bluuebunny/crossref_metadata_2025 huggingface dataset:
The Hagedorn transition in noncommutative open string theory
Authors: Gubser S. S. , Gukov S. , Klebanov I. R. , Rangamani M. , Witten E.
Published: July 2002

The Hagedorn transition in noncommutative open string theory (NCOS) is relatively simple because gravity decouples. For NCOS theories in no more than five space–time dimensions, the Hagedorn transition is second order, and the high temperature phase involves long, nearly straight fundamental strings separating from the D-brane on which the NCOS theory is defined. Above five spacetime dimensions interaction effects become important below the Hagedorn temperature. Although this complicates studies of the transition, we believe that the high temperature phase again involves long strings liberated from the bound state.
Optimized contact configuration for the study of transport phenomena in ropes of single-wall carbon nanotubes
Authors: Appenzeller J. , Martel R. , Avouris P. , Stahl H. , Lengeler B.
Published: July 2002

The study of the intrinsic transport properties of carbon nanotubes suffers from the difficulties in fabricating noninvasive contacts. Here, we present a scheme for the investigation of transport phenomena in metallic single-wall carbon nanotubes by means of a special four-terminal measurement configuration. To suppress the impact of the contacts on the measured conductance in a tube, we found a combination of top and bottom contacts to the rope of single-wall nanotubes to be most appropriate. Our experimental findings demonstrate that a linear decrease of the sample resistance can be observed under these circumstances without the common increase of resistance for decreasing temperatures.
Extended objects from warped compactifications of M theory
Authors: Silverstein Eva
Published: July 2002

We study the massive spectrum of fully wrapped branes in warped M-theory compactifications, including regimes where these states are parametrically lighter than the Planck scale or string scale. We show that many such states behave classically as extended objects in the noncompact directions in the sense that their mass grows with their size as measured along the Poincare slices making up the noncompact dimensions. On the other hand, these states can be quantized in a nontrivial regime: in particular, their spectrum of excitations in a limited regime can be obtained by a warped Kaluza–Klein reduction from ten dimensions. We briefly discuss scattering processes and loop effects involving these states, and also note the possibility of an exponential growth in the number of bound states of these objects as a function of energy.
The design and performance of an effusive gas source of conical geometry for photoionization studies
Authors: Seccombe D. P. , Collins S. A. , Reddish T. J.
Published: July 2002

The design, construction, and performance of an effusive gas source of conical geometry is reported. This gas jet enables experiments that require the gas and photon beams to be essentially collinear and has a focusing multicapillary array with the central portion left free for the photon beam. In total, the source comprises 90 “tubes” in three layers and has been designed by modeling free molecular gas flow and optimizing for highest gas density ∼2–3 mm from the jet’s exit plane. The nature of the gas flow through the source and its focusing properties are investigated theoretically and experimentally.
Finite N AdS/CFT correspondence for Abelian and non-Abelian orbifolds, and gauge coupling unification
Authors: Frampton Paul H.
Published: July 2002

Although the AdS/CFT correspondence is rigorous only for an infinite N→∞ stack of D3-branes, it can be fruitfully studied for finite N as a source of gauge structures and choices for chiral fermions and complex scalars which solve the hierarchy problem by a conformal fixed point. We emphasize orbifolds AdS5×S5/Γ where the resulting GFT has 𝒩=0 supersymmetry. The fact that the complex scalars are prescribed by the construction limits the possible spontaneous symmetry breaking. Both Abelian and non-Abelian Γ are illustrated by simple examples. An accurate sin2 θ in electroweak unification can be obtained, suggesting that this approach merits further study.
Decaying and kicked turbulence in a shell model
Authors: Hooghoudt Jan-Otto , Lohse Detlef , Toschi Federico
Published: July 2002

Decaying and periodically kicked turbulence are analyzed within the Gledzer–Ohkitani–Yamada shell model, to allow for sufficiently large scaling regimes. Energy is transferred towards the small scales in intermittent bursts. Nevertheless, mean field arguments are sufficient to account for the ensemble averaged energy decay E(t)∼t−2 or the parameter dependencies for the ensemble averaged total energy in the kicked case. Within numerical precision, the inertial subrange intermittency remains the same, whether the system is forced or decaying.
Identification of ternary boron–carbon–nitrogen hexagonal phases by x-ray absorption spectroscopy
Authors: Gago R. , Jiménez I. , Albella J. M. , Terminello L. J.
Published: July 2002

Boron carbon nitride (BCN) films have been grown by B4C evaporation with concurrent N2+ ion assistance, and have been characterized by x-ray absorption near edge (XANES) spectroscopy. Upon the nitrogen insertion, the film structure evolves from BxC-like to h-BN-like. The hexagonal structure corresponds to a true ternary BCN compound that can be understood as h-BN with carbon incorporated in substitutional sites. The C(1s)XANES presents π* states characteristic of the BCN arrangement. The basal planes of the h-BCN phase are oriented perpendicular to the substrate, as derived from the angle dependence of the XANES signal.
CO
2
laser based two-volume collective scattering instrument for spatially localized turbulence measurements
Authors: Saffman M. , Zoletnik S. , Basse N. P. , Svendsen W. , Kocsis G. , Endler M.
Published: July 2002

