bluuebunny/crossref_metadata_2025 huggingface dataset:
Ablation characteristics of cubic-boron nitride ceramic with femtosecond and picosecond laser pulses
Authors: Hirayama Yoichi , Obara Minoru
Published: July 2002

Ablation of cubic-boron nitride (c-BN) ceramic irradiated by Ti:sapphire laser (110 fs, 790 nm) and Nd:YAG laser (100 ps, 1.064 μm) pulses is comparatively studied. In the case of Ti:sapphire laser ablation, it is observed that the ablation depth per pulse has two different ablation regimes, being similar to the fs laser ablation of metals. The ablation threshold fluence for the 110 fs Ti:sapphire laser is estimated to be 15 mJ/cm2 for low fluence ablation, and 180 mJ/cm2 for high fluence ablation. With x-ray photoelectron spectroscopy signal and scanning electron microscope observation it is evident that the surface ablated with the 110 fs Ti:sapphire laser pulses remains as the BN layer itself and is not melted. With the 100 ps Nd:YAG laser ablation, the boron semiconductor layer is formed on the ablated surface.
Poly(α-methylstyrene) in methylcyclohexane: Densities and viscosities near the liquid–liquid critical point
Authors: Pendyala Krishna S. , Greer S. C. , Jacobs D. T.
Published: July 2002

We present new measurements of a thermodynamic property, the mass density, and of a transport property, the shear viscosity, of a solution of linear poly(α-methylstyrene), with a number average molecular weight of 32 300, in the solvent methylcyclohexane, in the one and two phase regions within 3 K of the liquid–liquid critical temperature, Tc. Both the density and the viscosity were measured with a magnetic suspension densimeter/rheometer, with precisions of 56 ppm and 4%, respectively. We find no critical anomalies in the density above Tc, the average density below Tc, the viscosity above Tc, or the average viscosity below Tc. The behavior of the difference in density below Tc is consistent with the Ising model.
Rotationally resolved photoionization: Influence of the 4σ→kσ shape resonance on CO+(B 2Σ+) rotational distributions
Authors: Farquar George R. , Miller J. Scott , Poliakoff E. D. , Wang Kwanghsi , McKoy V.
Published: July 2002

We present experimental and theoretical results on rotational distributions of CO+(B 2Σ+) photoions. Rotational distributions were determined for both the v+=0 and v+=1 vibrational levels following photoionization of cold (T0≈9 K) neutral CO target molecules. Data were generated using dispersed ionic fluorescence over a wide range of photoelectron kinetic energies, 0⩽Ek⩽120 eV, which allows one to interrogate the ionization dynamics. This wide spectral coverage permits illustrative comparisons with theory, and calculated spectra are presented to interpret the data. In particular, the comparison between theory and experiment serves to identify the strong continuum resonant enhancement at hνexc≈35 eV in the l=3 partial wave of the 4σ→kσ ionization channel, as this feature has profound effects on the ion rotational distributions over a wide range of energy. Second, there are differences between the rotational substructure for the v+=0 and v+=1 vibrational levels. All of the experimentally observed features and trends are reproduced by theory, and the consequences of these comparisons are discussed.
Electronegativity of the constituent rare-earth metals as a factor stabilizing the supercooled liquid region in Al-based metallic glasses
Authors: Louzguine Dmitri V. , Inoue Akihisa
Published: July 2002

The letter describes influence of the electronegativity of the constituent rare-earth metals on the supercooled liquid region in the Al–(Gd, Dy or Er)–Ni–Co metallic glasses. The samples were studied by x-ray diffractometry and differential scanning calorimetry. Calorimetry data for the La and Sm—bearing glasses studied earlier were also taken in consideration for comparison. It is found that supercooled liquid range in the Al85RE8Ni5Co2 alloys strongly depends upon electronegativity of the RE metal.
Classical phase-space analysis of vibronically coupled systems
Authors: Dilthey Stefan , Mehlig Bernhard , Stock Gerhard
Published: July 2002

Based on a recently introduced mapping formulation [G. Stock and M. Thoss, Phys. Rev. Lett. 78, 578 (1997)], a classical phase-space description of vibronically coupled molecular systems is developed. In this formulation the problem of a classical treatment of discrete quantum degrees of freedom such as electronic states is bypassed by transforming the discrete quantum variables to continuous variables. Here the mapping formalism is applied to a spin-boson-type system with a single vibrational mode, e.g., representing the situation of a photo-induced electron transfer promoted by a high-frequency vibrational mode. Studying various Poincaré surfaces-of-section, a detailed phase-space analysis of the mapped two-state problem is given, showing that the model exhibits mixed classical dynamics. Furthermore, a number of periodic orbits (PO’s) of the nonadiabatic system are identified. In direct extension of the usual picture of trajectories propagating on a single Born-Oppenheimer surface, these vibronic PO’s describe nuclear motion on several coupled potential-energy surfaces. A quasiclassical approximation is derived that expresses time-dependent quantities of a vibronically coupled system in terms of the PO’s of the system. As an example, it is demonstrated that vibronic PO’s may be used to calculate the time-dependent population probability of the initially excited electronic state. For the system under consideration, already two PO’s are sufficient to qualitatively describe the short-time evolution of the nonadiabatic process.
Photocontrol of protein conformation in a Langmuir monolayer
Authors: Cicuta Pietro , Hopkinson Ian , Petrov Peter G.
Published: July 2002

