bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlations between bonding, size, and second hyperpolarizability (γ) of small semiconductor clusters: Ab initio study on AlnPn clusters with n=2, 3, 4, 6, and 9
Authors: Karamanis Panaghiotis , Leszczynski Jerzy
Published: April 2008

A comprehensive investigation of the correlation between the second hyperpolarizability and the bonding and structural characteristics of stoichiometric aluminum phosphide clusters up to 18 atoms is presented. Several aluminum phosphide species displaying different types of configurations and bonding have been studied. The obtained ab initio and density functional finite field results suggest that the ionic AlP clusters are considerably less hyperpolarizable than the covalent bonded species. Other structural features such as symmetry, atoms’ arrangement, and shape also play an important role on the hyperpolarizabilities of those species. However, they are only noticeable among clusters characterized by the same bonding patterns. Furthermore, the results of this study demonstrate that the bonding which is determined by the atoms’ arrangement of a cluster has a more profound effect on the second hyperpolarizability than the cluster’s composition or size. In addition, the mean second hyperpolarizability increases with the increasing number of atoms, assuming that the bonding characteristics among the clusters of increasing size are similar. On the other hand, the hyperpolarizability per atom rapidly decreases with the increase of atoms’ number in the cluster and converges to values of ∼900e4a04 and ∼1300e4a04Eh−3 at the HF/cc-pVDZ and MP2/cc-pVDZ levels of theory respectively. Lastly, this work provides the first systematic study on the hyperpolarizabilities of small aluminum phosphide clusters which, in their covalent forms, exhibit larger second order hyperpolarizabilities than the well studied small gallium arsenide clusters.
Quasiparticle virtual orbitals in electron propagator calculations
Authors: Flores-Moreno R. , Ortiz J. V.
Published: April 2008

The computational limits of accurate electron propagator methods for the calculation of electron binding energies of large molecules are usually determined by the rank of the virtual orbital space. Electron density difference matrices that correspond to these transition energies in the second-order quasiparticle approximation may be used to obtain a virtual orbital space of reduced rank that introduces only minor deviations with respect to the results produced with the full, original set of virtual orbitals. Numerical tests show the superior accuracy and efficiency of this approach compared to the usual practice of omission of virtual orbitals with the highest energies.
Colossal dielectric constants in single-crystalline and ceramic CaCu3Ti4O12 investigated by broadband dielectric spectroscopy
Authors: Krohns S. , Lunkenheimer P. , Ebbinghaus S. G. , Loidl A.
Published: April 2008

In the present work, the authors report results of broadband dielectric spectroscopy on various samples of CaCu3Ti4O12 (CCTO), also including single-crystalline material, which so far was only rarely investigated. The measurements extend up to 1.3 GHz, covering more than nine frequency decades. We address the question of the origin of the colossal dielectric constants and of the relaxational behavior in this material, including the second relaxation reported in several recent works. For this purpose, the dependence of the temperature- and frequency-dependent dielectric properties on different tempering and surface treatments of the samples and on ac-field amplitude is investigated. Broadband spectra of a single crystal are analyzed by an equivalent circuit description by assuming two highly resistive layers in series to the bulk. Good fits could be achieved, including the second relaxation, which also shows up in single crystals. The temperature- and frequency-dependent intrinsic conductivity of CCTO is consistent with the variable range hopping model. The second relaxation is sensitive to surface treatment and, in contrast to the main relaxation, is also strongly affected by the applied ac voltage. Concerning the origin of the two insulating layers, we discuss a completely surface-related mechanism by assuming the formation of a metal-insulator diode and a combination of surface and internal barriers.
A charge coupled device camera with electron decelerator for intermediate voltage electron microscopy
Authors: Downing Kenneth H. , Mooney Paul E.
Published: April 2008

Electron microscopists are increasingly turning to intermediate voltage electron microscopes (IVEMs) operating at 300–400kV for a wide range of studies. They are also increasingly taking advantage of slow-scan charge coupled device (CCD) cameras, which have become widely used on electron microscopes. Under some conditions, CCDs provide an improvement in data quality over photographic film, as well as the many advantages of direct digital readout. However, CCD performance is seriously degraded on IVEMs compared to the more conventional 100kV microscopes. In order to increase the efficiency and quality of data recording on IVEMs, we have developed a CCD camera system in which the electrons are decelerated to below 100kV before impacting the camera, resulting in greatly improved performance in both signal quality and resolution compared to other CCDs used in electron microscopy. These improvements will allow high-quality image and diffraction data to be collected directly with the CCD, enabling improvements in data collection for applications including high-resolution electron crystallography, single particle reconstruction of protein structures, tomographic studies of cell ultrastructure, and remote microscope operation. This approach will enable us to use even larger format CCD chips that are being developed with smaller pixels.
Silicon thickness fluctuation scattering dependence of electron mobility in ultrathin body silicon-on-insulator n-metal-oxide-semiconductor field-effect transistors
Authors: Lee Yong-Seon , Shim Tae-Hun , Yoo Sang-Dong , Park Jea-Gun
Published: April 2008