We describe and demonstrate a two-volume collective scattering system for localized measurements of plasma turbulence. The finite crossfield correlation length of plasma turbulence combined with spatial variations in the magnetic field direction are used to obtain spatially localized turbulence measurements at the Wendelstein 7-AS fusion experiment. By comparing with localized measurements obtained using the single wide beam technique [Truc et al., Rev. Sci. Instrum. 63, 3716 (1992)] we provide a proof-of-principle demonstration of the two-beam method. An optimized configuration with large toroidal separation of the measurement volumes is predicted to be capable of providing spatial resolution better than 5 cm.
Low-gravity sideways double-diffusive instabilities
Authors: Zebib Abdelfattah
Published: July 2002

Instabilities of a double-diffusive system in a slot are considered and relevance to the microgravity environment is identified. Formulation of the problem including the effects of g-jitter is given, and a two-time-scale analysis, valid in the limit of high-frequency modulation, is employed to derive the equations governing the mean field. Two-dimensional linear stability results show that the effective rapid gravity modulation is destabilizing, resulting in a dramatically wider range of parameters where this double-diffusive instability is expected to prevail in suitable space experiments.
Microphotoluminescence mapping on CdZnTe: Zn distribution
Authors: Li Z.-F. , Lu W. , Huang G. S. , Yang J. R. , He L. , Shen S. C.
Published: July 2002

We report microphotoluminescence (μ-PL) mappings of CdZnTe wafers on micrometer and millimeter scales. The acquired PL spectra have been fitted to a model based on band-to-band transition including the contribution of localized states in the energy gap. The fitting yields energy gap Eg, which is correlated to the Zn fraction x in Cd1−xZnxTe. The statistics of large numbers of fitted Eg reveal the inhomogeneity of the Zn composition while the map of Eg gives the distribution of Zn atoms. The comparison between the PL mappings before and after epi-ready chemomechanical processing show a great improvement in homogeneity due to the removal of surface defects and damage by the processing. Our results demonstrate the feasibility of PL mapping in determining the Zn composition homogeneity and distribution in a CdZnTe wafer.
Large enhancement of the third-order optical susceptibility in Cu-silica composites produced by low-energy high-current ion implantation
Authors: Olivares J. , Requejo-Isidro J. , del Coso R. , de Nalda R. , Solis J. , Afonso C. N. , Stepanov A. L. , Hole D. , Townsend P. D. , Naudon A.
Published: July 2002

Low-energy high-current ion implantation in silica at a well-controlled substrate temperature has been used to produce composites containing a large concentration of spherical Cu clusters with an average diameter of 4 nm and a very narrow size distribution. A very large value for the third-order optical susceptibility, χ(3)=10−7 esu, has been measured in the vicinity of the surface plasmon resonance by degenerate four-wave mixing at 585 nm. This value is among the largest values ever reported for Cu nanocomposites. Additionally, the response time of the nonlinearity has been found to be shorter than 2 ps. The superior nonlinear optical response of these implants is discussed in terms of the implantation conditions.
Growth of high mobility GaN and AlGaN/GaN high electron mobility transistor structures on 4H-SiC by ammonia molecular-beam epitaxy
Authors: Webb James B. , Tang H. , Bardwell J. A. , Coleridge P.
Published: July 2002

Ammonia molecular-beam epitaxy has been used to grow high-quality epilayers of GaN and AlGaN/GaN heterostructure field-effect transistor (HFET) structures on insulating 4H-SiC. The growth process, which used a magnetron sputter epitaxy deposited buffer layer of AlN, has been described previously. Ex situ pretreatment of the SiC substrate was found to be unnecessary. For a single 2.0 μm thick silicon doped epilayer, a room temperature (RT) electron mobility of 500 cm2/Vs was measured at a carrier density of 6.6×1016 cm−3. For the HFET structure, a room temperature mobility of 1300 cm2/Vs at a sheet carrier density of 3.3×1012 cm−2 was observed, increasing to 11 000 cm2/Vs at 77 K. The surface morphology of the layers indicated a coalesced mesa structure similar to what we observed for growth on sapphire, but with a lower overall defect density and correspondingly larger grain size. The observation of well-resolved Shubnikov de Haas oscillations at fields as low as 3 T indicated a relatively smooth interface.
Photoabsorption of NaCl clusters at the Na K-edge: Development of the bond length with the cluster size
Authors: Riedler M. , de Castro A. R. B. , Kolmakov A. , Löfken J. O. , Nowak C. , Soldatov A. V. , Wark A. , Yalovega G. , Möller T.
Published: July 2002

Photoabsorption spectra of free and deposited NaCl clusters at the Na K-edge are presented. The absorption spectra were recorded using the yield of photoelectrons and cluster ions. Above the K-shell ionization limit, multi-scattering-type oscillations could be identified which allow the determination of bond length in the cluster. The variation of the bond length with cluster size is in reasonable agreement with model calculations, assuming a Coulomb interaction and a Born–Meyer type repulsion of the ions.
Optical metastability of subband gap (2.2 eV) yellow luminescence in GaN
Authors: Chang Y. C. , Oberhofer A. E. , Muth J. F. , Kolbas R. M. , Davis R. F.
Published: July 2002

Optical metastability has been studied in undoped GaN films grown on SiC substrates having a previously deposited AlN buffer layer. Brief exposures to a higher intensity ultraviolet light resulted in temporary changes in the optical properties of the GaN layer. The photoinduced changes created high contrast patterns on samples that could be observed under an optical microscope with lower intensity ultraviolet excitation. The subband gap yellow photoluminescence peak at 2.2 eV increased significantly after the patterns were created. This change slowly returned (hours) to its initial value at room temperature. The retention time decreased to a few seconds at temperatures above 100 °C. The data showed that a 1.34 eV thermal activation energy exists, which suggests that the cause of these metastable properties is related to the subband gap yellow luminescence.
Superconductivity in ultrathin artificial cuprate structures
Authors: Balestrino G. , Lavanga S. , Medaglia P. G. , Orgiani P. , Paoletti A. , Pasquini G. , Tebano A. , Tucciarone A.
Published: July 2002

