bluuebunny/crossref_metadata_2025 huggingface dataset:
The elastic constants and related properties of the energetic material cyclotrimethylene trinitramine (RDX) determined by Brillouin scattering
Authors: Haycraft James J. , Stevens Lewis L. , Eckhardt Craig J.
Published: January 2006

The acoustic phonons of cyclotrimethylene trinitramine (RDX) have been studied using Brillouin scattering. The analysis of the acoustic-phonon velocities allowed determination of the complete stiffness tensor for this energetic material. The results are compared to other recent experimental and theoretical determinations of the RDX elastic constants, bulk moduli, and shear moduli. The observed ordering of elastic constants, C11>C22>C33, is qualitatively associated with a (001) cleavage plane and molecular packing. This interpretation is further corroborated by the linear compressibilities plotted in three crystallographic planes, and a comparison to recent theoretical and experimental hydrostatic compression studies on RDX. Finally, the elasticity of RDX is compared to a recently published report on the beta polymorph of cyclotetramethylene tetranitramine’s elasticity, and is related to several proposed mechanisms for detonation initiation.
An atomistic simulation scheme for modeling crystal formation from solution
Authors: Kawska Agnieszka , Brickmann Jürgen , Kniep Rüdiger , Hochrein Oliver , Zahn Dirk
Published: January 2006

We present an atomistic simulation scheme for investigating crystal growth from solution. Molecular-dynamics simulation studies of such processes typically suffer from considerable limitations concerning both system size and simulation times. In our method this time-length scale problem is circumvented by an iterative scheme which combines a Monte Carlo-type approach for the identification of ion adsorption sites and, after each growth step, structural optimization of the ion cluster and the solvent by means of molecular-dynamics simulation runs. An important approximation of our method is based on assuming full structural relaxation of the aggregates between each of the growth steps. This concept only holds for compounds of low solubility. To illustrate our method we studied CaF2 aggregate growth from aqueous solution, which may be taken as prototypes for compounds of very low solubility. The limitations of our simulation scheme are illustrated by the example of NaCl aggregation from aqueous solution, which corresponds to a solute/solvent combination of very high salt solubility.
Elongation/compaction of giant DNA caused by depletion interaction with a flexible polymer
Authors: Kojima M. , Kubo K. , Yoshikawa K.
Published: January 2006

Structural changes in giant DNA induced by the addition of the flexible polymer Polyethylene Glycol (PEG) were examined by the method of single-DNA observation. In dilute DNA conditions, individual DNA assumes a compact state via a discrete coil-globule transition, whereas in concentrated solution, DNA molecules exhibit an extended conformation via macroscopic phase segregation. The long-axis length of the stretched state in DNA is about 103 times larger than that of the compact state. Phase segregation at high DNA concentrations occurs at lower PEG concentrations than the compaction at low DNA concentrations. These opposite changes in the conformation of DNA molecule are interpreted in terms of the free energy, including depletion interaction.
Accurate magnetic exchange couplings in transition-metal complexes from constrained density-functional theory
Authors: Rudra Indranil , Wu Qin , Van Voorhis Troy
Published: January 2006

We demonstrate an accurate method for extracting Heisenberg exchange-coupling constants (J) from density-functional theory (DFT) calculations. We note that the true uncoupled low-spin state of a given molecule should be identified with the ground state of the system subject to a constraint on the spin density of the atoms. Using an efficient optimization strategy for constrained DFT we obtain these states directly, leading to a simple, physically motivated formula for J. Our method only depends on state energies and their associated electron densities and assigns no unphysical meaning to the Kohn-Sham determinant or individual orbitals. We study several bimetallic transition-metal complexes and find that the constrained DFT approach is competitive with, if not better than, the best broken symmetry DFT results. The success of constrained DFT in these cases appears to result from a balanced elimination of self-interaction error and static correlation from the simulation.
The permanent electric dipole moment and hyperfine interaction in ruthenium monoflouride (RuF)
Authors: Steimle Timothy C. , Virgo Wilton L. , Ma Tongmei
Published: January 2006

Ruthenium monofluoride, RuF, has been detected using low-resolution laser-induced fluorescence (LIF) in the visible and near infrared spectral regions. A visible band, designated as [18.2]5.5−XΦ9∕24, has been recorded field-free and in the presence of a static electric field using high-resolution LIF spectroscopy. The r0 internuclear distances for the [18.2]5.5 and XΦ9∕24 states were determined to be 1.911 and 1.916Å, respectively. The vibrational interval ΔG1∕2 of 534(15)cm−1 for the XΦ9∕24 state was determined from the analysis of the dispersed LIF. The Stark shifts of the visible band were analyzed to produce permanent electric dipole moments of 1.97(8) and 5.34(7)D for the [18.2]5.5 and XΦ9∕24, states, respectively. The fluorine magnetic hyperfine structure associated with spectral features was analyzed. The hyperfine structure and dipole moments are interpreted using a molecular-orbital correlation model and compared with FeF and other ruthenium-containing molecules.
Optimal dynamic discrimination of similara quantum systems in the presence of decoherence
Authors: Li Baiqing , Zhu Wusheng , Rabitz Herschel
Published: January 2006

