bluuebunny/crossref_metadata_2025 huggingface dataset:
Room temperature terahertz emission from grating coupled two-dimensional plasmons
Authors: Meziani Y. M. , Handa H. , Knap W. , Otsuji T. , Sano E. , Popov V. V. , Tsymbalov G. M. , Coquillat D. , Teppe F.
Published: May 2008

Room temperature terahertz (far infrared) radiation emission from double grating coupled GaInAs∕AlGaAs∕GaAs heterojunctions is reported. Theoretical calculations of plasmon absorption spectrum are performed using a first principles electromagnetic approach. They correctly reproduce the frequency range and overall shape of the main (broad-band) part of the experimental spectra. The results clearly indicate that important part of the observed room temperature terahertz emission spectra can be due to the radiative decay of hot two-dimensional plasmons in the grating structure.
Numerical study of educed coherent structures in the near-wall region of a particle-laden channel flow
Authors: Dritselis Chris D. , Vlachos Nicholas S.
Published: May 2008

The interaction of small heavy solid particles with turbulence near the wall of a vertical downward channel flow is investigated by using direct numerical simulation (DNS) and Lagrangian particle tracking. The interest is focused on the effect of the particles on the near-wall coherent structures obtained by conditional sampling of DNS results of a particle-laden turbulent channel flow. The coherent structures are detected from instantaneous flow fields by using the vortex definition of Jeong and Hussain [J. Fluid Mech. 285, 69 (1995)]. The Reynolds number of the particle-free flow is Reτ≈180 based on the friction velocity and the wall half distance. The particle response time is 200 wall units and the average mass and volume fractions φm=0.5 and φv=6.8×10−5, respectively. The particle diameter is smaller than the Kolmogorov length scale and the grid spacing, the latter being small enough to adequately resolve the smaller fluid flow scales. The feedback effect of the particles on the carrier phase is taken into account by a point-force model. Purely elastic interparticle collisions are also considered. For both particle-free and particle-laden flows, the dominant coherent structures in the near-wall region are elongated quasistreamwise vortices. The addition of particles results in a weaker mean structure, with larger diameter and longer streamwise extent. The qualitative characteristics of the velocity distributions around the mean coherent structures are similar, independent of the particles. However, the coherent velocity fluctuations in the wall-normal and spanwise directions considerably decrease, and the low-speed streak is damped by the particles. The educed results show that the particles create a torque of opposite sign to the rotation of the mean vortex, which in turn reduces the streamwise vorticity of the structure. Consequently, the magnitude of fluid pressure decreases and the redistribution of turbulent kinetic energy from the streamwise to the other velocity components is significantly reduced.
Free-energy analysis of the molecular binding into lipid membrane with the method of energy representation
Authors: Matubayasi Nobuyuki , Shinoda Wataru , Nakahara Masaru
Published: May 2008

A statistical-mechanical treatment of the molecular binding into lipid membrane is presented in combination with molecular simulation. The membrane solution is viewed as an inhomogeneous, mixed solvent system, and the free energy of solvation of a solute in membrane is computed with a realistic set of potential functions by the method of energy representation. Carbon monoxide, carbon dioxide, benzene, and ethylbenzene are adopted as model solutes to analyze the binding into 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) membrane. It is shown that the membrane inside is more favorable than bulk water and that the solute distribution is diffuse throughout the membrane inside. The membrane-water partition coefficient is then constructed with the help of the Kirkwood-Buff theory from the solvation free energy obtained separately in the hydrophobic, glycerol, headgroup, and aqueous regions. To discuss the role of repulsive and attractive interactions, the solvation free energy is partitioned into the DMPC and water contributions and the effect of water to stabilize the benzene and ethylbenzene solutes within the membrane is pointed out.
Study of the interaction in clusters formed by phenol and CH3X (X=CN,F,Cl) molecules
Authors: Cabaleiro-Lago Enrique M. , Peña-Gallego Ángeles , Rodríguez-Otero Jesús
Published: May 2008

The characteristics of the interaction between phenol and acetonitrile, methyl fluoride and methyl chloride were studied. The most stable structures for clusters containing one or two CH3X molecules and one phenol moiety were located by means of ab initio and density functional theory calculations. Phenol-acetonitrile dimer presents two almost equally stable structures; one of them is a typical linearly hydrogen bonded minimum, whereas in the other one, a C–H⋯π contact is established accompanied by a distorted O–H⋯N hydrogen bond. Although the latter minimum presents the larger interaction energy, deformation effects favor the formation of the linear hydrogen bonded one. In complexes with methyl fluoride and methyl chloride, this arrangement is the most stable structure and no linear hydrogen bonded structures were located. Our best estimates for the interaction energies amount to −27.8, −21.6, and −19.7kJ∕mol for clusters of phenol with acetonitrile, methyl fluoride, and methyl chloride, respectively. The main contribution to the stabilization of these clusters is of electrostatic nature, although in structures where a C–H⋯π contact is present, the dispersion contribution is also significant. In clusters formed by phenol and two CH3X units, the most stable arrangement corresponds to a head to tail disposal with O–H⋯X, C–H⋯X, and C–H⋯π contacts forming a cycle. Only for this type of arrangement, three body effects are non-negligible even though they constitute a minor effect. The results also indicate that interactions with methyl fluoride and methyl chloride are of similar intensity, although weaker than with acetonitrile. Significant frequency shifts are predicted for the O–H stretching, which increase when increasing the number of CH3X molecules.
The parity-adapted basis set in the formulation of the photofragment angular momentum polarization problem: The role of the Coriolis interaction
Authors: Shternin Peter S. , Vasyutinskii Oleg S.
Published: May 2008

