bluuebunny/crossref_metadata_2025 huggingface dataset:
Micromachined photoplastic probe for scanning near-field optical microscopy
Authors: Genolet G. , Despont M. , Vettiger P. , Staufer U. , Noell W. , de Rooij N. F. , Cueni T. , Bernal M.-P. , Marquis-Weible F.
Published: July 2002

We present a hybrid probe for scanning near-field optical microscopy (SNOM), which consists of a micromachined photoplastic tip with a metallic aperture at the apex that is attached to an optical fiber, thus combining the advantages of optical fiber probes and micromachined tips. The tip and aperture are batch fabricated and assembled to a preetched optical fiber with micrometer centering precision. Rectangular apertures of 50 nm×130 nm have been produced without the need of any postprocessing. Topographical and optical imaging with a probe having an aperture of 300 nm demonstrate the great potential of the photoplastic probe for SNOM applications.
Extracting composition and alloying information of coherent Ge(Si)/Si(001) islands from [001] on-zone bright-field diffraction contrast images
Authors: Liao X. Z. , Zou J. , Cockayne D. J. H. , Jiang Z. M. , Wang X.
Published: July 2002

Ge(Si)/Si(001) coherent islands grown at 700 °C by molecular beam epitaxy were investigated using transmission electron microscopy. [001] on-zone bright-field diffraction contrast imaging and image simulation techniques were used to investigate the structure of these coherent islands. Comparison of simulated and experimental images indicates nonuniform composition distribution within the coherent islands when the islands were grown at high temperatures (700 °C), but uniform composition for growth at lower temperatures (600 °C).
Generation of calcium waves in living cells by pulsed-laser-induced photodisruption
Authors: Smith Nicholas I. , Fujita Katsumasa , Kaneko Tomoyuki , Katoh Kentaro , Nakamura Osamu , Kawata Satoshi , Takamatsu Tetsuro
Published: July 2002

Here, we present an optical technique that can induce waves of calcium ion concentration in live biological cells. Ca2+ waves were induced by femtosecond pulsed-laser illumination. Living HeLa cells were exposed to focused 140 fs pulses of 780 nm wavelength at 30 mW average power. Ca2+ waves were imaged by fluorescence and were observed to propagate from the laser focal point inside the cell. Photoinduced generation of Ca2+ waves can be performed at any point inside the cell, an improvement over previous mechanical or biochemical stimulation techniques.
Reactivity of vibrationally excited methane on nickel surfaces
Authors: Halonen Lauri , Bernasek Steven L. , Nesbitt David J.
Published: July 2002

Four-dimensional variational calculations have been performed for modeling energy flow between methane (CH4) stretching vibrational energy states as the molecule adiabatically approaches a metallic surface. The model is based on a local mode Hamiltonian for an isolated CH4 molecule and a London-Eyring-Polanyi-Sato potential describing surface–molecule interactions. The results suggest the possibility of mode specific effects on chemical reactivity. Specifically, the symmetric A1 stretch fundamental adiabatically correlates with the localized excitation in the unique CH bond pointing towards the surface. Conversely, the antisymmetric F2 stretch fundamental excitation correlates with A and E vibrations in the CH3 radical, and therefore this degree of freedom is localized away from the reactive CH bond. Landau–Zener semiclassical analysis of nonadiabatic curve crossings predicts a significant velocity dependence to the state specific energy flow dynamics. Since excitation localized in active versus spectator bonds is expected to be more efficient in accelerating CH bond cleavage and adsorption reactivity, these results offer insight into interpreting velocity and vibrationally mediated reaction dynamics of CH4 on catalytic surfaces.
Efficient electromigration testing with a single current source
Authors: Kim Choong-Un , Michael N. L. , Jiang Qing-Tang , Augur Rod
Published: July 2002

This article introduces a simple and effective technique for conducting electromigration testing of a number of samples using a single current source. It is based on a configuration where all samples are serially connected to a single current source, allowing them to be subjected to identical current conditions. In this design, each sample has a current bypass circuit, consisting essentially of a computer controlled shunt relay and a Zener diode, to enable continuation of testing without any interruption in the test current when samples fail. With this technique, a large number of samples can be tested with the same current and excellent current stability, making it suitable for both reliability assessment and scientific investigation of electromigration mechanisms. Initial results show high correlation with industry standard testing systems.
Pulsed x-ray generator for commercial gas lasers
Authors: Bollanti S. , Bonfigli F. , Di Lazzaro P. , Flora F. , Giordano G. , Letardi T. , Murra D. , Schina G. , Zheng C. E.
Published: July 2002

We have designed and tested a 1-m-long x-ray diode based on innovative plasma cathodes, which exploit commercial spark plugs as electron emitters. Based on the results of a numerical study, we optimized both diode geometry (e.g., the angle between anode and cathode surfaces, the thickness of the Al window) and electrical circuitry (e.g., the capacitance in series to each spark plug, the peak voltage of the anode) of our x-ray generator. The overall result is a simple and efficient circuitry, giving a total diode current in excess of 2.1 kA with a breakdown voltage of 70 kV, which generates a 50 ns rise-time x-ray pulse with a spatially averaged dosage of up to 6×10−4 Gy when using a Pb-wrapped anode. The double-diode x-ray generator was operated for 1.5×106 shots at a repetition rate of up to 30 Hz, and the lifetime test was interrupted without any fault. During the lifetime test, it was not necessary to adjust any working parameter. At the end of the lifetime test, the x-ray emission uniformity was better than 80% along the longitudinal axis. This x-ray generator has a lifetime, reliability, and cost fitting the requirements of industrial users. Among the broad range of potential applications, this x-ray generator is particularly suitable to ionize discharge pumped gas lasers, like TEA CO2 and excimer lasers, including those operated by x-ray triggered discharges.
Elemental analysis using hard x-ray emission from a laser-produced plasma, induced by a femtosecond laser pulse
Authors: Takeyasu N. , Hirakawa Y. , Imasaka T.
Published: July 2002

