bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultrafast transient dynamics of Zn(II) porphyrins: Observation of vibrational coherence by controlling chirp of femtosecond pulses
Authors: Yoon Min-Chul , Jeong Dae Hong , Cho Sung , Kim Dongho , Rhee Hanju , Joo Taiha
Published: December 2002

Femtosecond coherence spectroscopic study on porphyrin molecules has demonstrated that the oscillatory features residing at the transient absorption and fluorescence decay profiles are strongly correlated with the lifetimes of the excited states and the displacements of the minima of the potential energy surfaces that are involved in the pump and probe laser pulses. We have attained a greater degree of control in the wave packet dynamics in the transient absorption by controlling the chirp of the ultrashort optical pulses. This feature provides a clue to the excited potential energy surface such as its curvature and displacement. For two representative porphyrin monomers, ZnII tetraphenylporphyrin and ZnII octaethylporphyrin, we were able to obtain detailed information on the excited state dynamics and subsequent structural changes based on the comparison between the frequency spectra retrieved from the oscillatory features in the transient absorption and fluorescence temporal profiles and the ground state Raman spectra.
Collisionally enhanced isotopic selectivity in multiphoton dissociation of vibrationally excited CF3H
Authors: Boyarkin O. V. , Kowalczyk M. , Rizzo T. R.
Published: December 2002

We have studied infrared multiphoton dissociation of CF3H pre-excited to the second C–H stretch overtone under collisional conditions in view of developing a laser isotope separation scheme for carbon-13. This single stage process results in a C2F4 product that has been enriched in carbon-13 to a level as high as 99% starting from a naturally abundant sample, implying an isotopic selectivity in excess of 9000. While most of the selectivity is gained at the pre-excitation step, it can be increased up to a factor of 16 by collisions of the pre-excited CF313H species with room temperature molecules. This collision-induced enhancement in selectivity becomes evident from the dependence of the isotopic enrichment on both the total sample pressure and the time-delay between the two lasers, and we propose two different models that can account for this behavior. Finally, we evaluate the practical relevance of this two-laser scheme for isotope separation.
Dissociative photoionization of CH2Cl2 and enthalpy of formation of CHCl+: Experiments and calculations
Authors: Chiang Su-Yu , Bahou Mohammed , Sankaran K. , Lee Yuan-Pern , Lu Hsiu-Feng , Su Ming-Der
Published: December 2002

The dissociative photoionization of CH2Cl2 in a region ∼11–20 eV was investigated with photoionization mass spectrometry using a synchrotron radiation source. Major fragment ions CH2Cl+, CHCl+, and CCl+ were observed with their respective appearance energies at 12.08, 12.46, and 15.96 eV. Relative branching ratios as a function of photon energy were determined. Dissociative photoionization channels for formation of these three fragment ions are proposed based on comparison of determined appearance energies and energies predicted with the Gaussian-2 calculations. The principal dissociative process is direct breakage of the C–Cl bond to form CH2Cl++Cl; at greater energies formation of CHCl+ takes place via the CHCl++HCl channel. Formation of CCl+ near the threshold at 15.96 eV may proceed via the CCl++HCl+H channel rather than CCl++H2+Cl channel, but at greater energies the latter dominates. Enthalpy of formation at 0 K of CHCl+ is revised to 288.0±0.7 kcal mol−1 based on our experimental results; the adiabatic ionization energy of CHCl is accordingly revised to 9.12±0.12 eV.
Solvation structure, thermodynamics, and conformational dependence of alanine dipeptide in aqueous solution analyzed with reference interaction site model theory
Authors: Cui Qizhi , Smith Vedene H.
Published: December 2002

With the CHARMM22 (Chemistry at Harvard Macromolecular Mechanics) all-atom nonbonded potential parameters for alanine dipeptide solute and the transferable intermolecular potential model water for the solvent, the reference interaction site model (RISM) integral equations with the hypernetted chain closure are solved to obtain all the atomic solvent–solute radial distribution functions. The solvation structures of alanine dipeptide in its seven conformations: C7eq, C7ax, C5, αR, β, αL and PII, in aqueous solution are analyzed at the atomic level in terms of the atomic solute–solvent radial distribution functions. At a temperature of T=298.15 K and bulk water density ρ=0.9970 g cm−3, the corresponding solvation free energies are calculated by using Singer and Chandler’s analytic solvation free energy formulation [Mol. Phys. 55, 621 (1985)]. Solvation energies, enthalpies, and entropies are also calculated in the RISM theory framework. The conformational dependence of solvation for alanine dipeptide in aqueous solution is analyzed. The solvation thermodynamcs for alanine dipeptide in aqueous solution are mainly controlled by the strong hydrophobic groups: CH3 and CH, which make alanine dipeptide show strong hydrophobicity. But the differences in the solvation thermodynamics for different alanine dipeptide conformers are controlled by the carbonyl groups and amide groups, which make alanine dipeptide show some hydrophilicity and exist in various conformations in aqueous solution. Solvation of alanine dipeptide in aqueous solution is determined by the competition among the molecular packing effects, intramolecular hydrogen bonds, and intermolecular hydrogen bonds. Due to the intramolecular hydrogen bonds in the two folded C7 conformations, the two most favorable conformations in gas phase become the least solvated in aqueous solution. Due to the intermolecular hydrogen bonds, C5, PII, αL, αR, and β become more solvated in aqueous solution.
Supramolecular structure of helical ribbons self-assembled from a β-sheet peptide
Authors: Hwang Wonmuk , Marini Davide M. , Kamm Roger D. , Zhang Shuguang
Published: December 2002

