bluuebunny/crossref_metadata_2025 huggingface dataset:
Nanometer patterning of epitaxial CoSi2/Si(100) for ultrashort channel Schottky barrier metal–oxide–semiconductor field effect transistors
Authors: Zhao Q. T. , Klinkhammer F. , Dolle M. , Kappius L. , Mantl S.
Published: July 2002

A nanometer patterning method, based on local oxidation of silicide layers, was used to pattern epitaxial CoSi2 layers. A feature size as small as 50 nm was obtained for 20 nm epitaxial CoSi2 layers on Si(100) after patterning by local rapid thermal oxidation in dry oxygen. A Schottky source/drain metal–oxide–semiconductor field effect transistor with epitaxial CoSi2 on p-Si(100) was fabricated using this nanopatterning method to make the 100 nm gate. The device shows good I–V characteristics at 300 K.
Theoretical modeling of thermoelectricity in Bi nanowires
Authors: Sun X. , Zhang Z. , Dresselhaus M. S.
Published: July 2002

A theoretical model based on the basic electronic band structure of bulk Bi is developed to predict the dependence of the band structure and thermoelectric properties on nanowire width. By carefully tailoring the Bi wire size and carrier concentration, substantial enhancement in the thermoelectric figure of merit is expected for small nanowire widths.
Heterostructure P-channel metal–oxide–semiconductor transistor utilizing a Si1−x−yGexCy channel
Authors: John S. , Ray S. K. , Quinones E. , Oswal S. K. , Banerjee S. K.
Published: July 2002

The dc characteristics of Si1−x−yGexCy P-channel metal–oxide–semiconductor field-effect transistors (PMOSFETs) were evaluated between room temperature and 77 K and were compared to those of Si and Si1−xGex PMOSFETs. The low-field effective mobility in Si1−x−yGexCy devices is found to be higher than that of Si1−xGex (grown in the metastable regime) and Si devices at low gate bias and room temperature. However, with increasing transverse fields and with decreasing temperatures, Si1−x−yGexCy devices show degraded performance. The enhancement at low gate bias is attributed to the strain stabilization effect of C. This application of Si1−x−yGexCy in PMOSFETs demonstrates potential benefits in the use of C for strain stabilization of the binary alloy.
Negative luminescence from type-II InAs/GaSb superlattice photodiodes
Authors: Olafsen L. J. , Vurgaftman I. , Bewley W. W. , Felix C. L. , Aifer E. H. , Meyer J. R. , Waterman J. R. , Mason W.
Published: July 2002

Strong negative luminescence is displayed by type-II InAs/GaSb superlattice diodes under reverse bias. The negative emittance at room temperature is as high as 1.5 μW/cm2 meV at 4.9 μm, and the negative efficiency at 3.5 μm is 41% of the emission from a perfect blackbody at that temperature. The main features of the data are reproduced by a detailed photodiode simulation.
Second-harmonic generation in silicon carbide polytypes
Authors: Niedermeier S. , Schillinger H. , Sauerbrey R. , Adolph B. , Bechstedt F.
Published: July 2002

We report simultaneous measurements and ab initio calculations of the second-order nonlinear coefficients for SiC polytypes. Our measured values for χzzz (χzxx) are 18 (−4) pm/V for 4H SiC and 24 (−4) pm/V for 6H SiC. They are in good agreement with our theoretical results.
The interplay of sputtering and oxidation during plasma diffusion treatment
Authors: Parascandola S. , Kruse O. , Möller W.
Published: July 2002

Metals that form dense native surface oxide layers challenge plasma diffusion treatment techniques. Experimental results obtained during nitriding of stainless steel from real-time depth-resolved compositional analysis by elastic recoil detection give insight into the transport kinetics. In agreement with semiquantitative considerations on the oxide removal and the oxide growth, the interplay of sputtering and oxidation emerges as a key parameter. On this background, suggestions for practical applications and optimization of the modification processes are given for different plasma diffusion treatment techniques.
Structure determination of nanostructured Ni–Co films by anomalous x-ray scattering
Authors: Chow G. M. , Goh W. C. , Hwu Y. K. , Cho T. S. , Je J. H. , Lee H. H. , Kang H. C. , Noh D. Y. , Lin C. K. , Chang W. D.
Published: July 2002

Conventional x-ray diffraction failed to provide correct information on alloying of materials made of elements with close lattice parameters, even for elements commonly accepted to have miscibility. Using anomalous x-ray scattering, we showed that nanostructured NiCo films did not necessarily form solid solution as expected from their phase diagram or suggested by the results of conventional x-ray diffraction.
Single-wall carbon nanotubes as molecular pressure sensors
Authors: Wood Jonathan R. , Wagner H. Daniel
Published: July 2002

Specific peaks of the Raman spectrum of single-wall nanotubes shift significantly upon immersion of the tubes in a liquid, relative to the corresponding peaks in air. This observation means that nanotubes are sensitive to molecular forces, and is interpreted by relating the corresponding molecular strain to a thermodynamic parameter, the cohesive energy density (or more loosely, the surface tension) for a range of liquids. We find that nanotubes deform by a different amount for each liquid. Calibration of this phenomenon enables the construction of a compressive stress–strain curve for carbon nanotubes.
Transition from simple to complex behavior of single molecule line shapes in disordered condensed phase
Authors: Barkai E. , Silbey R. , Zumofen G.
Published: July 2002

