bluuebunny/crossref_metadata_2025 huggingface dataset:
Solutions of the matched KV envelope equations for a “smooth” asymmetric focusing channel
Authors: Reiser Martin , Li Hui
Published: June 2004

In many particle accelerators the applied focusing forces may differ in two or three directions either by design in order to avoid resonances or for other reasons, such as design constraints, bunch compression/expansion, dispersion, etc. At high intensities, space charge effects and related collective forces may cause unwanted emittance growth via instabilities and equipartitioning (relaxation of temperature anisotropy). For the transverse two-dimensional case, such asymmetric (anisotropic) systems are described by the coupled, matched envelope equations of the Kapchinskij–Vladimirskij distribution with different focusing strengths and emittances in the x and y directions, which must be solved numerically for a periodic lattice. In this article, we present results for a “smooth” asymmetric focusing channel, in which case one obtains a set of two coupled algebraic equations for the envelopes X and Y. Though the algebraic equations can easily be solved numerically, the scaling with the physics parameters is usually obscured by the numerical procedures. We derived an approximate solution as well as a general, more accurate solution, both of which represent results that exhibit the scaling with the applied focusing, space-charge, and emittance terms. The accuracy of the approximate solution is in the range of a few percent for a channel with a small degree of asymmetry. The general solution is obtained by solving for the aspect ratio A=Y/X by an iteration method that yields results to any desired degree of accuracy. More importantly, to facilitate the comparison between systems with different particle species and/or operating parameters, the envelope equations in this general treatment are written in dimensionless form. This is accomplished by expressing the envelopes X and Y in terms of the “average radius” as, and by introducing dimensionless parameters, v and w, which measure the degrees of focusing and emittance asymmetries, and the ratios of the space charge to the external focusing forces, defined by the intensity parameter χ. The results are then used to obtain formulas for the frequencies, or wave numbers, of the betatron oscillations and the tune depressions due to space-charge forces in the x and y directions, which are of fundamental importance for understanding the beam physics. These dimensionless relations exhibit the desired beam physics scaling and represent mathematically convenient forms for calculating, tabulating or plotting both exact as well as approximate solutions for the various quantities of interest. Two examples, including figures, are presented to illustrate the practical use of the theoretical relations. Our results should be useful for machine designers, theorists and experimentalists.
Optimization of transmission properties of two-dimensional photonic crystal channel waveguide bends through local lattice deformation
Authors: Ntakis Iraklis , Pottier Pierre , De La Rue Richard M.
Published: June 2004

We present an extended study of different topologies for lattice-deformed two-dimensional single line-defect (W1) photonic crystal channel waveguide bends and their effect on the optimization of the bend transmission properties. An enhancement of the spectral response by a factor of six was obtained, with the optimal design providing transmission greater than 95% over a 5.2% relative bandwidth. Experimental results for devices realized in III–V semiconductor epitaxial structure show a transmission efficiency of 95%.
Photoluminescence energy and linewidth in GaN/AlN stackings of quantum dot planes
Authors: Kalliakos S. , Bretagnon T. , Lefebvre P. , Taliercio T. , Gil B. , Grandjean N. , Damilano B. , Dussaigne A. , Massies J.
Published: June 2004

We analyze the room temperature photoluminescence properties of several multilayer stackings of GaN/AlN quantum dots. We report drastic differences of emission energies and linewidths between continuous wave and time-resolved photoluminescence experiments. In continuous wave experiments, the screening of internal electric fields by accumulation of e-h pairs in quantum dot planes induces a blue-shift as well as an unexpected narrowing of the emission line, when the laser intensity is increased. Under intense, pulsed excitation, in time-resolved photoluminescence, a substantial blue-shift is induced, due to the partial cancelation of the quantum confined Stark effect. When the system is again free to relax, we observe a time-dependent red-shift of the line, which maintains a fairly constant width. We attribute the observed behavior of energies and linewidths to the intricate contributions of the in-plane distribution of dot sizes and of the depth-dependent decrease of the degree of excitation of the different planes. We support our interpretations by the use of a model based on a self-consistent solution of the Schrödinger and Poisson equations within the envelope function approximation.
Resonance coupling in the fourth OH-stretching overtone spectrum of formic acid
Authors: Howard Daryl L. , Kjaergaard Henrik G.
Published: June 2004

The room-temperature vibrational overtone spectra of the formic acid isotopomers HCOOH and DCOOH have been recorded in the third and fourth OH-stretching overtone regions with intracavity laser photoacoustic spectroscopy. Resonance coupling between the OH- and CH-stretching vibrations in HCOOH is clearly identified in the fourth overtone region. This is an example of strong coupling across bonds. In the third overtone region, no resonance is observed. Vibrational energies and intensities of the OH- and CH-stretching overtones and combination bands have been calculated with an anharmonic oscillator local mode model. The pure OH-stretching bright state carries almost all the intensity prior to resonance coupling.
Spin relaxation times of two-dimensional holes from spin sensitive bleaching of intersubband absorption
Authors: Schneider Petra , Kainz J. , Ganichev S. D. , Danilov S. N. , Rössler U. , Wegscheider W. , Weiss D. , Prettl W. , Bel’kov V. V. , Glazov M. M. , Golub L. E. , Schuh D.
Published: June 2004