A new approach for elemental analysis, which is based on the measurement of the hard x ray emitted from a laser-produced plasma is reported here. Since the K-shell emission is characteristic of the element in a sample, Cu, Zn, Fe, Co, and Ni were readily confirmed to be present in brass and kovar alloys by measuring the x-ray emission spectrum over an energy range from 2 to 30 keV.
Gradient corrections for semiclassical theories of atoms in strong magnetic fields
Authors: Hainzl Christian
Published: July 2002

This paper is divided into two parts. In the first one the von Weizsäcker term is introduced to the magnetic Thomas–Fermi theory and the resulting MTFW functional is mathematically analyzed. In particular, it is shown that the von Weizsäcker term produces the Scott correction up to magnetic fields of order B≪Z2, in accordance with a result of Ivrii on the quantum mechanical ground state energy. The second part is dedicated to gradient corrections for semiclassical theories of atoms restricted to electrons in the lowest Landau band. We consider modifications of the Thomas–Fermi theory for strong magnetic fields (STF), i.e., for B≪Z3. The main modification consists in replacing the integration over the variables perpendicular to the field by an expansion in angular momentum eigenfunctions in the lowest Landau band. This leads to a functional (DSTF) depending on a sequence of one-dimensional densities. For a one-dimensional Fermi gas the analogue of a Weizsäcker correction has a negative sign and we discuss the corresponding modification of the DSTF functional.
A posteriori corrections to systematic failures of standard density functionals: The dissociation of two-center three-electron systems
Authors: Chermette H. , Ciofini I. , Mariotti F. , Daul C.
Published: July 2002

The method we proposed recently [J. Chem. Phys., 114, 1447 (2000)] to a posteriori correct the unphysical dissociation behavior of radical homonuclear diatomic cations obtained in density functional theory calculations has been enlarged to nonsymmetric three-center two electrons systems. This approach, which is derived from Slater’s transition state technique, allows to remove most of the self-interaction energy error contained in the current exchange functionals. It has been shown that this is the main contribution to the overestimation of the bonding energy of systems with delocalized charges. Although approximate, the method yields a better agreement with experimental bonding energies than more sophisticated methods.
Potential energy surfaces for the photochemical reactions Ca*+H2→CaH+H
Authors: Kim Kyoung Hoon , Lee Hyo Sug , Lee Yoon Sup , Jeung Gwang-Hi
Published: July 2002

Six A′1 potential energy surfaces of the Ca+H2 system have been calculated using an effective-core potential and a core-polarization potential for Ca. Large configuration interactions were performed to ensure accurate energies and wave functions. The 2 A′1 surface, made from Ca(4s3d, 1D), has a very stable intermediate, and crosses with the ground state (1 A′1) potential energy surface in C2v geometry. A diabatic coupling between these two can result in the CaH product. Three potential surfaces originating from Ca(4s3d, 1D) and two potential surfaces originating from Ca(4s4p, 1P) are well separated at low energy region, suggesting that the reaction product CaH from the Ca(4s4p, 1P) state via nonadiabatic transition to the Ca(4s3d, 1D) surface is very unlikely. Other possible routes of producing CaH from the Ca(4s4p, 1P) state also appear quite improbable in the single collision process. The wave function analysis does not show any sign of electron transfer from the metal atom to the hydrogen molecule in the reactant stage, which implies that the harpooning model does not apply to this reaction.
Dissociative electron attachment to C2N2 molecules at surface and in gas phase
Authors: Deng Z. W. , Souda R.
Published: July 2002

We report an observation of strong emission of CN− ions during heating of a nitrogen ion-irradiated graphite surface. An activation energy of 5.0±0.2 eV for thermionic CN− emission is derived from its temperature dependence. From the results of thermal desorption of neutral molecules and thermionic electron emission, it is proposed that the thermally induced CN− ions may originate from the dissociative negative ionization of desorbing C2N2 molecules on the surface via a thermionic electron attachment mechanism. This mechanism is supported by the dissociative negative ionization of the desorbed C2N2 molecules in the gas phase, as observed by negative ion RGA measurement. The large negative ionization cross section of C2N2 molecules at an electron energy of 0.8–0.9 eV, which also leads to the formation of CN− ions, is believed to be responsible for the CN− emission during heating of the nitrogen ion irradiated graphite.
A K-dependent adiabatic approximation to the Renner–Teller effect for triatomic molecules
Authors: Yu Hua-Gen , Muckerman James T. , Sears Trevor J.
Published: July 2002

