bluuebunny/crossref_metadata_2025 huggingface dataset:
Cubic phase stabilization in nanoparticles of hafnia-zirconia oxides: Particle-size and annealing environment effects
Authors: Lu Chih-Hsin , Raitano Joan M. , Khalid Syed , Zhang Lihua , Chan Siu-Wai
Published: June 2008

Amorphous hafnia (HfO2−y), zirconia (ZrO2−y), and hafnia-zirconia (xHfO2−y–(1−x)(ZrO2−y)) nanoparticles were prepared by combining aqueous solutions of hexamethylenetetramine (HMT) with hafnium dichloride oxide (HfOCl2⋅8H2O), zirconium dichloride oxide (ZrOCl2⋅8H2O), or a mixture of these two salts at room temperature. For pure hafnia, transmission electron microscopy showed that the lower cation concentration (0.01M) resulted in the precipitation of smaller amorphous nanoparticles relative to higher concentrations (0.015M–0.04M). Consequently, the lower concentration preparation route coupled with a reducing environment (H2:N2=9:91) during annealing at temperatures between 650 and 850°C allowed for nanoparticles with a cubic structure to be prepared as determined by x-ray diffraction. The structurally cubic hafnia nanoparticles were 6nm or less in diameter and equiaxed. Using the same method (0.01M total metal cation concentration and reducing environment during annealing), nanoparticles of cubic structure were prepared across the entire hafnia-zirconia compositional spectrum, with a critical particle size for the cubic structure of about 6nm. Nanoparticles of tetragonal and monoclinic structure were prepared by increasing the annealing temperature and/or using a less reducing environment. The unique role of HMT in sample preparation is discussed as well.
Analysis of orientational dynamics of single fluorophore trajectories from three-angle polarization experiments
Authors: Lu Chun-Yaung , Vanden Bout David A.
Published: June 2008

An algorithm of single fluorophore orientation reconstruction based on a recently proposed method [J. T. Fourkas, Opt. Lett. 26, 211 (2001)] is studied, which converts three measured intensities {I0,I45,I90} to the dipole orientation {IT,θ,ϕ}. Fluctuations in the detected signals {δI0,δI45,δI90} caused by the shot noise results in different profiles in δθ and δϕ, causing the originally equivalent coordinates (X,Y,Z) to separate into in-plane (X,Y) and out-of-plane (Z) components. The overall fluctuation in δθ turns out to be higher than δϕ, and thus noise has a greater effect on the Z component of the signal than on the X and Y components. Therefore, care should be taken not to interpret differences in the in-plane and out-of-plane dynamics as being evidence of nonisotropic rotational motion. For some molecular orientations around Θ=π∕2, the total signal intensity cannot be inverted directly to angular coordinates. An optimization method is proposed that calculates the corrected angular coordinates for the points in the trajectory. To test the effects of this recovery scheme, the covariance/correlation functions for reconstructed angular trajectories were calculated for the case of isotropic rotational diffusion. Rotational correlation functions of rank ℓ were found to deviate from the ideal single exponential decay as a result of the noise. This effect becomes more significant for large ℓ cases. The correlation functions were fitted to a stretch exponential to characterize their deviation from the true single exponential decay. Correlation functions of Z have larger deviations from the true correlation function due to the larger noise in the Z component. The trends and the distributions of stretched exponential parameters {τF} and {βF} fitted from trajectories of a given size T also exhibit the influences from noise. Again, large ℓ cases show a greater effect from the noise which eliminates the benefit of calculating higher rank correlation functions because of the smaller time constants. Due to the errors in estimating the correlation functions, significant differences between correlation functions of different orders can result from the statistics rather than being an indication of a nondiffusive behavior.
Neutron scattering study of the vibrations in vitreous silica and germania
Authors: Fabiani E. , Fontana A. , Buchenau U.
Published: June 2008

The incoherent approximation for the determination of the vibrational density of states of glasses from inelastic neutron or x-ray scattering data is extended to treat the coherent scattering. The method is applied to new room temperature measurements of vitreous silica and germania on the thermal time-of-flight spectrometer IN4 at the High Flux Reactor in Grenoble. The inelastic dynamic structure factor at the boson peak turns out to agree reasonably well with simulation results, but the long-wavelength fraction exceeds the expectation of the Debye model, in particular, in germania.
Introducing phase transitions to quantum chemistry: From Trouton’s rule to first principles vaporization entropies
Authors: Spickermann Christian , Lehmann Sebastian B. C. , Kirchner Barbara
Published: June 2008

In the present study, we employ quantum cluster equilibrium calculations on a small water cluster set in order to derive thermochemical equilibrium properties of the liquid phase as well as the liquid-vapor phase transition. The focus is set on the calculation of liquid phase entropies, from which entropies of vaporization at the normal boiling point of water are derived. Different electronic structure methods are compared and the influences of basis set size and of cooperative effects are discussed. In line with a previous study on the subject [B. Kirchner, J. Chem. Phys. 123, 204116 (2005)], we find that the neglect of cooperativity leads to large errors in the equilibrium cluster populations as well as in the obtained entropy values. In contrast, a correct treatment of the intermolecular many-body interaction yields liquid phase entropies and phase transition entropies being in very good agreement with the experimental reference, thus demonstrating that the quantum cluster equilibrium partition function intrinsically accounts for the shortcomings of the ideal gas partition function often employed in first principles entropy calculations. Comparing the calculated vaporization entropies to the value predicted by Trouton’s rule, it is observed that for entropy calculations the consideration of intracluster cooperative effects is more important than the explicit treatment of the intercluster association even in a highly associated liquid such as water. The decomposition of entropy into contributions due to different degrees of freedom implies the need for the accurate treatment of particle indistinguishability and free volume of translation, whereas minor influences should be expected from the vibrational and rotational degrees of freedom and none from the electronic degrees of freedom.
X-ray absorption and natural circular dichroism spectra of C84: A theoretical study using the complex polarization propagator approach
Authors: Jiemchooroj Auayporn , Norman Patrick
Published: June 2008

