bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature-induced structural changes in glassy, supercooled, and molten silica from 77 to 2150 K
Authors: Kalampounias A. G. , Yannopoulos S. N. , Papatheodorou G. N.
Published: December 2005

In situ polarized and depolarized Raman spectra of glassy, supercooled, and molten SiO2 have been measured over the broad temperature range 77–2150 K in an effort to examine possible structural changes caused by temperature variation. A new experimental setup using a CO2 laser for heating the sample has been designed allowing measurement with controllable blackbody radiation background at temperatures up to 2200 K. Careful and systematic relative intensity measurements and the use of the isotropic and anisotropic Raman representation of the spectra revealed hidden bands in the bending mode region and resolved bands in the stretching region of the spectra. Overall the spectra behavior shows similarities with the spectra of the recently studied tetrahedral glasses/melts of ZnCl2 and ZnBr2. Increasing temperature causes subtle changes of the relative intensities within the silicon-oxygen stretching region at ∼750-850cm−1 and gives rise to a new band at ∼930cm−1. The spectral behavior is interpreted to indicate that the “SiO4∕2” tetrahedra are bound to each other to form the network by apex-bridging and partly by edge-bridging oxygens. The network structure of the glass/melt is formed by mixing a variety of tetrahedra participating in “open” (cristobalitelike), “cluster” (supertetrahedra), and “chain” edge-bridged substructures bound to each other by bridging oxygens. A weak in intensity but strongly polarized composite band is resolved at ∼1400cm−1 and is assigned to SiO terminal bond frequency. Temperature rise increases the concentration of the terminal bonds by breaking up the network. These structural changes are reminiscent of the polyamorphic transformations occurring in silica as has recently been predicted by computer simulations. At low frequencies the Raman spectra reveal the presence of the Boson peak at ∼60cm−1 which is well resolved even above melting temperature up to 2150 K.
Correlated product distributions from ketene dissociation measured by dc sliced ion imaging
Authors: Komissarov Anatoly V. , Minitti Michael P. , Suits Arthur G. , Hall Gregory E.
Published: December 2005

Speed distributions of spectroscopically selected CO photoproducts of 308nm ketene photodissociation have been measured by dc sliced ion imaging. Structured speed distributions are observed that match the clumps and gaps in the singlet CH2 rotational density of states. The effects of finite time gates in sliced ion imaging are important for the accurate treatment of quasicontinuous velocity distributions extending into the thickly sliced and fully projected regime, and an inversion algorithm has been implemented for the special case of isotropic fragmentation. With accurate velocity calibration and careful treatment of the velocity resolution, the new method allows us to characterize the coincident rotational state distribution of CH2 states as a smoothly varying deviation from an unbiased phase space theory (PST) limit, similar to a linear-surprisal analysis. High-energy rotational states of CH2 are underrepresented compared to PST in coincidence with all detected CO rotational states. There is no evidence for suppression of the fastest channels, as had been reported in two previous studies of this system by other techniques. The relative contributions of ground and first vibrationally excited singlet CH2 states in coincidence with selected rotational states of CO (υ=0) are well resolved and in remarkably good agreement with PST, despite large deviations from the PST rotational distributions in the CH2 fragments. At 308nm, the singlet CH2 (υ2=0) and (υ2=1) channels are 2350 and 1000cm−1 above their respective thresholds. The observed vibrational branching is consistent with saturation at increasing energies of the energy-dependent suppression of rates with respect to the PST limit, attributed to a tightening variational transition state.
Analytical calculations of molecular integrals for multielectron R-matrix methods
Authors: Wong Bryan M. , Altunata Serhan N. , Field Robert W.
Published: December 2005

Closed-form analytical expressions for one- and two-electron integrals between Cartesian Gaussians over a finite spherical region of space are developed for use in ab initio molecular scattering calculations. In contrast with some previous approaches, the necessary integrals are formulated solely in terms of finite summations involving standard functions. The molecular integrals evaluated over the finite region of space are computed by subtracting the contributions outside the region from the integrals over all space. The latter integrals can be efficiently and accurately obtained from existing bound-state algorithms. Our approach incorporates molecular scattering calculations into current quantum chemistry programs and facilitates the unification of bound- and continuum-state calculations for both diatomic and polyatomic molecules. Multidimensional Monte Carlo numerical integrations validate the high accuracy of our closed form results for the two-electron integrals.
Light-scattering study of vibrational relaxation in liquid xylenes
Authors: Rubio J. E. F. , Taravillo M. , Baonza V. G. , Núñez J. , Cáceres M.
Published: December 2005

Brillouin spectra obtained in dynamic light-scattering experiments are reported for the three isomeric xylenes (ortho-, meta-, and paradimethylbenzenes) between 288 and 363 K. Limiting sound velocities and relaxation times, as obtained from the polarized spectra using the theory developed by Mountain [J. Res. Natl. Bur. Stand. 70A, 207 (1966)], reveal the existence of a relaxation process. Our results suggest that the relaxation process in liquid xylenes has a purely vibrational nature. Vibrational-translational energy exchanges in xylenes are analyzed in terms of available molecular models and compared to those previously obtained for toluene and benzene. The results presented here confirm the important role played by the molecular geometry in the vibrational relaxation process, as the relative arrangement of the methyl groups has significant effect in determining the relaxing vibrational modes.
High-pressure Brillouin scattering of amorphous BeH2
Authors: Ahart Muhtar , Yarger Jeffery L. , Lantzky Kristina M. , Nakano Satoshi , Mao Ho-kwang , Hemley Russell J.
Published: December 2005