We present spin relaxation times of two-dimensional holes obtained by spin sensitive bleaching of the absorption of infrared radiation in p-type GaAs/AlGaAs quantum wells (QWs). It is shown that the saturation of intersubband absorption of circularly polarized radiation is mainly controlled by the spin relaxation time of the holes. The saturation behavior has been determined for different QW widths and in a wide range of temperatures with the result that the saturation intensity substantially decreases with narrowing of the QWs. Spin relaxation times are derived from the measured saturation intensities by making use of calculated (linear) absorption coefficients for direct intersubband transitions. It is shown that spin relaxation is due to the D’yakonov–Perel’ mechanism governed by hole–hole scattering. The problem of selection rules is addressed.
Atomistic simulations of spontaneous etching of silicon by fluorine and chlorine
Authors: Humbird David , Graves David B.
Published: June 2004

Updated interatomic potential energy functions for Si–F and Si–Cl are used in molecular dynamics simulations of spontaneous etching of Si. Steady halogen uptake and spontaneous silicon etching are predicted as F and Cl atoms impact initially crystalline Si. At 300 K, the simulated etch probability (silicon atoms etched per incident F atom) is 0.03 for F atoms and 0.005 for Cl. The major etch products are SiF4 and Si2F6 for F etching and SiCl4 for Cl. Etching is not observed with F2 or Cl2. At 300 K and below, the simulation predictions are within the range of reported experimental measurements of the surface coverage, etch reaction probability, and etch product distribution. Etch products that remain weakly bound to the surface are detected in significant quantities. At higher temperature (T>450 K), SiF2 and SiCl2 become the dominant etch products as the temperature is increased, in agreement with experiment, but the simulation underpredicts the etch reaction probability. The atomistic mechanisms of etch product formation are examined. At T>450 K, internal decomposition of the halogenated silicon layer becomes a dominant mechanism.
Neutron transmutation doped far-infrared p-Ge laser
Authors: Nelson E. W. , Dolguikh M. V. , Muravjov A. V. , Flitsiyan E. S. , Du Bosq T. W. , Peale R. E. , Kleckley S. H. , Fredricksen C. J. , Vernetson W. G.
Published: June 2004

A far-infrared p-type germanium laser with active crystal prepared from ultra pure single-crystal Ge by neutron transmutation doping (NTD) is demonstrated. Calculations show that the high uniformity of Ga acceptor distribution achieved by NTD significantly improves average gain. The stronger ionized impurity scattering due to high compensation in NTD Ge is shown to have insignificant negative impact on the gain at the moderate doping concentrations sufficient for laser operation. Experimentally, this first NTD laser is found to have lower current-density lasing threshold than the best of a number of melt-doped laser crystals studied for comparison.
Propagation of a shear-horizontal surface acoustic mode in a periodically grooved AlN/Al2O3 microstructure
Authors: Xu J. , Thakur J. S. , Zhong F. , Ying H. , Auner G. W.
Published: June 2004

We investigate the shear-horizontal surface acoustic waves (SH-SAWs) generated on an AlN/Al2O3 microstructure by laser-micromachined grooves on the AlN film. In the absence of grooves, the AlN/Al2O3 device shows resonance for only a lower velocity SAW mode. However, when grooves of periodicity smaller than half the wavelength of the surface acoustic wave are micromachined, a higher velocity resonance due to a SH-SAW mode is observed in the device. It is found that our SH-SAW mode remains undamped and is able to propagate across the device when loaded with a biofluidic mass. We have also measured the mass sensitivity of the SH-SAW mode under biofludic load in terms of frequency shift. Measurements show that this mode has a very high mass sensitivity of the order 1.35 ng/ml, which suggests that the device can be applied for the detection of a small concentration of antigens in a biofluid. In this article, the fabrication and grooving techniques of the device are also addressed.
An ab initio correlated study of the potential energy surface for the HOBr.H2O complex
Authors: Santos Cristina Maria P. , Faria Roberto , Machado Sérgio P. , De Almeida Wagner B.
Published: June 2004

The potential energy surface (PES) for the HOBr.H2O complex has been investigated using second- and fourth-order Møller–Plesset perturbation theory (MP2, MP4) and coupled cluster theory with single and doubles excitations (CCSD), and a perturbative approximation of triple excitations (CCSD-T), correlated ab initio levels of theory employing basis sets of triple zeta quality with polarization and diffuse functions up to the 6-311++G(3dp,3df ) standard Pople’s basis set. Six stationary points being three minima, two first-order transition state (TS) structures and one second-order TS were located on the PES. The global minimum syn and the anti equilibrium structure are virtually degenerated [ΔEele-nuc≈0.3 kcal mol−1, CCSD-T/6-311++G(3df,3pd) value], with the third minima being ∼4 kcal mol−1 away. IRC analysis was performed to confirm the correct connectivity of the two first-order TS structures. The CCSD-T/6-311++G(3df,3pd)//MP2/6-311G(d,p) barrier for the syn⇔anti interconversion is 0.3 kcal mol−1, indicating that a mixture of the syn and anti forms of the HOBr.H2O complex is likely to exist.
Multiple “time step” Monte Carlo simulations: Application to charged systems with Ewald summation
Authors: Bernacki Katarzyna , Hetényi Balázs , Berne B. J.
Published: June 2004

