bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational spectroscopic investigation of carbonyl sulfide solvated with helium atoms
Authors: Xu Yunjie , Jäger Wolfgang
Published: August 2003

Rotational spectra of small- to medium-sized HeN–OCS clusters with N from 2 to 8 were investigated using a pulsed molecular beam Fourier transform microwave spectrometer and a microwave-microwave double resonance spectrometer. Spectra of five to eight isotopomers were measured for each HeN–OCS cluster. Nuclear quadrupole hyperfine structures were observed for S33 and O17 containing isotopomers. The isotopic data, together with the microwave-microwave double resonance experiments, were used to establish unambiguous assignments for these clusters. The assignment of N, the number of solvating helium atoms in a particular cluster, was supported by the observed intensity change under different backing pressure and nozzle cooling conditions and in particular by the systematic infrared vibrational band shifts from the concurrent infrared study. This sequence of clusters contains some of the largest clusters that have thus far been investigated with high-resolution spectroscopic techniques. This study provides detailed and important information for the understanding of the quantum solvation process of dopant molecules in helium nanodroplets.
High-resolution infrared spectra of carbonyl sulfide solvated with helium atoms
Authors: Tang Jian , McKellar A. R. W.
Published: August 2003

Infrared spectra of HeN–OCS clusters with N up to about 20 have been studied in the 2062 cm−1 region of the O–C stretching vibration using a tunable diode laser spectrometer to probe pulsed supersonic expansions from moderately high-pressure (<35 atm) cooled (>−60 °C) jet sources. Resolved rotation-vibration transitions have been assigned for the clusters with N=2–8, and these assignments have been confirmed in detail by observations of the corresponding pure rotational transitions in the microwave region. The vibrational frequencies (band origins) were observed to move to higher frequencies (blueshift) for N=2–5, continuing the shift previously observed for the binary complex, He–OCS. Then, for N=6–8, the vibrational frequency moves back in the direction of lower frequencies (redshift), leading towards the limiting redshift previously observed in larger helium nanodroplets with N≈103–104. These vibrational shifts are consistent with a model in which the first five helium atoms fill a “ring” around the “equator” of the OCS molecule, with the subsequent heliums then taking positions closer to the ends. The cluster rotational constants decrease monotonically (increasing rotational moments of inertia) for N=1–8, falling below the value previously observed for the “free” rotation of OCS in helium nanodroplets. Strong, sharp spectral lines from clusters in the size range N≈9–20 were also observed, but not assigned. This difficulty in extending the analysis for N>8 may be related to the onset of partially free internal rotation of the OCS within the helium cluster, which in turn has interesting implications for the nature of superfluid-type behavior in this finite scale system.
Short-range order in linear systems
Authors: Võ T. T. M. , Chen L.-J. , Robert M.
Published: August 2003

For linear systems in the continuum in which the particles interact with nearest-neighbor forces, the pair distribution function at short distances can be expressed exactly in a simple form. Results are given for various hard-core interaction potentials, in which the attractive potential is either constant (square-well potential), linear, or V-shaped. The second form is related to the depletion interaction.
Micellization behavior of π-shaped copolymers in a selective solvent: A Brownian dynamics simulation approach
Authors: Kim Kwang Hee , Kim Seung Hyun , Huh June , Jo Won Ho
Published: August 2003

Micellization for various types of π-shaped copolymers in a selective solvent (poor for backbone chain and good for graft chain) is investigated by using Brownian dynamics simulation. The architecture of starblock, regular π-shaped, and triblock copolymers are described in terms of the distance between junction points made by two branches and their micellization behaviors are compared with each other. It is observed that the critical micelle concentration (cmc) of π-shaped copolymers increases in the order of starblock<triblock<regular π-shaped copolymer. The micellar properties reveal that micelle formation of regular π-shaped copolymer is more difficult than that of other π architectures. These simulation results qualitatively agree with the theoretical prediction formulated by the simple mean field theory.
N
-reference, M-state coupled-cluster method: Merging the state-universal and reduced multireference coupled-cluster theories
Authors: Li Xiangzhu , Paldus Josef
Published: August 2003

We propose a generalization of the reduced multireference coupled-cluster method with singles and doubles (CCSD) to the genuine MR, state-universal (SU) CC approaches. Two key ingredients of this generalization are (i) the algorithm for the cluster analysis of general MR configuration interaction (CI) wave functions that is based on the SU cluster ansatz of Jeziorski and Monkhorst, and (ii) the formulation of the SU CC method employing a general (incomplete) model space. These recent developments enable us to employ modest size MR CISD wave functions that are based on an N-dimensional reference space M1 as a source of higher-than-pair-cluster amplitudes in the externally corrected SU CCSD method that is based on an M-dimensional model space M0, forming a subspace of M1. An appropriate choice of M0 and M1 makes it then possible to avoid the most severe intruder-state problems. The method is illustrated on the often-investigated H4 and H8 model systems.
The general-model-space state-universal coupled-cluster method exemplified by the LiH molecule
Authors: Li Xiangzhu , Paldus Josef
Published: August 2003

