bluuebunny/crossref_metadata_2025 huggingface dataset:
Stochastic chemical reactions in microdomains
Authors: Holcman D. , Schuss Z.
Published: March 2005

Traditional chemical kinetics may be inappropriate to describe chemical reactions in microdomains involving only a small number of substrate and reactant molecules. Starting with the stochastic dynamics of the molecules, we derive a master-diffusion equation for the joint probability density of a mobile reactant and the number of bound substrate in a confined domain. We use the equation to calculate the fluctuations in the number of bound substrate molecules as a function of initial reactant distribution. A second model is presented based on a Markov description of the binding and unbinding and on the mean first passage time of a molecule to a small portion of the boundary. These models can be used for the description of noise due to gating of ionic channels by random binding and unbinding of ligands in biological sensor cells, such as olfactory cilia, photoreceptors, hair cells in the cochlea.
Optical control of light in a waveguide by high-Q cavity modulation
Authors: Israel Abraham , Lewis Aaron
Published: March 2005

High Q factor whispering gallery mode resonators with their ultrahigh selectivity for optical resonances are used to demonstrate switching of cw light with cw light. This switching can result in either higher or lower throughput of a light beam in a waveguide channel and has implications for optical computation and communications. The effect in the present letter is based on small index of refraction alterations induced by a thermo-optic effect. The sign of the effect can be controlled by the relative position of sphere to the waveguide. The geometry used in our demonstration, which has been able to detect an index of refraction alteration of 10−7, is potentially capable of monitoring such changes as small as 10−10 and can be generalized to other means of optical switching and modulation for optical device and computing applications.
Combining smart darting with parallel tempering using Eckart space: Application to Lennard–Jones clusters
Authors: Nigra Pablo , Freeman David L. , Doll J. D.
Published: March 2005

The smart-darting algorithm is a Monte Carlo based simulation method used to overcome quasiergodicity problems associated with disconnected regions of configurations space separated by high energy barriers. As originally implemented, the smart-darting method works well for clusters at low temperatures with the angular momentum restricted to zero and where there are no transitions to permutational isomers. If the rotational motion of the clusters is unrestricted or if permutational isomerization becomes important, the acceptance probability of darting moves in the original implementation of the method becomes vanishingly small. In this work the smart-darting algorithm is combined with the parallel tempering method in a manner where both rotational motion and permutational isomerization events are important. To enable the combination of parallel tempering with smart darting so that the smart-darting moves have a reasonable acceptance probability, the original algorithm is modified by using a restricted space for the smart-darting moves. The restricted space uses a body-fixed coordinate system first introduced by Eckart, and moves in this Eckart space are coupled with local moves in the full 3N-dimensional space. The modified smart-darting method is applied to the calculation of the heat capacity of a seven-atom Lennard–Jones cluster. The smart-darting moves yield significant improvement in the statistical fluctuations of the calculated heat capacity in the region of temperatures where the system isomerizes. When the modified smart-darting algorithm is combined with parallel tempering, the statistical fluctuations of the heat capacity of a seven-atom Lennard–Jones cluster using the combined method are smaller than parallel tempering when used alone.
Effects of pulsed low frequency electromagnetic fields on water using photoluminescence spectroscopy: Role of bubble/water interface
Authors: Vallée Philippe , Lafait Jacques , Mentré Pascale , Monod Marie-Odile , Thomas Yolène
Published: March 2005

The effects of a pulsed low frequency electromagnetic field were investigated on photoluminescence of well characterized water and prepared under controlled conditions (container, atmospheric, electromagnetic, and acoustic environments). When reference water samples were excited at 260 nm, two wide emission bands centered at 345 nm (3.6 eV) and 425 nm (2.9 eV) were observed. By contrast under 310 nm excitation, only one band appeared at 425 nm. Interestingly, electromagnetic treatment (EMT) induced, at both excitation wavelengths, a decrease (around 70%) in the 425 nm band relative photoluminescence intensity. However, no difference between reference and treated sample was observed in the 345 nm band. Other experiments, performed on outgassed samples (reference and treated), show that the emission bands (position, shape, intensity) under excitation at 260 nm and 310 nm were similar and close to the corresponding bands of the treated nonoutgassed samples. Similar effects were observed on photoluminescence excitation of water samples. Two excitation bands monitored at 425 nm were observed at 272 nm and 330 nm. After EMT and/or outgassing, a decrease (>60%) was observed in the intensity of these two bands. Altogether, these results indicate that electromagnetic treatment and/or outgassing decrease in a similar fashion the photoluminescence intensity in water samples. They also suggest that this effect is most likely indirectly attributed to the presence of gas bubbles in water. The possible role of hydrated ionic shell around the bubbles in the observed extraluminescence is discussed.
Using pulsed neutron transmission for crystalline phase imaging and analysis
Authors: Steuwer A. , Withers P. J. , Santisteban J. R. , Edwards L.
Published: March 2005