We present a theoretical framework for calculating the recoil-angle dependence of the photofragment angular momentum polarization taking into account both radial and Coriolis nonadiabatic interactions in the diatomic/linear photodissociating molecules. The parity-adapted representation of the total molecular wave function has been used throughout the paper. The obtained full quantum-mechanical expressions for the photofragment state multipoles have been simplified by using the semiclassical approximation in the high-J limit and then analyzed for the cases of direct photodissociation and slow predissociation in terms of the anisotropy parameters. In both cases, each anisotropy parameter can be presented as a linear combination of the generalized dynamical functions fK(q,q′,q̃,q̃′) of the rank K representing contribution from different dissociation mechanisms including possible radial and Coriolis nonadiabatic transitions, coherent effects, and the rotation of the recoil axis. In the absence of the Coriolis interactions, the obtained results are equivalent to the earlier published ones. The angle-recoil dependence of the photofragment state multipoles for an arbitrary photolysis reaction is derived. As shown, the polarization of the photofragments in the photolysis of a diatomic or a polyatomic molecule can be described in terms of the anisotropy parameters irrespective of the photodissociation mechanism.
Microscopic model for the nonlinear behavior of high-k metal-insulator-metal capacitors
Authors: Wenger Ch. , Lupina G. , Lukosius M. , Seifarth O. , Müssig H.-J. , Pasko S. , Lohe Ch.
Published: May 2008

The achievement of sufficient capacitance-voltage linearity in metal-insulator-metal (MIM) capacitors with high permittivity (high-k) dielectrics is still a challenge, as the origin of the nonlinear behavior is still unclear. Based on fundamental physical mechanisms, such as electrostriction, Coulomb interaction between electrodes, and nonlinear optical effects, a microscopic model, which describes the nonlinearities in capacitance-voltage characteristics of high-k MIM capacitors, will be presented. The extended model, which includes stacked high-k dielectrics and interfacial layers, is able to predict the quadratic voltage capacitance coefficients as a function of the dielectric constant. The calculated coefficients are in suitable agreement with the experimental values of Al2O3-, Y2O3-, HfO2-, and Pr2Ti2O7-based MIM capacitors.
HgTe: A potential thermoelectric material in the cinnabar phase
Authors: Chen Xin , Wang Yi , Cui Tian , Ma Yanming , Zou Guangtian , Iitaka Toshiaki
Published: May 2008

We present the calculations of the electronic structure and transport properties on the zinc-blende (ZB) and cinnabar phases of HgTe using the full-potential linearized augmented plane-wave method and the semiclassical Boltzmann theory. Our results show that n-doped cinnabar HgTe has a significant larger Seebeck coefficient and electrical conductivity along the z axis than those of the n-doped ZB phase. This is mainly attributed to the large structural anisotropy originated from its chainlike bonding characters along the z axis, resulting in the anisotropic energy distribution in the lowest conduction band of cinnabar structure. The resulting ZT values along the z axis of the n-doped cinnabar HgTe are predicted to reach very high values of 0.61 at room temperature and 1.74 at 600K. Therefore, the current theory suggests that the cinnabar structure of HgTe could be a good thermoelectric material. Future experiments are thus demanded to explore its thermoelectric performance by making use of the high ZT.
Lithiated assemblies of metal chalcogenide nanowires
Authors: Murugan P. , Kumar Vijay , Kawazoe Yoshiyuki , Ota Norio
Published: May 2008

We study hexagonal assemblies of M6X6 (M=Mo and W and X=S, Se, and Te) nanowires from first-principles calculations to understand their structural stability, electronic properties, and the effects of Li intercalation. It is shown that due to van der Waals interactions between the nanowires, the intercalation is achieved without a significant change in their atomic structure. With an increase in Li concentration, we predict a new phase for Li3Mo6S6 compound, in which the hexagonal assembly transforms to a monoclinic structure by a change in the orientation of nanowires. The LixMo6S6 assemblies are electrically conductive and can be potentially used as cathode materials in Li-ion batteries for nanoscale applications. The voltage of such a battery, calculated to be 1.7V, can be manipulated such as by iodine doping without a significant change in the atomic structure.
Self-aligned n-channel metal-oxide-semiconductor field effect transistor on high-indium-content In0.53Ga0.47As and InP using physical vapor deposition HfO2 and silicon interface passivation layer
Authors: Ok InJo , Kim H. , Zhang M. , Zhu F. , Park S. , Yum J. , Zhao H. , Garcia Domingo , Majhi Prashant , Goel N. , Tsai W. , Gaspe C. K. , Santos M. B. , Lee Jack C.
Published: May 2008

In this work, we present the electrical and material characteristics of TaN∕HfO2∕In0.53Ga0.47As and InP substrate metal-oxide-semiconductor capacitors and self-aligned n-channel metal-oxide-semiconductor field effect transistor (n-MOSFET) with physical vapor deposition Si interface passivation layer. Excellent electrical characteristics, thin equivalent oxide thickness (∼1.7nm), and small frequency dispersion (<2%) were obtained. n-channel high-k InGaAs- and InP-MOSFETs with good transistor behavior and good split capacitance-voltage (C-V) characteristics on In0.53Ga0.47As and InP substrates have also been demonstrated.
Quantum Monte Carlo study of the ground state and low-lying excited states of the scandium dimer
Authors: Matxain Jon M. , Rezabal Elixabete , Lopez Xabier , Ugalde Jesus M. , Gagliardi Laura
Published: May 2008