The salient features of the recently introduced general-model-space (GMS) state-universal (SU) coupled-cluster (CC) method are illustrated on the case of the LiH molecule. Describing the breaking of the Li–H bond by relying on an open-shell-type GMS reveals the importance of the connectivity conditions (C conditions), which represent a crucial new ingredient of the GMS SU CC theory. Only when we properly account for these C conditions can we uniquely represent the full configuration interaction (FCI) wave functions in terms of the multireference SU exponential cluster ansatz and recover the FCI energies via the GMS SU CC method, assuming that all the relevant clusters at a given level of the theory are considered. Drawing on various GMSs, we compute the potential energy curves for three Σ+1, two Σ+3, three Π,1 and three Π3 states, using the GMS SU CC method truncated at the singly- and doubly-excited level (GMS SU CCSD), as well as the externally corrected (N,M)-CCSD method that exploits the NR-CISD wave functions as the external source of higher-than-pair clusters in the MR SU CCSD method. In all cases we obtain excellent results: For Σ+ states, the maximum difference between the FCI and various SU CCSD energies is about 0.5 millihartree. These errors are further reduced when we employ the (N,M)-CCSD methods. For the Π states, the deviations of the SU CCSD energies relative to FCI amount to at most a few hundreds of a millihartree. We also report on the size-extensivity tests and the exactness of the formalism for two-electron systems.
The relationships between the electronic structures and the normal and inverse isotope effects in the electron–phonon interactions in charged cubane
Authors: Kato Takashi , Yamabe Tokio
Published: August 2003

Electron–phonon coupling and possible normal and inverse isotope effects in the monoanion and cation of fully deuterated cubic cluster such as deutero-cubane (CD)8 are studied. The calculational results for charged deutero-cubane are compared with those for charged cubane. The calculated total electron–phonon coupling constants for the monoanion (lLUMO) and cation (lHOMO) of deutero-cubane are 0.631 and 0.777 eV, respectively. The lLUMO value increases much more significantly than the lHOMO value as a consequence of deuteration in cubane. Our calculational results show that inverse (normal) isotope effects as a consequence of full deuteration can be expected in the monoanion (monocation) of cubane. Significant phase patterns difference between the t1u lowest unoccupied molecular orbitals (LUMO) rather localized on carbon atoms and delocalized t2g highest occupied molecular orbitals (HOMO), and the larger displacements of carbon atoms in the Eg mode of 1072 cm−1 (ωD6) as a consequence of deuteration are the main reason for these results. The general relationships between the electronic structures and the normal and inverse isotope effects in superconductivity in charged molecular systems are discussed. We find from our calculations that inverse and no isotope effects as well as normal isotope effects are possible to be observed in molecular superconductivity if we assume that molecular superconductivity is caused by the electron–phonon interactions; the normal isotope effect in superconductivity would be observed when the atoms, the electron density on which is higher, are substituted by their heavier isotopes, while inverse and no isotope effects as well as normal isotope effects would be observed when the atoms, the electron density on which is lower, are substituted by their heavier isotopes. But the possibility that inverse isotope effect is observed is high in the latter case. Therefore, the electronic structures as well as the molecular weights are closely related to the isotope effects.
An empirical potential energy surface for He–Cl2(B 3Πu) based on a multiproperty fit
Authors: Garcı́a-Vela A.
Published: August 2003

An empirical interaction surface for the He–Cl2(B 3Πu) complex based on additive pairwise potentials is reported. A novelty of the present surface with respect to previous empirical potentials is that a dependence on the Cl–Cl separation is introduced in some of the potential parameters, which makes more flexible the analytic form used. The parameters of the surface are adjusted in order to reproduce the available data for several properties such like spectral blueshifts, predissociation lifetimes, and Cl2 product state distributions. The fitted surface yields very good agreement with experiment for most of the properties measured. As compared with previous surfaces, the present potential is found to improve significantly on the description of the vibrational dependence of the lifetime, in all the range of vibrational excitations probed by the experiment. In previous works this property has revealed difficult to describe accurately for He–Cl2(B). For the remaining properties, the current surface provides a level of accuracy as good (or better in some cases) as the best one achieved by earlier interaction potentials. The features of the proposed potential surface and its range of validity are discussed. It is found that validity of the potential is essentially limited to the range of vibrational levels probed experimentally.
Optimal reduced dimensional representation of classical molecular dynamics
Authors: Dey Bijoy K. , Rabitz H. , Askar Attila
Published: August 2003

An optimal reduced space method for capturing the low-frequency motion in classical molecular dynamics calculations is presented. This technique provides a systematic means for carrying out reduced-dimensional calculations in an effective set of reduced coordinates. The method prescribes an optimal reduced subspace linear transformation for the low frequency motion. The method is illustrated with a dynamics calculation for a model potential, where the original six-dimensional space is reduced to two (three) dimensions, depending on the desired frequency cutoff value.
Vibrational analysis of small Hn+ hydrogen clusters
Authors: Barbatti M. , Nascimento M. A. C.
Published: August 2003

Anharmonic vibrational frequencies are calculated for the Hn+ clusters (n=5,7,9) employing the correlation-corrected vibrational self-consistent field method. The present results confirm previous experimental assignments and also allow us to assign the 6690 cm−1 band of the H5+ cluster. The results explain the lack of features in the photodissociation spectra of the H7+ and H9+ clusters, in the region of 3000–3600 cm−1. The calculation also provides enough reliable data for the identification of these clusters in hydrogen atmospheres. The nature of the excitations near 4000 cm−1 in the vibrational spectra of the H7+ and H9+ clusters and their implication in the mechanism of photodissociation of these cluster are also discussed.
Theoretical simulations on photoexcitation dynamics of the silver atom embedded in helium clusters
Authors: Wada Akira , Takayanagi Toshiyuki , Shiga Motoyuki
Published: August 2003

