bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural properties of hydrogenated silicon nanocrystals and nanoclusters
Authors: Yu D. K. , Zhang R. Q. , Lee S. T.
Published: December 2002

The structures of fully and partially hydrogenated Si nanocrystals and nanoclusters are studied by geometric optimizations and molecular dynamics simulations based on an empirical tight-binding approach. It is shown that the structural properties of the hydrogen saturated Si nanocrystals have little size effect, contrary to their electronic properties. The surface relaxation is quite small in the fully hydrogen saturated Si nanocrystals, with a lattice contraction of 0.01 to 0.02 Å residing in the outermost two or three layers. Inside the hydrogenated Si nanocrystals, there is only very small strain (lattice expansion) of the order 10−4 to 10−3, in agreement with the x-ray diffraction measurement. The fully hydrogenated Si nanocrystals are the most stable structures compared to those partially hydrogenated. For the smaller SimHx (m⩽151) nanocrystals, removing up to 50% of the surface terminating H atoms only causes distortions to the crystal structure, with the basic tetrahedral structural features still retained. However, removal of more than 70%–80% of the surface terminating H atoms will lead to a change to more compact structures.
Driven Newton equations and separable time-dependent potentials
Authors: Lundmark Hans , Rauch-Wojciechowski Stefan
Published: December 2002

We present a class of time-dependent potentials in Rn that can be integrated by separation of variables: by embedding them into so-called cofactor pair systems of higher dimension, we are led to a time-dependent change of coordinates that allows the time variable to be separated off, leaving the remaining part in separable Stäckel form.
Pseudo-Hermiticity for a class of nondiagonalizable Hamiltonians
Authors: Mostafazadeh Ali
Published: December 2002

We give two characterization theorems for pseudo-Hermitian (possibly nondiagonalizable) Hamiltonians with a discrete spectrum that admit a block-diagonalization with finite-dimensional diagonal blocks. In particular, we prove that for such an operator H the following statements are equivalent: (1) H is pseudo-Hermitian; (2) the spectrum of H consists of real and/or complex-conjugate pairs of eigenvalues and the geometric multiplicity and the dimension of the diagonal blocks for the complex-conjugate eigenvalues are identical; (3) H is Hermitian with respect to a positive-semidefinite inner product. We further discuss the relevance of our findings for the merging of a complex-conjugate pair of eigenvalues of diagonalizable pseudo-Hermitian Hamiltonians in general, and the PT-symmetric Hamiltonians and the effective Hamiltonian for a certain closed FRW minisuperspace quantum cosmological model in particular.
Analysis of the Fenton–Karma model through an approximation by a one-dimensional map
Authors: Tolkacheva E. G. , Schaeffer D. G. , Gauthier D. J. , Mitchell C. C.
Published: December 2002

The Fenton–Karma model is a simplification of complex ionic models of cardiac membrane that reproduces quantitatively many of the characteristics of heart cells; its behavior is simple enough to be understood analytically. In this paper, a map is derived that approximates the response of the Fenton–Karma model to stimulation in zero spatial dimensions. This map contains some amount of memory, describing the action potential duration as a function of the previous diastolic interval and the previous action potential duration. Results obtained from iteration of the map and numerical simulations of the Fenton–Karma model are in good agreement. In particular, the iterated map admits different types of solutions corresponding to various dynamical behavior of the cardiac cell, such as 1:1 and 2:1 patterns.
Accounting for Auger yield energy loss for improved determination of molecular orientation using soft x-ray absorption spectroscopy
Authors: Genzer Jan , Kramer Edward J. , Fischer Daniel A.
Published: December 2002

Partial (Auger) yield near edge x-ray absorption fine structure (NEXAFS) is a structural analytical technique that has been primarily used to measure the spatial orientation and chemical bonding of small molecules on solid (i.e., inorganic or semiconductor) surfaces. In this article we demonstrate that the building block (BB) scheme proposed by Outka and co-workers [Phys. Rev. Lett. 59, 1321 (1987)] for analyzing NEXAFS spectra can be applied to model the molecular orientation of larger molecules, provided one accounts properly for kinetic energy losses of the Auger electrons traversing through the sample and hence the attenuation in measured Auger yield. We test the applicability of the proposed “modified” BB (MBB) model by measuring the orientation of a self-assembled monolayer (SAM) of –O1.5Si–(CH2)2–(CF2)8F, SF–SAM (SiOx), deposited on top of SiOx-covered silicon wafer as a function of the entrance grid bias (EGB) of the channeltron photoelectron detector. Our measurements of the EGB-dependent electron escape depth reveal that a crude depth profiling within the top ≈5 nm of the sample is possible by increasing the negative EGB on the channeltron detector, at the highest bias thus selecting only the Auger electrons, which have suffered negligible energy loss. In addition, we discuss how the order parameter method introduced recently by Stöhr and Samant [J. Electron Spectrosc. Relat. Phenom. 98–99, 189 (1989)] can be used to determine the molecular orientation of large organic molecules on surfaces. We also show that by accounting for energy losses of the NEXAFS Auger electrons (attenuation of measured Auger yield), the corrected order parameter (COP) approach gives good estimates of the orientation of molecules. We present a comparison between the MBB and COP models using experimental data collected from NEXAFS experiments from semifluorinated (SF) mesogens, –(CH2)x(CF2)yF, which are attached to: (1) the isoprene backbone of polyisoprene or a styrene–isoprene diblock copolymer and (2) a SiOx-covered solid substrate. We show that on both surfaces, the SF groups are oriented and on average are tilted by an angle 〈τF-helix〉 from the sample normal. We show that at higher 〈τF-helix〉 the results from the COP approach agree almost quantitatively with those extracted using the MBB model.
Cluster properties in relativistic quantum mechanics of N-particle systems
Authors: Polyzou W. N.
Published: December 2002