A matrix element for thickness fluctuation (δTSOI) scattering was derived to demonstrate the mobility dependence on Eeff, especially at a low inversion charge concentration, with an ultrathin channel thickness below 7 nm. This case contrasts other research with a lack of such dependence on either thick silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistors (MOSFETs) or bulk MOSFETs. Our matrix element implies that the quantized energy fluctuation associated with δTSOI varies with the average distance of electrons from the surface interface. In particular, it implies the effects of the change in transverse potential on the subband energy fluctuation for each subband, which results in a different matrix element for each applied gate bias. By using this matrix element model, the low-field electron mobility was shown to depend not only on TSi but also on Eeff. Furthermore, the results of a simulation of electron mobility degradation as a function of TSi at low Eeff agreed well with previous experimental results.
The electrical and mechanical properties of Au–V and Au–V2O5 thin films for wear-resistant RF MEMS switches
Authors: Bannuru Thirumalesh , Brown Walter L. , Narksitipan Suparut , Vinci Richard P.
Published: April 2008

To explore alternatives to the use of pure Au in Ohmic contact RF microelectromechanical switches, we have measured changes in the electrical resistivity and nanoindentation hardness of a series of sputter deposited Au–V and Au–V2O5 thin films. Increasing V content in the Au–V alloys increases resistivity and hardness, which is consistent with solid solution strengthening. In the Au–V2O5 films, the increase in resistivity is greatly reduced and the hardness is further increased as expected for dispersion strengthening with V2O5 particles. These two phenomena are explained in terms of solute and particle effects on electron scattering and bowing of dislocations, respectively.
Modified interfacial statistical associating fluid theory: Application to tethered polymer chains
Authors: Jain Shekhar , Jog Prasanna , Weinhold Jeffrey , Srivastava Rakesh , Chapman Walter G.
Published: April 2008

Modified interfacial statistical associating fluid theory density functional theory is extended to tethered polymer chains in the absence or presence of free polymer chains. The structures of the “dry” and “wet” polymer brushes have been calculated and compared with simulation results available in the literature. The comparisons show that the theory accurately predicts the structure of the tethered polymer brush. The average brush heights calculated from the theory agree with well-established scaling theories for tethered polymers. However, these scaling theories cannot predict the detailed structure, accurately. The effects of the segment-segment interactions of the tethered polymer and the free polymer have been effectively captured by the theory.
Monocrystalline diamond detector for ionizing radiation emitted by high temperature laser-generated plasma
Authors: Torrisi L. , Margarone D. , Laska L. , Marinelli M. , Milani E. , Verona-Rinati G. , Cavallaro S. , Ryc L. , Krasa J. , Rohlena K. , Ullschmied J.
Published: April 2008

A monocrystalline diamond detector was used for measurements of soft x-ray and ion emission from laser plasma obtained with the use of the PALS Asterix laser at intensities on the order of 1016 W/cm2 and pulse duration of 300 ps. Measurements were performed by varying the laser intensity and the focal position of the laser beam with respect to the target position. The spectra were obtained with the use of a diamond detector, which was without a filter, and showed not only the photopeak due to UV and soft x rays but also the ions emitted from the plasma. The detector was employed with absorbers of different thicknesses to determine, as a first approximation, the energy distribution of soft x-ray emission from the plasma. The time-of-flight technique was employed to determine the ion kinetic energies.
Half-metallic properties for the Mn2FeZ (Z=Al, Ga, Si, Ge, Sb) Heusler alloys: A first-principles study
Authors: Luo H. Z. , Zhang H. W. , Zhu Z. Y. , Ma L. , Xu S. F. , Wu G. H. , Zhu X. X. , Jiang C. B. , Xu H. B.
Published: April 2008

The electronic structure and magnetism of the Mn2FeZ (Z=Al, Ga, Si, Ge, Sb) Heusler alloys have been studied by density functional calculations. Two half-metallic ferromagnets, namely, Mn2FeAl and Mn2FeSb, are predicted. It is found that a small expansion of the crystal lattice can restore the half-metallicity in Mn2FeSi. The calculated total magnetic moments Mtot are 1μB/f.u. for Mn2FeAl and Mn2FeGa, 2μB/f.u. for Mn2FeSi and Mn2FeGe, and 3μB/f.u. for Mn2FeSb, which agree with the Slater–Pauling curve quite well. The moments of Mn (A) and Mn (B) are large and antiparallel to each other, which is indicative of ferrimagnetism in Mn2FeZ alloys. Fe shows only a small moment and its moment is parallel to that of Mn (B). By investigating the effect of lattice distortion on the half-metallicity and magnetic moments of Mn2FeZ, it is found that the half-metallic properties of Mn2FeSb are insensitive to the lattice distortion and a 100% spin polarization can be obtained within the wide range of 5.4–6.05 Å. This is preferable in practical applications.
Ferroelectric and structural properties of Ba2−xSrxNaNb5O15 thin films prepared on La0.05Sr0.95TiO3 substrates
Authors: Higuchi Tohru , Yamasaki Taro , Suzuki Yuko , Gotoh Keisuke , Hattori Takeshi , Tsukamoto Takeyo
Published: April 2008

