bluuebunny/crossref_metadata_2025 huggingface dataset:
Liquid-helium temperature long-path infrared spectroscopy of molecular clusters and supercooled molecules
Authors: Bauerecker Sigurd , Taraschewski Michael , Weitkamp Claus , Cammenga Heiko K.
Published: July 2002

Collisional cooling and supersonic jet expansion both allow us to perform infrared spectroscopy of supercooled molecules and atomic and molecular clusters. Collisional cooling has the advantage of higher sensitivity per molecule and enables working in thermal equilibrium. A new powerful method of collisional cooling is presented in this article. It is based on a cooling cell with integrated temperature-invariant White optics and pulsed or continuous sample-gas inlet. The system can be cooled with liquid nitrogen or liquid helium and operated at gas pressures between <10−5 and 13 bar. Temperatures range from 4.2 to 400 K and can be adjusted to an accuracy of ±0.2 K over most of the useable range. A three-zone heating design allows homogeneous or inhomogeneous temperature distributions. Optical path lengths can be selected up to values of 20 m for Fourier transform infrared (FTIR) and 40 m for laser operation. The cell axis is vertical, so optical windows are at room temperature. Diffusive trapping shields and low-power electric heating keep the mirrors free from perturbing deposits. The cell can be operated in a dynamic buffer-gas flow-cooling mode. A comprehensive review of existing collisional cooling cells is given. The formation of CO clusters from the gas phase was investigated using FTIR spectroscopy. For the isotope mixture consisting of C1613O,13C18O, and C1612O, a conspicuous change in the main spectroscopic structure of the clusters was observed between 20 and 5 K. The cluster bandwidth of the main isotope C1613O triples. This behavior could be interpreted as a change from the crystalline to the amorphous state or as a decrease in size to smaller clusters with relatively larger surfaces. To our knowledge, this is the first IR investigation of molecular clusters obtained by collisional cooling in this temperature range. For CO2 the change from the monomer to crystalline clusters was investigated. The observed spectra vary considerably with temperature. FTIR spectra of CO2 clusters observed previously by other researchers could be reproduced. The system allows us to determine various gases with a FTIR detection limit in the lower ppb range. With these concentrations and at temperatures <10 K the monomers can be supercooled, and small clusters can be obtained.
Two-photon vibronic spectra of vinyl chloride at 7.3–10 eV
Authors: Chang Jia-lin , Li Runhua , Wu Jen-Chieh , Shieh Jau-Chin , Chen Yit-Tsong
Published: July 2002

The vibronic spectra of vinyl chloride at 7.3–10 eV have been studied using 2+1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The geometries and vibrational frequencies of vinyl chloride and its cation calculated with ab initio methods are utilized to compute the Franck–Condon factors. The transitions to the Rydberg states of π→ns, npσ, npπ, nd, nf, and nCl→3s are responsible for the observed REMPI spectra of vinyl chloride. The complex vibrational structures for the observed transitions are successfully identified with the aid of calculated Franck–Condon factors.
Nanoscale selective growth of GaAs by molecular beam epitaxy
Authors: Lee Seung-Chang , Malloy Kevin J. , Brueck S. R. J.
Published: July 2002

Selective growth of GaAs on a nanoscale SiO2-patterned GaAs(001) substrate by molecular beam epitaxy is reported. Reduction of the lateral dimensions of a SiO2 pattern below the surface diffusion length of an incident Ga atom results in preferential migration from the SiO2 surface to stable bonding configurations at nearby open GaAs substrate surfaces. This intrinsic selectivity is achieved under high growth temperature with low growth rate where surface migration on the SiO2 surface is highly extended. A large-area nanoscale SiO2 pattern is realized by interferometric lithography. A 330-nm period two-dimensional array of GaAs disks having a height of 40 nm and a diameter of about 240 nm, selectively grown on a GaAs(001) substrate is presented. Variation of the nanoscale selective growth mode depending on the diameter of GaAs disks is also observed.
Electron diode with a large area ferroelectric plasma cathode
Authors: Dunaevsky A. , Krasik Ya. E. , Felsteiner J. , Sternlieb A.
Published: July 2002

The operation of a planar diode with a large area (64 cm2) ferroelectric plasma cathode based on BaTiO3 ceramics is studied. Diode parameters and parameters of the generated electron beam are investigated under accelerating voltage of 200–350 kV and pulse duration of 250 ns. Parameters of the plasma, which forms on the surface of the ferroelectric cathode due to incomplete discharge, are reported as well. It is shown that the application of this cathode allows one to produce an electron beam simultaneously with application of the accelerating pulse. The generation of an electron beam with current density up to 20 A/cm2 and divergence of less than 1° is demonstrated. It is shown that diode operation is governed by parameters of the cathode plasma which fills the anode–cathode gap prior to application of the accelerating voltage pulse. By proper adjustment of the cathode and diode parameters the operation of the diode with almost constant impedance is achieved. Measurements of the potential distribution in the diode confirm that the diode operates in a plasma-prefilled mode. The cathode is found to be capable of operating for up to 105 pulses at a repetition rate of few Hz without any maintenance and with relatively small vacuum deterioration.
A flow-through x-ray absorption spectroscopy cell for characterization of powder catalysts in the working state
Authors: Odzak J. F. , Argo A. M. , Lai F. S. , Gates B. C. , Pandya K. , Feraria L.
Published: July 2002

