bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectroscopic imaging of nanoparticles in laser ablation plume by redecomposition and laser-induced fluorescence detection
Authors: Muramoto Junichi , Inmaru Takahiro , Nakata Yoshiki , Okada Tatsuo , Maeda Mitsuo
Published: July 2002

We propose and demonstrate an imaging technique to observe clusters formed in a laser ablation plume, which are difficult to be detected by conventional laser-induced fluorescence (LIF) and ultraviolet Rayleigh scattering techniques. The clusters were decomposed by the irradiation of another laser beam, and the disintegrated atoms were visualized by a two-dimensional LIF technique. This technique was applied to visualize the formation process of clusters in a laser ablation plume. Based on this imaging diagnostics, the onset time of the clustering in the plume is discussed.
Electrically induced Bragg-diffraction grating composed of periodically inverted domains in lithium niobate crystals and its application devices
Authors: Yamada Masahiro
Published: July 2002

Bragg diffraction by an electrically induced and controlled grating composed of periodically inverted domains in a lithium niobate crystal is discussed theoretically and experimentally. As the applications of the Bragg-diffraction grating, experimental results of an optical switch with one input port and six output ports and a wide band optical amplitude modulator for a violet-blue light with the wide frequency bandwidth from direct current to 1 GHz are presented.
Frequency-tunable micromechanical oscillator
Authors: Zalalutdinov M. , Ilic B. , Czaplewski D. , Zehnder A. , Craighead H. G. , Parpia J. M.
Published: July 2002

An experimental method, employing a scanning tunneling microscope (STM) as an actuator and a scanning electron microscope (SEM) as a motion detector, was developed to study microelectromechanical systems (MEMS) and has been applied to study microfabricated cantilever beams. Vibrations actuated by an ac voltage applied to the piezodrive are transferred to the sample by the STM tip, which also provides a constraint at the drive location, altering the fundamental mode of the oscillation. A continuous change in the resonant frequency of the cantilever is achieved by varying the position of the STM tip. In contrast to the few percent tunability previously demonstrated for MEMS oscillators, we have varied the cantilever frequency over a 300% range.
Nuclear magnetic resonance study of the relaxor ferroelectric Pb(Sc1/2Nb1/2)O3
Authors: Blinc R. , Gregorovič A. , Zalar B. , Pirc R. , Laguta V. V. , Glinchuk M. D.
Published: July 2002

The Pb207, Sc45, and Nb93 nuclear magnetic resonance spectra have been measured for the relaxor ferroelectric single crystal Pb(Sc1/2Nb1/2)O3 both above and below Tc. The Sc45 spectra suggest that the “disordered” part of the crystal occupies about 60% and the “ordered” cubic part about 40% of the crystal volume. In agreement with the spherical random bond-random field (SRBRF) model the square of the polarization linearly increases with decreasing T below Tc in both the ordered as well as in the disordered regions. The Edwards–Anderson order parameter q similarly increases linearly with decreasing T below Tc. For the disordered region we find J0≈328 K and J≈227 K whereas Δ/J02≈0.006. In the ordered region J0≈297 K, whereas J and Δ are too small to be determined from this experiment. The results demonstrate that the SRBRF model works not only for pure relaxors but also for relaxor ferroelectrics.
Quantum-confined Stark effects of InAs/GaAs self-assembled quantum dot
Authors: Li Shu-Shen , Xia Jian-Bai
Published: July 2002

Quantum-confined Stark effects in InAs/GaAs self-assembled quantum dots are investigated theoretically in the framework of effective-mass envelope function theory. The electron and hole energy levels and optical transition energies are calculated in the presence of perpendicular and parallel electric field. In our calculation, the effect of finite offset, valence band mixing, and strain are all taken into account. The results show that the perpendicular electric field weakly affects the electron ground state and hole energy levels. The energy levels are affected strongly by the parallel electric field. For the electron, the energy difference between the ground state and the first excited state decreases as electric field increases. The optical transition energies have clear redshifts in electric field. The theoretical results agree well with the available experimental data. Our calculated results are useful for the application of quantum dots to photoelectric devices.
Combined dynamic adhesion and friction measurement with the scanning force microscope
Authors: Krotil H.-U. , Stifter Th. , Marti O.
Published: July 2002

In this letter, we present a promising scanning probe microscopy mode, called combined dynamic X mode or short CODYMode®. In CODYMode® scanning force microscopy, at least two modulations with sufficiently different frequencies and amplitudes are superpositioned and interact with the sample surface. This enables the concurrent measurement of the topography, adhesive and frictional forces, and further mechanical surface properties of the sample. The general advantages of CODYMode® are discussed. This technique is predestined for investigation of delicate samples (polymers, biological samples, etc.) in which common scanning force microscope techniques are not adequate. An ABC-triblock copolymer system served as sample system.
Aluminum, magnesium, and gold contacts to contamination free n-GaN surfaces
Authors: Wu C. I. , Kahn A. , Wickenden A. E. , Koleske D. , Henry R. L.
Published: July 2002

