bluuebunny/crossref_metadata_2025 huggingface dataset:
The intermolecular potential energy surface of the He⋅NO+ cationic complex
Authors: Soldán Pavel , Lee Edmond P. F. , Wright Timothy G.
Published: July 2002

Close-coupling calculations of bound rotational and vibrational states are carried out on a new intermolecular potential energy function based on 200 energies of the He⋅NO+ cationic complex calculated at the coupled-cluster single double (triple)/aug-cc-pV5Z ab initio level of theory at a range of geometries and point-by-point corrected for basis set superposition error. The potential energy function is constructed by combining the reciprocal power reproducing kernel Hilbert space interpolation with Gauss–Legendre quadrature. The best estimate of the intermolecular dissociation energy, De, is 198±4 cm−1, obtained by extrapolations to the complete basis set limit, and calculating estimates for relativistic effects and core and core-valence correlation effects.
Abstraction and exchange mechanisms for the D2+NH3+ reaction at hyperthermal collision energies
Authors: Coutinho-Neto Maurı́cio , Deumens Erik , Öhrn Yngve
Published: July 2002

Minimal Electron Nuclear Dynamics theory is applied to D2+NH3+ reaction at collision energies from 6 to 16 eV in the center-of-mass frame. This method for direct nonadiabatic dynamics describes the electrons with a family of complex determinantal wave functions in terms of nonorthogonal spin orbitals and treats the nuclei as classical particles. There are no geometrical constraints imposed on this six-atom system. Emphasis is put on the details of the abstraction and exchange reaction mechanisms for ground-state reactants. Comparisons are made to recent molecular-beam experiments.
Local electric-field-driven repoling reflected in the ferroelectric polarization of Ce-doped Sr0.61Ba0.39Nb2O6
Authors: Granzow T. , Dörfler U. , Woike Th. , Wöhlecke M. , Pankrath R. , Imlau M. , Kleemann W.
Published: July 2002

We present pyroelectric measurements with the relaxor-ferroelectric strontium barium niobate in the phase transition regime. It is demonstrated that domains poled at high temperatures are more stable than those oriented at room temperature. This yields a higher phase-transition temperature when heating the sample and a strong repoling during the cooling process. We explain this difference within the framework of the random field model for relaxor ferroelectrics.
Electronic polarizability, optical basicity, and interaction parameter of La2O3 and related glasses
Authors: Honma T. , Benino Y. , Fujiwara T. , Komatsu T. , Sato R. , Dimitrov V.
Published: July 2002

The electronic polarizability and optical basicity of La2O3 and related glasses have been determined from ultraviolet absorption spectra and calculations based on the Lorentz–Lorenz equation. The optical basicity for La2O3 oxide is found to be 1.07, being much larger compared with typical glass-forming oxides such as B2O3 (0.42) and SiO2 (0.48) but being similar to heavy element oxides such as TeO2 (0.93). The Yamashita and Kurosawa’s interaction parameter of La2O3 is 0.03 Å−3, indicating that La2O3 is classified as a normal ionic (basic) oxide, i.e., an ionic bonding character in the La3+–O bond is proposed. Close correlations are confirmed among optical basicity, interaction parameter, and oxygen 1s binding energy in x-ray photoelectron (XPS) spectra for La2O3–P2O5 and other La2O3-containing glasses. It is found from XPS and Raman spectra that La3+ ions in La2O3–P2O5 glasses act as network modifiers, supporting an ionic bonding character in the La3+–O bond. The parameters related to electronic polarizability in La2O3 determined in the present study would be useful for the design of rare-earth containing optical functional glasses.
Electron plasmas in a “partial” torus
Authors: Stoneking M. R. , Fontana P. W. , Sampson R. L. , Thuecks D. J.
Published: July 2002

Electron plasmas are trapped in a novel “partially” toroidal (or “C”-shaped) trap designed to study issues of equilibrium, stability, and confinement of toroidal nonneutral plasmas. Plasmas with densities as high as 3.3×106 cm−3 are trapped and decay on a 100 μs timescale in a 196 G magnetic field. Successful trapping of dense electron plasmas requires the application of a strong horizontal electric field (5–10 V/cm). The confinement time scales as the 32 power of the magnetic field. Oscillations in the image charge current to a grounded probe have a frequency that is proportional to 1/B and roughly proportional to the vacuum horizontal electric field. However, the frequency is independent of the amount of charge in the trap. Some evidence points toward the ion resonance instability as the driving mechanism for this new toroidal electron mode.
Model of the magnetic properties of FePt granular media
Authors: Chantrell R. W. , Weller D. , Klemmer T. J. , Sun S. , Fullerton E. E.
Published: July 2002

We have used a Monte Carlo model of an interacting fine particle system to study the magnetic properties of FePt nanoparticle arrays. The model is capable of calculating the temperature dependence of the hysteresis loop and dc demagnetization remanence curves. We have made a detailed fit of the theory to experimental data over the temperature range 10–350 K. The value of the best-fit anisotropy is K=2.35×107 erg/cc. This value is around one third the value expected for bulk FePt, possibly due to chemical disorder. The determination of K by application of Sharrock’s law to the temperature dependence of Hc gives a value of 7×107 erg/cc. It is shown that this arises from a breakdown of Sharrock’s law for systems containing a significant fraction of superparamagnetic particles.
One-color multiphoton threshold photoelectron spectra of methyl bromide, and their comparison with methyl iodide
Authors: Urban Bernhard , Bondybey Vladimir E.
Published: July 2002

