bluuebunny/crossref_metadata_2025 huggingface dataset:
Energy level tunneling spectroscopy and single electron charging in individual CdSe quantum dots
Authors: Alperson Boaz , Rubinstein Israel , Hodes Gary , Porath Danny , Millo Oded
Published: July 2002

We directly show the evolution of the electronic structure of semiconductor quantum dots (QDs) with QD size in the strong confinement regime by employing low-temperature tunneling current–voltage spectroscopy to individual electrodeposited CdSe QDs. From the spectra we measure the values of Eg, map discrete energy levels in both valence and conduction “bands,” and show the occurrence of single-electron tunneling effects for isolated QDs of different sizes (2, 3, and 4.5 nm diameter). Since tunneling is not limited by optical selection rules, we are able to directly measure energy level spacings not necessarily accessible by optical spectroscopy. Our spectra demonstrate the interplay between charging and energy level spacing, resulting in a rich and controllable structure that forms a basis for QD nanoelectronic devices.
Femtosecond laser-induced periodic structure writing on diamond crystals and microclusters
Authors: Ozkan A. M. , Malshe A. P. , Railkar T. A. , Brown W. D. , Shirk M. D. , Molian P. A.
Published: July 2002

Three-dimensional (3D), periodic nanowriting on diamond clusters is reported in this letter. Concentric circular rings were observed on diamond microclusters, nucleated near the periphery of a laser-irradiated region, when chemical-vapor deposited diamond was processed in air, with laser pulses of 380 fs duration and at a wavelength of 248 nm. Periodic ripples also have been observed on single-crystal and polycrystalline diamond surfaces. Further, it is experimentally shown that the periodicity of these corrugated two-dimensional and 3D structures is shorter than that of the laser wavelength used (248 nm for the excimer fs laser and 825 nm for the Ti–sapphire fs laser).
High-resolution submicron retarding field energy analyzer for low-temperature plasma analysis
Authors: Blain M. G. , Stevens J. E. , Woodworth J. R.
Published: July 2002

A retarding potential energy analyzer having 750 nm diameter, self-aligned grid apertures and micron scale grid separation has been fabricated using polycrystalline silicon and silicon dioxide. High-resolution in situ measurements of ion velocity distributions have been demonstrated in inductively coupled argon plasmas. Measurement results agree well with those from a macroscopic analyzer. Important differences are observed in the energies of plasma ions when measured with respect to chamber wall versus those measured with respect to the plasma floating potential. Preliminary measurements under rf bias conditions have also been made and results follow the expected trends.
GaN microdisk light emitting diodes
Authors: Jin S. X. , Li J. , Li J. Z. , Lin J. Y. , Jiang H. X.
Published: July 2002

Microdisk light-emitting diodes (μ-LEDs) with diameter of about 12 μm have been fabricated from InGaN/GaN quantum wells. Photolithographic patterning and inductively coupled plasma dry etching have been employed to fabricate these μ-LED devices. Device characteristics, such as the current–voltage characteristics, light output power, and electroluminescence (EL) spectra have been measured and compared with those of conventional broad-area LEDs. Our results showed that, for an identical area, the quantum efficiencies of μ-LED are enhanced over the conventional broad-area LEDs due to an enhanced current density and possibly microsize effects. The implications of our results on the design of future UV/blue microoptoelectronic devices are discussed.
Energy level alignment at the conjugated phenylenevinylene oligomer/metal interface
Authors: Veenstra S. C. , Stalmach U. , Krasnikov V. V. , Hadziioannou G. , Jonkman H. T. , Heeres A. , Sawatzky G. A.
Published: July 2002

In this letter we report an investigation of the interfacial electronic structure formed by metals and conjugated oligomers using ultraviolet photoelectron spectroscopy. Au and Ag were used as metal substrates for two five-ring phenylenevinylene oligomers: unsubstituted p-bis[(p-styryl)styryl]benzene (P5V4) and the analogous oligomer with 2-methoxy-5-(2’-ethyl-hexyloxy) substitution on the central ring (MEH-P5V4). We found for all interfaces a lowering of the energy levels of the organic overlayer by 0.4–1.2 eV. Remarkably, this energy lowering, presumably due to interface dipole layers, was always such as to keep the hole injection barrier nearly constant and therefore at most weakly sensitive to the work function of the metal or the ionization potential of the oligomer.
Load transfer and deformation mechanisms in carbon nanotube-polystyrene composites
Authors: Qian D. , Dickey E. C. , Andrews R. , Rantell T.
Published: July 2002

Multiwall carbon nanotubes have been dispersed homogeneously throughout polystyrene matrices by a simple solution-evaporation method without destroying the integrity of the nanotubes. Tensile tests on composite films show that 1 wt % nanotube additions result in 36%–42% and ∼25% increases in elastic modulus and break stress, respectively, indicating significant load transfer across the nanotube-matrix interface. In situ transmission electron microscopy studies provided information regarding composite deformation mechanisms and interfacial bonding between the multiwall nanotubes and polymer matrix.
Critical temperature depression and persistent photoconductivity in ion irradiated YBa2Cu3O7−x films and YBa2Cu3O7−x/PrBa2Cu3O7 superlattices
Authors: Navacerrada M. A. , Arias D. , Sefrioui Z. , Loos G. , Lucı́a M. L. , Santamarı́a J. , Sánchez-Quesada F. , Varela M.
Published: July 2002

