bluuebunny/crossref_metadata_2025 huggingface dataset:
Ab initio calculation of hydrogen bonds in liquids: A sequential Monte Carlo quantum mechanics study of pyridine in water
Authors: Malaspina Thaciana , Coutinho Kaline , Canuto Sylvio
Published: July 2002

A systematic procedure based on the sequential Monte Carlo quantum mechanics (S-MC/QM) methodology has been used to obtain hydrogen bond strength and structures in liquids. The system considered is pyridine in water. The structures are generated by NVT Monte Carlo simulation, of one pyridine molecule and 400 water molecules. The hydrogen bonds are obtained using a geometric and energetic procedure. Detailed analysis shows that 62% of the configurations have one hydrogen bond. In the average, pyridine in liquid water makes 1.1 hydrogen bonds. The sampling of the structures for the quantum mechanical calculations is made using the interval of statistical correlation obtained by the autocorrelation function of the energy. A detailed statistical analysis is presented and converged results are obtained. The QM calculations are performed at the ab initio MP2/6-31+G(d) level and the results are compared with the optimized 1:1 cluster. Our results using QM calculations on 155 structures making one hydrogen bond gives an average binding energy of 3.7 kcal/mol, after correcting for basis set superposition error, indicating that in the liquid the binding energy is about 2/3 of the corresponding binding in the optimized cluster.
Electric field effects on fluorescence quenching due to electron transfer. II. Linked donor–acceptor systems
Authors: Hilczer Maria , Tachiya M.
Published: July 2002

A theoretical model has been proposed and applied to explain recent experimental results concerning the external electric field effect on fluorescence quenching due to intramolecular electron transfer in donor–acceptor pairs linked by a spacer. These results show that for some systems with a short length of a spacer and for a very low concentration of donor–acceptor pairs, the fluorescence quenching of the photoexcited donor is reduced by the electric field instead of being enhanced. The model predicts that the reducing effect of the external electric field on the fluorescence quenching alternates with the enhancing effect when the standard free energy change or the donor–acceptor separation is varied. For systems with a high concentration of the donor–acceptor pairs the resultant effect of the field is determined additionally by competition between the intramolecular and intermolecular electron transfers.
CoCr/Pt multilayers with perpendicular anisotropy and texture-controlled coercivity
Authors: Svedberg Erik B.
Published: July 2002

Co 1−x Cr x /Pt multilayers have been deposited by magnetron sputtering onto different seed layers with simultaneous gradients in both Co1−xCrx thickness and composition x. These gradients make it possible to perform magnetic and structural measurements for different thicknesses and compositions on one single wafer. Hence, the depositions are not sensitive to variations in background conditions such as temperature, pressure, or contaminants that could otherwise vary between runs. The study concluded that the maximum for the coercivity of the multilayers is, in this case, dependent on the microstructure, namely the degree of c-axis orientation in the films. Specifically, the coercivity maximum coincides with the maximum of the x-ray diffraction peak full-width at-half-maximum for the two separate cases shown.
Interface reactions in ZrO2 based gate dielectric stacks
Authors: Gribelyuk M. A. , Callegari A. , Gusev E. P. , Copel M. , Buchanan D. A.
Published: July 2002

Interface reactions in Si/SiOx(Ny)/ZrO2 and Si/SiOx(Ny)/ZrO2/poly-Si gate stacks have been studied by high-resolution transmission electron microscopy. In the case of an uncapped stack ZrSi and ZrSi2 phases form during an ultrahigh vacuum anneal at temperatures above 900 °C. Both phases show an island-type growth with an epitaxial relationship with Si (100). Gate dielectric stacks with a poly-Si cap are found to be thermally unstable at T=1000 °C, so that the reaction is initiated at the ZrO2/poly-Si interface. Here a different reaction mechanism is identified, which involves the reduction of ZrO2 and the growth of a bottom interfacial layer between ZrOx and Si. Replacement of the bottom SiO2 layer by an ultrathin Si oxinitride does not completely suppress these interfacial reactions at T⩾1000 °C. We suggest that control of the poly-Si/ZrO2 interfacial reactions may be an important factor in modifying the thermal stability of a stack. These results shed a new light on understanding the material challenges involved in the integration of ZrO2 for the next generation of complementary metal–oxide–semiconductor technologies.
Effect of cracks on the thermal resistance of aligned fiber composites
Authors: Dryden J. , Deakin A. , Zok F.
Published: July 2002

The axial heat flow within an aligned fiber composite containing matrix cracks bridged by the fibers is analyzed. A unit cell is defined and an exact expression for the constriction resistance of the unit cell is found. For values of fiber volume fraction and crack spacing that occur in actual composites, the thermal interaction between cracks is found to be almost negligible when the fiber/matrix interface is perfect. The asymptotic behavior under conditions of severe interface debonding is found.
Subthreshold characteristics of field effect transistors based on poly(3-dodecylthiophene) and an organic insulator
Authors: Scheinert S. , Paasch G. , Schrödner M. , Roth H.-K. , Sensfuß S. , Doll Th.
Published: July 2002