We report a method to control the conformation of a weak polyampholyte (the protein β-casein) in Langmuir monolayers by light, even though the protein is not photosensitive. Our approach is to couple the monolayer state to a photochemical reaction excited in the liquid subphase. The conformational transition of the protein molecule is triggered through its sensitivity to a subphase bulk field (pH in this study), changing in the course of the photochemical process. Thus, reaction of photoaquation of the ferrocyanide ion, which increases the subphase pH from 7.0 to about 8.3, produces a change in the surface monolayer pressure, ΔΠ, between −0.5 and +1.5 mN/m (depending on the surface concentration), signalling a conformational switch. The approach proposed here can be used to selectively target and influence different interfacial properties by light, without embedding photosensitizers in the matrix.
Dissociation rates of benzene at VUV excitation
Authors: Hsu Tzu Chien , Shu Jinian , Chen Yang , Lin Jim J. , Lee Yuan T. , Yang Xueming
Published: July 2002

Photodissociation dynamics of benzene under 193 nm and 157 nm photon excitation have been studied using the molecular beam photofragmentation spectroscopic technique. By detecting the H atom products produced away from the photolysis region, the lifetime of the excited benzene molecule as well as the kinetic energy distribution of the H atom elimination process can be determined simultaneously. Experimental results show that the H atom elimination from benzene occurs on the ground electronic state in which the excess energy is likely fully randomized.
Dipole-assisted self-assembly of light-emitting p-nP needles on mica
Authors: Balzer F. , Rubahn H.-G.
Published: July 2002

We report on dipole-assisted, self-assembled formation of p-6P and p-5P needles on cleaved and heated mica (0001) surfaces. Low-energy electron diffraction (LEED) reveals that the needles are single crystalline with the (11̄1̄) face parallel to the surface, consisting of parallel stacks of laying molecules oriented along the direction of microscopic dipoles on the mica surface. They have submicrometer cross-sectional dimensions and lengths as large as millimeters. Moreover, due to the strong dipole confinement of individual molecules, the needles form large domains with parallel oriented entities. A pronounced optical dichroism agrees with the findings from LEED.
Design of an ion trap apparatus for nuclear astrophysics research
Authors: Itahashi T. , Kudomi N. , Donets E. D. , Donets E. E.
Published: July 2002

A design of an ion trap apparatus for measurements of excitation functions of d(d,p)t and He3(d,p)He4 reactions with bare beam and target at energies below the Coulomb barrier is presented. It aims to deduce the exact astrophysical S factor (cross section) without any modification due to the atomic electrons of target or both projectile and/or target particles in the region 1.5<Ecm<12 keV and 6.5<Ecm<15 keV. The technology of electron beam ion sources and traps has been applied substantially to solve luminosity and background problems for the measurement of nuclear reactions with less than a nanobarn cross section for astrophysical interest.
Band-gap tuning of PbS nanoparticles by in-flight sintering of size classified aerosols
Authors: Nanda K. K. , Kruis F. E. , Fissan H. , Acet M.
Published: July 2002

We report about the band-gap tuning of PbS nanoparticles by in-flight sintering of size-classified aerosols. Band gaps in the range of 0.5–2.0 eV were obtained. The band gap first decreases and then increases upon increasing the sintering temperature. The decrease in the band gap is associated with the sintering of primary particles in aggregates leading to larger crystalline domains. The increase in the band gap is associated with the partial evaporation of the particles, which sets in after quasispherical and monocrystalline particles were formed by sintering. The same band gap is found for large spherical particles and aggregates of smaller primary particles, suggesting that an intimate contact between primary particles by means of sinter necks leads to a weaker quantum confinement effect than when the particles are merely touching each other.
Photoassisted scanning tunneling microscopy
Authors: Grafström Stefan
Published: July 2002

The combination of scanning tunneling microscopy (STM) with optical excitation adds new information to STM. A review is presented covering the work done on light-induced effects in STM during the past 15 years. Effects discussed include thermal effects, nonlinear effects, field enhancement at the STM tip, various effects on semiconductor surfaces, excitation of surface plasmons, detection of photoelectrons, spin-polarized tunneling, as well as light-induced nanomodifications, local optical spectroscopy, the use of ultrashort laser pulses for time-resolved STM, and the combination of STM and scanning near-field optical microscopy.
Manipulation of single coiled DNA molecules by laser clustering of microparticles
Authors: Hirano K. , Baba Y. , Matsuzawa Y. , Mizuno A.
Published: July 2002

We have developed a method of manipulating single DNA molecules for application in single-molecule analysis. Using a bead cluster formed by laser trapping, the technique allows single DNA molecules to be manipulated at any point on the molecule without the need for prior chemical modification as in DNA-bead complex techniques. We describe the method and the characteristics of cluster formation, and present examples of actual DNA molecule manipulation.
Microwave electronic spectrum of the Ne⋯Ne+ long-range complex: The interaction potential
Authors: Carrington Alan , Gammie David I. , Page Josephine C. , Shaw Andrew M. , Hutson Jeremy M.
Published: July 2002

