bluuebunny/crossref_metadata_2025 huggingface dataset:
Piezoelectric properties of nonstoichiometric Sr1−xBi2+2x∕3Ta2O9 ceramics
Authors: Jain Rajni , Chauhan Arun Kumar Singh , Gupta Vinay , Sreenivas K.
Published: June 2005

The effect of poling on the structural, dielectric, and piezoelectric properties has been investigated for sol-gel-derived strontium bismuth tantalate (SBT) [Sr1−xBi2+2x∕3Ta2O9] ceramics with x=0.0,0.15,0.30,0.45. The dielectric and ferroelectric properties are found to improve with increase in x up to 0.3. Beyond x>0.3 the properties are found to degrade due to the limited solid solubility and the presence of a mixed phase of bismuth tantalate (BiTaO4) is detected with x=0.45. Poling treatment reduces the dielectric dispersion and dielectric loss in the frequency range (0.1–100kHz). The resonance and antiresonance frequencies increase with increase in x (x=0–0.30), and the corresponding minimum impedance decreases. The measured coupling coefficients (kp) are small (0.0967–0.1) for x=0–0.30, and the electromechanical quality factor (Qm=915) is a maximum for the Sr0.7Bi2.2Ta2O9 composition (x=0.30). The estimated piezoelectric charge coefficient (d31) and piezoelectric voltage coefficient (g31) are 5.2pC∕N and 5.8×10−3Vm∕N, respectively. The positive values of d31 and g31 and the low dielectric permittivity of SBT yield a high value for the hydrostatic coefficients, despite the low charge coefficient of d33=24pC∕N. The maximum values of charge coefficient (dh=34pC∕N) and voltage coefficient (gh=39×10−3Vm∕N) are obtained for Sr0.7Bi2.2Ta2O9 composition, and the estimated hydrostatic figure of merit (dhgh×10−15=1215m2/N) is high.
Measurement of metastable He*(2S13) density in dielectric barrier discharges with two different configurations operating at around atmospheric pressure
Authors: Tachibana K. , Kishimoto Y. , Sakai O.
Published: June 2005

We have measured the density of metastable He atoms in the lowest triplet state (2S13) with a diode-laser absorption spectroscopic technique in atmospheric pressure plasmas produced by dielectric barrier discharge schemes. Two different types of electrode configuration are employed: one is a conventional parallel-plate system and the other is a microdischarge integrated system with stacked metal-mesh electrodes covered by insulating films. We have analyzed the pressure-broadened spectral line corresponding to the 2S13→2PJ3 (J=0–2) transition to derive the broadening coefficient and to calibrate absolute densities. The measured density ranges from 1011 to 1012cm−3, but the values in the mesh-type system are larger than those in the parallel-plate system by about one order of magnitude. The density, however, depends strongly on the gas flow rate, showing the influence of quenching by the Penning-ionization process with impurities. Those behaviors are consistent with the variation of the electron density estimated by millimeter-wave transmittance measurement.
Quasi-donor-acceptor pair transitions in GaAsSb and AlGaAsSb on InP
Authors: Toginho Filho D. O. , Dias I. F. L. , Laureto E. , Duarte J. L. , Lourenço S. A. , Poças L. C. , Prabhu S. S. , Klem J.
Published: June 2005

We identify quasi-donor-acceptor pair transitions in the photoluminescence spectra of GaAsSb and AlGaAsSb layers, lattice matched to InP, and grown by molecular-beam epitaxy. These alloys show compositional inhomogeneity due to phase separation resulting from miscibility gaps. The presence of Al in the quaternary alloy increases the fluctuation of the electrostatic potential in the epitaxial layer, increasing the variation of recombination energy as a function of intensity excitation in the range of low temperatures.
Molecular simulation of loading-dependent diffusion in nanoporous materials using extended dynamically corrected transition state theory
Authors: Dubbeldam D. , Beerdsen E. , Vlugt T. J. H. , Smit B.
Published: June 2005

A dynamically corrected transition state theory method is presented that is capable of computing quantitatively the self-diffusivity of adsorbed molecules in confined systems at nonzero loading. This extention to traditional transition state theory is free of additional assumptions and yields a diffusivity identical to that obtained by conventional molecular-dynamics simulations. While molecular-dynamics calculations are limited to relatively fast diffusing molecules, our approach extends the range of accessible time scales significantly beyond currently available methods. We show results for methane, ethane, and propane in LTL- and LTA-type zeolites over a wide range of temperatures and loadings, and demonstrate the extensibility of the method to mixtures.
Nuclear-magnetic-resonance shielding constants calculated by pseudospectral methods
Authors: Cao Yixiang , Beachy Michael D. , Braden Dale A. , Morrill Laurie , Ringnalda Murco N. , Friesner Richard A.
Published: June 2005

We have developed an algorithm based upon pseudospectral (PS) ab initio electronic structure methods for evaluating nuclear magnetic shielding constants using gauge-including atomic orbitals (GIAOs) in the spin-restricted and spin-unrestricted formalisms of Hartree–Fock (HF) theory and density-functional theory (DFT). The nuclear magnetic shielding constants for both H1 and C13 calculated using PS methodology for 21 small molecules have absolute mean errors of less than 0.3ppm in comparison with analytic integral results. CPU timing comparisons between PS methods and conventional methods carried out for seven large molecules ranging from 510 to 1285 basis functions demonstrate that the PS methods are an order of magnitude more efficient than the conventional methods. PS-HF was between 9 and 26 times faster than conventional integral technology, and PS-DFT (Becke three-parameter Lee–Yang–Parr) was between 6 and 21 times faster.
Molecular-dynamics studies of bending mechanical properties of empty and C60-filled carbon nanotubes under nanoindentation
Authors: Jeng Yeau-Ren , Tsai Ping-Chi , Fang Te-Hua
Published: June 2005