We report the design and demonstration of an x-ray absorption spectroscopy (XAS) cell used for the characterization of solid (powder) catalysts in operation with gas-phase reactants. The use of powder samples removes complications arising from mass transfer limitations in pressed wafer samples, the typical form of catalyst used in other in situ XAS cells. The new cell allows collection of XAS data at temperatures ranging from about 230 to 470 K, gas flow rates ranging from about 10 to 500 ml min−1, and pressures ranging from about 1 to 3 atm. The cell is designed to function nearly as a plug flow reactor.
Radiative recombination characteristics in GaAs multilayer n+−i interfaces
Authors: Shen W. Z. , Jiang L. F. , Yu G. , Lai Z. Y. , Wang X. G. , Shen S. C. , Cao X.
Published: July 2002

In this communication, we have carried out a detailed investigation of radiative recombination in n-GaAs homojunction far-infrared detector structures with multilayer emitter (n+)-intrinsic (i) interfaces by temperature-dependent steady-state photoluminescence measurements. The observation of the emitter-layer luminescence structures has been identified from their luminescence characteristics, in combination with high density theoretical calculation. A photogenerated carrier transferring model has been proposed, which can well explain the dependencies of the luminescence intensities on the laser excitation intensity and temperature. Furthermore, the obtained radiative recombination behavior helps us to offer a proposal to improve the operating temperature of the detector.
Pulsed mirror electron microscope: A fast near-surface imaging probe
Authors: Kleinschmidt H. , Bostanjoglo O.
Published: July 2002

A pulsed mirror electron microscope was developed for imaging laser-induced processes on the nanosecond time scale. Variations of the electronic structure of the surface are imaged. The evaporation or deposition of an atomic monolayer is readily detected. Pulsed mirror electron microscopy is an alternative technique to pulsed photoelectron microscopy for tracking chemical reactions. In addition, gas flow above the surface and laser-induced ablation of absorbing layers at the back surface of a dielectric can be probed, exploiting electron scattering and capacitive coupling, respectively. Presently, the space/time resolution is limited by the competition between electron shot noise and illumination aperture to 1 μm/5 ns. The illuminating electron pulses are supplied by a laser-driven robust ZrC-covered Re cathode. This device can be operated in a high vacuum of 10−5 mbar either as a photo or a thermal electron emitter.
Measurement of grid heat loading in a negative ion source for producing intense H− ion beams by aperture displacement focusing technique
Authors: Takayanagi T. , Ikehata T. , Okumura Y. , Watanabe K. , Hanada M. , Amemiya T.
Published: July 2002

Heat loading of acceleration grids has been investigated using a JAERI 400 keV negative hydrogen ion source, which has a three-stage multiaperture accelerator. To investigate the effect of beamlet steering on the heat loading, the apertures in the grounded grid were displaced up to 5 mm. The steering angle increased linearly with the displacement distance of the grid. The obtained steering angle was 7.5 mrad at the displacement distance of 5 mm. It was confirmed that the steering angle agrees well with the thin lens theory. Degradation of beam optics due to the steering was not observed. The heat load of the grounded grid was scarcely changed up to the displacement distance of 2 mm and then increased from 2.5% to 8% of the total input power when the grid was displaced from 2.5 to 5 mm. Heat loads of the other unsteered grids were not changed.
Phase transition-governed opal–VO2 photonic crystal
Authors: Golubev V. G. , Davydov V. Yu. , Kartenko N. F. , Kurdyukov D. A. , Medvedev A. V. , Pevtsov A. B. , Scherbakov A. V. , Shadrin E. B.
Published: July 2002

Three-dimensional opal–VO2 photonic crystals were prepared by the chemical bath deposition technique. The x-ray diffraction and Raman spectroscopic data confirm the crystalline perfection of VO2 impregnated into synthetic opal pores. It is shown from the optical reflectivity measurements that the photonic bandgap of the opal–VO2 composite is governed by the phase transition in VO2.
Measuring normal and friction forces acting on individual fine particles
Authors: Ecke Stefan , Raiteri Roberto , Bonaccurso Elmar , Reiner Christof , Deiseroth Hans-Jörg , Butt Hans-Jürgen
Published: July 2002

Interparticle and surface forces are of great importance in many fields of pure and applied science. We present an apparatus to measure the normal and friction forces acting between a particle (radius of 0.5–20 μm) and another solid surface. The apparatus is based on the principle of an atomic force microscope. For quantitative friction measurements we propose a method to determine the lateral spring constants of atomic force microscope cantilevers with attached spherical particles.
Channeling on boron clusters in silicon
Authors: Selen L. J. M. , van IJzendoorn L. J. , van Loon A. , de Voigt M. J. A.
Published: July 2002

Low energy ion implantation at high doses of boron (>1015 cm−2) in Si is necessary for the fabrication of ultrashallow junctions but can result in the undesirable presence of boron clusters. Values for the dimensions of the lattice distortions in the implanted Si are obtained by comparing the enhanced dechanneling and the direct scattering peak in the region with clusters in a channeled Rutherford backscattering spectrometry spectrum to those from Monte Carlo calculations on a curved crystal structure. Values of about 0.17 and 65 nm are found for the maximum deformation and the length of the distortions in the crystal, respectively, which implies that the lattice distortions extend significantly outside the layer in which the B clusters are supposed to be present.
In-plane electrical transport in n-type selectively doped GaSb/AlGaSb multiquantum wells
Authors: Ghezzi C. , Cioce B. , Magnanini R. , Parisini A.
Published: July 2002