Current–voltage (I–V) characteristics of Al, Mg, and Au Schottky contacts to atomically clean n-GaN(0001)-1×1 surfaces prepared in an ultrahigh vacuum were investigated. The Al/n-GaN contact is rectifying at room temperature and becomes Ohmic after annealing at 500 °C. Coupled with previous photoemission spectroscopy data, this result demonstrates that the origin of the Ohmicity is the reaction-induced doping of the interface. For nonannealed interfaces, the Schottky barrier heights determined from I–V characteristics are in qualitative agreement with the results obtained by photoemission spectroscopy. We find that the ideality factor of the barrier is close to unity for the unreactive interface i.e. Au/GaN, but significantly higher for the reactive interfaces, i.e., Al/GaN and Mg/GaN. Our experimental results suggest that the reaction-induced defects and thermionic field emission play an important role in the electrical behavior of these interfaces.
Microscopic carrier dynamics of quantum-well-based light storage cells
Authors: Zhang S. K. , Santos P. V. , Hey R. , Garcia-Cristóbal A. , Cantarero A.
Published: July 2002

The dynamics of storage and recombination of electron–hole pairs in quantum-well-based light memory cells driven by high-frequency electric fields is investigated by spatially and time-resolved photoluminescence spectroscopy. In these cells, a laterally modulated potential is used to ionize and spatially separate photogenerated electron–hole pairs. The temporal and spatial dependence of the carrier dynamics depends not only on the modulated potential, but also on the transport properties of electrons and holes.
The nematic-isotropic phase transition in semiflexible fused hard-sphere chain fluids
Authors: Jaffer K. M. , Opps S. B. , Sullivan D. E. , Nickel B. G. , Mederos L.
Published: July 2002

A density-functional theory of the isotropic–nematic phase transition in both rigid and semiflexible hard-sphere chain fluids is described. The theory is based on an exact analytical evaluation of the excluded volume and second virial coefficient B2 for rigid chain molecules, which demonstrates that B2 in these cases is equivalent to that of a binary mixture of hard spheres and hard diatomic molecules. It is assumed that the same binary-mixture representation applies to semiflexible chains, while scaled particle theory is used to obtain the properties of the fluid at arbitrary densities. The results of the theory are in very good agreement with Monte Carlo (MC) simulation data for rigid tangent hard-sphere chains, but in lesser agreement with available MC studies of rigid fused hard-sphere chains. We find that there is reasonable agreement between the theory and MC data for semiflexible tangent chains, which improves with increasing chain length. The behavior predicted by the theory for semiflexible chains is contrasted with that given by the Khokhlov and Semenov theory of nematic ordering of wormlike polymer chains.
1/f
noise in metal–oxide–semiconductor transistors biased in weak inversion
Authors: Rhayem J. , Rigaud D. , Eya’a A. , Valenza M. , Hoffmann A.
Published: July 2002

An approach to model 1/f noise in the weak inversion range of metal–oxide–semiconductor transistors (MOST) is proposed, based on Hooge’s theory (mobility fluctuation model). Starting from conduction equations in the subthreshold regime, a method to evaluate the total number of carriers under the gate is presented and allows us to deduce the Hooge parameter αH. This model is applied to p-channel MOSTs. With the proposed model, the value of αH obtained in weak inversion is quite similar to this extracted in strong inversion allowing a unique description of the 1/f noise.
Combined formation of a self-pumped phase-conjugate mirror and spatial subharmonics in photorefractive sillenites
Authors: Nippolainen Ervin , Kamshilin Alexei A. , Prokofiev Victor V. , Jaaskelainen Timo
Published: July 2002

We found that the formation of self-pumped phase-conjugate mirrors (SPPCM) in photorefractive Bi12TiO20 crystals under external alternating electric field is accompanied by strong enhancement of spatial subharmonics. This combined formation of the fundamental grating and its spatial subharmonics results in an expansion of the spatial-frequency band allowed for SPPCM recording.
Diffusion, release, and uptake of hydrogen in magnesium-doped gallium nitride: Theory and experiment
Authors: Myers S. M. , Wright A. F. , Petersen G. A. , Wampler W. R. , Seager C. H. , Crawford M. H. , Han J.
Published: July 2002

The diffusion and release of H and its uptake from the gas phase are modeled for Mg-doped, wurtzite GaN using formation energies and vibration frequencies from the density-function theory. Comparison is made with rates of deuterium release and uptake measured by nuclear-reaction analysis of deuterium concentration. Good agreement is found when account is taken of a surface permeation barrier.
Electric-field-induced strain and piezoelectric properties near the morphotropic phase boundary of Pb(Mg1/3Nb2/3)O3–PbZrO3–PbTiO3 ceramics
Authors: Yoon Ki Hyun , Lee Hong Ryul
Published: July 2002

The dielectric, piezoelectric properties, and electric-field-induced strains of the 0.2Pb(Mg1/3Nb2/3)O3–xPbZrO3–(0.8−x)PbTiO3 (x=0.30–0.50) compositions close to the morphotropic phase boundary (MPB) have been systematically investigated as a function of PbZrO3 content (x). The phase approached from the tetragonal toward the rhombohedral phase when the PbZrO3 contents (x) increased. The maximum remnant polarization of 32.3 μC/cm2 was observed at the composition of x=0.40 near the MPB, and the specimen of x=0.39 showed the highest transverse strain value of 1.34×10−3 at 23 kV/cm. The values of Kp and d33 (Kp=0.65 and d33=454) showed maximum at the composition x=0.38 which was a tetragonal-rich phase, indicating the enhanced piezoelectric response.
Production and measurements of individual single-wall nanotubes and small ropes of carbon
Authors: Arepalli Sivaram , Nikolaev Pavel , Holmes William , Files Bradley S.
Published: July 2002

