bluuebunny/crossref_metadata_2025 huggingface dataset:
Implementation of a three-quantum-bit search algorithm
Authors: Vandersypen Lieven M. K. , Steffen Matthias , Sherwood Mark H. , Yannoni Costantino S. , Breyta Gregory , Chuang Isaac L.
Published: July 2002

We report the experimental implementation of Grover’s quantum search algorithm on a quantum computer with three quantum bits. The computer consists of molecules of C13-labeled CHFBr2, in which the three weakly coupled spin-1/2 nuclei behave as the bits and are initialized, manipulated, and read out using magnetic resonance techniques. This quantum computation is made possible by the introduction of two techniques which significantly reduce the complexity of the experiment and by the surprising degree of cancellation of systematic errors which have previously limited the total possible number of quantum gates.
Thermal amplification of photoinduced optical anisotropy of p-cyanoazobenzene polymer films monitored by temperature scanning ellipsometry
Authors: Kidowaki Masatoshi , Fujiwara Takenori , Morino Shin’ya , Ichimura Kunihiro , Stumpe Joachim
Published: July 2002

The in situ observation of thermal alteration of photoinduced optical anisotropy of ultrathin films of polymethacrylates having p-cyanoazobenzene side chains exhibiting amorphous, crystalline, and liquid-crystalline phases was performed by means of temperature scanning ellipsometry. Whereas photogenerated optical anisotropy of amorphous polymer films disappeared above glass transition temperature, the photoinduced anisotropy of films of liquid crystalline and crystalline polymer films was significantly amplified by heat treatment. In case of crystalline polymers, amplified optical anisotropy was maintained even at 300 °C.
Enhancement of p-type doping of ZnSe using a modified (N+Te)δ -doping technique
Authors: Lin W. , Guo S. P. , Tamargo M. C. , Kuskovsky I. , Tian C. , Neumark G. F.
Published: July 2002

Delta doping techniques have been investigated to enhance the p-type doping of ZnSe. Tellurium was used as a codopant for improving the nitrogen doping efficiency. The net acceptor concentration (NA−ND) increased to 1.5×1018 cm−3 using single δ doping of N and Te (N+Te), while it was limited to 8×1017 cm−3 by δ doping of N alone. A promising approach was developed in which three consecutive δ-doped layers of N+Te were deposited for each δ-doping cycle. An enhancement in the (NA−ND) level to 6×1018 cm−3 has been achieved in ZnSe using this technique. The resultant layer has an average ZnTe content of only about 3%. This doping method shows potential for obtaining highly p-type doped ohmic contact layers without introducing significant lattice mismatch to ZnSe. Low-temperature photoluminescence spectra reveal some Te-related emissions.
Temperature dependent polarization switching and band-gap anomalies in strained GaxIn1−xAs quantum wire heterostructures
Authors: Wohlert D. E. , Cheng K. Y.
Published: July 2002

We report on the polarized photoluminescence (PPL) properties of strained GaxIn1−xAs quantum wire (QWR) heterostructures formed in situ by the strain-induced lateral-layer ordering process. It is found that the PPL spectra of these QWRs have unique properties that depend on temperature and orientation of the pump polarization with respect to the QWR direction. In particular, the dominant polarization switches when the sample is warmed from 77 to 300 K provided the pump polarization is parallel to the QWRs. This indicates that the light-hole (LH) and heavy-hole (HH) bands cross with increasing temperature, which implies that the multiaxial strain in this material is a function of temperature. Furthermore, this effect is only observed in GaxIn1−xAs QWR heterostructures that display anomalous band-gap stability with respect to temperature. It is believed that the strain induced temperature dependent LH–HH crossing as evidenced by the polarization switching switching effect is responsible for this anomaly.
Epitaxial Fe (001) micro tiling: Size and interaction effects
Authors: Costa-Krämer J. L. , Martı́n J. I. , Menéndez J. L. , Cebollada A. , Anguita J. V. , Briones F. , Vicent J. L.
Published: July 2002

The magnetic properties of 200 Å epitaxial Fe (001)/MgO (001) tiling are studied as a function of tile size and separation. For edge sizes above ∼3 μm, the individual tiles maintain the single domain behavior while below ∼3 μm the tiles break into domains due to demagnetizing effects. In addition, below an inter tile separation threshold of about 0.9 μm, a single tile magnetization switch provokes a reversal cascade in all the tiles; while, above this threshold, the individual tile’s magnetization switches independently. In this last case, we have experimental access to the distribution of nucleation sites. Thus, we have found a clear signature of the magnetic interaction between patterned epitaxial micro tiles with in-plane magnetization.
Two-photon readout of three-dimensional memory in silica
Authors: Watanabe M. , Juodkazis S. , Sun H.-B. , Matsuo S. , Misawa H.
Published: July 2002

We report the readout of three-dimensional (3D) optical memory in silica by detecting the photoluminescence (PL) of the bits (voxels). A broad defect-related PL band at 400–700 nm was excited by two-photon absorption of femtosecond (pulse duration of 120 fs) illumination at 795 nm. We employed a simple reflection-type scanning readout without the use of a confocal detection scheme to read 3D memory by recording the PL of the bits (the same objective lens was used for the excitation and the collection of the PL). Bit plane separation as small as 3 μm was resolved without cross talk, when the theoretical limit of the axial resolution evaluated as a full-width at half maximum measure of a bit size was 1.4 μm at the fabrication conditions employed.
Poloidal force generation by applied radio frequency waves
Authors: Myra J. R. , D’lppolito D. A.
Published: July 2002