This paper utilizes molecular-dynamics simulations to investigate the mechanical characteristics of a suspended (10, 10) single-walled carbon nanotube (SWCNT) during atomic force microscopy (AFM) nanoindentation at different temperatures. Spontaneous topological transition of the Stone–Wales (SW) defects is clearly observed in the indentation process. The present results indicate that under AFM-bending deformation, the mechanical properties of the SWCNT, e.g., the bending strength, are dependent on the wrapping angle. In addition, it is also found that the radial dependence of the reduced formation energy of the SW defects is reasonably insensitive only for the small tubes. However, for tube diameters greater than 2.4 nm [corresponding to the (18, 18) CNT], the SW defects tend to be more radius sensitive. The results indicate that the bending strength decreases significantly with increasing temperature. This study also investigates the variation in the mechanical properties of the nanotube with the density of C60 encapsulated within the nanotube at various temperatures. It is found that, at lower temperatures, the bending strength of the C60-filled nanotube increases with C60 density. However, the reverse tendency is observed at higher temperatures. Finally, the “sharpest tip” phenomena between the probe and the tube wall and the elastic recovery of the nanotube during the retraction process are also investigated.
Mapping rotational coherences onto time-resolved photoelectron imaging observables
Authors: Suzuki Yoshi-ichi , Seideman Tamar
Published: June 2005

We explore the information content of time-resolved photoelectron imaging, a potentially powerful pump-probe technique whose popularity has been rapidly growing in recent years. To that end, we identify a mapping of the alignment properties of time-evolving wave packets onto the moments of the photoelectron images and investigate its origin and consequences theoretically and numerically.
The reaction rate for dissociative adsorption of N2 on stepped Ru(0001): Six-dimensional quantum calculations
Authors: van Harrevelt Rob , Honkala Karoliina , Nørskov Jens K. , Manthe Uwe
Published: June 2005

Quantum-mechanical calculations of the reaction rate for dissociative adsorption of N2 on stepped Ru(0001) are presented. Converged six-dimensional quantum calculations for this heavy-atom reaction have been performed using the multiconfiguration time-dependent Hartree method. A potential-energy surface for the transition-state region is constructed from density-functional theory calculations using Shepard interpolation. The quantum results are in very good agreement with the results of the harmonic transition-state theory. In contrast to the findings of previous model calculations on similar systems, the tunneling effect is found to be small.
Alternating-current electroluminescence from an organic heterojunction sandwiched between two amorphous SiO2 layers
Authors: Yang S. Y. , Qian L. , Teng F. , Xu Z. , Xu X. R.
Published: June 2005

We report alternating-current electroluminescence (ACEL) from an organic heterojunction sandwiched between two amorphous SiO2(α-SiO2) layers, i.e., ITO\α-SiO2\poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene](MEH-PPV)\Alq3\α-SiO2\Al. Besides the exciton radiation from Alq3 (peak at 520 nm) and MEH-PPV (peak at 587 nm), an additional broad blue emission around 410 nm is observed in its ACEL spectrum. By studying the ACEL and brightness wave form of the device, we found both hot-electron-impact excitation and injection carriers recombination are combined in its emission mechanism for this hybrid organic-inorganic structure, and the origin of the broad blue emission around 410 nm is discussed.
Biexcitonic blue Stark shift in semiconductor quantum dots
Authors: Banerjee S. , Shore K. A.
Published: June 2005

The effect of biexcitons on the Stark effect in GaAs and CdS quantum dots (QDs) has been investigated theoretically using a time-dependent perturbation technique. Our results show that the redshift, which usually occurs in QD energy levels, changes to a blueshift when account is taken of biexciton effects. The results are obtained for a single-quantum dot as well as for an inhomogeneous distribution of quantum dots. A comparison has also been made between the Stark effect in GaAs and in CdS crystals and it has been found that the Stark effect in GaAs is stronger than in CdS quantum dots. This can be attributed to the difference in biexciton binding energies of the two materials.
Ion irradiation of inhomogeneous Schottky barriers on silicon carbide
Authors: Roccaforte F. , Libertino S. , Giannazzo F. , Bongiorno C. , La Via F. , Raineri V.
Published: June 2005

In this paper, the effects of ion irradiation on Schottky barriers formed on silicon carbide are discussed. After Si-ion irradiation at the near-interface region in Ti/4H-SiC contacts an increase of the Schottky barrier height from 1.05 to 1.21 eV was observed, accompanied by a lowering of the reverse leakage current. The combination of several methods allowed us to determine the physical properties of the Schottky barrier and to explain the mechanism responsible for the barrier height changes. In particular, the structural and electrical modifications of the interfacial region, both of Ti layer and SiC (i.e., different orientation of the Ti layer, irradiation-induced defects in the epilayer, dopant deactivation, and the consequent reduction of the surface electric field) are responsible for the increase of the Schottky barrier height and the reduction of the leakage current. The electrical characterization of the contacts at different temperatures also suggested that ion irradiation induced modifications in the inhomogeneous nature of the Ti Schottky barrier.
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phase transformation and magnetic behaviors of (Fe, FePt, FePtCu)–C nanocomposite films
Authors: Mi W. B. , Liu Hui , Li Z. Q. , Wu P. , Jiang E. Y. , Bai H. L.
Published: June 2005