The full interaction potential between Ne(1S) and Ne+(2P) is determined by least-squares fitting of potential parameters to spectroscopic data, principally from the near-dissociation microwave spectra of the Ne2+ complex. The potential obtained in this way incorporates the potential curves for all six electronic states correlating with Ne(1S)+Ne+(2P) and the couplings between them. Coupled-channel calculations on the potential take account of breakdown of the Born–Oppenheimer approximation and provide an accurate description of the microwave rovibronic spectrum involving levels within ∼10 cm−1 of the first dissociation limit. The Ne2+ ions are both vibrationally and rotationally hot: the spectrum involves levels up to at least J=25/2 and there is evidence for transitions involving levels near the second dissociation limit. The long-range levels involved have 〈r〉 up to 12 Å, compared with an equilibrium bond length of 1.756 Å for the ground electronic state. The long-range parameters of the interaction can be extracted from the fit and are compared with recent theoretical values.
Density matrix functional theory of weak intermolecular interactions
Authors: Cioslowski Jerzy , Pernal Katarzyna
Published: July 2002

The known asymptotic behavior of the total energy of two weakly interacting systems imposes stringent conditions on the exchange-correlation energy as a functional of the one-electron reduced density matrix. Although the first-order conditions that involve Coulomb-type two-electron integrals are relatively trivial to satisfy, the exact functional should also conform to two second-order expressions, and consequently to certain sum rules. The primitive natural spin-orbital functionals satisfy the first-order conditions but, lacking terms quadratic in two-electron integrals, are found to be incapable of recovering the dispersion component of the interaction energy. Violating the sum rules, the recently proposed Yasuda functional yields nonvanishing dispersion energy with spurious asymptotic terms that scale like inverse fourth and fifth powers of the intersystem distance.
Smoothing of bit transition irregularity in coupled granular/continuous perpendicular media
Authors: Goodman A. M. , Greaves S. J. , Sonobe Y. , Muraoka H. , Nakamura Y.
Published: July 2002

Coupled granular/continuous (CGC) perpendicular media increases thermal stability without compromising SNR. However, increasing thermal stability by using a continuous layer leads to concerns regarding transition noise. Investigating these concerns, we examine bit transition irregularity in CGC media using a 3D micromagnetic model with sub-grain discretization. Irregularity is introduced by writing tracks diagonally to the x-y axes of the cubic computational cell. Initially we model the granular layer as ideal, with exchange decoupled grains. The investigation is then extended to a nonideal system by varying intergranular exchange coupling in a system of comprising clusters of 10 nm grains. The thickness of the granular layer, g, and the continuous layer, c, are varied while maintaining a constant media thickness. For a range of thickness ratio, R(=c/g), that depends on intergranular exchange coupling in the granular layer, it is found that bit transition irregularity is reduced to less than that of the underlying granular physical structure. In CGC media with 50 nm grains, the irregularity is reduced to that of a granular media with ≈10 nm grains. Thus, in addition to enhancing thermal stability, CGC media may provide a way to reduce noise, thereby extending the limit to the areal density of conventional media.
Rearrangement pathways of the water trimer and tetramer anions
Authors: Alfonso Dominic R. , Jordan Kenneth D.
Published: July 2002

Minimum energy pathways for the rearrangement of the anions of the water trimer and tetramer anions between their cyclic and chain structures were investigated by means of ab initio electronic structure calculations, coupled with nudged elastic band optimizations. The rearrangements of both anions are found to proceed by opening of the cyclic structure and reorientation of the water molecules as the excess electron migrates to the terminal water fragment with the dangling hydrogens. The activation energies for the cyclic→chain rearrangements are calculated to be 0.11 and 0.32 eV for (H2O)3− and (H2O)4−, respectively.
Effect of ion-neutral charge exchange on radial electric fields in the edge plasma
Authors: D’Ippolito D. A. , Myra J. R.
Published: July 2002

A fluid treatment of the perpendicular plasma conductivity is extended to include the effects of arbitrary ion-neutral collision frequency ν and large perpendicular drifts. It is shown that ion-neutral friction (charge-exchange) can give a nonmonotonic relationship between the nonambipolar radial current and the radial electric field, leading to a bifurcated plasma equilibrium. This bifurcation may influence the properties of the low-to-high-confinement (L-H) transition in the presence of neutrals. The physical origin of this effect is an interplay between the surface-averaged equilibrium drifts and the small poloidal variations in the pressure and parallel flow around the flux surface generated by the geodesic curvature.
The Fourier method for tri-atomic systems in the search for the optimal coordinate system
Authors: Katz Gil , Yamashita Koichi , Zeiri Yehuda , Kosloff Ronnie
Published: July 2002

The Fourier grid method has been implemented on four tri-atomic coordinate systems. The explicit forms of the kinetic-energy operators are presented and the method of implementation described. The coordinates tested are the perimetric, Eckart, Jacobi, and conformal Euclidean. A comparative study was carried out using propagation techniques to obtain the lowest vibrational eigenvalues on the H3+ and the van der Waals Na⋅⋅⋅FH molecules for J=0. Converged eigenvalues were obtained for all the coordinate systems tested. The wavepacket representation methods were also compared for the nonadiabatic photodissociation dynamics of the Na⋅⋅⋅FH. When the coordinate systems matched the topology of the potential-energy surface, significant reduction in the grid size was obtained. The analysis has led to a method for evaluating grid efficiency and optimization based on a classical phase space approach.
Ion-beam nanotexturing of buffer layers for near-single-crystal thin-film deposition: Application to YBa2Cu3O7-δ superconducting films
Authors: Reade R. P. , Berdahl P. , Russo R. E.
Published: July 2002