We have investigated the supramolecular structure of helical ribbons formed during self-assembly of a β-sheet peptide using computer simulation. We tested a wide range of molecular packing geometries consistent with the experimental dimensions to identify the most stable structure, and then systematically changed the helical geometry to investigate its energy landscape. The effect of pH was incorporated by scaling the amount of charge on the side chains based on the electrostatic double layer theory. Our results suggest that these left-handed helical ribbons are comprised of a double β-sheet and that the experimentally measured dimensions correspond to a local energy minimum. Side chain interactions are found to be critical in determining the stability and curvature of the helix. Our approach has general applicability to the study of self-assembled nanostructures from β-sheet peptides where high resolution data are not yet available.
An interpretation of the low-frequency spectrum of liquid water
Authors: Padró Joan Àngel , Martı́ Jordi
Published: December 2002

The controversy concerning the microscopic mechanical interpretation of the main spectral features appearing in the low-frequency spectrum of liquid water (0–400 cm−1) at room temperature is addressed. Information obtained from molecular dynamics simulations indicates the connection of the highest frequency band of the low-frequency spectrum of liquid water at ambient conditions (centered around 170 cm−1) with the existence of the hydrogen-bond network characteristic of liquid water, whereas no relation to hydrogen bonding is found for the lowest frequency mode, located at 60 cm−1.
Magnetic field quenching of individual rotational levels of the Ã 1Au, 2v3′ state of acetylene
Authors: Makarov Vladimir I. , Quiñones Edwin
Published: December 2002

Following optical excitation of the Ã 1Au, 2v3′, Ka=0 and 2 states of acetylene under collisionless conditions, population is transferred to the continuum spectrum of the ground state (the dominant process), to neighboring triplet “dark” manifolds as well as to low vibrational states in the ground state (as fluorescence). These states undergo the singlet–triplet crossing reversibly and intramolecular relaxation from the triplet manifold is not significant. Upon applying H in the presence of collisions we observe shortening in the fluorescence decay times and reduction in the amplitudes of decay signals. The magnetic quenching originates from the enhanced participation of triplet levels in the collisional deactivation, caused by the Zeeman mixing of triplet fine and hyperfine levels. The ratio of the singlet to triplet collisional cross sections for individual rotational states of σT/σS≈2, is estimated based on the response of the fluorescence decays to the external magnetic field.
Effects of sizes of charged species on the flocculation of biocolloids: Absorption of cations in membrane layer
Authors: Kuo Yung-Chih
Published: December 2002

The rate of flocculation of suspended biocolloidal particles each covered with an ion-penetrable charged membrane immersed in an electrolyte solution is investigated theoretically. We take into account the effects of sizes of charged species, including cations, anions, and functional groups in the membrane layer, where the absorption of cations by the functional groups leads to the variations in fixed charge density of the membrane phase of a biocolloid. The simulated results reveal that the sizes of cations, anions, and fixed groups have negative effect on the rate of biocolloidal flocculation for an originally negatively charged membrane (ONCM), and the classical point-charge model overestimates this rate. The discrepancy between an ONCM and an originally uncharged membrane (OUCM) for the time required for flocculation increases with the number of functional groups involved in the absorption or chelation of mobile cations. The rate of flocculation produced by an OUCM is faster than that by an ONCM under the same conditions. For an ONCM, the stronger the effect of the cationic absorption by the functional groups, the faster the rate of flocculation, and the reverse is true for an OUCM. Comparing with an OUCM, the effects of membrane thickness, concentration of functional groups, ionic strength of the bulk liquid and relative permittivity of the solution on the variations of flocculation time ratio are more considerable for an ONCM. A thicker membrane, a lower concentration of functional groups, a higher ionic strength of the bulk liquid and a larger relative permittivity of the solution cause a faster rate of flocculation.
Temperature dependence of the viscosity of nonpolymeric liquids
Authors: Hsu Jyh-Ping , Lin Sung-Hwa
Published: December 2002

The temperature dependence of the viscosity of a normal nonpolymeric liquid is analyzed theoretically based on a free volume model. We show that the viscosity is mainly a function of the activation energy for molecular collision. The activation energy is closely related to the temperature, and the relation between the two can be derived based on a cell theory. The applicability of the result derived is justified by fitting it to the experimental data for n-paraffins. We show that, in addition to providing a physically more realistic analytical expression than those in the literature, the analytical result derived in our study is capable of improving the performance of the empirical relation proposed by Doolittle [J. Appl. Phys. 22, 1031 (1951)]. The applicability of the present model to predict the basic properties of glass-forming liquids is also illustrated.
Geometry optimization of molecules in solution: Joint use of the mean field approximation and the free-energy gradient method
Authors: Galván I. Fdez. , Sánchez M. L. , Martín M. E. , Olivares del Valle F. J. , Aguilar M. A.
Published: December 2002