As-deposited (Fe, FePt, FePtCu)–C nanocomposite films with fixed C atomic fraction xc=47 fabricated using facing-target sputtering method at room temperature are composed of ∼2–3-nm amorphous metal granules buried in a-C matrix. Annealing at high temperatures turns the amorphous granules into α-Fe, α-Fe- and L10-structured FePt, and L10-ordered FePtCu for Fe–C, FePt–C, and FePtCu–C films, respectively, and makes a-C preferential graphitization. As-deposited granules are superparamagnetic at 300K, and ferromagnetic at 5K. The zero-field-cooled (ZFC) and field-cooled (FC) curves reveal that there exist strong intergranular interactions at temperatures below 300K, and the size distribution of granules becomes broad by Pt and Cu addition. The M-H loop of annealed Fe31Pt22C47 films exhibits a two-step saturation behavior because of the coexistence of soft and hard ferromagnetic phases. As the Cu atomic fraction is 14%, the coercivity of annealed Fe23Pt16Cu14C47 films reaches a large value of ∼11.2kOe at 5K and decreases to ∼7.2kOe at 300K.
Observation of the Verwey transition in thin magnetite films
Authors: Chapline Michael G. , Wang Shan X.
Published: June 2005

Magnetite (Fe3O4) has attracted much interest due to its unique properties such as the Verwey transition and 100% spin polarization at the Fermi surface. In this study thin films of epitaxial magnetite were grown on MgO and MgAl2O4 substrates using pulsed laser deposition. Previous studies suggested that the Verwey transition is typically not seen for film thicknesses less than 50nm. The existence of a sharp Verwey transition may indicate that the film has properties similar to that of bulk magnetite. We have investigated how stress, antiphase boundaries, imhomogenities, and deviations from stoichiometry affect the Verwey transition in thin films.
Enhancement of post-thaw viability of cells in suspension via pulsed laser heating prior to immersion in liquid nitrogen
Authors: Kandra Deepak , Charalampopoulos T. T. , Devireddy Ram V.
Published: June 2005

In the present study, we compared the structure of ice formation in cryobiologically relevant solutions immersed in liquid nitrogen with solutions that were laser heated prior to immersion in a liquid-nitrogen bath. The results suggest that there is a distinct and observable difference in the type of ice formed. Furthermore, we also present data that suggest pulsed laser heating prior to immersion in liquid nitrogen enhances post-thaw survival of cells in suspension. The laser used in our experimental approach was a Q-switched Quanta-Ray DCR-3 laser (Nd:yttrium aluminum garnet laser) operating in the second harmonic at 532nm. The laser irradiance consisted of pulses with a pulse width of 5–7ns and a pulse energy stability of 3%. The pulse energy at a pulse repetition rate of 1Hz was about 360mJ and the peak power delivered was about 60MW. The samples were held in a copper- (sheet thickness of 0.5mm) molded rectangular box of 5mm thick, 25mm wide, and 15mm long. Preliminary experiments were carried out using adipose-tissue-derived adult stem cells. The pre- and postfreeze∕thaw viability of ADAS cells was assessed using membrane-excluded fluorescent dyes. ADAS cells enclosed in a copper box, laser irradiated, and exposed to liquid nitrogen had a viability of 69%±5%. Similarly, when the cells were exposed directly to liquid nitrogen (note that as before the cell suspension was enclosed and sealed in the copper box), the viability reduced significantly to 9%±10%. These results are very encouraging and demonstrate the feasibility of using lasers and liquid nitrogen to cryopreserve cells in suspension.
Thermal-expansion behavior of a directionally solidified NiAl–Mo composite investigated by neutron diffraction and dilatometry
Authors: Bei H. , George E. P. , Brown D. W. , Pharr G. M. , Choo H. , Porter W. D. , Bourke M. A. M.
Published: June 2005

The thermal expansion of directionally solidified NiAl–Mo eutectic alloys consisting of nanoscale Mo fibers embedded in a NiAl matrix was analyzed by neutron diffraction and dilatometry. From room temperature to 800°C, perpendicular to the fiber direction, the NiAl and Mo phases expand independently with average coefficients of thermal expansion (CTEs) of 16.0×10−6°C−1 and 5.8×10−6°C−1, respectively. Parallel to the fiber direction, they coexpand up to 650°C with an average CTE of 12.8×10−6°C−1, but above this temperature the Mo fibers expand more than the NiAl matrix. This anomalous behavior is the result of the load transfer to the Mo fibers when the NiAl matrix softens. The average CTE of the composite parallel to the fiber direction was determined by dilatometry to be 13.0×10−6°C−1, which is approximately 11% lower than the value predicted by a simple rule of mixtures using the CTEs of the constituent phases.
Ductile relaxation in cracked metal-organic chemical-vapor-deposition-grown AlGaN films on GaN
Authors: Bethoux J.-M. , Vennéguès P.
Published: June 2005

A study of the strain relaxation of cracked (Al,Ga)N∕GaN films grown by metal-organic chemical-vapor deposition on (0001) sapphire is presented. The strain measured by x-ray diffraction is shown to depend both on the film thickness and on the high-temperature exposure time (including both growth duration and annealing). These macroscopic observations are linked to the film microstructure by using transmission electron microscopy and cathodoluminescence observations. Relaxation may occur either by the introduction of long, straight, a+c misfit dislocations or by the introduction of additional cracks bordered by bowed a-type misfit dislocations. The nucleation mechanism and the efficiency in relaxing the misfit strain of these two features are discussed.
Temperature dependence of the absorption and refraction of Mg-doped congruent and stoichiometric LiNbO3 in the THz range
Authors: Pálfalvi L. , Hebling J. , Kuhl J. , Péter Á. , Polgár K.
Published: June 2005

