bluuebunny/crossref_metadata_2025 huggingface dataset:
Infrared spectroscopy of aqueous ionic salt mixtures at low concentrations: Ion pairing in water
Authors: Max Jean-Joseph , Chapados Camille
Published: September 2007

The analysis by infrared spectroscopy of aqueous mixtures of NaI and CsCl was made in order to obtain information at the molecular level of the mixing of these two salts taken as model systems of strong electrolytes in water. In previous papers [J.-J. Max and C. Chapados, J. Chem. Phys. 115, 2664 (2001) and J.-J. Max et al., ibid. 126, 184507 (2007)] it was reported that a pure salt in water forms pairs of monoions to which are attached a fixed number of water molecules, giving solvated water species. Due to their interaction with the ion pairs, the solvated water molecules are strongly perturbed, modifying the IR water spectrum being monitored. After taking the IR spectrum of pure water, a small volume of NaI 2M was added and the IR spectrum taken. Then a small volume of CsCl 2M was added and a new IR spectrum taken. This procedure was repeated to obtain a series of 38 spectra in the 0.05M–0.83M concentration range. Factor analysis made on the series revealed the presence of three types of water: pure water and two salt solvated waters. The number of solvated water molecules on the two salts taken together is ten. Since NaI and CsCl have, respectively, 3.5 and 3.0 solvated water molecules, it was concluded that a reaction occurred in the solutions forming NaCl and CsI that have, respectively, five water molecules each for a total of ten. The analysis of the spectra of the orthogonal factors supports this attribution. These results provide additional proof of ion pairing in water. Furthermore, comparing the band displacements and intensity variations observed on the solvated water species to that of pure water indicates that the dielectric milieu surrounding the ion pairs is not constant. These results do not support the classical view of Debye-Hückel that considers that the ions are independent and the dielectric milieu constant. The present results give some in situ information on the reaction that goes on in “simple” electrolyte systems whose reactivity and molecular organization are still not completely mastered.
Dalitz plot analysis of Coulomb exploding O3 in ultrashort intense laser fields
Authors: Matsuda Akitaka , Takahashi Eiji J. , Hishikawa Akiyoshi
Published: September 2007

The three-body Coulomb explosion of O3, O33+→O++O++O+, in ultrashort intense laser fields (2×1015W∕cm2) is studied with two different pulse durations (9 and 40fs) by the coincidence momentum imaging method. In addition to a decrease in the total kinetic energy release, a broadening in the Dalitz plot distribution [Philos. Mag. 44, 1068 (1953)] is observed when the pulse duration is increased from 9 to 40fs. The analysis based on a simple Coulomb explosion model shows that the geometrical structure of O3 remains almost unchanged during the interaction with the few-cycle intense laser fields, while a significant structural deformation along all the three vibrational coordinates, including the antisymmetric stretching coordinate, is identified in the 40fs intense laser fields. The observed nuclear dynamics are discussed in terms of the population transfer to the excited states of O3.
Interpretation of the vibrational relaxation of H2 in H2 within the semiclassical effective mass approach
Authors: Dashevskaya E. I. , Litvin I. , Nikitin E. E. , Troe J.
Published: September 2007

The temperature dependence of the rate coefficients for vibrational relaxation of H2 in neat H2 is interpreted within the semiclassical effective mass approach. Across the temperature range of 80–3000K, the experimental rate coefficients vary by five orders of magnitude and fall onto a strongly nonlinear Landau-Teller plot. This behavior is explained by the nonclassical nature of the energy release and by a substantial participation of rotation of the colliding partners in inducing the vibrational transition. A single fitting parameter, the optimal reduced mass, permits one to represent the temperature dependence of the rate coefficient within a factor of 2. This parameter is found to be close to that obtained from a simple model suggested by Sewell et al. [J. Chem. Phys. 99, 2567 (1993)].
Computing stationary distributions in equilibrium and nonequilibrium systems with forward flux sampling
Authors: Valeriani Chantal , Allen Rosalind J. , Morelli Marco J. , Frenkel Daan , Rein ten Wolde Pieter
Published: September 2007

We present a method for computing stationary distributions for activated processes in equilibrium and nonequilibrium systems using forward flux sampling. In this method, the stationary distributions are obtained directly from the rate constant calculations for the forward and backward reactions; there is no need to perform separate calculations for the stationary distribution and the rate constant. We apply the method to the nonequilibrium rare event problem proposed by Maier and Stein, to nucleation in a 2-dimensional Ising system, and to the flipping of a genetic switch.
Comment on “Denaturation of hen egg white lysozyme in electromagnetic fields: A molecular dynamics study” [J. Chem. Phys. 126, 091105 (2007)]
Authors: Swicord Mays L. , Sheppard Asher R. , Balzano Quirino
Published: September 2007

The authors showed temperature-independent effects of microwave and far-IR electric fields on the secondary structure of proteins. However, the requisite extreme field strengths are unachievable in the environment or laboratory. Because protein bond effects are amplitude dependent, exposures to subthreshold field strengths would have no effect regardless of exposure duration.
Response to “Comment on ‘Denaturation of hen egg white lysozyme in electromagnetic fields: A molecular dynamics study’” [J. Chem. Phys. 126, 091105 (2007)]
Authors: English Niall J. , Mooney Damian A.
Published: September 2007