A general technique is presented for constructing a quantum theory of a finite number of interacting particles satisfying Poincaré invariance, cluster separability, and the spectral condition. Irreducible representations and Clebsch–Gordan coefficients of the Poincaré group are the central elements of the construction. A different realization of the dynamics is obtained for each basis of an irreducible representation of the Poincaré group. Unitary operators that relate the different realizations of the dynamis are constructed. This technique is distinguished from other solutions [S. N. Sokolov, Dokl. Akad. Nauk USSR 233, 575 (1977); F. Coester and W. N. Polyzou, Phys. Rev. D 26, 1348 (1982)] of this problem because it does not depend on the kinematic subgroups of Dirac’s forms [P. A. M. Dirac, Rev. Mod. Phys. 21, 392 (1949)] of dynamics. Special basis choices lead to kinematic subgroups.
Water vapor millimeter wave foreign continuum: A Lanczos calculation in the coordinate representation
Authors: Ma Q. , Tipping R. H.
Published: December 2002

The water vapor foreign-continuum absorption has been calculated theoretically from first principles for the millimeter wave spectral region as a function of frequency f and temperature T. The calculations are made using the Lanczos algorithm by writing the resolvent operator (ω−L)−1 as continued fractions. In order to guarantee the quick convergence of the continued fractions, the line space of H2O is divided into two subspaces: one consists of the positive resonance lines and the other the negative ones. By ignoring the coupling between them, (ω−L)−1 is expressed as a sum of two continued fractions. The parameters appearing in each of the fractions are functions of the matrix elements of powers of the Liouville operator ℒ between the starting vectors spanning the corresponding subspaces. In the present work, we have taken into account all powers of ℒ up to 5. With the coordinate representation in which the orientations of the H2O–N2 collision pair are chosen as the basis functions in Hilbert space, the anisotropic interaction potential is diagonal, and calculations of the matrix elements are transformed to multidimensional integrations. The latter are evaluated with the Monte Carlo method. In order to reduce the lengthy calculations, we assume that the anisotropic potential has rotational symmetry about the Z axis of H2O, and consists of the long-range dipole–quadrupole part and a short-range repulsive site–site model. Once the parameters of the continued fractions are known, one can calculate the poles and residues and then carry out the ensemble average over the translational motion. Within the quasistatic approximation, one can treat the latter classically and obtain contributions to the absorption coefficient at the poles. Finally, the absorption coefficient at frequency f can be derived by an interpolation method. The results are fitted to a simple function of f and T, and are compared with experimental data and with two different versions of Liebe’s empirical model. In general, the theoretical results are in good agreement with the experiment. Meanwhile, the magnitudes of the theoretical absorption are between those of the 1989 and 1993 versions, but the temperature dependence is closer to the latter one.
Development of a hand-held forensic-lidar for standoff detection of chemicals
Authors: Thomson Grant , Batchelder David
Published: December 2002

The design of a forensic-lidar is reported, enabling a single operator to perform the spectral identification of a labeled object at 5 m distance using a compact, hand-held instrument, low-power excitation (1 mW) and short sampling times (under 2 s) from a sample less than 100-μm thick. The system integrates a modified single-lens-reflex (SLR) telephoto camera with an optical-fiber coupled Raman probe head. Spectral analysis was achieved using an optical-fiber coupled spectrograph employing a volume-phase holographic grating, charge coupled device array detection and visible excitation. Thus, spectra may be acquired from objects encased in transparent packaging or behind glass windows. The use of a modified SLR camera enables accurate sample alignment using the visible spectrum and collection of surface-enhanced resonance Raman scattered flux through the SLR camera lens using epi-illumination. A single operator may acquire fingerprint recognition of a material using a single hand-held unit and spectrum-matching software. The instrument design is reported with preliminary performance such as spectrograph resolution and transmission. A preliminary spectrum is presented at a standoff distance of 5 m.
Picosecond laser-pump, x-ray probe spectroscopy of GaAs
Authors: Adams B. W. , DeCamp M. F. , Dufresne E. M. , Reis D. A.
Published: December 2002

A laser-pump, x-ray probe spectroscopic experiment is described, and the results are shown. The Ga Kα x-ray fluorescence following x-ray absorption, at the Ga K absorption edge was measured, and its increase due to excitation with subpicosecond pulses of laser light at 4.6 eV photon energy was determined. The x-ray absorption, and thus the fluorescence, is increased for about 200 ps after the laser pulse because additional final states for the x-ray absorption are cleared in the valence band by the laser excitation. The technique could eventually lead to a femtosecond pump-probe spectroscopy with an absolute reference energy level and also to a femtosecond x-ray detector. This is of particular importance to future short-pulse x-ray sources, such as free-electron lasers.
Carbon velvet field-emission cathode
Authors: Shiffler D. , Ruebush M. , Haworth M. , Umstattd R. , LaCour M. , Golby K. , Zagar D. , Knowles T.
Published: December 2002