The absorption coefficient and the index of refraction of undoped and Mg-doped stoichiometric and congruent LiNbO3 crystals were determined for polarization parallel to the z axis in the far-infrared (FIR) 30−180-cm−1 frequency range for different Mg-doping levels and temperatures down to 10 K. For stoichiometric LiNbO3, smaller absorption and index of refraction were found, than for congruent samples. At a Mg content near the photorefractive threshold, the FIR absorption coefficient has the lowest value in the stoichiometric crystals, which are most suitable for efficient pulse generation in the terahertz range.
Population inversion and threshold current densities: A comparison of GaAs∕(Al,Ga)As quantum-cascade structures with different barrier heights
Authors: Schrottke L. , Lu S. L. , Hey R. , Giehler M. , Kostial H. , Grahn H. T.
Published: June 2005

The population of the laser levels in undoped GaAs∕AlxGa1−xAs quantum-cascade structures (QCSs) is investigated by interband photoluminescence spectroscopy. We compare similar QCSs with different barrier heights (x=0.33 and x=0.45), for which the calculated population ratios ρpca are equal. While the experimental value ρpme for x=0.45 agrees with the theoretical one, ρpme for x=0.33 is much smaller than ρpca. At the same time, the threshold current densities jth are significantly smaller for x=0.45 than for x=0.33. In the framework of a linear rate equation model, we estimate the effect of the experimentally observed reduction of the population ratio on jth. We show that the increased value of jth for x=0.33 cannot only be attributed to a larger leakage current due to the lower barriers, but also to the reduced population ratio.
The effect of lithography processing on the I–V characteristics of Al–Al2O3–Ag junctions
Authors: Kreimer A. , Frydman A.
Published: June 2005

We present a detailed study of room-temperature current–voltage characteristics of Al–Al2O3–Ag junctions in which the Al electrode was treated by photolithography processing and chemical etching prior to the Al2O3 layer growth. The I–V curves exhibit polarity-dependent irreversible conductance changes and regions of negative differential resistance. These phenomena take place only if the bias voltage exceeds 0.8 V. We discuss a qualitative mechanism for such behavior based on charge accumulation in traps present in the barrier, due to the chemical treatment.
Investigation of benzene-hexafluorobenzene dynamics in liquid binary mixtures
Authors: Elola M. Dolores , Ladanyi Branka M.
Published: June 2005

The structure and microscopic dynamics of liquid mixtures of benzene and hexafluorobenzene at room temperature and several compositions have been studied by molecular-dynamics simulations. In this implementation we have rescaled the intermolecular H–F cross potential parameters obtained from the Lorentz–Berthelot combining rules, in order to avoid the substantial overestimation of the energy of mixing predicted by the model when the usual rules are employed. We found that a reduction in the strength of cross H–F interactions by 50% relative to the geometric mean is required in order to get a good agreement with experiments. Radial-angular pair-correlation functions between like and unlike species have been computed and analyzed, by comparing them with the correlations in the corresponding neat liquids. We have also studied the microscopic intermolecular momentum transfer, by computing the time correlation function between the initial velocity of a central molecule and later velocities of neighboring molecules. Structural and dynamical information extracted from the mentioned functions seem to be consistent with the picture of relatively long-lived benzene–hexafluorobenzene (Bz–Hf) complexes present in the mixtures, which would be responsible for the considerable perturbation of the structure in the first shell of like species, and would be moving within the liquid in a parallel face-to-face configuration. Using the tools developed originally to estimate hydrogen-bond lifetimes in liquids, we have computed the lifetimes of the Bz–Hf complexes as a function of the mixture composition, by two different methods: the direct time-averaging scheme and from the autocorrelation function of bond occupation numbers. The obtained lifetimes are strongly dependent on the scheme chosen to compute the characteristic times. We have obtained for the Bz–Hf dimer in solution, at room temperature, lifetimes in the range of 30–40ps from averaging schemes and around 60–120ps from autocorrelation function methods. In the latter case, the longest times correspond to the equimolar mixture.
Photoionization of vibrationally hot CH3 and CF3
Authors: Aguirre F. , Pratt S. T.
Published: June 2005

Vibrationally hot CH3 and CF3 were produced by the 266nm photodissociation of CH3I and CF3I, respectively, and probed by single-photon ionization at 118nm. Comparison of the ion images of the CH3 and CF3 fragments with those of the complementary I atoms, and with previous measurements of the product branching fractions, allowed the determination of the relative photoion yields as a function of the vibrational energy of the molecular radical. Some general ideas about the internal-energy dependence of photoionization cross sections are also discussed.
Finding pathways between distant local minima
Authors: Carr Joanne M. , Trygubenko Semen A. , Wales David J.
Published: June 2005

We report a new algorithm for constructing pathways between local minima that involve a large number of intervening transition states on the potential energy surface. A significant improvement in efficiency has been achieved by changing the strategy for choosing successive pairs of local minima that serve as endpoints for the next search. We employ Dijkstra’s algorithm [E. W. Dijkstra, Numer. Math. 1, 269 (1959)] to identify the “shortest” path corresponding to missing connections within an evolving database of local minima and the transition states that connect them. The metric employed to determine the shortest missing connection is a function of the minimized Euclidean distance. We present applications to the formation of buckminsterfullerene and to the folding of various biomolecules: the B1 domain of protein G, tryptophan zippers, and the villin headpiece subdomain. The corresponding pathways contain up to 163 transition states and will be used in future discrete path sampling calculations.
A theoretical investigation of gadolinium (III) solvation in molten salts
Authors: Hazebroucq Sandrine , Picard Gérard S. , Adamo Carlo
Published: June 2005