The Sr-substituted Ba2NaNb5O15 (Ba2−xSrxNaNb5O15) thin films were prepared on the La0.05Sr0.95TiO3 substrates by pulsed laser deposition. When the substrate temperature was fixed at 700°C, the thin films exhibited highly c-axis orientation and smooth surface. The c-axis orientation and surface roughness do not depend much on Sr concentration. These thin films consisted of small grains with a diameter of 50–80nm against the film thickness of 400nm. The postannealed Ba2−xSrxNaNb5O15 thin films exhibited good P-E hysteresis loops. The remanent polarization (Pr) and coercive field (Ec) at x=1.4 were 2Pr=59.4μC∕cm2 and 2Ec=182.0kV∕cm, respectively. Its leakage current was 4.0×10−7A∕cm2. The low leakage current contributes to the increase in band gap with Sr substitution.
Real-time plasma control in a dual-frequency, confined plasma etcher
Authors: Milosavljević V. , Ellingboe A. R. , Gaman C. , Ringwood J. V.
Published: April 2008

The physics issues of developing model-based control of plasma etching are presented. A novel methodology for incorporating real-time model-based control of plasma processing systems is developed. The methodology is developed for control of two dependent variables (ion flux and chemical densities) by two independent controls (27 MHz power and O2 flow). A phenomenological physics model of the nonlinear coupling between the independent controls and the dependent variables of the plasma is presented. By using a design of experiment, the functional dependencies of the response surface are determined. In conjunction with the physical model, the dependencies are used to deconvolve the sensor signals onto the control inputs, allowing compensation of the interaction between control paths. The compensated sensor signals and compensated set–points are then used as inputs to proportional-integral-derivative controllers to adjust radio frequency power and oxygen flow to yield the desired ion flux and chemical density. To illustrate the methodology, model-based real-time control is realized in a commercial semiconductor dielectric etch chamber. The two radio frequency symmetric diode operates with typical commercial fluorocarbon feed-gas mixtures (Ar/O2/C4F8). Key parameters for dielectric etching are known to include ion flux to the surface and surface flux of oxygen containing species. Control is demonstrated using diagnostics of electrode-surface ion current, and chemical densities of O, O2, and CO measured by optical emission spectrometry and/or mass spectrometry. Using our model-based real-time control, the set-point tracking accuracy to changes in chemical species density and ion flux is enhanced.
Diluting the hydrogen bonds in viscous solutions of n-butanol with n-bromobutane: A dielectric study
Authors: El Goresy T. , Böhmer R.
Published: April 2008

Glass-forming monohydroxy alcohols exhibit not only a structural relaxation but also a slower, single-exponential Debye-type relaxation process which already freezes in the liquid phase. By using dielectric spectroscopy, we study how these relaxations evolve when the aprotic alkyl halide n-bromobutane is added to n-butanol, thereby diluting the hydrogen-bond network. The structural relaxation times smoothly vary over the concentration range of this completely miscible binary system. The Debye process remains unaffected by the dilution of the OH groups up to n-bromobutane mole fractions of about 50%. For larger hydroxy dilutions, it turns rather abruptly into a feature which develops significant spectral broadening and it becomes faster. In the dilute limit, the decoupling between the time scale of the Debye process and that of the structural relaxation amounts to almost 6 decades when extrapolated to the glass transition temperature. This relatively large, strongly concentration dependent decoupling is interpreted in analogy to normal modes in polymers. The present results suggest that the structural and the Debye-like responses of monohydroxy alcohols are unrelated.
Ab initio potential energy surfaces and nonadiabatic collision dynamics in H++O2 system
Authors: Amaran Saieswari , Kumar Sanjay
Published: April 2008

The adiabatic potential energy surfaces for the lowest five electronic states of A″3 symmetry for the H++O2 collision system have been obtained at the multireference configuration interaction level of accuracy using Dunning’s correlation consistent polarized valence triple zeta basis set. The radial nonadiabatic coupling terms and the mixing angle between the lowest two electronic states (1A″3 and 2A″3), which adiabatically correlate in the asymptotic limit to H(S2)+O2+(XΠg2) and H++O2(XΣg−3), respectively, have been computed using ab initio procedures at the same level of accuracy to yield the corresponding quasidiabatic potential energy matrix. The computed strengths of the vibrational coupling matrix elements reflect the trend observed for inelastic vibrational excitations of O2 in the experiments at collision energy of 9.5eV. The quantum dynamics has been preformed on the newly obtained coupled quasidiabatic potential energy surfaces under the vibrational close-coupling rotational infinite-order sudden framework at the experimental collision energy of 9.5eV. The present theoretical results for vibrational elastic/inelastic excitations of O2 are in overall good agreement with the available experimental data obtained from the proton energy-loss spectra in molecular beam experiments [F. A. Gianturco et al., J. Phys. B 14, 667 (1981)]. The results for the complementary charge transfer processes are also presented at this collision energy.
Ultrafast intermolecular dynamics of liquid water: A theoretical study on two-dimensional infrared spectroscopy
Authors: Yagasaki Takuma , Saito Shinji
Published: April 2008