We have studied the effect of He+ irradiation with doses in the range 1012–2×1015 cm−2 on two high-temperature superconducting structures: YBa2Cu3O7−x (YBCO) films and YBa2Cu3O7−x/PrBa2Cu3O7 (YBCO/PBCO) superlattices. In particular we have focused on superlattices [YBCON/PBCOM]1000 Å with N=1, 8 unit cells of YBCO, and M=5 unit cells of PBCO, with a total thickness of 1000 Å. The analysis is presented in terms of depression of the critical temperature (Tc) and modification of the crystalline structure using X ray refinement technique. Single films show a systematic increase in the c-lattice parameter upon irradiation, which is not observed in thin one unit cell YBCO layers in superlattice structures. However, Tc depression resulting from irradiation is deeper in superlattices. These results are explained considering the strained nature of the as-grown [YBCON/PBCO5]1000 Å superlattices with low values of N. Both structures show persistent photoconductivity, indicating that defects are related to oxygen displacements in the Cu–O chains.
Patterned growth of individual and multiple vertically aligned carbon nanofibers
Authors: Merkulov V. I. , Lowndes D. H. , Wei Y. Y. , Eres G. , Voelkl E.
Published: July 2002

The results of studies of patterned growth of vertically aligned carbon nanofibers (VACNFs) prepared by plasma-enhanced chemical vapor deposition are reported. Nickel (Ni) dots of various diameters and Ni lines with variable widths and shapes were fabricated using electron beam lithography and evaporation, and served for catalytic growth of VACNFs whose structure was determined by high resolution transmission electron microscopy. It is found that upon plasma pre-etching and heating up to 600–700 °C, thin films of Ni break into droplets which initiate the growth of VACNFs. Above a critical dot size multiple droplets are formed, and consequently multiple VACNFs grow from a single evaporated dot. For dot sizes smaller than the critical size only one droplet is formed, resulting in a single VACNF. In the case of a patterned line, the growth mechanism is similar to that from a dot. VACNFs grow along the line, and above a critical linewidth multiple VACNFs are produced across the line. The mechanism of the formation of single and multiple catalyst droplets and subsequently of VACNFs is discussed.
Positively charged defects associated with self-assembled quantum dot formation
Authors: Belyaev A. E. , Stoddart S. T. , Martin P. M. , Main P. C. , Eaves L. , Henini M.
Published: July 2002

Capacitance measurements are used to investigate a series of single-barrier n-i-n GaAs/AlAs/GaAs heterostructures incorporating a layer of self-assembled InAs quantum dots into the AlAs barrier. They reveal a low-density, excess positive charge in the AlAs barrier which we attribute to defects associated with quantum dot formation. The quantity of positive charge is proportional to the amount of AlAs deposited on top of the dots.
Air-bridged lateral epitaxial overgrowth of GaN thin films
Authors: Kidoguchi Isao , Ishibashi Akihiko , Sugahara Gaku , Ban Yuzaburoh
Published: July 2002

A promising technique of selective lateral epitaxy, namely air-bridged lateral epitaxial overgrowth, is demonstrated in order to reduce the wing tilt as well as the threading dislocation density in GaN thin films. A seed GaN layer was etched to make ridge-stripe along 〈11̄00〉GaN direction and a GaN material was regrown from the exposed (0001) top facet of the ridged GaN seed structures, whose sidewalls and etched bottoms were covered with silicon nitride mask, using low-pressure metalorganic vapor phase epitaxy. The density of dislocations in the wing region was reduced to be <107 cm−2, which was at least two orders of magnitude lower than that of underlying GaN. The magnitude of the wing tilt was determined to be 0.08° by x-ray diffraction (XRD) measurements, which was smaller than other lateral epitaxial overgrown GaN thin films. The full width at half maximum of XRD for the wing region was 138 arc sec, indicating high uniformity of c-axis orientation.
Evaluation of Schottky contact parameters in metal–semiconductor–metal photodiode structures
Authors: Averine S. , Chan Y. C. , Lam Y. L.
Published: July 2002

The electrical behavior of metal–semiconductor–metal (MSM) Schottky barrier photodiode structures is analyzed by means of current–voltage measurements at different temperatures. The reverse characteristics of the Schottky contact are examined by taking into account the barrier height dependence on the electric field and tunneling through the barrier. It is shown that, under these conditions, the logarithmic dependence of the reverse current on the reverse bias is a linear function and allows us to evaluate the barrier height, saturation current density, and junction ideality factor of the MSM-photodiode Schottky contact. The results are well consistent with experiment.
Associative formation of Rydberg state clusters from collisions between a Rydberg state species and a ground state neutral atom
Authors: Peng Xianzhao , Abbott James E. , Kong Wei
Published: July 2002

Using a mass analyzed threshold ionization (MATI) spectrometer, the spectra of the monomer, dimer, and trimer of sodium were measured. Intense dimer signals both at the ionization threshold and at Rydberg resonances of the monomer were observed. Below the adiabatic threshold of the dimer, a trimer spectrum which showed vibrational progressions similar to those of the dimer was also recorded. In both cases, the high energy edge of the heavier cluster was shifted to a lower energy compared with that of the lighter cluster. The monomer time-of-flight spectrum at the Rydberg resonances was diffuse, while the time-of-flight spectrum of the dimer was sharp and intense. Based on the continuity of oscillator strength and further energetic analysis, we conclude that the dimer signal at the Rydberg resonances of the monomer originated from collisional association between a Rydberg state atom and a ground state atom. Similarly, the trimer signal was from the combination between a Rydberg state dimer and a ground state atom. The combined product from collision was in a Rydberg state that can be field ionized during MATI detection. This work offers direct experimental evidence for the formation of high Rydberg state species through collisional recombination.
Spectral properties of resonant-cavity, polyfluorene light-emitting diodes
Authors: Fletcher R. B. , Lidzey D. G. , Bradley D. D. C. , Bernius M. , Walker S.
Published: July 2002