Recently, we have proposed an efficient scheme for Monte Carlo simulations, the multiple “time step” Monte Carlo (MTS-MC) [J. Chem. Phys. 117, 8203 (2002)] based on the separation of the potential interactions into two additive parts. In this paper, the structural and thermodynamic properties of the simple point charge water model combined with the Ewald sum are compared for the MTS-MC real-/reciprocal-space split of the Ewald summation and the common Metropolis Monte Carlo method. We report a number of observables as a function of CPU time calculated using MC and MTS-MC. The correlation functions indicate that speedups on the order of 4.5–7.5 can be obtained for systems of 108–500 waters for n=10 splitting parameter.
Local surface elastic constants by resonant-ultrasound microscopy
Authors: Tian Jiayong , Ogi Hirotsugu , Tada Toyokazu , Hirao Masahiko , Ledbetter Hassel
Published: June 2004

We report a method—resonant-ultrasound microscopy—for measuring elastic-constant distribution over a solid’s surface. Applying an oscillating electric field to a rectangular-parallelepiped oscillator of langasite (La3Ga5SiO14) crystal by a surrounding solenoid coil, we generated and detected vibrations of the crystal without electrodes and without wires. Acoustic coupling of the specimen to the oscillator is only made at an antinodal vibration point on the crystal’s bottom surface. The crystal’s resonance-frequency shift reflects elastic constants of the specimen in the contacting area. Point-contact measurement permits sensitive, quantitative evaluation of a material’s local elastic constants. As an illustrating example, we measured the elastic-stiffness distribution of a Nb–Ti/Cu resin superconductive wire. We compared our measurements with both static-contact and dynamic-contact models.
Features of self-supporting tungsten nanowire deposited with high-energy electrons
Authors: Liu Z. Q. , Mitsuishi K. , Furuya K.
Published: June 2004

The features of self-supporting tungsten nanowire fabricated by electron-beam-induced deposition using 200 keV electrons were investigated. The width of wire first decreases with the increase of the scan speed, then saturates at about 7–10 nm when the scan speed is higher than 10 nm/s. The wire has belt-shipped morphology elongated along the incident beam. The wire parallel to the substrate surface was fabricated at the beam scan speed of 4.0 nm/s, while those with upward and downward features were obtained by changing the scan speed. Nanobelt, nanorod, and nanotip with high aspect ration and small lateral size were fabricated with this method. Considering the forward scattering of electrons and the beam Gaussian profile, a model was developed for the growth of wire using high-energy electrons.
Annealing of defect density and excess currents in Si-based tunnel diodes grown by low-temperature molecular-beam epitaxy
Authors: Chung Sung-Yong , Jin Niu , Pavlovicz Ryan E. , Berger Paul R. , Yu Ronghua , Fang Zhaoqiang , Thompson Phillip E.
Published: June 2004

Deep-level transient spectroscopy (DLTS) measurements were performed in order to investigate the effects of post-growth heat treatment on deep level defects in Si layers grown by low-temperature molecular-beam epitaxy (LT-MBE) at 320 °C. In the LT-MBE as-grown samples, two dominant divacancy-related complex defects, of which the possible origins are suggested as P–V (E center)+V–V (0/−) and V–V (−2/−) and others, were observed in P-doped n layers. When the as-grown samples were annealed at 700, 800, and 900 °C for 60 s by rapid thermal annealing, the total density of defects were decreased without generating other defects and most defects were annihilated at 900 °C. This study also compared the DLTS trends with performance of Si-based resonant interband tunnel diodes (RITDs) in terms of peak current density, valley current density, and peak-to-valley current ratio, which are closely related to the deep-level defects. The active regions of the RITDs were grown at the same substrate growth temperature and annealed at similar temperatures used in this DLTS study.
A theory of electrical conductivity of molten salt. II
Authors: Koishi T. , Tamaki S.
Published: June 2004

In molten equivalent salts, a unified relation that the ratio of partial dc conductivities is always equal to the inverse mass ratio, viz. σ+/σ−=m−/m+, has been confirmed in view of microscopic point starting from the generalized Langevin equation. It has been newly proved that the memory functions assumed to be an exponentially decaying form in the Langevin equation for cation and anion are equal to each other. Furthermore, the short time behaviors of moving ions are analyzed in terms of their velocity correlation functions, according to the combination of generalized Langevin equation and damped Einstein oscillator equation. A minor correction for the expression of common friction constant derived before has been given. Estimated results for conductivities are close to those obtained by computer simulation and also comparable to experimental values.
GeO
2
fibers: Preparation, morphology and photoluminescence property
Authors: Viswanathamurthi P. , Bhattarai N. , Kim H. Y. , Khil M. S. , Lee D. R. , Suh E.-K.
Published: June 2004

Nanomicron to submicron fibers of GeO2 have been prepared using poly(vinyl acetate) and germanium dioxide sol by electrospinning followed by high temperature calcination. The morphology of the fibers have been studied by scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. X-ray diffraction indicates that the fibers are single crystal with hexagonal α-phase quartz-like structure. At room temperature, the fibers show photoluminescence under excitation at 325 nm. The fibers may have potential applications in one-dimensional optoelectronic nanodevices.
The interaction of oxygen with TiC(001): Photoemission and first-principles studies
Authors: Rodriguez J. A. , Liu P. , Dvorak J. , Jirsak T. , Gomes J. , Takahashi Y. , Nakamura K.
Published: June 2004

