bluuebunny/crossref_metadata_2025 huggingface dataset:
Barrier properties of superconducting structures based on niobium nitride and a molybdenum–rhenium alloy
Authors: Shaternik V. E. , Belogolovskiı̆ M. A.
Published: December 2003

Stable superconducting structures of the metal–insulator–metal type are made using niobium nitride. A comparison of the measured current–voltage characteristics with the theoretical curves calculated with allowance for possible fluctuations in the transparency of the insulating spacer layer shows that the potential barrier formed as a result of the oxidation of NbN is quite uniform. The possibility of making niobium nitride Josephson junctions for superconducting micro- and nanoelectronics is discussed.
Phase-sensitive quantum effects in the Andreev conductance of an SNS system of metals with a macroscopic phase-breaking length
Authors: Chiang Yu. N. , Shevchenko O. G.
Published: December 2003

A study is made of the dissipative component of the electron transport in a doubly connected Andreev S–N–S (indium–aluminum–indium) interferometer with elastic mean free paths lel in the metals of around 100 μm and a macroscopic phase-breaking length Lφ that is two or more orders of magnitude larger than Lφ in disordered nanostructures, including two-dimensional ones. The studies are done under conditions not studied before in such interferometers: m̄<lel (m̄ is the transverse size of the NS boundaries). At helium temperatures the samples are found to exhibit new phase-sensitive effects of a quantum-interference nature. Conductance oscillations with a period Φ0/A (Φ0 is the flux quantum and A is the aperture area of the interferometer) are observed in the non-domain (normal) state of the indium narrowing near the NS boundary. In the domain intermediate state of the narrowing, magneto-temperature resistive oscillations are observed, with a period 2Φ0/ξHc(T)2 (ξHc(T) is the coherence length in a magnetic field close to critical). At sufficiently low temperatures (T≈2 K) the conductance of a macroscopic N region of the system has an oscillatory component of resonance shape which undergoes phase inversion relative to the phase of the nonresonance oscillations. An explanation for the effects is suggested in terms of the contribution to the Josephson current from coherent quasiparticles with energies of the order of the Thouless energy. The temperature behavior of the dissipative transport in a pure normal metal near an isolated NS point contact is investigated.
Surface and bulk spin excitations in ferromagnetic semiconductor superlattices
Authors: Tanriverdiyev V. A. , Tagiyev V. S. , Seyid-Rzayeva S. M.
Published: December 2003

The propagation of surface and bulk spin excitations in ferromagnetic semiconductor superlattices consisting of alternating monoatomic layers of two types of cubic Heisenberg ferromagnets is considered. The Green’s function method is used to obtain the dispersion relations describing the propagation of surface and bulk spin waves in the superlattices and also the temperature dependence of the magnetization of the localized spins. The results are integrated numerically and presented graphically.
Low-temperature magnetic properties of the monoclinic magnet RbDy(WO4)2
Authors: Khatsko E. N. , Cherny A. S. , Rykova A. I. , Borowiec M. T. , Szymczak H. , Dyakonov V. P.
Published: December 2003

The temperature dependence of the magnetic susceptibility along the principal magnetic axes of rubidium–dysprosium tungstate RbDy(WO4)2 is measured in the temperature interval 0.5–300 K. Strong anisotropy of the susceptibility is observed at all temperatures in that range. At TN=0.8 K the susceptibility along all three magnetic axes exhibits a sharp anomaly due to the phase transition to a magnetically ordered state. At 0.5 K the measured field dependence of the magnetization along all three axes demonstrates a metamagnetic orientational phase transition in the directions of the magnetic axes y and z. Another type of anomaly of the magnetic susceptibility is observed at temperatures of around 5, 8, and 50 K. Analysis shows that these anomalies correspond to structural phase transitions in this compound.
Heat capacity of solid solutions of deuteromethane in krypton. Nuclear spin conversion in CD4 molecules
Authors: Bagatskii M. I. , Manzhelii V. G. , Mashchenko D. A. , Dudkin V. V.
Published: December 2003

A study is made of the heat capacity of the binary solid solutions (CD4)0.01Kr0.99 (in the temperature interval ΔT=0.9–7 K) and (CD4)0.05Kr0.95 (ΔT=0.7–20 K) and of the ternary solid solutions (CD4)0.01Kr0.988(O2)0.002 (ΔT=0.7–11 K) and (CD4)0.05Kr0.948(O2)0.002 (ΔT=0.8–4 K). The contribution of the rotational subsystem to the heat capacity of the solutions is separated out. The influence of temperature, oxygen impurities, and the interaction between the deuteromethane molecules on the effective conversion rate is studied. The energy differences between the lowest-lying levels of the nuclear spin species A and T of deuteromethane is determined, and the effective characteristic conversion times are found. Rapid conversion of isolated CD4 molecules is observed in CD4–Kr solutions. It is shown that in the CD4–Kr solutions, as in CH4–Kr solutions, a hybrid mechanism of conversion is dominant at low temperatures (T<1.4 K), and the “bottleneck” governing the conversion rate is the intermolecular effective octupole–octupole interaction and the related probability of transfer of the conversion energy to phonons.
Influence of a boron oxide admixture on the magnetic and electrical properties of the high-Tc superconductor Bi1.7Pb0.3Sr2Ca2Cu4Oy
Authors: Jgamadze V. V. , Kokhreidze R. G. , Margiani N. G. , Mestvirishvili A. N. , Odenov S. V. , Papunashvili N. A. , Tsintsadze G. A. , Shurgaia G. A.
Published: December 2003