Layer-by-layer pulsed-laser deposition has been used to grow ultrathin superconducting (Ba0.9Nd0.1)CuO2+x/CaCuO2 artificial structures with various thicknesses of the two constituent layers. Superconductivity was observed in the ultrathin structure consisting of a single CaCuO2 superconducting block sandwiched between two (Ba0.9Nd0.1)CuO2+x blocks. The results obtained allowed an experimental demonstration of the currently accepted structural model for high TC superconductivity, which requires the presence in the same structure of two blocks with different functional properties: the superconducting infinite layer block and the nonsuperconducting charge reservoir block.
Effect of water absorption on the residual stress in fluorinated silicon-oxide thin films fabricated by electron-cyclotron-resonance plasma-enhanced chemical-vapor deposition
Authors: Kim S. P. , Choi S. K. , Park Youngsoo , Chung Ilsub
Published: July 2002

Fluorinated silicon-oxide (SiOF) films were produced by electron-cyclotron-resonance plasma-enhanced chemical-vapor deposition. The effect of water absorption on the film properties was studied by measuring the residual stress as a function of exposure time to room air. The residual stress shows an increase of the compressive component as the film absorbs water. However, the chemical bonding structure does not change after the water absorption. The residual stress returns to the initial value when the film is dried. It is suggested that the water absorption occurs entirely by physical adsorption of H2O molecules to Si–F bonds.
O
(
1
D)
reaction with cyclopropane: Evidence of O atom insertion into the C–C bond
Authors: Shu Jinian , Lin Jim J. , Wang Chia C. , Lee Yuan T. , Yang Xueming , Nguyen Thanh Lam , Mebel Alexander M.
Published: July 2002

The O(1D)+C3H5 reaction has been investigated using the universal crossed molecular beam method. A number of reaction pathways have been observed. One of the most interesting channels is the H2CO/HCOH+C2H4 process, in which H2CO products are clearly identified. Experimental results indicate that the H2CO products are likely produced through a long-lived complex formation process, for which insertion of O(1D) into the C–C bond should be responsible.
Nanostructural study of the thermal transformation of diamond-like amorphous carbon into an ultrahard carbon nanocomposite
Authors: Martı́nez-Miranda L. J. , Siegal M. P. , Provencio P. P.
Published: July 2002

We studied the structural transformation of diamond-like amorphous carbon (a-C) films into ultrahard carbon nanocomposites via postannealing to 600 °C using transmission electron microscopy, x-ray reflectivity, and small-angle scattering. Film density decreases monotonically above 200 °C. Film surfaces roughen upon annealing to 300 °C; however, a-C recovers its smoothness with higher temperature annealing. Finally, there exists some quasiperiodic nanostructural feature with a lattice spacing that increases with annealing, correlating well with purely a-C nanocomposite structures imaged from samples annealed at 600 °C. We propose that these annealing-induced nanostructural changes are a derivative of localized stress fields in as-grown a-C films.
Two-dimensional capsule-hohlraum designs for the National Ignition Facility
Authors: Bradley P. A. , Wilson D. C.
Published: July 2002

This article describes 250, 280, and 350 eV drive temperature copper-doped Be [Be(Cu)] two-dimensional (2-D) capsule-hohlraum designs for the National Ignition Facility (NIF) [Paisner et al., Laser Focus World 30, 75 (1994)]. These capsule-hohlraum designs are a follow-on to the previous one-dimensional capsule designs of Bradley and Wilson [Phys. Plasmas 6, 4293 (1999)]. It is shown that a 2-D 350 eV capsule-hohlraum design scaled from the successful 330 eV design does not ignite, mostly due to poor symmetry. In addition, the 350 eV capsule hohlraum design requires the full 500 TW of the NIF design and 1.66 MJ of the maximum 1.8 MJ designed energy output. It is possible to design a capsule-hohlraum combination that achieves ignition and burn with peak radiation temperatures of 250, 280, and 330 eV. These designs use from 1.3 to 1.6 MJ of laser energy and the successful designs have yields of 16–20 MJ. Changes in symmetry and yield due to changing the focal point of the inner and outer laser cones were examined. The 280 eV capsule can tolerate pointing changes of 40–100 μm before the yield drops by 50%, while even a 40 μm pointing change for the 250 eV capsule causes the yield to drop by a factor of 2 to 100.
Thermoelectric properties of Pb- and Ca-doped (Bi2Sr2O4)xCoO2 whiskers
Authors: Funahashi Ryoji , Matsubara Ichiro
Published: July 2002

