bluuebunny/crossref_metadata_2025 huggingface dataset:
Perturbational ab initio calculations of relativistic contributions to nuclear magnetic resonance shielding tensors
Authors: Manninen Pekka , Lantto Perttu , Vaara Juha , Ruud Kenneth
Published: July 2003

We present perturbational ab initio calculations of the leading-order one-electron relativistic contributions to the nuclear magnetic resonance shielding tensor based on the Pauli Hamiltonian. The scalar relativistic and spin–orbit interaction effects, including both relativistic corrections to the wave function (“passive” relativistic effects) and relativistic magnetic perturbation operators (“active” effects), are considered for H2X (X=O, S, Se, Te, Po), HX (X=F, Cl, Br, I, At), and noble gas (Ne, Ar, Kr, Xe, Rn) systems. The perturbational corrections are calculated using linear and quadratic response theory applied to nonrelativistic reference states. We use the uncorrelated self-consistent field as well as correlated, multiconfigurational complete active space self-consistent field models. Results for the 1H and heavy-atom shielding constants and anisotropies are compared with Dirac–Hartree–Fock and quasirelativistic data.
Conformations of indan and 2-indanol: A combined study by UV laser spectroscopy and quantum chemistry calculation
Authors: Das Aloke , Mahato K. K. , Panja Sujit S. , Chakraborty Tapas
Published: July 2003

Three conformational isomers of 2-indanol are identified by use of resonance enhanced two-photon ionization (R2PI) and single vibronic level dispersed fluorescence spectroscopy in a supersonic jet expansion. By combining the experimental results with the predictions of the ab initio quantum chemistry calculations at the MP2/6-311++G(d,p) level of theory, the major species is identified as a conformational isomer in which the hydroxyl hydrogen is involved in an intramolecular hydrogen bonding with the π-electrons of the aromatic ring. The theoretical estimate of the hydrogen bond energy is ∼6.5 kJ/mol. A comparative investigation with indan reveals that this weak hydrogen bonding in the former significantly affects the puckering potential of the five-member side ring. The dispersed fluorescence data indicate for a much higher ring-puckering barrier in the ground state than what has been suggested recently by measuring rotational spectra of the unsubstituted indan.
Formation of ozone: Metastable states and anomalous isotope effect
Authors: Babikov Dmitri , Kendrick Brian K. , Walker Robert B. , T Pack Russell , Fleurat-Lesard Paul , Schinke Reinhard
Published: July 2003

A clear explanation for an anomalous isotope effect in ozone formation is given in terms of the energy transfer mechanism, where the metastable states of ozone are formed first, and then stabilized by collisions with other atoms. Unusual nonstatistical properties of metastable states spectra discovered earlier [J. Chem. Phys. 118, 6298 (2003)] are incorporated into the kinetics model, where different metastable states are treated as different species, and the stabilization step is treated approximately. The population of the ozone metastable states builds up and decays through three possible O2+O channels. When different isotopes of oxygen are involved the three channels become open at different energies because of the differences in the quantum zero-point-energies (ΔZPE) of the different O2 molecules. The spectrum of metastable states is anomalously dense below the ΔZPE threshold and these states are accessible only from the lower entrance channel. Also, these low-lying metastable states are stabilized very efficiently (by collisions with third body) because they are energetically close to the bound O3 states. Such processes significantly enhance the formation rates of ozone isotopologues through the lower channels over the formation rates through the upper channels. Numerical results obtained for J=0 give isotope effects in the right direction and of the right order of magnitude. Consideration of J>0 should improve the comparison with experiment.
First determination of the NO2 Ã 2B2 stretching frequencies by jet cooled intracavity laser absorption spectroscopy around 11 000 cm−1
Authors: Jost R. , Garcia Vergniory M. , Campargue A.
Published: July 2003

Using intracavity laser absorption spectroscopy combined with a supersonic slit jet, we have observed and analyzed seven vibronic cold bands of NO2 between 10 800 and 11 680 cm−1. The vibronic energies, relative intensities, A, B, and C rotational constants, and the spin-rotation constants have been determined. The rotational constants play a crucial role in the vibronic assignments. The seven observed states are vibronically mixed. However, three of them have a dominant Ã 2B2 electronic character while the four others have a dominant X̃ 2A1 character. The vibrational assignments and energies of the three Ã 2B2 levels are: (1, 0, 0) at 10 999.42 cm−1, (0, 2, 0) at 11 210.50 cm−1, and (0, 0, 2) at 11 283.15 cm−1. The frequencies of the symmetric stretch, (ω1≅1265 cm−1), and bending, (ω2≅738 cm−1) modes derived from the (1, 0, 0) and (0, 2, 0) levels are in agreement with ab initio calculations. In contrast, the frequency of the antisymmetric stretch, ω3≅775 cm−1, derived from the observed (0, 0, 2) level, agrees only with the ab initio value obtained by Kaldor while other ab initio values ranging from 390 to 1750 cm−1 were reported. The four levels with a dominant X̃ 2A1 character have been tentatively vibrationally assigned. The present experimental data allows for a first experimental determination of the two Ã 2B2 stretch vibrational frequencies which are necessary to model the X̃ 2A1–Ã 2B2 vibronic interactions, i.e., the conical intersection between X̃ 2A1 and Ã 2B2 potential energy surfaces.
Atomic and molecular stabilization in two-frequency laser fields
Authors: Datta Avijit , Bhattacharyya S. S. , Lee Sungyul , Kim Bongsoo
Published: July 2003