The average solvent electrostatic potential/molecular dynamics (ASEP/MD) and the free-energy gradient methods are applied together with the multidimensional geometry optimization of molecules in solution. The systems studied were formamide in aqueous solution and water and methanol in liquid phase. The solute molecules were described through ab initio quantum mechanics methods (density dunctional theory or Møller–Plesset second order perturbation theory) while the solvent structure was obtained from Molecular Dynamics calculations. The method is very efficient; the increase in computation time is minimal with respect to previous ASEP/MD versions that worked at a fixed geometry. Despite the use of the mean field approximation in the calculation of the solvent reaction potential the agreement with previous theoretical calculations was satisfactory. Large changes were observed in the solute charge distribution induced by the solvent, and the solute polarization was accompanied by an increase in the solvent structure around the solute.
The role of different reorganization energies within the Zusman theory of electron transfer
Authors: Casado-Pascual Jesús , Morillo Manuel , Goychuk Igor , Hänggi Peter
Published: December 2002

We consider the kinetics of electron transfer reactions in condensed media with different reorganization energies for the forward and backward processes. The starting point of our analysis is an extension of the well-known Zusman equations to the case of parabolic diabatic curves with different curvatures. A generalized master equation for the populations as well as formal expressions for their long-time limit is derived. We discuss the conditions under which the time evolution of the populations of reactants and products can be described at all times by a single exponential law. In the limit of very small tunnel splitting, a novel rate formula for the nonadiabatic transitions is obtained. It generalizes previous results derived within the contact approximation. For larger values of the tunnel splitting, we make use of the consecutive step approximation leading to a rate formula that bridges between the nonadiabatic and solvent-controlled adiabatic regimes. Finally, the analytical predictions for the long-time populations and for the rate constant are tested against precise numerical solutions of the starting set of partial differential equations.
Free energy determination of phase coexistence in model C60: A comprehensive Monte Carlo study
Authors: Costa D. , Pellicane G. , Abramo M. C. , Caccamo C.
Published: December 2002

The free energy of the solid and fluid phases of the Girifalco C60 model are determined through extensive Monte Carlo simulations. In this model the molecules interact through a spherical pair potential, characterized by a narrow and attractive well, adjacent to a harshly repulsive core. We have used the Widom test particle method and a mapping from an Einstein crystal in order to estimate the absolute free energy in the fluid and solid phases, respectively; we have then determined the free energy along several isotherms, and the whole phase diagram, by means of standard thermodynamic integrations. The dependence of the simulation’s results on the size of the sample is also monitored in a number of cases. We highlight how the interplay between the liquid–vapor and the liquid–solid coexistence conditions determines the existence of a narrow liquid pocket in the phase diagram, whose stability is assessed and confirmed in agreement with previous studies. In particular, the critical temperature follows closely an extended corresponding-state rule recently outlined by Noro and Frenkel [J. Chem. Phys. 113, 2941 (2000)]. We discuss the emerging “energetic” properties of the system, which drive the phase behavior in systems interacting through short-range forces [A. A. Louis, Philos. Trans. R. Soc. London, Ser. A 359, 939 (2001)], in order to explain the discrepancy between the predictions of several structural indicators and the results of full free energy calculations, to locate the fluid phase boundaries. More generally, we aim to provide extended reference data for calculations of the free energy of the C60 fullerite in the low temperature regime, as for the determination of the phase diagram of higher order Cn>60 fullerenes and other fullerene-related materials, whose description is based on the same model adopted in this work.
Theory of intrapolymer excimer-formation kinetics
Authors: Sung Jaeyoung , Lee Jinuk , Lee Sangyoub
Published: December 2002

We generalize the Wilemski–Fixman theory for reversible polymer cyclization to treat the kinetics of intrachain excimer-formation reactions. While most previous theories for intrachain reactions dealt with the end-to-end reaction case, we consider the general situation in which the reacting groups are located at any place on the chain backbone. Various aspects of the reaction kinetics, such as the effect of hydrodynamic interaction and the dependence of reaction rate on the positions of reacting groups as well as on the chain length, are investigated.
Vibrational spectroscopy of HOD in liquid D2O. III. Spectral diffusion, and hydrogen-bonding and rotational dynamics
Authors: Lawrence C. P. , Skinner J. L.
Published: December 2002

Time-resolved infrared spectroscopy has the potential to provide unprecedented information about molecular dynamics in liquids. In the case of water, one of the most exciting techniques being developed is transient hole-burning. From experiments on dilute HOD in D2O one can obtain the transition frequency time-correlation function for the OH stretch vibration, finding that it decays on a time scale of between 0.5 and 1 ps. In this paper we provide a molecular-level interpretation of this spectral diffusion time-correlation function. First, we verify that for hydrogen-bonded HOD molecules the instantaneous OH frequency is highly correlated with the distance to the (hydrogen-bonded) D2O molecule. Second, we show that the instantaneous OH frequency is highly correlated with whether or not the HOD molecule has a hydrogen bond. Finally, we show that the short-time dynamics of the spectral diffusion time-correlation function is due to hydrogen-bond stretching motions, while the longer-time decay observed in the experiments is due to the dynamics of forming and breaking hydrogen bonds. We also present theoretical results that describe recent polarization anisotropy experiments, which measure frequency-dependent rotational dynamics.
Time-dependent laminar electrokinetic slip flow in infinitely extended rectangular microchannels
Authors: Yang Jun , Kwok Daniel Y.
Published: December 2002