Explosive field emission cathodes comprise an important class of cathodes for high power microwave tubes, having the advantages of light weight as well as requiring no heater for electron emission. Generally, however, this class of cathodes suffers from large amounts of outgassing, nonuniform emission, and very high emittance. This article describes a new class of carbon velvet cathodes that have been coated with a cesium iodide (CsI) salt. We discuss two manifestations of the cathode. We review the lifetime and operation of the cathodes with two different pulse durations, as well as the outgassing from the cathodes during operation. Lifetimes in excess of 980 000 pulses have been obtained, with an outgassing rate of 3.5 atoms per electron. Finally, we discuss the uniformity and emittance of tufted carbon cathodes that have been coated with CsI salt. For comparison, we relate these results to those previously obtained from other cathodes in this class. The cathodes have an emittance of 2.5π mm rad, as compared to the theoretical value, based on computation, of 2.3π mm rad. These new cathodes differ greatly from cathodes such as polymer velvet and tufted carbon fiber cathodes in that no volatiles reside on the cathode and in that a unique coating technique allows the cathodes to function. This new class of cathodes offers a potential replacement for existing thermal cathodes, in that no heater is required for superior operation with low outgassing and long lifetime.
Inverse problem theory in the optical depth profilometry of thin films
Authors: Power J. F.
Published: December 2002

The problem of nondestructive measurement of composition with depth on the scale of ∼0.1–500 μm, in polymers and related materials, has many applications in traditional and recent areas of thin film processing. This article reviews the optical depth profilometry techniques operating on this scale based on optical absorption, photoluminescence, elastic, and inelastic scattering. These methods include photoacoustic and photothermal imaging (including pulsed laser opto–acoustic profiling), attenuated total reflectance infrared, integrated optical spectroscopy methods (based on excitation of planar waveguide structures), confocal scanning microscopy, and the recent technique of light profile microscopy. The profiling of planar structures is emphasized. A common element of all of these methods is that depth mapping requires the solution of a linear inverse problem, where a map of the sample properties is mathematically reconstructed from a set of experimental measurements. This problem is to some extent ill conditioned in some or all regimes of measurement, with the result that depth maps may show sensitivity to data errors. A method is presented for assessing performance of the above experimental depth profilometry techniques in terms of ill conditioning as indicated by: spatial resolution, sensitivity to data errors, and apparent multiplicity of solutions. This method is applied a priori given a knowledge of the linear response theory and measurement parameters Application is made to individual profiling techniques, the performance of each in applications is reviewed, and an inter-comparison is made based on the conditioning of the inverse problem.
An undulator-based spherical grating monochromator beamline for low energy electron-molecule scattering experiments
Authors: Hoffmann Søren V. , Lunt Stuart L. , Jones Nykola C. , Field David , Ziesel Jean-Pierre
Published: December 2002

A new synchrotron radiation undulator beamline, SGM2, with an energy range of 12–20 eV, has been commissioned on the ASTRID storage ring at the University of Aarhus. Using a spherical grating monochromator, the beamline is presently optimized for an energy of 15.76 eV (78.65 nm), for the 3p5(2P1/2)11s resonance in argon, for use in electron-molecule scattering experiments. Using this beamline in conjunction with an electron-molecule scattering apparatus, a beam of electrons down to kinetic energies of a few meV with a resolution of ∼1 meV full width at half maximum is routinely produced.
Instrumentation for plasma immersion ion implantation
Authors: López-Callejas R. , Valencia-Alvarado R. , Muñoz-Castro A. E. , Godoy-Cabrera O. G. , Tapia-Fabela J. L.
Published: December 2002

Plasma immersion ion implantation (PIII) has proved to be a good method to implant ions into materials in order to modify their surface properties. In this article, we describe the design and construction of a small and low cost PIII device. The instrumentation consists of: (i) a simple plasma immersion experimental setup for ion implantation based on direct current glow discharges, (ii) a 25 kV pulse generator, (iii) an electrical probe system endowed with a guard to carry out diagnostics of the plasma parameters, and (iv) an automatic spectroscopy system for determining the plasma temperature. A study of the sheath expansion has been considered in order to fulfill the requirements of electron temperature, plasma density, high voltage bias, pulse frequency, and pulse duration for an adequate PIII process.
Boundary states, extended symmetry algebra, and module structure for certain rational torus models
Authors: Smyrnakis Ioannis
Published: December 2002

The massless bosonic field compactified on the circle of rational R2 is reexamined in the presense of boundaries. A particular class of models corresponding to R2=1/2k is distinguished by demanding the existence of a consistent set of Neumann boundary states. The boundary states are constructed explicitly for these models and the fusion rules are derived from them. These are the ones prescribed by the Verlinde formula from the S-matrix of the theory. In addition, the extended symmetry algebra of these theories is constructed, which is responsible for the rationality of these theories. Finally, the chiral space of these models is shown to split into a direct sum of irreducible modules of the extended symmetry algebra.
High-frequency vibrational energy relaxation in liquids: The foundations of instantaneous-pair theory and some generalizations
Authors: Deng Yuqing , Ladanyi Branka M. , Stratt Richard M.
Published: December 2002