We examine the suppression of 1+1 photodissociation or photoionization by laser fields. We find that atoms and molecules become more stable against photoionization or photodissociation when the intensity of the nonresonant laser field is increased above a limiting value for a two level plus continuum model system. We treat the ionization or dissociation dynamics analytically and discuss the conditions for optimum yields in a two-frequency square pulse laser field. We observe similar suppression against photodissociation for realistic sine-squared laser pulses. We demonstrate such a type of molecular suppression for 1+1 photodissociation of H2.
Intermolecular Fermi resonance
Authors: Stride John A. , Dallin Paul H. , Jayasooriya Upali A.
Published: July 2003

The exceptionally broad feature at ∼3025 cm−1 observed in the Raman spectrum of chloroform dissolved in liquid sulfur dioxide is shown to be due to the triple combination mode, ν1+ν2+ν3, of sulfur dioxide gaining intensity by mixing with the fundamental C–H stretching mode of chloroform. Investigation of a number of similar systems shows that this broadening is unique to this system and is certainly not heterogeneous broadening due to C–H hydrogen bonding to SO2. This therefore is probably the first observation of the phenomenon of intermolecular Fermi resonance, between molecularly distinct species.
Absolute hydration free energies of ions, ion–water clusters, and quasichemical theory
Authors: Asthagiri D. , Pratt Lawrence R. , Ashbaugh H. S.
Published: July 2003

Experimental studies of ion–water clusters have provided insights into the microscopic aspects of hydration phenomena. One common view is that extending those experimental studies to larger cluster sizes would give the single-ion absolute hydration free energies not obtainable by classical thermodynamic methods. This issue is reanalyzed in the context of recent computations and molecular theories of ion hydration, particularly considering the hydration of H+, Li+, Na+, and HO− ions, and thence the hydration of neutral ion pairs. The hydration free energies of neutral pairs computed here are in good agreement with experimental results, whereas the calculated absolute hydration free energies and the excess chemical potentials deviate consistently from some recently tabulated hydration free energies based on ion–water cluster data. We show how the single-ion absolute hydration free energies are not separated from the potential of the phase in recent analyses of ion–water cluster data, even in the limit of large cluster sizes. We conclude that naive calculations of ion–water clusters ought to agree with results obtained from experimental studies of ion–water clusters because both values include the contribution, somewhat extraneous to the local environment of the ion, from the potential of the phase.
Self-assembled aggregates in the gravitational field: Growth and nematic order
Authors: Baulin Vladimir A.
Published: July 2003

The influence of the gravitational field on the reversible process of assembly and disassembly of linear aggregates is the focus of this paper. Even the earth gravitational field can affect the equilibrium properties of heavy biological aggregates such as microtubules or actin filaments. The gravity gives rise to the concentration gradient which results in the distribution of aggregates of different lengths with height. Strong enough gravitational field induces the overall growth of the aggregates. The gravitational field facilitates the isotropic to nematic phase transition reflecting in a broader transition region. Coexisting phases have notedly different length distributions and the phase transition represents the interplay between the growth in the isotropic phase and the precipitation into nematic phase. The fields in an ultracentrifuge can only reinforce the effect of gravity, so the present description can be applied to a wider range of systems.
Quantum-classical approximation beyond Redfield theory
Authors: Neufeld A. A.
Published: July 2003

A quantum-classical approximation, capable of describing the evolution of open quantum systems well beyond the applicability limits of Redfield theory is suggested. The theory is based on the short lifetime of the quantum correlations between the quantum and the classical subsystem, caused by energy dispersion (per degree of freedom) of the canonical bath. The resulting quantum-classical approximation has the form of two auxiliary differential equations and fully accounts both for the arbitrary long memory of the heat reservoir and detailed balance. These equations allow direct solution in the time domain without constructing/diagonalizing Liouville space operators, and, in combination with molecular dynamics techniques to simulate bath dynamics, may be applied to quantum subsystems with a fairly large number of levels. A simple example of a two-level system, coupled to a single correlation time canonical bath, was considered to demonstrate different regimes of approaching the canonical equilibrium state.
Molecular dynamics approach to vibrational energy relaxation: Quantum-classical versus purely classical nonequilibrium simulations
Authors: Neufeld A. A. , Schwarzer D. , Schroeder J. , Troe J.
Published: July 2003

We present an efficient method for the direct solution in the time domain of the equations of a novel recently proposed non-Markovian quantum-classical approximation, valid well beyond the applicability limits of both Redfield theory and Fermi’s Golden Rule formula. The method is based on an ab initio molecular dynamics description of the classical bath and is suitable for applications to systems with a fairly large number of quantum levels. A simple model of the breathing sphere in a Lennard-Jones fluid was used to compare the results of the quantum-classical and purely classical treatments of vibrational energy relaxation.
Molecular dynamics simulations of crystallization under confinement at triple point conditions
Authors: Cámara Luis G. , Bresme Fernando
Published: July 2003