The influence of an admixture of boron oxide B2O3 on the magnetic and electrical properties of a high-Tc superconducting ceramic with the nominal composition Bi1.7Pb0.3Sr2Ca2Cu4Oy is investigated. A nonmonotonic dependence of the critical temperature Tc (ρ=0) and magnetic susceptibility on the boron concentration is found.
Time-resolved analysis of the white photoluminescence from SiO2 films after Si and C coimplantation
Authors: Pellegrino P. , Pérez-Rodriguez A. , Garrido B. , González-Varona O. , Morante J. R. , Marcinkevic̆ius S. , Galeckas A. , Linnros J.
Published: December 2003

The analysis of the white photoluminescence (PL) from Si+ and C+ coimplanted SiO2 is reported as a function of the implanted dose. By both steady and time-resolved measurements, the presence of several components in the emission between 2 and 3.3 eV has been resolved. The decays of the PL transients are characterized by short lifetimes, below 2 ns. For the emission at 2.1–2.3 eV, photoluminescence decay transients have been measured, obtaining a fast relaxation component of about 50–70 ps, followed by a slower component of the order of 1 ns. These values contrast with the very slow behavior, characteristic for the light emission from Si nanocrystals, and make carbon-related emitting centers interesting for optoelectronic applications where fast switching behavior is important.
An interaction energy decomposition approach for the supermolecule density functional theory calculations
Authors: Cybulski Sławomir M. , Seversen Christopher E.
Published: December 2003

A method is proposed for partitioning the supermolecule interaction energy obtained from density functional theory calculations. Exploratory calculations for Ar2 reveal serious shortcomings of several commonly used correlation functionals. There are also problems with exchange functionals resulting in unphysical behavior at long range. These findings clearly demonstrate the limitations of some commonly used functionals.
Optical properties of metallic nanoparticles in Ni-ion-implanted α-Al2O3 single crystals
Authors: Xiang X. , Zu X. T. , Zhu S. , Wang L. M.
Published: December 2003

64 keV Ni ion implantation was performed at room temperature up to a dose of 1×1017 cm−2 in α-Al2O3 single crystals. The charge states, structure, and optical properties of metallic embedded Ni nanoparticles were studied by using x-ray photoelectron spectroscopy (XPS), transmission electron microscopy, and optical spectroscopy, respectively. XPS analysis showed that implanted Ni ions are mainly in charge state of metallic Ni0. Nanoparticles distributed from the surface to 30 nm below the surface were observed in a high-angle annular dark-field image. The size of nanoparticles ranges from 1 to 5 nm in diameter. A high-resolution electron microscopy image indicated the Ni-implanted area had been entirely amorphized. A new broad absorption band centered at 400 nm appeared in the optical absorption spectrum of the as-implanted crystal, due to surface plasma resonance of Ni nanoparticles. We did not find any emission band in the as-implanted sample under a Xe lamp excitation wavelength of 250–430 nm in a spectrophotometer.
Silicon oxide thickness-dependent growth of carbon nanotubes
Authors: Cao Anyuan , Ajayan P. M. , Ramanath G. , Baskaran R. , Turner K.
Published: December 2003

Recent discovery of substrate-selective growth of carbon nanotubes on SiO2 in exclusion to Si, has opened up the possibility of organizing nanotubes on Si/SiO2 patterns in premeditated configurations for building devices. Here, we report the strong dependence of nanotube growth on the SiO2 layer thickness, and the utility of this feature to build three-dimensional architectures. Our results show that there is no detectable nanotube growth on SiO2 layers with thickness (TSiO2) less than ∼5–6 nm. For 6 nm<TSiO2<24 nm, the nanotube growth rate increases monotonically with increasing oxide thickness, and then saturates as TSiO2 approaches >50 nm. We grew nanotubes with multiple lengths at close proximity in a single step by using substrates with regions of different TSiO2. Such processing strategies would be attractive for creating nanotube mesoscale architectures for device applications.
Near-term aging and thermal behavior of polyfluorene in various aggregation states
Authors: Cheun H. , Tanto B. , Chunwaschirasiri W. , Larson B. , Winokur M. J.
Published: December 2003

Steady-stage photoluminescence and photoabsorption have been used to characterize the impact of aggregation in various poly(di-n-alkylfluorene) thin films. Highly aggregated films exhibit systematic changes in the photoluminescence while aging. These are indicative of a gradual increase in interchain interactions. Nonaggregated films do not exhibit this behavior. However the mesomorphic properties are altered and these films undergo irreversible changes on thermal cycling. All these properties appear to be correlated with changes in the local short-range molecular packing.
Valley splitting in strained silicon quantum wells
Authors: Boykin Timothy B. , Klimeck Gerhard , Eriksson M. A. , Friesen Mark , Coppersmith S. N. , von Allmen Paul , Oyafuso Fabiano , Lee Seungwon
Published: December 2003

A theory based on localized-orbital approaches is developed to describe the valley splitting observed in silicon quantum wells. The theory is appropriate in the limit of low electron density and relevant for quantum computing architectures. The valley splitting is computed for realistic devices using the quantitative nanoelectronic modeling tool NEMO. A simple, analytically solvable tight-binding model reproduces the behavior of the splitting in the NEMO results and yields much physical insight. The splitting is in general nonzero even in the absence of electric field in contrast to previous works. The splitting in a square well oscillates as a function of S, the number of layers in the quantum well, with a period that is determined by the location of the valley minimum in the Brillouin zone. The envelope of the splitting decays as S−3. The feasibility of observing such oscillations experimentally in Si/SiGe heterostructures is discussed.
Room-temperature hydrogen storage characteristics of ZnO nanowires
Authors: Wan Q. , Lin C. L. , Yu X. B. , Wang T. H.
Published: December 2003