An oxide single crystalline whisker has been prepared which displays superior thermoelectric properties at high temperature in air. The whisker has an average composition of (Bi, Pb)2.2(Sr, Ca)2.8Co2Oy (abbreviated to BC-232 whisker) and a layered structure in which two different sublattices, CoO2 and rocksalt (Bi, Pb)2(Sr, Ca)2O4, alternate in the c-axis direction. Edge-sharing CoO2 layers act as conducting and thermoelectric units. Although a plateau is observed at around 500 K, the Seebeck coefficient of the whiskers is about 100 μV K−1 at 100 K, basically increases with temperature up to 773 K, and is saturated at temperatures higher than 773 K. Temperature dependence of electric resistivity shows semiconducting-like behavior but its value is very low compared with general semiconductors. These electrical phenomena would indicate pseudogap formation with its width of a few meV at the Fermi level. The power factor of the BC-232 whiskers is estimated at over 0.5 mWm−1 K−2 at temperatures higher than 650 K and reaches 0.9 mWm−1 K−2 at 973 K.
Ti/Al/Pt/Au and Al ohmic contacts on Si-substrated GaN
Authors: Zhao Z. M. , Jiang R. L. , Chen P. , Xi D. J. , Yu H. Q. , Shen B. , Zhang R. , Shi Y. , Gu S. L. , Zheng Y. D.
Published: July 2002

Al and Ti/Al/Pt/Au ohmic contacts on GaN epitaxial layers were studied. The epilayers were grown on Si (111) substrates by low-pressure metalorganic chemical vapor deposition. Al/GaN contacts achieved a minimum contact resistivity of 7.5×10−3 Ω cm2 after annealing in N2 ambient at 450 °C for 3 min. Further annealing degraded the contacts. Ti/Al/Pt/Au and GaN contacts achieved a minimum contact resistivity of 8.4×10−5 Ω cm2 after annealing in N2 at 650 °C for 20 s. The Ti/Al/Pt/Au contacts on GaN showed a better thermal stability than Al/GaN contacts. After annealing at 600 °C for 30 min. they were still ohmic contacts. The mechanisms for ohmic contact formation of Ti/Al/Pt/Au contacts were also analyzed.
Quantum efficiency of 1460 nm transition and energy transfer in Tm3+ doped glass
Authors: Quimby R. S.
Published: July 2002

A self-calibrating quantum efficiency measurement technique is applied to the Tm3+ H43→F43 transition at 1460 nm. The method requires only relative fluorescence measurements to determine the absolute quantum efficiency. Overlapping spectral features are separated using phase-shifted fluorescence from different levels at high modulation frequency. The effect of energy transfer between Tm3+ ions is studied, and it is found that Tm3+ concentrations on the order of 1018 cm−3 are needed to avoid complications from Tm3+–Tm3+ cross relaxation. Application of the technique to Tm3+ doped fluorozirconate glass gives good agreement with the expected quantum efficiency based on multiphonon relaxation theory.
Time-resolved measurements of stress effects on solid-phase epitaxy of intrinsic and doped Si
Authors: Barvosa-Carter W. , Aziz M. J.
Published: July 2002

The effect of externally applied in-phase stresses on the solid-phase epitaxial growth rate of both intrinsic and B-doped Si has been measured using time-resolved reflectivity. The data are described phenomenologically by a product of a function of concentration, an Arrhenius function of temperature, and a Boltzmann factor in the product of the stress and the activation strain V*, with V11*=(+0.14±0.04) and (+0.17±0.02) times the atomic volume, in intrinsic and B-doped material, respectively.
Empirical potentials for rovibrational energy transfer of hydrogen fluoride in collisions with argon
Authors: Shroll Robert M. , Lohr Lawrence L. , Barker John R.
Published: July 2002

Rovibrational energy transfer of hydrogen fluoride in collisions with argon was investigated by using the coupled-states approximation to the quantum scattering problem. Empirically determined 3-D ab initio potential energy surfaces (PES) for the interaction between hydrogen fluoride and argon are presented. Second-order Møller-Plesset perturbation theory (MP2) was used to provide an initial approximate PES for the complex. The MP2 PES was subsequently modified to compensate for the underestimated dispersion interaction and adjusted until the desired agreement between calculated and observed spectroscopic quantities was achieved. Calculated rotational cross sections are in good agreement with experimental results as well as those obtained with a highly accurate vibrationally averaged empirical PES [J. M. Hutson, J. Chem. Phys. 96, 6752 (1992)]. The rate constants for the collision induced relaxation of the first vibrational state of hydrogen fluoride are presented as functions of temperature. The rate constants show structure at low temperature corresponding to cross-section resonances. The calculated rate constants are in good agreement with available high temperature experimental results. The calculations provide lower temperature rate constants and a wealth of detailed state-to-state information that are not available from experiment.
Band-tail parameter in polysilicon materials
Authors: Meng Fanying , Cui Rongqiang , Shen W. Z.
Published: July 2002

We develop a band-tail model in polysilicon by analyzing the energy band structure on the basis of the band-tail parameter with Urbach exponential behavior due to the trap states and the strain in grain bulks and grain boundaries. A quantitative analytical formulation for the band-tail parameter E0 has been deduced by taking into account the carrier–impurity interaction, the carrier–phonon interaction, and the structural disorder. The dependence of E0 and the electric field F in the grain boundary of polysilicon on carrier concentration n0 and temperature T has been studied within the framework of the model. The results indicate that E0 and F increase with T. Furthermore, the increase in E0 at n0=1.0×1018 cm−3 is much larger than that in E0 at n0=1.0×1016 cm−3 at the same temperature. We have demonstrated that the theoretical results are consistent with the experiment, and our model is therefore valuable for both describing polysilicon materials and improving the performance of polysilicon devices.
Structural and vibrational study of chromium doped elpasolite crystals Cs2NaAlF6
Authors: Bordallo Heloisa N. , Henning Robert W. , Sosman Lilian P. , da Fonseca Raul J. M. , Tavares Armando Dias , Hanif Khalid M. , Strouse Geoffrey F.
Published: July 2002