Molecular dynamics computer simulations of crystallization of a Lennard-Jones system under confinement conditions in the vicinity of the triple point are reported. We calculate the force exerted on a crystal by a melt when it crystallizes. The force due to crystallization is reflected in the disjoining pressure isotherms as a characteristic peak. We find that at conditions of high confinement, i.e., pore thicknesses of ≈1 nm, the disjoining pressure can rise up to ≈108 Pa. We also analyze the dependence of the crystallization under confinement as a function of temperature. Confinement can stabilize the crystal phase at temperatures significantly higher than the melting temperature. For the systems studied in this work, a pore of 1 nm thickness stabilizes the crystal phase at temperatures up to 45% higher than the normal melting temperature. In addition we consider the disjoining pressure profile along confining pore slits of finite lengths. The finite size effects due to the pore length modifies the value of the force close to the pore edge. There exist a reduction of the total disjoining pressure in short pores with respect to long pores. The simulations show that these effects are more noticeable for pore lengths below 3 nm.
Interfacial tension of a salty droplet: Monte Carlo study
Authors: Ho Ching-Hung , Tsao Heng-Kwong , Sheng Yu-Jane
Published: July 2003

The interfacial tension of an aqueous droplet with 1:1 strong electrolytes is investigated by Monte Carlo (MC) simulations. The electrostatic interactions among ions are calculated directly by solving the Poisson equation based on the restricted primitive model. The surface excess and chemical potential are evaluated from MC. The latter can be reasonably described by the mean spherical model. The interfacial tension is then obtained through Gibbs adsorption isotherm. The addition of a strong electrolyte leads to the increase of the interfacial tension and the MC result accounts reasonably well for the observed interfacial tensions up to relatively high salt concentrations (∼1.0 M). For very dilute solutions, our results agree with the Onsager–Samaras (OS) limiting law. However, the OS theory underestimates the surface excess and the interfacial tension for high salt concentrations. It is found that hydration around ions plays a very important role in interfacial tension increment.
Statistical mechanics of pseudoknot polymers
Authors: Lucas Adam , Dill Ken A.
Published: July 2003

We present the theory for the conformational free energies of RNA molecules and other polymers that have pseudoknots. We derive an expression for the partition function by counting conformational loops that we call pseudoknot core units, using the theory of self-avoiding walks. We show that the thermal transitions of pseudoknot polymers to their denatured states are not two-state; they are continuous. We also find that the entropy of a pseudoknot loop depends not only on its chain length, as is assumed in most current RNA models, but also on the length of the adjacent stem, because stems are stiff and the stiffness determines the starting and ending points of the loop.
Electronic transport in disordered n-alkanes: From fluid methane to amorphous polyethylene
Authors: Cubero David , Quirke Nicholas , Coker David F.
Published: July 2003

We use a fast Fourier transform block Lanczos diagonalization algorithm to study the electronic states of excess electrons in fluid alkanes (methane, ethane, and propane) and in a molecular model of amorphous polyethylene (PE) relevant to studies of space charge in insulating polymers. We obtain a new pseudopotential for electron–PE interactions by fitting to the electronic properties of fluid alkanes and use this to obtain new results for electron transport in amorphous PE. From our simulations, while the electronic states in fluid methane are extended throughout the whole sample, in amorphous PE there is a transition between localized and delocalized states slightly above the vacuum level (∼+0.06 eV). The localized states in our amorphous PE model extend to −0.33 eV below this level. Using the Kubo–Greenwood equation we compute the zero-field electron mobility in pure amorphous PE to be μ≈2×10−3 cm2/V s. Our results highlight the importance of electron transport through extended states in amorphous regions to an understanding of electron transport in PE.
Density functional theory study of HfCl4, ZrCl4, and Al(CH3)3 decomposition on hydroxylated SiO2: Initial stage of high-k atomic layer deposition
Authors: Jeloaica L. , Estève A. , Djafari Rouhani M. , Estève D.
Published: July 2003

The initial stage of atomic layer deposition of HfO2, ZrO2, and Al2O3 high-k films, i.e., the decomposition of HfCl4, ZrCl4, and Al(CH3)3 precursor molecules on an OH-terminated SiO2 surface, is investigated within density functional theory. The energy barriers are determined using artificial activation of vibrational normal modes. For all precursors, reaction proceeds through the formation of intermediate complexes that have equivalent formation energies (∼−0.45 eV), and results in HCl and CH4 formation with activation energies of 0.88, 0.91, and 1.04 eV for Hf, Zr, and Al based precursors, respectively. The reaction product of Al(CH3)3 decomposition is found to be more stable (by −1.45 eV) than the chemisorbed intermediate complex compared to the endothermic decomposition of HfCl4 and ZrCl4 chemisorbed precursors (0.26 and 0.29 eV, respectively).
Examination of the correlation energy and second virial coefficients from accurate ab initio calculations of rare-gas dimers
Authors: Giese Timothy J. , Audette Vanessa M. , York Darrin M.
Published: July 2003

Calculations of rare-gas dimers (He–He, Ne–Ne, Ar–Ar, He–Ne, He–Ar, and Ne–Ar) at the coupled-cluster single double (triple) level of theory with large basis sets including bond functions and counterpoise corrections are reported over a wide range of 100 internuclear separations. These results are compared to experimental curves obtained from fitting to rovibrational spectra, and to second virial coefficients and Boyle temperatures. Accurate analytic potentials are developed for the total interaction energy, Hartree–Fock (exchange) energy, and correlation (dispersion) energy; the transferability of the latter is demonstrated to very high accuracy even in the region of considerable wave function overlap. These calculations represent an important set of benchmarks that can be used to develop improved empirical molecular mechanical force fields and new quantum models.
Eigenstates and transmission coefficients of finite-sized carbon nanotubes
Authors: Compernolle S. , Chibotaru L. , Ceulemans A.
Published: July 2003