A rectangular microchannel is the most popular shape to be widely used in microelectromechanical devices. However, analytical solutions of flow in this shape of microchannels are seldom reported because of its two-dimensional problem. Furthermore, microflows in microchannels are more difficult to describe. In this paper, we addressed analytically the problem of laminar electrokinetic slip flow in microchannels with a rectangular cross section subjected to a time-dependent pressure gradient and a time-dependent electric field. The analytical solution has been determined based on the Debye–Hückel approximation and its solution is useful to precisely control microflow with slip in rectangular cross-section microchannels.
Spin exchange interactions of a spin dimer: Analysis of broken-symmetry spin states in terms of the eigenstates of Heisenberg and Ising spin Hamiltonians
Authors: Dai Dadi , Whangbo Myung-Hwan
Published: December 2002

We examined the eigenstates of the Heisenberg spin Hamiltonian Ĥ=−JŜ1⋅Ŝ2 and the Ising spin Hamiltonian ĤIsing=−JŜ1zŜ2z for a general spin dimer consisting of M unpaired spins at one spin site and N unpaired spins at the other spin site, and then analyzed how the broken-symmetry spin state of a spin dimer is related to the eigenstates of Ĥ and ĤIsing. Our work shows that the description of the highest-spin and broken-symmetry spin states of a spin dimer by Ĥ is the same as that by ĤIsing. For the analysis of spin exchange interactions of a magnetic solid on the basis of density functional theory, the use of the Heisenberg spin Hamiltonian in the “cluster” approach is consistent with that of the Ising spin Hamiltonian in the “noncluster” approach.
Gibbs ensemble simulation of symmetric mixtures composed by the homopolymers AA, BB and their common block copolymer AB
Authors: Poncela Antonio , Rubio Ana M. , Freire Juan J.
Published: December 2002

A Gibbs ensemble algorithm implemented previously for mixtures of homopolymers and copolymers in a cubic lattice with coordination number z=26 is now used to characterize the complete phase separation diagram of the ternary mixtures formed by AA and BB homopolymers and their common symmetric diblock copolymer AB, all chains of the same length. We consider two alternative types of systems with repulsions between neighboring A and B units or with attractions between A and A or B and B neighboring units. A certain proportion of voids is included in both cases. The 3-phase region of the diagram is obtained by using a 3-box algorithm that has not been previously employed for polymer mixtures. The 3-phase region is composed of two homopolymer-rich asymmetric phases, each one mainly composed of a different homopolymer component together with a small proportion of copolymer. These two phases are in equilibrium with a third phase rich in copolymer. The 3-phase region connects smoothly with the 2-phase region covered by our 2-box simulations reported earlier for low copolymer composition. Similarly, it also connects with two asymmetric 2-phase regions characterized now through 2-box simulations performed with a low initial proportion of one of the homopolymer components. The data obtained for the systems with repulsions are in agreement with earlier single box Monte Carlo estimations for ternary mixtures with a symmetric homopolymer composition. They show a non-clearly structured copolymer-rich phase with a substantial amount of homopolymer, while the presence of copolymer in the AA or BB enriched phases is small. The systems with attractions, however, exhibit important differences with respect to this picture, due to a more specific role of the voids. Some differences with respect to the mean-field theory predictions are also discussed.
Liquid–liquid phase separation in solutions of living semiflexible polymers
Authors: Rane Sagar S.
Published: December 2002

We consider a model of living semiflexible polymers in a solution and obtain results for the liquid–liquid phase separation, which has been a subject of growing interest in the literature and has been studied in some recent experiments. The tetrahedral lattice model is solved exactly on a Husimi cactus of coordination number q=4. The exact solution on the Husimi cactus forms the approximate theory for the original lattice. We consider the end groups to be a different species from the middle groups. This allows us to incorporate the end-group effects in our calculations, which become important at low molecular weights. We investigate the effect of chain rigidity, end-group/middle-group interactions, solvent quality, and the chemical potential of the end-group on the liquid–liquid coexistence curve. We also calculate the average molecular weights of the coexisting liquid phases. The bending penalty ε is found to shift the phase diagram only slightly. Attractive end-group/middle-group interaction enhances the phase separation. Due to the exactness of our calculations, the theory never violates thermodynamics and is applicable in the entire parameter space. Our theory also improves upon the classical theories and hence our results should prove useful.
High frequency acoustic modes in vitreous beryllium fluoride probed by inelastic x-ray scattering
Authors: Scopigno T. , Yannopoulos S. N. , Kastrissios D. Th. , Monaco G. , Pontecorvo E. , Ruocco G. , Sette F.
Published: December 2002

Inelastic x-ray scattering measurements of the dynamics structure factor have been performed on vitreous beryllium fluoride (v-BeF2) at T=297 K in the momentum transfer, Q, range Q=1.5–10 nm−1. We find evidence of well defined high frequency acoustic modes. The energy position and linewidth of the excitations disperse with Q as ∝Q and ∝Q2, respectively, up to about one half of the first maximum of the static structure factor. Their magnitude compares favorably with low-frequency sound velocity and absorption data. The results indicate worth mentioning similarities of the high frequency collective dynamics of different network forming glasses such as v-B2O3 and v-SiO2.
Electron-stimulated hydrogen desorption from diamond surfaces and its influence on the low-pressure synthesis of diamond
Authors: Goeden C. , Dollinger G.
Published: December 2002

A total cross section σD=(5±2.6)×10−18 cm2 is measured for electron-stimulated desorption (ESD) of deuterium from a boron-doped, deuterated diamond(100) surface at 5 eV incident electron energy. This large ESD cross section means a significant contribution of ESD to hydrogen abstraction reactions in microwave-driven chemical vapor deposition of diamond. The ESD cross section decreases when changing to a nitrogen-doped diamond. This change is suggested to be the reason for the reported influence of small concentrations of nitrogen or boron added to the process gas on diamond growth.
Colloidal particle foams: Templates for Au nanowire networks?
Authors: O’Shea J. N. , Phillips M. A. , Taylor M. D. R. , Moriarty P. , Brust M. , Dhanak V. R.
Published: December 2002

