bluuebunny/crossref_metadata_2025 huggingface dataset:
Switching behavior of semiconducting carbon nanotubes under an external electric field
Authors: Rochefort Alain , Di Ventra Massimiliano , Avouris Phaedon
Published: July 2002

We investigate theoretically the switching characteristics of semiconducting carbon nanotubes connected to gold electrodes under an external (gate) electric field. We find that the external introduction of holes is necessary to account for the experimental observations. We identify metal-induced gap states (MIGS) at the contacts and find that the MIGS of an undoped tube would not significantly affect the switching behavior, even for very short tube lengths. We also explore the miniaturization limits of nanotube transistors, and, on the basis of their switching ratio, we conclude that transistors with channels as short as 50 Å would have adequate switching characteristics.
Transform-limited, narrow-linewidth, terahertz-wave parametric generator
Authors: Kawase Kodo , Shikata Jun-ichi , Imai Kazuhiro , Ito Hiromasa
Published: July 2002

An injection-seeded nanosecond terahertz (THz) wave parametric generator was demonstrated using nonlinear crystals that were pumped by a single frequency Nd:yttrium–aluminum–garnet laser. Spectrum narrowing to the Fourier transform limit (ν=1.58 THz, Δν<200 MHz) was achieved by injection seeding the idler wave (near-infrared Stokes). This resulted in a THz-wave output power (900 pJ/pulse, >100 mW peak) approximately 300 times higher than that of a conventional THz-wave parametric generator, which has no injection seeder. This compact system operates at room temperature and promises to be a widely tunable THz-wave source that will compete with free-electron lasers and p-Ge lasers.
Role of oxygen at the grain boundary of metal oxide varistors: A potential barrier formation mechanism
Authors: Bueno P. R. , Leite E. R. , Oliveira M. M. , Orlandi M. O. , Longo E.
Published: July 2002

A model is proposed here to explain how the chemical features of metal oxide varistors can alter their nonohmic physical behavior, based on nonohmic similarities in the electrical properties of ZnO- and SnO2-based varistors. The proposed model explains the electrical properties of ZnO- and SnO2-based varistors before and after thermal treatments in oxygen- and nitrogen-rich atmospheres, which cause similar changes in the nonohmic feature of these polycrystalline ceramics with greatly differing chemical compositions and microstructures. The model is based on the key role that oxygen plays in varistor grain boundaries, independently of the type of ceramic system (ZnO-, SnO2- or even SrTiO3-based varistors) involved.
Propagation of intense ultrashort laser pulses in a plasma filled capillary tube: Simulations and experiments
Authors: Courtois C. , Couairon A. , Cros B. , Marquès J. R. , Matthieussent G.
Published: July 2002

Recent experiments [F. Dorchies et al., Phys. Rev. Lett. 82, 4655 (1999)] have demonstrated monomode guiding over 100 Rayleigh lengths (10 cm) of high intensity, ultrashort laser pulses in a dielectric capillary tube filled with low pressure He gas. A model for the propagation of femtosecond laser pulses in a capillary tube, coupled with the tunnel ionization of the gas filling the capillary tube, is derived and solved numerically using the parameters of these experiments. The simulations accurately reproduce experimentally measured damping lengths, transmitted pulse energy and spectrum at the output of the capillary tube. They show that a few cm long, homogeneous, plasma column with an electron density of the order of 1017 cm−3 is created for an incident intensity of 4×1015 W/cm2.
Ab initio simulation of charged slabs at constant chemical potential
Authors: Lozovoi A. Y. , Alavi A. , Kohanoff J. , Lynden-Bell R. M.
Published: July 2002

We present a practical scheme for performing ab initio supercell calculations of charged slabs at constant electron chemical potential μ, rather than at constant number of electrons Ne. To this end, we define the chemical potential relative to a plane (or “reference electrode”) at a finite distance from the slab (the distance should reflect the particular geometry of the situation being modeled). To avoid a net charge in the supercell, and thus make possible a standard supercell calculation, we restore the electroneutrality of the periodically repeated unit by means of a compensating charge, whose contribution to the total energy and potential is subtracted afterwards. The “constant μ” mode enables one to perform supercell calculation on slabs, where the slab is kept at a fixed potential relative to the reference electrode. We expect this to be useful in modeling many experimental situations, especially in electro-chemistry.
Comparison of solid-state thermionic refrigeration with thermoelectric refrigeration
Authors: Ulrich Marc D. , Barnes Peter A. , Vining Cronin B.
Published: July 2002