High-resolution photoemission and first-principles density-functional slab calculations were used to study the interaction of oxygen with a TiC(001) surface. Atomic oxygen is present on the TiC(001) substrate after small doses of O2 at room temperature. A big positive shift (1.5–1.8 eV) was detected for the C 1s core level. These photoemission studies suggest the existence of strong O↔C interactions. A phenomenon corroborated by the results of first-principles calculations, which show a CTiTi hollow as the most stable site for the adsorption of O. Ti and C atoms are involved in the adsorption and dissociation of the O2 molecule. In general, the bond between O and the TiC(001) surface contains a large degree of ionic character. The carbide→O charge transfer is substantial even at high coverages (>0.5 ML) of oxygen. At 500 K and large doses of O2, oxidation of the carbide surface occurs with the removal of C and formation of titanium oxides. There is an activation barrier for the exchange of Ti–C and Ti–O bonds which is overcome only by the formation of C–C or C–O bonds on the surface. The mechanism for the removal of a C atom as CO gas involves a minimum of two O adatoms, and three O adatoms are required for the formation of CO2 gas. Due to the high stability of TiC, an O adatom alone cannot induce the generation of a C vacancy in a flat TiC(001) surface.
The effect of atomic substitution on electron–phonon interactions in negatively charged B, N-substituted acenes
Authors: Kato Takashi , Yamabe Tokio
Published: June 2004

Electron–phonon interactions in the monoanions of B, N-substituted acenes such as B3N3F6 (1f) and B5N5F8 (2f) are studied, and compared with those in the monoanions of B3N3H6 (1h) and B5N5H8 (2h), and B3N3D6 (1d) and B5N5D8 (2d). The low frequency modes around 500 cm−1 as well as the frequency modes higher than 1000 cm−1 strongly couple to the lowest unoccupied molecular orbitals (LUMO) in 1f and 2f. The total electron–phonon coupling constants (lLUMO) are estimated to be 2.710 and 2.054 eV for 1f and 2f, respectively, and those are estimated to be 0.342 and 0.235 eV for 1d and 2d, respectively, while those were estimated to be 0.340 and 0.237 eV for 1h and 2h, respectively. That is, the lLUMO value increases much more significantly by H–F substitution than by H–D substitution in B, N-substituted acenes. The larger displacements of B and N atoms in the vibronic active modes in 1f and 2f than those in 1d and 2d due to larger atomic mass of fluorine than that of deuterium, and the phase patterns difference between the LUMO in 1f and 2f, in which the atomic orbitals between N and its neighboring F atoms form strong σ-antibonding interactions, and that in 1d and 2d, in which the atomic orbitals between two neighboring B and N atoms form weak π-bonding and π-antibonding interactions, are the main reason why the lLUMO value increases much more significantly by H–F substitution than by H–D substitution. The reorganization energies between the neutral molecules and the corresponding monoanions are estimated to be 0.122, 0.063, 0.733, and 0.830 eV for 1h, 2h, 1f, and 2f, respectively. Therefore, the estimated reorganization energies between the neutral molecules and the corresponding monoanions for 1f and 2f are much larger than those for 1h and 2h.
Ab initio prediction of the potential energy surface and vibrational-rotational energy levels of dialuminum monoxide, Al2O
Authors: Koput Jacek , Gertych Artur
Published: June 2004

The equilibrium structure and potential energy surface of dialuminum monoxide, Al2O, have been determined from large-scale ab initio calculations using the coupled-cluster method, CCSD(T), in conjunction with basis sets of triple- through quintuple-zeta quality. The effects of core-electron correlation on the calculated molecular parameters were investigated. The vibrational-rotational energy levels of the Al2 16O and Al2 18O isotopic species were calculated by a variational approach. The predicted energy levels are in remarkably good agreement with the available experimental spectroscopic data (from laser-induced fluorescence), demonstrating that the Al2O molecule is linear at equilibrium in its ground electronic state. The reported theoretical data settle controversies between the experimental studies about the equilibrium structure and assignment of vibrational fundamentals of the Al2O molecule.
The trust-region self-consistent field method: Towards a black-box optimization in Hartree–Fock and Kohn–Sham theories
Authors: Thøgersen Lea , Olsen Jeppe , Yeager Danny , Jørgensen Poul , Sałek Paweł , Helgaker Trygve
Published: June 2004

The trust-region self-consistent field (TRSCF) method is presented for optimizing the total energy ESCF of Hartree–Fock theory and Kohn–Sham density-functional theory. In the TRSCF method, both the Fock/Kohn–Sham matrix diagonalization step to obtain a new density matrix and the step to determine the optimal density matrix in the subspace of the density matrices of the preceding diagonalization steps have been improved. The improvements follow from the recognition that local models to ESCF may be introduced by carrying out a Taylor expansion of the energy about the current density matrix. At the point of expansion, the local models have the same gradient as ESCF but only an approximate Hessian. The local models are therefore valid only in a restricted region—the trust region—and steps can only be taken with confidence within this region. By restricting the steps of the TRSCF model to be inside the trust region, a monotonic and significant reduction of the total energy is ensured in each iteration of the TRSCF method. Examples are given where the TRSCF method converges monotonically and smoothly, but where the standard DIIS method diverges.
Electron density and two-channel neutron emission measurements in steady-state spherical inertial-electrostatically confined plasmas, with review of the one-dimensional kinetic model
Authors: Dobson Chris C. , Hrbud Ivana
Published: June 2004

