing the interfacial tension. Increasing the block copolymer concentration reduces interfacial tension. This effect is more prominent for shorter block copolymer chains.
Photoluminescence studies on Al and Ga interdiffusion across (Al,Ga)Sb∕GaSb quantum well interfaces
Authors: Gonzalez-Debs M. , Cederberg J. G. , Biefeld R. M. , Kuech T. F.
Published: May 2005

The thermal interdiffusion of AlSb∕GaSb multiquantum wells was measured and the intrinsic diffusivities of Al and Ga determined over a temperature range of 823–948 K for 30–9000 s. The 77-K photoluminescence (PL) was used to monitor the extent of interdiffusion through the shifts in the superlattice luminescence peaks. The chemical diffusion coefficient was quantitatively determined by fitting the observed PL peak shifts to the solution of the Schrödinger equation, using a potential derived from the solution of the diffusion equation. The value of the interdiffusion coefficient ranged from 5.2×10−4 to 0.06nm2∕s over the conditions studied and was characterized by an activation energy of 3.0±0.1eV. The intrinsic diffusion coefficients for Al and Ga were also determined with higher values for Al than for Ga, described by activation energies of 2.8±0.4 and 1.1±0.1eV, respectively.
Modeling the morphology and mechanical properties of sheared ternary mixtures
Authors: Travasso Rui D. M. , Buxton Gavin A. , Kuksenok Olga , Good Kevin , Balazs Anna C.
Published: May 2005

Through a combination of simulation techniques, we determine both the structural evolution and mechanical properties of blends formed from immiscible ternary mixtures. In this approach, we first use the lattice Boltzmann method to simulate the phase separation dynamics of A∕B∕C fluid mixtures for varying compositions within the spinodal region. We also investigate the effect of an imposed shear on the phase ordering of the mixture. We assume that the fluid is quenched sufficiently rapidly that the phase-separated structure is preserved in the resultant solid. Then, the output from our morphological studies serves as the input to the lattice spring model, which is used to simulate the elastic response of solids to an applied deformation. These simulations reveal how the local stress and strain fields and the global Young’s modulus depend on the composition of the blend and the stiffness of the components. By comparing the results for the sheared and unsheared cases, we can isolate optimal processing conditions for enhancing the mechanical performance of the blends. Overall, the findings provide fundamental insight into the relationship between structure, processing, and properties for heterogeneous materials and can yield guidelines for formulating blends with the desired macroscopic mechanical behavior.
Origin of large magnetocurrent in three-terminal double-barrier magnetic tunnel junctions
Authors: Ladak S. , Hicken R. J.
Published: May 2005

Double-barrier magnetic tunnel junctions (DBMTJs) of composition Co∕AlOx∕Co∕AlOx∕Ni81Fe19 have been fabricated by magnetron sputtering through shadow masks. Two terminal measurements made upon the individual tunnel barriers revealed nonlinear I–V curves and significant room-temperature tunnel magnetoresistance (TMR) in all cases. Measurements were also performed with connections made to all three electrodes. The TMR of a particular tunnel barrier within the DBMTJ can be strongly modified by applying a bias voltage to the other barrier, while the TMR measured across the two barriers in series decreases more slowly with increasing bias voltage than for a single barrier. With zero bias applied between the central Co base electrode and the Co collector electrode, the collector current was measured as electrons were injected from the Ni81Fe19 electrode. For structures grown on Si∕SiO2 substrates, the collector current showed a nonmonotonic dependence upon the emitter-base bias voltage, and collector magnetocurrent values in excess of 100% were observed at nonzero emitter-base bias values. For structures grown on quartz the collector current increased while the magnetocurrent decreased with increasing emitter-base voltage. We suggest that the enhanced TMR and magnetocurrent effects can be explained by substrate leakage and geometrical artifacts rather than by transport of spin-polarized hot electrons across the base layer.
The impact of frequency mixing on sheath properties: Ion energy distribution and Vdc∕Vrf interaction
Authors: Shannon Steven , Hoffman Daniel , Yang Jang-Gyoo , Paterson Alex , Holland John
Published: May 2005

A dual frequency rf sheath is analyzed using a simple rf sheath model to study the interaction between the two driving rf currents and their effect on sheath parameters. A symmetric rf discharge with defined electron density and dc sheath potential is modeled using a sharp boundary sheath approximation. Three results of this study are reported: (1) reproduction of trends in ion energy distribution functions predicted and measured in previous studies, (2) a frequency-mixing-dependent relationship between the dc sheath potential and applied rf potential, and (3) an additional asymmetry in the ion energy distribution function generated by the intermodulation components resulting from the nonlinear sheath.
Numerical and experimental study of flow of a stratified fluid over a sill towards a sink
Authors: Jamali Mirmosadegh , Seymour Brian , Kasaiian Roozbeh
Published: May 2005

