bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical properties of epitaxial lateral overgrowth GaN structures studied by Raman and cathodoluminescence spectroscopies
Authors: Martínez O. , Avella M. , Jiménez J. , Gérard B. , Cuscó R. , Artús L.
Published: September 2004

The properties of epitaxial lateral overgrowth (ELO) GaN layers were studied by means of cathodoluminescence (CL), micro-Raman spectroscopies, and transmission electron microscopy (TEM). CL shows a strong enhancement of the luminescence emission in the ELO regions, where TEM showed the absence of dislocations. The CL enhancement observed is mostly due to the yellow luminescence (YL) band, which contrasts with the good crystal quality observed in the ELO regions by TEM and Raman spectroscopy. Local CL spectra in the ELO regions showed a different behavior of the near band edge emission in relation to the buffer layers and central part of the vertical growth region. Donor-acceptor pair recombination is enhanced in the ELO regions. The Raman spectra indicate a low strain level in the ELO regions. Plasmon-coupled modes are not observed in these areas, indicating the absence of free carriers. CL and Raman observations could thus be explained in terms of the good crystalline quality of the ELO regions, in which the incorporation of impurities is electrically compensated by the formation of deep acceptors, probably VGa, responsible as well for the enhancement of the YL band.
Electric conduction and dielectric relaxation processes in solid oxide fuel cell electrolyte La0.5Sr0.5Ga0.6Ti0.4O3−δ
Authors: Iguchi E. , Mochizuki S.
Published: September 2004

Electric conduction in the solid oxide fuel cells electrolyte, La0.5Sr0.5Ga0.6Ti0.4O3−δ, has been elucidated by dc and ac measurements as a function of temperature from 528to1103K. The sum of resistances in the bulk and boundary, Rb and Rgb, estimated in impedance analyses are nearly in agreement with dc resistance. Though Rb<Rgb at T<650K, their relative relation is reversed at T>650K. Since reciprocal resistance is proportional to conductivity, temperature dependencies of T∕Rb and T∕Rgb yield activation energies for conductions in the bulk and boundary. Two relaxation processes show up in dielectric properties, i.e., low-frequency relaxation peak due to conduction in the boundary and high-frequency peak resulting from bulk conduction. Temperature dependencies of resonance frequencies and maxima of dielectric parameters in these relaxation peaks are used to estimate migration energies of O2− ion, EM, and energies required for creation of a free oxygen vacancy assisting O2− migration, EO. The sums of EM and EO in high- and low-frequency peaks are mostly in agreement with activation energies estimated from temperature dependencies of T∕Rb and T∕Rgb.
Singular and nonsingular three-body integrals for exponential wave functions
Authors: Harris Frank E. , Frolov Alexei M. , Smith Vedene H.
Published: September 2004

Integrals which are individually singular, but which may be combined to yield convergent expressions, are needed for computations of relativistic effects and various properties of atomic and quasiatomic systems. As computations become more detailed and precise, more such integrals are required. This paper presents general formulas for the radial parts of the singular and nonsingular (regular) integrals that occur when three-body systems are described using wave functions that include exponentials in all three interparticle coordinates. Our results are compared with those found in the literature for some of the integrals, and are also shown to be consistent with previously reported results for Hylleraas functions (a limiting case in which one of the exponential parameters is set to zero).
Parameter space minimization methods: Applications to Lennard-Jones–dipole-dipole clusters
Authors: Oppenheimer Craig A. , Curotto E.
Published: September 2004

The morphology of the uniform Lennard-Jones–dipole-dipole cluster with 13 centers (LJDD)13 is investigated over a relatively wide range of values of the dipole moment. We introduce and compare several necessary modifications of the basin-hopping algorithm for global optimization to improve its efficiency. We develop a general algorithm for T=0 Brownian dynamics in curved spaces, and a graph theoretical approach necessary for the elimination of dissociated states. We find that the (LJDD)13 cluster has icosahedral symmetry for small to moderate values of the dipole moment. As the dipole moment increases, however, its morphology shifts to an hexagonal antiprism, and eventually to a ring.
Dissipative particle dynamics simulations of polymer melts. I. Building potential of mean force for polyethylene and cis-polybutadiene
Authors: Guerrault X. , Rousseau B. , Farago J.
Published: September 2004

We present the derivation of coarse-grained force fields for two types of polymers, polyethylene (PE), and cis-polybutadiene (cis-PB), using the concept of potential of mean force. Coarse-grained force fields were obtained from microscopic simulations for several coarse-graining levels, i.e., different number of monomers λ per mesoscopic unit called “bead.” These force fields are then used in dissipative particle dynamics (DPD) simulations to study structural and dynamical properties of polymer melts of PE and cis-PB. The radial distribution functions g(R), the end-to-end distance R0, the end-to-end vector relaxation time τ, and the chain center of mass self-diffusion DCM, are computed for different chain lengths at different coarse-graining factor λ. Scaling laws typical of the Rouse regime are obtained for both polymers for chain lengths ranging from 6 to 50 beads. It is found that the end-to-end distance R0 obtained from DPD simulations agree well with values obtained from both microscopic simulations and experiments. The dependence of the friction coefficient used in DPD simulations versus the coarse-graining level is discussed in view of the overall scaling of the dynamical properties.
Isotope effects and Born-Oppenheimer breakdown in excited singlet states of the lithium dimer
Authors: Adohi-Krou A. , Martin F. , Ross A. J. , Linton C. , Le Roy R. J.
Published: September 2004