Massive ZnO nanowires with the mean diameter of 20 nm have been rapid synthesized by evaporation of metal zinc at 900 °C in the quartz tube. Metal catalyst, graphite additive, and vacuum are not necessary, and, more importantly, it is very convenient and effective for large-scale industrialization. The hydrogen storage characteristics of the synthesized ZnO nanowires are investigated at room temperature. The highest storage capacity of 0.83 wt% is achieved under the pressure of about 3.03 Mpa, and about 71% of the stored hydrogen can be released under ambient pressure at room temperature.
Self-frequency tripling from two cascaded second-order nonlinearities in GdAl3(BO3)4:Nd3+
Authors: Brenier Alain , Tu Chaoyang , Zhu Zhaojie , Li Jianfu , Wang Yan , You Zhenyu , Wu BaiChang
Published: December 2003

We combine inside a single noncentrosymmetric laser crystal, GdAl3(BO3)4:Nd3+, a stimulated emission and two second-order nonlinearities in cascade. The first one self-doubles the 1062.8 nm Nd3+ laser emission corresponding to the F3/24→I11/24 channel, despite the fact that it is not phase-matched. The second one can be phase-matched or not and mixes the laser wave and its second harmonic (SH) in order to produce the third harmonic (TH) at 354.3 nm.
Mg-doped C60 thin film as improved n-type organic semiconductor for a solar cell
Authors: Chikamatsu Masayuki , Taima Tetsuya , Yoshida Yuji , Saito Kazuhiro , Yase Kiyoshi
Published: December 2003

Mg doping effect on a [2-methoxy, 5-(2′-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV)/C60 bilayer solar cell has been investigated. We find that the Mg doping occurs during the deposition of the Mg top electrode onto the C60 film and that the solar cell property is markedly improved by the doping. The power conversion efficiency of the Mg-doped device under 100 mW/cm2 white light illumination is 0.54%, which is approximately 400 times larger than that of the nondoped device.
Enhancement at low temperatures of the critical current density for Au-coated MgB2 thin films
Authors: Choi Eun-Mi , Lee Hyun-Sook , Kim Heon-Jung , Lee Sung-Ik , Kim Hyeong-Jin , Kang W. N.
Published: December 2003

We measured the superconducting critical current densities (Jc) from the magnetization hysteresis (M–H) loop for Au-coated MgB2 thin films. The purpose of this experiment was to determine whether the vortex avalanche phenomenon which suppresses the Jc for low temperature (T<15 K) and low field (H⩽1000 Oe) could be cured by gold deposition. This avalanche, called flux noise, has been a headache in applications of MgB2 thin films. Fortunately, the flux noise in the M–H loop is suppressed with increasing Au-film thickness and finally disappears when the thickness of the gold becomes 2.55 μm. We found a way to remove one obstacle for applications of MgB2 thin films as an superconducting device.
Photoemission study of hole-injection enhancement in organic electroluminescent devices with Au/CFx anode
Authors: Tang J. X. , Li Y. Q. , Hung L. S. , Lee C. S.
Published: December 2003

We demonstrate hole-injection enhancement from Au into N,N′-bis-(1-naphyl)-N,N′diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) by inserting between them a plasma-polymerized fluorocarbon (CFx) buffer layer. Ultraviolet and x-ray photoemission spectroscopic study indicates that chemically tailoring the Au surface with CFx can reduce the hole-injection barrier to ∼1 eV with respect to bare Au. This effect is ascribed to the buildup of an interfacial electric field, arising from the molecular dipolar characteristic of CFx. The present results suggest that CFx can function as a hole-injection enhancement layer for organic optoelectronic/electronic devices which use a metallic anode.
Observation of the discrete transition of optically trapped particle position in the vicinity of an interface
Authors: Fujiwara Hideki , Takasaki Hidehisa , Hotta Jun-ichi , Sasaki Keiji
Published: December 2003

Discrete transitions in the position of a particle optically trapped in the vicinity of an interface are observed using a nanometer position sensing system. These discrete transitions in the position with distance from the interface are attributed to interference effects due to reflection and scattering of the trapping laser beam from the interface and the particle. The discrete transitions occurred at uniform intervals of ∼400 nm, corresponding to the spacing of standing wave nodes of the trapping laser beam.
Coulomb explosion-induced formation of highly oriented nanoparticles on thin films of 3C–SiC by the femtosecond pulsed laser
Authors: Dong Yuanyuan , Molian Pal
Published: December 2003

We report the formation of highly oriented, uniform, and spherical nanoparticles of 3C–SiC as a result of Coulomb explosion during the interaction of near-infrared ultrafast laser pulses with 3C–SiC thin films grown on Si substrate. Experiments were performed at laser fluences well below the single shot, thermal modification threshold.
Low-divergence coherent soft x-ray source at 13 nm by high-order harmonics
Authors: Takahashi Eiji J. , Nabekawa Yasuo , Midorikawa Katsumi
Published: December 2003

We have generated a few tens of nanojoules of coherent soft x-ray pulses at a wavelength around 13 nm with high-order harmonics. Output energy was estimated to be 25 nJ/pulse at the 59th harmonic (13.5 nm), and the beam divergence was measured to be 0.35 mrad full width at half maximum. Since a Mo/Si mirror covers two harmonics in the high reflectivity region, we can use ∼50 nJ harmonic energy per pulse in the 13 nm wavelength. This high-coherent low-emittance x-ray source is useful for the characterization and inspection of an optical system for extreme-ultraviolet lithography.
Thermal stability and decomposition of the HfO2–Al2O3 laminate system
Authors: Chang Hyo Sik , Hwang Hyunsang , Cho Mann-Ho , Moon Dae Won , Doh Seok Joo , Lee Jong Ho , Lee Nae-In
Published: December 2003