A theoretical analysis of single-barrier thermionic emission cooling in semiconducting materials is performed using Fermi–Dirac statistics. Both maximum cooling and coefficient of performance are evaluated. It is shown that the performance of a thermionic refrigerator is governed by the same materials factor as thermoelectric devices. For all known materials, single-barrier thermionic refrigeration is less effective and less efficient than thermoelectric refrigeration.
Visible-blind ultraviolet photodetectors based on ZnMgBeSe Schottky barrier diodes
Authors: Vigué F. , Tournié E. , Faurie J.-P. , Monroy E. , Calle F. , Muñoz E.
Published: July 2002

Planar geometry Schottky barrier photodiodes designed for visible-blind ultraviolet detection have been fabricated. They are based on ZnMgBeSe alloys grown by molecular-beam epitaxy. High crystalline quality is achieved, which leads to a high responsivity (0.17 A/W at 375 nm) and a sharp cutoff of more than three orders of magnitude. As attested by the linear variation of the photocurrent with the optical excitation, there is no internal gain mechanism. A detectivity of 2×1010 mHz1/2 W−1 is obtained showing that low-noise devices with high sensitivity have been fabricated.
Density functional study of structural and electronic properties of NanMg (1⩽n⩽12) clusters
Authors: Zope Rajendra R. , Blundell S. A. , Baruah Tunna , Kanhere D. G.
Published: July 2002

Low-lying equilibrium geometric structures of NanMg (n=1–12) clusters obtained by an all-electron linear combination of atomic orbital approach, within spin-polarized density functional theory, are reported. The binding energy, dissociation energy, and stability of these clusters are studied within the local spin density approximation (LSDA) and the three-parameter hybrid generalized gradient approximation (GGA) due to Becke-Lee-Yang-Parr (B3LYP). Ionization potentials, electron affinities, hardness, and static dipole polarizabilities are calculated for the ground-state structures within the GGA. It is observed that for clusters with n<9, symmetric structures with the magnesium atom occupying the internal position are higher in energy (typically by less than 1 eV) than those where Mg occupies a peripheral position. It is found that the relative ordering of the isomers is influenced by the nonlocal exchange-correlation effects for small clusters. Generalized gradient approximation extends bond lengths and widens the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), as compared to the LSDA gap. The odd-even oscillations in the dissociation energy, the second differences in energy, the HOMO–LUMO gaps, the ionization potential, the electron affinity, and the hardness are more pronounced within the GGA. The stability analysis based on the energetics clearly shows the Na6Mg cluster to be endowed with special stability, which occurs because of an electronic shell closure.
Fine tuning work function of indium tin oxide by surface molecular design: Enhanced hole injection in organic electroluminescent devices
Authors: Ganzorig Chimed , Kwak Kwang-Joo , Yagi Kazuto , Fujihira Masamichi
Published: July 2002

Characteristics of electroluminescent (EL) devices were improved dramatically using indium tin oxide (ITO) chemically modified with H-, Cl-, and CF3-terminated benzoyl chlorides. By the use of reactive –COCl groups, ITO surfaces were modified quickly and the work function of the modified ITO was changed widely depending upon the permanent dipole moments introduced in the para- position of benzoyl chloride. We also compared the performance of the EL devices with ITO modified with different binding groups (–SO2Cl, –COCl, and –PO2Cl2) of p-chlorobenzene derivatives. Finally, we examined the correlation between the change in the work function and the performance of the EL devices by the chemical modification and found that the enormous increase in ITO work function up to 0.9 eV is possible using phenylphosphoryl dichloride with a CF3-terminal group in the para-position.
Large transport critical currents in unsintered MgB2 superconducting tapes
Authors: Grasso Giovanni , Malagoli Andrea , Ferdeghini Carlo , Roncallo Scilla , Braccini Valeria , Siri Antonio S. , Cimberle Maria R.
Published: July 2002

