bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron Distribution in Molecules. IV. Phosphorus Magnetic Resonance Shifts
Authors: Gutowsky H. S. , McCall D. W.
Published: April 2005

Chemical shifts in the phosphorus nuclear magnetic resonance absorption have been measured in twenty compounds. The shifts arise from differences in the magnetic shielding of the phosphorus nuclei by electrons, and the values observed are interpreted qualitatively in terms of the molecular electronic structures. The range in shifts in 7×10—4 which is somewhat larger than observed for fluorine on which more extensive data are available. The phosphorus resonance shifts fall into two distinct groups one of which contains compounds with trivalent phosphorus and the other, pentavalent. The shifts in pentavalent compounds fall in a relatively narrow range, generally with greater nuclear magnetic shielding than in the trivalent. The resonance shifts between similar or homologous compounds can be accounted for by considering changes in the importance of ionic and double-bonded electronic structures.
Singlet-Triplet Absorption Bands in Some Halogen Substituted Aromatic Compounds
Authors: McClure Donald S. , Blake Norman W. , Hanst Philip L.
Published: April 2005

Singlet-triplet absorption bands are reported for a number of halogen benzenes and naphthalenes. Combining the results with other data, the rates of radiationless triplet to ground state transitions are found. The effect of the atomic number of a substituted halogen atom on the absorption strength is shown to agree with a simple theory. It is also shown that the spectroscopic moments of substituents do not add vectorially for singlet-triplet transitions.
The Near Infrared Spectrum and the Internuclear Distances of Nitrous Oxide
Authors: Douglas A. E. , Mo/ller C. K.
Published: April 2005

The near infrared spectra of N214O16 and N15N14O16 have been investigated using a high-resolution grating spectrometer. The α's and hence the Be values for both isotopic species have been determined. From the Be values, the equilibrium N–N and N–O internuclear distances have been found to be 1.126A and 1.186A, respectively.
Infrared Spectrum of σ2 for B10F3 and B11F3
Authors: Nielsen Alvin H.
Published: April 2005

The infrared grating spectra of σ2 for B10F3 and B11F3 have been reexamined from 650 to 750 K. Two samples of BF3, one composed of 93.4 percent B10F3 and 6.6 percent of B11F3, and the other of the normal abundance ratio 20 percent B10F3 and 80 percent B11F3 were secured for the investigation. Complete resolution of the rotational fine structure of this parallel-type band was achieved. The following constants were obtained by means of the usual sort of rotational analysis: for B10F3, σ0 = 718.23 K, B0 = 0.35272 K, B(v2 = 1) = 0.35288 K, DJ = 13.72×10—7 K. For B11F3, σ0 = 691.45 K and the rotational constants are the same for the two isotopic species.
Statistical Mechanical Theory of Transport Processes. VII. The Coefficient of Thermal Conductivity of Monatomic Liquids
Authors: Zwanzig Robert W. , Kirkwood John G. , Oppenheim Irwin , Alder Berni J.
Published: April 2005

A molecular theory of the coefficient of thermal conductivity is developed from the general theory of transport processes presented in the first article of this series. The thermal conductivity of liquid argon at its normal boiling point is evaluated using the Lennard-Jones intermolecular potential and a theoretically determined radial distribution function. The theory leads to an explicit expression for the product of the thermal conductivity and the friction constant of the theory of Brownian motion. With a reasonable estimate of the friction constant, the results of the theory agree satisfactorily with experiment.
Infrared Spectrum of S16O18O and the Potential Constants of SO2
Authors: Polo Santiago R. , Wilson M. Kent
Published: April 2005

An investigation of the infrared spectrum of S16O18O results in the following frequency assignment: ν1=1122±1 cm−1,  ν2=506.8±0.5 cm−1,  ν3=1341±0.5 cm−1. These data combined with the fundamental frequencies of S16O2 have been used to calculate the potential constants for sulfur dioxide. These are fd=10.02,  fdd=0.03,  1dfdα=0.20,  1d2fα=0.793.
The Vapor Pressure of Zinc between 150° and 350°C
Authors: McKinley John D. , Vance John E.
Published: April 2005

The vapor pressure of zinc has been measured by the Knudsen effusion method using the radioisotope Zn65. Measurements have been made from 150° to 350°C, and the temperature-vapor pressure relation over this range was found to be log10Pmm=—7198/T+9.664. The heat of vaporization was calculated to be 31.35 kcal per mole at 0°K. A test has been made of the Clausing probability factor regarding the dependence of the effusion rate on the dimensions of the effusion orifice.
Theory of Thermal Diffusion in Liquids and the Use of Pressure to Investigate the Theory
Authors: Rutherford W. M. , Drickamer H. G.
Published: April 2005

A theory of thermal diffusion in liquids is presented which involves: (1) a molecular interpretation due to Denbigh of the results of the thermodynamics of irreversible processes in terms of regular solution theory; (2) a correction for differences in shape and size of the molecules.