Quantum molecular dynamics simulations have been performed to study the photoexcitation Ag(5p 2PJ)←Ag(5s 2S1/2) in size-selected helium clusters considering electronically nonadiabatic transitions. We employed the hybrid method in which the electronic degree of freedom of Ag(2P) was treated quantum mechanically while the motions of helium atoms were described by the semiclassical path integral centroid molecular dynamics method in order to take the quantum fluctuation effect into account. It has been found that the dynamics after photoexcitation is dominantly nonadiabatic in all cluster sizes studied, and that nonadiabatic transitions are enhanced by quantum fluctuation of helium motions. Most of the photoexcited AgHen clusters decompose into an isolated Ag atom and free helium atoms within several picoseconds. However, Ag*Hen (n=1–5) exciplex formation was also found to occur for all helium cluster sizes studied, although this process is found to be minor.
Photodissociation of methyl iodide clusters in the A-band excitation: Photofragmentation excitation spectra of (CH3I)n by ultraviolet pump-CRD probe measurement
Authors: Ito Fumiyuki , Nakanaga Taisuke
Published: August 2003

Methyl iodide clusters (CH3I)n in a supersonic jet are photolyzed in an excitation of the valence A band with a tunable ultraviolet light source, and the iodine molecule (I2) thus produced is detected by cavity ring-down (CRD) spectroscopy. A production yield of I2 is measured in the region of 245–275 nm to obtain photofragment excitation (PHOFEX) spectra of (CH3I)n. The PHOFEX spectra show maximum at 250 nm (∼1000 cm−1 blueshifted from monomer) at low concentration, and a new peak at 260–265 nm (∼450 cm−1 redshifted) at high concentration. These two peaks are assigned to dimer and higher cluster (hexamer to decamer), respectively, based on our infrared-CRD study [Chem. Phys. 286, 337 (2003)]. The redshifted peak of the higher clusters indicates that perturbation of the excited state due to intermolecular interaction is stronger than in the dimer.
Exclusion surfaces for molecules in argon and helium
Authors: Bentley John
Published: August 2003

Molecular exclusion surfaces [J. Bentley, J. Phys. Chem. A 104, 9630 (2000)] have been determined for a number of molecules and molecular ions interacting with argon or helium atoms. These surfaces represent the boundaries between the molecules and their environment and have application, for instance, to continuum solvation models. To make these surfaces useful for such applications, we present some simple models for exclusion surfaces which depend only on properties of the electron densities of the isolated molecules.
Optimized deployment of heat-activated surgical staples using thermography
Authors: Ng Y. , Song C. , McLean D. , Shimi S. M. , Frank T. G. , Cuschieri A. , Campbell P. A.
Published: August 2003

We have developed a suture analogue, suitable for use in laparoscopic surgery, in the form of a staple constructed from NiTi shape memory alloy (SMA). Closure of the staple is effected by resistive heating via a (50–100 ms) pulse of electrical current (≈5 A). In order to optimize deployment protocols and minimize thermal collateral damage to tissue, the heat sink effect of the electrical contact rails, as well as the presence of contact hotspots must be considered. Here, we have employed high-resolution thermal imaging to observe the dynamic temperature distributions in SMA staples as a function of the pulse parameters. This has facilitated process optimization and also provided data from which to validate computational finite-element models of the heat transport phenomena.
Spatiotemporal stochastic forcing effects in an ensemble consisting of arrays of diffusively coupled Lorenz cells
Authors: Lorenzo Maria Nieves , Santos Miguel A. , Pérez-Muñuzuri Vicente
Published: August 2003

Spatiotemporal stochastic forcing of an ensemble system consisting of chaotic Lorenz cells diffusively coupled is analyzed. The nontrivial effects of time and length correlations on the ensemble mean error and spread are studied and the implications to new trends in weather forecast methodologies are discussed. A maximum for the forecast scores is observed to occur for specific values of time and length correlations. This maximum is studied in terms of an interplay between the natural scales occurring in the system and the noise parameters.
Dynamic synchronization and chaos in an associative neural network with multiple active memories
Authors: Raffone Antonino , van Leeuwen Cees
Published: August 2003

Associative memory dynamics in neural networks are generally based on attractors. Retrieval based on fixed-point attractors works if only one memory pattern is retrieved at the time, but cannot enable the simultaneous retrieval of more than one pattern. Stable phase-locking of periodic oscillations or limit cycle attractors leads to incorrect feature bindings if the simultaneously retrieved patterns share some of their features. We investigate retrieval dynamics of multiple active patterns in a network of chaotic model neurons. Several memory patterns are kept simultaneously active and separated from each other by a dynamic itinerant synchronization between neurons. Neurons representing shared features alternate their synchronization between patterns, thus multiplexing their binding relationships. Our model includes a mechanism for self-organized readout or decoding of memory pattern coherence in terms of short-term potentiation and short-term depression of synaptic weights.
Temperature dependent spectroscopic studies of the electron delocalization dynamics of excited Ce ions in the wide band gap insulator, Lu2SiO5
Authors: van der Kolk E. , Basun S. A. , Imbusch G. F. , Yen W. M.
Published: August 2003

