bluuebunny/crossref_metadata_2025 huggingface dataset:
X-ray scattering patterns of model liquid crystals from computer simulation: Calculation and analysis
Authors: Bates Martin A. , Luckhurst Geoffrey R.
Published: March 2003

We have simulated the x-ray scattering patterns for the isotropic, nematic, smectic A, and smectic B phases formed by the Gay–Berne model mesogen GB(4.4,20.0,1,1). We have used these scattering patterns for two quite different purposes. First, they have been employed to confirm the previous identification of the phases formed by this Gay–Berne mesogen, building on the considerable knowledge base created from x-ray scattering studies of real liquid crystal phases. Second, we have analyzed the simulated scattering patterns to obtain quantitative structural information about the mesophases using several approximate routes. The structural parameters include the nearest neighbor separation, the layer spacing, and translational order parameters of the smectic phase and the singlet orientational distribution function of the nematic phase. Since these quantities have already been determined from previous simulations, we are able to provide a unique test of the approximations used to extract them from the scattering patterns. The success of this investigation has required the use of sensibly large systems and so we have sought to understand, in a quantitative manner, how the size and geometry of the system influences the scattering pattern.
Enhancement of ultraviolet upconversion in Yb3+ and Tm3+ codoped amorphous fluoride film prepared by pulsed laser deposition
Authors: Qin Guanshi , Qin Weiping , Wu Changfeng , Huang Shihua , Zhang Jisen , Lu Shaozhe , Zhao Dan , Liu Huangqing
Published: March 2003

Zn 0.3 Al 0.25 Pb 0.3 Li 0.098 Yb 0.1 Tm 0.002 F 2.354 amorphous fluoride film was prepared by pulsed laser deposition. Ultraviolet and blue up-conversion emissions were observed under infrared excitation at 978 nm. In comparison with the upconversion of the target, the ultraviolet emissions are enhanced greatly. The enhancement is attributed to a decrease of the Judd–Ofelt parameter Ω2 induced by pulsed laser deposition, which precludes the transition rate from 3F2 to 3F4 and enhances the energy transfer process and populates the 1D2 level: 3F2→3H6(Tm3+):3H4→1D2(Tm3+).
Writing optical waveguides in fused silica using 1 kHz femtosecond infrared pulses
Authors: Saliminia A. , Nguyen N. T. , Nadeau M.-C. , Petit S. , Chin S. L. , Vallée R.
Published: March 2003

We have investigated the writing of waveguides in bulk pure fused silica glass with femtosecond Ti:Sapphire laser at 1 kHz repetition rate. The photoinduced tracks were characterized in terms of writing geometry (parallel and perpendicular), pulse duration (45 fs, 140 fs, and 200 fs), pulse energy (1–10 μJ), and translation speed (5–150 μm/s) of the sample. Under specific writing conditions, uniform buried waveguides with circular cross section, core diameter of 3–4 μm, and refractive-index change as large as 5×10−3 between core and cladding were achieved.
Modeling of the compressible vapor flow induced in a keyhole during laser welding
Authors: Amara E. H. , Fabbro R. , Bendib A.
Published: March 2003

The compressible vapor flow inducing a keyhole in the deep penetration laser welding process is studied. Suitable boundary conditions are deduced by the mean of a ray-tracing procedure, and the finite volume method is used to solve the hydrodynamic equations governing the vapor flow. A steady-state solution gives the distribution of the vapor parameters, consisting of the density, the pressure, the temperature, the velocity, and the Mach number. The influence of the ambient pressure on the vapor flow is mainly investigated, and the occurrence of friction phenomena between the vapor and the keyhole walls resulting in a more uniform distribution of the pressure is observed. We note that the stabilizing effect of frictions is important at low ambient pressures.
Field dependent permittivity of composite materials containing ferromagnetic wires
Authors: Makhnovskiy D. P. , Panina L. V.
Published: March 2003

A type of a composite material is proposed, the microwave permittivity of which changes under the effect of a dc magnetic field applied to the whole composite sample. The composite consists of short ferromagnetic wires embedded into a dielectric matrix. A strong field dependence of the permittivity is seen in the vicinity of the antenna resonance, where the dispersion behavior can experience a transformation from a resonant spectrum to a relaxation one under the effect of the field. This permittivity behavior is due to a high sensitivity of the ac surface impedance of a ferromagnetic wire to a magnetic field, known as the magnetoimpedance (MI) effect. If the resonance-like dispersion behavior is realized, the real part of the effective permittivity can be made negative past the resonance for wire inclusion concentrations well below the percolation threshold. Applying a magnetic field, the negative peak continuously decreases as the dispersion tends to become of a relaxation type. The effective permittivity is analyzed within a one-particle approximation, by considering a wire piece as an independent scatterer and solving the scattering problem with the impedance boundary condition. A magnetic field is assumed to be applied in parallel to the wire. A new integrodifferential equation for the current distribution in a wire is obtained, which is valid for the surface impedance matrix of a general form. This work demonstrates the possibility of using the MI effect to design field-controlled composites and band-gap structures.
Dielectric relaxation of shallow donor in polycrystalline Mn-doped ZnO
Authors: Han Jiaping , Senos A. M. R. , Mantas P. Q. , Cao Wenwu
Published: March 2003