Physical and chemical properties of liquid water are dominated by hydrogen bond structure and dynamics. Recent studies on nonlinear vibrational spectroscopy of intramolecular motion provided new insight into ultrafast hydrogen bond dynamics. However, our understanding of intermolecular dynamics of water is still limited. We theoretically investigated the intermolecular dynamics of liquid water in terms of two-dimensional infrared (2D IR) spectroscopy. The 2D IR spectrum of intermolecular frequency region (<1000cm−1) is calculated by using the equilibrium and nonequilibrium hybrid molecular dynamics method. We find the ultrafast loss of the correlation of the libration motion with the time scale of approximately 110fs. It is also found that the energy relaxation from the libration motion to the low frequency motion takes place with the time scale of about 180fs. We analyze the effect of the hindered translation motion on these ultrafast dynamics. It is shown that both the frequency modulation of libration motion and the energy relaxation from the libration to the low frequency motion significantly slow down in the absence of the hindered translation motion. The present result reveals that the anharmonic coupling between the hindered translation and libration motions is essential for the ultrafast relaxation dynamics in liquid water.
Photoelectron spectroscopy of cluster anions of naphthalene and related aromatic hydrocarbons
Authors: Ando Naoto , Mitsui Masaaki , Nakajima Atsushi
Published: April 2008

The electronic structures and structural morphologies of naphthalene cluster anions, (naphthalene)n− (n=3–150), and its related aromatic cluster anions, (acenaphthene)n− (n=4–100) and (azulene)n− (n=1–100), are studied using anion photoelectron spectroscopy. For (naphthalene)n− clusters, two isomers coexist over a wide size range: isomers I and II-1 (28⩽n⩽60) or isomers I and II-2 (n⩾∼60). Their contributions to the photoelectron spectra can be separated using an anion beam hole-burning technique. In contrast, such an isomer coexistence is not observed for (acenaphthene)n− and (azulene)n− clusters, where isomer I is exclusively formed throughout the whole size range. The vertical detachment energies (VDEs) of isomer I (7⩽n⩽100) in all the anionic clusters depend linearly on n−1∕3 and their size-dependent energetics are quite similar to one another. On the other hand, the VDEs of isomers II-1 and II-2 produced in (naphthalene)n− clusters with n⩾∼30 remain constant at 0.84 and 0.99eV, respectively, 0.4–0.6eV lower than those of isomer I. Based upon the ion source condition dependence and the hole-burning photoelectron spectra experiments for each isomer, the energetics and characteristics of isomers I, II-1, and II-2 are discussed: isomer I is an internalized anion state accompanied by a large change in its cluster geometry after electron attachment, while isomers II-1 and II-2 are crystal-like states with little structural relaxation. The nonappearance of isomers II-1 and II-2 for (acenaphthene)n− and (azulene)n− and a comparison with other aromatic cluster anions indicate that a highly anisotropic and symmetric π-conjugated molecular framework, such as found in the linear oligoacenes, is an essential factor for the formation of the crystal-like ordered forms (isomers II-1 and II-2). On the other hand, lowering the molecular symmetry makes their production unfavorable.
Resonant 2-photon ionization study of the conformation and the binding of water molecules to 2-phenylethanethiol (PhCH2CH2SH)
Authors: Martin Danielle E. , Robertson Evan G. , Thompson Christopher D. , Morrison Richard J. S.
Published: April 2008

The structures of 2-phenylethanethiol (PET, PhCH2CH2SH) and its 1:1 water clusters have been studied using resonant two-photon ionization spectroscopy including band contour analysis and UV-UV holeburning, combined with extensive ab initio calculations on ground and excited states. The most populated conformer, labeled Ggπ, has a gauche arrangement about the SCCC and HSCC bonds that permits a stabilizing SH⋯π type of hydrogen bond. The other observed conformer, Ag, is anti with respect to the SCCC bond. In the dominant 1:1 water cluster, a water molecule binds to the Ggπ conformer via an OH⋯S hydrogen bond and two significant CH⋯O interactions. There is also evidence for water binding to conformer Ag with a similar arrangement, and for a second Ggπ cluster where water inserts between the SH and the aromatic ring. The additional interactions to the water molecules result in net De binding energies approximately double those resulting from a single thiol-water hydrogen bond. The (π,π*)1 excited state lifetimes in the bare molecules are very short because of internal conversion to a dissociative (n,π*)1 state related to the thiol. In the dominant Gw1 cluster, the lifetime is significantly increased from <1to∼4ns. Hydrogen bonding to the thiol, which raises the energy of the dissociative (n,π*)1 state, accounts for this behavior.
Infrared spectroscopic study of C2F6 monolayers and bilayers on graphite
Authors: Hopkins Todd A. , Boyd David A. , Xia Yu , Shifflett G. Michael , Hess Frank M. , Hess George B.
Published: April 2008

We report an experimental study of adsorbed films of C2F6 on graphite by using infrared reflection absorption spectroscopy supplemented by ellipsometry. The vibrational C–F stretch modes ν5 (parallel to the molecular axis) and ν7 (perpendicular) in the film are strongly blueshifted by dynamic dipole coupling, and these shifts are sensitive to lattice spacing and molecular tilt. The relative strength of the absorption peaks mainly depends on the tilt angle relative to the surface normal. We use the strength data to estimate the tilt angle across the known monolayer phases, information that is difficult to obtain by other techniques. Although only the surface-normal component of the induced dipole moment appreciably couples to the external infrared field, surface-parallel components contribute to the intralayer coupling and hence to the frequency shifts for tilted molecules. Comparison to model calculations for a range of herringbone tilt configurations allows us to draw conclusions regarding the pattern of tilt azimuths. On this basis, we offer a revised interpretation of the origin of the Ising-type ordering transition found by Arndt et al. [Phys. Rev. Lett. 80, 1686 (1998)] in heat capacity measurements. Our phase boundaries for monolayer phases above 80K are in good agreement with earlier results of the Saarbrücken group. We identify three distinct bilayer phases near saturation in isothermal pressure scans from ellipsometric steps and spectroscopic signatures. In temperature scans, we find evidence for several monolayer phases more dense than the well-established 2×2 commensurate phase and for a stable trilayer phase below about 60K.
Velocity fluctuations and hydrodynamic diffusion in finite-Reynolds-number sedimenting suspensions
Authors: Yin Xiaolong , Koch Donald L.
Published: April 2008