The tight-binding eigenstates of isolated finite-sized zigzag and armchair nanotubes are obtained analytically using the transfer matrix method. Edge states are encountered for zigzag tubes but not for armchair tubes. Inclusion of curvature leads to a decaying-oscillating behavior of the highest occupied and lowest unoccupied molecular orbital gap in armchair tubes as a function of length. For zigzag tubes the inclusion of curvature induces the conversion of an extended state at nonbonding energy to an edge state. In addition the transmission coefficient of zigzag and armchair tubes with featureless leads is analytically obtained. The existence of a transmission peak at the Fermi level that decays exponentially with nanotube length for zigzag tubes is explained.
Temperature-programmed time of flight secondary ion mass spectrometry study of hydration of ammonia and formic acid adsorbed on the water–ice surface
Authors: Souda Ryutaro
Published: July 2003

The interactions between condensed H2O, NH3, and HCOOH molecules have been investigated on the basis of secondary ion mass spectrometry (SIMS). The protonated molecular ions are created during energetic collisions of hydrogen-bonded neutral molecules as a consequence of proton transfer reactions. The NH4+ yield from the NH3 molecules adsorbed on the HCOOH surface is about 30 times as high as that from the pure ammonia surface since the ions are formed in the course of the dissociation of the NH4HCO2 molecule or its precursors. The hydration or reorganization of hydrogen bonding between the adsorbed molecules and the water–ice surface is discussed as a function of temperature (15–300 K). The adsorbed NH3 or HCOOH molecule forms a bound state on the water–ice surface at a temperature of 60–140 K. In the same temperature regime, the coadsorbed NH3 and HCOOH molecules react with each other to create the NH4HCO2 molecule prior to hydration. The hydration occurs drastically for all these molecules above 140 K as evidenced by the occurrence of rapid and almost complete H/D exchange.
Dynamics of Li ions in an intercalated polymer electrolyte: A 7Li nuclear magnetic resonance study
Authors: Arun N. , Vasudevan S. , Ramanathan K. V.
Published: July 2003

The 7Li nuclear magnetic resonance spectra and spin-lattice relaxation rates of lithium ions in an intercalated polymer electrolyte, Li-polyethylene-oxide confined within the galleries of an inorganic layered solid CdPS3, are reported. The spectra show an unusual feature, the appearance above 330 K of quadrupolar satellites, whose splitting increases linearly with temperature, and their absence below this temperature. This behavior is also reflected in the temperature variation of the spin-lattice relaxation rates, which show a minimum at 300 K but above 330 K start decreasing again. Using an anisotropic quantum oscillator model, the observed variation in the 7Li quadrupolar coupling constant is shown to arise from changes in the time average of the electric field gradient as seen by a 7Li nucleus associated with mobile segments of the intercalated polymer.
Equation of state for hard-spheres
Authors: Miandehy Mohsen , Modarress Hamid
Published: July 2003

A new equation of state is presented for hard-spheres, which can reproduce the exact values of the first seven virial coefficients. These values are in agreement with the molecular dynamic calculations of Alder and Wainwright [J. Chem. Phys. 33, 1439 (1960)]. Comparisons with the existing equations of state show that the new equation possesses superior ability to describe the hard-sphere phase behavior.
Wavelength selective charge storage in self-assembled InGaAs/GaAs quantum dots
Authors: Kroutvar M. , Ducommun Y. , Finley J. J. , Bichler M. , Abstreiter G. , Zrenner A.
Published: July 2003

We present a wavelength selective optical memory device based on optically-induced charge storage in an inhomogeneously broadened ensemble of InGaAs quantum dots. We show that electrons and holes can be efficiently and resonantly stored without thermal redistribution of charge between dots over timescales much longer than 25 μs at 10 K. Direct information on the absorption mechanisms are obtained for self-assembled dots. The maximum efficiency of the charge storage process is estimated to be close to unity.
Experimental and computational estimate of bipolar flow parameters in an explosive field emission cathode
Authors: Shiffler D. , Cartwright K. L. , Lawrence Kim , Ruebush M. , LaCour M. , Golby K. , Zagar D.
Published: July 2003

Explosive field emission cathodes constitute an important class of cathodes for high power microwave tubes. These cathodes have the advantages of being light weight and the capability of operating cold. In the past, this class of cathodes suffered from large amounts of outgassing, nonuniform emission, and very high emittance. These effects tended to dominate the diode performance, masking effects due to the anode. However, a type of carbon cathode has enabled the role of the anode in the diode to be better determined. In this letter, we compare experimental results with simulated diode performance, allowing an estimate of the bounds on the secondary emission coefficient from the anode surface as well as the amount of neutral gas liberated from this surface. In general, secondary electrons and neutral atoms lead to plasma formation in high power microwave devices, which in turn deleteriously affect the tube performance. Hence, an estimate of such quantities for use in particle-in-cell codes can prove pivotal for accurate modeling of experimental devices.
Room-temperature electrical conductivity studies of sulfur-modified microcrystalline diamond thin films
Authors: Gupta S. , Weiner B. R. , Morell G.
Published: July 2003