We report on the fabrication and device properties of light-emitting resonant cavity conjugated-polymer diodes. The microcavity structures were constructed using a dielectric mirror, an indium–tin–oxide anode, a green light-emitting polyfluorene blend, and a metal (cathode) mirror. The best performance was obtained for a composite calcium–aluminum cathode, that combines high reflectivity with a low workfunction. This structure emitted electroluminescence with a linewidth of 12 nm and a maximum brightness of 300 cd m−2 at a bias of 35 V with an efficiency of 0.95 cd A−1.
Consequences of propene and propane on plasma remediation of NOx
Authors: Dorai Rajesh , Kushner Mark J.
Published: July 2002

Dielectric barrier discharge (DBD) reactors are being investigated for plasma remediation of NOx from the exhaust of internal combustion engines and diesel emissions, in particular. In earlier works, it was found that unburned hydrocarbons (UHCs), inevitably present in exhausts, play a significant role in altering NOx remediation pathways and increasing the oxidation of NO. In this study, the DBD processing of NOx in simulated diesel exhausts with hydrocarbons [propane (C3H8) and propene (C3H6)] has been investigated. In general, the presence of UHCs improved the energy efficiency of remediation. For example, at 56 J/L, NOx remediation improved from 12% without UHCs to 32% in the presence of 175 ppm propene. The W-values for NO remediation at 56 J/L decreased from 160 eV/molecule in the absence of UHCs to 96 eV/molecule with 175 ppm propene, though the majority of this improvement is a result of conversion to NO2. Temperature dependencies of NOx remediation were investigated and it was found that higher temperatures improve NO remediation whereas the effect on total NOx remediation is not significant, implying that NO remediation at higher temperatures results from increased NO2 formation.
Charge transfer between ND3+(ν2+) and phenol
Authors: Kim Ho-Tae , Green Richard J. , Anderson Scott L.
Published: July 2002

Reactions of vibrationally state-selected ND3+ with phenol were studied in a guided beam arrangement. There are four exoergic channels, of which only charge transfer (CT) has significant intensity. The dominant mechanism requires intimate collisions, with little long-range electron hopping. Despite the presence of deep hydrogen-bonded wells, only a few percent of collisions form long-lived complexes. ND3+ vibration has no effect on CT at low energies, with weak inhibition at high energies. Charge transfer with H/D exchange is a minor channel, inhibited by ND3+ vibration and collision energy. The small vibrational effects are in contrast to the large effects observed in reaction of PhOH+(ν6a,ν12) with ND3.
Self-biased dielectric bolometer from epitaxially grown Pb(Zr,Ti)O3 and lanthanum-doped Pb(Zr,Ti)O3 multilayered thin films
Authors: Liu Weiguo , Jiang Bin , Zhu Weiguang
Published: July 2002

A self-biased dielectric bolometer has been realized by epitaxially grown lead zirconate titanate Pb1.1(Zr0.3Ti0.7)O3 (PZT 30/70) and lanthanum-doped lead zirconate titanate Pb0.83La0.17(Zr0.3Ti0.7)0.9575O3 (PLZT 17/30/70) multilayered thin films using the sol-gel technique. The high-resolution transmission electron microscopy images and electron diffraction pattern showed that the multilayered film had its preferred (111) orientation and the epitaxial growth between the PZT 30/70 and PLZT 17/30/70 layers. The self-biased phenomenon and a large pyroelectric current were observed in the prepoled sample without any external bias at the PLZT 17/30/70 phase transition temperature of 120 °C. Pyroelectric coefficient as high as 990 μC/m2K is obtained in the multilayer thin films around 120 °C. It is obvious that there exists an induced polarization in the PLZT 17/30/70 layer. We attribute this self-biased effect to the strong remnant polarization in the immediately contacted tetragonal PZT 30/70 layers.
Thickness-dependent transport properties of Nd2/3Sr1/3MnO3 thin films
Authors: Barman A. , Koren G.
Published: July 2002

A systematic study is reported on the thickness dependence of the electrical resistivity in thin films of the giant magnetoresistance manganite Nd2/3Sr1/3MnO3. We observed a first-order phase transition versus thickness in these films, which is seen as a jump of about 30 K in the metal-to-insulator transition temperature (Tp) at film thickness of 50–60 nm. This phenomenon is attributed to a sudden release of strain in the film as its thickness increases. We also observed at low temperatures, 5–30 K, another transition from localized-to-metallic behavior versus film thickness, which is also related to the strain relief in the films.
Stark effects on the spectra of trivalent praseodymium ions in strontium fluorapatite laser host
Authors: Sardar Dhiraj K. , Salinas Felipe S. , Yow Raylon M.
Published: July 2002

