bluuebunny/crossref_metadata_2025 huggingface dataset:
On the conductance and the conductivity of disordered quantum wires
Authors: Haque A. , Khondker A. N.
Published: July 2002

We present a model, based on the Keldysh formalism, to study the transport properties of disordered quantum wires of finite lengths. Unlike the phenomenological models, we estimate the electron in- and out-scattering rates using the local density of states of various transverse modes that allow electrons to change their directions. The formulation, therefore, guarantees the conservation of both the charge and the current at any cross section of the device. Using the model we calculate the average two-probe conductances of quantum wires that are terminated at nonideal contacts.
Absorption spectrum of silver bromide crystals and fibers in the 9–11 μm wavelength range
Authors: Bunimovich D. , Nagli L. , Shalem Sh. , Katzir A.
Published: July 2002

The absorption spectrum in the 9–11 μm range of both undeformed and plastically deformed AgBr crystals and polycrystalline optical fibers was investigated by tunable laser calorimetry. The observed spectral features are discussed in terms of the crystal defect structure.
Focused ion beam assisted chemically etched mesas on GaAs(001) and the nature of subsequent molecular beam epitaxial growth
Authors: Kalburge A. , Konkar A. , Ramachandran T. R. , Chen P. , Madhukar A.
Published: July 2002

Focused ion beam assisted chemical etching is examined as an ultrahigh vacuum compatible in situ direct-write approach to patterning substrates to create mesas with arbitrary shapes (i.e., sidewall angles) independent of crystallographic orientation. Ga+ ion beam assisted Cl2 etching of GaAs(001) is employed as a vehicle. A phenomenological model is used to predict mesa profiles as a function of ion beam conditions. Mesas created under various conditions are characterized via atomic force microscopy and good agreement is found between the measured and predicted profiles. Examination of the growth profile evolution on such mesas with as-patterned sidewall angles between ∼10° and ∼60° reveal that mesa top size reduction suitable for nanostructure fabrication on stripes oriented along the [11̄0] and [100] directions occurs only for as-patterned mesa sidewall angles greater than ∼19° and ∼45°, respectively, which are the angles subtended by the {114} and {101} facets that emerge during growth and cause mesa top pinch-off for the two mesa stripe orientations, respectively. An as-patterned surface roughness in the sidewall and valley regions of ∼20 nm is found to heal with typically ∼50 nm of GaAs buffer growth via molecular beam epitaxy. Cross-section transmission electron microscope studies evidence the high quality of growth on the mesas and the lack of extended defects in the as-patterned substrate as well as in the overlayers, thus making such in situ prepared mesas suitable for creation of nanostructures via the technique of size-reducing epitaxy.
Preferential nucleation of Ge islands at self-organized pits formed during the growth of thin Si buffer layers on Si(110)
Authors: Weil J. D. , Deng X. , Krishnamurthy M.
Published: July 2002

The epitaxial growth of thin (∼20–40 nm) Si buffer layer on Si(110) leads to the formation of ∼100-nm-wide, uniformly sized faceted pits. The cause of these rhombohedral pits is revealed to be the overgrowth of a homoepitaxial layer over clusters of coherent contaminant particles, possibly SiC. Deposition of Ge on such “pitted” surfaces shows highly selective nucleation of pairs of coherent islands at the opposite corners of the pits along the 〈110〉 direction. Continued deposition leads to strain relaxation of one or both of the islands within the pit which then rapidly coarsen to form a single Ge island within the pit. Our observations offer insight into heterogeneous nucleation mechanisms important for producing controlled arrays of self-assembled quantum dots.
Band alignment engineering for high speed, low drive field quantum-confined Stark effect devices
Authors: Yip R. Y.-F. , Desjardins P. , Isnard L. , Aït-Ouali A. , Marchand H. , Brebner J. L. , Currie J. F. , Masut R. A.
Published: July 2002

An analysis and discussion of the device physics for the quantum-confined Stark effect based on barrier height and band alignment considerations is presented. It identifies two important design principles for band structure engineering of the multi-quantum well stack: (1) Due to the counterbalance relationship between field-induced redshift and field-induced polarization of the quantum well eigenstates, design strategies must look to attain an optimal balance or compromise between a minimum drive field and maximum absorption coefficient change. This can be achieved with an appropriate choice of the valence band discontinuity. (2) In III–V semiconductors, the strong asymmetry in the field response of the conduction and valence band eigenstates is due directly to the asymmetry of the conduction and valence band effective masses. As a result, optimum device performance is obtained by using a heterostructure with a disproportionately large conduction band offset to compensate the effective mass asymmetry and balance the field-induced wave function leakage in the conduction band to that in the valence band. The relative wave function leakage between conduction and valence bands is compared by examining tunneling currents through the quantum well barriers as a function of the electric field and barrier height. For conduction and valence band effective masses of, respectively, 0.055 and 0.5 times the free electron mass, the optimal band alignment requires a conduction band discontinuity 3–9 times greater than the valence band discontinuity. Applying these design principles for high speed, low drive voltage optical modulators shows that the overall performance of these devices may be improved by using a combination of balanced band alignments and low valence band barriers. The low valence band barriers reduce the drive field required to operate the devices, which has direct effects upon the drive voltage, device capacitance, attenuation coefficient, and optical coupling and propagation losses. The analysis and discussion is supported by experimental modulation depth and drive field data obtained from strained-layer multiple quantum well InAsP/InP and strain-compensated InAsP/InGaP optical modulators fabricated with layers grown on InP(001) by metalorganic vapor phase epitaxy.
Coalescence and overgrowth of diamond grains for improved heteroepitaxy on silicon (001)
Authors: Jiang X. , Schiffmann K. , Klages C.-P. , Wittorf D. , Jia C. L. , Urban K. , Jäger W.
Published: July 2002