The powder-in-tube process has been employed to fabricate tape-like conductors with a strong metallic sheath and based on the newly discovered MgB2 superconducting phase. Long superconducting tapes have been prepared by packing reacted MgB2 powders inside pure Ag, Cu, and Ni tubes which are then cold worked by drawing and rolling. Such tapes have shown transport properties as good as bulk MgB2 samples sintered in high pressure and high temperature conditions. At 4.2 K, the highest critical current density of 105 A/cm2 has been achieved on nickel-sheathed single-filament conductors. A direct correlation between the sheath tensile strength and the critical current of the unsintered tape has been observed.
Damage in hydrogen plasma implanted silicon
Authors: Wang Lianwei , Fu Ricky K. Y. , Zeng Xuchu , Chu Paul K. , Cheung W. Y. , Wong S. P.
Published: July 2002

The damage and defects created in silicon by hydrogen plasma immersion ion implantation (PIII) are not the same as those generated by conventional beamline ion implantation due to the difference in the ion energy distribution and lack of mass selection in PIII. Defect generation must be well controlled because damage in the implanted and surface zones can easily translate into defects in the silicon-on-insulator structures synthesized by the PIII/wafer bonding/ion-cut process. The defect formation and its change with annealing temperature were investigated experimentally employing channeling Rutherford backscattering spectrometry, secondary ion mass spectrometry, and atomic-force microscopy. We also calculated the damage energy density of the three dominant hydrogen species in the plasma (H+, H2+, and H3+) as well as displacement of silicon atoms in the silicon wafer. H2+ creates the most damage because its damage energy density is very close to the silicon threshold energy. The effects of atmospheric gaseous impurities unavoidably coimplanted from the overlying plasma are also modeled. Even though their concentration is usually small in the plasma, our results indicate that these gaseous impurities lead to significant silicon atom displacement and severe damage in the implanted materials.
The branching ratio between reaction and relaxation in the removal of H2O from its |04〉− vibrational state in collisions with H atoms
Authors: Barnes Peter W. , Sims Ian R. , Smith Ian W. M. , Lendvay György , Schatz George C.
Published: July 2002

The removal of H2O molecules from their |04〉− vibrational state in collisions with H atoms can occur both by reaction, producing OH(v=0)+H2, and by nonreactive relaxation. We report an experimental measurement of the fraction (freac) that occurs by reaction. The value of freac is determined by comparing the yields of OH from three experiments in which the same concentration of H2O(|04〉−) is prepared by overtone absorption of pulsed laser radiation and OH(v=0) is produced: (i) solely by the H+H2O(|04〉−) reaction; (ii) solely by the photodissociation of H2O(|04〉−) at 266 nm; and (iii) both by the photodissociation of H2O(|04〉−) and by the subsequent reaction of a fraction of the remaining H2O(|04〉−) with H atoms. Analysis of these experiments shows that freac=(0.34±0.11). The experimental results are compared with the results of two kinds of scattering calculations performed on a potential energy surface developed recently, specifically with this problem in mind. Using the vibrational coupled-channel infinite-order-sudden (VCC-IOS) method, rate coefficients have been calculated for individual vibrationally inelastic processes and then summed to find the rate coefficient (krelaxH) for total nonreactive relaxation from the |04〉− state. The quasiclassical trajectory (QCT) method has been used to calculate the rate coefficient (kreac) for reaction between H atoms and H2O(|04〉−). Both the calculated rate coefficient (i.e., krelaxH+kreac) for total loss from H2O(|04〉−) and the calculated branching ratio, freac=kreac/(krelaxH+kreac)=0.38, are in quite good agreement with the experimental values.
Improving the performance of polymer light-emitting diodes using polymer solid solutions
Authors: Liu Jie , Shi Yijian , Yang Yang
Published: July 2002

Optical and electronic properties of conjugated polymers can be modified significantly by arranging the polymer chains differently. For example, it is well known that polymer solutions of lower concentrations have different photoluminescence spectra from solutions of higher concentrations. In this manuscript, we demonstrated that such effects can also be achieved in solid states, i.e., in polymer solid solutions consisting of a narrower band-gap material—poly(2-methoxy-5(2′-ethyl-hexyloxy)-1,4-phenylene vinylene (MEH-PPV) with a wider band-gap polymer–poly(9,9-dioctylfluorene). At very low MEH-PPV concentrations, emission spectra of these solid solutions are similar to the photoluminescence spectrum of a diluted MEH-PPV solution in an ordinary organic solvent, suggesting the absence of a significant amount of interchain species. In addition, the electroluminescence efficiency is significant enhanced. Furthermore, the full width at half maximum (FWHM) is dramatically reduced at lower MEH-PPV concentrations, as a result of suppressed interchain interactions. This approach is particularly important for the application to display technology, in which well-defined narrow FWHM emission spectra of red, green, and blue colors are required.
Optimized particle-mesh Ewald/multiple-time step integration for molecular dynamics simulations
Authors: Batcho Paul F. , Case David A. , Schlick Tamar
Published: July 2002