We use the Kubo–Anderson sudden jump approach to investigate line shapes of single molecules (SMs) interacting with randomly distributed two level systems (TLSs). Depending on their random environment, SMs exhibit a wide variety of behaviors. Under certain conditions, given in the text, line shapes exhibit simple behavior, e.g., cases where lines are Lorentzian with a width which varies from one molecule to the other. As control parameters are changed a transition to complex line shape phenomena is observed (i.e., the line shapes have random structures, each with a random number of peaks). We investigate these behaviors for two cases—(i) the case when all TLSs are identical though randomly distributed in space and (ii) the standard tunneling model of low temperature glass where the TLSs are nonidentical. We show that, in certain limits, both models can be analyzed using Lévy-stable laws. For the glass model we compute the distribution of line shape variance and discuss a previous proposition, that distribution of variance and the distribution of linewidth measured in experiment are related. For the line shape problem of SMs in glass we show that background TLSs, defined in the text, can be treated collectively using a simple Gaussian approximations. The Gaussian approximation for the background reduces the number of TLSs needed for a full size simulation of the SM glass system.
Electrohydrodynamic flow of a dielectric liquid around a blade electrode
Authors: Higuera F. J.
Published: July 2002

Injection of charge into a dielectric liquid, and a Coulomb force that sets the liquid into motion, may be obtained by applying a dc voltage to a blade-shaped, metallic electrode immersed in the liquid. An analysis of this motion and its influence on the transport of electric charge is carried out for a simple charge injection law. It is shown that the liquid motion, the electric field, and the charge distribution in a region around the electrode tip of size of the order of the electrode curvature radius determine the injected current as a function of the far electric potential seen by this region. The current increases exponentially with the potential when the contribution of the space charge to the electric field is negligible and algebraically when it is dominant, and presents a range of multiplicity in between. When the inertia of the liquid matters, the region around the electrode tip is also the origin of an electrohydrodynamic plume. An oscillatory current regime is found in which the space charge in the interelectrode space rearranges into many discrete lumps that, under constant voltage bias and small current, induce oscillations of the electric field at the injecting electrode and thus fire new lumps. An order of magnitude analysis and numerical computations for this regime give results in line with known experimental data. In conjunction with the hydrodynamic instability of the plume, this pulse firing mechanism is seen to lead to more complex, nonperiodic oscillations.
Direct imaging of highly charged ions in an electron beam ion trap
Authors: Porto J. V. , Kink I. , Gillaspy J. D.
Published: July 2002

We have directly observed the ion cloud distribution in an electron beam ion trap using visible and ultraviolet fluorescence from lines in the ground term of Ar13+, Xe31+ and Xe32+ ions. Using a gated intensified charge coupled device camera, we have the capability to measure both static and dynamic ion cloud distributions. The images provide information about the trapped highly charged ions which is difficult to obtain by other methods. To demonstrate the usefulness of the technique, we took images of static ion clouds under different conditions and compared the distributions to a simple model. We also recorded time resolved images which show that we can monitor the relaxation of the ion cloud toward equilibrium when the trapping conditions are suddenly changed. The information provided by such measurements can be used to improve models of ion cloud dynamics and, combined with modeling, these techniques can help improve measurements of atomic data using electron beam ion traps.
Phonons and exciton recombination in CdSe/ZnSe self-assembled quantum dots
Authors: Rho H. , Robinson L. M. , Smith L. M. , Jackson Howard E. , Lee S. , Dobrowolska M. , Furdyna J. K.
Published: July 2002

We report resonant photoluminescence from CdSe/ZnSe self-assembled quantum dots. When CdSe quantum dots are resonantly excited, excitonic sharp micro-photoluminescence peaks originating from individual quantum dots are strongly enhanced in the region corresponding to optical phonon energies below the excitation. The phonons active in this process are identified as the longitudinal optical (LO) phonons from the CdSe dots, as the interface phonons, and tentatively as the LO phonons from the two-dimensional-like precursor layers. These observations suggest that exciton recombination via phonons is a major relaxation process under resonant excitation.
The HCO2 potential energy surface: Stationary point energetics and the HOCO heat of formation
Authors: Duncan Timothy V. , Miller Charles E.
Published: July 2002

The energies of six stationary points on the OH+CO→HOCO→H+CO2 potential energy surface have been calculated using the G3 and CBS-QB3 methods. An analysis combining ab initio and experimental enthalpies yielded ΔHf298 K (trans-HOCO)=−42.9±1.5 kcal mol−1 (−43.8±1.4 kcal mol−1) at the G3(CBS-QB3) level of theory. These results confirm the revised HOCO heat of formation derived from photoionization spectroscopy and suggest that the HOCO potential well is 8.8 kcal mol−1 shallower than previously thought. We discuss the implications of these results for accurate Rice–Ramsperger–Kassel–Marcus modeling or quantum mechanical scattering calculations of the OH+CO reaction. © 2000 American Institute of Physics.
New approach to the design of density functionals
Authors: Pérez-Jiménez Angel J. , Moscardó Federico , Sancho-Garcı́a Juan C. , Abia Luis Pastor , San-Fabián Emilio , Pérez-Jordá José M.
Published: July 2002

The prevailing approach in density-functional theory makes use of universal functionals, valid for any number of electrons (N). In this article and following the work of Lieb [E. H. Lieb, Int. J. Quantum Chem. 24, 243 (1983)], we argue that the use of N-dependent functionals could be an equally valid approach. Size consistency (dissociation into proper fragments) puts restrictions on the form that N-dependent functionals may adopt. We propose a simple procedure for correcting the size-consistency problem of existing N-dependent functionals that also provides an original way of designing new ones.
Symmetry breaking of the flow in a cylinder driven by a rotating end wall
Authors: Blackburn H. M. , Lopez J. M.
Published: July 2002