The selective withdrawal of a linearly stratified fluid through a line sink at the base of a reservoir with a sill at the bed is investigated theoretically and experimentally. The nonlinear equations of motion are solved using a finite-difference technique in curvilinear coordinates in two dimensions. The evolution of selective withdrawal and the flow properties in the steady state are studied. Using the numerical results, an improvement is made to a previously proposed equation for the withdrawal-layer thickness upstream of a sill. Empirical equations are given for the critical Froude number and the density thickness. For comparison, experiments were conducted in a flume and particle image velocimetry technique was used to capture the flow field upstream of a sill during evolution of selective withdrawal. In general, good agreement is obtained between the numerical and experimental results. It is found that the presence of a sill causes the withdrawal layer to be thicker and to shift upward compared to the flat-bed case. The proposed equation for the withdrawal-layer thickness is verified experimentally.
Effect of dielectric materials on stress-induced damage modes in damascene Cu lines
Authors: Paik Jong-Min , Park Hyun , Joo Young-Chang , Park Ki-Chul
Published: May 2005

The stress of damascene Cu integrated with silicon dioxide (SiO2) and a low-k material was analyzed by x-ray diffraction, and that of the via-line structure was evaluated using finite element analysis. In both cases, the hydrostatic stress of the Cu line embedded in SiO2 was greater than that of the Cu line with low-k dielectric, whereas the opposite was true for the von Mises stress. In particular, the von Mises stress in the via embedded in the low-k dielectric was large. It was also shown that the yield strength of the via embedded in the low-k material is severely reduced compared with that of the via embedded in SiO2. Therefore, the deformation of the via, due to high von Mises stress and low yield strength, is expected to be the important failure mode in the interconnects made with low-k dielectrics which have higher coefficient of thermal expansion and lower elastic modulus.
Contribution of oxygen partial pressures investigated over a wide range to SrRuO3 thin-film properties in laser deposition processing
Authors: Yoo Y. Z. , Chmaissem O. , Kolesnik S. , Dabrowski B. , Maxwell M. , Kimball C. W. , McAnelly L. , Haji-Sheikh M. , Genis A. P.
Published: May 2005

Sr Ru O 3 (SRO) thin films were grown on SrTiO3 (100) substrates using the pulsed laser deposition method. In order to fully exploit the correlation between their properties and the partial (working) pressures of oxygen in which they are grown, a wide range of pressures—from 0.1to300mTorr—was investigated. SRO ablation plumes showed a shape transition from forward directed to confined isotropic at 60mTorr. Also, all measured properties of SRO films displayed significant changes at 60mTorr. Out-of-plane lattice constants and strains of SRO films were quite large for low-pressure growth (up to 10mTorr), but became notably minimized at 60mTorr and continued to change gradually with further pressure increases. Ru deficiencies seemed to occur regardless of working pressure value. The island growth mode was dominant for low pressures up to 10mTorr, followed by the step flow growth mode at 60mTorr and step flow plus two-dimensional growth at 200mTorr, reverting then back into island growth at 300mTorr. Only those SRO films exhibiting the step flow growth mode showed a high TC. The correlation between oxygen pressure and film properties is discussed in terms of strain and compositional effects.
Microwave properties of granular amorphous carbon films with cobalt nanoparticles
Authors: Lutsev L. V. , Yakovlev S. V. , Zvonareva T. K. , Alexeyev A. G. , Starostin A. P. , Kozyrev S. V.
Published: May 2005

We have studied losses of microwave electromagnetic radiation in granular amorphous hydrogenated carbon films a-C:H(Co) containing cobalt nanoparticles at cobalt concentrations below the percolation threshold and in the percolation threshold region. The difference between the experimental values of the losses in granular structures sputtered on corundum-based substrates and those on aramide tissues is observed. It is found that this is determined by the difference in localization sizes of cluster electron states in a-C:H(Co) films. We have found that losses grow with increasing concentration of cobalt nanoparticles and with increasing frequency. The frequency dependence of losses is determined by the fast spin relaxation of nanoparticles. The anisotropy of losses in a-C:H(Co) covers on aramide tissues is observed.
Current interruption limit and resistance of the self-similar electric arc
Authors: Christen Thomas , Seeger Martin
Published: May 2005