High-pressure micro-Brillouin scattering is employed to investigate the pressure dependence of the sound velocity, refractive index, equation of state, and mechanical properties of amorphous BeH2. The refractive index n has been determined by using two scattering geometries (70° and 180°). The equation of state is deduced from the pressure dependences of the sound velocity. The bulk modulus is 14.2 (±3.0)GPa and its pressure derivative is 5.3 (±0.5). The polarizability is calculated from the refractive index and the density of the material. It increases with pressure while Poisson’s ratio decreases with pressure.
Solubility of simple, nonpolar compounds in TIP4P-Ew
Authors: Krouskop Peter E. , Madura Jeffry D. , Paschek Dietmar , Krukau Aliaksei
Published: December 2005

We find that the TIP4P-Ew water model simulates the solubility of small, nonpolar solutes better than the other water models currently employed when using Ewald summation. This is in agreement with the findings of Paschek [J. Chem. Phys.120, 6674 (2004)] that the water models that best represent the experimental water density are the best representing the experimental solubility of these species.
Evidence of a two-stage thermal denaturation process in lysozyme: A Raman scattering and differential scanning calorimetry investigation
Authors: Hédoux A. , Ionov R. , Willart J.-F. , Lerbret A. , Affouard F. , Guinet Y. , Descamps M. , Prévost D. , Paccou L. , Danéde F.
Published: December 2005

Raman spectroscopy (in the low-frequency range and the amide I band region) and modulated differential scanning calorimetry investigations have been used to analyze temperature-induced structural changes in lysozyme dissolved in H21O and H22O in the thermal denaturation process. Low-frequency Raman data reveal a change in tertiary structure without concomitant unfolding of the secondary structure. Calorimetric data show that this structural change is responsible for the configurational entropy change associated with the strong-to-fragile liquid transition and correspond to about 13 of the native-denaturated transition enthalpy. This is the first stage of the thermal denaturation which is a precursor of the secondary structure change and is determined to be strongly dependent on the stability of the hydrogen-bond network in water. Low-frequency Raman spectroscopy provides information on the flexibility of the tertiary structure (in the native state and the transient folding state) in relation to the fragility of the mixture. The unfolding of the secondary structure appears as a consequence of the change in the tertiary structure and independent of the solvent. Protein conformational stability is directly dependent on the stability of the native tertiary structure. The structural transformation of tertiary structure can be detected through the enhanced H1∕H2 exchange inhibited in native proteins. Taking into account similar features reported in the literature observed for different proteins it can be considered that the two-stage transformation observed in lysozyme dissolved in water is a general mechanism for the thermal denaturation of proteins.
Pulse shaping effect on two-photon excitation efficiency of α-perylene crystals and perylene in chloroform solution
Authors: Otake I. , Kano S. S. , Wada A.
Published: December 2005

We demonstrated that the two-photon excitation efficiency of perylene in chloroform solution as well as that of crystalline perylene was dramatically increased by optimizing the shape of intense femtosecond laser pulses of a regenerative amplifier output. The efficiency was three times higher than for an unshaped single femtosecond pulse with the pulse width of shorter than 50fs. The pulse shape optimized for the solution sample was a pulse train with a repetition frequency of about 340cm−1, and the pulse shape optimized for crystalline perylene was very similar. These results supported our previous findings on α-perylene crystals using weak femtosecond pulses from a mode-locked laser oscillator [T. Okada et al. J. Chem. Phys. 121, 6385 (2004)]. Furthermore, it was confirmed that the shaped pulse optimized for the liquid sample could also increase the two-photon excitation efficiency of α-perylene crystals and vice versa. We concluded that the mechanism for the increase in excitation efficiency of perylene in chloroform was almost the same as that for α-perylene crystal, and that the efficiency increased mainly through intramolecular dynamical processes. Processes involving intermolecular interactions and/or energy states delocalized over the crystal cannot play the major role.
On the effect of a radiation field in modifying the intermolecular interaction between two chiral molecules
Authors: Salam A.
Published: December 2005

The change in the mutual energy of interaction between a pair of chiral molecules coupled via the exchange of a single virtual photon and in the presence of an electromagnetic field is calculated using nonrelativistic quantum electrodynamics. The particular viewpoint adopted is one that has an intuitive physical appeal and resembles a classical treatment. It involves the coupling of electric and magnetic dipole moments induced at each center by the incident radiation field to the resonant dipole-dipole interaction tensor. The energy shift is evaluated for fixed as well as random orientations of the molecular pair with respect to the direction of propagation of the field. A complete polarization analysis is carried out for the former situation by examining the effect of incident radiation that is linearly or circularly polarized and traveling in a direction that is parallel or perpendicular to the intermolecular distance vector. After tumble averaging, all polarization dependence of the energy shift vanishes. In both cases the interaction energy is directly proportional to the irradiance of the applied field, and is discriminatory, changing sign when one optically active species is replaced by its enantiomer. The asymptotic behavior of the energy shift at the limits of large and small separations is also studied.
Scalable implementation of analytic gradients for second-order Z-averaged perturbation theory using the distributed data interface
Authors: Aikens Christine M. , Fletcher Graham D. , Schmidt Michael W. , Gordon Mark S.
Published: December 2005