The solvation of lanthanides [here Gd(III)] in molten LiCl and KCl has been studied using a classical solvation approach, based on clusters of increasing size. In particular, density-functional calculations have been carried out on charged and neutral clusters, containing up to 35 chlorine halide molecules. A number of properties have been then evaluated and analyzed, including structural, vibrational, and thermochemical data. Special attention has also been devoted to the analysis of the local structure of the solvent surrounding the Gd3+ cation, a problem deeply investigated by experimentalists. Our results show that the charged clusters are not suitable to model the first solvation shell in such species, since their structures are strongly affected by the large electrostatic contribution. In contrast, more reliable simulations are obtained using the neutral clusters. In the latter, the coordination of Gd(III) in molten salts is computed to be 8 or 6, according to the salt LiCl or KCl. Furthermore, a good agreement is found with the experimental structural data and Raman spectra. Finally, preliminary results of potential interest for the estimation of solvation thermodynamics, a key parameter for exploiting molten salt chemistry, are reported for neutral clusters.
Validation of intermolecular pair potential model of SiH4: Molecular-dynamics simulation for saturated liquid density and thermal transport properties
Authors: Sakiyama Yukinori , Takagi Shu , Matsumoto Yoichiro
Published: June 2005

We demonstrate a validation of the intermolecular pair potential model of SiH4, which is constructed from ab initio molecular-orbital calculations and expressed as the sum of the exponential and the London dispersion terms. The saturated liquid densities of SiH4 are calculated for temperatures from 100to225K by molecular-dynamics (MD) simulation. The average deviation between the experiment and the MD simulation using the present potential model is 3.9%, while the deviations exceed 10% for other well-known potential models such as the five-center Lennard-Jones (LJ) model. Subsequently, the shear viscosity, the thermal conductivity, and the self-diffusion coefficient of liquid SiH4 are calculated by an equilibrium MD simulation with the Green–Kubo formula from 100to225K. The average deviations from experiment are 11.8% and 13.7% for the shear viscosity and the thermal conductivity, respectively. Comparing the present model with an empirical one-center LJ model, it turns out that the rotational energy transfer through the intermolecular potential energy, which comes from the anisotropic potential energy, plays an important role in the thermal conductivity of liquid SiH4. These results indicate that the present intermolecular potential model has an ability to give realistic pictures for liquid SiH4 through molecular simulations.
“Glass transition” in peptides: Temperature and pressure effects
Authors: Papadopoulos P. , Floudas G. , Schnell I. , Klok H.-A. , Aliferis T. , Iatrou H. , Hadjichristidis N.
Published: June 2005

We report on the origin of the liquid-to-glass transition in a series of oligopeptides of γ-benzyl-L-glutamate up to the polymer (PBLG), and in Poly-Z-L-lysine (PZLL) and Polyglycine (PGly) using dielectric spectroscopy as a function of temperature and pressure. We show that temperature is the dominant control variable of the dynamics associated with the peptidic “glass transition.” This is an intrinsic feature of the peptide dynamics, irrespective of the type of amino acid and of the peptide secondary structure. The influence of the type of secondary structure (α helix vs β sheet) on the liquid-to-glass dynamics is discussed.
Sunlight stability of organic light-emitting diodes
Authors: Heil H. , Andress G. , Schmechel R. , von Seggern H. , Steiger J. , Bonrad K. , Sprengard R.
Published: June 2005

This paper reports on the photodegradation of organic light-emitting diodes (OLEDs) due to exposure to visible and near-ultraviolet light. Such exposure affects strongly the device performance, e.g., the electroluminescence intensity and the device current decrease considerably, however, the photoluminescence remains unaffected. This photodegradation was investigated on various production-relevant classes of high-quality polymer semiconductors with different energy gaps, i.e., a yellowish-green polyphenylenevinylene derivative, a red polyfluorene derivative, and a blue polyspiro derivative. It will be demonstrated that the action spectrum of the photodegradation is strongly correlated with the fundamental absorption of the polymer itself. The indium-doped tin oxide (ITO) polymer interface was identified as the interface where photodegradation takes place, while the bulk of each layer (ITO or polymer) turned out to be insensitive to irradiation with visible or near-ultraviolet light. Furthermore, it is demonstrated that the photoinduced damage can be suppressed by inserting appropriate interlayer systems.
On the uniqueness of solutions to gauge covariant Poisson equations with compact Lie algebras
Authors: Cronström Christofer
Published: June 2005

It is shown, under rather general smoothness conditions on the gauge potential, which takes values in an arbitrary semisimple compact Lie algebra g, that if a (g-valued) solution to the gauge-covariant Laplace equation exists, which vanishes at spatial infinity, in the cases of 1,2,3,…, space dimensions, then the solution is identically zero. This result is also valid if the Lie algebra is merely compact. Consequently, a solution to the gauge-covariant Poisson equation is uniquely determined by its asymptotic radial limit at spatial infinity. In the cases of one or two space dimensions a related result is proved, namely that if a solution to the gauge-covariant Laplace equation exists, which is unbounded at spatial infinity, but with a certain dimension-dependent condition on the asymptotic growth of its norm, then the solution in question is a covariant constant.
Scheme for adding electron–nucleus cusps to Gaussian orbitals
Authors: Ma A. , Towler M. D. , Drummond N. D. , Needs R. J.
Published: June 2005