The properties of field effect transistors with organic insulator and semiconducting regions, fabricated with a top-gate architecture, have been investigated. Thin films (d≈30 nm) of regioregular poly(3-dodecylthiophene) were employed as the active semiconductor and the gate insulator was formed by a 500-nm-thick layer of poly(4-vinylphenol). Both were solution-processed on top of poly(ethylenetherephthalate) films, which were used as substrates. The output characteristics show a pronounced saturation behavior with an unconventional nonquadratic saturation current dependence on the gate voltage. Hence the (hole) mobility of 0.002–0.005 cm2/Vs has been estimated from the linear region of the transfer characteristics. The transistor turn-on occurs at a threshold voltage of approximately Vth=0 V, and the device can be operated with a supply voltage of between 15 and 20 V. As is usually observed for organic transistors, the inverse subthreshold slope (S) is very high, in our case S≈7 V/dec, by contrast with S≈200 mV/dec obtained for the similar material poly(3-octylthiophene) (P3OT) with silicon dioxide (SiO2) as an insulator. Furthermore, the subthreshold current depends on the drain voltage even though the transistor is electrically a long channel device with L=2 μm, notwithstanding the fact that this channel length is rather small for the present organic devices. To clarify these peculiarities numerical simulations have been carried out with a systematic variation of the relevant material parameters and assuming the existence of interface or bulk trap states. It turns out that both the high inverse subthreshold slope and the drain voltage dependence can be explained by recharging of trap states either at the interface or in the bulk. Considering the difference to the P3OT device with SiO2 as insulator it is proposed that interface traps are responsible for these effects, although one excludes the possibility that the film formation either on an organic substrate or on SiO2 leads to different bulk properties.
Temperature dependence of absorption in photorefractive iron-doped lithium niobate crystals
Authors: Panotopoulos G. , Luennemann M. , Buse K. , Psaltis D.
Published: July 2002

We present experimental data showing a significant dependence of light absorption on temperature in photorefractive LiNbO3:Fe crystals. The results are successfully explained by assuming that the widths of the Fe2+ absorption bands in the visible and in the infrared spectral region depend on temperature. The findings are of relevance for thermal fixing of holograms. Furthermore, a temperature-induced increase of the infrared absorption is promising for improved infrared recording.
Density functional study of surface forces in athermal polymer solutions with additive hard sphere interactions: Solvent effects, capillary condensation, and capillary-induced surface transitions
Authors: Forsman Jan , Woodward Clifford E. , Freasier Ben C.
Published: July 2002

A density functional theory for polymer solutions is generalized to cases where the monomers have a different diameter to the solvent. An appropriate free energy functional is obtained by integration of the generalized Flory equation of state for such systems. This functional predicts that entropic demixing may occur in polymer solutions in which the solvent particles are smaller than the monomers. Demixing is promoted not only by a large size disparity, but also by a high pressure as well as by polymer length. The existence of two separate phases in the bulk solution suggests the possibility of capillary-induced phase transitions, even when the confining surfaces are hard, but otherwise inert. We examine such phase transitions and their relation to surface forces and colloidal stability. The density functional theory also predicts that under certain conditions, layering transitions will occur at hard and flat surfaces. A transition from a thin to a thick polymer-rich surface layer may take place as the separation between two surfaces is decreased, and we study the concomitant change on the surface force. Stable thick phases are predicted even at very large undersaturations, and they give rise to a profound increase of the range and strength of the surface force. We furthermore include comparisons with predictions from a model in which the solvent only enters the description implicitly. Responses of the surface forces to changes in monomer diameter, solvent diameter, polymer density, and chain length are investigated.
Onset of the second wake: Dependence on the Reynolds number
Authors: Vorobieff Peter , Georgiev Daniel , Ingber Marc S.
Published: July 2002

The second wake transition occurs in the far wake of a bluff body. This transition destroys the Bénard–von Kármán vortex street originating in the near wake and produces a secondary vortex street with a lower characteristic frequency. We characterize the onset of the second wake for Reynolds numbers 50<Re<800 in a nearly two-dimensional soap film flow. The dimensionless distance between the cylinder and the onset of the second wake decreases with Reynolds number consistently with Re−1/2 power law. Our two-dimensional far-wake numerical simulations are in good agreement with the experiment.
Ion imaging studies of Cl(2P3/2) fragments arising in the visible photolysis of BrCl: Measurement of orientation, alignment, and alignment-free anisotropy parameters
Authors: Wouters Eloy R. , Beckert Marco , Russell Lucy J. , Rosser Keith N. , Orr-Ewing Andrew J. , Ashfold Michael N. R. , Vasyutinskii Oleg S.
Published: July 2002

The photodissociation dynamics of jet-cooled BrCl molecules have been investigated at four different wavelengths in the range 425–485 nm by high-resolution velocity map ion imaging. Four images of the Cl(2P3/2) atomic fragments are recorded at each photolysis wavelength with the probe laser polarization, respectively, linearly aligned and vertical (i.e., perpendicular to the detection axis), right circularly polarized, horizontally linearly polarized (i.e., parallel to the detection axis) and left circularly polarized on successive laser shots, thereby ensuring automatic mutual self-normalization. Appropriate linear combinations of these images allow quantification of the angular momentum alignment of the Cl(2P3/2o) fragments [i.e., the correlation between their recoil velocity (v) and their electronic angular momentum (J)] in terms of the alignment anisotropy parameters s2, α2, η2, and γ2, and determination of the “alignment-free” recoil anisotropy parameter, β0, as a function of parent excitation wavelength. Both incoherent and coherent contributions to the alignment are identified, with both simultaneous parallel and perpendicular excitations to the B 3Π(0+) and C 1Π(1) states and excitations to the Ω=±1 components of the C state contributing to the latter. The deduced values of the alignment-free β parameters indicate (wavelength dependent) contributions from both parallel and perpendicular parent absorptions in this wavelength range. Such a conclusion accords with approximate deconvolutions of the parent absorption spectrum that are currently available, and with determinations of the orientation parameter γ1′ obtained by fitting the difference image obtained when using left and right circularly polarized radiation to probe the ground state Cl atoms arising in the 480.63 nm photodissociation of BrCl when the photolysis laser radiation is polarized linearly at 45° to the detection axis.
Near-field cathodoluminescence studies on n-doped gallium nitride films
Authors: Nogales E. , Joachimsthaler I. , Heiderhoff R. , Piqueras J. , Balk L. J.
Published: July 2002

