bluuebunny/crossref_metadata_2025 huggingface dataset:
Conductance spikes in single-walled carbon nanotube field-effect transistor
Authors: Liu Kun , Burghard Marko , Roth Siegmar , Bernier Partrick
Published: July 2002

Nanoscale field-effect transistor (FET) has been fabricated from single-walled carbon nanotubes (CNTs). At ∼5 K, the transistor shows pronounced field effect. Most interestingly, reproducible spikes are observed in the channel conductance as a function of the gate voltage. These conductance spikes are attributed to van Hove singularities in the electronic density of states in the carbon nanotubes. Based on the observation of these conductance spikes, a mechanism is proposed for the charge transport in CNT FETs.
Laser driven shock wave acceleration experiments using plastic foams
Authors: Koenig M. , Benuzzi-Mounaix A. , Philippe F. , Faral B. , Batani D. , Hall T. A. , Grandjouan N. , Nazarov W. , Chieze J. P. , Teyssier R.
Published: July 2002

In this letter, we present an experimental study of shock propagation in a decreasing density gradient which is a common feature in many astrophysical objects. An original scheme for the targets has been designed to investigate the case of a discrete density step. Using foams in a wide density range (20–400 mg/cm−3), we obtained accurate data on shock acceleration at the solid-foam interface. Results are discussed and compared with models.
Heteroepitaxial electrodeposition of zinc oxide films on gallium nitride
Authors: Pauporté Th. , Lincot D.
Published: July 2002

Epitaxial zinc oxide films have been prepared on gallium nitride (0002) substrates by cathodic electrodeposition in an aqueous solution containing a zinc salt and dissolved oxygen at 85 °C. The films have the hexagonal structure with the c axis parallel to that of GaN and the [100] direction in ZnO parallel to the [100] direction in GaN in the (0002) basal plane. The structural quality is attested by the values of the full width at half maximum in θ/2θ x-ray diffraction (XRD) diagrams [0.07° for the (0002) peak] and in five circles XRD diagrams [0.74° for the ZnO (101̄1) planes compared to 0.47° for the GaN (101̄1) planes]. The morphology of the layers has been studied by scanning electron microscopy. Before coalescence, arrays of epitaxial single crystalline hexagonal columns are observed with a low dispersion in size, indicating instantaneous tridimensional nucleation. Preliminary results on luminescence properties of the films before and after annealing are presented.
Enhanced intergrain tunneling magnetoresistance in double perovskite Sr2FeMoO6 polycrystals with nanometer-scale particles
Authors: Yuan C. L. , Wang S. G. , Song W. H. , Yu T. , Dai J. M. , Ye S. L. , Sun Y. P.
Published: July 2002

Polycrystalline ordered double perovskite Sr2FeMoO6 bulk samples with grain size in the range of 29–45 nm have been synthesized at temperatures from 900 to 1000 °C, using a sol-gel method. We find that the intergrain magnetoresistance is closely correlated with the grain size. The sample with the grain size of 29 nm shows large magnetoresistance Δρ/ρ0, 30%–20% at a low magnetic field of 4 kG over a wide temperature range from 20 to 300 K. The results can be explained in terms of spin-dependent intergrain tunneling model.
1/f
noise in carbon nanotubes
Authors: Collins Philip G. , Fuhrer M. S. , Zettl A.
Published: July 2002

The electrical noise characteristics of single-walled carbon nanotubes have been investigated. For all three cases of individual isolated nanotubes, thin films of interconnected nanotubes, and bulk nanotube mats, anomalously large bias-dependent 1/f noise is found. The noise magnitude greatly exceeds that commonly observed in metal films, carbon resistors, or even carbon fibers with comparable resistances. A single empirical expression describes the noise for all nanotube samples, suggesting a common noise-generating mechanism proportional only to the number of nanotubes in the conductor. We consider likely sources of the fluctuations, and consequences for electronic applications of nanotubes if the excessive noise cannot be suppressed.
Midinfrared studies of the contact region at superconductor–semiconductor interfaces
Authors: Eckhause T. A. , Tsujino S. , Lehnert K. W. , Gwinn E. G. , Allen S. J. , Thomas M. , Kroemer H.
Published: July 2002

InAs quantum wells (QWs) have been used as weak links in many recent studies of novel superconductor-normal metal–superconductor junctions. The degree of coupling between the superconducting electrodes depends sensitively on both the superconductor/InAs interface and the QW material in the weak link, factors that are difficult to separate in dc transport studies. Here we used midinfrared spectroscopy to investigate the superconductor/semiconductor contact region. The remnant intersubband absorption we observe in Nb-clad InAs shows that the superconductor/InAs interface produced some confinement of electrons in the InAs. This confinement is, however, consistent with phase coherent transport in the InAs. We find no evidence for charge transfer from the superconductor to the InAs on cooling below the critical temperature of Nb.
Electron-beam-induced conduction in a ruthenium carbonyl nanoparticle polymer
Authors: Thomas M. D. R. , Ahmed H. , Sanderson K. M. , Shephard D. S. , Johnson B. F. G. , Zhou W.
Published: July 2002