When the relevant frequencies get high enough, vibrational energy relaxation in liquids should, in principle, be governed by instantaneous-pair theory. The basic idea is that in any significantly contributing liquid configuration there is a single critical solvent molecule and that solute relaxation rates are determined by the time evolution of that molecule’s distance from the solute. The theory posits, moreover, that dynamics can always be modeled as a simple one-dimensional, two-body, scattering process with the liquid playing a role only in determining the initial conditions for the scattering. In this article we reformulate this theory so that it can address both polyatomic solutes and molecular solvents and we show that fundamental assumptions and basic approach remain valid even with multiple solute and solvent sites and with long-ranged intermolecular forces. We further show that while the corrections are often not large, it is possible to make systematic improvements by allowing for the multidimensionality of the solute–solvent scattering. We then turn to the instantaneous-normal-mode (INM) interpretation and implementation of the theory. At the lowest level, INM analysis enables us to define the “high frequencies” relevant to the theory as being outside the INM band of the liquid’s intermolecular vibrations and to think of the liquid as generating these frequencies from the overtones of a single INM mode. This kind of analysis predicts a temperature dependence to high-frequency vibrational relaxation remarkably similar to that of solid-state multiphonon models. However, by systematically improving this INM formulation we find that we can also explore the steps a liquid has to take to handle the relaxation of frequencies within its natural band. As the frequency decreases, a liquid evidently needs to invoke more and more of its band to drive the important solvent dynamics. Nonetheless, we continue to find that none of this important dynamics ever seems to involve anything more than the solute’s first solvation shell.
Metrics on the real quantum plane
Authors: Fiore G. , Maceda M. , Madore J.
Published: December 2002

Using the frame formalism we determine some possible metrics and metric-compatible connections on the noncommutative differential geometry of the real quantum plane. By definition, a metric maps the tensor product of two 1-forms into a “function” on the quantum plane. It is symmetric in a modified sense, namely in the definition of symmetry one has to replace the permutator map with a deformed map σ fulfilling some suitable conditions. Correspondingly, also the definition of the Hermitian conjugate of the tensor product of two 1-forms is modified (but reduces to the standard one if σ coincides with the permutator). The metric is real with respect to such modified  * -structure.
Infinite symmetries and conservation laws
Authors: Rosenhaus V.
Published: December 2002

We will consider partial differential equations of a variational problem whose symmetry group generators contain arbitrary function(s) of one or more independent variables. Unlike the Second Noether Theorem we will be interested in the case of arbitrary functions of not all base variables. We will study the relations between infinite symmetries and local conservation laws. We will demonstrate that infinite symmetries may lead to a finite number of conservation laws through appropriate boundary conditions, or to a set of additional constraints for the function and its derivatives.
Are the maximum hardness and minimum polarizability principles always obeyed in nontotally symmetric vibrations?
Authors: Torrent-Sucarrat Miquel , Luis Josep M. , Duran Miquel , Solà Miquel
Published: December 2002

In a recent paper [J. Am. Chem. Soc. 123, 7951 (2001)] we have shown for the first time the existence of molecules with nontotally symmetric vibrational modes that break the maximum hardness (MHP) and minimum polarizability (MPP) principles. We present here an extension of this previous work by devising a mathematical procedure that helps to determine the nontotally symmetric molecular distortions of a given molecule that do not follow the MPP or the MHP. This methodology is based on the diagonalization of the Hessian matrix of the polarizability or the hardness with respect to the vibrational normal coordinates. For a relatively large series of molecules, we have carried out diagonalizations of the Hessian matrix of the polarizability to determine the molecular distortions with a more marked MPP or anti-MPP character. From the results obtained, we have derived a set of simple rules that allow to predict a priori without calculations the existence of vibrational modes that break the MPP. With respect to the MHP, the results strongly depend on the method of calculation, but the same rules are useful to predict the existence of vibrational modes that disobey the MHP when the Koopmans’ approximation is used to calculate the hardness.
Electrochemical preparation of cobalt tips for scanning tunneling microscopy
Authors: Albonetti Cristiano , Bergenti Ilaria , Cavallini Massimiliano , Dediu Valentin , Massi Massimiliano , Moulin Jean-François , Biscarini Fabio
Published: December 2002

Here we describe a straightforward electrochemical method for fabricating sharp cobalt tips. Such tips are particularly useful for those scanning tunneling microscopy (STM) experiments where the focus is on magnetic properties of the surface and the spin polarized (SP) tunneling current is the relevant property, such as in SPSTM and SP scanning tunneling spectroscopy.
Phase diagram, chemical bonds, and gap bowing of cubic InxAl1−xN alloys: Ab initio calculations
Authors: Teles L. K. , Scolfaro L. M. R. , Leite J. R. , Furthmüller J. , Bechstedt F.
Published: December 2002