The thermal stability of the HfO2–Al2O3 laminate gate stack grown by atomic layer chemical vapor deposition was investigated using medium-energy ion scattering spectroscopy and high-resolution x-ray photoelectron spectroscopy. The laminate structure was maintained up to 800 °C under ultrahigh vacuum conditions, while it was drastically degraded at 850 °C, resulting in silicide formation on the film surface. Dissociated oxygen in the Hf–Al-oxide preferentially diffuses out through the film and desorbing at the surface. Volatile SiO species and Al–O components desorb through the sample surface, while HfO2 contributes to Hf silicide formation on the film surface.
Time-resolved photoluminescence measurements of InAs self-assembled quantum dots grown on misorientated substrates
Authors: Karachinsky L. Ya. , Pellegrini S. , Buller G. S. , Shkolnik A. S. , Gordeev N. Yu. , Evtikhiev V. P. , Novikov V. B.
Published: December 2003

Time-resolved photoluminescence decay measurements have been performed on samples with varying-sized self-assembled InAs/GaAs quantum dot ensembles, formed by substrate misorientation. Ground-state radiative recombination lifetimes from 0.8 to 5.3 ns in the incident power density range of 0.05–3400 W cm−2 at a temperature of 77 K have been obtained. It was found that a reduction of the quantum dot size led to a corresponding reduction of the radiative lifetime. The evident biexponential decay was obtained for the ground state emission of the quantum dot array, with the slower second component attributed to a carrier recapturing process.
Monte Carlo simulation of hot-carrier phenomena in open quantum devices: A kinetic approach
Authors: Zaccaria Remo Proietti , Iotti Rita Claudia , Rossi Fausto
Published: December 2003

An alternative simulation strategy for the study of nonequilibrium carrier dynamics in quantum devices with open boundaries is presented. In particular, we propose replacing the usual modeling of open quantum systems based on phenomenological injection/loss rates with a kinetic description of the system-reservoir thermalization process. More specifically, in this simulation scheme the partial carrier thermalization induced by the device spatial boundaries is treated within the standard Monte Carlo approach via an effective scattering mechanism between the highly nonthermal device electrons and the thermal carrier distribution of the reservoir. The proposed simulation strategy is applyed to state-of-the-art semiconductor nanostructures.
Structural stability of BeH2 at high pressures
Authors: Vajeeston P. , Ravindran P. , Kjekshus A. , Fjellvåg H.
Published: December 2003

The electronic structure and structural stability of BeH2 are studied using first-principles density-functional calculation. The calculated structural parameters for α-BeH2 at the equilibrium volume are in very good agreement with experiments. At higher pressures α-BeH2 successively undergoes four structural transitions: (i) α- to β-BeH2 at 7.07 GPa; (ii) β- to γ-BeH2 at 51.41 GPa; (iii) γ- to δ-BeH2 at 86.56 GPa; and (iv) δ- to ε-BeH2 at 97.55 GPa [an effective two-phase (γ and δ) region is found at 73.71–86.56 GPa]. Density of states studies reveal that BeH2 remains insulating up to 100 GPa whereupon anomalous changes are seen in the band-gap region with increasing pressure.
Millimeter wave emission from GaN high electron mobility transistor
Authors: Deng Yanqing , Kersting Roland , Xu Jingzhou , Ascazubi Ricardo , Zhang Xi-Cheng , Shur Michael S. , Gaska Remis , Simin Grigory S. , Khan M. Asif , Ryzhii Victor
Published: December 2003

We report on millimeter wave electromagnetic radiation from a GaN high electron mobility transistor with the gate length of 1.5 μm at 8 K. The emission takes place at gate and drain voltages in the linear regime of operation but close to the saturation voltage with the principal emission peak at approximately 75 GHz, which is much higher than the device cut-off frequency. An explanation of this effect involves the “shallow water” plasma wave instability, with the frequency of the plasma waves decreased by the ungated regions of the device.
Facile, on-demand electronic nanodevice fabrication from photo- and electro-active silver oxide
Authors: Lee Tae-Hee , Hladik Chad R. , Dickson Robert M.
Published: December 2003

Formed from pure, single-component silver oxide films, nanoscale heterojunctions are electrically written through electromigration. Instantly formed through applying dc current, the nanogap junctions have different oxygen contents at either electrode. This direct writing of junction asymmetry yields diode behavior with a forward to reverse bias current ratio of 71. Two different, electrically written diode logic gates were implemented based on these in situ formed heterojunctions. Together with the photoreduction of silver oxides to silver nanoclusters, this easy fabrication method of writing nanoscale wires and heterojunctions may provide useful instant device fabrication schemes utilizing only light and electricity.
Controlled-junction superconducting quantum interference device via phonon injection
Authors: Hutchinson G. D. , Qin H. , Hasko D. G. , Kang D. J. , Williams D. A.
Published: December 2003

The direct current superconducting quantum interference device (DC-SQUID), using controlled Josephson junction technology, provides a mechanism to modify the characteristics of the device post-fabrication. We report on the fabrication and measurement of a micron sized DC-SQUID using two Dayem bridge weak-link Josephson junctions with integrated “heaters.” The weak link critical current is controlled by hot phonons from the current biased titanium, normal metal heater. By using the heaters, control over the critical current oscillations of the SQUID was observed at 4.2K.
Thermal stability of plasma-nitrided aluminum oxide films on Si
Authors: Bastos K. P. , Pezzi R. P. , Miotti L. , Soares G. V. , Driemeier C. , Morais J. , Baumvol I. J. R. , Hinkle C. , Lucovsky G.
Published: December 2003