The x-ray absorption and circular dichroism K-edge spectra for the D2-isomer of C84 have been determined using the complex polarization propagator method in conjunction with Kohn–Sham density functional theory. The circular dichroism spectrum is rich in details and, in comparison to the absorption spectrum, it provides a superior resolution of the electronic transitions below the ionization threshold.
Charge recombination in distributed heterostructures of semiconductor discotic and polymeric materials.
Authors: Clark Jenny , Archer Robert , Redding Tim , Foden Clare , Tant Julien , Geerts Yves , Friend Richard H. , Silva Carlos
Published: June 2008

Control of microstructure and energetics at heterojunctions in organic semiconductors is central to achieve high light-emitting or photovoltaic device efficiency. We report the observation of an emissive exciplex formed between an electron-accepting discotic material (hexaazatrinaphthylene or HATNA-SC12) and a hole accepting conjugated polymer {poly[9,9- dioctylfluorene-co-N-(4-butylphenyl)diphenylamine] or TFB}. In contrast to polymer-polymer systems, we find here that the exciplex is strongly localized at the interface, acting as an energy bottleneck with inefficient transfer to bulk exciton states and with low yield of charge separation.
Exact dynamics of dissipative electronic systems and quantum transport: Hierarchical equations of motion approach
Authors: Jin Jinshuang , Zheng Xiao , Yan YiJing
Published: June 2008

A generalized quantum master equation theory that governs the exact, nonperturbative quantum dissipation and quantum transport is formulated in terms of hierarchically coupled equations of motion for an arbitrary electronic system in contact with electrodes under either a stationary or a nonstationary electrochemical potential bias. The theoretical construction starts with the influence functional in path integral, in which the electron creation and annihilation operators are Grassmann variables. Time derivatives on the influence functionals are then performed in a hierarchical manner. Both the multiple-frequency dispersion and the non-Markovian reservoir parametrization schemes are considered for the desired hierarchy construction. The resulting hierarchical equations of motion formalism is in principle exact and applicable to arbitrary electronic systems, including Coulomb interactions, under the influence of arbitrary time-dependent applied bias voltage and external fields. Both the conventional quantum master equation and the real-time diagrammatic formalism of Schön and co-workers can be readily obtained at well defined limits of the present theory. We also show that for a noninteracting electron system, the present hierarchical equations of motion formalism terminates at the second tier exactly, and the Landuer–Büttiker transport current expression is recovered. The present theory renders an exact and numerically tractable tool to evaluate various transient and stationary quantum transport properties of many-electron systems, together with the involving nonperturbative dissipative dynamics.
The effects of high-pressure compression on transport and thermoelectric properties of TiS2 at low temperatures from 5 to 310 K
Authors: Li D. , Qin X. Y. , Li H. J. , Zhang J. , Hoon Hng Huey
Published: June 2008

The effects of high-pressure compression on the transport and thermoelectric properties of TiS2 were investigated at temperatures ranging from 5 to 310 K. The results indicated that compression under the pressure of 6 GPa caused a significant decrease (16-fold at 300 K) in the absolute thermopower |S| and the thermal conductivity (5-fold at 300 K). At the same time, the electrical resistivity ρ increased by two orders of magnitude after the compression. A transition from metallic state (dρ/dT>0) to semiconductorlike state (dρ/dT<0) was found to occur after the compression. This transition to the semiconductorlike state could be caused by the substantially enhanced grain boundary (GB) scattering due to the refinement of its grains to the nanoscale range, which should also be responsible for the remarkable increase in the resistivity and large decrease in thermal conductivity. Moreover, Mott’s two-dimensional variable range hopping law, ln ρ∝T−1/3, was observed at T<∼100 K for TiS2 after the compression, suggesting that substantial potential disorder was produced by the high-pressure compression. The significant decrease of |S| could originate from the possible compositional disorder in the GBs of TiS2 after compression. The thermoelectric figure of merit of TiS2 decreased after the compaction due to the large decrease in |S| and increase in ρ, indicating that high-pressure compression is not beneficial to the thermoelectric performance of TiS2.
Ionicity in disordered GeSe2: A comparison of first-principles and atomistic potential models
Authors: Wilson Mark , Sharma Bevan K. , Massobrio Carlo
Published: June 2008

The structural properties of liquid GeSe2, generated using two distinct computational methodologies, are compared. The results of molecular dynamics simulations, utilizing both first-principles density functional and a potential model which account for aspects of many-body interactions, are considered. The potential model favors ionic character in the bonding, resulting in a structure with very little chemical disorder and no homopolar bonds, in contrast to experimental observation. The use of a relatively simple potential model is shown to be useful in order to understand differences between the observed experimental structure and those obtained from the first-principles approach, the latter being affected by insufficient account of ionic character in the bonding. Both computational schemes are able to predict the appearance of the first sharp diffraction peak in the total neutron structure factor and in some of the partial structure factors as well as the concomitant presence of corner- and edge-sharing tetrahedral connections. For the potential model, this holds true provided the system temperatures are set to values high enough to allow for diffusion properties typical of a liquid. Structural properties obtained for the two sets of configurations are in closer agreement when the potential model is applied at very high temperatures.
Influence of ion bombardment induced patterning of exchange bias in pinned artificial ferrimagnets on the interlayer exchange coupling
Authors: Höink V. , Schmalhorst J. , Reiss G. , Weis T. , Lengemann D. , Engel D. , Ehresmann A.
Published: June 2008