A large set of electronic states of scandium dimer has been calculated using high-level theoretical methods such as quantum diffusion Monte Carlo (DMC), complete active space perturbation theory as implemented in GAMESS-US, coupled-cluster singles, doubles, and triples, and density functional theory (DFT). The Σu3 and Σu5 states are calculated to be close in energy in all cases, but whereas DFT predicts the Σu5 state to be the ground state by 0.08eV, DMC and CASPT2 calculations predict the Σu3 to be more stable by 0.17 and 0.16eV, respectively. The experimental data available are in agreement with the calculated frequencies and dissociation energies of both states, and therefore we conclude that the correct ground state of scandium dimer is the Σu3 state, which breaks with the assumption of a Σu5 ground state for scandium dimer, believed throughout the past decades.
Phase behavior of parallel hard cylinders
Authors: Capitán José A. , Martínez-Ratón Yuri , Cuesta José A.
Published: May 2008

We test the performance of a recently proposed fundamental measure density functional of aligned hard cylinders by calculating the phase diagram of a monodisperse fluid of these particles. We consider all possible liquid-crystalline symmetries, namely, nematic, smectic, and columnar, as well as the crystalline phase. For this purpose we introduce a Gaussian parametrization of the density profile and use it to numerically minimize the functional. We also determine, from the analytic expression for the structure factor of the uniform fluid, the bifurcation points from the nematic to the smectic and columnar phases. The equation of state, as obtained from functional minimization, is compared to the available Monte Carlo simulation. The agreement is very good, nearly perfect in the description of the inhomogeneous phases. The columnar phase is found to be metastable with respect to the smectic or crystal phases, its free energy though being very close to that of the stable phases. This result justifies the observation of a window of stability of the columnar phase in some simulations, which disappears as the size of the system increases. The only important deviation between theory and simulations shows up in the location of the nematic-smectic transition. This is the common drawback of any fundamental measure functional of describing the uniform phase just with the accuracy of scaled particle theory.
Six-dimensional quantum dynamics of H2 dissociative adsorption on the Pt(211) stepped surface
Authors: Olsen R. A. , McCormack D. A. , Luppi M. , Baerends E. J.
Published: May 2008

Results of experimental studies, and theoretical calculations utilizing classical trajectories, have shown that dissociation of H2 on the Pt(211) stepped surface is enhanced at low energies by a molecular trapping mechanism. Because quantum effects can play a large role at the low energies and long lifetimes that characterize molecular trapping, we have undertaken quantum dynamics calculations for this system, the first to treat all molecular degrees of freedom of a gas molecule reacting on a stepped metallic surface. The calculations show that molecular trapping persists in the quantum system, but only at much lower energies than experimentally seen, pointing to possible deficiencies in the potential energy surface. Classical and quasiclassical trajectory calculations on the same potential provide a reasonable picture of reaction overall, but many of the finer details are inaccurate, and certain classical reaction mechanisms are entirely invalid. We conclude that some skepticism should be shown toward any classical study for which long-lived trapping states play a role.
Theoretical investigation of the relationships between magnetic circular dichroism signals and Gilbert damping coefficient in magnetic films
Authors: Lu Jie , Yan Peng
Published: May 2008

Inspired by the traditional ferromagnetic resonance (FMR) approach, the relationships between two kinds of magnetic circular dichroism signals and the Gilbert damping coefficient in magnetic films are theoretically investigated within linear response framework. These results may provide inspirations on potential experimental strategies to remeasure the Gilbert damping coefficient, which is traditionally obtained from FMR technique.
Effective work function tunability and interfacial reactions with underlying HfO2 layer of plasma-enhanced atomic layer deposited TaCxNy films
Authors: Park Tae Joo , Kim Jeong Hwan , Jang Jae Hyuck , Na Kwang Duk , Hwang Cheol Seong , Kim Gee-Man , Choi Kang Jun , Jeong Jae Hak
Published: May 2008

This study examined the interfacial reaction of plasma-enhanced atomic layer deposited TaCxNy films with underlying SiO2 and HfO2 layers, as well as their effective work functions (EWFs). The adoption of Ar∕H2 plasma as a reducing agent suppressed the interfacial reactions resulting in a lower electrical thickness. However, it increased the interface state density due to the massive Ar+ plasma damage on the dielectric films. The interfacial reactions were suppressed in TaCxNy on HfO2 compared with that on SiO2. The EWF of TaCxNy with the H2 plasma and Ar∕H2 plasma on HfO2 was ∼4.9–5.2 and ∼4.6eV, respectively.
Generalized gradient approximation model exchange holes for range-separated hybrids
Authors: Henderson Thomas M. , Janesko Benjamin G. , Scuseria Gustavo E.
Published: May 2008

We propose a general model for the spherically averaged exchange hole corresponding to a generalized gradient approximation (GGA) exchange functional. Parameters are reported for several common GGAs. Our model is based upon that of Ernzerhof and Perdew [J. Chem. Phys. 109, 3313 (1998)]. It improves upon the former by precisely reproducing the energy of the parent GGA, and by enabling fully analytic evaluation of range-separated hybrid density functionals. Analytic results and preliminary thermochemical tests indicate that our model also improves upon the simple, local-density-based exchange hole model of Iikura et al. [J. Chem. Phys. 115, 3540 (2001)].
A spherical electron cloud hopping model for studying product branching ratios of dissociative recombination
Authors: Yu Hua-Gen
Published: May 2008