Thermal diffusion measurements have been made to 10 000 atmospheres pressure on a series of binary mixtures in a single stage system. The results indicate that the theory is satisfactory.
Lattice Parameters of the FeFe(2—x)CrxO4 Spinel System
Authors: Yearian H. J. , Kortright J. M. , Langenheim R. H.
Published: April 2005

Spinels of the FeFe(2−x)CrxO4,0⩽x⩽2, system have been made by reduction of the corresponding sesquioxide solid solution at 1180°C, 1100°C and 950°C and the lattice parameters determined at room temperature. The solutions of approximately stoichiometric composition have lattice parameters which vary with x in a nonlinear manner from 8.396±0.002A at x=0 to 8.376±0.002A at x=2. The four essentially linear segments comprising the total curve are interpreted as follows: from x=0 to x=0.3 the structure is completely inversed; from x=1.28 to x=2.0 it is completely normal; between these regions reversion from the inversed to the normal form occurs in two successive stages. Oxygen-rich spinels crystallized at 950°C show the same general variations of parameter but the region of complete inversion extends to approximately x=0.6.
Electron Scattering and Internal Rotation
Authors: Karle J.
Published: April 2005

Theoretical functions are developed which facilitate the analysis of electron-scattering data from molecules which undergo internal rotation. The probability distributions for distances whose equilibrium positions occur anywhere on the circle of rotation are derived taking into account the combined effect of the over-all vibration of the molecular frame and the torsional motion. The extremes of small oscillations and free rotation are treated in detail.
The Box Model and Electron Densities in Conjugated Systems
Authors: Platt John R.
Published: April 2005

Part I describes the pi-electron density distributions of some conjugated systems, computed by generalizing Schmidt's model, with the electrons free to move in a properly shaped box of constant potential. Only products and sums of sine functions are needed for rectangular boxes (polyacenes, porphine) and Bessel functions for circular (coronene) or circular-sector boxes (phenanthrene). Plaster models of the pi-electron density surface are shown for 10 molecules. In aromatics, ring-shaped regions of high density are found at the same locations as the rings of the carbon skeleton.

In Part II it is shown that the density maxima in the boxes usually coincide with the high-order bonds or high-density atoms predicted by resonance theory or LCAO theory or the free-electron-network theory, with similar correspondence for low-density bonds or atoms. This suggests that the common success of these three other theories rests not on their various special assumptions, but on primitive general requirements of wave continuity and orthogonality in space.

In Part III this point is elaborated into a theorem that the density and transition energy of a closed-shell system are stable under large changes in the one-particle potential function if the changes occur below the highest-filled orbital. Taken together with electron repulsion and antisymmetrization, this helps explain why Coulomb and short-range forces can be successfully approximated by the smoothed-potential model of the nucleus and the free-electron model of metals and molecules.

In Part IV it is shown that the box model leads to a ``periodic table'' of stabilities of convex conjugated hydrocarbons. Their possible spectral types are discussed. Some new hydrocarbons are described which may be stable.
Influence of a Magnetic Field on the Microwave Dielectric Constant of a Liquid Crystal
Authors: Carr E. F. , Spence R. D.
Published: April 2005

The real and imaginary parts of the complex dielectric constant of the liquid crystal and normal liquid phases of para-azoxyanisole and para-azoxyphenetole have been measured at a frequency of 15 300 Mc. A static magnetic field applied to the sample is capable of producing large changes in the observed values of the complex dielectric constant. The extent of this effect depends on the orientation and intensity of the static magnetic field and on the temperature.
Fluorescence and Thermoluminescence of Ice
Authors: Grossweiner Leonard I. , Matheson Max S.
Published: April 2005

Experiments on the x-ray excited fluorescence and thermoluminescence of pure ice and ice containing certain additives are described. The luminescent spectra are explained by a model in which the primary process is the radiative capture of electrons by impurity cations. The first-order buildup of fluorescence and temperature dependence of luminous intensity are interpreted by the breakdown and formation of the dipole sheath surrounding the dissolved ions. From this viewpoint, the activation energy for rotation of a sheath ice dipole from the normal lattice position to the aligned configuration is calculated to be 0.07 ev. The principal trap depth determined from the glow curve is 0.32 ev, a correction for the luminous efficiency being incorporated in the calculation; the corresponding s value is 2×108 sec—1. Possible mechanisms for the first-order thermoluminescence are given.
Optic and acoustic modes measured in a cobalt/Permalloy exchange spring bilayer using inductive magnetometry
Authors: Crew D. C. , Kennewell K. J. , Lwin M. J. , Woodward R. C. , Prasad S. , Stamps R. L.
Published: April 2005

The resonance frequencies of a cobalt (100nm)/Permalloy (50nm) exchange spring bilayer have been measured using a pulsed inductive microwave magnetometer. Two field dependent frequencies are detected which vary in relative intensity as a function of applied magnetic field. The two frequencies are identified as the optic and acoustic spin wave modes of coupled ferromagnetic films. Only over a limited range of fields are both frequencies observable. In low fields the acoustic mode is dominant, and in higher fields the optic mode gives the largest intensity. These intensity variations are explained in terms of off-resonant driving of cobalt modes by the uniform mode in Permalloy and a field dependent interfacial pinning.
Scanning photoelectron microscopy study of Ge1−xTx (T=Cr, Fe) diluted ferromagnetic semiconductor single crystals
Authors: Kim G. , Wi S. C. , Lee S. S. , Kang J.-S. , Choi S. Y. , Cho Sunglae , Han S. W. , Kim K. H. , Song H. J. , Shin H. J.
Published: April 2005