A method of producing biaxially textured template layers for near-single-crystal-quality film growth on substrates that do not provide a template for oriented crystalline growth is described and compared to existing methods. This technique, ion-beam nanotexturing (ITEX), produces a biaxially textured layer by oblique ion irradiation of an amorphous film surface. Using in situ reflection high-energy electron diffraction and ex situ x-ray diffraction, an yttria-stabilized zirconia (YSZ) template layer fabricated by ITEX is shown to have the appropriate surface texture for YBa2Cu3O7-δ coated conductor fabrication. A YBa2Cu3O7-δ thin film deposited on an ITEX YSZ layer has a critical current of 2.5×105 A/cm2 (77 K, 1 μV/cm). ITEX produces texture rapidly and should be ideally suited for future low-cost manufacturing.
Second-harmonic generation from chemically modified Ge(111) interfaces
Authors: Fomenko Vasiliy , Bodlaki Dora , Faler Catherine , Borguet Eric
Published: July 2002

Second-harmonic generation (SHG) was used to investigate chemically modified surfaces of Ge(111). Chemical modification was achieved by wet-chemical covalent binding of decyl and sulfur directly to the Ge interface after oxide stripping. Chemical modification of the interface substantially changes the second harmonic response. The decyl and sulfur terminations are stable in ambient during several weeks, as judged by SHG and XPS measurements. The SHG rotational anisotropy patterns were analyzed to estimate the relative values of the nonlinear susceptibilities describing the surface and bulk response. The choice of fundamental/SHG polarization combinations for accessing various nonlinear coefficients is presented. The factors affecting the relative values of the surface-to-bulk contributions to SHG and their changes upon chemical modification of the surface are discussed. In particular, it was found that the higher the electronegativity of chemically attached species, the higher the contribution of the surface-originating nonlinear terms to the overall response. Also, it was found that the relative contribution of surface versus bulk to SHG is different for different polarization combinations: the surface contribution to the p-in/p-out response is the greatest.
Physics and control of resistive wall modes
Authors: Bondeson A. , Liu Y. Q. , Gregoratto D. , Fransson C. M. , Lennartson B. , Breitholtz C. , Gribov Y. , Pustovitov V. D.
Published: July 2002

Active feedback stabilization of resistive wall modes in tokamaks is studied both analytically, using large aspect ratio theory, and by means of toroidal computations. Extensive studies show that robust stabilization, with respect to variations in plasma current, pressure and flow velocity, can be achieved with a simple control system using poloidal sensors inside the first wall. The required coil voltages are modest, even for the two-wall structure of a tokamak reactor.
Comparative study of magnetoresistance and magnetization in nano-oxide specular and nonspecular MnIr/CoFe/Cu/CoFe spin valves from 10 to 300 K
Authors: Sousa J. B. , Ventura J. O. , Salgueiro da Silva M. A. , Freitas P. P. , Veloso A.
Published: July 2002

The temperature and magnetic field dependences of the magnetoresistance (MR), superconducting quantum interference device magnetization M, its electrical resistance R and temperature derivative dR/dT (10–300 K) are reported for nonspecular CoFe/Cu/CoFe spin valves and specular spin valves formed by nano-oxidation of the pinned and free CoFe layers. The MR(T) increases linearly with decreasing temperature on both spin valves, and data extrapolation converges to zero MR practically at the same Curie temperature. At temperatures below ∼175 K the specular spin valve MR(H) curves present two anomalous bumps not seen in the nonspecular curves. Also, for the nonspecular spin valve a clear relation is visible between M and MR curves, which is not the case for the specular spin-valve curves. In the specular spin valve, dR/dT presents an anomalous enhancement after T∼150 K, which is discussed in terms of electron scattering in the nano-oxide layers. The application of a saturating magnetic field suppresses most of the anomaly, indicating its magnetic origin.
State-resolved differential and integral cross sections for the reaction H+D2→HD(v′=3,j′=0–7)+D at 1.64 eV collision energy
Authors: Bean Brian D. , Ayers James D. , Fernández-Alonso Félix , Zare Richard N.
Published: July 2002

A 212.8 nm laser initiates the reaction H+D2→HD+D in a mixture of HBr and D2. A second laser state-selectively ionizes the HD(v′=3,j′) reaction product, allowing a determination of the speed distribution and the relative cross section in a velocity-sensitive time-of-flight mass spectrometer. From these measurements we construct differential and integral cross sections for H+D2→HD(v′=3,j′=0–7)+D at 1.64±0.05 eV collision energy. Although the integral cross sections do not show any unusual features, the differential cross sections reveal forward-scattered features that have not been observed in crossed-beam experiments. An analysis of the scattering features in HD(v′=3,j′=1–4) suggests that these states are dominated by classical hard-sphere scattering. This hard-sphere (direct recoil) mechanism, however, cannot account for the dominant forward scattering observed in HD(v′=3,j′=0).
On the application of virtual Gibbs ensembles to the direct simulation of fluid–fluid and solid–fluid phase coexistence
Authors: Shetty Ritesh , Escobedo Fernando A.
Published: July 2002