Electron delocalization processes of optically excited states of Ce3+ impurities in Lu2SiO5 were investigated by means of a temperature and spectrally resolved photoconductivity study. By monitoring separately the strength of the photocurrent resulting from excitation into each of the Ce3+ 5d absorption bands, over a broad temperature region, three different delocalization processes, namely direct photoionization, thermal ionization, and tunneling, have been identified. The relative probabilities and temperature dependencies of each of these processes are discussed. The observed exponential temperature increase in the photocurrent, which spans six orders of magnitude, allows for the exact placement of the lowest energy 5d levels of the Ce3+ ions within the band gap. For Lu2SiO5:Ce3+, the lowest 5d state is determined to be 0.45 eV below the conduction band edge.
Nanoheteroepitaxy of GaN on a nanopore array Si surface
Authors: Liang J. , Hong S.-K. , Kouklin N. , Beresford R. , Xu J. M.
Published: August 2003

We report the growth by molecular beam epitaxy and the optical characterization of GaN films nucleated on a Si(111) surface that has been patterned by dry etching an ordered array of nanometer-scale pores prior to the growth. The etching is performed using an anodized aluminum oxide membrane as a mask. The nanopore array surface with the pore diameter of 60 nm and periodicity of 110 nm exhibits significant effects on emissivity and the optical properties of the resulting film. Room-temperature photoluminescence intensity increases by a factor of 5 for GaN grown on nanoporous Si. Peak shifts in photoluminescence and Raman spectroscopy suggest that the material grown on nanopores may be more relaxed than films grown on flat substrates. The effects of nanopore topography on the nucleation of GaN films offer a potential path to significant improvement of III-nitride heteroepitaxy for device applications.
Electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy of nitric oxide
Authors: Hanna Sherif F. , Kulatilaka Waruna D. , Arp Zane , Opatrný Tomas , Scully Marlan O. , Kuehner Joel P. , Lucht Robert P.
Published: August 2003

A dual-pump, electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy (CARS) technique for the measurement of minor species concentrations has been demonstrated. The frequency difference between a visible Raman pump beam and Stokes beam is tuned to a vibrational Q-branch Raman resonance of nitric oxide (NO) to create a Raman polarization in the medium. The second pump beam is tuned into resonance with rotational transitions in the (1,0) band of the A2Σ+–X2Π electronic transition at 236 nm, and the CARS signal is thus resonant with transitions in the (0,0) band. We observe significant resonant enhancement of the NO CARS signal and have obtained good agreement between calculated and experimental spectra.
Transition from island formation to pseudomorphic growth in the submonolayer CdSe/ZnSe multilayer system
Authors: Kim M. , Furdyna J. K. , Dobrowolska M. , Lee S. , Cheon M. , Luo H.
Published: August 2003

Transmission electron microscopy and photoluminescence measurements have been performed on samples with submonolayer (0.3 up to 0.9 ML) CdSe insertions in ZnSe in both single- and multilayer geometries. We observe a significant difference in Cd distribution in the layer plane between these two geometries for the same Cd coverage. While Cd-rich islands can be seen for a single layer of Cd, a layered growth is achieved in the multilayer samples. This effect is attributed to the reduction of lattice mismatch in the multilayer samples due to a high level of diffusion of Cd atoms along the growth direction.
Convergence of hot-carrier-induced saturation-region drain current and linear-region drain current degradation in advanced n-channel metal–oxide–semiconductor field-effect transistors
Authors: Chen Jone F. , Tsao Chih-Pin
Published: August 2003

The relationship between hot-carrier-induced drain current degradation and characterization drain voltage is investigated in n-channel metal–oxide–semiconductor field-effect transistors fabricated using 0.18 μm technology. The results show that the maximum drain current degradation occurs at a characterization drain voltage that is higher than 0.1 V. This finding is attributed to two competing mechanisms as the characterization drain voltage increases: the reduction in channel inversion charges and the reduction in charged interface states. The convergence of the saturation-region drain current and linear-region drain current degradation is more evident when the device is under stress at higher temperature and forward body bias.
Observation of MnP magnetic clusters in room-temperature ferromagnetic semiconductor Zn1−xMnxGeP2 using nuclear magnetic resonance
Authors: Hwang Taesoon , Shim Jeong Hyun , Lee Soonchil
Published: August 2003

We investigated chalcopyrite Zn1−xMnxGeP2 polycrystals, which have been reported as a room-temperature ferromagnetic semiconductor, with Mn concentrations of x=0.08 and 0.15 using Mn55 and P31 nuclear magnetic resonance spectroscopy. The samples were made by the same process and showed the same crystallographic and magnetic behavior as in the previous report, but the experimental results indicated that more than 90% of Mn atoms were in a MnP impurity phase and the MnP cluster size was tens of nanometers. No evidence of Mn atom substitution in the host ZnGeP2 lattice was observed and the magnetic property of Zn1−xMnxGeP2 was determined to be that of the MnP magnetic clusters, at least in the bulk. The inconsistency of the conclusions with x-ray diffraction data is a result of the weak crystallinity of MnP phase.
Super-resolution by elliptical bubble formation with PtOx and AgInSbTe layers
Authors: Kim Jooho , Hwang Inoh , Yoon Duseop , Park Insik , Shin Dongho , Kikukawa Takashi , Shima Takayuki , Tominaga Junji
Published: August 2003