Heteroepitaxial [001]-oriented diamond films with considerably increased lateral grain size and strongly improved orientational perfection could be prepared by microwave plasma-assisted chemical vapor deposition using a [001]-textured growth process on Si (001) substrates followed by a [110] step-flow growth process. The diamond films were characterized by atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. The results indicate that the diamond crystals increase their lateral dimensions at the (001) film surface either by coalescence of grains combined with a termination of the propagation of grain boundaries or by changing the grain boundary plane orientations from preferentially vertical to preferentially parallel directions with respect to the (001) growth faces. In the second case, the grains with relatively large angle deviation from the ideal epitaxial orientation are overgrown by those with relatively small angle deviation. As a result, the degree of orientational perfection of the films improves considerably in comparison to that of films prepared by the established process of [001]-textured growth. The presence of boron in the gas phase was found to strongly enhance the step-flow lateral grain growth. It was possible to achieve deposition of a thin boron-doped diamond film characterized by a full width at half maximum value of the measured tilt angle distribution of only 2.1°.
H–T
phase diagram for the giant magnetic flux jumps in low temperature superconductors and high temperature superconductors
Authors: Chabanenko V. V. , D’yachenko A. I. , Chabanenko A. V. , Szymczak H. , Piechota S. , Nabialek A. , Dung N. D.
Published: July 2002

We have studied magnetothermal instabilities both theoretically and experimentally. Magnetostriction and magnetization hysteresis loops with flux jumps were calculated over a wide range of experimental parameters employing three critical state models: the original Bean model, the Kim–Anderson model, and the exponential model. The influence of the magnetic prehistory on the flux jumps, magnetostriction, and the magnetization was investigated for the LaSrCuO crystal. H–T diagrams of flux instabilities were constructed from calculation and from experimental results. One can see good qualitative agreement between these two diagrams.
Fabrication of Co/Si nanowires by ultrahigh-vacuum scanning tunneling microscopy on hydrogen-passivated Si(100) surfaces
Authors: Palasantzas G. , Ilge B. , De Nijs J. , Geerligs L. J.
Published: July 2002

We have fabricated nanometer width Co/Si metal lines on Si(100) surfaces by ultrahigh-vacuum scanning tunneling microscopy (UHVSTM) based nanolithography on the hydrogen-passivated surface, combined with vapor deposition of Co at room temperature and subsequent annealing. The STM tip was used to define depassivated lines (<10 nm in width) by electron stimulated hydrogen desorption, and subsequently Co was deposited at a submonolayer coverage. Annealing of the substrate at 410 °C (just below hydrogen desorption) improves the structure of the wire due to silicidation, whereas the as-deposited wire is very granular (comparable to other materials in previous studies).
Feasibility of molecular-resolution fluorescence near-field microscopy using multi-photon absorption and field enhancement near a sharp tip
Authors: Kawata Yoshimasa , Xu Chris , Denk Winfried
Published: July 2002

Aperture-based near-field microscopy suffers from fundamental limitations imposed by the electromagnetic skin depth of the aperture material and a rapidly decreasing throughput as the aperture is made smaller. Apertureless approaches without these limitations have been demonstrated for coherent imaging but are not easily applicable to incoherent processes such as fluorescence or Raman scattering and to photochemical surface modification. Using multi-photon processes in conjunction with the field enhancement that occurs at a sharp tip in close apposition to a substrate should permit substantial localization of absorption and excitation to a nm sized volume. The expected enhancement of the optical field at the tip edge is calculated here for various combinations of metallic and nonmetallic tip and substrate materials. It is estimated that when using 100 fs pulses repeating at 100 MHz average laser powers of about 10 mW should be sufficient to reach saturating field strengths for three-photon absorption. Steady state and instantaneous temperature rises at the tip are estimated and found likely not to be a limiting factor. Fluorescence quenching is expected to limit the resolution achievable with metallic tips to about 5 nm, but tips made from highly refracting insulators or semiconductors should allow truly molecular resolution.
Lithium trapping at excess oxygen in sputter-deposited a-WO3 films
Authors: Vink T. J. , Boonekamp E. P. , Verbeek R. G. F. A. , Tamminga Y.
Published: July 2002

The initial stages of electrochemical intercalation of lithium in sputter-deposited tungsten oxide thin films were studied. The as-deposited films show a substantial irreversible uptake of lithium without an electrochromic effect. This trapping of lithium is associated with the presence of excess oxygen in the film. The experimental results are consistent with a reaction of lithium and interstitial oxygen to form a neutral Li2−Oi complex. Our findings are relevant for the manufacture of electrochromic devices, and the electrochemical titration method used may be applied to probe excess oxygen in other oxidic films as well.
Elastic properties by Brillouin spectroscopy of sol–gel (Pb,Ca)TiO3 films
Authors: Riobóo R. J. Jiménez , Calzada M. L. , Krüger J. K. , Alnot P.
Published: July 2002

