bluuebunny/crossref_metadata_2025 huggingface dataset:
Second-order quadrupole-shielding effects in magic-angle spinning solid-state nuclear magnetic resonance
Authors: Wi Sungsool , Ashbrook Sharon E. , Wimperis Stephen , Frydman Lucio
Published: February 2003

We investigate the nature of higher-order effects arising in solid-state nuclear magnetic resonance (NMR) when quadrupolar nuclei are subject to significant chemical shift anisotropies. It is shown that the quadrupole interaction can give rise to shielding-derived terms that are not entirely averaged away by conventional magic-angle spinning (MAS). These terms are proportional to the square of the z component of the spin angular momentum and therefore leave unaffected both the central and other −mI↔+mI symmetric multiple-quantum transitions, yet lead to noticeable effects when monitoring other nonsymmetric transitions within the spin manifold. The recently-developed satellite-transition (ST) MAS NMR method for the simultaneous averaging of the first- and second-order quadrupole effects makes such quadrupole-shielding cross terms observable. Although this may present a resolution limitation to this averaging scheme, it opens up new possibilities for determining the coupling parameters of the quadrupolar nucleus—particularly the relative orientation between its quadrupole and shielding tensors. Average Hamiltonian derivations of these effects are explored, and employed to derive analytical expressions for their resulting splittings. These predictions are then successfully compared with variable-field STMAS NMR spectra of a 59Co-containing sample. A brief discussion of potential complications arising from third-order quadrupole effects when trying to analyze such line shapes is also presented.
Diffusion of Si and Ge dimers on Ge (001) surfaces
Authors: Afanasieva T. V. , Bulavenko S. Yu. , Koval I. F. , Zandvliet H. J. W.
Published: February 2003

We have studied the various diffusion pathways of Si and Ge dimers on the Ge (001) surface using scanning tunneling microscopy. The adsorbed dimers can be classified into two categories: Dimers adsorbed on top of the substrate rows and dimers adsorbed in the troughs between the substrate rows. There are three different diffusion pathways for the dimers: Along the substrate rows, across the substrate rows, and in the troughs between the substrate rows. The activation barriers for diffusion of these three pathways have been determined for both Ge and Si dimers on Ge (001). The barriers for dimer diffusion of the system Ge/Ge (001) are slightly lower than for the Si/Ge (001) system. As compared to Si on Si (001) the activation barriers for dimer diffusion on Ge (001) are significantly lower.
Photoluminescence study of crosslinked reactive mesogens for organic light emitting devices
Authors: Contoret A. E. A. , Farrar S. R. , Khan S. M. , O’Neill M. , Richards G. J. , Aldred M. P. , Kelly S. M.
Published: February 2003

We study the spectroscopic properties of luminescent liquid crystals which show a glassy nematic phase at room temperature and then form polymer networks by polymerization using ultraviolet light. The reactive mesogens possess fluorene-based aromatic cores with either diene or acrylate photoreactive groups at the end of aliphatic spacers. The photoluminescence quantum efficiency is enhanced when a rigid polymer backbone is formed by crosslinking of the diene endgroups. Spectral shifts of the vibronic transitions confirm an increase in the viscosity of the matrix following photopolymerization. Continuous and time-resolved photoluminescence measurements show that the quantum efficiency is limited by exciton diffusion to traps. Either the diffusion constant or the density of traps is reduced by photopolymerization.
Thin films of double perovskite Sr2FeMoO6: Growth, optimization, and study of the physical and magnetotransport properties of films grown on single-crystalline and polycrystalline SrTiO3 substrates
Authors: Shinde S. R. , Ogale S. B. , Greene R. L. , Venkatesan T. , Tsoi Ken , Cheong S.-W. , Millis A. J.
Published: February 2003

Thin films of Sr2FeMoO6 have been deposited on single-crystalline and polycrystalline SrTiO3 substrates by pulsed laser deposition from a stoichiometric target. In order to obtain high-quality films, the deposition parameters were systematically optimized. The films grown under optimized conditions show properties comparable to those of bulk Sr2FeMoO6 single crystals. Surprisingly, polycrystalline films obtained by depositing on polycrystalline SrTiO3 substrates do not show any significant low-field magnetoresistance (MR), contrary to expectations for a spin-polarized material. However, after controlled annealing-induced alteration of the grain-boundary properties, a development of the low-field MR feature is clearly observed. The occurrence of the low-field MR is accompanied by nonlinearity in the current-voltage characteristics. These results are analyzed in the light of the prevalent understanding of the properties of this double perovskite material.
Complete suppression of filamentation and superior beam quality in quantum-dot lasers
Authors: Ribbat Ch. , Sellin R. L. , Kaiander I. , Hopfer F. , Ledentsov N. N. , Bimberg D. , Kovsh A. R. , Ustinov V. M. , Zhukov A. E. , Maximov M. V.
Published: February 2003