Threshold photoelectron spectra (ZEKE) of methyl iodide, bromide, and chloride are investigated using a one-color, two-photon ionization. Resonances due to a dissociative intermediate state are responsible for differences between spectra of the three halides, with long progressions in the C–I stretching vibration being observed in CH3I, where the one photon absorption is near the maximum of the dissociative continuum, while the vibrational structure is much less prominent in the lighter halides, where this does not occur. The spectra provide accurate ionization potentials of the three compounds as well as insights into their dissociation dynamics, and extensive information about the vibrational structure of their cations.
Metal-encapsulated icosahedral superatoms of germanium and tin with large gaps: Zn@Ge12 and Cd@Sn12
Authors: Kumar Vijay , Kawazoe Yoshiyuki
Published: July 2002

Metal (M)-encapsulated clusters of Ge and Sn, Zn@Ge12 and Cd@Sn12, are obtained from total energy calculations using ab initio pseudopotential plane wave method and generalized gradient approximation for the exchange-correlation energy. These have perfect icosahedral symmetry and large highest occupied–lowest unoccupied molecular orbital gap of about 2 eV. It lies in the optical region and makes these species attractive for cluster assembled optoelectronic materials. Calculations on silicon clusters doped with Be show a different behavior.
Epitaxial growth and characterization of Fe3O4 on Ru underlayer
Authors: Aoshima Ken-ichi , Wang Shan X.
Published: July 2002

The magnetic and transport properties of magnetite (Fe3O4) thin films grown epitaxially with a Ru underlayer on single crystal MgO (110) have been investigated. Epitaxial growth has been confirmed by x-ray diffraction (XRD). It has been shown that addition of a Ru underlayer affects in-plane magnetic anisotropy: Fe3O4 grown directly on MgO exhibits an in-plane anisotropy, while Fe3O4 grown on MgO/Ru does not. XRD results show that presence of Ru underlayer reduces tensile stress in the Fe3O4 film. This difference in the stress explains the difference in the anisotropy of the Fe3O4 films. Resistivity of the Fe3O4 films with and without Ru underlayer both increases at low temperatures, but no clear Verway transition at 120 K was observed.
The effect of imperfections on the emergence of three-dimensionality in stationary vortex breakdown bubbles
Authors: Ventikos Yiannis
Published: July 2002

We conduct detailed simulations in order to investigate the structure of steady vortex breakdown bubbles. Careful examination of all experimental visualizations reveals a number of ubiquitous features, such as the existence of dye inside the bubble and asymmetries in its downstream end. We show numerically that such features can be attributed to imperfections in the setup of the experimental apparati, imperfections that may be minimized with careful manufacturing and setting up of the rig, but are never completely eliminated. We study such forced breaking of symmetry for imperfections of an order of magnitude typical to those of realistic experimental setups.
A nanofabrication scheme for InAs/AlSb heterostructures
Authors: Yang M. J. , Cheng K. A. , Yang C. H. , Culbertson J. C.
Published: July 2002

We report a technique for nanofabrication in the InAs/GaSb/AlSb 6.1 Å material system that utilizes the large difference in the surface Fermi level pinning position for InAs [Efs(InAs)] compared with that for AlSb. An InAs/AlSb single quantum well is capped with a 3 nm, intentionally p-doped InAs layer. As a result of its construction and a relatively low Efs(InAs) there are no free carriers in the InAs/AlSb single quantum well making the quantum well insulating as-grown. Simply by selectively removing the thin p-doped InAs cap layer with a wet etch, the surface Fermi level becomes pinned on AlSb and shifted upward by half an electron volt. This results in a drastic change in band bending and creates a conducting electron channel in the buried InAs quantum well. We demonstrate with experiment and the support of a self-consistent band bending calculation that this scheme is highly effective for nanofabrication.
Collisional dynamics of Bi2 A(0u+). II. State-to-state rotational energy transfer
Authors: Franklin Robert E. , Perram Glen P.
Published: July 2002

Rotational-to-translational (R–T) energy transfer within v′=1 of the A(0u+) state of Bi2 has been investigated using spectrally resolved, laser induced fluorescence techniques. Spectrally resolved emissions from collisionally populated rotational levels of Bi2(A,v′=1) were observed for helium, neon, and argon collision partners after laser excitation of the high rotational levels J′=171, 201, and 231. Total rotational removal rates from the initially prepared state range from 2.8–8.9×10−10 cm3/molecule s. Collisional population of rotational states with |ΔJ|⩽56 was observed at pressures of 0.09–1.4 Torr. The state-to-state rates are adequately modeled by the energy based statistical power gap law.
Crystal growth and interface relaxation rates from fluctuations in an equilibrium simulation of the Lennard-Jones (100) crystal-melt system
Authors: Tepper H. L. , Briels W. J.
Published: July 2002

The kinetic coefficient of crystallization is calculated according to a previously introduced equilibrium method [Phys. Rev. Lett. 79, 5074 (1997)]. The existence of two regimes of interface relaxation and macroscopic growth, such as they were found in previous nonequilibrium simulations, is fully confirmed by the results of the equilibrium method. Special attention is given to the relation between pressure fluctuations and fluctuations of the amount of crystalline material. Furthermore, we investigate the density and order parameter profiles of the interface and make a clear distinction between the instantaneous structure and the time-averaged profile which is usually presented.
Magnetic properties of Nd–Fe binary alloys produced by a metallic mold casting method
Authors: Saito Tetsuji , Ozawa Shunpei , Motegi Tetsuichi
Published: July 2002