A direct evaluation of elastic properties and refractive indices of sol–gel derived calcium modified lead titanate films has been made by means of Brillouin spectroscopy using a new experimental configuration. Amorphous films deposited on platinized substrates were treated at temperatures between 350 and 650 °C, using rapid thermal annealing. A variation of the sound velocity in the films with the annealing temperature was measured. A clear decrease at 375 °C and a continuous softening of the sound velocity up to 475 °C were observed. The latter is followed by a jump toward higher values of the sound velocity when the samples are treated at higher temperatures. Grazing incidence x-ray diffraction and profilemetry have been used to monitor structural changes and variation of film thickness with temperature, inferring relations among these parameters and properties obtained from the Brillouin measurements.
Interfacial electrostatic phenomena and capacitance–voltage characteristics of ultrathin polyimide Langmuir–Blodgett films
Authors: Itoh Eiji , Iwamoto Mitsumasa
Published: July 2002

The capacitance–voltage (C–V) characteristics of ultrathin polyimide (PI) Langmuir–Blodgett (LB) films were examined with taking into account the interfacial electrostatic phenomena and the presence of the interfacial electric states at the metal/PI LB film interface. It was revealed that the apparent film thickness of metal/PI LB film/metal elements decreases due to the charge exchange at the metal/film interface. It was also shown that the current–voltage (I–V) characteristics of Au/PI LB film/Al elements were ruled by the electric field of an order of 108–109 V/m built at the metal/film interface due to the excess charges transferred from metals into PI LB films.
Type-II quantum-well “W” lasers emitting at λ=5.4–7.3 μm
Authors: Stokes D. W. , Olafsen L. J. , Bewley W. W. , Vurgaftman I. , Felix C. L. , Aifer E. H. , Meyer J. R. , Yang M. J.
Published: July 2002

A series of optically pumped type-II quantum-well “W” lasers with wavelengths ranging from 5.4 to 7.3 μm operated at temperatures up to at least 220 K for pulsed operation. The peak output power at 80 K was 1.1 W/facet for a device emitting at λ=7.0 μm. Internal losses were characterized for the temperature range between 40 and 190 K. Auger coefficients determined from an analysis of the threshold pump intensities were found to be suppressed by up to an order of magnitude compared to type-I III–V semiconductors with the same energy gaps.
Intervalence band absorption loss coefficients of the active layer for InP-based long wavelength laser diodes
Authors: Kakimoto Syouichi , Watanabe Hitoshi
Published: July 2002

We have calculated the intervalence band absorption loss coefficients of the active layer for InP-based long wavelength laser diodes lasing in the range of 1.2–1.55 μm. The calculated results show that, as the wavelength of laser diode becomes longer, the loss coefficient of the laser diode rapidly increases toward 1.55 μm, which coincides with the experimental data reported. We also describe the dependence of the loss coefficient on temperature.
Photocurrent study of molecular beam epitaxy GaAs grown at low temperature
Authors: Hozhabri N. , Montoya J. C. , Alavi K.
Published: July 2002

Photocurrent spectroscopy measurement has been utilized to study low temperature behavior of molecular beam epitaxy GaAs grown at low temperature. Photocurrent spectra of the as-grown sample show a dip near 50–60 meV below the conduction edge for sample temperatures of 18–275 K. The magnitude of this dip is observed to be temperature dependent and one possible explanation of this new phenomenon is the trapping of electrons by arsenic vacancies. An annealed sample does not show this feature at any sample temperature.
Spatial variation of the current in grain boundary Josephson junctions
Authors: Carmody M. , Moeckly B. H. , Merkle K. L. , Marks L. D.
Published: July 2002

The spatial variation of the current across the boundary in several YBa2Cu3O7−x grain boundary Josephson junctions was determined using direct methods. A phase retrieval algorithm was used to calculate the positional critical current density J(x) from critical current versus applied magnetic field, Ic(B), measurements. The current distributions were highly nonuniform along the length of the junctions. These measurements are consistent with existing filamentary grain boundary models, low temperature scanning microscopy studies, and laser scanning microscopy studies of high Tc grain boundaries. The very large scatter in the critical currents reported in the literature for grain boundaries of the same macroscopic geometry appear to be due to the underlying variations in local critical currents.
Structural, magnetotransport, and micromagnetic properties of sputtered Ir(111)/Co superlattices
Authors: Luo Yuansu , Pfeifer Birgit , Kaeufler Andrea , Moske Michael , Samwer Konrad
Published: July 2002

This article presents a further study on the structural and magnetotransport properties in sputtered Ir(111)/Co superlattices. The magnetization loops and magnetoresistance curves show a correlation, which can be written as ΔR/R∝−(M/Ms)2 and indicates that the resistive change in external magnetic field is due to spin dependent scattering of conduction electrons, although the largest ΔR/R value observed is only 4% (10 K). The low effect can be attributed to band matching between Ir and Co subbands. Annealing effects on structural and magnetic properties were observed by the results that interdiffusion of the components takes place at an annealing temperature above 350 °C and leads to degradation of the superlattice, followed by a decrease both in magnetic moment and in magnetoresistance. The images of Kerr microscopy and magnetic force microscopy show micromagnetic structures, which can be well understood in comparison with the magnetization and magnetoresistance measurements.
Polarization gain dependence of vertical-cavity surface-emitting lasers: Phenomenological modeling and experiments
Authors: Plouzennec Loig
Published: July 2002