The velocity variance and the hydrodynamic diffusivity for a finite-Reynolds-number settling suspension are determined from lattice-Boltzmann simulations of many particles in cubic cells with periodic boundary conditions. The velocity variance is found to grow logarithmically with the size of the computational domain in contrast to the algebraic growth found in comparable Stokes-flow simulations. The growth rate and size of the velocity variance are found to be smaller than the theoretical prediction for a random suspension owing to a deficit in particle pair probability distribution in the wake of a test particle that screens the velocity disturbance felt by other particles. The particle velocity variance is smaller than the fluid velocity variance because a particle does not follow fluid motions on length scales comparable to or smaller than its own size. The hydrodynamic diffusivity of particles is proportional to the product of the root-mean-square velocity and the size of the computational domain.
Higher-codimension bifurcations in a discrete unidirectional neural network model with delayed feedback
Authors: Peng Mingshu , Huang Lihong , Wang Guanghui
Published: April 2008

In this paper, we give a detailed study of the codimension-1/3/4 bifurcations, which there may occur in a unidirectional neural network model proposed by S. Guo, X. Tang, and L. Huang [Neurocomputing 71, 1422–1435 (2008)].
Microcrystalline silicon formation by silicon nanoparticles
Authors: Schierning G. , Theissmann R. , Wiggers H. , Sudfeld D. , Ebbers A. , Franke D. , Witusiewicz V. T. , Apel M.
Published: April 2008

Thin silicon films are of great importance for large-area electronic applications, for example, as the basis for switching electronics in flat-panel display devices or as the active layer of solar cells. In this paper, we show that silicon nanoparticles have the potential to be used as raw material for further processing toward a microcrystalline silicon film. This can be done by thermal treatment with a reduced thermal budget because the melting point of the nanoparticles is much lower with only 60% of the equilibrium melting temperature of silicon. Coagulation processes of liquid droplets then lead to the growth of microcrystalline silicon in agglomerated nanoparticles. We demonstrate by in situ transmission electron microscopy (TEM) and differential thermal analysis that silicon nanoparticles with a size of approximately 20nm start melting at around 1000K; furthermore, the TEM observations directly demonstrate the details of the coagulation process leading to microcrystalline silicon.
Molecular dynamics analysis of multiple site growth and coalescence effects on homogeneous and heterogeneous nucleations
Authors: Suh D. , Yoon W. , Shibahara M. , Jung S.
Published: April 2008

Homogeneous and heterogeneous nucleations were simulated by molecular dynamics (MD). The behavior of Lennard-Jones molecules was studied inside a liquid-gas system where all dimensions of the wall were periodic and a soft core carrier gas within the system controlled the temperature. In this study, the classical nucleation theory was found to underestimate the homogeneous nucleation rate by five orders of magnitude, which complies with other MD studies. The discrepancy in the nucleation rate between theory and simulation was mainly caused by the fundamental assumption that there are no volumetric interactions in the growth process. In this particular case, however, growth was observed at multiple sites due to Ostwald ripening and coalescence between nuclei by Brownian motion. Furthermore, even though the supersaturation ratio is inadequate for homogeneous nucleation, once a seed is introduced to the system, a cluster can be created. The addition of seeds not only enhances nucleation but also renders coalescence as an important nucleation mechanism in the earlier stages compared to homogeneous nucleation.
Multiple histogram reweighting method for the surface tension calculation
Authors: Ghoufi A. , Goujon F. , Lachet V. , Malfreyt P.
Published: April 2008

The multiple histogram reweighting method takes advantage of calculating ensemble averages over a range of thermodynamic conditions without performing a molecular simulation at each thermodynamic point. We show that this method can easily be extended to the calculation of the surface tension. We develop a new methodology called multiple histogram reweighting with slab decomposition based on the decomposition of the system into slabs along the direction normal to the interface. The surface tension is then calculated from local values of the chemical potential and of the configurational energy using Monte Carlo (MC) simulations. We show that this methodology gives surface tension values in excellent agreement with experiments and with standard NVT MC simulations in the case of the liquid-vapor interface of carbon dioxide.
Relaxation time dispersions in glass forming metallic liquids and glasses
Authors: Wang Li-Min , Liu Riping , Wang Wei Hua
Published: April 2008