The flow driven by a rotating end wall in a cylindrical container with aspect ratio H/R=2.5 is time dependent for Reynolds numbers Re=ΩR2/ν>2700. For Reynolds numbers up to 4000 three solution branches have been identified, and we examine a solution on each one. At Re=3000, the flow is axisymmetric and time periodic. At Re=3500, the flow is quasiperiodic with a low-frequency modulation and supports a modulated rotating wave with azimuthal wave number k=5. At Re=4000, the flow is time periodic with a qualitatively different mode of oscillation to that at Re=3500. It also supports a modulated rotating wave, with k=6. The peak kinetic energy of the nonaxisymmetric modes is associated with the jet-like azimuthal flow in the interior.
Theory for determining alignment parameters of symmetric top molecule using (n+1) LIF
Authors: Cong Shu-Lin , Han Ke-Li , Lou Nan-Quan
Published: July 2002

Expressions used for extracting the population and alignment parameters of a symmetric top molecule from (n+1) laser-induced fluorescence (LIF) are derived by employing the tensor density matrix method. The molecular population and alignment are described by molecular state multipoles. The LIF intensity is a complex function of the initial molecular state multipoles, the dynamic factors, and the excitation-detection geometrical factors. The problem of how to extract the initial molecular state multipoles from (2+1)LIF, as an example, is discussed in detail.
Characterization of magnetron sputtering deposited thin films of TiN for use as a metal electrode on TiN/SiO2/Si metal–oxide–semiconductor devices
Authors: Evangelou E. K. , Konofaos N. , Aslanoglou X. A. , Dimitriadis C. A. , Patsalas P. , Logothetidis S. , Kokkoris M. , Kossionides E. , Vlastou R. , Groetschel R.
Published: July 2002

Titanium nitride (TiN) thin films were deposited by dc magnetron sputtering on SiO2/n-Si substrates in order to study their use as gate electrodes in metal–oxide–semiconductor (MOS) devices. Rutherford backscattering spectroscopy was used to determine the composition of the films and the results were correlated to those obtained by electrical measurements of the constructed MOS devices. Oxygen contamination of the TiN layers was observed, with percentage and spatial variations depending on the various deposition parameters such as the deposition temperature and the substrate bias. The best electrical performance was achieved for devices where the exposed TiN surface had low oxygen contamination. From these samples, the TiNx–Si barrier height was calculated to be equal to 0.52 eV.
Compact soft x-ray reflectometer based on a line-emitting laser-plasma source
Authors: Johansson G. A. , Berglund M. , Eriksson F. , Birch J. , Hertz H. M.
Published: July 2002

We describe a compact soft x-ray reflectometer for in-house characterization of water-window multilayer optics. The instrument is based on a line-emitting, liquid-jet, laser-plasma source in combination with angular scanning of the studied multilayer optics. With a proper choice of target liquid and thin-film filters, one or a few lines of well-defined wavelength dominate the spectrum and multilayer periods are measured with an accuracy of 0.003 nm using a multi-line calibration procedure. Absolute reflectivity may also be estimated with the instrument. The typical measurement time is currently 10 min. Although the principles of the reflectometer may be used in the entire soft x-ray and extreme ultraviolet range, the current instrument is primarily directed towards normal-incidence multilayer optics for water-window x-ray microscopy, and is thus demonstrated on W/B4C multilayers for this wavelength range.
Innershell absorption spectroscopy on CdS: Free clusters and nanocrystals
Authors: Nowak C. , Döllefeld H. , Eychmüller A. , Friedrich J. , Kolmakov A. , Löfken J. O. , Riedler M. , Wark A. , Weller H. , Wolff M. , Möller T.
Published: July 2002

A study of the innershell absorption of free CdS clusters in a beam and of CdS nanocrystals deposited on an Al substrate is presented. CdS clusters in a beam were prepared by a pick-up type cluster source. CdS nanocrystals with a very narrow size distribution were prepared by colloidal chemical synthesis. At the onset of the S 2p absorption a detailed structure is observed which can be assigned to excitonic absorption, variations in the density of states and XANES. The strength of the first absorption band increases with increasing cluster size in contrast to exciton absorption at the valence band. The absorption of CdS nanocrystals is compared with that of free CdS Clusters. The results give evidence that the Frenkel exciton model is more suitable for the description than the Wannier model.
Thermal stability of 2.4 nm period Ni–Nb/C multilayer x-ray mirror
Authors: Vitta Satish , Yang Ping
Published: July 2002

The structure of Ni–Nb/C multilayers as a function of annealing at 200 and 320 °C has been studied. The layered structure is found to be extremely stable even after annealing at 320 °C. The interdiffused layer present at the two interfaces Ni1/2Nb1/2/C and C/Ni1/2Nb1/2 before annealing undergoes a reverse diffusion on annealing leading to an increase in repeat unit thickness as well as reflectivity enhancement. The repeat unit thickness increases from 2.36 to 2.56 nm and the reflectivity at the first order peak increases by 5 times after annealing at 320 °C. Only the Ni1/2Nb1/2 layers in the multilayer undergo a crystalline transformation into an equilibrium NiNb compound without increasing the interface roughness.
Production and characterization of a fully ionized He plasma channel
Authors: Gaul E. W. , Le Blanc S. P. , Rundquist A. R. , Zgadzaj R. , Langhoff H. , Downer M. C.
Published: July 2002