Previous work has shown that the overlap between Fabry–Perot resonances and gain spectra explain some polarization state behaviors in vertical-cavity surface-emitting lasers (VCSELs). In this article, this approach is complemented for devices whose resonance is blueshifted relative to the peak gain. A simple phenomenological model is introduced that assumes the existence of a uniaxial strain in the quantum well layers making the material gain anisotropic. Measurements of the polarization anisotropy in spontaneous emission spectra confirm the hypothesis. Using rate equations, which include the demonstrated anisotropy, the polarization-state behavior of even externally coupled cavities VCSELs is predicted.
Characterization of poly-Si thin films deposited by magnetron sputtering onto Ni prelayers
Authors: Guliants E. , Anderson W. A.
Published: July 2002

A method of producing a polycrystalline silicon thin film on a foreign substrate without subsequent annealing has been developed. Thermally evaporated 5–100 nm thick Ni films served as prelayers for magnetron sputtered Si thin films. A continuous film was obtained as a result of metal induced growth of polysilicon during low temperature (below 600 °C) deposition. The film uniformity is promising for large area device applications. The influence of the Ni prelayer thickness on the grain size of thus obtained films was investigated. Atomic force microscopy and cross-sectional scanning electron microscopy studies revealed features in the 150–600 nm size range while x-ray diffraction and Raman spectra analysis predicted 50–100 nm diam randomly oriented grains and a complete absence of an amorphous phase. The carrier lifetime was evaluated to be 11 μs.
Influence of doping on the emission and scintillation characteristics of PbWO4 single crystals
Authors: Nikl M. , Boháček P. , Mihóková E. , Martini M. , Meinardi F. , Vedda A. , Fabeni P. , Pazzi G. P. , Kobayashi M. , Ishii M. , Usuki Y.
Published: July 2002

X-ray excited luminescence spectra, wavelength-resolved thermoluminescence glow curves, ultraviolet and 22Na excited emission decays, and 60Co excited light yield were measured on a set of high purity (99.9999%) PbWO4:X (X=Lu3+, Y3+, Gd3+, Sc3+, Nb5+, Pb2+) samples grown in an equivalent way. Positive influence of trivalent doping (Lu, Gd, Y) was noted, which consists in suppressing the deeper trapping states in the PbWO4 structure. Such states are related to the radiative recombination processes in the green and red part of the spectra. The presence of the green emission centers also results in the increase of the slow recombination decay components in the microsecond time scale. High concentration of the dopant ions (670 atomic ppm for PbWO4:Nb) results in the creation of new nonradiative recombination sites, which suppress the recombination decay components and the light yield as well.
Microwave properties of pulsed laser deposited Sc-doped barium hexaferrite films
Authors: Shi P. , Yoon S. D. , Zuo X. , Kozulin I. , Oliver S. A. , Vittoria C.
Published: July 2002

The properties of thick BaScxFe12−xO19 (0.3<x<0.6) scandium hexaferrite films were measured by static and microwave field techniques. Films were deposited by pulsed laser ablation onto c-plane sapphire at oxygen pressures of 20 and 50 mTorr. Vibrating sample magnetometer measurements as corroborated by x-ray data showed that the films below 3 μm had easy axis of magnetization (c-axis) normal to the film plane with saturation magnetization values of 3.0–3.8 kG. From the ferrimagnetic resonance frequency versus external magnetic field, we deduced a g value of 1.96±0.03 and uniaxial anisotropy field of ∼10 kOe. The ferrimagnetic resonance linewidth for the film thicker than 5 μm was maximum at 32 GHz and decreased with increasing frequency, indicating evidence for nonuniform magnetic field scattering internally. However, the linewidths were lower for films having thickness below 3 μm.
Molecular-dynamics simulations of energetic C60 impacts on (2×1)-(100) silicon
Authors: Hu Xiaoyuan , Albe Karsten , Averback Robert S.
Published: July 2002

Single impacts of energetic C60 clusters on (2×1)-(100) silicon substrates are studied by molecular-dynamics simulations. The role of impact energies and internal cluster energy are investigated in detail. Six different energy regimes can be identified at the end of the ballistic phase: At thermal energies below 20 eV the fullerene cages undergo elastic deformation, while impinging on the surface, and are mostly chemisorpted on top of the (2×1)-dimer rows. Between 20 and 100 eV the cage structure is preserved after the collision, but the cluster comes to rest within a few monolayers of the silicon surface. At energies of 100–500 eV the cluster partially decomposes and small coherent carbon caps are embedded in the surface. At higher energies up to 1.5 keV complete decomposition of the fullerene cluster occurs and an amorphous zone is formed in the subsurface area. At energies greater than approximately 1.5 keV craters form and above 6 keV sputtering becomes significant. In all cases the substrate temperature is of minor influence on the final result, but the projectile temperature is important for impacts at lower energies (<1.5 keV). For high energy impacts the ballistics resemble that of single atom impacts. Nearly 1:1 stoichiometry is obtained for impact energies around 1 keV. These results reveal an interesting possibility for controlled implantation of C in Si at high local concentrations, which might allow the formation of silicon carbide.
Influence of strain and quantum confinement on the optical properties of InGaAs/GaAs V-groove quantum wires
Authors: Constantin C. , Martinet E. , Lelarge F. , Leifer K. , Rudra A. , Kapon E.
Published: July 2002

