bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical and experimental studies of CIDNP kinetics in recombination of radical pairs by the method of switched external magnetic field. III. Free radicals in homogeneous solution
Authors: Fedin M. V. , Bagryanskaya E. G. , Purtov P. A. , Makarov T. N. , Paul H.
Published: September 2002

The method of chemically induced dynamic nuclear polarization in a switched external magnetic field (SEMF CIDNP) is applied here for the first time to an experimental study of short-lived neutral radicals in homogeneous solutions. With three photochemical reactions it is exemplified, that SEMF CIDNP allows investigations of the kinetics of the transient species with high time-resolution as well as a determination of their spin relaxation times in low magnetic fields. A theoretical approach is developed, which permits simulation and analysis of the experimental data. In weak magnetic fields (0.5–2.0 mT) the effective spin-lattice relaxation times for the decay of the chemically induced spin polarizations in benzyl, tert-butyl, and 2-hydroxy-2-propyl radicals are found to be T1=(3.8±0.5) μs, T1=(7.8±0.5) μs, and T1=(2.5±0.5) μs, respectively, in benzene solution at room temperature. They are in fair agreement with relaxation times determined by time-resolved X-band electron paramagnetic resonance spectroscopy at strong magnetic fields (≈350 mT).
Magnetic entropy change and its temperature variation in compounds La(Fe1−xCox)11.2Si1.8
Authors: Hu F. X. , Qian X. L. , Sun J. R. , Wang G. J. , Zhang X. X. , Cheng Z. H. , Shen B. G.
Published: September 2002

Magnetic entropy change ΔS of compounds La(Fe1−xCox)11.2Si1.8 with the cubic NaZn13-type structure was investigated around their Curie temperature TC. It is found that the phase transition is completely reversible, indicating a nature of second order phase transition. The maximum value of |ΔS|∼13.0 J/kg K under a field of 5 T was achieved in compound LaFe11.2Si1.8 at its TC of ∼222 K, which exceeds that of most materials involving a second order transition at the corresponding temperature. With increasing substitution of Co for Fe from x=0 to x=0.8, TC shifts from 222 to 307 K and entropy change decreases. However, |ΔS| still has a considerable magnitude near room temperature. The large magnetic entropy change is believed to be due to the abrupt change of magnetization at TC, which is associated with the strong structural and magnetic interplay in the compounds.
Fluorene–fluorenone copolymer: Stable and efficient yellow-emitting material for electroluminescent devices
Authors: Panozzo S. , Vial J-C. , Kervella Y. , Stéphan O.
Published: September 2002

We have synthesized and characterized a new fluorene copolymer exhibiting bright yellow luminescence. In order to ensure a complete π-stacking of the active layer, a 9-fluorenone monomeric unit (FOne) has been used as comonomer in conjunction with the more classical 9,9-di-n-nonylfluorene unit. As expected with fluorene-based materials, when excited at 370 nm, the corresponding dilute copolymer solution photoluminescence spectra exhibit a main peak centered at 450 nm in the blue part of the visible spectrum. However, in the solid state, immediate structural reorganization of the layer occurs, leading to a red-shifted emission (bright yellow emission) centered at 550 nm. The origin of the emitted light has been attributed to excimers and/or aggregates based on short FOne segments and involves mainly exciton transfer between nonaggregated fluorene segments and aggregated ones. It is noteworthy that organic light-emitting devices based on these new materials exhibit no spectral evolution upon device operation. However, although stacking leads generally to a detrimental quenching of the luminescence in the solid state, as for regular poly(alkyl-fluorene), the luminescence efficiency of the fluorene–fluorenone copolymer remains suitable for device preparation. High material stability is attributed to an efficient and fast structural reorganization of the active layer, triggered by the small proportion of fluorenone. High electroluminescence efficiency, when compared to aggregated regular poly(alkyl-fluorene), results from an improved electron injection, a better carrier transport, and the conjunction of an efficient energy transfer from fluorene segments to excimers and/or aggregates with the implication of spin triplet, which is often lacking when using regular semiconducting polymers.
Micro-Raman investigation of strain in GaN and AlxGa1−xN/GaN heterostructures grown on Si(111)
Authors: Tripathy S. , Chua S. J. , Chen P. , Miao Z. L.
Published: September 2002

Using micro-Raman spectroscopy, we have studied the vibrational properties of GaN and Al0.5Ga0.5N/GaN long period superlattices (SLs) grown on Si(111). Crack-free areas of GaN layers grown on Si(111) exhibit residual tensile stress, which is evidenced by the red shift of the frequency of E2(TO) phonon. We have derived the strain cartography in GaN and Al0.5Ga0.5N/GaN long period SLs, which shows that cracking leads to strain relaxation. In addition, the AlGaN layers on GaN introduce an additional component of compressive strain into the GaN layers in these SLs. The amount of strain is quantified using micro-Raman analyses and by taking into account the elastic properties of GaN and AlGaN. By introducing a thin, low temperature InGaN interlayer, we could significantly reduce the crack density of the GaN layer.
Influence of nitrogen on diamond growth in oxyacetylene combustion chemical vapor deposition
Authors: Okkerse M. , de Croon M. H. J. M. , Kleijn C. R. , Marin G. B. , van den Akker H. E. A.
Published: September 2002