Observation of infrared electronic transitions involving the 1 1Δg state of Li27 has instigated an investigation of Born-Oppenheimer breakdown in four singlet electronic states correlating with (2s+2s), (2s+2p), and (2p+2p) lithium atoms. The 1 1Δg state, which correlates at long range with (2p+2p) atoms, has been observed in emission from the (5p) Πu1 Rydberg state and in 1 1Δg-B 1Πu bands, in both instances following optical-optical double-resonance excitation. The latter transition was observed previously for the lighter isotopomer, Li26 [C. Linton, F. Martin, P. Crozet, A. J. Ross, and R. Bacis, J. Mol. Spectrosc. 158, 445 (1993)]. By analyzing multiple-isotopomer data for several electronic systems simultaneously, we have determined the electronic isotope shifts and the leading vibrational and/or rotational Born-Oppenheimer breakdown terms for the X 1Σg+, A 1Σu+, B 1Πu, and 1 1Δg states of the lithium dimer. This paper also reports Fourier transform measurements of the B-X absorption spectra of Li26 and Li27, which were required to better define the bottom portion of the B 1Πu state potential.
Terahertz radiation from CdxHg1−xTe photoexcited by femtosecond laser pulses
Authors: Krotkus A. , Adomavičius R. , Molis G. , Urbanowicz A. , Eusebe H.
Published: September 2004

Terahertz radiation from CdxHg1−xTe samples excited by femtosecond Ti:sapphire laser pulses were measured by using an ultrafast photoconductive antenna manufactured from low-temperature grown GaAs. Terahertz fields radiated by the samples of all three investigated alloy compositions with x=0, 0.2, and 0.3 were of the same order of magnitude. No azimuthal angle dependence of the radiated signal was detected, which evidences that linear current surge effect is dominating over nonlinear optical rectification.
Damage and recovery in arsenic doped silicon after high energy Si+ implantation
Authors: Solmi S. , Ferri M. , Nobili D. , Bianconi M.
Published: September 2004

Electrical measurements were used to study the irradiation effects and the annealing behavior of heavily As doped silicon on insulator samples implanted with 2MeV Si+ ions. It is found that implantation induces a strong reduction of the carrier density, which markedly depends on the concentration of As. Annealing at temperatures in the range 600–800°C, by rapid thermal treatments or heating in furnace, showed that recovery takes place in two stages. The kinetics of the former, which should involve point defect-dopant complexes or small defect clusters, is rapid, while more stable defects demanding prolonged heating recover in the latter stage. It is concluded that these more stable defects should originate by the aggregation with an Ostwald ripening mechanism of the dopant-defect complexes and small point defect clusters, a phenomenon which competes with their annihilation. These processes, which ultimately determine the carrier density trapped in the stable defects, can also partially take place under the Si+ implantation. The effects of irradiation dose, temperature of the samples in the course of the irradiation, dopant concentration, and annealing temperature on defect structure and carrier concentration are reported and discussed.
Determination of absolute bond strength from hydroxyl groups at oxidized aluminum-epoxy interfaces by angle beam ultrasonic spectroscopy
Authors: Cantrell John H.
Published: September 2004

The hydrogen bonding (H-bond) contribution to the adhesive strength of oxidized aluminum-epoxy interfaces is calculated from the Morse potential for the interaction of hydroxyl groups formed on the Al2O3 surface with epoxy receptors. A strength parameter called the normal spring constant flux KN is defined and calculated to be 3.8×1017Nm−3, corresponding to a tensile strength of 26MPa, for values of the H-bond parameters and bond densities typical of moderate strength epoxies. The KN value is in very good agreement with values determined experimentally from an inversion of ultrasonic spectral data [A. I. Laverntyev and S. I. Rokhlin, J. Appl. Phys. 76, 4643 (1994); J. Acoust. Soc. Am. 102, 3467 (1997)]. It is also in agreement with the value KN=3.6×1017Nm−3 obtained by a reinterpretation of the periodic crack model [F. J. Margetan et al., J. Nondestr. Eval. 7, 131 (1988)] when applied to adhesive interfaces. The agreement between theory and experiment establishes KN as a quantitative nondestructive measure of adhesive strength. The relationship between the normal and transverse (shear) spring constant fluxes is derived and the effects of thermal fluctuations are addressed.
High-resolution spectroscopy of the 22Πu←X4Σg− forbidden transitions of C2+
Authors: Tarsitano Christopher G. , Neese Christopher F. , Oka Takeshi
Published: September 2004

The electronic absorption spectrum of the (0,2), (1,3), and (6,9) bands of the B4Σu−−X4Σg− system of C2+ was obtained using the velocity modulation technique in conjunction with heterodyne detection. The rotationally resolved spectrum shows perturbations, which are attributed to the 22Πu state. The mixing between the B4Σu− state and the 22Πu state for nearly degenerate levels generated enough intensity borrowing to observe twenty 22Πu←X4Σg− forbidden transitions. The parameters of a model Hamiltonian were fit to the bands and their corresponding forbidden transitions. Line position measurements, line strength factors, and expectation values for the orbital angular momentum 〈Λ′〉 for the forbidden transitions are reported. Molecular parameters from the global fit of each band, including their corresponding forbidden transitions, are reported.
High frequency response of p-i-n photodiodes analyzed by an analytical model in Fourier space
Authors: Konno K. , Matsushima O. , Navarro D. , Miura-Mattausch M.
Published: September 2004