The analytic gradient expression for second-order Z-averaged perturbation theory is revised and its parallel implementation is described in detail. The distributed data interface is used to access molecular-orbital integral arrays stored in distributed memory. The algorithm is designed to maximize the use of local data and reduce communication costs. The iterative solution and the preconditioner used to induce the convergence of the coupled-perturbed Hartree-Fock equations are presented. Several illustrative timing examples are discussed.
Molecular-dynamics simulation of a ceramide bilayer
Authors: Pandit Sagar A. , Scott H. Larry
Published: December 2005

Ceramide is the simplest lipid in the biologically important class of glycosphingolipids. Ceramide is an important signaling molecule and a major component of the strateum corneum layer in the skin. In order to begin to understand the biophysical properties of ceramide, we have carried out a molecular-dynamics simulation of a hydrated 16:0 ceramide lipid bilayer at 368K (5° above the main phase transition). In this paper we describe the simulation and present the resulting properties of the bilayer. We compare the properties of the simulated ceramide bilayer to an earlier simulation of 18:0 sphingomyelin, and we discuss the results as they relate to experimental data for ceramide and other sphingolipids. The most significant differences arise at the lipid/water interface, where the lack of a large ceramide polar group leads to a different electron density and a different electrostatic potential but, surprisingly, not a different overall “dipole potential,” when ceramide is compared to sphingomyelin.
Behavior of rodlike polyelectrolytes near an oppositely charged surface
Authors: Messina René
Published: December 2005

The behavior of highly charged short rodlike polyelectrolytes near oppositely charged planar surfaces is investigated by means of Monte Carlo simulations. A detailed microstructural study, including monomer and fluid charge distributions and chain orientation, is provided. The influence of chain length, substrate’s surface-charge density, and image forces is considered. Due to the lower chain entropy (compared to flexible chains), our simulation data show that rodlike polyelectrolytes can, in general, better adsorb than flexible ones do. Nonetheless, at low substrate-dielectric constant, it is found that repulsive image forces tend to significantly reduce this discrepancy.
K and mixed K+O adlayers on Rh(110)
Authors: Günther S. , Hoyer R. , Marbach H. , Imbihl R. , Esch F. , Africh C. , Comelli G. , Kiskinova M.
Published: December 2005

The evolution of the structure of the adlayers and the substrate during adsorption of K and coadsorption of K and O on Rh(110) is studied by scanning tunneling microscopy and low-energy electron diffraction. The K adsorption at temperature above 450K leads to consecutive (1×4), (1×3), and (1×2) missing-row reconstructions for coverage up to 0.12 ML, which revert back to (1×3) and (1×4) with increasing coverage up to 0.21 ML. The coadsorption of different oxygen amount at T>450K and eventually following reduction-reoxidation cycles led to a wealth of coadsorbate structures, all involving substrate missing-row-type reconstructions, some including segmentation of Rh rows along the [11¯0] direction. The presence of K stabilizes the (1×2) missing-row reconstruction, which facilitates the formation of a great variety of very open (10×2)-type reconstructions at high oxygen coverage, not observed in the single adsorbate systems.
Molecular-dynamics simulations of binary structure II hydrogen and tetrahydrofurane clathrates
Authors: Alavi Saman , Ripmeester J. A. , Klug D. D.
Published: December 2005

The binary structure II hydrogen and tetrahydrofurane (THF) clathrates are studied with molecular-dynamics simulations. Simulations are done at pressures of 120 and 1.013bars for temperatures ranging from 100to273K. For the small cages of the structure II unit cell, H2 guest molecule occupancies of 0, 16 (single occupancy), and 32 (double occupancy) are considered. THF occupancies of 0–8 in the large cages are studied. For cases in which THF does not occupy all large cages in a unit cell, the remaining large cages can be occupied with sets of four H2 guest molecules. The unit-cell volumes and configurational energies are compared in the different occupancy cases. Increasing the small cage occupancy leads to an increase in the unit-cell volume and thermal-expansion coefficient. Among simulations with the same small cage occupancy, those with the large cages containing 4H2 guests have the largest volumes. The THF guest molecules have a stabilizing effect on the clathrate and the configurational energy of the unit cell decreases linearly as the THF content increases. For binary THF+H2 clathrates, the substitution of the THF molecules in the large cages with sets of 4H2 molecules increases the configurational energy. For the binary clathrates, various combinations of THF and H2 occupancies have similar configurational energies.
On the emergence of molecular structure from atomic shape in the 1∕r2 harmonium model
Authors: Müller-Herold Ulrich
Published: December 2005