Optimal dynamic discrimination (ODD) of a mixture of similar quantum systems with time series signals enables the extraction of the associated concentrations with reasonable levels of laser-pulse noise, signal detection errors, and imperfect signal detector resolution [Li et al., J. Chem. Phys. 122, 154103 (2005)]. The ODD paradigm is reexpressed in the density-matrix formulation to allow for the consideration of environmental decoherence on the quality of the extracted concentrations, along with the above listed factors. Simulations show that although starting in a thermally mixed state along with decoherence can be detrimental to discrimination, these effects can be counteracted by seeking a suitable optimal control pulse. Additional sampling of the temporal data also aids in extracting more information to better implement ODD.
Line positions and line strengths for the 3← electric quadrupole band of H2Σg+1
Authors: Robie Daniel C. , Hodges Joseph T.
Published: January 2006

Several rotational lines in the S and Q branches [including the previously unobserved Q(2) and Q(3) lines] of the 3-0 electric quadrupole band of H2 have been detected by cavity ring-down spectroscopy. Line strengths were measured at densities between 2.7×1018 and 7.5×1019moleculescm−3 at room temperature. The observed line strengths in the S branch are consistent with earlier measurements, and systematically below theoretical calculations [relative differences of approximately 10% for the S(1),S(2), and S(3) lines, and nearly 30% for the S(0) line]. Line strength measurements for the Q branch range from 25% to 33% below theoretical calculations.
On the theory of the reaction rate of vibrationally excited CO molecules with OH radicals
Authors: Chen Wei-Chen , Marcus R. A.
Published: January 2006

The dependence of the rate of the reaction CO+OH→H+CO2 on the CO-vibrational excitation is treated here theoretically. Both the Rice-Ramsperger-Kassel-Marcus (RRKM) rate constant kRRKM and a nonstatistical modification knon [W.-C. Chen and R. A. Marcus, J. Chem. Phys. 123, 094307 (2005).] are used in the analysis. The experimentally measured rate constant shows an apparent (large error bars) decrease with increasing CO-vibrational temperature Tv over the range of Tv’s studied, 298–1800K. Both kRRKM(Tv) and knon(Tv) show the same trend over the Tv-range studied, but the knon(Tv) vs Tv plot shows a larger effect. The various trends can be understood in simple terms. The calculated rate constant kv decreases with increasing CO vibrational quantum number v, on going from v=0 to v=1, by factors of 1.5 and 3 in the RRKM and nonstatistical calculations, respectively. It then increases when v is increased further. These results can be regarded as a prediction when v state-selected rate constants become available.
A simple model for the water o-H2 complex
Authors: Ceponkus Justinas , Nelander Bengt
Published: January 2006

The infrared spectrum of the complex between o-H2 and H2O, D2O, or HDO, isolated in a matrix of solid p-H2, has been studied between 20 and 4500cm−1. In addition the infrared spectrum of the complex between p-D2 and H2O in solid o-D2 has been studied. The spectral shifts are interpreted as the result of the quadrupole-dipole interaction between hydrogen and water.
Orbital-free tensor density functional theory
Authors: Ovchinnikov Igor V. , Neuhauser Daniel
Published: January 2006

We propose a family of time-dependent orbital-free density-based theories that go beyond the usual current-density description of electrons or other particles. The theories deal with physical quantities that characterize the one-particle density matrix and consequently the kinetics of the particles. We analyze the first two theories in the family. The “lowest-order” theory is quantum hydrodynamics. The second one yields not only the longitudinal plasmon collective excitations, but also the transverse phonon modes that are associated with elementary excitations in Fermi liquids. The theories should make it feasible to do large orbital-free simulations of time-dependent and stationary systems.
Deforming biological membranes: How the cytoskeleton affects a polymerizing fiber
Authors: Daniels D. R. , Wang J. C. , Briehl R. W. , Turner M. S.
Published: January 2006

We give a theoretical treatment of the force exerted by a fluctuating membrane on a polymer rod tip, taking into account the effects of an underlying biological cytoskeleton by way of a simple harmonic dependence on displacement. We also consider theoretically and experimentally the dynamics of a growing fiber tip under the influence of such a fluctuation-induced membrane force, including the effects of an underlying cytoskeletal network. We compare our model with new experimental data for the growth of hemoglobin fibers within red blood cells, revealing a good agreement. We are also able to estimate the force and membrane/cytoskeletal displacement required to stall growth of, or buckle, a growing fiber. We discuss the significance of our results in a biological context, including how the properties of the membrane and cytoskeleton relate to the thermodynamics of rod polymerization.
Dynamics of OH radical generation in laser-induced photodissociation of tetrahydropyran at 193 nm
Authors: SenGupta Sumana , Upadhyaya Hari P. , Kumar Awadhesh , Naik Prakash D. , Bajaj Parmanand
Published: January 2006

Tetrahydropyran (THP) undergoes photodissociation on excitation with ArF laser at 193 nm, generating OH radical as one of the transient photoproducts. Laser-induced fluorescence technique is used to detect the nascent OH radical and measure its energy state distribution. The OH radical is formed mostly in the ground vibrational level (v″=0), with low rotational excitation. The rotational distribution of OH (v″=0,J) is characterized by a temperature of 433±31K, corresponding to a rotational energy of 0.86±0.06kcal∕mol. Two Λ-doublet levels, Π+2(A′) and Π−2(A″), and the two spin-orbit states, the Π3∕22 and Π1∕22, of OH are populated statistically for all rotational levels. The relative translational energy associated with the photoproducts in the OH channel is calculated to be 21.9±3.2kcalmol−1, from the Doppler-broadened linewidth, giving an ft value of ∼43%, and most of the remaining 57% of the available energy is distributed in the internal modes of the other photofragment, C5H9. The observed distribution of the available energy is explained well, using a hybrid model of energy partitioning, with an exit barrier of 40kcalmol−1. The potential-energy surface of the reaction channel was mapped by ab initio molecular-orbital calculations. Based on experimental and theoretical results, a mechanism for OH formation is proposed. Electronically excited THP relaxes to the ground electronic state, and from there, a sequence of reactions takes place, generating OH. The proposed mechanism first involves C–O bond scission, followed by a 1,3 H atom migration to O atom, and finally, the C–OH bond cleavage giving OH.
A thermodynamic approach to the fragility of glass-forming polymers
Authors: Cangialosi D. , Alegría A. , Colmenero J.
Published: January 2006