A polymer composed of ruthenium carbonyl and of the formula [Ru6C(CO)15Ph2PCCPPh2]n has been synthesized. It is found to behave as a negative electron-beam resist with a sensitivity of 400 C/m2. Upon exposure to the electron beam, the electrical conductivity of the patterned films is found to vary over seven orders of magnitude according to a power-law dependence on dose. Temperature dependence of the conductivity is studied, and the conduction is attributed to variable-range hopping between ruthenium superclusters in two dimensions.
InGaAsN/AlGaAs P-n-p heterojunction bipolar transistor
Authors: Chang P. C. , Baca A. G. , Li N. Y. , Sharps P. R. , Hou H. Q. , Laroche J. R. , Ren F.
Published: July 2002

We have demonstrated a functional P-n-p heterojunction bipolar transistor (HBT) using InGaAsN. The metalorganic-vapor-phase-epitaxy-grown Al0.3Ga0.7As/In0.03Ga0.97As0.99N0.01 HBT takes advantage of the narrower band gap energy (Eg=1.2 eV) of In0.03Ga0.97As0.99N0.01, which is lattice matched to GaAs. Compared with the Al0.3Ga0.7As/GaAs material system, the Al0.3Ga0.7As/In0.03Ga0.97As0.99N0.01 material system has a larger conduction-band offset, while the valence-band offset remains comparable. This characteristic band alignment is very suitable for P-n-p HBT applications. The device’s peak current gain is 23, and it has a turn-on voltage of 0.77 V, which is 0.25 V lower than in a comparable P-n-p Al0.3Ga0.7As/GaAs HBT.
Investigation of β-SiC precipitation in Si1−yCy epilayers by x-ray scattering at grazing incidence
Authors: Kovats Z. , Metzger T. H. , Peisl J. , Stangl J. , Mühlberger M. , Zhuang Y. , Schäffler F. , Bauer G.
Published: July 2002

We have investigated molecular-beam-epitaxy-grown, pseudomorphic Si1−yCy epilayers (y⩽0.015) on Si(001) after ex situ annealing by x-ray scattering at grazing incidence. The diffuse intensity around the Si (220) surface reflection consists of Huang scattering due to the long-range displacement field of substitutional carbon atoms and of the form-factor-induced small angle scattering of holes created by β-SiC precipitates in the Si matrix. Even in the as-grown samples, where other methods gave no indication of β-SiC precipitates, grazing incidence diffraction clearly showed their presence. The precipitates with a mean size of 5 Å are stable against annealing up to at least 600 °C; at 800 °C carbon leaves substitutional sites and the number of precipitates increases, whereas at 1000 °C a significant increase of the precipitate size (up to ≈15 Å) is detected as well.
Real-time measurements of the pseudodielectric function of low-temperature-grown GaAs
Authors: Gajewski Donald A. , Guyer Jonathan E. , Pellegrino Joseph G.
Published: July 2002

We present real-time in situ spectroscopic ellipsometry (SE) measurements of the pseudodielectric function of low-temperature-grown GaAs as a function of growth temperature Tg, As2:Ga flux ratio R, and thickness. We show that the interband critical point E1 amplitude and sharpness decrease monotonically with decreasing Tg and/or increasing R for layers thinner than the critical epitaxial thickness hepi. We used in situ SE to reveal distinct signatures of the onset of polycrystalline or amorphous growth above hepi, which depends strongly on Tg. We revealed these systematic trends using in situ SE in conjunction with diffuse reflectance spectroscopy for active feedback temperature control.
From parametric gap solitons to chaos by means of second-harmonic generation in Bragg gratings
Authors: Trillo Stefano , Conti Claudio , Assanto Gaetano , Buryak Alexander V.
Published: July 2002

We review the theory of light localization due to the combined action of single or double Bragg coupling between dichromatic counterpropagating envelopes and parametric mixing nonlinearities. We discuss existence, stability, and excitation of such localized envelopes. We also investigate the link between stationary gap solitons and input–output response of nonlinear quadratic Bragg gratings. Frustrated transmission and multistable switching is expected to occur under suitable integrable (cascading) limits. Substantial deviations from these conditions lead to the onset of spatial chaos.
A technique for accurate measurements of ion beam current density using a Faraday cup
Authors: Sosolik C. E. , Lavery A. C. , Dahl E. B. , Cooper B. H.
Published: July 2002

We have performed measurements of the spatial distribution of current in various alkali and reactive ion beams over the energy range 5–600 eV using a Faraday cup. Ion beam current densities have been extracted from these measurements using a simple deconvolution procedure. Our results reveal that the beams are Gaussian in shape with a constant width, σ, for energies greater than approximately 75 eV and for all ion species investigated. This width is consistent with that determined from the distribution of oxygen on a Cu(001) crystal after an O+ ion beam deposition, measured using Auger electron spectroscopy. Using the measurement technique outlined in this article, together with the linear relationship between current density and Faraday cup current, it is possible to determine the beam current density using a single current measurement.
Local electron distribution of N2 adsorbed on a Ni(111) surface probed by metastable impact electron spectroscopy
Authors: Masuda Shigeru , Suzuki Ryochi , Aoki Masaru
Published: July 2002

Electron emission spectra resulting from thermal collisions of He*(23S) metastable atoms with N2 on a Ni(111) surface in the physisorbed, chemisorbed, and condensed phases were measured as well as the gas-phase spectrum. The ionization cross sections for the 3σg-, 1πu-, and 2σu-derived states depend on the orientation of N2 with respect to the metastable beam, reflecting the spatial electron distribution. Our data also show that upon chemisorption the 3σg- and 2σu-derived states are modified by mixing with each other to yield a strong charge localization. This finding indicates that, even if the metal contribution is small in the chemisorbed states, it plays a crucial role in the topology of the adsorbate wave functions.
Femtosecond self-guided atmospheric light strings
Authors: Moloney J. V. , Kolesik M. , Mlejnek M. , Wright E. M.
Published: July 2002