Artificial ferrimagnets have many applications as, e.g., pinned reference electrodes in magnetic tunnel junctions. It is known that the application of ion bombardment (IB) induced patterning of the exchange bias coupling of a single layer reference electrode in magnetic tunnel junctions with He ions is possible. For applications as, e.g., special types of magnetic logic, a combination of the IB induced patterning of the exchange bias coupling and the implementation of an artificial ferrimagnet as reference electrode is desirable. Here, investigations for a pinned artificial ferrimagnet with a Ru interlayer, which is frequently used in magnetic tunnel junctions, are presented. It is shown that in this kind of samples the exchange bias can be increased or rotated by IB induced magnetic patterning with 10keV He ions without a destruction of the antiferromagnetic interlayer exchange coupling. An IrMn∕Py∕Co∕Cu∕Co stack turned out to be more sensitive to the influence of IB than the Ru based artificial ferrimagnet.
Structural and thermodynamic properties of different phases of supercooled liquid water
Authors: Jedlovszky Pál , Pártay Lívia B. , Bartók Albert P. , Voloshin Vladimir P. , Medvedev Nikolai N. , Garberoglio Giovanni , Vallauri Renzo
Published: June 2008

Computer simulation results are reported for a realistic polarizable potential model of water in the supercooled region. Three states, corresponding to the low density amorphous ice, high density amorphous ice, and very high density amorphous ice phases are chosen for the analyses. These states are located close to the liquid-liquid coexistence lines already shown to exist for the considered model. Thermodynamic and structural quantities are calculated, in order to characterize the properties of the three phases. The results point out the increasing relevance of the interstitial neighbors, which clearly appear in going from the low to the very high density amorphous phases. The interstitial neighbors are found to be, at the same time, also distant neighbors along the hydrogen bonded network of the molecules. The role of these interstitial neighbors has been discussed in connection with the interpretation of recent neutron scattering measurements. The structural properties of the systems are characterized by looking at the angular distribution of neighboring molecules, volume and face area distribution of the Voronoi polyhedra, and order parameters. The cumulative analysis of all the corresponding results confirms the assumption that a close similarity between the structural arrangement of molecules in the three explored amorphous phases and that of the ice polymorphs Ih, III, and VI exists.
Prediction of nitroxide hyperfine coupling constants in solution from combined nanosecond scale simulations and quantum computations
Authors: Houriez Céline , Ferré Nicolas , Masella Michel , Siri Didier
Published: June 2008

We present a combined theoretical approach based on analyzing molecular dynamics trajectories (at the nanosecond scale) generated by use of classical polarizable force fields and on quantum calculations to compute averaged hyperfine coupling constants. That method is used to estimate the constant of a prototypical nitroxide: the dimethylnitroxide. The molecule is embedded during the simulations in a cubic box containing about 500 water molecules and the molecular dynamics is generated using periodic conditions. Once the trajectories are achieved, the nitroxide and its first hydration shell molecules are extracted, and the coupling constants are computed by considering the latter aggregates by means of quantum computations. However, all the water molecules of the bulk are also accounted for during those computations by means of the electrostatic potential fitted method. Our results exhibit that in order to predict accurate and reliable coupling constants, one needs to describe carefully the out-of-plane motion of the nitroxide nitrogen and to sample trajectories with a time interval of 400fs at least to generate an uncorrelated large set of nitroxide structures. Compared to Car–Parrinello molecular dynamics techniques, our approach can be used readily to compute hyperfine coupling constants of large systems, such as nitroxides of great size interacting with macromolecules such as proteins or polymers.
Water induced weakly bound electrons in DNA
Authors: Berashevich Julia , Chakraborty Tapash
Published: June 2008

We have studied the effect of humidity on the electronic properties of DNA base pairs. We found that the hydrogen links of the nucleobases with water molecules lead to a shift of the π electron density from carbon atoms to nitrogen atoms and can change the symmetry of the wave function for some nucleobases. As a result, the orbital energies are shifted which leads to a decrease in the potential barrier for the hole transfer between the G-C and A-T pairs from 0.7eV for the dehydrated case to 0.123eV for the hydrated. More importantly, the π electron density redistribution activated by hydration is enhanced by the intrastrand interactions. This leads to a modification of the nucleobase chemical structures from the covalent type to a resonance structure with separated charges, where some π electrons are not locked up into the covalent bonds. Within the (G-C)2 sequences, there is overlapping of the electronic clouds of such unlocked electrons belonging to the stacked guanines, that significantly increases the electron coupling between them to VDA=0.095eV against the VDA=0.025eV for the dehydrated case. Consequently, the charge transfer between two guanines within the (G-C)2 sequences is increased by 250 times due to hydration. The presence of nonbonded electrons suppress the band gap up to ∼3.0eV, that allows us to consider DNA as a narrow band gap semiconductor.
Electrostatic modification of infrared response in gated structures based on VO2
Authors: Qazilbash M. M. , Li Z. Q. , Podzorov V. , Brehm M. , Keilmann F. , Chae B. G. , Kim H. T. , Basov D. N.
Published: June 2008