We have connected the dynamic fragility, namely, the steepness of the relaxation-time variation upon temperature reduction, to the excess entropy and heat capacity of a large number of glass-forming polymers. The connection was obtained in a natural way from the Adam-Gibbs equation, relating the structural relaxation time to the configurational entropy. We find a clear correlation for a group of polymers. For another group of polymers, for which this correlation does not work, we emphasize the role of relaxation processes unrelated to the α process in affecting macroscopic thermodynamic properties. Once the residual excess entropy at the Vogel temperature is removed from the total excess entropy, the correlation between dynamic fragility and thermodynamic properties is reestablished.
Surface reconstructions and stability of X-shaped carbon nanotube junction
Authors: Meng F. Y. , Shi S. Q. , Xu D. S. , Chan C. T.
Published: January 2006

A complete surface reconstruction takes place after a local connection between two crossed tubes is established, leading to the creation of an extended X-shaped junction constituted by topological defects with smooth negative curvature. Molecular-dynamics simulations show that the surface reconstructions occur through (1) generalized Stone-Wales transformation and (2) the movement of sp and sp3 atoms and their transformation to sp2 atoms by bond rearrangement. Based on both the principle of energy minimization and a generalized Euler’s rule, it is demonstrated that the most stable structure for X junctions contains only 12 heptagons. The annealing temperature influences the topological structure and stability of junctions.
Structure and shape variations in intermediate-size copper clusters
Authors: Yang Mingli , Jackson Koblar A. , Koehler Christof , Frauenheim Thomas , Jellinek Julius
Published: January 2006

Using extensive, unbiased searches based on density-functional theory, we explore the structural evolution of Cun clusters over the size range n=8–20. For n=8–16, the optimal structures are plateletlike, consisting of two layers, with the atoms in each layer forming a trigonal bonding network similar to that found in smaller, planar clusters (n⩽6). For n=17 and beyond, there is a transition to compact structures containing an icosahedral 13-atom core. The calculated ground-state structures are significantly different from those predicted earlier in studies based on empirical and semiempirical potentials. The evolution of the structure and shape of the preferred configuration of Cun, n⩽20, is shown to be nearly identical to that found for Na clusters, indicating a shell-model-type behavior in this size range.
Three-pulse photon echo of an excitonic dimer modeled via Redfield theory
Authors: Kjellberg Pär , Pullerits Tõnu
Published: January 2006

In this article the third-order response of an excitonically coupled dimer is studied. The three-pulse photon echo signals were calculated by extracting polarization components from the total polarization in the corresponding phase-matched directions. The total nonlinear response was obtained by numeric propagation of the density matrix, with the exciton-vibrational coupling modeled via Redfield relaxation theory. The full two-dimensional three-pulse photon echo signals and the peak shift were analyzed in terms of the density-matrix dynamics of coherence dephasing and population relaxation. The location of the two-exciton state was found to be essential for proper modeling of the three-pulse photon echo. In particular, an oscillation in the three-pulse photon echo peak shift is found if the two-exciton state is displaced. The oscillations can be related to the dynamics of the one-exciton coherences.
Postcollision relaxation of small atomic clusters
Authors: Napari Ismo , Vehkamäki Hanna
Published: January 2006

Molecular-dynamics simulations are performed to investigate the effects caused by the lack of internal equilibration on the dynamics and properties of atomic clusters. The studied systems consist of Lennard-Jones clusters of five to ten atoms and a colliding vapor monomer. Cluster radius and potential energy are shown to reach a time-independent value within 30ps after a collision with a vapor monomer. The relaxation in terms of rotational energy takes at least 200ps. During the first couple of picoseconds after the collision time-dependent cluster decay rates are observed. The unrelaxed cluster states are expected to have minimal effect on gas-liquid nucleation rates.
Interaction energies in hydrogen-bonded systems: A testing ground for subsystem formulation of density-functional theory
Authors: Kevorkyants R. , Dulak M. , Wesolowski T. A.
Published: January 2006

The formalism based on the total energy bifunctional (E[ρI,ρII]) is used to derive interaction energies for several hydrogen-bonded complexes (water dimer, HCN–HF, H2CO–H2O, and MeOH–H2O). Benchmark ab initio data taken from the literature were used as a reference in the assessment of the performance of gradient-free [local density approximation (LDA)] and gradient-dependent [generalized gradient approximation (GGA)] approximations to the exchange-correlation and nonadditive kinetic-energy components of E[ρI,ρII]. On average, LDA performs better than GGA. The average absolute error of calculated LDA interaction energies amounts to 1.0kJ∕mol. For H2CO–H2O and H2O–H2O complexes, the potential-energy curves corresponding to the stretching of the intermolecular distance are also calculated. The positions of the minima are in a good agreement (less than 0.2Å) with the reference ab initio data. Both variational and nonvariational calculations are performed to assess the energetic effects associated with complexation-induced deformations of molecular electron densities.
The effect of surface relaxation on the N2 dissociation rate on stepped Ru: A transition state theory study
Authors: van Harrevelt Rob , Honkala Karoliina , Nørskov Jens K. , Manthe Uwe
Published: January 2006

