bluuebunny/crossref_metadata_2025 huggingface dataset:
On the role of back reaction in the stochastic model of electron transfer
Authors: Barzykin A. V. , Frantsuzov P. A.
Published: July 2002

A classical stochastic model of electron transfer is considered with a slow Markovian solvent polarization mode and some faster (solvent or intramolecular) mode. It is shown that when the rate of nonadiabatic transitions between the reactant and the product states is comparable with or exceeds the relaxation rate of the slow mode, back reaction should be included in the stochastic formulation of the overall kinetics even if the free energy change of reaction, −ΔG, is ≫kBT. As a result, dynamical solvent control of electron transfer is to be expected in the normal region even in the presence of fast modes. In contrast, participation of fast modes leads to the reaction becoming effectively irreversible in the inverted region. The energy gap law is thus significantly modified. The relationship to the variational transition state theory is established. It is demonstrated that spectral separation of the reaction coordinate into fast and slow counterparts is generally more consistent than temporal separation. As checked against numerical results for biexponential relaxation, the variational transition state theory offers a reasonable approximation for the rate constant in the normal region, provided that the activation energy is sufficiently high.
Structure and energetics of Lin(OH)n−1 (n=2–5) clusters deduced from photoionization efficiency curves
Authors: Tanaka Hiromasa , Yokoyama Keiichi , Kudo Hiroshi
Published: July 2002

Ionization efficiency curves (IECs) were measured for Lin(OH)n−1 (n=2–5) clusters generated by reactions of laser-ablated lithium metal with water vapor, using photoionization time-of-flight mass spectrometry. The measured IECs are well-reproduced with a simulation involving Franck–Condon factors, enabling to identify the global-minimum structure of the clusters predicted in our theoretical study [Tanaka et al., J. Chem. Phys. 113, 1821 (2000)]. Hyperlithiated structures (HLS), in which an excess electron is delocalized, are identified for the cluster with n=2 and 3, while electron-localized or segregated structure (ELS) are for those with n=3–5. Adiabatic ionization energies were determined as 4.053±0.003 eV for HLS of Li2OH, 3.687±0.003 eV for HLS of Li3(OH)2, 4.133±0.003 eV for ELS of Li3(OH)2, and 3.418±0.009 eV for ELS of Li4(OH)3. The ionization energy of Li5(OH)4 was determined as 3.60±0.11 eV by a linear extrapolation of IEC. For Li3(OH)2, the energetics of two lowest-energy isomers is discussed based on the observed abundance ratio.
Collision induced fragmentation of small ionic alkali clusters. III. Heteronuclear clusters
Authors: Barat M. , Brenot J. C. , Dunet H. , Fayeton J. A. , Picard Y. J.
Published: July 2002

Fragmentation of small heteronuclear NapKn–p+ clusters (n=3–7, p=0–2) induced by collision with helium atoms is investigated at a laboratory energy of 4800 eV. Populations of the various fragmentation pathways are measured. Detailed analysis of the collision is performed with a multicoincidence technique allowing the determination of the velocity vectors of the fragments. Fragmentation is predominantly induced by momentum transferred in binary collisions between He and alkali cores, electronic excitation accounting only for about 10% of the process. Experiments with heteronuclear clusters allow the study of the various fragmentation mechanisms following the initial momentum transfer. Two-step fragmentation processes are governed by the endothermicity of the pathway while direct ejection of an alkali atom in binary collisions is directly dependent on the relative number of Na and K cores.
In situ observation of moisture-induced swelling of leather in an environmental scanning electron microscope
Authors: Tuckermann Martin , Mertig Michael , Pompe Wolfgang
Published: July 2002

The correlation of microscopic and macroscopic effects of moisture-induced swelling of the collagen fiber structure has been investigated by measuring both mechanical stress development and stress-free deformation of leather in an environmental scanning electron microscope. Swelling of the collagen matrix is observed at microscopic scale by imaging local rearrangement of individual fibers. Microscopic swelling and macroscopic stress development exhibit the same exponential time response, when exposing the leather samples to changing water vapor pressure. The equilibrium stress levels are reached about ten times faster than observed for comparable humidity changes at ambient conditions. This acceleration is attributed to a reduced diffusion layer above the collagen surface at low pressures (≈10 Torr).
Impact of growth polar direction on the optical properties of GaN grown by metalorganic vapor phase epitaxy
Authors: Chichibu S. F. , Setoguchi A. , Uedono A. , Yoshimura K. , Sumiya M.
Published: July 2002

The growth polar direction during metalorganic vapor phase epitaxy of wurtzite GaN was shown to affect the optical properties in terms of impurity and vacancy incorporation during the growth. The GaN film grown toward the Ga (0001) face (+c polarity) exhibited clear excitonic features in its optical absorption and luminescence spectra up to room temperature. Conversely, the film with the N (0001̄) face (−c polarity) exhibited a broad emission band, which locates in the broad absorption tail. The difference between the two was explained in terms of the presence of impurity-induced band-tail states in −c GaN due to increased impurity density and incorporation of large volume vacancy-type defects, which were confirmed by secondary ion mass spectrometry [Sumiya et al., Appl. Phys. Lett. 76, 2098 (2000)] and monoenergetic slow positron annihilation technique.
Capillary waves on a Eulerian jet emerging from a channel
Authors: Osborne T. J. , Stump D. M.
Published: July 2002