The total scattering cross section of polycrystalline materials in the thermal neutron region contains valuable information about the scattering processes that neutrons undergo as they pass through the sample. In particular, it displays characteristic discontinuities or Bragg edges of selected families of lattice planes. We have developed a pixelated time-of-flight transmission detector able to record these features and in this paper we examine the potential for quantitative phase analysis and crystalline phase imaging through the examination of a simple two-phase test object. Two strategies for evaluation of the absolute phase volumes (path lengths) are examined. The first approach is based on the evaluation of the Bragg edge amplitude using basic profile information. The second approach focuses on the information content of certain regions of the spectrum using a Rietveld-type fit after first identifying the phases via the characteristic edges. The phase distribution is determined and the coarse chemical species radiographic image reconstructed. The accuracy of this method is assessed.
Direct evidence of weak-link grain boundaries in a polycrystalline MgB2 superconductor
Authors: Khare Neeraj , Singh D. P. , Gupta A. K. , Sen Shashawati , Aswal D. K. , Gupta S. K. , Gupta L. C.
Published: March 2005

The observation of voltage-flux oscillations due to the radio-frequency–superconducting quantum interference device (rf-SQUID) effect in polycrystalline bulk MgB2 superconductor is reported. This provides a direct evidence of the presence of Josephson junction-type weak-link grain boundaries in MgB2. Analysis of the SQUID modulation indicates that only a few grain boundaries are of weak-link type whereas others are strong links. Generation of even and odd harmonics are observed when the MgB2 sample is subjected to both ac and dc fields. This also indicates the presence of weak-link grain boundaries. Flux noise in the bulk rf-SQUID in MgB2 is studied and compared with the flux noise in bulk high-Tc rf-SQUID.
Conditions for multiplicativity of maximal ℓp-norms of channels for fixed integer p
Authors: Giovannetti Vittorio , Lloyd Seth , Ruskai Mary Beth
Published: March 2005

We introduce a condition for memoryless quantum channels which, when satisfied guarantees the multiplicativity of the maximal ℓp-norm with p a fixed integer. By applying the condition to qubit channels, it can be shown that it is not a necessary condition, although some known results for qubits can be recovered. When applied to the Werner-Holevo channel, which is known to violate multiplicativity when p is large relative to the dimension d, the condition suggests that multiplicativity holds when d⩾2p−1. This conjecture is proved explicitly for p=2,3,4. Finally, a new class of channels is considered which generalizes the depolarizing channel to maps which are combinations of the identity channel and a noisy one whose image is an arbitrary density matrix. It is shown that these channels are multiplicative for p=2.
A note on positive energy theorem for spaces with asymptotic SUSY compactification
Authors: Dai Xianzhe
Published: March 2005

We extend the higher dimensional positive mass theorem in [Dai, X., Commun. Math. Phys. 244, 335–345 (2004)] to the Lorentzian setting. This includes the original higher dimensional positive energy theorem whose spinor proof is given in [Witten, E., Commun. Math. Phys. 80, 381–402 (1981)] and [Parker, T., and Taubes, C., Commun. Math. Phys. 84, 223–238 (1982)] for dimension 4 and in [Zhang, X., J. Math. Phys. 40, 3540–3552 (1999)] for dimension 5.
Contribution of the metal∕SiO2 interface potential to photoinduced switching in molecular single-electron tunneling junctions
Authors: Noguchi Yutaka , Kubota Tohru , Mashiko Shinro , Wakayama Yutaka
Published: March 2005

Photoinduced switching of the Coulomb staircase in molecular single-electron tunneling junctions was previously observed. These junctions consisted of evaporated SiO2 insulator (∼5nm), with tetrakis-3,5-di-t-butylphenyl-porphyrin (H2-TBPP) molecules as Coulomb islands, sandwiched between top and bottom electrodes. The reversible response and the relaxation time of the photoinduced switching suggest that this phenomenon depends on the properties of the metal∕SiO2 interface rather than those of the H2-TBPP molecule or SiO2 tunneling layer. We analyzed the photoinduced switching according to the theory of single-electron tunneling taking into account the discrete molecular energy states and the metal∕SiO2 interfacial electrostatic phenomena. We conclude that the main contributor to the photoinduced shift was the electrostatic potential formed through the space-charge exchange at the metal∕SiO2 interface.
High resolution emission Fourier transform infrared spectra of the 4p-5s and 5p-6s bands of ArH
Authors: Baskakov O. I. , Civiš S. , Kawaguchi K.
Published: March 2005

In the 2500–8500cm−1 region several strong emission bands of ArH40 were observed by Fourier transform spectroscopy through a dc glow discharge in a mixture of argon and hydrogen. Rotational-electronic transitions of the two previously unstudied 4p-5s and 5p-6s,v=0-0, bands of ArH40 were measured and assigned in the 6060 and 3770cm−1 regions, respectively. A simultaneous fit of the emission transitions of the 4p-5s and 5p-6s bands and an extended set of transitions of the 6s-4p band observed by Dabrowski, Tokaryk, and Watson [J. Mol. Spectrosc. 189, 95 (1998)] and remeasured in the present work yielded consistent values of the spectroscopic parameters of the electronic states under investigation. In the branch of the 4p-5s band with transitions of type Qf3eQ we observed a narrowing in the linewidths with increasing rotational quantum number N. The rotational dependence of the linewidth is caused by predissociation of the 5s state by the repulsive ground 4s state through homogeneous coupling and changes in overlap integrals of the vibrational wave functions with the rotational level. Analysis was based on the Fermi’s golden rule approximation model. In the 4p-5s band region a vibrational sequence ofv′-v″=1-1, 2-2, and 3-3 were recorded and a number of transitions belonging to the strongest Qf3eQ form branch of the 1-1 band were analyzed.
A new force field for atomistic simulations of aqueous tertiary butanol solutions
Authors: Lee Maeng Eun , van der Vegt Nico F. A.
Published: March 2005