Addition of di-nitrogen to the feed gas has been shown to greatly influence growth rates and morphology of the deposited layer in various diamond chemical vapor deposition (CVD) techniques. In this article, several hypotheses for these phenomena, as presented in literature, are tested for the case of diamond combustion CVD with the aid of an atmospheric pressure oxyacetylene flame. For this purpose, one-dimensional and two-dimensional simulations are performed of the hydrodynamics, the combustion and deposition chemistry, and the nitrogen chemistry. Based on the simulation results, several proposed hypotheses can be ruled out as possible explanations for the observed phenomena. It is concluded, that the most likely hypotheses are: (i) the presence of nitrogen atoms in the diamond lattice, enhancing diamond growth by acting on the electron structure of surface dimer bonds, and (ii) selective adsorption of nitrogen-containing species on the surface, selectively increasing growth in the (100) direction. It is found that possible gas phase candidates for affecting diamond growth are NH, NH2, NH3, CN, HCN, H2CN, and NCO.
Phonon behaviors and electronic structures of the filled skutterudite YbyCo4Sb12 compounds: An electron tunneling study
Authors: Nagao Jiro , Nataraj Devaraj , Ferhat Marhoun , Uchida Tsutomu , Takeya Satoshi , Ebinuma Takao , Anno Hiroaki , Matsubara Kakuei , Hatta Eiji , Mukasa Kōichi
Published: September 2002

Electron tunneling experiments were performed on YbyCo4Sb12–Al oxide–Al junctions for y=0–0.25 at 4.2 K. In the second derivative tunneling spectrum of CoSb3 compound (y=0), three peaks were observed at around 5, 20, and 33 mV, which are closely related to an optical phonon mode with a rigid rectangle, Sb–Sb bond bending and bond stretching, and a large Co atomic motion, respectively. Appearance of the strong peak at 7 mV observed in Yb-filled samples corresponds to a rattled phonon mode of Yb ions. The peak energy due to the Sb–Sb bonds is unchanged, whereas the one due to Co motions shifts to lower with increasing Yb concentration. This fact indicates that the filled Yb ions strongly interact to the host framework Co atoms, which were clearly observed in the change of tunneling conductance.
Simulating quantum transport in nanoscale transistors: Real versus mode-space approaches
Authors: Venugopal R. , Ren Z. , Datta S. , Lundstrom M. S. , Jovanovic D.
Published: September 2002

In this article, we present a computationally efficient, two-dimensional quantum mechanical simulation scheme for modeling electron transport in thin body, fully depleted, n-channel, silicon-on-insulator transistors in the ballistic limit. The proposed simulation scheme, which solves the nonequilibrium Green’s function equations self-consistently with Poisson’s equation, is based on an expansion of the active device Hamiltonian in decoupled mode space. Simulation results from this method are benchmarked against solutions from a rigorous two-dimensional discretization of the device Hamiltonian in real space. While doing so, the inherent approximations, regime of validity and the computational efficiency of the mode-space solution are highlighted and discussed. Additionally, quantum boundary conditions are rigorously derived and the effects of strong off-equilibrium transport are examined. This article shows that the decoupled mode-space solution is an efficient and accurate simulation method for modeling electron transport in nanoscale, silicon-on-insulator transistors.
Magnetic resonance studies of chemically intercalated LixV2O5 aerogels
Authors: Stallworth P. E. , Johnson F. S. , Greenbaum S. G. , Passerini S. , Flowers J. , Smyrl W.
Published: September 2002

Li 7 ,  V51 solid-state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) measurements have been performed upon chemically lithiated LixV2O5 aerogels, with compositions of 1.00<x<5.84. These compounds can intercalate reversibly large amounts of Li+ and, therefore, are of interest as battery cathodes. Still, the mechanism regarding the electron transfer from an inserted lithium metal to a host aerogel V2O5 and details regarding the lithium cation environments are not fully understood. LixV2O5 crystals are known to exhibit various structural phase changes and, when multiple phases are present, the capability of the material to intercalate reversibly appears to be adversely affected. On the other hand, aerogels have no such multiphase behavior and aerogel based cathodes exhibit greater stability upon cycling. NMR shows that neither the structure nor the dynamics vary greatly with the amount of lithium content, and that the lithiated aerogel is best described as a single-phase material. Characterization of lithium and vanadium sites is performed through analysis of both NMR and EPR spectra. Li7 line shapes are affected by first-order quadrupolar, magnetic dipolar interactions and motional narrowing. At and above room temperature, relaxation is governed primarily by a quadrupolar mechanism. NMR derived activation energies and diffusion coefficients are different from those of bronzes and electrochemically intercalated V2O5. V51 NMR lines, indicative of the presence of V5+ at all compositions, undergo diamagnetic shifts of up to about 50 ppm with an increase in lithium content. These results imply the presence of oxidized impurities or electronic charge delocalization. Additionally, EPR measurements provide evidence of VO2+ impurities and indirect evidence of nonbridging oxygen at high lithium contents.
Chemi-ionization in neon plasma
Authors: Sheverev V. A. , Stepaniuk V. P. , Lister G. G.
Published: September 2002