This paper considers the flow of a Eulerian fluid jet emerging out of a slightly compressed circular or rectangular cross-section channel. The emerging fluid is subject to surface tension, which provides a restoring force and produces steady waves along the free surface. In this paper, the full nonlinear problem for the jet is formulated and a dimensionless surface tension parameter is identified. The linearized problem is then solved by the Wiener–Hopf technique. The free surface shape consists of a steady sinusoidal oscillation and a component which decays exponentially with distance from the channel opening. Asymptotic methods are used to find the surface profile near the channel exit.
Defect and nanocrystal cathodoluminescence of synthetic opals infilled with Si and Pt
Authors: Dı́az-Guerra C. , Kurdyukov D. A. , Piqueras J. , Sokolov V. I. , Zamoryanskaya M. V.
Published: July 2002

Synthetic opals—composed of 250 nm amorphous silica spheres closed packed in a face centered cubic structure—have been infilled with silicon, platinum, and with Si and different Pt contents. The luminescent properties of these composites have been investigated by cathodoluminescence (CL) microscopy and spectroscopy. CL emission is influenced by the material used to infill the pores of the opal matrix. CL spectra of all the samples investigated show two well-known bands, associated with the defect structure of the silica spheres, centered at about 1.9 and 2.7 eV, respectively. Emission in the 2.15–2.45 eV range, particularly intense in opal-based composites with a high Pt content, is tentatively associated with SiO2 defects involving silicon clusters. A CL band peaked at about 3.4 eV as well as a band in the 1.50–1.75 eV range, whose peak position seems to be affected by the Pt content of the samples, are associated with the presence of Si nanocrystals. The behavior of these emissions suggests that both are related to defect states at the interface between Si nanocrystals and SiO2 forming the opal spheres.
Atom probe analysis of roughness and chemical intermixing in CoFe/Cu films (invited)
Authors: Larson D. J. , Cerezo A. , Clifton P. H. , Petford-Long A. K. , Martens R. L. , Kelly T. F. , Tabat N.
Published: July 2002

Three-dimensional atom probe analyses of the interfaces between CoFe and Cu layers has shown that both roughness and chemical intermixing can occur independently. Interfaces formed by the deposition of Cu onto CoFe mimic the roughness present in previously deposited interfaces, but have a very small amount of interfacial mixing. In contrast, interfaces formed by the deposition of CoFe onto Cu are less rough, but more chemically intermixed. The region of chemical intermixing formed when CoFe is deposited onto Cu (0.7–1.0 nm) is approximately two times larger than that when Cu is deposited onto CoFe (0.3–0.5 nm).
Stripping and bulk CO electro-oxidation at the Pt–electrode interface: Dynamic Monte Carlo simulations
Authors: Saravanan Chandra , Markovic N. M. , Head-Gordon M. , Ross P. N.
Published: July 2002

We perform dynamic Monte Carlo simulations to understand the kinetics of CO oxidation on Pt surface. Our simulations are performed on a model for CO electro-oxidation, where oxygen-containing species (adsorbed OH) formed on the Pt surface reacts with adsorbed CO by a Langmuir–Hinshelwood mechanism to form CO2. A systematic study of the model parameters shows that we obtain better electrocatalytic activity by increasing the surface diffusion coefficient of CO (kD) or the rate of reaction between adsorbed CO and adsorbed OH. We also show that if the initial state of adsorbed CO is a closely packed structure, increasing the surface coverage can result in negative voltammetry peak shifts because of the formation of weakly adsorbed states of CO. We have also modeled bulk CO voltammetry by introducing a CO adsorption rate, k4. We show that near-zero currents are obtained for the potentials (E) where the rate of OH adsorption, k1(E), is much smaller than k4. We also note that kD plays no significant role in CO oxidation when kD≪k4. Our results also show that bulk CO voltammetry peaks shift to more positive potentials compared to the stripping, as observed in experiments. Our potential step simulations show no difference in bulk and stripping current transients in the potential regions where k1(E)≫k4, whereas they vary significantly when k4≅k1(E).
Superconductivity and flux pinning in Y and heavily Pb codoped Bi-2212 single crystals
Authors: Wang X. L. , Liu H. K. , Dou S. X. , Horvat J. , Millikon D. , Heine G. , Lang W. , Luo H. M. , Ding S. Y.
Published: July 2002

Studies of superconductivity and flux pinning were carried out on (Bi1.64Pb0.36)Sr2Ca1−xYxCu2O8+y (x=0, 0.05, 0.11, 0.33) single crystals grown by the self-flux method. X-ray diffraction, transport, and magnetic measurements were performed for purposes of characterization. X-ray analysis revealed that the c lattice parameter systemically decreases as the Y doping level increases. The superconducting transition temperature Tc decreases from 80 to 30 K as x increases. A strong annealing effect on Tc and superconducting volume has been observed. Resistance measurements show that x=0.33 samples are semiconductive over a wide temperature range between 4.2 and 300 K for the as-grown state, but become metallic with Tc of 65–70 K after air or oxygen annealing. Flux pinning was studied by measuring the hysteresis loop at different temperatures and different fields. A peak effect was observed in all the codoped samples. Results show that at low temperatures, the peak field is smaller than in solely Pb doped crystals and decreases as x increases (x>0.1). However, the peak field at high temperature for the x=0.05 sample is higher than in heavily Pb doped Bi2212 crystals, indicative of a strong pinning due to the codoping.
Reduction of interlayer coupling in bottom synthetic spin valves through a gas exposure process
Authors: Makino E. , Ishii S. , Syoji M. , Furukawa A. , Hosomi M. , Matuzono A.
Published: July 2002