Virtual Gibbs ensembles (VGEs), a class of pseudo-ensemble Monte Carlo methods, are specialized in this work to simulate phase equilibrium for systems for which conventional direct methods (such as two-box Gibbs ensembles and one-box interfacial ensembles) are inadequate. It is shown that by removing the mass/volume balance constraints of conventional Gibbs ensembles, the resulting VGEs can be used to effectively simulate systems wherein (a) the number of molecules or the composition of one of the phases is to be kept constant and (b) the initial choices for the volume and number of molecules would preclude direct methods to convergence to a stable two-phase state. Applications of VGEs are presented for the simulation of vapor–liquid, vapor–solid, and liquid–solid equilibrium in single component systems and multicomponent sorption equilibria of gases in polymer melts. VGE simulations of solid–fluid coexistence entail the combination of elements of both interfacial ensembles and Gibbs ensembles.
Dissipative mixed quantum-classical simulation of the aqueous solvated electron system
Authors: Wong Kim F. , Rossky Peter J.
Published: July 2002

A direct integration of quantum decoherence into the mixed quantum-classical (MQC) molecular dynamics (MD) method mean field with surface hopping (MF/SH) is explored from the context of the aqueous solvated electron system. Within this framework, the time evolution of the reduced density matrix via the MQC Liouville–von Neumann equation includes dissipation of the off-diagonal elements according to some prescribed decoherence time scale. This fixed parameter corresponds, for example, to the characteristic thermal average decay time of nuclear overlaps of the bath. The MF/SH implementation includes decoherence only within the evolution of the primary subsystem that is responsible for transition probabilities but not within the auxiliary equations governing environmental molecular dynamics. Within this implementation, adiabatic MQC propagation is independent of decoherence rate, and only transition times are affected. Simulations with an average decoherence parameter of 6 fs extend the excited-state lifetime of the solvated electron by three and a half times compared to coherent evolution. Since condensed phase environments typically destroy the electronic coherence on such femtosecond time scales, standard MQC methods based on coherent propagation will, in general, overestimate the true transition rate.
Hydrogen-defect complexes formed by neutron irradiation of hydrogenated silicon observed by optical absorption measurement
Authors: Fukata N. , Ohori T. , Suezawa M. , Takahashi H.
Published: July 2002

Neutron-irradiation-induced defects in hydrogenated Si were investigated by detecting optical absorption due to their complexes with hydrogen. Specimens were doped with hydrogen by heating in H2 gas at 1300 °C followed by quenching in water. They were then irradiated with neutrons. The optical absorption spectra were measured at about 5 K with a Fourier transform infrared spectrometer. We investigated the dopant dependence and the annealing behaviors of H–interstitial (I) and H–vacancy (V) complexes. From the dopant dependence, we classified the peaks observed as I-related complexes or V-related complexes. In the annealing experiment, we observed numerous peaks after annealing above 300 °C in the region from 1940 to 2040 cm−1, whereas no such peaks were observed in the case of electron irradiation. This result shows that agglomerations of I and of V form more easily in neutron-irradiated Si than in electron-irradiated Si because of higher local concentrations of V and I in neutron-irradiated specimens.
In-line dielectric monitoring during extrusion of filled polymers
Authors: Bur Anthony J. , Roth Steven C. , McBrearty Michael
Published: July 2002

Real-time monitoring of the dielectric properties of polymer melts and filled polymer melts has been carried out during extrusion. The measurements are obtained using a dielectric cell that is placed directly in line with the extruder machine. The dielectric cell consists of interdigitating electrodes that are deposited on the inside of a ceramic ring that is electrically insulated and temperature controlled to the set point of extrusion. As the processed resin passes through the ring, its permittivity and conductivity are measured. The spatial sensitivity of the cell was determined experimentally and was biased to the resin flowing near the electrodes. Using the spatial sensitivity function, we examined the time profile of the transition from one composition to another during extrusion. We demonstrate the operation of the cell during the processing of polystyrene filled with aluminum oxide and calcium carbonate and of polyethylene-ethyl vinyl acetate copolymer filled with montmorillonite clay.
Theory of open quantum systems
Authors: Xu Ruixue , Yan YiJing
Published: July 2002

A quantum dissipation theory is constructed with the system–bath interaction being treated rigorously at the second-order cumulant level for both reduced dynamics and initial canonical boundary condition. The theory is valid for arbitrary bath correlation functions and time-dependent external driving fields, and satisfies correlated detailed-balance relation at any temperatures. The general formulation assumes a particularly simple form in driven Brownian oscillator systems in which the correlated driving-dissipation effects can be accounted for exactly in terms of local-field correction. Remarks on a class of widely used phenomenological quantum master equations that neglects the bath dispersion-induced dissipation are also made in contact with the present theory.
Measurement of transverse electric and transverse magnetic spontaneous emission and gain in tensile strained GaInP laser diodes
Authors: Lewis G. M. , Smowton P. M. , Blood P. , Jones G. , Bland S.
Published: July 2002

We have measured the modal gain and spontaneous emission spectra of a 0.8% tensile-strained GaInP laser structure in transverse electric (TE) and transverse magnetic (TM) polarizations by analysis of the single-pass amplified spontaneous emission from the end of a segmented-contact structure. The absolute values of the true spontaneous emission rates have been determined for both polarizations. Data for the TEy and TM peak gain as functions of the total intrinsic radiative current can be fitted with the usual logarithmic relation with a transparency current density of 99 A cm−2 and with the (electron–light-hole) TM/TEy gains in the ratio 4:1 as predicted from the oscillator strengths. Spontaneous emission into the TM modes accounts for 48% of the total radiative current.
Praseodymium oxide coated anode for organic light-emitting diode
Authors: Qiu Chengfeng , Chen Haiying , Xie Zhiliang , Wong Man , Kwok Hoi Sing
Published: July 2002