We develop an efficient multiple time step (MTS) force splitting scheme for biological applications in the AMBER program in the context of the particle-mesh Ewald (PME) algorithm. Our method applies a symmetric Trotter factorization of the Liouville operator based on the position-Verlet scheme to Newtonian and Langevin dynamics. Following a brief review of the MTS and PME algorithms, we discuss performance speedup and the force balancing involved to maximize accuracy, maintain long-time stability, and accelerate computational times. Compared to prior MTS efforts in the context of the AMBER program, advances are possible by optimizing PME parameters for MTS applications and by using the position-Verlet, rather than velocity-Verlet, scheme for the inner loop. Moreover, ideas from the Langevin/MTS algorithm LN are applied to Newtonian formulations here. The algorithm’s performance is optimized and tested on water, solvated DNA, and solvated protein systems. We find CPU speedup ratios of over 3 for Newtonian formulations when compared to a 1 fs single-step Verlet algorithm using outer time steps of 6 fs in a three-class splitting scheme; accurate conservation of energies is demonstrated over simulations of length several hundred ps. With modest Langevin forces, we obtain stable trajectories for outer time steps up to 12 fs and corresponding speedup ratios approaching 5. We end by suggesting that modified Ewald formulations, using tailored alternatives to the Gaussian screening functions for the Coulombic terms, may allow larger time steps and thus further speedups for both Newtonian and Langevin protocols; such developments are reported separately.
Molecular dynamics simulation of stick-slip
Authors: Li Bin , Clapp P. C. , Rifkin J. A. , Zhang X. M.
Published: July 2002

Molecular dynamics simulation was carried out to investigate the stick-slip on an atomic scale by using the embedded atom method potential for Ni–Al. The analyses of the dynamic features of the atoms in the sliding block clearly show that the elastic deformation of the surface layers is the main cause for the stick-slip phenomenon, which is consistent with the macroscopic stick-slip. The simulation results also indicate that phonons are emitted during stick-slip, and a commensurate fit between the contacting surfaces is not significant for the stick-slip friction.
Rotational dynamics of coumarins in nonassociative solvents: Point dipole versus extended charge distribution models of dielectric friction
Authors: Dutt G. B. , Krishna G. Rama , Raman Sumathi
Published: July 2002

Rotational diffusion of three structurally similar coumarins; coumarin 6, coumarin 7, and coumarin 30 has been studied in two nonassociative solvents; dimethylsulfoxide (DMSO) and octanenitrile using steady-state fluorescence depolarization technique. The experimentally measured reorientation times of these coumarins are almost identical in a given solvent at a particular temperature. However, these coumarins are found to rotate slower in octanenitrile compared to DMSO especially at higher values of viscosity over temperature. The mechanical friction experienced by the coumarins has been modeled using the Stokes–Einstein–Debye hydrodynamic theory with slip boundary condition and the dielectric friction using the point dipole as well as the extended charge distribution models. Point dipole models of Nee–Zwanzig and van der Zwan–Hynes fail to explain the experimentally observed trend even in a qualitative way, whereas the extended charge distribution model of Alavi–Waldeck is successful in predicting the observed behavior at least qualitatively in these nonassociative solvents.
The helium–, neon–, and argon–cyclopropane van der Waals complexes: Ab initio ground state intermolecular potential energy surfaces and intermolecular dynamics
Authors: Pedersen Thomas Bondo , Fernández Berta , Koch Henrik , Makarewicz Jan
Published: July 2002