We present a new tert-butanol force field parametrized to reproduce the mixture thermodynamics of tert-butanol/water over a wide range of solution compositions at room temperature and atmospheric pressure. The experimental Kirkwood–Buff integrals, which quantify preferential solvation of solution components by the same species or by the other components, were used as target values to be reproduced. Water was modeled using the simple point charge model. In the range of alcohol mole fractions between 0.02 and 0.98, our optimized model satisfactorily reproduces alcohol-alcohol, water-water, and alcohol-water aggregation behavior. As a consequence, the solution activity derivatives are reproduced as well. A comparison has been made with solution activities obtained by free energy calculations (i.e., thermodynamic integration). It clearly shows that the Kirkwood–Buff based approach performs superior in predicting solution activities of liquid mixtures. The new tert-butanol model has been used to examine the solution structure and hydrophobic interactions in aqueous tert-butanol at the various mixture compositions. A comparison is made with structural data obtained by neutron diffraction.
Current through SiO2 gate oxide and its low frequency fluctuations: Trapping on charged dangling bonds with negative Hubbard U
Authors: Moyzhes Boris , Geballe Theodore H. , Jeong Steve , Gitlin Daniel , Karp James
Published: March 2005

An estimate of Hubbard U supports instability of neutral one-electron Si dangling bonds in SiO2 and the formation of charged two-electron and two-hole negative U centers through the reaction Si•+Si•→Si++Si−••. The trapping on these negative U centers creates and annihilates “dents” in the thin barrier for electron and hole tunneling through the gate oxide. Such dents are visible as gate current low frequency fluctuations (1∕f noise). The longer trapping time of holes causes irreversible Si−••→Si+ conversion, which leads to stress-induced leakage current and accumulation of positive charge in the oxide under voltage stress.
Adiabatic release measurements in aluminum from 240-to500-GPa states on the principal Hugoniot
Authors: Knudson M. D. , Asay J. R. , Deeney C.
Published: March 2005

Adiabatic release measurements were performed on aluminum (6061-T6) from ∼240-to500-GPa(∼2.4–5Mbar) states on the principal Hugoniot. Using a magnetically accelerated flyer plate technique, capable of launching macroscopic aluminum flyer plates (approximately 12×25mm in lateral dimension and ∼300μm in thickness) to velocities in excess of 20km∕s, direct impact experiments were performed on a low shock impedance, 200-mg∕cc silica aerogel to determine the aerogel Hugoniot in the pressure range of ∼30–75GPa. Release experiments were then performed in which the aerogel was mounted onto an aluminum base plate, and the initial shock in the base plate was transmitted to the aerogel sample. Given the measured aerogel shock velocities from the release experiments and the measured aerogel Hugoniot from the direct impact experiments, release states on release adiabats of aluminum were ascertained. The results were compared to several equation of state models for aluminum, and a rigorous statistical analysis was performed. The statistical analysis enabled the suitability of these various models in describing the release response of hot, liquid states to be determined. This study enhances our understanding of the release response of aluminum for high-pressure states on the Hugoniot and lends confidence to the use of aluminum as a standard material in impedance-matching experiments.
Evaluation of elastic properties of submicrometer thin films using slanted finger interdigital transducers
Authors: Wu Tsung-Tsong , Chen Yung-Yu , Huang Guo-Tsai , Chang Pei-Zen
Published: March 2005

Accurate evaluation of film properties with thickness in the submicrometer range is very important throughout microelectronic industry as well as nano/microelectromechanical system industry. In this study, we developed a nondestructive evaluation technique, which is not only suitable for dielectric films but also valid for metallic films, to measure the elastic properties of submicrometer thin films. Firstly, we established the forward solution of the inverse evaluation and analyzed the dispersion of surface acoustic wave (SAW) in a SiO2∕YZ–LiNbO3 coated solid based on the effective permittivity approach. To measure the dispersion of SAW in the coated solid, a slanted finger interdigital transducer (SFIT) was employed to generate wide band SAW signals. The SFIT was designed by using the coupling of modes method to obtain the optimal frequency response. SiO2 films with submicrometer thickness were deposited on the piezoelectric YZ–LiNbO3 substrate via the plasma-enhanced chemical-vapor deposition process. Pairs of the SFITs were also fabricated on the substrate. The measured frequency responses were then processed using the spectral analysis of surface wave to obtain the SAW dispersion in the coated solid. Finally, based on the forward solution and measured dispersion, we determined inversely the elastic properties of the SiO2 film through the use of the Simplex algorithm. The inversion result shows that the elastic properties of the submicrometer SiO2 film were measured successfully. We note that the results of this study provide an important basis for developing a SAW sensor which can be adopted to measure in situ film properties.
Growth of ferromagnetic semiconductor Ge1−xMnxTe films on BaF2 (111) by ionized cluster beam deposition
Authors: Fukuma Y. , Arifuku M. , Asada H. , Koyanagi T.
Published: March 2005