Capping an indium–tin–oxide anode with a thin layer of praseodymium oxide (Pr2O3) has been found to enhance not only hole-injection, quantum, and power efficiencies but also the lifetime of organic light-emitting diode made using copper (II) phthalocyanine as the anode buffer layer, N, N′-diphenyl-N,N′ bis(3-methylphenyl-1,1′-biphenyl-4,4′-diamine as the hole-transport layer and tris-8-hydroxyquinoline aluminum as the electron-transport/emission layer. The best results have been obtained on diodes with ∼1 nm thick Pr2O3. A possible mechanism behind the improvement is discussed.
Vector correlations and alignment parameters in the photodissociation of HF and DF
Authors: Balint-Kurti G. G. , Orr-Ewing A. J. , Beswick J. A. , Brown Alex , Vasyutinskii O. S.
Published: July 2002

Orientation and alignment parameters have been computed from first principles for the photodissociation of the HF and DF diatomic molecules. The calculations are entirely ab initio and the break-up dynamics of the molecule is treated rigorously taking account of the electronically nonadiabatic dynamics on three coupled adiabatic electronic potential energy curves. The potential energy curves and spin–orbit interactions, which have been previously reported [J. Chem. Phys. 113, 1870 (2000)], are computed using ab initio molecular electronic structure computer codes. These are then used to compute photofragmentation T matrix elements using a time-dependent quantum mechanical wave packet treatment and from these a complete set of anisotropy parameters with rank up to K=3 is computed. The predicted vector correlations and alignment parameters are presented as a function of energy for HF and DF initially in both their ground and first excited vibrational states. The parameters predicted for the molecules which are initially in their excited vibrational states display a pronounced sharp energy dependence arising from the nodal structure of the initial vibrational wavefunction. The theoretical results are analyzed using a simple model of the dynamics and it is demonstrated how the magnitude and relative phases of the photofragmentation T matrix elements can be deduced from the experimentally measured alignment parameters. No experimental measurements have yet been made of alignment parameters for hydrogen halide diatomics and the present results provide the first predictions of these quantities which may be compared with future experimental observations.
Enhanced dielectric properties of low-temperature sintered SrBi2Nb2O9/Ag composites
Authors: Wang Can , Fang Q. F. , Zhu Z. G.
Published: July 2002

Strontium bismuth niobate (SBN) matrix composites containing Ag particles were prepared by the conventional solid-state reaction method. With the addition of Ag particles to the SBN matrix, the single-phase layered perovskite structure of the matrix was preserved and the sintering temperature of the system was lowered about 200 °C compared with monolithic SBN. The SBN/Ag composites showed a significant increase of the dielectric constant and the loss tangent of SBN/Ag composites decreased in certain Ag content. With increasing Ag content, a gradual increase of the dielectric constant was observed in the temperature range from room temperature to 600 °C. The Curie temperature was found to decrease from 439 °C for the monolithic SBN to 435 °C for the composite with 3.5 vol % Ag, and then with increasing Ag content, the Curie temperature gradually increased to 445 °C for the composite with 14 vol % Ag. The results show that the SBN/Ag composites have enhanced dielectric properties and an improved processing condition, which could be further exploited in capacitor applications.
Features of the low-temperature plasticity of Pb–In single crystals
Authors: Isaev N. V. , Fomenko V. S. , Pustovalov V. V. , Braude I. S.
Published: July 2002

The temperature dependence of the plasticity parameters of Pb–In single crystals with indium concentrations of 1–20 at. % are investigated under tensile deformation at a constant strain rate in the temperature interval 4.2–295 K. From an analysis of the experimental data, empirical estimates are obtained for the main parameters of the dislocation–impurity interaction and the dynamic drag coefficient of the dislocations. The calculated values of these parameters are consistent with the idea of a gradual transition on cooling, from a thermally activated motion of dislocations through local impurity barriers to a thermal–inertial motion. With increasing indium concentration (to 20 at. %) the low-temperature mechanisms of dislocation motion begin to be affected substantially by regions of short-range order (clusters), the presence of which is detected by the diffuse x-ray scattering method.
Mechanisms of stochastic phase locking
Authors: Longtin André
Published: July 2002

Periodically driven nonlinear oscillators can exhibit a form of phase locking in which a well-defined feature of the motion occurs near a preferred phase of the stimulus, but a random number of stimulus cycles are skipped between its occurrences. This feature may be an action potential, or another crossing by a state variable of some specific value. This behavior can also occur when no apparent external periodic forcing is present. The phase preference is then measured with respect to a time scale internal to the system. Models of these behaviors are briefly reviewed, and new mechanisms are presented that involve the coupling of noise to the equations of motion. Our study investigates such stochastic phase locking near bifurcations commonly present in models of biological oscillators: (1) a supercritical and (2) a subcritical Hopf bifurcation, and, under autonomous conditions, near (3) a saddle-node bifurcation, and (4) chaotic behavior. Our results complement previous studies of aperiodic phase locking in which noise perturbs deterministic phase-locked motion. In our study however, we emphasize how noise can induce a stochastic phase-locked motion that does not have a similar deterministic counterpart. Although our study focuses on models of excitable and bursting neurons, our results are applicable to other oscillators, such as those discussed in the respiratory and cardiac literatures.
Internal symmetries of cellular automata via their polynomial representation
Authors: Salazar Gelasio , Urı́as Jesús
Published: July 2002