The effect of post-deposition rapid thermal annealing in vacuum and in dry O2 on the stability of remote plasma-assisted nitrided aluminum oxide films on silicon is investigated. The areal densities of Al, O, N, and Si were determined by nuclear reaction analysis and their concentration versus depth distributions by narrow nuclear reaction resonance profiling, with subnanometric depth resolution. Annealing in both vacuum and O2 atmospheres produced partial loss of N from the near-surface regions of the films and its transport into near-interface regions of the Si substrate. Oxygen from the gas phase was incorporated in the AlON films in exchange for O and N previously existing therein, as well as in the near-interface regions of the Si substrate, leading to oxynitridation of the substrate. Al and Si remained essentially immobile under rapid thermal processing, confirming that the presence of nitrogen improves the thermal stability characteristics of the AlON/Si structures in comparison with non-nitrided Al2O3/Si.
Effect of acidic and basic surface dipoles on the depletion layer of indium tin oxide as measured by in-plane conductance
Authors: Swint A. L. , Bohn P. W.
Published: December 2003

Four-point probe measurements of indium tin oxide (ITO) films demonstrate the ability to decrease or increase in-plane resistance by surface treatment with acids or bases, respectively. Treatment of ITO surfaces with solutions of varying pH indicates that the degree of protonation or hydroxylation of basic or acidic surface sites, respectively, alters the n-type depletion layer depth and consequently affects the in-plane conductance. Modifying the ITO surface with a series of nearly constant-pH tetraalkylammonium hydroxide species reveals that the in-plane conductance is also modulated by the size of the adsorbed surface dipole layer. The overall change in in-plane conductance upon acid or base treatment is attributed to the net effect of surface charge and adsorbed dipole layer magnitude. Due to the intrinsically high signal–to–noise ratio of the in-plane resistance, these observations have implications for the use of ITO in condensed phase sensing applications.
Domain structure and polarization properties of lanthanum-substituted bismuth titanate single crystals
Authors: Soga Masayuki , Noguchi Yuji , Miyayama Masaru , Okino Hirotake , Yamamoto Takashi
Published: December 2003

Single crystals of bismuth titanate (BiT) and lanthanum-substituted BiT [Bi4−xLaxTi3O12 (BLT)] were grown by a self-flux method. BLT (x=0.85) showed a smaller remanent polarization (2Pr) along the a(b) axis of 41 μC/cm2 than 98 μC/cm2 for BiT, which mainly originates in its smaller ionic displacements along the a axis. Striped 90° domain walls as well as head-to-head and tail-to-tail 180° domain walls were observed for both crystals, and La substitution led to a marked decrease in the 90° domain width from 8.5 μm (BiT) to 2.5 μm for BLT (x=1.20). Piezoelectric-force microscope observations suggested that antiphase domain boundary (ADB) is present only for BLT and the ADB plays an important role in the formation of the 90° domain.
Highly efficient electrophosphorescence devices based on rhenium complexes
Authors: Li Feng , Zhang Ming , Cheng Gang , Feng Jing , Zhao Yi , Ma Yuguang , Liu Shiyong , Shen Jiacong
Published: December 2003

Highly efficient electrophosphorescence from light-emitting devices based on two types of rhenium(I) diimine complexes, (1,10-phenanthroline)Re(CO)3Cl (phen-Re) and (2,9-dimethyl-1,10-phenanthroline)Re(CO)3Cl (dmphen-Re), is reported. N, N′-di-1-naphthyl-N, N′-diphenylbenzidine is used as the hole-transporting layer. 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline is used to confine excitons within the luminescence zone. phen-Re and dmphen-Re are doped into the host material (4,4′-N-N′-dicarbazole)biphenyl with mass ratios of 2–20% as the light-emitting layer, respectively. The maximum efficiency and brightness achieved from the devices based on phen-Re and dmphen-Re are 6.67 cd/A and 2769 cd/m2, and 7.15 cd/A and 3686 cd/m2, respectively.
High-efficiency solar cells on edge-defined film-fed grown (18.2%) and string ribbon (17.8%) silicon by rapid thermal processing
Authors: Rohatgi A. , Kim D. S. , Nakayashiki K. , Yelundur V. , Rounsaville B.
Published: December 2003

Solar cell efficiencies of 18.2 and 17.8% were achieved on edge-defined film-fed grown and string ribbon multicrystalline silicon, respectively. Improved understanding and hydrogenation of defects in ribbon materials contributed to the significant increase in bulk lifetime from 1–5 μs to as high as 90–100 μs during cell processing. It was found that SiNx-induced defect hydrogenation in these ribbon materials takes place within one second at 740–750 °C. The bulk lifetime decreases at annealing temperatures above 750 °C or annealing times above one second due to the enhanced dissociation of the hydrogenated defects coupled with the decrease in hydrogen supply from the SiNx film deposited by plasma enhanced chemical vapor deposition.
Pressure dependence of the fundamental band-gap energy of CdSe
Authors: Shan W. , Walukiewicz W. , Ager J. W. , Yu K. M. , Wu J. , Haller E. E.
Published: December 2003

Pressure-dependent photomodulation spectroscopic studies of the optical transition associated with the fundamental bandgap of single-crystal bulk CdSe are presented. Photomodulated transmission (PT) measurements were performed at room temperature as a function of applied hydrostatic pressure using the diamond-anvil-cell technique. The sharp, derivative-like PT spectral features corresponding to the band-gap transition in CdSe were found to shift linearly toward higher energy with increasing pressure. By examining the pressure dependence of the PT spectra, the pressure coefficient for the direct band gap of wurtzite CdSe was determined to be 43.1 meV/GPa. The hydrostatic deformation potential of the band gap was found to be −2.3 eV.
Pulse-induced quantum interference of intersubband transitions in coupled quantum wells
Authors: Müller T. , Parz W. , Strasser G. , Unterrainer K.
Published: December 2003