The interaction between N2 and the surface results in a considerable rearrangement of the structure of the surface near the steps. The present work investigates the effect of this interaction on the rate of dissociative adsorption within the framework of harmonic transition state theory. Employing different models where selected Ru degrees of freedom are included, we show that the dominant effect of surface relaxation is a shift of the classical barrier height. Therefore, the “relaxed/N2” model, which takes relaxation of the surface into account but neglects vibrations of the surface atoms explicitly, can be used for reliable rate constant calculations. Sticking coefficients calculated employing the relaxed/N2 model based on different density-functional theory (DFT) calculations are then compared with experiment. Comparison with the experiment suggests that for the best DFT calculations the barrier height (0.49eV) is about 0.15eV too high.
High-resolution infrared spectroscopy andab initiostudies of the cyclopropane-carbon dioxide interaction
Authors: Su Zheng , Tam Wai Shun , Xu Yunjie
Published: January 2006

A jet-cooled high-resolution infrared spectrum of the cyclopropane-carbon dioxide complex was detected for the first time, using a rapid scan infrared spectrometer with an astigmatic multipass sample cell. The spectrum was recorded in the vicinity of the CO2 asymmetric stretching band (ν3) and exhibits a b-dipole selection rule. Altogether, over 200 lines were observed, assigned, and fitted to Watson’s S-reduction Hamiltonian. Rotational and quartic distortion constants were obtained. The band origin was located at 2347.6263(2)cm−1, redshifted by 1.5230(2)cm−1 from the corresponding frequency of the CO2 monomer. The experimentally determined structure shows that CO2 lies next to a C–C bond edge and is perpendicular to the C3 ring, indicating that the interaction is characterized by the bonding between the carbon atom of CO2 and the pseudo-π system of cyclopropane. The intermolecular distance between the carbon atom of CO2 and the center of mass of cyclopropane was determined to be 3.667(2)Å. Complete ab initio geometry optimizations and harmonic frequency calculations were carried out at the level of second-order Møller-Plesset perturbation theory with four different basis sets: cc-pVDZ, 6-311++G(d,p), aug-cc-pVDZ, and aug-cc-pVTZ. The lowest-energy structure identified with the three larger basis sets is in accord with the experimental finding. In addition, a transition state was identified and the tunneling barrier height was computed.
Local-structural diversity and protein folding: Application to all-β off-lattice protein models
Authors: Pan Patricia Wang , Gordon Heather L. , Rothstein Stuart M.
Published: January 2006

Global measures of structural diversity within a distribution of biopolymers, such as the radius of gyration and percent native contacts, have proven useful in the analysis of simulation data for protein folding. In this paper we describe a statistical-based methodology to quantify the local structural variability of a distribution of biopolymers, applied to 46- and 69-“residue” off-lattice, three-color model proteins. Each folds into β-barrel structures. First we perform a principal component analysis of all interbead distance variables for a large number of independent, converged Boltzmann-distributed samples of conformations collected at each of a wide range of temperatures. Next, the principal component vectors are subjected to orthogonal (varimax) rotation. The results are displayed on so-called “squared-loading” plots. These provide a quantitative measure of the contribution to the sample variance of the position of each residue relative to the others. Dominant structural elements, those having the largest structural diversity within the sampled distribution, are responsible for peaks and shoulders observed in the specific heat versus temperature curves, generated using the weighted histogram analysis method. The loading plots indicate that the local-structural diversity of these systems changes gradually with temperature through the folding transition but radically changes near the collapse transition temperature. The analysis of the structural overlap order statistic suggests that the 46-mer thermodynamic folding transition involves the native state and at least three other nearly native intermediates. In the case of the 46-mer protein model, data are generated at sufficiently low temperatures that squared-loading plots, coupled with cluster analysis, provide a local and energetic description of its glassy state.
Cluster kinetics and dynamics during spinodal decomposition
Authors: Yang Jiao , McCoy Benjamin J. , Madras Giridhar
Published: January 2006

Spinodal decomposition (barrierless phase transition) is a spontaneous phase separation caused by conditions that force the system to become thermodynamically unstable. We consider spinodal decomposition to occur under conditions of large supersaturation S and/or small ratio of interfacial to thermal energies ω, such that the computed number of monomers in a critical nucleus ξ*=(ω∕lnS)3 is less than unity. The small critical nucleus size is consistent with a negligible energy barrier for initiating condensation. Thus, in contrast to conventional opinion, it is suggested that the spinodal decomposition is related to the homogeneous nucleation of metastable fluids. Population balance equations show how clusters aggregate and rapidly lead to phase separation. Different mass dependences of aggregation rate coefficients are proposed to investigate the fundamental features of spinodal decomposition. When the mass dependency is an integer, the equations are solved by the moment technique to obtain analytical solutions. When the mass dependency is a noninteger, the general cases are solved numerically. All solutions predict the two time regimes observed experimentally: the average length scale of condensed-phase domains increases as a power law with an exponent of 1∕3 at early times, followed by a linear increase at longer times.
Lifetimes of C602− and C702− dianions in a storage ring
Authors: Tomita S. , Andersen J. U. , Cederquist H. , Concina B. , Echt O. , Forster J. S. , Hansen K. , Huber B. A. , Hvelplund P. , Jensen J. , Liu B. , Manil B. , Maunoury L. , Brøndsted Nielsen S. , Rangama J. , Schmidt H. T. , Zettergren H.
Published: January 2006