A formulation of carrier transport in vertical p-i-n photodiodes is presented in Fourier space, taking into account diffusion effects of carriers outside the intrinsic region. High frequency response of photodiodes is investigated using the model. Calculated results show that diffusion limits the cutoff frequency characteristics of photodiodes with short intrinsic region length. Photocurrent calculations via the spectral method are in excellent agreement with two-dimensional device simulator results. The model can be utilized in circuit simulation because of its reduced computational runtime.
Atomic force algorithms in density functional theory electronic-structure techniques based on local orbitals
Authors: Miyazaki T. , Bowler D. R. , Choudhury R. , Gillan M. J.
Published: September 2004

Electronic structure methods based on density-functional theory, pseudopotentials, and local-orbital basis sets offer a hierarchy of techniques for modeling complex condensed-matter systems with a wide range of precisions and computational speeds. We analyze the relationships between the algorithms for atomic forces in this hierarchy of techniques, going from empirical tight-binding through ab initio tight-binding to full ab initio. The analysis gives a unified overview of the force algorithms as applied within techniques based either on diagonalization or on linear-scaling approaches. The use of these force algorithms is illustrated by practical calculations with the CONQUEST code, in which different techniques in the hierarchy are applied in a concerted manner.
Highly ordered phthalocyanine thin films on a technically relevant polymer substrate
Authors: Peisert H. , Liu X. , Olligs D. , Petr A. , Dunsch L. , Schmidt T. , Chassé T. , Knupfer M.
Published: September 2004

We have studied the molecular orientation of well-known representatives of organic semiconductors from the family of the phthalocyanines [copper phthalocyanine (CuPc) and its perfluorinated relative (CuPcF16)] on a conducting polymer thin film using polarization-dependent x-ray absorption spectroscopy. As a polymer substrate PEDOT:PSS [a mixture of poly-3,4-ethylenedioxy-thiophene (PEDOT) and polystyrenesulfonate (PSS), which is often applied as an electrode material in (all-)organic semiconductor devices] was spin coated onto indium-tin-oxide substrates. Even if the interfaces themselves are relatively ill defined (we found recently a mixing of the two organic materials and charge-transfer processes), a very high degree of molecular ordering is observed in the 20–50nm thick phthalocyanine films.
Experimental measurement and model analysis of damping effect in nanoscale mechanical beam resonators in air
Authors: Yum Kyungsuk , Wang Zhaoyu , Suryavanshi Abhijit P. , Yu Min-Feng
Published: September 2004

The damping effect in the nanoscale mechanical beam resonators operated under ambient conditions was studied. Experimental measurement of the viscous air damping in the nanowire cantilever resonators was carried out using the electric-field-induced resonance method; and a theoretical model, which accounts for the effects of dimension and material property of the nanowires and the air viscosity of the environment, was developed for describing the mechanical resonance and damping. The study showed that the damping effect in the nanoscale beam resonators operated in air could be as high as that in the microscale resonators operated in liquids, and scaled with the geometric dimension of the studied nanowire cantilevers.
An exactly solvable model for a ternary solution with three-body interactions and orientationally dependent bonding
Authors: Buzatu Florin D. , Lungu Radu P. , Huckaby Dale A.
Published: September 2004

A model is considered in which the bonds of a honeycomb lattice are covered by rodlike molecules of types AA, BB, and AB. Neighboring molecular ends have three-body and orientation-dependent interactions. The model is shown to be equivalent to a spin-1/2 Ising model on the same lattice with a field, but with only pairwise interactions. Symmetric and asymmetric coexistence surfaces for the separation into an AA-rich and a BB-rich phase are calculated exactly.
The effect of electron detachment on the structure and properties of the chlorine-acetonitrile anionic complex
Authors: Rode Michal F. , Roszak Szczepan , Szymczak Jaroslaw J. , Sadlej Joanna , Leszczynski Jerzy
Published: September 2004

The results of the theoretical study of ground state potential energy surfaces for the chlorine-acetonitrile anion and its photodetachment product are presented. The shallow potential surfaces allow for the nondefinitive position of the chlorine within the complex. The dissociation energy of the neutral complex, estimated through the thermodynamic cycle, indicates significant structural changes due to the photodetachment process. The excess negative charge is localized mostly on the chlorine atom, and the electron detachment proceeds as an electron is removed from chlorine. The process leads to drastic changes in the electrostatic interactions within the complex. The first electronic excited state corresponds to the excess electron transfer from chlorine to acetonitrile fragment. This state is a precursor of the observed charge-transfer-to-solvent state.
Theoretical prediction of the structure and properties of Sn3N4
Authors: Huang M. , Feng Y. P.
Published: September 2004

First-principles calculations based on plane-wave pseudopotential and local-density approximation (LDA) were carried out to study the structure and properties of Sn3N4. In agreement with experimentally determined crystal structure, γ-Sn3N4, which has a high density, was found to be most energetically favored among the structures considered. We also searched for other possible polymorphs of Sn3N4, which may be stable at higher pressure, by studying the properties of six additional high-pressure structures. Phase transition from the spinel structure to the CaFe2O4 and CaTi2O4-type structures can be expected at pressures of 60GPa and 40GPa, respectively. Sn3N4 is predicted to be a semiconductor with a band gap of 1.15eV within the LDA.
Converged quantum dynamics calculations of vibrational energies of CH4 and CH3D using an ab initio potential
Authors: Yu Hua-Gen
Published: September 2004