In response to the comments of Swicord et al., the use of high intensity external electromagnetic fields in the original paper is justified to allow the study of dynamical system response for a reasonable “signal to noise” ratio within the limited time scales amenable to molecular simulation. It is also stated that it is not our intention to correlate our work directly with lower-field intensities available either experimentally, industrially, or in communication devices: rather, our intention was that it may be reasonable to postulate that lower-field intensities could have biological impacts, when acting over much longer time scales.
Accurate ab initio potentials at low cost via correlation scaling and extrapolation: Application to CO(AΠ1)
Authors: Varandas A. J. C.
Published: September 2007

A recently proposed scheme that enables high quality molecular potentials to be obtained from small basis set calculations via scaling and extrapolation of the electron correlation to the complete basis set limit plus extrapolation to the complete basis set limit of the complete-active-space self-consistent field energy has been applied to the AΠ1 electronic state of CO. Based on standard multireference configuration interaction raw energies calculated with correlation-consistent basis sets of the aug-cc-pVXZ and aug-cc-pCVXZ types for the scaling and extrapolation procedures, a barrier separating the equilibrium region from the dissociation asymptote of 594±46cm−1 has been predicted at an internuclear distance of 2.25±0.01Å in reasonable agreement with some previous theoretical work and a recent direct least-squares fit to available spectroscopic data, while being 38% smaller than an early experimental upper limit of 950±150cm−1. Good agreement with experiment is also obtained for the minimum well depth and location at equilibrium: 25532±165cm−1 at 1.2346±0.0015Å (theoretical) versus 25597cm−1 at 1.2346±0.0006Å (experimental). The method can be extended to other wave function models, providing a general strategy for accurate potentials of larger dimensionality at costs that can be drastically smaller than traditional ones.
Self-assembly of diblock copolymers confined in cylindrical nanopores
Authors: Yu Bin , Sun Pingchuan , Chen Tiehong , Jin Qinghua , Ding Datong , Li Baohui , Shi An-Chang
Published: September 2007

Self-assembly of AB diblock copolymers confined in cylindrical nanopores is studied using a simulated annealing technique. The pore diameter and surface preference are systematically varied to examine their effects on the self-assembled morphologies and the chain conformations. For bulk lamella-forming and cylinder-forming diblock copolymers, novel structures such as helices and concentric (perforated) lamellae spontaneously form when the copolymers are confined in cylindrical pores. The observed equilibrium morphologies are compared with that obtained from experiments, theory, and other simulations. A simple model is proposed for symmetric diblock copolymers, which gives a reasonable description of the layer thickness for the concentric lamellae. It is found that chains near the pore surfaces are compressed relative to the bulk chains, which can be attributed to the existence of the surfaces. The dependence of the chain conformation on the degree of confinement and strength of the surface preference are reasonably explained. The energetics is discussed qualitatively and used to account for the appearance of the complex phase behavior observed for certain intermediate conditions.
Stress relaxation in polymer networks: Equilibrium behavior and dynamics
Authors: Adams J. M. , Mao Y. , Vandoolaeghe W. L.
Published: September 2007

The elastic and relaxational properties of a polymer network have been calculated using a stress based formulation based on the Rouse mode expansion [W. L. Vandoolaeghe and E. M. Terentjev, J. Chem. Phys. 123, 34902 (2005)]. In this article, we propose an improved Rouse mode expansion incorporating appropriate boundary conditions. In contrast to the previous work, this improved formulation provides a smooth crossover from the classical equilibrium result of rubber elasticity to the shorter-time-scale Rouse relaxation of a polymer melt. Our results are compared with the classical phantom network approach in equilibrium, as well as both equilibrium and dynamic elongation experiments. The model captures the qualitative features of the data well and some of the quantitative aspects, such as the exponents seen in the dynamic modulus G(ω).
Vibronic structure in C2H and C2D from anion slow electron velocity-map imaging spectroscopy
Authors: Zhou Jia , Garand Etienne , Neumark Daniel M.
Published: September 2007

The C2H and C2D radicals are investigated by slow electron velocity-map imaging (SEVI) of the corresponding anions. This technique offers considerably higher resolution (<0.5meV) than photoelectron spectroscopy. As a result, SEVI spectra of the two isotopomers yield improved electron affinities and reveal many new structures that are particularly sensitive to vibronic coupling between the ground Σ+2 and low-lying excited Π2 states. These structures, which encompass more than 5000cm−1 of internal excitation, are assigned with the aid of previous experimental and theoretical work. We also show that SEVI can be applied to photodetachment transitions resulting in ejection of an electron with orbital angular momentum l=1, a p wave, in contrast to anion zero-electron kinetic energy spectroscopy which is restricted to s-wave detachment.
Significant increase in the stability of rare gas hydrides on insertion of beryllium atom
Authors: Jayasekharan T. , Ghanty Tapan K.
Published: September 2007