A spherical electron cloud hopping (SECH) model is proposed to study the product branching ratios of dissociative recombination (DR) of polyatomic systems. In this model, the fast electron-captured process is treated as an instantaneous hopping of a cloud of uniform spherical fractional point charges onto a target M+q ion (or molecule). The sum of point charges (−1) simulates the incident electron. The sphere radius is determined by a critical distance (ReMc) between the incoming electron (e−) and the target, at which the potential energy of the e−−M+q system is equal to that of the electron-captured molecule M+q−1 in a symmetry-allowed electronic state with the same structure as M+q. During the hopping procedure, the excess energies of electron association reaction are dispersed in the kinetic energies of M+q−1 atoms to conserve total energy. The kinetic energies are adjusted by linearly adding atomic momenta in the direction of driving forces induced by the scattering electron. The nuclear dynamics of the resultant M+q−1 molecule are studied by using a direct ab initio dynamics method on the adiabatic potential energy surface of M+q−1, or together with extra adiabatic surface(s) of M+q−1. For the latter case, the “fewest switches” surface hopping algorithm of Tully was adapted to deal with the nonadiabaticity in trajectory propagations. The SECH model has been applied to study the DR of both CH+ and H3O+(H2O)2. The theoretical results are consistent with the experiment. It was found that water molecules play an important role in determining the product branching ratios of the molecular cluster ion.
Spreading of droplets on lubricant-patterned substrates
Authors: Li Xin , Hu Yuanzhong , Jiang Lan , Zhang Jun
Published: May 2008

Droplet spreading behaviors on lubricant-patterned substrates are investigated by using molecular dynamics simulations to explore application potentials in magnetic storage drive systems. Microscopic spreading processes are studied by both potential fields of lubricant-patterned substrates and single molecule movements in lubricant droplets. The potential fields indicate that the wall molecules patterned on the substrates attract the mobile ones in the lubricant droplets. Due to the attraction force, the mobile molecules experience difficulties in diffusing freely along the substrates. The single molecule movements in lubricant droplets demonstrate that during the diffusion process, the mobile molecules encounter, adsorb, encompass, and disengage the wall ones. The spreading behaviors are significantly impacted by the bonded ratio. The potential fields indicate that as the bonded ratio increases, the attractive regions of wall molecules merge to overlap, which indicate combined interactions formed by the adjacent wall molecules.
Configuration control of quantum dot molecules by droplet epitaxy
Authors: Sablon K. A. , Lee J. H. , Wang Zh. M. , Shultz J. H. , Salamo G. J.
Published: May 2008

We demonstrate that by changing the substrate temperature at which Ga droplets form and by varying the InAs deposition, we are able to control the configuration of quantum dots per GaAs mound. The size of the Ga droplets increases with increasing substrate temperature and resulting configurations show a very strong correlation with the size of initial GaAs islands. In distinction from previous reports, we attained two structures: quadmolecules and quantum rod pairs. Quadmolecules are elongated along the [011] crystallographic direction due to strain-driven processes and are directly formed at the edges of the GaAs mounds. On the other hand, quantum rod pairs formed along the [01−1] direction due to higher anisotropic diffusion.
Oscillator strength and polarization of the forbidden n→π* band of trans-azobenzene: A computational study
Authors: Cusati Teresa , Granucci Giovanni , Persico Maurizio , Spighi Gloria
Published: May 2008

The trans-azobenzene molecule is thought to prefer a planar C2h geometry, in gas phase as well as in solution, according to the most recent computational studies. As a consequence, the weak n→π* absorption band is forbidden by symmetry at the equilibrium geometry, and its intensity depends on the effect of the vibrational motions on the electronic structure. In this computational study, we determine the contribution of the vibrational modes to the oscillator strength, taking into account the anharmonicity, the thermal distributions, and the solvent effects. The good agreement of our results with the measured absorption spectrum confirms the C2h equilibrium structure of trans-azobenzene, with a relatively easy torsion of the phenyl groups around the NC bonds. We also address the question of the polarization of this transition, which is a preliminary step to interpret the time-resolved fluorescence anisotropy measurements [C.-W. Chang et al., J. Am. Chem. Soc., 126, 10109 (2004)], a very sensitive probe of solvent effects on the excited state dynamics.
Comment on “Calculation of the mean first passage time tested on simple two-dimensional models” [J. Chem. Phys. 126, 194708 (2007)]
Authors: Mon K. K.
Published: May 2008

In a recent paper [J. Chem. Phys.126, 194708 (2007)] on mean first passage time, Kalinay cites the simulation results of Bowles et al. [J. Chem. Phys.121, 10668 (2004)] for two diffusing disks to support the validity of the Fick–Jacobs type of method used in his paper. This comment points out that Kalinay’s reference to the results of Bowles et al. is inaccurate and incomplete. The kind of support for the Fick–Jacobs method as suggested by Kalinay does not exist in the data of Bowles et al.
Current-induced domain wall motion in a nanowire with perpendicular magnetic anisotropy
Authors: Jung Soon-Wook , Kim Woojin , Lee Taek-Dong , Lee Kyung-Jin , Lee Hyun-Woo
Published: May 2008

We theoretically study the current-induced magnetic domain wall motion in a metallic nanowire with perpendicular magnetic anisotropy. The anisotropy can reduce the critical current density of the domain wall motion. We explain the reduction mechanism and identify the maximal reduction conditions. This result facilitates both fundamental studies and device applications of the current-induced domain wall motion.
Subgap states in transparent amorphous oxide semiconductor, In–Ga–Zn–O, observed by bulk sensitive x-ray photoelectron spectroscopy
Authors: Nomura Kenji , Kamiya Toshio , Yanagi Hiroshi , Ikenaga Eiji , Yang Ke , Kobayashi Keisuke , Hirano Masahiro , Hosono Hideo
Published: May 2008