The formal similarity of the three-body Hamiltonians for helium and the hydrogen molecule ion is used to demonstrate the unfolding of a rotating dumbbell-like proton distribution from a (1s)2-type electron distribution by smooth variation of the particles’ masses in the 1∕r2 harmonium model. The 1∕r2 harmonium is an exactly solvable modification of the harmonium model (also known as Hooke’s law atom) where the attraction between different particles is harmonic and the repulsion between the two equal particles is given by a 1∕r2 potential. The dumbbell-like molecular structure appears as an expression of increasing spatial correlation due to increasing mass. It gradually appears in the one-density distribution of the two equal particles if their mass exceeds a critical value depending on the mass of the third particle. For large mass of the equal particles, their one-density distribution approaches an asymptotic form derived from the Born-Oppenheimer treatment of H2+ in the 1∕r2 harmonium model. Below the critical value, the one density is a spherical, Gaussian-type atomic density distribution with a maximum at the center of mass. The topological transition at the critical value separates molecular structure and atomic shape as two qualitatively different manifestations of spatial structure.
Periodical forcing for the control of chaos in a chemical reaction
Authors: Córdoba A. , Lemos M. C. , Jiménez-Morales F.
Published: December 2005

Control of the chaotic behavior of a chemical system can be achieved perturbing periodically some control parameters of the system. This procedure based on external forcing, which is based on the phenomenon of resonance, can change a chaotic behavior into a periodical one by means of the application of a sinusoidal perturbation. In this paper, the influence of a periodical modulation added to the parameter controlling the oxygen adsorption rate in a cellular automaton (CA) model studying CO oxidation is analyzed. This CA model considers the oxidation reaction of CO on a catalytic surface, taking into account the catalyst temperature variation in order to analyze the reaction time oscillatory behavior. Simulations of the CA model exhibit chaotic and quasiperiodical behaviors, and it can be shown that the periodical forcing strategy can suppress the chaotic dynamics by means of the stabilization of periodical solutions.
The dependence of nanostructures on the molecule rigidity of A2(B4)2-type miktoarm block copolymer
Authors: Qian Hu-Jun , Chen Li-Jun , Lu Zhong-Yuan , Li Ze-Sheng , Sun Chia-Chung
Published: December 2005

Using the dissipative particle dynamics simulation technique, we have studied the influence of the molecule rigidity on the nanostructures of the A2(B4)2-type miktoarm block copolymers. A typical spherical micellar ordered structure is obtained for a coil-coil miktoarm block copolymer in melt. By introducing a bond angle potential in our model to enhance the molecule rigidity systematically, we find, respectively, a hexagonal cylindrical structure and a parallel ellipsoid in lamellae structure which is discovered for the first time.
The equivalence problem for fourth order differential equations under fiber preserving diffeomorphisms
Authors: Dridi Raouf , Neut Sylvain
Published: December 2005

In the present work we use the method of equivalence to determine necessary and sufficient conditions for a general fourth order ordinary differential equation to be equivalent to the flat model under a fiber preserving transformation. As a result, explicit and simple conditions are obtained.
Possible quantum kinematics
Authors: Gromov N. A. , Kuratov V. V.
Published: December 2005

The quantum group and space theory is reformulated from the standard skew-symmetric basis to an arbitrary one. The N-dimensional quantum Cayley–Klein spaces are described in Cartesian basis and the quantum analogs of (N−1)-dimensional constant curvature spaces are introduced. Part of the four-dimensional constant curvature spaces are interpreted as the noncommutative analogs of (1+3) space-time models. As a result the quantum (anti) de Sitter, Minkowski, Newton, Galilei, Carroll kinematics with the fundamental length and the fundamental time are suggested.
Plasma illumination devices enhanced by carbon nanotubes
Authors: Tsai J. T. H. , Ko H. C.
Published: December 2005

In this paper, we report the fabrication and testing of a flat visible light source by discharging Ar gas at carbon nanotubes tips. The electrodes comprised of indium tin oxide coated glass plates on which a paste of UV excited phosphor powders blended with the carbon nanotubes were screen printed. When applying voltages on such electrodes, the sharp tips of carbon nanotubes generate very high electric fields at relatively low voltages. The results show that these electrodes reduce the plasma ignition voltages effectively compared to conventional metal electrodes. A prototype light panel was also fabricated. This panel was operated at 180 V DC with a current of 20 mA. The 25cm2 device has a uniform visible light emission with a luminance of ∼500cd∕m2.
The structure of the SiO2∕Si(100) interface from a restraint-free search using computer simulations
Authors: Fischer Dominik , Curioni Alessandro , Billeter Salomon , Andreoni Wanda
Published: December 2005

The structure of the interface between SiO2 and Si(100) is investigated using the replica-exchange method driven by classical molecular dynamics simulations based on ab initio-derived interatomic potentials. Abrupt interfaces are shown to be unstable, whereas a substoichiometric oxide forms at the transition between the two materials that exhibits Si atoms in all three intermediate oxidation states, in agreement with experiment. A number of physical characteristics are found to be consistent with experimental data, including the distribution of Si atoms with different oxidation states, the increase in atom density and the stability of a pseudo-cristobalite pattern at the interface as well as the presence of Si–O–Si bridge bonds between the substrate and the suboxide.
Experimental demonstration of the relationship between the OH− content and photoluminescence in Tb-doped YPO4∙2H2O
Authors: Di Weihua , Wang Xiaojun , Chen Baojiu , Lu Shaozhe , Ren Xinguang
Published: December 2005