This work focuses on the size and spatial dependence of single-wall carbon nanotubes produced by the pulsed-laser vaporization technique. The study indicates that very long (tens of microns) individual nanotubes form in the vicinity of the target, and subsequently coalesce into bundles. The role of the inner flow tube is confirmed to restrict plume expansion and improve interactions between carbon atoms resulting in nanotube and rope formation. The effect of the flowing buffer gas seems to influence the dispersion of particulate contaminant material in the nanotube product. More particulate matter is produced at lower oven temperatures.
New theoretical insight into the thermal cis–trans isomerization of azo compounds: Protonation lowers the activation barrier
Authors: Sokalski W. A. , Góra R. W. , Bartkowiak W. , Kobyliński P. , Sworakowski J. , Chyla A. , Leszczyński J.
Published: July 2002

The detailed mechanism of thermal cis–trans inversion of protonated diazenes and azobenzenes has been investigated for the first time using ab initio Hartree–Fock (HF), density functional theory (DFT), second order Mo/ller–Plesset (MP2), and complete active space self consistent field (CASSCF) approaches. Protonation of one of the nitrogens at the N=N bond leads to a considerable (5–20 kcal/mol) reduction of the activation barrier. The magnitude of this effect depends on the electron withdrawing nature of the substituents and the differential stabilization of the transition state by positively charged functional groups. In the case of 4-phenylazopirydine protonated at pyridine nitrogen, the barrier is dramatically reduced to just 7 kcal/mol. This result establishes the lowest (close to an experimental estimate) ab initio value for the cis–trans isomerization for a substituted azobenzene. The structures of neutral and protonated transition states are similar, and the correlation effects for molecular parameters as well as the energy are negligible for extended basis set calculations.
On bidirectional fifth-order nonlinear evolution equations, Lax pairs, and directionally dependent solitary waves
Authors: Dye J. M. , Parker A.
Published: July 2002

In this paper, Lax pairs are constructed for two fifth-order nonlinear evolution equations of “Boussinesq”-type which govern wave propagation in two opposite directions. One of the equations is related to the well-known Sawada–Kotera (SK) equation and, through its bilinear form, is identified with the Ramani equation. The second equation—about which very little seems to be known—may be considered a bidirectional version of the Kaup–Kupershmidt (KK) equation and is the main focus of this study. The “anomalous” solitary wave of this latter equation is derived and is found to possess the remarkable property that its profile depends on the direction of propagation. This type of directional dependence would appear to be quite unusual and, to our knowledge, has not been reported in the literature before now. By taking an appropriate undirectional (long wave) limit, it is shown that neither the Ramani, nor the bidirectional Kaup–Kupershmidt (bKK) equation can be classified as truly “Boussinesq” in character (a distinction that is made precise in the study). Recursion formulas are given for generating an infinity of conserved densities for both equations. These are used to obtain the first few conservation laws of the bKK and Ramani equations explicitly; not surprisingly, they exhibit the same lacunary behavior as their unidirectional counterparts. In conclusion, a canonical interpretation of the N-soliton solution of the bKK equation is proposed which provides a basis for constructing these anomalous solitons in a future work.
State-to-state rate constants for the rotational relaxation of CH(B 2Σ−,v=0,J) in inelastic collisions with Ar
Authors: Kind M. , Stuhl F.
Published: July 2002

A state-resolved experimental investigation of inelastic collisions of single electronically excited CH(B 2Σ−,v=0,N,Fi) states with Ar is presented by monitoring population distributions in a partly relaxed system. Fluorescence detection of the collision induced population permits spin resolution at low N. Single-collision state-to-state rate constants were determined for low values of N. At the lowest values of N, the most dominant process is spin unit change, F1↔F2, at ΔN=0. The efficiency of the ΔN=0 transitions decreases markedly with increasing N. For ΔN≠0 steps, conservation of the spin label is more important than its change. The very same trends have been recently observed in our laboratory for the fine structure resolved states of CH(A 2Δ,v=0). These results are discussed with respect to propensity rules previously derived and observed for Σ+2 states.
Manipulation of magnetic microbeads in suspension using micromagnetic systems fabricated with soft lithography
Authors: Deng Tao , Whitesides George M. , Radhakrishnan Mala , Zabow Gary , Prentiss Mara
Published: July 2002

Current-carrying microcircuits can generate strong magnetic-field gradients; these gradients, in turn, can control the position of magnetic microbeads in aqueous suspension. Micromagnetic systems were prepared using two representative soft lithography techniques—micromolding in capillaries and microtransfer molding—combined with electrodeposition. They can capture microbeads from solution, hold them in a fixed position, and move them along complex paths. Simply changing the current flowing in the systems can dynamically reconfigure the paths. The manipulation of magnetic microbeads using microfabricated circuits should expand current sample handling capabilities for biotechnology and combinational technology with or without the presence of net fluid flow and microfluidic channels.
The α-relaxation process in simple glass forming liquid m-toluidine. II. The temperature dependence of the mechanical response
Authors: Cutroni Maria , Mandanici Andrea
Published: July 2002

We have measured, as a function of temperature and frequency, the velocity and attenuation of sound down to the glass transition of the fragile glass-forming molecular liquid m-toluidine. The mechanical relaxation process is compared with the dielectric one recently studied on the same substance. The Arrhenius plot of log(τ) vs 1/T shows that the relaxation time associated with ultrasonic measurements is the same as that previously found by the dielectric spectroscopy in the same temperature region.
Integral equation theory approach to rodlike polyelectrolytes: Counterion condensation
Authors: Hofmann T. , Winkler R. G. , Reineker P.
Published: July 2002