Electron density measurements have been made in steady-state plasmas in a spherical inertial electrostatic confinement (IEC) discharge using microwave interferometry. Plasma cores interior to two cathodes, having diameters of 15 and 23 cm, respectively, were probed over a transverse range of 10 cm with a spatial resolution of about 1.4 cm for buffer gas pressures from 0.2 to 6 Pa in argon and deuterium. The transverse profiles are generally flat, in some cases with eccentric symmetric minima, and give mean densities of from ≈0.4 to 7×1010 cm−3, the density generally increasing with the neutral gas pressure. Numerical solutions of the one-dimensional Poisson equation for IEC plasmas are reviewed and energy distribution functions are identified which give flat transverse profiles. These functions are used with the plasma approximation to obtain solutions which also give densities consistent with the measurements, and a double potential well solution is obtained which has minima qualitatively similar to those observed. Explicit consideration is given to the compatibility of the solutions interior and exterior to the cathode, and to grid transparency. Deuterium fusion neutron emission rates were also measured and found to be isotropic, to within the measurement error, over two simultaneous directions. Anisotropy was observed in residual emissions during operation with nonfusing hydrogen-1. The deuterium rates are consistent with predictions from the model. 2004 American Institute of Physics.
Effect of p-type activation ambient on acceptor levels in Mg-doped GaN
Authors: Nakano Yoshitaka , Fujishima Osamu , Kachi Tetsu
Published: June 2004

The effect of p-type activation ambient on acceptor levels in Mg-doped GaN grown by metalorganic chemical-vapor deposition has been investigated electrically. GaN samples with Mg concentrations of ∼2.7×1019 cm−3 were activated at 700 °C for 5 min in pure O2 and N2 gas ambients, respectively. From room-temperature Hall-effect measurements, the sheet carrier concentration of the O2-annealed sample was ∼2.3×1013 cm−2, a value about three times higher than that of the N2-annealed sample. However, capacitance–voltage, thermal admittance spectroscopy, and current deep-level transient spectroscopy measurements on Schottky diodes fabricated showed no significant difference in Mg acceptor level between them. In addition, secondary ion mass spectrometry and atomic force microscopy measurements revealed that the O2 annealing enhanced the removal of residual H atoms and the formation of Ga-oxide islands with surface roughening, compared to conventional N2 annealing. These results indicate that the O2 anneal induces lowering of the surface resistance via the conductive Ga-oxide phase rather than the enhancement of the p-type doping characteristics in the near surface region.
Two-color resonance-enhanced multiphoton ionization study of the lowest Rydberg p state of bis(η6-benzene)chromium and its deuterated derivatives
Authors: Ketkov Sergey Yu. , Selzle Heinrich L. , Schlag Edward W.
Published: June 2004

Two-color resonance-enhanced multiphoton ionization (REMPI) spectra of jet-cooled (η6-C6H6)2Cr(1), (η6-C6D6)2Cr(2), and (η6-C6D6)(η6-C6D5H)Cr(3) have been measured with use of the 3dz2→R4px,y Rydberg transition as the first step of the electronic excitation. The 000 Rydberg component shifts by 59 and 54 cm−1 to red when one goes from 1 to 2 and 3, respectively. Surprisingly, the REMPI spectra of 1–3 show very rich vibronic structures revealing both totally symmetric vibrations and degenerate vibrational modes. Presence of intense peaks corresponding to the e2g modes in the spectra of 1 and 2 is indicative of Jahn–Teller coupling in the R4px,y Rydberg state. Additional REMPI resonances appear on going from 1 and 2 to 3 as a result of the symmetry reduction. The vibronic components in the spectra of 1–3 were assigned on the basis of the selection rules and comparison with the vibrational frequencies of the 1 and 2 ground-state molecules. The frequencies of over 10 normal vibrations have been determined for the gas-phase 1–3 Rydberg-state molecules from the REMPI experiment. The wavenumber corresponding to the lowest-energy mode (the ring torsion vibration) appears to be 40 cm−1 in 1 and 35 cm−1 in the deuterated complexes. The REMPI peaks are homogeneously broadened. The lower lifetime limits for the upper-state components increase on going from the vibrationless level to higher-lying vibronic states and on going from 1 to the deuterated derivatives.
Interface formation of pentacene on Al2O3
Authors: Watkins N. J. , Zorba S. , Gao Yongli
Published: June 2004

We examined the interface formed by pentacene deposition onto an Al2O3 substrate. We found that upon pentacene deposition onto Al2O3 the pentacene vacuum level aligns with that of Al2O3. We observe the immediate appearance of a measurable pentacene molecular orbital near the Fermi level upon deposition of as little as 2 Å of pentacene onto the Al2O3 surface. This suggests that there are no chemical bonds at this interface. The vacuum level change at the interface dipole is found to be less than 0.25 eV, and at least part of it can be attributed to the oxygen adsorbates at the in situ prepared aluminum oxide surface.
Nucleation sites of recombination-enhanced stacking fault formation in silicon carbide p-i-n diodes
Authors: Ha S. , Skowronski M. , Lendenmann H.
Published: June 2004