We investigated the electronic states in amorphous In–Ga–Zn–O films with high carrier concentrations by optical absorption and hard x-ray photoelectron spectroscopy (HX-PES). Films having different Hall mobilities were prepared and their annealing effects were examined. All HX-PES spectra showed Fermi edge structures and extra subgap densities of states (DOSs). Tail-like structures observed in the optical spectra originate from subgap DOSs (⪢1020cm−3) near valence band maximas (VBMs). Subgap DOSs near VBMs provide a reason why In–Ga–Zn–O thin film transistors show hard saturation in off states and are difficult to operate in an inversion p-channel mode.
Colossal nonlinear optical magnetoelectric effects in multiferroic Bi2FeCrO6
Authors: Ju Sheng , Guo Guang-Yu
Published: May 2008

Within density functional theory with the generalized gradient approximation plus on-site Coulomb repulsion method, we investigate magnetic ordering dependence of linear and nonlinear optical responses in multiferroic Bi2FeCrO6. We find that in spite of the minor difference in the linear dielectric function between ferrimagnetic state and ferromagnetic state, the second-harmonic generation susceptibility and its related linear electro-optical coefficient dramatically change. Such colossal magnetoelectric effects come mainly from the enhanced contrast in the double-photon resonance absorption between two magnetic configurations, and will open a direction in the application of conventional multiferroics via nonlinear magnetoelectric effect.
Efficient production of a collimated MeV proton beam from a polyimide target driven by an intense femtosecond laser pulse
Authors: Nishiuchi M. , Daido H. , Yogo A. , Orimo S. , Ogura K. , Ma J. , Sagisaka A. , Mori M. , Pirozhkov A. S. , Kiriyama H. , Bulanov S. V. , Esirkepov T. Zh. , Choi I. W. , Kim C. M. , Jeong T. M. , Yu T. J. , Sung J. H. , Lee S. K. , Hafz N. , Pae K. H. , Noh Y.-C. , Ko D.-K. , Lee J. , Oishi Y. , Nemoto K. , Nagatomo H. , Nagai K. , Azuma H.
Published: May 2008

High-flux energetic protons whose maximum energies are up to 4MeV are generated by an intense femtosecond titanium:sapphire laser pulse interacting with 7.5, 12.5, and 25μm thick polyimide tape targets. Laser pulse with an energy of 1.7J and with a duration of 34fs is focused with an f/3.4 parabolic mirror giving an intensity of 3×1019Wcm−2. The main pulse to amplified spontaneous emission (ASE) intensity contrast ratio is 2.5×107. The conversion efficiency from the laser energy into the proton kinetic energies is achieved to be ∼3%, which is comparable to or even higher than those achieved in the previous works; using nanometer-thick targets, in combination with the short-pulse lasers that have almost the same pulse width and the intensity but different main pulse to ASE intensity contrast of ∼1010 [Neely et al., Appl. Phys. Lett. 89, 021502 (2006)], in which the authors claim that the main mechanism is target normal sheath acceleration; or using the 7.5μm thick polyimide target, in combination with the short-pulse laser, which has almost the same pulse width and the intensity, but the main pulse to ASE intensity contrast ratio was controlled to be 2.5×105 [Yogo et al., Phys. Rev. E 77, 016401 (2008)], in which the authors claim the efficient acceleration by the mechanism of the underdense plasma model. The contrast ratio of the present experiment is in between these two experiments. The possible mechanism of this regime is discussed.
Edge-truncated cubic platinum nanoparticles as anode catalysts for direct methanol fuel cells
Authors: Liu Y. G. , Shi S. L. , Xue X. Y. , Zhang J. Y. , Wang Y. G. , Wang T. H.
Published: May 2008

The edge-truncated cubic platinum nanoparticles (ECPs) are synthesized with the addition of silver ions. The nanoparticle is closed by 6 {100} facets and 12 {110} facets, confirmed from the transmission electron microscopy and cyclic voltammogram (CV) results. The current density increases up to a maximum of 1.05mAcm−2 (Jf) during the forward sweep. A current density ratio value of 1.12 and the sharp initial current drop in the CV reveal that the tailored facets of the ECPs dominate the methanol oxidation behaviors. Our results show promising anode catalysts of truncated-edge cubic platinum nanoparticles for direct methanol fuel cells.
Apparent volume dependence of 1∕f noise in thin film structures: Role of contacts
Authors: Barone C. , Pagano S. , Méchin L. , Routoure J.-M. , Orgiani P. , Maritato L.
Published: May 2008

The experimental investigation of low-frequency noise properties in new materials is very useful for the understanding of the involved physical transport mechanisms. In this paper it is shown that, when contact noise is present, the experimental values of the normalized Hooge parameter show a fictitious linear dependence on the volume of the analyzed samples. Experimental data on noise measurements of La0.7Sr0.3MnO3 thin films are reported to demonstrate the validity of the analysis performed.
Resistivity reduction of boron-doped multiwalled carbon nanotubes synthesized from a methanol solution containing boric acid
Authors: Ishii Satoshi , Watanabe Tohru , Ueda Shinya , Tsuda Shunsuke , Yamaguchi Takahide , Takano Yoshihiko
Published: May 2008