The dielectric properties of Mn-doped ZnO ceramics with electrically active grain boundaries at low temperatures of 10–70 K were investigated by admittance spectroscopy. It was observed that the dielectric relaxation of the main shallow donors, zinc interstitial, in these samples occurred in the temperature range of 20–50 K. Based on theoretical analysis, an expression for the admittance of an electrically active grain boundary at low temperatures was obtained. It was concluded that the relaxation of the shallow donors is Debye type with a thermally activated relaxation time and is related to the ionization process of these donors. The activation energy for the relaxation, i.e., the ionization energy of the shallow donors, was determined to be about 0.04 eV, the capture cross section of the shallow donors to be around 10−12 cm2, and the relaxation time at room temperature to be about 10−13 s.
Micropatterning of small molecular weight organic semiconductor thin films using organic vapor phase deposition
Authors: Shtein Max , Peumans Peter , Benziger Jay B. , Forrest Stephen R.
Published: March 2003

Using both analytical and experimental methods, we show that micron scale patterned growth of small molecular weight organic semiconductor thin films can be achieved by the recently demonstrated process of organic vapor phase deposition (OVPD). In contrast to the conventional process of vacuum thermal evaporation, the background gas pressure during OVPD is typically 0.1–10 Torr, resulting in a molecular mean free path (mfp) of from 100 to 1 μm, respectively. Monte Carlo simulations of film growth through apertures at these gas densities indicate that when the mfp is on the order of the mask-to-substrate separation, deposit edges can become diffuse. The simulations and deposition experiments discussed here indicate that the deposited feature shape is controlled by the mfp, the aperture geometry, and the mask-to-substrate separation. Carefully selected process conditions and mask geometries can result in features as small as 1 μm. Furthermore, based on continuum and stochastic models of molecular transport in confined geometries, we propose the in situ direct patterning growth technique of organic vapor jet printing. The high pattern definition obtained by OVPD makes this process attractive for the growth of a wide range of structures employed in modern organic electronic devices.
Analysis of 1/f noise current sources in InP/InGaAs heterojunction bipolar transistors
Authors: G.-Jarrix S. , Penarier A. , Pascal F. , Delseny C. , Chay C. , Blayac S.
Published: March 2003

The 1/f noise of double InP/InGaAs heterojunction bipolar transistors is measured and analyzed. Standard mesa transistors, transistors with an air-bridge-connected base and hexagonal shaped transistors conceived for digital circuits are studied. These differences in the technology will have an influence on the origin of the noise sources. Regarding noise analysis, the base and collector internal current noise sources ib and ic are assumed to be correlated for all devices. This is highlighted by the voltage noise correlation function between the input and the output of the devices presenting an unusual behavior versus bias and geometry. The collector current noise source is divided into a correlated and uncorrelated part with base current. These parts give rise to spectral densities Sc and Snc. They are shown to have distinct origins for the different types of transistor geometry.
Avalanche speed in thin avalanche photodiodes
Authors: Ong D. S. , Rees G. J. , David J. P. R.
Published: March 2003

The duration of the avalanche multiplication process in thin GaAs avalanche photodiodes is investigated using a full band Monte Carlo (FBMC) model. The results are compared with those of a simple random path length (RPL) model which makes the conventional assumptions of a displaced exponential for the ionization path length probability distribution function and that carriers always travel at their saturated drift velocities. We find that the avalanche duration calculated by the RPL model is almost twice of that predicted by the FBMC model, although the constant drift velocities used in the former model are estimated using the latter. The faster response predicted by FBMC model arises partly from the reduced dead space but mainly from the velocity overshoot of ionizing carriers. While the feedback multiplication processes forced by the effects of dead space extend the avalanche duration in short structures, the effects of velocity overshoot in the realistic model more than compensate, significantly improving multiplication bandwidth.
Influence of strain, surface diffusion and Ostwald ripening on the evolution of nanostructures for erbium on Si(001)
Authors: Fitting Lena , Zeman M. C. , Yang W.-C. , Nemanich R. J.
Published: March 2003

This study explores the evolution of nanoscale islands and wire structures during deposition and surface ripening. Ultraviolet photoelectron emission microscopy has been employed to study the real time growth process of individual erbium silicide nanostructures on Si(001) surfaces at temperatures up to 1050 °C. During the initial island formation process compact islands form and some undergo a shape transition to elongated islands oriented along the 〈110〉 directions of the Si substrate. The initial island formation is driven by the surface and interface energies of the silicide/Si structure. The widths of the growing islands remain essentially constant while the lengths increase. The observed elongated islands are ∼150 nm wide, which is larger than the width of prior reported erbium silicide nanowire structures. We propose that the ∼150 nm elongated islands are partially relaxed, possibly through the formation of misfit dislocations. The results indicate a temperature regime where island growth is mainly governed by surface diffusion of the deposited Er adatoms and a higher temperature regime where Ostwald ripening contributes to the island morphology.
Dissociative recombination of NO+: Dynamics of the X 1Σ+ and a 3Σ+ electronic states
Authors: Hellberg Fredrik , Rosén Stefan , Thomas Richard , Neau Anita , Larsson Mats , Petrignani Annemieke , van der Zande Wim J.
Published: March 2003