Near-field cathodoluminescence (NFCL) has been used to characterize hydride vapor phase epitaxy grown n-GaN films. This technique can obtain high resolution luminescence images and perform local measurements of the diffusion length for minority carriers in different parts of the sample. NFCL contrast observed in round growth hillocks at the sample surface, with a diameter of less than 10 μm, is compared with that observed by conventional cathodoluminescence in scanning electron microscope (CLSEM) techniques. In particular NFCL images reveal features not detected by CLSEM which is explained by the fact that under near field conditions the signal arises from a depth of only several tens of nanometers and is then directly related to the surface hillocks. Diffusion lengths of about 0.4 and 4 μm have been found for the holes in different regions of the samples at room temperature. The order of magnitude of these minority carriers diffusion lengths is in good agreement with previous measurements performed at different GaN samples with other techniques. The NFCL contrast and the differences in the measured diffusion lengths are discussed and explained by variations in local trap concentrations.
Reflective reversed-mode polymer stabilized cholesteric texture light switches
Authors: Ren Hongwen , Wu Shin-Tson
Published: July 2002

Light-switching behavior of reflective reserved-mode polymer-stabilized-cholesteric-texture (PSCT) was analyzed and validated experimentally. Our results indicate that the reflective reversed-mode PSCT offers a much higher contrast ratio and lower operating voltage than its transmissive counterpart. Effects of cell gap, birefringence, and pitch length on the contrast ratios of the reversed-mode PSCT cells were investigated. Potential applications of this device as variable optical attenuator and light shutter for telecommunication are emphasized.
Addendum: “Space charge effect and the step voltages in metal/polyimide/rhodamine–dendorimer/polyimide/metal junctions” [J. Appl. Phys. 90, 1368 (2001)]
Authors: Noguchi Yutaka , Iwamoto Mitsumasa , Kubota Tohru , Mashiko Shinro
Published: July 2002

Recently, we reported the observation of step structure, similar to Coulomb staircase, in the current–voltage characteristic of metal/polyimide(PI)/rhodamine-dendrimer(Rh-G2)/PI/metal junctions prepared by the Langmuir–Blodgett(LB) technique [J. Appl. Phys. 90, 1368 (2001)]. Calculating the additional apparent capacitance formed by electrostatic charge at the interface, the equation of threshold voltage of the step structure was derived and the interfacial space charge effect was pointed out. However, this analysis is not sufficient. In this Addendum, the equation of step voltage is derived taking into account both electrostatic charges and residual charges by electron tunneling. It was found that the derived equation is more satisfactory to explain our previous experimental result.
Plasma-enhanced chemical vapor deposition and characterization of high-permittivity hafnium and zirconium silicate films
Authors: Kato Hiromitsu , Nango Tomohiro , Miyagawa Takeshi , Katagiri Takahiro , Seol Kwang Soo , Ohki Yoshimichi
Published: July 2002

Deposition of hafnium silicate films with various hafnium contents was tried by plasma-enhanced chemical vapor deposition using tetraethoxysilane and a hafnium alkoxide. From x-ray photoelectron spectroscopy, the deposited films are confirmed to be silicate with Hf–O–Si bonds but without any Hf–Si bonds. The permittivity calculated from the capacitance of the accumulation layer increases monotonically with an increase in the hafnium content, whereas the optical band gap energy estimated from vacuum ultraviolet absorption spectra decreases. Similar results were obtained from zirconium silicate films deposited using tetraethoxysilane and a zirconium alkoxide. If we compare the films with the same hafnium or zirconium content, the hafnium silicate exhibits a higher permittivity and a larger band gap energy than the zirconium silicate.
Barrier lowering and reorientation of dipoles grafted at indium–tin–oxide/polymer interfaces
Authors: Sigaud Philippe , Chazalviel Jean-Noël , Ozanam François
Published: July 2002

Dipolar molecules have been grafted on the surface of indium–tin–oxide plates. Using these modified substrates as anodes for organic diodes made from poly–vinylcarbazole enhances the hole current by 1–2 orders of magnitude as compared to diodes with nonmodified anodes. The organization and surface concentration of the grafted monolayer, determined by infrared spectroscopy, show that the grafted layer is not densely packed. The influence of the dipolar molecules on the energy barrier height for the injection of hole current into the organic layer is also measured by internal photoemission, evidencing that the barrier is lowered by the molecular grafting. The free space around the grafted molecules allows for their reorientation under electric field. The energy barrier height has been measured as a function of time during this reorientation.
Discrete-path transport theory of physical sputtering
Authors: Stepanova M. , Dew S. K.
Published: July 2002