Comparative near-field and beam-quality (M2) measurements on narrow stripe quantum-dot (QD) and quantum-well (QW) lasers of identical structure, both emitting at 1100 nm, are presented. Intrinsic suppression of filamentation in the QD lasers is observed. QD lasers emitting at 1300 nm again show no filamentation. For a 6-μm-stripe, QW laser, M2 increases from 2.6 to 6.1 with output power increasing from 5 to 60 mW and with increasing stripe width (20 mW, 3→10 μm, M2=2.6→4.7). In the QD lasers, filamentation is suppressed up to 8 μm (1100 nm) and 9 μm (1300 nm) stripe width and no dependence on output power is observed.
Metal organic vapor phase epitaxial growth of heavily carbon-doped GaAs using a dopant source of CCl3Br and quantitative analysis of the compensation mechanism in the epilayers
Authors: Bhunia S. , Uchida K. , Nozaki S. , Sugiyama N. , Furiya M. , Morisaki H.
Published: February 2003

Heavy carbon doping of GaAs by metal organic vapor phase epitaxy has been carried out using a dopant source of carbon trichloro bromide (CCl3Br), an intersubstituted compound of the two highly efficient dopant sources of CCl4 and CBr4. Results are being reported in the doping range of 1.76×1019–1.12×1020 cm−3, achieved at growth temperatures between 570 and 600 °C and V/III ratios between 10 and 50. The compensation mechanism of the carriers in the samples and its effect on the electrical and optical properties were systematically studied using double crystal x-ray diffraction, mobility, and photoluminescence measurements. A data analysis technique has been presented to quantitatively calculate the level of compensation in the layers from conventional lattice mismatch measurements. The antisite incorporation of carbon was found to be the dominant compensation mechanism for hole concentrations above 7.36×1019 cm−3. Room temperature mobility data of the samples showed a sharp deviation from the usual Hilsum’s fitting after the carrier concentration of 7.36×1019 cm−3, due to increased self-compensation of the epilayers. The optical properties of the samples were correlated to the self-compensation effect by the photoluminescence measurements carried out in the temperature range of 10–140 K.
Quantitative microdiffraction from deformed crystals with unpaired dislocations and dislocation walls
Authors: Barabash R. I. , Ice G. E. , Walker F. J.
Published: February 2003

This article describes how unpaired dislocations alter white-beam Laue patterns for either isolated dislocations, dislocation walls, or combinations of dislocation walls and isolated dislocations. The intensity distribution of Laue diffraction is analyzed as a function of local misorientation. We show how to quantitatively determine the dislocation structure of single crystals and polycrystals with plastic deformation. The technique is applied to interpret the complicated plastic–elastic field in an iridium weld sample.
Magnetic-field effects in defect-controlled ferromagnetic Ga1−xMnxAs semiconductors
Authors: dos Santos Raimundo R. , d’Albuquerque e Castro José , Oliveira Luiz E.
Published: February 2003

We have studied the magnetic-field and concentration dependences of the magnetizations of the hole and Mn subsystems in diluted ferromagnetic semiconductor Ga1−xMnxAs. A mean-field approximation to the hole-mediated interaction is used, in which the hole concentration p(x) is parametrized in terms of a fitting (of the hole effective mass and hole/local moment coupling) to experimental data on the Tc critical temperature. The dependence of the magnetizations with x, for a given temperature, presents a sharply peaked structure, with maxima increasing with applied magnetic field, which indicates that application to diluted-magnetic-semiconductor devices would require quality control of the Mn-doping composition. We also compare various experimental data for Tc(x) and p(x) on different Ga1−xMnxAs samples and stress the need of further detailed experimental work to assure that the experimental measurements are reproducible.
Transparent thin film transistors using ZnO as an active channel layer and their electrical properties
Authors: Masuda Satoshi , Kitamura Ken , Okumura Yoshihiro , Miyatake Shigehiro , Tabata Hitoshi , Kawai Tomoji
Published: February 2003

Bottom-gate-type thin film transistors using ZnO as an active channel layer (ZnO–TFT) have been constructed. The ZnO layers were deposited using pulsed laser deposition at 450 °C at an oxygen pressure of 3 m Torr, and the material that was formed had a background carrier concentration of less than 5×1016 cm−3. A double layer gate insulator consisting of SiO2 and SiNx was effective in suppressing leakage current and enabling the ZnO–TFT to operate successfully. The Ion/Ioff ratio of ZnO–TFTs fabricated on Si wafers was more than 105 and the optical transmittance of ZnO–TFTs fabricated on glass was more than 80%. These results show that it is possible to fabricate a transparent TFT that can even be operated in the presence of visible light.
Growth condition dependence of morphology and electric properties of ZnO films on sapphire substrates prepared by molecular beam epitaxy
Authors: Ohgaki Takeshi , Ohashi Naoki , Kakemoto Hirofumi , Wada Satoshi , Adachi Yutaka , Haneda Hajime , Tsurumi Takaaki
Published: February 2003

Zinc oxide (ZnO) films were grown on sapphire (112̄0) substrates by molecular beam epitaxy under oxygen radical irradiation. The effect of the growth conditions, including the Zn/O ratio supplied to the film surface, on the electrical properties of ZnO films was studied in relation to the film morphology. We found that the growth rate strongly depended on the Zn flux from the Knudsen cell and the optimum condition for high growth rate was very narrow. The grain size in the lateral direction increased with increasing growth rate in the thickness direction. The increase in growth rate, especially in the lateral direction, resulted in the carrier mobility increasing up to 42 cm2 V−1 s−1. The carrier concentration N was sensitive to the substrate temperature, while the value of N was not sensitive to the source supplying ratio Zn/O. We discuss the decrease of the carrier concentration with increasing substrate temperature in regard to the formation of nonequilibrium defects.
Intersubband absorption linewidth in GaAs quantum wells due to scattering by interface roughness, phonons, alloy disorder, and impurities
Authors: Unuma Takeya , Yoshita Masahiro , Noda Takeshi , Sakaki Hiroyuki , Akiyama Hidefumi
Published: February 2003