An optical characterization of the Stark components of the 1D2 and 3H4 manifolds has been performed using the fluorescence spectra for the 1D2→3H4 transition of Pr3+ in Sr5(PO4)3F laser host at 10 K. The temperature dependencies of the spectral width, position, and shift of the sharp, intense inter-Stark transition of the 623.6 (X2→Y5) nm line within the 1D2→3H4 inter-manifold transition have been also investigated. The linewidth of this transition was found to increase with increasing crystal temperature. Also, the 623.6 nm line was found to shift toward the longer wavelength (redshift) as temperature was increased. The experimental results of the temperature dependent width and shift of this line are explained in light of the Debye model, which invokes phonon-ion interaction in crystalline solids.
Determination of the energy levels of a phosphorescent guest in organic light emitting devices
Authors: Hill I. G. , Mäkinen A. J. , Kafafi Z. H.
Published: July 2002

We have used photoelectron spectroscopy to study the relative binding energies of the highest occupied molecular orbitals (HOMOs) of the phosphorescent guest, 2, 3, 7, 8, 12, 13, 17, 18octaethyl-21H,23H-porphine platinum (PtOEP), and the organic electron transport host, tris(8-hydroxyquinolinato) aluminum (III) (Alq3) in PtOEP:Alq3 composite films. The PtOEP HOMO position was found to be 0.50±0.13 eV above that of the Alq3 host, independent of the guest molecule concentration (0.8%–22% by mass). These results are consistent with the assumption that the vacuum levels of the guest and host align.
Space charge relaxation in polyetherimides by the electric modulus formalism
Authors: Mudarra M. , Belana J. , Cañadas J. C. , Diego J. A. , Sellarès J. , Dı́az-Calleja R. , Sanchı́s M. J.
Published: July 2002

Dynamic electrical analysis shows that at high temperatures (above the glass transition temperature), the electrical properties of polyetherimide are strongly influenced by space charge. In this article we have studied the relaxation of space charge in two commercial varieties of polyetherimide: Ultem 1000 and Ultem 5000. Their conductive properties were determined by dynamic electrical analysis, using the electric modulus formalism. The complex part of the electric modulus was fitted to Coelho’s model which considers ohmic conductivity and diffusion as the prevailing charge transport mechanisms. The complex part of the electric modulus exhibits a peak in the low frequency range that can be associated with space charge. A good agreement between experimental and calculated data is observed after the fitting process to Coelho’s model. Differences in the electrode behavior were required: blocking electrode conditions in the case of Ultem 5000 and partially blocking electrode for Ultem 1000. In both cases the conductivity determined is thermally activated and it increases with the temperature due to an increasing mobility, as their carrier densities do not vary significantly in the temperature range studied.
Insights into the structure of BN nanotubes
Authors: Golberg D. , Bando Y. , Bourgeois L. , Kurashima K. , Sato T.
Published: July 2002

The following features of multiwalled BN nanotubes were discovered using a field emission high-resolution analytical transmission electron microscope: (i) coexistence of hexagonal and rhombohedral stacking in nanotube shell assembly; (ii) flattening of nanotube cross section, which makes possible clear atomic resolution of the core structure in a three-shelled nanotube; and (iii) change in chirality of tubular layers from armchair to zigzag arrangement in a 30° double-walled nanotube kink, as revealed by atomically resolved images of tube wall segments.
Transitions from Taylor vortex flow in a co-rotating Taylor–Couette system
Authors: Antonijoan J. , Sánchez J.
Published: July 2002

The stability of the Taylor vortex flow in the periodic Taylor–Couette problem with co-rotating cylinders is examined. Transitions to twisted and wavy twisted vortices and wavy inflow and outflow boundary flows are considered. Marginal stability curves for the transition from Taylor to twisted and wavy twisted vortices have been calculated. The azimuthal wave number and the phase velocity at their onset have also been obtained. To compare with experiments and previous numerical works for the narrow gap approximation, the case of radius ratio 0.883 is analyzed in detail. An explanation for the increase in the azimuthal wave number of the twisted vortices as the Reynolds number of the inner cylinder is increased is provided. The velocity fields of twisted vortices, wavy twisted vortices, wavy inflow, and wavy outflow boundary flows at their onset are also shown.
Structure, thermodynamics, and transition kinetics of nonadecylcyclohexane
Authors: Sirota E. B. , Herhold A. B. , Varma-Nair M.
Published: July 2002

The kinetics of crystallization in nonadecylcyclohexane, an asymmetric alkane, was studied using x-ray scattering, differential scanning calorimetry (DSC), and optical microscopy. A transient mesophase was found with a crystallization/melting temperature 13 °C below the melting point of the stable crystal phase. In a bulk sample, nucleation from the melt proceeds though the transient mesophase, which subsequently converts to the stable phase with rather slow kinetics. When quenched to low temperatures, the mesophase does not anneal and can be described as a two-dimensional (2D) Ising-like glass with regard to the up-down orientation of the asymmetric molecules. A second metastable mesomorph was also detected at low temperatures. Using an emulsified sample, homogeneous nucleation was also shown to proceed through the transient phase. The crystal structure of the transient phase is determined.
A high-power picosecond near-infrared laser transmitter module
Authors: Vainshtein Sergey N. , Maslevtsov Andrey V. , Kilpelä Ari J. , Kostamovaara Juha T.
Published: July 2002