A theoretical framework is developed for calculating the nonlinear rf forces that can drive sheared poloidal flow in a tokamak plasma. It is shown that the rf-induced flow drive can be calculated without first obtaining an explicit result for the nonlinear distribution function. Instead, for modes satisfying the eikonal approximation, the flow drive can be expressed entirely in terms of moments of the linearized plasma responses. The method is applied to obtain explicit results for poloidal force generation for sheared flow drive applications in a hot plasma slab that supports rf waves of arbitrary polarization. The theory is fully electromagnetic and retains k⊥ρi∼1 (Bessel function) effects for the ion dynamics without approximation. An illustrative application to the ion Bernstein wave is presented.
Stability of E×B zonal flow in electron temperature gradient driven turbulence
Authors: Idomura Y. , Wakatani M. , Tokuda S.
Published: July 2002

The electron temperature gradient driven turbulence in a slab configuration modeling the negative shear tokamak is studied using a gyrokinetic finite element particle-in-cell code. It is found that quasisteady Er×B zonal flows are generated in finite magnetic shear regions in both sides of the qmin-surface, where the electron thermal transport is reduced substantially compared with the qmin-surface region. Stability analyses of the electrostatic Kelvin–Helmholtz (KH) mode show that the quasisteady Er×B zonal flow pattern is closely related to the q profile or the magnetic shear, which has a stabilizing effect on the KH mode. By changing the q profile to reduce the magnetic shear, the KH mode becomes unstable for the quasisteady Er×B zonal flow, and the Er×B zonal flows disappear in the weak magnetic shear region. Numerical results show a possibility of controlling Er×B zonal flows with the magnetic shear, which depends on the stability of the KH mode.
Fragmentation and ion-scattering in the low-energy collisions of small silver cluster ions (Agn+: n=1−4) with a highly oriented pyrolytic graphite surface
Authors: Tai Yutaka , Yamaguchi Wataru , Maruyama Yutaka , Yoshimura Kazuki , Murakami Junichi
Published: July 2002

Fragmentation and ion-scattering processes in the low-energy (0–200 eV) collisions of Agn+ (n=1–4) with a highly oriented pyrolytic graphite (HOPG) surface have been investigated by employing a tandem time-of-flight mass spectrometer. It was found that the fragmentation of scattered cluster ions is due to unimolecular dissociation in the energy range studied. A marked difference between the fragmentation pattern of incident Ag3+ and that of Ag4+ has been found: The intact scattered cluster ion was observed for Ag3+ while only fragment ions for Ag4+. From the incident energy dependence of fragment ion intensities, it was deduced that internal energies of the scattered parent ions have upper and lower limits. These limitations are probably due to the adsorption and the implantation of the projectile clusters on the surface. Ion-scattering yield was found to increase with cluster size. Both the incident energy and cluster size dependencies of ion-scattering yields were reproduced by a model calculation in which the hole survival and sticking probabilities of the clusters, and the incident energy spread of the projectile ions are incorporated. The hole survival probability was evaluated from the time-dependent electron transfer rate for a elastically scattered sphere with an equivalent volume to that of the incident cluster ion. The calculation indicates that the average hole–surface distance at the moment of impact, which is determined by the dimensions of the incident cluster, is important for the hole survival in the cluster–surface collisions.
Magnetic circular dichroism of symmetry and spin forbidden transitions of high-spin metal ions
Authors: Oganesyan Vasily S. , Thomson Andrew J.
Published: July 2002

Recently we have developed a general method of analyzing magnetic circular dichroism (MCD) spectra and magnetization curves of high-spin metal ions for spin-allowed transitions [Oganesyan et al., J. Chem. Phys. 110, 762 (1999)]. In the present article this approach is extended to cover the cases of spin- and symmetry-forbidden transitions. At high ligand fields many low-energy ligand field transitions become spin-forbidden. Extraction of information content about the electronic structure of the ground state can be obtained through the analysis and correlation of the positions, signs, and intensities of the MCD bands and magnetization curves of these transitions. The casting of the theory in terms of the irreducible tensor method allows full advantage to be taken of any symmetry elements and simplifies multielectron calculations. The theory is valid over the entire range of magnetic field strength and, therefore, allows the information content of spectra over the full field and temperature range to be exploited. The method is applied to the analysis of the recorded MCD spectra and magnetization curves of the lowest energy spin-forbidden ligand field transitions of pseudo-tetrahedral high-spin Fe(III), S=5/2, in the protein rubredoxin from Methanobacterium thermoautotrophicum (strain Marburg). The predicted signs, intensities, and magnetization curves for these transitions are in excellent agreement with experimental data. We also show that when the anisotropy of the ground state is larger than the Zeeman splitting the MCD of both spin-forbidden and allowed transitions can become comparable in magnitude. Hence caution is needed in order to avoid the misinterpretation of experimental results.
Effect of additives on the structure and magnetic properties of 1:7 type Sm2Fe15Ga2C3 permanent magnets
Authors: Zhang Junxian , Kleinschroth I. , Cuevas F. , Cheng Z. H. , Kronmüller H.
Published: July 2002