A simple scheme is described for introducing the correct cusps at nuclei into orbitals obtained from Gaussian basis set electronic structure calculations. The scheme is tested with all-electron variational quantum Monte Carlo (VMC) and diffusion quantum Monte Carlo (DMC) methods for the Ne atom, the H2 molecule, and 55 molecules from a standard benchmark set. It greatly reduces the variance of the local energy in all cases and slightly improves the variational energy. This scheme yields a general improvement in the efficiency of all-electron VMC and DMC calculations using Gaussian basis sets.
Strong polaronic effects on rutile TiO2 electronic band edges
Authors: Persson Clas , Ferreira da Silva Antonio
Published: June 2005

Thin TiO2 films are prepared by dc magnetron sputtering as well as by the sol-gel technique, and the optical band edge absorption α(ω) is obtained from transmission spectroscopy. The electronic structure and optical properties are studied employing a first-principle linearized augmented plane-wave method within the local density approximation (LDA), improved by an on-site Coulomb self-interaction potential (LDA+USIC). We show that the correction potential, the polaronic screening, and the spin-orbit interaction are crucial for determining the TiO2 effective electron and hole masses. The dielectric function ε(ω)=ε1(ω)+iε2(ω) and the high-frequency constantε(0⪡ω⪡Eg∕ℏ) show pronounced anisotropy. The electron-optical phonon interaction affectsε(ω≈0) strongly.
Quasiballistic, nonequilibrium electron distribution in inhomogeneous semiconductor structures
Authors: Csontos Dan , Ulloa Sergio E.
Published: June 2005

We report on a study of quasiballistic transport in deep submicron, inhomogeneous semiconductor structures, focusing on the analysis of signatures found in the full nonequilibrium electron distribution. We perform self-consistent numerical calculations of the Poisson-Boltzmann equations for a model n+−n−−n+ GaAs structure and realistic, energy-dependent scattering. We show that, in general, the electron distribution displays significant, temperature dependent broadening and pronounced structure in the high-velocity tail of the distribution. The observed characteristics have a strong spatial dependence, related to the energy dependence of the scattering, and the large inhomogeneous electric field variations in these systems. We show that in this quasiballistic regime, the high-velocity tail structure is due to pure ballistic transport, whereas the strong broadening is due to electron scattering within the channel, and at the source(drain) interfaces.
Admittance spectroscopy for phosphorus-doped n-type diamond epilayer
Authors: Koide Yasuo , Koizumi S. , Kanda H. , Suzuki M. , Yoshida H. , Sakuma N. , Ono T. , Sakai T.
Published: June 2005

Capture and emission processes of electron at phosphorus (P) donor in a n-diamond epilayer are analyzed by admittance spectroscopy. Dependence of temperatures on capacitance and conductance-frequency curves is well explained by Shockley-Read-Hall statistics. Thermal ionization energy and capture cross section of P donor are evaluated to be 0.54±0.02eV and (4.5±2.0)×10−17cm2, respectively. Broadening of the conductance-frequency curve is observed, which is believed to be due to a large Debye tail of electron distribution at a depletion layer edge.
InAsN
∕
GaAs
(
N
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quantum-dot and InGaNAs∕GaAs quantum-well emitters: A comparison
Authors: Bais G. , Cristofoli A. , Jabeen F. , Piccin M. , Carlino E. , Rubini S. , Martelli F. , Franciosi A.
Published: June 2005

We compare the luminescence from InAsN∕GaAs and InAsN∕GaAsN quantum dots with that obtained from InGaNAsN∕GaAs quantum wells grown in the same experimental system. All structures were engineered to emit near 1.3μm at room temperature. Quantum-dot emitters were found to exhibit higher thermal stability and did not require postgrowth annealing. The use of GaAsN barriers as opposed to GaAs barriers provided for narrower and more intense quantum-dot luminescence.
Room-temperature spin coherence in ZnO
Authors: Ghosh S. , Sih V. , Lau W. H. , Awschalom D. D. , Bae S.-Y. , Wang S. , Vaidya S. , Chapline G.
Published: June 2005

Time-resolved optical techniques are used to explore electron spin dynamics in bulk and epilayer samples of n-type ZnO as a function of temperature and magnetic field. The bulk sample yields a spin coherence time T2* of 20 ns at T=30K. Epilayer samples, grown by pulsed laser deposition, show a maximum T2* of 2 ns at T=10K, with spin precession persisting up to T=280K.
Linear surface smoothening of (Ti0.48Al0.52)N thin films grown on rough substrates
Authors: Liu Z.-J. , Shum P. W. , Shen Y. G.
Published: June 2005

The evolution of surface roughness during the growth of sputter-deposited solid solution (Ti0.48Al0.52)N films on rough high-speed-steel (HSS) substrates has been studied by atomic force microscopy. It has been revealed that the growing (Ti0.48Al0.52)N∕HSS film experiences a continuous surface smoothening. Scaling analyses along with surface power spectra calculation of the (Ti0.48Al0.52)N films grown on smooth Si(100) substrates under the same deposition conditions indicate that this surface smoothening is linear and can be explained by a simple linear equation with surface diffusion as the smoothening mechanism and shot noise as the roughening effect. The observed linear surface smoothening in (Ti0.48Al0.52)N∕HSS films has also been confirmed by our numerical simulations of the film growth using real HSS and Si(100) substrates as their initial growth conditions and can be understood in terms of the competition between the surface-diffusion-induced decrease in substrate roughness contribution and the noise-driven roughening effect.
Uncooled operation of type-II InAs∕GaSb superlattice photodiodes in the midwavelength infrared range
Authors: Wei Yajun , Hood Andrew , Yau Haiping , Gin Aaron , Razeghi Manijeh , Tidrow Meimei Z. , Nathan Vaidya
Published: June 2005