Results are reported regarding in-plane electrical transport in n-type selectively doped GaSb/AlGaSb multiquantum wells. In the samples, which were grown by molecular beam epitaxy, only the central regions of the Al0.40Ga0.60Sb barriers were Te doped. Low-field, low-temperature Hall measurements in the dark demonstrated the presence in the GaSb wells of a degenerate electron gas with nonzero occupancy only for the lowest miniband. A positive persistent photoconductivity effect, related to the DX character of the Te impurity, was also observed. This behavior enabled the μ electron mobility to be measured at T=10 K as a function of the nS sheet carrier density. Since the experimental data were consistent with a dominant role of the interface roughness scattering in the limiting of μ, the height, Δ, and the lateral size, Λ, of the interface roughness were determined from the analysis of the μ=μ(nS) dependence. Acceptable values of Δ were obtained, consistent with results of structural investigations in single quantum well samples of GaSb/Al0.40Ga0.60Sb [E. Kh. Mukhamedzhanov, C. Bocchi, S. Franchi, A. Baraldi, R. Magnanini, and L. Nasi, J. Appl. Phys. 87, 4234 (2000)].
Effects of growth interruption on the optical and the structural properties of InGaN/GaN quantum wells grown by metalorganic chemical vapor deposition
Authors: Cheong M. G. , Yoon H. S. , Choi R. J. , Kim C. S. , Yu S. W. , Hong C.-H. , Suh E.-K. , Lee H. J.
Published: July 2002

Effects of growth interruption on the optical and the structural properties of InGaN/GaN quantum wells were investigated by using photoluminescence, transmission electron microscopy, optical microscopy, and high resolution x-ray diffraction. The InxGa1−xN/GaN (x>0.2) quantum wells used in this study were grown on c-plane sapphire by using metalorganic chemical vapor deposition. The interruption was carried out by closing the group-III metalorganic sources before and after the growths of the InGaN quantum well layers. The transmission electron microscopy images show that with increasing interruption time, the quantum-dot-like regions and well thickness decreased due to indium reevaporation or the thermal etching effect. As a result the photoluminescence peak position was blueshifted and the intensity was reduced. Temperature- and excitation-power-dependent photoluminescence spectra support the results of transmission electron microscopy measurements. The sizes and the number of V defects did not differ with the interruption time. The interruption time is not directly related to the formation of defects. The V defect originates at threading dislocations and inversion domain boundaries due to higher misfit strain.
Solid state supercapacitor materials: Layered structures of yttria-stabilized zirconia sandwiched between platinum/ yttria-stabilized zirconia composites
Authors: Hendriks M. G. H. M. , Heijman M. J. G. W. , van Zyl W. E. , ten Elshof J. E. , Verweij H.
Published: July 2002

Layered structures of cubic yttria-stabilized zirconia (YSZ) sandwiched between two layers of platinum/YSZ composites with the platinum concentration varying between 20 and 55 vol % were prepared. Large double-layer capacitances were obtained due to the presence of large three-dimensional electrode surface areas in the composite layers. The layers were subjected to image analysis obtained from scanning electron microscopy micrographs. The specific surface area of the platinum particles decreased from ∼17×103 to ∼13.5×103 cm2/cm3 with increasing Pt concentration. Image analyses obtained with an in-lens detector revealed that the effective surface area of the Pt particles that were part of a conducting percolative network increased with Pt concentration to ca. 11×103 cm2/cm3 at a Pt metal volume fraction of 0.50. Impedance spectroscopy measurements performed at 550 °C demonstrated that the capacitance increased with increasing Pt concentration in the composite layer and optimal values as high as 4.3±1.3 kF/m2 at 50 vol % Pt were recorded. The specific electrode surface area calculated from this data was in good agreement with the data obtained from image analysis.
Raman scattering study of (GaAs)1−x(Si2)x alloys epitaxially grown on GaAs
Authors: Rodrı́guez A. G. , Navarro-Contreras H. , Vidal M. A.
Published: July 2002

(GaAs) 1−x (Si 2 ) x metastable alloys grown on (001), (110), (112), and (111) GaAs substrates, with Si fractions in the range 0⩽x⩽0.43, were studied by Raman scattering. Two modes near the LO and TO modes of GaAs, besides two local modes associated either with Si–As or Si–Ga and Si–Si bonds, are observed in the Raman spectra. The ratio of integrated intensities of TO-like and LO-like modes (ITO/ILO) is used to evaluate the short range order. It is observed that the zinc blende to diamond transition reported in the literature for these alloys does not influence the Si fraction dependence of the short range order.
Nonresonant nonlinearity due to intersubband transitions in nonparabolic semiconductor quantum wells
Authors: Mukhopadhyay Sanghamitra
Published: July 2002

We have calculated the contribution of intersubband transitions to the nonresonant third-order optical nonlinear susceptibility. It is shown that partial quantum confinement, rather than the band nonparabolicity, is responsible for high values of intersubband nonlinearity. The effects of finite barrier height, energy-band nonparabolicity, and energy-dependent effective mass are included. As examples, we consider InAs/AlSb, GaInAlAs/AlInAs, and GaAs/GaAlAs quantum wells.
Stochastic resonance of elastic string motion
Authors: Dikshtein Igor E. , Polzikova Natalia I. , Kuznetsov Dmitri V. , Schimansky-Geier Lutz
Published: July 2002