Spin coating a dilute solution of thiol-passivated Au nanoparticles onto silicon produces nanostructured cellular networks. Photoemission measurements, coupled with atomic force microscopy imaging and a statistical crystallography analysis, show that although annealing in the 500–600 K range removes the thiol surfactants surrounding the nanoparticles, the cellular morphology of the nanocrystal foam is preserved following annealing. Thus, self-assembled nanocrystal arrays may be exploited as templates for (bare) Au nanostructures on Si. Although appreciable particle diffusion during annealing does not occur, significant sintering of Au nanocrystals within the cellular network branches is observed.
Microstructure and field emission properties of coral-like carbon nanotubes
Authors: Shang N. G. , Li C. P. , Wong W. K. , Lee C. S. , Bello I. , Lee S. T.
Published: December 2002

Coral-like carbon nanotubes (CNTs) have been synthesized by using chemical vapor deposition. Unlike conventional CNTs, the as-deposited CNT consisted of a high density of interlaced graphitic nanoflakes of about 5 nm in thickness. The CNTs had a bamboo-like internal structure with their outer walls consisting of many open graphite layers, and the nanoflakes stemming from the “bamboo knots.” The growth mechanism of CNTs was discussed. The field emission characteristics of CNT films showed a turn-on field as low as 4 V/μm. This special CNT might extend mechanical and electronic properties and applications of the CNTs.
Direct imaging of the depletion region of an InP p–n junction under bias using scanning voltage microscopy
Authors: Ban D. , Sargent E. H. , Dixon-Warren St. J. , Calder I. , SpringThorpe A. J. , Dworschak R. , Este G. , White J. K.
Published: December 2002

We directly image an InP p–n junction depletion region under both forward and reverse bias using scanning voltage microscopy (SVM), a scanning probe microscopy (SPM) technique. The SVM results are compared to those obtained with scanning spreading resistance microscopy (SSRM) measurements under zero bias on the same sample. The SVM and SSRM data are shown to agree with the results of semiclassical calculations. The physical basis of the SVM measurement process is also discussed, and we show that the measured voltage is determined by the changes in the electrostatic potential and the carrier concentration at the SVM tip with and without the applied bias.
High-Curie-temperature Ga1−xMnxAs obtained by resistance-monitored annealing
Authors: Edmonds K. W. , Wang K. Y. , Campion R. P. , Neumann A. C. , Farley N. R. S. , Gallagher B. L. , Foxon C. T.
Published: December 2002

We show that by annealing Ga1−xMnxAs thin films at temperatures significantly lower than in previous studies, and monitoring the resistivity during growth, an unprecedented high Curie temperature TC and conductivity can be obtained. TC is unambiguously determined to be 118 K for Mn concentration x=0.05, 140 K for x=0.06, and 120 K for x=0.08. We also identify a clear correlation between TC and the room temperature conductivity. The results indicate that Curie temperatures significantly in excess of the current values are achievable with improvements in growth and post-growth annealing conditions.
Charge redistribution in YBa2Cu3O7−d probed by Raman spectroscopy: CuO2-plane phonon as a probe of carrier dynamics in the CuO2 plane
Authors: Osada Minoru , Kakihana Masato , Käll Mikael , Börjesson Lars
Published: December 2002

Raman-scattering spectra were investigated for the Y1−xCaxBa2Cu3−yCoyO7−d system which undergoes a metal-to-insulator transition under variation of carrier concentration. As the system approaches the metal-to-insulator phase boundary, the intensity of CuO2-plane phonon modes decreases. We found that the intensity of the CuO2-plane mode scales with the inverse of the hole concentration within the CuO2 conducting planes. A remarkable correlation is found between the CuO2-plane mode intensity and Tc for Y1−xCaxBa2Cu3−yCoyO7−d. We suggest that this correlation as well as the CuO2-plane mode intensity offer a simple and useful probe for metal-to-insulator transition and carrier dynamics in the CuO2 plane for substituted materials.
Optical spectroscopy of single InAs/InGaAs quantum dots in a quantum well
Authors: Kaiser S. , Mensing T. , Worschech L. , Klopf F. , Reithmaier J. P. , Forchel A.
Published: December 2002

We have grown self-assembled InAs quantum dots embedded in the center of an InGaAs quantum well by molecular-beam epitaxy. Using electron-beam lithography and wet etching techniques, small mesas with only a few quantum dots were fabricated. At room temperature, the quantum dots have an emission wavelength of 1.3 μm. By photoluminescence spectroscopy at low temperatures, we observe the emission lines of excitons and biexcitons in single-dot structures. The assignment of exciton and biexciton recombination is based on the characteristic excitation intensity dependence of these states. A biexciton binding energy of about 3.5 meV is obtained for the present dots.
Metal–insulator–metal transition in Sr2Rh1−xRuxO4(0⩽x⩽1)
Authors: Koida T. , Wakisaka T. , Itaka K. , Koinuma H. , Matsumoto Y.
Published: December 2002