Tb-doped YPO4∙2H2O was synthesized via a simple wet-chemical route. Both the crystallinity of samples and the concentration of OH− adsorbed at the surface were experimentally controlled by adjusting the reaction time. An abnormal phenomenon that photoluminescence efficiency decreases with the increase of crystallinity demonstrates enormously adverse effect of OH− on photoluminescence, especially, as the OH− content reaches a critical value, the luminescent efficiency decreases more drastically. This strong dependence of photoluminescence on the OH− content indicates that to control the OH− content in a limited range or to reduce the OH− effect as much as possible is an efficient way to improve photoluminescence efficiency.
Acoustomagnetic pulse experiments in LiNbO3∕Mn12 hybrids
Authors: Hernandez J. M. , Santos P. V. , Macià F. , García-Santiago A. , Tejada J.
Published: December 2005

We report here on the influence of surface acoustic waves (SAWs) on the magnetization of a Mn12-acetate single crystal. The crystal was mounted on the surface of a piezoelectric LiNbO3 substrate containing an interdigital transducer for the excitation of SAWs. The magnetization of the crystal was measured using a rf superconducting quantum interference device with a time resolution of 1μs. The piezoelectric material was excited by SAW pulses of different frequencies produced by applying microwave pulses to the transducer. Our data show that molecular magnets onto the LiNbO3 surface can be used as very sensitive detectors of the SAW frequency and intensity.
Electron-beam-induced domain poling in LiNbO3 for two-dimensional nonlinear frequency conversion
Authors: Glickman Yinnon , Winebrand Emil , Arie Ady , Rosenman Gil
Published: December 2005

A bulk of LiNbO3 was periodically poled in two dimensions to achieve noncollinear second-harmonic generation. The sample was fabricated using an electron-beam indirect exposure. This method used a dielectric buffer layer deposited on LiNbO3 Z−-polar face to trap the incident electrons. These localized electrons formed a charge droplet which generated a strong electric field and triggered the domain inversion process in the LiNbO3. Tailored two-dimensional domain configurations enabled quasi-phase-matched frequency doubling of a Nd:YLF laser at 15 different input angles. The measured angular dependence was in agreement with calculations based on the nonlinear reciprocal lattice vectors of the poled structure.
GaInN light-emitting diode with conductive omnidirectional reflector having a low-refractive-index indium-tin oxide layer
Authors: Kim Jong Kyu , Gessmann Thomas , Schubert E. Fred , Xi J.-Q. , Luo Hong , Cho Jaehee , Sone Cheolsoo , Park Yongjo
Published: December 2005

Enhancement of light extraction in a GaInN light-emitting diode (LED) employing a conductive omnidirectional reflector (ODR) consisting of GaN, an indium-tin oxide (ITO) nanorod low-refractive-index layer, and an Ag layer is presented. An array of ITO nanorods is deposited on p-type GaN by oblique-angle electron-beam deposition. The refractive index of the nanorod ITO layer is 1.34 at 461nm, significantly lower than that of dense ITO layer, which is n=2.06. The GaInN LEDs with GaN∕low-n ITO/Ag ODR show a lower forward voltage and a 31.6% higher light-extraction efficiency than LEDs with Ag reflector. This is attributed to enhanced reflectivity of the ODR that employs the low-n ITO layer.
High-quality m-plane GaN thin films deposited on γ-LiAlO2 by ion-beam-assisted molecular-beam epitaxy
Authors: Gerlach J. W. , Hofmann A. , Höche T. , Frost F. , Rauschenbach B. , Benndorf G.
Published: December 2005

Ga N ( 1 1 ¯ 00 ) thin films are deposited on γ-LiAlO2(100) by low-energy-ion-beam-assisted molecular-beam epitaxy. Structural properties of the epitaxial GaN films are investigated by x-ray diffraction, transmission electron microscopy, and atomic force microscopy. X-ray diffraction measurements give evidence for a high crystalline quality far better than previously reported in literature. Cross-section transmission electron microscopy and atomic force microscopy show an anisotropy in defect structure and surface topography parallel and perpendicular to the GaN c axis. Optical properties are examined by photoluminescence spectroscopy at various temperatures. The spectra exhibit a strong and sharp near-band-gap transition, as well as a donor-acceptor pair transition.
Nonlinear characteristics of a superconducting receiver
Authors: Salkola M. I.
Published: December 2005

Superconducting filter technology enables highly selective and sensitive base-station receivers for wireless communications. The performance analysis of a superconducting receiver, which is inherently a nonlinear system, is simplified by introducing the concept of an effective frequency response. With this approach, superconducting filters are shown to be ideal for applications that require steep rejection at the edges of a communication band. In contrast, filter nonlinearities decrease the order of the effective frequency response of a cascaded bandpass filter, thus reducing its stop-band rejection.
Synthesis of electron transmission in nanoscale semiconductor devices
Authors: Schmidt Petra , Haas Stephan , Levi A. F. J.
Published: December 2005

Adaptive design may be used to synthesize a conduction band potential profile to obtain desired nonequilibrium electron transmission-voltage characteristics. Our methodology is illustrated by designing a two-terminal linear element in which electron motion is limited by quantum mechanical transmission through a potential profile.
Coherent detection of pulsed narrowband terahertz radiation
Authors: Cao Hua , Nahata Ajay
Published: December 2005