The density of electrons formed in binary collisions of 2p53s neon atoms was measured in the afterglow of a low pressure glow discharge by observing the high energy tail of the electron energy distribution function (EEDF) using a Langmuir probe. In the afterglow, the bulk plasma electrons thermalize but the density of 2p53s neon atoms remains significant. In an ionizing collision, a pair of these atoms releases high energy (∼11.6 eV) electrons which form a characteristic peak in the EEDF. Simultaneously with the chemi-ionization electrons, the densities of 1s2, 1s3, 1s4, and 1s5 neon atoms were independently measured using diode laser absorption spectroscopy. It was found that the data obtained are described well by a single chemi-ionization reaction when the 2p53s configuration is considered a single state. The corresponding rate coefficient, found to be (3.2±0.4)×10−10 cm3 s−1 at a temperature of 310 K, is recommended for use in discharge modeling.
Penetrability in model colloid–polymer mixtures
Authors: Schmidt Matthias , Fuchs Matthias
Published: September 2002

In order to study the effects of penetrability in mixtures of dissimilar particles we consider hard (colloidal) spheres and penetrable spheres. The latter may be taken to represent ideal, noninteracting polymer coils. Polymers and colloids interact by means of a repulsive step-function pair potential, which allows for insertion of colloids into the polymer coil. The potential strength is obtained from scaling arguments for the cross virial coefficient of true colloid–polymer systems. For this model we construct a geometry-based density functional and apply it to bulk fluid demixing. We find that taking into account penetrability leads to a significant stabilization of the mixed phase for large polymer-to-colloid size ratio.
Optimal pump–dump control and time-frequency resolved spectroscopy of ground-state wave-packet focusing
Authors: Shen Zhenwen , Engel Volker , Xu Ruixue , Cheng Jixin , Yan YiJing
Published: September 2002

The theory of optimal control together with that of transient probe absorption spectroscopy are applied for control and detection of molecular wave packet dynamics in the weak response regime. We obtain a globally optimal pump–dump control field which drives the initial state to the predefined target, focused at the inner-turning point region of the electronic ground state potential. As for the detection, we calculate both the integrated and the dispersed transient pump–dump–probe absorption signals and adopt the difference detection schemes to reveal the optimally controlled molecular dynamics.
Verification of the nucleation theorem for the internally constrained one-dimensional hard rod fluid
Authors: Punnathanam Sudeep , Corti David S.
Published: September 2002

The generalized nucleation theorem as derived by Bowles et al. [J. Chem. Phys. 115, 1853 (2001)] had been verified for nonuniformities generated by a cavity and a cluster within the one-dimensional hard rod fluid. We further verify the nucleation theorem for nonuniformities created by a cavity and by a cluster containing m particles within the internally constrained hard rod fluid. The given results provide additional evidence of the validity of the generalized nucleation theorem even down to the molecular level.
Sample size autocorrelation analysis of kinetic data: Resolving reaction path heterogeneity
Authors: Murugan R.
Published: September 2002

Detailed theory on a method of analysis of kinetic data based on sample size autocorrelation function was developed to identify the presence of reaction path heterogeneity based on the master equation approach. The behavior of such functions under ideal conditions as well as under heterogeneity conditions was discussed and some of the potential applications of this method have been summarized. Application of this theory to renaturation kinetics of DNA not only revealed the presence of path heterogeneity in the second zipping phase and quantified it but also proved that it was due to nonreactive modes of dynamics on conformational energy landscape, which agreed well with earlier studies.
Experimental determination of the site–site radial distribution functions of supercooled ultrapure bulk water
Authors: Botti A. , Bruni F. , Isopo A. , Ricci M. A. , Soper A. K.
Published: September 2002

Neutron diffraction data on three isotopically substituted samples of liquid water in the supercooled regime are presented. Supercooling of ultrapure water has been achieved by means of a polytetrafluoroethylene coating on a standard vanadium sample container. Supercooling water to T=267 K, with density ρ=100.2 atoms/nm3, at ambient pressure gives a shift of the first peak of the three partial structure factors toward lower wave numbers, while the second peak of the oxygen–oxygen partial structure factor moves outwards compared to ambient water. The present data in Q space are compatible with previous x-ray experiments performed on supercooled water. Moreover the partial radial distribution functions obtained here for the first time for the bulk liquid at this thermodynamic state compare well with previous experiments performed at higher pressure.
Burrowing of Pt nanoparticles into SiO2 during ion-beam irradiation
Authors: Hu Xiaoyuan , Cahill David G. , Averback Robert S.
Published: September 2002

Atomic force microscopy and cross-sectional transmission electron microscopy are used to characterize the evolution of nanoparticle/substrate interfaces during heavy-ion bombardment. Pt nanoparticles, prepared by annealing 3 Å Pt films on SiO2, embed into the substrates following 800 keV Kr+ irradiation. For Pt particles with diameters 5–20 nm, the depth of the embedding increases with an ion dose until the particles are fully submerged at a dose of ∼1016 cm−2. The results are explained by capillary driving forces and an ion-induced viscous flow of amorphous SiO2. The irradiation-induced viscosity of SiO2 needed to explain our results is ∼0.9×1023 Pa ion cm−2, consistent with previous measurements using stress relaxation. Similar results are obtained for 10 keV He+ irradiation, suggesting that ion-induced viscosity arises from localized defects rather than from the creation of large melt zones. The embedding of Pt particles is inhibited, however, for energetically unfavorable substrates such as alumina.
Optical studies of ZnS:Mn films grown by pulsed laser deposition
Authors: Yeung K. M. , Tsang W. S. , Mak C. L. , Wong K. H.
Published: September 2002