The influence of site-symmetry and electron phonon coupling in Cs2NaAlF6:Cr3+ is probed by correlation of optical and structural measurements. Based on neutron and x-ray analysis the structure is a distorted R3m¯, exhibiting a unique stacking interaction. Selection rules have been used to assign the Raman-active zone-center vibrations. Although local and bulk mode frequencies differ slightly, indicating that the guest Cr3+ ion does not significantly perturb the host structure, vibrational analysis indicates the Cr3+ ion is coupled to the first coordination sphere of the Al–F lattice.
Effect of orientation on the dielectric and piezoelectric properties of 0.2Pb(Mg1/3Nb2/3)O3–0.8Pb(Zr0.5Ti0.5)O3 thin films
Authors: Yoon Ki Hyun , Lee Byoung Duk , Park Jihoon
Published: July 2002

0.2 Pb(Mg 1/3 Nb 2/3 )O 3 –0.8Pb(Zr 0.5 Ti 0.5 )O 3 (0.2PMN–0.8PZT) thin films were deposited on Pt(111)/Ti/SiO2/Si [Pt(111)] and Pt(200)/SiO2/Si [Pt(200)] substrates by a sol–gel method, and the effect of orientation on the piezoelectric and dielectric properties of 0.2PMN–0.8PZT thin films was investigated. The 0.2PMN–0.8PZT thin films on Pt(111) and Pt(200) showed strong (111) and (100) preferred orientations, respectively. The spontaneous polarization of the (111) oriented film was higher than that of the (100) oriented film. However, the (100) oriented film showed a higher dielectric constant (K) and transverse piezoelectric coefficients (d31) than the (111) oriented film. Because the spontaneous polarization direction of the (100) oriented film is more tilted away from the normal to the film surface than that of the (111) oriented film, the dielectric constant and the transverse piezoelectric coefficient (d31) of the (100) oriented film were enhanced.
Role of metallic neutrals and gaseous molecular ions in a copper cathodic arc operated with oxygen gas
Authors: Lepone A. , Kelly H. , Márquez A.
Published: July 2002

The plasma-neutral gas structure generated in a copper cathodic arc operated with oxygen gas at a constant current level of 100 A has been investigated by employing a Langmuir spherical probe and a calorimetric technique. The plasma potential and electron temperature at several axial distances (between 10 and 25 cm) from the cathode surface and for different O2 pressure values (between 0.5 and 5 Pa) have been measured. A simplified one-dimensional model which incorporates the relevant atomic processes of the problem (charge-exchange, lateral diffusion loses, electron impact ionization, and dissociative recombination) is developed. This model allows calculating densities and drift velocities of the different species and, in particular, predicts the creation of a considerable number of metallic neutrals and gaseous ions. A comparison of the experimental ion saturation probe current and the heat source term derived from the calorimetric measurements with the corresponding quantities predicted by the model is performed. We found good agreement between theory and experiment, notwithstanding the fact that the physical quantities presented large variations, and the atomic reaction rates varied for several orders of magnitude, over the range of gas pressure and axial position values investigated.
Xenon excimer emission from pulsed microhollow cathode discharges
Authors: Moselhy M. , Shi W. , Stark R. H. , Schoenbach K. H.
Published: July 2002

By applying electrical pulses of 20 ns duration to xenon microplasmas, generated by direct current microhollow cathode discharges, we were able to increase the xenon excimer emission by more than an order of magnitude over direct current discharge excimer emission. For pulsed voltages in excess of 500 V, the optical power at 172 nm was found to increase exponentially with voltage. Largest values obtained were 2.75 W of vacuum-ultraviolet optical power emitted from a single microhollow cathode discharge in 400 Torr xenon with a 750 V pulse applied to a discharge. Highest radiative emittance was 15.2 W/cm2. The efficiency for excimer emission was found to increase linearly with pulsed voltages above 500 V reaching values of 20% at 750 V.
Fluorescence depolarization in organized media. Two-excited-state reactions controlled by orientation-dependent kinetic rates. I. Theory
Authors: Fisz J. J. , Budziński M. P.
Published: July 2002

A model describing fluorescence depolarization of fluorophores undergoing the two-ground- and two-excited-state reactions in ordered systems, is discussed. It is assumed that the rates for state-to-state kinetic relaxation process are dependent on angular orientation of fluorophores, and that this process is controlled by fluorophore-matrix ground- and excited-state aligning interactions and by rotational dynamics. The evolution of the two-excited-state system is described by the equation of motion composed of a set of two differential equations for the Green functions, coupling state-dependent molecular ordering and rotational dynamics with the orientation-dependent kinetic relaxation. A method of solving the motion equation is outlined. The main properties of the solution to motion equation and its information content on angular alignment and potential-restricted rotational dynamics of fluorophores, are discussed in detail. An illustrative example manifesting the effect of orientation-dependent excited-state reversible interconversion process on fluorescence depolarization, is presented. Two experimental arrangements enabling the recovery of the model parameters are presented and discussed. Possible applications of the model to different cases important from the experimental point of view, are indicated and discussed.
Time-resolved electroabsorption measurement of the transient electron velocity overshoot in GaN
Authors: Wraback M. , Shen H. , Carrano J. C. , Collins C. J. , Campbell J. C. , Dupuis R. D. , Schurman M. J. , Ferguson I. T.
Published: July 2002