A K-dependent adiabatic approach is suggested to study the Renner–Teller effect in triatomic molecules using a two-dimensional Hilbert space approximation in the electronic degree of freedom. The theory is developed in hyperspherical coordinates, and approximately includes the electronic spin–orbit interaction. The adiabatic Hamiltonians are expressed in terms of a K-dependent effective potential energy surface. Eigenpairs are calculated by solving the time-independent Schrödinger equation, which is represented in a mixed grid/basis set. The method is applied to the Ã←X̃ band system of bromomethylene (H/DCBr). The results obtained show that the Renner–Teller effect in this molecule is pronounced, particularly in the excited Ã 1A″ state. The results are compared to recent experimental measurements and good agreement is found.
Time-resolved vibrational optical activity measurement by the infrared-visible sum-frequency-generation with circularly polarized infrared light
Authors: Cho Minhaeng
Published: July 2002

A theoretical description of the time-resolved infrared-visible sum-frequency-generation (IV-SFG) process when the incident infrared pulsed beam is either left- or right-circularly-polarized (CP) is presented. Even for isotropic chiral molecular liquids, the CP IV-SFG polarization does not vanish because the chiral component of the associated response function is nonzero due to the breakdown of the Born–Oppenheimer approximation as well as to the finite polarizability-electric-quadrupole response function, which is a fourth-rank tensor. For a specific perpendicular detection scheme, it is shown that the three different contributions, originated from (1) all-electric-dipole-allowed polarization, (2) polarizability-quadrupole-induced polarization, and (3) interference between the above two polarizations, to the CP IV-SFG signal can be sperately measured. Also, the circular intensity difference, which is the difference between the left-CP IV-SFG intensity and the right-CP IV-SFG intensity, is theoretically investigated and found to be solely determined by the interference between the all-electric-dipole-allowed polarization and the polarizability-quadrupole-induced polarization. Thus, it is shown that the time-resolved CP IV-SFG methods are useful for the investigation of the molecular chirality of liquids and vibrational optical activity.
Thermal effects in self-assembled FePt nanoparticles with partial chemical ordering
Authors: Harrell J. W. , Wang S. , Nikles D. E. , Chen M.
Published: July 2002

We have studied the thermal stability of self-assembled FePt nanoparticles with partial chemical ordering. The magnetic viscosity was measured at zero field after partial dc demagnetization as a function of the initial remanent magnetization. The viscosity curves are in good agreement with a calculation using a noninteracting Stoner–Wohlfarth, Arrhenius–Néel model with a broad distribution of anisotropy fields and randomly oriented easy axes. The shape of the zero field viscosity curve differs from curves measured for thin film media with in-plane and perpendicular anisotropy, in which interactions are known to be important.
Parametrized direct dynamics study of rate constants of H with CH4 from 250 to 2400 K
Authors: Pu Jingzhi , Truhlar Donald G.
Published: July 2002

Four implicit potential energy surfaces (PESs) with specific-reaction-parameters (SRP) are developed and tested for the reaction CH4+H→CH3+H2. The first is called MPW60 and is based on the modified Perdew–Wang hybrid density-functional method with the percentage of the Hartree–Fock exchange equal to 60%. The other three PESs are constructed with multi-coefficient correlation methods (MCCMs). The second is called MCOMP2-SRP, and the third is called MC-QCISD-SRP. Both of them are parametrized for this specific reaction by starting with their corresponding global parameters. The fourth is called MCG3-SRP and is based on the MCG3-CHO semiglobal parametrization with further refinement for this specific reaction. All four SRP surfaces have a classical forward barrier height of 14.8 kcal/mol, and all three MCCM SRP surfaces have a classical endoergicity of 3.3 kcal/mol. The stationary point geometries, vibrational frequencies, and zero-point-energies are reported for several standard single-level methods and MCCMs with global parameters as well as for the four new SRP surfaces. Direct dynamics calculations are carried out using variational transition state theory with multidimensional tunneling contributions on the proposed SRP surfaces. We calculate forward reaction rate constants for the title reaction from 250 to 2400 K and compare them with the latest re-analyzed experimental results over the temperature range from 348 to 1950 K. The calculated rate constants using canonical variational theory with the small-curvature tunneling approximation carried out on the MPW60, MC-QCISD-SRP, and MCG3-SRP surfaces show good agreement with the experimental results.
Three-dimensional trapping of Mie metallic particles by the use of obstructed laser beams
Authors: Gu Min , Morrish Dru
Published: July 2002

In this article, we propose a method for three-dimensional optical trapping of metallic Mie particles using an obstructed laser beam. It is found from the ray-optics model that Mie gold, silver, and copper particles can be trapped against gravity in the focal region of a high numerical-aperture objective illuminated by a centrally obstructed Gaussian (TEM00-mode) beam. The axial trapping force of the three types of metallic particles is maximized in the near-infrared wavelength region. The maximum axial trapping efficiency increases with the size of the center obstruction and the aperture angle of an objective. Axial trapping force on Mie metallic particles is enhanced by a factor of two if an obstructed doughnut (TEM01*-mode) beam is employed. The experimental condition for achieving three-dimensional trapping is also discussed.
Superintegrability in a two-dimensional space of nonconstant curvature
Authors: Kalnins E. G. , Kress J. M. , Winternitz P.
Published: July 2002

A Hamiltonian with two degrees of freedom is said to be superintegrable if it admits three functionally independent integrals of the motion. This property has been extensively studied in the case of two-dimensional spaces of constant (possibly zero) curvature when all the independent integrals are either quadratic or linear in the canonical momenta. In this article the first steps are taken to solve the problem of superintegrability of this type on an arbitrary curved manifold in two dimensions. This is done by examining in detail one of the spaces of revolution found by G. Koenigs. We determine that there are essentially three distinct potentials which when added to the free Hamiltonian of this space have this type of superintegrability. Separation of variables for the associated Hamilton–Jacobi and Schrödinger equations is discussed. The classical and quantum quadratic algebras associated with each of these potentials are determined.
Evidence of the influence of phonon density on Tm3+ upconversion luminescence in tellurite and germanate glasses
Authors: Tsang W. S. , Yu W. M. , Mak C. L. , Tsui W. L. , Wong K. H. , Hui H. K.
Published: July 2002