C 60 2 − and C702− dianions have been produced by electrospray of the monoanions and subsequent electron pickup in a Na vapor cell. The dianions were stored in an electrostatic ring and their decay by electron emission was measured up to 1 s after injection. While C702− ions are stable on this time scale, except for a small fraction of the ions which have been excited by gas collisions, most of the C602− ions decay on a millisecond time scale, with a lifetime depending strongly on their internal temperature. The results can be modeled as decay by electron tunneling through a Coulomb barrier, mainly from thermally populated triplet states about 120 meV above a singlet ground state. At times longer than about 100 ms, the absorption of blackbody radiation plays an important role for the decay of initially cold ions. The tunneling rates obtained from the modeling, combined with WKB estimates of the barrier penetration, give a ground-state energy 200±30meV above the energy of the monoanion plus a free electron and a ground-state lifetime of the order of 20 s.
Remote two-dimensional imaging of giant magnetoresistance with spatial resolution
Authors: Stirk S. M. , Thompson S. M. , Mennicke R. T. , Matthew J. A. D. , Lee A. F.
Published: January 2006

Spatially resolved two-dimensional images of giant magnetoresistance (GMR) are demonstrated in Co∕Cu and CoFe∕Cu multilayers by remote imaging of the sample temperature using an infrared charge coupled device camera. We have previously shown that the emissivity of a giant magnetoresistive material at infrared wavelengths is related to its electrical resistance [S. M. Stirk et al., Appl. Phys. Lett. 86, 102505 (2005)]. Here, the change in emissivity in an applied magnetic field is sensed by the camera as a field dependent temperature image. Using the inherent spatial resolution of the camera, we characterize the GMR on a spatial scale of 200 microns across the sample surface. In a small sputtered Co∕Cu multilayer sample, a significant variation of the GMR over the sample surface is observed, a characteristic previously undetectable by conventional electrical measurements. By contrast, no such spatial variation of the GMR is observed from a commercially grown CoFe∕Cu wafer. These measurements demonstrate the potential for quick and remote imaging of magnetoresistance with spatial resolution with possible applications such as the whole wafer characterization of GMR materials and the magnetoresistance in patterned arrays.
Determination of surface tension in binary mixtures using transition-matrix Monte Carlo
Authors: Shen Vincent K. , Errington Jeffrey R.
Published: January 2006

We present a methodology based on grand-canonical transition-matrix Monte Carlo and finite-size scaling analysis to calculate surface tensions in binary mixtures. In particular, mixture transition-matrix Monte Carlo is first used to calculate apparent, system-size-dependent free-energy barriers separating coexisting fluid phases. Finite-size scaling is then used to extrapolate these values to the infinitely large system limit to determine the true thermodynamic surface tension. A key distinction of the methodology is that it yields the entire isothermal surface-tension curve for a binary mixture in a relatively small number of simulations. We demonstrate the utility of the method by calculating surface-tension curves for three binary Lennard-Jones mixtures. While we have only examined the surface tension of simple fluids in this work, the method is general and can be extended to molecular fluids as well as to determine interfacial tensions of liquid-liquid interfaces.
Photoluminescence from gold nanoplates induced by near-field two-photon absorption
Authors: Imura Kohei , Nagahara Tetsuhiko , Okamoto Hiromi
Published: January 2006

We have investigated two-photon-induced photoluminescence (PL) properties of single gold nanoplates by using an apertured scanning near-field optical microscope. The nanoplates show PL much stronger than nanorods. The near-field PL images show characteristic spatial features, which depend on the incident polarization. These PL images are in good agreement with the calculated spatial distribution of the electric fields adjacent to the particles at the excitation wavelength. We attribute the observed images to spatial characteristics of plasmon modes.
Secondary electron imaging of nonconductors with nanometer resolution
Authors: Toth Milos , Knowles W. Ralph , Thiel Bradley L.
Published: January 2006

The resolution of secondary electron (SE) images in scanning electron microscopy (SEM) is limited by the SE diffusion length. However, most materials are poor electrical conductors and in practice, resolution and image information content are often limited by charging. We demonstrate how charging can be eliminated as the resolution-limiting factor using a gaseous SE detector for magnetic immersion electron lenses. Charging is stabilized by ions produced in a magnetic field-assisted gas ionization cascade. The charge control self-regulation process does not quench the SE imaging signal, thereby enabling high resolution image contrast mechanisms that are suppressed in high vacuum SEM.
Epitaxial Bi∕GaAs(111) diodes via electrodeposition
Authors: Bao Zhi Liang , Kavanagh Karen L.
Published: January 2006

Bismuth films formed by electrodeposition on n-GaAs (111) at 70°C are found to be single crystalline, (0001) oriented, with trigonal surface morphologies typical of high quality single crystals. Diode current-voltage characteristics display low reverse-bias leakage currents and average barrier heights of 0.77±0.02eV (n=1.07). A necessary requirement for single crystalline growth is the presence of ammonium sulfate in the electrolyte.
Source of low-energy coherent electron beams
Authors: Daineche R. , Degiovanni A. , Grauby O. , Morin R.
Published: January 2006