We calculate the intersubband absorption linewidth 2Γop in quantum wells (QWs) due to scattering by interface roughness, LO phonons, LA phonons, alloy disorder, and ionized impurities, and compare it with the transport energy broadening 2Γtr=2ℏ/τtr, which corresponds to the transport relaxation time τtr related to the electron mobility μ. Numerical calculations for GaAs QWs clarify the different contributions of each individual scattering mechanism to the absorption linewidth 2Γop and transport broadening 2Γtr. Interface roughness scattering contributes about an order of magnitude more to the linewidth 2Γop than to the transport broadening 2Γtr, because the contribution from the intrasubband scattering in the first excited subband is much larger than that in the ground subband. On the other hand, LO phonon scattering (at room temperature) and ionized impurity scattering contribute much less to the linewidth 2Γop than to the transport broadening 2Γtr. LA phonon scattering makes comparable contributions to the linewidth 2Γop and transport broadening 2Γtr, and so does alloy disorder scattering. The combination of these contributions with significantly different characteristics makes the absolute values of the linewidth 2Γop and transport broadening 2Γtr very different, and leads to the apparent lack of correlation between them when a parameter, such as temperature or alloy composition, is changed. Our numerical calculations can quantitatively explain the previously reported experimental results.
Macromolecule formation in low density CF4 plasmas: The influence of H2
Authors: Schabel M. J. , Peterson T. W. , Muscat A. J.
Published: February 2003

High molecular weight fluorocarbon species are regarded as important contributors to the nucleation of films and particulates in fluorocarbon plasmas. The chemical reaction mechanisms by which fluorocarbon macromolecules form within a plasma are generally unknown. To elucidate these mechanisms, experiments were conducted in a rf capacitively coupled discharge in a Gaseous Electronics Conference reference cell. The relationships between macromolecule growth and plasma pressure, power, flow rate, and the fraction of H2 in the CF4 gas feed are identified. Macromolecule growth was found to increase with increased pressure and rf power, and decreased flow rate. A set of electron-induced dissociation and radical-recombination reactions are simulated using Chemkin–Aurora, a commercially available plasma chemistry model, and are in good agreement with the experimental results of macromolecule growth. We show that a primary mechanism by which fluorocarbon macromolecules form in a plasma occurs by electron-induced dissociation of a fluoroalkane to produce a fluoroalkyl radical and a fluorine atom, followed by a three-body radical–radical recombination reaction with CF3. Hydrogen is shown to have a profound effect on this reaction sequence by reducing the gas phase atomic fluorine concentration through the formation of HF which in turn increases the CF3 concentration available to participate in the macromolecule growth process. At moderate levels of hydrogen in the feed gas (<20%), macromolecule growth is directly correlated with the fraction of hydrogen in the feed gas. At high concentrations of hydrogen, hydrofluorocarbon and hydrocarbon growth occurs in the plasma at the expense of fluorocarbon macromolecule growth. The conditions under which the formation of these species occurs is consistent with observations in the literature of dramatic reductions in silicon dioxide etching rate. The transition between the formation of fluorocarbon macromolecules and hydrocarbon species in a CF4/H2 plasma is shown to be fundamental to understanding the growth process of each class of species within the plasma.
Polarization recovery of fatigued Pb(Zr,Ti)O3 thin films: Switching current studies
Authors: F. Schloss Lawrence , C. McIntyre Paul
Published: February 2003

In the process of fatiguing Ir/Pb(Zr0.35Ti0.65)O3/Pt capacitors we have observed the splitting and separation of both the positive and negative switching currents into two distinct peaks. By measuring the current response to a post-fatigue, triangular, voltage waveform, we have observed a shift of the majority of the switching current to higher voltages and, eventually, beyond the voltage testing range. At high fatigue cycle numbers, this current peak shift is large enough to reveal a smaller switching current peak whose position remains invariant. Subsequent higher amplitude switching pulses access the high coercive voltage switching peaks, returning the remanent polarization values, though not the overall switching profile, to unfatigued levels. Pulse polarization measurements reveal that the switchable polarization that appears lost at lower testing voltages is highly recoverable when higher voltages are applied. Thus, the fatigue cycling appears to primarily increase the polarization switching resistance while inducing very little irrecoverable switching loss. Finally, we measured increased resistance to switching with fatigue cycling even when the majority of the capacitor volume was no longer switching during the fatigue pulses, suggesting that only voltage cycling, and not the concomitant polarization switching, are necessary to induce fatigue.
Electron–phonon coupling in negatively charged cubane and octasilacubane
Authors: Kato Takashi , Yamabe Tokio
Published: February 2003