A compact laser diode based transmitter was designed and tested for laser radar and various laboratory applications. Single optical pulses with a peak power of up to 200 W, 23–65 ps pulse duration, and a repetition rate of up to 50 kHz were measured. Transient mode spectral filtering suppressed after pulsing modes by a factor of 104–105 with respect to the peak power. A control module was developed which provided a jitter value between electrical triggering and the optical pulses as low as 14 ps. Averaging of 103 events allows 1.5 ps stability between the triggering and the optical pulses to be achieved within a delay range from 5 to 250 ns.
Symmetry breaking and spectra of diphenyloctatetraene in n-alkanes
Authors: Walser Daniel , Zumofen Gert , Plakhotnik Taras
Published: July 2002

One- and two-photon excitation spectra, as well as absorption and emission spectra of diphenyloctatetraene (DPOT) in n-alkanes are investigated at low temperatures. For DPOT in n-octane we report on the measurements of one-photon excitation and emission spectra and for DPOT in n-tetradecane (TD) on the measurements of one- and two-photon excitation spectra and emission spectra. The spectra are governed by the transitions between the electronic ground (S0) and the two lowest electronic excited singlet states (S1,S2). The interpretation is based on allowed transitions and transitions induced by the S1–S2 coupling due to Herzberg–Teller promoting modes or due to static lattice-induced distortions of DPOT. A single noncentrosymmetric site is observed for DPOT in n-octane. For DPOT in TD a centrosymmetric and a noncentrosymmetric site are reported. The analysis indicates that there is a dynamical equilibrium in the population of these two sites. The experimental data are quantitatively studied by comparison with simulated spectra. The simulation calculations are based on the coupling between nonadiabatic S1 and S2 states, harmonic modes, and suitable transition moments and line-shape functions. For DPOT in TD the calculations reveal an interesting interference pattern in the vibronic progressions observed in two-photon excitation spectra.
Temperature dependence of the electronic properties of K3C60 and K4C60 single-phase films investigated by means of electron spectroscopies
Authors: Goldoni A. , Sangaletti L. , Friedmann S. L. , Shen Z.-X. , Peloi M. , Parmigiani F. , Comelli G. , Paolucci G.
Published: July 2002

The normal state temperature dependence of a metallic K3C60 phase pure film has been investigated by means of core levels and valence band photoemission, carbon K edge x-ray absorption, and electron energy loss spectroscopies and compared with that of a Mott–Hubbard insulating K4C60 film. The anomalous temperature behavior of K3C60, in the range 30 K–600 K, can be consistently interpreted considering the presence of orientational disorder of the C60 molecules together with the inhomogeneity of the conduction electron distribution at the molecular level. In particular, the changes observed in the photoemission spectra near the Fermi level, are consistent with the behavior predicted for strongly correlated metallic systems in presence of disorder. Also in the case of K4C60 we observe anomalies in the temperature dependence of core levels photoemission data. These anomalies can be explained with the freezing of the molecular motions at low temperature and point to large thermal fluctuations of the alkali atoms around their equilibrium position at high temperature.
Surface plasmon enhanced substrate mediated photochemistry on roughened silver
Authors: Kidd Robert T. , Lennon David , Meech Stephen R.
Published: July 2002

The wavelength dependent photochemical cross sections for three adsorbates (OCS, NO, SO2) on roughened silver have been measured, and contrasted with the behavior on Ag(111). Surface roughness leads to significant enhancements of the photochemical cross sections for all three adsorbates. The enhancement exhibits a maximum at 350±5 nm. Competing enhancement mechanisms are considered. Temperature programmed desorption measurements show that new adsorption sites are available on the surface, but that these are not uniquely associated with the enhanced cross section. The coincidence of the peak enhancement for both photodissociation of OCS and photodesorption of NO and SO2 suggests a substrate mediated mechanism. It is proposed that the enhancement arises from surface plasmon excitation on the roughened surface. This mechanism may contribute to an enhanced cross section in two ways. First the collective surface plasmon excitation can decay to single particle, hot electron, excitations. The hot electrons so generated may attach to the adsorbates, to cause the photochemistry observed. Secondly, the enhanced local electric field at the surface can generate electron–hole pair excitations, which may also attach to the adsorbate.
Chaotic advection and relative dispersion in an experimental convective flow
Authors: Boffetta G. , Cencini M. , Espa S. , Querzoli G.
Published: July 2002

Lagrangian motion in a quasi-two-dimensional, time-dependent, convective flow is studied at different Rayleigh numbers. The particle tracking velocimetry technique is used to reconstruct Lagrangian trajectories of passive tracers. Dispersion properties are investigated by means of the recently introduced finite size Lyapunov exponent analysis. Lagrangian motion is found to be chaotic with a Lyapunov exponent which depends on the Rayleigh number as Ra1/2. The power law scaling is explained in terms of a dimensional analysis on the equation of motion. A comparative study shows that the fixed scale method makes more physical sense than the traditional way of looking at the relative dispersion at fixed times.
A theoretical study of the polarized neutron scattering from Cs3CoCl5
Authors: Li Zucheng , Jayatilaka Dylan , Figgis Brian N. , Chandler Graham S.
Published: July 2002