Thermodynamic, structural, and electronic properties of cubic InxAl1−xN alloys are studied by combining first-principles total energy calculations and the generalized quasichemical approach. Results for bond-lengths, second-nearest-neighbors distances, and bond angles in the alloy are presented. The calculated phase diagram of the alloy shows a broad and asymmetric miscibility gap. The gap fluctuations in the alloy allow for the definition of a minimum gap and an average gap with different bowing parameters, that can provide an explanation for the discrepancies found in the experimental values for the bowing parameter. It is also found that lattice matched In0.2Al0.8N with GaN is suitable to form a barrier material for electronic and optoelectronic nitride based devices.
Wigner functions for curved spaces. I. On hyperboloids
Authors: Alonso Miguel Angel , Pogosyan George S. , Bernardo Wolf Kurt
Published: December 2002

We propose a Wigner quasiprobability distribution function for Hamiltonian systems in spaces of constant curvature, in this article on hyperboloids, which returns the correct marginals and has the covariance of the Shapiro functions under SO(D,1) transformations. To the free systems obeying the Laplace–Beltrami equation on the hyperboloid, we add a conic-oscillator potential in the hyperbolic coordinate. As an example, we analyze the one-dimensional case on a hyperbola branch, where this conic-oscillator is the Pöschl–Teller potential. We present the analytical solutions and plot the computed results. The standard theory of quantum oscillators is regained in the contraction limit to the space of zero curvature.
Vacuum and cryogenic station for microelectromechanical systems probing and testing
Authors: Legrand B. , Quévy E. , Stefanelli B. , Collard D. , Buchaillot L.
Published: December 2002

We have developed a flexible station for probing and testing microelectromechanical systems (MEMS) and relative integrated circuits. The system allows testing devices under vacuum condition ranging from 10−6 to 760 Torr and at variable temperatures from −100 to +150 °C. Electrical measurements up to 3 GHz and optical measurements through glass windows can be performed simultaneously on MEMS devices. Either packaged devices mounted on printed circuit boards or diced or undiced chips are acceptable. Chips are probed using manual probe heads or probe cards. If needed, two separable parts of a MEMS device, for example, an electromechanical part and an electronic integrated circuit, can be tested simultaneously in different temperature conditions. The setup has been successfully used to characterize electromechanical resonators which exhibit quality factors close to 100 000 in high vacuum.
A compact versatile femtosecond spectrometer
Authors: Nagarajan V. , Johnson E. , Schellenberg P. , Parson W. , Windeler R.
Published: December 2002

A compact apparatus for femtosecond pump–probe experiments is described. The apparatus is based on a cavity-dumped titanium:sapphire laser. Probe pulses are generated by focusing weak (∼1 nJ) pulses into a microstructure fiber that produces broadband continuum pulses with high efficiency. With the pump pulses compressed and probe pulses uncompressed, the rise time of the pump–probe signals is <100 fs. The 830 nm pump pulses are also frequency doubled to generate light for excitation at 415 nm. The versatility of the spectrometer is demonstrated by exciting molecules at either 830 or 415 nm, and probing at wavelengths ranging from 500 to 950 nm. Some results on the green fluorescent protein are presented.
First measurement of neutron emission profile on JT-60U using Stilbene neutron detector with neutron-gamma discrimination
Authors: Ishikawa M. , Nishitani T. , Morioka A. , Takechi M. , Shinohara K. , Shimada M. , Miura Y. , Nagami M. , Kaschuck Yu. A.
Published: December 2002

This article describes a diagnostic for measuring neutron emission profile in JT-60U. The Stilbene neutron detector, developed by TRINITI laboratory in Russia, has been installed on the JT-60U Tokamak to measure the neutron emission profile for the first time. The Stilbene neutron detector is a detector which combines a Stilbene crystal scintillator with a neutron-gamma pulse shape discrimination circuit, with a very compact size. Performance tests were carried out using neutron and gamma-ray source prior to installation on JT-60U. Good gamma suppression of the Stilbene neutron detector was verified. Though the neutron emission profile obtained by Stilbene neutron detectors has error of 30% in innermost channel with a calculation using measured plasma parameters, there is an agreement within 10% error in the other channels.
Optical study of redox process of Ag nanoparticles at high temperature
Authors: Bi Huijuan , Cai Weiping , Kan Caixia , Zhang Lide , Martin D. , Träger F.
Published: December 2002

An optical study of the redox of Ag nanoparticles was carried out by loading the particles into the pores of monolithic mesoporous silica. The optical absorption of silver nanoparticles was measured after annealing in different atmospheres. It has been found that treatment in H2 or N2 induces a strong surface plasmon resonance (SPR) as usually expected, but air-annealing results in disappearance of such SPR. Alternate annealing in air and H2 (or N2) induces alternant disappearance and appearance of the SPR absorption, showing reversibility of optical changes. By additionally considering the surface and curvature effects of a particle, thermodynamic analysis for the oxidation of a Ag nanoparticle was conducted. Unlike bulk metal silver, which is stable and not oxidized in air at above 200 °C, for spherical Ag particle there exists a critical size for the oxidation (say, 2 nm at 500 °C). When the particle size is smaller than the critical value, oxidation could occur in air at above 200 °C and even at 700 °C. The reversible optical changes are attributed to the redox process of Ag particles in different atmospheres, which has been confirmed by further experiments.
Dangling bonds with “negative Hubbard U ”: Physical model for degradation of SiO2 gate dielectric under voltage stress
Authors: Gitlin Daniel , Karp James , Moyzhes Boris
Published: December 2002