The fundamentals of low-energy physical sputtering currently attract increasing interest in relation to ion-based and plasma-based micro- and nanofabrication technologies. However, explaining sputtering in the sub-keV bombardment regime has long been a challenge for the kinetic theory, partly because of a simplistic treatment of the surface. Here, transport theory of sputtering is formulated, which includes a more realistic analytical model of the surface in a semi-infinite target. In contrast to the traditional theory, which is based on the linearized Boltzmann equation, the new approach uses the discrete-path master equation for particle transport in matter. Starting from the discrete-path formalism, the deflection of incoming projectiles and focusing of emitted particles when they pass the surface are described by a unified few-collision approach and combined with the bulk master equation through sophisticated boundary conditions. The numerical results are compared with available experiments. Major anisotropies that affect differential sputtering yields in the sub-keV bombardment regime are considered as obtained from the kinetic modeling.
Barrier-controlled carrier transport in microcrystalline semiconducting materials: Description within a unified model
Authors: Weis T. , Lipperheide R. , Wille U. , Brehme S.
Published: July 2002

A recently developed model that unifies the ballistic and diffusive transport mechanisms is applied to the carrier transport across potential barriers at grain boundaries in microcrystalline semiconducting materials. In the unified model, the conductance depends on the detailed structure of the band edge profile and in a nonlinear way on the carrier mean free path. Equilibrium band edge profiles are calculated within the trapping model for samples made up of a linear chain of identical grains. Quantum corrections allowing for tunneling are included in the calculation of electron mobilities. The dependence of the mobilities on carrier mean free path, grain length, number of grains, and temperature is examined, and appreciable departures from the results of the thermionic-field-emission model are found. Specifically, the unified model is applied in an analysis of Hall mobility data for n-type μc-Si thin films in the range of thermally activated transport. Owing mainly to the effect of tunneling, potential barrier heights derived from the data are substantially larger than the activation energies of the Hall mobilities. The specific features of the unified model, however, cannot be resolved within the rather large uncertainties of the analysis.
An exact variational method to calculate vibrational energies of five atom molecules beyond the normal mode approach
Authors: Yu Hua-Gen
Published: July 2002

A full dimensional variational algorithm to calculate vibrational energies of penta-atomic molecules is presented. The quantum mechanical Hamiltonian of the system for J=0 is derived in a set of orthogonal polyspherical coordinates in the body-fixed frame without any dynamical approximation. The vibrational Hamiltonian has been obtained in an explicitly Hermitian form. Variational calculations are performed in a direct product discrete variable representation basis set. The sine functions are used for the radial coordinates, whereas the Legendre polynomials are employed for the polar angles. For the azimuthal angles, the symmetrically adapted Fourier–Chebyshev basis functions are utilized. The eigenvalue problem is solved by a Lanczos iterative diagonalization algorithm. The preliminary application to methane is given. A comparison with previous results is made.
Interaction-site model description of the reorientational relaxation of molecular liquids: Incorporation of the interaxial coupling into the site–site generalized Langevin/mode-coupling theory
Authors: Yamaguchi T. , Hirata F.
Published: July 2002

The reorientational relaxation of nonlinear molecules in liquids is treated using the site–site generalized Langevin/mode-coupling theory. We found an inconsistency between the rank-1 reorientational correlation functions of different vectors on a molecule when the molecule is nearly planar. We show that it is because the coupling between the torque and the acceleration of different rotational modes is missing in the theory. A modification of the theory is proposed to incorporate this coupling, and the inconsistency between the reorientational correlation functions is remedied by the modification. We also apply the modified theory to the reorientational motion of water. The rotational part of the memory function becomes greater compared with the conventional theory, and it approaches to that from the molecular dynamics simulation. The charge-current spectrum of water is also shown to be improved by the modification.
Patterning surfaces with colloidal particles using optical tweezers
Authors: Hoogenboom J. P. , Vossen D. L. J. , Faivre-Moskalenko C. , Dogterom M. , van Blaaderen A.
Published: July 2002

A method for positioning colloidal particles on surfaces in any designed pattern is described. Optical tweezers are used to bring particles from a reservoir to the substrate where opposite surface charges are used to immobilize particles on the surface. Both chemical surface modification and polyelectrolyte coating of either substrate or colloids make the method generally applicable. We show that using this technique large, two-dimensional patterns can be created that can be dried without distortions by critical point drying. As an example we show the positioning of 79 nm radius metallodielectric particles and we show how two-dimensional patterns can be used to direct three-dimensional epitaxial crystal growth. The method is inexpensive, relatively fast, and can be fully automated.
Resonant-enhanced two photon ionization and mass-analyzed threshold ionization spectroscopy of jet-cooled 2-aminopyridines (2AP–NH2,–NHD,–NDH,–ND2)
Authors: Baek Sun Jong , Choi Kyo-Won , Choi Young S. , Kim Sang Kyu
Published: July 2002