A femtosecond time-resolved electroabsorption technique employing an AlGaN/GaN heterojunction p–i–n diode with a p-type AlGaN window layer and a semitransparent p contact has been used to measure the transient electron velocity overshoot in GaN. A peak transient electron velocity of 7.25×107 cm/s within the first 200 fs after photoexcitation has been observed at a field of 320 kV/cm. The increase in electron transit time across the device with increasing field beyond 320 kV/cm provides experimental evidence for a negative differential resistivity region of the steady-state velocity-field characteristic in this high field range.
Vibrational and collision energy effects on the reaction of CH3CHO+ with methanol
Authors: Kim Ho-Tae , Liu Jianbo , Anderson Scott L.
Published: July 2002

Integral cross sections and product velocity distributions were measured for reaction of acetaldehyde cation with methanol over a center-of-mass collision energy range from 0.1 to 2.2 eV. Reactivity is dominated by exoergic proton transfer (PT), which is strongly suppressed by collision energy, and mildly suppressed by CH3CHO+ vibrational excitation. PT is complex-mediated at low energies, switching to a direct stripping mechanism at high energies. Of the two possible PT channels, it appears that transfer of the aldehyde proton dominates. Hydrogen abstraction (HA) is a minor channel at low collision energies, also complex-mediated. Abstraction is observed from both hydroxyl and methyl sites on methanol, and the two channels have different, and counterintuitive collision energy dependence. Despite being exoergic, with no barriers, the HA channel shows apparent threshold behavior, attributed to competition with the dominant PT channel. The competition indicates that different intermediate complexes must interconvert efficiently, at least for low collision energies. At low energies, HA is strongly enhanced by collision energy, while vibrational excitation has no effect. Finally, there is a minor product channel corresponding to methyl elimination (ME) from a complex. Despite a relatively complicated reaction coordinate, the ME channel shows substantial recoil energy release and an asymmetric velocity distribution. A series of ab initio and RRKM calculations were performed to help interpret the results.
Commensurate vortex lattices in thin vanadium films and in V/Si superconducting superlattices
Authors: Fogel N. Ya. , Buchstab E. I. , Cherkasova V. G. , Yuzephovich O. I. , Mikhailov M. Yu. , Stetzenko A. N.
Published: July 2002

The features of the Shubnikov phase in thin films of type-II superconductors are investigated in the case when the magnetic field is parallel to the surface of the film. Measurements of the nonmonotonic dependence of the critical current Ic on the magnetic field H∥ reveal commensurate vortex lattices with different numbers of vortex chains in the film. It is proved experimentally that in homogeneous films the commensurability effect between the vortex lattice parameter and the thickness of the film can be observed only for the ideal state of the film boundaries, admitting the formation of an infinite lattice of the vortices and their images. Worsening of the film surface smoothness and of the plane-parallel precision of the two surfaces of the film leads to vanishing of the oscillations of Ic and to a sharp decrease of the critical current. The lock-in transition due to the influence of the surface barrier is observed in the films for the first time. It is found that a thin-film layered sample exhibits an interplay between two types of commensurability effects: with the period of the superstructure and with the total thickness of the sample. The H–T phase diagram is considered for a homogeneous film in a parallel magnetic field.
Analysis of the sign reversal of the second-order molecular polarizability in polymethineimine chains
Authors: Jacquemin Denis , Beljonne David , Champagne Benoı̂t , Geskin Victor , Brédas Jean-Luc , André Jean-Marie
Published: July 2002

The second-order polarizability tensor (β) components of polymethineimine oligomers have been computed with the semiempirical intermediate neglect of differential overlap (INDO) coupled either to sum-over-states (SOS) or finite-field real space approach. Our goal is to rationalize the sign reversal of the longitudinal β component with chain length. The INDO results are shown to compare well with the values obtained from previous ab initio calculations. Using a four-state SOS model that reproduces correctly the evolution of β with chain length, we demonstrate that the shape of the β curve can be explained as resulting from the competition between two contributions of opposite signs. In the framework of a real-space approach, those contributions are found to correspond to bond charge and bond polarization phenomena.
Freezing of hard spheres in confinement
Authors: Kegel Willem K.
Published: July 2002

The influence of confinement on the freezing transition of hard spheres is investigated. Two limiting cases are considered: (1) large systems, where walls weakly perturb the bulk system, and (2) small systems where the influence of geometry becomes important. In the first situation, the shift in coexisting densities is a linear function of the area to volume ratio in the system. This is a manifestation of the Kelvin equation, and the phenomenon is thermodynamically equivalent to capillary condensation. A claim (by others) of “prefreezing” of hard spheres at a smooth hard wall is quantitatively attributed to capillary crystallization. It is shown that the coexistence region narrows as a function of the area to volume ratio. In the second limit two different confined geometries are studied. In these limits, widening of the coexistence region is observed, pointing to an upper and lower critical point at intermediate values of the area to volume ratio, or no critical point at all. In a slit geometry buckling transitions interfere with the freezing transition. In a box geometry, at large values of the area to volume ratio, fluctuations become important. These fluctuations determine the fate of the freezing transition at intermediate values of the area to volume ratio.
Optical transparency of metallic La0.5Sr0.5TiO3+δ thin films
Authors: Cho J. H. , Cho H. J.
Published: July 2002

We prepared for Mott–Hubbard-type perovskite La0.5Sr0.5TiO3+δ thin films under an oxygen reduced environment, 5% H2/Ar. Control of the partial pressure during deposition could change the conductivity of the films, and induces a metal–insulator transition. In addition, we observed an excellent transparency at visible light with a metallic behavior, where this property could be utilized as a transparent conductor.
A comparative study of electron and positron scattering from molecules. IV. CH3Cl, CH3Br, and CH3I molecules
Authors: Kimura M. , Sueoka O. , Makochekanwa C. , Kawate H. , Kawada M.
Published: July 2002