Chemical binding between a rare gas atom with other elements leading to the formation of stable chemical compounds has received considerable attention in recent years. With an intention to predict highly stable novel rare gas compounds, the process of insertion of beryllium atom into rare gas hydrides (HRgF with Rg=Ar, Kr, and Xe) has been investigated, which leads to the prediction of HBeRgF species. The structures, energetic, and charge distributions have been obtained using MP2, density functional theory, and CCSD(T) methods. Analogous to the well-known rare gas hydrides, HBeRgF species are found to be metastable in nature; however, the stabilization energy of the newly predicted species has been calculated to be significantly higher than that of HRgF species. Particularly, for HBeArF molecule, it has been found to be an order of magnitude higher. Strong chemical binding between beryllium and rare gas atom has also been found in the HBeArF, HBeKrF, and HBXeF molecules. In fact, the basis set superposition error and zero-point energy corrected Be–Ar bond energy calculated using CCSD(T) method has been found to be 112kJ∕mol, which is the highest bond energy ever achieved for a bond involving an argon atom in any chemically bound neutral species. Vibrational analysis reveals a large blueshift (∼200cm−1) of the H–Be stretching frequency in HBeRgF with respect to that in BeH and HBeF species. This feature may be used to characterize these species after their preparation by the laser ablation of Be metal along with the photolysis of HF precursor in a suitable rare gas matrix. An analysis of the nature of interactions involved in the present systems has been performed using theory of atoms in molecules (AIM). Geometric as well as energetic considerations along with the AIM results suggest a substantial covalent nature of Be–Rg bond in these systems. Thus, insertion of a suitable metal atom into rare gas hydrides is a promising way to energetically stabilize the HRgX species, which eventually leads to the formation of a new class of insertion compounds, viz., rare gas metallohydrides.
Effects of block copolymer’s architecture on its association with lipid membranes: Experiments and simulations
Authors: Frey Shelli L. , Zhang Dongsheng , Carignano Marcelo A. , Szleifer Igal , Lee Ka Yee C.
Published: September 2007

Triblock copolymers of the form poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) have been shown to effectively interact with and restore activity of damaged cell membranes. To better understand the interaction between these polymers and cell membranes, we have modeled the outer leaflet of a cell membrane with a lipid monolayer spread at the air-water interface and injected poloxamers of varying architectures into the subphase beneath the monolayer. Subsequent interactions of the polymer with the monolayer upon compression were monitored with concurrent Langmuir isotherm and fluorescence microscopy measurements. Monte Carlo simulations were run in parallel using a coarse-grained model to capture interactions between lipids and poloxamers. Changing the ratio of the PEO to PPO block lengths (NPEO:NPPO) affects the equilibrium spreading pressure of the polymer. Poloxamers with a relatively longer central hydrophobic block are less soluble, resulting in more polymer adsorbed to the interface and therefore a higher equilibrium spreading pressure. Simulation results show that changing the poloxamer structure effectively affects its solubility. This is also reflected in the degree of lipid corralling as poloxamers with a higher chemical potential (and resulting higher equilibrium spreading pressure) cause the neighboring lipid domains to be more ordered. Upon lateral compression of the monolayers, the polymer is expelled from the film beyond a certain squeeze-out pressure. A poloxamer with a higher NPEO:NPPO ratio (with either NPEO or NPPO held constant in each series) has a lower squeeze-out pressure. Likewise when the total size of the polymer is varied with a constant hydrophilic:hydrophobic ratio, smaller poloxamers are squeezed out at a lower pressure. Our simulation results capture the trends of our experimental observations, both indicating how the interactions between lipids and poloxamers can be tuned by the polymer architecture.
Heterogeneous dynamics and dynamic heterogeneities at the glass transition probed with single molecule spectroscopy
Authors: Adhikari Aashish N. , Capurso Noah A. , Bingemann Dieter
Published: September 2007

We studied the temperature dependence of the structural relaxation in poly(vinyl acetate) near the glass transition temperature with single molecule spectroscopy from Tg−1K to Tg+12K. The temperature dependence of the observed relaxation times matches results from bulk experiments; the observed relaxation times are, however, 80-fold slower than those from bulk experiments at the same temperature. We attribute this factor to the size of the probe molecule. The individual relaxation times of the single molecule environments are distributed normally on a logarithmic time scale, confirming that the dynamics in poly(vinyl acetate) is heterogeneous. The width of the distribution of individual relaxation times is essentially independent of temperature. The observed full width at half maximum (FWHM) on a logarithmic time axis is approximately 0.7, corresponding to a factor of about 5-fold, significantly narrower than the dielectric spectrum of the same material with a FWHM of about 2.0 on a logarithmic time axis, corresponding to a factor of about 100-fold. We explain this narrow width as the effect of temporal averaging of single molecule fluorescence signals over numerous environments due to a limited lifetime of the probed heterogeneities, indicating that heterogeneities are dynamic. We determine a loose upper limit for the ratio of the structural relaxation time to the lifetime of the heterogeneities (the rate memory parameter) of Q<80 for the range of investigated temperatures.
Quantum state resolved scattering dynamics of F+HCl→HF(v,J)+Cl
Authors: Zolot A. M. , Nesbitt D. J.
Published: September 2007

State-to-state reaction dynamics of the reaction F+HCl→HF(v,J)+Cl have been studied under single-collision conditions using an intense discharge F atom source in crossed supersonic molecular beams at Ecom=4.3(1.3)kcal∕mol. Nascent HF product is monitored by shot-noise limited direct infrared laser absorption, providing quantum state distributions as well as additional information on kinetic energy release from high resolution Dopplerimetry. The vibrational distributions are highly inverted, with 34(4)%, 44(2)%, and 8(1)% of the total population in vHF=1, 2, and 3, respectively, consistent with predominant energy release into the newly formed bond. However, there is a small [14(1)%] but significant formation channel into the vHF=0 ground state, which is directly detectable for the first time via direct absorption methods. Of particular dynamical interest, both the HF(v=2,J) and HF(v=1,J) populations exhibit strongly bimodal J distributions. These results differ significantly from previous flow and arrested-relaxation studies and may signal the presence of microscopic branching in the reaction dynamics.
Collective diffusion coefficient of proteins with hydrodynamic, electrostatic, and adhesive interactions
Authors: Prinsen Peter , Odijk Theo
Published: September 2007