The chemical distributions and electronic structures of Ge1−xTx (T=Cr, Fe) diluted ferromagnetic semiconductors have been investigated by employing scanning photoelectron microscopy (SPEM), x-ray absorption spectroscopy (XAS), and photoemission spectroscopy (PES). The SPEM images and PES spectra of Ge1−xTx show the stripe-shaped regions due to the inhomogeneous concentration distributions of T ions. T2p XAS spectra indicate that T ions are mainly in the divalent states, and that they do not form pure metal clusters. This study reveals that Ge1−xTx crystals are chemically phase-separated, suggesting that the observed ferromagnetic ordering in Ge1−xTx arises from the magnetic properties of the T-rich Ge1−xTx phases.
Electronic and magnetic properties of SmFe1−xMnxO3 orthoferrites (x=0.1, 0.2, and 0.3)
Authors: Bouziane K. , Yousif A. , Abdel-Latif I. A. , Hricovini K. , Richter C.
Published: April 2005

The electronic and magnetic properties of the orthoferrites SmFe1−xMnxO3 (x=0.1, 0.2, and 0.3) have been investigated. The Mössbauer data show that the iron ions at different nonequivalent octahedral sites have essentially a trivalent state. Resonant inelastic x-ray emission measurements performed at Fe L3 edge and Mn L3 edge show clearly that Fe ions and Mn ions have the same electronic structure. The temperature-dependent magnetization shows that the Curie temperature of the weak-ferromagnetic SmFeO3 decreases as Fe ions are substituted by Mn ions. A spin-reorientation transition from weak-ferromagnetism to antiferromagnetism seems to take place at low temperature, and the corresponding transition temperature essentially decreases as the Mn content increases. The effect of the substitution of Fe ions by Mn ions in SmFeO3 on the double exchange and the critical temperatures is analyzed and discussed.
Photoemission spectroscopy and x-ray absorption spectroscopy study of delafossite AgTO2 (T=Fe,Co,Ni)
Authors: Lee S. S. , Kim J. H. , Wi S. C. , Kim G. , Kang J.-S. , Shin Y. J. , Han S. W. , Kim K. H. , Song H. J. , Shin H. J.
Published: April 2005

Electronic structures of delafossite AgTO2 (T=Fe,Co,Ni) have been investigated by using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). The T 2p XAS spectra of AgTO2 show that Fe and Co ions are in the trivalent states but that Ni ions are mainly in the divalent states, suggesting that the metallic nature of AgNiO2 arises from hole carriers due to divalent Ni ions. The valence-band PES study of AgTO2 (T=Co,Ni) reveals that T 3d states are located close to EF and that the Ni 3d band is more metalliclike than the Co 3d band.
Influence of a nonmagnetic dilution on the magnetic properties of a ZnxCd1−xCr2S4 system by means of microwave magnetic resonance
Authors: Mendoza G. A. , Guzmán O. , Ariza–Calderón H. , Prieto P.
Published: April 2005

The influence of a nonmagnetic dilution on the magnetic properties of a ZnxCd1−xCr2S4 system (0.1<x<0.9) has been studied by means of microwave magnetic resonance at temperatures between 10 and 293K. Experimental results for the peak-to-peak linewidth ΔHpp and the slope dΔHpp∕dT have been discussed by the reduction of the exchange interaction between magnetic ions through the Zn substitution. For 0.6<x<0.9 at T<50K, the electron-paramagnetic-resonance line tends toward the Lorentzian shape, indicating a medium with competing interactions with strong exchange coupling. For x<0.4 at T<50K the line shapes have a rich Gaussian line indicating an increment of the interaction cluster medium.
Scattering of spin current injected in Pd(001)
Authors: Foros J. , Woltersdorf G. , Heinrich B. , Brataas A.
Published: April 2005

We have studied spin pumping in Pd∕Fe(001) ultrathin crystalline films prepared on GaAs(001) by ferromagnetic resonance (FMR). FMR measurements show that the Pd(001) overlayers lead to an appreciable attenuation of the spin current, which was generated by the precessing magnetization of Fe. Pd overlayers thicker than about 10 nm act as perfect spin sinks. It is argued that the loss of spin coherence in Pd is caused by scattering with spin fluctuations.
Soft x-ray magnetic circular dichroism study of Cr tellurides
Authors: Yaji Koichiro , Kimura Akio , Koyama Michie , Hirai Chiyuki , Sato Hitoshi , Shimada Kenya , Tanaka Arata , Taniguchi Masaki
Published: April 2005

Ferromagnetic chromium tellurides Cr5Te6(δ=0.17) and Cr2Te3(δ=0.33) have been investigated by Cr 2p x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD). The observed XMCD spectra have been analyzed by means of a configuration-interaction cluster model calculation. From calculated results, we suggest that the doped holes created by the Cr deficiency exist mainly in the Te 5p orbital of Cr1−δTe.
Bénard–Marangoni convection in a differentially heated cylindrical cavity
Authors: Hoyas S. , Mancho A. M. , Herrero H. , Garnier N. , Chiffaudel A.
Published: April 2005