We investigate the changes in the infrared response due to charge carriers introduced by electrostatic doping of the correlated insulator vanadium dioxide (VO2) integrated in the architecture of the field effect transistor. Accumulation of holes at the VO2 interface with the gate dielectric leads to an increase in infrared absorption. This phenomenon is observed only in the insulator-to-metal transition regime of VO2 with coexisting metallic and insulating regions. We postulate that doped holes lead to the growth of the metallic islands thereby promoting percolation, an effect that persists upon removal of the applied gate voltage.
Efficiency and reliability enhancement of silicon nanocrystal field-effect luminescence from nitride-oxide gate stacks
Authors: Perálvarez M. , Carreras Josep , Barreto J. , Morales A. , Domínguez C. , Garrido B.
Published: June 2008

We report on a field-effect light emitting device based on silicon nanocrystals in silicon oxide deposited by plasma-enhanced chemical vapor deposition. The device shows high power efficiency and long lifetime. The power efficiency is enhanced up to ∼0.1% by the presence of a silicon nitride control layer. The leakage current reduction induced by this nitride buffer effectively increases the power efficiency two orders of magnitude with regard to similarly processed devices with solely oxide. In addition, the nitride cools down the electrons that reach the polycrystalline silicon gate lowering the formation of defects, which significantly reduces the device degradation.
Magnetic properties of HoFe2Hx
Authors: Schurer P. J. , Bennett B.
Published: June 2008

The magnetic hyperfine field distribution that is associated with the number of hydrogen nearest neighbors of iron atoms in HoFe2Hx has been determined. From an analysis of the isomer shift values the 4s electron transfer from iron to hydrogen and the intra-atomic 3d-4s electron transfer has been calculated.
Dielectric breakdown in Co–Fe–B/MgO/Co–Fe–B magnetic tunnel junction
Authors: Khan Ayaz Arif , Schmalhorst J. , Thomas A. , Schebaum O. , Reiss G.
Published: June 2008

The time-dependent dielectric breakdown has been investigated in Co–Fe–B/MgO/Co–Fe–B junctions by voltage ramp experiments and focused on its dependence on the barrier thickness, junction area, polarity of the applied voltage, ramp speed, and annealing temperature. The results suggest that the breakdown voltage strongly depends both on the polarity of the applied voltage and the annealing temperature. Magnetic tunnel junctions (MTJs) with positive bias on the top electrode show higher breakdown voltage than MTJs with negative bias. We found that there is a significant decrease in the breakdown voltage when the annealing temperature is increased above 350 °C.
Comparativeab initiostudies on the molecular structure and spectroscopic properties of FeF2: Single reference versus multireference methods
Authors: Solomonik Victor G. , Stanton John F. , Boggs James E.
Published: June 2008

The electronic excitation energies, molecular geometry, quadratic force fields, and vibrational frequencies in the ground Δg5 and low-lying excited Σg+5 and Πg5 electronic states of iron difluoride are studied at sophisticated levels of theory. Two families of basis sets, nonrelativistic and Douglas–Kroll–Hess relativistic, are used that range in quality from triple-ζ to quintuple-ζ. These are augmented by additional diffuse functions (on fluorine atoms) and tight functions (on all atoms) for the description of core-valence correlation and utilized to determine complete basis set molecular properties. The quality of electron correlation treatment using conventional single reference coupled cluster methods CCSD and CCSD(T) is compared to that attained at the multiconfigurational quasidegenerate second-order perturbation theory (CASSCF+MCQDPT2) and the electron attachment equation-of-motion coupled cluster (EOMEA-CCSD) levels. Spin-orbit coupling effects are studied by the SO-MCQDPT2 method using the full Breit–Pauli spin-orbit operator. Effects of spin contamination in the coupled cluster molecular calculations are carefully analyzed. Results of the single reference CCSD(T) and multireference calculations are found to be in a remarkable agreement. The calculations indicate that the EOMEA-CC approach provides a suitable tool for an accurate treatment of FeF2 and other systems where delicate electron correlation effects have to be carefully dealt with. The inclusion of relativistic effects is shown to be necessary for an accurate description of the molecular geometry and excitation energies of FeF2. The results of calculations are in good agreement with the experimental data available. The predicted FeF2 molecular properties are compared to those of the related FeF3.
White light emission on amplified spontaneous emission with dye content controlled polymer system
Authors: Zhang Dingke , Chen Zhenyu , Ma Dongge
Published: June 2008

White light emission from amplified spontaneous emission (ASE) was realized by optically pumping fluorescent dye 4-(dicy-anomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl−9-enyl)-4H-pyran (DCJTB) doped semiconducting poly(9,9-dioctylfluorene) (PFO) polymer thin films. Two individual ASE peaks originating from DCJTB and PFO were observed by carefully controlling the DCJTB concentration in PFO. The studies of the ASE characteristics of DCJTB:PFO thin films lead to the conclusion that the DCJTB:PFO system with 0.3% w/w DCJTB dopant concentration in PFO showed the best ASE performance. In the DCJTB:PFO system with 0.3% w/w DCJTB dopant concentration, the net gains, loss coefficients, and threshold reached 36.3 and 22.35 cm−1, 7.39 and 15.88 cm−1, and 0.072 and 0.035 mJ pulse−1, for DCJTB emission and PFO emission, respectively. Our results predict an approach to achieve white light emission through amplified spontaneous emission in host polymer systems via controlled dye content.
Thickness and coupling effects in bilayered multiferroic CoFe2O4/Pb(Zr0.52Ti0.48)O3 thin films
Authors: Sim C. H. , Pan A. Z. Z. , Wang J.
Published: June 2008