The magnetoresistance (MR) ratio of spin valves can be improved by reducing the thickness of the nonmagnetic interlayer, such as Cu, due to not only reducing the shunt, but also increasing the probability of electrons scattered through the Cu. However, at small thickness, interlayer coupling between the free and pinned layer is increased, which makes it difficult to control the bias point. The minimum thickness of the interlayer Cu was thereby limited to around 3 nm. On the other hand, it is reported that the ferromagnetic interlayer coupling that arises from the film roughness can be reduced by controlling the residual gas inside the deposition chamber. The same effect can also be achieved by exposing the wafer into an oxygen gas atmosphere right after the deposition of the high conductive Cu interlayer. By this method the thickness of the interlayer Cu was reduced successfully from 3.0 to 2.0 nm without increasing the interlayer coupling. As a result, the MR ratio and dRsq was improved by 12% and 48%, and showed 10% and 1.82 Ω, respectively. Furthermore, by inserting a Cu backlayer to form a spin filter spin valve structure an antiferromagnetic interlayer coupling was also observed.
Diffusion behavior of the spin valve structure
Authors: Huang Rong-Tan , Chen Fu-Rong , Kai Ji-Jung , Tsu I-Fei , Mao Sining , Kai W.
Published: July 2002

NiMn/NiFe/Co/Cu/Co/NiFe/seed layer (sample No. 1) and NiFe/CoFe/Cu/CoFe/Ru/CoFe/NiFe/NiMn/Seed layer (sample No. 2), are investigated by using high resolution analytical transmission electron microscopy and an imaging filter. The compositional analysis demonstrated that the diffusions of the Mn and Ni into the Cu/Co bilayer are only observed in sample No. 1. This result indicated that the Ru layer in sample No. 2 might not only act as the spacer of the synthetic antiferromagnet but also behaves as a good diffusion barrier for the Ni and Mn element in the spin valve structure. The diffusion coefficients of constituent elements are simply investigated using the Matano–Boltzmann method. The diffusion mechanisms of Cu in Co layer and Co in Cu layer were primarily dominated by the grain boundary.
Monte Carlo simulations of ferromagnetic–antiferromagnetic grains
Authors: Heinonen O.
Published: July 2002

The effects of finite temperature on exchange bias are investigated using Monte Carlo simulations of model systems. Thermal effects may introduce phase slips of domain walls wound into the antiferromagnet. Furthermore, spin–flop coupling may be unstable with respect to domain formation at the interface.
Temperature dependence of the spin wave spectrum in the double exchange model
Authors: Slepoy A. , Singh R. R. P. , Shender E.
Published: July 2002

We examine the spin dynamics of a classical spin system coupled with a double exchange interaction cos(θ/2). The spin wave spectrum of the system displays anomalously weak temperature dependence. This behavior is first demonstrated in an exact calculation of a two-spin system. A leading order spin wave calculation also gives a vanishing change in the spectrum with temperature. A spin dynamics simulation confirms that while dynamical structure factor peaks broaden with temperature, their position does not shift downward. These results may help to explain the slower reduction of spin stiffness in colossal magnetoresistance materials as temperature increases.
Joule heating generated by spin current through Josephson junctions
Authors: Takahashi S. , Yamashita T. , Koyama T. , Maekawa S. , Imamura H.
Published: July 2002

We theoretically study the spin-polarized current flowing through a Josephson junction (JJ) in a spin injection device. When the spin-polarized current is injected from a ferromagnet in a superconductor (SC), the charge current is carried by the superconducting condensate (Cooper pairs), while the spin-up and spin-down currents flow in equal magnitude but in the opposite direction in a SC, because of no quasiparticle charge current in the SC. This indicates that not only the Josephson current but also the spin current flow across JJ at zero bias voltage, thereby generating Joule heating by the spin current. The result provides a new method for detecting the spin current by measuring Joule heating at JJ.
Unstable equilibrium point in chaotic domain-wall motion and Ott–Grebogi–Yorke control
Authors: Okuno H. , Takemura Y.
Published: July 2002

A method for finding the unstable equilibrium points in Bloch wall motion is proposed, which is important for controlling the chaotic domain-wall motion by using the Ott–Grebogi–Yorke (OGY) method. The dynamics of Bloch wall motion are expressed by a nonlinear differential equation with the terms of inertia, damping, restoring, and an external magnetic drive force. An equation is transformed into the difference equations by following the OGY method, approximating linearly around an unstable equilibrium point (a saddle point), and adding a controlling input. The unstable equilibrium points are obtained by using the return map and the condition of hyperbolic fixed point. The time series of domain-wall motion successfully controlled on the unstable equilibrium points by the OGY method is shown.
Subloops, Barkhausen noise, and disorder induced critical behavior
Authors: Carpenter John H. , Dahmen Karin A. , Sethna James P. , Friedman Gary , Loverde Sharon , Vanderveld Ali
Published: July 2002