The recording and retrieval of signals below 100 nm mark length were attempted with elliptical bubble-type super-resolution technology with platinum oxide (PtOx) and ductile AgInSbTe layers, using the same optical system as that of a digital versatile disk (a 635 nm wavelength red laser system). The carrier-to-noise ratio (CNR) of over 47 dB for 100 nm mark length signals (over 43 dB for 80 nm mark length signals) was obtained, which can be considered as a commercially acceptable level of CNR. The recording mechanism of the sample disk was shown through the transmission electron microscopy cross-section image observation to be by rigid elliptical bubble formation at the PtOx layer located between the AgInSbTe layers. The results of this report represent the potential for a much higher-density storage using the red laser system and a subterabyte optical storage using the blue laser system.
X-ray photoemission electron microscopy investigation of magnetic thin film antidot arrays
Authors: Heyderman L. J. , Nolting F. , Quitmann C.
Published: August 2003

Periodic square arrays of antidots in 10 nm-thick cobalt films have been investigated with antidot periods, p, ranging from 2 μm down to 200 nm and various ratios of antidot size to antidot separation, w/d. For p=2 μm, the extent of modification of the thin film magnetic domain structure increases with increasing w/d, forming domains pinned diagonally between antidots for w/d⩾0.2 and resulting in a two-dimensional periodic checked domain contrast commensurate with the antidot lattice for w/d⩾0.9. As p is decreased while maintaining d≈w, we observe a dramatic change in the magnetic domain configuration at p=400 nm resulting in chains of magnetic domains running parallel to the intrinsic hard axis and with lengths corresponding to a multiple of the antidot period.
Electrostrictive and dielectric properties of stretched poly(vinylidene fluoride–trifluoroethylene) copolymers at cryogenic temperatures
Authors: Ang Chen , Yu Zhi , Cross L. E.
Published: August 2003

The electrostrictive property and dielectric behavior of the stretched poly (vinylidene fluoride–trifluoroethylene) copolymers, denoted as P(VDF–TrFE), have been studied at cryogenic temperatures. Polarization hysteresis loops and high electrostrictive performance (strain ∼1%) are obtained at room temperature. An obvious dielectric relaxation process is observed and the relaxation rate follows the Vogel–Fulcher relation with TVF=160 K, U=0.18 eV, ν0=5.7×1012 Hz. The dielectric relaxation behavior and high strain level decrease quickly with decreasing temperature, and below 160 K only a small strain level (∼0.1%) and an “intrinsic” dielectric constant sustains. The correlation of dielectric behavior and strain activity and the physical mechanism of the quick suppression of the strain with decreasing temperature are briefly discussed. The results indicate that a high dielectric constant is critical for obtaining high electrostrictive strains in the polymers at a reasonable electric field.
Antiferromagnetic interlayer exchange coupling across epitaxial, Ge-containing spacers
Authors: Gareev R. R. , Bürgler D. E. , Schreiber R. , Braak H. , Buchmeier M. , Grünberg P. A.
Published: August 2003

We give experimental evidence of antiferromagnetic interlayer exchange coupling of Fe(001) layers across epitaxial, Ge-containing spacers consisting of either Ge wedges embedded between two Si boundary layers or Si–Ge-multilayers. The coupling strengths are of the order of 1 mJ/m2 and decay on a length scale below 2 Å as determined from magneto-optic Kerr effect and Brillouin light scattering. The coupling evolves with the spacer thickness from ferromagnetic to prevailing 90° or antiferromagnetic for Ge wedges and Si–Ge multilayers, respectively. The bilinear coupling is comparable in both cases, but the biquadratic contribution is suppressed for Si–Ge-multilayer spacers. Thus, Si–Ge-multilayer spacers give rise to perfect antiparallel alignment of the Fe film magnetizations.
Near-field scanning optical microscopy of standing waves in fiber Bragg gratings
Authors: Gates J. C. , Mills J. D. , Brocklesby W. S.
Published: August 2003

Near-field scanning optical microscopy (NSOM) can provide direct information about the electric fields inside optoelectronic devices with subwavelength resolution. This letter describes direct, wavelength-resolved measurements of the amplitude and phase of the standing waves within a fiber Bragg grating, using a heterodyne interference variant of NSOM that works at telecommunications wavelengths. The amplitudes of forward- and backward-going components of the standing wave can be measured separately, and the position shift of the standing wave antinodes as the wavelength varies across the stop band is imaged directly.
Hidden resonant excitation of photoluminescence in bilayer arrays of InAs/GaAs quantum dots
Authors: Mazur Yu. I. , Wang Z. M. , Salamo G. J. , Xiao Min , Tarasov G. G. , Zhuchenko Z. Ya. , Masselink W. T. , Kissel H.
Published: August 2003

Photoluminescence (PL) of self-organized quantum dots (QDs) in bilayer InAs/GaAs structures is studied with a fixed seed layer and spacer, but variable second-layer coverage. Careful line shape analysis reveals modulation in the high-energy tail of the seed-layer PL spectrum. The oscillation-like behavior is reproducible with variations in both the temperature and optical excitation energy. These oscillations are attributed to carrier relaxation through inelastic phonon scattering from the wetting layer to the QD excited states.
FePt (001) texture development on an Fe–Ta–C magnetic soft underlayer with SiO2/MgO as an intermediate layer
Authors: Zhang Z. G. , Kang K. , Suzuki T.
Published: August 2003