Spectral properties of blue upconversion luminescences in Tm3+ doped tellurite (PWT, PbF2-WO3-TeO2) glasses and germanate (PWG, PbF2-WO3-GeO2) glasses pumped by a tunable dye laser were studied at room temperature. Two emission bands centered at 453 and 477 nm, corresponding to the D21→H43 and G41→H63 transitions of Tm3+ ions respectively, were observed. The two-photon absorption mechanism responsible for the 477 nm luminescence was confirmed by a quadratic dependence of luminescent intensities on the excitation power. Tellurite glasses showed a weaker upconversion luminescence than germanate glasses. This observation was inconsistent with the prediction from the phonon sideband measurement. In this article, Raman spectroscopy and transmittance measurement were employed to investigate the origin of the difference in upconversion luminescences in the two glasses. Compared with phonon sideband spectroscopy, Raman spectroscopy extracts more information, including both phonon energy and phonon density, and therefore, is a more effective analytical tool for understanding upconversion luminescence. Our results showed that the phonon energy as well as phonon density of the host glass is important in determining the upconversion efficiency.
On quantum effects near the liquid–vapor transition in helium
Authors: Müser Martin H. , Luijten Erik
Published: July 2002

The liquid–vapor transition in He3 and He4 is investigated by means of path-integral molecular dynamics and the quantum virial expansion. Both methods are applied to the critical isobar and the critical isochore. While previous path-integral simulations have mainly considered the lambda transition and superfluid regime in He4, we focus on the vicinity of the critical point and obtain good agreement with experimental results for the molar volume and the internal energy down to subcritical temperatures. We find that an effective classical potential that properly describes the two-particle radial distribution function exhibits a strong temperature dependence near the critical temperature. This contrasts with the behavior of essentially classical systems like xenon, where the effective potential is independent of temperature. It is conjectured that, owing to this difference in behavior between classical and quantum-mechanical systems, the crossover behavior observed for helium in the vicinity of the critical point differs qualitatively from that of other simple liquids.
Alignment in angular distribution of photofragments in multiphoton above threshold dissociation of HD+ by linearly and circularly polarized intense laser fields
Authors: Sen Sanjay , Ghosh Swaralipi , Bhattacharyya S. S. , Saha Samir
Published: July 2002

We have investigated the angular distribution of photofragments resulting from multiphoton above threshold dissociation (ATD) of HD+ from initial bound levels νi=0, 1, 2 and Ji=0 induced by linearly (↕) and circularly (↻) polarized laser fields of intensities I=1, 5, and 10 TW/cm2 and frequency ω=30 333 cm−1 (λ=329.7 nm). The time-independent close coupling (CC) method has been used. Molecular rotation has been taken into account by including 72 channels with J=0–7 and number of photons n=−1–7, for ↕ polarization. In case of ↻ polarization, the channel number reduced to 48. All the radiative couplings including those due to the intrinsic dipole moments of HD+ have been considered in a truncated length gauge form of interaction. For (net) one-photon dissociation (from νi=1, 2) by bond softening, the alignment of the angular distribution of photofragments increases with intensity (1–10 TW/cm2), for both ↕ and ↻ polarizations. The angular distributions for (net) two-photon fragmentation via the adiabatic paths are sharper, for both ↕ and ↻ polarizations, than those for (net) three-photon dissociation by nonadiabatic transition. With increase of laser intensity, the alignments in the angular distribution of the two- and three-photon peaks (from νi=0) change differently for ↕ and ↻ polarizations. In general, at a given intensity, the photofragment angular distributions about the space fixed Z-axis are different for ↕ and ↻ polarizations. The Z-axis is along the direction of polarization vector for ↕ polarization, but along the propagation vector for ↻ polarization. The degree of alignment in the angular distribution of the photofragments for ↕ polarization at a lower intensity is expected to be the same as that at a higher intensity for ↻ polarization. We have endeavored to interpret our results of the sharpness of the photofragment angular distributions of HD+ as functions of intensity for different open photon channels within the framework of the dressed state picture and the bond softening, adiabatic path, and nonadiabatic transition mechanisms. We have also computed the branching ratios of the photofragments of HD+ to different open photon channels for both ↕ and ↻ polarizations of the laser fields.
Transient absorption of vibrationally excited water
Authors: Bakker H. J. , Nienhuys H.-K. , Gallot G. , Lascoux N. , Gale G. M. , Leicknam J.-C. , Bratos S.
Published: July 2002

We study the spectral response of the transition between the first and the second excited state of the O–H stretch vibration of HDO dissolved in liquid D2O with two-color femtosecond mid-infrared spectroscopy. The spectral response of this transition differs strongly from the fundamental absorption spectrum of the O–H stretch vibration. In addition, excitation of the O–H stretch vibration is observed to lead to a change of the hydrogen-bond dynamics of liquid water. We show that both these observations can be described with a refined quantum-mechanical version of the Lippincott–Schroeder model for hydrogen-bonded OH⋯O systems.
Towards a voltage tunable two-color quantum-well infrared photodetector
Authors: Majumdar Amlan , Choi K. K. , Rokhinson L. P. , Tsui D. C.
Published: July 2002