Nd–Fe binary alloys with the composition of Fe–23 at. % Nd (∼Nd5Fe17) and Fe–80 at. % Nd (the eutectic composition) were produced by the metallic mold casting method. The microstructures and magnetic properties of the resultant cylindrical specimens with 1–5 mm in diameter and 50 mm in length were investigated. The cylindrical specimens of Fe–23 at. % Nd alloy consisted of Nd and Nd2Fe17 phases together with a small amount of Nd5Fe17 phase. The coercivity of the cylindrical specimens was as low as the original alloy ingot. On the other hand, the cylindrical specimens of Fe–80 at. % Nd alloy consisted mainly of Nd and Nd5Fe17 phases and showed a high coercivity over 5 kOe. It was found that the coercivity was dependent on the grain size of the cylindrical specimens produced by the metallic mold casting method.
Structures, spectra, and electronic properties of halide-water pentamers and hexamers, X−(H2O)5,6 (X=F,Cl,Br,I): Ab initio study
Authors: Lee Han Myoung , Kim Dongwook , Kim Kwang S.
Published: July 2002

Various structures of halide-water pentamers and hexamers have been investigated using extensive ab initio calculations. Then, we compare the structures, spectra, and electronic properties of the hydrated fluoride, chloride, bromide, and iodide systems. Although some of the fluoride and iodide systems have been investigated earlier, we have carried out more accurate calculations on an enlarged conformational ensemble. The chloride-, bromide-, and iodide-water pentamers and hexamers behave somewhat similarly, but differently from the fluoride-water pentamer and hexamer. Fluoride-water clusters show semisurface (or semi-internal) structures, while chloride-, bromide-, and iodide-water clusters show surface structures. We substantiate our findings by evaluating various electronic properties such as ionization potentials, natural bond orbital charges, dipole moments, and charge-transfer-to-solvent energies, as well as vibrational frequencies of the low-energy halide-water pentamers and hexamers.
Optical absorption measurements of silica containing Si nanocrystals produced by ion implantation and thermal annealing
Authors: Elliman R. G. , Lederer M. J. , Luther-Davies B.
Published: July 2002

Optical absorption spectra from silicon-implanted silica slides are shown to contain features due to optical interference. These features, which result from the modified refractive index profile produced by the implant, can readily lead to misinterpretation of absorption spectra. To demonstrate the importance of such effects, silica samples were implanted with 80, 400, and 600 keV Si ions to fluences in the range 0.6–3.0×1017 Si.cm−2 and annealed at 1100 °C for 1 h to form Si nanocrystals. Optical absorption/transmittance spectra from these samples show considerable structure that is characteristic of the particular implant conditions. This structure is shown to correlate with the transmittance of the samples as calculated from the modified refractive index profile for each implant. The lack of such structure in absorption spectra measured by photodeflection spectrometry is used to confirm this interpretation.
Overcritical Meissner current densities in YBa2Cu3O7 films in soft magnetic environments
Authors: Jarzina H. , Jooss Ch. , Freyhardt H. C.
Published: July 2002

Magnetic flux density and current density distributions of YBa2Cu3O7 films in soft magnetic environments are investigated by magneto-optics. Next to the magnets flux penetration is strongly modified. Current density distributions obtained by an inversion of the law of Biot–Savart show overcritical current densities in the current domains next to the magnets.
Vibration-rotation spectra of HCl in rare-gas liquid mixtures: Molecular dynamics simulations of Q-branch absorption
Authors: Medina A. , Roco J. M. M. , Calvo Hernández A. , Velasco S. , Bulanin M. O. , Herrebout W. A. , van der Veken B. J.
Published: July 2002

New experimental results are presented on the fundamental IR band shape of HCl dissolved in neat liquid Ar and Ar doped with Kr and Xe. A strong enhancement of the absorption in the range of a central Q-branch is observed in the spectra of doped solutions. Semiclassical molecular dynamics simulations of the spectral band profile are carried out using (12-6) Lennard-Jones site–site interaction potentials. The parameters of these model potentials were deduced by fitting the available anisotropic interaction surfaces, accurately describing the structure of binary rare-gas-HCl van der Waals complexes. Simulations realistically reproduce the observed triplet band structure and its evolution with changing thermodynamic conditions. The analysis of the influence of anisotropic interactions on the orientational dynamics of solutes and orientation-dependent radial distribution functions reveals the mechanisms that contribute to appearance of the Q-branches. It is shown that long-living solute-solvent spatial correlations present in liquid solutions retain to some extent the properties of van der Waals complexes.
D
1
(
2
B
2g
)→D
(
2
A
u
)
fluorescence from the matrix-isolated perylene cation following laser excitation into the D5(2B3g) and D2(2B3g) electronic states
Authors: Chillier Xavier D. F. , Stone Bradley M. , Joblin Christine , Salama Farid , Allamandola Louis J.
Published: July 2002

Fluorescence spectra of the perylene cation, isolated in an argon matrix and pumped by direct laser excitation via the D2(2B3g)←D0(2Au) and D5(2B3g)←D0(2Au) transitions, are presented. Direct excitation into the D5 or D2 states is followed by rapid nonradiative relaxation to D1 that, in turn, relaxes radiatively. Excitation spectroscopy across the D2(2B3g)←D0(2Au) transition near 730 nm shows that site splitting plays little or no role in determining the spectral substructure in the ion spectra. Tentative assignments for ground state vibrational frequencies are made by a comparison of spectral intervals with calculated normal mode frequencies, with the strongest IR bands leading to the most intense vibronic bands.
Low-shear viscosity of nondilute polymer solutions from a generalized Kirkwood–Riseman model
Authors: Phillies George D. J.
Published: July 2002