In order to develop the FePt (001) texture on an Fe–Ta–C magnetic soft underlayer (SUL), a combined SiO2/MgO intermediate layer is used between the SUL and FePt/MgO laminated recording layer. The function of the amorphous SiO2 layer is to cut the epitaxial growth relationship between the SUL and recording layer and to establish the MgO (200) texture. The thin FePt layers are epitaxially grown on the MgO layers and transformed into (001)-textured L10 FePt films upon annealing. Increasing the surface roughness of the Fe–Ta–C SUL from 0.74 to 2.35 nm does not significantly affect the FePt (001) texture, but has an effect on the FePt grain c-axis dispersion and the FePt L10 phase ordering.
Properties of lanthanum-doped Bi4Ti3O12–SrBi4Ti4O15 intergrowth ferroelectrics
Authors: Zhu Jun , Chen Xiao-Bing , Lu Wang-Ping , Mao Xiang-Yu , Hui Rong
Published: August 2003

Lanthanum-doped intergrowth ferroelectrics, (Bi,La)4Ti3O12–Sr(Bi,La)4Ti4O15 [BLT–SBLT(x)], have been prepared. It is found that the remnant polarization (2Pr) of Bi4Ti3O12–SrBi4Ti4O15 (BIT–SBTi) is enlarged. The 2Pr of BLT–SBLT(0.50) has a maximum value of 25.6 μC/cm2. Assuming that the Curie temperature of BLT–SBLT(x) is the average value of the two constituents, La content in both BLT and SBLT can be estimated on the basis of the fact that the crystal structure of BLT–SBLT(x) is not affected by doping. The result shows that when x is not larger than 1.25, La content in BIT is higher than that in SBTi, but when x is 1.50, La content in SBTi is higher. It coincides with the variation of d(118) and the relaxation characteristics of BLT–SBLT(1.50).
Electrical conductivity of a single conducting polyaniline nanotube
Authors: Long Yunze , Chen Zhaojia , Wang Nanlin , Ma Yongjun , Zhang Ze , Zhang Lijuan , Wan Meixiang
Published: August 2003

We report the electrical properties of a single conducting polyaniline nanotube measured by a standard four-terminal technique. Camphor sulfonic acid doped polyaniline nanotubes were self-assembled by a template-free method. The directly measured conductivity of the single polyaniline nanotube is very high (∼31.4 S/cm), and its temperature dependence follows the three-dimensional variable range hopping model. However, the bulk conductivity of the polyaniline nanotube pellets is much smaller than the nanotube itself (only 3.5×10−2 S/cm) and ln ρ(T) is linear in T−1/2, which is due to the large intertubular contact resistance. These results will help us to understand the conduction mechanism in conducting polymers.
Unidirectional electron flow in a nanometer-scale semiconductor channel: A self-switching device
Authors: Song A. M. , Missous M. , Omling P. , Peaker A. R. , Samuelson L. , Seifert W.
Published: August 2003

By tailoring the boundary of a narrow semiconductor channel to break its symmetry, we have realized a type of nanometer-scale nonlinear device, which we refer to as self-switching device (SSD). An applied voltage V not only changes the potential profile along the channel direction, but also either widens or narrows the effective channel depending on the sign of V. This results in a diode-like characteristic but without the use of any doping junction or barrier structure. The turn-on voltage can also be widely tuned from virtually zero to more than 10 V, by simply adjusting the channel width. The planar and two-terminal structure of the SSD also allows SSD-based circuits to be realized by only one step of lithography.
Pulsed-laser vibrometer using photoelectromotive-force sensors
Authors: Rodriguez Ponciano , Trivedi Sudhir , Jin Feng , Wang Chen-Chia , Stepanov Serguei , Elliott Greg , Meyers James F. , Lee Joseph , Khurgin Jacob
Published: August 2003

We demonstrate experimentally significant improvement in the sensitivity of photoelectromotive-force (photo-EMF) laser vibrometers using pulsed-light sources. The vibrating surface is discretely sampled by individual laser pulses and recorded by the photo-EMF sensor via the generation of photocurrent pulses whose magnitudes are proportional to the instantaneous surface displacements. With a sufficiently high sampling rate, reconstruction of the vibration wave form can be achieved by conducting envelope (or peak) detection of the resultant series of photocurrent pulses. Significantly higher peak optical power levels of the probe laser pulses, which can be orders of magnitude greater than those of continuous-wave interrogation lasers with the same average power, lead to proportional enhancement in the photo-EMF response and remarkable improvement in detection sensitivity when the photodetection process is initially amplifier noise current limited.
Morphology evolution during strained (In,Ga)As epitaxial growth on GaAs vicinal (100) surfaces
Authors: Wang Z. M. , Shultz J. L. , Salamo G. J.
Published: August 2003

Molecular-beam-epitaxy growth of strained (In,Ga)As on GaAs vicinal (100) surfaces is investigated by scanning tunneling microscopy. Surface roughing as the consequence of step bunching driven by strain is explored. By tuning the In content over the range from 0.05 to 0.2, the step bunching is observed to exhibit considerable uniformity and order. These results experimentally demonstrate that strain-driven step bunching is a viable approach to provide templates for nanostructure growth.
Chaotic itinerancy
Authors: Kaneko Kunihiko , Tsuda Ichiro
Published: August 2003