A model for the motion of an elastic string is studied numerically and analytically. An elastic string in two dimensions and restricted by two pinning centers is considered. We consider two stable configurations (positively or negatively curved) with pinned ends due to the action of a bistable potential. It is further assumed that the string is driven externally by periodic and white noisy forces. The noise enables the string to flip between the two configurations. The small temporally periodic force synchronizes these flippings and the phenomenon of stochastic resonance is observed. The signal-to-noise ratio (SNR) of the output is investigated and shows a maximum for a nonvanishing intensity of the applied noise. Its dependence on the stiffness of the string is studied. The peak of SNR versus the noise intensity D is found to be more pronounced and to be shifted to small values of D with an increase in the effective stiffness of a string. The calculation is extended to stochastic resonance of vortex motion in a type II superconductor. For vortices restricted by two pinning centers, the characteristic time scale τR relevant to the vortex dynamics is shown to depend crucially on the effective vortex stiffness, with the time τR being extremely small for flexible vortices. Therefore, the effects of noise color on the vortex dynamics should be taken into account in many practical situations.
Optimization of the signal-to-noise ratio for photoreflectance spectroscopy
Authors: Behn U. , Thamm A. , Brandt O. , Grahn H. T.
Published: July 2002

The amplitude and the signal-to-noise ratio of photoreflectance (PR) spectra are experimentally and theoretically investigated as a function of the pump and probe intensity. The model calculations of the PR amplitude and the signal-to-noise ratio based on a simple transport model taking the shot noise of the photodetector as the only noise source confirm the experimentally observed dependencies. Increasing the probe light intensity leads to a decrease of the absolute PR background noise. At the same time, the PR amplitude decreases. This may, in particular for comparable probe and pump intensities, result in a decrease of the signal-to-noise ratio.
Dead space effect in space-charge region of collector of AlGaAs/InGaAs p-n-p heterojunction bipolar transistors
Authors: Yan B. P. , Wang X. Q. , Yang E. S.
Published: July 2002

Hole-initiated avalanche multiplication is investigated using an AlGaAs/InGaAs p-n-p heterojunction bipolar transistor (HBT). Both experimental measurements and theoretical calculation are used to determine the avalanche multiplication factor. A large departure is observed at low electric field when comparison is made between the measured data and theoretical results obtained from the standard ionization model. The comparison shows that the conventional impact ionization model, based on local electric field, substantially overestimates the hole avalanche multiplication factor Mp−1 in the AlGaAs/InGaAs p-n-p HBT, where a significant dead space effect occurs in the collector space-charge region. A simple correction model for the dead space is proposed, that allows the multiplication to be accurately predicted, even in a heavily doped structure. Based on this model, multiplication characteristics for different threshold energy of the hole are calculated. A threshold energy of 2.5 eV was determined to be suitable for describing the hole-initiated impact ionization process.
Electron and hole components of tunneling currents in metal–oxide–semiconductor diodes
Authors: Faigon Adrián N. , Vercik Andrés
Published: July 2002

Electron and hole contributions to tunneling current have been identified through measurements on a simple two terminal Al–SiO2–nSi metal–oxide–semiconductor structure. Under reverse bias, the tunnel current is dominated by hole conduction at low voltage, and electron conduction at high voltage. At 0.9 V across the oxide the contributions are equal.
Spectroscopic ellipsometry and electrical studies of as-grown and rapid thermal oxidized Si1−x−yGexCy films
Authors: Choi W. K. , Feng W. , Bera L. K. , Yang C. Y. , Mi J.
Published: July 2002

Transmission electron microscopy results showed the formation of SiC precipitation in a rapid thermally oxidized (RTO) Si1−x−yGexCy sample with high-C content. The spectroscopic ellipsometry results showed that the E1 gap increased and E2 gap decreased as the C concentration increased. For the oxidized samples, the amplitude of the E2 transitions reduced rapidly and the E1 transition shifted to a lower energy. The reduction in the E2 transitions was due to the presence of the oxide layer. A high-Ge content layer and the low-C content in the RTO films accounted for the E1 shift to lower energy. The electrical measurements showed that RTO at 800 °C did not improve the oxide quality as compared to 1000 °C.
Field-induced sharp ferroelectric phase transition in Sr0.66Ba0.34Nb2O6 relaxor ferroelectric
Authors: Santos I. A. , Garcia D. , Eiras J. A. , Manoel E. R. , Hernandes A. C.
Published: July 2002

Field-induced sharp ferroelectric phase transition was observed in Sr0.66Ba0.34Nb2O6 single crystals. The peaks of the real and imaginary parts of dielectric permittivity arise upon cooling under weak dc bias electric fields and subsequently heating under zero fields or upon cooling under zero fields and subsequently heating under weak electric fields. The applied electric field cancels the random fields that destroy the phase transition and give rise to low temperature domain state if E=0. These results strongly suggest that the SBN system belongs to the random field Ising universality class, a rare case among structural phase transitions.
Magnetic and optical anisotropy of Clostridium pasteurianum rubredoxin from optically detected electron paramagnetic resonance
Authors: Börger Birgit , Suter Dieter
Published: July 2002