The onset and recurrence of multiple filamentary structures in the long-distance propagation of high power femtosecond laser pulses in air displays features analogous to strong turbulence events observed in many areas of physics. However, dissipation is not a key player in regularizing the underlying singular dynamical events. The collapse singularity of the two-dimensional (2D) nonlinear Schrödinger equation is identified as the robust nonlinear mode which both initiates and sustains the waveguide. Physical collapse regularization mechanisms include normal group velocity dispersion and plasma generation in the high intensity nonlinear focal regions. Plasma absorption is weak and the dominant process is the transient generation of a highly localized strongly defocusing lens which evacuates the light behind the leading intense light filament. Consequently the nonlinear waveguide formed is highly dynamic.
Mode-dependent vibrational autoionization in aniline
Authors: Raptis C. A. , Pratt S. T.
Published: July 2002

High-resolution photoelectron spectroscopy is used to study the branching ratios for vibrational autoionization of Rydberg states of aniline (C6H5NH2) converging to the ground electronic state of the ion. By using two-color double-resonance excitation, it is possible to prepare autoionizing resonances in which two different vibrational modes are excited. Determination of the vibrational state distribution in the product ion provides information on the relative rates of autoionization for the two modes. It is found that some normal modes appear to be especially effective at promoting vibrational autoionization, while others appear to be completely ineffective.
Guided-ion beam investigations of the reactions CO++N2 and N2++CO
Authors: Lu Wenyun , Tosi Paolo , Bassi Davide
Published: July 2002

We have investigated seven different reaction channels for each of the two symmetric charge-state reactants CO+N2 and N2++CO. For all the 14 reactions we have measured the integral cross sections as a function of relative energy and determined the corresponding reaction enthalpies. We make use of these values to find the heats of formation of NCO, NCO+, CNN, and CNN+. Similar branching ratios are observed for both charge-state reactions. This gives some hints that all reactions proceed through the same intermediate N2CO+.
A phase transition in water coupled to a local external perturbation
Authors: Volchenkov D. , Lima R.
Published: July 2002

A flux of ideal fluid coupled to perturbation is investigated by nonperturbative methods of the quantum field theory. Asymptotic behavior of the flux coupled to perturbation turns out to be similar to that of superfluids.
Permanent electric dipoles in B 1Π and D 1Π states of NaRb: Experiment and theory
Authors: Nikolayeva O. , Klincare I. , Auzinsh M. , Tamanis M. , Ferber R. , Pazyuk E. A. , Stolyarov A. V. , Zaitsevskii A. , Cimiraglia R.
Published: July 2002

The paper presents experimentally obtained permanent electric dipole moment values (μ) in electronically excited B 1Π and D 1Π states of Na8523Rb and Na8723Rb isotopomer molecules for a number of vibrational and rotational levels (v′,J′). The method is based on measuring relative intensities of “forbidden” fluorescence lines appearing due to dc Stark effect induced e/f parity mixing for a particular (v′,J′)-level, combined with electric radio frequency–optical double resonance measurement of Λ-splitting energy Δe,f. The measured D 1Π state μ values are close to 6 D, representing minor changes with the vibrational level v′ varying from 0 to 12 and J′ in the region between 7 and 50, while the measured B 1Π state μ values are about 3 D for v′=4, 5 and 6. The X 1Σ+, B 1Π, and D 1Π dipole moment functions μ(R) are calculated ab initio using the many body multipartitioning perturbation theory for explicit treatment of core-valence correlations. The theoretical and experimental dipole moment estimates are in a perfect agreement for the ground state and the D 1Π state, differing by 15%–25% for the B 1Π state.
Growth temperature dependence of transport properties of InAs epilayers grown on GaP
Authors: Souw Victor , Gopal V. , Chen E.-H. , Kvam E. P. , McElfresh M. , Woodall J. M.
Published: July 2002

Undoped InAs was grown by molecular-beam epitaxy directly on GaP at a set of different substrate temperatures. Transport properties were characterized by means of Hall-effect and resistivity measurements at temperatures between 3 and 300 K. It was observed that samples grown at higher temperatures had lower carrier concentrations, consistent with a decrease of ionized defects. In addition, samples grown at higher temperatures also had higher mobility, consistent with a smaller number of scattering centers. Samples grown at higher temperatures also showed much higher sensitivity of the mobility to the measurement temperature, suggesting a drop in neutral scattering defects. Transmission electron microscopy showed that the samples grown at higher temperatures had a significantly different dislocation microstructure. The observed dislocation microstructure is consistent with the mechanisms proposed for the influence of growth temperature on the variation of carrier concentration and mobility.
Newtonian viscosity of supercooled liquid in a Pd40Ni40P20 metallic glass
Authors: Kawamura Yoshihito , Inoue Akihisa
Published: July 2002