We report on the impact of quantum confinement and strain effects on the optical properties of state-of-the-art, densely stacked, In0.15Ga0.85As/GaAs V-groove quantum wires. High uniformity and efficient carrier capture lead to narrow (6 meV) and intense emission from the wires. Large optical polarization anisotropy is obtained thanks to the combined effects of lateral quantum confinement and triaxial strain. Band filling in the fundamental subband occurs at a modest carrier density (∼9×105 cm−1), and is accompanied by a small spectral blueshift of the emission. Several sharp excitonic resonances associated with two dimensionally confined subbands of dominant heavy-hole character are observed in photoluminescence excitation spectroscopy, together with a remarkably small Stokes shift (3 meV). The subband separations (∼24 meV) are nearly independent of the wire thickness, as the nonuniform Indium composition across the structure is found to dominate the lateral confinement for thick wires. Such strained quantum wires are promising for the realization of advanced nanostructure devices.
Bandwidth considerations in modulated and transient photoconductivity measurements to determine localized state distributions
Authors: Reynolds S. , Main C. , Webb D. P. , Grabtchak S.
Published: July 2002

This work examines the influence of limited instrumental bandwidth on the accuracy of recovery of the density of localized states in semiconductors from transient and modulated photoconductivity data. Paradoxically, knowledge of the short-time transient photoresponse can be vital in the estimation, via a Fourier transform, of the density of deep-lying states. We demonstrate that retention of the natural response of a bandwidth limited system, although subject to distortion at short times, can lead to much improved accuracy in density of states determination than simple truncation of the short-time response. It is shown that this improvement arises simply from the integrating effect of a bandwidth limited system over short time intervals, which makes it possible to access and exploit information originating at times much shorter than the instrumentation rise time. These concepts are exemplified using computer simulated transient photoconductivity for several model systems including one which mimics the expected density of states in amorphous silicon.
Influence of magnetic field on cooling by normal-insulator–superconductor junctions
Authors: Arutyunov K. Yu. , Suppula T. I. , Suoknuuti J. K. , Pekola J. P.
Published: July 2002

Cooling by normal-insulator–superconductor junctions in external magnetic field has been studied experimentally. For all orientations of magnetic field the cooling performance correlates with the magnetic field dependent superconducting energy gap Δ(H). In perpendicular orientation of magnetic field with respect to the sample plane, additional degradation of the cooling power originates from scattering of nonequilibrium quasiparticles in the superconductor on magnetic vortices. The effect is hysteretic and its magnitude depends on the shape of the superconducting probes.
Analysis of the growth modes for gallium arsenide metalorganic vapor-phase epitaxy
Authors: Law D. C. , Li L. , Begarney M. J. , Hicks R. F.
Published: July 2002

The surface roughness of gallium arsenide (001) films produced by metalorganic vapor-phase epitaxy has been studied as a function of temperature and growth rate by in situ scanning tunneling microscopy. Height–height correlation analysis reveals that the root-mean-height difference follows a power-law dependence on lateral separation, i.e., Γ(L)=kLa, up to a critical distance Lc, after which it remains constant. For layer-by-layer growth, the roughness exponent, α, equals 0.25±0.05, whereas the critical distance increases from 50 to 150 nm as the substrate temperature increases from 825 to 900 K. The roughness exponent jumps to 0.65±0.1 upon transitioning to three-dimensional island growth. By relating the height–height correlation analysis to the Einstein diffusivity relationship, the activation energy for gallium surface diffusion has been estimated: Ed=1.35±0.1 eV.
Behavior of hydrogen intentionally introduced by plasma into metalorganic vapor phase epitaxy grown CdTe:As layers
Authors: Pelletier H. , Theys B. , Lusson A. , Cote D. , Rommeluère J. F. , Jomard F. , Dolin C.
Published: July 2002

Arsenic doped CdTe layers have been successfully exposed to a hydrogen or deuterium plasma to study the diffusion of these species and their interactions with arsenic. It is shown that hydrogen behaves similarly when unintentionally (during growth) or intentionally (plasma exposure) introduced. Infrared absorption measurements show that in both cases, identical As–H complexes are formed. The IR absorption line of deuterium–arsenic complexes has also been detected. The concentration of active arsenic acceptors decreases after intentional hydrogen or deuterium diffusion, as a consequence of the formation of these As–H or As–D complexes. But, the diffusion profiles of deuterium show that deuterium in the semiconductor binds also to other As-related centers which have not been identified unambiguously.
Gas phase kinetics of the reactions of Na and NaO with O3 and N2O
Authors: Ager Joel W. , Talcott Carol L. , Howard Carleton J.
Published: July 2002