Boron-doped multiwalled carbon nanotubes (MWNTs) were synthesized using a methanol solution of boric acid as a source material. Accurate measurements of the electrical resistivity of an individual boron-doped MWNT was performed with a four-point contact, which was fabricated using an electron beam lithography technique. The doped boron provides conduction carriers, which reduces the resistivity of the MWNT.
Terahertz detection by two dimensional plasma field effect transistors in quantizing magnetic fields
Authors: Sakowicz M. , Łusakowski J. , Karpierz K. , Grynberg M. , Knap W. , Köhler K. , Valušis G. , Gołaszewska K. , Kamińska E. , Piotrowska A.
Published: May 2008

Detection of terahertz radiation by two dimensional electron plasma in high electron mobility GaAs∕GaAlAs transistors was investigated at cryogenic temperatures in quantizing magnetic fields. Shubnikov–de Haas oscillations of the detection signal were observed. A double (optical and electrical) modulation technique applied allowed us to study the influence of gated and ungated parts of the transistor channel on the detection. Our results provide a direct experimental evidence that both the gated and ungated plasma participate in the detection and clearly show the necessity to improve theoretical models that usually take into account only the gated part of the channel.
Anisotropic optical properties of free and bound excitons in highly strained A-plane ZnO investigated with polarized photoreflectance and photoluminescence spectroscopy
Authors: Nam Yoon Sung , Lee Sang Wook , Baek K. S. , Chang S. K. , Song Jae-Ho , Song Jung-Hoon , Han Seok Kyu , Hong Soon-Ku , Yao Takafumi
Published: May 2008

We have investigated the polarization dependence of the near-band-edge photoluminescence and photoreflectance spectra in nonpolar (A-plane) ZnO films under strong biaxial compressive strain. We show that anisotropic strain and the orientation of the nonpolar plane play an important role in determining the polarization selectivity and properties of excitonic transitions. We identified four distinct band-edge transitions at 3.449, 3.420, 3.386, and 3.326eV. They were identified as E2 and E1 free excitons, E1 excitons bound to a donor, and free-electron-to-bound-hole transition, respectively. Unlike previously reported results on relatively thick nonpolar films, the E1 exciton (lowest energy) was mainly polarized to E⊥c and weakly polarized to E∥c under strong biaxial compressive strain in the 100nm thick film. The E2 exciton (next higher energy) was exclusively polarized to E∥c. The localization energy of DX is 34meV, which is much larger than that in polar ZnO, and the DX was not thermally delocalized even at room temperature.
Vacancy engineering by optimized laser irradiation in boron-implanted, preamorphized silicon substrate
Authors: Tan D. X. M. , Pey K. L. , Ong K. K. , Colombeau B. , Ng C. M. , Yeong S. H. , Wee A. T. S. , Liu C. J. , Wang X. C.
Published: May 2008

In this letter, the effect of vacancies generated by preirradiated laser on dopant diffusion and activation in preamorphized silicon substrate has been studied. Laser-induced melting in silicon was used to generate excess vacancies near the maximum melt depth before silicon substrate amorphization and subsequent boron implantation. We demonstrate that by matching the preirradiated laser melt depth with the implant amorphize depth, it can effectively reduce the silicon self-interstitials released from the end-of-range defect band. The results show great suppression in boron transient enhanced diffusion and significant removal of end-of-range defects. This is attributed to the recombination of laser-generated excess vacancies with preamorphizing induced free silicon interstitials at the end-of-range region.
The absorption of light by rough metal surfaces—A three-dimensional ray-tracing analysis
Authors: Bergström D. , Powell J. , Kaplan A. F.H.
Published: May 2008

The laser absorptance of rough surfaces has been investigated by using Monte Carlo simulations based on three-dimensional (3D) ray tracing. The influence of multiple scattering, shadowing, and the Fresnel-equation based angle dependence is discussed. The 3D results are compared to previously published results from a two-dimensional ray-tracing analysis and the different applications of the two models are explained.
Orbital magnetization of graphene and graphene nanoribbons
Authors: Liu Junfeng , Ma Zhongshui , Wright A. R. , Zhang Chao
Published: May 2008

We present a quantitative analysis on orbital magnetization of graphene under a magnetic field. The energy spectra were obtained by solving Harper equation for a honeycomb lattice. The effect of the next-nearest-neighbor hopping (NNNH) is to increase the period of the magnetic subbands in ϕ/ϕ0 from 1 to 6 (where ϕ is the magnetic flux through a unit cell and ϕ0 is the flux quanta). The shifts of the energy levels due to the NNNH vary with the magnetic field. The Holmholtz free energy at the points ϕ/ϕ0=6n+1 is lowered as compared to that with the nearest neighbor hopping only. For graphene nanoribbons, the magnetic susceptibility is very sensitive to the ribbon width and chirality. The zero-field susceptibility increase with the ribbon width.
Buckling of microtubules under bending and torsion
Authors: Yi Lijun , Chang Tienchong , Ru Chongqing
Published: May 2008

Microtubules (MTs) in living cells are frequently bend, e.g., with a mean curvature of about 0.4 rad/μm in fibroblast cells [Odde et al., J. Cell Sci. 112, 3283 (1999)]. This raises a natural question whether bending buckling can occur in a MT. In this paper, an orthotropic model is developed to investigate buckling of MTs upon bending and torsion. A critical buckling curvature for a bent MT is predicted to be about 0.03 rad/μm (to which the corresponding bending moment is 0.85 nN nm), indicating that MTs in living cells are likely buckled. Buckling behavior of torsional MTs is also studied, and a critical buckling torque of 0.077 nN nm is obtained. Comparison to the results from an isotropic model shows that anisotropic properties of the MT wall have severe effect on the mechanical behavior of MTs.
Ce
3
+
activated LaBr3−xIx: High-light-yield and fast-response mixed halide scintillators
Authors: Birowosuto M. D. , Dorenbos P. , Krämer K. W. , Güdel H. U.
Published: May 2008