The high-spin Fe(III)-center of oxidized rubredoxin from Clostridium pasteurianum shows a complicated, temperature-dependent EPR spectrum. We combine conventional EPR spectroscopy with optically detected EPR (ODEPR) to elucidate the electronic structure of this protein metal center. The ODEPR experiment, which can be considered as coherent Raman scattering or modulated magnetic circular dichroism (MCD), yields spectra that depend on the relative orientation of optical and magnetic dipole moments. A detailed analysis of the spectra shows that they correspond to a zero-field splitting of D=+46.3 GHz and a strong rhombic distortion with E/D=0.25. In the frozen solution, conformational strain gives rise to variation of the rhombicity, which can be measured quantitatively from the EPR line shape. Analysis of the ODEPR line shapes yields the orientation of the optical anisotropy with respect to the magnetic g-tensor. We compare the results from this study to published results on EPR, optical spectroscopy, and MCD.
Temperature-dependent roughness of electronically excited InP surfaces
Authors: Singh J. P. , Singh R. , Mishra N. C. , Kanjilal D. , Ganesan V.
Published: July 2002

Topographical evolution of 100 MeV Au8+-ion-irradiated InP surfaces was studied using atomic-force microscopy (AFM). The surfaces were roughened under dense electronic excitations. Root-mean-square roughness measured from AFM studies showed an exponential saturation behavior with fluence. Sample temperature during irradiation was found as a parameter to control the amount of roughness on the surface and the evolution of irradiated surface topography is discussed in terms of thermal spike model. © 2001 American Institute of Physics.
Noise spectroscopy of randomly modulated atoms
Authors: Vemuri Gautam
Published: July 2002

The coupling between a chromophore and its surroundings can, in the event of environmental perturbations, result in a stochastic modulation of the chromophore’s transition frequency, which in turn can cause fluctuations in the population of the excited state. We demonstrate that by using these fluctuations in population inversion as the spectroscopic variable, one can extract information on whether the random modulation of the chromophore follows a diffusion process or a jump process. The population fluctuations also discriminate between Markov and non-Markov fluctuations of the atomic frequency. In addition, we show that this observable allows, in certain regimes, to unambiguously determine whether the population fluctuations are due to atomic frequency noise, or due to noise in the irradiating laser field. Finally, through a dressed state analysis, we demonstrate that the response of a randomly modulated atom is similar to that of an unmodulated atom driven by a noisy field that has amplitude fluctuations in only one quadrature.
Noncommuting limits in homogenization theory of electromagnetic crystals
Authors: Felbacq D.
Published: July 2002

We study the homogeneous properties of metallic 2D photonic crystals. We give a rigorous proof that the limits when the ratio period over wavelength tends to zero and the conductivity tends to infinity do not commute.
Semiclassical properties and chaos degree for the quantum Baker’s map
Authors: Inoue Kei , Ohya Masanori , Volovich Igor V.
Published: July 2002

We study the chaotic behavior and the quantum-classical correspondence for the Baker’s map. Correspondence between quantum and classical expectation values is investigated and it is numerically shown that it is lost at the logarithmic timescale. The quantum chaos degree is computed and it is demonstrated that it describes the chaotic features of the model. The correspondence between classical and quantum chaos degrees is considered.
Remarks on the lattice Green’s function: The Glasser case
Authors: Sakaji A. , Hijjawi R. S. , Shawagfeh N. , Khalifeh J. M.
Published: July 2002

We have investigated the lattice Green’s function for the Glasser cubic lattice. Expressions for its density of states, phase shift, and scattering cross section in terms of complete elliptic integrals of the first kind are derived.
Development of transferable interaction models for water. I. Prominent features of the water dimer potential energy surface
Authors: Burnham Christian J. , Xantheas Sotiris S.
Published: July 2002

We present an analysis of the morphology of the water dimer potential energy surface (PES) obtained from ab initio electronic structure calculations and perform a quantitative comparison with the results from various water potentials. In order to characterize the morphology of the PES we have obtained minimum energy paths (MEPs) as a function of the intermolecular O–O separation by performing constrained optimizations under various symmetries (Cs, Ci, C2, and C2v). These constitute a primitive map of the dimer PES and aid in providing an account for some of its salient features such as the energetic stabilization of “doubly hydrogen-bonded” configurations for R(O–O)<2.66 Å. Among the various interaction potentials that are examined, it is found that the family of anisotropic site potential (ASP) models agrees better with the ab initio results in reproducing the geometries along the symmetry-constrained MEPs. It is demonstrated that the models that produce closest agreement with the morphology of the ab initio PES, tend to better reproduce the experimental data for the second virial coefficients. We finally comment on the functional forms of simple water models and discuss how effects such as charge overlap can be incorporated into such models.
Asymptotics of bound states and bands for laterally coupled waveguides and layers
Authors: Popov I. Yu.
Published: July 2002

The asymptotics (in the width of windows) of eigenvalues and bands for two-dimensional waveguides and three-dimensional layers coupled through small windows is obtained. The technique is matching of asymptotic expansions of the solutions of boundary value problems.
Hyper-complex four-manifolds from the Tzitzéica equation
Authors: Dunajski Maciej
Published: July 2002

It is shown how solutions to the Tzitzéica equation can be used to construct a family of (pseudo) hyper-complex metrics in four dimensions.
Fedosov quantization on symplectic ringed spaces
Authors: Vaisman Izu
Published: July 2002

We expose the basics of the Fedosov quantization procedure, placed in the general framework of symplectic ringed spaces. This framework also includes some Poisson manifolds with nonregular Poisson structures, presymplectic manifolds, complex analytic symplectic manifolds, etc.
Chain statistics and the changes in the entropy and heat capacity during melt polymerization
Authors: Wang J. , Johari G. P.
Published: July 2002