The room-temperature electrical conductivity of sulfur-incorporated microcrystalline diamond (μc-D:S) thin films synthesized by hot-filament chemical vapor deposition was investigated as a function of sulfur concentration. The films were prepared using a 0.3% CH4/H2 gas mixture and hydrogen sulfide (H2S) as dopant source on intrinsic Si(001) substrates. The μc-D:S films exhibited an increase in n-type conductivity with increase in H2S concentration from 0 to 200 ppm, followed by a decrease in conductivity and sign reversal for the films grown with 500 ppm of H2S. These films were also characterized using scanning electron microscopy, atomic force microscopy, and Raman spectroscopy techniques. The findings are discussed in terms of the role of sulfur in the films. The films grown at the highest [H2S] possess the highest carrier concentration (∼1.07×1017/cm3) and the lowest carrier Hall mobility (0.01 cm2 V−1 s−1). Since the conductivity is affected by carrier concentration and crystallinity, the relatively low experimental conductivity values are connected to low mobilities arising from crystal defects and grain boundaries induced by sulfur addition, and to low carrier concentrations due to incomplete ionization of S atoms.
Negative-U property of the oxygen vacancy defect in SiO2 and its implication for the E1′ center in α-quartz
Authors: Chadi D. J.
Published: July 2003

The +1 charged state of an oxygen vacancy V(O)+ in α-quartz is found to be unstable with respect to the reaction 2V(O)+→V(O)0+V(O)2+, which lowers the total energy by 2.9 eV, making it highly unlikely that V(O)+ is the source of the electron-paramagnetic-resonance (EPR) active E1′ center in α-quartz. Results from ab initio total energy calculations indicate that V(SiO3)+ and V(SiO4)+ are the smallest EPR active vacancy complexes. The atomic and electronic structures of these two multivacancy clusters are surprisingly similar to those of V(O)+.
Characteristics and field emission of conducting poly (3,4-ethylenedioxythiophene) nanowires
Authors: Kim B. H. , Kim M. S. , Park K. T. , Lee J. K. , Park D. H. , Joo J. , Yu S. G. , Lee S. H.
Published: July 2003

Conducting poly (3,4-ethylenedioxythiophene) nanowires were synthesized by using an electrochemical polymerization method with a nanoporous template. Scanning and transmission electron microscopy confirmed the formation of conducting polymer nanowires (CPNWs) with an open end. The formation and the electrical properties of the CPNWs formed were dependent on synthetic conditions, such as the doping level, the polymerization time, and the applied current. The measured electrical conductivity of a single strand of CPNW was ∼3.4×10−3 S/cm at room temperature. From the ultraviolet and visible absorbance spectra, we observed a π–π* transition at ∼2.1 eV for the de-doped systems. A field emission cell of CPNW nanotips was fabricated. The turn-on field of the CPNWs was 3.5∼4 V/μm at 10 μA/cm2, and the current density increased up to 100 μA/cm2 at ∼4.5 V/μm. The field enhancement factor of CPNW nanotips was ∼1200, which is comparable to those of carbon nanotubes.
Friction force microscopy using silicon cantilevers covered with organic monolayers via silicon–carbon covalent bonds
Authors: Ara Masato , Tada Hirokazu
Published: July 2003

Cantilevers covered with hydrocarbon (CH) and fluorocarbon (CF) monolayers via Si–C covalent bonds were prepared and used for adhesion force measurements and friction force microscopy of the surface patterned also with CH and CF areas. The adhesion and friction forces on CF areas were larger than those on CH areas, especially using CF cantilevers. Large polarizabilities of CF molecules compared to CH molecules are found to enhance the contrast in adhesion and friction images. The cantilevers covered with organic monolayers via covalent bonds are useful for chemical force microscopy with contact and noncontact mode atomic force microscopy in various atmospheres since the interface between molecules and cantilevers is thermally and chemically stable.
Conductivity-based contact sensing for probe arrays in dip-pen nanolithography
Authors: Zou Jun , Bullen David , Wang Xuefeng , Liu Chang , Mirkin Chad A.
Published: July 2003

This letter reports a contact sensing method for dip-pen nanolithography which functions by monitoring the conductivity between the lithography probe and the substrate. Experimental results show that this method has high sensitivity and is suitable for dip-pen nanolithography applications with large probe arrays.
Monodomain versus polydomain piezoelectric response of 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 single crystals along nonpolar directions
Authors: Damjanovic Dragan , Budimir Marko , Davis Matthew , Setter Nava
Published: July 2003

Using recently published experimental data we calculated the piezoelectric response of rhombohedral 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 monodomain single crystals along arbitrary directions. The calculations indicate that the value of the longitudinal piezoelectric coefficient along the [001] axis of the pseudocubic system ([111] axis of the rhombohedral system) of a monodomain crystal is close to the value determined experimentally along the same direction of a multidomain crystal with a so-called “engineered domain state.” The results thus show that the large piezoelectric response in this material is dominated by intrinsic crystal lattice effects and that the multidomain state has a relatively minor effect (<20%). In addition, calculations predict a transverse piezoelectric coefficient >1900 pC/N for certain directions of the monodomain crystal.
Band alignment on a nanoscopically patterned inorganic–organic interface
Authors: Koller G. , Netzer F. P. , Ramsey M. G.
Published: July 2003