A theory is presented for λC, the coefficient of the first-order correction in the density of the collective diffusion coefficient, for protein spheres interacting by electrostatic and adhesive forces. An extensive numerical analysis of the Stokesian hydrodynamics of two moving spheres is given so as to gauge the precise impact of lubrication forces. An effective stickiness is introduced and a simple formula for λC in terms of this variable is put forward. A precise though more elaborate approximation for λC is also developed. These and numerically exact expressions for λC are compared with experimental data on lysozyme at pH 4.5 and a range of ionic strengths between 0.05M and 2M.
Direct identification and decongestion of Fermi resonances by control of pulse time ordering in two-dimensional IR spectroscopy
Authors: Donaldson Paul M. , Guo Rui , Fournier Frederic , Gardner Elizabeth M. , Barter Laura M. C. , Barnett Chris J. , Gould Ian R. , Klug David R. , Palmer D. Jason , Willison Keith R.
Published: September 2007

We show that it is possible to both directly measure and directly calculate Fermi resonance couplings in benzene. The measurement method used was a particular form of two-dimensional infrared spectroscopy (2D-IR) known as doubly vibrationally enhanced four wave mixing. By using different pulse orderings, vibrational cross peaks could be measured either purely at the frequencies of the base vibrational states or split by the coupling energy. This capability is a feature currently unique to this particular form of 2D-IR and can be helpful in the decongestion of complex spectra. Five cross peaks of the ring breathing mode ν13 with a range of combination bands were observed spanning a region of 1500–4550cm−1. The coupling energy was measured for two dominant states of the ν13+ν16 Fermi resonance tetrad. Dephasing rates were measured in the time domain for ν13 and the two (ν13+ν16) Fermi resonance states. The electronic and mechanical vibrational anharmonic coefficients were calculated to second and third orders, respectively, giving information on relative intensities of the cross peaks and enabling the Fermi resonance states of the combination band ν13+ν16 at 3050–3100cm−1 to be calculated. The excellent agreement between calculated and measured spectral intensities and line shapes suggests that assignment of spectral features from ab initio calculations is both viable and practicable for this form of spectroscopy.
Effects of three-dimensional Ehrlich-Schwoebel barrier on texture selection during Cu nanorod growth
Authors: Johansen Christopher G. , Huang Hanchen , Lu Toh-Ming
Published: September 2007

This work explores the effectiveness of the three-dimensional Ehrlich-Schwoebel (3D ES) barrier in the selection of textures during nanorod growth. The authors use molecular dynamics simulations to show that the 3D ES barrier, together with geometrical shadowing and the thermal dynamics of surface faceting, defines the texture of nanorods. The results offer insights into when and how the 3D ES barrier will be effective in texture selection, and may be applied to texture design during growth of various nanorods.
Refined potential model for atomistic simulations of ionic liquid [bmim][PF6]
Authors: Bhargava B. L. , Balasubramanian S.
Published: September 2007

Refined parameters of an atomistic interaction potential model for the room temperature ionic liquid 1-n-butyl,3-methylimidazolium hexafluorophosphate are presented. Classical molecular dynamics simulations have been carried out to validate this fully flexible all-atom model. It predicts the density of the liquid at different temperatures between 300 and 500K within 1.4% of the experimental value. Intermolecular radial distribution functions and the spatial distribution functions obtained from the new model are in close agreement with ab initio simulations. The calculated diffusion coefficients of ions and the surface tension of the liquid agree well with experiment.
Cutoff radius effect of isotropic periodic sum method for transport coefficients of Lennard-Jones liquid
Authors: Takahashi Kazuaki , Yasuoka Kenji , Narumi Tetsu
Published: September 2007

Molecular dynamics simulations of a Lennard-Jones (LJ) liquid were applied to compare the isotropic periodic sum (IPS) method [X. Wu and B. R. Brooks, J. Chem. Phys. 122, 044107 (2005)], which can reduce the calculation cost of long-range interactions, such as the Lennard-Jones and Coulombic ones, with the cutoff method for the transport coefficients which includes the self-diffusion coefficient, bulk viscosity, and thermal conductivity. The self-diffusion coefficient, bulk viscosity, and thermal conductivity were estimated with reasonable accuracy if the cutoff distance of the LJ potential for the IPS method was greater than 3σ. The IPS method is an effective technique for estimating the transport coefficients of the Lennard-Jones liquid in a homogeneous system.
H
2
S
ultrafast dissociation probed by energy-selected resonant Auger electron–ion coincidence measurements
Authors: Le Guen K. , Miron C. , Céolin D. , Guillemin R. , Leclercq N. , Simon M. , Morin P. , Mocellin A. , Björneholm O. , Naves de Brito A. , Sorensen S. L.
Published: September 2007

We have studied the ultrafast dissociation of the H2S molecule upon S 2p3∕2→6a1 inner-shell excitation by combining high-resolution resonant Auger spectroscopy and energy-selected Auger electron–ion coincidence measurements. Auger final states have been correlated to the different fragmentation pathways (S+, HS+, and H2S+ ions). As an original result, we evidence a three-step mechanism to describe the resonant production of S+: the Auger recombination in the HS* fragment is followed for the AΠ3 and cΠ1 states by the S++H fragmentation mechanism.
Amorphous silica modeled with truncated and screened Coulomb interactions: A molecular dynamics simulation study
Authors: Carré Antoine , Berthier Ludovic , Horbach Jürgen , Ispas Simona , Kob Walter
Published: September 2007