The coherence of the electron beam emitted at a low applied field from a single micrometer size mineral particle deposited on a carbon membrane is demonstrated by holography experiments. The experimental design combining this electron source with a conventional electrostatic lens system opens a way to holographic microscopy with low-energy electrons.
Enhancement of both direct-current and microwave characteristics of AlGaN∕GaN high-electron-mobility transistors by furnace annealing
Authors: Arulkumaran S. , Ng G. I. , Tan C. L. , Liu Z. H. , Bu J. , Radhakrishnan K. , Aggerstam T. , Sjödin M. , Lourdudoss S.
Published: January 2006

The enhancement of both direct-current (dc) and microwave characteristics of AlGaN∕GaN high-electron-mobility transistors (HEMTs) were demonstrated by conventional furnace annealing at 400°C for 5min. Due to the improvement in Ni∕Au Schottky contact properties by furnace annealing, about 17%, 34%, 23%, and 25% of enhancements in maximum drain current density, maximum extrinsic transconductance (gmmax), cutoff frequency and maximum oscillation frequency were observed, respectively. A positive threshold voltage shift and the increase in gmmax can also be correlated to the improved Schottky parameters such as ideality factor and barrier height. The annealed devices exhibited low reverse gate-leakage-current by more than three orders of magnitude and low drain-leakage-current by two orders of magnitude. Correspondingly, the devices exhibited 55% of higher breakdown voltage after annealing. The furnace annealing is an effective and viable means to enhance both dc and microwave characteristics of AlGaN∕GaN HEMTs.
Improved laser triggering and guiding of meqavolt discharges with dual fs-ns pulses
Authors: Méjean Guillaume , Ackermann Roland , Kasparian Jérôme , Salmon Estelle , Yu Jin , Wolf Jean-Pierre , Rethmeier Kay , Kalkner Wilfried , Rohwetter Philipp , Stelmaszczyk Kamil , Wöste Ludger
Published: January 2006

We demonstrate that the capacity of ultrashort high-power laser pulses to trigger and guide high-voltage discharges can be significantly enhanced by a subsequent visible nanosecond laser pulse. The femtosecond pulse induces a bundle of filaments, which creates a conducting channel of low density and cold plasma connecting the electrodes. The subsequent laser pulse photodetaches electrons from O2− ions in the electrode leader. The resulting electrons allow efficient heating by Joule effect in a retroaction loop, resulting in a 5% reduction of the breakdown voltage.
Transport of ionized metal atoms in high-power pulsed magnetron discharges assisted by inductively coupled plasma
Authors: Konstantinidis S. , Dauchot J. P. , Ganciu M. , Hecq M.
Published: January 2006

Transporting metallic ions from the magnetron cathode to the substrate is essential for an efficient thin-film deposition process. This letter examines how inductively coupled plasma superimposed onto a high-power pulsed magnetron discharge can influence the mobility of titanium ions. To this effect, time-resolved optical emission and absorption spectrometry are conducted and the current at the substrate is measured. With this new hybrid technique, ions are found to reach the substrate in two successive waves. Metal ions, only present in the second wave, are found to accelerate proportionally to the power supplied to the inductively coupled plasma. All the measurements in this study are made at 10 and 30mTorr, with 10μs long pulses at the magnetron cathode.
Temperature-dependent hole detrapping for unprimed polycrystalline chemical vapor deposited diamond
Authors: Wang Sigen , Sellin Paul , Lohstroh Annika
Published: January 2006

Rise-time distribution spectra of a polycrystalline chemical vapor deposited diamond detector were directly measured from alpha-particle induced pulse shapes over a temperature range of 240–280K. Pulses due to hole-dominated charge transport showed a strong delayed component due to thermal detrapping of charge from a shallow level, with a mean rise time that decreased strongly with increasing temperature. The activation energy of this shallow hole trap was directly measured using an Arrhenius plot, with a value of 0.31±0.03eV. No priming or pre-irradiation of the device was required in order to observe thermal detrapping, indicating that the concentration of shallow hole traps in this sample is relatively high. In contrast, no delayed component was observed from electron transport, indicating that only deep electron-trapping levels are active.
Quantum confinement effect in ZnO∕Mg0.2Zn0.8O multishell nanorod heterostructures
Authors: Jang Eue-Soon , Bae Jun Young , Yoo Jinkyoung , Park Won Il , Kim Dong-Wook , Yi Gyu-Chul , Yatsui T. , Ohtsu M.
Published: January 2006

We report on photoluminescence measurements of Mg0.2Zn0.8O∕ZnO∕Mg0.2Zn0.8O multishell layers on ZnO core nanorods. Dominant excitonic emissions in the photoluminescence spectra show a blueshift depending on the ZnO shell layer thickness attributed to the quantum confinement effect in the nanorod heterostructure radial direction. Furthermore, near-field scanning optical microscopy clearly shows sharp photoluminescence peaks from the individual nanorod quantum structures, corresponding to subband levels.
Optically driven bacterial screw of Archimedes
Authors: Robbins J. R. , Tierney D. A. , Schmitzer H.
Published: January 2006

The linear momentum transfer from photons to asymmetrically shaped structures generates an optomechanical rotation, but these micron-sized structures require costly, high-precision fabrication. Nature, however, provides a great diversity of engineered forms and dead, but rigid, asymmetrically shaped bacteria can provide a low cost alternative. In this work, we show that helical bacteria rotate very quickly in optical tweezers. Their rotation is fast enough to create a strong whirl, such that they may act as micropumps in thin capillaries.
Enhancement of the power factor of the transition metal pentatelluride HfTe5 by rare-earth doping
Authors: Lowhorn Nathan D. , Tritt Terry M. , Abbott Edward E. , Kolis J. W.
Published: January 2006