Resonantly-enhanced two photon ionization and mass-analyzed threshold ionization (MATI) spectra of 2-aminopyridine (2AP–NH2) and its deuterated analogs have been obtained using two-photon (1+1′) excitation process via S1 intermediate states for ionization. Ionization energies of 2AP–NH2 and 2AP–ND2 are both precisely and accurately determined to be 8.1086±0.0005 and 8.1027±0.0005 eV, respectively. Two geometrical isomers, 2AP–NHD or 2AP–NDH, of which H or D is hydrogen-bonded to the nitrogen atom on the pyridine ring, respectively, are spectroscopically well isolated using the hole-burning spectroscopy in the S1 states. Corresponding ionization energies are thus separately determined to be 8.1067±0.0005 or 8.1048±0.0005 eV for 2AP–NHD or 2AP–NDH, respectively. Vibrational bands of 2-aminopyridine ions associated with various aromatic ring-skeletal modes are identified in the MATI spectra and appropriately assigned with the aid of ab initio calculation. All of the ring-skeletal vibrational frequencies observed in this work become slightly higher than those in the S1 states when the molecules are ionized, consistent with the fact that the S1–S0 excitation is due to π*–π transition. According to ab initio calculation, the amino group is in the molecular plane both in the S1 and D0 states, while it is slightly distorted in the ground state of 2-aminopyridine. Inversion modes in 2AP–ND2 and 2AP–NDH in S1 states are split into two bands due to their strong coupling with the other mode, which is most probably due to torsional motion of the amino group. Strong mode couplings are clearly manifested in interferencelike patterns observed in vibrational band structures of MATI spectra taken via those two bands in S1 states as intermediate states. A new spectroscopic scheme, in which MATI signals are used for obtaining mode-resolved spectra for the intermediate state is introduced. The vibrational band at 911 cm−1 from the S1–S0 origin that has been previously assigned as the inversion mode of 2AP–NH2 is found to actually consist of two closely-spaced different modes giving two clearly-resolved different Franck–Condon active modes in corresponding MATI spectra.
On the behavior of the ideal lines of thermodynamic functions
Authors: Sarkisov G. N.
Published: July 2002

In order to analyze the properties of the ideal lines of thermodynamic parameters use is made of some analytical equations of state, approximate integral equations of the theory of liquids, and computer simulations data. The findings of this study reveal that the ideal lines are not linear functions in the density–temperature phase diagram as was previously believed. It is also shown that the nonlinear behavior of the ideal lines is associated with the structure of the fluid, that is with the inherent features of correlation functions in different domains of the phase diagram. Changes from one linear branch of the ideal line to another are found to occur in the vicinity of the Fisher–Widom line, around which the asymptotically monotonic damping of the total correlation function alters to damping oscillations.
Magnetoabsorption spectra of intraexcitonic transitions in GaAs-(Ga,Al)As semiconductor quantum wells
Authors: Barticevic Z. , Pacheco M. , Duque C. A. , Oliveira L. E.
Published: July 2002

We present a theoretical study, within the effective-mass approximation, of the magnetoabsorption spectra of intraexcitonic terahertz transitions of light-hole and heavy-hole confined magnetoexcitons in GaAs-(Ga,Al)As quantum wells. The semiconductor quantum wells are studied under magnetic fields applied in the growth direction of the semiconductor heterostructure. The various magnetoexciton states are obtained in the effective-mass approximation by an expansion of the exciton-envelope wave functions in terms of products of hole and electron quantum-well states with appropriate Gaussian functions for the various excitonic states. Intramagnetoexciton transitions are theoretically studied by exciting the allowed excitonic transitions with σ+ (or σ−) far-infrared radiation circularly polarized in the plane of the GaAs-(Ga,Al)As quantum well. Theoretical results are obtained for the intramagnetoexciton transition energies and magneto-absorption spectra associated with excitations from 1s-like to 2p±, and 3p±-like magnetoexciton states, and found in overall agreement with optically detected resonance measurements.
Carbon in silicon: Modeling of diffusion and clustering mechanisms
Authors: Pinacho R. , Castrillo P. , Jaraiz M. , Martin-Bragado I. , Barbolla J. , Gossmann H.-J. , Gilmer G.-H. , Benton J.-L.
Published: July 2002

Carbon often appears in Si in concentrations above its solubility. In this article, we propose a comprehensive model that, taking diffusion and clustering into account, is able to reproduce a variety of experimental results. Simulations have been performed by implementing this model in a Monte-Carlo atomistic simulator. The initial path for clustering included in the model is consistent with experimental observations regarding the formation and dissolution of substitutional C–interstitial C pairs (Cs–Ci). In addition, carbon diffusion profiles at 850 and 900 °C in carbon-doping superlattice structures are well reproduced. Finally, under conditions of thermal generation of intrinsic point defects, the weak temperature dependence of the Si interstitial undersaturation and the vacancy supersaturation in carbon-rich regions also agree with experimental measurements.
Thermal effects on the Cl−+ClCH2CN reaction by Car-Parrinello molecular dynamics
Authors: Pagliai Marco , Raugei Simone , Cardini Gianni , Schettino Vincenzo
Published: July 2002

The thermal effects on the ClCH2CN+Cl−SN2 reaction at 300 K have been studied by ab initio molecular dynamics. The role of the cyano-substituent is explained by the formation of a hydrogen bond and is discussed by comparison with the reactions of ClCH3 and Cl2CH2. It is shown that the interactions occurring in the prereactive complex are sufficiently strong to be effective also at high temperature. It is also shown that the temperature effects on the activation barrier are significant.
Negative differential resistance related to self-organization phenomena in a dc gas discharge
Authors: Lozneanu E. , Popescu V. , Sanduloviciu M.
Published: July 2002