Electron–phonon coupling in the monoanions of cubic molecules such as cubane (CH)8 and octasilacubane (SiH)8 (cubane-series) are studied. The vibrational frequencies and the orbital vibronic coupling constants are computed and analyzed and the electron–phonon coupling constants are estimated. The results for cubane-series are compared with those for acene- (acene-series) and phenanthrene-edge-type hydrocarbons (phenanthrene-series). The T2g modes of 687 and 848 cm−1, the Eg modes of 922 and 1125 cm−1, and the A1g mode of 1021 cm−1 strongly couple to the t1u lowest unoccupied molecular orbital (LUMO) in cubane, on the other hand, only T2g mode of 197 cm−1 and the Eg mode of 403 cm−1 strongly couple to the t1u LUMO in octasilacubane. The calculated total electron–phonon coupling constant for the monoanion of cubane (0.495 eV) is much larger than that for the monoanion of octasilacubane (0.262 eV). The relationships between the HOMO–LUMO gaps and the total electron–phonon coupling constants in the monoanions of acene-, phenanthrene-, and cubane-series are investigated. The plot of the total electron–phonon coupling constants against the HOMO–LUMO gap is found to be linear and the slope of the linear plots would be approximately 0.05 for the monoanions of nanosized molecular systems. We suggest the precondition under which the monoanions of nanosized molecular crystals can exhibit high-temperature superconductivity on the basis of the hypothesis that vibronic interactions between the LUMO and intramolecular vibrations would play an essential role in the occurrence of possible superconductivity in negatively charged nanosized molecules.
Neat macroscopic membranes of aligned carbon nanotubes
Authors: Casavant M. J. , Walters D. A. , Schmidt J. J. , Smalley R. E.
Published: February 2003

Thick macroscopic membranes of magnetically aligned single-wall carbon nanotubes (SWNT) have been produced via high pressure filtration of aqueous surfactant-suspended SWNT in a magnetic field, resulting in membrane thicknesses of 10 μm and surface areas of 125 cm2. Field strengths of 7 and 25 T were used. Polarized Raman spectroscopy shows an anisotropy of (2.5±0.5), indicating good alignment. A similar degree of alignment was seen at both intensities of magnetic field. Furthermore, the degree of alignment is comparable to that achieved in previous work that produced smaller and thinner membranes at 25 T. The membranes also exhibit uniform anisotropy across their surface and throughout their thickness.
Correlation between electronic and molecular structure distortions and vibrational properties. I. Adiabatic approximations
Authors: Cho Minhaeng
Published: February 2003

Distortions of the electronic and molecular structures of a polarizable molecule in an inhomogeneous electrostatic potential were investigated by using perturbation theory. The second-order electronic energy correction term is directly related to the charge response kernel, which is a site representation of the nonlocal charge response susceptibility. Instead of using the sum-over-state expression for the charge response kernel, we discuss how a finite-field method using point charges can be used to calculate the charge response kernel. By invoking various levels of adiabatic approximations, four different coupled differential equations for the vibrational wave functions were obtained. From the effective vibrational potential functions thus obtained, the molecular structural distortion induced by the external electrostatic potential was shown to be calculable. We also present a discussion on how vibrational properties are affected by the presence of the electrostatic potential. The relationship between the vibrational frequency shift and molecular structural distortion, when a polarizable molecule is exposed to an electrostatic potential, was elucidated.
New optimization method for intermolecular potentials: Optimization of a new anisotropic united atoms potential for olefins: Prediction of equilibrium properties
Authors: Bourasseau Emeric , Haboudou Mehalia , Boutin Anne , Fuchs Alain H. , Ungerer Philippe
Published: February 2003

In this study, we propose a new global procedure to perform optimization of semiempirical intermolecular potential parameters on the basis of a large reference database. To obtain transferable parameters, we used the original method proposed by Ungerer [Ungerer et al., J. Chem. Phys. 112, 5499 (2000)], based on the minimization of a dimensionless error criterion. This method allows the simultaneous optimization of several parameters from a large set of reference data. However, the computational cost of such a method limits its application, because it implies the calculation of an important number of partial derivatives, calculated by finite differences between the results of several different simulations. In this work, we propose a new method to evaluate partial derivatives, in order to reduce the computing time and to obtain more consistent derivatives. This method is based on the analysis of statistical fluctuations during a single simulation. To predict equilibrium properties of olefins, we optimize the Lennard-Jones potential parameters of the unsaturated hydrocarbon groups using the anisotropic united atoms description. The resulting parameters are consistent with those previously determined for linear and branched alkanes. Test simulations have been performed at temperatures ranging from 150 to 510 K for several α-olefins (ethylene, propene, 1-butene, 1-pentene, 1-hexene, 1-octene), several β-olefins (trans-2-butene, cis-2-butene, trans-2-pentene), isobutene, and butadiene. Equilibrium properties are well predicted, and critical properties can be evaluated with a good accuracy, despite the fact that most of the results constitute pure predictions. It is concluded that the AUA potential, due to a relevant physical meaning, can be transferred to a large range of olefins with good success.
Strain relaxation correlated with the transport properties of AlN/GaN heterostructure grown by plasma-assisted molecular-beam epitaxy
Authors: Jeganathan Kulandaivel , Ide Toshihide , Shimizu Mitsuaki , Okumura Hajime
Published: February 2003