We show that finite-range alternatives to the standard long-range pair potential for silica by van Beest et al. [Phys. Rev. Lett. 64, 1955 (1990)] might be used in molecular dynamics simulations. We study two such models that can be efficiently simulated since no Ewald summation is required. We first consider the Wolf method, where the Coulomb interactions are truncated at a cutoff distance rc such that the requirement of charge neutrality holds. Various static and dynamic quantities are computed and compared to results from simulations using Ewald summations. We find very good agreement for rc≈10Å. For lower values of rc, the long-range structure is affected which is accompanied by a slight acceleration of dynamic properties. In a second approach, the Coulomb interaction is replaced by an effective Yukawa interaction with two new parameters determined by a force fitting procedure. The same trend as for the Wolf method is seen. However, slightly larger cutoffs have to be used in order to obtain the same accuracy with respect to static and dynamic quantities as for the Wolf method.
Patterning of silicon by indentation and chemical etching
Authors: Rao R. , Bradby J. E. , Williams J. S.
Published: September 2007

An array of features on Si (100) is fabricated by a new maskless pattering process involving a combination of indentation and anisotropic wet chemical etching. Indentation is carried out in order to induce transformation to the high-pressure phases, Si III and Si XII, before etching in a KOH solution. The pressure-induced phases are found to be highly resistant to etching in the KOH solution, with an etch rate more than an order of magnitude slower than that of Si (100). The possibility of exploiting this mechanism for a maskless nanoscale patterning process in Si using indentation is discussed.
Length-dependent thermal conductivity of an individual single-wall carbon nanotube
Authors: Wang Zhao Liang , Tang Da Wei , Li Xiao Bo , Zheng Xing Hua , Zhang Wei Gang , Zheng Li Xin , Zhu Yuntian T. , Jin Ai Zi , Yang Hai Fang , Gu Chang Zhi
Published: September 2007

The thermal conductivity of single-wall carbon nanotubes (SWCNTs) is predicted to increase with length, but this has never been proved experimentally because of limitations in previous measurement methods. Here, the authors report the measurement of the length-dependent thermal conductivities of individual SWCNTs on a Si substrate using a four-pad 3ω method. An increase in thermal conductivity with length was observed at room temperature, which is consistent with a theoretical prediction that considers higher order three-phonon processes. When SWCNTs are longer than the phonon mean path, they showed dissipative thermal transport. The observed increase of thermal conductivity with length makes SWCNTs ideal for thermal management.
Adsorption and freezing of diblock copolymers on stripe-patterned surfaces: A scaling analysis
Authors: Sumithra K. , Straube E.
Published: September 2007

We present the results of scaling analysis of diblock copolymers adsorbed on stripe-patterned surfaces of various widths. Our previous studies [K. Sumithra and E. Straube, J. Chem. Phys. 125, 154701 (2006)] show that the adsorption of diblock copolymer on patterned surfaces yields two peaks in the specific heat capacity, thereby indicating two transition. In the current study, we characterize these two transitions. The scaling of the adsorption energy data proves that the first peak in the heat capacity curve is, in fact, associated with the adsorption transition. We found that for this transition the classical scaling laws are obeyed and that the critical crossover exponent is unaltered with respect to the case of homogeneous polymers. However, we found a change in the scaling exponent in the case of parallel component of the radius of gyration. It is evident from the scaling analysis of the parallel component of the radius of gyration that the chain is stretched along the direction of the stripes. The scaling plot shows, for ⟨Rg‖2⟩∕Nν, an exponent of approximately 0.55 which is much different from that expected of a self-avoiding chain (νd=2−ν)∕ϕ which is 0.25. The observed value is closer to an exponent of (νd=1−ν)∕ϕ=0.69, for a completely stretched chain in one dimension. The perpendicular component of the radius of gyration ⟨Rg⊥2⟩ shows deviation from the power law and the slope is steeper than the expected value of −2. We have also defined an order parameter to characterize the second transition and have found that it corresponds to a freezing transition where there are only a few dominant conformations. The perpendicular component of the radius of gyration also supports this information.
Study of the structural damage in the (0001) GaN epilayer processed by laser lift-off techniques
Authors: Chen W. H. , Kang X. N. , Hu X. D. , Lee R. , Wang Y. J. , Yu T. J. , Yang Z. J. , Zhang G. Y. , Shan L. , Liu K. X. , Shan X. D. , You L. P. , Yu D. P.
Published: September 2007

The structural influences of the laser lift-off (LLO) techniques on the created (0001) GaN surface region are characterized by cross-sectional high-resolution transmission electron microscopy and fitted using the model of stress waves caused by a longitudinal impact at the end of a cylindrical bar extending to infinity. The authors study reveals that, in addition to the superficial damage caused by laser absorption, the stress saltation in GaN crystal where the shock waves come into being induces deformation of the lattices and generates a cluster of half loops above the LLO interface. After that, the lattice deformation will be induced every time the partial dissipation of the steady-state shock waves takes place until the shock wave is dissipated to elastic mode.
Inverse spin Hall effect by spin injection
Authors: Liu S. Y. , Horing Norman J. M. , Lei X. L.
Published: September 2007