The effect of M(M=Zr, Cu, Nb) additives on the stability and magnetic properties of melt-spun nanocrystalline Sm2Fe15Ga2C3 ribbons with the 1:7-type structure has been investigated. The addition of 1 at. % of Zr or Nb was found to improve the thermal stability significantly, but the addition of Cu was found to decrease the thermal stability of the 1:7 metastable phase. After an appropriate heat treatment, M (Zr, Nb, or Cu)-containing samples exhibit better hard magnetic properties than the M-free sample. The intrinsic coercivity and maximum energy product were found to increase from 3.6 kOe and 59 kJ/m3 for the M-free sample to 4.8 kOe and 88 kJ/m3 for the Zr-containing one. The exchange interaction between the grains or the “exchange spring” behavior is studied according the Kneller and Hawig model and Henkel’s plot. The enhancements of magnetic properties originate from the improvement of the intrinsic magnetic properties as well as the refinement of the microstructure.
Electronic structure and magnetic coupling in sodium electro sodalite: All-electron density functional calculations
Authors: Windiks René , Sauer Joachim
Published: July 2002

Spin density functional calculations employing the full potential linearized augmented plane wave method (FLAPW) are performed on the periodic structure of sodium electro sodalite. The density functional adopted (PW91) includes gradient corrections for exchange and correlation. A body-centered cubic lattice of bare Na43+ clusters is found to be metallic and diamagnetic. The presence of the aluminosilicate framework makes it an antiferromagnetic material with a gap of about 0.1 eV between valence and conduction band. The antiferromagnetic state is more than 110–170 kJ mol−1 more stable than a ferromagnetic state. The Heisenberg exchange integrals between nearest and next nearest neighboring Na43+ clusters, Jnn and Jnnn, are derived from an extended Hückel tight-binding approach. The parameters of this Hamiltonian were chosen such that the density functional band structure is reproduced. The Heisenberg exchange integrals between nearest and next nearest neighboring Na43+ sites, Jnn and Jnnn, are negative, i.e., both nn and nnn sites are coupled antiferromagnetically. In absolute terms Jnnn is 0.6 meV. Estimates of Jnn are between 6.4 and 9.5 meV, the most likely value being 8.1±0.5 meV. Using these values for Jnn and Jnnn the molecular field approximation yields Weiss temperatures between −160 and −230 K. The most likely result, −200±10 K, fits well to the experimental value of about −200 K.
Temperature-dependent carrier trapping processes in short period quantum wire superlattices grown by flow rate modulation epitaxy
Authors: Liu X. Q. , Sasaki A. , Ohno N. , Wang Xue-Lun , Ogura Mutsuo
Published: July 2002

We report the first systematic investigation of the temperature-dependent carrier transferring processes of very short period V-grooved GaAs/AlGaAs quantum wire superlattice structures grown by flow rate-modulated metal-organic vapor phase epitaxy. The one monolayer (1 ML) fluctuation causes carrier confinement in sidewall (111) facet superlattice structures, and is shown to play an important role in the carrier transferring process. At low temperatures, the carrier transfer is blocked by the barriers of 1 ML fluctuation, while at high temperatures the carrier transfer from (111) superlattice to the wire region is shown to be very efficient after thermally overcoming the barriers. The temperature-dependent decay times of the different parts give direct evidence of the carrier transferring process, which demonstrates that the carriers can trap into the wire region within 50 ps at temperatures higher than 100 K.
High throughput aperture near-field scanning optical microscopy
Authors: Minh Phan Ngoc , Ono Takahito , Esashi Masayoshi
Published: July 2002

This article presents a simple measurement setup for characterization of a combined near-field scanning optical and atomic force microscopy (NSOM/AFM) using an aperture Si based probe. A technological approach has been found for the fabrication of a miniature aperture at the apex of a SiO2 tip on a Si cantilever for NSOM using a “low temperature oxidation and selective etching” (LOSE) technique. The optical transmission efficiency (throughput) of the fabricated probe was measured to be about 10−2 when the aperture size was approximately 100 nm, which is several orders of magnitude higher than that of conventional optical fibers and is also higher than other published throughput on micromachined tips due to a large curvature angle of the SiO2 tip. An extension of the LOSE technique for the fabrication of a tip with a tiny aperture having a metal nanowire at the center of the aperture for a multipurpose NSOM probe is also presented. Using the proposed measurement setup and the fabricated probe, NSOM and corresponding AFM images in contact and tapping modes of several surfaces are obtained.
Effect of oxidation method and post-oxidation annealing on interface properties of metal–oxide–semiconductor structures formed on n-type 4H-SiC C(0001̄) face
Authors: Fukuda K. , Cho W. J. , Arai K. , Suzuki S. , Senzaki J. , Tanaka T.
Published: July 2002

The C(0001̄) face of silicon carbide (SiC) has superior properties such as a faster oxidation ratio and a smoother surface compared with the Si(0001) face. We have investigated the oxidation and post-oxidation annealing effects on the capacitance–voltage and the interface state density (Dit) of n-type SiC metal–oxide–semiconductor (MOS) structures formed on the C(0001̄) face. It was found that pyrogenic oxidation and hydrogen annealing above 700 °C reduced Dit near the conduction-band edge. The value of Dit at Ec−E=0.2 eV is 1×1012 eV−1 cm−2, which is comparable with that of the MOS structure formed on the Si(0001) face. However, the value of Dit around the deep level at Ec−E=0.6 eV is one order of magnitude higher than that of n-type MOS structures formed on the Si(0001) face. It is very important to reduce Dit at the deep level for a high-quality SiO2/SiC interface on the 4H-SiC C(0001̄) face.
Reduced coercivity in ferromagnetic Co–Cu coevaporated epitaxial films on Cu(111)
Authors: Prieto J. E. , Camarero J. , de Miguel J. J. , Miranda R. , Rath Ch. , Müller S. , Hammer L. , Heinz K.
Published: July 2002