The band alignment of organic semiconductors on a nanoscopically patterned surface is investigated for submonolayer coverages to thin molecular films of bithiophene, using ultraviolet photoelectron spectroscopy and work function measurements. The Cu (110)–(2×1)O stripe phase, used as a substrate, consists of alternating stripes of clean and oxygen passivated copper, with a respective stripe diameter of three to four times the molecular length. For the first molecular layer, a superposition of bithiophene spectra, offset from each other by 1 eV, reflecting the interface dipole differences on Cu (110) and Cu (110)–p(2×1)O, was found. However, after completion of the second layer, the observed band alignment between the substrate and the overlayer is determined by the average interface potential and the underlying substrate pattern is obscured.
n-
AlGaAs
/p-
GaAs
/n
-GaN heterojunction bipolar transistor wafer-fused at 550–750 °C
Authors: Estrada Sarah , Huntington Andrew , Stonas Andreas , Xing Huili , Mishra Umesh , DenBaars Steven , Coldren Larry , Hu Evelyn
Published: July 2003

We recently reported an initial AlGaAs/GaAs/GaN heterojunction bipolar transistor (HBT), formed via wafer fusion of a p-GaAs base to an n-GaN collector. The device was formed by fusion at a high temperature (750 °C) and demonstrated low output current (∼100 A/cm2) and low common-emitter current gain (0.5). This letter describes a systematic variation of fusion temperature (550–750 °C) in the formation of the HBT, and reveals the correlation between fusion temperature, base–collector leakage, and emitter–base degradation. With reduced fusion temperatures, devices demonstrate improvements in leakage, output current (∼1 kA/cm2), and common-emitter current gain (>1). Optimization of device structure should further improve performance.
Conductivity of macromolecular networks measured by electrostatic force microscopy
Authors: Lei C. H. , Das A. , Elliott M. , Macdonald J. E.
Published: July 2003

High-resolution electrostatic force microscopy (EFM)-phase measurements are reported on molecular networks of semiconductor polymer poly-hexylthiophene (P3HT) and DNA molecules. A lateral resolution of better than 20 nm is demonstrated in EFM-phase images of the P3HT network by detecting the phase shift of the tip along the molecules under electrical bias. Strands of λ-DNA are shown to be highly insulating in comparison to the semiconductor polymer P3HT, with a minimum resistance of ∼1×107 Ω cm.
In situ lateral growth control of optically efficient quantum structures
Authors: Schallenberg T. , Faschinger W. , Karczewski G. , Molenkamp L. W. , Türck V. , Rodt S. , Heitz R. , Bimberg D. , Obert M. , Bacher G. , Forchel A.
Published: July 2003

We present a versatile method for in situ lateral growth control of optically efficient quantum structures. The method is based on molecular-beam epitaxy through an epitaxial shadow mask. Lateral control is achieved by selective area growth of short period superlattices. We demonstrate how the method can be applied to both lattice-matched and unmatched material systems. In the former, selective growth is employed in the formation of quantum wires. In the latter, this growth concept was also successfully applied to control the self-assembly of quantum dots within selected areas. The excellent quality of the quantum structures is demonstrated by their bright cathodoluminescence at room temperature.
Linear relationship between H+-trapping reaction energy and defect generation: Insight into nitrogen-enhanced negative bias temperature instability
Authors: Tan Shyue Seng , Chen T. P. , Soon Jia Mei , Loh Kian Ping , Ang C. H. , Teo W. Y. , Chan L.
Published: July 2003

In this letter, we report a first-principles calculation which is well correlated to experiment on the role of nitrogen at Si/SiOxNy interface in negative bias temperature instability (NBTI). Our calculation shows that nitrogen’s lone pair electrons can trap dissociated hydrogen species more easily than oxygen. After trapping, a positive charge complex is formed and weakening of bond strength is observed at trapping site. Furthermore, as nitrogen concentration goes beyond 8 at. %, the neighboring effect from nitrogen starts to play a role in further degradation. The interfacial nitrogen dependence of the NBTI-induced defect generation is found to coincide with that of the H+-trapping reaction energy. Eventually, a linear correlation is found between the reaction energy and the defect generation. This provides an insight into nitrogen-enhanced NBTI.
III-nitride ultraviolet light-emitting diodes with delta doping
Authors: Kim K. H. , Li J. , Jin S. X. , Lin J. Y. , Jiang H. X.
Published: July 2003

We present the results on the fabrication and characterization of 340 nm UV light-emitting diodes (LEDs) based on InAlGaN quaternary alloys grown by metalorganic chemical vapor deposition. By employing δ doping in the n- and p-type layers, we have demonstrated enhanced LED structural quality and emission efficiency. Combining with our interconnected microdisk LED architecture, the output power of a 300×300 μm2 bare LED chip measured from the sapphire side reached 50 μW under a standard dc operation condition (20 mA) at 4.6 V and 1.6 mW under a pulsed driving current.
1.3 μm light amplification in dye-doped hybrid sol-gel channel waveguides
Authors: Casalboni M. , De Matteis F. , Merlo V. , Prosposito P. , Russo R. , Schutzmann S.
Published: July 2003

We report on light amplification of dye-doped hybrid channel waveguides in the near-infrared region. Films have been grown both on glass and silicon substrates by a spin-coating process that uses a hybrid organic/inorganic sol-gel route. Doped films have been obtained by incorporation of an organic dye (IR1051) emitting in the 1.1–1.3 μm spectral region. Active channel waveguides have been synthesized by photolitographic and ion-milling techniques. An optical gain of about 11 cm−1 was estimated by amplified spontaneous emission measurements. An estimation of the optical losses of the hybrid waveguides is also reported.
Photostimulated luminescence in Eu-doped fluorochlorozirconate glass ceramics
Authors: Schweizer Stefan , Hobbs Linn W. , Secu Mihail , Spaeth Johann-Martin , Edgar Andrew , Williams Grant V. M.
Published: July 2003