Relaxation time dispersions in glass forming metallic liquids of diverse fragility characters were reviewed mainly based on mechanical relaxations. The compilation of the stretching exponents revealed the common nonexponential dynamic features among the metallic liquids. The time-temperature-superposition law of the relaxation profiles was identified with an average stretching exponent around 0.5 at low frequency regions near the glass transitions. No notable correlation of the stretching parameter with alloy composition was discerned. The construction of the frequency dependence of the stretching exponent across the whole range of liquid dynamics revealed a striking similarity of the nonexponential dynamics between metallic and fragile molecular liquids.
Intermolecular zero-quantum coherence NMR spectroscopy in the presence of local dipole fields
Authors: Balla David Z. , Faber Cornelius
Published: April 2008

NMR experiments detecting intermolecular zero-quantum coherences (iZQCs) allow for observation of homogeneous line shapes under inhomogeneous magnetic fields. Local dipole fields impair the refocusing capacity of such experiments and render the available theoretical description of signal evolution invalid. In this article, the impact of local dipole fields on two-dimensional iZQC spectroscopy experiments was assessed by performing extensive numerical simulations, which solved the nonlinear Bloch equations for a binary solution in a magnetization array of 643 spatial points. Local dipole fields were simulated using spherical volumes with different magnetic susceptibility values corresponding to either a glass sphere or an air inclusion with a diameter of 100μm. The local field resulted in a broadened distribution of difference frequencies between locally interacting spins and led to the dominating effect of decreasing the amplitude of the solute peak, before line broadening was observed in the spectra. From simulations using a magnetic field strength of 17.6T, the smallest ratio of sample to inclusion volume that still allowed for observation of the solute peak was determined to be ηlimit=215 and ηlimit=392 for glass and air inclusions, respectively. Experimental data acquired with a 100μm diameter glass sphere embedded in agar gel yielded a value of ηlimit=252 and confirmed the order of magnitude obtained from the simulations. From these data, it was concluded that iZQC spectroscopy is possible as long as the relative volume occupied by air inclusions does not exceed the order of 0.1% of the sample volume. This limit, in contrast to the previous speculations, strongly excludes materials or tissues with high density of strong inhomogeneities from the investigation by iZQC spectroscopy.
Effect of dust on an amplitude modulated electromagnetic beam in a plasma
Authors: Sharma Suresh C. , Gahlot Ajay , Sharma R. P.
Published: April 2008

A large amplitude modulated Gaussian electromagnetic beam propagating in a dusty plasma has been studied. The electrons are heated nonuniformly by the beam. For the nonsteady state, we obtain nonlinear current density in the presence of dust grains. This expression has been used to study the nonstationary self-focusing and resulting self-distortion of the amplitude modulated electromagnetic beam. It has been observed that the effect of dust is drastic on the modulation index in comparison to dust free plasma. A simple scaling showing the dependence of modulation index on dust grain size and charge has been presented.
Two-stream instability in free electron lasers with a planar wiggler and an axial guide magnetic field
Authors: Mehdian H. , Saviz S. , Hasanbeigi A.
Published: April 2008

The linearized theory of two-stream free electron lasers (FELs) consisting of a relativistic electron beam transported along the axis of a planar wiggler in the presence of an axial guiding magnetic field is proposed and investigated. The dispersion relation is derived employing linear fluid theory. The characteristics of the dispersion relation are analyzed by numerical solutions. The results show that the growth rate is considerably enhanced on suitable ranges of normalized axial guiding magnetic field and normalized wave number. The effect of the difference between the velocities of the two beams, Δv=v1−v2, in this configuration of FELs is also considered and found that the growth rate is remarkably affected by velocity differences. It is also shown that the effect on the electrostatic mode is stronger than electromagnetic.
Insights into the room temperature magnetism of ZnO∕Co3O4 mixtures
Authors: Martín-González M. S. , Fernández J. F. , Rubio-Marcos F. , Lorite I. , Costa-Krämer J. L. , Quesada A. , Bañares M. A. , Fierro J. L. G.
Published: April 2008

The origin of room temperature (RT) ferromagneticlike behavior in ZnO-based diluted magnetic semiconductors is still an unclear topic. The present work concentrates on the appearance of RT magnetic moments in just mixed ZnO∕Co3O4 mixtures without thermal treatment. In this study, it is shown that the magnetism seems to be related to surface reduction of the Co3O4 nanoparticles, in which, an antiferromagnetic Co3O4 nanoparticle (core) is surrounded by a CoO-like shell. This singular superficial magnetism has also been found in other mixtures with semiconductors such as TiO2 and insulators such as Al2O3.
Comments on the Hartree–Fock description of Hooke’s atom and suggestion for an accurate closed-form orbital
Authors: Ragot Sébastien
Published: April 2008

The ground-state Hartree–Fock (HF) wavefunction of Hooke’s atom is not known in closed form, contrary to the exact solution. The single HF orbital involved has thus far been studied using expansion techniques only, leading to slightly disparate energies. Therefore, the present letter aims at proposing alternative definitions of the HF wavefunction. First, the HF limit is ascertained using a simple expansion, which makes it possible to formulate explicit expressions of HF properties. The resulting energy, 2.038 438 871 8 Eh, is found stable at the tenth digit. Second and more instructive, an analysis of the Hartree equation makes it possible to infer a remarkably simple and accurate HF orbital, i.e., φHF(r)=nHFe−αr2r2+β2, leading to an energy exceeding by 5.76×10−7 Eh only the above HF limit. This orbital makes it possible to obtain (near) Hartree–Fock properties in closed form, which in turn enables handy comparisons with exact quantities.
Beyond the dynamic density functional theory for steady currents: Application to driven colloidal particles in a channel
Authors: Tarazona P. , Marini Bettolo Marconi Umberto
Published: April 2008