The Newtonian viscosity of the supercooled liquid measured by the continuous-strain-rate tensile test was investigated in the Pd40Ni40P20 metallic glass. It was found that continuous-strain-rate tests were applicable to determine the viscosity over a wide strain-rate region, and that the measured Newtonian viscosity η0 corresponded to the equilibrium one. The viscosity η over the entire strain-rate interval could be well described by one master curve expressed with a stretched exponential function η/η0=1−exp(C/ε̇β) in which β was 0.82. The free-volume-theory relation proposed by Cohen and Grest reproduced the equilibrium viscosity data over the entire temperature region better than the famous Vogel–Fulcher–Tammann relation. The supercooled liquid of the Pd40Ni40P20 metallic glass was classified into the relatively strong liquids as well as the other metallic glasses. The Pd40Ni40P20 metallic glass that is a metal-metalloid alloy exhibited to be more fragile than metal–metal glasses.
Ab initio investigation of the diaza-dicarbon CCNN molecule
Authors: Hochlaf M.
Published: July 2002

The six dimensional potential energy surface of the ground state X 1Σ+ of CCNN has been generated by the CCSD(T) approach. The quartic force field in dimensionless normal coordinates has been evaluated. This force field has been used to calculate a set of spectroscopic constants. The lowest singlet states are also investigated. The bound ground state has a potential energy barrier to dissociation due to an avoided crossing with the excited d 1Σ+ state. The other singlet states are found to be repulsive with respect to the dissociation into C2 and N2.
Microstructure evolution, magnetic domain structures, and magnetic properties of Co–C nanocomposite films prepared by pulsed-filtered vacuum arc deposition
Authors: Wang H. , Wong S. P. , Cheung W. Y. , Ke N. , Chiah M. F. , Liu H. , Zhang X. X.
Published: July 2002

Co-based nanocrystals encapsulated in carbon have potential applications in ultra-high-density magnetic recording media. In this work, CoxC1−x (x=60, 65, and 70 at. %) nanocomposite films were prepared by pulsed-filtered vacuum arc deposition. Subsequent thermal annealing was performed in vacuum at various temperatures. The films were characterized by non-Rutherford backscattering spectrometry, transmission electron microscopy, Raman spectroscopy, atomic-force microscopy, and magnetic-force microscopy. The as-deposited films were found to be amorphous. After annealing at appropriate temperatures, the films were found to be consisting of hexagonal close-packed nanocrystalline Co grains encapsulated in graphite-like carbon. Clear magnetic-force microscopy images were only observed in those films annealed at sufficiently high temperatures, indicating that there was perpendicular magnetic anisotropy in these films. The magnetic hysteresis loops of the films were measured by a superconducting quantum interference device magnetometer. The optimum annealing temperature for the maximum coercivity was found to depend on the cobalt concentration. For a Co65C35 sample about 20 nm thick after annealing at 350 °C in vacuum for 1 h, the saturation magnetization was 500 emu/cm3, the coercivity was 460 Oe, and the ratio of the remanence to the saturation magnetization was 0.68 at 300 K. Our results are consistent with those of the sputtered Co–C films recently reported in the literature.
The role of the local-multiplicative Kohn–Sham potential on the description of occupied and unoccupied orbitals
Authors: Garza Jorge , Nichols Jeffrey A. , Dixon David A.
Published: July 2002

The optimum local-multiplicative exchange potential was found using as input the Hartree–Fock electron density, for the molecular systems: H2, LiH, HF, NH3, CH4, H2O, N2, CO, F2, C2H2 and C2H4. The Zhao and Parr method was used to obtain the local-multiplicative potential where the kinetic energy is minimized using a constrained-search formulation of density functional theory. Two orbital sets were compared, those obtained with the nonlocal Hartree–Fock potential and those obtained with the local-multiplicative potential, both sets yielding the same electron density. As expected, the highest occupied molecular orbital (HOMO) energy was similar in both orbital sets. In contrast, the virtual orbital energies, and in particular the lowest unoccupied molecular orbital (LUMO), exhibited considerable differences. The Hartree–Fock LUMO energy goes to zero in a complete basis set limit and to nearly zero with reasonably large basis sets (e.g., augmented triple zeta) with sufficient diffuse functions added. The LUMO provided by the local-multiplicative potential using the same large basis set goes to a bounded energy not equal to zero. The nonlocal Hartree–Fock potential generates a large gap between the HOMO and LUMO energies; this difference is equal to the negative of the HOMO energy at the complete basis set limit. Contrary to this behavior, the gap obtained with the local-multiplicative potential is a reasonable approximation to the lowest experimental vertical excitation energy. For some of the molecules tested, the ordering of the orbitals corresponding to the HF and local-multiplicative potential are different.
Spectral analysis of photons emitted during scratching of an insulator surface by a diamond in air
Authors: Miura T. , Nakayama K.
Published: July 2002

Spectral analysis of photons emitted during scratching of insulator surfaces of Si3N4, Al2O3, ZrO2 and soda-lime glass with a diamond was performed under relatively slight frictional conditions, i.e., with a normal force of less than 1 N and a sliding velocity of less than 16 cm/s in ambient air. All the spectra showed similar profiles with strong sharp peaks in the ultraviolet region and photon energies of 2.8–4 eV. The wavelengths of the photons were perfectly matched to those for the second positive band of N2, demonstrating that an electric discharge of N2 gas occurred at the frictional contact.
Transition path sampling of cavitation between molecular scale solvophobic surfaces
Authors: Bolhuis Peter G. , Chandler David
Published: July 2002