We report the synthesis of Eu2+- and chlorine-doped fluorozirconate glass-ceramics that show an intense photostimulated luminescence (PSL) after x-ray irradiation at room temperature. The PSL efficiency is up to 80% of that found in the well-known crystalline x-ray storage phosphor BaFBr:Eu2+, and it is the largest thus far reported for a glass-ceramic. We attribute the PSL to crystallites of orthorhombic BaCl2 that are formed after annealing above the glass temperature. Hexagonal BaCl2 crystallites are also observed after short annealing times, but they do not provide a measurable PSL signal. The photoluminescence peak from glass-ceramics containing orthorhombic BaCl2 crystallites occurs at 402 nm, and the stimulation band is centered at about 560 nm.
Short-period SinGem strained-layer superlattices grown from gas sources by synchrotron-radiation-excited chemical-beam epitaxy
Authors: Akazawa Housei
Published: July 2003

Short-period SinGem strained-layer superlattices were successfully grown on Si (100) substrate from gas sources by synchrotron-radiation-excited chemical-beam epitaxy at 250 °C. Atomically sharp interfaces between the Si and Ge layers and two-dimensional morphology were obtained. With thinner Ge layers, the misfit strain is relieved by atomic-scale roughness at the interfaces; as the Ge layers become thicker, misfit dislocations become the main relief mechanism. The thickness of Si and Ge layers as derived from spectroscopic ellipsometry agreed nicely with images of the lattices. The dielectric constants of the growing Si and Ge top layers were much lower than those of Si and Ge bulk crystals, which is consistent with the mechanism of photoepitaxy, that is, the formation of a hydrogenated network as the precursor state followed by photolytic crystallization.
Improvement of holographic recording sensitivities in the green in SiO2 nanoparticle-dispersed methacrylate photopolymers doped with pyrromethene dyes
Authors: Tomita Yasuo , Nishibiraki Hiroshi
Published: July 2003

We demonstrate more than one-order-of-magnitude improvement of holographic recording sensitivities in the green by doping pyrromethene dyes into SiO2 nanoparticle-dispersed methacrylate photopolymer films. Holographic recording dynamics are measured for several dye concentrations and writing intensities. It is found that there exists the optimum writing intensity for a given dye concentration to maximize the diffraction efficiency. Noticeable hologram-apodization phenomena caused by the longitudinal refractive-index changes are also observed in the Bragg-angle selectivity curve.
Non-uniform carrier distribution in multi-quantum-well lasers
Authors: Smowton P. M. , Lewis G. M. , Sobiesierski A. , Blood P. , Lutti J. , Osbourne S.
Published: July 2003

We describe an approach to detect the presence of a nonuniform distribution of carriers between the different wells of multi-quantum-well laser diodes by measuring the gain and spontaneous emission spectra and demonstrate its application to a five-well sample that has a nonuniform carrier distribution at low temperatures.
Elastic–stiffness mapping by resonance-ultrasound microscopy with isolated piezoelectric oscillator
Authors: Ogi Hirotsugu , Tian Jiayong , Tada Toyokazu , Hirao Masahiko
Published: July 2003

A resonance-ultrasound microscopy has been developed for mapping a material’s elastic constant in a localized surface region. It detects the effective elastic modulus through a resonance frequency of free vibrations of a solid probe touching the specimen via a small tungsten-carbide bearing. Langasite (La3Ga5SiO14) crystal is used as a probe because of the low sensitivity of its elastic constants to temperature and its high piezoelectric coefficients. The vibration of the probe is excited and detected with a surrounding solenoid coil. This noncontacting acoustic coupling isolates the probe vibration and measures the resonance frequency with an accuracy better than one part in 105. This microscopic method is applied to a composite material consisting of silicon-carbide (SiC) fibers in titanium-alloy matrix. The stiffness distribution inside a single fiber was determined.
Electrical noise in n- and p-type Ag2Te
Authors: Jiang L. , Nowak E. R.
Published: July 2003

The relatively large and linear magnetoresistance found in nonstoichiometric silver chalcogenides makes them attractive candidates for field sensing applications. We report on the intrinsic electrical noise in bulk, polycrystalline Ag2+δTe. Low-frequency noise is due to resistance fluctuations having a 1/f-like power spectrum. The temperature dependence of the noise magnitude and its spectral slope indicate thermally activated kinetics that we attribute to some form of charge trapping–detrapping process occurring in or near the intergranular regions. The effective magnetic field noise in Ag2+δTe is compared to other materials systems used in field sensing applications.
Simple enhancement of the magnetocaloric effect in giant magnetocaloric materials
Authors: Lewis L. H. , Yu M. H. , Gambino R. J.
Published: July 2003