The work described in this paper concerns the study of a Bénard–Marangoni convection problem in a differentially heated cylindrical cavity. The study had two main aims; first to justify from a numerical point of view the transitions that have been reported in several experiments as the aspect ratio is varied and, second, to study both theoretically and experimentally the role of vertical and horizontal temperature differences in lateral heating convection. Initially, we analyzed the role of the aspect ratio in layers where a dynamic flow is imposed through a nonzero temperature gradient at the bottom. The basic solutions are linear or return flows depending on different parameters. Depending on the vertical temperature difference and other heat-related parameters, the problem bifurcates either to stationary or oscillatory structures. Competing solutions at codimension two bifurcation points were found: stationary radial rolls with different wavenumbers and radial rolls together with hydrothermal waves. For small aspect ratios it was found that the Biot number does not influence the bifurcations, whereas for large aspect ratios it does. In the second part we present experimental results obtained at larger aspect ratios and for stronger surface tension effects. The role of horizontal gradients to determine the type of bifurcation both in experiments and in numerics approaching experimental conditions are discussed along with the role of vertical temperature gradients in comparison with previous theoretical works. Good agreement was obtained in terms of patterns, bifurcation sequences, and thresholds between theory, where eigenfunctions are obtained by tuning two parameters in a linear stability analysis, and experiments, where patterns are due to a nonlinear secondary bifurcation sequence caused by increasing one of the parameters.
IR/UV and UV/UV double-resonance study of guaiacol and eugenol dimers
Authors: Longarte Asier , Redondo Carolina , Fernández José A. , Castaño Fernando
Published: April 2005

Guaiacol (2-methoxyphenol) and eugenol (4-allyl-2-methoxyphenol) molecules are biologically active phenol derivatives with an intramolecular −OH⋯OCH3 hydrogen bond (H bond). Pulsed supersonic expansions of mixtures of either of the two molecules with He yield weakly bound homodimers as well as other higher-order complexes. A number of complementary and powerful laser spectroscopic techniques, including UV-UV and IR-UV double resonances, have been employed to interrogate the species formed in the expansion in order to get information on their structures and spectroscopic properties. The interpretation of the spectra of eugenol dimer is complex and required a previous investigation on a similar but simpler molecule both to gain insight into the possible structures and support the conclusions. Guaiacol (2-methoxyphenol) has been used for that purpose. The combination of the broad laser study combined with ab initio calculations at the Becke 3 Lee–Yang–Parr/6-31+G(d) level has provided the isomer structures, the potential-energy wells, and shed light on the inter- and intramolecular interactions involved. Guaiacol homodimer has been shown to have a single isomer whereas eugenol dimer has at least two. The comparison between the computed geometries of the dimers, their respective energies, and the vibrational normal modes permits the identification of the spectra.
Comparative studies of the spectroscopy of CuCl2: DFT versus standard ab initio approaches
Authors: Ramírez-Solís A. , Poteau R. , Vela A. , Daudey J. P.
Published: April 2005

The XΠg2-Σg+2, XΠg2-Δg2, XΠg2-Σu+2, XΠg2-Πu2 transitions on CuCl2 have been studied using several exchange-correlation functionals from the various types of density functional theory (DFT) approaches like local density approximation (LDA), generalized gradient approximation (GGA), hybrid and meta-GGA. The results are compared with the experience and with those coming from the most sophisticated nondynamic and dynamic electronic correlation treatments using the same relativistic effective core potentials and especially developed basis sets to study the electronic structure of the five lowest states and the corresponding vertical and adiabatic transition energies. The calculated transition energies for three of the hybrid functionals (B3LYP, B97-2, and PBE0) are in very good agreement with the benchmark ab initio results and experimental figures. All of the other functionals largely overestimate the XΠg2-Σg+2 and XΠg2-Δg2 transition energies, many of them even placing the Δg2 ligand field state above the charge transfer Πu2 and Σu+2 states. The relative weight of the Hartree-Fock exchange in the definition of the functional used appears to play a key role in the accurate description of the ΛSΣ density defined by the orientation of the 3d hole (σ, π, or δ) on Cu in the field of both chlorine atoms, but no simple connection of this weight with the quality of the spectra has been found. Mulliken charges and spin densities are carefully analyzed; a possible link between the extent of spin density on the metal for the XΠg2 state and the performance of the various functionals was observed, suggesting that those that lead to the largest values (close to 0.65) are the ones that best reproduce these four transitions. Most functionals lead to a remarkably low ionicity for the three ligand field states even for the best performing functionals, compared to the complete active space (SCF) (21, 14) ab initio values. These findings show that not only large variational ab initio calculations can produce reliable spectroscopic results for extremely complex systems where delicate electronic correlation effects have to be carefully dealt with. However, those functionals that were recently shown to perform best for a series of molecular properties [J. Chem. Phys. 121 3405 (2004)] are not the ones that produce the best transition energies for this complex case.
Search for global minimum geometries for medium sized germanium clusters: Ge12–Ge20
Authors: Bulusu S. , Yoo S. , Zeng X. C.
Published: April 2005

We have performed an unbiased search for the global minimum geometries of small-to-medium sized germanium clusters Gen(12⩽n⩽18) as well as a biased search (using seeding method) for Gen(17⩽n⩽20). We employed the basin-hopping algorithm coupled with the plane-wave pseudopotential density functional calculations. For each size, we started the unbiased search with using several structurally very different initial clusters, or we started the biased search with three different seeds. Irrespective of the initial structures of clusters we found that the obtained lowest-energy clusters of the size n=12–16 and 18 are the same. Among them, the predicted global minima of Gen(12⩽n⩽16) are identical to those reported previously [Shvartsburg et al., Phys. Rev. Lett. 83, 167 (1999)]. For n=17–20, we have identified two or three nearly isoenergetic low-lying isomers (for each size) that compete for the global minimum. Nearly all the low-lying clusters in the size range of 12⩽n⩽20 contain the tri-caped trigonal prism motif and are all prolate in geometry, in agreement with the experiment.
Reactive Monte Carlo and grand-canonical Monte Carlo simulations of the propene metathesis reaction system
Authors: Hansen Niels , Jakobtorweihen Sven , Keil Frerich J.
Published: April 2005