A polynomial representation of elementary cellular automata (ECA) is used to give a complete characterization of the local internal symmetries of all ECA. It is also shown that the polynomial representation is a natural choice for the study of local internal transformations of all cellular automata with two symbols. This is achieved by proving that local internal transformations are simply expressed in this representation as sums of polynomials.
Noise from underlayer of perpendicular magnetic recording medium
Authors: Uesaka Y. , Koizumi M. , Tsumita N. , Kitakami O. , Fujiwara H.
Published: July 2002

Noise from the underlayer is observed with a Mn-Zn ferrite head and a Co-Mo-Zr low coercivity amorphous underlayer. Three kinds of noise are observed, spike noise (s), medium noise (m), and low noise (l). s noise is generated through the domain walls of an underlayer. m noise originates from the interaction between an underlayer and the reproducing head’s residual magnetization. Three effective methods are tested for reducing the noise: (1) increase of coercivity of an underlayer, (2) use of an ac-erased head, and (3) use of a thin-film head.
Magnetic and Mössbauer studies on GdCo3B2 and DyCo3B2
Authors: Malik S. K. , Umarji A. M. , Shenoy G. K.
Published: July 2002

Magnetization and Mössbauer studies have been carried out on GdCo3B2 and DyCo3B2. These compounds are magnetically ordered with Curie temperatures of 56 and 21 K, respectively. The Co atoms are either nonmagnetic or carry a small moment in these compounds. The saturation moment of DyCo3B2 at 5 K is smaller than the Dy3+ free-ion value. From 161Dy Mössbauer studies, the measured hyperfine magnetic field at the Dy site is also observed to be smaller than the free-ion value. 155Gd Mössbauer measurements in GdCo3B2 reveal the presence of large crystalline electric fields at the rare-earth site. This causes the moment and the hyperfine field at the Dy site in DyCo3B2 to be reduced from its free-ion value.
Pyroelectric conversion cycle of vinylidene fluoride-trifluoroethylene copolymer
Authors: Olsen Randall B. , Bruno David A. , Briscoe Joseph M. , Jacobs Everett W.
Published: July 2002

Copolymers of vinylidene fluoride–trifluoroethylene P(VDF-TrFE) exhibit large piezoelectric and pyroelectric effects. In addition to the most common application of the pyroelectric effect (radiant detection) it is possible to convert heat directly into electrical energy by pyroelectric conversion. This study reports the first pyroelectric conversion cycle to be measured for the copolymer P(VDF-TrFE). It is found that standard isothermal D-E hysteresis loop measurements are not necessarily accurate predictors of pyroelectric conversion performance for this material. Conduction effects are found to obscure the observation of conversion cycles in most cases for the presently available materials. In spite of these difficulties, a conversion cycle was measured whose output electric energy density was 30 mJ/cm3. The output density is 15 times larger than any other polymer previously measured .
Lateral-mode analysis of gain-guided and index-guided semiconductor-laser arrays
Authors: Agrawal Govind P.
Published: July 2002

The lateral-mode behavior of a semiconductor-laser array is analyzed after including the effect of injected charge carriers on the active-layer dielectric constant. The wave equation is solved self-consistently and the lowest-threshold lateral supermode is obtained by treating the whole array as one unit. The resulting mode profile incorporates the effects of gain guiding, carrier-induced index antiguiding, and built-in index guiding. The analysis is therefore applicable to gain-guided as well as index-guided arrays; near- and farfields for both kinds of arrays are obtained and compared to those obtained without gain-loss considerations using the plane-wave diffraction theory or the coupled-mode theory. For gain-guided arrays, carrier-induced antiguiding plays an important role and leads to a transition from twin-lobe to three-lobe farfields for closely spaced, narrow array elements. For weakly index-guided arrays the in-phase or out-of-phase coupling between neighboring elements can occur depending on whether the gain is laterally homogeneous or not. The calculated results are in qualitative agreement with the reported experimental observations. The present analysis is useful to understand the role of various guiding mechanisms as well as to provide guidelines for the device optimization.
The structural and compositional characterization of InSb films prepared by metalorganic magnetron sputtering
Authors: Webb J. B. , Halpin C. , Noad J. P.
Published: July 2002

Metalorganic magnetron sputtering has been used to deposit polycrystalline and homoepitaxial films of indium antimonide. In this technique a metal antimony target is magnetron sputtered in a reactive vapor of trimethylindium (TMI). Stoichiometric layers of indium antimonide could be deposited for all growth temperatures studied (20–370 °C). However, below 250 °C the films were either amorphous or composed of very small crystallites. Above 290 °C the layers were epitaxial as determined from the electron channeling patterns observed. For these layers the growth rate was controlled by the TMI flow with the excess antimony flux acting to stabilize the surface. The surface morphology was excellent with ‘‘mirrorlike’’ surfaces except at high TMI flows where indium surface droplets were formed.
Determination of bulk minority-carrier lifetime and surface/interface recombination velocity from photoluminescence decay of a semi-infinite semiconductor slab
Authors: ‘t Hooft G. W. , van Opdorp C.
Published: July 2002