IV-VI ferromagnetic semiconductor Ge1−xMnxTe (x≈0.4) films were grown on BaF2 (111) substrates by an ionized cluster beam method. In neutral cluster beam deposition, Ge0.64Mn0.36Te films grown at substrate temperatures in the range from 250to300°C show an epitaxial relationship Ge0.64Mn0.36Te∕GeTe(111)‖BaF2(111). The crystallinity of the Ge0.64Mn0.36Te layer is improved with increasing substrate temperature. Further improvements of the crystal properties such as the crystallinity and the surface smoothness are accomplished by the proper acceleration of the ionized GeTe cluster, although the acceleration of the ionized MnTe cluster hardly gives rise to any noticeable improvement of the crystalline quality. The crystalline quality significantly affects the ferromagnetism: the enhancement of the spontaneous magnetization and the decrease of the coercive field are observed for the film grown using the ionized GeTe cluster at the acceleration voltage of 1kV due to the decreases of magnetic disorder caused by inhomogeneities in Mn distribution and pinning sites of domain-wall motion.
Robust discretizations versus increase of the time step for the Lorenz system
Authors: Letellier Christophe , Mendes Eduardo M. A. M.
Published: March 2005

When continuous systems are discretized, their solutions depend on the time step chosen a priori. Such solutions are not necessarily spurious in the sense that they can still correspond to a solution of the differential equations but with a displacement in the parameter space. Consequently, it is of great interest to obtain discrete equations which are robust even when the discretization time step is large. In this paper, different discretizations of the Lorenz system are discussed versus the values of the discretization time step. It is shown that the sets of difference equations proposed are more robust versus increases of the time step than conventional discretizations built with standard schemes such as the forward Euler, backward Euler, or centered finite difference schemes. The nonstandard schemes used here are Mickens’ scheme and Monaco and Normand-Cyrot’s scheme.
Electrothermal simulation of a defect in a solar cell
Authors: Breitenstein O. , Rakotoniaina J. P.
Published: March 2005

A local electrothermal simulation of a model solar cell is presented. A rigorous discussion of the heat dissipation mechanisms in a solar cell is performed, showing that the total dissipated heat splits into heating terms (thermalization, recombination, and Joule heat) and different Peltier cooling terms. Such simulations are important for interpreting lock-in thermography images of real solar cells. The simulated model cell consists of a circular noncontacted region surrounded by a grid line and a nonlinear edge shunt. Based on this simulation, a special lock-in thermography operation mode is proposed, which enables noncontacted regions in real solar cells to be imaged. Experimental results confirm the theoretical predictions.
High pressure regime of plasma enhanced deposition of microcrystalline silicon
Authors: Amanatides E. , Hammad A. , Katsia E. , Mataras D.
Published: March 2005

An investigation of the effect of the total gas pressure on the deposition of microcrystalline thin films form highly diluted silane in hydrogen discharges was carried out at two different frequencies. The study was performed in conditions of constant power dissipation and constant silane partial pressure in the discharge while using a series of plasma diagnostics as electrical, optical, mass spectrometric, and in situ deposition rate measurements together with a simulator of the gas phase and the surface chemistry of SiH4∕H2 discharges. The results show that both the electron density and energy are affected by the change of the total pressure and the frequency. This in turn influences the rate of high energy electron–SiH4 dissociative processes and the total SiH4 consumption, which are favored by the frequency increase for most of the pressures. Furthermore, frequency was found to have the weakest effect on the deposition rate that was enhanced at 27.12MHz only for the lowest pressure of 1Torr. On the other hand, the increase of pressure from 1to10Torr has led to an optimum of the deposition rate recorded at 2.5Torr for both frequencies. This maximum is achieved when the rate of SiH4 dissociation to free radical is rather high; the flux of species is not significantly hindered by the increase of pressure and the secondary gas phase reactions of SiH4 act mainly as an additional source of film precursors.
Nanoelectromechanics of polarization switching in piezoresponse force microscopy
Authors: Kalinin S. V. , Gruverman A. , Rodriguez B. J. , Shin J. , Baddorf A. P. , Karapetian E. , Kachanov M.
Published: March 2005

Nanoscale polarization switching in ferroelectric materials by piezoresponse force microscopy in weak and strong indentation limits is analyzed using exact solutions for coupled electroelastic fields under the tip. Tip-induced domain switching is mapped on the Landau theory of phase transitions, with domain size as an order parameter. For a point charge interacting with a ferroelectric surface, switching by both first and the second order processes is possible, depending on the charge–surface separation. For a realistic tip, the domain nucleation process is first order in charge magnitude and polarization switching occurs only above a certain critical tip bias. In pure ferroelectric or ferroelastic switching, the late stages of the switching process can be described using a point charge model and arbitrarily large domains can be created. However, description of domain nucleation and the early stages of growth process when the domain size is comparable with the tip curvature radius (weak indentation) or the contact radius (strong indentation) requires the exact field structure. For higher order ferroic switching (e.g., ferroelectroelastic), the domain size is limited by the tip–sample contact area, thus allowing precise control of domain size.
Residual stress in thick low-pressure chemical-vapor deposited polycrystalline SiC coatings on Si substrates
Authors: Choi D. , Shinavski R. J. , Steffier W. S. , Spearing S. M.
Published: March 2005