We have studied the dissociation dynamics of NO+ ions in their ground, X 1Σ+, and first excited metastable, a 3Σ+ states, induced by the capture of electrons of variable collision energy in the dissociative recombination (DR) process. The branching over the different dissociation channels has been measured in a merged-beam experiment on the heavy-ion storage ring, CRYRING. In accord with previous observations, NO+ (X 1Σ+,v=0) ions dissociate dominantly to the N(2D)+O(3P) product limit at 0 and 1.2 eV collision energies. In contrast to earlier reports, the spin-forbidden N(4S)+O(1D) dissociation limit contributes 0(±2)% at 0 eV. At 5.6 eV a new channel coupled to the production of ground-state atoms becomes more important, but no increase in the production of ground-state product atoms was observed. All observed branching fractions compare very favorably with predictions from a simple statistical model, which is based on the multiplicity of each dissociation limit in combination with spin conservation during the dissociation and the initial electron capture. We also report the distribution of fragment pairs from the DR reaction involving the metastable a 3Σ+ state. This state is found to dissociate to nearly all of the energetically allowed product pairs. The lifetime of the a 3Σ+ state is found to be 730(±50) ms, in agreement with earlier, sometimes indirect, observations. The experimental observations have been complemented with ab initio calculations on the different radiative decay processes both for the X 1Σ+ and the a 3Σ+ states. It is found that vibrational relaxation via infrared radiation is faster for NO+ (a 3Σ+,v>0) ions than the electronic decay of these metastable-state ions to the ground state.
Photofragment angular momentum polarization from dissociation of hydrogen peroxide near 355 nm
Authors: Alexander Andrew J.
Published: March 2003

Hydrogen peroxide (H2O2) was photodissociated at around 355 nm, using both linearly and circularly polarized light. OH(2Π) rotational distributions, spin–orbit branching ratios, lambda doublet populations, and angular momentum J polarization in the laboratory and molecule frames were measured by polarized laser probing of the products using laser induced fluorescence. The effects of dynamical torsion and parent molecule bending vibrations on product rotational alignment is discussed, and evidence supporting preferential dissociation of ground-state molecules far from the equilibrium configuration is presented. Possible mechanisms for orientation of product angular momentum in the molecule frame are discussed, and evidence is presented that interference occurs between OH molecules dissociating via the Ã and B̃ electronic states of H2O2.
Photodissociation of nitrobenzene at 266 nm: Experimental and theoretical approach
Authors: Li Ya-Min , Sun Ju-Long , Yin Hong-Ming , Han Ke-Li , He Guo-Zhong
Published: March 2003

The 266 nm photodissociation of gaseous nitrobenzene has been studied by monitoring the NO X 2Π product using the single-photon laser-induced fluorescence technique. The rotational population and internal energy distributions of fragment NO product were determined. In the experiment, we only observed the rotational state distributions of the NO fragment for v″=0. The rotational state distributions of the nascent NO photofragment can be characterized by a Boltzmann temperature of 3300±300 K. Ab initio calculations were performed to characterize the transition state and to determine the barrier height for the rearrangement of nitrobenzene to phenylnitrite. A clear photodissociation picture has been proposed theoretically.
Alignment of CS2 in intense nanosecond laser fields probed by pulsed gas electron diffraction
Authors: Hoshina Kennosuke , Yamanouchi Kaoru , Ohshima Takashi , Ose Yoichi , Todokoro Hideo
Published: March 2003

A pulsed gas electron diffraction apparatus was developed and applied to investigate an alignment process of molecules in intense laser fields. A two-dimensional (2D) electron diffraction pattern of jet-cooled CS2 in intense nanosecond laser fields (1064 nm, ∼0.64 TW/cm2, 10 ns) was measured using short-pulsed 25 keV electron beam packets (∼7 ns) generated by irradiating a tantalum photocathode with the 4th harmonics of pulsed YAG laser light. The observed anisotropic 2D diffraction pattern was analyzed quantitatively by taking into account the spatio-temporal distributions of the laser pulses, the electron beam packets, and the molecular beam through a numerical simulation of the observed diffraction pattern. The anisotropy of the spatial distribution of molecular axes of CS2 in the laser polarization direction is accounted for by the effect of the intense laser fields. Considering the spatio-temporal averaging effect, the temporal pulse width of an electron packet required for real-time probing of the alignment process of molecules in intense nanosecond laser fields is discussed. A numerical simulation of temporal and spatial profiles of an electron packet is also performed to examine conditions for generating sub-picosecond ultrashort electron pulse for real-time probing of ultrafast molecular dynamics by the pulsed gas electron diffraction method.
Calculating free energies for diffusion in tight-fitting zeolite-guest systems: Local normal-mode Monte Carlo
Authors: Turaga Srinivas C. , Auerbach Scott M.
Published: March 2003

We present an efficient Monte Carlo algorithm for simulating diffusion in tight-fitting host–guest systems, based on using zeolite normal modes. Computational efficiency is gained by sampling framework distortions using normal-mode coordinates, and by exploiting the fact that zeolite distortion energies are well approximated by harmonic estimates. Additional savings are obtained by performing local normal-mode analysis, i.e., only including the motions of zeolite atoms close to the jumping molecule, hence focusing the calculation on zeolite distortions relevant to guest diffusion. We performed normal-mode analysis on various silicalite structures to demonstrate the accuracy of the harmonic approximation. We computed free energy surfaces for benzene in silicalite, finding excellent agreement with previous theoretical studies. Our method is found to be orders-of-magnitude faster than comparable Monte Carlo calculations that use conventional forcefields to quantify zeolite distortion energies. For tight-fitting guests, the efficiency of our new method allows flexible-lattice simulations to converge in less CPU time than that required for fixed-lattice simulations, because of the increased likelihood of jumping through a flexible lattice.
Transport in molecular wire with long-range Coulomb interactions: A mean-field approach
Authors: Pati Swapan K.
Published: March 2003