Hysteresis loops are often seen in experiments at first order phase transformations when the system goes out of equilibrium, such as in the supercooling of liquids and in magnets. The nonequilibrium, zero-temperature random-field Ising model has been studied as a model for the hysteretic behavior of these transformations. As disorder is added, one finds a transition where the jump in the saturation hysteresis loop (corresponding to an infinite avalanche) decreases to zero. At this transition the model exhibits power law distributions of noise (avalanches), universal behavior, and a diverging length scale [O. Perković, K. Dahmen, and J. P. Sethna, Phys. Rev. B 59, 6106 (1999)]. Interestingly, not only the saturation loops but also subloops reflect this critical point, and at the critical disorder one finds history-induced critical scaling. We present simulation results for histories in systems of almost 14 million spins. Concentric inner subloops are found to resemble rescaled saturation loops at effectively higher (possibly correlated) disorder. In addition, avalanche size distributions for the inner subloops are collapsed using Widom scaling methods. The resulting exponents and scaling functions are shown to differ from those corresponding to the saturation loop.
Effect of Fe doping on high field magnetoresistance and low field magnetoresistance at zero field in polycrystalline La0.7Sr0.3Mn1−xFexO3 (x=0–0.12) thin films
Authors: Huang Q. , Li Z. W. , Li J. , Ong C. K.
Published: July 2002

Polycrystalline La0.7Sr0.3Mn1−xFexO3 thin films, with x=0–0.12, have been prepared on (001)-Si substrates using pulsed laser deposition. The films consist of fine grains with an average size of 60–80 nm. For those films, the metal–insulator transition temperature, Tp, is much lower than the Curie temperature, TC. The high field magnetoresistance, HFMR, is nearly temperature independent for x<0.08, whereas the extrapolated low field magnetoresistance at zero field, LFMR*, decreases rapidly with increasing temperature. Moreover, Fe doping significantly decreases LFMR* and enhances HFMR at low temperatures. We propose that for the Fe-doped films, both the reduced spin polarization of conduction electrons and the increased spin-flip scattering are responsible for the decrease of LFMR*, while the weakened ferromagnetic spin interaction at the grain boundaries is responsible for the enhanced HFMR.
Magnetization reversal dynamics in exchange-coupled NiO–Co bilayers
Authors: Camarero J. , Pennec Y. , Bonfim M. , Vogel J. , Pizzini S. , Fontaine A. , Cartier M. , Fettar F. , Dieny B.
Published: July 2002

We performed a detailed study of the magnetization reversal in polycrystalline exchange-coupled NiO/Co bilayers over 10 decades of field sweep rate dH/dt for different NiO and Co thicknesses. For all sweep rates and thicknesses, the symmetry of the hysteresis loops shows that an identical pinning strength has to be overcome in both directions of the reversal. At low dH/dt the reversal is governed by domain wall displacement while domain nucleation is dominant at higher ones. The dH/dt at which the transition between the two regimes takes place depends on the relative thickness of the NiO and Co layers. It increases (decreases) when the Co (NiO) thickness is increased. Experimentally, it was found that the energy barrier varies linearly with the square root of the area corresponding to the activation (Barkhausen) volume which is consistent with a random walk model of the coupling between antiferromagnetic and ferromagnetic layers. The results can be explained in terms of a thermally activated switching of the NiO magnetization dragged by the Co reversal.
Temperature dependence of spin waves in Co/CoO bilayers
Authors: Lew W. S. , Ercole A. , Lauhoff G. , Kernohan E. T. M. , Lee J. , Bland J. A. C.
Published: July 2002

Brillouin light scattering measurements of spin-wave frequencies in an exchange coupled ferromagnet/antiferromagnet epitaxial Co/CoO bilayer are reported. A striking temperature dependence of the measured spin-wave frequencies in the cobalt layer in the range 77–300 K was observed which has been demonstrated to be due to exchange coupling to the ultrathin (7 Å) CoO layer as antiferromagnetic order develops. The temperature dependence of the spin-wave frequency demonstrates that interface exchange coupling occurs in the absence of the unidirectional anisotropy. A study of the mode line width shows a broadening with reducing temperature which indicates that locally ordered antiferromagnet regions persist above the Néel temperature and play a central role in determining the magnetic behavior of the bilayer system.
Structural and superconducting properties of La2−xRxBa2CayCu4+yO10+δ (R=Nd, Gd; y=2x) system
Authors: Kuberkar D. G. , Thampi R. S. , Shah Nikesh A. , Mavani Krushna , Rayaprol S. , Kulkarni R. G. , Malik S. K. , Yelon W. B.
Published: July 2002

A nonsuperconducting La2Ba2Cu4O9+δ system has been made superconducting by the addition of CaCuO2, along with rare earth doping at the La site, with the stoichiometric composition La2−xRxCayBa2Cu4+yO10+δ (R=Nd, Gd, and y=2x). All the samples with 0.0<x<0.5; y=2x were characterized by x-ray diffraction and neutron diffraction measurements for structural properties and by ac susceptibility, dc resistivity, and iodometric analysis for superconducting properties. The x=0.0 sample displays semiconducting behavior. Increasing x increases Tc from less than 15 K for Nd/Gd samples with x=0.1 to ∼78 K for x=0.5 samples. It is interesting to note that the hole concentration and Tc exhibit a strong correlation with increasing dopant concentration. Neutron diffraction studies confirm the occupancy of Ca and Nd ions at the La site with a concomitant displacement of La on to the Ba site.
Interlayer coupling in CoNi/Gd/CoNi trilayer
Authors: Altuncevahir B. , Koymen A. R.
Published: July 2002