The low shear viscosity η of a nondilute polymer solution is calculated from an extended Kirkwood–Riseman model that includes hydrodynamic interactions between chains. Hydrodynamic interaction tensors for chains in a weak shear field are obtained for interacting pairs and trios of chains. The model proposed here gives a general expansion for η(c) to arbitrary order in polymer concentration c. Divergent integrals encountered in some prior calculations using Stokes-level hydrodynamics are avoided. The model confirms empirical demonstrations that the intrinsic viscosity [η] is a good reducing variable for η(c).
Onset of magnetic anisotropy in epitaxial Fe films on GaAs(001)
Authors: Bensch F. , Moosbühler R. , Bayreuther G.
Published: July 2002

It was shown previously that ultrathin Fe films epitaxially grown on GaAs(001) exhibit a strong in-plane uniaxial magnetic anisotropy which turns out to be a pure interface contribution with an anisotropy constant KUS, expressed as an energy per unit area, which is constant in a wide thickness range. However, for films thinner than ∼10 monolayers (ML), KUS decreases with decreasing thickness when measured at 300 K. In order to eliminate effects of thermal excitations, Fe(001) films grown on GaAs(001) by molecular beam epitaxy were investigated by superconducting quantum interference device magnetometry at low temperature. The extrapolated room temperature values and the ground state data both indicate that KUS vanishes at t=2.5 ML. This is the thickness at which the onset of ferromagnetism takes place, i.e., where the Curie temperature TC becomes nonzero. This might be interpreted as a verification of the Mermin–Wagner theorem in the sense that long-range ferromagnetic order is stabilized by the magnetic anisotropy. It is discussed whether the onset of ferromagnetism is indeed triggered by the appearance of magnetic anisotropy or if there is a common origin of both phenomena. Finally, it is found that the uniaxial anisotropy does not vanish at TC, but persists up to temperatures of ∼1.5 TC. This means that KUS does not scale with a certain power of the spontaneous magnetization. The disparity between the persistence of the anisotropy above TC and its disappearance below the critical thickness of 2.5 ML is discussed.
Magnetic and structural properties for quarternary (Fe–Co–Ni)50Pt50 alloy thin films
Authors: Kanazawa Hiroshi , Suzuki Takao
Published: July 2002

The intrinsic perpendicular magnetic anisotropy constant Ku for (Fe–Co–Ni)50Pt50 quarternary alloy thin films as a function of structure and composition is discussed. The estimated Ku values obtained by the extrapolation to S=1 using the dependence of Ku on S increase for Fe content beyond 50 at. % in (Fe–Co)50Pt50 and (Fe–Ni)50Pt50 alloy thin films. It is found for both cases that Ku decreases with increasing the total number of valence electrons. The magnetic anisotropy is determined to the first approximation by the Fermi energy, which changes with the number of valence electrons, but not by the band structure itself.
Er-related luminescence in Er,O-codoped InGaAs/GaAs multiple-quantum-well structures grown by organometallic vapor phase epitaxy
Authors: Koizumi A. , Moriya H. , Watanabe N. , Nonogaki Y. , Fujiwara Y. , Takeda Y.
Published: July 2002

We have grown Er,O-codoped InGaAs/GaAs multiple-quantum-well (MQW:Er,O) structures by low-pressure organometallic vapor phase epitaxy (OMVPE), and investigated their luminescence properties. The MQW structures are designed to emit light at 0.98 μm for direct excitation of Er ions. Degradation of the structures due to introduction of Er and oxygen is not observed in x-ray diffraction patterns. Er ions doped with oxygen exhibit a sharp and well-ordered photoluminescence spectrum predominantly from one kind of Er center (Er-2O center). Photoluminescence excitation measurements on MQW:Er,O and Er,O-codoped GaAs samples reveal that under below-GaAs-band-gap excitation (830–940 nm), the Er-related luminescence is observed only in the MQW:Er,O sample. It indicates that the luminescence originates from Er ions in InGaAs QWs by means of trap-mediated excitation process.
High-mobility InSb epitaxial films grown on a GaAs (001) substrate using liquid-phase epitaxy
Authors: Dixit V. K. , Bansal Bhavtosh , Venkataraman V. , Bhat H. L. , Subbanna G. N. , Chandrasekharan K. S. , Arora B. M.
Published: July 2002

The growth of epitaxial InSb layers on highly lattice-mismatched semi-insulating GaAs substrates has been achieved via traditional liquid-phase epitaxy. Scanning and transmission electron microscopy show sharp interfaces even at 35 nm resolution. High-resolution x-ray diffraction studies reveal reflections even up to 2θ=153° with distinct layer and substrate peaks, indicating structural coherence. The films grown were n type and the highest electron mobility obtained was 3.96×104 cm2/V s at room temperature. The band gap varies from 0.17 to 0.23 eV in the temperature range of 300–10 K and is consistent with the expected variation. These results indicate that the films grown are comparable to those grown by other sophisticated techniques in terms of structural, optical and electrical properties.
Application of hollow channel between sustain electrodes to improve discharge characteristics in alternating current plasma display panels
Authors: Son Seung-Hyun , Park Yong-Suk , Bae Seong-Chan , Choi Sie-Young
Published: July 2002

A front-panel structure with a hollow channel between sustain electrodes for an alternating current plasma display panel is proposed to achieve a low breakdown voltage and short discharge time lag due to the strong electric field in the hollow channel. When compared with a conventional structure, the structure with a hollow channel produced a 15% firing voltage improvement, 8% minimum sustaining voltage improvement at a neon gas pressure of 150 Torr, and 8% discharge time lag improvement at a sustaining voltage of 270 V.
Spin polarization contrast observed in GaAs by force-detected nuclear magnetic resonance
Authors: Thurber Kent R. , Harrell Lee E. , Fainchtein Raúl , Smith Doran D.
Published: July 2002