Two-color quantum-well infrared photodetectors (QWIPs), based on electron transfer between coupled QWs, suffer from the presence of the shorter wavelength peak at all bias voltages Vb. We investigate this problem by studying the bias dependence of absorption coefficient α and photoconductive gain g as a function of wavelength at 10 K. We fabricate such a detector with a peak wavelength of 8 μm for both bias polarities but a voltage tunable cutoff wavelength (9 μm for Vb>0 and 11 μm for Vb<0). We use corrugated QWIPs with different corrugation periods to extract α and g for different values of Vb. We find α≈0.1 μm−1 in the 6–12 μm range with small peaks at 8 and 9.8 μm for Vb>0 and 10 μm for Vb<0. g has a pronounced peak at 7.8 μm for both bias polarities and determines the line shape of the QWIP spectral responsivity. These results are attributed to insufficient electron transfer between the coupled QWs and to low tunneling probability of the longer wavelength photoelectrons. A modified QWIP structure has been proposed for complete switching of spectral responsivity peak when the bias voltage polarity is reversed.
Laser nitriding of iron with laser pulses from femtosecond to nanosecond pulse duration
Authors: Schaaf P. , Han M. , Lieb K.-P. , Carpene E.
Published: July 2002

Pulsed-laser nitriding is an attractive method to improve metal surface properties, such as hardness, wear, and corrosion resistance, with the advantage of simple experimental setup, rapid treatment, and precise process control. Here, the dependence of the laser nitriding process on the laser pulse duration was investigated over five orders of magnitude in a series of experiments employing pulsed lasers ranging from nanosecond excimer laser pulses (55 ns) and Nd-doped yttrium aluminum garnet (Nd:YAG) laser pulses (8 ns), to ultrashort Ti:sapphire laser pulses (150 fs). The results revealed that for all laser pulse durations and different wavelengths a large nitriding effect was observed. The excimer laser shows the highest nitriding efficiency. The basic processes for the femtosecond pulsed laser are not well understood but seem to result at least partly from processes within the plasma, whereas nanosecond nitriding is based mainly on processes within the liquid/solid surface.
Difference in coercivity between Co/Fe and Fe/Co bilayers
Authors: Park M. H. , Hong Y. K. , Gee S. H. , Mottern M. L. , Jang T. W. , Burkett S.
Published: July 2002

A study of the deposition order and film thickness dependence on the coercivity of ferromagnetic bilayers, ∥Co/Fe and ∥Fe/Co, is presented (the sign, “∥,” is for indicating glass or Si substrate position). The magnetization of the Co layer is aligned with the in-plane direction during rf sputter deposition. The thickness is controlled in the range of 3–22 nm. Since there exists a strong exchange interaction between the two ferromagnetic layers, the magnetization reversal process occurs cooperatively. ∥Fe/Co shows an isotropic and hard-magnetic behavior, whereas ∥Co/Fe shows an anisotropic and soft-magnetic behavior. A sudden drop of coercivity in ∥Fe/Co observed at the Fe layer thickness below 5 nm is caused by a decrease in the saturation magnetization of the Fe layer. Due to the surface roughness, the bilayer on the glass substrate possesses a higher coercivity than that of the bilayer deposited on the silicon substrate. The magnetization reversal process of the ferromagnetic bilayers is discussed.
Visible photoluminescence of Ge dots embedded in Si/SiO2 matrices
Authors: Shklyaev Alexander A. , Ichikawa Masakazu
Published: July 2002

Ge island formation on ultrathin SiO2 films enabled us to fabricate multilayer structures of Ge dots ∼6–7 nm in diameter and with an extremely high dot density of 2×1012 cm−2. Each dot had a boundary with the SiO2 film and a Si spacer layer. The multilayer structures exhibited photoluminescence (PL) with a maximum in the range of 2–3 eV depending on the excitation energy. The PL was associated with recombination between holes confined within Ge dots and electrons localized in the radiative defect centers at the Ge-dot/SiO2 interfaces. The results suggest that this recombination is much more effective than that at the Si/SiO2 interface and supported by the hole migration from the Si spacer layers to the Ge dots.
A 0.5-μm-thick polycrystalline silicon Schottky diode with rectification ratio of 106
Authors: Guliants Elena A. , Ji Chunhai , Song Young J. , Anderson Wayne A.
Published: July 2002

Polycrystalline Si films, 0.5-μm thick, were obtained as a result of metal-induced growth by sputtering from a Si target on 25 nm thick Ni prelayers at 525 °C. Silicon grew heteroepitaxially on the NiSi2 layer formed due to the reaction between the sputtered Si atoms and Ni. Schottky diodes were fabricated on the Si films by deposition of a Schottky metal on the front surface of the film while Ni disilicide provided an intimate ohmic contact at the back. A Pd/n-Si diode using an n-Si film annealed for 2 h at 700 °C in forming gas demonstrated a rectification ratio of 106, while an as-deposited p-Si film provided an Al/p-Si diode with rectification of five orders of magnitude. Schottky barrier properties are briefly discussed.
Localized relaxation’s strength and its mimicry of glass-softening thermodynamics
Authors: Johari G. P. , Power G. , Vij J. K.
Published: July 2002