A simple method of enhancing the magnetocaloric effect (MCE)) in ferromagnetic materials is described. Thin layers of pure Fe of 0.1- and 0.2-μm thickness were evaporated onto both sides of polished 0.6-mm, slices of the giant magnetocaloric material Gd5Si1.5Ge2.5. The slices were stacked for magnetic measurement in the temperature range from 165 to 235 K to assess the MCE. Sample geometries with the iron layers oriented both parallel and perpendicular to the applied field were measured. In the metamagnetic transition temperature range, the Fe layer in the parallel geometry lowers the onset field of the metamagnetic transition by 4240 Oe for the 0.1-μm layer thickness and by 4940 Oe for the 0.2-μm layer thickness from the base value of 8700 Oe at 187 K in the absence of Fe layers. Furthermore, the 0.1-μm layer of Fe oriented perpendicular to the applied field is found to enhance the entropy change, and thus the magnetocaloric effect, by approximately 11% above its base value at 191.5 K; intriguingly, the 0.2-μm Fe layer does not obviously alter the MCE from its base value.
Vortex melting line and anisotropy of high-pressure-synthesized TlBa2Ca2Cu3O10−y high-temperature superconductor from third-harmonic susceptibility studies
Authors: Crisan A. , Iyo A. , Tanaka Y.
Published: July 2003

The fundamental and third-harmonic susceptibility response of YBa2Cu3O7−δ (YBCO) and TlBa2Ca2Cu3O10−y (Tl:1223) high-critical temperature (Tc) superconductors with preferentially oriented crystallites has been measured in various magnetic fields up to 14 T, and with very low ac field amplitude, to probe the melting transition of the vortex matter. This method does not require large single crystals, being therefore suitable for high-Tc superconductors with small crystallites, prepared with high-pressure synthesis techniques. We determine experimentally the melting line of vortices and the anisotropy factor in high-pressure-grown Tl:1223 with a high Tc of 133.5 K. Results from similar measurements on YBCO with preferentially oriented grains are consistent with both commonly accepted models of the melting line and values of the anisotropy factor, supporting the validity of our approach.
Abnormal electrical resistivity in γ-TiAl thin films deposited by magnetron sputtering
Authors: Alford T. L. , Gadre K. S. , Kim H. C. , Deevi S. C.
Published: July 2003

Thin films of γ-TiAl are being considered as a potential conductor and/or diffusion barrier for high temperature electronics because of their high melting points and high oxidation resistance. However, it is not possible to form pure γ-TiAl thin films by thermal annealing of Al/Ti bilayers. This study, however, demonstrates the formation of γ-TiAl thin films by dc magnetron sputtering of a compound target. X-ray diffractometry and Rutherford backscattering spectrometry analyses confirm the γ-TiAl phase formation, composition, and thermal stability in vacuum (up to 700 °C, 1 h) on SiO2. Four-point probe resistivity measurements in vacuum show an initial increase in the resistivity with temperature up to transition temperature for the γ-TiAl thin films. At higher temperatures a decrease in resistivity with additional heating (i.e., negative temperature coefficient of resistivity, TCR) is seen. The values of dρ/dT are typically on the order of −0.32 μΩ cm/°C between 200 and 550 °C. At the highest temperature, a minimum value of resistivity of ∼13 μΩ cm is obtained; this value is about one half the value of bulk TiAl at room temperatures. The negative TCR, low resistivity values at high temperatures, and temperature stability are not typically seen in bulk TiAl. This abnormal electrical property is explained using a modified model for a thermally activated polaron-hopping mechanism.
Electro-tunable laser action in a dye-doped nematic liquid crystal waveguide under holographic excitation
Authors: Matsui Tatsunosuke , Ozaki Masanori , Yoshino Katsumi
Published: July 2003

Optically pumped distributed feedback lasing has been demonstrated in a dye-doped nematic liquid crystal (NLC) waveguide by holographic excitation. The excitation was performed by two-beam interference using Lloyd mirror configuration. With an applied electric field, continuous tuning of the lasing wavelength was realized due to the change of the effective refractive index of the NLC core layer caused by the reorientation of NLC molecules.
Optical recording in silver-doped glasses by a femtosecond laser
Authors: Kaganovskii Yu. , Antonov I. , Ianetz D. , Rosenbluh M. , Ihlemann J. , Mueller S. , Marowsky G. , Lipovskii A. A.
Published: July 2003

Glass embedded with silver nanoclusters has been used to record interference patterns of two 500-fs, 496-nm, laser pulses. The recording is based on optical heating of metal clusters and their subsequent motion in temperature gradients. Estimates of the kinetics of cluster motion and growth are in good agreement with experiments.
Double incoherent light scattering induced by molecular interactions in binary mixtures
Authors: Le Duff Yves
Published: July 2003

A new Raman band is observed in two gaseous binary mixtures, CF4–CH4 and CF4–N2. It is attributable to a double Raman scattering process that takes place in pairs of unlike molecules. Spectral scattering intensities are given on an absolute scale. The zero order spectral moment is measured and compared with its long-range interaction value.
Measurement of the elastic constants of textured anisotropic thin films from x-ray diffraction data
Authors: Renault P.-O. , Le Bourhis E. , Villain P. , Goudeau Ph. , Badawi K. F. , Faurie D.
Published: July 2003

The elastic constants (compliances sij) of a textured anisotropic thin film deposited on a substrate have been determined. Using x-ray diffraction to measure the intragranular strain and a tensile machine to deform in situ the samples, an analytical method is described and has been developed for fiber textured thin films. The determination of thin film compliances only requires the knowledge of the substrate elastic constants. In the case of a 260-nm-thin gold film, the compliances were found to be slightly different from the corresponding bulk material ones.