Epitaxial films grown by coevaporation of Co and Cu on Cu(111) were investigated by low-energy electron diffraction and surface magneto-optical Kerr-effect measurements. The films are dominantly face-centered-cubic stacked up to high Co concentrations and exhibit ferromagnetism. Their coercivity is significantly reduced compared to pure Co films produced by thermal Co deposition on Cu(111) independent of the use of Pb as a surfactant.
Amorphous silicon air-gap resonators on large-area substrates
Authors: Boucinha M. , Brogueira P. , Chu V. , Conde J. P.
Published: July 2002

Air-gap resonators composed of bridges of hydrogenated amorphous silicon and aluminum bilayers on glass and polyethylene terephthalate substrates are realized using surface micromachining techniques. The resonance frequency of structures on plastic substrates was found to vary inversely with the square of the length of the bridge span. On glass substrates, this dependence is seen for lengths >100 μm, while for lengths <100 μm, a decrease in the frequency is attributed to compressive stress. Resonance frequencies of ∼500 kHz to ∼2 MHz were measured in bridges with lengths from 160 μm down to ∼80 μm.
Extreme ultraviolet laser/time-of-flight mass spectra of Kr2 near Kr*(4d,5p′,6s)
Authors: Mao D. M. , Hu X. K. , Shi Y. J. , Leech J. H. , Lipson R. H.
Published: July 2002

New single-photon spectra of jet-cooled Kr2 between ∼98 700 and 99 900 cm−1 have been recorded with an extreme ultraviolet laser generated by four-wave sum-mixing in Xe gas. Molecular ions created by (1+1′) resonance enhanced multiphoton ionization, and daughter atomic ions created by excited state predissociation prior to ionization were mass dispersed and detected in a time-of-flight mass spectrometer. Analyses of the subsequent single isotopic spectra have resulted in vibrational numbering and constants for eight band systems of Kr2 that dissociate to Kr(1S0)+Kr*(4d,5p′,6s). Excited state bond lengths were estimated by Franck–Condon factor intensity simulations. When possible, electronic symmetry assignments for the states were established by correlation diagrams constructed from angular momentum arguments. Possible reasons are discussed for the deviations observed for the shape of many of the observed Rydberg state potential energy curves from their expected nonbonding form.
Nitrogen dependence of the GaAsN interband critical points E1 and E1+Δ1 determined by spectroscopic ellipsometry
Authors: Leibiger G. , Gottschalch V. , Rheinländer B. , Šik J. , Schubert M.
Published: July 2002

The effects of the nitrogen concentrations on the E1 and E1+Δ1 transitions of tensile-strained GaAs1−yNy (0.1%⩽y⩽3.7%) grown pseudomorphically to GaAs by metalorganic vapor-phase epitaxy are studied by spectroscopic ellipsometry. Adachi’s critical-point composite model is employed for ellipsometry data analysis. Contrary to the well-known redshift of the band-gap energy E0, we observe linearly blueshifted E1 and E1+Δ1 transition energies with increasing nitrogen composition y. For nitrogen compositions of 0⩽y⩽1.65%, the observed blueshift of the E1 energy is well explained by the sum of the effects of biaxial (001) strain and alloying.
Rovibrational spectroscopy of the v=6 manifold in C212H2 and C213H2
Authors: Srivastava H. K. , Conjusteau A. , Mabuchi H. , Callegari A. , Lehmann K. K. , Scoles G. , Silva M. L. , Field R. W.
Published: July 2002

We recorded rovibrational spectra of the 006+ level of C212H2 and the 2131 11−1 level of C213H2 in the ground electronic state using a two-photon sequential double resonance technique with a resolution of 15 MHz. Owing to the g/u symmetry of acetylene, the levels that we observe are inaccessible from the ground state by single photon techniques, and observation of these levels is reported here for the first time. Upper state rotational constants were derived from whole band fits of the observed lines, and compare favorably with expected values. Both spectra exhibit signs of local perturbations, and a density of states analysis leads us to believe that we are observing couplings to the full density of vibrational states one would expect from acetylene in this energy region. Despite the high resolution of our spectrometer, and the high excitation energy, no evidence for acetylene hydrogen permutation exchange isomerization (which is predicted to proceed through the vinylidene minimum on the potential) has been observed, implying that the rate of exchange isomerization is more than four orders-of-magnitude below the rate predicted by RRKM (Rice, Ramsperger, Kassel, and Marcus) theory.
Probing the lowest coordination number of dianionic platinum–cyanide complexes in the gas phase: Dynamics of the charge dissociation process
Authors: Bojesen Gustav , Hvelplund Preben , Jo/rgensen Thomas J. D. , Nielsen Steen Bro/ndsted
Published: July 2002