Residual stress in thick coatings of polycrystalline chemical-vapor deposited SiC on Si substrates is a key variable that must be controlled if SiC is to be used in microelectromechanical systems. Studies have been conducted to characterize the residual stress level as a function of deposition temperature, Si wafer and SiC coating thickness, and the ratios of methyltrichlorosilane to hydrogen and hydrogen chloride. Wafer curvature was used to monitor residual stress in combination with a laminated plate analysis. Compressive intrinsic (growth) stresses were measured with magnitudes in the range of 200–300MPa; however, these can be balanced with the tensile stress due to the thermal-expansion mismatch to leave near-zero stress at room temperature. The magnitude of the compressive intrinsic stress is consistent with previously reported values of surface stress in combination with the competition between grain-boundary energy and elastic strain energy.
Effective conjugation and Raman intensities in oligo(para-phenylene)s: A microscopic view from first-principles calculations
Authors: Heimel Georg , Somitsch Dieter , Knoll Peter , Brédas Jean-Luc , Zojer Egbert
Published: March 2005

Electron-phonon coupling in oligo(para-phenylene)s is addressed in terms of the off-resonance Raman intensities of two distinct modes at 1220 and 1280cm−1. On the basis of Albrecht’s theory, vibrational coupling and Raman intensities are calculated from first-principles quantum-chemical methods. A few-state model is used to evaluate the dependence of the mode intensities on oligomer length, planarity, and excitation wavelength. The link between electron delocalization∕conjugation and Raman intensities is highlighted. Extending on prior studies, the present work focuses on providing an in-depth understanding of the origin of this correlation in addition to reproducing experimental findings. The model applied here allows us to interpret the results on a microscopic, quantum-mechanical basis and to relate the observed trends to the molecular orbital structure and nature of the excited states in this class of materials. We find quantitative agreement between the results of the calculations and those of measurements performed on oligo(para-phenylene)s of various chain lengths in the solid state and in solution.
Oxygen vacancies and ferromagnetism in CoxTi1−xO2−x−y
Authors: Jaffe John E. , Droubay Timothy C. , Chambers Scott A.
Published: March 2005

Cobalt-doped titanium dioxide, or CTO, has emerged in the past 2 years as a semiconducting, transparent, room-temperature ferromagnet. Very recently it has been shown that the magnetism in CTO often originates in surface nanoparticles or Co-rich regions that have a much-enhanced substitutional Co content up to 40% of Ti sites, so that magnetic CTO is not a true dilute magnetic semiconductor (DMS), but rather a fairly high-density spin system. In this work we describe a computational study of Co-rich CTO using the generalized gradient approximation to the density functional theory within the supercell model. Our total energy calculations show a strong tendency for Co-atom clustering or segregation on Ti sites. There is also a strong tendency for the oxygen vacancies to form complexes with the Co atoms. In addition, we find that the oxygen stoichiometry plays an essential role in determining the system’s magnetic order. The largest ordered moments require at least enough oxygen vacancies to put all of the Co atoms in the +2 charge state, as they indeed appear to be experimentally, so that the conventional DMS mechanism could only apply via n-type carriers. We find a small but not negligible spin density associated with Ti atoms near the vacancy sites, suggesting an F-center-mediated interaction between the much larger Co moments. We also present experimental data showing that the ferromagnetic remanence and coercive field increase with the n-type conductivity.
Adhesion energy in carbon nanotube-polyethylene composite: Effect of chirality
Authors: Al-Haik M. , Hussaini M. Y. , Garmestani H.
Published: March 2005

This work presents a study of the adhesion energy between carbon nanotube-polyethylene matrix based on molecular dynamics simulations. Specifically, the study focuses on the influence of carbon nanotube chirality on adhesion energy. It is observed that composites that utilize nanotubes with smaller chiral angles achieve higher adhesion energy, and tend to have smaller diameter and longer cylindrical axes compared to those with larger chiral angles. A zigzag nanotube (zero-chiral angle) undergoes considerable deformation to achieve an equilibrium configuration that has relatively maximum adhesion energy. On the other hand, the armchair nanotube (30° chiral angle) deforms moderately to reach equilibrium with minimal adhesion bonds to the polyethylene matrix.
A large low-field tunneling magnetoresistance of CrO2/(CrO2∕Cr2O3) powder compact with two coercivities
Authors: Wang Jingping , Che Ping , Feng Jing , Lu Minfeng , Liu Jianfen , Meng Jian , Hong Yuanjia , Tang Jinke
Published: March 2005

Two channels of the conductance G exist in cold-pressed powder compacts of CrO2/(CrO2∕Cr2O3), where two types of granules have different coercivities. One of the channels is the spin-dependent intergranular tunneling conductance, and the other one is the spin-independent higher-order inelastic hopping conductance. The conductance is mainly related with the spin-dependent tunneling channel at low temperature. A large low field tunneling magnetoresistance (TMR) of 30.4% is achieved in 950 Oe field at 2 K. The reason is the relative orientation of the magnetization that tends to be antiparallel between the two coercivities. Field dependence of the tunneling magnetoresistance shows two better-separated peaks than that of CrO2 powder compact. The results suggest that the introduction of CrO2/Cr2O3 improves low field sensitivity, TMR, and switching characteristics.
Co-doped anatase TiO2 heteroepitaxy on Si(001)
Authors: Kaspar T. C. , Droubay T. , Wang C. M. , Heald S. M. , Lea A. S. , Chambers S. A.
Published: March 2005