Here, we report the scintillation properties of LaBr3−xIx:5%Ce3+ with four different compositions of x, i.e., x=0.75, 1.5, 2, and 2.25. Radioluminescence spectra reveal a shift of the emission wavelength with the LaBr3 to LaI3 ratio. LaBr1.5I1.5:5%Ce3+ shows the highest scintillation light yield of 58 000 photons/MeV, whereas LaBr0.75I2.25:5%Ce3+ shows the fastest scintillation decay time of 12 ns under 662 keV γ-ray excitation. This decay time is faster than that of 16 ns in LaBr3:Ce3+. The temperature dependence of radioluminescence spectra is presented. The structures and lattice parameters of the materials were determined from powder x-ray diffraction.
On the structure of the B2 phase in Ti–Al–Mo alloys
Authors: Singh A. K. , Banumathy S. , Sowjanya D. , Rao M. H.
Published: May 2008

The structure of the B2 phase has been investigated in Ti-26Al-20Mo and Ti-37.5Al-12.5Mo alloys using Rietveld refinement of x-ray diffraction data in homogenized conditions. Different initial structure models have been used for the refinement. The site occupancy of the B2 phase has been calculated and compared to those of earlier experimental and theoretical investigations.
Effect of ultraviolet irradiation on electrical resistance and phase transition characteristics of thin film vanadium oxide
Authors: Ko Changhyun , Ramanathan Shriram
Published: May 2008

Metal-insulator transitions in strongly correlated oxides such as vanadium oxide (VO2) are of great scientific and technological interest. Due to the presence of multiple oxidation states, synthesis of high-quality VO2 films on substrates with the desired phase transition characteristics such as large jumps in phase transition resistance is a challenge. We show that the resistance ratio across the metal-insulator transition as well as the resistance of thin film VO2 can be modulated at relatively low temperatures by the use of ultraviolet irradiation. The enhanced oxygen incorporation due to creation of excited oxygen species enables controllably tunable stoichiometry.
Significantly reducing electrical resistivity by forming conductive Ni chains in a polyurethane shape-memory polymer/carbon-black composite
Authors: Leng J. S. , Huang W. M. , Lan X. , Liu Y. J. , Du S. Y.
Published: May 2008

We demonstrate an approach to significantly reduce the electrical resistivity in a polyurethane shape-memory polymer (SMP) filled with randomly distributed carbon black (CB). With an additional small amount of randomly distributed Ni microparticles (0.5vol%) in the SMP/CB composite, its electrical resistivity is only reduced slightly. However, if these Ni particles are aligned into chains (by applying a low magnetic field on the SMP/CB/Ni solution before curing), the drop of the electrical resistivity is significant. This approach, although demonstrated in a SMP, is applicable to other conductive polymers.
Fragility and glassy dynamics of 2Ca(NO3)2⋅3KNO3 under pressure: Molecular dynamics simulations
Authors: Ribeiro Mauro C. C. , Scopigno Tullio , Ruocco Giancarlo
Published: May 2008

Molecular dynamics simulations of the glass-forming liquid 2Ca(NO3)2⋅3KNO3 (CKN) were performed from high temperature liquid states down to low temperature glassy states at six different pressures from 10−4to5.0GPa. The temperature dependence of the structural relaxation time indicates that the fragility of liquid CKN changes with pressure. In line with recent proposal [Scopigno et al., Science 302, 849 (2003)], the change on liquid fragility is followed by a proportional change of the nonergodicity factor of the corresponding glass at low temperature.
Integrating superconductive and optical circuits
Authors: Stella F. , Casalboni M. , Cirillo M. , Merlo V. , Palazzesi C. , Pepe G. P. , Prosposito P. , Salvato M.
Published: May 2008

We have integrated on oxidized silicon wafers superconductive films and Josephson junctions along with sol-gel optical channel waveguides. The fabrication process is carried out in two steps that result to be solid and noninvasive. It is demonstrated that 660nm light, coupled from an optical fiber into the channel sol-gel waveguide, can be directed toward superconducting tunnel junctions whose current-voltage characteristics are affected by the presence of the radiation. The response of the junction biased at various currents as a function of the optical pumping is presented and discussed according to a nonequilibrium superconductivity model.
Icosahedral ordering in Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass
Authors: Hui X. , Fang H. Z. , Chen G. L. , Shang S. L. , Wang Y. , Liu Z. K.
Published: May 2008

This paper presents a computational evidence of icosahedral short and medium range ordering in Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass using ab initio molecular dynamics simulation. It is found that 1551, 1541, and 1431 types of bond pairs are pronounced in both the liquid and glass states, resulting in icosahedral coordinate polyhedra at low temperatures. By linking the individual icosahedra through vertex-, edge-, face-, and intercross-shared atoms, icosahedral medium range ordering is formed. The predicted homogenized structure factor and pair correlation function of the glass structure have been confirmed to be in agreement with the experimental results.
Combinatorial discovery of a lead-free morphotropic phase boundary in a thin-film piezoelectric perovskite
Authors: Fujino S. , Murakami M. , Anbusathaiah V. , Lim S.-H. , Nagarajan V. , Fennie C. J. , Wuttig M. , Salamanca-Riba L. , Takeuchi I.
Published: May 2008