Four CoFe2O4/Pb(Zr0.52Ti0.48)O3 (CFO/PZT) bilayered thin films with increasing PZT constituent layer thicknesses have been fabricated in order to investigate the optimized PZT layer thickness and the thickness effect on the magnetic and ferroelectric behaviors of the heterolayered multiferroic thin films. These heterostructured thin films were fabricated via a combined route of spin coating and rf sputtering. Both magnetic CFO and ferroelectric PZT phases were successfully retained in the heterostructured thin film. The improved saturation magnetization (Ms) of the CFO/PZT thin films has been observed and the enhancement increases with increasing PZT layer thickness. Both ferroelectric and dielectric measurements suggest the presence of interfacial space charge polarization arising from the ferroelectric/magnetic interface. Among the multiferroic thin films containing different PZT layer thicknesses fabricated, the thin film that consisted of six PZT constituent layers demonstrated the highest Ms value of 397 emu/cm3 and the best ferroelectric property with Pr and Ec of 29.91 μC/cm2 and 138.67 kV/cm, respectively, at 500 kV/cm, together with εr and tan δ values of 116.420 and 0.717, respectively, at 100 kHz. Investigation into its magnetoelectric effect demonstrated an αE,33 of 238 mV/cm Oe, which was measured by superimposing an ac magnetic field of 5 Oe at 5 kHz onto a dc magnetic field of 4.4 kOe.
Analysis and improvement of an X-band magnetically insulated transmission line oscillator
Authors: Fan Yu-wei , Zhong Hui-huang , Yang Han-wu , Li Zhi-qiang , Shu Ting , Zhang Jun , Wang Yong , Luo Ling
Published: June 2008

An X-band magnetically insulated transmission line oscillator has been investigated theoretically and experimentally in our laboratory. However, severe pulse shortening and electrode erosion are observed in the experiments. The theoretical analyses show that anode plasma formation in the load region is the essential reason for the pulse shortening and electrode erosion. In order to eliminate or at least minimize anode plasma formation in the load region, an improved beam dump is presented. The theoretical analyses show that anode plasma formation can be eliminated or at least minimized in the improved beam dump.
Defect structures in B12As2 epitaxial layers grown on (0001) 6H-SiC
Authors: Chen H. , Wang G. , Dudley M. , Zhang L. , Wu L. , Zhu Y. , Xu Z. , Edgar J. H. , Kuball M.
Published: June 2008

A detailed analysis of the microstructure in B12As2 epitaxial layers grown by chemical-vapor deposition on (0001) 6H-SiC substrates is presented. Synchrotron white beam x-ray topography enabled macroscopic characterization of the substrate/epilayer ensembles and revealed the presence of a quite homogeneous solid solution of twin and matrix epilayer domains forming a submicron mosaic structure. The basic epitaxial relationship was found to be (0001)B12As2⟨112¯0⟩B12As2∥(0001)6H-SiC⟨112¯0⟩6H-SiC and the twin relationship comprised a 180° (or equivalently 60°) rotation about [0001]B12As2 in agreement with previous reports. Cross-sectional high resolution transmission electron microscopy revealed the presence of a ∼200 nm thick disordered transition layer which was shown to be created by the coalescence of a mosaic of translationally and rotationally variant domains nucleated at various types of nucleation sites available on the (0001) 6H-SiC surface. In this transition layer, competition between the growth of the various domains is mediated in part by the energy of the boundaries created between them as they coalesce. Boundaries between translationally variant domains are shown to have unfavorable bonding configurations and hence high-energy. These high-energy boundaries can be eliminated during mutual overgrowth by the generation of a 1/3[0001]B12As2 Frank partial dislocation which effectively eliminates the translational variants. This leads to an overall improvement in film quality beyond thicknesses of ∼200 nm as the translational variants grow out leaving only the twin variants. (0003) twin boundaries in the regions beyond 200 nm are shown to possess fault vectors such as 1/6[11¯00]B12As2, which are shown to originate from the mutual shift between the nucleation sites of the respective domains.
Theory of competitive counterion adsorption on flexible polyelectrolytes: Divalent salts
Authors: Kundagrami Arindam , Muthukumar M.
Published: June 2008

The counterion distribution around an isolated flexible polyelectrolyte in the presence of a divalent salt is evaluated using the adsorption model [M. Muthukumar, J. Chem. Phys. 120, 9343 (2004)] that considers the Bjerrum length, salt concentration, and local dielectric heterogeneity as physical variables in the system. Self-consistent calculations of effective charge and size of the polymer show that divalent counterions replace condensed monovalent counterions in competitive adsorption. The theory further predicts that at modest physical conditions for a flexible polyelectrolytes such as sodium polystyrene sulfonate in aqueous solutions polymer charge is compensated and reversed with increasing divalent salt. Consequently, the polyelectrolyte shrinks and reswells. Lower temperatures and higher degrees of dielectric heterogeneity between chain backbone and solvent enhance condensation of all species of ions. Complete diagrams of states for the effective charge calculated as functions of the Coulomb strength and salt concentration suggest that (a) overcharging requires a minimum Coulomb strength and (b) progressively higher presence of salt recharges the polymer due to either electrostatic screening (for low Coulomb strengths) or coion condensation (for high Coulomb strengths). Consideration of ion-bridging by divalent counterions leads to a first-order collapse of polyelectrolytes in modest presence of divalent salts and at higher Coulomb strengths. The authors’ theoretical predictions are in agreement with the generic results from experiments and simulations.
Gradings and contractions of affine Kac–Moody algebras
Authors: Daboul Claudia , Daboul Jamil , de Montigny Marc
Published: June 2008