We report the measurement of the transient intersubband absorption in a coupled quantum well semiconductor structure. Pulsed interband excitation is used to prepare a coherent population in the two lowest states of a three level system of electronic subbands. The time-dependent absorption to the upper state is probed by midinfrared time-domain spectroscopy. For certain pump–probe delay times we observe in the intersubband absorption spectra typical signatures of quantum interference: regions with reduced or enhanced absorption.
Stability and band offsets of nitrogenated high-dielectric-constant gate oxides
Authors: Shang G. , Peacock P. W. , Robertson J.
Published: December 2003

We find that addition of nitrogen to high-dielectric-constant gate oxides such as HfO2, La2O3, and silicates adds gap states up to 1.2 eV above their valence-band edge. This reduces the valence-band offset, but not by enough to reduce the hole injection barrier to unsafe levels. Nitrogen also leads to a decrease in average atomic coordination, which may help to increase their resistance to crystallization and ability to withstand high temperature processing.
Two-photon photoelectron spectroscopy of conjugated polymer thin films on gold
Authors: Sohn Y. , Richter J. , Ament J. , Stuckless J. Todd
Published: December 2003

We present results for two-photon photoelectron spectroscopy of thin regioregular poly-3-hexyl-thiophene films on a gold metal substrate. The photoemission quantum yields are orders of magnitude larger than that of bare gold at the same laser intensity. Furthermore, we observe a strong increase of the quantum yield with increasing film thickness accompanied by a decreasing yield slope with respect to laser power. Our findings can be understood by assuming the creation of a long-lived intermediate state excitation that couples to the metal surface. This view is further supported by the dependence of the electron kinetic energy spectra on the laser photon energy.
Lateral homogeneity of Schottky contacts on n-type ZnO
Authors: von Wenckstern H. , Kaidashev E. M. , Lorenz M. , Hochmuth H. , Biehne G. , Lenzner J. , Gottschalch V. , Pickenhain R. , Grundmann M.
Published: December 2003

The electrical properties of Schottky contacts (SCs) produced ex situ on n-type ZnO single crystals and epitaxial thin films were investigated. Electron beam induced current imaging was used to study lateral variations of the current induced in the space charge region of the SC. Further, the effective barrier height was determined by current–voltage and capacitance–voltage measurements. Pd contacts prepared on ZnO thin films that had undergone treatment in a plasma-enhanced chemical vapor deposition with nitrous oxide (N2O) as ambient gas are laterally homogeneous with an effective barrier height of (600±30) meV.
High-gain photorefractive reflection gratings in layered photoconductive polymers
Authors: Kwon O-Pil , Montemezzani Germano , Günter Peter , Lee Suck-Hyun
Published: December 2003

Large two-wave mixing gain in reflection grating geometry is obtained in layered organic photorefractive polymers doped with the chromophore piperidinodicyanostyrene. With an applied field of 60 V/μm and for a grating spacing of 0.205 μm, one measures a two-wave mixing gain coefficient of 104 cm−1 and a diffraction efficiency of 4.8% in 100-μm-thick samples. Our photorefractive and electrochemical investigations suggest that the excellent material performance in reflection grating geometry is associated with an increased effective number of trapping sites.
Observation of backflow in the switch-on dynamics of a hybrid aligned nematic
Authors: Jewell S. A. , Sambles J. R.
Published: December 2003

The optical convergent-beam technique is used to measure, in 0.3 ms steps, the response of the director in a 4.6-μm-thick ZLI-2293 filled hybrid aligned nematic cell when a 10 kHz, 7 Vrms ac voltage is applied to the cell. The total time taken for the reorientation process is 2.4 ms, with backflow observed during the first 1.5 ms after the application of the voltage. The measured director profiles show excellent agreement with theoretical profiles produced from the Leslie–Eriksen–Parodi theory using typical values for the viscosity coefficients. Fluid velocity profiles within the cell are also modeled.
The Gravitational Radius of an Irradiated Disk
Authors: Liffman Kurt
Published: December 2003

We derive an updated, analytic formula for the gravitational radius, rg, of an irradiated, thin disk. For such a disk, the region outside the gravitational radius will produce a thermal wind, while the region interior to the gravitational radius will be stable. We find that rg ≈ 1.4 (M*/M⊙)/(T0/104K) AU. The value of 1.4 AU is approximately one fifth of currently used values.

The analysis uses the adiabatic approximation. We argue that the same formula applies for the non-adiabatic, isothermal case.
Understanding AGB Carbon Star Nucleosynthesis from Observations
Authors: Abia C. , Domínguez I. , Gallino R. , Busso M. , Straniero O. , de Laverny P. , Wallerstein G.
Published: December 2003

Recent advances in the knowledge of the evolutionary status of asymptotic giant branch (AGB) stars and of the nucleosynthesis processes occurring in them are discussed, and used to interpret abundance determinations for s-process elements, lithium and CNO isotopes in several types of AGB stars. We focus our attention mainly on carbon-rich AGB stars. By combining these different constraints we conclude that most carbon stars in the solar neighborhood are of low mass (M≤3 M⊙), their abundances being a consequence of the operation of thermal pulses and the third dredge-up. However, the observed abundances in carbon stars of the R and J types cannot be explained by this standard scenario. These stars may not be on the AGB, but possibly in the core-He burning phases; their envelopes may have been polluted with nuclear ashes of the core-He flash, followed by CNO re-processing enhancing 13C. Observational evidence suggesting the operation of non-standard mixing mechanisms during the AGB phase is also discussed.
Nucleosynthesis in Binary Populations
Authors: Izzard Robert G. , Tout Christopher A.
Published: December 2003