The dynamics of a cavitation transition between repulsive plates in a Lennard-Jones system is studied using transition path sampling. It is found that the critical nucleus for the transition coincides with the formation of a vapor tube connecting the two plates. The number of particles between the plates and the tube radius are relevant order parameters. In the transition state ensemble, the distributions of these parameters have widths that are roughly 20% of their respective means. Committor distributions of constrained ensembles show that of these two important parameters, the tube radius is the most significant component in the reaction coordinate for the cavitation transition.
Electrical characterization of conducting polypyrrole by THz time-domain spectroscopy
Authors: Jeon Tae-In , Grischkowsky D. , Mukherjee A. K. , Menon Reghu
Published: July 2002

Using an optoelectronic THz beam system for THz time-domain spectroscopy (THz TDS), we have measured the absorption and index of refraction of a conducting polypyrrole film from low frequencies to 2.5 THz. From these measurements, the dc conductivity of 215/(Ω cm) and the complex conductance were obtained over this frequency range. All of the results were well fit by Drude theory, which gives a carrier scattering time of only 12.6 fs, less than 1/10 that of the semiconductors, thereby illustrating the disorder and low mobility of the polymer.
A high-quality x-ray scattering experiment on liquid water at ambient conditions
Authors: Hura Greg , Sorenson Jon M. , Glaeser Robert M. , Head-Gordon Teresa
Published: July 2002

We report a new, high-quality x-ray scattering experiment on pure ambient water using a synchrotron beam line at the Advanced Light Source at Lawrence Berkeley National Laboratory. Several factors contribute to the improved quality of our intensity curves including use of a highly monochromatic source, a well-characterized polarization correction, a Compton scattering correction that includes electron correlation, and more accurate intensities using a modern charge coupled device (CCD) detector. We provide a comprehensive description of the data processing that we have used for correcting systematic errors, and we provide an estimate of our remaining random errors. The resulting error estimates of our data are smaller then the discrepancies between data sets collected in past x-ray experiments. We find that the older x-ray curves support a family of gOO(r)’s that exhibit a smaller first peak (∼2.2), while the current data is better fit with a family of gOO(r)’s with a first peak height of 2.8, and systematic shifts in all peak positions to smaller r.
Ru doping of the A-type antiferromagnet Pr0.5Sr0.5MnO3: Conversion to a metallic ferromagnet
Authors: Maignan A. , Martin C. , Hervieu M. , Raveau B.
Published: July 2002

The doping of Mn sites of the A-type antiferromagnet Pr0.5Sr0.5MnO3 with ruthenium shows that ferromagnetism and metallicity are induced similarly to the doping of the CE-type phases Ln0.5Ca0.5MnO3. This effect, similar to the Cr doping, differs from the latter by the fact that TC=TIM increases significantly with the Ru level. The different behaviors of Ru and Cr doped Pr0.5Sr0.5MnO3 samples are interpreted on the basis of two factors, ferromagnetic superexchange introduced by the magnetic dopants, and valency effect which contributes to increase the Mn3+ content for Ru(IV) and Ru(V) and does the opposite for Cr(III).
Nonlinear electrical transport in nc-Si/CaF2 multilayer structures with ultrathin CaF2 layers
Authors: Ioannou-Sougleridis V. , Ouisse T. , Nassiopoulou A. G. , Bassani F. , Arnaud d’Avitaya F.
Published: July 2002

We present a study of the electrical transport in (Si/CaF2)n superlattices with n=100 and with Si, CaF2 thickness in each period in the range of 1.2–1.6 nm (Si) and below 1 nm (CaF2), respectively. The results suggest that at gate voltages higher than ±4 V a Poole–Frenkel-type mechanism accounts for the observed electric-field-assisted conduction through the layers.
Electron mobility in tris(8-hydroxy-quinoline)aluminum thin films determined via transient electroluminescence from single- and multilayer organic light-emitting diodes
Authors: Barth S. , Müller P. , Riel H. , Seidler P. F. , Rieß W. , Vestweber H. , Bässler H.
Published: July 2002

Transient electroluminescence (EL) from single- and multilayer organic light-emitting diodes (OLEDs) was investigated by driving the devices with short, rectangular voltage pulses. The single-layer devices consist of indium-tin oxide (ITO)/tris(8-hydroxy-quinoline)aluminum (Alq3)/magnesium (Mg):silver (Ag), whereas the structure of the multilayer OLEDs are ITO/copper phthalocyanine (CuPc)/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB)/Alq3/Mg:Ag. Apparent model-dependent values of the electron mobility (μe) in Alq3 have been calculated from the onset of EL for both device structures upon invoking different internal electric field distributions. For the single-layer OLEDs, transient experiments with different dc bias voltages indicated that the EL delay time is determined by the accumulation of charge carriers inside the device rather than by transport of the latter. This interpretation is supported by the observation of delayed EL after the voltage pulse is turned off. In the multilayer OLED the EL onset—dependent on the electric field—is governed by accumulated charges (holes) at the internal organic-organic interface (NPB/Alq3) or is transport limited. Time-of-flight measurements on 150-nm-thin Alq3 layers yield weak field-dependent μe values of the order of 1×10−5 cm2/Vs at electrical fields between 3.9×105 and 1.3×106 V/cm.
Studies on carbon/sulfur cluster anions produced by laser vaporization: Experiment (collision-induced dissociation) and theory (ab initio calculation). II C4Sm− (4⩽m⩽10)
Authors: Chen Hong , Huang Rong-bin , Xie Zhao-xiong , Chen Li-hua , Zheng Lan-sun
Published: July 2002