We applied the technique of force-detected nuclear magnetic resonance to observe Ga71, Ga69, and As75 in GaAs. The nuclear spin-lattice relaxation time is 21±5 min for Ga69 at ∼5 K and 4.6 T. We have exploited this long relaxation time to first create and then observe spatially varying nuclear spin polarization within the sample, demonstrating a form of contrast for magnetic resonance force microscopy. Such nuclear spin contrast could be used to indirectly image electron spin polarization in GaAs-based spintronic devices.
Acousto-optic diffraction of blue and red light in GaN
Authors: Ciplys D. , Rimeika R. , Shur M. S. , Gaska R. , Deng J. , Yang J. W. , Khan M. A.
Published: July 2002

The diffraction of guided optical waves on surface acoustic waves in a GaN layer grown by metalorganic chemical-vapor deposition on a (0001) sapphire substrate has been experimentally studied. The measurements have been performed at optical wavelengths 442 and 633 nm for the acoustic wavelength of 16 μm. The acousto-optic diffraction regime was close to the pure Bragg diffraction regime. The advantage of considerably lower acoustic power required at the shorter optical wavelength for diffraction is demonstrated. Our results show the potential of GaN-based structures for the development of blue acousto-optical devices.
Motion of droplets along thin fibers with temperature gradient
Authors: Yarin Alexander L. , Liu Wenxia , Reneker Darrell H.
Published: July 2002

Liquid n-decane, n-undecane, n-dodecane, and n-hexadecane formed tiny symmetrical droplets on a partially wettable cylindrical fiber. When a temperature gradient was created along the fiber, the droplets began to move along the fiber toward the cold region. An explanation of the phenomenon is related to the thermocapillary motion. Other possible mechanisms were ruled out. The theoretical results and experimental data agree reasonably well.
Relaxation dynamics of deuterated formyl and isoformyl cations
Authors: Wester R. , Hechtfischer U. , Knoll L. , Lange M. , Levin J. , Scheffel M. , Schwalm D. , Wolf A. , Baer A. , Vager Z. , Zajfman D. , Mladenović M. , Schmatz S.
Published: July 2002

Vibrational relaxation and isomerization of internally excited deuterated formyl and isoformyl cations has been investigated on the time scale of 2 ms to 12 s using the nearly interaction-free environment of an ion storage ring. De-excitation of the v2 bending modes of DCO+ and DOC+ due to spontaneous radiative transitions was observed as a function of the storage time by measuring their foil-induced Coulomb explosion using three-dimensional coincident fragment imaging. No isomerization of low-lying vibrational levels of DOC+ ions was observed on the time scales considered. By comparing the Coulomb explosion data to molecular bond angle distributions obtained from vibrational wave function calculations, the time evolution of the mean v2 population is deduced for both isomers. The stored DOC+ ions are found to thermalize with the 300 K black-body radiation, while relaxation of the DCO+ bending vibrations was found to require considerably longer times, in agreement with a predicted very small transition moment of the v2=1 level.
Four-center energy transfer and interaction pairs: Molecular quantum electrodynamics
Authors: Jenkins Robert D. , Andrews David L.
Published: July 2002

In the photophysics of complex macromolecules, resonance energy transfer is the key mechanism for the migration of electronic excitation. As the ability to engineer specific architectures for such molecules improves, environments for new forms of energy migration are being envisioned. Set in this context, one of our aims in this paper is to expound a new, general way of representing complex energy transfer systems, to obviate semantic and conceptual problems in addressing multicenter interactions. The theory of four-center energy transfer is developed within this framework, through the application of molecular quantum electrodynamics. A variety of mechanisms is identified by which four-center energy transfer may proceed, and a recently developed diagrammatic technique is employed to calculate relevant quantum amplitudes. Symmetry considerations are addressed, and key features of the ensuing rate equations are discussed with regard to their potential exploitation in novel photoactive devices.
The importance of the diabatic channels in the chemi-ionization reaction O(3P)+CH(a 4Σ−)→HCO+(X 1Σ+)+e−
Authors: Metropoulos Aristophanes
Published: July 2002

We have investigated the importance of the diabatic channels in the chemi-ionization reaction O(3P)+CH(a 4Σ−)→HCO+(X 1Σ+)+e−. These channels arise due to an avoided crossing along the C–O coordinate at ∼4.7 bohr, between the 2 2Π and 1 2Π adiabatic HCO states as well as between the states that correlate to these two upon bending (4&3 2A′ and 4&3 2A″). We have estimated the nonadiabatic transition probability between these states to be of the order of ∼90%. This indicates that the diabatic channels are indeed very important.
Band gaps and lattice parameters of 0.9 μm thick InxGa1−xN films for 0⩽x⩽0.140
Authors: Beach J. D. , Al-Thani Hamda , McCray S. , Collins R. T. , Turner J. A.
Published: July 2002

The c0 lattice parameter, band gap, and photoluminescence spectra of n-type 0.9 μm thick InxGa1−xN films with x=0, 0.045, 0.085, and 0.140 were examined. The c0 lattice parameter followed Vegard’s law using c0=0.5185 nm for GaN and c0=0.569 nm for InN. Band gap measurements by photocurrent spectroscopy fit well with data published by one other research group, with the combined set being described by the equation Eg=3.41−7.31x+14.99x2 for 0⩽x⩽0.15. Photoluminescence measurements with a pulsed nitrogen laser showed a peak well below the measured band gap, as well as significant luminescence above the measured band gap. The above-gap luminescence appears to be due to band filling by the high intensity laser pulses.
Photoinduced reorientation and thermal effects in an oligothiophene-doped liquid crystal system
Authors: Zhang Haichao , Shiino Shigeaki , Kanazawa Akihiko , Tsutsumi Osamu , Shiono Takeshi , Ikeda Tomiki
Published: July 2002