Oscillations stimulated by an S-shaped negative differential resistance originating from the self-assembly and decay of a self-organized space charge configuration in front of the anode of a dc gas discharge were observed. The oscillations appear in a resonant circuit, appropriate for the discharge, which contains a capacitor having a plate charged by secondary electron emission. The plate can be a metallic or a dielectric one because the negative differential resistance operates at zero dc current to the plate.
Improvement of the photovoltaic performance of solid-state dye-sensitized device by silver complexation of the sensitizer cis-bis(4,4′-dicarboxy-2,2′bipyridine)-bis(isothiocyanato) ruthenium(II)
Authors: Krüger Jessica , Plass Robert , Grätzel Michael , Matthieu Hans-Jörg
Published: July 2002

The photovoltaic performance of solid-state dye-sensitized solar cells based on 2,2′7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9′-spiro-bifluorene has been improved to 3.2% overall conversion efficiency under air mass (AM) 1.5 illumination by performing the dye adsorption in the presence of silver ions in the dye solution. The enhancement in overall device efficiency is a result of increased open circuit potential and short circuit current. Different spectroscopic methods, such as x-ray photoelectron, Fourier-transform infrared and UV-visible spectroscopy have been employed to scrutinize the impact of the silver on the dye-sensitized device. From spectroscopic evidence it is inferred that the silver is mainly binding to the sensitizer via the amphidentate thiocyanate, allowing the formation of ligand-bridged dye complexes.
Controlled magnetic switching in single narrow rings probed by magnetoresistance measurements
Authors: Kläui M. , Vaz C. A. F. , Bland J. A. C. , Wernsdorfer W. , Faini G. , Cambril E.
Published: July 2002

We present anisotropic magnetoresistance measurements of magnetic switching processes in narrow ferromagnetic permalloy rings fabricated with six nonmagnetic electrical contacts. We demonstrate that measuring the resistance between different contacts allows the determination of the domain wall positions. Furthermore, these measurements also yield the possibility of determining the local switching fields of different parts of the ring. This provides a very useful tool to explore the complete switching process of a single ring. We show that, by using notches of suitable size and by applying fields along appropriate directions, it is possible to select the circulation direction of the vortex state using a uniform field only.
In situ vectorial magnetization reversal study of ultrathin Co films on Pd (111) using magneto-optical Kerr effects
Authors: Shin Sung-Chul , Lee Jeong-Won , Kim Sang-Koog , Kim Jonggeol
Published: July 2002

We have investigated magnetization reversal of ultrathin Co/Pd (111) films via in situ determination of three-dimensional magnetization orientations using magneto-optical Kerr effects of p and s waves. We find that with increasing the Co thickness the easy axis of magnetization is switching from normal-to-plane to in-plane through a stable canted phase and magnetization reversal under an applied magnetic field shows very contrastive behavior depending on the easy axis of magnetization. In particular, magnetization reversal via spiral motion is observed in the Co film of the canted phase which could be explained by an existence of in-plane anisotropy and an applied field slightly tilted from the film normal.
Organic light emitting devices with enhanced operational stability at elevated temperatures
Authors: Aziz Hany , Popovic Zoran D. , Hu Nan-Xing
Published: July 2002

Temperature dependence of electroluminescence degradation is studied in organic light emitting devices containing an emitting layer composed of a mixture of N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine and tris(8-hydroxyquinoline)aluminum (AlQ3), doped with quinacridone green emitter. The emitting layer is sandwiched between hole and electron transport layers. Electroluminescence degradation in time is measured on devices operated at temperatures ranging from 22 to 100 °C. The devices demonstrated remarkable stability, even at elevated temperatures. From accelerated degradation tests, a device half-life of about 78 500, 18 700, and 8600 h can be projected for devices operated at 22, 70, and 100 °C, respectively, at an initial device luminance of 100 cd/m2. Activation energy for device degradation of 0.27 eV is consistent with the recently proposed degradation mechanism based on the unstable cationic AlQ3 species.
Manifestation of two-dimensional behavior of YBCO films in a study of their complex susceptibility
Authors: Khokhlov A. V. , Prokhorov A. Yu. , Drobotko V. F. , Levchenko G. G. , Klimov A. V.
Published: July 2002

The temperature dependence of the complex magnetic susceptibility χ′+χ″ of various YBCO films is investigated at different amplitudes of the exciting field Hac. It is found that when the temperature Tm at which the maximum is observed on the χ″(T) curve is plotted as a function of Hacα (the parameter α=1, 1/2, or 2/3, depending on the character of the coupling between crystallites), a kink appears at a certain field Hac=H1*. It is suggested that the temperature Tc1 obtained by extrapolating the Tm(Hac2/3) curve from the high-field region to zero field is the Berezinskii–Kosterlitz–Thouless transition temperature TBKT. This suggestion is based on a comparison of the present results with published data from a study of the complex susceptibility of a GdBa2Cu3O6.75 single crystal for directions of Hac parallel to the c axis of the single crystal and in its ab plane, and also on the coincidence of the calculated values of certain characteristic temperatures near the BKT transition with our experimental values.
Two-quantum electron spin–lattice relaxation in amorphous solids
Authors: G. Zakharov L. , Chotorlishvili L. L. , Buishvili T. L.
Published: July 2002