We demonstrate the generation and coherent detection of narrowband terahertz radiation using a Q-switched laser pumped optical parametric oscillator as the optical source. Narrowband terahertz radiation is produced using conventional difference frequency mixing and coherently detected via a frequency domain technique that relies on coherent upconversion of the terahertz field combined with optical homodyning to suppress background noise. GaSe crystals are used for both generation and coherent detection processes. Although we provide a proof-of-principle demonstration at 2THz, the infrared radiation may be tuned from the far-infrared through the midinfrared by simply tuning the idler wavelength of the optical parametric oscillator and the orientation of the two crystals.
Strongly frequency dependent focusing efficiency of a concave lens based on two-dimensional photonic crystals
Authors: Foca E. , Föll H. , Carstensen J. , Sergentu V. V. , Tiginyanu I. M. , Daschner F. , Knöchel R.
Published: December 2005

Results of an experimental study of a concave lens based on a two-dimensional microwave photonic crystal with neff<1 are shown. We demonstrate that the lens focuses electromagnetic radiation for transverse electric (TE) and transverse magnetic (TM) polarizations. Intensity gains as high as 5.4 for TE polarization and 6.3 for TM polarization were achieved for definite frequencies lying in the explored interval from 6to15GHz, the smallest area of the focal spot being equal to 0.24λ2 and 1.02λ2 for TE and TM polarizations respectively. The proposed lens serves as a model system that can be scaled to THz and optical frequencies.
Co–Fe–B–Si–Nb bulk glassy alloys with superhigh strength and extremely low magnetostriction
Authors: Chang Chuntao , Shen Baolong , Inoue Akihisa
Published: December 2005

Co-based bulk glassy alloys with diameters up to 4mm were formed in a [(Co1−xFex)0.75B0.2Si0.05]96Nb4 system. The bulk glassy alloys exhibit a superhigh fracture strength of 3980–4170MPa and Young’s modulus of 190–210GPa. The bulk glassy alloys also exhibit excellent soft-magnetic properties, i.e., high saturation magnetization of 0.71–0.97T, low coercive force of 0.7–1.8A∕m, high permeability of 1.48–3.25×104, and extremely low saturation magnetostriction of 0.55–5.76×10−6. The first successful synthesis of the Co–Fe–B–Si-based bulk glassy alloys exhibiting superhigh fracture strength and excellent soft-magnetic properties with extremely low magnetostriction is encouraging for future development of Co-based bulk glassy alloys as new engineering and functional materials.
Experimental analysis of multimode guided waves in stratified media
Authors: Lu Laiyu , Wang Chenghao , Zhang Bixing
Published: December 2005

The dispersive characteristics of multimode guided waves in stratified media are studied experimentally. Ultrasonic surface detections are performed for two layered specimens: lucite/steel half-space and aluminum/lucite/steel half-space. Characteristics of the multimode dispersion curves are analyzed using the frequency-wave number method. The dispersion curves of two modes are obtained for the first specimen of a low-velocity layer on a fast substrate. The dispersion curves for the second specimen containing a low-velocity midlayer are discontinuous and correspond to different modes branches due to mode jumping. Further analysis demonstrates that the mode jumping is caused by the different surface displacement distribution with frequency for each mode. This indicates that the surface displacement distributions of the modes should be also accounted for in the inversion problems, especially in stratified media with a low-velocity midlayer.
Evolution of luminance by voltage in organic light-emitting diodes
Authors: Weon Byung Mook , Kim Soo Young , Lee Jong-Lam , Je Jung Ho
Published: December 2005

We study degradation behaviors of luminance and voltage in organic light-emitting diodes. We find that normalized luminance and inverse normalized voltage with time, L(t) and V(t)−1, follow the stretched exponential decay. On this basis, we derive a general relation of luminance and voltage with time as L(t)=V(t)−δ(t), where δ(t) indicates a decay exponent, which is attributed to time-dependent space-charge limitation. Here the observation of higher δ(t) at higher initial luminance explains why luminance decay is faster at higher initial luminance.
Submicron sensors of local electric field with single-electron resolution at room temperature
Authors: Barbolina I. I. , Novoselov K. S. , Morozov S. V. , Dubonos S. V. , Missous M. , Volkov A. O. , Christian D. A. , Grigorieva I. V. , Geim A. K.
Published: December 2005

We describe probes of a local electric field, which are capable of detecting an electric charge as small as the charge of one electron e, operational under ambient conditions and having a spatial resolution down to 100nm. The submicron-sized probes were made from a high-density high-mobility two-dimensional electron gas, which is sensitive to the presence of electric charges near its surface. We demonstrate the possibility of using such microprobes for life-science applications by measuring an electric response of individual yeast cells to abrupt changes in their environment.
Regular MgO nanoflowers and their enhanced dielectric responses
Authors: Fang Xiao-Sheng , Ye Chang-Hui , Xie Ting , Wang Zhen-Yang , Zhao Jian-Wei , Zhang Li-De
Published: December 2005

Regular MgO nanoflowers were synthesized via a simple chemical route. The as-synthesized nanoflowers are single crystalline and consist of MgO nanofibers which have lengths up to several micrometers and diameters in the range of 20–80nm, with an average of 50nm. The frequency spectra of the relative dielectric constant and the dielectric loss were measured in the frequency range of 50Hzto5MHz. Temperature spectra of the relative dielectric constant were also measured and the experimental results showed that MgO nanoflowers have a much higher relative dielectric constant as compared with MgO micropowders, especially at low frequencies at room temperature.
Influence of structural nonuniformity and nonradiative processes on the luminescence efficiency of InGaAsN quantum wells
Authors: Geelhaar L. , Galluppi M. , Jaschke G. , Averbeck R. , Riechert H. , Remmele T. , Albrecht M. , Dworzak M. , Hildebrant R. , Hoffmann A.
Published: December 2005