Pure anatase TiO2 and CoxTi1−xO2(0.01<x<0.04) epitaxial thin films were deposited by oxygen-plasma-assisted molecular beam epitaxy on Si(001) for evaluation as a potential dilute magnetic semiconductor material suitable for Si-based spintronic devices. Epitaxial growth on Si(001) was facilitated by the deposition of 1∕4 or 1∕2ML Sr metal on the clean Si(001) surface to form an oxidation resistant silicide layer, followed by deposition of a thin SrTiO3 buffer layer. Using 1∕2ML Sr metal to form the silicide allowed the deposition of 10 ML SrTiO3 without oxidation of the Si interface. Epitaxial anatase could be grown on this heterostructure, although use of the oxygen plasma during deposition resulted in significant SiO2 formation. Pure anatase films consisted of epitaxial anatase surface particles on a continuous anatase film. For Co-doped films, Co segregation to surface particles of epitaxial anatase was observed by Auger electron spectroscopy and transmission electron microscopy (TEM); faceting of the particles was observed for low Co doping concentrations. Although no secondary phases containing Co were observed in Co-doped anatase films by x-ray diffraction or TEM, x-ray absorption near edge spectroscopy indicated Co was present in the films as a mixture of Co(0), Co(II), and Co(III). All samples were ferromagnetic at room temperature; for lower Co concentrations, the ferromagnetic remanence (9%) and coercive field (100 Oe) were similar to phase-pure Co:TiO2∕LaAlO3. However, the presence of Co(0) under strongly oxidizing growth conditions known to oxidize the Si interface implies that under no deposition conditions can Co metal be eliminated while simultaneously protecting the Si interface from oxidation.
Electron spin resonance investigation of impurity and intrinsic defects in Nb-doped BaTiO3 single crystal and ceramics
Authors: Laguta V. V. , Slipenyuk A. M. , Bykov I. P. , Glinchuk M. D. , Maglione M. , Bilous A. G. , V’yunov O. I. , Rosa J. , Jastrabik L.
Published: March 2005

Electron spin resonance (ESR) spectra of impurity and intrinsic defects have been measured at 4.2–295 K in Nb-doped BaTiO3 single crystals and ceramics to clarify their role in the conductivity and positive temperature coefficient of resistivity (PTCR effect). The measurements revealed a small amount of Fe3+, Cr3+, and Mn2+ impurities, which change their valence state (for example, Cr5+→Cr3+, Mn4+→Mn2+) with increased Nb concentration due to the compensation of the excess charge of Nb5+ ions. Besides the 3d-metal impurities, several types of Ti3+ polaronic and possible fluctuon states, where electrons can be localized near ferroelectric domain boundaries, have been revealed as well. All of them are associated with Ti3+ lattice ions and not with Nb5+∕4+ impurity, which apparently represents rather a very shallow donor level. The data obtained strongly support the polaronic origin of Nb-doped BaTiO3 conductivity at T<300K. Comparative investigations of ESR spectra in single crystals and ceramics of the same kind of BaTiO3 together with computer simulation allowed us unambiguously to ascribe complex ESR signals observed in ceramic samples to Cr3+, Mn2+, and Fe3+ ions and Ti3+ polarons and/or fluctuons. The role of manganese ions at grain boundaries in the PTCR effect is discussed as well.
Finite depth square well model: Applicability and limitations
Authors: Pellegrini Giovanni , Mattei Giovanni , Mazzoldi Paolo
Published: March 2005

An investigation of the finite depth square well model is presented in this article: model features and limitations, concerning size dependent band gap of semiconductor quantum structures, are presented and discussed. Model predictions are compared with large sets of experimental data for III–V, II–VI and lead salt semiconductor quantum dots and quantum wires. Matrix influence on the confinement is studied by modeling experimental results for colloidal CdS, CdSe, CdTe and InP quantum dots. The effect of quantum structure dimensionality is investigated and successfully simulated for colloidal CdSe and InP experimental data and Si first-principle calculations. Finally, model limitations for narrow band gap semiconductors are studied and discussed.
Photoelectron spectroscopy study of metallic nanocluster arrangement at the surface of reactively sputtered amorphous hydrogenated carbon
Authors: Videnović Ivan R. , Oelhafen Peter
Published: March 2005

We report on the results of the arrangement of isolated surface metallic nanoclusters embedded in amorphous hydrogenated carbon (a-C:H) thin films, studied by photoelectron spectroscopy. As a model system we used gold-containing amorphous hydrogenated carbon (a-C:H∕Au), due to the lack of reactivity between carbon and gold. The a-C:H∕Au samples are obtained by simultaneous magnetron sputtering of Au target by argon and plasma-enhanced chemical vapor deposition of methane. Photoelectron spectroscopy with x-ray and ultraviolet excitation has been employed for surface studies that comprise as-deposited sample spectra recordings, measurements at off-normal takeoff angle, in situ in-depth profiling by Ar+ ion etching, and thiophene adsorption at the sample surface. The results of these extended studies firmly support previously drawn conclusions [I. R. Videnović, V. Thommen, P. Oelhafen, D. Mathys, M. Düggelin, and R. Guggenheim, Appl. Phys. Lett 80, 2863 (2002)] that by deposition on electrically grounded substrates one obtains samples with topmost Au clusters covered with a thin layer of a-C:H. Introducing a dc substrate bias voltage results in bald Au clusters on the surface and increased sp2∕sp3 coordinated carbon ratio in the a-C:H matrix.
Optical near-field simulation of Sb thin film thermal lens and its application in optical recording
Authors: Wei Jingsong , Zhou Fei , Wang Yang , Gan Fuxi , Wu Yiqun , Gu Donghong
Published: March 2005