Exact variational calculations of vibrational energies of CH4 and CH3D are carried out using a two-layer Lanczos algorithm based on the ab initio potential energy surface of D. W. Schwenke and H. Partridge, Spectrochim. Acta, Part A 57, 887 (2001). The convergence of the calculated vibrational energies is discussed in detail. In addition, we report all well converged vibrational energy levels up to 6600 cm−1 for CH4, and those up to 5000 cm−1 for CH3D, respectively. These results clearly outperform previous theoretical calculations. And a comparison with experimental results available is also made.
Homoepitaxial diamond growth for the control of surface conductive carrier transport properties
Authors: Williams Oliver A , Jackman Richard B
Published: September 2004

Growth of high quality diamond for surface conductive device applications is demonstrated. Mobility values higher than 140cm2V−1s−1 at sheet carrier concentrations of 2.5×1012cm−2 were achieved using a high growth rate process. Furthermore, control over the carrier transport statistics is demonstrated on both single crystal and polycrystalline diamond. This process allows the production of high quality electronic grade diamond with ability to tune carrier transport statistics. The mechanism behind this process is discussed.
Temperature dependence of density, thermal expansion coefficient and shear viscosity of supercooled glycerol as a reflection of its structure
Authors: Blazhnov Ivan V. , Malomuzh Nikolay P. , Lishchuk Sergey V.
Published: September 2004

The relationship of the microstructure of supercooled, highly viscous glycerol to the temperature dependence of its density, thermal expansion coefficient, and shear viscosity are discussed. The character of this temperature dependence at the transition from low viscosity state to the solid amorphous state (solidified state without nuclei) is described with help of function ψ, which can be interpreted as the effective number of degrees of freedom responsible for the change of viscosity of glycerol over a broad range; these degrees of freedom are those related to the α-relaxation process. It is shown that the change in effective activation energy of the viscosity is completely determined by the parameter ψ. The change in the shear viscosity of glycerol due to the influence of the solid-phase nuclei is considered. It is shown that the introduction of the parameter φ, equal to the specific volume occupied by the nuclei of the solid phase, together with ψ provides a natural explanation of the temperature dependence of density and thermal expansion coefficients of glycerol in its liquid, solid amorphous, glassy, and crystal states. The peculiarities of the temperature dependence of φ(T) and ψ(T) for glycerol and o-terphenyl are compared.
The influence of surface segregation on the optical properties of quantum wells
Authors: de la Cruz G. Gonzalez
Published: September 2004

Segregation of column III atoms during molecular beam epitaxy of III-V semiconductor compounds result in nonabrupt interfaces and surface compositions different from the bulk. This effect modifies the electronic states in the quantum well and the emission energy in the photoluminescence spectrum. In this work, we have solved analytically the Schrödinger equation taking into account the shape changes in the quantum well due to the segregation of atoms during the growth process of the semiconductor heterostructures. We apply this model to the case of indium segregation in the InGaAs/GaAs system. The transition energy calculations between the confined electron and hole states as function of the well width for different In composition and growth temperature are in agreement with the measured photoluminescence energy peaks.
In-plane switching liquid crystal gel for polarization-independent light switch
Authors: Ren Hongwen , Lin Yi-Hsin , Fan Yun-Hsing , Wu Shin-Tson
Published: September 2004

An in-plane switching (IPS) liquid crystal (LC) gel for light shutter is demonstrated. In the voltage-off state, the LC gel is transparent. As the voltage exceeds a threshold, light scattering occurs due to microdomain formations. This light scattering is found to be polarization independent. Such an IPS LC gel is a broadband device and has a wide viewing angle and a relatively fast response time. Potential applications for switchable windows, variable optical attenuators, and reflective displays are considered.
Microwave investigation of the CO-CH4 van der Waals complex
Authors: Liu Yaqian , Jäger Wolfgang
Published: September 2004

Rotational spectra of eight isotopomers of the weakly bound van der Waals complex CO-CH4 were recorded in the frequency range from 4 to 19 GHz using a pulsed molecular beam Fourier transform microwave spectrometer. For the isotopomers containing methane monomers of Td symmetry, namely, C1612O-12CH4, C1612O-13CH4, C1612O-12CD4, C1613O-12CH4, and C1813O-12CH4, three rotational progressions were observed that correlate to the jm=0, 1, and 2 rotational levels of free methane. For those containing partially deuterated methane monomers with C3V symmetry, namely, C1612O-12CH3D and C1612O-12CHD3, only two progressions were recorded, correlating to the jk=00 and 11 rotational levels of free CH3D and CHD3, respectively. The van der Waals bond distance R, intermolecular stretching frequency νs, and the corresponding stretching force constant ks were derived from the obtained spectroscopic results. The results obtained for the jm=0 ground state are compared to the previous infrared and millimeter wave data. A O17 nuclear quadrupole coupling constant was determined from the resolved hyperfine structure of C1713O-12CH4 and was used to obtain angular information about the carbon monoxide subunit. A Coriolis interaction was deduced from the irregular spectral pattern involving levels with jm=1. Qualitative information about the extent of the perturbation was obtained from a comparison of spectroscopic constants of different isotopomers.
Influences of hydrostatic pressure during casting and Pd contenton as-cast phase in Zr-Al-Ni-Cu-Pd bulk alloys
Authors: Kato Hidemi , Inoue Akihisa , Saida Junji
Published: September 2004