The analytical expression is derived for the time dependence of the total minority-carrier content in a semi-infinite semiconductor slab, after excitation with a delta function light pulse. It is shown that to this end it is sufficient to solve the continuity equation for the minority carriers in one dimension only. This implies that the excitation may have an arbitrary lateral spatial distribution. The photoluminescence, probing the total minority-carrier content, turns out to be a nonexponential function of time when the surface/interface recombination velocity differs from zero. A practical method is developed to deduce accurate values of the bulk lifetime and the interface recombination velocity from such a nonexponential decay curve, based on a special way of plotting. Moreover, the analytical expressions are derived for the photoluminescence decay in the case of finite slab thicknesses and finite interface recombination velocities. From these results it is evident that only for large ratios (>10) of slab thickness and minority-carrier diffusion length can the semiconductor be considered as being semi-infinite.
Ferroelectric (Pb,La)(Zr,Ti)O3 epitaxial thin films on sapphire grown by rf-planar magnetron sputtering
Authors: Adachi Hideaki , Mitsuyu Tsuneo , Yamazaki Osamu , Wasa Kiyotaka
Published: July 2002

Ferroelectric (Pb,La)(Zr,Ti)O3 (PLZT) thin films have been epitaxially grown on the c plane of sapphire by rf-planar magnetron sputtering. The sputtering conditions were investigated to obtain epitaxial and transparent films. Dielectric, piezoelectric, and electro-optic properties of the films were measured. Piezoelectricity of the PLZT(28/0/100) film was confirmed and was as strong as that of BaTiO3. Excellent quadratic electro-optic effects for PLZT(28/0/100) and PLZT(9/65/35) films and a linear electro-optic effect for PLZT(21/0/100) film were observed at 0.633-μm wavelength. Epitaxial PLZT thin film on sapphire is presently the most promising material for new functional devices.
Dose rate dependence and time constant of the ion-beam-induced crystallization mechanism in silicon
Authors: Linnros J. , Holmén G.
Published: July 2002

The influence of dose rate on the ion-beam-induced crystallization of amorphous layers in silicon has been investigated. The amorphous layers were produced by self-ion implantation both in bulk silicon and in silicon on sapphire. Subsequent recrystallization was induced at 200 to 400 °C by Ne, Si, Ar, and Kr ion beams of 300 keV energy passing through the amorphous layers. Rutherford backscattering/channeling measurements showed that the regrowth rate decreased with increasing dose rate. This behavior was more pronounced for heavy ions where high dose rates and/or low temperatures could reverse the recrystallization and induce further amorphous growth of the layer. In this new solid-phase growth regime, the amorphous/crystalline interface moved inwards into the crystal in a manner similar to an epitaxial process. An intermittent beam experiment yielded a time constant for the ion beam induced crystallization mechanism of the order of 0.3 s. The time constant and a scaling law for different ions support a model where the planar growth is caused by the accumulation of divacancies in the interface region.
Multigigawatt microwave generation by use of a virtual cathode oscillator driven by a 1–2 MV electron beam
Authors: Burkhart S.
Published: July 2002

Microwave power generated by a virtual cathode oscillator is measured using independent diagnostics to levels exceeding 4 GW at 6.5 GHz. Variation of the cathode radius is shown to have little effect on the oscillation frequency, however, changing the anode–cathode gap is shown to have a strong effect on the frequency. Electron beam power to microwave power conversion efficiency is measured at 3.3%.
Magnetic properties of Fe-implanted sapphire
Authors: Ohkubo Masataka , Hioki Tatsumi , Kawamoto Jun-ichi
Published: July 2002

Vibrating sample magnetometry has been used to investigate the magnetic properties of sapphire implanted with 400-keV 56Fe ions to a dose of 1×1017 ions/cm2 and isochronally annealed at temperature ranging from 700 to 1450 °C in vacuum for 1 h. The specimens are superparamagnetic or ferromagnetic, depending upon the annealing temperature or the size of α-Fe particles precipitated inside sapphire. The precipitation of α-Fe has been confirmed by x-ray diffraction. From x-ray diffraction line broadening, it has been confirmed that the size of α-Fe particles monotonically increases from 3.5 to 38 nm with annealing temperature. The critical size for superparamagnetic behavior has been roughly determined to be 7 nm. The saturation magnetic moment suggests that after the annealings at 850 to 1200 °C, 92%±8% of the implanted Fe ions exist as α-Fe.
Point radiographic source characterization
Authors: Zigler A. , Lee R. W. , Kilkenny J. D. , Kolbe G. , Nathel H.
Published: July 2002

A study was undertaken to find the optimal conditions for generating x rays using a high-power frequency-doubled Nd laser pulse for x-ray backlighting application. More than 30 laser shots on the Lawrence Livermore National Laboratory JANUS Research Laser system have been studied. The following specific questions are addressed: (a) X-ray yield dependence on the laser parameters: (i) temporal pulse width and (ii) focusing conditions. (b) X-ray source size versus target dimensions by using a tungsten wire with various diameters. (c) Influence of background plasma on the x-ray yield from the wire using 25-μm wires surrounded by plastic.