We report guiding of intense (I=1.3±0.7×1017 W/cm2) 80 fs laser pulses with negligible spectral distortion through 1.5-cm-long preformed helium plasma channels. Channels were formed by axicon-focused laser pulses of either 0.3 J energy, 100 ps duration, after preionizing a 200–700 Torr backfill of He gas to ne∼1016 cm−3 with a pulsed electrical discharge; or 0.6–1.1 J energy, 400 ps duration, which required neither preionization nor intentional impurities for seeding. Transverse interferometry showed that He was fully ionized on the channel axis in both cases. Identical femtosecond pulses suffered substantial ionization-induced blueshifts after propagating through Ar and Ne channels of similar dimensions.
The ab initio model potential method: Lanthanide and actinide elements
Authors: Seijo Luis , Barandiarán Zoila , Harguindey Eduardo
Published: July 2002

In this paper we present relativistic core ab initio model potentials based on atomic Cowan–Griffin calculations, together with Wood–Boring spin-orbit operators and optimized Gaussian valence basis sets, for the lanthanide elements Ce to Lu and for the actinide elements Th to Lr. This completes the chemically relevant part of the Periodic Table. A [Kr,4d] core was chosen for Ce–Lu and a [Xe,4f,5d] core was chosen for Th–Lr. Minimal (14s10p9d8f )/[2s1p1d1f] and (14s10p11d9f )/[2s1p1d1f] valence basis sets were, respectively, optimized for Ce–Lu and Th–Lr, and a [6s5p5d4f] contraction is recommended for all these 28 elements in molecular calculations. The atomic and molecular results show the same good quality already observed for the main-group elements and the transition metal elements.
Quantum time correlation functions from complex time Monte Carlo simulations: A maximum entropy approach
Authors: Krilov Goran , Sim Eunji , Berne B. J.
Published: July 2002

We present a way of combining real-time path integral Monte Carlo simulations with a maximum entropy numerical analytic continuation scheme in a new approach for calculating time correlation functions for finite temperature many body quantum systems. The real-time dynamics is expressed in the form of the symmetrized time correlation function, which is suitable for Monte Carlo methods, and several simulation techniques are presented for evaluating this function accurately up to moderate values of time. The symmetrized time correlation function is then analytically continued in combination with imaginary time data to obtain the real-time correlation function. We test this approach on several exactly solvable problems, including two one-dimensional systems, as well two cases of vibrational relaxation of a system coupled to a dissipative environment. The computed time correlation functions are in good agreement with exact results over several multiples of the thermal time βℏ, and exhibit a significant improvement over analytic continuation of imaginary time correlation functions. Moreover, we show how the method can be systematically improved.
Electrical and optical properties of strongly reduced epitaxial BaTiO3−x thin films
Authors: Zhao Tong , Chen Zheng-Hao , Chen Fan , Lu Hui-Bin , Yang Guo-Zhen , Cheng Huan-Sheng
Published: July 2002

A series of BaTiO3−x (x=0, 0.07, 0.18, and 0.48) thin films with c-axis oriented tetragonal perovskite structures were epitaxially grown on SrTiO3 (100) and MgO (100) substrates by laser molecular-beam epitaxy under various oxygen pressures. The electrical conductivity of the deposited thin films increases with decreasing the oxygen pressure. A metallic conduction is observed in the oxygen most deficient BaTiO2.52 thin film whose oxygen content (2.52) and electrical resistivity (6.0×10−5 Ω cm) are both the minimum values among literature. An optical absorption peak is observed in each oxygen deficient BaTiO3−x thin film. The peak position shifts to lower energy, and the peak width becomes larger with decreasing the oxygen pressure. The influence of oxygen pressure on the electrical and optical properties in the reduced BaTiO3−x thin films is discussed.
Efficiently computing bound-state spectra: A hybrid approach of the multi-configuration time-dependent Hartree and filter-diagonalization methods
Authors: Beck M. H. , Meyer H.-D.
Published: July 2002

We present a new approach for determining bound-state spectra of molecules or clusters. In our approach a wave packet propagation is performed, which exploits the efficiency of the multi-configuration time-dependent Hartree scheme, to produce an autocorrelation function. From this, an accurate spectrum is extracted employing the filter-diagonalization procedure. The accuracy of this hybrid method is demonstrated by applying it to the spectrum of carbon dioxide. Compared with the filter-diagonalization scheme based on a numerically exact wave packet propagation and with a matrix diagonalization using the Lanczos algorithm, our approach turns out to be more efficient. The method can easily be generalized to the treatment of resonant states.
Solutions of the Yang–Mills–Higgs equations in 2+1-dimensional anti-de Sitter space–time
Authors: Zhou Zixiang
Published: July 2002

The solutions of the Bogomolny equation in 2+1-dimensional anti-de Sitter space–time are obtained by using Darboux transformations with both constant spectral parameters and variable “spectral parameters.” These solutions give the Yang–Mills–Higgs fields in 2+1-dimensional anti-de Sitter space–time. Some examples in the SU(2) case are considered and qualitative asymptotic behaviors of the solutions as t→∞ are discussed in detail.
Observation of multiple step de-excitation in luminescent single conjugated polymers
Authors: White J. D. , Hsu J. H. , Yang Shu-Chun , Fann W. S. , Pern G. Y. , Chen S. A.
Published: July 2002