Motivated by a recent experiment [S. O. Valenzuela and M. Tinkham, Nature (London) 442, 176 (2006)], the authors present a quantitative microscopic theory to investigate the inverse spin-Hall effect with spin injection into aluminum considering both intrinsic and extrinsic spin-orbit couplings using the orthogonalized-plane-wave method. Their theoretical results are in good agreement with the experimental data. It is also clear that the magnitude of the anomalous Hall resistivity is mainly due to contributions from extrinsic skew scattering.
Cryogenic amplifier for fast real-time detection of single-electron tunneling
Authors: Vink I. T. , Nooitgedagt T. , Schouten R. N. , Vandersypen L. M. K. , Wegscheider W.
Published: September 2007

The authors employ a cryogenic high electron mobility transistor (HEMT) amplifier to increase the bandwidth of a charge detection setup with a quantum point contact (QPC) charge sensor. The HEMT is operating at 1K and the circuit has a bandwidth of 1MHz. The noise contribution of the HEMT at high frequencies is only a few times higher than that of the QPC shot noise. The authors use this setup to monitor single-electron tunneling to and from an adjacent quantum dot. The authors measure fluctuations in the dot occupation as short as 400ns, 20 times faster than in previous work.
Electron transport and recombination in polycrystalline TiO2 nanowire dye-sensitized solar cells
Authors: Enache-Pommer Emil , Boercker Janice E. , Aydil Eray S.
Published: September 2007

Electron transport and recombination time constants in dye-sensitized solar cells made from TiO2 nanowires were determined using transient photocurrent and photovoltage measurements. The magnitude of the electron transport time (10−2to10−3s) and its dependence on the illumination intensity were similar to those reported for dye-sensitized solar cells made from TiO2 nanoparticles indicating that electron capture and release by surface traps determine transport times even in nanowires. However, the ratio of the electron recombination time to the electron collection time in nanowire solar cells was ∼150, larger than those observed in nanoparticle dye-sensitized solar cells.
Electrical characterization of MgO tunnel barriers grown on InAs (001) epilayers
Authors: Magnus F. , Clowes S. K. , Gilbertson A. M. , Branford W. R. , Barkhoudarov E. D. , Cohen L. F. , Singh L. J. , Barber Z. H. , Blamire M. G. , Buckle P. D. , Buckle L. , Ashley T. , Eustace D. A. , McComb D. W.
Published: September 2007

The authors examine the electrical properties of ultrathin MgO barriers grown on (001) InAs epilayers and the dependence on InAs surface pretreatment and growth conditions. Pretreatment improves the yield of tunnel junctions and changes the roughness of the interface between oxide and semiconductor. Electrical characterization confirms that tunnel barriers with appropriate values of interface resistance for efficient spin injection/detection have been achieved. Using the Rowell criteria and various tunneling models, the authors show that single step tunneling occurs above 150K. Incorporating a thermal smearing model suggests that tunneling is the dominant transport process down to 10K.
Magnetothermopower and magnetoresistivity of RuSr2Gd2−xCexCu2O10+δ (x=0.6,1.0)
Authors: Liu Chia-Jyi , Nayak Pradipta K. , Williams G. V. M.
Published: September 2007

We report measurements of magnetothermopower and magnetoresistivity as a function of temperature on RuSr2Gd2−xCexCu2O10+δ (x=0.6,1). In a magnetic field of 5T, the size of thermopower decreases significantly, whereas the normal-state resistivity decreases slightly, which behaves similarly to RuSr2Gd1−xLaxCu2O8 (x=0,0.1). A significant decrease of thermopower for both samples could be attributed to the decrease of the spin entropy contribution associated with the carriers in the RuO2 layers, as the Ru moments are aligned with the magnetic field. It is also found that the nonmetallic temperature dependence at low T follows a variable range hopping process for both samples.
Noninvasive optical amplification and detection of Faraday rotation
Authors: LaForge J. M. , Steeves G. M.
Published: September 2007

The authors present a Mach-Zehnder-type interferometer with orthogonally polarized arms that passively, noninvasely, and optically amplifies Faraday rotation. At ambient temperatures, the interferometer, with an optical gain factor of ∼4.5, outperforms the optical bridge (OB) technique with a detection limit of approximately 50mrad, compared to 250mrad for the OB, when both techniques are used for time resolved faraday rotation under similar experimental conditions. Isolation and active stabilization of the interferometer have reduced sources of noise so that the interferometer’s signal is also quieter than the OB.
Miniaturized superconducting quantum interference magnetometers for high sensitivity applications
Authors: Granata C. , Vettoliere A. , Russo M.
Published: September 2007

A miniaturized niobium based dc superconducting quantum interference device (SQUID) magnetometer for high magnetic field sensitivity applications has been developed. The sensing coil consists of an integrated square superconducting coil with a length of 3mm, involving a device area much smaller with respect to the standard SQUID magnetometers with a comparable magnetic field sensitivity; so it allows increasing the spatial resolution keeping the magnetic field sensitivity unaltered. Furthermore, a small pickup coil minimizes its antenna gain, reducing the radio frequency interference. At T=4.2K, the sensors have shown smooth and resonance free V-Φ characteristics and an intrinsic white magnetic field noise spectral density as low as 5.8fT∕Hz1∕2, measured in flux locked loop configuration. The good agreement with the theoretical predictions guarantees the reliability and the controllability of the sensors. Due to their compactness and good characteristic parameters, such sensors are suitable for large multichannel systems used in biomagnetic imaging.
Controlled fabrication of 1–2nm nanogaps by electromigration in gold and gold-palladium nanowires
Authors: Hadeed F. O. , Durkan C.
Published: September 2007