CoNi/Gd/CoNi transition metal-rare earth trilayers were deposited by dc magnetron sputtering at 3 mTorr Ar pressure at room temperature. The interlayer coupling strength was measured by minor hysteresis loop shifts and calculated from major hysteresis loops. The strong ferromagnetic coupling up to 3 nm Gd film thickness can be explained by the development of ferrimagnetism in the Gd layer caused by the diffusion of CoNi into the Gd film or by pinholes. For other interlayer thicknesses, the coupling strength decreases rapidly and cannot be explained by Néel’s “orange-peel” coupling and domain wall coupling. An exchange type coupling needs to be considered. The e–e exchange interaction among d electrons may be the reason for the exponentially decaying coupling.
Magnetization reversal in the pinned layer of PtMnCr/NiFe exchange biased bilayers
Authors: Hughes T. , Laidler H. , O’Grady K. , Petford-Long A. K. , Mao Sining , Kief M. , Linville E.
Published: July 2002

A study has been made on the effect of annealing time on magnetization reversal of the pinned layer in NiFe(100 Å)/PtMnCr(500 Å) bilayers. In the as-deposited state, the PtMnCr layer is in a metastable, nonmagnetic, disordered fcc phase. Heating progressively transforms the alloy to the stable fct phase which is antiferromagnetic, providing the pinning layer for the soft ferromagnetic NiFe layer. The samples were annealed in a magnetic field at 250 °C for 1, 1.5, 2, and 8 h. The effect of annealing is to both increase the shift of the loops along the field axis and the coercivity of the pinned ferromagnetic layer. Although the widening of the loop is correlated with the degree of antiferromagnet transformation, the exact mechanism for the increased coercivity is unclear. Measurements of loops made at different field sweep rates and after different waiting times at saturation, on the sample annealed for 1.5 h, indicate two possible mechanisms for the increased coercivity: (i) thermally activated reversal of some of the antiferromagnetic layer (AFM) during the measurement of the hysteresis loop and/or (ii) spin-flop coupling between the AFM and ferromagnet moments at a partially compensated interface.
Spin polarized tunneling as a probe for quantitative analysis of field dependent domain structure in magnetic tunnel junctions
Authors: Tiusan C. , Hehn M. , Dimopoulos T. , Ounadjela K.
Published: July 2002

Micromagnetic features appearing during the reversal of an artificial ferrimagnet used as a hard layer of a magnetic tunnel junction are quantitatively analyzed using the high sensitivity of the spin polarized tunnel current to magnetization fluctuations in the electrodes of the magnetic junctions. We propose an analytical model which takes into account different tunneling paths associated with local magnetization configurations. The model allows a quantitative correlation between the spin polarized transport characteristics and the field-dependent domain structure. The results extracted from the tunnel magnetoresistance measurements are found to be in good agreement with the magnetic domain wall density extracted from magnetic force microscopy experiments.
Hierarchy of events in the folding of model proteins
Authors: Broglia R. A , Tiana G.
Published: July 2002

The functional properties of proteins depend upon their three-dimensional structure (native state). All the information needed to specify this structure is contained within its amino acid sequence. Under suitable conditions, most small proteins will spontaneously fold to their native states. To understand the biological function of proteins one would, therefore, like to be able to deduce or predict the three-dimensional structure from the amino acid sequence. This we cannot do. On the other hand, simple models have been developed to design sequences which are able to fold to a given conformation. Good folder sequences are characterized by a large gap, compared with the standard deviation of contact energies among the amino acids, between the energy of the sequence in the native conformation and the lowest energy (threshold Ec) of the conformations structurally dissimilar to the native one. Designed sequences which conserve (in any way) this energy gap share a common set of (conserved) contacts, contacts which form the folding nucleus of the protein. At the basis of the folding of designed sequences, lattice model representation of the folding of small, single domain, real proteins, we systematically observe the presence of local elementary structures, elementary structures which are formed at the very early stages of the compaction process, and which build the folding nucleus when they assemble together. All designed sequences which conserve the energy gap, display the same folding mechanism, although those displaying larger gaps fold faster than those whose native energy lies closer to Ec.
Argon inclusion in sputtered films and the effect of the gas on molybdenum field emitter arrays
Authors: Chalamala Babu R. , Reuss Robert H.
Published: July 2002

Residual gas analysis of a number of field emission displays showed that argon desorbed from molybdenum metal lines was the dominant gas in sealed vacuum packages. We present experimental results on the emission characteristics of molybdenum field emitter arrays in argon ambient. In argon, the emission current dropped rapidly similar to that in oxygenic gas ambients. Existing degradation models do not provide an adequate explanation for this behavior. Rather, we suggest a model based on shallow implantation of argon into the field emitter tips that increases the effective width of the tunneling barrier. Experimental support for this model comes from the following observations: emission current degraded only when the device was turned on; after gas exposure, significant current recovery which followed diffusion type behavior was noted; degradation and recovery rates were functions of partial pressure; and no detectable effects associated with sputtering were observed. This mechanism is also consistent with ion pumping known to occur in field emission displays.
Role of magnetostatic interactions in assemblies of Fe nanoparticles
Authors: Schulthess T. C. , Benakli M. , Visscher P. B. , Sorge K. D. , Thompson J. R. , Modine F. A. , Haynes T. E. , Boatner L. A. , Stocks G. M. , Butler W. H.
Published: July 2002

It has recently become possible to synthesize a class of nanostructured materials by ion implantation. The implanted ions aggregate into crystallographically oriented nanoscale inclusions in the host material. We have performed simulations of the magnetization curves for such assemblies of nanoscale Fe inclusions in a nonmagnetic host. We use random positions for the magnetic particles (not a regular grid) and include magnetostatic interactions in detail. We find that these materials are not adequately described by standard noninteracting theories—interactions have a significant effect. In particular, interactions can mask the effects of crystallite orientation, producing nearly isotropic hysteresis curves. The use of a noninteracting model could thus lead one to conclude, incorrectly, that the inclusions are randomly oriented.
Coordinate Bethe ansatz for the one-dimensional SU(n) Hubbard model with open boundary conditions
Authors: Li Guang-Liang , Yue Rui-Hong , Shi Kang-Jie
Published: July 2002