High-quality ZnS:Mn thin films have been deposited on (001)Si substrates at various temperatures using pulsed laser deposition. Their structural properties were characterized by x-ray diffraction. Optical studies by spectroscopic ellipsometry, optical transmittance, and photoluminescence measurements were systematically carried out on all film samples. In the analysis of the measured SE spectra, a modified double-layer Sellmeier model was adopted to represent the optical properties of the ZnS:Mn films. In this model, the films were assumed to consist of two layers—a bottom bulk ZnS:Mn layer and a surface layer composed of bulk ZnS:Mn as well as void. Good agreement was obtained between the measured spectra and the model calculations. Changes in the refractive indices of the ZnS:Mn films as a function of growth temperature were investigated. The PL and absorption measurements revealed that the orange-yellow emission band at ∼590 nm and the absorption edge at ∼370 nm upshifted to shorter wavelengths for films deposited at higher substrate temperatures. These results imply that the energy gap of the ZnS:Mn films increases with growth temperature. The observed changes of optical properties in these films are correlated to their structural qualities.
Hole scattering near the valence band edge in wurtzite gallium nitride
Authors: Albrecht J. D. , Ruden P. P. , Reinecke T. L.
Published: September 2002

Scattering rates for holes in wurtzite GaN are investigated theoretically taking into account six valence bands including the spin-orbit interaction. Scattering rates for acoustic deformation potential, piezoelectric acoustic phonon, polar optical phonon, and impurity scattering are formulated. Results are given for intra- and interband processes, and their role in understanding transport measurements is discussed. The scattering probabilities are anisotropic owing to incomplete initial and final state overlap, to nonuniform final state density, and angular dispersion of the phonon modes. In addition, the level of detail obtained is sufficient to examine the modifications of the scattering rates with the application of uniaxial stress, and numerical results are presented.
Application of Fourier harmonic analysis to the electromechanical response of an electroactive material
Authors: DiAntonio C. B. , Williams F. A. , Pilgrim S. M. , Schulze W. A.
Published: September 2002

Signal decomposition through Fourier analysis can aid quantification of the electromechanical properties (induced strain and polarization) of electroactive materials. Spectral analysis of the strain and polarization, obtained from the Fourier transform, provides a unique characterization tool that better conveys material response than can be accomplished with polynomial fitting. The derived coefficients can be mapped onto those in the Devonshire phenomenology. The technique is demonstrated by analysis of a lead magnesium niobate relaxor ferroelectric [0.9875(0.935PMN–0.065PT)–0.0125BT or 0.9233PMN–0.06419PT–0.0125BT] operating in the electrostrictive regime. Fourier analysis, applied to a materials response, provides the first quantitative linkage to materials coefficients. A generalized mathematical approach has been derived that equates a Fourier series expression, from the transform of a time-domain electromechanical response, to the basic underlying physics developed in Devonshire theory. Thus, the electrostrictive strain and polarization coefficients are calculated directly from the harmonic spectrum of the response. A benefit of the Fourier transform approach is the direct calculation of electrostrictive (and piezoelectric) coefficients with quantitative criterion for truncation and a “goodness of fit” criterion related to the zero frequency component. The real strength of the approach lies in its ability to accommodate electromechanical hysteresis and provide a distinct quantification of a given strain response. This is accomplished while faithfully describing the true harmonic content of the signal. The coefficients provide a descriptive fingerprint for use of the material under varying conditions.
Effect of geometrical shape on magneto–Peltier and Ettingshausen cooling in Bi and Bi0.88Sb0.12 polycrystals
Authors: Yamashita Osamu , Tomiyoshi Shoichi
Published: September 2002

The cooling temperatures of rectangular parallelepiped Bi and Bi0.88Sb0.12 polycrystals of various dimensions were measured at 298 K as a function of electric current in magnetic fields up to 2.17 T, where the magnetic field was aligned along the thickness (t) direction of the sample and copper current leads were soldered to the centers of two ridges along the thickness direction on two diagonally opposite corners of a rectangle with length L and width W. With this configuration, the temperature measurements were carried out at both end surfaces with the cross section (W×t). The thermoelement was not in contact with a heat sink. The maximum cooling temperature at the cooled surface increases with increasing magnetic field, but it depends strongly on not only the aspect ratio of the sample but also its thickness. It was found that geometrically optimum shaping of a thermoelement is very important for achieving high-cooling efficiency. A thermoelement made of Bi0.88Sb0.12 with optimum dimensions of L=15 mm, W=2 mm, and t=3 mm effects the high-cooling temperature of 4.2 K in a magnetic field of 2.17 T. As a result, the cooling temperature reached 74% of the cooling temperature of 5.7 K obtained with p-type Bi2Te3 of L=12 mm and W=t=4.8 mm which has a thermoelectric figure of merit of ZT=0.87 at 298 K.
Shear viscosity of a supercooled polymer melt via nonequilibrium molecular dynamics simulations
Authors: Varnik F. , Binder K.
Published: September 2002