A fast flow reactor, with an oven source and resonant fluorescence detection, was used to study the kinetics of Na atoms in the gas phase. The rate constant for Na+O3 is (7.3±1.4)×10−10 cm3 molecule−1 s−1 at 286 K and NaO+O2 is the predominant product channel. The rate constant for Na+N2O over the temperature range 240 to 429 K is (3.7±0.9)×10−10 exp[(−1690±180)/T] cm3 molecule−1 s−1. The kinetics of the NaO radical were measured directly. NaO is made in the flow tube by the reaction Na+N2O → NaO+N2 with N2O in large excess. NaO is detected by conversion to Na atoms in the detection region by NaO+NO → Na+NO2. NaO+O3 has two exothermic product channels, Na+2O2 and NaO2+O2. The rate constants are ∼5×10−11 and (1.8±0.4)×10−10 cm3 molecule−1 s−1 at 296 K, respectively. Upper limits for NaO+N2O →Na+N2+O2 and NaO+N2O → NaO2+N2 are 1×10−16 and 2×10−15 cm3 molecule−1 s−1, respectively. The rate constant for NaO+NO → Na+NO2 is ∼1.5×10−10 cm3 molecule−1 s−1. Since wall collisions remove NaO with near unit efficiency, NaO diffusion coefficients can be measured. DNaO,He =640±340 cm2 Torr s−1 and DNaO,N2O =48±24 cm2 Torr s−1 at 295 K. The error limits in all cases represent the 95% confidence level, including an estimate of systematic errors.
Observation of dissociative and radiative states of N2H by neutralized ion beam techniques
Authors: Selgren Susan F. , McLoughlin Patrick W. , Gellene Gregory I.
Published: July 2002

The N2 H(D) radical has been studied experimentally by measurement of the kinetic energy release in its unimolecular dissociation following formation by electron transfer from metal atoms to high velocity, mass-resolved ion beams and theoretically by ab initio techniques. Calculations of the dissociation coordinate of the ground state radical at the MP4/6-311G**//MP3/6-311G** level of theory indicate that the radical is unstable with respect to N2 and H by 0.6 eV but separated from the dissociation products by a 0.4 eV barrier. One dimensional tunneling lifetimes are determined to be 7.0×10−12 s for N2 H and 3.6×10−10 s for N2 D. Neutralization of the ion by Zn targets produces predominantly radicals in the 2 A′ ground state with dissociative lifetimes τ<0.5 μs, in agreement with the calculations. Mg targets produce the radical in a mixture of the 2A′ ground and 2A″(π) excited states with a branching ratio dependent on the internal energy of the precursor ion. A higher excited state of the radical, suggested to be an n=3 Rydberg level, is produced with K targets and is inferred to undergo radiative transitions, probably containing some discrete structure, to the lower 2A′ and 2A″(π) states in the wavelength range of 2700–4500 Å. Observations of these transitions may constitute the first spectroscopic observation of the radical.
A further study of HCO+ dissociative recombination
Authors: Rowe B. R. , Gomet J. C. , Canosa A. , Rebrion C. , Mitchell J. B. A.
Published: July 2002

The rate coefficient for the dissociative recombination of HCO+ has been measured using a new flowing afterglow technique which employs a movable Langmuir probe to measure electron density and a movable mass spectrometer to measure ion density, both as a function of distance along the flow. A value of 2.2×10−7 cm3 s−1 has been found at 300 K. An analysis of the excitation state of the ions indicates that more than 93% are in the v=0 state while the rest have ∼0.1 eV of internal energy. A discussion of recent theoretical controversy concerning this ion is given.
Fourier transform emission spectroscopy of ScN
Authors: Ram R. S. , Bernath P. F.
Published: July 2002

The near-infrared electronic emission spectrum of ScN has been observed in the 1.6–1.8 μm spectral region using a Fourier transform spectrometer. The three bands with origins at 5820.0944(5), 6051.2858(20), and 6266.1290(13) cm−1 have been assigned as the 0-0, 1-1, and 2-2 bands of the A 1Σ+−X 1Σ+ electronic transition. The principal equilibrium molecular constants for the ground state obtained from a fit of the observed lines are B″e=0.554 609(17) cm−1, r″e=1.687 23(3) Å and the corresponding values for the excited state are B′e=0.549 277(17) cm−1, re=1.695 40(3) Å. Although ScN is a well-known refractory solid, our work is the first observation of the gas-phase ScN molecule.
Comment on: Fluid distributions in two-phase random media: Arbitrary matrices
Authors: Given James A. , Stell George
Published: July 2002

It is pointed out that the Ornstein–Zernike equations recently used by Madden in treating quenched-annealed mixtures are approximate. The exact equations are given and briefly discussed.
Mean-field theory of phase transitions in liquid-crystalline polymers
Authors: Gupta Anshu M. , Edwards Sam F.
Published: July 2002