The influence of silicalite-1 pores on the reaction equilibria and the selectivity of the propene metathesis reaction system in the temperature range between 300 and 600K and the pressure range from 0.5to7bars has been investigated with molecular simulations. The reactive Monte Carlo (RxMC) technique was applied for bulk-phase simulations in the isobaric-isothermal ensemble and for two phase systems in the Gibbs ensemble. Additionally, Monte Carlo simulations in the grand-canonical ensemble (GCMC) have been carried out with and without using the RxMC technique. The various simulation procedures were combined with the configurational-bias Monte Carlo approach. It was found that the GCMC simulations are superior to the Gibbs ensemble simulations for reactions where the bulk-phase equilibrium can be calculated in advance and does not have to be simulated simultaneously with the molecules inside the pore. The confined environment can increase the conversion significantly. A large change in selectivity between the bulk phase and the pore phase is observed. Pressure and temperature have strong influences on both conversion and selectivity. At low pressure and temperature both conversion and selectivity have the highest values. The effect of confinement decreases as the temperature increases.
Fluctuation-dissipation theorem density-functional theory
Authors: Furche Filipp , Van Voorhis Troy
Published: April 2005

Using the fluctuation-dissipation theorem (FDT) in the context of density-functional theory (DFT), one can derive an exact expression for the ground-state correlation energy in terms of the frequency-dependent density response function. When combined with time-dependent density-functional theory, a new class of density functionals results that use approximations to the exchange-correlation kernel fxc as input. This FDT-DFT scheme holds promise to solve two of the most distressing problems of conventional Kohn–Sham DFT: (i) It leads to correlation energy functionals compatible with exact exchange, and (ii) it naturally includes dispersion. The price is a moderately expensive O(N6) scaling of computational cost and a slower basis set convergence. These general features of FDT-DFT have all been recognized previously. In this paper, we present the first benchmark results for a set of molecules using FDT-DFT beyond the random-phase approximation (RPA)—that is, the first such results with fxc≠0. We show that kernels derived from the adiabatic local-density approximation and other semilocal functionals suffer from an “ultraviolet catastrophe,” producing a pair density that diverges at small interparticle distance. Nevertheless, dispersion interactions can be treated accurately if hybrid functionals are employed, as is demonstrated for He2 and HeNe. We outline constraints that future approximations to fxc should satisfy and discuss the prospects of FDT-DFT.
Origin of nonlocal interactions in adsorption of polar molecules on Si(001)-2×1
Authors: Wang Yun , Hwang Gyeong S.
Published: April 2005

Using density functional theory slab calculations, we have investigated (i) the origin of nonlocal interactions occurring in the adsorption of small polar molecules (H2O,NH3,CH3OH,CH3NH2) on the clean Si(001)-2×1 surface and (ii) the nonlocal effects on two-dimensional arrangement of adsorbates. Our results show the adsorption properties are significantly altered in the presence of adsorbates on an adjacent dimer along a row. We have identified that the coverage dependent behavior arises from a combination of (i) surface polarization change, (ii) adsorbate-induced charge delocalization, (iii) adsorbate-adsorbate repulsion, and (iv) hydrogen bonding. The nucleophilic-electrophilic molecular adsorption involves charge delocalization to neighboring dimers along a row, which in turn undermines molecular adsorption on the neighboring dimers. Nonlocal effects associated with polar interactions with neighboring dimers and adsorbates vary with adsorption system. While such polar interactions are unimportant in CH3OH adsorption, hydrogen bonding and adsorbate-adsorbate repulsion play an important role in determining the adsorption structures of H2O and NH3∕CH3NH2, respectively. In addition, the electrostatic attraction with the buckled-up Si atoms of adjacent dimers contributes to stabilization of H2O, NH3, and CH3NH2 adsorption. We also discuss kinetic effects on two-dimensional ordering of adsorbates, in conjunction with surface phase transition and adsorption-dissociation rates.
Multidimensional quantum trajectories: Applications of the derivative propagation method
Authors: Trahan Corey J. , Wyatt Robert E. , Poirier Bill
Published: April 2005

In a previous publication [J. Chem. Phys. 118, 9911 (2003)], the derivative propagation method (DPM) was introduced as a novel numerical scheme for solving the quantum hydrodynamic equations of motion (QHEM) and computing the time evolution of quantum mechanical wave packets. These equations are a set of coupled, nonlinear partial differential equations governing the time evolution of the real-valued functions C and S in the complex action, S¯=C(r,t)+iS(r,t)∕ℏ, where Ψ(r,t)=exp(S¯). Past numerical solutions to the QHEM were obtained via ensemble trajectory propagation, where the required first- and second-order spatial derivatives were evaluated using fitting techniques such as moving least squares. In the DPM, however, equations of motion are developed for the derivatives themselves, and a truncated set of these are integrated along quantum trajectories concurrently with the original QHEM equations for C and S. Using the DPM quantum effects can be included at various orders of approximation; no spatial fitting is involved; there is no basis set expansion; and single, uncoupled quantum trajectories can be propagated (in parallel) rather than in correlated ensembles. In this study, the DPM is extended from previous one-dimensional (1D) results to calculate transmission probabilities for 2D and 3D wave packet evolution on coupled Eckart barrier/harmonic oscillator surfaces. In the 2D problem, the DPM results are compared to standard numerical integration of the time-dependent Schrödinger equation. Also in this study, the practicality of implementing the DPM for systems with many more degrees of freedom is discussed.
Dielectric behavior and microstructure of (Bi1∕2Na1∕2)TiO3–(Bi1∕2K1∕2)TiO3–BaTiO3 lead-free piezoelectric ceramics
Authors: Wang X. X. , Choy S. H. , Tang X. G. , Chan H. L. W.
Published: April 2005