A model proposed to explain the phenomenon of current increase and its fluctuation under voltage stress in a SiO2 gate dielectric is based on the amorphous nature and presence of dangling bonds in SiO2. Dangling bonds D0 are thought to be negative-U centers, where their neutral state is unstable and therefore a spontaneous reaction of charge disproportionation take place: D0+D0→D++D−. As a result, a SiO2 amorphous network has diamagnetic positive D+ and negative D− centers. Due to a large difference in mobility between electrons and holes, hopping current in SiO2 is mainly electron current on D+ centers. Current increase and fluctuation under a voltage stress is due a conversion of D− into D+ centers by the hole component of current through SiO2 gate dielectric. This conversion is an irreversible process accelerated by temperature and electric field.
Spontaneous buildup of giant surface potential by vacuum deposition of Alq3 and its removal by visible light irradiation
Authors: Ito Eisuke , Washizu Yuuki , Hayashi Naoki , Ishii Hisao , Matsuie Noritaka , Tsuboi Kazuma , Ouchi Yukio , Harima Yutaka , Yamashita Kazuo , Seki Kazuhiko
Published: December 2002

We observed high and persistent spontaneous buildup of the surface potential (SP) upon vacuum deposition of tris(8-hydroxyquinolinato) aluminum(III) (Alq3) on an Au substrate under dark conditions. SP determined by the Kelvin probe method reached 28 V at a thickness of 560 nm and the surface of the Alq3 film was positively charged. We propose a model in which preferential orientation of the dipole moments of Alq3 molecules is the origin of this buildup of the SP. The intensity of second-harmonic generation was also dramatically increased by the deposition of Alq3 under dark conditions, which supports the notion of a buildup of dipole layers. This giant surface potential was almost completely removed by irradiation of Alq3 molecules with visible light, and irradiation during deposition also prevented the buildup of SP.
Structure of the water ice surface studied by x-ray absorption spectroscopy at the O K-edge
Authors: Parent Ph. , Laffon C. , Mangeney C. , Bournel F. , Tronc M.
Published: December 2002

Vapor-deposited H2O ice films grown between 38 and 150 K under ultrahigh vacuum conditions have been investigated using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy at the oxygen K-edge, in conventional mode—which is bulk sensitive-, and using the photon-stimulated desorption mode (PSD-NEXAFS), which is surface sensitive. By recording simultaneously those two signals, we have evidenced the differences between the surface and bulk electronic and atomic structures, for both amorphous porous ice condensed at 40 K and crystalline ice condensed at 150 K. We have also followed the bulk and surface evolutions of an amorphous ice film annealed from 38 to 147 K. A steep change in the local atomic structure of the bulk is observed, likely related to the high-density amorphous ice→low-density amorphous ice phase transition between 38 and 55 K. We have shown that the surface of crystalline ice is well ordered, but this order is different from that of the bulk. We have evidenced that the H2O–H2O intermolecular distance at the surface of ice is always longer than in the bulk, and that this difference increases with temperature, as the thermally induced reordering of the surface proceeds. SCF-Xα multiple scattering calculations allow us to figure out those structural differences, both in the bulk and at the surface of amorphous ice, but further calculations are necessary for crystalline ice. We have shown that the PSD-NEXAFS signal is sensitive to the surface morphology that changes with temperature because of the micropores collapse. We have used a model [E. Vichnevetski, A. D. Bass, and L. Sanche, J. Chem. Phys. 113, 3874 (2000)] that quantitatively describes the effect of the surface porosity on the ion yield. The surface of ice at 38 K is well described by a network of vertical cylindrical pores of 20 Å of diameter, separated by 6 Å, collapsing when annealing the film. This model also properly accounts of the peculiar temperature evolution of the PSD signal at the O1s−14a1+1 excitation, and therefore, allows to establish the relation between the PSD-NEXAFS signal and the surface porosity.
Nonstoichiometry and chemical purity effects in thermoelectric Ba8Ga16Ge30 clathrate
Authors: Bryan J. Daniel , Blake Nick P. , Metiu Horia , Stucky Galen D. , Iversen Bo B. , Poulsen Rasmus D. , Bentien Anders
Published: December 2002

Zone melting purification experiments have been carried out on the clathrate, Ba8Ga16Ge30. The impurities present have been identified and their approximate concentrations measured. Trace impurities were determined to be approximately 240 parts per million (ppm) in the most impure sample to 17 ppm in the most pure sample. The temperature-dependent Seebeck coefficient, thermal conductivity, and electrical conductivity are reported as a function of sample purity as well as the room-temperature Hall coefficient. Microprobe analysis suggests that the samples are nonstoichiometric with excess Ge relative to Ga, and there are indications of the presence of defects. Single-crystal x-ray investigations as well as synchrotron powder diffraction measurements support the presence of defects, but the x-ray data cannot accurately determine the relative amounts of Ga and Ge. Band-structure calculations in the generalized gradient approximation show that the measured Hall and Seebeck coefficients are consistent with a defect lattice of approximate stoichiometry Ba8Ga14Ge31. Although the figure of merit (ZT) is found to be the highest for the purest sample, the dominant contribution to transport is conjectured to arise from deviations from the ideal stoichiometry and not impurities.
Membrane deformation and the elastic energy of insertion: Perturbation of membrane elastic constants due to peptide insertion
Authors: Partenskii Michael B. , Jordan Peter C.
Published: December 2002