We report high performance uncooled midwavelength infrared photodiodes based on interface-engineered InAs∕GaSb superlattice. Two distinct superlattices were designed with a cutoff wavelength around 5μm for room temperature and 77 K. The device quantum efficiency reached more than 25% with responsivity around 1A∕W. Detectivity was measured around 109cmHz1∕2∕W at room temperature and 1.5×1013cmHz1∕2∕W at 77 K under zero bias. The devices were without antireflective coating. The device quantum efficiency stays at nearly the same level within this temperature range. Additionally, Wannier–Stark oscillations in the Zener tunneling current were observed up to room temperature.
ZnO hexagonal arrays of nanowires grown on nanorods
Authors: Wang R. C. , Liu C. P. , Huang J. L. , Chen S.-J.
Published: June 2005

ZnO single-crystalline nanowire-type nanostructures were synthesized on silicon by thermal chemical vapor deposition without catalysts through a two-step pressure-controlled vapor-reflected process at a low temperature of 550 °C where self-organized hexagonal crystalline or porous nanowire arrays were grown on nanorods. The nanowire diameter is around 20 nm and number of nanowires is selected by the nanorod size. Cathodoluminescence spectra exhibit strong green emissions, indicative of high oxygen-vacancy density, which sheds a light on further applications for multichannel nanoconductors in nanodevices.
Neutral base recombination in InP∕GaAsSb∕InP double-heterostructure bipolar transistors: Suppression of Auger recombination in p+ GaAsSb base layers
Authors: Bolognesi C. R. , Liu H. G. , Tao N. , Zhang X. , Bagheri-Najimi S. , Watkins S. P.
Published: June 2005

We report on the tradeoff between current gain β and the base sheet resistance RSH in metalorganic chemical vapor deposition-grown NpN InP∕GaAs1−xSbx∕InP double-heterojunction bipolar transistors (DHBTs) with heavy base carbon-doping levels resulting in hole concentrations NB ranging from 4×1019 to 12×1019∕cm3. In contrast to Ga0.47In0.53As and GaAs–based transistors, which both display gain variations proportional to 1∕(NB×WB)2 due to Auger recombination at high doping levels, neutral base recombination in InP∕GaAsSb∕InP DHBTs is not limited by Auger processes, and the measured current gain is proportional to 1∕(NB×WB). We show that GaAs1−xSbx base layers offer a growing lifetime advantage over Ga0.47In0.53As with increasing doping levels. Potential explanations for the observed suppression of Auger recombination in the InP–GaAsSb system are proposed.
Inversion of critical angle and Brewster angle in anisotropic left-handed metamaterials
Authors: Grzegorczyk Tomasz M. , Thomas Zachary M. , Kong Jin Au
Published: June 2005

We study the inversion of critical angle when a wave propagates through an anisotropic and a bianisotropic metamaterial exhibiting negative permittivity and negative permeability along some specific directions. This inversion of critical angle is explained by the dispersion relation of the medium, which shows that waves are totally reflected for low incident angles and transmitted for high incident angles. In addition, analytical formulae for the loci of the Brewster angles are derived for both half-space and slabs of both anisotropic and bianisotropic media, where the bianisotropy is the one exhibited by some split-ring resonators. Experimental results are provided and corroborate the theoretical predictions.
Isolated SiC nanocrystals in SiO2
Authors: Makkai Zs. , Pécz B. , Bársony I. , Vida Gy. , Pongrácz A. , Josepovits K. V. , Deák P.
Published: June 2005

A simple method, compatible with conventional silicon technology, is introduced to generate SiC nanocrystals, embedded in SiO2 on Si, and aligned parallel with the interface. The nanometer-sized SiC grains were grown into SiO2 close to the Si∕SiO2 interface by a two-step annealing of oxide covered Si: first in a CO, then in a pure O2 atmosphere. The first (carbonization) step creates epitaxial SiC crystallites grown into the Si surface, while the second (oxidation) step moves the interface beyond them. Conventional and high-resolution cross-sectional electron microscopy shows pyramidal Si protrusions at the Si∕SiO2 interface under the grains. The size of the grains, as well as their distance from the Si∕SiO2 interface (peak of pyramids) can be controlled by the annealing process parameters.
Trapping of hydrogen in argon-implanted crystalline silicon
Authors: Ma Y. , Job R. , Düngen W. , Huang Y. L. , Fahrner W. R. , Beaufort M. F. , Rousselet S. , Horstmann J. T.
Published: June 2005

Crystalline silicon wafers are implanted with argon ions and subsequently hydrogenated by H-plasma treatments (Sample A). The control silicon samples are solely treated by the H-plasma under identical conditions (Sample B). Depth-resolved μ-Raman spectroscopy and cross-sectional transmission electron microscopy are carried out on both samples. In Sample A, two kinds of hydrogen molecules, namely H2(I) located in vacancies, and H2(II) located in platelets are observed. It is found that the depth profile of the H2(I) molecules is consistent with that of the vacancies created by the Ar implantation, but deeper than that of H2(II) molecules (∼0.4 versus ∼0.3μm). In Sample B, only the H2(II) molecules are observed. Its depth distribution extends much deeper than in Sample A (∼1 versus ∼0.3μm). These results indicate that the vacancies created during Ar implantation act as trap centers to block the hydrogen diffusion into deeper wafer regions during the subsequent H-plasma treatments.
Surface-plasmon-mediated emission from metal-capped ZnO thin films
Authors: Lai C. W. , An J. , Ong H. C.
Published: June 2005