We compare the luminescence efficiency (i.e., room-temperature photoluminescence intensity), fluctuations in composition and thickness, degree of localization, and luminescence decay times of In0.37Ga0.63As0.983N0.017 quantum wells grown by molecular-beam epitaxy at different temperatures and annealed under a comprehensive variety of conditions. Luminescence efficiency is not directly coupled to structural nonuniformity or localization, and even three-dimensional growth is not detrimental by itself. In contrast, there is always a correlation between luminescence efficiency and nonradiative decay time. Therefore, the luminescence efficiency of InGaAsN quantum wells depends almost exclusively on the density of nonradiative recombination centers, while the influence of structural nonuniformity is negligible.
Investigation of silicon diffusion into yttrium using x-ray photoelectron spectroscopy
Authors: Chiam S. Y. , Chim W. K. , Huan A. C. H. , Pan J. S. , Zhang J.
Published: December 2005

The reactions of yttrium (Y) metal on silicon (Si) are investigated by x-ray photoelectron spectroscopy (XPS). Low-temperature annealing studies are performed to investigate the diffusion of Si. It is found that Si diffusion occurs even under low annealing temperatures of <300°C in an ultrahigh vacuum environment. This is attributed to the weakening of the Si–Si covalent bonds by metallic Y. XPS depth profiling of room-temperature-oxidized films revealed a possible oxygen-mediated pathway which allowed significant Si diffusion at room temperature for silicate formation.
Observation of micromechanically controlled tuning of photonic crystal line-defect waveguide
Authors: Iwamoto Satoshi , Ishida Satomi , Arakawa Yasuhiko , Tokushima Masatoshi , Gomyo Akiko , Yamada Hirohito , Higo Akio , Toshiyoshi Hiroshi , Fujita Hiroyuki
Published: December 2005

We fabricated a photonic crystal (PC) line-defect waveguide integrated with a microelectromechanical actuator and demonstrated the optical switching operation. The device consisted of a PC line-defect waveguide fabricated in a silicon-on-insulator substrate and a polycrystalline-Si dielectric plate located above the PC waveguide. An applied voltage moved the dielectric plate towards the PC surface due to the electrostatic force. This motion increased out-of-plane scattering of the guided light through the evanescent interaction with the dielectric plate, and modulated the transmittance of the PC waveguide. With only a 5μm interaction length, an extinction ratio of ∼10dB was obtained at a wavelength of 1568nm under an applied voltage of 60V. The response time of the switching operation was approximately 1ms.
Detection of lipid bilayer and peptide pore formation at gigahertz frequencies
Authors: Olapinski Michael , Manus Stephan , George Michael , Brüggemann Andrea , Fertig Niels , Simmel Friedrich C.
Published: December 2005

A coplanar waveguide defined on a borosilicate glass chip is integrated with a microfluidic cartridge to allow for characterization of lipid bilayer membranes at gigahertz frequencies. The membranes are deposited on the waveguide using a vesicle fusion method. The deposition of the bilayers results in a significant change in the high-frequency transmission properties of the waveguide. We also embed alamethicin, an ion channel forming peptide, in the bilayers. The influence of the pore forming peptides can be clearly detected in the transmission signal.
Significant improvement of activation energy in MgB2∕Mg2Si multilayer films
Authors: Zhao Y. , Dou S. X. , Ionescu M. , Munroe P.
Published: December 2005

We obtained a MgB2∕Mg2Si multilayer structure by sequentially switching a stoichiometric MgB2 target and a Si target during off-axis pulsed-laser deposition. The transmission-electron-microscopic cross-sectional image of the resulting film exhibits a layered structure with each MgB2 layer being 40–50nm thick and the Mg2Si interlayers about 5nm thick. A clearly enhanced anisotropy in the irreversibility lines and the vortex activation energy was observed. Pinning and the flux flow activation energy are significantly increased in parallel applied fields.
Enhanced solid-state thermionic emission in nonplanar heterostructures
Authors: Bian Zhixi , Shakouri Ali
Published: December 2005

Conservation of transverse momentum during thermionic emission from planar structures is a key factor limiting the number of hot electrons emitted and the efficiency of solid-state thermionic energy conversion devices. In this letter, electron emission from nonplanar potential barrier structures is analyzed using a Monte Carlo transport model. Compared to the planar structures, nonplanar tall barriers can achieve much larger emission currents. Although the average energy of the transmitted electrons drops a little, the thermoelectric figure of merit can be increased with a nonplanar barrier structure. The improvement of the thermoelectric properties is attributed to the combined effects of increased effective interface area and reduced probability of total internal reflection at heterostructure interfaces.
Transparent indium zinc oxide top cathode prepared by plasma damage-free sputtering for top-emitting organic light-emitting diodes
Authors: Kim Han-Ki , Lee Kyu-Sung , Kwon J. H.
Published: December 2005