Carbon/sulfur binary cluster anions with various compositions can be produced from laser vaporization of a sample mixed with sulfur and carbon powders in a 20:1 molar ratio. After mass-selection, their molecular formula was determined by collision-induced dissociation. The cluster anions consist of even carbon atoms only and their number of clustering sulfur atoms equals or exceeds that of carbon atoms. In this paper, we focus on mass spectrometry investigation and ab initio calculations on the cluster anions containing four carbon atoms, C4Sm− (4⩽m⩽10). Geometry of various isomeric structures of the clusters was optimized at the ROHF/6-31G* level and their energies were compared to find the most stable isomers. The structures of C4Sm− clusters computed in this paper were designed based on the structural features learned from the previous study on C2Sm− and the results of mass spectrometry experiments. Among the structures, four low-lying forms with two dangling sulfur atoms and two closed rings have the lowest energy. Based on the experimental and theoretical investigations, a ring shrinkage scheme is proposed to account for the dissociation process of the binary cluster anions observed in the experiment.
Structural templating effects in molecular heterostructures grown by organic molecular-beam deposition
Authors: Heutz S. , Cloots R. , Jones T. S.
Published: July 2002

Powder x-ray diffraction has been used to study the structural properties of multilayered heterostructures grown by organic molecular-beam deposition and based on the molecular materials H2Pc, perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) and Alq3. Structural templating has been observed for multilayers of PTCDA and H2Pc, with the first layer completely disrupting the structure of subsequent layers. H2Pc adopts a new structure when deposited on PTCDA, in which the molecular plane is oriented parallel to the substrate and this configuration is retained for film thicknesses >380 nm. The templating effect extends through molecular thin-film structures involving several different layers, but is attenuated by insertion of an amorphous layer such as Alq3.
Energy loss mechanisms in the microdischarges in plasma display panels
Authors: Hagelaar G. J. M. , Klein M. H. , Snijkers R. J. M. M. , Kroesen G. M. W.
Published: July 2002

Low luminous efficacy is one of the major drawbacks of plasma display panels (PDPs), where the main limiting factor is the efficiency of the microdischarges in generating UV radiation. In this work we use a two-dimensional self-consistent fluid model to analyze the energy loss mechanisms in neon–xenon discharges in coplanar-electrode color PDPs and interpret experimental data on the luminous efficacy of these PDPs. The modeling results are in good agreement with the measured UV emission spectrum and measured trends in the efficacy. Most of the electrical input energy is transferred to ions and subsequently to the gas and the surface. The electrical energy transferred to electrons is mostly used for ionization and excitation, where the part used for xenon excitation largely ends up in UV radiation. The amplitude, frequency, and rise time of the driving voltage mainly affect the energy losses due to ion heating. The xenon content also affects the conversion of electron energy into UV energy.
Temperature resolved modeling of plasma abatement of perfluorinated compounds
Authors: Kiehlbauch Mark W. , Graves David B.
Published: July 2002

Plasma abatement of perfluorinated compounds (PFCs) is a technology of increasing interest as the semiconductor industry seeks to reduce emission of these global warming gases. This article reports a simulation of point-of-use plasma abatement, in which a high density abatement plasma is placed between the process tool turbomolecular pump and the roughing pump. Oxygen is added to convert the feed PFCs to oxidized species that can then be scrubbed downstream. The simulation is used to examine the transport and chemistry that occurs in these plasmas. Two-dimensional plasma and neutral transport models are coupled to a description of gas and surface plasma chemistry. The simulation shows that, at high electron density and low neutral density, abatement of CF4 is governed by electron impact dissociation. Both the model and experimental measurements predict high peak neutral temperatures (>1500 K) that play a large role in determining the characteristics of these plasmas.
Intra-molecular phase segregation in a single polyelectrolyte chain
Authors: Takagi S. , Tsumoto K. , Yoshikawa K.
Published: July 2002

We performed a theoretical study on the collapse transition of a single polyelectrolyte chain induced by multivalent cations, under the framework of the mean-field theory by taking into account the effect of small mobile ions. For a sufficiently long chain, the folding transition is found to be a two-step process; from coil to segregated and from segregated to collapsed, where the segregated state means the coexistence of an elongated coil part and a collapsed compact part in a single polymer chain in the equilibrium state. These transitions are considered to reflect competition between two states with double minima in the free energy; i.e., a first-order phase transition in a mesoscopic system. With the experimental technique of single chain observation by fluorescence microscopy, we monitored the folding transition of individual giant DNAs. The results of this experiment indicate the actual existence of the two-step folding transition in single DNA molecules. The translational entropy of ions may play an important role in this process.
Electron scattering from gaseous SF6: Comparing calculations with experiments
Authors: Gianturco F. A. , Lucchese R. R.
Published: July 2002

The dynamical observables associated with low-energy electron scattering from SF6 molecules in the gas-phase, e.g., elastic differential cross sections, integral elastic cross sections and momentum transfer cross sections, are computed using quantum methods and describing the full interaction between the molecule and the impinging electron without empirical parameters. The above quantities are obtained over an energy interval ranging from a few meV up to 100 eV and the results are compared with the available experiments. Various aspects of the theoretical method employed are analyzed in relation to their agreement with the experimental data discussed in this work.
Thermodynamic analysis of thermophotovoltaic efficiency and power density tradeoffs
Authors: Baldasaro P. F. , Raynolds J. E. , Charache G. W. , DePoy D. M. , Ballinger C. T. , Donovan T. , Borrego J. M.
Published: July 2002