We investigate the structural properties of rigid linear polyelectrolytes in dilute and semidilute solutions using an integral equation theory. The Polymer Reference Interaction Site Model together with the Reference Laria Wu Chandler Closure is solved numerically taking the counterions into account explicitly. The counterions and the polymer chains, modeled as linearly connected, charged hard spheres, interact through an unscreened Coulomb potential. The pair correlation functions between the monomers of different chains, the counterions, and the monomers and counterions, respectively, are calculated for various densities and Bjerrum lengths. Based upon these quantities, the effective potential among the monomers and the counterions, respectively, is extracted. In particular, a critical Bjerrum length is determined, which separates the regime of a repulsive interaction between the counterions from the regime of an attractive interaction transmitted by the polymer chains.
Thermal characterization of Bi2Te3/Sb2Te3 superlattices
Authors: Touzelbaev M. N. , Zhou P. , Venkatasubramanian R. , Goodson K. E.
Published: July 2002

Superlattices offer the potential to enhance the figure of merit for thermoelectric cooling by increasing the Seebeck coefficient while decreasing the thermal conductivity compared to bulk samples. The large bulk value of ZT makes superlattices containing Bi2Te3 attractive for demonstrating benefits of using low-dimensional materials in thermoelectric applications. The present work describes measurements of the effective thermal conductivity normal to Bi2Te3/Sb2Te3 superlattices deposited on GaAs using noncontact pulsed laser heating and thermoreflectance thermometry. The data show a strong reduction in the effective thermal conductivity of the Bi2Te3/Sb2Te3 superlattices compared to bulk Bi2Te3, which can further increase thermoelectric figure of merit. The dependence of thermal conductivity on superlattice period is found to be weak, particularly at periods above 60 Å. This indicates that disorder in Bi2Te3/Sb2Te3 superlattices may limit the heat conduction process at shorter periods than in Si/Ge superlattices, for which measurements were previously reported in the literature.
Electrical characterization of an operating Si pn-junction diode with scanning capacitance microscopy and Kelvin probe force microscopy
Authors: Buh G. H. , Chung H. J. , Yi J. H. , Yoon I. T. , Kuk Y.
Published: July 2002

Electrical characterization of an operating pn-junction diode is performed with scanning capacitance microscopy (SCM) and Kelvin probe force microscopy (KPFM) with submicron scale resolution. We image the spatial distribution of the carrier density inside a diode with SCM and the potential distribution on the surface of the operating diode with KPFM. The surface potential distribution measured at reverse bias is different from that in bulk. The potential drop is extended deep into a lightly p-doped region at reverse bias. The positive fixed oxide charge of 1–2×1011/cm2 would explain the modified potential drop: A known detrimental effect in such a device. The potential distribution at forward bias is nearly bulklike. The potential drops only near the metal–semiconductor junction.
Non-Hamiltonian molecular dynamics: Generalizing Hamiltonian phase space principles to non-Hamiltonian systems
Authors: Tuckerman Mark E. , Liu Yi , Ciccotti Giovanni , Martyna Glenn J.
Published: July 2002

The use of non-Hamiltonian dynamical systems to perform molecular dynamics simulation studies is becoming standard. However, the lack of a sound statistical mechanical foundation for non-Hamiltonian systems has caused numerous misconceptions about the phase space distribution functions generated by these systems to appear in the literature. Recently, a rigorous classical statistical mechanical theory of non-Hamiltonian systems has been derived, [M. E. Tuckerman, et al., Europhys. Lett. 45, 149 (1999)]. In this paper, the new theoretical formulation is employed to develop the non-Hamiltonian generalization of the usual Hamiltonian based statistical mechanical phase space principles. In particular, it is shown how the invariant phase space measure and the complete sets of conservation laws of the dynamical system can be combined with the generalized Liouville equation for non-Hamiltonian systems to produce a well defined expression for the phase space distribution function. The generalization provides a systematic, controlled procedure for designing non-Hamiltonian molecular dynamics algorithms which can be used to generate nonmicrocanonical ensembles, stationary nonequilibrium flows, and/or the dynamics of constrained systems. In light of this new general analysis, molecular dynamics algorithms for the canonical and isothermal–isobaric ensembles are examined, potential difficulties are illuminated, and the limitations of previous theoretical treatments are elucidated.
Identity of long-range surface plasmons along asymmetric structures and their potential for refractometric sensors
Authors: Pigeon F. , Salakhutdinov I. F. , Tishchenko A. V.
Published: July 2002

The field identity of the long-range surface plasmon (LRSP) mode in an asymmetric metal dielectric structure is elucidated and it is shown that it can be pictured as having a zero crossing of the longitudinal electric field at the middle of the metal film. A parametric dependence between the metal and the dielectric layer thicknesses leading to a LRSP mode in an asymmetric structure is given. The sensitivity of an asymmetric four layer structure supporting a grating excited LRSP mode regarding sensing objectives has been investigated. It is compared with the sensitivity of a usual plasmon mode propagating along a metal–dielectric interface. The existence of an anomalous increase of the reflection coefficient in the case of the LRSP is observed theoretically and experimentally. The comparative study is made on the basis of analytical expressions which reveal that the LRSP does not bring a decisive advantage over the standard plasmon for sensor application but its specific features can be advantageously used once well understood.
Polarization fatigue in ferroelectric films: Basic experimental findings, phenomenological scenarios, and microscopic features
Authors: Tagantsev A. K. , Stolichnov I. , Colla E. L. , Setter N.
Published: July 2002