Low-energy and high-energy collision induced dissociation techniques are used to study the dissociation behavior of the gaseous Pt(CN)62− and Pt(CN)42− dianion complexes in order to probe the smallest stable dianion complex. Loss of neutral molecules from Pt(CN)62− occurs resulting in Pt(CN)52− and Pt(CN)42−, but no indication of the existence of Pt(CN)32− was found. This indicates that the lifetime of Pt(CN)32− is less than 4 μs (the flight time from the collision region to the detector). In contrast, all monoanion platinum–cyanide complexes were observed, i.e., Pt(CN)n− (n=1–6). The kinetic energy released in processes where monoanions are formed is much lower than the barrier height for the reverse reaction (>1.5 eV), which indicates that electron detachment is involved, e.g., Pt(CN)3− and CN− are not formed in the same dynamical process from Pt(CN)42−, or that the dynamics of the dissociation process is slow. For comparison, the Ru(bipy)32+*→[Ru(bipy)2−H]++bipyH+ reaction is associated with a kinetic-energy release of ∼0.3 eV (bipy=2,2′-bipyridine).
A four-site model for elongated liquid crystal molecules
Authors: Lee J. C.
Published: July 2002

Molecular dynamics simulation is performed for linear liquid crystal molecules using a four-site model. The model consists of four Lennard-Jones centers which are bonded together via central potentials in a shish-kebab shape. When the molecules are left under their own vapor pressure, they exhibit isotropic and smectic B phases with no intervening nematic phase. A wall consisting of frozen Lennard-Jones centers is shown to anchor the molecules and to widen the range of the smectic regime. This demonstrates that the model can be used for molecular dynamics studies on liquid crystals in various polymer matrices.
Temperature dependence of atomic hydrogen-induced surface processes on Ge(100): Thermal desorption, abstraction, and collision-induced desorption
Authors: Shimokawa S. , Namiki A. , N.-Gamo M. , Ando T.
Published: July 2002

The temperature effect on the atomic hydrogen-induced surface processes on Ge(100) has been studied from a desorption point of view. The experiments are carried out for temperature-programmed-desorption (TPD) of H2 and D2 and collision-induced desorption (CID) of D2 and abstraction of HD in the reaction system H(g)+D/Ge(100). The D2 and H2 TPD spectra exhibit two clear peaks, assigned as β1 and β2 TPD arising from a monohydride and a dihydride phase, respectively. There are isotope effects on the TPD spectra; D2 TPD peaks shift to higher surface temperature (Ts) compared to the peaks of H2, and the ratio of the β2 to the β1 TPD peak intensity is smaller for H2 than for D2. It is found that the kinetics of the abstraction and CID reactions are similar to those on Si(100), indicating that the mechanism for the abstraction and CID on Ge(100) is same to that on Si(100). The observed D2 rate curves show up a strong Ts dependence. The CID of D2 versus Ts curve exhibits a peak at Ts≃480 K corresponding to the leading edge of the β2TPD spectra. For Ts⩾530 K, CID of D2 is fully replaced by the spontaneous desorption ascribed to the β1 TPD. This fact suggests that the mechanism of CID is same to that of the β2 TPD. For Ts∼480 K, the D2 rate curves can be fitted with a fourth-order kinetics in a momentary D adatom coverage. The transiently created dihydride species that are considered to be mobile across the surface via a dihydride–monohydride isomerization reaction to exchange their sites, are invoked to explain the observed fourth-order kinetics. Probing H atoms reveals that the thermal desorption from the isolated dideuterides is not allowed, suggesting that for the β1 TPD the concerted desorption from hydrogen-prepared Ge dimers is preferred to the desorption from isolated dihydrides.
“Detector walk” in position-sensitive detectors with biased microchannel plates
Authors: Tremsin A. S. , Vallerga J. V. , Siegmund O. H. W.
Published: July 2002

The spatial resolution of high-accuracy microchannel plate (MCP) detectors has reached the values, where the so-called detector walk (or image blurring) may start to limit any further improvements. Image blurring with gain is studied in detail for detectors incorporating angular-biased MCPs. It was found that the presence of the pore bias at the output MCP results in a variation of the charge footprint position for events with different gains. Events with higher gains are shifted in the direction of the pore bias and the absolute value of this shift is directly proportional to the absolute value of the detector gain. Variation of the detector modal gain from 7.5×106 to 2.5×107 resulted in a ∼100 μm image offset for a 13°-biased MCP positioned at a distance of 8.5 mm from the anode with an accelerating rear field of 75 V/mm. We also extended our previous study of another type of detector walk associated with fluctuations of the accelerating rear field. Image displacements as functions of the rear accelerating field for both 13°- and 19°-biased MCPs were measured and compared with the results of computer simulation based on our charge cloud propagation model presented earlier. A good agreement between the experimental and simulated data verifies the validity of the model for different MCPs.
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1
NMR study of the local magnetic field gradients in evoluting porous structures. An application to cement gels
Authors: Leventis A. , Papavassiliou G. , Fardis M. , Milia F. , Chaniotakis E. , Dolinsek J. , Apih T.
Published: July 2002