Using nonequilibrium molecular dynamics simulations, we compute the shear viscosity, ηs, of a glass forming polymer melt at temperatures ranging from the normal liquid state down to the supercooled state. For this purpose, the polymer melt is confined between two solid walls and a constant force pointing in direction parallel to the walls is applied on each monomer thus giving rise to a Poiseuille flow. It is shown that ηs(T) does not exhibit an Arrhenius-type behavior but can be described both by a power law (mode coupling theory) and by a Vogel–Fulcher–Tammann law. A similar behavior is observed in recent experiments above the glass transition temperature. The diffusion coefficient is computed using the mean square displacements in direction perpendicular to the flow. Combined with the knowledge of ηs(T), it is then shown that the Stokes–Einstein relation is valid at high temperatures, whereas deviations are observed in the supercooled regime in agreement with experiments. Moreover, the local viscosity, η(z), is also computed and its reliability is discussed. Using the sharp rise of η(z) close to the wall, we estimate zwall, the effective position of the wall. It is found that zwall moves towards the film center at lower T thus leading to a decrease of the (hydrodynamic) width of the system. Furthermore, we observe that the curves for η(z)/ηs at various temperatures superimpose if the data are depicted versus z−zwall(T). This suggests that the spatial and temperature dependence of the local viscosity separate if the effective position of the wall is chosen as a new reference plane.
Correspondence between configurational temperature and molecular kinetic temperature thermostats
Authors: Delhommelle Jerome , Evans Denis J.
Published: September 2002

Molecular fluids undergoing shear flow are often modeled using a homogeneous nonequilibrium molecular dynamics algorithm. To reach a steady state, this method must be used in conjunction with a thermostating mechanism which duplicates the heat dissipation in the experimental setup (e.g., by conduction to the shearing boundaries). The most commonly used type of thermostat involves fixing the center of mass kinetic (c.m.) temperature. Though perfectly valid, this approach does not seem to be the most realistic for a molecular fluid since heat is removed only through the 3 degrees of freedom of the center of mass for each molecule. The second type of thermostat involves fixing the “atomic” kinetic temperature and therefore takes into account all degrees of freedom. However, since the streaming velocity of atoms within their constituent molecules is unknown, the implementation of such a thermostat is problematic and relies on incorrect assumptions on the streaming velocity of atoms. The recently developed configurational temperature thermostat requires no assumption on the streaming velocity of atoms and takes into account all degrees of freedom. Using a configurational temperature thermostat to thermostat homogeneous shear flow thus seems to be a more realistic approach than the c.m. kinetic thermostat. In this work, we apply this configurational temperature thermostat to the study of linear alkanes (C10 and C20) undergoing shear flow. The results so obtained are compared with those obtained using a c.m. kinetic thermostat. Our aims are (1) to test the influence of the total number of degrees of freedom of the system, (2) to make a connection between the results obtained with the two types of thermostats. By carefully examining the energies of the internal modes, we have been able to characterize the loss of accuracy of a c.m. kinetic thermostat at high shear rates and for high molecular weight compounds. Finally, we establish a correspondence between the two types of thermostats by showing that, for the internal modes, a simulation at a fixed c.m. kinetic temperature is equivalent to a simulation at a fixed but higher configurational temperature.
Isotope effects in linear dihydrogen bonded complexes containing LiH
Authors: McDowell Sean A. C. , Forde Toni S.
Published: September 2002

An ab initio study of linear dihydrogen-bonded complexes containing LiH (acting as a proton acceptor) was undertaken. The complexes studied were LiH⋯HF, LiH⋯HCN, and LiH⋯HCCH. Equilibrium geometries and harmonic vibrational frequencies were computed at various levels of theory using a 6-31++G(d,p) basis set. It was found that the LiH⋯HF complex was not stable at all levels of theory. The relative stabilities of H-bonded and D-bonded isotopomers of LiH⋯HCN and LiH⋯HCCH were determined by differences in zero-point vibrational frequencies. For LiH⋯HCN isotopomers, it was found that the LiD⋯HCN was favored over LiH⋯DCN with a relative stability of 54 cm−1 calculated at the Quadratic Configuration Interaction—Singles and Doubles level. Similarly, LiD⋯HCCH is favored over LiH⋯DCCH, with a lower value for the relative stability. The relative stabilities of the H-bonded and D-bonded species LiH⋯HCCD and LiH⋯DCCH indicated that the D-bonded complex was energetically favored, in accordance with the Buckingham–Liu theory.
Excited states of beryllium atom from explicitly correlated wave functions
Authors: Gálvez F. J. , Buendı́a E. , Sarsa A.
Published: September 2002

A study of the first excited states of beryllium atom starting from explicitly correlated wave functions is carried out. Several properties are obtained and discussed focusing on the analysis of the Hund’s rules in terms of the single-particle and electron pair intracule and extracule densities. A systematic study of the differences on the electronic distributions of the singlet and triplet states is carried out. The trial wave function used to describe the different bound states consists of a generalized Jastrow-type correlation factor times a configuration interaction model wave function. This model wave function has been fixed by using a generalization of the optimized effective potential method to deal with multiconfiguration wave functions. The optimization of the wave function and the calculation of the different quantities is carried out by means of the Variational Monte Carlo method.
Suppression of interdiffusion in GaAs/AlGaAs quantum-well structure capped with dielectric films by deposition of gallium oxide
Authors: Fu L. , Wong-Leung J. , Deenapanray P. N. K. , Tan H. H. , Jagadish C. , Gong Bin , Lamb R. N. , Cohen R. M. , Reichert W. , Dao L. V. , Gal M.
Published: September 2002