Structural and electrical properties of c-axis oriented epitaxial Sr2Rh1−xRuxO4 (0⩽x⩽1) films were systematically investigated. The composition-spread films were fabricated on a temperature gradient (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate to quickly optimize the growth temperature for a high-quality crystalline film having each composition (x). An anomalous c-axis length dependence on x was observed, and it was accompanied by a distinct change in electric property, that is, a metal–insulator–metal transition.
Optical characteristics of hexagonal GaN self-assembled quantum dots: Strong influence of built-in electric field and carrier localization
Authors: Cho Yong-Hoon , Kwon B. J. , Barjon J. , Brault J. , Daudin B. , Mariette H. , Dang Le Si
Published: December 2002

Optical characteristics of hexagonal GaN self-assembled quantum dots (QDs) were systematically studied by photoluminescence (PL), PL excitation (PLE), time-resolved PL, and cathodoluminescence (CL). We observed a Stokes-like shift between PLE absorption edge and PL emission from the GaN QDs as well as from the Al(Ga)N base layer. With decreasing emission energy, the measured lifetime of the hexagonal GaN QDs emission increased, while that of the cubic GaN QDs kept almost constant. The optical emission from the GaN QDs was measured as a function of temperature from 10 to 300 K, and their properties were compared with GaN quantum-well structures. With increasing temperature, the PL intensity of Al(Ga)N base layer or GaN quantum wells was dramatically decreased, while that of GaN QDs was not changed much. We observed CL images showing strong carrier localization in GaN QDs. Therefore, we conclude that the GaN QD emissions are strongly influenced by built-in electric field as well as by carrier localization in the QDs.
Charge imaging and manipulation using carbon nanotube probes
Authors: Tzeng S.-D. , Wu C.-L. , You Y.-C. , Chen T. T. , Gwo S. , Tokumoto H.
Published: December 2002

Due to their high aspect ratio, well-defined cylindrical structure, and good electrical conductivity, carbon nanotubes (CNTs) are ideal probes for “true” local imaging of electric domain structures at the nanoscale. By performing force–distance measurements and tip-shape profiling with a uniformly charged oxide square, we clearly demonstrate the local nature of the CNT tip for electrostatic force microscopy. We show that CNTs can be used to probe long-range electrostatic forces with a lateral resolution better than 5 nm.
Evidence of the relaxor-paraelectric phase transition in Pb(Fe2/3W1/3)O3 ceramics
Authors: Mitoseriu Liliana , Carnasciali Maria M. , Piaggio Paolo , Nanni Paolo
Published: December 2002

The transition relaxor-paraelectric in Pb(Fe2/3W1/3)O3 ceramics was studied by x-ray diffraction and Raman scattering at temperatures in the range of 85–800 K. For high diffraction angles (160° and 162°), a slight difference between the two phases was found. Raman spectra show an evolution with temperature with anomalies of bands of 848 cm−1 and 147 cm−1 (shift and intensity) in the Curie range of temperatures. This behavior is interpreted as an experimental evidence of perturbations of the nanopolar ordering related to the phase transition.
Highly crystallized as-grown smooth and superconducting MgB2 films by molecular-beam epitaxy
Authors: van Erven A. J. M. , Kim T. H. , Muenzenberg M. , Moodera J. S.
Published: December 2002

We have investigated the growth of superconductive thin films of magnesium diboride (MgB2) by molecular-beam epitaxy. A Si(111) substrate with a seed layer of MgO was used for the growth of these films by varying parameters such as the growth temperature, Mg:B flux ratio and deposition rate as well as the background pressure. It was found that highly crystallized films could already form at 250 °C; however, only in a narrow window of growth parameters. The highest critical temperature of 35.2 K with a sharp transition (ΔTC of 0.5 K) was observed for films grown at 300 °C. Using a capping layer of MgO proved to be highly beneficial for the preservation and the smoothness of these films. Together with the fact that MgO proved to be a good seed layer for thin films of MgB2 makes it an ideal candidate for growing all epitaxial MgB2 Josephson junctions.
Optical study of the full photonic band gap in silicon inverse opals
Authors: Palacios-Lidón E. , Blanco A. , Ibisate M. , Meseguer F. , López C. , Sánchez-Dehesa J.
Published: December 2002

An optical study of the band structure of both silicon–silica composite and silicon inverse opals is presented. The study is aimed at demonstrating the development of a full photonic band gap for a system already revealed as paradigmatic. The characterization is based on the comparison between the band structure calculations and optical reflectance spectroscopy experiments. This study is carried out for various symmetry points in the Brillouin zone, some never explored before as K, (110) and W, (210). The results show that, in accordance with the band structure, there is a certain frequency range that produces a reflectance peak regardless of orientation and can be assigned to the band gap. Similarly all other reflectance peaks can be accounted for by other band structure features.
AlGaN layers grown on GaN using strain-relief interlayers
Authors: Chen C. Q. , Zhang J. P. , Gaevski M. E. , Wang H. M. , Sun W. H. , Fareed R. S. Q. , Yang J. W. , Khan M. Asif
Published: December 2002

We report on a study to compare the growth of thick AlGaN layers on GaN with different strain-relief interlayers. A set of ten period AlN/AlGaN superlattices was found to be the most efficient approach for the strain relief. The superlattice interlayer not only decreases the tensile strain but also improves the crystal structural quality. Thus, 2-μm-thick, high quality n+-Al0.2Ga0.8N layers can be grown on GaN epilayers without any cracks.
Hundred-micron-sized all-solid-state Li secondary battery arrays embedded in a Si substrate
Authors: Kushida K. , Kuriyama K. , Nozaki T.
Published: December 2002