Using the coupled cluster singles and doubles including connected triples model and the augmented correlation consistent polarized valence double zeta basis set extended with a set of 3s3p2d1f1g midbond functions, ab initio helium–, neon–, and argon–cyclopropane ground state intermolecular potential energies are evaluated and fitted to an analytic function including up to four-body interactions. These are the first ab initio potential energy surfaces available for these complexes and are characterized by an absolute minimum of −73.3 cm−1 at a distance on the cyclopropane C3-axis of 3.291 Å, −125.3 cm−1 at 3.435 Å, and −301.1 cm−1 at 3.696 Å for helium, neon, and argon, respectively. The bound van der Waals states are calculated. Two types of tunneling motion cause splittings of these levels: a C3 tunneling between the three equivalent local minima placed in the cyclopropane plane, and a C2 tunneling motion of the rare gas atom between the global minima above and below the cyclopropane plane.
Ab initio simulation of ammonia monohydrate (NH3⋅H2O) and ammonium hydroxide (NH4OH)
Authors: Fortes A. D. , Brodholt J. P. , Wood I. G. , Vočadlo L. , Jenkins H. D. B.
Published: July 2002

We report the results of the first pseudopotential plane-wave simulations of the static properties of ammonia monohydrate phase I (AMH I) and ammonium hydroxide. Our calculated fourth-order logarithmic equation of state, at zero pressure and temperature, has molar volume, V0=36.38(3) cm3 mol−1, bulk modulus, K0=9.59(9) GPa, and the first derivative of the bulk modulus with respect to pressure, K0′=5.73(21). Both this and the lattice parameters are in very good agreement with experimental values. The monohydrate transforms, via a solid-state proton transfer reaction, to ammonium hydroxide (NH4OH) at 5.0(4) GPa. The equation of state of ammonium hydroxide is, V0=31.82(5) cm3 mol−1, K0=14.78(62) GPa, K0′=2.69(48). We calculate the reaction enthalpy, ΔH(NH4OH,s→NH3⋅H2O,s)=−14.8(5) kJ mol−1 at absolute zero, and thus estimate the enthalpy of formation, ΔfH⊖(NH4OH,s)=−356 kJ mol−1 at 298 K. This result places an upper limit of 84 kJ mol−1 on the barrier to rotation of the ammonium cation, and yields an average hydrogen bond enthalpy of ∼23 kJ mol−1.
Second-order sound field during megasonic cleaning of patterned silicon wafers: Application to ridges and trenches
Authors: Deymier P. A. , Vasseur J. O. , Khelif A. , Raghavan S.
Published: July 2002

We report calculations of first-order pressure and second-order pressure gradient fields in the neighborhood of patterned silicon wafers. The patterned wafers consist of a single ridge and two parallel ridges separated by a trench on a planar substrate. The efficacy of megasonic waves for cleaning patterned wafers contaminated with micron to submicron silica particles is discussed by comparing a removal force arising from the second-order pressure gradient to a van der Waals adhesion force. The calculated second-order pressure gradient fields show that acoustic energy may be concentrated in small volumes in the vicinity of pattern features with dimensions significantly smaller than the wavelength of the incident acoustic wave. The angle the incident acoustic wave makes with the planar substrate has a strong impact on the second-order pressure gradient field. Grazing incident waves appear to provide a more efficient way of cleaning inside a trench. Excitation of a trench resonant vibrational mode enhances the magnitude of the first-order pressure, the second-order pressure gradient, and therefore the removal force.
Optical frequency synthesis based on mode-locked lasers
Authors: Cundiff Steven T. , Ye Jun , Hall John L.
Published: July 2002

The synthesis of optical frequencies from the primary cesium microwave standard has traditionally been a difficult problem due to the large disparity in frequency. Recently this field has been dramatically advanced by the introduction and use of mode-locked lasers. This application of mode-locked lasers has been particularly aided by the ability to generate mode-locked spectra that span an octave. This review article describes how mode-locked lasers are used for optical frequency synthesis and gives recent results obtained using them.
A solid-state dedicated circular dichroism spectrophotometer: Development and application
Authors: Kuroda Reiko , Harada Takunori , Shindo Yohji
Published: July 2002