The transition metal pentatellurides HfTe5 and ZrTe5 have been observed to possess interesting electrical transport properties with high thermopower and low resistivity values leading to high thermoelectric power factors. We have investigated the effect of doping HfTe5 with rare-earth elements by measuring the power factor data from about 10K to room temperature on single crystals of Hf1−xRxTe5, where R=Ce, Pr, Nd, Sm, Gd, Tb, Dy, and Ho. Samples that have been doped with Nd (Hf1−xNdxTe5) possess power factors more than a factor of 2 larger than that of the commonly used thermoelectric material Bi2Te3.
Ce
Mn
Ni
4
: A soft ferromagnet with a high degree of transport spin polarization
Authors: Singh Surjeet , Sheet Goutam , Raychaudhuri Pratap , Dhar Sudesh Kumar
Published: January 2006

In this letter we introduce a soft ferromagnetic compound, i.e., CeMnNi4, which exhibits a large moment (∼4.95μB∕Mn) and high degree of spin polarization. The system has a ferromagnetic transition temperature of 140K. Isothermal magnetization measurements at 5K reveal that the material is a soft ferromagnet with a magnetization saturating at about 1500Oe and a coercive field of <5Oe. We determine the transport spin polarization of this material from point contact Andreev reflection measurements to be 66% thereby making this material potentially important for spintronic applications.
Magnetic tunnel junctions using B2-ordered Co2MnAl Heusler alloy epitaxial electrode
Authors: Sakuraba Y. , Nakata J. , Oogane M. , Ando Y. , Kato H. , Sakuma A. , Miyazaki T. , Kubota H.
Published: January 2006

Magnetic tunnel junctions were fabricated with epitaxially grown Co2MnAl bottom electrodes combined with an Al–O tunnel barrier using a magnetron sputtering system. The epitaxial Co2MnAl electrode had very low surface roughness of 0.2nm and a highly ordered B2 structure. Magnetic tunnel junctions (MTJs) with a stacking structure of epitaxial-Co2MnAl∕Al–O∕CoFe∕IrMn exhibited large tunnel magnetoresistance (TMR) ratios of 65% at room temperature and 83% at 10K. The TMR ratios were larger than those of a MTJ with a Co2MnAl polycrystalline electrode.
Oblique Hanle measurements of InAs∕GaAs quantum dot spin-light emitting diodes
Authors: Itskos G. , Harbord E. , Clowes S. K. , Clarke E. , Cohen L. F. , Murray R. , Van Dorpe P. , Van Roy W.
Published: January 2006

We report on studies of electrical spin injection from ferromagnetic Fe contacts into semiconductor light emitting diodes containing single layers of InAs∕GaAs self-assembled quantum dots (QDs). An oblique magnetic field is used to manipulate the spin of the injected electrons in the semiconductor. This approach allows us to measure the injected steady-state spin polarization in the QDs, Pspin as well as estimate the spin losses in the QD spin detector. After subtraction of magneto-optical effects not related to spin injection, we measured a Pspin of 7.5% at 15 K and estimated an injected spin polarization before QD recombination of around 20%.
Optical study of hot electron transport in GaN: Signatures of the hot-phonon effect
Authors: Wang Kejia , Simon John , Goel Niti , Jena Debdeep
Published: January 2006

The hot-phonon lifetime in GaN is measured by temperature- and electric field-dependent photoluminescence studies of a n-type channel. The rate of increase of electron temperature with the external electric field provides a signature of nonquilibrium hot-phonon accumulation. Hot-electron temperatures are measured directly as a function of applied electric fields, and by comparing theoretical models for electron energy-loss into acoustic and optical phonons, a hot-phonon lifetime of τph=3 to 4 ps is extracted.
Direct femtosecond laser nanopatterning of glass substrate by particle-assisted near-field enhancement
Authors: Zhou Y. , Hong M. H. , Fuh JYH , Lu L. , Luk’yanchuk B. S. , Wang Z. B. , Shi L. P. , Chong T. C.
Published: January 2006

Direct femtosecond laser nanopatterning of glass substrate by particle-assisted near-field enhancement was demonstrated in this letter. The nanostructure was characterized by field-emission scanning electron microscopy and atomic force microscopy. No cracks were found on the glass surface. The hole size were measured from 200∼300nm. When laser fluence is close to the damage threshold, a trihole structure was observed. Nonlinear multiphoton absorption and near-field enhancement were the mechanisms of the nanofeature formation. Calculations based on particle-on-surface theory were carried out. The suggested method has potential applications in the nanolithography of a transparent glass substrate for nanostructure device fabrication.
Magnetophotonic crystals based on yttrium-iron-garnet infiltrated opals: Magnetization-induced second-harmonic generation
Authors: Murzina T. V. , Kim E. M. , Kapra R. V. , Moshnina I. V. , Aktsipetrov O. A. , Kurdyukov D. A. , Kaplan S. F. , Golubev V. G. , Bader M. A. , Marowsky G.
Published: January 2006