The morphology and nucleation sites of stacking faults formed during the forward operation of 4H silicon carbide p-i-n diodes were investigated using optical emission microscopy (OEM) and transmission electron microscopy (TEM). Partial dislocations bounding the stacking faults are mostly aligned to the 〈11–20〉 directions with Burgers vectors of the 1/3〈1–100〉 type. Arrays of dislocation half loops in the blocking layer serve as nucleation sites of double-rhombic stacking faults. The morphology of these stacking faults indicates that short basal plane segments associated with threading dislocations are the origin of rhombic stacking faults. All dislocations in a half-loop array have the same Burgers vector and nucleate on a single basal plane, which was evidenced by the merging of double-rhombic stacking faults. Most pre-existing basal plane dislocations within the blocking layer which are visible in OEM images dissociate to form stacking faults during the degradation. Basal plane dislocations aligned along the off-cut direction form rectangular stacking faults, while others break up into partial dislocation segments along the 〈11–20〉 directions, which are often wedge-shaped. Thus, all nucleation sites of the stacking faults correspond to pre-existing dislocation segments residing in basal planes. The morphology and evolution of double-rhombic stacking faults indicate that the p-i-n diode degradation cannot be driven by stress in the structure.
Importance of chirality and reduced flexibility of protein side chains: A study with square and tetrahedral lattice models
Authors: Zhang Jinfeng , Chen Yu , Chen Rong , Liang Jie
Published: June 2004

Side chains of amino acid residues are the determining factor that distinguishes proteins from other unstable chain polymers. In simple models they are often represented implicitly (e.g., by spin states) or simplified as one atom. Here we study side chain effects using two-dimensional square lattice and three-dimensional tetrahedral lattice models, with explicitly constructed side chains formed by two atoms of different chirality and flexibility. We distinguish effects due to chirality and effects due to side chain flexibilities, since residues in proteins are L residues, and their side chains adopt different rotameric states. For short chains, we enumerate exhaustively all possible conformations. For long chains, we sample effectively rare events such as compact conformations and obtain complete pictures of ensemble properties of conformations of these models at all compactness region. This is made possible by using sequential Monte Carlo techniques based on chain growth method. Our results show that both chirality and reduced side chain flexibility lower the folding entropy significantly for globally compact conformations, suggesting that they are important properties of residues to ensure fast folding and stable native structure. This corresponds well with our finding that natural amino acid residues have reduced effective flexibility, as evidenced by statistical analysis of rotamer libraries and side chain rotatable bonds. We further develop a method calculating the exact side chain entropy for a given backbone structure. We show that simple rotamer counting underestimates side chain entropy significantly for both extended and near maximally compact conformations. We find that side chain entropy does not always correlate well with main chain packing. With explicit side chains, extended backbones do not have the largest side chain entropy. Among compact backbones with maximum side chain entropy, helical structures emerge as the dominating configurations. Our results suggest that side chain entropy may be an important factor contributing to the formation of α helices for compact conformations.
Theoretical analysis and computer simulation of fluorescence lifetime measurements. II. Contour length dependence of single polymers
Authors: Yang Shilong , Cao Jianshu
Published: June 2004

Fluorescence lifetime measurements in a polymer chain are modeled using a memory function expansion, computer simulations, and simple scaling arguments. Unless the quenching rate is localized and infinitely fast, the fluorescence lifetime is generally not equivalent to the first passage time. The fluorescence lifetime distribution is decomposed into memory functions that can be measured separately in single-molecule experiments. The leading order of the expansion gives the Wilemski–Fixman (WF) approximation, and the convergence of higher order terms determines its validity. Simulations of the fluorescence quenching on a Rouse chain verify the accuracy of the WF approximation at small contact radii, short contour lengths, and small quenching rates. Detailed investigation of the average fluorescence lifetime reveals two competing mechanisms: the independent motion of end-to-end vector, which dominates at small contact radius, and the slowest relaxation of polymer, which dominates at large contact radius. The Wilemski–Fixman rate is used in combination with scaling arguments to predict the dependence of fluorescence lifetime on the contour length. Our predictions for the scaling of the average lifetime with the contour length are in good agreement with both simulations and recent experiments by Eaton and his group [L. J. Lapidus, W. A. Eaton, and J. Hofrichter, Proc. Natl. Acad. Sci. U.S.A. 97, 7220 (2000)].
Photodetachment spectrum of OHF−: Three-dimensional study of the heavy–light–heavy resonances
Authors: González-Sánchez Lola , Gómez-Carrasco Susana , Aguado Alfredo , Paniagua Miguel , Hernández M. Luz , Alvariño José M. , Roncero Octavio
Published: June 2004

In this work a simulation of the OHF− photodetachment spectrum is performed in a three-dimensional potential energy surface recently developed for OHF(3A″). The ground 2A′ state potential of the anion is calculated in three dimensions based on accurate ab initio calculations and the reaction dynamics is studied using a wave packet method. The calculated spectrum shows a sequence of bands associated to vibrational HF(v) up to v=3. Each band is formed by a continuous spectrum and resonant structures. These resonances are associated to the OH–F channel well of the 3A″ PES, in which fragmentation occurs through vibrational predissociation. Above the OH(v=0) threshold a new resonant pattern appears corresponding to heavy–light–heavy resonances. Special attention is paid to the assignment of these resonances because they mediate the reaction dynamics in the OH+F collision at low kinetic energies. The sequence of bands is in rather good agreement with that appearing in the experimental spectrum, especially at higher electron kinetic energies. At low kinetic energies, however, some other electronic states may contribute. The resonance structures might be washed out by the rotational average and the relatively low energy resolution of the experiment.
Rabi oscillations of ultrashort optical pulses in 1.55 μm InGaAs/InGaAsP quantum-well amplifiers
Authors: Zhang J.-Z. , Galbraith I.
Published: June 2004