Fabrication and charge/discharge behaviors of all-solid-state Li secondary battery arrays embedded into three Si trenches of 200×100 μm2 in area and 0.5∼2.0 μm in depth are presented. The battery arrays in a Si wafer, consisting of multiple layers (Al current collector/Li/SiO2–15 at. %P2O5/LiMn2O4/polycrystalline silicon current collector), are prepared by combining a sol-gel spin-coating method and Si very-large-scale integration technologies. Porous spin-on glass (SiO2–15 at. %P2O5) is adopted as an electrolyte layer, in which spatial paths for Li+ ions are artificially introduced into the glass. Each active battery area is isolated with double insulating walls (Si3N4/SiO2). The battery arrays demonstrate a constant capacity of ∼9.2 μA h cm−2 at ∼3.6 V up to 100 cycles.
Spinodal patterning in organic–inorganic hybrid layer systems
Authors: Müller-Wiegand M. , Georgiev G. , Oesterschulze E. , Fuhrmann T. , Salbeck J.
Published: December 2002

We demonstrate a general feature of organic films between a substrate and a deformable cladding layer: the capability of forming periodic structures by heating above the glass transition temperature of the organic film. This patterning process by self-organization is caused by the interplay of dispersion interactions and mechanical stress which leads to spinodal deformation. The dynamic evolution of the structures was investigated by time-resolved light diffraction. By prepatterning the substrate, the direction of the waves can be controlled.
Field-induced anisotropic nitrogen distribution as the source of uniaxial magnetic anisotropy in (Fe0.98Al0.02)1−δNδ films
Authors: Liu Y-K. , Kryder M. H. , Harris V. G.
Published: December 2002

Extended x-ray absorption fine structure is employed to describe a physical fingerprint that distinguishes the easy and hard in-plane magnetic axes in (Fe0.98Al0.02)1−δNδ films. This fingerprint manifests as an anisotropic strain field about the Fe atom and is shown to scale with magnetic anisotropy energy as δ increases. We propose this effect is due to an anisotropic distribution of N atoms caused by the application of a magnetic field during growth.
Observations of pressure-wave-excited contrast agent bubbles in the vicinity of cells
Authors: Wolfrum B. , Mettin R. , Kurz T. , Lauterborn W.
Published: December 2002

Levovist contrast agent bubble dynamics after spark-induced pressure wave exposure is investigated in the vicinity of normal rat kidney fibroblast cells. The bubble dynamics is calculated using the Keller–Miksis model and results are compared with the experimental radius time curves. High-speed microscopic images show rapid bubble expansion and collapse. Even at moderate peak negative pressure amplitudes of less than 2 MPa the contrast agent bubbles have been observed to expand to more than 30 times their original radius and to rupture cells upon collapse.
Two charge states of dominant acceptor in unintentionally doped GaN: Evidence from photoluminescence study
Authors: Reshchikov M. A. , Morkoç H. , Park S. S. , Lee K. Y.
Published: December 2002

Photoluminescence of the dominant deep-level acceptor in high-purity freestanding GaN is studied over a wide range of excitation intensities. A yellow luminescence (YL) band at about 2.2 eV saturates with increasing excitation intensity, whereas a green luminescence (GL) band at about 2.5 eV increases as a square of the excitation intensity. The YL and GL bands are attributed to two charge states of the same defect, presumably a gallium vacancy-oxygen complex.
Vacancy clustering model for Ti non-steady-state radiation-enhanced diffusion in MgO(100)
Authors: Lu M. , Lupu C. , Rabalais J. W.
Published: December 2002

Ti diffusion in MgO(100) prebombarded by Ar+, Cl+, and Cr+ has been studied by secondary ion mass spectroscopy under conditions for non-steady-state radiation-enhanced diffusion. The Ti diffusion behaviors of these samples are different, although the ballistic behaviors of the implanting ions are similar. For short annealing periods, the Ti diffusion enhancements follow the order Cr+>Ar+>Cl+, while for long annealing periods the order is Cl+>Ar+>Cr+. A model based on a vacancy clustering mechanism during thermal relaxation of the energetic cascade is proposed to explain these trends.
Comprehensive above-threshold analysis of large-aperture (8–10 μm) antiresonant reflecting optical waveguide diode lasers
Authors: Chang J. C. , Lee J. J. , Al-Muhanna A. , Mawst L. J. , Botez D.
Published: December 2002

An above-threshold analysis of 8–10-μm-core antiresonant reflecting optical waveguide (ARROW) lasers is performed, including the carrier-induced index depression, carrier diffusion, and gain spatial hole burning (GSHB). The study is done as a function of the (transverse) optical-mode confinement factor Γ and the core width. Just as for index-guided devices, it is found that ARROW devices (i.e., index-antiguided devices) are much less immune to multimoding via GSHB the smaller the value of Γ. For the case Γ=3%, the high-order mode of most concern reaches the threshold much earlier than for the case Γ=1%, due both to gain-profile distortion as well as to distortion of the effective-index profile (in the device core) with increasing drive level. Devices of 8.5-μm-wide cores and Γ=1%, are found to stay single-mode to at least 40× threshold, which in turn allows the projection of stable, single-mode operation to 1.2 W output power. In contrast, 10-μm-core devices become multimode at around 10× threshold. Preliminary experimental results from 10-μm-wide core ARROW lasers, with Γ=1.5%, are stable, single-mode operation to 300 mW at 10× threshold, in good agreement with the presented study.
GaN nanorods coated with pure BN
Authors: Han Wei-Qiang , Zettl A.
Published: December 2002