Using the finite-difference-time-domain method, the near-field optical distribution and properties of Sb thin film thermal lens are calculated and simulated. The results show as follows. Within the near-field distance to the output plane of thermal lens, the spot size is approximately 100nm, and its intensity is greatly enhanced, which is higher than that of incident light. The spot shape gradually changes from ellipse to round at the distance of more than 12nm to the output plane. The above-simulated results are further demonstrated by the static optical recording experiment.
The dynamics of one-dimensional Bloch electrons in constant electric fields
Authors: Bentosela F. , Duclos P. , Moldoveanu V. , Nenciu G.
Published: March 2005

We study the dynamics of a one-dimensional Bloch electron subjected to a constant electric field. The periodic potential is supposed to be less singular than the δ-like potential (Dirac comb). We give a rigorous proof of Ao’s result that for a large class of initial conditions (high momentum regime) there is no localization in momentum space. The proof is based on the mathematical substantiation of the two simplifying assumptions made in physical literature: the transitions between far away bands can be neglected and the transitions at the quasicrossing can be described by Landau–Zener-type formulas. Using the connection between the above model and the driven quantum ring (DQR) shown by Avron and Nemirovski, our results imply the increase of energy for weakly singular such DQR and appropiate initial conditions.
From molecular dynamics to hydrodynamics: A novel Galilean invariant thermostat
Authors: Stoyanov Simeon D. , Groot Robert D.
Published: March 2005

This paper proposes a novel thermostat applicable to any particle-based dynamic simulation. Each pair of particles is thermostated either (with probability P) with a pairwise Lowe–Andersen thermostat [C. P. Lowe, Europhys. Lett. 47, 145 (1999)] or (with probability 1−P) with a thermostat that is introduced here, which is based on a pairwise interaction similar to the Nosé–Hoover thermostat. When the pairwise Nosé–Hoover thermostat dominates (low P), the liquid has a high diffusion coefficient and low viscosity, but when the Lowe–Andersen thermostat dominates, the diffusion coefficient is low and viscosity is high. This novel Nosé–Hoover–Lowe–Andersen thermostat is Galilean invariant and preserves both total linear and angular momentum of the system, due to the fact that the thermostatic forces between each pair of the particles are pairwise additive and central. We show by simulation that this thermostat also preserves hydrodynamics. For the (noninteracting) ideal gas at P=0, the diffusion coefficient diverges and viscosity is zero, while for P>0 it has a finite value. By adjusting probability P, the Schmidt number can be varied by orders of magnitude. The temperature deviation from the required value is at least an order of magnitude smaller than in dissipative particle dynamics (DPD), while the equilibrium properties of the system are very well reproduced. The thermostat is easy to implement and offers a computational efficiency better than (DPD), with better temperature control and greater flexibility in terms of adjusting the diffusion coefficient and viscosity of the simulated system. Applications of this thermostat include all standard molecular dynamic simulations of dense liquids and solids with any type of force field, as well as hydrodynamic simulation of multiphase systems with largely different bulk viscosities, including surface viscosity, and of dilute gases and plasmas.
Quasiclassical trajectory study of formaldehyde unimolecular dissociation: H2CO→H2+CO, H+HCO
Authors: Zhang Xiubin , Rheinecker Jaime L. , Bowman Joel M.
Published: March 2005

We report quasiclassical trajectory calculations of the dynamics of the two reaction channels of formaldehyde dissociation on a global ab initio potential energy surface: the molecular channel H2CO→H2+CO and the radical H2CO→H+HCO. For the molecular channel, it is confirmed that above the threshold of the radical channel a second, intramolecular hydrogen abstraction pathway is opened to produce CO with low rotation and vibrationally hot H2. The low-jCO and high-νH2 products from the second pathway increase with the total energy. The competition between the molecular and radical pathways is also studied. It shows that the branching ratio of the molecular products decreases with increasing energy, while the branching ratio of the radical products increases. The results agree well with very recent velocity-map imaging experiments of Suits and co-workers and solves a mystery first posed by Moore and co-workers. For the radical channel, we present the translational energy distributions and HCO rotation distributions at various energies. There is mixed agreement with the experiments of Wittig and co-workers, and this provides an indirect confirmation of their speculation that the triplet surface plays a role in the formation of the radical products.
Alternative descriptions in quaternionic quantum mechanics
Authors: Blasi A. , Scolarici G. , Solombrino L.
Published: March 2005

We characterize the quasianti-Hermitian quaternionic operators in quaternionic quantum mechanics by means of their spectra; moreover, we state a necessary and sufficient condition for a set of quasianti-Hermitian quaternionic operators to be anti-Hermitian with respect to a uniquely defined positive scalar product in a infinite dimensional (right) quaternionic Hilbert space. According to such results we obtain two alternative descriptions of a quantum optical physical system, in the realm of quaternionic quantum mechanics, while no alternative can exist in complex quantum mechanics, and we discuss some differences between them.
Development and performance of the nanoworkbench: A four tip STM for conductivity measurements down to submicrometer scales
Authors: Guise Olivier , Marbach Hubertus , Yates John T. , Jung Moon-Chul , Levy Jeremy , Ahner Joachim
Published: March 2005