Using the Foreman effective mass Hamiltonian for InxGa1−xAs/InyGa1−yAszP1−z quantum wells, the propagation of a 150 fs pulse in an optical amplifier was calculated by solving the wave equation with the computed polarization as a source term. The multisubband carrier dynamics and heating as well as the polarization dynamics were taken into account. The intensity of the propagated pulse as well as its imposed frequency chirp, the carrier density, and temperature show strong Rabi oscillations. The Rabi oscillation imposes negative frequency chirp, redshifting the frequency components of a propagating pulse and thus its spectrum, as a result of the gain saturation-induced self-phase modulation.
Effects of surface forces and phonon dissipation in a three-terminal nanorelay
Authors: Jonsson L. M. , Nord T. , Kinaret J. M. , Viefers S.
Published: June 2004

We have performed a theoretical analysis of the operational characteristics of a carbon–nanotube-based three-terminal nanorelay. We show that short range and van der Waals forces have a significant impact on the characteristics of the relay and introduce design constraints. We also investigate the effects of dissipation due to phonon excitation in the drain contact, which changes the switching time scales of the system, decreasing the longest time scale by 2 orders of magnitude. We show that the nanorelay can be used as a memory element and investigate the dynamics and properties of such a device.
Observation of the gap blueshift on Gd2O3:Eu3+ nanoparticles
Authors: Mercier Bruno , Dujardin Christophe , Ledoux Gilles , Louis Cédric , Tillement Olivier , Perriat Pascal
Published: June 2004

We report on the characterization of Gd2O3 nanocrystals of different sizes doped with 2.5 at. % Eu3+ ions. The particles have been synthesized by a sol-lyophilisation process. This method allows the synthesis of 7–100 nm diameter cubic-phase particles. The photoluminescence properties have been studied from visible to vacuum ultraviolet (VUV) wavelengths. Compared to the bulk material properties, some important changes on the luminescence are observed. In particular, some lines are strengthened when the particle’s size is diminished. In another article we had ascribed these bands to doping ions located on sites close to the surface. Both contributions of volume and surface states are observed. VUV spectroscopy performed selectively on the volumes states has allowed in sesquioxide nanoparticles to point out a gap blueshift due to quantum confinement.
Time-resolved electron detachment imaging of the I− channel in I2Br− photodissociation
Authors: Mabbs Richard , Pichugin Kostyantyn , Surber Eric , Sanov Andrei
Published: June 2004

The evolution of the I− channel in I2Br− photodissociation is examined using time-resolved negative-ion photoelectron imaging spectroscopy. The 388 nm photodetachment images obtained at variable delays following 388 nm excitation reveal the transformation of the excess electron from that belonging to an excited trihalide anion to that occupying an atomic orbital localized on the I− fragment. With increasing pump-probe delay, the corresponding photoelectron band narrows on a ∼300 fs time scale. This trend is attributed to the localization of the excess-electron wave function on the atomic-anion fragment and the establishment of the fragment’s electronic identity. The corresponding band position drifts towards larger electron kinetic energies on a significantly longer, ∼1 ps, time scale. The gradual spectral shift is attributed to exit-channel interactions affecting the photodetachment energetics, as well as the photoelectron anisotropy. The time-resolved angular distributions are analyzed and found consistent with the formation of the asymptotic I− fragment.
Vibrational coherence transfer characterized with Fourier-transform 2D IR spectroscopy
Authors: Khalil M. , Demirdöven N. , Tokmakoff A.
Published: June 2004

Two-dimensional infrared (2D IR) spectroscopy of the symmetric and asymmetric C≡O stretching vibrations of Rh(CO)2acac in hexane has been used to investigate vibrational coherence transfer, dephasing, and population relaxation in a multilevel vibrational system. The transfer of coherence between close-lying vibrational frequencies results in extra relaxation-induced peaks in the 2D IR spectrum, whose amplitude depends on the coherence transfer rate. Coherence transfer arises from the mutual interaction of the bright CO stretches with dark states, which in this case reflects the mutual d-π* back bonding of the Rh center to both the terminal carbonyls and the acetylacenonate ligand. For 2D IR relaxation experiments with variable waiting times, coherent dynamics lead to the modulation of peak amplitudes, while incoherent population relaxation and exchange results in the growth of the relaxation-induced peaks. We have modeled the data by propagating the density matrix with the Redfield equation, incorporating all vibrational relaxation processes during all three experimental time periods and including excitation reorientation effects arising from relaxation. Coherence and population transfer time scales from the symmetric to the asymmetric stretch were found to be 350 fs and 3 ps, respectively. We also discuss a diagrammatic approach to incorporating all vibrational relaxation processes into the nonlinear response function, and show how coherence transfer influences the analysis of structural variables from 2D IR spectroscopy.
Nuclear quadrupole hyperfine structure in the microwave spectrum of HCl–N2O: Electric field gradient perturbation of N2O by HCl
Authors: Leung Helen O. , Cashion Winn T. , Duncan Katharine K. , Hagan Christine L. , Joo Sujin
Published: June 2004