When placed into an external magnetic field inhomogeneous porous structures, like rocks, cement gels, ceramics, etc., exhibit strong local magnetic field gradients (LMFG), which are inherently related with the size, shape, and distribution of their pores. In this paper, we present a method of measuring LMFG in a liquid-filled porous media, provided that the fast exchange model sufficiently describes water motion in the pores, which is based solely on the measurement of the water H1 NMR spin lattice relaxation rates and the knowledge of the unrestricted diffusion coefficient. The method is applied to two hydrating white cement samples with different hydration kinetics, and the results are correlated with the time evolution of the pore structure. It is shown that the measured LMFG have extremely high values, which qualitatively reflect the sharp needlelike morphology of the internal cement gel surface.
Bimolecular recombination quenching in Langmuir Blodgett multilayers
Authors: Elliott J. E. , Jeong I. S. , Scott K. , Donovan K. J. , Wilson E. G.
Published: July 2002

A model is developed that describes bimolecular recombination of photogenerated carriers in two dimensional systems. Carriers are free to diffuse in two dimensions and undergo bimolecular recombination, while drifting under the influence of an electric field in the third dimension. The model describes a competition between carrier loss due to transiting and loss due to bimolecular recombination. This model of recombination quenching is then used to obtain information on microscopic parameters associated with photogeneration efficiency and charge transport in organic quantum wells formed from Langmuir Blodgett films of conjugated molecules. The ratio of the intralayer to interlayer tunneling rates is found along with the quantum efficiency for photocarrier generation for two bis-phthalocyanine amphiphilic molecules.
Quantum cables as transport spectroscope for quasi-one-dimensional density of states of cylindrical quantum wires
Authors: Zeng Z. Y. , Xiang Y. , Zhang L. D.
Published: July 2002

We propose that quantum cables can be used as a kind of transport spectroscope to obtain directly the quasi-one-dimensional (Q1D) density of states (DOS) of cylindrical quantum wires. By simultaneously measuring the direct current through the cylindrical quantum wire and the leaky tunneling current into its neighboring wire at desirable temperatures, one can obtain detailed information about the Q1D DOS and subband structure of the cylindrical quantum wire.
Measurement of the electronic wave function: Separated atom wave function analysis of the R-dependent hyperfine constants of the iodine monochloride A state
Authors: Barney William S. , Western Colin M. , Janda Kenneth C.
Published: July 2002

New measurements are presented of the electric and magnetic hyperfine constants of the I and Cl nuclei for the A(3Π1) state of I35Cl, ν=16–27 and 33. This extends the existing set of measurements for the A state to cover most of the vibrational levels from ν=7 to the dissociation limit. These data are inverted to provide hyperfine constants as a function of internuclear distance. The constants are found to be strongly dependent on the internuclear distance. The changing values are modeled by a separated atom (or valence-bond) model of the wave function. This model contains only eight basis states, but successfully describes the observed changes in the hyperfine constants for internuclear separations ranging from 5.5 down to 2.9 Å. The model also allows the rehybridization of the bond to be described quantitatively as a function of internuclear separation. Two interesting effects of vibration are observed; first, the orientation of the unoccupied p orbitals varies considerably over the range of nuclear motion. Second, the contribution to the wave function of spin–orbit excited atomic chlorine states changes dramatically around 3.5 Å. As part of this analysis we present extensions to the current methods of estimating molecular hyperfine constants from atomic properties to allow a wider range of parameters to be covered.
Effects of end-of-range dislocation loops on transient enhanced diffusion of indium implanted in silicon
Authors: Noda T. , Odanaka S. , Umimoto H.
Published: July 2002

Transient enhanced diffusion of indium implanted in silicon is studied in the presence of the end-of-range (EOR) damage layer. To investigate the effect of EOR defects on the indium diffusion, the samples that were implanted with indium at a high dose (1×1013–5×1014/cm2) sufficient to produce the amorphous layer were prepared. Transmission electron microscopy measurements and Rutherford backscattering spectrometry reveal the amorphization threshold of indium implantation is around 5×1013/cm2 for 200 keV, In+115 implanted with 100 μA/cm2 beam current density at room temperature. These results are consistent with Monte Carlo simulation of implantation. Monte Carlo simulations indicate the deviation from the plus one model due to the mass effect of indium. After amorphization, following both RTA at 1000 °C and furnace anneal at low temperature (650 and 850 °C) in nitrogen ambient showed the formation of extrinsic EOR dislocation loops below the original amorphous/crystalline interface. During this process, strong segregation of indium toward the EOR dislocation loops is clearly observed. The profile shift of indium at a concentration of 1×1017 atoms/cm3 is not proportional to the implanted dose. Since most interstitials condense into EOR dislocation loops, diffusivity enhancement of indium is not proportional to the implant dose above amorphization threshold.
Two-dimensional vibrational spectroscopy. VII. Investigation of the vibronic and vibrational couplings by using novel triply resonant two-dimensional vibrational spectroscopies
Authors: Cho Minhaeng
Published: July 2002

A single vibrational coherence state can be created by using either vibrationally resonant infrared field-matter interactions or Raman transitions induced by the electronically off-resonant fields. Then, the two-dimensional (2D) vibrational coherence created by two such interactions can be probed by using electronically off-resonant optical pulse via scattering by the 2D transient grating. However, due to the off-resonant field-matter interactions during the probing process, the signal intensity is likely to be small. Thus, in this paper, the cases when the 2D transient grating is probed by electronically resonant optical pulse are theoretically considered and the associated nonlinear response functions are investigated. It turns out that these novel 2D vibrational spectroscopic methods utilizing an electronically resonant probe pulse can be used to study both the linear and quadratic vibronic coupling strengths of molecules in either isotropic or anisotropic condensed phases, in addition to the vibrational anharmonicity and nonlinearity of the coordinate dependencies of the dipole moment, polarizability, and hyperpolarizability.
Predictions of rotation–vibration effects in time-resolved photoelectron angular distributions
Authors: Althorpe Stuart C. , Seideman Tamar
Published: July 2002