Motivated by recent studies on the dynamics of colloidal solutions in narrow channels, we consider the steady state properties of an assembly of noninteracting particles subject to the action of a traveling potential moving at a constant speed, while the solvent is modeled by a heat bath at rest in the laboratory frame. Here, since the description we propose takes into account the inertia of the colloidal particles, it is necessary to consider the evolution of both positions and momenta and study the governing equation for the one-particle phase-space distribution. First, we derive the asymptotic form of its solutions as an expansion in Hermite polynomials and their generic properties, such as the force and energy balance, and then we particularize our study to the case of an inverted parabolic potential barrier. We numerically obtain the steady state density and temperature profile and show that the expansion is rapidly convergent for large values of the friction constant and small drifting velocities. On the one hand, the present results confirm the previous studies based on the dynamic density functional theory (DDFT): On the other hand, when the friction constant is large, it display effects such as the presence of a wake behind the barrier and a strong inhomogeneity in the temperature field which are beyond the DDFT description.
Reynolds number dependence of plane jet development in the transitional regime
Authors: Suresh P. R. , Srinivasan K. , Sundararajan T. , Das Sarit K.
Published: April 2008

The transitional characteristics of plane turbulent jets have been investigated in the present study. Hot wire measurements have been performed for a jet issuing from a rectangular nozzle of aspect ratio 20, for Reynolds number varying in the range 250⩽Re⩽6250. In this range, the characteristics of flow development are found to be Reynolds number dependent, in contrast to the fully developed turbulent jets which show features independent of initial conditions such as inlet Re. For low Re jets, the jet spread is significantly influenced by the low frequency oscillations caused by shear layer instability. Large sized vortices are formed in the shear layers at the fundamental frequency of the instability, which lead to subharmonic low frequency oscillations due to vortex pairing and merger, at larger axial distances. Consequently, the far field flow structure of a low Re jet is dominated by large size vortices which give rise to a higher level of flow intermittency, larger entrainment of ambient fluid, and faster jet decay, as compared to high Re jets. Also, in the absence of finer scales and broader spectrum of eddies, the mean flow field achieves self-similar structure much ahead of the fluctuating components and fully developed turbulent flow characteristics are not observed, even in the far field. In high Re jets, on the other hand, the vortex break-up processes also simultaneously occur along with vortex pairing and merger. Therefore, energy transfers to a broad spectrum of scales and finer scales are observed even in the near field of the jet. Although the achievement of self-similarity for the mean field is slightly faster than that for the fluctuating components, turbulence also attains a fully developed state at about a nondimensional axial distance of 20. The associated probability density functions of the fluctuating components evolve into Gaussian profiles, implying isotropic turbulence. Due to the dominance of finer scales, the overall entrainment level is less and decay is slower for a high Re jet.
The adsorption of O2 on Pb films and the effect of quantum modulation: A first-principles prediction
Authors: Yang Yu , Zhou Gang , Wu Jian , Duan Wenhui , Xue Qi-Kun , Gu Bing-Lin , Jiang Peng , Ma Xucun , Zhang S. B.
Published: April 2008

Using first-principles calculations based on density-functional theory, we systematically study the adsorption of O2 molecules on ultrathin Pb(111) films ranging from 3 to 11 monolayers (MLs). It is found that no matter how thick the film is, the O2 molecule prefers to adsorb at the threefold hcp hollow site where it lies parallel to the surface. The adsorption mechanism is discussed from the hybridization of p orbitals of O2 and Pb. The adsorption energy of O2 on the Pb(111) film, about several hundred meV, shows a 2 ML oscillation with the thickness. This study well confirms the modulation of the surface reactivity of Pb films induced by the quantum well states, which is compatible with the previous experimental observation.
Tuning the work function of ultrathin oxide films on metals by adsorption of alkali atoms
Authors: Martinez Umberto , Giordano Livia , Pacchioni Gianfranco
Published: April 2008

We report a theoretical investigation of the adsorption of alkali metal atoms deposited on ultrathin oxide films. The properties of Li, Na, and K atoms adsorbed on SiO2∕Mo(112) and of K on MgO∕Ag(100) and TiO2∕Pt(111) have been analyzed with particular attention to the induced changes in the work function of the system, Φ. On the nonreducible SiO2 and MgO oxide films there is a net transfer of the outer ns electron of the alkali atom to the metal substrate conduction band; the resulting surface dipole substantially lowers Φ. The change in Φ depends (a) on the adsorption site (above the oxide film or at the interface) and (b) on the alkali metal coverage. Deposition of K on reducible TiO2 oxide films results in adsorbed K+ ions and in the formation of Ti3+ ions. No charge transfer to the metal substrate is observed but also in this case the surface dipole resulting from the K–TiO2 charge transfer has the effect to considerably reduce the work function of the system.
Interfacial structure and defect analysis of nonpolar ZnO films grown on R-plane sapphire by molecular beam epitaxy
Authors: Vennéguès P. , Chauveau J. M. , Korytov M. , Deparis C. , Zuniga-Perez J. , Morhain C.
Published: April 2008