Photoinduced reorientation (photon-mode process) and thermal effects in an oligothiophene-doped liquid crystal (LC) system were investigated by self-diffraction measurements in different experimental configurations. The results obtained in homeotropic sample cells were compared with those obtained in homogeneous cells. Optical Freedericksz transition could be induced by low-intensity light in the homeotropic cell, and the induced birefringence was affected by the thermal effect. No photoinduced reorientation effect was observed in the homogeneous cell in the present experimental configuration; however, due to the thermal modulation of the refractive index of the LC, which arises from light absorption, self-diffraction phenomena could also be observed.
Growth of (103) fiber-textured SrBi2Nb2O9 films on Pt-coated silicon
Authors: Asayama G. , Lettieri J. , Zurbuchen M. A. , Jia Y. , Trolier-McKinstry S. , Schlom D. G. , Streiffer S. K. , Maria J-P. , Bu S. D. , Eom C. B.
Published: July 2002

(103) fiber-textured SrBi2Nb2O9 thin films have been grown on Pt-coated Si substrates using a SrRuO3 buffer layer. High-resolution transmission electron microscopy reveals that the fiber texture arises from the local epitaxial growth of (111) SrRuO3 grains on (111) Pt grains and in turn (103) SrBi2Nb2O9 grains on (111) SrRuO3 grains. The films exhibit remanent polarization values of 9 μC/cm2. The uniform grain orientation (fiber texture) should minimize grain-to-grain variations in the remanent polarization, which is important to continued scaling of ferroelectric memory device structures.
Optical fiber interferometer for measuring the d33 coefficient of piezoelectric thin films with compensation of substrate bending
Authors: Fernandes J. R. , de Sá F. A. , Santos J. L. , Joanni E.
Published: July 2002

An optical fiber interferometer for measuring the d33 coefficient of piezoelectric samples is described. Its configuration is based on the Mach–Zehnder interferometer, and a double incidence on the thin-film samples successfully suppresses the undesirable bending effect of the substrate. Detection of the small displacement is based on an active homodyne scheme. Results are reported for a bulk piezoelectric transducer (PZT) sample and a PZT thin-film incorporated in a microactuator.
Extending Wertheim’s perturbation theory to the solid phase of Lennard-Jones chains: Determination of the global phase diagram
Authors: Vega C. , Blas F. J. , Galindo A.
Published: July 2002

Wertheim’s first order thermodynamic perturbation theory (TPT1) [M. S. Wertheim, J. Chem. Phys. 87, 7323 (1987)] is extended to model the solid phase of chains whose monomers interact via a Lennard-Jones potential. Such an extension requires the free energy and contact values of the radial distribution function for the Lennard-Jones reference system in the solid phase. Computer simulations have been performed to determine the structural properties of the monomer Lennard-Jones system in the solid phase for a broad range of temperatures and densities. Computer simulations of dimer Lennard-Jones molecules in the solid phase have also been carried out. The theoretical results for the equation of state, the internal energy, and the sublimation curve of the dimer model in the solid phase are in excellent agreement with the simulation data. The extended theory is used to determine the global (solid–liquid–vapor) phase diagram of the LJ dimer model; the theoretical estimate of the triple point temperature for the LJ dimer is T*=0.653. Similarly, Wertheim’s TPT1 is used to determine the global phase diagram of chains formed by up to 8 monomer units. It is found that the calculated triple point temperature is hardly affected by the chain length, and that for large chain lengths the fluid–solid equilibrium coexistence densities are virtually independent of the number of monomers in the chain when the densities are expressed in monomer units. This is in agreement with experimental indications observed in polyethylene, where both the critical and the triple point temperatures tend to finite values for large molecular weights.
Assessment of time-dependent density-functional theory for the calculation of critical features in the absorption spectra of a series of aromatic donor–acceptor systems
Authors: Jamorski Christine , Foresman James B. , Thilgen Carlo , Lüthi Hans-Peter
Published: July 2002

Singlet and triplet vertical excitation energies of a series of acceptor parasubstituted N,N-dimethyl–anilines [NC–C6H4–N(CH3)2, NC–C6H4–NH2, OHC–C6H4–N(CH3)2, NC–C6H2(CH3)2–N(CH3)2, (H2N)OC–C6H4–N(CH3)2, (CH3)OC–C6H4–N(CH3)2, O2N–C6H4–N(CH3)2, named, respectively, 4DMAB–CN, 4AB–CN, 4DMAB–CHO, TMAB–CN, 4DMAB–CONH2, 4DMAB–COMe, and 4DMAB–NO2] have been calculated with TDDFT. Geometry optimization and excitation energy calculations have been performed, in most cases, with the B3LYP functional using a 6-31G(d) and a 6-311+G(2d,p) basis set (hereafter referred to as Sm and Bg, respectively). 4DMAB–CN and TMAB–CN have been investigated with particular care since gas-phase absorption spectra exist for those two molecules allowing thus a direct comparison with experimental results. The first and second singlet excited states of 4DMAB–CN, commonly named locally excited (LE) state and charge transfer (CT) state, are 0.1 and 0.04 eV higher than the experimental results at the B3LYP-Bg level, leading to a 0.06 eV underestimation of the gap between the two states. In the case of TMAB–CN, which is twisted in its ground state, B3LYP–(Sm/Bg) results show an error of 0.36 eV for the singlet CT state. Better agreement with experiment is obtained using the MPW1PW91 functional and Bg basis set with an underestimation of 0.17 eV for the singlet CT state and an overestimation of 0.16 eV for the second singlet state. Contrary to DFT/SCI results, the relative order and position of excitation energies of 4AB–CN and 4DMAB–CHO are well reproduced compared to solution spectra results. The singlet CT state using B3LYP and a Bg basis set is calculated 0.1 eV higher in energy than the experimental value obtained in isopentane for 4DMAB–CONH2, while the same excitation energy is predicted 0.08 and 0.28 eV too low compared to the gas-phase values for 4DMAB–COMe and 4DMAB–NO2, respectively. Finally, the CT excitation energy and its relative position to the LE state agrees with the acceptor strength concept.
Impedance match equation of state experiments using indirectly laser-driven multimegabar shocks
Authors: Rothman S. D. , Evans A. M. , Horsfield C. J. , Graham P. , Thomas B. R.
Published: July 2002