The authors report the electrical characterization of gold and gold-palladium nanowires failed by electromigration. Nanogaps 1–2nm in size are reliably made from metal nanowires by controlling the electromigration failure process, opening up the possibility of using these metal nanowires with nanogaps for molecular conduction studies and large-scale molecular junction device fabrication. Nanogaps are formed by applying a voltage sweep to the wires at a ramp rate of 4mV∕s. The interplay between Joule heating and electromigration means that reliable nanogaps can be formed without the need of a feedback circuit, rendering the technique relatively simple to implement.
Patterned nanoring magnetic tunnel junctions
Authors: Wen Z. C. , Wei H. X. , Han X. F.
Published: September 2007

Patterned nanoring (NR) magnetic tunnel junctions (MTJs) with outer diameters between 100 and 400nm and narrow ring widths between 25 and 30nm were fabricated. The NR structure consists of CoFeB electrodes and Al-oxide barrier. The tunneling magnetoresistance (TMR) ratio in the range of 20%–50% for the NR-MTJs with the resistance-area product lower than 50Ωμm2 were observed at room temperature. These NR-MTJs allow current-induced magnetization switching with a low switching current density of around 9×106A∕cm2. Due to the small stray field and high TMR ratio, NR-MTJs offer superior prospects for very high density magnetic random access memory, recording medium, and other spintronic devices.
Direct observation of a hypersonic band gap in two-dimensional single crystalline phononic structures
Authors: Gorishnyy Taras , Jang Ji-Hyun , Koh CheongYang , Thomas Edwin L.
Published: September 2007

Brillouin light scattering is employed to record the phonon dispersion relation of two-dimensional (2D) hypersonic phononic crystals with square lattice plane group p4mm symmetry. The samples are single crystalline arrays of cylindrical holes with a lattice constant of 750nm and 35% porosity patterned in epoxy using interference lithography. The dispersion relation reveals the presence of a phononic band gap between 1.21 and 1.57GHz at the edge of the first Brillouin zone for elastic waves propagating along the [10] direction and conclusively demonstrates a band gap in a single crystalline 2D polymer based phononic structure at hypersonic frequencies.
Microplasma synthesis of metal nanoparticles for gas-phase studies of catalyzed carbon nanotube growth
Authors: Chiang Wei-Hung , Sankaran R. Mohan
Published: September 2007

Catalytic properties of metal nanoparticles toward gas-phase carbon nanotube (CNT) growth are presented. Narrow dispersions of iron (Fe) and nickel (Ni) nanoparticles are prepared in a direct current microplasma reactor and subsequently introduced with acetylene (C2H2) and hydrogen (H2) into a heated flow furnace to catalyze CNT growth. Aerosol size classification and high-resolution transmission electron microscopy show that CNT growth occurs on Ni particles at lower temperatures than that for similarly produced Fe nanoparticles. Activation energies of 117 and 73kJ∕mol are found for Fe and Ni catalyst particles, respectively, suggesting that CNT growth occurs by carbon surface diffusion.
Reduced efficiency roll-off in phosphorescent organic light emitting diodes by suppression of triplet-triplet annihilation
Authors: Reineke Sebastian , Schwartz Gregor , Walzer Karsten , Leo Karl
Published: September 2007

The authors investigate phosphorescent organic light emitting diodes comprising mixed films of fac tris(2-phenylpyridine) iridium dispersed in 4,4′,4′-tris(N-carbazolyl)-triphenylamine (TCTA) as emission layer (EML). Based on the results of photoluminescence experiments, they intermit the EML with thin neat layers of TCTA acting as an exciton blocking layer inside the EML which suppresses triplet-triplet annihilation. They show that this EML structure leads to an improved roll-off behavior: Starting at the initial external quantum efficiencies (IEQE) of 15.8% and 14.4% at low brightness for the reference and interlayer device, respectively, those structures yield critical current densities jc of 140mA∕cm2 and 270mA∕cm2 defining the current density of half-value IEQE.
Near ultraviolet light emitting diode composed of n-GaN∕ZnO coaxial nanorod heterostructures on a p-GaN layer
Authors: An Sung Jin , Yi Gyu-Chul
Published: September 2007

The authors report on the fabrication and characteristics of near ultraviolet nanorod light emitting diodes (LEDs) composed of n-GaN∕ZnO nanorod heterostructures on p-GaN substrates. The nanorod LEDs consist of the vertically aligned n-GaN∕ZnO coaxial nanorod arrays grown on a p-GaN substrate. The LEDs demonstrated strong near ultraviolet emission at room temperature. The nanorod LEDs were turned on a forward-bias voltage of 5V, and exhibited a large light emitting area. From electroluminescent spectra, dominant emission peaks were observed at 2.96 and 3.24eV for an applied current of 2mA. The origins of the strong and large area light emission are also discussed in terms of enhanced carrier injection from n-GaN nanostructures to p-GaN substrates.
Improved color stability in white phosphorescent organic light-emitting diodes using charge confining structure without interlayer
Authors: Kim Sung Hyun , Jang Jyongsik , Lee Jun Yeob
Published: September 2007