The various contributions to the configurational entropy, Sconf, and heat capacity have been calculated within the precepts of the polymer chain statistics in the lattice-hole model for a liquid undergoing isothermal polymerization and remaining in an equilibrium state. Out of the five contributions from (i) the fractional occupancy of holes, (ii) coordination number or number of nearest neighbors, (iii) extent of polymerization, (iv) bond flexibility, and (v) entropy of mixing of entities, all but (i) shows a maximum in their plots against the extent of reaction, and of these only (i) and (iv) are temperature dependent. Thus the net Sconf and configurational heat capacity of a polymerizing liquid show a maximum in their plots against the extent of reaction. This maximum varies with the temperature of polymerization. If the polydispersity of a high temperature equilibrium state is frozen-in upon quenching the system to 0 K, the temperature-independent parts of the entropy persists at 0 K, as for isotopic mixtures of elements and chemical compounds. In real systems, this may occur if the liquid is impure or its molecules interact with its container’s wall, thus preventing the formation of an infinitely long chain in the equilibrium state near 0 K. Calculations are also done for the monodispersed system and modifications made to the approximations used in the earlier calculations. These lead to a decrease in Sconf to zero at 0 K for a system with an infinite long polymer chain. The conclusions agree with the experimental finding that the net heat capacity of a polymerizing liquid at a fixed temperature first increases slowly, and then decreases rapidly, thus showing a broad maximum, before configurational freezing vitrifies the melt. It is predicted that if polymerization is done at a sufficiently high temperature so that the ultimately polymerized state is a liquid, the plots of the experimental heat capacity against the extent of polymerization will show a broad asymmetric maximum. Implications of these findings for our current understanding of vitrification and energy landscape models are discussed.
Experiments on ion-acoustic shock waves in a dusty plasma
Authors: Nakamura Y.
Published: July 2002

Dust ion-acoustic shock waves have been investigated experimentally in a homogeneous unmagnetized dusty double-plasma device. An initial compressional wave with a ramp shape steepens to form oscillations at the leading part due to dispersion. The oscillation develops to a train of solitons when the plasma contains no dust grain. The wave becomes an oscillatory shock wave when the dust is mixed in the plasma and the density of the dust grains is smaller than a critical value. When the dust density is larger than the critical value, only steepening is observed at the leading part of the wave and a monotonic shock structure is observed. The velocity and width of the shock waves are measured and compared with results of numerical integrations of the modified Korteweg–de Vries–Burgers equation.
Impurity effect on charge-ordered and ferromagnetic–metallic state in manganese perovskites: Comparison between Cr and Al doping
Authors: Takenaka K. , Okuyama S. , Shiozaki R. , Fujita T. , Sugai S.
Published: July 2002

The role of impurities in melting of the charge-ordered (CO) state in half-doped manganese perovskites is discussed based on the systematic study of Cr and Al doping on the CO state as well as on the ferromagnetic–metallic (FM) state. The CO state in Nd0.5Sr0.5MnO3 is destroyed and the FM state is restored by 2% high-spin (S=3/2) Cr3+, whereas the CO state survives doping of spinless Al3+. On the other hand, the doping effects on the FM state in La1−xSrxMnO3 are almost identical between Cr and Al, except that Cr degrades Curie temperature less. The slight resistance of the CO state against Cr doping can be explained by the magnetic interaction between the Cr and the Mn t2g spins.
Effect of degree of amorphization of Si on the formation of titanium silicide
Authors: Tan C. C. , Lu L. , See A. , Chan L.
Published: July 2002

The mechanisms of the formation of titanium silicide from the reaction of a single thin film of Ti on the silicon substrate with different degrees of substrate amorphization were studied using differential scanning calorimetry. With increased amorphization, an exothermic reaction prior to C49-TiSi2 formation was observed. X-ray diffraction discovered an unidentified phase together with Ti5Si3 and Ti5Si4 that disappeared with subsequent formation of C49-TiSi2 at temperatures above 650 °C. The difference in Gibbs free energy was considered in explaining the formation of Ti5Si3 and Ti5Si4. Based on the Scherrer equation, the resultant C49-TiSi2 grains with higher degrees of silicon substrate amorphization were smaller than those without silicon substrate amorphization.
Comparative intra- versus extra-cavity laser cooling efficiencies
Authors: Heeg Bauke , Rumbles Garry , Khizhnyak Anatoliy , DeBarber Peter A.
Published: July 2002

Due to recent demonstrations of cooling by anti-Stokes fluorescence the optical geometries under which the cooling efficiency can be optimized are investigated. Since the cooling efficiency is proportional to the absorbed power of radiation, and in previously reported cooling experiments a single pass configuration was mostly used, two schemes for enhancing the absorbed power are compared: placing the cooling medium within the laser resonator and multipassing through an externally located medium. The point of departure in this comparative study is the intracavity circulating intensity, described in terms of the laser gain coefficient and the sum total of losses due to reflections, scatter, and absorption due to the presence of a cooling medium. Substituting measured values of the gain and loss factors for a practical cw pumped dye laser system, a comparison in cooling efficiencies between the two schemes is made for a range of optical densities of the cooling medium. The gain and loss coefficients of a dye laser are measured by introducing a varying loss mechanism by means of an acousto-optic modulator inside the cavity. For high optical densities (>0.1) it was found that when extrapolating the pump power to the dye laser up to 10 W the same cooling power can be achieved with an extra-cavity configuration using relatively few passes as with the intracavity configuration. For low optical densities (<0.01) the number of passes required for equivalent cooling power exceeds 10 and the intracavity configuration becomes a more efficient means for laser cooling.
The hydrogen-bonded heterodimer between methylene cyclobutane and hydrogen chloride: Observation of an endo conformer
Authors: Lesarri Alberto , Blanco Susana , López Juan C. , Alonso José L.
Published: July 2002