Measurements of equation of state (EOS) points on the principal Hugoniots of Cu, Au, Pb and the plastics Parylene-C and brominated CH at multimegabar pressures have been made using the 1 TW HELEN laser at AWE. The aim was 1% accuracy in shock velocity measurement (3%–4% in pressure) in order to compare with data from gas-gun and nuclear underground test experiments and the theoretical EOS’s based on, or supported by, these data. Experiments comprised a hohlraum heated by two 500 J, 0.53 μm wavelength, 1 ns Gaussian laser pulses generating an x-ray flux which drove a shock into a target consisting of a base, with steps of a known EOS material and of the material of unknown EOS. Shock breakout from base and steps was detected by monitoring light emission from the target with optical streak cameras and shock velocities were derived from the transit times across the known-height steps.
A low-dimensional approach to nonlinear plane–Poiseuille flow of viscoelastic fluids
Authors: Khayat Roger E. , Ashrafi Nariman
Published: July 2002

The nonlinear stability and bifurcation of the one-dimensional plane–Poiseuille flow is examined for a Johnson–Segalman fluid. The methodology used is closely related to that of Ashrafi and Khayat [Phys. Fluids 12, 345 (2000)] for plane–Couette flow. The velocity and stress are represented by orthonormal functions in the transverse direction to the flow. The flow field is obtained from the conservation and constitutive equations using the Galerkin projection method. Both inertia and normal stress effects are included. The stability picture is dramatically influenced by the viscosity ratio, ε. The range of shear rate or Weissenberg number for which the base flow is unstable increases (from zero) as the fluid deviates from the Newtonian limit (as ε decreases). Typically, two turning points are observed near the critical Weissenberg numbers. The transient response is heavily influenced by the level of inertia. It is found that the flow responds oscillatorily when the Reynolds number is small, and monotonically at large Reynolds number (when elastic effects are dominated by inertia).
Lead–zirconate–titanate-based metal/ferroelectric/insulator/semiconductor structure for nonvolatile memories
Authors: Liu Mingjiao , Kim Hong Koo , Blachere Jean
Published: July 2002

We have investigated the structural and electrical properties of the metal/ferroelectric/insulator/semiconductor (MFIS) structure that incorporates a MgO/SiO2 insulating buffer between a ferroelectric layer and Si substrate. Highly oriented lead–zirconate–titanate [Pb(Zr,Ti)O3, or PZT] films were grown on the MgO-buffered oxidized silicon substrates with a rf magnetron sputtering technique. The x-ray diffraction and energy-dispersive x-ray spectroscopy analysis results show that a MgO buffer serves well not only as a template layer for growing oriented PZT films on an amorphous surface but also as a diffusion barrier between PZT and Si substrates. The memory window of the MFIS structure was characterized with a capacitance-versus-voltage method. Numerical analyses were also carried out to simulate the MFIS capacitor characteristics. In this simulation, the PZT films were assumed to have a two-layer structure in which the dielectric and ferroelectric properties of an initial layer are significantly weaker than those of the main layer part. By comparing the measurement data with the simulation result, we have extracted the parameters of this two-layer model (dielectric constant and the polarization-versus-electric-field characteristics) of the PZT films in the MFIS structure. The scalability of the memory window of the MFIS structure was investigated by varying the ferroelectric (PZT) layer thickness. Both the experimental and simulation results show that the PZT-based MFIS structure is suitable for nonvolatile memory field-effect transistors with low-voltage requirement (3 V or less) and large memory window (1–2 V).
Domain structures in epitaxial (110) Fe3O4 particles studied by magnetic force microscopy
Authors: Pan Qi , Pokhil Taras G. , Moskowitz Bruce M.
Published: July 2002

Magnetic domain structures on single-crystalline magnetite (Fe3O4) particles, prepared by microfabrication techniques from molecular-beam epitaxial (110) magnetite films grown on MgO, were studied by magnetic force microscopy. The (110) magnetite film thickness was 250 nm and the patterned particles ranged in size from 2×2 to 10×10 μm. The patterned particles showed in-plane, stripe-like domain structures with ill-defined and fragmented walls mainly aligned along the in-plane [110] direction. In both the parent film and the patterned particles, an out-of-plane component of the stray field was observed within domain interiors as a fine-scale (100–300 nm) and spatially variable magnetic contrast present in both the remanent state and in applied fields. Individual wall sections were observed to be highly fragmented with variable widths (100–300 nm) and offsets and subdivided into opposite polarity segments of variable lengths. Remagnetization of a 10×10 μm particle in fields up to 500 Oe occurred by reverse spike domain nucleation at the edge of the particle followed by growth and propagation towards the interior of the particle similar to classical behavior of uniaxial materials. In contrast, the unusual domain wall structures are a consequence of the antiferromagnetically coupled, growth-induced, structural antiphase domains and antiphase boundaries (APB) know to form in epitaxial thin films of magnetite. Magnetically, the particles behave differently at the different length scales. A particle as a whole (micrometer length scale) behaves as a magnetically uniaxial object, but on a smaller length scale (submicron scale), the magnetic microstructure is strongly influenced by the antiphase structural domains. Analysis of the domain spacing as a function of particle size yields an estimate of the average exchange stiffness constant that is nearly 2 orders of magnitude lower than the value in bulk magnetite. This is consistent with the idea that exchange interactions across the APBs are severely suppressed due to spin frustration.
The singlet–triplet spectroscopy of 1,3-butadiene using cavity ring-down spectroscopy
Authors: Robinson Allison G. , Winter Paul R. , Zwier Timothy S.
Published: July 2002