The reduction in switchable polarization of ferroelectric thin films due to electrical stress (polarization fatigue) is a major problem in ferroelectric nonvolatile memories. There is a large body of available experimental data and a number of existing models which address this issue, however the origin of this phenomena is still not properly understood. This work synthesizes the current experimental data, models, and approaches in order to draw conclusions on the relative importance of different macro- and microscopic scenarios of fatigue. Special attention is paid to the role of oxygen vacancy migration and electron injection into the film and it is concluded that the latter plays the predominant role. Experiments and problems for theoretical investigations, which can contribute to the further elucidation of polarization fatigue mechanisms in ferroelectric thin films, are suggested.
Opposition control of near-wall turbulence with a piston-type actuator
Authors: Rebbeck Henry , Choi Kwing-So
Published: July 2002

Opposition control of near-wall turbulence was carried out using a wall-normal jet from a piston-type actuator, by selectively cancelling the downwash of high-momentum fluid during the sweep events. By employing an innovative methodology we were able to study the effectiveness of the scheme without carrying out real-time control, overcoming experimental difficulties associated with the high-frequency responses required for the turbulence control. Observed changes of the burst signature in the turbulent boundary layer suggest that there is a possibility of reducing the skin-friction drag when opposition control is carried out with appropriate timing.
Anisotropy in turbulence profiles of stratified wakes
Authors: Spedding G. R.
Published: July 2002

At sufficiently high values of the Reynolds number (Re⩾4.5×103) and internal Froude number (F⩾4), initially turbulent bluff body wakes evolve in the presence of a stable background density gradient with wake-averaged mean and turbulence length and velocity scales that are independent of Re and F for at least two orders of magnitude extension in both parameters. The way in which the initially three-dimensional motions transition to the characteristic (and Re- and F-independent) late wakes (where vertical velocities, w≪u,v) is both of great practical interest, and complex, hence somewhat unclear. Here, digital particle imaging velocimetry type measurements on towed-sphere wakes are described, so that the development of anisotropy can be measured by the time development of turbulence profiles in horizontal and vertical centerplanes. The observed anisotropies can be associated with energy transfer to internal wave modes, and suppression of other vertical displacements, that contrasts with sphere wakes at similar Re in a homogeneous fluid. Maximum Reynolds stresses occur at the boundary of a sinuous undulation of the wake, which increases in amplitude up to Nt≈60 (N is the buoyancy frequency that characterizes the strength of the stratification). Although an intrinsic wake profile instability cannot be excluded, the observed wake element spacings can be accounted for by known spiral and Kelvin–Helmholtz instabilities in the near wake.
Electronic energy density in chemical reaction systems
Authors: Tachibana Akitomo
Published: July 2002

The energy of chemical reaction is visualized in real space using the electronic energy density nE(r⃗) associated with the electron density n(r⃗). The electronic energy density nE(r⃗) is decomposed into the kinetic energy density nT(r⃗), the external potential energy density nV(r⃗), and the interelectron potential energy density nW(r⃗). Using the electronic energy density nE(r⃗) we can pick up any point in a chemical reaction system and find how the electronic energy E is assigned to the selected point. We can then integrate the electronic energy density nE(r⃗) in any region R surrounding the point and find out the regional electronic energy ER to the global E. The kinetic energy density nT(r⃗) is used to identify the intrinsic shape of the reactants, the electronic transition state, and the reaction products along the course of the chemical reaction coordinate. The intrinsic shape is identified with the electronic interface S that discriminates the region RD of the electronic drop from the region RA of the electronic atmosphere in the density distribution of the electron gas. If the R spans the whole space, then the integral gives the total E. The regional electronic energy ER in thermodynamic ensemble is realized in electrochemistry as the intrinsic Volta electric potential φR and the intrinsic Herring–Nichols work function ΦR. We have picked up first a hydrogen-like atom for which we have analytical exact expressions of the relativistic kinetic energy density nTM(r⃗) and its nonrelativistic version nT(r⃗). These expressions are valid for any excited bound states as well as the ground state. Second, we have selected the following five reaction systems and show the figures of the nT(r⃗) as well as the other energy densities along the intrinsic reaction coordinates: a protonation reaction to He, addition reactions of HF to C2H4 and C2H2, hydrogen abstraction reactions of NH3+ from HF and NH3. Valence electrons possess their unique delocalized drop region remote from those heavily localized drop regions adhered to core electrons. The kinetic energy density nT(r⃗) and the tension density τ⃗S(r⃗) can vividly demonstrate the formation of the chemical bond. Various basic chemical concepts in these chemical reaction systems have been clearly visualized in real three-dimensional space.
Nanoporous structure of methyl-silsesquioxane films using monoenergetic positron beams
Authors: Uedono Akira , Chen Zhi Quan , Suzuki Ryoichi , Ohdaira Toshiyuki , Mikado Tomohisa , Fukui Shiomi , Shiota Atsushi , Kimura Shin-ichi
Published: July 2002