We report on the discovery of a lead-free morphotropic phase boundary (MPB) in Sm doped BiFeO3 with a simple perovskite structure using the combinatorial thin film strategy. The boundary is a rhombohedral to pseudo-orthorhombic structural transition which exhibits a ferroelectric to antiferroelectric transition at approximately Bi0.86Sm0.14FeO3 with dielectric constant and out-of-plane piezoelectric coefficient comparable to those of epitaxial (001) oriented PbZr0.52Ti0.48O3 (PZT) thin films at the MPB. The discovered composition may be a strong candidate of a Pb-free piezoelectric replacement of PZT.
Surface recombination and facet heating in high-power diode lasers
Authors: Ziegler Mathias , Talalaev Vadim , Tomm Jens W. , Elsaesser Thomas , Ressel Peter , Sumpf Bernd , Erbert Götz
Published: May 2008

Surface recombination velocities and surface temperatures at front facets of standard broad-area lasers emitting at 808nm were investigated by time-resolved two-color photoluminescence and micro-Raman spectroscopy. Surface recombination velocities in the range between <105 and 106cm∕s are determined for devices with tailored surface properties. The results clearly show that increased surface recombination velocities are accompanied by increased facet temperatures. Reabsorption of light generated in the diode lasers leads to an additional enhancement of facet heating for surfaces of minor structural quality. The methodological approach presented here paves the way for improved analytical access to diode laser facet properties.
Vacancy-type defects in Er-doped GaN studied by a monoenergetic positron beam
Authors: Uedono A. , Shaoqiang C. , Jongwon S. , Ito K. , Nakamori H. , Honda N. , Tomita S. , Akimoto K. , Kudo H. , Ishibashi S.
Published: May 2008

A relationship between intra-4f transitions of Er and vacancy-type defects in Er-doped GaN was studied by using a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured for Er-doped GaN grown by molecular beam epitaxy. A clear correlation between the defect concentration and the photoluminescence (PL) intensity was observed. The major defect species detected by positrons was identified as a Ga vacancy VGa, and its concentration increased with increasing Er concentration [Er]. For the sample with [Er]=3.3at.%, the maximum integrated intensity of PL was observed. The VGa concentration was above 1018cm−3 and additional vacancies such as divacancies started to be introduced at this Er concentration. For the sample with higher [Er], the PL intensity decreased, and the mean size of vacancies decreased due to an introduction of precipitates and/or metastable phases.
Electronic structure and optical properties of Sn and SnGe quantum dots
Authors: Moontragoon Pairot , Vukmirović Nenad , Ikonić Zoran , Harrison Paul
Published: May 2008

Self-assembled quantum dots in a Si–Ge–Sn system attract research attention as possible direct band gap materials, compatible with Si-based technology, with potential applications in optoelectronics. In this work, the electronic structure near the Γ point and interband optical matrix elements of strained Sn and SnGe quantum dots in a Si or Ge matrix are calculated using the eight-band k⋅p method, and the competing L-valley conduction band states were found by the effective mass method. The strain distribution in the dots was found with the continuum mechanical model. The parameters required for the k⋅p or effective mass calculation for Sn were extracted by fitting to the energy band structure calculated by the nonlocal empirical pseudopotential method. The calculations show that the self-assembled Sn/Si dots, sized between 4 and 12 nm, have indirect interband transition energies between 0.8 and 0.4 eV and direct interband transitions between 2.5 and 2.0 eV. In particular, the actually grown, approximately cylindrical Sn dots in Si with a diameter and height of about 5 nm are calculated to have an indirect transition (to the L valley) of about 0.7 eV, which agrees very well with experimental results. Similar good agreement with the experiment was also found for SnGe dots grown on Si. However, neither of these is predicted to be direct band gap materials, in contrast to some earlier expectations.
Analytical evaluation of describing functions arising from harmonic balance analysis of tapping mode atomic force microscope
Authors: Mamedov B. A.
Published: May 2008

A new algorithm of harmonic balance analysis of tapping mode atomic force microscopes has been developed. The new algorithm is applicable to analytical evaluation of a large class of common tip-sample interaction potentials. The extensive test calculations show that the proposed algorithm in this work is the efficient one in practical computations. The comparative values presented in tables are acceptable and have the excellent agreement with the numerical results.
Near optical isotropy in noncubic SrI2: Density functional calculations
Authors: Singh D. J.
Published: May 2008

We report calculations of the electronic structure and optical properties of orthorhombic SrI2. These were done using the Engel–Vosko generalized gradient approximation. We find that the dielectric function and refractive indices are almost isotropic in this noncubic material. This suggests that low cost nonconventional routes for producing SrI2-based scintillators may be practical and, in particular, that ceramic processing may be used.
Tunable electronic band structures of hydrogen-terminated ⟨112⟩ silicon nanowires
Authors: Lu A. J. , Zhang R. Q. , Lee S. T.
Published: May 2008

The possibility of inducing indirect-to-direct band gap transition in silicon nanowires (SiNWs) by changing wire diameter is well known. Here, we show that for ⟨112⟩-oriented SiNWs indirect-to-direct band gap transition can be tuned simply by changing the wire cross-section shape or the cross-sectional aspect ratio of the (111) and (110) facets that enclose the wire, instead of changing the wire diameter. The cross-sectional aspect ratio must be smaller than 0.5 in order to maintain a direct band gap, indicating the important role of the (110) facet.