In peptide insertion, matching of the hydrophobic regions of both peptide and lipid molecules constrains the lipid molecules’ mobility and their ability to adjust position, orientation and conformation. This can be described as an insertion induced modification of membrane elastic constants close to the insertion. The perturbation’s correlation length (λc) must be comparable to the length of lipid molecules, ∼1.5 nm. We simulate this effect by introducing a “transition” function with decay length λc. The deformation profile u(r) and corresponding elastic free energy E are calculated using Euler-Lagrange equations. The proper choice of boundary conditions is discussed. Perturbation of the membrane’s compressional modulus is shown to have much greater influence than perturbation of the bending modulus. Experiments on gramicidin channels’ lifetime are discussed from this perspective. Possible implications of the nonuniformity of the elastic constants on the membrane-mediated interaction between the insertions is also considered.
Femtosecond excitation tuning and site energy memory of population transfer in poly(p-phenylenevinylene): Gated luminescence experiments and simulation
Authors: Sperling J. , Milota F. , Tortschanoff A. , Warmuth Ch. , Mollay B. , Bässler H. , Kauffmann H. F.
Published: December 2002

We present a comprehensive experimental and computational study on fs-relaxational dynamics of optical excitations in the conjugated polymer poly(p-phenylenevinylene) (PPV) under selective excitation tuning conditions into the long-wavelength, low-vibrational S1ν=0-density-of-states (DOS). The dependence of single-wavelength luminescence kinetics and time-windowed spectral transients on distinct, initial excitation boundaries at 1.4 K and at room temperature was measured applying the luminescence up-conversion technique. The typical energy-dispersive intra-DOS energy transfer was simulated by a combination of static Monte Carlo method with a dynamical algorithm for solving the energy-space transport Master-Equation in population-space. For various, selective excitations that give rise to specific S1-population distributions in distinct spatial and energetic subspaces inside the DOS, simulations confirm the experimental results and show that the subsequent, energy-dissipative, multilevel relaxation is hierarchically constrained, and reveals a pronounced site-energy memory effect with a migration-threshold, characteristic of the (dressed) excitation dynamics in the disordered PPV many-body system.
Frequency locking to a high-finesse Fabry–Perot cavity of a frequency doubled Nd:YAG laser used as the optical phase modulator
Authors: Bregant M. , Cantatore G. , Valle F. Della , Ruoso G. , Zavattini G.
Published: December 2002

We report on the frequency locking of a frequency doubled Nd:YAG laser to a 45 000 finesse, 87-cm-long, Fabry–Perot cavity using a modified form of the Pound–Drever–Hall technique. Necessary signals, such as light phase modulation and frequency correction feedback, are fed directly to the infrared pump laser. This is sufficient to achieve a stable locking of the 532 nm visible beam to the cavity, also showing that the doubling process does not degrade laser performances.
Monolayer topography resolution achieved in a scanning near-field optical microscope
Authors: Plake T. , Ramsteiner M. , Grahn H. T.
Published: December 2002

A low-temperature scanning near-field optical microscope has been adapted to achieve a high topographical sensitivity. This setup allows us to resolve morphological features on semiconductor surfaces of monolayer height, which is demonstrated for two different material systems, monolayer steps on an epitaxial GaN film and growth islands in a GaAs single quantum well (SQW) structure. Complementary photoluminescence measurements for the SQW structure reveal the potential of combining high-resolution topography with spatially resolved optical spectroscopy for investigating semiconductor nanostructures.
Role of a metal–dielectric structure for the high-charge-state-ion production in electron cyclotron resonance ion sources
Authors: Schächter L. , Dobrescu S. , Stiebing K. E.
Published: December 2002

The influence of a metal–dielectric (MD) cylindrical structure, covering the radial walls of the plasma chamber of an electron cyclotron resonance ion source (ECRIS), on the production of highly charged ions is discussed. Its influence on the main plasma parameters (plasma potential, electron density, and electron temperatures) was investigated using the Langmuir probe method. An increase of these parameters was observed when the MD structure was inserted into the plasma chamber of the Frankfurt 14 GHz ECRIS. In addition, the plasma potential was measured independently by determining the offset extraction voltage at zero magnetic field of the magnetic 90° analyzer field with high precision. Good agreement with the Langmuir probe results was obtained. The main influence of the MD structure is characterized by a significant increase of the plasma potential. This indicates that the MD structure helps to increase lifetimes of both, electrons as well as ions. The ion lifetime is known to be one of the essential parameters to influence the production of highly charged ions.
Measurements of the nonthermal x-ray emission in the T-10 tokamak using CdTe detectors
Authors: Savrukhin P. V.
Published: December 2002

A diagnostic technique based on sets of CdTe detectors is used for measurements of spatial localization and temporal evolution of the nonthermal (Eγ up to 150 keV) x-ray emission in plasma with high density in the T-10 tokamak. Various types of x-ray bursts observed during the density limit disruption appear as a result of the forward bremsstrahlung due to nonthermal electron interaction with the residual plasma as well as to nonuniform interaction of the runaway electrons with the plasma-facing constructions.
Quantitative evaluation of chemisorption processes on semiconductors
Authors: Rothschild A. , Komem Y. , Ashkenasy N.
Published: December 2002