We investigate the physical origin of direct reflection of rotation–vibration coupling in time-resolved photoelectron angular distributions. The theory is developed for a general rotation–vibration coupling mechanism in a polyatomic system and applied to the simplest instance of such interaction, namely centrifugal coupling in a diatomic molecule. Our results suggest the possibility of determining coupling strengths from the observed time dependence of the ionization asymmetry parameters.
Influence of growth direction on order–disorder transition in (GaAs)1−x(Ge)2x semiconductor alloys
Authors: Rodrı́guez A. G. , Navarro-Contreras H. , Vidal M. A.
Published: July 2002

We provide direct evidence of the dependence of the critical concentration of IV-type atoms at the order–disorder transition in ternary metastable (III–V)–IV, zincblende–diamond semiconductor alloys on the growth direction. The excellent agreement between the experimental and model predicted critical concentrations is evidence that the atomic ordering in these alloys is ruled almost entirely by substrate geometry. In this letter, we report the observation of the critical concentration dependence on substrate orientation in (GaAs)1−x(Ge2)x metastable alloys, epitaxially grown on (001), (111), (112), and (113) oriented GaAs. A different long-range order parameter behavior with Ge concentration is obtained for each growth direction. The Ge critical concentrations observed are 0.36, 0.96, 0.59, and 0.50±0.03, for the orientations listed above, values that compare well with those obtained from a Monte Carlo simulation of the growth, 0.33, 1.0, 0.64, and 0.54, respectively.
On the systematic improvement of fixed-node diffusion quantum Monte Carlo energies using pair natural orbital CI guide functions
Authors: Lüchow Arne , Fink Reinhold F.
Published: July 2002

While the diffusion quantum Monte Carlo method (DQMC) is capable, in principle, of calculating exact ground state energies, in practice the fixed-node (FN) approximation leads to node location errors which make FN-DQMC energies upper bounds. It is shown that the node location error can be reduced systematically and without prohibitive increase of computer time requirements by using nodes derived from pair natural orbital CI wave functions (PNO-CI). The reduction is demonstrated for the N atom and the molecules N2 and H2O. With the DQMC/PNOCI method, we obtain a variational energy of −109.520(3) H for the N2 molecule and −76.429(1) H for the ground state of the water molecule which is only 22 and 9 mH above the estimated nonrelativistic ground state energy, respectively.
Laser induced dispersed fluorescence spectroscopy of 107 vibronic levels of NO2 ranging from 12 000 to 17 600 cm−1
Authors: Delon Antoine , Jost Rémy , Jacon Marcel
Published: July 2002

We report the laser induced dispersed fluorescence spectra (LIDFS) of NO2 recorded when exciting 107 vibronic levels lying between 11 961 and 17 627 cm−1. These levels result from the vibronic interactions between the highly excited vibrational levels of the electronic ground state, X̃ 2A1, and the isoenergetic levels of the electronic excited state, Ã 2B2. One observes that the intensity patterns of almost half of the LIDFS can be interpreted in terms of Franck–Condon factors (FCFs) between a single bright parent level belonging to Ã 2B2 and the low-lying vibrational levels of X̃ 2A1. This means that the vibronic interactions occurring between the vibrational levels of X̃ 2A1 and Ã 2B2 are not strong enough to systematically mix together several bright parent states of Ã 2B2. We have also found that vibronic eigenstates scattering over several hundreds of cm−1 can be assigned to the same bright parent state. However, these features, which are characteristic of individual vibronic levels, must be faced with the statistical properties of the NO2 visible spectrum, well-known to exemplify molecular quantum chaos, at least above 16 500 cm−1 [J. Chem. Phys. 103, 1732 (1995)]. Complementarily, we briefly present results of the breakdown of vibrational and rotational selection rules, as observed in LIDFS. This phenomenon reflects the onset of strong vibronic and rovibronic interactions that progressively destroy the above mentioned one-bright-parent-state model and leads, at higher energies, to a regime of rovibronic chaos.
Theoretical analysis and experimental study of Fourier transformation of Franz–Keldysh oscillations in GaAs
Authors: Jin Peng , Pan S. H. , Liang Ji Ben
Published: July 2002

Fourier transformation (FT) has been used in the theoretical line shape analysis of Franz–Keldysh oscillations (FKOs) in detail by numerical simulations. FKOs from the surface-intrinsic-n+ GaAs structure were obtained in photoreflectance (PR) measurements with various modulation light intensities and with different strengths of bias light illumination, which were used to change the static electric field in the intrinsic layer of the sample. The FT spectra of the PR spectra, including the real part, imaginary part, and the modulus, were very consistent with the theoretical line shapes. The ratio of the square root of the reduced mass (μL/μH) and the ratio of transition strength of the electron heavy hole to the electron light hole were obtained from the FT spectra. In addition, the electric field in the intrinsic layer of the sample without and with bias illumination and the modulation field induced by photomodulation were also obtained.
Optimizing phonon-cooled Nb hot-electron bolometers
Authors: Araújo H. M. , Puplett E. F. , White G. J.
Published: July 2002