( 0.95 − x ) ( Bi 1 ∕ 2 Na 1 ∕ 2 ) TiO 3 – x ( Bi 1 ∕ 2 K 1 ∕ 2 ) TiO 3 – 0.05 BaTiO 3 lead-free piezoelectric ceramics (abbreviated as BNT–BKT–BT100x with x varying from 0 to 20 mol %) are prepared by a solid-state reaction process. Variation of the dielectric properties and microstructure of BNT–BKT–BT100x ceramics with BKT content is studied. The results indicate that the relative permittivity εr and loss tangent tanδ vary with the BKT amount. Scanning electron microscope observation also indicates that BKT in high amount affects the microstructure. X-ray diffraction analysis shows that the incorporated BKT diffuses into the BNT–BT lattice to form a solid solution during sintering.
Molecular-dynamics model of energetic fluorocarbon-ion bombardment on SiO2. II. CFx+ (x=1, 2, 3) ion etch characterization
Authors: Smirnov V. V. , Stengach A. V. , Gaynullin K. G. , Pavlovsky V. A. , Rauf S. , Stout P. , Ventzek P. L. G.
Published: April 2005

A molecular-dynamics-based model has been used to understand etching of SiO2, with and without a fluorocarbon-polymer layer, by energetic fluorocarbon (CFx+) ions. The test structures for computational experiments are prepared by starting with α-quartz ([001] orientation) and bombarding it with low-energy ions: Ar+ ion for amorphous and fluorocarbon ions for fluorocarbon-polymerized test structures. CF+, CF2+, and CF3+ ions with a range of energies and angles of impact are then bombarded on these test structures to characterize fluorocarbon-ion etching. Results show that aggregate Si and O etch yields increase with ion energy for all ions and all angles of impact. Etch yields, however, exhibit nonlinear dependence on angle of impact with a peak around 60°. This nonlinear behavior is attributed to the balance among the incident ion energy transfer fraction, depth of energy deposition, and cluster scattering direction during secondary scattering events. The Si etch yield increases going from CF+ to CF2+ and then decreases for CF3+. This etch yield dependence on the nature of ion is because the amount of F per ion increases but the energy per F atom decreases as one moves from CF+→CF2+→CF3+. Si and O etch yields decrease considerably without the presence of the fluorocarbon layer, emphasizing the importance of C and F in etching SiO2. Parent ion clusters are only observed in the etch effluent at low ion energy and near the grazing angle of incidence. Under other conditions, the incident ion fragments upon impact at the surface. If fragments of the incident ions make it out of the surface, they retain a substantial fraction of the incident ion energy and their angle of ejection from the surface is related to the ion angle of incidence. However, clusters that contain test structure atoms are ejected from the surface with only a few eV of energy, and their angular and energy distributions are relatively independent of incident ion characteristics.
Effect of material properties on low-energy electron transmission in thin chemical-vapor deposited diamond films
Authors: Yater J. E. , Shih A. , Butler J. E. , Pehrsson P. E.
Published: April 2005

Electron transmission spectroscopy is used to measure the electron escape depth in thin diamond films and to examine how the transport characteristics are affected by the crystal quality and B concentration. In the studies, energy distribution and yield measurements are taken from several films with thicknesses ranging from ∼1.5 to 4.2μm, and the data are characterized in both the conduction-band and grain-boundary transport regimes. In the conduction-band transport regime, the escape depth increases from 0.5 to 0.85 to 1.3μm as the film thickness increases from 1.5 to 2.0 to 2.5μm due to the higher crystal quality in the thicker diamond films. However, the escape depth is relatively insensitive to different B concentrations in films of similar thickness. On the other hand, the B concentration more strongly affects the transmission stability. At sufficiently high B concentration (⩾1020cm−3), the transmission yields and energy distributions are fairly insensitive to changes in Io. However, at lower B concentration, the measurements exhibit evidence of sample charging in spite of a reasonably long escape depth and high secondary (reflection) yields. In investigations of the grain-boundary transport regime, the transmission characteristics are found to be largely insensitive to the crystal quality or doping levels of the diamond films, with low yields and broad energy distributions observed in all of the measurements. Moreover, the yields and energy distributions vary little with beam energy or current, except for measurements from the thickest 4.2-μm film that exhibit irregular variation in intensity and energy.
Domain evolution in ferroelectric thin films during fatigue process
Authors: Liu J. S. , Zhang S. R. , Dai L. S. , Yuan Y.
Published: April 2005