The interfacial relationship and the microstructure of nonpolar (11−20) ZnO films epitaxially grown on (1−102) R-plane sapphire by molecular beam epitaxy are investigated by transmission electron microscopy. The already-reported epitaxial relationships [1−100]ZnO∥[11−20]sapphire and ⟨0001⟩ZnO∥[−1101]sapphire are confirmed, and we have determined the orientation of the Zn–O (cation-anion) bond along [0001]ZnO in the films as being uniquely defined with respect to a reference surface Al–O bond on the sapphire substrate. The microstructure of the films is dominated by the presence of I1 basal stacking faults [density=(1–2)×105cm−1] and related partial dislocations [density=(4–7)×1010cm−2]. It is shown that I1 basal stacking faults correspond to dissociated perfect dislocations, either c or a+c type.
Imaging mechanism of piezoresponse force microscopy in capacitor structures
Authors: Kalinin Sergei V. , Rodriguez Brian J. , Kim Seung-Hyun , Hong Suk-Kyoung , Gruverman Alexei , Eliseev Eugene A.
Published: April 2008

The image formation mechanism in piezoresponse force microscopy (PFM) of capacitor structures is analyzed. We demonstrate that the spatial resolution is a bilinear function of film and top electrode thicknesses and derive the corresponding analytical expressions. For many perovskites, the opposite contributions of d31 and d33 components can result in anomalous domain wall profiles. This analysis establishes the applicability limits of PFM for polarization dynamics studies in capacitors and applies them to other structural probes, including focused x-ray studies of capacitor structures.
Effects of grown-in defects on interdiffusion dynamics in InAs∕InP(001) quantum dots subjected to rapid thermal annealing
Authors: Dion C. , Desjardins P. , Shtinkov N. , Schiettekatte F. , Poole P. J. , Raymond S.
Published: April 2008

This work investigates the interdiffusion dynamics in self-assembled InAs∕InP(001) quantum dots (QDs) subjected to rapid thermal annealing in the 600–775°C temperature range. We compare two QD samples capped with InP grown at either optimal or reduced temperature to induce grown-in defects. Atomic interdiffusion is assessed by using photoluminescence measurements in conjunction with tight-binding calculations. By assuming Fickian diffusion, the interdiffusion lengths LI are determined as a function of annealing conditions from the comparison of the measured optical transition energies with those calculated for InP∕InAs1−xPx∕InP quantum wells with graded interfaces. LI values are then analyzed using a one-dimensional interdiffusion model that accounts for both the transport of nonequilibrium concentrations of P interstitials from the InP capping layer to the InAs active region and the P–As substitution in the QD vicinity. It is demonstrated that each process is characterized by a diffusion coefficient D(i) given by D(i)=D0(i)exp(−Ea(i)∕kBTa). The activation energy and pre-exponential factor for P interstitial diffusion in the InP matrix are Ea(P–InP)=2.7±0.3eV and D0(P–InP)=103.6±0.9cm2s−1, which are independent of the InP growth conditions. For the P–As substitution process, Ea(P–As)=2.3±0.2eV and (co∕no)D0(P–As)∼10−5−10−4cm2s−1, which depend on the QD height and concentration of grown-in defects (co∕no).
Reliability characteristics of NiSi nanocrystals embedded in oxide and nitride layers for nonvolatile memory application
Authors: Chen Wei-Ren , Chang Ting-Chang , Yeh Jui-Lung , Sze S. M. , Chang Chun-Yen
Published: April 2008

The authors provided the reliability characteristics of nonvolatile nickel-silicide nanocrystal memories embedded in oxide and nitride layers for next-generation nonvolatile memory application. The charge trapping layer was deposited by sputtering a commixed target in the argon and oxygen/nitrogen ambiances, and then using a low temperature rapid thermal annealing to form nanocrystals. Transmission electron microscope clearly shows the sharpness and the density of nanocrystals. These proposed memory structures were compared for the charge storage ability, retention, and endurance. In addition, we used a simple simulation of electric field for nonvolatile nanocrystals memory to explain the advantages by using the high-k dielectric.
Optical properties of polyelectrolyte quantum dot multilayer films prepared using the layer by layer self-assembly method
Authors: Lioudakis Emmanouil , Koupanou Elena , Kanari Constantina , Leontidis Epameinondas , Othonos Andreas
Published: April 2008

In this work, we have used spectroscopic ellipsometry to study the optical properties of polyelectrolyte-PbS quantum dot (QD) multilayer films prepared using the layer by layer self-assembly method. The optical results provide information about the absorption coefficients of the materials as a function of the number of layers deposited on a quartz substrate. We have found that the fundamental energy gap of the films decreases linearly upon addition of each layer due to the formation of nanoclusters at the surface. Furthermore, the influence of PbS QD concentration in colloidal dispersion on the energy gap of the materials is examined in detail, and it is found that the optical band gap in the films is in agreement with the linear absorption measurements in the PbS colloidal dispersion from which the film deposition takes place. Finally, the observed electronic transitions of the films corroborate that nanoparticles in the regime of strong quantum confinement are present in the films. This comprehensive fundamental study provides important information, necessary for photovoltaic applications, about the absorption tunability of these nanofilms.