The influences of sample diameter (D), Pd content (x), and hydrostatic pressure (P) in a chamber during casting on the structure of as cast Zr65Al7.5Ni10Cu17.5−xPdx(x=10,17.5at.%) bulk alloys were investigated. Zr65Al7.5Ni10Cu7.5Pd10 and Zr65Al7.5Ni10Pd17.5 alloys (D=3mm) cast in a vacuum chamber (P∼4.0×10−3Pa) were mainly of the tetragonal-Zr2Ni equilibrium phase and nanosize icosahedral primary phase, respectively, while the same alloys cast in inert argon gas at atmospheric pressure (P∼0.1MPa) were of the single glassy phase. Due to the higher cooling rate obtained by decreasing the sample diameter (D=2mm) even in the vacuum chamber, the Zr65Al7.5Ni10Pd17.5 alloy was still of the icosahedral phase, while the Zr65Al7.5Ni10Cu7.5Pd10 alloy froze into a single glassy phase. These results indicate that the temperature- and time- transformation curves for the icosahedral and subsequent equilibrium phase formations in the alloy system shifts to a shorter time side with decreasing P, and the pressure sensitivity of the icosahedral phase formation increases with x.
Peierls stress of a screw dislocation in a piezoelectric medium
Authors: Li Shaofan , Gupta Anurag
Published: September 2004

In this letter, the Peierls-Nabarro (PN) model is extended to describe dislocation mobility in piezoelectric materials. The Peierls stress of a screw dislocation in a piezoelectric material is calculated based on the generalized PN model and linear piezoelectricity theory.
Electromigration-driven motion of morphologically stable voids in metallic thin films: Universal scaling of migration speed with void size
Authors: Cho Jaeseol , Gungor M. Rauf , Maroudas Dimitrios
Published: September 2004

The dependence on void size of the migration speed of morphologically stable voids that translate along metallic thin films due to surface electromigration is analyzed in finite-width films through self-consistent numerical simulations taking surface diffusional anisotropy into account. It is shown that, as the morphological stability limit is approached, the void migration speed deviates substantially from being inversely proportional to the void size. A nonlinear “shape function” that includes both current crowding and diffusional anisotropy effects is derived and incorporated into the well-known theoretical result that is valid for infinite-conductor domains and isotropic surface diffusivity. Rescaling the void migration velocity with the corresponding, numerically evaluated shape function results in a universally valid relationship for the migration speed as a function of void size. This result is important in understanding electromigration-induced void dynamics in metallic interconnect lines.
Nanocrystal-size selective spectroscopy in SnO2:Eu3+ semiconductor quantum dots
Authors: Yanes A. C. , Del Castillo J. , Torres M. , Peraza J. , Rodríguez V. D. , Méndez-Ramos J.
Published: September 2004

Glass ceramics of composition 95SiO2-5SnO2 doped with 0.4mol% Eu3+ have been prepared by thermal treatment of sol-gel glasses. The segregated SnO2 nanocrystals present a mean size comparable to the bulk exciton Bohr radius (about 2.4nm), corresponding to a wide band-gap quantum-dot system in an insulator SiO2 glass. A fraction of the Eu3+ ions is incorporated to the SnO2 nanocrystals in the process. In these strong confinement conditions, the energy gap presents a high dependence on the nanocrystal size. Taking advantage of this effect, it has been possible to excite selectively the Eu3+ ions located in the SnO2 nanocrystals, by energy transfer from the host, obtaining emission spectra that depend on the nanocrystal size. The Eu3+ ions environment in small nanocrystals (radius under 2nm) are very distorted, meanwhile they are like crystalline for nanocrystals with a radius of some nanometers.
On the variation of the structure of liquid deuterium fluoride with temperature
Authors: McLain S. E. , Benmore C. J. , Siewenie J. E. , Molaison J. J. , Turner J. F. C.
Published: September 2004

The structure of liquid deuterium fluoride has been measured using pulsed neutron diffraction and high energy x-ray diffraction techniques as a function of temperature. The neutron experiments were performed at T=296±2 K, 246±2 K, and 193±2 K and the x-ray measurements carried out at 296±2 K and 195±2 K. The x-ray pair correlation functions, which are dominated by fluorine-fluorine interactions, show the first peak at ∼2.53±0.05 Å remains very nearly invariant with decreasing temperature. Peaks around 4.5 and 5.0 Å also appear at both temperatures in the x-ray data. In contrast, the intermolecular peaks in the total neutron pair correlation function show that significant systematic local structural changes occur as the temperature is lowered. The first intermolecular peak position shortens from 1.64±0.05 Å at 296 K to 1.56±0.05 Å at 195 K. Although there are overlapping contributions from the intermolecular hydrogen-fluorine and hydrogen-hydrogen correlations, it is clear that the temperature dependent structural changes are largely due to a rearrangement of the deuterium atom positions in the fluid. By comparison with partial structure factor data the hydrogen bonds appear to become more linear at lower temperatures.
Polarization-dependent electron affinity of LiNbO3 surfaces
Authors: Yang W.-C. , Rodriguez B. J. , Gruverman A. , Nemanich R. J.
Published: September 2004