In this work, different dielectric caps were deposited on the GaAs/AlGaAs quantum well (QW) structures followed by rapid thermal annealing to generate different degrees of interdiffusion. Deposition of a layer of GaxOy on top of these dielectric caps resulted in significant suppression of interdiffusion. In these samples, it was found that although the deposition of GaxOy and subsequent annealing caused additional injection of Ga into the SiO2 layer, Ga atoms were still able to outdiffuse from the GaAs QW structure during annealing, to generate excess Ga vacancies. The suppression of interdiffusion with the presence of Ga vacancies was explained by the thermal stress effect which suppressed Ga vacancy diffusion during annealing. It suggests that GaxOy may therefore be used as a mask material in conjunction with other dielectric capping layers in order to control and selectively achieve impurity-free vacancy disordering.
Structural and optical properties of thin zirconium oxide films prepared by reactive direct current magnetron sputtering
Authors: Venkataraj S. , Kappertz Oliver , Weis Hansjörg , Drese Robert , Jayavel R. , Wuttig Matthias
Published: September 2002

Thin films of zirconium oxide have been deposited onto glass and Si(100) substrates at room temperature by reactive dc magnetron sputtering of a metallic Zr target in an argon–oxygen atmosphere. The films were characterized by Rutherford backscattering spectroscopy, x-ray diffraction, x-ray reflectometry, atomic force microscopy, and optical spectroscopy to investigate the variation of stoichiometry, structure, density, and optical properties upon increasing oxygen partial pressure. The Zr target shows a well-defined hysteresis upon varying the oxygen pressure as determined by quartz crystal microbalance measurements. Rutherford backscattering experiments reveal that the films are metallic with a small amount of oxygen incorporation at oxygen flows well below 2.7 sccm. Stoichiometric ZrO2 films, however, are formed above 2.8 sccm O2 flow (oxidic mode). Grazing angle incidence x-ray diffraction studies show that crystalline ZrO2 films with monoclinic structure grow in this oxygen flow regime. X-ray reflectivity studies determine a constant density of 6.5 g/cm3 and a deposition rate of approximately 1.5 nm/s in the metallic mode. The transition to the oxidic mode is accompanied by a decrease of film density and a reduction in deposition rate to below 0.2 nm/s for a constant cathode current of 900 mA. In this oxygen flow regime a density of 5.2 g/cm3 is determined, which is approximately 90% of the bulk density of monoclinic ZrO2. With increasing O2 flow in the oxidic sputtering regime the surface roughness of the films increases as is also confirmed by atomic force microscopy. For these O2 flow rates fully transparent ZrO2 films are grown. From measurements of the optical transmittance and reflectance we have determined the optical constants such as the band gap Eg, refractive index n, and extinction coefficient k as well as the film thickness. While the refractive index of the films decreases upon increasing O2 flow, the band gap Eg increases simultaneously from 4.52 to 4.67 eV.
Refractive index profiles of LiB3O5 waveguides formed by MeV He ion irradiation
Authors: Wang Ke-Ming , Hu Hui , Lu Fei , Chen Feng , Shi Bo-Rong , Ma Chang-Qin , Lu Qing-Ming , Shen Ding-Yu
Published: September 2002

LiB 3 O 5 crystals were irradiated with MeV He+ ions of different fluences and energies. The refractive index profiles, nx, ny, and nz in LiB3O5 waveguides are given. The results show that: (1) the thickness of the ion irradiated waveguides can be estimated by the mean projected range of irradiated ions, and the refractive index changes in ion irradiated waveguides are mainly caused by nuclear collisions; (2) after annealing at 200 °C for 40 min., the height of the optical barrier in the LiB3O5 waveguide is decreased; (3) the attenuation measured is 0.87 dB/cm for m=0 in the LiB3O5 waveguide irradiated with 2.8 MeV He+ to 5×1016 ions/cm2; and (4) it is possible to make double waveguides in LiB3O5 by MeV He+ ion irradiation.
Trap filled limit of conducting organic materials
Authors: Jain S. C. , Kapoor Ashok. K. , Geens Wim , Poortmans J. , Mertens R. , Willander M.
Published: September 2002

According to the approximate theory of transport in a conducting organic material containing exponential traps, ln J versus ln V plots are straight lines with slope l=TC/T, where TC is the characteristic temperature of the trap distribution. It is assumed in this theory that the concentration pt of trapped holes is much larger than the concentration p of free holes. Our experiments and recent literature results show that at high applied voltages the observed ln J versus ln V plots deviate from the straight lines and bend down. The numerical solution presented in this article shows that at high voltages the contribution of p to the space charge does not remain negligible. Calculated ln J versus ln V plots do bend down consistent with our experimental results. The current approaches the trap-filled limit asymptotically as the applied voltage approaches infinity.
Effects of postbombardment annealing on Ti diffusion in ion prebombarded MgO(100)
Authors: Lu M. , Lupu C. , Rabalais J. W.
Published: September 2002