A combination of piezoresponse force microscopy and x-ray diffraction sin2Ψ stress measurement have been used to investigate domain evolutions in (111)-oriented Pb(Zr0.55Ti0.45)O3 (PZT) and (115)-oriented SrBi2Ta2O9 (SBT) thin films with the repeated switching cycles (fatigue process). In the PZT film, it has been found that 90° a-c domain walls form at the beginning of fatigue process and increase in amount with accumulation of switching cycles, accompanied by increase of stress. However, domain pattern and stress in the SBT film seem to be unchanged as the fatigue process is going on. The increase in stress with the repeated switching cycles in the PZT film can be attributed to the lattice distortion caused by polarization rotation. On the contrary, polarization switching in SBT involves little lattice distortion and lets it in a nearly stress-free status during the fatigue process. The difference in domain evolution during the fatigue process suggests the important role of stress in fatigue behaviors of PZT and SBT films.
Mathematical analysis of the Mandelstam–Tamm time-energy uncertainty principle
Authors: Gray John E. , Vogt Andrew
Published: April 2005

In the Mandelstam–Tamm version of the time-energy uncertainty principle Δt denotes the infimum of time intervals that elapse before the change in the mean of any observable has the same magnitude as its standard deviation. We clarify this interpretation, and show that the infimum is achieved for certain observables and thus that this famous inequality is actually an equality.
Global existence to Boltzmann equation with external force in infinite vacuum
Authors: Duan Renjun , Yang Tong , Zhu Changjiang
Published: April 2005

In this paper, we give a condition on the bicharacteristic which guarantees the global existence of the mild solution to the Boltzmann equation with an external force for the hard-sphere model and potentials with angular cutoff in infinite vacuum. This generalizes the previous results to the case when the force can have arbitrary strength. The constructive condition on the bicharacteristic is used to obtain the pointwise estimates on the collision operator so that the global existence comes from the contraction mapping theorem.
Enhanced luminescent and electrical properties of hydrogen-plasma ZnO nanorods grown on wafer-scale flexible substrates
Authors: Lin Chin-Ching , Chen Hung-Pei , Liao Hung-Chou , Chen San-Yuan
Published: April 2005

Photoelectronic characteristics are performed in well-aligned hydrogen-plasma ZnO nanorods grown on 4 in. flexible organic substrates buffered with ZnO film. Enhancement of photoluminescence (PL) properties due to H2 plasma treatment by a factor of 60 times for relative intensity ratio (ultraviolet emission to deep level emission) has been observed. X-ray photoelectron spectroscopy analysis reveals that the enhanced PL property is attributed to both defect passivation and modification on the surface region of ZnO nanorods due to the absorption of hydrogen ions. However, the PL spectra of H2 plasma ZnO nanorods can be restored to the original state of ZnO nanorods by thermal annealing process. The current-voltage measurements suggest that the n-type ZnO nanorods with H2 plasma treatment present a higher conductivity of about 5–6 orders of magnitude than the nonplasma ZnO nanorods.
Carbon nanotubes-semiconductor networks for organic electronics: The pickup stick transistor
Authors: Bo X.-Z. , Lee C. Y. , Strano M. S. , Goldfinger M. , Nuckolls C. , Blanchet Graciela B.
Published: April 2005

We demonstrate an alternative path for achieving high transconductance organic transistors in spite of relatively large source to drain distances. The improvement of the electronic characteristic of such a scheme is equivalent to a 60-fold increase in mobility of the underlying organic semiconductor. The method is based on percolating networks, which we create from a dispersion of individual single-wall carbon nanotubes and narrow ropes within an organic semiconducting host. The majority of current paths between source and drain follow the metallic nanotubes but require a short, switchable semiconducting link to complete the circuit. With these nanotube-semiconducting composites we achieve effectively a 60× reduction in source to drain distance, which is equivalent to a 60-fold increase of the “effective” mobility of the starting semiconducting material with a minor decrease of the on/off current ratio. These field-induced percolating networks allow for the fabrication of high-transconductance transistors having relatively large source to drain distances that can be manufactured inexpensively by commercially available printing techniques.
Room-temperature resonant tunneling of electrons in carbon nanotube junction quantum wells
Authors: Biswas Sujit K. , Schowalter Leo J. , Jung Yung Joon , Vijayaraghavan Aravind , Ajayan Pulickel M. , Vajtai Robert
Published: April 2005

Resonant tunneling structures [M. Bockrath, W. Liang, D. Bozovic, J. H. Hafner, C. B. Lieber, M. Tinkham, and H. Park, Science 291, 283 (2001)], formed between the junction of two single walled nanotubes and the conductive atomic force microscopy tip contact were investigated using current sensing atomic force microscopy. Oscillations in the current voltage characteristics were measured at several positions of the investigated nanotube. The oscillatory behavior is shown to follow a simple quantum mechanical model, dependent on the energy separation in the quantum well formed within the two junctions. Our model shows that these observations seen over several hundreds of nanometers, are possible only if the scattering cross section at defects is small resulting in long phase coherence length, and if the effective mass of the carrier electrons is small. We have calculated the approximate mass of the conduction electrons to be 0.003me.
Mechanism of structural transformation in bismuth titanate
Authors: Mallick Sudhanshu , Bowman Keith J. , King Alexander H.
Published: April 2005