The dielectric relaxation strength of the β process, Δεβ, in a rigid-molecular glass of 16.6 mol % chlorobenzene cis-decalin mixture increases slowly with temperature, T, until the glass-softening range is reached, and thereafter it increases rapidly, i.e., (dΔεβ/dT) increases at Tg relatively abruptly. This is remarkably similar to the volume and entropy against T plots of a glass being heated through its Tg and of the implied rise in the expansion coefficient and heat capacity near Tg. The β-relaxation rate increases smoothly through Tg according to the Arrhenius equation, and the distribution of relaxation times becomes narrower. Both the entropy and free volume affect Δεβ, but not the rate.
Stability analysis and molecular simulation of nanoscale lubricant films with chain-end functional groups
Authors: Izumisawa Satoru , Jhon Myung S.
Published: July 2002

In this article, the thermodynamic stability of ultrathin perfluoropolyether (PFPE) films were examined via experimentation and molecular simulation. Using measured surface energies of PFPE films and a virgin carbon overcoat, a mesoscopic model using the Gibbs free energy change of spreading PFPE films was developed, yielding a film stability diagram as a function of film thickness, surface polarity of the carbon overcoat, and molecular weight of the PFPEs. We predict that a lower PFPE molecular weight and a lower surface polarity of the carbon overcoat produce a less stable film. This stability analysis was enhanced with off-lattice Monte Carlo simulations, which were applied for the first time to examine PFPE film nanostructures. The simulation results produced oblate conformations for the PFPE molecules near the surface, layered polar endgroup orientations, and a rougher surface morphology for films with less stable conditions, as predicted by the Gibbs free energy model.
Highly efficient electrophosphorescent devices based on conjugated polymers doped with iridium complexes
Authors: Zhu Weiguo , Mo Yueqi , Yuan Min , Yang Wei , Cao Yong
Published: July 2002

Iridium complexes with alkyl substituted 2-phenylpyridine, Ir(Bu-PPy)3, were synthesized. Polymer light emitting diodes with Ir complexes as the guest materials and the substituted polyphenylenes as the host were fabricated. Ir(Bu-PPy)3-doped Poly(2-(6-cyano-6-methyl)- heptyloxy-1,4-phenylene) (CNPPP) device showed generally higher quantum efficiency (QE) than that of Ir(PPy)3-doped device for a given dopant concentration. More importantly, the addition of butyl group into phenylpyridine ligand significantly suppresses the decay of device efficiency at high current density. For instance, for devices made with Ir(Bu-PPy)3-doped CNPPP: the maximum external quantum efficiency, QE, and luminance efficiency reached 5.1% ph/el and 12 cd/A, respectively, at 800 cd/m2 and maintained at 4.2% ph/el and 10 cd/A, respectively, at 2500 cd/m2.
Separation of variables and subgroup bases on n-dimensional hyperboloids
Authors: Pogosyan G. S. , Winternitz P.
Published: July 2002

A graphical formalism is introduced for describing subgroup type coordinates on n-dimensional Lorentzian hyperboloids imbedded into n+1 dimensional Minkowski spaces. The O(n,1) group element is parametrized according to different subgroup chains, involving Lorentz, rotation, and Euclidean subgroups. The coordinates are then induced by the corresponding group action. Eigenfunctions of the Laplace–Beltrami operator are obtained as products of Jacobi functions, associated Legendre functions, and modified Bessel functions.
Quantum limit to phase coherence in thin superconducting wires
Authors: Tinkham M. , Lau C. N.
Published: July 2002

Spontaneous quantum phase slips by 2π in thin superconducting wires could cause errors in superconducting computers. The rate of such phase slips at T≪Tc is estimated, and shown to be very small as long as the wire is >20 nm in diameter.
Suppression of decomposition of aniline cation in intense laser fields by cluster formation with ammonia molecules
Authors: Watanabe Jun , Itakura Ryuji , Hishikawa Akiyoshi , Yamanouchi Kaoru
Published: July 2002

Mass-selected aniline cations and [aniline-(NH3)n]+ (n=1 and 2) cluster ions are exposed to the femtosecond laser fields (λ∼395 nm,I∼4×1015 W/cm2) and the nanosecond laser fields (λ=532 nm,I∼2.7×1010 W/cm2) by using a tandem type time-of-flight mass spectrometer. In the case of the bare aniline cation, the decomposition forming the five-membered ring compound, cyclopentadienyl cation (C5H6+), dominantly proceeds in both the femtosecond and nanosecond laser fields. When one or two ammonia molecules are attached to the aniline cation, the decomposition is significantly suppressed. This suppression was interpreted in terms of an intermolecular energy flow through the hydrogen bonding.
Interaction of HOCl with a chlorinated ice surface to produce molecular chlorine: An ab-initio study
Authors: Casassa Silvia , Pisani Cesare
Published: July 2002

The reaction between molecular HOCl and a chlorinated surface of ice to produce molecular chlorine has been theoretically investigated. Ab initio quantum mechanical tools (GAUSSIAN 98 and EMBED01) have been used for this purpose, based on a molecular cluster and on an embedded cluster model, respectively. Different reaction paths have been explored, requiring a pre-activation of the substrate. In all cases, important activation barriers have been found. Only if certain types of structural defects are present at the surface, the reaction has been found to take place almost spontaneously.
Surface modification of C60 by ion irradiation studied with photoelectron spectroscopy
Authors: Reinke P. , Oelhafen P.
Published: July 2002