This article presents a method for numerical computation of the degree of coverage of chemisorbates and the resultant surface band bending as a function of the ambient gas pressure, temperature, and semiconductor doping level. This method enables quantitative evaluation of the effect of chemisorption on the electronic properties of semiconductor surfaces, such as the work function and surface conductivity, which is of great importance for many applications such as solid- state chemical sensors and electro-optical devices. The method is applied for simulating the chemisorption behavior of oxygen on n-type CdS, a process that has been investigated extensively due to its impact on the photoconductive properties of CdS photodetectors. The simulation demonstrates that the chemisorption of adions saturates when the Fermi level becomes aligned with the chemisorption-induced surface states, limiting their coverage to a small fraction of a monolayer. The degree of coverage of chemisorbed adions is proportional to the square root of the doping level, while neutral adsorbates are independent of the doping level. It is shown that the chemisorption of neutral adsorbates behaves according to the well-known Langmuir model, regardless of the existence of charged species on the surface, while charged adions do not obey Langmuir’s isotherm. In addition, it is found that in depletive chemisorption processes the resultant surface band bending increases by 2.3kT (where k is the Boltzmann constant and T is the temperature) when the gas pressure increases by one order of magnitude or when the doping level increases by two orders of magnitude.
Growth of cubic-TaN thin films by plasma-enhanced atomic layer deposition
Authors: Kim H. , Kellock A. J. , Rossnagel S. M.
Published: December 2002

Low resistivity cubic-TaN thin films were grown by plasma-enhanced-atomic layer deposition using TaCl5 as the metal precursor and hydrogen/nitrogen plasma. The deposition has been performed by alternate exposures of TaCl5 and the plasma of hydrogen and nitrogen mixture. X-ray diffraction analyses show that the film is composed of cubic TaN grains, in contrast to the previously reported highly resistive Ta3N5 films grown by Ta3N5 grown by TaCl5 and NH3 as precursors. The composition and thickness were measured by Rutherford backscattering and hydrogen concentrations were obtained by forward recoil elastic spectrometry as a function of growth parameters. The N content of the cubic TaN films was controlled from N/Ta=0.7 up to 1.3 by changing nitrogen partial pressure. The resistivity and growth rate increase with increasing N concentration in the film. The Cl and H content were found to be strong functions of plasma exposure time and growth temperatures, and TaN films with resistivity as low as 350 μΩ cm were obtained at a low growth temperature of 300 °C.
Metal–germanium–metal ultrafast infrared detectors
Authors: Buca D. , Winnerl S. , Lenk S. , Mantl S. , Buchal Ch.
Published: December 2002

We demonstrate silicon-based ultrafast metal–semiconductor–metal (MSM) photodetectors for near infrared optocommunication wavelengths. They show a response time of 12.5 ps full width at half maximum (FWHM) at both 1300 and 1550 nm wavelengths. The overall external quantum efficiencies are 13% at 1320 nm and 7.5% at 1550 nm. The sensitive volumes are 270 nm thick Ge films, grown on Si(111) by molecular beam epitaxy. Interdigitated Cr metal top electrodes with 1.5–5 μm spacing and identical finger width form Schottky contacts to the Ge film. A Ti-sapphire femtosecond laser with an optical parametric oscillator and an electro-optic sampling system are used to evaluate the temporal response, which is limited by the transit time of the carriers between electrodes. In addition, results on Si–Ge MSM heterostructure detectors with plate capacitor geometry are presented. At 1550 nm an ultrafast response of 9.4 ps FWHM and an overall quantum efficiency of 0.9% are measured.
Optimization of quantum mechanical molecular mechanical partitioning schemes: Gaussian delocalization of molecular mechanical charges and the double link atom method
Authors: Das Debananda , Eurenius Kirsten P. , Billings Eric M. , Sherwood Paul , Chatfield David C. , Hodošček Milan , Brooks Bernard R.
Published: December 2002

Two new techniques for modeling chemical processes in condensed phases with combined quantum mechanical and molecular mechanical (QM/MM) potentials are introduced and tested on small, model compounds. The first technique, the double link atom (DLA) method, is an extension of the traditional, single link atom (SLA) method to avoid some of the problems with the latter method. These problems are primarily electrostatic, as the SLA method can produce an unphysical overall charge or dipole. The second technique, the delocalized Gaussian MM charge (DGMM) method, is an empirical way to include the delocalized character of the electron density of atoms in the MM region. This can be important for the electrostatic interaction of the QM region with nearby atoms in the MM region, and it can simplify the rules governing which classical interactions are included in the energies and forces. Even for very short distances, the DGMM method does not require the neglect of the MM host in the QM calculation. The DGMM method can be used for modeling reactions in solution, and it can be combined with methods such as the link atom, frozen orbital, or pseudopotential methods for terminating the QM region at a covalent bond. The DLA and the DGMM methods have been combined effectively. Presented here are tests on small, model systems that mimic properties important for reactions in proteins, in particular rotational barriers, proton affinities, and deprotonation energies. The new methods yield improved energetics for model compounds, vis-à-vis a point-MM-charge and SLA treatment.