Polar surfaces of a ferroelectric LiNbO3 crystal with periodically poled domains are explored using UV-photoelectron emission microscopy (PEEM). Compared with the positive domains (domains with positive surface polarization charges), a higher photoelectric yield is found from the negative domains (domains with negative surface polarization charges), indicating a lower photothreshold and a corresponding lower electron affinity. The photon-energy-dependent contrast in the PEEM images of the surfaces indicates that the photothreshold of the negative domains is ∼4.6eV while that of the positive domains is greater than ∼6.2eV. We propose that the threshold difference between the opposite domains can be attributed to a variation of the electron affinity due to opposite surface dipoles induced by surface adsorbates.
Growth and characterization of tungsten carbide nanowires by thermal annealing of sputter-deposited WCx films
Authors: Wang Shui-Jinn , Chen Chao-Hsuing , Chang Shu-Cheng , Uang Kai-Ming , Juan Chuan-Ping , Cheng Huang-Chung
Published: September 2004

In this letter, the growth of dense W2C nanowires by a simple thermal annealing of sputter-deposited WCx films in nitrogen ambient is reported. Straight nanowires with a density of 250–260μm−2 and length∕diameter in the range of 0.2–0.3μm∕13–15nm were obtained from the 700°C-annealed samples, which exhibit good electron field emission characteristics with a typical turn-on field of about 1.7V∕μm. The self-catalytic growth of W2C nanowires is attributed to the formation of α-W2C phase caused by carbon depletion in the WCx films during thermal annealing.
Direct evidence for strain inhomogeneity in InxGa1−xN epilayers by Raman spectroscopy
Authors: Correia M. R. , Pereira S. , Pereira E. , Frandon J. , Watson I. M. , Liu C. , Alves E. , Sequeira A. D. , Franco N.
Published: September 2004

This contribution is focused on Raman analysis of the InxGa1−xN alloy. It presents direct evidence that both strain and composition effects must be taken into account to interpret the Raman experimental results. Raman studies have been commonly discussed in view of composition inhomogeneity only, neglecting the possible existence of strain depth variations, recently shown to occur for layers grown above the critical layer thickness. The effects of this variation on the A1(LO) phonon frequency could only be investigated by combining both structural and Raman measurements. In this letter, a set of InxGa1−xN layers has been chemically etched during different periods, allowing the depth variation of the phonon frequency to be unambiguously evidenced. Comparing the Raman spectra before and after etching, two distinct InxGa1−xN regions, differing on their strain state, are identified: a relaxed one, found near the surface region; another one, grown coherently (i.e., pseudomorphic) to the GaN buffer layer. These results are in excellent agreement with an additional reciprocal space map analysis.
Probing nanosecond protein motions of calmodulin by single-molecule fluorescence anisotropy
Authors: Tan Xin , Hu Dehong , Squier Thomas C. , Lu H. Peter
Published: September 2004

We report a single-molecule fluorescence anisotropy study of calmodulin, a regulatory protein for calcium-dependent cell signaling. Calmodulin in this study contains a site-specifically inserted tetra-cysteine motif that reacted with FlAsH, a biarsenic fluorescein derivative that can be rotationally locked to the host protein. A photon time-stamping technique was employed that combined the capability for both subnanosecond time resolution of time-correlated single photon counting and single-molecule time trajectory recording. The study provided direct characterization of the nanosecond motions of calmodulin tethered to a biologically compatible surface under physiological buffer solution. The unique technical approaches are applicable to single-molecule study of protein conformational dynamics and protein–protein interactions at a wide range of time scales and without the signal convolution of probe-dye molecular motions.
Intracavity coherent addition of Gaussian beam distributions using a planar interferometric coupler
Authors: Ishaaya A. A. , Davidson N. , Shimshi L. , Friesem A. A.
Published: September 2004

We present a compact and practical combined laser resonator configuration in which several Gaussian beam distributions are efficiently combined. It is based on intracavity coherent addition of pairs of Gaussian beam distributions with a planar interferometric coupler. The principle, configuration, and experimental results using pulsed Nd:YAG laser beams are presented. The results reveal more than 92% combining efficiency with a nearly Gaussian output beam, in free running and Q-switched operation.
Effect of MnAs/GaAs(001) film accommodations on the phase-transition temperature
Authors: Iikawa F. , Brasil M. J. S. P. , Couto O. D. D. , Adriano C. , Giles C. , Däweritz L.
Published: September 2004

The phase-transition temperature of MnAs epitaxial films grown by molecular-beam epitaxy on GaAs(001) with different crystalline accommodations was studied by specular and grazing incidence x-ray diffraction. The transition temperature of MnAs films with tilted hexagonal c-axis orientations with respect to the GaAs substrate is higher than the most investigated nontilted films and reaches a value above room temperature, which is more suitable for device applications.
Pseudomorphic SiC alloys formed by Ge ion implantation
Authors: Dashiell M. W. , Xuan G. , Ansorge E. , Zhang X. , Kolodzey J. , DeSalvo G. C. , Gigante J. R. , Malkowski W. J. , Clarke R. C. , Liu J. , Skowronski M.
Published: September 2004

Pseudomorphic-strained layers containing from 0.07–1.25atomic% Ge were formed by ion implantation at 1000°C into 4H-SiC substrates. X-ray diffraction revealed high crystalline quality and coherent interfaces for strains up to 1.4%. Infrared reflectivity indicated a phonon mode at 948cm−1, attributed to Ge implantation disorder. Annealing above 1250°C caused the disappearance of the 948cm−1 disorder mode, and the strengthening of the phonon mode at 848cm−1, associated with the 4H stacking sequence. Structural measurements of the annealed samples revealed thermally stable, coherently strained layers of the 4H polytype, without precipitation, suggesting an isoelectronic Ge alloy compatible with SiC for heterostructure strained layer engineering.
Growth of epitaxial nanowires by controlled coarsening of strained islands
Authors: Shenoy V. B.
Published: September 2004