Thin films of C60 were subjected to irradiation by low energy Ar+ ions (E<2 keV) and the changes in the surface initiated by ion impact were monitored in situ with photoelectron spectroscopy in the ultraviolet (UPS) and x-ray regime (XPS). The ion irradiation leads to damage of the C60 cages and accumulation of defects in the surface regions. This is expressed in the UPS spectra by a gradual decrease in the intensity of the sharp features associated with the valence band of C60 and the appearance of additional states close to the Fermi energy. An analysis of the UPS-valence band spectra shows that the destruction of the cages is incomplete. The formation of defects and structural changes in the surface layer trigger a shift of the C60 related features in the valence band spectra and the C 1s core level by 0.4 eV to smaller binding energies. This is commensurate with a shift and pinning of the Fermi level in the gap. The C 1s peak parameters derived from a deconvolution of the peak likewise reflect the partial cage destruction, and the deconvolution indicates the presence of a single component. This might point to the presence of a structurally rather homogeneous surface layer, but different explanations for this experimental result can be envisaged and are discussed.
The influence of internal degrees of freedom on the unimolecular decay of the molecule–cluster compound Au8+CH3OH
Authors: Vogel M. , Hansen K. , Herlert A. , Schweikhard L. , Walther C.
Published: July 2002

Time-resolved photodissociation measurements of the sequential reaction Au8+CH3OH→Au8+→Au7+ and the direct reaction Au8+→Au7+ have been performed for several excitation energies. The production rates and yields of the final state Au7+ in the sequential process are strongly influenced by the excitation energy deposited into the evaporated methanol molecule during the initial fragmentation step. Both the rate constants and yields can be fitted with a single parameter, the cluster–methanol binding energy.
Thermodynamic properties of highly undercooled liquid TiAl alloy
Authors: Wang N. , Wei B.
Published: July 2002

The specific heat of an undercooled liquid TiAl alloy was measured by an electromagnetic levitation drop calorimeter and the related thermodynamic properties, difference in Gibbs free energy and change in entropy are calculated based on the experimental results and some well known models. Under the containerless processing condition, the liquid alloy was undercooled by up to 219 K (0.122TL). The change in enthalpy shows a linear relationship with the temperature and a nearly constant specific heat was determined even though this alloy was undercooled far below its liquidus temperature. The calculated results of the thermodynamic properties reveal that even though semiquantitative models fit with the experimental curve of the difference in Gibbs free energy well, they lead to different degrees of deviation in the entropy difference calculation.
Nonlinear coupling of whistler wave turbulence with magnetosonic perturbations
Authors: Das Amita , Singh Raghvendra , Kaw Predhiman , Champeaux Stephanie
Published: July 2002

The coupling between the short scale whistler turbulence and the long scale magnetosonic excitations are studied in both weak and strong turbulence regimes. The coupling occurs via the following mechanism. The nonlinear interactions among the whistler waves can lead to low frequency perturbations involving ion motion. The low frequency perturbation changes the background plasma density and the magnetic field, thereby altering the whistler wave trajectories. Wave kinetic description and Fourier representation are adopted for the whistler turbulence and the magnetosonic excitations, respectively, keeping in view the disparity in their scales. It is shown that the turbulent spectrum of whistler waves is modulationally unstable to magnetosonic excitations. For the weakly turbulent case, it is shown that the long scale magnetosonic scales backreact on the whistler spectrum, leading to a diffusion of the whistler spectrum towards shorter scales. However, for a strongly turbulent case, the whistler wave trajectories can become trapped inside the magnetosonic waves, thereby leading to the formation of a variety of nonlinear coherent structures. These structures are of relevance for understanding the phenomena of intermittency in magnetofluids.
Renormalization group theory of anomalous transport in systems with Hamiltonian chaos
Authors: Zaslavsky G. M.
Published: July 2002

We present a general scheme to describe particle kinetics in the case of incomplete Hamiltonian chaos when a set of islands of stability forms a complicated fractal space-time dynamics and when there is orbit stickiness to the islands’ boundary. This kinetics is alternative to the ‘‘normal’’ Fokker–Planck–Kolmogorov equation. A new kinetic equation describes random wandering in the fractal space-time. Critical exponents of the anomalous kinetics are expressed through dynamical characteristics of a Hamiltonian using the renormalization group approach. Renormalization transformation has been applied simultaneously for space and time and fractional calculus has been exploited.
A basic mathematical model of the immune response
Authors: Mayer H. , Zaenker K. S. , an der Heiden U.
Published: July 2002

Interaction of the immune system with a target population of, e.g., bacteria, viruses, antigens, or tumor cells must be considered as a dynamic process. We describe this process by a system of two ordinary differential equations. Although the model is strongly idealized it demonstrates how the combination of a few proposed nonlinear interaction rules between the immune system and its targets are able to generate a considerable variety of different kinds of immune responses, many of which are observed both experimentally and clinically. In particular, solutions of the model equations correspond to states described by immunologists as ‘‘virgin state,’’ ‘‘immune state’’ and ‘‘state of tolerance.’’ The model successfully replicates the so-called primary and secondary response. Moreover, it predicts the existence of a threshold level for the amount of pathogen germs or of transplanted tumor cells below which the host is able to eliminate the infectious organism or to reject the tumor graft. We also find a long time coexistence of targets and immune competent cells including damped and undamped oscillations of both. Plausibly the model explains that if the number of transformed cells or pathogens exeeds definable values (poor antigenicity, high reproduction rate) the immune system fails to keep the disease under control. On the other hand, the model predicts apparently paradoxical situations including an increased chance of target survival despite enhanced immune activity or therapeutically achieved target reduction. A further obviously paradoxical behavior consists of a positive effect for the patient up to a complete cure by adding an additional target challenge where the benefit of the additional targets depends strongly on the time point and on their amount. Under periodically pulsed stimulation the model may show a chaotic time behavior of both target growth and immune response.
Hierarchical structures in the phase space and fractional kinetics: I. Classical systems
Authors: Zaslavsky G. M. , Edelman M.
Published: July 2002