We report on the transport properties of two-dimensional electron gases confined at the AlN/GaN heterointerface grown by plasma-assisted molecular-beam epitaxy on (0001) sapphire substrates. Two-dimensional (2D) sheet carrier density has been found to vary with the AlN barrier layer thickness due to the existence of strain-induced piezoelectric polarization field at the heterointerface. The highest 2D sheet carrier density achieved was 2.3×1013 cm−2 for a 35-Å-thick AlN barrier layer. Further, with the increase of the AlN barrier width about 50∼60 Å, a drop in 2D sheet carrier density was noticed due to the annihilation of piezoelectric polarization, caused by the partial tensile-strain relaxation of the AlN barrier layer. Inconsistently, the 2D electron mobility was high: about 853 cm2/V s. The interface roughness of the heterostructure was estimated to be 6 Å rms for a 50-Å-thick AlN barrier layer, using grazing incidence x-ray reflectivity studies.
Local treatment of electron excitations in the EOM-CCSD method
Authors: Korona Tatiana , Werner Hans-Joachim
Published: February 2003

The Equation-of-Motion coupled cluster method restricted to single and double excitations (EOM-CCSD) and singlet excited states is formulated in a basis of nonorthogonal local orbitals. In the calculation of excited states only electron promotions from localized molecular orbitals into subspaces (excitation domains) of the local basis are allowed, which strongly reduces the number of EOM-CCSD amplitudes to be optimized. Furthermore, double excitations are neglected unless the excitation domains of the corresponding localized occupied orbitals are close to each other. Unlike in the local methods for the ground state, the excitation domains cannot be simply restricted to the atomic orbitals that are spatially close to the localized occupied orbitals. In the present paper the choice of the excitation domains is based on the analysis of wave functions computed by more approximate (and cheaper) methods like, e.g., configuration-interaction singles. The effect of various local approximations is investigated in detail, and it is found that a balanced description of the local configuration spaces describing the ground and excited states is essential to obtain accurate results. Using a single set of parameters for a given basis set, test calculations with the local EOM-CCSD method were performed for 14 molecules and 49 electronically excited states. The excitation energies computed by the local EOM-CCSD method reproduce the conventional EOM-CCSD excitation energies with an average error of 0.06 eV.
A semiclassical study of the effects of rotation on the unimolecular dissociation of HN2 and DN2
Authors: Guo Yin , Thompson Donald L.
Published: February 2003

The influence of rotation on the unimolecular dissociation of HN2 and DN2 is investigated. The semiclassical method we have been using for treating tunneling within classical trajectory simulations is employed to compute the level widths of selected states in both low and high energy regions. The rotational motion is considered by assigning different rotational energies and orientations of the angular momentum. It is found that the level widths for many of the states studied depend strongly on the magnitude and orientation of the angular momentum, and the variation is more pronounced in the low-energy tunneling regime.
An analysis of poly(lactic acid) with varying regio regularity
Authors: Kang Shuhui , Zhang Guangzhao , Aou Kaoru , Hsu Shaw Ling , Stidham Howard D. , Yang Xiaozhen
Published: February 2003

Poly(L-lactic acid) samples of varying regio regularity were studied by static and dynamic laser light scattering in conjunction with Raman spectroscopy. Light scattering results allow an estimate of the characteristic ratio, C∞. Experimental values were compared to the characteristic ratio computed from structural parameters determined from Raman spectroscopy. C∞ for poly(L-lactic acid) was determined to be 12.4, a value consistent with an inherently rigid chain. As expected, C∞ declines as regio regularity decreases. The Raman spectrum of solid amorphous samples of poly(lactic acid) of varying D content was compared with the simulated Raman spectrum calculated from an ensemble of 1000 different chains. Structural parameters, force constants and intensity parameters were transferred from similar molecules. The results reinforce the concept that poly(lactic acid) forms stiff chains. Upon introduction of D units, the copolymerization is more consistent with Markovian rather than Bernoullian statistics. Rotational isomeric states associated with the D to L junctions were also analyzed.
HfO
2
-based insulating stacks on 4H–SiC(0001)
Authors: Afanas’ev V. V. , Stesmans A. , Chen F. , Campbell S. A. , Smith R.
Published: February 2003

Depositing HfO2 layers on ultrathin thermally grown SiO2 on 4H–SiC(0001) is demonstrated to yield an insulator with good properties. The stack combines the high quality of the ultrathin SiO2/SiC interface and associated high energy barriers for electron and hole injection from SiC with the high dielectric permittivity of HfO2 (≈20). The latter allows application of high electric fields to the SiC surface (up to 3 MV/cm), while keeping the strength of the field in the insulator at a moderate level.
Magnetization reversal and magnetic anisotropies in epitaxial Fe/MgO and Fe/MgO/Fe heterostructures grown on Si(001)
Authors: Martı́nez Boubeta C. , Cebollada A. , Calleja J. F. , Contreras C. , Peiró F. , Cornet A.
Published: February 2003