We consider a one-dimensional molecular wire described by long-range Coulomb interactions, attached to two metal electrodes on either side of it. The long-range interactions are approximated using Mataga–Nishimoto potential, and we solve the Hamiltonian in the mean-field limit. Using scattering formalism in the Coulomb blockade regime, we obtain the current–voltage characteristics for this system. We find that the long-range interaction neutralizes the voltage induced localization of the electronic states at off-resonant voltages. Furthermore, the current increases for decreasing interaction strengths for both positive and negative charging. Our mean-field model does not preserve particle–hole symmetry, and as such we find that the hole transport is preferred over the electron transport in this system.
The effect of common gases on nucleation of metal islands: The role of oxygen in Ag(100) homoepitaxy
Authors: Layson A. R. , Evans J. W. , Fournée V. , Thiel P. A.
Published: March 2003

Preexposure to molecular oxygen gas, O2,g, can have a strong effect on the nucleation and growth of Ag islands on Ag(100) at 250 K. At this temperature, molecular oxygen dissociates efficiently at kink sites on steps. Subsequent deposition of Ag produces a far lower density of Agad islands than without oxygen. There is an associated increase in the Ag flux-scaling exponent, from 0.28 for the oxygen-free surface to 0.9 for the preexposed surface. Two-step deposition experiments show that species containing atomic oxygen diffuse freely across terraces and steps at this temperature and on the time scale of deposition. We hypothesize that the nucleating species contains both Ag and O, and that nucleation of islands is highly reversible (critical size i≫1). The diffusion of small islands, if it occurs, is not sufficient to explain the data.
Nanoadhesion of elastic bodies: Roughness and temperature effects
Authors: Zilberman S. , Persson B. N. J.
Published: March 2003

We present a simple model which illustrates the nature of the contact between an elastic solid and a hard surface with cosine-corrugation profile. In the continuum limit, the contact mechanics depends only on two dimensionless parameters, namely the ratio between the height and wavelength of the substrate corrugation, and the ratio between a surface energy and an elastic energy. The theory shows that the complete contact state is always a local energy minima (in the zero temperature limit), but for large enough surface roughness the global minima correspond to a partial contact state. We show that at nonzero temperature, the contribution to the free energy from the vibrational entropy is very important, and favors the detached state. Computer simulations results are also presented where we study more complicated roughness geometries and the influence of temperature on the adhesion. Simulation results agrees well with the analytical predictions.
Phase control of chromium oxide in selective microregions by laser annealing
Authors: Yu T. , Shen Z. X. , He J. , Sun W. X. , Tang S. H. , Lin J. Y.
Published: March 2003

Laser annealing has been employed to decompose half-metallic CrO2 into insulating Cr2O3 in air ambient. While both Cr2O3 and CrO2 are present, the relative fraction of each phase was controlled by changing the duration and power of laser irradiation. Glancing angle x-ray diffraction and micro-Raman scattering were used to characterize samples before and after laser annealing. The laser-induced decomposition of CrO2 into Cr2O3 that leads to a threefold enhancement of the low-field magnetoresistance and the realization of phase control of the CrO2/Cr2O3 system in selective microregions by laser irradiation implies: (i) optical lithography can be used as a potential method to directly control the magnetotransport properties which are strongly depended on the interface tunneling barrier and (ii) The CrO2 polycrytalline phase could be much more attractive as a high-density magnetic storage medium.
High dielectric permittivity in AFe1/2B1/2O3 nonferroelectric perovskite ceramics (A=Ba, Sr, Ca; B=Nb, Ta, Sb)
Authors: Raevski I. P. , Prosandeev S. A. , Bogatin A. S. , Malitskaya M. A. , Jastrabik L.
Published: March 2003

AFe 1/2 B 1/2 O 3 (A=Ba , Sr, Ca; B=Nb, Ta, Sb) ceramics were synthesized and temperature dependences of dielectric permittivity were measured at different frequencies. The experimental data obtained show very high values of dielectric permittivity in a wide temperature interval that is inherent to so-called high-k materials. Analyses of these data establish a Maxwell–Wagner mechanism as a main source for the phenomenon observed.
Effects of substrate annealing and post-crystallization thermal treatments on the functional properties of preferentially oriented (Pb,Ca)TiO3 thin films
Authors: Poyato R. , Calzada M. L. , Pardo L.
Published: March 2003