The one-dimensional (1D) SU(n) Hubbard model with open boundary condition is solved by using the coordinate Bethe ansatz method. The energy and integrable boundary conditions are obtained. At the same time, the corresponding Bethe ansatz equations are achieved by diagonalizing the inhomogeneous transfer matrix of the open SU(2n−2) XXX vertex model. When n=2, our result comes back to that of the 1D Hubbard model given by Deguchi and Yue (con-mat/9704138).
Role of a buried ultrathin amorphous interlayer on the growth of Co films on different metal substrates
Authors: Sarkar D. K. , Falke M. , Giesler H. , Teichert S. , Beddies G. , Hinneberg H.-J.
Published: July 2002

Thin films of different metals M (M=Ti, Zr, Hf, Nb, Fe, and Ni) of thickness around 10 nm are deposited on Si(100) substrates and a ∼30 nm Co film is deposited on these metal films using the magnetron sputtering method. Cross-sectional transmission electron microscopy (XTEM) shows the presence of a ∼2 nm buried ultrathin amorphous interlayer at the interface between the Co layer and the M layers (M=Ti, Zr, Hf, and Nb). X-ray reflectivity is used to determine the electron density of this buried ultrathin amorphous interlayer. X-ray diffraction (XRD) is used to determine the crystalline quality of the deposited Co film on these various metal film substrates. The XRD peaks of Co(111) and Co(222) are observed when Ti, Zr, Hf, and Nb are used as substrates. Pole figure measurements confirm that the Co film is highly textured on such metallic substrates. On the other hand no characteristic XRD peaks of cobalt are observed when Ni and Fe are used as the substrate, however, XTEM shows the presence of the Co film on those substrates. Theoretically, the heat of mixing (−ΔH) has been calculated for Co–M systems. The local temperature rise is estimated using the average heat capacity and the calculated heat of mixing of the amorphous interface compound (Co3M). The local temperature rise due to the amorphous phase formation and hence better mobility of the further deposited metal atoms is the cause of highly textured Co thin films on such metal substrates.
Mechanism of microlens formation in quantum dot glasses under continuous-wave laser irradiation
Authors: Kaganovskii Yuri , Antonov Irena , Bass Fredrick , Rosenbluh Michael , Lipovskii Audrey
Published: July 2002

The exposure of glass, embedded with copper nanoparticles, to a focused continuous wave laser results in controllable formation of microlenses on the surface of the glass. The dependence of the shape and the size of the microlenses on the exposure time and the laser power is discussed. The mechanism for heat and mass transfer under laser irradiation is proposed and is used to explain the relatively low powers required for the process, the shape of the microlenses, and the kinetics of the lens shape evolution during exposure. The fabrication of the microlenses of 2–50 μm in diameter is demonstrated
A simple and fast transimpedance amplifier for microchannel plate detectors
Authors: Schwartze P. , Baumgärtel H. , Eisenhardt C. G.
Published: July 2002

A simple and fast transimpedance amplifier to couple a microchannel plate detector with an oscilloscope with a low impedance input has been developed. The amplifier protects the connected electronic equipment from destructive high voltage bursts. Examples of the detection of electrons are shown. The influence of the amplifier on pulse shape and intensity is discussed.
Refraction contrast imaging with a scanning microlens
Authors: Fletcher D. A. , Crozier K. B. , Quate C. F. , Kino G. S. , Goodson K. E. , Simanovskii D. , Palanker D. V.
Published: July 2002

We demonstrate subwavelength spatial resolution with a scanning microlens operating in collection mode with a large-area detector. Optical contrast is created by refraction of off-axis light rays at angles larger than the maximum collection angle. With a microfabricated silicon microlens 10 μm in diameter, we measure spatial resolution due to refraction contrast of λ/4.3 at a wavelength of λ=10.7 μm. A model based on ray tracing is developed to explain our result, and we show that lens diameter and index of refraction limit resolution for large emission and collection angles.
Spectroscopic study of surfactant enhanced organometallic vapor phase epitaxy growth of GaInP
Authors: Fetzer C. M. , Lee R. T. , Chapman D. C. , Stringfellow G. B.
Published: July 2002

Samples of GaxIn1−xP grown by organometallic vapor phase epitaxy on (001) GaAs substrates by addition of TESb demonstrating a lateral superlattice compositional modulation (CM) have been studied by low temperature polarized photoluminescence (PL), power dependent PL, and photoluminescence excitation (PLE) spectroscopy. Strong polarization is observed in the low temperature PL and PLE spectra at Sb concentrations below that where CuPtB ordering is removed and triple period ordering is produced. Low temperature polarized PL is shown to be the most sensitive optical technique for detecting the presence of CM. The radiative recombination mechanism at low temperature is excitonic, originating from the exponential tail of band gap states observed in the PLE spectra. From the measured band gaps, a continuum model of the band structure allows an estimate of an upper limit of the percent modulation present in the samples. Above Sb/III(v)=0.01, compositional modulation is the dominant factor determining the low temperature optical properties. The percent fluctuation of composition increases monotonically with increasing Sb during growth.
X-ray study of antiphase boundaries in the quadruple-period ordered GaAs0.87Sb0.13 alloy
Authors: Zhong Zhenyang , Holý V. , Li J. H. , Kulik J. , Bai J. , Golding T. D. , Moss S. C.
Published: July 2002