The formation of a hydrogen bond between methylene cyclobutane and hydrogen chloride has been investigated in the region 8–18 GHz using molecular-beam Fourier transform microwave spectroscopy. The rotational spectrum of an endo conformer in which methylene cyclobutane retains a puckered conformation and HCl is bonded in T-shape to the methylenic group has been observed. Searches for the exo form using Ar, Ne or He as carrier gases were unsuccessful. A Cs symmetry r0-like structure has been obtained for this conformer from the rotational data of three observed isotopomers (C5H8⋯H35Cl, C5H8⋯H37Cl, and C5H8⋯DCl). Ab initio calculations at MP2/6-311+G(d,p) level have been used to complement the analysis. These calculations predict the existence of both endo and exo conformers close in energy.
Contrasting hysteresis behavior between the magneto-optical Kerr rotation and ellipticity in NiFe/FeMn/Co trilayer films
Authors: Kim Sang-Koog , Lee Jeong-Won , Shin Sung-Chul , Kim Kwang Youn
Published: July 2002

We report an experimental observation of strikingly contrasting hysteresis behaviors between the longitudinal magneto-optical Kerr rotation, θK, and ellipticity, εK, in NiFe/FeMn/Co films. For example, one is likely to reproduce a typical magnetic hysteresis loop obtainable with vibrating sample magnetometry while the other one does reproduce magnetoresistance (MR)-like curves in some cases. Interestingly, θK/εK loops form inverse MR curves. The contrasting hysteresis behavior could be explained by the discriminate sensitivity of θK and εK to the ferromagnetic NiFe as well as Co layers. Using a simple model, we determine the sensitivity ratios of the Co to NiFe layer for both signals, which are far from unity as much as θKCo/θKNiFe=−1.4 and εKCo/εKNiFe=8.5. These values may allow one to understand the anomaly of the remarkably contrasting loops observed in NiFe/FeMn/Co trilayer films.
Study of photoluminescence in lead tungstates doped with Pr3+, Sm3+, and Er3+ ions
Authors: Chang K. D. , Lee H. J. , Jang H. S. , Choi K. S. , Lee S. Y. , Choi S. D.
Published: July 2002

Photoluminescence (PL) in PbWO4 single crystals doped with Pr3+, Sm3+, and Er3+ ion, which were grown by the Czochralski method, was studied. By investigating the PL for incident radiation with wavelength of 325 nm (He–Cd Laser), the energy levels of the PL centers were found: The PL levels in the Pr doped single crystal are P3o and D12, that in the Sm-doped one is G45/2, and those in the Er doped one are H211/2, S43/2, and F49/2. In conclusion, the characteristics of these single crystals are governed solely by those of the rare-earth dopants, not by the lead tungstate.
Geometrical confinement of a domain wall in a nanocontact between two NiFe wires
Authors: Miyake K. , Shigeto K. , Mibu K. , Shinjo T. , Ono T.
Published: July 2002

A nanocontact structure (typically 22×34 nm2) between two NiFe wires was fabricated by an electron-beam lithography and a lift-off method, and the magnetoresistance was measured. The magnetization switching process was artificially controlled by engineering the sample geometry to realize a magnetic structure with a single domain wall (DW) trapped in the nanocontact area. This domain structure was confirmed by magnetic force microscopy observations. The magnetization rotation of 180° was realized within the nanocontact area. The contribution of the DW to the resistance was negative, which can be understood on the basis of anisotropic magnetoresistance.
Carrier-enhanced ferromagnetism in Ge1−xMnxTe
Authors: Fukuma Y. , Asada H. , Arifuku M. , Koyanagi T.
Published: July 2002

We have studied the dependence of magnetic properties on the carrier concentration of IV–VI diluted magnetic semiconductor Ge1−xMnxTe prepared by ionized-cluster beam technique. With increasing carrier concentration, the magnetic properties drastically change; saturation magnetization increases and coercive field decreases in hysteresis loop, and the Curie temperature increases. These results obviously show carrier-enhanced ferromagnetic order. The similar effect is also observed in magnetoresistance behavior.
Determination of the occupancy level of the DX center resonant with the conduction band in Te-doped AlGaSb
Authors: Ghezzi C. , Magnanini R. , Parisini A. , Franchi S. , Gombia E. , Mosca R.
Published: July 2002

The analysis of the amplitude of the deep level transient spectroscopy signal due to DX centers is exploited to determine the EDX occupancy level of the DX center in Te-doped AlxGa1−xSb in the range of low values of x where EDX is resonant with the conduction band. We take advantage of a small but still detectable change in the occupancy factor of the DX level induced by the filling pulse. It is shown that EDX is very close to the L conduction band edge for x⩽0.20. This behavior is different from the one at x⩾0.30 where EDX lies in the forbidden energy gap and exhibits an x dependence similar to the X edge. These results are discussed at the light of different atomic configuration for DX centers at an anion-substitutional impurity.
Conformational properties of randomly flexible heteropolymers
Authors: Debnath Pallavi , Cherayil Binny J.
Published: July 2002