〈111〉 and 〈001〉,〈100〉 preferentially oriented calcium-modified lead titanate thin films have been studied. Effects of the substrate annealing and post-crystallization thermal treatment of the films on the texture and ferro-, piezo-, and pyroelectric properties have been analyzed and discussed. The annealing of the substrate has effect on the texture of 〈001〉,〈100〉-oriented films and, as a consequence, produces an increase in the net polarization in the perpendicular direction to the plane of the film and in the pyroelectric coefficient. The annealing of the substrate does not affect the texture of 〈111〉-oriented films, but the electrical properties at the electrode–substrate interface and, as a consequence, gives place to high internal electric fields leading to the highest spontaneous piezo- (5 pm V−1) and pyroelectric coefficients (25.0×10−9 C cm−2 K−1). The asymmetry that characterizes the functional properties of 〈111〉-oriented films under poling with negative or positive polarity is also consequence of such a high internal field. This is accompanied with a remarkable field stability of the piezoelectric d33 coefficient when poling in the sense of the spontaneous polarization (d33∼45 pm V−1 under ±200 kV cm−1). The application of post-crystallization thermal treatment results in an enhancement of the ferroelectric and pyroelectric properties in all the films, by liberation of domains that were clamped by charged defects. Both the substrate and post-crystallization film annealing treatments lead to reach the highest values of remanent polarization (43 μC cm−2), pyroelectric coefficient (42.0×10−9 C cm−2 K−1), and figure of merit [13.3×10−6 (N m−2)−1/2] reported to date for sol-gel PTC films.
Magnetic properties of Co nanocolumns fabricated by oblique-angle deposition
Authors: Tang F. , Liu D.-L. , Ye D.-X. , Zhao Y.-P. , Lu T.-M. , Wang G.-C. , Vijayaraghavan A.
Published: March 2003

The magnetic properties of columnar Co films on SiO2 substrates fabricated by oblique-angle incident thermal evaporation at room temperature were systematically examined by multiple techniques, including magnetic force microscopy (MFM), magneto-optical Kerr effect (MOKE), and scanning electron microscopy (SEM). Films with thickness ranging from 50 to 500 nm were deposited at the incident angles θ (with respect to substrate normal) from 0° to 85°. For films with thickness of ∼500 nm, the SEM shows the column tilt angle β increases as the θ angle increases and β<θ. The MFM images show that for 55°<θ<75°, stripe domains are formed and are nearly parallel to the direction of incident vapor beam. The hysteresis loops obtained from MOKE show that along the direction perpendicular to the incident vapor beam the coercivity Hc stays almost constant for all θ angles and the squareness decreases as the θ increases. This is in contrast to the increase of Hc and the increase of squareness in the direction parallel to the incident vapor beam for θ>60° . The result implies that for θ>60°, the axis parallel to the incident beam behaves more like the in-plane easy axis. These magnetic anisotropies are correlated to the angular-dependent columnar structure of Co films.
Nucleation, growth, and graphitization of diamond nanocrystals during chlorination of carbides
Authors: Welz Sascha , Gogotsi Yury , McNallan Michael J.
Published: March 2003

Synthesis of nano- and microcrystalline sp3-bonded carbon (diamond) with cubic and hexagonal structure by extraction of silicon from silicon carbide in chlorine-containing gases has been reported recently. This process is attractive because it can produce diamond at ambient pressure and temperatures below 1000 °C. No plasma or other high-energy activation is required, thus providing an opportunity for large-scale synthesis. However, the mechanism of diamond formation has not been previously analyzed. This work reports on the formation mechanisms of diamond as well as the transformation of diamond to graphite and onionlike carbon upon heating. Study of SiC/carbon interfaces showed that direct epitaxial growth of diamond on SiC is possible, in agreement with previous molecular-dynamics simulation. However, random nucleation of diamond from amorphous sp3-bonded carbon produced as the result of extraction of Si from SiC has also been demonstrated. It has been shown that the presence of hydrogen in the environment is not required for diamond synthesis. However, hydrogen can stabilize the nanocrystals and lead to the growth of thick diamond layers. If no hydrogen is added, diamond nanocrystals transform to graphite, forming carbon onions and other curved graphitic nanostructures.
Texture and strain analysis of the ferroelastic behavior of Pb(Zr,Ti)O3 by in situ neutron diffraction
Authors: Rogan Robert C. , Üstündag Ersan , Clausen Bjørn , Daymond Mark R.
Published: March 2003

In situ uniaxial compression experiments on Pb(Zr,Ti)O3 or PZT-based polycrystalline electroceramics were conducted using time-of-flight neutron diffraction. Elastic lattice strain and texture evolution were observed in PZT’s near the edge of the morphotropic phase boundary (with tetragonal and rhombohedral phases present). Multiphase Rietveld analysis yielded anisotropic lattice strain evolution curves in directions parallel and perpendicular to the loading axis for both phases. A quantitative analysis of the domain switching under applied stress was possible through application of a March–Dollase model for texture.
Lattice models of ionic systems with charge asymmetry
Authors: Artyomov Maxim N. , Kobelev Vladimir , Kolomeisky Anatoly B.
Published: March 2003

The thermodynamics of a charge-asymmetric lattice gas of positive ions carrying charge q and negative ions with charge −zq is investigated using Debye–Hückel theory. Explicit analytic and numerical calculations, which take into account the formation of neutral and charged clusters and cluster solvation by the residual ions, are performed for z=2, 3, and 4. As charge asymmetry increases, the predicted critical point shifts to lower temperatures and higher densities. This trend agrees well with the results from recent Monte Carlo simulations for continuum charge-asymmetric hard-sphere ionic fluids and with the corresponding predictions from continuum Debye–Hückel theory.
Density functional theory for the elastic moduli of a model polymeric solid
Authors: Sushko Nazar , van der Schoot Paul , Michels M. A. J.
Published: March 2003