Epitaxial Fe/MgO heterostructures have been grown on Si(001) by a combination of sputtering and laser ablation deposition techniques. The growth of MgO on Si(001) is mainly determined by the nature of the interface, with large lattice mismatch and the presence of an amorphous layer of unclear origin. Reflection high energy electron diffraction patterns of this MgO buffer layer are characteristic of an epitaxial, but disordered, structure. The structural quality of subsequent Fe and MgO layers continuously improves due to the better lattice match and the burial of defects. A weak uniaxial in-plane magnetic anisotropy is found superimposed on the expected cubic biaxial anisotropy. This additional anisotropy, of interfacial nature and often found in Fe/MgO and Fe/MgO/GaAs(001) systems, is less intense here due to the poorer MgO/Si interface quality compared with that of other systems. From the evolution of the anisotropy field with film thickness, magnetic anisotropy is also found to depend on the crystal quality. Kerr measurements of a Fe/MgO multilayered structure grown on Si show two different switching fields, suggesting magnetic coupling of two of the three Fe layers. Nevertheless, due to the little sensitivity to the bottom Fe film, independent switching of the three layers cannot be ruled out.
Room-temperature growth of crystalline indium tin oxide films on glass using low-energy oxygen-ion-beam assisted deposition
Authors: Liu C. , Matsutani T. , Asanuma T. , Murai K. , Kiuchi M. , Alves E. , Reis M.
Published: February 2003

One of the long-standing problems to improve the resolution of organic electroluminescence devices has been related to the fabrication of very smooth, high-quality indium tin oxide (ITO) layers at room temperature. It seems that this problem could be solved by low-energy oxygen-ion-beam assisted electron-beam evaporation of ITO bulk material in vacuum. The oxygen ions were produced in an electron cyclotron resonance source with energies varied between 50 and 1000 eV. The growth rate changes from 0.04 to 0.23 nm/s. The structural, electrical and optical properties were characterized by x-ray diffraction, Rutherford backscattering, atomic force microscopy, Hall-effect and optical transmittance measurements. Crystalline structure, which depends only on the thickness of the deposited ITO films, can be easily obtained at room temperature. A very smooth surface of only 0.6 nm roughness (root mean square), almost one order smaller than that prepared by other methods, low resistivity of 7.0×10−4 Ω cm, high carrier density of 6.1×1020 cm−3, and high optical transmittance of 85% at wavelength 550 nm (including the glass substrate) could be repeatedly achieved at room temperature.
Multichromophore excitons and resonance energy transfer: Molecular quantum electrodynamics
Authors: Jenkins Robert D. , Andrews David L.
Published: February 2003

Resonance energy transfer in multichromophore arrays, such as light-harvesting complexes and dendrimers, is well documented. The theory involved in the migration of energy to an acceptor from one excited donor, or concertedly from two of three such donors, has also been thoroughly investigated. In cases where the initial excitations form a delocalized exciton amongst the donors the corresponding theory describing transfer to an acceptor is less well-developed. By considering a model dendrimeric system we analyze the configuration and energy transfer properties of excitonic states formed by the absorption of one and two photons. Using molecular quantum electrodynamics and interaction-pair notation we quantify these effects in terms of quantum amplitudes. New insights result from our description in terms of state sequences. In particular it is possible for the first time to identify quantum interconnections between different exciton relaxation routes.
Ab initio molecular dynamics simulation of hydrogen fluoride at several thermodynamic states
Authors: Kreitmeir Markus , Bertagnolli Helmut , Mortensen Jens Jørgen , Parrinello Michele
Published: February 2003

Liquid hydrogen fluoride is a simple but interesting system for studies of the influence of hydrogen bonds on physical properties. We have performed ab initio molecular dynamics simulations of HF at several thermodynamic states, where we examine the microscopic structure of the liquid as well as its static and dynamic properties. The results obtained show good agreement with well established data, and, moreover, we were able to show significant changes within the structure depending on the system’s temperature and density.
Charge trapping in DNA due to intrinsic vibrational hot spots
Authors: Kalosakas G. , Rasmussen K. O/. , Bishop A. R.
Published: February 2003

We study temperature effects on the characteristic time for which charge carriers remain spatially confined while interacting with fluctuational openings (bubbles) of double stranded DNA. Using semiclassical molecular-dynamics simulations, we find that in the low-temperature regime this characteristic time decreases in a power-law fashion with temperature and coincides with the polaronic lifetime. However, above 50–70 K the confinement time exhibits an exponential increase with temperature. We demonstrate that this enhanced trapping is a result of intrinsic dynamical structural disorder resulting from thermal fluctuations. Specifically, nonlinearity-induced hot spots in the lattice subsystem form breathing potential barriers confining the charge for substantially longer times.
Biological calcium phosphates and Posner’s cluster
Authors: Yin Xilin , Stott Malcolm J.
Published: February 2003

A calcium phosphate amorphous to x-ray diffraction (ACP) exists in bone mineral in addition to the main bone apatite component, such as hydroxyapatite (HA). Experimental studies found that ACP has definite local atomic order and contains microcrystallites about 9.5 Å in extent rather than a random network structure. Experimental evidence indicates that Posner’s cluster (PC), Ca9(PO4)6, could be the basic component of ACP. In addition, it is present in various simulated body fluids and could be the growth unit of HA. In the transformation from ACP to HA, ACP need only dissociate into the clusters rather than undergo complete ionic solvation. Although PC could bridge the biologically important gap between ACP and HA, the form it is likely to take in body fluids is not known. In this study, we have performed ab initio density functional calculations to investigate the structure and stability of PC alone in vacuum and in the presence of H+, OH−, Na+, and Cl− ions mimicing the interaction with water and other constituents of body fluids. We find that the cluster with C1 symmetry is the most stable isomer in vacuum. The interaction of PC with sodium ions and especially with protons leads to a great increase in its stability and surprisingly, the cluster with six protons and six OH− recovers the C3 symmetry and similar atomic arrangement it has as a structural unit in HA crystal. This may be a key factor in the transformation from ACP to HA crystal.
Molecular concepts of normal and superconducting states in acenes and B, N-substituted acenes: A theoretical study
Authors: Kato Takashi , Yamabe Tokio
Published: February 2003