Random copolymers made up of subunits with arbritary degrees of flexibility are useful as models of biomolecules with different kinds of secondary structural motifs. We show that the mean square end-to-end distance 〈R2〉 of a two-letter A–B random heteropolymer in which the constituent polymeric subunits are represented as continuum wormlike chains and the randomness is described by the two-state Markov process introduced by Fredrickson, Milner, and Leibler [Macromolecules 25, 6341 (1992)] can be obtained in closed form. The expression for 〈R2〉 is a function of several parameters, including the number n of subunits, the fraction f of one kind of subunit, the persistence lengths lA and lB of the two subunits, and the degree of correlation λ between successive subunits. The variation of 〈R2〉 with each of these parameters is discussed.
Valence band structures of the InAs/GaAs quantum ring
Authors: Li Shu-Shen , Xia Jian-Bai
Published: July 2002

In the framework of effective-mass envelope function theory, the valence energy subbands and optical transitions of the InAs/GaAs quantum ring are calculated by using a four-band valence band model. Our model can be used to calculate the hole states of quantum wells, quantum wires, and quantum dots. The effect of finite offset and valence band mixing are taken into account. The energy levels of the hole are calculated in the different shapes of rings. Our calculations show that the effect of the difference between effective masses of holes in different materials on the valence subband structures is significant. Our theoretical results are consistent with the conclusion of the recent experimental measurements and should be useful for researching and making low-dimensional semiconductor optoelectronic devices.
Analysis of spectral eddy viscosity and backscatter in incompressible, isotropic turbulence using statistical closure theory
Authors: Schilling Oleg , Zhou Ye
Published: July 2002

The spectral eddy viscosity and backscatter viscosity in three-dimensional, incompressible, unforced, nonhelical, isotropic turbulence are decomposed into a sum of contributions corresponding to the Reynolds and cross-stresses, and studied numerically as a function of different assumed kinetic energy spectra. The eddy viscosities and backscatter viscosities are computed using the kinetic energy transfer obtained from the eddy-damped quasinormal Markovian (EDQNM) closure model as a function of k/kc (where kc is the cutoff wave number) using the sharp Fourier cutoff filter. The behavior of the Reynolds and cross-contributions is studied using a Kolmogorov kinetic energy spectrum, a family of spectra with small wave number scaling proportional to k, and a spectrum from an EDQNM calculation that includes both a k4 energy production subrange and a dissipation subrange. The principal results of this theoretical investigation and sensitivity study are (1) the main contributions from the Reynolds and cross-components of the eddy viscosity arise from modes with k/kc≪1 and k/kc≲1, respectively; (2) the contributions from the Reynolds and cross-components of the backscatter viscosity are of the same order, which are nearly zero for k/kc<1 and rise sharply near the cusp k/kc↑1, and; (3) for both the eddy and backscatter viscosity, the Reynolds components are more sensitive to the details of the production subrange than are the cross-components. The implications of these results for subgrid-scale modeling in spectral large-eddy simulations of incompressible, isotropic turbulence are discussed.
Superlattice-like structure for phase change optical recording
Authors: Chong T. C. , Shi L. P. , Qiang W. , Tan P. K. , Miao X. S. , Hu X.
Published: July 2002

In order to increase the crystallization speed and data transfer rate (DTR), a superlattice-like structure (SLL) was applied to the recording layer of phase change optical disks. Unlike the conventional phase change layer, the recording layer with the SLL structure consisted of alternating thin layers of two different phase change materials, i.e., GeTe and Sb2Te3. Although neither GeTe nor Sb2Te3 could be used as a phase change layer material for practical applications, present experimental results revealed that the phase change optical disk with the SLL structure demonstrated an excellent recording property that could meet practical recording requirements. X-ray photoelectron spectroscopy was employed to confirm that the SLL structure could be preserved after many times of melting and quenching. Dynamic properties of the optical recording disk with the SLL structure were investigated with a 1 T pulse duration of 8 ns and a constant linear velocity of 19 m/s. A clear eye pattern was observed. The carrier-to-noise ratio was about 58 dB and a DTR of 47 Mbit/s was achieved. The DTR would be as high as 140 Mbit/s if the blue light is used. It has been proven that the SLL structure is a useful means to increase the DTR of phase change optical recording disks.
Recording characteristics and thermal stability comparisons between antiferromagnetically coupled and conventional media (invited)
Authors: Choe G. , Zhou J. N. , Weng R. , Johnson K. E.
Published: July 2002

We report on the recording performance and thermal stability of antiferromagnetically coupled (AFC) media with varying top and bottom layer thickness as compared with those of highly oriented conventional longitudinal recording media. AFC media offers the expected merit of low Mrt designs such as narrow pulse width (PW50), while maintaining good thermal stability. However, the magnetostatic field resulting from thicker total magnetic thickness in AFC media results in poor overwrite (OW) and a degradation in nonlinear transition shifts (NLTS). Thermal stability of AFC media is mainly determined by intrinsic coercivity of the top layer plus the AF exchange field and the magnetostatic field acting on the top layer through a Ru layer. Low noise conventional media with high orientation ratio (OR) can be produced with an acceptable thermal decay equivalent to AFC media at Mrt values above 0.25 memu/cm2.