We present a self-consistent-field structure for the thermodynamic description of concentrated solutions of liquid-crystal polymers. The polymers are assumed to be locally stiff but capable of curvature over large distances. The formulation of the chain geometry is that of a wormlike polymer, but the final analysis is simpler than the Kratky–Porod form. The phase behavior is analyzed for a cylindrically symmetric mean-field nematic ordering interaction. As expected, in two dimensions the system undergoes a second-order phase transition and in three dimensions the nematic transition is first-order uniaxial. The three relevant physical parameters which describe the phase behavior of the polymers are the molecular mass of the polymers, a measure of the chain stiffness and strength of the nematic interaction. The formulation in this paper is for infinitely long polymers. If the chain stiffness is measured by the bending energy ε, with u as the strength of the hardcore repulsion and U that of the soft interactions, we find the following relationship at the isotropic-nematic transition: ccβcε(u+Uβc)=6. cc is the critical concentration and βc=1/kBTc is the inverse critical temperature. At the transition we find that the order parameter has a universal value of 0.25. We present universal plots for the order parameter and free energy of the system as a function of polymer concentration and temperature.
Dissociation and ionization of vibrationally autoionizing Rydberg states of H2 in a static electric field
Authors: Glab W. L. , Qin K.
Published: July 2002

We have studied the spectroscopic and dynamic properties of several autoionizing Rydberg states of molecular hydrogen which converge to the vibrationally excited v+=1 state of the ion, in the presence of a static electric field. Using the techniques of multiphoton ionization and time-of-flight mass spectrometry, we are able to separately observe the yield of dissociation and ionization after excitation of resolved Stark components of the Rydberg states. The energies of the Stark components can be modeled accurately by an energy matrix diagonalization calculation, when rotational interactions between states converging to different ion rotational states are taken into account. The dynamics of the different states in the Stark manifolds can be partly explained by assuming that they are governing by l mixing of predominantly predissociative s and d states and a dominantly autoionizing p state.
End-evaporation kinetics in living-polymer systems
Authors: Marques C. M. , Turner M. S. , Cates M. E.
Published: July 2002

We study theoretically the process of ‘‘end-evaporation’’ in living polymer systems, such as wormlike surfactant micelles. End-evaporation occurs when single monomers either break away from, or join onto, a chain end, the rates being described by the (mean-field) rate constants k and k′, respectively. Thus the chains can exchange material with one-another via a bath of free monomers. The relaxation of a system of living polymers after a small temperature jump (T-jump) is studied theoretically. The effect of a T-jump is to prepare the system with the wrong mean chain length, which relaxes to its equilibrium value L̄ by end-evaporation. It is found that the number of free monomers in the system relaxes almost completely in a time of order 1/kL̄, while the weight-average chain length, which is the quantity measured in light scattering experiments, relaxes on a time scale τD=4L̄2/k, which is three powers of L̄ longer. We also predict that the stress relaxation after a step strain is dominated by end-evaporation whenever τD≲τrep, where τrep is the reptation (disengagement) time for a chain of length L̄. In this case the stress relaxation is found to be ‘‘stretched exponential’’ for times smaller than τD and single exponential for longer times.
Self-diffusion and heat flow in isotropic and liquid crystal phases of the Gay–Berne fluid
Authors: Sarman Sten , Evans Denis J.
Published: July 2002

We use the so-called Evans heat flow algorithm and the color conductivity algorithm to calculate the thermal conductivity, λ, and self-diffusion, D, tensors of a model liquid crystal forming fluid, the Gay–Berne fluid. We compared the conductivities of the isotropic phase, the nematic liquid crystal phase, and the smectic B phase and found that the transverse components of λ and D in the nematic phase are about the same as λ and D for the isotropic phase at the same temperature and slightly lower density. The parallel components of λ and D are about two and four times as large as the respective transverse components. In the smectic B phase the parallel thermal conductivity ratio increases to about three, whereas the diffusion coefficient is virtually zero because of the solid like nature of this phase. We cross checked the results by deriving and evaluating the Green–Kubo relations for λ and D using conventional equilibrium molecular dynamics. We also discovered that in the presence of a temperature gradient, the director of the nematic phase, spontaneously aligns itself into the plane perpendicular to the temperature gradient. This effect which is nonlinear in the temperature gradient, minimizes the entropy production in the heat conducting steady state.
Structure of confined alkane liquids
Authors: Walley K. P. , Schweizer K. S. , Peanasky J. , Cai L. , Granick S.
Published: July 2002

Polymer integral equation theory is employed to calculate structural properties of undecane and short polymer liquids confined between parallel plates. A strong dependence is found on the strength of fluid–surface interactions which is consistent with solvation force measurements using both strongly attractive mica and weakly repulsive hydrocarbon surfaces.
Vibrational predissociation rate from dynamics of a full collision: A test of the Landau method against the exact results
Authors: Karni Y. , Nikitin E. E.
Published: July 2002

We applied the Landau method of calculating the coupling matrix element for a bound–free transition to the evaluation of the vibrational predissociation rate constant of the van der Waals complex ICl*–Ne. Use of the adiabatic separation of the radial and angular motion of the van der Waals modes complemented with the angular averaging with the help of the dominant natural expansion wave function of the bending mode, calculated by Lipkin, Moiseyev, and Leforestier [J. Chem. Phys. 98, 1888 (1993)], yields the rate constant which is about 10%–15% lower compared to the best theoretical estimates. This disagreement is ascribed to the true dynamical interaction between the stretching and bending modes which cannot be incorporated into the adiabatic separation of motion of these modes. Different approximations for treating bending motion are discussed and the sensitivity of the rate constant to the variation of principle parameters of the potential energy surface is studied.
Stability of the nematic phase of a mixture of aligned cylinders with respect to the smectic and columnar phases
Authors: Sear Richard P. , Jackson George
Published: July 2002