The effect of postbombardment annealing, or postannealing, has been investigated for Ti diffusion in ion prebombarded MgO(100). MgO postannealing was performed in ultrahigh vacuum after ion prebombardment of MgO and prior to Ti evaporation and diffusion. For postannealing temperatures of T=800, 900, and 1000 °C, Ti diffusion in 7 keV Ar+ prebombarded MgO exhibits the expected exponential decays with postannealing time and more rapid decays with higher temperatures. Cationic diffusion in the ion prebombarded sample proceeds in a nonsteady state condition of extra defects known as nonsteady state radiation enhanced diffusion (NSRED). Thermal dissociation of small vacancy clusters is responsible for NSRED. From the data of T⩽1000 °C, the derived dissociation energy of the vacancy clusters is only ∼10% of the normal value. For some specific postannealing times at T=1100 °C, the subsequent Ti diffusion was more enhanced than that without postannealing, a phenomenon which is contrary to the usual understanding of annealing effects. This same effect was also observed for Ti diffusion in 7 keV Cl+ prebombarded MgO postannealed at 1100 °C. Previous work indicates that the small vacancy clusters for NSRED are mainly Mg divacancies. We propose a divacancy creation mechanism that is due to the thermal dissociation of larger vacancy clusters during postannealing. The dissociation energy ratio of the effective large vacancy clusters (LVC) to that of the divacancies (DIV) is ELVC/EDIV∼1.4. The monovacancy coalescence was found to be unimportant for divacancy creation. A model based on this mechanism explains the unusual diffusion enhancement observed.
Longitudinal-optical-phonon-assisted energy relaxation in self-assembled CdS quantum dots embedded in ZnSe
Authors: Kumano H. , Yoshida H. , Tawara T. , Suemune I.
Published: September 2002

The energy relaxation processes of CdS self-assembled quantum dots (QDs) embedded in ZnSe were investigated. Longitudinal-optical (LO)-phonon resonant structures were discerned in the photoluminescence spectra under the CdS selective excitation, especially under nearly resonant excitation conditions. Strong energy selectivity among the Gaussian distributed energy levels of the QDs are observed in energy relaxation processes, in which excitons are dominantly injected into the QDs whose ground state energies with respect to the excitation energy are equal to the multi-LO-phonon energy of ZnSe. Detection-energy dependent photoluminescence excitation (PLE) measurements also suggest that the coupling strength between the excitons and LO phonons is strongly size-dependent, i.e., coupling is strongly enhanced in smaller QDs. In addition, type-II band alignment of the CdS/ZnSe heterostructure is experimentally confirmed directly by PLE measurements.
Thermoelectric effect spectroscopy of deep levels in semi-insulating GaN
Authors: Desnica U. V. , Pavlović M. , Fang Z.-Q. , Look D. C.
Published: September 2002

The report of thermoelectric effect spectroscopy (TEES) applied on semi-insulating GaN was presented. The type of TEES setup, especially suitable for film-on-substrate samples, was devised. TEES enabled determination of sign of observed deep traps. Using TEES and thermally stimulated current spectroscopy measurements in combination with the simultaneous multiple peak analysis formalism all important trap parameters were determined. The shallowest identified electron and hole traps had activation energies Ec−0.09 eV and Ev+0.167 eV, respectively. Results indicate that both these traps, oppositely charged are present in the studied material in relatively high concentrations causing the electrical compensation and high resistivity.
Ohmic contact technology in III nitrides using polarization effects of cap layers
Authors: Gessmann Th. , Graff J. W. , Li Y.-L. , Waldron E. L. , Schubert E. F.
Published: September 2002

A technology for low-resistance ohmic contacts to III nitrides is presented. The contacts employ polarization-induced electric fields in strained cap layers grown on lattice-mismatched III-nitride buffer layers. With appropriate choice of the cap layer, the electric field in the cap layer reduces the thickness of the tunnel barrier at the metal contact/semiconductor interface. Design rules for polarization-enhanced contacts are presented giving guidance for composition and thickness of the cap layer for different III-nitride buffer layers. Experimental results for ohmic contacts with p-type InGaN and GaN cap layers are markedly different from samples without a polarized cap layer thus confirming the effectiveness of polarization-enhanced ohmic contacts.
Effective electron mass and plasma filter characterization of n-type InGaAs and InAsP
Authors: Metzger W. K. , Wanlass M. W. , Gedvilas L. M. , Verley J. C. , Carapella J. J. , Ahrenkiel R. K.
Published: September 2002

We measured the infrared reflectance of thin films of degenerate n-type InxGa1−xAs and n-InAsyP1−y as a function of doping for compositions that correspond to x=0.53, 0.66, and 0.78 (band gaps of 0.74, 0.60, and 0.50 eV, respectively) and y=0.00, 0.31, 0.52, and 0.71 (band gaps of 1.34, 1.00, 0.75, and 0.58 eV, respectively). We then used the Drude theory and Hall measurements to determine the effective electron mass for these samples, and checked the results using Raman spectroscopy. The effective electron mass for these compositions increases abruptly as a function of free-electron density and converges at 5×1019 electrons/cm3. Consequently, it is difficult to attain plasma edges at wavelengths shorter than 5 μm using these materials, and the plasma edge is nearly independent of composition at large electron density levels. Results from similar studies on InP, InAs, and GaAs have been compiled and compared with our data. It is clear that the Kane band model offers an accurate description of the conduction-band nonparabolicity.
Evolution of structural and optical properties of ion-beam synthesized GaAsN nanostructures
Authors: Weng X. , Clarke S. J. , Ye W. , Kumar S. , Goldman R. S. , Daniel A. , Clarke R. , Holt J. , Sipowska J. , Francis A. , Rotberg V.
Published: September 2002