A two-quantum process of electron spin–lattice relaxation in amorphous solids is investigated. A relaxation mechanism involving two-level tunneling systems and phonons from the region of the boson peak is considered. It is shown that this mechanism is effective under certain conditions.
Self-consistent field model for spatially inhomogeneous Bose systems
Authors: Poluéktov Yu. M.
Published: July 2002

A self-consistent field (SCF) model is constructed for spatially inhomogeneous Bose systems with broken symmetry. A system of self-consistent equations is obtained for the wave functions of the quasiparticles and the wave function of the condensate particles and another system of equations for the normal and anomalous single-particle density matrices. The many-particle wave function is found. The thermodynamics of multiparticle Bose systems is constructed on the basis of a microscopic treatment in the SCF approximation. It is stressed that Bose condensation in the ideal gas model is substantially different from Bose condensation in a system of interacting Bose particles because a pair condensate is necessarily present as well as the single-particle condensate, even for an arbitrarily weak interaction. The role of single-particle and collective excitations in Bose systems is discussed.
Multidimensional and surface solitons in a nonlinear elastic medium
Authors: Kovalev A. S. , Syrkin E. S. , Maugin J. A.
Published: July 2002

Nonlinear shear waves in two-dimensional systems (in particular, surface waves) are investigated with allowance for the spatial dispersion of the elastic medium. It is shown that the dispersion plays an important role in the structural and modulational stability of the nonlinear waves and to a large degree determines the directions of localization of phonons in a nonlinear localized wave and, in particular, the possibility of existence of elastic surface solitons. By means of an asymptotic procedure, solutions are found for small-amplitude two-dimensional elastic shear solitons of the one-parameter stationary-profile type and for envelope solitons and also for surface solitons localized near an ideal surface of an elastic half space. Localized excitations of this kind can exist only in a medium with a “focusing” (soft) nonlinearity and positive dispersion ∂2ω/∂k2>0, where ω(k) is the dispersion relation for linear waves. A procedure is proposed for finding solutions for surface envelope solitons localized near a surface covered with a layer of another substance. A comparison is made between the structures of the surface shear solitons at an ideal surface and at a surface with a film coating.
Cyclometalated Ir complexes in polymer organic light-emitting devices
Authors: Lamansky Sergey , Djurovich Peter I. , Abdel-Razzaq Feras , Garon Simona , Murphy Drew L. , Thompson Mark E.
Published: July 2002

Several new iridium based cyclometalated complexes were investigated as phosphorescent dopants for molecularly doped polymeric organic light-emitting diodes. Specifically, the complexes used in this study were iridium (III) bis(2-phenylpyridinato-N,C2′) (acetylacetonate) [ppy], iridium (III) bis(7,8-benzoquinolinato-N,C3′) (acetylacetonate) [bzq], iridium (III) bis(2-phenylbenzothiazolato-N,C2′) (acetylacetonate) [bt], iridium (III) bis(2-(2′-naphthyl)benzothiazolato-N,C2′) (acetylacetonate) [bsn] and iridium (III) bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3′) (acetylacetonate) [btp]. Single layer devices of doped polyvinylcarbazole: 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole give maximum external quantum efficiencies that varied from 3.5% for the ppy dopant to 0.4% for the btp dopant. Several different device heterostructure architectures were explored, and the best quantum efficiency of the devices reached 4.2% for the heterostructures.
Improving solar cell efficiencies by down-conversion of high-energy photons
Authors: Trupke T. , Green M. A. , Würfel P.
Published: July 2002

One of the major loss mechanisms leading to low energy conversion efficiencies of solar cells is the thermalization of charge carriers generated by the absorption of high-energy photons. These losses can largely be reduced in a solar cell if more than one electron–hole pair can be generated per incident photon. A method to realize multiple electron–hole pair generation per incident photon is proposed in this article. Incident photons with energies larger than twice the band gap of the solar cell are absorbed by a luminescence converter, which transforms them into two or more lower energy photons. The theoretical efficiency limit of this system for nonconcentrated sunlight is determined as a function of the solar cell’s band gap using detailed balance calculations. It is shown that a maximum conversion efficiency of 39.63% can be achieved for a 6000 K blackbody spectrum and for a luminescence converter with one intermediate level. This is a substantial improvement over the limiting efficiency of 30.9%, which a solar cell exposed directly to nonconcentrated radiation may have under the same assumption of radiative recombination only.
Pseudodielectric function of ZnGeP2 from 1.5 to 6 eV
Authors: Blickle V. , Flock K. , Dietz N. , Aspnes D. E.
Published: July 2002

We report pseudodielectric function data 〈ε〉=〈εa1〉+i〈εa2〉 and 〈ε〉=〈εc1〉+i〈εc2〉 for the optically uniaxial material ZnGeP2, critical point energies of structures in these data, and dielectric function data for the natural oxide. Annealing reduces the values of the peaks of 〈εa2〉.
High-performance epitaxial Na0.5K0.5NbO3 thin films by magnetron sputtering
Authors: Blomqvist Mats , Koh Jung-Hyuk , Khartsev Sergey , Grishin Alex , Andréasson Johanna
Published: July 2002