The fluorescence intensity fluctuation and polarization modulation of spin-cast single fluorescent conjugated polymers of 2,5-dioctyloxy p-phenylene vinylene (DOO-PPV) with length of the order of the persistence length have been studied. The results indicate that this type of polymer behaves like a collection of semi-independent excitons, with dipoles aligned predominately in the same direction. This suggests that this type of polymer can be considered as a linearly extended rigid structure (i.e., a straight rod). The fluorescent transient exhibits a multiple-step-like behavior with the count rate at each step being a multiple of a common step-size. The fluorescence variation follows the excitation polarization with a DC background observable in some molecules. This DC background indicates that the chromophores in some polymers are not aligned in the same direction. Variations in the Huang–Rhys factor at a single polymer level are also observed.
The linear stability of a ridge of fluid subject to a jet of air
Authors: McKinley I. S. , Wilson S. K.
Published: July 2002

In this paper we investigate the linear stability of an initially symmetric two-dimensional thin ridge of Newtonian fluid of finite width on a horizontal planar substrate acting under the influence of a symmetric two-dimensional jet of air normal to the substrate. Ridges both with and without a dry patch at their center are considered. For both problems we examine both the special case of quasistatic motion (corresponding to zero capillary number) analytically and the general case of nonzero capillary number numerically. In all cases the ridge is found to be unconditionally unstable, but the nature and location of the most unstable mode are found to depend on the details of the specific problem considered.
Self-generation of bright microwave magnetic envelope soliton trains in ferrite films through frequency filtering
Authors: Scott Mark M. , Kalinikos Boris A. , Patton Carl E.
Published: July 2002

Resonant ring feedback with frequency filtering has been used for the self-generation of bright soliton trains. The solitons were produced from magnetostatic backward volume spin waves propagated in an in-plane magnetized magnetic film delay line as part of the resonant ring structure. The amplitude and phase time profiles, together with the power spectra of the self-generated pulses, confirm their bright soliton nature.
A density functional theory study of the reaction of C+O, C+N, and C+H on close packed metal surfaces
Authors: Michaelides A. , Hu P.
Published: July 2002

Density functional theory (DFT) has been used to determine reaction pathways for several reactions taking place on Pt(111) and Cu(111) surfaces. On Pt(111), the reactions of C+O and C+N were studied, and on Cu(111) we investigated the reaction of C+H. The structures of the transition states accessed in each reaction are similar. An equivalent distance separates the reactants with the first located at a three-fold hollow site and the second close to a bridge site. Previous DFT studies have, in fact, often identified transition states of this type and in every case it is the reactant with the weaker chemisorption energy that is located close to the bridge site. An explanation as to why this is so is provided.
Simple microcalorimeter for measuring microgram samples at low temperatures
Authors: Doettinger-Zech S. G. , Uhl M. , Sisson D. L. , Kapitulnik A.
Published: July 2002

An innovative microcalorimeter has been developed for measuring specific heat of very small microgram samples in the temperature range from 1.5 to 50 K and in magnetic fields up to 11 T. The device is built from a commercial sapphire temperature chip (Cernox), which is modified by means of standard microfabrication techniques and which is used as a sample holder, temperature sensor, and sample heater. Compared to existing microcalorimeters the simple design of our instrument allows a fabrication of the device in a few process steps by using facilities present in a standard laboratory clean room. As an illustrative example for the performance of our device, the specific heat of an underdoped (La1−xSrx)2CuO4 and CaRuO3 single crystal has been measured by means of the relaxation time method as well as the ac method.
Relationship between the geometries, electronic structures, and dopant atom of C35B and C35N
Authors: Changgeng Ding , Jinlong Yang , Rongsheng Han , Kelin Wang
Published: July 2002

The geometrical and electronic structures of substitutionally doped fullerenes C35B and C35N have been studied using the density-functional theory with the local spin density approximation and generalized gradient approximation methods. It is found that the dopant atom has a tendency to substitute the site where the substituted carbon atom has a significant contribution for the frontier orbitals of C36 and that the ground state of C35B is the D2d structure whereas C35N prefers the D6h structure. The reactivities towards a nucleophile or electrophile attack are discussed and the binding energies, vertical ionization potentials, electron affinities and chemical hardnesses are predicted for all the clusters.
Spectra of N2–HF from symmetry-adapted perturbation theory potential
Authors: Jankowski Piotr , Tsang Susy N. , Klemperer William , Szalewicz Krzysztof
Published: July 2002

Symmetry-adapted perturbation theory has been used to calculate the interaction energy for the N2–HF van der Waals complex at two H–F separations corresponding to average values for vHF=0 and vHF=3 vibrational states and the N–N separation corresponding to vN2=0. The total of 228 and 197 grid points have been computed for the vHF=0 and vHF=3 case, respectively. A basis set containing 119 spdf-symmetry orbitals and including bond functions has been used. An analytical fit of the four-dimensional ab initio potential energy surface at the H–F separation corresponding to vHF=0 has a global minimum depth De of 762.4 cm−1 at the intermolecular separation R=6.73 bohr for the linear geometry with the H atom pointing towards the N2 molecule. The surface corresponding to the vHF=3 vibrational state has De of 897.9 cm−1 at R=6.71 bohr and the same orientation of HF relative to N2 as in the vHF=0 case. Exact quantum rovibrational calculations have been performed on both surfaces and the rotational constants and the lowest rovibrational frequencies of the complex have been compared to experimental data. The agreement between theory and experiment for vHF=0 potential is substantially better than achieved previously, while for the vHF=3 state our results constitute the first theoretical prediction.
Single quantum dots emit single photons at a time: Antibunching experiments
Authors: Zwiller Valéry , Blom Hans , Jonsson Per , Panev Nikolay , Jeppesen Sören , Tsegaye Tedros , Goobar Edgard , Pistol Mats-Erik , Samuelson Lars , Björk Gunnar
Published: July 2002