The size and depth distributions of pores in silica-based intermetal-dielectric materials were studied using monoenergetic positron beams. Doppler broadening spectra of the annihilation radiation and lifetime spectra of positrons were measured for methyl-silsesquioxane (MSSQ) spin-on-glass films. The size distribution of pores in the MSSQ films fabricated with 8% porogen load was found to be bimodal, with the major peaks located at 3 and 8 nm3. Increasing the porogen load from 8% to 40% caused the smaller pores (3 nm3) to disappear and 30-nm3 ones to appear; these pores were considered to be interconnected, and this structure makes it possible for positronium (Ps) atoms to find paths towards the surface and to escape into vacuum. The 8%-porogen MSSQ films had low porosity near the Si substrate. From measurements of the temperature dependence of the self-annihilation rate of ortho-Ps, we discuss the relationship between o-Ps emission into vacuum and the pore structure.
Accurate relativistic Gaussian basis sets for H through Lr determined by atomic self-consistent field calculations with the third-order Douglas–Kroll approximation
Authors: Tsuchiya Takashi , Abe Minori , Nakajima Takahito , Hirao Kimihiko
Published: July 2002

Highly accurate relativistic Gaussian basis sets are developed for the 103 elements from H(Z=1) to Lr (Z=103). Orbital exponents are optimized by minimizing the atomic self-consistent field (SCF) energy with the scalar relativistic third-order Douglas–Kroll approximation. The basis sets are designed to have equal quality and to be appropriate for the incorporation of relativistic effects. The basis set performance is tested by calculations on prototypical molecules, hydrides, and dimers of copper, silver, and gold using SCF, Møller–Plesset theory, and the singles and doubles coupled-cluster methods with and without perturbative triples [CCSD, CCSD(T)]. Spectroscopic constants and dissociation energies are reported for the ground state of each species. The effects of relativity, electron correlation, and the basis set superposition error (BSSE) are investigated. At the BSSE corrected CCSD(T) level, the mean absolute error relative to experiment in De for three dimers (hydrides) is 0.13 (0.09) eV; for Re the error is 0.024 (0.003) Å, and for ωe it is 2 (13) cm−1. These illustrative calculations confirm that the present basis sets fulfill their design objectives.
Spectroscopic investigation of tetrahydroisoquinoline in supersonic jet
Authors: Chakraborty Abhijit , Guchhait Nikhil , Banerjee Sudip , Nath Debnarayan , Patwari G. Naresh , Chowdhury Mihir
Published: July 2002

Fluorescence excitation and dispersed emission spectra of tetrahydroisoquinoline are presented here. Two bands at 36 781 and 36 884 cm−1 are confirmed from the spectral hole burning studies as two origins for two conformers. These bands correspond to the inequivalent twist conformers with the hydrogen atom attached to the nitrogen atom at axial and equatorial positions, respectively. The former is found to be the most stable one. SVL spectra are explained on the basis of two theoretically calculated low-frequency vibrations. These correspond to the butterfly and puckering motions of the benzene chromophore, respectively.
Ballistic hole transport in pentacene with a mean free path exceeding 30 μm
Authors: Schön Jan Hendrik , Kloc Christian , Batlogg Bertram
Published: July 2002

Low temperature ballistic hole transport in high-quality pentacene single crystals is studied using macroscopic gated four-terminal van-der-Pauw geometry and two-terminal conventional field-effect transistor structures. Well-pronounced magnetic focusing peaks are observed in the magnetoresistance of van-der-Pauw samples indicating ballistic transport up to at least 45 μm. At low temperature the field-effect transistors show saturation currents independent of the channel length and proportional to the channel width. The transistor characteristics are consistently analyzed assuming ballistic transport of holes throughout the whole length of the channel.

At the request of the authors and editor, this article is being retracted effective 1 March 2003.
Following transformation in self-assembled alkanethiol monolayers on Au(111) by in situ scanning tunneling microscopy
Authors: Vericat C. , Andreasen G. , Vela M. E. , Martin H. , Salvarezza R. C.
Published: July 2002

Transformations taking place in alkanethiol adlayers on Au(111) in contact with an electrolyte solution in the potential range −1.1 V<E<−0.4 V have been investigated by in situ scanning tunneling microscopy (STM). Two different types of processes have been identified: potential-dependent transformations involving electrodesorption/electroadsorption of molecules, and structure and defect fluctuations that take place without the influence of the electric potential. Potential-dependent transformations involve two steps: electrodesorption of chemisorbed alkanethiol molecules from Au(111) terraces producing unstable physisorbed aggregates of alkanethiolates, and electrodesorption of alkanethiol molecules that remained chemisorbed at step edges due to the increased coordination number at these substrate sites.
Anisotropic electrical conductivity of delafossite-type CuAlO2 laminar crystal
Authors: Lee M. S. , Kim T. Y. , Kim D.
Published: July 2002

Delafossite-type CuAlO2 laminar crystals (R3̄m) were prepared through melt by a cooling method from 1423 K. The layer-by-layer structure of the crystal was observed. Because of the structural anisotropy of the crystal, electrical conductivity along the ab plane (σab) was higher than that along the c axis (σc), σab≳25σc. The anisotropy unveiled that the main conduction path of the crystal is closed-packed Cu+ layers. The values of the activation energies which were estimated from the Arrhenius plot were ∼0.20 and ∼0.13 eV for σc and σab, respectively. The linearity in the log σ vs (1/T)1/4 plot and the positive thermoelectric power (>+300 μV/K) of the crystal suggested p-type variable-range hopping conduction.
Non-Heisenberg magnetic behavior of a triangular bridged heterometallic Fe2(III)Co(II) complex: Evidence of strong orbital contributions
Authors: Tsukerblat B. S. , Palii A. V. , Mirovitskii V. Yu. , Ostrovsky S. M. , Turta K. , Jovmir T. , Shova S. , Bartolome J. , Evangelisti M. , Filoti G.
Published: July 2002