We have investigated the evolution of structural and optical properties of GaAsN nanostructures synthesized by N ion implantation into epitaxial GaAs, followed by rapid thermal annealing. Transmission electron microscopy and x-ray diffraction indicate the formation of nanometer-sized crystallites with lattice parameters close to those of pure zincblende GaN. The average crystallite size increases with annealing temperature while the size distribution is self-similar and the volume fraction remains constant, suggesting a coarsening process governed by Ostwald ripening. These GaAsN nanostructures exhibit significant photoluminescence in the near infrared range. The apparent lowering of the fundamental band gap is likely due to the incorporation of a small amount of As in GaN.
Intracavity second harmonic generation in the green from a diode-end-pumped Nd3+:Ca3Ga2Ge3O12 laser garnet crystal
Authors: Jaque D. , Romero J. J. , Garcı́a Solé J.
Published: September 2002

A diode-end-pumped intracavity frequency doubled Nd3+:Ca3Ga2Ge3O12/KTiOPO4 green laser is demonstrated. The infrared laser output and pump-to-green conversion efficiency have been investigated as a function of crystal length and Nd3+ concentration. Results show that optimum ion concentration is around 2–3 at. %, in excellent agreement with theoretical predictions. Due to the broad absorption bands of this crystal, output power does not depend on diode temperature within a 12 °C range. The influence of Nd3+ concentration on the F3/24 quantum efficiency has been also analyzed. We have found that for optimum ion concentration quantum efficiency is as high as 0.7. Finally, the spectral distribution of green light is explained in terms of a combination of both second harmonic and frequency mixing processes between the two infrared oscillating lines.
The global phase diagram of the Gay–Berne model
Authors: de Miguel Enrique , Vega Carlos
Published: September 2002

The phase diagram of the Gay–Berne model with anisotropy parameters κ=3, κ′=5 has been evaluated by means of computer simulations. For a number of temperatures, NPT simulations were performed for the solid phase leading to the determination of the free energy of the solid at a reference density. Using the equation of state and free energies of the isotropic and nematic phases available in the existing literature the fluid–solid equilibrium was calculated for the temperatures selected. Taking these fluid–solid equilibrium results as the starting points, the fluid–solid equilibrium curve was determined for a wide range of temperatures using Gibbs–Duhem integration. At high temperatures the sequence of phases encountered on compression is isotropic to nematic, and then nematic to solid. For reduced temperatures below T=0.85 the sequence is from the isotropic phase directly to the solid state. In view of this we locate the isotropic–nematic–solid triple point at TINS=0.85. The present results suggest that the high-density phase designated smectic B in previous simulations of the model is in fact a molecular solid and not a smectic liquid crystal. It seems that no thermodynamically stable smectic phase appears for the Gay–Berne model with the choice of parameters used in this work. We locate the vapor–isotropic liquid–solid triple point at a temperature TVIS=0.445. Considering that the critical temperatures is Tc=0.473, the Gay–Berne model used in this work presents vapor–liquid separation over a rather narrow range of temperatures. It is suggested that the strong lateral attractive interactions present in the Gay–Berne model stabilizes the layers found in the solid phase. The large stability of the solid phase, particularly at low temperatures, would explain the unexpectedly small liquid range observed in the vapor–liquid region.
Comment on “A reaction-path Hamiltonian described with quasirectilinear vibrational coordinates constructed from a nonlinear combination of curvilinear internal coordinates: Application to examination of the reaction CH4+F→CH3+HF” [J. Chem. Phys. 113, 3136 (2000)]
Authors: Natanson G.
Published: September 2002

The comment presents explicit relations between Okuno’s projected covariant Hessian matrix [J. Chem. Phys. 113, 3130 (2000)] and the projected Cartesian force constant matrix utilized in Miller–Handy–Adams’ projection technique [J. Chem. Phys. 72, 99 (1980)]. The derived formulas provide a direct proof for the author’s conclusion [Chem. Phys. Lett. 178, 49 (1991)] that nonzero eigenvalues of the two matrices must coincide at each point of the intrinsic reaction path, contrary to results of Okuno et al. [J. Chem. Phys. 113, 3136 (2000)].
Constrained dynamics in supramolecular structures of poly(p-phenylenes) with ethylene oxide side chains: A combined dielectric and nuclear magnetic resonance investigation
Authors: Mierzwa M. , Floudas G. , Neidhöfer M. , Graf R. , Spiess H. W. , Meyer W. H. , Wegner G.
Published: September 2002

The complex dynamics in supramolecular structures of poly(p-phenylenes) with short ethylene oxide (EO) side chains (known as “hairy rods”) have been studied by combining temperature and pressure dependent dielectric spectroscopy (DS) with different nuclear magnetic resonance (NMR) techniques. The system forms a periodic lattice of layers of the main chains separated by layers of the amorphous EO chains. Since the dipole moment is located on the EO side chain, DS is probing mainly the side-chain motions and only indirectly the backbone dynamics. On the other hand, the different NMR techniques are also sensitive to the backbone mobility and provide the molecular origin of the dynamic processes. Two dielectrically active processes exist, a “fast” β (which differs from the usual Johari–Goldstein β process found in homopolymers and low molecular weight glass-forming liquids) and a “slow” α process with the latter carrying most of the intensity. Comparison of the NMR and dielectric results identifies the outer EO units as solely responsible for the dielectric β process whereas both the short EO segments and the unfreezing of the internal backbone dynamics including bending motions contribute to the α process. The unfreezing of the backbone dynamics does not lead to the dissolution of the structure that only happens at even higher temperatures. Hairy rod molecules thus can serve as model systems which provide the means of studying the effect of confinement on the dynamics. In this respect, the effect of confinement is mainly on the temperature dependence of the α-process relaxation times.