We show that elongated nanowires can be grown on crystal surfaces by allowing large strained two-dimensional islands to desorb by varying the adatom supersaturation or chemical potential. The width of the wires formed in this process is determined by a competition between the repulsive elastic interactions of the long edges of the wires and the thermodynamic driving force which tends to decrease the distance between these edges. The proposed mechanism allows for control of the wire sizes by changing the growth conditions, in particular, the vapor pressure of the material that is being deposited.
Optimization of a tunneling barrier in magnetic tunneling junction by tilted-plasma oxidation
Authors: Nam C. H. , Shim Heejae , Kim K. S. , Cho B. K.
Published: September 2004

Oxidation of an AlOx insulating barrier in a magnetic tunneling junction (MTJ) was carried out by a tilted-plasma oxidation method. It was found that the tilted-plasma oxidation induced a gradual change in the extent of oxidation of an insulating layer, which consequently led to a gradual change in the tunneling magnetoresistance (TMR) and specific junction resistance (RA) of the MTJ. We found a linear relation in the TMR versus RA curve with positive and negative slopes for less- and overoxidized junctions, respectively, and a parabolic relation for optimally oxidized junctions. The crossover in the TMR versus RA curves provides an effective and useful way to optimize (and monitor) the oxidation condition of a tunneling barrier in MTJs especially of a tunneling barrier less than 10Å thick. The tunneling junctions were also investigated after thermal annealing at various temperatures. The observations after thermal annealing were found to be consistent with transmission electrons microscopy images and a scenario of the partial formation of an additional ultrathin tunneling barrier at the top surface of the bottom magnetic layer.
Selective area growth of InAs quantum dots formed on a patterned GaAs substrate
Authors: Birudavolu S. , Nuntawong N. , Balakrishnan G. , Xin Y. C. , Huang S. , Lee S. C. , Brueck S. R. J. , Hains C. P. , Huffaker D. L.
Published: September 2004

We describe the growth and characterization of InAs quantum dots (QDs) on a patterned GaAs substrate using metalorganic chemical vapor deposition. The QDs nucleate on the (001) plane atop GaAs truncated pyramids formed by a thin patterned SiO2 mask. The base diameter of the resulting QDs varies from 30 to 40nm depending on the size of the mask. With specific growth conditions, we are able to form highly crystalline surface QDs that emit at 1.6μm under room-temperature photopumped conditions. The crystalline uniformity and residual strain is quantified in high-resolution transmission electron microscopy images and high-resolution x-ray reciprocal space mapping. These strained QDs may serve as a template for selective nucleation of a stacked QD active region.
Stable plasma configurations in a cylindrical magnetron discharge
Authors: Levchenko I. , Romanov M. , Keidar M. , Beilis I. I.
Published: September 2004

Transition between different plasma configurations is studied in a system with negative biased cylindrical target in crossed E×B fields. It was found that the diffuse plasma torus formed around the cylindrical target in relatively small magnetic field (0.02T on target surface) changes the shape with magnetic field to form a thin disk with a width lower than 1cm when target voltage is less than −400V. The target current decreases sharply when the magnetic field reaches some critical value. When the target voltage exceeds 400V, the target current increases with the magnetic field and the plasma has always toroidal shape. The plasma behavior can be understood taking in account the interaction of the drift currents and the magnetic field.
Guided-wave acousto-optic diffraction in AlxGa1−xN epitaxial layers
Authors: Bu G. , Shur M. S. , Čiplys D. , Rimeika R. , Gaska R. , Fareed Q.
Published: September 2004

The acousto-optic (AO) diffraction of guided optical waves from surface acoustic waves in AlxGa1−xN layers grown on sapphire substrates by Migration Enhanced Metalorganic Chemical Vapor Deposition (MEMGCVD™) technique was studied at the optical wavelengths of 442 and 633nm and acoustic wavelength of 16μm. In the near-to-Bragg diffraction regime, the diffraction efficiency from 90% to 95% was attained at SAW powers of 0.28 and 0.72W for the blue and red light, respectively. The simulation using photo-elastic and electro-optic constants reported in literature revealed the prevailing contribution of the photo-elastic effect to the AO diffraction. The calculated SAW power required to attain the diffraction maximum was about seven times larger than the measured values. This discrepancy implies that the photo-elastic constants of AlN and GaN available from literature are underestimated. The increase in the diffraction efficiency with the decreasing optical wavelength is in a good agreement with the theoretical prediction. This feature makes AlGaN very promising for AO applications in the deep UV region.
Effect of high-frequency driving current on magnetization reversal in Co-rich amorphous microwires
Authors: Chizhik A. , Zhukov A. , Gonzalez J. , Blanco J. M.
Published: September 2004

Influence of high frequency electric current on the magnetization reversal in Co-rich glass covered amorphous microwires has been studied. The strong correlation between the coercivity and the circular magnetization in the outer shell of the wire has been found. The change of the mechanism of magnetization reversal in the presence of high-frequency circular magnetic field, which is related with the impedance properties, is presented.
Silicon-containing organic matrices as hosts for highly photostable solid-state dye lasers
Authors: Costela A. , García-Moreno I. , del Agua D. , García O. , Sastre R.
Published: September 2004