Electron–phonon coupling and the normal and possible superconducting states in the monoanions of B, N-substituted acenes such as B3N3H6, B5N5H8, and B7N7H10 are studied. The results for B, N-substituted acenes are compared with those for acenes. The B–N stretching modes around 1500 cm−1 and the low-frequency modes, less and more, respectively, strongly couple to the lowest unoccupied molecular orbitals (LUMO) with an increase in molecular size from B3N3H6 to B7N7H10. The relationship between the intrinsic intramolecular conductivity and the intramolecular electronic structures is investigated, and it is found that the intrinsic high conductivity needs small energy difference between the highest occupied molecular orbitals and the LUMO, the high frequency modes which play an essential role in the electron–phonon interactions, and large number of atoms. The relationship between the normal and superconducting states in acenes is compared with that in B, N-substituted acenes. Both possible superconducting transition temperatures and the intrinsic normal conductivity in acenes are estimated to be larger than those in the same size of B, N-substituted acenes. These results argue against the interesting apparent paradox in conventional superconductivity; the higher resistivity at room temperature, the more likely it is that a metal will be a superconductor when cooled. That is because the frequencies of the vibrational modes which play an important role in the electron–phonon interactions in negatively charged B, N-substituted acenes are much lower than those in negatively charged acenes because of electronegativity perturbation. In conventional superconductivity, such frequencies depend mainly on the atomic masses. However, such frequencies depend on the detailed intramolecular electronic structures as well as the molecular weights in nano-sized molecular systems.
Photothermal imaging of localized delamination between organic coatings and metallic substrates using a scanning photopyroelectric microscope
Authors: van den Brand J. , Chirtoc M. , Wübbenhorst M. R. , de Wit J. H. W.
Published: February 2003

We introduce the scanning photopyroelectric microscope and demonstrate its application for the investigation of delamination sites below organic coatings applied onto metallic substrates. The technique is based on a photothermal method, i.e., the photopyroelectric one, and uses a flexible polyvinylidene-difluoride sensor in contact with the sample surface. It is suitable for studying with high spatial resolution subsurface features like air gaps in multilayer systems and determining their depth below the surface. Using a one-dimensional approach, we derived an analytical expression for the signal contrast arising from subsurface thermal barriers. We also developed a three-dimensional finite element model that allows describing the thermal response of more complex systems and the lateral resolution in the actual measurement. The theoretical models are validated using both, model and real-life samples, and we show that it is possible to detect air gaps with thickness down to 1 μm below organic coatings applied onto metallic substrates.
Shear force detection by using bimorph cantilever with the enhanced Q factor
Authors: Lei F. H. , Nicolas J.-L. , Troyon M. , Sockalingum G. D. , Rubin S. , Manfait M.
Published: February 2003

An improved nonoptical shear force detection system based on a rectangular bimorph cantilever incorporating the force feedback technique has been developed for tip–sample distance regulation in shear force microscopy. The force feedback amplifier consisting of a phase shifter and a linear amplifier is adjusted in such a way that the motion of the cantilever is mechanically amplified, resulting in a great enhancement of quality factor Q. Driving a fiber attached bimorph cantilever at its first harmonic resonance, with a phase shift φ=π/2 and an appropriate amplifier gain, allows one to obtain a Q factor greater than 103 in air, which corresponds to a Q enhancement of more than 1 order of magnitude. The effect of Q enhancement leads to an increase in the signal to noise ratio and thus the force detection sensitivity of the system. Typically, the minimum interaction force that can be sensed by the system is about 2 pN/√Hz. Topographic images of a human aorta tissue section in its natural state, taken with the described shear force detection system, show that it is very sensitive, reliable, and particularly suitable for biological applications.
Optical emission spectroscopy and time-of-flight investigations of plasmas generated from AlN targets in cases of pulsed laser deposition with sub-ps and ns ultraviolet laser pulses
Authors: Ristoscu Carmen , Mihailescu Ion N. , Velegrakis Michalis , Massaouti Maria , Klini Argyro , Fotakis Costas
Published: February 2003