We report on plasma damage-free sputtering of an indium zinc oxide (IZO) top cathode layer for top-emitting organic light-emitting diodes (TOLEDs) by using a box cathode sputtering (BCS) technique. A sheet resistance of 42.6Ω∕cm and average transmittance above 88% in visible range were obtained even in IZO layers deposited by BCS at room temperature. The TOLED with the IZO top cathode layer shows electrical characteristics and lifetime comparable to a TOLED with only thermally evaporated Mg–Ag cathode. In particular, it is shown that the TOLED with the IZO top cathode film shows very low leakage current density of 1×10−5mAcm2 at reverse bias of −6V. This suggests that there is no plasma damage caused by the bombardment of energetic particles during IZO sputtering using the BCS system.
Real-time phase-shift measurement during formation of a volume holographic grating in nanoparticle-dispersed photopolymers
Authors: Suzuki Naoaki , Tomita Yasuo
Published: December 2005

We describe the real-time measurement of the phase shift between the light interference pattern and a volume holographic grating recorded in dry photopolymers. The time evolution of the refractive index modulation including its magnitude and phase shift is measured for one-component and nanoparticle-dispersed photopolymers. It is confirmed that phase shifts in nanoparticle-dispersed photopolymers depend on the magnitude of the refractive index of nanoparticles relative to that of polymerized monomers.
Spatially tunable laser emission in dye-doped photonic liquid crystals
Authors: Huang Yuhua , Zhou Ying , Wu Shin-Tson
Published: December 2005

A spatially tunable laser emission of the dye-doped cholesteric liquid crystal (CLC) cell using a one-dimensional temperature gradient is demonstrated. The photoexcitation of dye-doped CLC device using a frequency-doubled pulsed Nd: yttrium–aluminium–garnet laser gives rise to laser emission in the yellow-red spectral range. The lasing wavelength is widely tunable from 577 to 670 nm by shifting the position of the dye-doped CLC cell with respect to the pumping beam. The lowest excitation energy and maximum lasing efficiency occur at λ∼605nm which corresponds to the peak fluorescence emission of the dye.
Thermomechanical criteria for overlay alignment in flexible thin-film electronic circuits
Authors: Gleskova Helena , Cheng I-Chun , Wagner Sigurd , Suo Zhigang
Published: December 2005

A simple mechanical model for a deposited film∕substrate couple is presented to describe how film deposition at an elevated temperature induces change in the substrate’s in-plane dimensions at room temperature. The model provides a quantitative guideline for reducing, or completely eliminating, this elongation, by tailoring the tensile built-in stress in the deposited film. The dimensional stability so achieved is necessary for accurate overlay alignment of photomasks during the fabrication of thin-film electronic circuits.
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Thermo-optical switch utilizing total internal reflection
Authors: Yu Hui , Jiang Xiaoqing , Yang Jianyi , Tang Yi , Wang Minghua
Published: December 2005

A photonic switch of 2×3 structure was designed and fabricated using the thermo-optical effect of polymer materials. By setting sectional electrode and exerting voltage to proper section, we can control the reflection region length and ray propagation trace. Input light can be switched freely among three output ports. The experiment results show that, for the “half-reflection” state, the extinction ratio is 16.6dB for the half-reflection port; and for the “bar” state, the extinction ratio for the reflection port is 26.8dB. The power consumption is on the scale of 100mW and varies with different function mode.
Laser-controlled stress of Si nanocrystals in a free-standing Si∕SiO2 superlattice
Authors: Khriachtchev Leonid , Räsänen Markku , Novikov Sergei
Published: December 2005

We report laser manipulations with stress at the nanoscale level. The continuous-wave Ar+ laser radiation melts Si nanocrystals in a free-standing Si∕SiO2 superlattice. Silicon crystallization from the liquid phase leads to a compressive stress, which can be accurately tuned in the 3GPa range using laser annealing below the Si melting temperature and then recovered by laser annealing above the melting temperature. This allows investigations of various phenomena as a function of stress and makes a case of Si-nanocrystal memory with very long retention time, which can be written, erased, and read by optical means.
Fluorescence upconversion in Sm-doped Gd2O3
Authors: Dosev D. , Kennedy I. M. , Godlewski M. , Gryczynski I. , Tomsia K. , Goldys E. M.
Published: December 2005

We report the observation of efficient fluorescence upconversion in Sm-doped Gd2O3 nanopowders prepared by the spray pyrolysis method. The blue upconversion emission was observed with low-power continuous-wave excitation at 514, 561, 594, and 633nm and with a pulsed femtosecond at 710nm, in a laser scanning confocal microscope. This result indicates that Sm-doped Gd2O3 has the potential as a fluorescent label that may be excited in red, yellow, and green with blue emission.
Large broadband visible to infrared plasmonic absorption from Ag nanoparticles with a fractal structure embedded in a Teflon AF® matrix
Authors: Biswas A. , Eilers H. , Hidden F. , Aktas O. C. , Kiran C. V. S.
Published: December 2005

An unusual almost flat broadband plasmonic absorption, ranging from 400nm to well beyond 2500nm, was observed in a 150nm thin film of Ag nanoparticles embedded in a Teflon AF® matrix. The nanocomposites were synthesized by a simple single-step vapor-phase codeposition method. The Ag nanoparticles of various sizes and shapes, and thus various resonance frequencies, form a fractal percolating network. The broadband absorption, attributed to plasmon excitations within the nanoparticles, could be useful for multicolor applications in the visible and infrared wavelengths region.