The x-ray diffraction intensity from antiphase boundaries (APBs) in a quadruple-period ordered GaAsSb alloy is derived introducing statistical probabilities for these APBs and their associated phase shifts. Through experimental fits, we obtain an average distance between the neighboring APBs of approximately 130, 400, and 5 nm along [110], [−110], and [001] directions, respectively. The short distance along the [001] growth direction leads to a narrow streak along [001] in an intensity contour map. In addition, we find that the APBs broaden the ordering peaks, while their effect on the intensity ratio between the ordering peaks is negligible. Static atomic displacements, associated with bond length disparities are also included in the calculations.
Homotopy arguments for quantized Hall conductivity
Authors: Richter T. , Schulz-Baldes H.
Published: July 2002

Using the strong localization bounds obtained by the Aizenman–Molcanov method for a particle in a magnetic field and a disordered potential, we show that the zero-temperature Hall conductivity of a gas of such particles is quantized and constant as long as both Fermi energy and disorder coupling parameter vary in a region of strong localization of the corresponding two-dimensional phase diagram.
Noninvasive measurement of electron-beam size with diamagnetic loops
Authors: Ekdahl Carl
Published: July 2002

Diamagnetic loops can be used as a noninvasive method for measurements of beam size in electron beam accelerators that use solenoidal magnetic transport. A comprehensive theory for interpreting data from a diamagnetic loop is developed. It is shown that the change in flux through a diamagnetic loop can be simply related to the rms beam radius to high accuracy, regardless of the details of the current profile, when the ratio of beam current to Alfven current Ib/IA is small. The difficulty in making this measurement lies in the fact that the diamagnetic-loop signal is also small to the same order.
Hartree approximation II: The thermodynamic limit
Authors: Prodan E. , Nordlander P.
Published: July 2002

We investigate the thermodynamic limit of the Hartree approximation for periodic background potentials and short range two-body interactions. We prove that, for any finite volume, the Hartree problem has a unique solution among the periodic densities of particles provided the coupling constant is smaller than a certain value. This value is independent of volume. We also prove that these solutions converge as the thermodynamic limit is considered.
Singular and nonsingular eigenvectors for the Gaudin model
Authors: Garajeu Daniela , Kiss Annamaria
Published: July 2002

We present a method to construct a basis of singular and nonsingular common eigenvectors for Gaudin Hamiltonians in a tensor product module of the Lie algebra SL(2). The subset of singular vectors is completely described by analogy with covariant differential operators. The relation between singular eigenvectors and the Bethe Ansatz is discussed. In each weight subspace the set of singular eigenvectors is completed to a basis, by a family of nonsingular eigenvectors. We discuss also the generalization of this method to the case of an arbitrary Lie algebra.
Classical Lie subalgebras of the Lie algebra of matrix differential operators on the circle
Authors: Boyallian Carina , Liberati Jose I.
Published: July 2002

We give a complete description of the anti-involutions of the algebra DN of N×N-matrix differential operators on the circle, preserving the principal ℤ gradation. We obtain, up to conjugation, two families σ±,m with 1⩽m⩽N, getting two families D±,mN of simple Lie subalgebras fixed by −σ±,m. We also give a geometric realization of σ±,m, concluding that D+,mN is a subalgebra of DN of type o(m,n) and D−,mN is a subalgebra of DN of type osp(m,n) (ortho-symplectic). Finally, we study the conformal algebras associated with D+,mN and D−,mN.
Mass-selective resonance ion ejection from the external quadrupole static attraction ion trap
Authors: Hiura Hidefumi , Kanayama Toshihiko
Published: July 2002

We have developed a method to mass selectively eject ions confined in a novel ion trap; an external quadrupole static attraction ion trap (EQSIT). The resonance ion ejection method is based on the resonance between the secular oscillation of trapped ions and a periodic external field to modulate the trapping potential. We applied the method to a system of hydrogenated silicon cluster ions, SinHx+ and SinHx2+ grown in the EQSIT, and found that only predetermined cluster ions were eliminated from the EQSIT. The method makes it possible not only to eject the confined ions mass selectively from the trap, but also to observe resonance ion ejection spectra of the secular oscillation of the trapped ions. For example, we measured resonance ion ejection spectra of rare gas ions, He+ and Ar+, and thereby analyzed the secular oscillation of the trapped ions.
Self-assembly of submicron particles between electrodes
Authors: Gu Z.-Z. , Meng Q.-B. , Hayami S. , Fujishima A. , Sato O.
Published: July 2002

A method for the fabrication of opal films between parallel transparent electrodes is described. Monodispersed particles are assembled by taking advantage of the rheological force induced by the evaporation of a solvent in a thin capillary cell. Three-dimensional opal films with controllable thickness could be fabricated, in which a regular hexagonal arrangement of particles parallel to the substrates over a large area was observed. Such a sandwich-like photonic device may find applications in fabricating electrically tunable photonic crystals.
Molecular spectroscopy of dye aggregates by scanning-tunneling-microscope-induced light emission
Authors: Touhari F. , Stoffels E. J. A. J. , Gerritsen J. W. , van Kempen H. , Callant P.
Published: July 2002