We report a method to efficiently synthesize gallium nitride (GaN) nanorods coated with insulating boron nitride (BN) layers. The GaN core is crystalline (with either a cubic zincblende or hexagonal wurtzite structure) and has diameters ranging from 10 to 85 nm and lengths up to 60 μm. The outer encapsulating BN shells with typical thicknesses less than 5 nm extend fully over, and adhere well to, the entire nanorod surface.
Reversible adsorption-enhanced quantum confinement in semiconductor quantum dots
Authors: Sarkar Shaibal K. , Chandrasekharan Nirmala , Gorer Sasha , Hodes Gary
Published: December 2002

Semiconductor quantum dots possess two important properties: Size-dependent increase in band gap (size quantization) and a very high surface-to-volume ratio. We show that cyanide adsorption on this high surface further increases the size quantization in films of CdSe and CdS quantum dots and that removal of the cyanide reverses this effect. This effect is explained by increased localization due to compression of the electron wave function in the dots by the strongly adsorbed [on both Cd and Se(S)] negatively charged cyanide.
Lasing from a single-quantum wire
Authors: Hayamizu Yuhei , Yoshita Masahiro , Watanabe Shinichi , Akiyama Hidefumi , Pfeiffer Loren N. , West Ken W.
Published: December 2002

A laser with an active volume consisting of only a single-quantum wire in the one-dimensional (1D) ground state is demonstrated. The single wire is formed quantum mechanically at the T-shaped intersection of a 14 nm Al0.07Ga0.93As quantum well and a 6 nm GaAs quantum well, and is embedded in a 1D single-mode optical waveguide. We observe single-mode lasing from the quantum-wire ground state by optical pumping. The laser operates from 5 to 60 K, and has a low threshold pumping power of 5 mW at 5 K.
Melt-textured YBa2Cu3O7 based material doped with Li and Zn: Comparison of high trapped fields and pinning
Authors: Shlyk L. , Krabbes G. , Fuchs G. , Nenkov K. , Verges P.
Published: December 2002

The pinning forces and trapped fields obtained in bulk YBa2Cu3O7 (YBCO)-based material doped with Li are compared with results reported for melt-processed samples doped with Zn. The increase of the volume pinning force of Li doped material is twice that of Zn doped YBCO at 77 K. This leads to 20% higher magnetic fields trapped in the Li-doped sample at 77 K.
Optimal rubbing angle for reflective in-plane-switching liquid crystal displays
Authors: Sun Yubao , Zhang Zhidong , Ma Hongmei , Zhu Xinyu , Wu Shin-Tson
Published: December 2002

Rubbing angle effects on the response time and optical threshold voltage of the reflective in-plane-switching homogeneous liquid crystal displays were analyzed. As the rubbing angle increases, the rise time decreases except that its operating voltage also increases. The optimal rubbing angle is around 30°–40°.
Post-growth p-type doping enhancement for ZnSe-based lasers using a Li3N interlayer
Authors: Schulz Oliver , Strassburg Matthias , Rissom Thorsten , Pohl Udo W. , Bimberg Dieter , Klude Matthias , Hommel Detlef
Published: December 2002

A method to increase decisively the p-type doping level in ZnSe-based laser diodes is described. Upon Li3N indiffusion, the formation of a stable acceptor complex is observed. Free hole concentrations of 8×1018 cm−3 are obtained. This value is at least one order of magnitude larger than typical p-type doping levels achieved by molecular-beam epitaxy of ZnSe. In addition, no compensation effects occur, as usually observed for p-type doping using either Li or N. ZnSe-based lasers processed by applying this post-growth p-doping enhancement technology show significantly improved properties.
Simulation of interface states effect on the scanning capacitance microscopy measurement of p–n junctions
Authors: Yang J. , Kong F. C. J.
Published: December 2002

A two-dimensional numerical simulation model of interface states in scanning capacitance microscopy (SCM) measurements of p–n junctions is presented. In the model, amphoteric interface states with two transition energies in the Si band gap are represented as fixed charges to account for their behavior in SCM measurements. The interface states are shown to cause a stretch-out and a parallel shift of the capacitance–voltage characteristics in the depletion and neutral regions of p–n junctions, respectively. This explains the discrepancy between the SCM measurement and simulation near p–n junctions, and thus modeling interface states is crucial for SCM dopant profiling of p–n junctions.
Miniaturized, highly sensitive single-chip multichannel quartz-crystal microbalance
Authors: Hung Vu Ngoc , Abe Takashi , Minh Phan Ngoc , Esashi Masayoshi
Published: December 2002

A miniaturized highly sensitive single-chip multichannel quartz-crystal microbalance prepared by deep reactive ion etching is presented. In the present work, quartz resonators in a single-chip with the diameters in the range 0.05–1.0 mm and thicknesses in the range 18–82 μm were fabricated. The conductance measurements carried out on the resonators showed that the Q factor is inversely proportional to resonator thickness. The Q-factor value as high as ∼30 000 has been observed in case of a 94 MHz resonator whose diameter is 1 mm and the thickness 17.8 μm. The Q factor of a resonator of very small diameter (0.1 mm) reached the value 5700.