A Nb hot-electron bolometer designed for operation as a sensor of millimeter waves and far-infrared radiation has been analyzed. This optically coupled detector has a very wide spectral bandwidth, a response speed of ∼1 ns determined by electron-phonon scattering, and an impedance of 50 Ω. The geometry employed here has traditionally been affected by stability problems manifesting as random voltage fluctuations, which have hindered the usefulness of these bolometers. This issue has been addressed by exploring the underlying physical causes and adapting the fabrication technique. Analytical results on substrate-induced bolometric effects are summarized and other factors concerning detector performance addressed, and the reality of an isothermal regime of operation is questioned. A noise equivalent power of 4 pW/Hz1/2 was measured at 100 μm, with a coupling efficiency close to unity that was achieved by reducing the optical reflection losses. Operation of a similar device is shown able to resolve the macropulses of FELIX, the Free-Electron Laser for Infrared eXperiments, into its 40 ns spaced micropulses.
Quasimicrowave spectroscopy of nonpolar diatomic molecules by using optical phase-locked lasers
Authors: Kumagai Motohiro , Kanamori Hideto , Matsushita Michio , Kato Tatsuhisa
Published: July 2002

A new type of optical–optical double resonance (OODR) experiment using an optical phase-lock loop was applied to a molecular system. The difference frequency between two single-mode lasers was locked to a tunable MW reference using an optical heterodyne technique. An OODR signal was observed in the sub-Doppler condition when three rovibronic levels in the B 1Πu–X 1Σg+ transitions of the Cs2 molecule interacted simultaneously with the two lasers. This optical phase-locked OODR spectroscopy achieves absolute frequency measurements of the rotational structure. It is especially advantageous to the ground state of nonpolar molecules because the resonance signal is free from the radiative lifetime of the electronic excited state, and then the accuracy would be as precise as that obtained by MW spectroscopy.
X-ray absorption and infrared reflectance of poled silica glass for second harmonic generation
Authors: Nazabal V. , Fargin E. , Le Flem G. , Briois V. , Cartier dit Moulin C. , Buffeteau T. , Desbat B.
Published: July 2002

Structural alterations induced by a thermal poling treatment which is at the origin of second harmonic generation in a Herasil silica glass are studied by x-ray absorption near-edge spectroscopy (XANES). Multiple scattering calculations are performed to explain the experimental O–K edge XANES shape of unpoled surfaces of the glass and then to interpret modifications resulting from poling. The observations are consistent with emergence of disorder by breaking Si–O bridging bonds which are not completely removed by a thermal depoling treatment. Fourier transform infrared spectroscopy shows modifications of nonbridging oxygen bonds which are coherent with a model involving a protons migration during poling. Moreover, a partial anisotropy in the direction of the electrostatic field which is applied during the poling treatment is observed.
Spin-polarized transport through a ZnSe/Zn1−xMnxSe heterostructure under an applied electric field
Authors: Guo Yong , Wang Hao , Gu Bing-Lin , Kawazoe Yoshiyuki
Published: July 2002

We investigate spin-dependent transport through a band-gap-matched ZnSe/Zn1−xMnxSe heterostructure with a single paramagnetic layer under influence of both an electric and a magnetic field. It is shown that the magnetic-field tunable potential and its symmetry in this system can be significantly modulated by the electric field, which results in obvious dependence of the magnitude of the electronic spin polarization on the electric field magnitude. The results obtained in this work might shed light on the development of new electron-polarized devices.
Mixtures of charged colloids and nonadsorbing flexible polyelectrolytes: An integral equation study
Authors: Ferreira Paula G. , Dymitrowska Magdalena , Belloni Luc
Published: July 2002

We study the equilibrium and structural properties of mixtures of like-charged colloids and nonadsorbing flexible polyelectrolytes using liquid state integral equations. The polymer chains in the semidilute regime are treated within the polymer reference interaction site model. The monomers and the colloids interact via screened Coulomb potentials (Debye–Hückel level). The electrostatic interactions between the different macromolecules inside the solution are responsible for complex local structures and rich phase diagrams, which depend on the charges and on the Debye screening length. The pair correlations between colloids are interpreted in terms of the effective, polymer-averaged, colloid–colloid pair potential. A first-order picture indicates that the Coulombic depletion attraction due to the osmotic pressure of the semidilute network against the colloidal particles dominates the direct repulsion at high enough monomer concentration and induces the liquid–liquid phase separation. Higher-order effects involving correlations inside the polyelectrolyte system alter this simple picture.
Transient response of organic light-emitting devices: A Monte Carlo simulation study
Authors: Das Sharmistha , Talapatra G. B. , Chowdhury Arabinda , Pal Amlan J.
Published: July 2002

A Monte Carlo simulation method has been conducted to study transient response of electroluminescence in single-layer light-emitting devices. The results from the numerical method have been compared with our recent (transient) experimental results. The simulation results supported the role and contribution of accumulated charges at the metal–semiconductor interfaces in device operation. Several parameters like barrier heights and mobilities of charge carriers, width and distribution of the accumulated space charge regions in the device, have been used in the simulation work. Additionally, barriers for charge injection have been considered to be modified by the presence of space charges near the injecting interfaces. A recipe has been established to estimate different parameters of device operation from the comparison between simulation and experimental results.