The effects of metal coating on the near-band-edge emission of ZnO thin films have been studied by photoluminescence and atomic force microscopy. Large enhancement in emission intensity has been observed from ZnO films when they are capped by Ag while negligible effect is seen on Au-coated films. In addition, the enhancement is found to increase with Ag thickness and the intensity eventually saturates at thickness of 200nm. By introducing MgO as a spacer between the metal coating and ZnO, the enhancement is shown to decrease with increasing the spacer thickness, which suggests the presence of the local fields induced by surface plasmons. As the emission energy of ZnO matches closely with the surface plasmon of Ag, it is speculated that the resonant coupling of the spontaneous emission in ZnO into the surface plasmons enhances the emission efficiency.
Acoustic phonon scattering in a low density, high mobility AlGaN∕GaN field-effect transistor
Authors: Henriksen E. A. , Syed S. , Ahmadian Y. , Manfra M. J. , Baldwin K. W. , Sergent A. M. , Molnar R. J. , Stormer H. L.
Published: June 2005

We report on the temperature dependence of the mobility μ of the two-dimensional (2D) electron gas in a variable density AlGaN∕GaN field-effect transistor, with carrier densities ranging from 0.4×1012to3.0×1012cm−2 and a peak mobility of 80000cm2∕Vs. Between 20 and 50K we observe a linear dependence μac−1=αT, indicating that acoustic phonon scattering dominates the temperature dependence of the mobility, with α being a monotonically increasing function of decreasing 2D electron density. This behavior is contrary to predictions of scattering in a degenerate electron gas, but consistent with calculations that account for thermal broadening and the temperature dependence of the electron screening. Our data imply a deformation potential D=12–15eV.
Serpins in fruit and vegetative tissues of apple (Malus domestica): expression of four serpins with distinct reactive centres and characterisation of a major inhibitory seed form, MdZ1b
Authors: Hejgaard Jørn , Laing William A. , Marttila Salla , Gleave Andrew P. , Roberts Thomas H.
Published: June 2005

Most serpins irreversibly inhibit serine proteinases of the chymotrypsin family using a suicide-substrate-based mechanism. Serpins are present in all domains of life, but physiological functions in the plant kingdom are yet to be elucidated. Inhibitory properties of many abundant cereal grain serpins are well characterised, but serpins have not been identified in eudicot seeds. In apple (Malus domestica Borkh.), the origin of 88 serpin expressed sequence tags (ESTs) identified among 160 000 ESTs from 30 cultivar-, tissue- and time-specific libraries showed that serpin genes are expressed in a wide variety of tissues, including developing and mature fruits, seeds and vegetative buds as well as developing, mature and senescing leaves. Analysis of 46 sequences, most full-length, identified serpins with four distinct reactive centres belonging to two subfamilies (MdZ1 and MdZ2) with ~85% amino acid sequence identity. MdZ1 included three molecular forms with identical reactive centre loop (RCL) sequences except for three different, but related, residues at P2 (Asp, Asn or Glu). A major seed serpin, MdZ1b, with P2–P1′ Glu–Arg–Arg was purified from decorticated seeds and characterised kinetically. MdZ1b was a fast inhibitor of bovine and porcine trypsin (second-order association rate constant k a ~4 × 106 m –1 s–1 and stoichiometry of inhibition SI = 1). Human plasmin and urokinase-type plasminogen activator (u-PA), but not thrombin, were inhibited at lower rates (k a ~104 m –1 s–1). Chymotrypsin was inhibited at the same site (k a~4 × 103 m –1 s–1), but a significant part of MdZ1b was cleaved as substrate (SI > 2). Unexpectedly, the MdZ1b-trypsin complex was relatively short-lived with a first-order dissociation rate constant k d in the order of 10−4 s–1. The bulk of mature seed MdZ1b was localised to the cotyledons. The content of MdZ1b in ripe apples was 5–26 µg per seed, whereas MdZ1b could not be detected in the cortex or skin. Localisation and inhibitory specificity of serpins in monocot and eudicot plants are compared and putative functions are discussed.
Improved procedures for in vitro regeneration and for phenotypic analysis in the model legume Lotus japonicus
Authors: Barbulova Ani , D'Apuzzo Enrica , Rogato Alessandra , Chiurazzi Maurizio
Published: June 2005

As a prerequisite for the development of an efficient gene transfer methodology, the possibility of inducing direct somatic embryogenesis in Lotus japonicus (Regel) K. Larsen explants was investigated. Petiole bases, cotyledons, hypocotyls and stem segments were cultivated in the presence of different amounts of benzylaminopurine (BAP) and / or thidiazuron (TDZ). Regeneration was achieved differentially in the different explants and a higher efficiency of shoot formation was obtained with TDZ. By maintaining the same TDZ regime a second cycle of morphogenesis was achieved and the histological analysis of these structures indicated unambiguously their somatic embryogenic nature. Thidiazuron was also tested as an agent to improve the kinetics of shoot formation in a Lotus japonicus transformation–regeneration procedure based on indirect organogenesis. A very significant, highly reproducible, increase in the rate of the shoot formation was observed in independent transformation experiments. We also present an extensive analysis of the feasibility and reproducibility of an in vitro procedure, which can be very useful for the screening of symbiotic phenotypes in transgenic Lotus plants and for the analysis of the cascade of molecular and cytological events occurring soon after Mesorhizobium loti infection.