A multiple-tip ultrahigh vacuum (UHV) scanning tunneling microscope (MTSTM) with a scanning electron microscope (SEM) for imaging and molecular-beam epitaxy growth capabilities has been developed. This instrument (nanoworkbench) is used to perform four-point probe conductivity measurements at μm spatial dimension. The system is composed of four chambers, the multiple-tip STM∕SEM chamber, a surface analysis and preparation chamber, a molecular-beam epitaxy chamber, and a load–lock chamber for fast transfer of samples and probes. The four chambers are interconnected by a unique transfer system based on a sample box with integrated heating and temperature-measuring capabilities. We demonstrate the operation and the performance of the nanoworkbench with STM imaging on graphite and with four-point-probe conductivity measurements on a silicon-on-insulator (SOI) crystal. The creation of a local FET, whose dimension and localization are, respectively, determined by the spacing between the probes and their position on the SOI surface, is demonstrated.
Perpendicular magnetic recording: Playback
Authors: Litvinov Dmitri , Khizroev Sakhrat
Published: March 2005

For the past several years, perpendicular magnetic recording has been under intense scrutiny as the primary alternative to magnetic data storage technologies in place today. Major system components, write heads and media in particular, have been the subject of extensive studies. Less attention, however, has been devoted to the playback processes in perpendicular recording systems. The playback heads used in technology demonstrations remain largely unchanged from their longitudinal recording counterparts. It is an open question whether the longitudinal playback-head design is optimal for perpendicular recording. For example, application of longitudinal playback heads in perpendicular recording leads to undesirable phenomena associated with modified playback response, increased flying height sensitivity, adjacent track interference, and calls for major modifications of the existing read channels. The subject of this work is a detailed discussion of the playback physics, in perpendicular recording systems; the focus being to establish the design guidelines for optimized perpendicular playback heads, which are equivalent or superior in their performance characteristics to conventional shielded readers used in longitudinal recording. Conformal mapping is applied to demonstrate the playback wave form equivalency between a shielded and dual-pole readers when applied in longitudinal and perpendicular recording, respectively. Utilizing extensive three-dimensional modeling and reciprocity principle to evaluate the performance of various playback-head configurations, it is demonstrated that differential reader configurations possess advantageous playback characteristics, such as higher playback amplitude, improved spatial resolution, and reduced dependence on flight-height variations as compared to conventional shielded readers. Modified design of differential readers with a single magnetoresistive sensor is proposed to overcome the manufacturability issues associated with a conventional dual-sensor differential reader.
Matching-pursuit∕split-operator Fourier-transform simulations of nonadiabatic quantum dynamics
Authors: Wu Yinghua , Herman Michael F. , Batista Victor S.
Published: March 2005

A rigorous and practical approach for simulations of nonadiabatic quantum dynamics is introduced. The algorithm involves a natural extension of the matching-pursuit∕split-operator Fourier-transform (MP∕SOFT) method [Y. Wu and V. S. Batista, J. Chem. Phys. 121, 1676 (2004)] recently developed for simulations of adiabatic quantum dynamics in multidimensional systems. The MP∕SOFT propagation scheme, extended to nonadiabatic dynamics, recursively applies the time-evolution operator as defined by the standard perturbation expansion to first-, or second-order, accuracy. The expansion is implemented in dynamically adaptive coherent-state representations, generated by an approach that combines the matching-pursuit algorithm with a gradient-based optimization method. The accuracy and efficiency of the resulting propagation method are demonstrated as applied to the canonical model systems introduced by Tully for testing simulations of dual curve-crossing nonadiabatic dynamics.
On dynamical tunneling and classical resonances
Authors: Keshavamurthy Srihari
Published: March 2005

This work establishes a firm relationship between classical nonlinear resonances and the phenomenon of dynamical tunneling. It is shown that the classical phase space with its hierarchy of resonance islands completely characterizes dynamical tunneling and explicit forms of the dynamical barriers can be obtained only by identifying the key resonances. Relationship between the phase space viewpoint and the quantum mechanical superexchange approach is discussed in near-integrable and mixed regular-chaotic situations. For near-integrable systems with sufficient anharmonicity the effect of multiple resonances, i.e., resonance-assisted tunneling, can be incorporated approximately. It is also argued that the presumed relation of avoided crossings to nonlinear resonances does not have to be invoked in order to understand dynamical tunneling. For molecules with low density of states the resonance-assisted mechanism is expected to be dominant.
The topology of the electroweak interaction
Authors: Tolksdorf Jürgen
Published: March 2005

In this paper we show that the Higgs boson of the (minimal) standard model has at most three gauge inequivalent ground states. One of these states is related to ordinary electromagnetism and the other two to electromagnetism within magnetically charged vacua. If space–time is assumed to be rotationally symmetric then the charged electroweak vacua may be identified with Dirac monopoles of magnetic charge g=±1∕2. This offers a physical interpretation of magnetic monopoles and Dirac’s quantization condition of electric charge in terms of the electroweak interaction. Moreover, in the case of the (minimal) standard model the three possible gauge inequivalent ground states of the Higgs boson are shown to fully determine the topological structure of the gauge bundle which underlies the electroweak interaction.