We performed a comparative study of the plasma generated from AlN targets under sub-ps vs ns UV (λ=248 nm) excimer laser pulses. Optical emission and time-of-flight spectra recorded in cases of samples irradiated with ns laser pulses showed the presence of Al lines, which became prevalent after the first laser pulse was incident on the target. These observations are congruent with the metallization of AlN targets inside each crater under multipulse ns laser action at laser fluences above the ablation threshold, observed by visual inspection and optical microscopy. Metallization was not observed when working with sub-ps laser pulses. Moreover, our studies confirmed the predominant presence of AlN positive molecular ions in the plasma generated in front of AlN targets submitted to sub-ps multipulse laser irradiation. The optical emission data are in good agreement with time-of-flight mass analysis. We emphasize that all investigations support the experimental evidence reported by György et al. [E. György et al., J. Appl. Phys. 90, 456 (2001)], according to which thin films obtained by pulsed laser deposition with ns laser pulses contain a significant amount of metallic Al, while only AlN is detected in films obtained with sub-ps laser pulses. Measurements of the velocity and kinetic energy distributions of AlN+ indicate that in the case of ns-laser ablation the ions are emitted with thermal energy, while in the case of sub-ps-laser ablation a bimodal distribution exists and has thermal (1 eV) and hyperthermal (10 eV) energy components. This points to different plasma formation mechanisms for the two cases.
Erbium-related band gap states in 4H– and 6H–silicon carbide
Authors: Pasold G. , Albrecht F. , Grillenberger J. , Grossner U. , Hülsen C. , Witthuhn W. , Sielemann R.
Published: February 2003

The band gap states of erbium (Er) in 4H– and 6H–silicon carbide (SiC) were investigated by means of deep level transient spectroscopy (DLTS). The samples were doped with Er by ion implantation followed by thermal annealing procedures. The DLTS measurements with the stable Er167 isotope reveal donorlike levels located at ET=EV+0.75(2) eV in 4H–SiC and ET=EV+0.78(2) eV in 6H–SiC. A definite chemical correlation of these bandgap states to Er was obtained by employing the radioactive isotope Er160 as a tracer in 4H–SiC.
Free-electron maser driven by a two-stage ferroelectric electron gun
Authors: Einat M. , Jerby E. , Rosenman G.
Published: February 2003

A two-stage ferroelectric electron gun is employed in a free-electron maser (FEM) oscillator experiment. This gun produces a pulsed electron beam of a 5–15 keV energy, ∼0.5 A current, and ∼3% energy spread. The FEM output microwave pulse train is coupled out with a 66 W peak power. The microwave frequency is tunable in the range of 2.9–3.3 GHz by varying the electron beam energy. The interaction mechanism is identified by a comparison to the known FEM tuning relation. The energy spread of the two-stage ferroelectric electron gun satisfies the FEM acceptance parameter in the microwave regime.
Using self-consistent fields to bias Monte Carlo methods with applications to designing and sampling protein sequences
Authors: Zou Jinming , Saven Jeffery G.
Published: February 2003

For complex multidimensional systems, Monte Carlo methods are useful for sampling probable regions of a configuration space and, in the context of annealing, for determining “low energy” or “high scoring” configurations. Such methods have been used in protein design as means to identify amino acid sequences that are energetically compatible with a particular backbone structure. As with many other applications of Monte Carlo methods, such searches can be inefficient if trial configurations (protein sequences) in the Markov chain are chosen randomly. Here a mean-field biased Monte Carlo method (MFBMC) is presented and applied to designing and sampling protein sequences. The MFBMC method uses predetermined sequence identity probabilities wi(α) to bias the sequence selection. The wi(α) are calculated using a self-consistent, mean-field theory that can estimate the number and composition of sequences having predetermined values of energetically related foldability criteria. The MFBMC method is applied to both a simple protein model, the 27-mer lattice model, and an all-atom protein model. Compared to conventional Monte Carlo (MC) and configurational bias Monte Carlo (BMC), the MFBMC method converges faster to low energy sequences and samples such sequences more efficiently. The MFBMC method also tolerates faster cooling rates than the MC and BMC methods. The MFBMC method can be applied not only to protein sequence search, but also to a wide variety of polymeric and condensed phase systems.
Resonant cavity enhancement in heterojunction GaAs/AlGaAs terahertz detectors
Authors: Esaev D. G. , Matsik S. G. , Rinzan M. B. M. , Perera A. G. U. , Liu H. C. , Buchanan M.
Published: February 2003

The room-temperature absorption and reflection spectra in the range of 5–100 μm (3–60 THz) for multilayer heterojunction interfacial work function internal photoemission (HEIWIP) GaAs/AlGaAs far-infrared (FIR) detectors are presented. Calculated results based on the free carrier absorption and interaction with optical phonons are found to be in good agreement with the experimental results. Experimental responsivity spectra demonstrate the expected maxima from the absorption measurements due to resonant cavity effects. It is shown that the resonance cavity architecture enhances the performance of the FIR HEIWIP detectors and further improvement is proposed through the use of n++ and p++ bottom contact layers or doped substrates.
Simulations of single grafted polyelectrolyte chains: ssDNA and dsDNA
Authors: Crozier Paul S. , Stevens Mark J.
Published: February 2003

The structure of a single, grafted polyelectrolyte, DNA, is investigated by molecular dynamics simulations. The polyelectrolyte is treated as a bead–spring model with explicit charges using parametrizations of both flexible (ssDNA) and stiff (dsDNA) polyelectrolytes. In this single chain limit with no added salt, the flexible ssDNA is always highly extended. Counterion condensation on both molecules is found to be chain length dependent. The counterion distribution is not localized to the chain volume as in related polyelectrolyte brush states. Even at large chain lengths, where the majority of counterions are condensed, a significant fraction of counterions reside far from the chain. The distributions of positions of the nongrafted end monomer for ssDNA and dsDNA differ significantly, indicating a possibility for distinguishing the two states in DNA array technologies.