A bromide-modified silver (111) surface, covered by Jelly-type aggregates of a cyanine dye, is investigated by scanning tunneling microscopy (STM) in air. We show that, under appropriate conditions, light is emitted out of the junction by inelastic electron tunneling. Study of the relation between the light intensity and the tunnel bias clearly reveals a molecular influence on the light emission. Furthermore, through a spectral analysis, we show that sharp resonances appear for dye-covered surfaces by coupling of tip-induced plasmons with molecular excitations. This demonstrates that through STM-induced luminescence local spectroscopic information can be obtained on molecules.
Unitary irreducible representations of a Lie algebra for matrix chain models
Authors: Jakobsen H. P. , Lee C.-W. H.
Published: July 2002

There is a decomposition of a Lie algebra for open matrix chains akin to the triangular decomposition. We use this decomposition to construct unitary irreducible representations. All multiple meson states can be retrieved this way. Moreover, they are the only states with a finite number of non-zero quantum numbers with respect to a certain set of maximally commuting linearly independent quantum observables. Any other state is a tensor product of a multiple meson state and a state coming from a representation of a quotient algebra that extends and generalizes the Virasoro algebra. We expect the representation theory of this quotient algebra to describe physical systems at the thermodynamic limit.
Study of phase changes of the water octamer using parallel tempering and multihistogram methods
Authors: Nigra Pablo , Carignano Marcelo A. , Kais Sabre
Published: July 2002

Parallel tempering Monte Carlo and multihistogram methods are combined to study the phase changes of the water octamer. The heat capacity is calculated continuously from very low temperatures up to T=230 K. We find the melting temperature to be 178.5 K. In addition, a solid–solid phase change is found at 12 K. We introduce an order parameter to monitor this low temperature phase change.
Transformation of a classical voltage controlled oscillator into a voltage controlled quadrature sinusoidal oscillator
Authors: Bayard J. , Ayachi M.
Published: July 2002

A setup to be added onto a classical voltage controlled oscillator (VCO) is proposed. It transforms this classical VCO into a voltage controlled quadrature sinusoidal oscillator, which gives two sinusoidal signals simultaneously, vs=Vm sin ωt and vc=Vm cos ωt, with the same amplitude Vm whatever the frequency value. The described setup is particularly easy to add to a classical VCO during the making of the integrated circuit. So, a new generation of low cost integrated voltage controlled quadrature sinusoidal oscillators would be available for electronic designers. It can also be added to an already existing classical integrated VCO.
Subband characteristics of Si δ-doped pseudomorphic In0.2Ga0.8As/GaAs heterostructures
Authors: Huang Zhiming , Jiang Chunping , Zhang Zhanhong , Lin Tie , Chu Junhao , Yu Roger
Published: July 2002

Subband properties of Si δ-doped pseudomorphic In0.2Ga0.8As/GaAs heterostructures have been investigated by solving the Schrödinger–Kohn–Sham equation and the Poission equation self-consistently, and by the density–density dynamical response function. Different δ-doping configurations, in which the same Si δ-doped layer is placed at different positions with respect to the In0.2Ga0.8As well, have been studied to find their effect on subband electron densities and mobilities. High electron densities of greater than 3.6×1012 cm−2 are obtained when a δ-doping density of 4.5×1012 cm−2 is placed at the well center or at the well-barrier interface. However, the electron density in the well for Si δ-modulation-doped In0.2Ga0.8As/GaAs heterostructure is only about 1.3×1012 cm−2. The change of the position of a Si δ-doped layer from the well center to the barrier does not change the mobility of electrons in the lowest subband significantly. The Si δ doping in both barriers leads to an increase of the electron density by almost a factor of 2. The mobilities in the well for the modulation-doped structures are always much greater than those for the well-doped structures. The calculated results are also compared to the corresponding experimental data.
Low temperature photoluminescence of Ga0.84In0.16As0.22Sb0.78 solid solutions lattice matched to InAs
Authors: Moiseev K. D. , Mikhailova M. P. , Yakovlev Yu. P. , Šimeček T. , Hulicius E. , Oswald J.
Published: July 2002

Low-temperature photoluminescence (PL) study of liquid phase epitaxy grown undoped and Sn doped GaIn0.16As0.22Sb layers lattice matched to InAs is reported. The quaternary solid solutions Ga1−xInxAsySb1−y are promising materials for the fabrication of optoelectronics devices operating in the spectral range 3–5 μm because these alloys can form type II heterojunctions both with staggered and broken-gap alignment. The band structure engineering of these devices requires the knowledge of energy gaps and mechanism of radiative recombination transitions in the forbidden gap of cladding layers. The high quality quaternary GaIn0.16As0.22Sb epitaxial layers with low native defect concentration were grown lattice matched to InAs and their photoluminescence was studied at low temperatures. The emission band related to bound exciton was dominant. While the emission bands associated with the first ionization state of VGaGaSb vacancy-antisite defect with activation energy ΔEA=22 meV and unknown deep defect with ΔEB=46 meV were found besides the main PL peak ΔEBE=16 meV. It was established that Sn as an amphoteric impurity can form shallow donor levels.
Dynamic flow, broken surface anchoring, and switching bistability in three-terminal twisted nematic liquid-crystal displays
Authors: Qian Tiezheng , Xie Zhiliang , Kwok Hoi S. , Sheng Ping
Published: July 2002

We investigate the switching bistability between the Φ and Φ+π twist states in liquid-crystal cells with a three-terminal electrode structure. Based on general nematohydrodynamic equations and a modeling potential of surface anchoring, numerical calculation shows that dynamic shear flow and electric-field-induced breaking of surface azimuthal anchoring are essential to the realization of the switching bistability observed experimentally.