This report presents an assessment of the efficiency and power density limitations of thermophotovoltaic (TPV) energy conversion systems for both ideal (radiative limited) and practical (defect-limited) systems. Thermodynamics is integrated into the unique physics of TPV conversion, and used to define the intrinsic tradeoff between power density and efficiency. The results of the analysis reveal that the selection of diode band gap sets a limit on achievable efficiency well below the traditional Carnot level. In addition it is shown that filter performance dominates diode performance in any practical TPV system and determines the optimum band gap for a given radiator temperature. It is demonstrated that for a given radiator temperature, lower band-gap diodes enable both higher efficiency and power density when spectral control limitations are included. The goal of this work is to provide a better understanding of the basic system limitations that will enable successful long-term development of TPV energy conversion technology.
Density-functional study of homogeneous bubble nucleation in the stretched Lennard-Jones fluid
Authors: Shen Vincent K. , Debenedetti Pablo G.
Published: July 2002

Density-functional theory is used to study homogeneous bubble nucleation in the stretched Lennard-Jones liquid. We show that the ratio of density-functional to classical nucleation theory free energy barriers should scale with the quantity Δμ/Δμspin, the difference in chemical potential between the bulk superheated and the saturated liquid divided by the chemical potential difference between the liquid spinodal and the saturated liquid. The critical bubble changes from classical near coexistence (sharp interface, uniform density that decreases with penetration into the coexistence region) to nonclassical beyond Δμ/Δμspin≈0.5 (diffuse interface, increasing density with increasing penetration into the metastable region). The density at the center of the bubble, the mean bubble density, the bubble size, the interfacial thickness, and the free energy cost of forming a critical bubble all scale with Δμ/Δμspin in temperature-independent fashion. This precise measure of the degree of metastability should emerge as a natural parameter in data correlation, as well as in the development of improved theories of nucleation.
One-dimensional overdriven detonations with branched-chain kinetics
Authors: Sánchez Antonio L. , Carretero Manuel , Clavin Paul , Williams Forman A.
Published: July 2002

The dynamics of time-dependent, planar propagation of gaseous detonations is addressed on the basis of a three-step chemistry model that describes branched-chain processes. Relevant nondimensional parameters are the ratio of the heat release to the thermal enthalpy at the Neumann state, the nondimensional activation energies for the initiation and branching steps, the ratio of the branching time to the initiation time and the ratio of the branching time to the recombination time. The limit of strong overdrive is considered, in which pressure remains constant downstream from the leading shock in the first approximation, and the ratio of specific heats γ is taken to be near unity. A two-term expansion in the strong overdrive factor is introduced, and an integral equation is derived describing the nonlinear dynamics and exhibiting a bifurcation parameter, the reciprocal of the product of (γ−1), the nondimensional heat release and the nondimensional branching activation energy, with an acoustic correction. A stability analysis shows that, depending on values of the parameters, either the mode of lowest frequency or a mode of higher frequency may be most unstable. Numerical integrations exhibit different conditions under which oscillations die, low-frequency oscillations prevail, high-frequency oscillations prevail, highly nonlinear oscillations persist, or detonation failure occurs. This type of parametric analysis is feasible because of the relative simplicity of the model, which still is more realistic than a one-step, Arrhenius chemical approximation. In particular, by addressing the limit of slow radical recombination compared with branching, explicit results are derived for the critical value of the bifurcation parameter, involving the ratio of the recombination time to the induction time. The results help to clarify the general nature of one-dimensional detonation instability and provide simplifications that can be employed in efficiently relating gaseous detonation behavior to the true underlying chemistry.
The hot carbon, oxygen, and nitrogen echelle spectrograph
Authors: Indebetouw Rémy , McLean Ryan , Wilkinson Erik , Green James , Beasley Matthew
Published: July 2002

The hot carbon, oxygen, and nitrogen echelle spectrograph (HotCONes) sounding rocket payload allows a unique probe of the local interstellar medium through R=λ/δλ≳200 000 spectroscopy of C+3, N+4, and O+5. It is a practical proof of the low-order echelle spectrograph design, which may prove extremely useful in conjunction with emerging technology. We present the scientific motivation, instrument design, and measured performance.
CN
x
/carbon nanotube junctions synthesized by microwave chemical vapor deposition
Authors: Ma Xucun , Wang E. G.
Published: July 2002

The CNx/carbon nanotube junctions were successfully synthesized by microwave plasma-assisted chemical vapor deposition method from the mixture of N2/CH4 and H2/CH4 gases in a continuous growth process. High resolution transmission electron microscopy revealed that these junctions were of heterostructure between CNx nanotubes with polymerized nanobells and cylindrical carbon nanotubes. The growth process is quite simple and can be easily scaled up. The intimate correlation between the electronic structure and the chemical composition at the both sides of the junction indicate some interesting properties and offers potential applications for future nanodevices.
Recombination lifetimes in undoped, low-band gap InAsyP1−y/InxGa1−xAs double heterostructures grown on InP substrates
Authors: Ahrenkiel R. K. , Johnston S. W. , Webb J. D. , Gedvilas L. M. , Carapella J. J. , Wanlass M. W.
Published: July 2002