We have studied the photoluminescence correlation from a single InAs/GaAs self-assembled Stranski–Krastanow quantum dot under continuous, as well as under pulsed excitation. Under weak continuous excitation, where the single dot luminescence is due primarily to single exciton recombinations, antibunching is observed in the single dot emission correlation. Under weak pulsed excitation, the number of photons emitted by the quantum dot per pulse is close to one. We present data obtained under both conditions and are able to show that devices based on single quantum dots can be used to generate single photons.
Diamond-like carbon films as electron-injection layer in organic light emitting diodes
Authors: Lmimouni K. , Legrand C. , Dufour C. , Chapoton A. , Belouet C.
Published: July 2002

In this letter a thin film of diamond-like carbon (DLC) deposited by pulse laser deposition (PLD) is used as an electron injection layer in organic light emitting diodes. The heterojunction structures of these devices consist of: indium tin oxide (ITO)/conducting polymer/DLC/nickel, or ITO/DLC/P3OT/Pt. Poly(3-octylthiophene) conjugated polymer (P3OT) is used as the emission layer. In all the realized diodes, the current is two orders of magnitude larger than in the conventional ITO/P3OT/aluminum structure, and the driving voltage is drastically reduced. However, the light emission is observed only in the ITO/DLC/P3OT/Pt structure. These results can be interpreted in terms of a highly efficient electron injection from the DLC into the conducting polymer and a DLC/P3OT interface.
Multiphoton ionization and photoelectron spectroscopy of formaldehyde via its 3p Rydberg states
Authors: Liu Jianbo , Kim Ho-Tae , Anderson Scott L.
Published: July 2002

The resonance-enhanced multiphoton ionization (REMPI) spectrum of formaldehyde, two photon resonant in the region of the A21(3px), A11(3py), and B21(3pz) states, is reported. The A21(3px) state spectrum is dominated by the ν3′ (CH2 scissors), ν4′ (CH2 out-of-plane bending), ν5′ (CH2 asymmetric stretching), and ν6′ (CH2 rock) modes, with weaker bands observed for excitation of the ν2′ (CO stretching) mode. Vibrational analysis of the spectrum provides many new frequencies for the A21(3px) state, not resolved or accessible in single photon spectroscopic measurements. Photoelectron spectroscopy is used to probe the nature of the vibronic levels associated with the A21(3px) intermediate state, to measure vibrational frequencies of the resulting cations, and to identify useful routes for preparing vibrational state-selected H2CO+. It is found that H2CO[1A2(3px)] is a well-behaved Rydberg state, generating cations in the same vibrational level that was populated in the intermediate. Cations with mode-selective excitation of up to 0.62 eV can be produced. Ab initio calculations are used to help assign the cation vibrations. In contrast to the well-behaved A21(3px) state, the A11(3py) and B21(3pz) states are strongly mixed with each other and with valence states.
Single- and double-photoionization cross sections of sulfur dioxide (SO2) and ionic fragmentation of SO2+ and SO22+
Authors: Masuoka Toshio
Published: July 2002

Single- and double-photoionization processes of sulfur dioxide (SO2) have been studied in the photon energy region of 37–120 eV by use of time-of-flight mass spectrometry and the photoion–photoion-coincidence method together with synchrotron radiation. The single- and double-photoionization cross sections of SO2 are determined. Ion branching ratios and the partial cross sections for the individual ions, respectively, produced from the parent SO2+ and SO22+ ions are determined separately at excitation energies where the molecular and dissociative single- and double-photoionization processes occur simultaneously, thus enabling more detailed study of the dissociation processes of the parent SO2+ and SO22+ ions.
Noncommutative tachyons and K-theory
Authors: Harvey Jeffrey A. , Moore Gregory
Published: July 2002

We show that the relation between D-branes and noncommutative tachyons leads very naturally to the relation between D-branes and K-theory. We also discuss some relations between D-branes and K-homology, provide a noncommutative generalization of the ABS construction, and give a simple physical interpretation of Bott periodicity. In addition, a framework for constructing Neveu–Schwarz fivebranes as noncommutative solitons is proposed.
Hyperfine structure and the Stark effect in the electronic spectrum of the SiCH radical with implications for microwave spectroscopy and radioastronomy
Authors: Smith Tony C. , Clouthier Dennis J. , Steimle Timothy C.
Published: July 2002

A skimmed molecular beam of silicon methylidyne, SiCH, was produced using a pulsed discharge jet source with methyltrichlorosilane as the precursor. Linewidths of 35 MHz were achieved using laser-induced fluorescence detection. The field-free fine and magnetic hyperfine interactions and Stark effects were analyzed for the Ã 2Σ+-X̃ 2Π3/2 component of the 301 band at 12 934.428 cm−1. The observed proton hyperfine structure was satisfactorily reproduced with a single upper state Fermi contact parameter bF=117(2) MHz. The determined permanent electric dipole moments are μ(Ã 2Σ+)=1.51(2) D and μ(X̃ 2Π3/2)=0.066(2) D. Our ab initio predictions of the Fermi contact parameter and dipole moments at the B3LYP and quadratic configuration interaction including single and double substitutions levels of theory are in reasonable accord with experiment. An interpretation of the magnitudes of the determined parameters is given based on the highest occupied molecular orbitals. The very small ground state dipole moment will make microwave and radioastronomy studies of SiCH very challenging.
Kinetic evidence for the structural similarity between a supercooled liquid and an icosahedral phase in Zr65Al7.5Ni10Cu12.5Ag5 bulk metallic glass
Authors: Chen M. W. , Dutta I. , Zhang T. , Inoue A. , Sakurai T.
Published: July 2002