We study contractions of affine Kac–Moody algebras (KMAs) with respect to their twisted and untwisted Kac–Moody subalgebras and also contract twisted KMA with respect to their affine subalgebras. Our work generalizes the Inönü–Wigner contraction of KMA by Majumdar [J. Math. Phys. 34, 2059 (1993)] in several directions. We compare Inönü–Wigner contractions with respect to twisted subalgebras to another kind of contractions, which we call modulo contractions. We also introduce a novel type of contractions, based on Z-gradings of KMA, which we call level contractions. Along the way we use an algorithm for constructing “Kac–Moody–like algebras” ĝ for any (and not just semisimple) finite-dimensional Lie algebra g.
Cold atmospheric pressure air plasma jet for medical applications
Authors: Kolb J. F. , Mohamed A.-A H. , Price R. O. , Swanson R. J. , Bowman A. , Chiavarini R. L. , Stacey M. , Schoenbach K. H.
Published: June 2008

By flowing atmospheric pressure air through a direct current powered microhollow cathode discharge, we were able to generate a 2cm long plasma jet. With increasing flow rate, the flow becomes turbulent and temperatures of the jet are reduced to values close to room temperature. Utilizing the jet, yeast grown on agar can be eradicated with a treatment of only a few seconds. Conversely, animal studies show no skin damage even with exposures ten times longer than needed for pathogen extermination. This cold plasma jet provides an effective mode of treatment for yeast infections of the skin.
Stability of suspended gold and silver alloy monatomic chains
Authors: Fa Wei , Dong Jinming
Published: June 2008

Using the first-principles plane wave pseudopotential method, we have studied the structures and stability of gold and silver alloy monatomic chains. It is found that minimizing system’s enthalpy instead of its energy is critical to identify the stability of stretched alloy chains at zero temperature since the string tension can efficiently suppress the self-purification. Our simulations show that all the gold-containing chains do exhibit a local enthalpy minimum, giving a reasonable interpretation for the experimental observations of gold and silver alloy chains with different Ag concentrations [Bettini et al., Nat. Nanotechnol. 1, 182 (2006)]. These alloy chains are stabilized by the combined actions of the gold’s relativistic effect and the string tension applied by the tip contacts, having similar geometrical structures to those of the pure gold chains.
Does natural frequency quantify the mass flow rate of fluid conveying single-walled carbon nanotubes?
Authors: Reddy C. D. , Lu C.
Published: June 2008

The structural vibration analysis of fluid conveying single-walled carbon nanotubes (SWCNTs) is carried out to study natural frequencies. Continuum modeling of SWCNTs required Young’s modulus and wall thickness of SWCNTs. To circumvent the individual selection of these parameters from the large scattered (called Yakobsons paradox) values reported from the prior studies, atomistic simulations are used to compute the equivalent bending rigidity. This analysis is particularly useful to estimate the mass flow rate of fluid using the natural frequency of SWCNTs. A numerical expression is developed to quantify the mass flow rate in terms of natural frequency and dimensionless parameters of the fluid conveying SWCNTs.
Electrical resistivity in Co-rich pseudobinary Ni–Cr ferromagnetic metallic glasses from 1.5to300K
Authors: Banerjee Mitali , Chakraborty Swapan , Majumdar A. K.
Published: June 2008

Electrical resistivity ρ(T) has been measured in two series of melt-spun amorphous ferromagnets Fe5Co50Ni17−xCrxB16Si12 (x=0,5,10,15) and Fe7.8Co31.2Ni39−xCrxB14Si8 (x=5,10,15) and Fe7.8Co31.2Ni24Mn15B14Si8 between 1.5 and 25K. ρ(T) of all these high-resistive, soft ferromagnetic samples with wide device applications show pronounced minima at Tmin. In the region well below Tmin, it shows very conclusively a (−√T) dependence coming from electron-electron interaction effects in the presence of weak localization due to their high structural disorder. ρ(T), measured over a wider temperature range of 1.5–300K in the second series, shows in the region well above Tmin a T2 dependence coming from either the dynamical structure factor or magnetic scattering or from both. At still higher temperatures, ρ(T)∼T through the structural term only. The above interpretation is severely restricted by various characteristic temperatures such as Debye, Curie, and Tmin in the relevant theories used for the analysis. Reasonable isolations of different contributions were only possible because of the very high resolution of the ρ(T) data.
Time-dependent density-functional theory in the projector augmented-wave method
Authors: Walter Michael , Häkkinen Hannu , Lehtovaara Lauri , Puska Martti , Enkovaara Jussi , Rostgaard Carsten , Mortensen Jens Jørgen
Published: June 2008