High-quality, thin-film, lattice-matched (LM) InAsyP1−y/InxGa1−xAs double heterostructures (DHs) have been grown lattice mismatched on InP substrates using atmospheric-pressure metalorganic vapor-phase epitaxy. The low-band gap InxGa1−xAs layers in the DHs have room-temperature band gaps that range from 0.47 to 0.6 eV. Both the optical and electronic properties of these films have been extensively measured. The band-to-band photoluminescence is quite strong and comparable to that found for LM InP/In0.53Ga0.47As DHs grown on InP. Recombination lifetime measurements of undoped DH structures show minority-carrier lifetimes in excess of 1 μs in most cases. The earlier properties make the band gap-flexible InAsyP1−y/InxGa1−xAs DH system attractive for applications in high-performance, infrared-sensitive devices.
Establishment of strong velocity shear and plasma density profile modification with associated low frequency fluctuations
Authors: Shinohara Shunjiro , Matsuoka Norikazu , Matsuyama Shoichiro
Published: July 2002

Strong shear of azimuthal plasma rotation velocity and a large density profile modification from a hollow to peaked profiles were successfully obtained in a cylindrical magnetized plasma, using voltage biased electrodes. The shear region and density profiles could be finely controlled by changing the voltage biased position as well as by the magnetic field configurations. A low frequency (<4 kHz) density oscillation was identified as a drift wave type: propagation in the electron diamagnetic direction with a rigid rotation (Mach number M∼−1 at the edge), opposite to the direction of the edge plasma rotation with M∼1.
Molecular simulations of sound wave propagation in simple gases
Authors: Hadjiconstantinou Nicolas G. , Garcia Alejandro L.
Published: July 2002

Molecular simulations of sound waves propagating in a dilute hard sphere gas have been performed using the direct simulation Monte Carlo method. A wide range of frequencies is investigated, including very high frequencies for which the period is much shorter than the mean collision time. The simulation results are compared to experimental data and approximate solutions of the Boltzmann equation. It is shown that free molecular flow is important at distances smaller than one mean free path from the excitation point.
Effect of catalyst film thickness on carbon nanotube growth by selective area chemical vapor deposition
Authors: Wei Y. Y. , Eres Gyula , Merkulov V. I. , Lowndes D. H.
Published: July 2002

The correlation between prepatterned catalyst film thickness and carbon nanotube (CNT) growth by selective area chemical vapor deposition (CVD) was studied using Fe and Ni as catalyst. To eliminate sample-to-sample variations and create a growth environment in which the film thickness is the sole variable, samples with continuously changing catalyst film thickness from 0 to 60 nm were fabricated by electron-gun evaporation. Using thermal CVD CNTs preferentially grow as a dense mat on the thin regions of the catalyst film. Moreover, beyond a certain critical film thickness no tubes were observed. The critical film thickness for CNT growth was found to increase with substrate temperature. There appears to be no strong correlation between the film thickness and the diameter of the tubes. In contrast, using plasma enhanced CVD with Ni as catalyst, vertically oriented CNTs grow in the entire range of catalyst film thickness. The diameter of these CNTs shows a strong correlation with the catalyst film thickness. The significance of these experimental trends is discussed within the framework of the diffusion model for CNT growth.
Auger recombination in heavily carbon-doped GaAs
Authors: Ahrenkiel R. K. , Ellingson R. , Metzger W. , Lubyshev D. I. , Liu W. K.
Published: July 2002

The recombination parameters in heavily carbon-doped GaAs are of considerable importance to current bipolar transistor technology. Here, we used time-resolved photoluminescence and quantum-efficiency techniques in parallel to measure the very short lifetimes expected at high doping. The samples were isotype double heterostructures, with the structure Al(0.4)Ga(0.6)As/GaAs/Al(0.4)Ga(0.6)As, grown by molecular-beam epitaxy. The doping level was varied from 5×1018 to 1×1020 cm−3 for the samples described here. For doping levels greater than 1×1019 cm−3, the lifetime decreased as the inverse of the cube of the hole density, indicating that phonon and impurity-assisted Auger processes are dominant.
Monte Carlo and numerical self-consistent field study of systems with end-grafted and free polymers in good solvent
Authors: Pépin M. P. , Whitmore M. D.
Published: July 2002

We present a systematic Monte Carlo and numerical self-consistent field (NSCF) study of thin films consisting of grafted and free polymers in good solvent, for the range of densities found in most experiments. Above the overlap threshold for the grafted polymer, the two approaches agree well. Even at low densities, the agreement is surprisingly good. The NSCF results are also directly compared with experiments. The systematic results are interpreted in the context of the regimes and behavior predicted by scaling and analytic SCF theories. We find that the grafted layer is generally thinner, and the penetration of the free polymer into the grafted layer is generally greater than predicted, and that the overall behavior is not in accord with the earlier theories. We find it useful to introduce and distinguish between two measures of the penetration, and we find that one of them can increase with the concentration of grafted polymer.
Relativistic Lagrange formulation
Authors: Geroch Robert , Nagy G. , Reula O.
Published: July 2002

It is well-known that the equations for a simple fluid can be cast into what is called their Lagrange formulation. We introduce a notion of a generalized Lagrange formulation, which is applicable to a wide variety of systems of partial differential equations. These include numerous systems of physical interest, in particular, those for various material media in general relativity. There is proved a key theorem, to the effect that, if the original (Euler) system admits an initial-value formulation, then so does its generalized Lagrange formulation.