By differential scanning calorimetric measurement, the kinetics of the phase transformation present in Zr65Al7.5Ni10Cu12.5Ag5 bulk metallic glass during continuous heating was investigated. It was found that the effective activation energy from a supercooled liquid to an icosahedral quasicrystalline phase is much lower than that from the supercooled liquid to eutectic crystalline phases. In addition, the activation energy from the icosahedral phase to the crystalline phases is almost the same as that from the supercooled liquid to the crystalline phases. Both of them support that the local atomic structure is similar for the supercooled liquid and the icosahedral phase in the bulk metallic glass.
Solute–solute potential of mean force in supercritical solvents: A nonlocal integral equation study
Authors: Egorov S. A. , Rabani Eran
Published: July 2002

We employ Monte Carlo simulations and two versions of integral equation theory to study the potential of mean force between two dilute solutes in a supercritical solvent. The nonlocal integral equation theory is shown to be far superior compared to its homogeneous counterpart. Important differences in the behavior of the potential of mean force between medium and high density solvents are pointed out.
Delayed ionization of C60: The competition between ionization and fragmentation revisited
Authors: Rohmund F. , Hedén M. , Bulgakov A. V. , Campbell E. E. B.
Published: July 2002

We reconsider the competition between delayed ionization and neutral fragmentation of C60 as observed on the microsecond time scale. Experimental data are compared with numerically calculated ionization rates allowing for three competing decay channels: thermionic emission, neutral fragmentation, and radiative cooling. The value of the neutral dissociation energy extracted from the analysis of the power law behavior of the ionization rate for delays up to 10 μs was found to be significantly larger than previously reported. The unreasonably high values obtained in combination with the strong deviation from power law behavior for delay times longer than 10 μs has prompted a reinterpretation of the experimental results in terms of delayed ionization from electronically excited fullerenes in the lowest triplet state.
Influence of microstructure on electrical properties of diluted GaNxAs1−x formed by nitrogen implantation
Authors: Jasinski J. , Yu K. M. , Walukiewicz W. , Washburn J. , Liliental-Weber Z.
Published: July 2002

Structural studies of GaAs implanted with N or coimplanted with other elements showed that, in addition to typical postimplant defects, small voids were present in the implanted region in such materials. Comparison of the microstructure found in these layers with electrical results indicates that these voids are responsible for the low activation efficiency of N implanted into GaAs. The results show that the N-induced enhancement of the donor activation efficiency can be achieved only in a void-free region of the implanted sample.
Fourier transform millimeter-wave spectroscopy of chlorocarbene (HCCl)
Authors: Yamamoto Satoshi , Habara Hideta , Kim Eunsook , Nagasaka Hiroyuki
Published: July 2002

The 101–000 and 202–101 rotational transitions of HC35Cl and HC37Cl in the X̃ 1A′ ground vibronic state have been observed with a Fourier transform millimeter-wave spectrometer. The HCCl molecule is produced by discharging a gaseous sample of CH2Cl2 diluted in Ar with a pulsed discharge nozzle. The effective rotational constant (B+C)/2, the centrifugal distortion constant ΔJ, the nuclear quadrupole interaction constants, and the nuclear-spin rotation interaction constant are determined for each isotopic species. The nuclear-spin rotation interaction is found to make a significant contribution to the hyperfine structure of this molecule, which originates from the relatively low-lying electronic excited state. The nuclear quadrupole interaction tensor is highly asymmetric, indicating a significant π character of the C–Cl bond. This can be interpreted in terms of the backdonation of π electrons from the chlorine atom to the carbon atom.
Structure and spectroscopy of Pa4+ defects in Cs2ZrCl6. An ab initio theoretical study
Authors: Seijo Luis , Barandiarán Zoila
Published: July 2002

In this paper we present the results of spin–orbit relativistic ab initio model potential embedded cluster calculations on (PaCl6)2− embedded in a reliable representation of the Cs2ZrCl6 host. Totally symmetric local distortions and vibrational frequencies are calculated for all the states of the 5f1 and 6d1 manifolds, as well as the corresponding 5f↔6d transition energies and the shape of the 5f(Γ8u)←6d(Γ8g) fluorescence band. An excellent overall agreement with available experimental data is observed which allows us to conclude that the quality of the spin–orbit operators used is very high for actinide elements, as was already known for transition metal and lanthanide elements. Furthermore, it is concluded that the structural and spectroscopic information produced here is very reliable and that the 6d(Γ8g′) state is around 10 000 cm−1 higher in energy than it was thought; our calculations suggest a value of 30 000 cm−1 for the 10Dq parameter of Pa4+ in Cs2ZrCl6, which would be compatible with the lower limit of 20 000 cm−1 accepted for Ce3+ in Cs2NaYCl6.
Gas-phase ion/molecule reactions in octafluorocyclobutane
Authors: Hiraoka Kenzo , Mizuno Takayuki , Eguchi Daisuke , Takao Kiyotoshi , Iino Tomoyuki , Yamabe Shinichi
Published: July 2002