Chaotic itinerancy is universal dynamics in high-dimensional dynamical systems, showing itinerant motion among varieties of low-dimensional ordered states through high-dimensional chaos. Discovery, basic features, characterization, examples, and significance of chaotic itinerancy are surveyed.
Optical parametric spectral broadening of picosecond laser pulses in β-barium borate
Authors: Lee Seung Mook , Rhee Bum Ku , Choi Moongoo , Park Seung-Han
Published: August 2003

We present that optical parametric luminescence generated near the degenerate point of a type I optical parametric tuning curve in β-BaB2O4 can be used as a spectrally broadened probe beam for a picosecond pump-probe spectroscopy. In particular, we show that it is possible to change the position of the central peak and spectral range in visible and infrared wavelengths by varying the incident angle of pump pulse with respect to the optical axis of barium borate crystal. The picosecond time-resolved diffuse-reflectance experiment is also performed in order to demonstrate usefulness of these spectrally broadened pulses as a probe source.
In-stream uptake dampens effects of major forest disturbance on watershed nitrogen export
Authors: Bernhardt E. S. , Likens G. E. , Buso D. C. , Driscoll C. T.
Published: September 2003

Between January 4 and 10, 1998, a severe ice storm impacted large areas of
northern New York, New England, and eastern Canada. This storm struck the
Hubbard Brook Experimental Forest in New Hampshire on January 7-8, 1998, and
caused extensive forest crown damage (>30%) in a narrow elevation band
(600-740 m) across the south-facing experimental watersheds. Stream water has
been collected and chemically analyzed since 1963 in six experimental
watersheds at Hubbard Brook Experimental Forest; thus, we were able to examine
the effect of this severe natural disturbance on watershed nutrient export and
changes in instream nitrate (NO
3
-
) processing. The ice
storm caused large increases in watershed export of NO
3
-
for 2 years after the disturbance, but our examination of in-stream processing
suggests that NO
3
-
losses would have been much more
dramatic had there not been an increase in in-stream, nitrogen-processing
efficiency after the ice storm. The canopy damage that resulted from the ice
storm led to increased light availability and large inputs of woody debris to
the stream. We suspect that increases in algal production and storage and
processing of terrestrial litter account for the increase in inorganic
nitrogen processing in these streams. Our results indicate that, without
in-stream processing, export of NO
3
-
from the damaged
watersheds would have been 80-140% higher than was observed. These results
point to an intriguing negative feedback mechanism whereby the same
disturbance that causes watershed NO
3
-
loss may
simultaneously lead to increased in-stream retention and transformation.
Dielectrophoretic dynamic light-scattering (DDLS) spectroscopy
Authors: Halaka Folim G.
Published: September 2003

Dielectrophoretic dynamic light-scattering (DDLS) spectroscopy is
presented. DDLS identifies macromolecules based on their dielectric, or
polarizability, properties. DDLS measurements are carried out in an
oscillating, nonuniform electric field. The field induces macromolecules to
undergo dielectrophoretic motion, which is detected by the modulation in the
dynamic light-scattering autocorrelation function. The DDLS experimental
setup, data analysis, and data on latex particles and yeast cells are
presented.
Haplotype variation in bovine Toll-like receptor 4 and computational prediction of a positively selected ligand-binding domain
Authors: White Stephen N. , Taylor Kristen H. , Abbey Colette A. , Gill Clare A. , Womack James E.
Published: September 2003

Toll-like receptor 4 (TLR4) is a cell-surface receptor that activates innate and adaptive immune responses. Because it recognizes a broad class of pathogen-associated molecular patterns presented by lipopolysaccharides and lipoteichoic acid, TLR4 is a candidate gene for resistance to a large number of diseases. In particular, mouse models suggest TLR4 as a candidate gene for resistance to major agents in bovine respiratory disease and Johne's disease. The coding sequence of bovine TLR4 is divided into three exons, with intron/exon boundaries and intron sizes similar to those of human TLR4 transcript variant 1. We amplified each exon in 40 individuals from 11 breeds and screened the sequence for single-nucleotide polymorphisms (SNPs). We identified 32 SNPs, 28 of which are in the coding sequence, for an average of one SNP per 90 bp of coding sequence. Eight SNPs were nonsynonymous and potentially alter specificity of pathogen recognition or efficiency of signaling. To evaluate the functional importance of these SNPs, we used codon-substitution models to detect diversifying selection in an extracellular region that may physically interact with ligands. One nonsynonymous SNP is located within this region, and other substitutions are in adjacent regions that may interact with coreceptor molecules. The 32 SNPs were found in 20 haplotypes that can be assigned to geographic ranges of origin. Haplotype-tagging SNP analysis indicated that 12 SNPs need to be genotyped to distinguish these 20 haplotypes. These data provide a basic understanding of bovine TLR4 sequence variation and supply haplotype markers for disease association studies.
Stress promotes maleness in hermaphroditic modular animals
Authors: Hughes R. N. , Manríquez P. H. , Bishop J. D. D. , Burrows M. T.
Published: September 2003

Sex-allocation theory developed for hermaphroditic plants predicts that
impaired phenotype or reduced parental survivorship caused by environmental
stress should induce relatively greater allocation to the male function. We
provide experimental evidence of stress-induced maleness, already well
documented in flowering plants, in a modular animal. By using cloned copies of
replicate genotypes, we show that the marine bryozoan
Celleporella
hyalina
increases the ratio of male to female modules in response to
diverse environmental stressors. Mating trials confirmed that paternity is
determined by fair-raffle sperm competition, which should obviate local mate
competition at characteristic population density and promote the advantage of
increased male allocation. The demonstrated similarity to plants transcends
specific physiological pathways and suggests that stress-induced bias toward
male function is a general response of hermaphroditic modular organisms to
impaired prospects for parental productivity or survival.
Liquid-solid transition in nuclei of protein crystals
Authors: Lomakin Aleksey , Asherie Neer , Benedek George B.
Published: September 2003