A model for the axially blown cylindrical arc is derived. In contrast to earlier theories, the model is gauge invariant with respect to energy, which is crucial for investigating current interruption. We determine from our model the dependence of the maximum interruptible current rate, (dI∕dt)L, on the pressure, on the parallel capacitance, and on the line impedance for an SF6 arc. (dI∕dt)L scales, approximately independent of the gas type, with the square root of the pressure. The arc resistance, at current zero with current rate equal to (dI∕dt)L, is pressure independent. As a consequence, the arc resistance at current zero can serve as a figure of merit for the interruption performance of gas circuit breakers.
Kelvin–Helmholtz instability in a Hele-Shaw cell: Large effect from the small region near the meniscus
Authors: Hinch E. J. , Plouraboué F.
Published: May 2005

In an attempt to improve the poor prediction of our previous theory, we examine corrections from the small region in a Hele-Shaw cell near the meniscus where the flow is three dimensional. At larger Reynolds numbers, we find an O(1) change to the effective boundary condition for mass conservation which is to be applied to the large scale flow outside the small region.
Effects of Landau quantization on the equations of state in intense laser plasma interactions with strong magnetic fields
Authors: Eliezer Shalom , Norreys Peter , Mendonça José T. , Lancaster Kate
Published: May 2005

Recently, magnetic fields of 0.7(±0.1) gigaGauss (GG) have been observed in the laboratory in laser plasma interactions. From scaling arguments, it appears that a few gigaGauss magnetic fields may be within reach of existing petawatt lasers. In this paper, the equations of state (EOS) are calculated in the presence of these very large magnetic fields. The appropriate domain for electron degeneracy and for Landau quantization is calculated for the density-temperature domain relevant to laser plasma interactions. The conditions for a strong Landau quantization, for a magnetic field in the domain of 1–10 GG, are obtained. The role of this paper is to formulate the EOS in terms of those that can potentially be realized in laboratory plasmas. By doing so, it is intended to alert the experimental laser-plasma physics community to the potential of realizing Landau quantization in the laboratory for the first time since the theory was first formulated.
The effect of autocorrelation length on the real area of contact and friction behavior of rough surfaces
Authors: Zhang Yilei , Sundararajan Sriram
Published: May 2005

Autocorrelation length (ACL) is a surface roughness parameter that provides spatial information of surface topography that is not included in amplitude parameters such as root-mean-square roughness. This paper presents a relationship between ACL and the friction behavior of a rough surface. The influence of ACL on the peak distribution of a profile is studied based on Whitehouse and Archard’s classical analysis [Whitehouse and ArchardProc. R. Soc. London, Ser. A 316, 97 (1970)] and their results are extended to compare profiles from different surfaces. The probability density function of peaks and the mean peak height of a profile are given as functions of its ACL. These results are used to estimate the number of contact points when a rough surface comes into contact with a flat surface, and it is shown that the larger the ACL of the rough surface, the less the number of contact points. Based on Hertzian contact mechanics, it is shown that the real area of contact increases with increasing of number of contact points. Since adhesive friction force is proportional to the real area of contact, this suggests that the adhesive friction behavior of a surface will be inversely proportional to its ACL. Results from microscale friction experiments on polished and etched silicon surfaces are presented to verify the analysis.
High-energy x-ray production with pyroelectric crystals
Authors: Geuther Jeffrey A. , Danon Yaron
Published: May 2005

The invention of pyroelectric x-ray generator technology has enabled researchers to develop ultraportable, low-power x-ray sources for use in imaging, materials analysis, and other applications. For many applications, the usefulness of an x-ray source is determined by its yield and endpoint energy. In x-ray fluorescence, for example, high-energy sources enable the excitation of the K-shell x-ray peaks for high-Z materials as well as the lower-energy L-shell peaks, allowing more positive sample identification. This report shows how a paired-crystal pyroelectric source can be used to approximately double the endpoint x-ray energy, in addition to doubling the x-ray yield, versus a single-crystal source. As an example of the advantage of a paired-crystal system, we present a spectrum showing the fluorescence of the K shell of thorium using a pyroelectric source, as well as a spectrum showing the fluorescence of the K shell of lead. Also shown is an x-ray spectrum with an endpoint energy of 215 keV.
The influence of an extrinsic interfacial layer on the polarization of sputtered BaTiO3 film
Authors: Cho Y. W. , Choi S. K. , Rao G. Venkata
Published: May 2005