An experimental study of electron and positron scattering from CH3Cl, CH3Br, and CH3I molecules has been carried out, and total cross sections (TCSs) for both projectiles were determined. Several strong structures due to resonances in the TCSs have been observed for electron impact, while weak but not negligible structures have also been seen for positron impact. A strong variation for the dominant resonance peak seen at around 10 eV was found to depend on a type of halogen atoms, and a detailed study of this dependence on molecular species has been performed to understand the origin and nature of these resonances. The continuum multiple-scattering method has been employed for the analysis of experimental results in addition to the evaluation of the elastic cross section. For larger halogen atoms, TCSs tend to possess larger magnitudes at energies above 100 eV than for smaller halogen atoms suggesting that the halogen atoms in fact dominate the dynamics, and their magnitudes increase in the order of CH3I>CH3Br>CH3Cl. A comparative study of CH4 was also performed to provide insight on the effects of molecular geometrical structure and electronic state.
Ge/Si interdiffusion in the GeSi dots and wetting layers
Authors: Wan J. , Luo Y. H. , Jiang Z. M. , Jin G. , Liu J. L. , Wang Kang L. , Liao X. Z. , Zou J.
Published: July 2002

The Ge/Si interdiffusion in GeSi dots grown on Si (001) substrate by gas-source molecular beam epitaxy is investigated. Transmission electron microscopy images show that, after annealing, the aspect ratio of the height to base diameter increases. Raman spectra show that the Si–Ge mode redshifts and the intensity of the local Si–Si mode increases with the increase of annealing temperature, which suggests the Ge/Si interdiffusion during annealing. The photoluminescence peaks from the dots and the wetting layers show blueshift due to the atomic intermixing during annealing. The interdiffusion thermal activation energies of GeSi dots and the wetting layers are 2.16 and 2.28 eV, respectively. The interdiffusion coefficient of the dots is about 40 times higher than that of wetting layers and the reasons were discussed.
A fourth-order real-space algorithm for solving local Schrödinger equations
Authors: Auer J. , Krotscheck E. , Chin Siu A.
Published: July 2002

We describe a rapidly converging algorithm for solving the Schrödinger equation with local potentials in real space. The algorithm is based on solving the Schrödinger equation in imaginary time by factorizing the evolution operator e−εH to fourth order with purely positive coefficients. The wave functions |ψj〉 and the associated energies extracted from the normalization factor e−εEj converge as O(ε4). The energies computed directly from the expectation value, 〈ψj|H|ψj〉, converge as O(ε8). When compared to the existing second-order split operator method, our algorithm is at least a factor of 100 more efficient. We examine and compare four distinct fourth-order factorizations for solving the sech2(ax) potential in one dimension and conclude that all four algorithms converge well at large time steps, but one is more efficient. We also solve the Schrödinger equation in three dimensions for the lowest four eigenstates of the spherical analog of the same potential. We conclude that the algorithm is equally efficient in solving for the low-lying bound-state spectrum in three dimensions. In the case of a spherical jellium cluster with 20 electrons, our fourth-order algorithm allows the use of very large time steps, thus greatly speeding up the rate of convergence. This rapid convergence makes our scheme particularly useful for solving the Kohn–Sham equation of density-functional theory and the Gross–Pitaevskii equation for dilute Bose–Einstein condensates in arbitrary geometries.
Transition state dynamics of the OH+OH→O+H2O reaction studied by dissociative photodetachment of H2O2−
Authors: Deyerl Hans-Jürgen , Clements Todd G. , Luong A. Khai , Continetti Robert E.
Published: July 2002

Photoelectron-photofragment coincidence (PPC) spectroscopy has been used to study the dissociative photodetachment of H2O2− and D2O2−. The observed partitioning of photoelectron and photofragment translational energies provides information on the dynamics in the transition state region of the reaction between two hydroxyl radicals: OH+OH→O(3P)+H2O. The data reveal vibrationally resolved product translational energy distributions for both the entrance channel OH+OH and the exit channel O(3P)+H2O upon photodetachment. The total translational energy distribution shows a convoluted vibrational progression consistent with antisymmetric stretch excitation of H2O in the exit channel and OH stretch in the entrance channel. The photoelectron spectra are compared to two-dimensional time-dependent wave packet dynamics simulations based on an anharmonic potential in the anion and a model collinear potential energy surface for the neutral complex. The PPC spectra also yield the dissociation energies D0(H2O2−→H2O+O−)=1.15±0.08 eV and D0(D2O2−→D2O+O−)=1.05±0.08 eV.
Time-resolved photoluminescence of ytterbium-doped nanocrystalline Si thin films
Authors: Zhao Xinwei , Komuro Shuji
Published: July 2002

Ytterbium-doped nanocrystalline silicon (nc-Si) thin films were formed on Si and quartz substrates by ablating a Si:Yb2O3 mixture target. The Yb-doped nc-Si showed sharp emission peaks at wavelengths around 1 μm. Time-resolved photoluminescence measurements indicated that the Yb3+ ions were excited through an energy transfer process due to the photoinduced carriers in the host nc-Si. A clear delay on the rise time of the Yb emission from the recombination of the host carriers was observed, also suggesting an indirect excitation of the Yb3+ ions. These results showed a possibility of exciting Yb3+ ions by carrier injection.
Relative stabilities of H- and D-bonded isotopomers of HCN⋯HF and HNC⋯HF
Authors: McDowell Sean A. C.
Published: July 2002