We apply a recently developed density functional theory for freely hinged, hard polymeric chains to calculate the elastic moduli of an idealized polymeric solid lacking long-range bond order. We find that for such a model packing effects dominate the elastic behavior of the polymeric solid in a similar way as is the case in the hard-sphere crystal, which we reexamine. Our calculations show that the elastic stiffness of the model polymeric solid is essentially determined by how far one is removed from its melting point. The main role of the chain connectivity is to destabilize the solid relative to the equivalent solid of hard monomers. Comparison of our results with experimental data on semicrystalline polymers shows order-of-magnitude agreement.
Performance of Ornstein–Uhlenbeck diffusion quantum Monte Carlo for first-row diatomic dissociation energies and dipole moments
Authors: Lu Shih-I
Published: March 2003

In this paper, calculation of nine first-row diatomic dissociation energies (De) and six first-row heterodiatomic equilibrium dipole moments (μe) highlights the strength of Ornstein–Uhlenbeck diffusion quantum Monte Carlo with the fixed-node approximation. By combining explicitly correlated wave functions based on spherical Gaussian-type orbitals and geminals obtained by an optimization of energy functional, it was possible to achieve satisfied agreement with experiment in most cases. Calculations on the molecules LiH, Li2, BH, BF, HF, LiF, CO, N2, and F2 are presented. This work gives an accuracy that is comparable to or better than previous theoretical estimates. We also compare the performance of three different approximations in estimating dipole moments of molecules.
Comparison of two ways to decompose intermolecular interactions for hydrogen-bonded dimer systems
Authors: Langlet J. , Caillet J. , Bergès J. , Reinhardt P.
Published: March 2003

In this work we test two ab initio methodologies which allow the decomposition of the total intermolecular interaction energy into physically meaningful contributions, namely, the symmetry adapted perturbation theory (SAPT) and the use of localized orbitals within a Møller–Plesset perturbation scheme. The accuracy of the two different methods is compared to supermolecular results, within MP2 and coupled-cluster theory within single and double excitations, with perturbative estimates of the amplitudes of triple excitations [CCSD(T)]. Some relations between the different approaches are conjectured from theoretical considerations, and are confirmed by numerical results. The corresponding calculations have been performed for three model dimers: two NH3⋯H2O dimers, with NH3 acting once as a proton acceptor and once as a proton donor, and the NH4+⋯H2O considered as a prototype of the ion–molecule interaction. We may conclude that third-order terms in SAPT help significantly to reproduce the Hartree–Fock induction and the relaxed, total dispersion in the LMP2 decomposition.
Extension of complete basis set model chemistries to molecules containing third row atoms Ga–Kr
Authors: Ramakrishna Vinutha , Duke Brian J.
Published: March 2003

The complete basis set methods, CBS-4, CBS-q, CBS-Q and CBS-QB3, have been extended to molecules containing the third row elements Ga–Kr and their performance is assessed on 40 energies (atomization energies, ionization energies, electron affinities, and proton affinities) from the third row G2 test set. The mean absolute deviation from experiment for the highly accurate methods, CBS-Q and CBS-QB3, are 1.15 kcal/mol and 1.12 kcal/mol, respectively, both lower than for the G2 method, but not as low as the G3 method. The mean absolute deviation from experiment for the more affordable methods, CBS-4 and CBS-q, are 2.23 kcal/mol and 1.81 kcal/mol, respectively. The set of methods, CBS-4(d), CBS-q(d), CBS-Q(d), and CBS-QB3(d), with the d orbitals included in the correlation space is also investigated, giving results in poorer agreement with experiment. Problems with the CBS extrapolation step when the d orbitals are included in the correlation space are also discussed.
Curvy steps for density matrix-based energy minimization: Application to large-scale self-consistent-field calculations
Authors: Shao Yihan , Saravanan Chandra , Head-Gordon Martin , White Christopher A.
Published: March 2003

A unitary transformation approach to avoiding the diagonalization step in density functional theory calculations is developed into an algorithm that can scale linearly with molecular size. For target accuracy of 10−5 in the rms rotation gradient, the average number of matrix multiples required per self-consistent field iteration is between about 35 (STO-3G) and 50 (6-31G**). This compares favorably to the existing canonical purification method. Crossovers with direct diagonalization are demonstrated for 1D alkane chains and 2D water clusters.
An ab initio model of electron transport in hematite (α-Fe2O3) basal planes
Authors: Rosso Kevin M. , Smith Dayle M. A. , Dupuis Michel
Published: March 2003

Transport of conduction electrons through basal planes of the hematite lattice was modeled as a valence alternation of iron cations using ab initio molecular orbital calculations and electron transfer theory. A cluster approach was successfully implemented to compute electron-transfer rate-controlling quantities such as the reorganization energy and electronic coupling matrix element. Localization of a conduction electron at an iron lattice site is accompanied by large iron–oxygen bond length increases that give rise to a large internal component of the reorganization energy (1.03 eV). The internal reorganization energy calculated directly is shown to differ from Nelsen’s four-point method due to the short-range covalent bridge interaction between the Fe–Fe electron transfer pair in the hematite structure. The external reorganization energy arising from modification of the lattice polarization surrounding the localization site is predicted to contribute significantly to the total reorganization energy. The interaction between the reactants and products electronic states near the crossing-point configuration is 0.20 eV and is consistent with an adiabatic electron-transfer mechanism. Electron transfer is predicted to possess a small positive activation energy (0.11 eV) that is in excellent agreement with values deduced from conductivity measurements. Measured electron mobility can be explained in terms of nearest-neighbor electron hops without significant contribution from iron atoms further away. Comparison of the predicted maximum polaron binding energy with the predicted half bandwidth indicates compliance with the small-polaron condition. Therefore the localized electron treatment is appropriate to describe electron transport in this system.
Crystallographic characterization of epitaxial Pb(Zr,Ti)O3 films with different Zr/Ti ratio grown by radio-frequency-magnetron sputtering
Authors: Kanno Isaku , Kotera Hidetoshi , Wasa Kiyotaka , Matsunaga Toshiyuki , Kamada Takeshi , Takayama Ryouichi
Published: March 2003