Hamiltonian chaotic dynamics is not ergodic due to the infinite number of islands imbedded in the stochastic sea. Stickiness of the islands’ boundaries makes the wandering process very erratic with multifractal space–time structure. This complication of the chaotic process can be described on the basis of fractional kinetics. Anomalous properties of the chaotic transport become more transparent when there exists a set of islands with a hierarchical structure. Different consequences of the described phenomenon are discussed: a distribution of Poincaré recurrences, characteristic exponents of transport, nonuniversality of transport, log periodicity, and chaos erasing.
An electromagnetic microscopic positioning device for the scanning tunneling microscope
Authors: Corb B. W. , Ringger M. , Güntherodt H.-J.
Published: July 2002

We have developed a device that is capable of moving with steps as small as 70 Å in any direction over a flat surface. The device that we call the Maggot consists of two permanent magnets attached to the corners of a triangular plate that stands on a flat base with three fixed ball-bearing feet. The Maggot is propelled across the base by the magnetic fields from two coils that are powered by computer-controlled current pulses. Steering of the Maggot is made by vector addition of the pulses by the computer. A complete electromagnetic force theory is given in order to find the best magnet and solenoid geometries for maximum performance. The Maggot is UHV compatible, operates from 500 to 4.2 °K, and is most suitable for operation on a vibration-isolated base. Movement data is measured with both optical and scanning tunneling microscopy.
Conductivity of inhomogeneous materials represented by discrete resistor networks
Authors: Söderberg Martin , Grimvall Göran
Published: July 2002

We examine the effective resistivity of inhomogeneous materials, in particular two-phase materials, which are statistically homogeneous and isotropic. A review of methods used to determine the effective resistivity is given. Comparisons are made between the traditional methods for continuous materials and resistor network calculations. The purpose is to clarify to what extent it is possible to describe a continuous, inhomogeneous material, with significantly differing constituent resitivities by discrete resistor models. Four discretization procedures, the aim of which are to determine the effective resistivity of such materials, are proposed and discussed. The applicability of discretization procedures is tested against a specific example, the checkerboard. It is shown that none of the discretization models may be expected to give a good description of the material. This fact is intimately related to the difference in duality properties of continuous and discrete systems.
Soft x-ray emission from ω0, 2ω0, and 4ω0 laser-produced plasmas
Authors: Kodama R. , Okada K. , Ikeda N. , Mineo M. , Tanaka K. A. , Mochizuki T. , Yamanaka C.
Published: July 2002

The soft x-ray spectra from Au plasmas in 0.1–1.6 keV range and x-ray conversion efficiencies of Au (high Z) and Al (low Z) targets with three laser wavelengths of 0.26, 0.53, and 1.05 μm are measured. In addition, the intensity dependence of the x-ray conversion efficiency at 0.26 μm is obtained for Au targets. For 0.26-μm laser light, the conversion efficiency of Au targets can be as high as 80% at an intensity of 2.4×1013 W/cm2.
A time-resolved x-ray absorption study of amorphous Si during pulsed laser irradiation
Authors: Gerritsen H. C. , van Brug H. , Bijkerk F. , Murakami K. , van der Wiel M. J.
Published: July 2002

We report the first time-resolved x-ray absorption measurements on pulsed laser irradiated Si in the photon energy range from 90 to 300 eV, at irradiation energy densities up to 4 J/cm2, and at several delay times between irradiation pulse and x-ray probe pulse. The absorption spectra recorded at a delay time of 12 ns can be classified in three categories. Below ≊0.17 J/cm2 only thermal damping of extended x-ray absorption fine-structure (EXAFS) oscillations is observed. Between ≊0.17 J/cm2 and ≊1 J/cm2 annealing takes place. Several changes are observed, among which a ≊7-eV LII,III edge shifts towards higher energy, while the LI edge remains approximately at the same position. This edge shift behavior is explained by a largely reduced 3s density of states of the conduction band. This supports the idea that pulsed laser molten Si has a metalliclike structure. Thermal damping in this region is so strong that EXAFS oscillations are no longer visible. This is consistent with a lattice temperature above or equal to the melting temperature of Si. Above 1 J/cm2, the damage region, further edge shifts and the development of several peaks are observed. Clear cooling effects are visible at long delay times (60 ns) between irradiation and x-ray probe pulse. Cooling on this time scale can only be explained by evaporation of Si.
Thickness dependence of the critical solution temperature of hydrogen in Pd films
Authors: Nicolas M. , Dumoulin L. , Burger J. P.
Published: July 2002

The isotherms of very thin PdHx films (60 Å<d<600 Å) have been determined below room temperature by equilibrium potential measurements. One finds that the critical temperature Tc is lowered when the film thickness decreases: for the 60-Å film Tc would be near 173 K. This marked drop of Tc is attributed to clamping effects of the sample on the substrate, which decreases the effective H–H interaction and favors short-range coherency effects.