Ion/molecule reactions in octafluorocyclobutane (c-C4F8) were studied using a high-pressure mass spectrometer. The thermochemical stabilities of the cluster ions of halide ions (X−) with c-C4F8 were measured. While the F− ion forms a covalent bond with c-C4F8, the interaction between other halide ions with c-C4F8 is mainly electrostatic. Theoretical calculation revealed that the halide ions interact not with the lowest unoccupied molecular orbital but with the next lowest unoccupied molecular orbital of the c-C4F8 molecule in the most stable cluster ions X−(c-C4F8). The electron affinity of c-C4F8 was measured to be 24.2±2.3 kcal/mol (1.05±0.10 eV) by observing the equilibria for reaction of SF6−+c-C4F8=c-C4F8−+SF6. While the sound equilibrium for that reaction was established in the temperature region from ∼350 K down to the lowest temperature measured (∼150 K), that was not established in the higher temperature region above 350 K. This was attributed to the existence of an isomer for c-C4F8− whose electron detachment energy is smaller than 1.05±0.10 eV. By the measurement of thermochemical stabilities of [(O2)m(c-C4F8)n]− (m+n=1–3), the lower electron affinity of c-C4F8 was determined to be 12.0±1.2 kcal/mol (0.52±0.05 eV). The lower limit of the proton affinity of c-C4F8 was estimated to be 130 kcal/mol.
Collective theory of shot noise in gyroklystrons
Authors: Antonsen Thomas M. , Fliflet Arne , Calame Jeffrey P. , Levush Baruch
Published: July 2002

The discrete nature of the electron charge gives rise to what is known as shot noise on electron beams. In this paper the spectrum of shot noise fluctuations on a spiraling electron beam is calculated. Particular attention is paid to the collective effects of the electron beam that can either decrease or increase the noise level. Shielding of the electron charge tends to decrease the noise level while the electrostatic cyclotron maser instability tends to increase the noise level. The formulation describes the competition of these effects for a radially inhomogeneous beam in a nonuniform magnetic field. Results of the calculations are compared with recently obtained experimental measures of the noise spectrum. The experiments indicate that noise is suppressed by collective effects. The calculations indicate that, while suppression is possible for a sufficiently cold electron beam, it is not expected for a beam with several percent velocity spread. The observed suppression is thus due to some collective effect not included in the present analysis.
High β compact toroidal equilibria
Authors: Srinivasan R. , Avinash K. , Kaw P. K.
Published: July 2002

The relationship of the recently proposed tokamak with spheromak shell (STSS) with other compact equilibria in the low aspect ratio A regime, e.g., spherical tokamaks, field reversed configurations, is studied. It is shown that these equilibria are complementary to equilibria with a magnetic hole studied earlier by Cowley et al. [S. C. Cowley, P. K. Kaw, R. S. Kelly, and R. M. Kulsrud, Phys. Fluids B 3, 2066 (1991)] in the large A regime. The former is perfectly paramagnetic while the latter is perfectly diamagnetic. Relevance of these results to the study of compact equilibria conducted recently on Tokyo University Spherical Torus(TS)-3 and TS-4 [M. Inomoto, Y. Ueda, Y. Ono, T. Murakami, M. Tsurda, M. Yamada, and M. Katsurai, Proceedings of the 17th Conference on Fusion Energy, Yokohama, 1998 (International Atomic Energy Agency, Vienna, 1998), Vol. 3, p. 927] is briefly discussed.
Classification of particle orbits in high-β spherical tokamaks
Authors: Belikov V. S. , Yakovenko Yu. V.
Published: July 2002

Orbits of charged particles in magnetic configurations with high β (ratio of the plasma pressure to the pressure of the magnetic field) and strong elongation typical for modern spherical tokamaks are analyzed. Using a model representation of the magnetic field, the dependence of the topology and the shape of particle orbits on constants of motion (the magnetic moment and the canonical angular momentum of a particle) is systematically studied. The resulting classification of particle orbits takes into account the possible existence of the so-called “magnetic valley” (local minimum of the magnetic field, B) and the nonmonotonic dependence of B on the poloidal angle. Special attention is paid to the orbit types that do not exist in standard tokamaks (“tear drops,” “dumb-bells”). It is found that some particle orbits, including banana and passing ones, have unusual shape with the maximum and/or minimum radial coordinate reached outside the equatorial plane of the torus. The boundaries between the regions of orbits of usual and unusual shape on the plane of constants of motions are delineated.
Path integral centroid molecular-dynamics evaluation of vibrational energy relaxation in condensed phase
Authors: Poulsen Jens Aage , Rossky Peter J.
Published: July 2002

We present a method based on centroid molecular dynamics (CMD) to calculate nonlinear quantum force correlation functions important in the golden rule approach for studying vibrational energy relaxation (VER) in condensed phases. We consider a model of a diatomic molecule in a two-dimensional neon liquid and also a diatomic coupled to a small Helium cluster. The predictions of the theory for the neon bath are compared and found in close agreement with available theories for VER based on the Egelstaff correction factor and Feynman–Kleinert variational theory. For the Helium cluster, the force spectrum obtained from CMD is found to be in slightly better agreement with the exact result than a method based on a cumulant approach. The results support the use of CMD in condensed phase studies of VER when quantum effects are important.