We report the synthesis, magnetization and ac susceptibility of the triangular μ3-oxo-bridged heterometallic complex [Fe2CoO(CH3COO)6(3-Cl-Py)3], in powder form. The model Hamiltonian is elaborated involving isotropic exchange interactions Fe(III)-Fe(III) and Co(II)-Fe(III) dealing with the real spins of high-spin Co(II) and Fe(III) ions, and also spin-orbit coupling and low-symmetry crystal field (C2v) acting within the T1g4 ground manifold of Co(II) ion. The outlined general model takes into account strong anisotropic orbital contribution to the magnetic characteristics of the whole system arising from Co(II) ion with unquenched orbital angular momentum. The treatment of the Hamiltonian is performed with the use of the irreducible tensor operator technique. The influence of the exchange and spin-orbital interactions on the magnetic properties of Fe2(III)Co(II) clusters is revealed. The result of calculations in the framework of the isotropic model (neglecting low-symmetry crystal field) are in a good agreement with the experimentally observed magnetization versus magnetic field, as well as with the unusual temperature behavior of the magnetic susceptibility of the powder samples (1.8–250 K). The states of the system are enumerated by the quantum number of the total angular momentum J, these hybrid states are combined from the sets of full and intermediate spins of Fe2Co triangle and orbital angular momentum of Co(II), (with fictious L=1). The best fit to the experimental data is achieved for the exchange parameters JFe-Fe=−87.5 cm−1, JFe-Co=−17 cm−1 and orbital reduction factor for Co(II) ion κ=0.93. The ground state and the first excited one are found to possess total angular momentum J=1/2, Effective g-factors for the ground- and first-excited states calculated using the hybrid spin-orbital functions prove to be 4.3 and 0.75, that are distinctly different from the pure spin value 2 peculiar for the Heisenberg model. The comparison of the magnetic measurements with the theory clearly demonstrates the insufficiency of a simple Heisenberg spin-coupling model as evidenced by the discovered strong orbital magnetic contribution.
Green emission from cerium hydroxide layers formed in Si/In/CeO2/Si structures
Authors: Kim Chong-Geol
Published: July 2002

Green photoluminescence (PL) was observed from the Si/In/CeO2 on Si substrates at room temperature. Indium was very thinly deposited on CeO2 at 400 °C. X-ray photoelectron spectroscopy indicated the formation of a cerium hydroxide in the indium-included Si/CeO2 structure. The reacting CeO2 with In became the defective CeO2−x, and then the defective CeO2−x was changed to Ce(OH)4 in poor vacuums and air. The green PL was due to Ce(OH)4. The luminescence disappeared by high-temperature annealing. Another PL peak appeared from the sample annealed at 1000 °C.
Multipeak negative-differential-resistance device by combining single-electron and metal–oxide–semiconductor transistors
Authors: Inokawa Hiroshi , Fujiwara Akira , Takahashi Yasuo
Published: July 2002

A multipeak negative-differential-resistance device is proposed. The device comprises a single-electron transistor (SET) and a metal–oxide–semiconductor field-effect transistor (MOSFET), and can, in principle, generate an infinite number of current peaks. Operation of the proposed device is verified at 27 K with a SET fabricated by the pattern-dependent oxidation process and a MOSFET on the same silicon-on-insulator wafer. Six current peaks and a peak-to-valley current ratio of 2.1 are obtained, and multiple-valued memory operation is successfully demonstrated.
Space-charge-limited currents in materials with Gaussian energy distributions of localized states
Authors: Arkhipov V. I. , Heremans P. , Emelianova E. V. , Adriaenssens G. J.
Published: July 2002

A model is formulated describing trap-controlled space-charge-limited currents (SCLCs) in an organic material with a Gaussian density-of-states (DOS) distribution. It is shown that SCLC is not always controlled by carrier release from localized states around the Fermi level and, therefore, a Gaussian DOS can serve as either shallow or deep distribution of localized states depending upon the carrier and/or current density.
Structural and thermoelectric transport properties of Sb2Te3 thin films grown by molecular beam epitaxy
Authors: Kim Yunki , DiVenere Antonio , Wong George K. L. , Ketterson J. B. , Cho Sunglae , Meyer Jerry R.
Published: July 2002

We have studied the structural and transport properties of Sb2Te3 thin films prepared by molecular beam epitaxy as a function of the Te/Sb flux ratio during deposition. Both the crystallinity and the transport properties are found to be strongly affected by nonstoichiometry. The most stoichiometric sample (prepared with a Te/Sb ratio of 3.6) had a high degree of crystallinity, high thermopower, and high carrier mobility. However, Sb2Te3 films with excess Sb or Te had poorer crystallinity, reduced magnitude of the thermopower, and reduced mobility as a result of the formation of antisite defects. These antisite defects were able to be reduced by controlling the relative flow rate ratio of Te to Sb during growth.
Neutralization of static surface charges by an ac ionizer in a nitrogen and dry air environment
Authors: Chang J. S. , Berezin A. A. , Noll C. G.
Published: July 2002