We investigate the effect of duplicity on stellar yields of carbon, nitrogen, and oxygen. Populations of single and binary stars are modelled and the yields calculated for the whole population. The effects of explosive nucleosynthesis in novae and supernovae are included but by artificially removing these effects from our populations we determine the influence of a binary companion on asymptotic giant branch yields of the CNO elements.
Constraining Models of Classical Nova Outbursts with the Murchison Meteorite
Authors: José Jordi , Hernanz Margarita , Amari Sachiko , Zinner Ernst
Published: December 2003

Infrared observations of nova light curves reveal that classical novae form grains in the expanding shells, ejected into the interstellar medium as a consequence of a violent outburst. Such grains contain nucleo-synthetic fingerprints of the nova explosion. In this paper, we analyse different isotopic signatures expected to be present in nova grains on the basis of detailed hydrodynamic calculations of CO and ONe novae and compare them with recent determinations of presolar nova grains from the Acfer 094 and Murchison meteorites.
The Nucleosynthesis of the Light Elements C to Al by Core Collapse Supernovae
Authors: Chieffi Alessandro , Limongi Marco
Published: December 2003

We discuss the production sites of the nuclei from C to Al in solar metallicity stars in the range 13–35 M⊙. We will show how, contrary to current beliefs, the advanced burning phases and the passage of the blast wave play a pivotal role in determining the final yields of quite a few ‘light’ nuclei. We will also show how the relative contributions of the hydrostatic and explosive burning depend on the initial mass of the star: the smaller the mass the larger the importance of the explosive burning.
Presolar Graphite: Noble Gases and their Origins
Authors: Amari Sachiko
Published: December 2003

Presolar graphite contains a 22Ne-rich component called Ne-E(L). Noble gas studies on graphite aggregates and single grains have shown that although a dominant source of the 22Ne is 22Na, 22Ne in the He-shell of asymptotic giant branch stars have also contributed to the Ne-E(L). In addition to novae that have been considered to be a possible source of 22Na, supernovae are a likely source as well. Krypton isotopic ratios of the separates indicate that part of graphite formed in low-mass (≤3 M⊙) asymptotic giant branch stars of low metallicity (Z ≤ 0.006).
Galactic Chemical Evolution
Authors: Gibson Brad K. , Fenner Yeshe , Renda Agostino , Kawata Daisuke , Lee Hyun-chul
Published: December 2003

The primary present-day observables upon which theories of galaxy evolution are based are a system’s morphology, dynamics, colour, and chemistry. Individually, each provides an important constraint to any given model; in concert, the four represent a fundamental (intractable) boundary condition for chemodynamical simulations. We review the current state-of-the-art semi-analytical and chemodynamical models for the Milky Way, emphasising the strengths and weaknesses of both approaches.
Synthesis, Biological Activity, and QSAR Studies of Antimicrobial Agents Containing Biguanide Isosteres
Authors: Wernert Gregory T. , Winkler David A. , Holan George , Nicoletti Gina
Published: January 2004

Analogues of chlorhexidine and chemically related antimicrobial compounds were synthesized, based on a model in which the bisbiguanide moieties were replaced by conformationally restricted cyclic isosteres. This model was tested by measuring the antimicrobial activities of the compounds. Quantitative structure–activity relationship (QSAR) studies showed a parabolic dependence of antimicrobial activity on the lipophilicity of the compounds. The basicity of the functional groups in the molecules was also very important, as uncharged molecules were not able to disrupt the microbial phospholipid bilayer and cause an antimicrobial effect. We compared our QSAR results to those reported in other studies of antimicrobials of diverse structure. We found very similar QSAR models for all compounds studies with a log P (octanol/water partition constant) optimum at 5.5 (neutral log P value). The form of the QSAR equations were similar, suggesting a common mode of action for these agents.
Taxane Diterpene Synthesis Studies. Part 1: Chemoenzymatic and Enantiodivergent Routes to AB-ring Substructures of Taxoids and ent-Taxoids
Authors: Banwell Martin G. , Darmos Penny , Hockless David C. R.
Published: January 2004

The microbially derived cis-1,2-dihydrocatechol 2 has been converted, via reaction sequences including Diels–Alder cycloaddition and anionic oxy-Cope rearrangement steps, into enantiopure bicyclo[5.3.1]undecenes 21 and 34, which correspond to AB-ring substructures of ent-taxoids and taxoids, respectively.
Circumstellar Calcium Chemistry
Authors: Petrie Simon
Published: January 2004

High-level ab initio calculations are used to explore several aspects of calcium chemistry of direct relevance to formation and reaction of calcium-containing molecules in circumstellar envelopes. Counterpoise-corrected G2 and G2(MP2) calculations have been used to determine the bond dissociation energies (BDEs) of Ca+/NC2n+1H complexes (n = 0, 1, 2); these complexes, and CaNC7H+, are also assessed through counterpoise-corrected MP2(thaw)/6-311 + G(3df,2p) calculations. The relative energies of isomers of the feasible Ca(CN), Ca(C3N), and Ca(C5N) products (which may arise from dissociative recombination of the Ca+/NC2n+1H complexes) are obtained from G2 and G2(MP2) calculations; these calculations also permit evaluation of the thermochemistry of the dissociative recombination reactions in question. Thermochemical data are presented for a possible loss mechanism for calcium-containing neutrals by reaction with molecular ions in circumstellar envelopes. Finally, we provide an empirical assessment of the prospects for detecting the Ca(CN), Ca(C3N), and Ca(C5N) radicals within circumstellar environments.
Ethyl 2-(2-Ethoxy-2-oxoethylidene)-1,3-diselenole-4-carboxylate—an Example of Unconventional Bonding in Organic Selenides
Authors: Carland Michael W. , Schiesser Carl H. , White Jonathan M.
Published: January 2004