As an origin of degradation of remnant polarization in Pt∕BaTiO3∕Pt capacitor structure, an interfacial layer formed at the interface of BaTiO3 film and a Pt bottom electrode is considered. BaTiO3 films were deposited on two types of bottom electrodes (La0.5Sr0.5CoO3 and Pt) by the radio frequency magnetron sputtering method and both capacitors showed a microstructural similarity with strong preferred orientations. However, a Pt∕BaTiO3∕La0.5Sr0.5CoO3 capacitor exhibited a saturated hysteresis loop with the remnant polarization (2Pr) of 6μC∕cm2, and for the Pt∕BaTiO3∕Pt structure, the polarization-voltage curve revealed a linear dielectric characteristic. From a cross-sectional high-resolution transmission electron microscope analysis of the Pt∕BaTiO3∕Pt capacitor showing the linear dielectric property, an interfacial layer with an amorphous structure as well as a multidomain structure in the interior of the BaTiO3 film were observed. It is concluded that the interfacial layer might help degradation of polarization and its origin can be classified as being extrinsic.
Layzer type models for pressure driven shells
Authors: Hurricane O. A.
Published: May 2005

Models for the nonlinear instability of finite thickness shells driven by pressure are constructed in the style of Layzer. Equations for both Cartesian and cylindrically convergent/divergent geometries are derived. The resulting equations are appropriate for incompressible shells with unity Atwood number. Predictions from the equations compare well with two-dimensional simulations.
Carrier dynamics in self-assembled ErAs nanoislands embedded in GaAs measured by optical-pump terahertz-probe spectroscopy
Authors: Prasankumar R. P. , Scopatz A. , Hilton D. J. , Taylor A. J. , Averitt R. D. , Zide J. M. , Gossard A. C.
Published: May 2005

We use optical-pump terahertz (THz)-probe spectroscopy to study carrier dynamics in self-assembled ErAs nanoislands embedded in GaAs and deposited in a superlattice structure. Measurements are performed at several pump fluences on samples with different superlattice periods, enabling a determination of the time-dependent conductivity. Subpicosecond carrier capture times are obtained, indicating the potential of these devices as time-domain THz detectors with performance comparable to low-temperature grown GaAs and superior control of material parameters.
Effect of metal chelate layer on electroluminescent and current-voltage characteristics
Authors: Xu Xinjun , Yu Gui , Liu Yunqi , Tang Rupei , Xi Fu , Zhu Daoben
Published: May 2005

We report the effects of a metal chelate, bis[2-(2-hydroxyphenyl)benzothiazolate]zinc [Zn(BTZ)2] and 8-tris-hydroxyquinoline aluminum (Alq3) used as electron-transporting layers, and various cathodes on electroluminescent and current-voltage characteristics of a polymer light-emitting diode with phenyl-substituted poly(phenylene vinylene) as an emissive layer. We find that the metal chelate layers also play a role in hole blocking. For a double-layer device with the structure of indium-tin oxide (ITO)/polymer/Zn(BTZ)2 or Alq3/Al, the metal chelate layer lowers the hole current and improves the electroluminescent properties. Inserting a LiF layer between the metal chelate layer and the Al cathode enhances the electron injection and improves the elctroluminescent efficiency. The device with the structure of ITO/polymer/Zn(BTZ)2/Alq3/LiF/Al can reach the highest luminous efficiency of 5.0 cd/A.
Polarization conversion and “focusing” of light propagating through a small chiral hole in a metallic screen
Authors: Krasavin A. V. , Schwanecke A. S. , Zheludev N. I. , Reichelt M. , Stroucken T. , Koch S. W. , Wright E. M.
Published: May 2005

Propagation of light through a thin flat metallic screen containing a hole of twisted shape is sensitive to whether the incident wave is left or right circularly polarized. The transmitted light accrues a component with handedness opposite to the incident wave. The efficiency of polarization conversion depends on the mutual direction of the hole’s twist and the incident light’s wave polarization handedness and peaks at a wavelength close to the hole overall size. We also observed a strong transmitted field concentration at the center of the chiral hole when the handedness of the chiral hole and the wave’s polarization state are the same.
Femtosecond laser-drilled capillary integrated into a microfluidic device
Authors: Kim Tyson N. , Campbell Kyle , Groisman Alex , Kleinfeld David , Schaffer Chris B.
Published: May 2005

Recent growth in microfluidic technology is, to a large extent, driven by soft lithography, a high-throughput fabrication technique where polymer materials, such as poly(dimethyl) siloxane (PDMS), are molded to form microscopic channel networks. Nevertheless, the channel architectures that can be obtained by molding are limited. We address this limitation by using femtosecond laser micromachining to add unmoldable features to the microfluidic devices. We apply laser ablation to drill microcapillaries, with diameters as small as 0.5μm and aspect ratios as high as 800:1, in the walls of molded PDMS channels. Finally, we use a laser-drilled microcapillary to trap a polystyrene bead by suction and hold it against a shear flow.