Modified polymeric matrices incorporating silicon atoms in their structure and doped with lasing dyes have been synthesized and demonstrated to render improved laser performance. When transversely pumped at 532nm, highly stable laser operation with reasonable, not-optimized efficiencies was obtained. Materials were prepared where dyes pyrromethene 567 and pyrromethene 597 emitted laser radiation with no sign of degradation in the laser output, albeit with some oscillations, after 100 000pump pulses in the same position of the sample at 10Hz repetition rate.
Phonon deformation potentials of the E2(high) phonon mode of AlxGa1−xN
Authors: Sarua A. , Kuball M. , Van Nostrand J. E.
Published: September 2004

Micro-Raman spectroscopy was applied to study the E2(high) phonon deformation potentials in AlxGa1−xN material, which are required to convert phonon frequency shifts into stress values. AlxGa1−xN layers were grown by molecular beam epitaxy directly on (111)-oriented Si substrates. Mechanical bending was applied to introduce biaxial stress in the AlxGa1−xN layers and Raman shifts were measured as a function of the applied deformation. The Si phonon mode provided a reference for the applied stress and allowed determination of phonon deformation potential values for AlxGa1−xN. Deformation potentials of the GaN- and AlN-like E2(high) phonon mode of AlGaN alloys were found to be similar to the corresponding values of binary compounds.
Continuous-wave operation of λ∼4.8μm quantum-cascade lasersat room temperature
Authors: Evans A. , Yu J. S. , Slivken S. , Razeghi M.
Published: September 2004

Continuous-wave (cw) operation of quantum-cascade lasers emitting at λ∼4.8μm is reported up to a temperature of 323K. Accurate control of layer thickness and strain-balanced material composition is demonstrated using x-ray diffraction. cw output power is reported to be in excess of 370mW per facet at 293K, and 38mW per facet at 323K. Room-temperature average power measurements are demonstrated with over 600mW per facet at 50% duty cycle with over 300mW still observed at 100% (cw) duty cycle.
Effects of orientation transition on exchange anisotropy of Co∕NiMn films by biorientation epitaxial Cu∕Au∕Cu underlayers
Authors: Lai Chih-Huang , Wang Yung-Hung , Huang R. T.
Published: September 2004

A biorientation structure of Co∕NiMn films, composed of columnar (001) and (111) epitaxial grains, was developed by using Cu∕Au∕Cu underlayers on Si substrates. When the Au layer is thinner than 7.5Å, Co∕NiMn films are epitaxial (001) and show exchange fields of 50Oe for 150ÅCo. When the Au layer is thicker than 7.5Å, parts of the Co∕NiMn films change to the (111) orientation, and the (111) volume fraction increases with increasing Au thickness. The exchange field of Co∕NiMn can be manipulated by altering the relative amount of epitaxial (001) and (111) grains and the exchange field reaches 150Oe, an enhancement of 300%, by altering the orientation from (001) to (111). The formation of (111) grains in the biorientation films may decrease (001) crystallite diameter or introduce multidomains in the NiMn films, which plausibly increase uncompensated spins or frustrated spins in NiMn films, leading to the increase in the exchange fields.
Ballistic hole emission luminescence
Authors: Appelbaum Ian , Russell K. J. , Shalish I. , Narayanamurti V. , Hanson M. P. , Gossard A. C.
Published: September 2004

Using a method complementary to ballistic electron emission luminescence (BEEL), we demonstrate tunnel-junction injection of sub-band-gap hot holes into the valence band of a semiconductor heterostructure to generate band-gap luminescence. This mechanism can be used in a scanning-probe geometry for the development of a simultaneous hole transport and luminescence microscopy of p-type Schottky devices.
Clarifying the origin of near-infrared electroluminescence peaksfor nanocrystalline germanium in metal-insulator-silicon structures
Authors: Kan E. W. H. , Chim W. K. , Lee C. H. , Choi W. K. , Ng T. H.
Published: September 2004

The origin of room-temperature near-infrared electroluminescence (EL) of nanocrystalline germanium (Ge) embedded in oxide was investigated. The nanocrystals were synthesized by partial oxidation of silicon-germanium, Si0.54Ge0.46, films. Under constant current density bias in accumulation and inversion, Ge nanocrystals with diameters of 5 and 10nm exhibit strong luminescence at 1350nm. The 1350nm EL peak is only observed in the presence of elemental Ge and is enhanced with the formation of Ge nanocrystals. The introduction of hydrogen during annealing passivates the dangling bonds at the interfaces of nanocrystals, thus minimizing the energy spread in the 1350nm peak. The 1350nm peak intensity is a function of the injected carrier density, while the peak location remains constant in energy and independent of the applied bias. The results are a clear indication that the luminescence peak originates from radiative recombination of excitons confined in the nanocrystals.
Liquid hyperpolarized Xe129 produced by phase exchange in a convection cell
Authors: Su T. , Samuelson G. L. , Morgan S. W. , Laicher G. , Saam B.
Published: September 2004

We present a method for the production of liquid hyperpolarized Xe129 that employs spin-exchange optical pumping in the gas phase and subsequent phase exchange with a column of xenon liquid. A convection loop inside the sealed glass cell allows efficient transfer of magnetization between the gas and liquid phases. By condensing to liquid a large fraction of the sample, this scheme permits the polarization of many more Xe129 atoms in a given sealed-cell volume than would otherwise be possible. We have thus far produced a steady-state polarization of 8% in 0.1mL of liquid with a characteristic rise time of ≈15min.