Reaction of ethyl propiolate with lithium hexamethyldisilazide in tetrahydrofuran followed by the addition of elemental selenium provides ethyl 2-(2-ethoxy-2-oxoethylidene)-1,3-diselenole-4-carboxylate in 75% yield and as a mixture of geometrical isomers 2, 3. X-Ray crystallographic studies, as well as density functional calculations reveal close oxygen–selenium contacts that are the result of unconventional bonding interactions.
An Alternative Synthesis of Some Carbohydrate α-Amino Acids
Authors: Forman Grant S. , Scaffidi Adrian , Stick Robert V.
Published: January 2004

Several carbohydrate ketones have been converted into their trichloromethyl-branched tertiary alcohols. A subsequent treatment of these alcohols under Corey–Link conditions (base, sodium azide, methanol) has given rise to α-azido esters, transformable into azido acids, amino esters, and amino acids. An amino ester and an azido acid have been coupled to form a dipeptide.
Metal Ion Assisted Folding and Supramolecular Organization of a De Novo Designed Metalloprotein
Authors: Vandermeulen Guido W. M. , Tziatzios Christos , Schubert Dieter , Andres Philip R. , Alexeev Alexander , Schubert Ulrich S. , Klok Harm-Anton
Published: January 2004

This paper describes the supramolecular organization of a novel de novo designed metalloprotein, which consists of two N-terminal terpyridine modified coiled-coil protein folding motif sequences held together by an iron(II) ion. The self-assembly of the metalloprotein is the result of the interplay of metal ion complexation and protein folding, and can be manipulated by changes in concentration, temperature, and solvent. At low concentrations, folding and organization of the metalloprotein resembles that of the native coiled-coil peptide. Besides unimeric species, also dimeric and tetrameric metalloprotein assemblies were found. Several indications suggest that at least part of these unimeric species may exist as intramolecularly folded coiled-coils, however, unambiguous proof is lacking at the moment. At higher concentrations, folding and organization is dominated by the large octahedral [FeII(terpy)2] complexes (terpy = 2,2′:6′,2″-terpyridine) and considerable amounts of large, ill-defined aggregates are formed.
Spirocyclic Restriction of Nucleosides
Authors: Paquette Leo A.
Published: January 2004

The concept of spirocyclic restriction as applied broadly to the field of nucleoside mimics makes possible the generation of diastereomeric pairs configured with a syn- or anti-oriented hydroxyl substituent at C5′. The development of concise synthetic routes to spiro-fused nucleosides bearing all possible natural bases and expectedly capable of enforcing a conformation favourable for duplex formation on incorporation into oligomers represents a significant new direction in the design of antisense molecules. The present overview describes the convenient approaches that have been developed in this laboratory for accessing varied members of this class, including analogues that feature sulfur and carbon at the apical position.
Gradual evolution of male genitalia in a sibling species complex of millipedes (Diplopoda : Spirobolida : Rhinocricidae : Anadenobolus)
Authors: Bond Jason E. , Beamer David A. , Hedin Marshal C. , Sierwald Petra
Published: December 2003

Jamaican millipedes in the Anadenobolus species complex provide an unusual case study of arthropods having undergone speciation in the absence of conspicuous divergence of male genitalia. Using landmark-based morphometrics, we examined shape deformation of the male anterior copulatory device in three genetically divergent yet morphologically cryptic species. A multivariate analysis of variance and relative warp analysis of nonuniform components show that although male genitalic shape is statistically different among species, many specimens are 'misplaced' in morphological space, perhaps consistent with a condition analogous to incomplete lineage sorting. A simulation of neutral nuclear gene coalescence suggests that such incomplete sorting is expected, given the depth of mtDNA divergences observed across species. The pronounced contrast between deep molecular v. incomplete genitalic divergence is at odds with the paradigm of selection-driven rapid change in male copulatory structure during arthropod speciation. Alternatively, we suggest that male genitalic divergence is evolving neutrally or in concert with other components of the genome (pleiotropy). Although we recognise the empirical validity of rapid genitalic divergence via sexual selection or sexual conflict, such models must be empirically tested using multiple lines of evidence. Accepting the rapid and divergent hypothesis without such multiple evidence scrutiny may result in a gross underestimation of evolutionary diversity and, subsequently, the misinterpretation of processes shaping genitalic change.
Speciation of Antechinus stuartii and A. subtropicus (Marsupialia : Dasyuridae) in eastern Australia: molecular and morphological evidence
Authors: Crowther M. S. , Sumner J. , Dickman C. R.
Published: December 2003

This paper evaluates the systematic status of the Antechinus populations of northern New South Wales and southern Queensland, using a combined morphological and molecular (allozymes and mitochondrial DNA) approach. Analysis of the d-loop section of the mitochondrial DNA control region revealed two highly supported clades within A. stuartii sensu lato that were sympatric in the Border Ranges of northern New South Wales. However, genetic distances between these clades were small (approximately 3%), indicating that time of divergence was probably relatively recent. Allozyme electrophoresis also showed very small differences between clades/species. Analyses of cranial and dental characters showed that the members of each of these clades differed morphologically and that the clades corresponded to A. stuartii and the recently described A.�subtropicus. The combined results support the species status of A. stuartii and A.�subtropicus, and suggest that speciation was likely a result of a recent vicariant event.