Color stability of white organic light-emitting diodes was improved by using a charge confining device structure, which can confine charges at the center of the emitting layer. A red phosphorescent emitting layer was sandwiched between blue phosphorescent emitting layers and there was little change of color coordinate from 100to10000cd∕m2.
Dissolution and diffusion behavior of Al2O3 in a CaO–Al2O3–SiO2 liquid: An experimental-numerical approach
Authors: Verhaeghe F. , Liu J. , Guo M. , Arnout S. , Blanpain B. , Wollants P.
Published: September 2007

A technique to study the dissolution and diffusion behavior of Al2O3 in CaO–Al2O3–SiO2 liquids is presented. The dissolution of spherical Al2O3 particles at elevated temperatures is observed using confocal scanning laser microscopy and interpreted by means of numerical simulations with a lattice Boltzmann dissolution model. The dissolution mechanism is identified as diffusion-controlled and an estimate of the effective binary diffusion coefficient and its activation energy is obtained. The technique is readily applicable to other systems.
Fracture initiation mechanisms in α-alumina under hypervelocity impact
Authors: Zhang Cheng , Kalia Rajiv K. , Nakano Aiichiro , Vashishta Priya
Published: September 2007

Atomistic mechanisms of crack initiation and fracture in α-alumina under hypervelocity impact are investigated using large-scale molecular-dynamics simulations involving 540×106 atoms. The simulations reveal crack initiation due to amorphization, structural phase changes, and interaction between high strain rate deformation modes, which can advance at sustained speeds of 8km∕s. Fracture toughness in the impact-damaged substrate is estimated to be around 2MPam. Spallation at the interface of the amorphous region and twins causes eventual failure. The favorable cleavage planes normal to basal planes are found to be {21¯1¯0}.
Significant suppression of ferromagnetism by hydrostatic pressure in the diluted magnetic semiconductor Sb2−xVxTe3 with x⩽0.03
Authors: Dyck J. S. , Mitchell T. J. , Luciana A. J. , Quayle P. C. , Drašar Č. , Lošťák P.
Published: September 2007

The authors report on the hydrostatic pressure dependence of the magnetotransport properties of ferromagnetic Sb2−xVxTe3 single crystals with x=0.02–0.03. Pressure significantly increases the free hole concentration in these compounds. In turn, the Curie temperature is suppressed by roughly 40%, which goes against many models that would predict an increase in Curie temperature with increasing carrier concentration. These results indicate that the ferromagnetism in these materials is carrier mediated and that a full Ruderman-Kittel-Kasuya-Yosida model that takes into account the oscillatory nature of the indirect exchange interaction among localized spins is needed in order to explain the data.
Semianalytical evaluation of linear and nonlinear piezoelectric potentials for quantum nanostructures with axial symmetry
Authors: Even J. , Doré F. , Cornet C. , Pedesseau L. , Schliwa A. , Bimberg D.
Published: September 2007

A model is proposed to describe the linear and quadratic piezoelectric potentials in quantum nanostructures with axial symmetry. Semianalytical expressions of the charge densities are given to illustrate the symmetry reduction. The results for InAs∕InP quantum dots (QDs) are compared to previous full three-dimensional calculation and to the full isotropic elastic model. The model is well adapted to study various effects for QD: the lattice mismatch, the QD materials composition or alloying, and the QD size or shape. Special emphasis is put on variations of the vertical aspect ratio and the role of the wetting layer.
Fowler-Nordheim hole tunneling in metal-Er2O3-silicon structures
Authors: Zhu Y. Y. , Fang Z. B. , Chen S. , Liao C. , Wu Y. Q. , Fan Y. L. , Jiang Z. M.
Published: September 2007

Fowler-Nordheim (FN) tunneling of holes in metal-Er2O3–Si structures is confirmed. The effective mass of holes in Er2O3 films is estimated ranging from 0.068m to 0.092m, where m is the free electron mass. The film shows a high breakdown electric field of about 70MV∕cm for an Er2O3 film thickness of 8.5nm, implying that the film which is epitaxially grown on Si substrate has smooth interface and surface.
Dielectric constants and phonon modes of amorphous hafnium aluminate deposited by metal organic chemical vapor deposition
Authors: Bundesmann C. , Buiu O. , Hall S. , Schubert M.
Published: September 2007

Dielectric constants and long-wavelength optical phonon modes of amorphous hafnium aluminate films with a maximum aluminum content of 19at.% are studied by infrared spectroscopic ellipsometry (IRSE). The hafnium aluminate films were prepared by metal organic chemical vapor deposition on silicon substrates. IRSE revealed one polar lattice mode and one impurity-type mode, which show all a systematic shift in frequency with varying Al content. The static dielectric constant decreases from 10.1 for 4.6at.% Al to 8.1 for 19at.% Al. The absolute values were found to be between 50% and 70% smaller than the values obtained from electrical measurements.
Optical characterization of a strained silicon quantum well on SiGe on insulator
Authors: Munguía J. , Bremond G. , Marty O. , Bluet J.-M. , Mermoux M.
Published: September 2007

An 8nm thick strained silicon layer embedded in relaxed Si0.8Ge0.2 has been grown on SiGe on insulator substrate in order to reduce the optical response of dislocations present in the SiGe virtual substrate. Photoreflectance measurement shows bandgap shrinkage at Γ point of 0.19eV which corresponds to a 0.94% strain value close to the one measured in Raman spectroscopy. The luminescence arising only from the strained Si quantum well in high injection conditions reveals clearly two optical transitions observed at 0.959 and 1.016eV.