Crystallographic structure of as-grown epitaxial Pb(Zr,Ti)O3 (PZT) films was investigated with regard to the Zr/Ti ratio and crystalline orientation. PZT films with (001) and (111) orientation were epitaxially grown on (100) and (111)SrTiO3 substrates respectively using radio-frequency (rf) sputtering. Four circle x-ray diffraction measurements revealed that the crystallographic dependence on Zr/Ti composition in PZT films was much different from bulk PZT. In particular, (001)-oriented PZT films showed tetragonal structure even in the Zr/Ti composition of 70/30 where the bulk PZT ceramics are rhombohedral phase. In addition, although (001)-oriented PZT films with Zr/Ti ratio of 53/47 and 70/30 showed tetragonal structure, (111)-oriented PZT films with the same Zr/Ti ratio were identified as the rhombohedral structure. The cell volume of the PZT films with both orientations increased, suggesting the excess Pb atoms in the films due to the impinging energetic sputtered particles induces the anomalous crystalline structure of the PZT films. Dielectric properties of the PZT films exhibited stable value independent of Zr/Ti ratio and characteristic increase of dielectric constant near Zr/Ti=53/47 could not be observed. These results suggest that the internal stress due to the sputter deposition plays an important roll in the unique characteristics of crystallographic and electrical properties of the epitaxial PZT films.
Polarization behavior of ferroelectric multilayered composite structures
Authors: Or Y. T. , Wong C. K. , Ploss B. , Shin F. G.
Published: March 2003

We model the polarization behavior of ferroelectric multilayered composite structures including the double layer and the triple layer under the action of variable fields. The model takes into consideration the ceramic/polymer interfacial charge and the nonlinear hysteresis nature of the ferroelectric constituent materials to study the poling process of multilayered structures. The results obtained are compared with available experimental results on double-layer lead zirconate titanate/polyvinylidene fluoride-trifluoroethylene and triple-layered triglycine sulphate/polyvinylidene fluoride-trifluoroethylene/triglycine sulphate systems. In general the broad features of the experimental results were reproduced, and fairly good agreement between the predictions and the experimental results was found. The work also demonstrates that electrical conductivity in the ferroelectric materials is an important factor controlling the poling process.
Investigations on magnetic refrigeration: Application to RNi2 (R=Nd, Gd, Tb, Dy, Ho, and Er)
Authors: von Ranke P. J. , Grangeia Daniel F. , Caldas A. , de Oliveira N. A.
Published: March 2003

In this article we report the thermodynamic investigations on the Ericsson cycle with application on RNi2 (R=Nd, Gd, Tb, Dy, Ho, and Er) series. Besides the Zeeman and exchange interactions, these compounds present an important contribution from crystalline electrical field interaction. The Ericsson coefficient of performance and refrigerant capacity was investigated under the crystal field influence. An optimum molar composite of Er–Dy–TbNi2 was proposed to work as refrigerant material in the temperature interval from 7 to 22 K.
Crack formation in GaAs heteroepitaxial films on Si and SiGe virtual substrates
Authors: Yang V. K. , Groenert M. , Leitz C. W. , Pitera A. J. , Currie M. T. , Fitzgerald E. A.
Published: March 2003

We have determined the critical cracking thickness, or the thickness beyond which crack formation is favored, in GaAs films grown on Si and SiGe virtual substrates analytically and experimentally. The analytical model predicts a critical cracking thickness proportional to the biaxial modulus and the crack resistance of the GaAs film, and inversely proportional to the square of the thermal stress and a nondimensional crack resistance number Z. This Z number is determined by the mechanical properties of the GaAs film for a system without substrate damage, and is also determined by the mechanical properties of the substrate for a system with substrate damage. The experimentally determined critical thicknesses were in general greater than the analytically derived values due to the kinetic barriers to crack nucleation, which were not taken into consideration in the models. In addition, we have observed an asymmetric crack array formation, where arrays running in the 〈110〉 substrate off-cut direction are favored. We have also performed finite element modeling of the crack systems to study the evolution of thermal stress around crack planes in the GaAs film.
Microstructural characterization of MgO thin films grown by radio-frequency sputtering. Target and substrate-temperature effect
Authors: Cáceres D. , Vergara I. , González R.
Published: March 2003