Spin-orbit interactions play an essential part in elucidating the magnetic structures which are measured by the polarized neutron diffraction technique. This work extends our previous Hartree–Fock theory, with a one-electron spin-orbit term limitation, to incorporate both one- and two-electron terms exactly. This new theory, which is based on the current density, has been applied to calculate the low-temperature magnetic structure factors of the Cs3CoCl5 crystal. The crystal is assumed to be assembled from noninteracting molecular fragments. The calculated structure factors were compared directly with those observed from experiment. The agreement between these theoretical results and the experimental data shows great improvement compared with those from the usual unrestricted Hartree–Fock theory and with those from the previous generalized Hartree–Fock theory including one-electron terms. To examine the electron correlation effects, we use ab initio wave-function-based correlation methods, unrestricted second-order Møller–Plesset perturbation (UMP2) and quadratic configuration interaction including single and double excitations (QCISD), and also density functional theoretical methods, Xα, local-spin density approximation, generalized gradient approximation, Becke’s three-parameter hybrid methods, and modified half-and-half hybrid methods, to calculate magnetic structure factors. Results indicate that the effects of spin-orbit interactions are comparable to those of electron correlation, and both must be included in reproducing the experimental data. To consider both these effects simultaneously we employed a simple model. With only three parameters, this model reproduced the observed data almost exactly as reflected by a χ2 goodness of fit close to unity.
Bonding in Nb3O, Nb3S and Nb3Se: A topological analysis of the electrostatic potential
Authors: Martı́nez Ana , Calaminici Patrizia , Köster Andreas M. , Salahub Dennis R.
Published: July 2002

The structures of different neutral and cationic isomers of Nb3O, Nb3S and Nb3Se were optimized within the framework of Kohn–Sham density functional theory. For Nb3O, neutral and cationic, the lowest minimum is a planar C2v structure with an edge-bound oxygen atom. The binding energy of the oxygen to the Nb3 cluster is 184.9 kcal/mol, 185.4 kcal/mol for the cationic system. For Nb3S and Nb3Se, neutral and cationic, the lowest minima are three-dimensional structures. The corresponding binding energies are 137.9, 138.1, 131.8 and 132.3 kcal/mol for Nb3S, Nb3S+, Nb3Se and Nb3Se+, respectively. In order to explain the different ground state structures of Nb3O, on the one hand, and Nb3S and Nb3Se, on the other, in terms of the nature of the bonding in each of them a topological analysis of the molecular electrostatic potential was performed. The different relative stabilities of two- and three-dimensional structures can be explained on the basis of bond critical points in the molecular electrostatic potential.
Rotating Bianchi type V dust models generalizing the k=−1 Friedmann model
Authors: Krasiński Andrzej
Published: July 2002

The Einstein equations are investigated for a rotating Bianchi type V dust model in which one of the Killing fields is spanned on velocity and rotation (case 1.2.2.2 in the classification scheme of the earlier papers). A first integral of the field equations is found, and with a special value of this integral coordinate transformations are used to eliminate two components of the metric. The k=−1 Friedmann model is shown to be contained among the solutions in the limit of zero rotation. The field equations for the simplified metric are reduced to 3 second-order ordinary differential equations that determine 3 metric components plus a first integral that algebraically determines the fourth component. First derivatives of the metric components are subject to a constraint (a second-degree polynomial with coefficients depending on the functions). It is shown that the set does not follow from a Lagrangian of the Hilbert type. The group of Lie point-symmetries of the set is found, it is two-dimensional noncommutative. Finally, a method of searching for first integrals (for sets of differential equations) that are polynomials of degree 1 or 2 in the first derivatives is applied. No such first integrals exist. The method is used to find a constraint (of degree 1 in first derivatives) that could be imposed on the metric, but it leads to a vacuum solution, and so is of no interest for cosmology.
Near-field and far-field optical properties of thin metallic films
Authors: Dumay B. , Richard N. , David T. , Bourillot E. , Scheurer F. , Beaurepaire E. , Dereux A. , Lacroute Y.
Published: July 2002

Reflectivity measurements in an attenuated total internal reflection (ATIR) geometry and the variation of the transmitted intensity associated to the total electric field in a frustrated ATIR configuration using a photon scanning tunneling microscope (PSTM) of multilayered metallic films are presented as a function of the angle of incidence. Eigenmodes for total reflection and absorption associated with the optical properties of magnetic and nonmagnetic materials are theoretically and experimentally determined from the reflected and the transmitted spectra. Good agreement between theory and experiment demonstrates that PSTM is a powerful technique to characterize accurately and locally drastic variations of the intensity around optical modes of a multilayer system.
Euler number of instanton moduli space and Seiberg–Witten invariants
Authors: Sako A. , Sasaki T.
Published: July 2002

We show that a partition function of topological twisted N=4 Yang–Mills theory is given by Seiberg–Witten invariants on a Riemannian four manifolds under the condition that the sum of the Euler number and the signature of the four manifolds vanishes. The partition function is the sum of the Euler number of instanton moduli space when it is possible to apply the vanishing theorem. Also we obtain a relation of the Euler number labeled by the instanton number k with Seiberg–Witten invariants. All calculations in this article are done without assuming duality.
Efficient calculation of molecular constants and transition intensities in weakly bound species from J=0 eigenstates: Benzene-Ar as test case
Authors: Felker Peter M. , Neuhauser Daniel , Kim Wousik
Published: July 2002