The theory for the stability of the nematic phase of parallel cylinders due to Mulder [Phys. Rev. A 35, 3095 (1987)] is generalized to binary mixtures. It is shown that adding cylinders of a different length to a one-component nematic phase of cylinders postpones the nematic–smectic A transition, enhancing the range of the nematic. This is a result of the two different length scales each favoring a density modulation of a different wavelength. When the ratio of the lengths of the cylinders is large a cusp appears in the limit of stability of the nematic phase. This cusp corresponds to a large jump in the wavelength of the density modulation from a wavelength characteristic of ordering of the short rods to that characteristic of the longer rods. The cusp is, however, masked by a transition to a columnar phase unless the diameters of the two species of rod are significantly different. The limit of stability with respect to the smectic phase bears a striking resemblance to the phase behavior of a mixture of two mesogens studied some time ago by Sigaud et al. [J. Phys. (Paris) Colloq. 40, C3 (1979)]. Their results have hitherto only been described by Landau theory and via lattice ‘‘analogs’’ which have no obvious connection with the underlying behavior at the molecular level. In contrast, theories of specific model molecules such as the cylinders studied here offer insight into behavior at the molecular level.
Photodissociation dynamics of state-selected resonances of HCO X̃ 2A′ prepared by stimulated emission pumping
Authors: Neyer David W. , Luo Xin , Burak Itamar , Houston Paul L.
Published: July 2002

Metastable resonances on the ground electronic state of the HCO radical have been prepared by stimulated emission pumping. The resonances have energies 5000 to 10 000 cm−1 above the dissociation limit of HCO and can be assigned by their vibrational and asymmetric-top rotational character. The transition linewidths of the resonances and the rotational and vibrational distributions of the CO dissociation products have been measured. The linewidths show a strong dependence on the vibrational character of the resonance rather than a monotonic dependence on energy, and thus provide an important example of nonstatistical behavior. CO(v=2) was produced in the decay of all six resonances studied, while only the three highest energy resonances produced measurable amounts of CO(v=3). CO rotational distributions with population in low-J states, which often showed nonstatistical structure, were characteristic of the products from all the resonances studied. The rotational distributions depend both on the vibrational character of the parent state and on the rotational state prepared in the HCO. The experimental results are compared and contrasted with previous quantum mechanical calculations and analyzed in the context of a modified Franck–Condon model for the dissociation.
Spinodals in a polymer dispersed liquid crystal
Authors: Shen Chungsea , Kyu Thein
Published: July 2002

Thermodynamic phase equilibria of a polymer dispersed liquid crystal (PDLC) consisting of monomeric liquid crystals and a polymer have been investigated theoretically and experimentally. The equilibrium limits of phase separation as well as phase transition of a PDLC system were calculated by taking into consideration the Flory–Huggins (FH) theory for the free energy of mixing of isotropic phases in conjunction with the Maier–Saupe (MS) theory for phase transition of a nematic liquid crystal. The correspondence between the Landau–de Gennes expansion and the Maier–Saupe theory was found and the coefficients were evaluated. The calculation based on the combined FH-MS theory predicted a spinodal line within the coexistence of the nematic–isotropic region in addition to the conventional liquid–liquid spinodals. The cloud point phase diagram was determined by means of polarized optical microscopy and light scattering for a polybenzyl methacrylate/E7 (PBMA/E7) PDLC system. The calculated phase diagrams were tested with the experimental cloud points, assuming the Flory–Huggins interaction parameter simply to be a function of temperature.
Electronic structure and electric properties of the alkali metal dimers
Authors: Urban Miroslav , Sadlej Andrzej J.
Published: July 2002

Dipole moments and dipole polarizabilities of a series of the alkali metal atom dimers involving Li, Na, K, and Rb are calculated at the level of different coupled cluster approximations as well as the complete active space self-consistent-field approach followed by the perturbation treatment of the dynamical correlation effects. All reported calculations have been carried out with recently developed polarized basis sets. The electron correlation effects on these electric properties are studied within the valence and valence plus next-to-valence shell approximations. Also the relativistic contribution is calculated by using the quasirelativistic scalar (mass–velocity+Darwin terms) approximation. A comparison of results of different methods indicates that the values calculated by the coupled clusters method in which one- and two-electron clusters are treated iteratively and the three-body terms perturbatively [as defined in the CCSD(T) method] are well saturated in terms of the electron correlation contribution to molecular electric properties of the dimers. The achieved agreement with available experimental data is for most molecules fairly good. The interpretation of the electric property data is based on the analysis of the valence and core-polarization contributions and the observed regularities are elucidated in terms of simple molecular orbital ideas. The dipole moments of the heteronuclear dimers follow the pattern predicted by electronegativities of the two atoms. The perpendicular component of the electric dipole polarizability has been found to mostly reflect its atomlike origin, while the parallel one is intimately related to the electronic structure of the week bonding in the dimers. The relativistic effect becomes of certain importance only for the rubidium compounds. The results of our calculations are compared with ab initio and empirical pseudopotential data and with more recent calculations using the density functional theory methods. Our results indicate that the density functional methods lead to fairly reliable polarizability data, though they depend on the choice of the exchange and exchange-correlation potentials.