The microwave spectra of six isotopomers of HCl–N2O have been obtained in the 7–19 GHz region with a pulsed molecular beam, Fourier transform microwave spectrometer. The nuclear quadrupole hyperfine structure due to all quadrupolar nuclei is resolved and the spectra are analyzed using the Watson S-reduced Hamiltonian with the inclusion of nuclear quadrupole coupling interactions. The spectroscopic constants determined include rotational constants, quartic and sextic centrifugal distortion constants, and nuclear quadrupole coupling constants for each quadrupolar nucleus. Due to correlations of the structural parameters, the effective structure of the complex cannot be obtained by fitting to the spectroscopic constants of the six isotopomers. Instead, the parameters for each isotopomer are calculated from the A and C rotational constants and the chlorine nuclear quadrupole coupling constant along the a-axis, χaa. There are two possible structures; the one in which hydrogen of HCl interacts with the more electronegative oxygen of N2O is taken to represent the complex. The two subunits are approximately slipped parallel. For H 35Cl–14N2O, the distance between the central nitrogen and chlorine is 3.5153 Å and the N2O and HCl subunits form angles of 72.30° and 119.44° with this N–Cl axis, respectively. The chlorine and oxygen atoms occupy the opposite, obtuse vertices of the quadrilateral formed by O, central N, Cl, and H. Nuclear quadrupole coupling constants show that while the electric field gradient of the HCl subunit remains essentially unchanged upon complexation, there is electronic rearrangement about the two nitrogen nuclei in N2O.
Elastic–plastic wave profiles in cyclotetramethylene tetranitramine crystals
Authors: Dick J. J. , Hooks D. E. , Menikoff R. , Martinez A. R.
Published: June 2004

The explosive molecular crystal cyclotetramethylene tetranitramine was studied in three orientations in a set of plate impact experiments; the orientations studied were {110}, {011}, and {010} in P21/n space group. The elastic–plastic shock response was measured using laser interferometry. The measured particle velocity profiles showed elastic precursor decay typical of a stress relaxing material. There is anisotropy in elastic shock strength and decay. The amount of precursor decay with propagation distance and stress relaxation behind the elastic shock varied among the orientations. The {010} orientation had larger elastic precursors than did the other two orientations; the {010} crystal does not have the regular plastic deformation mechanisms available to it. Elastic Hugoniots were obtained from the measurements. The inelastic deformation mechanisms may vary with orientation.
Magnetic force microscopy with work function compensation
Authors: Yu Jun , Ahner Joachim , Weller Dieter
Published: June 2004

Electrostatic interaction due to the work function difference between the tip of a magnetic force microscope (MFM) tip and the sample surface competes with the magnetic interaction at a lower tip–sample distance. This results in the superposition of sample topographic and magnetic images. As a consequence, the interpretation of MFM images becomes difficult, and the spatial resolution is limited. In this work, it is demonstrated that by applying an external voltage to the tip, the electrostatic tip–sample interaction can be compensated and the influence of topography on the magnetic images can be significantly reduced. The minimum tip–sample distance without topography influence is dramatically reduced and the detected signal is increased by 50%. The impact on the spatial resolution of the MFM is discussed.
Strain-enhanced phase separation affecting electro- and magnetotransport in La0.67Ca0.33MnO3 films
Authors: Boikov Yu. A. , Gunnarsson R. , Claeson T.
Published: June 2004

Biaxial strain during nucleation influences phase separation into ferromagnetic (metallic) and nonferromagnetic (insulating) regions and that, in turn, markedly affects the electric transport of a manganite film. A 40-nm-thick La0.67Ca0.33MnO3 film, coherently constrained by a (001)LaAlO3 substrate, possesses a noticeably contracted unit cell volume (Veff≈56.70 Å3) as compared with that of a stoichiometric bulk sample. It corresponds to a higher relative concentration (45%) of tetravalent manganese ions in the manganite layer than that in the target (33%). The resistivity ρ(T) curve of the strained film peaks twice in the range 4.2–300 K. The charge transport of strained La0.67Ca0.33MnO3 films is non-ohmic at T<130 K. A magnetic field H linearizes the current–voltage characteristic, but its impact on ρ(T) and I–V decreases at low temperature. The unusual features in the electro- and magnetotransport properties of thin La0.67Ca0.33MnO3/(001)LaAlO3 films are ascribed to a strain-enhanced phase separation, which is also responsible for the large magnetoresistance (up to 90%) at 5 T within a broad temperature range. Thicker films experience a relaxation, a smaller resistivity, and less non-linear properties.
Manifestation of grain boundaries in the transport properties of p-sexiphenyl films
Authors: Tkaczyk S. , Kityk I. V. , Viennois R.
Published: June 2004

Direct current conductivity of p-sexiphenyl films with 0.2–2.5 μm thickness deposited on glass substrates was investigated at temperatures between 10 and 300 K. The molecular dynamics simulations and quantum chemical calculations of the experimentally observed conductivity have shown that there exist at least three different states of the films effectively contributing to the observed features of the dc conductivity: conductivity caused by proper crystalline states, conductivity originated from amorphous-like inter-grain region and caused by grain’s boundary. Comparison of the experimental data and theoretically calculated dependences shows that the main contribution to the observed dc conductivity gives grain boundaries between polycrystalline grains and amorphous-like background, though the latter possesses a relatively low part of the total volume.