We present the implementation of the time-dependent density-functional theory both in linear-response and in time-propagation formalisms using the projector augmented-wave method in real-space grids. The two technically very different methods are compared in the linear-response regime where we found perfect agreement in the calculated photoabsorption spectra. We discuss the strengths and weaknesses of the two methods as well as their convergence properties. We demonstrate different applications of the methods by calculating excitation energies and excited state Born–Oppenheimer potential surfaces for a set of atoms and molecules with the linear-response method and by calculating nonlinear emission spectra using the time-propagation method.
Complex dynamics and enhanced photosensitivity in a modified Belousov–Zhabotinsky reaction
Authors: Li Nan , Zhao Jinpei , Wang Jichang
Published: June 2008

This study presents an experimental investigation of nonlinear dynamics in a modified Belousov–Zhabotinsky (BZ) reaction, in which the addition of 1,4-benzoquinone induced various complex behaviors such as mixed-mode oscillations and consecutive period-adding bifurcations. In addition, the presence of 1,4-benzoquinone significantly enhanced the photosensitivity of the ferroin-catalyzed BZ system, in which light-induced transitions between simple and complex oscillations have been achieved. Mechanistic study suggests that the influence of benzoquinone may arise from its interactions with the metal catalyst ferroin/ferriin, where cyclic voltammograms illustrate that the presence of benzoquinone causes an increase in the redox potential of ferroin/ferriin couple, which may consequently alternate the oxidation and reduction paths of the catalyst.
An intraline of conical intersections for methylamine
Authors: Levi C. , Halász G. J. , Vibók Á. , Bar I. , Zeiri Y. , Kosloff R. , Baer M.
Published: June 2008

In this article are considered the conical intersections (ci’s) related to the N–H bond in the methylamine, CH3NH2, molecule. The novel feature that was revealed is that the two lowest states 1A′ and 1A″ are coupled by a line of cis located in HC–NHH plane—a line that is formed by moving a single hydrogen on that plane while fixing the (six) other atoms. The validity of this line was proven first by studying the singularities of the (angular) nonadiabatic coupling terms and then by revealing the degeneracy points formed by the two interacting adiabatic potential energy surfaces (PESs). A theoretical analysis indicated that the line has to be a finite closed line. We also calculated the Berry phase for a contour that surrounds this line and found it to be 3.127rad, namely, a value reasonably close to π. The existence of such lines of cis—instead of isolated cis (as exhibited by other n-atomic (n>3) molecules such as HNCO or C2H2)—may enhance significantly the transition rate from an upper adiabatic state to a lower one. There are also numerical advantages in such situations, that is, if such a line is properly placed in that plane (like in the present case) the wave-packet treatment of the nuclei can be carried out employing a single diabatic PES instead of having to consider two coupled PESs.
Enhanced magnetoelectric effect in longitudinal-transverse mode Terfenol-D∕Pb(Mg1∕3Nb2∕3)O3–PbTiO3 laminate composites with optimal crystal cut
Authors: Wang Yaojin , Or Siu Wing , Chan Helen Lai Wa , Zhao Xiangyong , Luo Haosu
Published: June 2008

Magnetoelectric (ME) laminate composite consisting of optimal crystal cut thickness-polarized piezoelectric 0.7Pb(Mg1∕3Nb2∕3)O3–0.3PbTiO3 (PMN-PT) single crystal and the length-magnetized magnetostrictive Tb0.3Dy0.7Fe1.92 (Terfenol-D) alloy has been fabricated. The cut optimization of PMN-PT crystal greatly enhances the longitudinally magnetized-transversely polarized (L-T) mode ME effect, which has a superior ME voltage coefficient αE of ∼3.02V∕cmOe in low frequency band. Near the resonance frequency of 95kHz, the coefficient dramatically increases and reaches the maximized value of 33.2V∕cmOe, which is almost two times larger than the previously reported ⟨001⟩-oriented PMN-PT crystal based L-T mode laminate composite.
Nano Au-decorated boron nitride nanotubes: Conductance modification and field-emission enhancement
Authors: Chen Hua , Zhang Hongzhou , Fu Lan , Chen Ying , Williams James S. , Yu Chao , Yu Dapeng
Published: June 2008

This letter reports the electrical and field-emission properties of Au-decorated boron nitride nanotubes (Au-BNNTs). The insulating BNNTs become metallic after Au coating as the Au coverage exceeds a critical value. The Au decoration modifies the work function of the BNNTs and, as a consequence, the field-emission current densities of Au-BNNTs are significantly enhanced. Correspondingly, the turn-on field of the Au-BNNTs is reduced to one third and the emission current density is increased by four orders in contrast to pure BNNTs. The experimental results demonstrate that such Au-BNNTs are promising electron field emitters.
Phase transition and dielectric properties of nanograin BaTiO3 ceramic under high pressure
Authors: Zhu Jinlong , Jin Changqing , Cao Wenwu , Wang Xiaohui
Published: June 2008

Temperature dependence of the dielectric constant of nanograin BaTiO3 ceramic has been investigated under hydrostatic pressure σ up to 5GPa. We found that the paraelectric-ferroelectric phase transition temperature TC decreases with σ at a rate of dTC∕dσ=−40.0(±1.1)K∕GPa for coarse-grain ceramic and −34.3(±1.4)K∕GPa for ceramic with 60nm grains. The variation of intergranular stresses with reducing grain size is considered as the main factor causing the decrease of TC and the change of dTC∕dσ.
Growth and enhanced light emission of hybrid structures of ZnO∕Si nanocrystals
Authors: Kim Sung , Kim Chang Oh , Hwang Sung Won , Choi Suk-Ho
Published: June 2008