The T1←S0 absorption spectrum of gas-phase 1,3-butadiene (C4H6) has been investigated over the region from 20 500 to 23 000 cm−1 using cavity ring-down spectroscopy. Resolved vibrational structure and partially resolved rotational structure have been observed for the first time in the gas phase. The T1←S0 origin transition is located at 20 777 cm−1, with a peak absorption cross section of 2.5×10−26 cm2/molecule. Vibronic bands appear 249, 491, 1166, and 1617 cm−1 above the origin. This structure is observed on top of a rising background whose absolute magnitude and wavelength dependence is quantitatively accounted for as Rayleigh scattering. Using the recent calculations of Brink et al. [J. Phys. Chem. A 102, 6513 (1998)] as a guide, the bands 491, 1166, and 1617 cm−1 above the origin can be assigned as totally symmetric fundamentals, while the band 249 cm−1 above the origin is the first overtone of the bg symmetry CH2 torsion (calculated at 129.6 cm−1) of a planar T1 excited state. The rotational band contour of the origin transition shows several sharp band-heads that appear in doublets with a splitting of 2 cm−1. Only part of this structure can be accounted for as a single vibronic band. The possible explanations for the remaining band heads and the observed Franck–Condon intensities are discussed in terms of the shape of the T1 potential energy surface.
Photodissociation of the hydroxymethyl radical. I. The role of conical intersections in line broadening and decomposition pathways
Authors: Hoffman Brian C. , Yarkony David R.
Published: July 2002

When the 3 2A state of D2COH is excited broadened lines are observed and both D and H atoms are produced. When the 2 2A state is excited the spectrum is quite diffuse. These experimental observations can be explained by conical intersections connecting the 3 2A and 2 2A states and the 2 2A and 1 2A states. The 2 2A←1 2A excitation leads to rapid dissociation as a result of a seam of conical intersections connecting the 2 2A and 1 2A states. The relevant region of the 2 2A–1 2A seam of conical intersections occurs for long R(O–H) and can lead to ground state D2CO+H. Following the 3 2A←1 2A excitation a low energy portion of the 2 2A–3 2A seam of conical intersection can be accessed. This portion of the 2 2A–3 2A seam occurs for long R(D–C). From this seam of conical intersections, paths on the 2 2A potential energy surface lead to portions of the 2 2A–1 2A seam from which either of the ground state dissociation products DCOH+D, or D2CO+H are accessible.
Intraband-transition-induced dipoles in self-assembled InAs/GaAs(001) quantum dots
Authors: Chen Zhonghui , Kim Eui-Tae , Madhukar Anupam
Published: July 2002

We present experimental evidence for the existence and directionality of dipoles induced by intraband transitions from the electron ground states to high, bound excited states in self-assembled InAs/GaAs quantum dots (QDs). Moreover, the orientation of the interband transition induced dipoles is also determined for the same QDs. The findings indicate the potential use of intraband dipoles in asymmetric QDs in proposed quantum gates.
An emissive probe with a rhenium filament for measuring plasma potential in a radio frequency oxygen plasma
Authors: Wilson E. H. , Jeong Jongtae , Hershkowitz N.
Published: July 2002

Plasma potential measurements in a rf inductively coupled oxygen plasma were carried out using an emissive probe with rhenium filaments. Rhenium was chosen because of the remarkable electrical conductivity of its oxide which is 108 times higher than tungsten oxide. Using 75 μm diam filaments hot-wire emissive probe measurements of an oxygen plasma potential were performed in pressures of 3–100 mTorr. Due to surface contaminants, filament conditioning was performed in order to allow electron emission. Analysis of time-averaged current–voltage curves yielded the maximum and minimum of the rf fluctuation in the plasma. Axial profiles of the plasma potential fluctuations at 3.3 mTorr show a nearly linear decrease with the distance from the wall, and demonstrate the feasibility for further investigation of oxygen plasmas using rhenium filaments.
Suppression of spontaneous emission in incomplete opaline photonic crystal
Authors: Romanov S. G. , Maka T. , Torres C. M. Sotomayor , Müller M. , Zentel R.
Published: July 2002

Angular-resolved photoluminescence spectra have been obtained from polymer-made opaline photonic crystals impregnated with Coumarin dye. Spectral changes with optical excitation power and the direction of detection have been analyzed. The anisotropy in the suppression of the spontaneous emission is shown to be in accord with the Bragg stop-band dispersion.
System for fast time-resolved measurements of c-axis quasiparticle conductivity in intrinsic Josephson junctions of Bi2Sr2CaCu2O8+δ
Authors: Fenton J. C. , Thomas P. J. , Yang G. , Gough C. E.
Published: July 2002

A wide-band cryogenic amplifier measurement system for time-resolved four-point current-voltage (I-V)-characteristic measurements on Bi2Sr2CaCu2O8+δ mesa structures is described. We present measurements which demonstrate the importance of self-heating on ∼50 ns time scales. Such heating is likely to have been very significant in many previously published measurements, where the reported nonlinear I-V characteristics have been used to derive superconducting energy gaps.