To elucidate the dynamics of static charge elimination, we measured charge decay and residual potential (balance or offset voltage) in gases with various ion mobilities. It was observed that surface charge decay, especially for positive charges, occurs much faster in nitrogen than in air. The residual potential on the probe is negative in pure nitrogen and increases toward positive values with the injection of small quantities of air in front of the ionizer. The fluctuations in the residual potential are generally less than 3 V peak-to-peak. For ionizer operations in nitrogen environments, the charge decay rate increases with superficial gas-flow rate. The results are consistent with a theory of the interaction between bipolar ions and a charged object.
Isotope effects in the electron impact ionization of H2/D2, H2O/D2O, and C6H6/C6D6 near threshold
Authors: Hanel G. , Gstir B. , Fiegele T. , Hagelberg F. , Becker K. , Scheier P. , Snegursky A. , Märk T. D.
Published: July 2002

Appearance energies of all parent ions and several fragment ions produced by electron impact ionization of the isotope systems H2/D2, H2O/D2O, and C6H6/C6D6 were determined with high precision using a dedicated high-resolution electron impact ionization mass spectrometer. The determination of the appearance energies from scans of the ion signal as a function of electron energy in the near-threshold region of each ion utilized a fitting and analysis procedure that has recently been successfully applied to the determination of appearance energies of singly and multiply charged rare-gas ions and several molecular ions and cluster ions. The experimentally determined appearance energies are in good agreement (i) with theoretical calculations that we carried out using standard quantum chemistry codes and (ii) with appearance energy values listed in standard reference data tables (to the extent that tabulated values are available). We find isotope shifts for all three systems ranging from a few meV for the parent ions to a few hundred meV for the fragment ions. The deuterated species always have the higher appearance energy. The present results for H2O/D2O and C6H6/C6D6 do not confirm the larger isotope effects that were reported earlier by Snegursky and Zavilopulo [Nucl. Instrum. Meth. Phys. Res. B 126, 301 (1997)].
Pyrolysis approach to the synthesis of gallium nitride nanorods
Authors: Han Wei-Qiang , Zettl Alex
Published: July 2002

Herein we describe a pyrolysis route to the synthesis of gallium nitride (GaN) nanorods. GaN nanorods have been grown by the pyrolysis of gallium dimethylamide and ferrocene under an ammonia atmosphere. High-resolution transmission electron microscopy and energy dispersive x-ray spectrometer show that they are GaN single crystals, the sizes of which vary from 3 to 30 μm in length and 15 to 70 nm in diameter. Iron acts as an important catalyst for the GaN nanorod growth.
Low-energy resonant structures in electron scattering from C20 fullerene
Authors: Gianturco F. A. , Kashenock G. Yu. , Lucchese R. R. , Sanna N.
Published: July 2002

We have performed the first ab initio computational investigation of the elastic scattering of electrons by the isolated cluster which was described with its symmetry lowered (due to Jahn–Teller distortion) to the C2 and Ci point groups. The energy range considered was 1–10 eV. The geometry and electronic energy of the molecule were taken to be those of its ground state. The total and partial cross sections were calculated through a coupled-channel dynamics with inclusion of a parameter-free model exchange and correlation-polarization potentials. The scattering process has been found to exhibit a rather complex resonant structure due to the special “hollow” framework of the molecular cage. One distinguishing feature of the cross sections is the presence of strong near-threshold peaks which we attribute to a series of C20− metastable negative ions. The present results therefore provide a benchmark calculation which could be of guidance to future experiments on the very recently produced fullerene C20 species. In fact, the analysis carried out in the present work allows us to assign each scattering resonance to a specific molecular state, their symmetries and parameters obtained from our calculations.
Electron–phonon coupling in negatively charged acene- and phenanthrene-edge-type hydrocarbon crystals
Authors: Kato Takashi , Yoshizawa Kazunari , Hirao Kazuyuki
Published: July 2002

Vibronic interaction and its role in the occurrence of possible superconductivity in the monoanions of phenanthrene-edge-type aromatic hydrocarbons are studied. The vibrational frequencies and the vibronic coupling constants are computed and analyzed and the electron–phonon coupling constants are estimated. The results for phenanthrene-edge-type hydrocarbons are compared with those for acene-edge-type hydrocarbons. The lowest frequency mode and the C–C stretching modes of 1400–1600 cm−1 afford large electron–phonon coupling constants in the monoanions of acene- and phenanthrene-edge-type hydrocarbons. The total electron–phonon coupling constants decrease with an increase in the number of carbon atoms in both acene- and phenanthrene-edge-type hydrocarbons, but those for the monoanions of phenanthrene-edge-type hydrocarbons are larger than those for the monoanions of acene-edge-type hydrocarbons. Possible superconducting transition temperatures Tcs for the monoanions are estimated. The monoanions of phenanthrene-edge-type hydrocarbons would have higher Tcs than the monoanions of acene-edge- type hydrocarbons if phenanthrene-edge-type hydrocarbons exhibit superconductivity. These results suggest that molecular edge structures as well as molecular sizes have relevance to the strength of electron–phonon coupling and Tcs. The fragment molecular-orbital method (FMO) method successfully characterizes the distinct electronic structures of the two small polynuclear aromatic hydrocarbons (PAHs) with different type of edges such as anthracene and phenanthrene.