Ab initio calculations were performed at the SCF, B3LYP, MP2, QCISD, and CCSD(T) levels of theory using the cc-pVTZ basis set to obtain optimized geometries, dipole moments, binding energies, and harmonic vibrational frequencies for the linear complexes HCN⋯HF and HNC⋯HF. It was found that HNC⋯HF has a slightly larger binding energy [29 kJ mol−1 at CCSD(T)] than HCN⋯HF [27 kJ mol−1 at CCSD(T)]. The relative stabilities of the monodeuterated isotopomers DCN⋯HF (DNC⋯HF) and HCN⋯DF (HNC⋯DF) were also determined from their zero-point energies; vibrational frequency shifts of HF and HCN (HNC) on bonding were also calculated and used to gain insight into these relative stabilities. It was found that the D-bonded isotopomer is more thermodynamically stable than the H-bonded isotopomer. Previous studies on the weakly bound linear complexes CO–acetylene and N2–acetylene also found that the D-bonded species was more stable than the H-bonded species.
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2
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Ga2O3(Gd2O3), Y2O3, and Ga2O3, as high-k gate dielectrics on SiGe: A comparative study
Authors: Pal S. , Ray S. K. , Chakraborty B. R. , Lahiri S. K. , Bose D. N.
Published: July 2002

We report a comparative study of the electrical properties of some oxides e.g., Gd2O3, Ga2O3(Gd2O3), Y2O3, and Ga2O3 as gate dielectric for strained Si0.74Ge0.26 metal–oxide–semiconductor devices. Secondary ion mass spectroscopy spectra of the Ga2O3(Gd2O3)/SiGe sample showed a significant amount of GaO and GdO along with Ga and Gd signals. The depth profile taken for O, Si, SiO, Ga, Ge, Gd, and GdO showed sharp interface at about 20 nm. Though Gd2O3 and Y2O3 showed the highest resistivity and breakdown strength, Ga2O3(Gd2O3) was found to be most effective for surface passivation of SiGe giving lowest interface state density while pure Ga2O3 was incapable of passivating the SiGe surface. The positive fixed oxide charge and interface state density for Ga2O3(Gd2O3) film were found to be 8.4×1010 cm−2 and 4.8×1011 eV−1 cm−2, respectively, which are the lowest among all the oxide films of the present study.
Impact of domain wall displacements on the dielectric permittivity of epitaxial Ba0.5Sr0.5TiO3 films
Authors: Boikov Yu. A. , Claeson T.
Published: July 2002

Contributions of ferroelectric domain wall oscillations to the loss factor, tan δ, and the real part of the dielectric permittivity, ε′, of 700 nm thick Ba0.5Sr0.5TiO3 layers were considered. The domain wall related relaxation in the ferroelectric layer exhibited thermally activated behavior with a hindering barrier φ≈0.08 eV. The tan δ(T) of the layer peaked and there was a hump anomaly in ε′(T) at a temperature where the relaxation rate matched the frequency of measurement.
Squeezing out hidden force information from scanning force microscopes
Authors: Todd Brian A. , Eppell Steven J. , Zypman Fredy R.
Published: July 2002

A method to measure force-separation curves with a scanning force microscope is presented. Forces within the “snap to contact” are obtained by high-speed (MHz) measurement of cantilever deflection signals analyzed using the generalized beam theory. Numerical simulation is used to demonstrate the effectiveness of the method. Experimental results show that the method yields complete continuous force-separation curves with flimsy cantilevers in fluids allowing for sensitive force measurements in nonvacuum environments.
Piezoelectric hysteresis analysis and loss separation
Authors: Robert G. , Damjanovic D. , Setter N.
Published: July 2002

Losses in piezoelectrics are approached by considering a description of direct stress–charge displacement hysteresis. Shortcomings of the usual loss models (viscous and Rayleigh, taken separately) are briefly demonstrated. Then, a mixed model for piezoelectric losses superimposing viscous and Rayleigh types of descriptions is proposed and validated for a modified lead titanate and lead zirconate titanate (PZT). It is shown for PZT that viscous and Rayleigh-like loss contributions can be effectively separated by fitting the piezoelectric hysteresis loop with an expression combining the two lossy responses. The possibility to obtain all the necessary parameters that describe piezoelectric stress nonlinearity from one single piezoelectric loop is also demonstrated in the case of PZT. The origins of the mixed loss behavior are interpreted as arising from domain wall motion in terms of internal friction mechanisms and of pinning energy ranges.
Bistable characteristic and current jumps in field electron emission of nanocrystalline diamond films
Authors: Wu Kehui , Wang X. R. , Liu S. , Wang E. G.
Published: July 2002

Field electron emission of nanocrystalline diamond films with controllable grain size and graphite content, grown by microwave plasma-assisted chemical vapor deposition using N2/CH4/H2 as precursors, is studied. A bistable current–voltage characteristic is found. Current jumps (“steps”) are observed when a point probe of Mo is used as the anode. However, when the point probe is replaced by a large quartz plate coated with indium–tin–oxide (ITO), emission domains, which consist of many lighting spots, appear on the ITO plate. Bistable behavior may be understood in terms of the negative differential conductance induced instability. Current jumps can be attributed to the generation and elimination of conducting channels. It is suggested that the electron emission from individual diamond nanocrystallites is a switching process. It is noticed that, when the microstructure of the films is uniform enough, the micro-scale switching behavior can even be amplified to macroscale.