Hybrid nanostructures composed of ZnO nanocrystals (NCs) and Si NCs have been fabricated by annealing double layers of ZnO and SiOx on Si (100) wafer at 1100°C for 20min. High-resolution transmission electron microscopy images demonstrate the coexistence of 4–5nm ZnO NCs and 2–10nm Si NCs in the range of x from 1.0 to 1.8. The photoluminescence intensity of the hybrid structures is almost 10 times larger at x=1.0 than that of the ZnO single layer and decreases with increasing x above 1.0, exactly consistent with the x-dependent intensity behaviors of the near-edge x-ray absorption fine structure features. These results are very promising in view of the strong enhancement in the luminescence from ZnO by forming hybrid structures of ZnO∕Si NCs.
Analysis of V defects in GaN-based light emitting diodes by scanning transmission electron microscopy and electron beam induced current
Authors: Progl C. L. , Parish C. M. , Vitarelli J. P. , Russell P. E.
Published: June 2008

In the fabrication of InGaN∕GaN multiple quantum well light emitting diodes so-called V defects are common, but little is known about their electrical activity. Scanning transmission electron microscopy is capable of directly observing these defects, while electron beam induced current (EBIC) techniques can be used to probe electronic behavior of semiconductor defects. These techniques were combined to obtain localized measurements and our results indicate that V defects suppress the EBIC signal near the core of the defect and produce a displacement in the p-n junction location. Furthermore, the EBIC profile suggests that minority carrier diffusion lengths are longer inside the defect.
Air-stable inverted flexible polymer solar cells using zinc oxide nanoparticles as an electron selective layer
Authors: Hau Steven K. , Yip Hin-Lap , Baek Nam Seob , Zou Jingyu , O’Malley Kevin , Jen Alex K.-Y.
Published: June 2008

The performance and stability of unencapsulated inverted bulk-heterojunction solar cells with zinc oxide (ZnO) made by different processes as the electron selective contact are compared to conventional bulk-heterojunction solar cells. The low temperature processed inverted devices using ZnO nanoparticles on indium tin oxide plastic substrates showed high power conversion efficiency of ∼3.3%. This inverted device structure possessed much better stability under ambient conditions retaining over 80% of its original conversion efficiency after 40days while the conventional one showed negligible photovoltaic activity after 4days. This is due to the improved stability at the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/Ag interface.
Green light emitting diodes on a-plane GaN bulk substrates
Authors: Detchprohm Theeradetch , Zhu Mingwei , Li Yufeng , Xia Yong , Wetzel Christian , Preble Edward A. , Liu Lianghong , Paskova Tanya , Hanser Drew
Published: June 2008

We report the development of 520–540nm green light emitting diodes (LEDs) grown along the nonpolar a axis of GaN. GaInN∕GaN-based quantum well structures were grown in homoepitaxy on both, a-plane bulk GaN and a-plane GaN on r-plane sapphire. LEDs on GaN show higher, virtually dislocation-free crystalline quality and three times higher light output power when compared to those on r-plane sapphire. Both structures show a much smaller wavelength blue shift for increasing current density (<10nm for 0.1to12.7A∕cm2) than conventional LEDs grown along the polar c axis.
Design of Co∕Pd multilayer system with antiferromagnetic-to-ferromagnetic phase transition
Authors: Thiele Jan-Ulrich , Hauet Thomas , Hellwig Olav
Published: June 2008

Among the known magnetic material systems, most are either purely antiferromagnetic or purely ferromagnetic at temperatures up to their critical temperature. There are only very few examples of materials that undergo a temperature dependent phase transition from an antiferromagnetic to a ferromagnetic phase or vice versa, and of these, only the chemically ordered alloy FeRh exhibits this transition near room temperature. Here we present a perpendicular anisotropy multilayer structure that by careful tuning of the constituent layers and their coupling mimics the magnetic behavior of FeRh, yet allows great flexibility in tuning its magnetic properties over a wide temperature range.
Growth and characterization of crack-free semipolar {1-101}InGaN∕GaN multiple-quantum well on V-grooved (001)Si substrates
Authors: Chen Guan-Ting , Chang Shih-Pang , Chyi Jen-Inn , Chang Mao-Nan
Published: June 2008

This work elucidates the two-stage growth of GaN on V-grooved (001)Si substrates using metal-organic chemical vapor deposition. The first growth stage proceeds on the {111}Si sidewalls until GaN fills the V grooves. Then the second stage continues and leads to a semipolar surface with the {1-101}GaN facets. GaN films with thickness of over 1μm can be obtained without cracks by this two stage-growth. Excitation-power-dependent and time-resolved photoluminescence measurements confirm that the internal electric field in the InGaN∕GaN multiple-quantum well (MQW) grown on this GaN template is indeed smaller than that of the MQW grown on (0001)GaN.
Antimonide-based diodes for terahertz mixers
Authors: Magno R. , Champlain J. G. , Newman H. S. , Ancona M. G. , Culbertson J. C. , Bennett B. R. , Boos J. B. , Park D.
Published: June 2008

Antimonide-based p+N junctions have been grown by molecular beam epitaxy and processed into diodes. The diodes have good rectification with ideality factors near 1, and high saturation current densities of 2.5×10−2A∕cm2. S-parameter measurements to 50GHz indicate a 1Ω series resistance and a capacitance of 1.2fF∕μm2 for a 5μm diameter mesa diode. A cutoff frequency of 6.5THz is estimated from the RC product. The high saturation current indicates that this diode will reach forward bias currents at substantially lower voltages than GaAs Schottky diodes. These properties suggest using the diode as a terahertz mixer.