In application to benzene-Ar we test a procedure that makes use of conveniently calculated J=0 intermolecular eigenstates to compute molecular constants and transition intensities without requiring explicit diagonalization of the full rotational/intermolecular vibrational Hamiltonian of the species. The approach relies on the ability to calculate the orientation of an Eckart body-fixed frame for each point on a grid over which a J=0 state is represented. That ability allows one to compute vibrational matrix elements of operators referred to the Eckart axes while working with J=0 eigenstates obtained in an entirely different body-fixed frame. Generally excellent agreement is found between the Eckart results and those obtained by others via diagonalization of the full rotational/intermolecular vibrational Hamiltonian. Finally, a general prescription for the construction of an efficient rovibrational basis from J=0 solutions in a convenient body-fixed frame is presented.
Projective symmetry in null Einstein–Maxwell space-times
Authors: Hall G. S. , Khan S.
Published: July 2002

A discussion of projective symmetry in general relativity is given. Techniques are developed and used to show that null Einstein–Maxwell fields in general relativity cannot admit any proper projective symmetry.
Studies of boron diffusivity in strained Si1−xGex epitaxial layers
Authors: Rajendran K. , Schoenmaker W.
Published: July 2002

Diffusion of boron in compressively strained Si1−xGex alloy layers grown by rapid pressure chemical vapor deposition has been studied as a function of Ge fractions for 0.0006⩽x⩽0.15 and annealing temperature. The comparison of the Si1−xGex samples to the Si samples after rapid thermal and furnace anneals revealed a retarded B diffusion inside the strained Si1−xGex epitaxial layers. The influence of the Ge content on the dopant diffusion was also measured and simulated, demonstrating that the diffusion of B and the extracted bang-gap change was found to decrease with the Ge alloy content, and the extracted activation energies for diffusion were found to increase with increasing Ge content in the investigated composition range. The influence of various factors on the B diffusivity is studied and their importance is discussed elaborately. A simple empirical expression for the B retardation is presented and incorporated into a diffusion model for dopants in heterostructures. Good agreement between the measured and simulated diffusivity that includes the model for strain and chemical effects is obtained.
Energy relaxation by hot electrons in n-GaN epilayers
Authors: Stanton N. M. , Kent A. J. , Akimov A. V. , Hawker P. , Cheng T. S. , Foxon C. T.
Published: July 2002

The energy relaxation rate for hot electrons in n-type GaN epilayers has been measured over the temperature range 1.5–300 K. Several samples grown by molecular-beam epitaxy and having different electron concentrations have been studied. At low electron temperatures (Te<20 K), the energy relaxation is via acoustic phonon emission. The magnitude and temperature dependence of the energy relaxation are found to be in good agreement with theoretical calculations using appropriate values of the deformation potential and piezoelectric coupling constants and ignoring screening. For Te⩾70 K, the dominant mechanism of energy loss is optic phonon emission. For the several samples studied, consistent values of the optic phonon energy and electron-optic phonon relaxation time, 90±4 meV and 5–10 fs, respectively, are measured. The energy agrees well with values obtained by other methods and the relaxation time is consistent with theoretical calculations of the Fröhlich interaction and indicate that hot phonon effects are absent up to 10−8 W/electron dissipation.
Energy gap of a molecularly doped polymer measured by a new method of electron spectroscopy: Measurement of charge transfer force
Authors: Manabe Chikara , Shigematsu Taishi , Watanabe Hiroyuki , Shimizu Masaaki
Published: July 2002

We have proposed a new method of electron spectroscopy which is measuring charge transfer (CT) force by atomic force microscopy. We have applied this new method to a typical molecularly doped polymer system [N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′ diamine (TPD) and polycarbonate] to elucidate the electronic structure of the system. CT force spectra obtained show that the energy gap (Eg) continuously decreases with increasing the TPD concentration. We discuss the change in Eg from two different points of view: homogeneous and inhomogeneous distributions of TPD.
Theory and experiment of coherent wave packet dynamics in rare earth solids: Absorption spectrum vs femtosecond fringe-resolved interferogram
Authors: Luo Q. , Dai D. C. , Wang G. Q. , Ninulescu V. , Yu X. Y. , Luo L. , Zhou J. Y. , Yan YiJing
Published: July 2002

Coherent dynamic property of neodymium yttrium aluminum garnet (Nd:YAG) crystal at 77 K is studied via the conventional absorption, the femtosecond fringe-resolved wave packet interferometry, and the related difference-phase spectrum. The recorded interferogram exhibits beatings in subpicosecond time scale arising from the interferences among various weakly split 4f-electronic states and the coupled vibronic optical phonon sidebands. The electron–phonon coupling in Nd:YAG can be well described by multiple Brownian oscillators model involving in each individual electronic transition. The parameters for characterizing material coherence and relaxation are determined via the theoretical modelings of both the frequency and the time-domain experimental signals.
Significant improvement of electrical and thermal properties of low dielectric constant plasma polymerized paraxylene thin films by postdeposition H2+He plasma treatment
Authors: Quan Yong Chun , Yeo Sanghak , Shim Cheonman , Yang Jaeyoung , Jung Donggeun
Published: July 2002

Electrical and thermal properties of plasma polymerized paraxylene (PPpX) thin films were significantly improved by postdeposition H2+He plasma treatment. Plasma treatment decreased dielectric constants and increased thermal stability. As the plasma treatment time increased from 0 to 6 min, the relative dielectric constant k decreased from 3.23 to 2.77. Suppression of C=O and O–H group formation and increase of C–H groups in the PPpX film were thought to contribute to the reduction of the k values. Plasma treatment enhanced the thermal stability of PPpX thin films. While the untreated PPpX thin film was stable only to 400 °C, plasma-treated films were stable up to 450 °C. H2+He plasma treatment did not increase the leakage current through the PPpX films notably and did not degrade the surface smoothness, either.
Role of specimen thickness on the electrical conductivity of single crystalline alumina under electron irradiation
Authors: Howlader M. M. R. , Kinoshita C. , Shiiyama K. , Higuchi T.
Published: July 2002