Epitaxial Na0.5K0.5NbO3 (NKN) thin films have been grown on LaAlO3 substrates by rf magnetron sputtering of a stoichiometric, high-density, ceramic target. X-ray diffraction analysis showed c-axis oriented cube-on-cube growth. Micrometer size interdigital capacitor (IDC) structures were defined on the surface of the NKN film using photolithography. The electrical characterization at 1 MHz showed dissipation factor tan δ of 0.010, tunability 16.5% at 200 kV/cm and dielectric permittivity εr=470. The frequency dispersion of εr between 1 kHz and 1 MHz was 8.5% and the IDCs showed very good insulating properties with leakage current density on the order of 30 nA/cm2 at 400 kV/cm. The polarization loop exhibits weak ferroelectric hysteresis with maximum polarization 23.5 μC/cm2 at 600 kV/cm. These results are promising for tunable microwave devices based on rf sputtered NKN thin films.
Studies of phase segregation in Cd1−xZnxTe using surface photovoltage spectroscopy
Authors: Zidon Y. , Yang Jihua , Shapira Yoram
Published: July 2002

Cd 1−x Zn x Te wafers used for x-ray detector arrays have been failure analyzed using x-ray diffraction, x-ray electron spectroscopy (XPS), energy dispersive spectroscopy (EDS), and surface photovoltage spectroscopy (SPS). The last shows ZnTe segregation in failed pixels while the precipitant phase is too small to be observed by the other techniques. The Zn concentration, measured using EDS and XPS, was higher than that deduced from SPS data, confirming the conclusion. The segregation can be revealed only by SPS since it is sensitive to the electronic structure and thus to x in each phase while the other techniques average x over their measurement volume.
State switching in Bi-doped La0.67Ca0.33MnO3 and the effects of current
Authors: Sun J. R. , Gao J. , Kang L.
Published: July 2002

Electronic transport and magnetic properties of La0.477Bi0.193Ca0.33MnO3 have been experimentally studied. Different resistive behaviors are observed in the cooling and warming processes. The system first stays at a high resistive state, and switches to a state of lower resistivity when it is cooled below a critical temperature. However, keeping the sample at a temperature below ∼60 K, a relaxation to the high resistive state takes place. This process is current dependent, and the application of a large current slows down the relaxation greatly. There is a strong competition between the two resistive states, which causes a switch of the system between states.
Miniature vapor-cell atomic-frequency references
Authors: Kitching J. , Knappe S. , Hollberg L.
Published: July 2002

We propose a sub-millimeter-scale vapor-cell atomic-frequency reference based on a micromachined vapor cell, all-optical excitation, and advanced diode-laser technology. We analyze theoretically the performance of such a device as a function of cell size. Initial measurements on small-scale vapor cells support the theoretical treatment.
Tuning the electronic behavior of Au nanoparticles with capping molecules
Authors: Zhang P. , Sham T. K.
Published: July 2002

The electronic behavior of gold nanoparticles (NPs) of ∼2 nm capped with dendrimer and thiol molecules was studied with Au L3,2-edge x-ray absorption near-edge structure (XANES). The results reveal the tunability of the d-electron distribution in the Au NPs by selective capping. That is, that the Au atoms in the NPs gain 5d electrons (relative to the bulk) when capped with weakly interacting dendrimers and lose 5d electrons when capped with strongly interacting thiol molecules. A semiquantitative analysis of the d-charge (holes) distribution is presented. This work demonstrates the important role of the capping molecules in the d-charge distribution of Au NPs and the usefulness of XANES in probing the electronic behavior of transition metal NPs.
Silicon-based optical waveguide polarizer using photonic band gap
Authors: Zhao Dengtao , Shi Bin , Jiang Zuimin , Fan Yongliang , Wang Xun
Published: July 2002

Based on different photonic band structures of TE and TM polarization modes in periodic multilayers, a method to realize the waveguide polarizer is proposed. The waveguide structure contains a SiO2 core layer sandwiched between two multilayers of alternately stacked poly-Si and SiO2, and the whole structure can be grown on a Si substrate. Its propagation characteristics are studied theoretically. High extinction ratio over 40 dB at a light wavelength of 1.3 μm is expected in the waveguide of only 40 μm long, accompanied with very low propagation loss of the passive TE mode. These characteristics are very suitable for the applications in integrated optics. The fabrication of this polarizer structure by using the magnetron sputtering method is demonstrated.
Giant exciton-light coupling in ZnO quantum dots
Authors: Gil Bernard , Kavokin Alexey V.
Published: July 2002

We investigate the strength of the coupling of the electronic states with the electromagnetic field in ZnO nanospheres, taking into account the retardation effect. We show that the coupling strength is particularly strong: the bulk properties are so enhanced that the radiative decay time can reach some 200 ps for quantum dot sizes of some 30 nm.
Experimental studies of the electron–phonon interaction in InGaAs quantum wires
Authors: Sugaya T. , Bird J. P. , Ferry D. K. , Sergeev A. , Mitin V. , Jang K.-Y. , Ogura M. , Sugiyama Y.
Published: July 2002

Electron-heating measurements are used to compare the form of the electron–phonon interaction in two-dimensional, and quasi-one-dimensional, InGaAs quantum wires. Evidence for a strongly enhanced interaction is found in the quasi-one-dimensional wire, and is suggested to result from the presence of the singularities in its electronic density of states. The Bloch–Gruneisen criterion is easily violated in this wire, and its energy-loss function is found to show a weak temperature dependence, which is argued to result from a saturation of scattering processes in the uppermost one-dimensional subband.