The microstructure of thin films deposited by radio-frequency (rf)-sputtering on a silicon substrate at several temperatures and with two different targets was investigated by x-ray diffractometry (XRD) and scanning electron microscopy. XRD spectra reveal that films deposited at room temperature from either an MgO or an Mg target contain small (∼5 nm) periclase MgO crystallites. Thermal treatments in air followed by a fast cooling improved the degree of crystallinity and increased the grain size. The films grown from an Mg target at high temperatures are polycrystalline with a rock-salt structure. However, in thin films deposited from a sintered MgO target at T⩾873 K, the cubic spinel structure due to a mix of periclase (MgO) and brucite [Mg(OH)2] is observed; hydrogen comes from the target contamination. Thermal treatments in air at high temperatures improved the degree of crystallinity and texturing. The film structure depends on the cooling rate from elevated temperatures. Nanoindentation measurements yield a hardness value independent of whether the film is amorphous or polycrystalline, and close to the value obtained for bulk MgO single crystals. However, the Young's modulus strongly depends on the atomic structure of the film, being much lower for amorphous films.
Electrical properties of Y2O3 high-κ gate dielectric on Si(001): The influence of postmetallization annealing
Authors: Ioannou-Sougleridis V. , Vellianitis G. , Dimoulas A.
Published: March 2003

In this work we report on the electrical properties of Y2O3 dielectrics grown on silicon (001) substrates at low oxygen background pressure by molecular beam epitaxy. Using metal-insulator-semiconductor capacitors, it is shown that as-grown samples are characterized by a large number of interface states coupled with insulator traps. Annealing in forming gas improves the electrical behavior of the samples, especially those grown at low temperatures, having typical interface state density of the order of 1012 cm−2 eV−1. Overall, the electrical response depends on the structural quality of the interfaces and especially on the presence of a uniform SiOx interfacial layer. The permittivity κ of the Y2O3 films was found to be rather low (κ∼9), which is attributed mainly to deviations from stoichiometry and the associated generation of oxygen deficiencies in the metal oxide.
Buckling phenomena in Y2O3 thin films on GaAs substrates
Authors: Paumier F. , Gaboriaud R. J. , Coupeau C.
Published: March 2003

Worm-like buckling structures have been investigated by atomic force microscopy to estimate the relevant parameters for the mechanical stability of Y2O3 thin films deposited by ion beam sputtering technique on GaAs substrates. The internal stresses involving in the observed buckling phenomena have been estimated to be around 8.5 GPa in compression. The critical thicknesses up to which the various damaging mechanisms occur have also been determined.
Effect of Mn substitution on magnetoelectric properties of bismuth ferrite system
Authors: Palkar V. R. , Kundaliya Darshan C. , Malik S. K.
Published: March 2003

The effect of Mn substitution on ferroelectric and antiferromagnetic properties of the Bi0.9La0.1Fe1−xMnxO3 system has been studied. It is seen that, with the increase in Mn content, there is a contraction in unit cell volume. Since the cell volume contraction is isotropic, lattice distortion (c/a) remains the same and so is the ferroelectric transition temperature TC. Further, it is observed that the presence of Mn does not affect the loss tangent (tan δ) of the samples. On the contrary, with increase in Mn content, a small but linear enhancement in the magnetization is observed.
Strain and Ge concentration determinations in SiGe/Si multiple quantum wells by transmission electron microscopy methods
Authors: Benedetti A. , Norris D. J. , Hetherington C. J. D. , Cullis A. G. , Robbins D. J. , Wallis D. J.
Published: March 2003

SiGe/Si multiple quantum wells, nominally 4 nm thick, were grown by low pressure chemical vapor deposition and the Ge distribution within the wells was studied using a variety of transmission electron microscope-based techniques. Energy-dispersive x-ray spectroscopy and electron energy-loss imaging were used to directly measure the Ge compositional profile across the SiGe wells. In addition, the average Ge concentration was deduced indirectly from measurement of the strain-induced lattice displacements in high resolution images, obtained from the relative phase shift of the Si lattice planes on either side of a SiGe well. The results from both the direct and indirect measurement techniques were compared and found to be in good agreement with one another. The Ge profiles exhibited an asymmetric shape consistent with the occurrence of Ge segregation during growth. However, the amplitude of the asymmetry indicated that an additional factor, in particular gas dwell times within the reactor, also needed to be taken into account. Based upon this approach, a successful theoretical model of the growth process was derived.
Analysis of ultraviolet photoconductivity in ZnO films prepared by unbalanced magnetron sputtering
Authors: Sharma Parmanand , Sreenivas K. , Rao K. V.
Published: March 2003

Photoresponse characteristics of polycrystalline ZnO films prepared by the unbalanced magnetron sputtering technique have been analyzed for ultraviolet photodetector applications. Changes in the crystallographic orientation and the microstructure of the films due to in situ bombardment effects during film growth have been studied. Variations in photoresponse are correlated with the observed changes in the optical properties and the defect concentration in the films. ZnO films with (100) and (101) orientation possessing a small grain size exhibited a slow response with a rise time=1.99 s, whereas porous ZnO films with a mixed orientation (100), (002), and (101) and a larger grain size exhibited a fast response speed with a rise time=792 ms. The influence of trap levels on the slow and fast rising components of the photoresponse characteristics and the origin for a fast and a stable response have been identified. A slow rise in the photocurrent directly relates to the adsorption and desorption of oxygen on the film surface, and the fast rise is due to a bulk-related phenomena involving embedded oxygen. The magnitude of the photocurrent and the rise time are found to decrease considerably with increasing number of trap levels.