The electrical conductivity of 125-, 332-, and 545-μm-thick single crystalline Kyocera alpha alumina has been measured before, during, and after irradiation with 1 MeV electrons in an applied electric field of 300 kV/m at temperatures up to 723 K. Simultaneous measurements of the bulk and surface conductivity to a total fluence of 8.0×1022 e/m2 (9.4×10−5 dpa and 5.0×109 Gy) at 723 K show no bulk and no surface degradation in the specimen, rather than only a sort of decrease of the conductivity with total dpa. Strong thickness dependence of radiation induced conductivity (RIC) is found and is believed to be due to the effect of electron charge deposition and the production of charged point defects during irradiation. Finally it is suggested that the thickness dependent RIC of the insulating materials must be considered carefully before designing the coating and window materials of fusion reactors.
Observation of the FeNC molecule by laser fluorescence excitation spectroscopy
Authors: Lie Jie , Dagdigian Paul J.
Published: July 2002

A new molecular electronic band system has been observed in the near UV by laser fluorescence excitation in a free-jet supersonic expansion of photolyzed Fe(CO)5 diluted in seed gases containing acetonitrile. The three observed bands displayed the rotational structure of a Ω′=7/2←Ω″=9/2 transition and could be assigned as an excited-state progression in a single vibrational mode. The rotational constant B″ and its change upon isotopic substitution in acetonitrile [CH313CN] was consistent with the assignment of the molecular carrier as the linear FeNC molecule. In analogy with the iron monohalides, which have electronic band systems in the same wavelength range, the transition can be described as a 4p←4s excitation. The derived structure is compared with that of other metal-containing cyanide/isocyanide molecules.
Structural characteristics of Y2O3 films grown on oxidized Si(111) surface
Authors: Cho M.-H. , Ko D.-H. , Choi Y. K. , Lyo I. W. , Jeong K. , Kim T. G. , Song J. H. , Whang C. N.
Published: July 2002

We investigated the characteristics of Y2O3 films grown on an oxidized Si(111) surface, using x-ray diffraction, Rutherford backscattering spectroscopy, and high-resolution transmission electron microscopy. The films grown on the oxidized Si show drastically improved crystallinity, compared with the film grown on clean Si surfaces: channeling minimum yield (Xmin) of 2.5% and full width at half maximum of rocking curve lower than 0.03°. The improvement of the crystallinity was due to the difference of the crystalline structure at the interface between the films grown on the oxidized and clean Si surfaces. Crystalline orientation of Y2O3 islands at the interfacial region was misaligned from the normal substrate direction. The misalignment decreased with increasing the substrate temperature. In particular, the ordering of the oxygen atom in the film grown on oxidized Si was improved compared to that of the Y atom, indicating that the crystallinity of the film is dominantly determined by the arrangement of the oxygen atom in the unit cell. These characteristics of crystalline structure are influenced by the interfacial interactions among SiO2, Y, and Si. The interfacial SiO2 layer can be removed at high growth temperature above 800 °C using the reaction process; the high crystalline Y2O3 film without any interlayer oxide can be obtained.
Chemical modification of sputtered amorphous-carbon surfaces
Authors: Leezenberg Pieter B. , Johnston William H. , Tyndall George W.
Published: July 2002

Methods to chemically passivate the surfaces of amorphous-carbon films (a-C) produced by dc magnetron sputtering were studied. The chemical composition of carbon surfaces produced via sputtering are dependent upon the environment to which the carbon is exposed immediately following deposition. When the sputtered film is vented to ambient conditions, free radicals produced at the surface during the deposition process are quenched by reaction with oxygen and/or water to form an oxidized, hydrophilic surface. If the sputtered carbon film is, however, exposed to a reactive gas prior to venting to ambient, the chemical nature of the resulting surface can be modified substantially. Specifically, a less highly oxidized and much more hydrophobic carbon surface is produced when the surface free radicals are quenched via either an addition reaction (demonstrated with a fluorinated olefin) or a hydrogen abstraction reaction (demonstrated with two alkyl amines). Chemical modification of amorphous-carbon films can also be accomplished by performing the sputtering in a reactive plasma formed from mixtures of argon with molecular hydrogen, amines, and perfluorocarbons. The elemental composition of these films, and the relative reactivity of the surfaces formed, were investigated via x-ray photoelectron spectroscopy and contact-angle goniometry, respectively. In the case of sputtering with a mixture of argon and hydrogen, increasing the hydrogen flow results in an increase in the amount of hydrogen incorporated into the carbon film and a decrease in the surface free energy. Sputtering in diethylamine produces an amorphous-carbon film into which nitrogen is incorporated. The free energies of the a-C:N surfaces produced in this process are similar to those of the a-C:H films. Sputtering in a fluorocarbon vapor results in the incorporation of fluorine into the film structure and the formation of very low free-energy surfaces. Increasing the concentration of the fluorocarbon in the sputtering plasma increases the amount of F incorporated into the film. At the highest fluorocarbon flow rates employed, a-C films were produced with stoichiometries and surface free energies comparable to those of bulk Teflon.