Sodium-doped bismuth titanate undergoes a transformation from Bi4Ti3O12 to Na0.5Bi4.5Ti4O15 on heating in air at temperatures exceeding 800 °C. This transformation proceeds through the intermediate Na0.5Bi8.5Ti7O27 structure which is an intergrowth phase of the two. High-resolution transmission electron microscopy was used to study this transformation. From the Moiré pattern that was obtained, the crystallographic orientation of the transformation front has been determined and a mechanism is proposed for this structural transformation.
Terahertz bound-to-continuum quantum-cascade lasers based on optical-phonon scattering extraction
Authors: Scalari Giacomo , Hoyler Nicolas , Giovannini Marcella , Faist Jérôme
Published: April 2005

A design for a terahertz quantum-cascade laser emitting at λ≃82μm is presented. A bound-to-continuum active region is coupled to an optical phonon extraction stage in order to improve the population inversion at high temperatures. Device performances in a double-metal waveguide reach 116K in pulsed operation and 53K in continuous wave. A detailed characterization of two waveguide configurations is also presented together with a comparison with previous bound-to-continuum design.
Leakage conduction behavior in electron-beam-cured nanoporous silicate films
Authors: Liu Po-Tsun , Tsai T. M. , Chang T. C.
Published: April 2005

This letter explores the application of electron-beam curing on nanoporous silicate films. The electrical conduction mechanism for the nanoporous silicate film cured by electron-beam radiation has been studied with metal-insulator-semiconductor capacitors. Electrical analyses over a varying temperature range from room temperature to 150°C provide evidence for space-charge-limited conduction in the electron-beam-cured thin film, while Schottky-emission-type leaky behavior is seen in the counterpart typically cured by a thermal furnace. A physical model consistent with electrical analyses is also proposed to deduce the origin of conduction behavior in the nanoporous silicate thin film.
Enhanced coercive field of cobalt film deposited on noodle-like porous silicon substrates
Authors: Sun Z. G. , Akinaga H.
Published: April 2005

Cobalt films were deposited on special porous silicon substrate composing of nanosized noodle-like channels with large length-to-axis ratio. Modulation effects due to the substrate on microstructure, domain structures, and magnetic properties of the Co films were studied. Magnetic images and hysteresis loops indicate that a large uniaxial in-plane shape anisotropy is induced in the magnetic cobalt films. Accordingly, the magnetization of the magnetic cobalt films lies in the film plane, along the noodle direction. The coercivity of the cobalt films is enhanced as compared to reference cobalt film that was grown on a flat silicon substrate.
Effects of annealing and temperature on acoustic dissipation in a micromechanical silicon oscillator
Authors: Haucke Hans , Liu Xiao , Vignola J. F. , Houston B. H. , Marcus M. H. , Baldwin J. W.
Published: April 2005

The temperature dependence (15–320K) of the acoustic dissipation was studied for some lower vibrational modes of a suspended silicon plate 1.5μm thick. Our oscillator was exposed to the laboratory environment prior to measurement, laser annealed while in a cryogenic vacuum, and remeasured. We find a dissipation peak at 160K, similar to results by others, and a second dissipation peak near 30K. Annealing reduced the dissipation at 160K by as much as a factor of 10, and gave quality factors as high as 1.4×106 at 470kHz and our lowest temperature. Our data support the idea that the 160K peak is related to adsorbates, and show this mechanism is important at room temperature. Post-anneal room-temperature dissipation appears to be limited by thermoelastic loss for certain modes.
Strain and anisotropy effects on spin waves in epitaxial La0.67Sr0.33MnO3 films
Authors: Talbayev D. , Zhao H. , Lüpke G. , Chen J. , Li Qi
Published: April 2005

Uniform magnetization precession in the ferromagnetic state of La0.67Sr0.33MnO3 films grown on different substrates is investigated in the 50–300K temperature range in magnetic fields of up to 1.3T. The precession was induced by an optical pump pulse and probed by a time-delayed pulse using magneto-optical Kerr effect. The measured field dependence of the precession frequency agrees well with calculation from the Landau-Lifshitz-Gilbert equation using different uniaxial anisotropies for different substrates. The field-dependent relaxation times of the exponentially decaying precession are in 100–1400ps range.
Experimental investigation of interface states and photovoltaic effects on the scanning capacitance microscopy measurement for p-n junction dopant profiling
Authors: Yang J. , Kopanski J. J. , Postula A. , Bialkowski M.
Published: April 2005

Controlled polishing procedures were used to produce both uniformly doped and p-n junction silicon samples with different interface state densities but identical oxide thicknesses. Using these samples, the effects of interface states on scanning capacitance microscopy (SCM) measurements could be singled out. SCM measurements on the junction samples were performed with and without illumination from the atomic force microscopy laser. Both the interface charges and the illumination were seen to affect the SCM signal near p-n junctions significantly. SCM p-n junction dopant profiling can be achieved by avoiding or correctly modeling these two factors in the experiment and in the simulation.
Temperature-dependent light intensity controlled optical switching in azobenzene polymers
Authors: Ivanov M. , Ilieva D. , Minchev G. , Petrova Ts. , Dragostinova V. , Todorov T. , Nikolova L.
Published: April 2005

We have investigated the dependence of the photoinduced birefringence in a low-molecular azopolymer on the exciting light intensity at elevated temperatures. We have found that in the temperature range 44–56 °C the stationary photobirefringence is higher for a low pump intensity(1 mW) than for a higher intensity (20 mW). At some fixed temperatures, the difference between the two values of the birefringence is as high as 0.07. By alternating the light intensity, the anisotropic phase shift in the polymer films can be switched repeatedly between two values differing by more than 130 deg. This can be used for light-intensity controlled optical switching.