It is generally assumed that crystallization begins with a small,
crystalline nucleus. For proteins this paradigm may not be valid. Our
numerical simulations show that under conditions typically used to produce
protein crystals, small clusters of model proteins (particles with
short-range, attractive interactions) cannot maintain a crystalline structure.
Protein crystal nucleation is therefore an indirect, two-step process. A
nucleus first forms and grows as a disordered, liquid-like aggregate. Once the
aggregate grows beyond a critical size (about a few hundred particles) crystal
nucleation becomes possible.
Divergent impact of progesterone and medroxyprogesterone acetate (Provera) on nuclear mitogen-activated protein kinase signaling
Authors: Nilsen Jon , Brinton Roberta Diaz
Published: September 2003

The impact of progestins on estrogen-inducible mechanisms of neuroprotection was investigated. Previously, we showed that estrogen and progesterone are neuroprotective against excitotoxicity, whereas the synthetic progestin medroxyprogesterone acetate (MPA; Provera) is not. Here, we demonstrate that 17β-estradiol (E2) and progesterone (P4) treatment of hippocampal neurons attenuated the excitotoxic glutamate-induced rise in intracellular calcium concentration. Although MPA had no effect alone, MPA completely antagonized E2-induced attenuation of intracellular calcium concentration. Activation of extracellular receptor kinase (ERK) is required for estrogen-induced neuroprotection and calcium regulation. Paradoxically, E2, P4, and MPA all elicited similar rapid and transient activation of ERK, presenting a contradiction between the dependence on ERK for gonadal hormone-induced neuroprotection and the lack of neuroprotection induced by MPA. Subcellular analysis of ERK demonstrated that the phospho-ERK signal is transduced to the nucleus only by E2and P4, not by MPA. These results indicate that the profile of nuclear translocation of ERK is consistent with the neuroprotective profile. Further, the E2-induced nuclear translocation of ERK was blocked by coadministration of MPA. Results of this study reveal that nuclear ERK induction by ovarian steroids is predictive of the neuroprotective effects of estrogen and progestin treatments, revealing a hitherto unrecognized divergence of progestin signaling through the src/MAPK pathway. These results have much broader implications encompassing the impact of progestins on estrogen-mediated effects in multiple tissues. The recent results from the Women's Health Initiative trial, which used MPA as the progestinal agent, indicate that differences between progestin formulations are crucial to health outcomes in women.
Role and location of the unusual redox-active cysteines in the hydrophobic domain of the transmembrane electron transporter DsbD
Authors: Katzen Federico , Beckwith Jon
Published: September 2003

The central hydrophobic domain of the membrane protein DsbD catalyzes the transfer of electrons from the cytoplasm to the periplasm ofEscherichia coli. Two cysteine residues embedded in transmembrane segments are essential for this process. Our results, based on cysteine alkylation and site-directed proteolysis, provide strong evidence that these residues are capable of forming an intramolecular disulfide bond. Also, by using a combination of two complementary genetic approaches, we show that both cysteines appear to be solvent-exposed to the cytoplasmic side of the inner membrane. These data are inconsistent with earlier topological models that place these residues on opposite sides of the membrane and permit the formulation of alternate hypotheses for the mechanism of this unusual transmembrane electron transfer.
Targeted disruption of GPR7, the endogenous receptor for neuropeptides B and W, leads to metabolic defects and adult-onset obesity
Authors: Ishii Makoto , Fei Hong , Friedman Jeffrey M.
Published: September 2003

Gold-thioglucose (GTG) induces lesions in the ventromedial nucleus of the hypothalamus, resulting in hyperphagia and obesity. To identify genes involved in the hypothalamic regulation of energy homeostasis, we used a screen for genes that are dysregulated in GTG-induced obese mice. We found that GPR7, the endogenous G protein-coupled receptor for the recently identified ligands neuropeptide B and neuropeptide W, was down-regulated in hypothalamus after GTG treatment. Here we show that male
GPR7
-/- mice develop an adult-onset obese phenotype that progressively worsens with age and was greatly exacerbated when animals are fed a high-fat diet.
GPR7
-/- male mice were hyperphagic and had decreased energy expenditure and locomotor activity. Plasma levels of glucose, leptin, and insulin were also elevated in these mice.
GPR7
-/- male mice had decreased hypothalamic neuropeptide Y RNA levels and increased proopiomelanocortin RNA levels, a set of effects opposite to those evident in
ob
/
ob
mice. Furthermore,
ob
/
ob GPR7
-/- and
Ay
/
a GPR7
-/- double mutant male mice had an increased body weight compared with normal
ob
/
ob
or
Ay
/
a
male mice, suggesting that the obesity of
GPR7
-/- mice is independent of leptin and melanocortin signaling. Female mice did not show any significant weight increase or associated metabolic defects. These data suggest a potential role for GPR7 and its endogenous ligands, neuropeptide B and neuropeptide W, in regulating energy homeostasis independent of leptin and melanocortin signaling in a sexually dimorphic manner.