Three-dimensional magnetophotonic crystals (MPCs) based on artificial opals infiltrated by yttrium iron garnet (YIG) are fabricated and their structural, optical, and nonlinear optical properties are studied. The formation of the crystalline YIG inside the opal matrix is checked by x-ray analysis. Two templates are used for the infiltration by YIG: bare opals and those covered by a thin platinum film. Optical second-harmonic generation (SHG) technique is used to study the magnetization-induced nonlinear-optical properties of the composed MPCs. A high nonlinear magneto-optical Kerr effect in the SHG intensity is observed at the edge of the photonic band gap of the MPCs.
Ultraviolet photoluminescence and Raman properties of MgZnO nanopowders
Authors: Bergman Leah , Morrison John L. , Chen Xiang-Bai , Huso Jesse , Hoeck Heather
Published: January 2006

We report on the ultraviolet photoluminescence (UV-PL) and Raman properties of wurtzite MgxZn1−xO nanopowders of average size ∼30nm that were synthesized via the thermal decomposition method. For the studied composition range of 0⩽x⩽0.26, the room-temperature UV-PL was found to be tuned by ∼0.24eV towards the UV spectral range, and the PL emission was established to be due to an excitonic-type recombination mechanism. The first-order longitudinal optical (LO) Raman mode was found to exhibit a blueshift of ∼33cm−1 and the second-order LO a shift of ∼60cm−1. The LO mode of the nanopowders is discussed in terms of a mixed A1-E1 symmetry phonon known as a quasi-LO mode. The observed 30cm−1 blueshift indicates that the E1 is the principal symmetry component in the Raman scattering of the MgxZn1−xO nanopowders.
H-induced platelet and crack formation in hydrogenated epitaxial Si∕Si0.98B0.02∕Si structures
Authors: Shao Lin , Lin Yuan , Swadener J. G. , Lee J. K. , Jia Q. X. , Wang Y. Q. , Nastasi M. , Thompson Phillip E. , Theodore N. David , Alford T. L. , Mayer J. W. , Chen Peng , Lau S. S.
Published: January 2006

An approach to transfer a high-quality Si layer for the fabrication of silicon-on-insulator wafers has been proposed based on the investigation of platelet and crack formation in hydrogenated epitaxial Si∕Si0.98B0.02∕Si structures grown by molecular-beam epitaxy. H-related defect formation during hydrogenation was found to be very sensitive to the thickness of the buried Si0.98B0.02 layer. For hydrogenated Si containing a 130nm thick Si0.98B0.02 layer, no platelets or cracking were observed in the B-doped region. Upon reducing the thickness of the buried Si0.98B0.02 layer to 3nm, localized continuous cracking was observed along the interface between the Si and the B-doped layers. In the latter case, the strains at the interface are believed to facilitate the (100)-oriented platelet formation and (100)-oriented crack propagation.
Grain boundary sliding mechanisms in ZrN-Ag, ZrN-Au, and ZrN-Pd nanocomposite films
Authors: Aouadi S. M. , Basnyat P. , Zhang Y. , Ge Q. , Filip P.
Published: January 2006

Nanocomposite films of ZrN-Me (Me=Ag, Au, or Pd) were produced by reactive unbalanced magnetron sputtering and were found to form a dense and homogeneous microstructure whereby nanocrystals of Me are distributed evenly throughout the ZrN matrix. Interestingly, the Young’s modulus was found to decrease much more dramatically with the increase in metal content for the ZrN-Ag system. A systematic ab initio study was undertaken to understand the mechanism of grain boundary sliding in these nanostructures. The maximum energy variation during the sliding was found to be the largest and the smallest for ZrN-Pd and ZrN-Ag, respectively.
Factors affecting the survival of Australian wild rabbits exposed to rabbit haemorrhagic disease
Authors: Henzell Robert P. , Cunningham Ross B. , Neave Helen M.
Published: January 2006

Rabbit haemorrhagic disease virus (RHDV) is foreign to Australia, and first entered populations of Australian wild rabbits (Oryctolagus cuniculus L.) in Australia in late 1995. Rabbits are serious environmental and agricultural pests in Australia, and RHDV, a major new pathogen, was introduced as a biological control agent to reduce their numbers. Our study evaluated some of the factors affecting survival of wild rabbits exposed to rabbit haemorrhagic disease (RHD) at 78 sites across Australia.Our data on rabbit numbers consist of the number of rabbits per spotlight kilometre present shortly before and shortly after an RHD outbreak at each site. They are a direct measure of survival rather than mortality. By reducing the interval between the pre- and post-RHD counts to the minimum possible, we sought to minimise the influence on the analysis of other causes of change in rabbit numbers. We calculated proportional survival as the ratio (number of rabbits present after RHD)/(number present before RHD), and used regression analysis to relate it to environmental and other variables. Proportional survival was lower at higher densities of rabbits; was lower if RHDV arrived naturally at the site rather than if it was deliberately released; was lower in areas with hot, dry climates than in areas with cold, wet climates; was lower in southern, inland areas than in warm, coastal areas; and, if the outbreak occurred during summer, was lower in areas of winter rainfall than in areas of summer rainfall. Rainfall seasonality was not correlated with survival at other times of the year. Only in the last effect was there a significant interaction with the time of the year that the outbreak occurred.Our statistical model describes correlations among the data, but does not in itself establish cause and effect. We interpret the properties of our statistical model to draw the following conclusions. First, the effectiveness of RHD is reduced in cold, wet areas and warm, coastal areas, because of the prevalence in these areas of one or more pre-existing caliciviruses in rabbits that impart year-round resistance to RHD. Second, we conclude that the poor summertime performance of RHD in areas that are wet in summer could result from poor survival of RHDV exposed to the combination of high temperature and high relative humidity, although it is also possible that during summer the effectiveness of vectors declines.