A solid-state dedicated circular dichroism (CD) spectrophotometer (J-800KCM) was designed and constructed. As a CD spectrophotometer is a polarization–modulation instrument, CD spectra are necessarily accompanied by artifacts due to macroscopic anisotropies such as linear birefringence (LB) and linear dichroism (LD) which are unique to the solid state. A photomultiplier with the least polarization characteristics and a photoelastic modulator (PEM) with the least residual static birefringence were selected for the new instrument, which was based on the electrical and optical systems of a commercially available spectrophotometer. A phased-locked loop circuit was introduced to a PEM driver, and a sample rotation holder, a stage controller, and an analyzer were installed. We have designed and built a special solid-state sample holder to enable the cancellation of artifact CD, and a lens unit for smaller samples. A set of procedures for obtaining true CD has been devised based on the Mueller matrix method, and a program was written to facilitate the otherwise cumbersome measurements. Using the J-800KCM instrument and this approach, we could obtain nearly zero CD for achiral polyvinyl alcohol (PVA) film dyed with achiral Congo Red by cancelling the substantial apparent CD arising from large macroscopic anisotropies. Experiments on chiral single crystals of α-Ni(H2O)6⋅SO4, one with larger and one with smaller LB, have established that, by our method, we could also eliminate parasitic artifacts to obtain the true CD.
Intersubband absorption in degenerately doped GaN/AlxGa1−xN coupled double quantum wells
Authors: Gmachl Claire , Ng Hock M. , Cho Alfred Y.
Published: July 2002

Intersubband absorption in coupled GaN/AlGaN double quantum wells (DQWs) has been measured. The samples were grown by molecular-beam epitaxy on a sapphire substrate and with large (0.65 or 0.9) AlN-mole fraction in the barriers. Peak absorption wavelengths as short as 1.35 and 1.52 μm were measured for a symmetric DQW of 12 Å wide wells coupled by a 10 Å wide barrier, which also showed evidence of excited-state anticrossing. As expected, asymmetric DQWs displayed no such anticrossing, and the ground-state anticrossing energies were found to be much smaller, as a result of the comparatively large effective electron mass, than the energy broadening of individual transitions. Degenerate doping of the DQWs was used to establish a common reference energy at the Fermi level, which allows overcoming uncertainties related to intrinsic internal electric fields. The asymmetric DQWs displayed peak absorption wavelengths between 1.5 and 2.9 μm.
Analysis of the linear response function along the adiabatic connection from the Kohn–Sham to the correlated system
Authors: Savin Andreas , Colonna François , Allavena Marcel
Published: July 2002

Careful calculations are performed to obtain the radial density–density response function for the He and the Be series. This is also done along the adiabatic connection of the density functional theory (as the system evolves from the real, physical system to the Kohn–Sham one). In this process the electron density is kept constant, while the strength of the interaction between electrons changes. The response functions are analyzed in terms of their eigenvalues and eigenfunctions. The latter change only little along this process. The absolute value of the eigenvalues is in general reduced by the interaction: A screening effect is present. For the near-degenerate systems, we notice that the opposite effect can appear (antiscreening).
Quantum effects in liquid water: Path-integral simulations of a flexible and polarizable ab initio model
Authors: Stern Harry A. , Berne B. J.
Published: July 2002

We examine quantum effects in liquid water at ambient conditions by performing path-integral molecular dynamics simulations of a flexible, polarizable water model that was parameterized from ab initio calculations. The quantum liquid is less structured and has a smaller binding energy, in accord with previous simulations. The difference between the quantum and classical liquid binding energies (∼1.5 kcal/mol) is in reasonable agreement with a simple harmonic model, and is somewhat larger than previous estimates in the literature. Quantum effects do not appear to significantly modify the average induced dipole moment for a polarizable model, although the distribution is broader, especially for the component along the C2 axis of symmetry.
Reflection high-energy electron diffraction studies of epitaxial oxide seed-layer growth on rolling-assisted biaxially textured substrate Ni(001): The role of surface structure and chemistry
Authors: Cantoni C. , Christen D. K. , Feenstra R. , Goyal A. , Ownby G. W. , Zehner D. M. , Norton D. P.
Published: July 2002

We present a study of the {100}〈100〉 biaxially textured Ni(001) surface and seed-layer growth using in situ reflection high-energy electron diffraction and Auger electron spectroscopy. Our observations are consistent with formation of a c(2×2) two-dimensional superstructure due to the surface segregation of sulfur contained in the metal. We show that this superstructure can have a dramatic effect on the heteroepitaxial growth of oxide seed layers. In particular, the surface superstructure promotes the (200) epitaxial oxide growth of Y2O3-stabilized ZrO2, which is necessary for the development of high-Jc superconducting films for coated conductors.
Absolute concentration and loss kinetics of hydrogen atom in methane and hydrogen plasmas
Authors: Takashima Seigou , Hori Masaru , Goto Toshio , Kono Akihiro , Yoneda Katsumi
Published: July 2002

A measurement technique of the absolute concentration of hydrogen (H) atoms in methane (CH4) and/or hydrogen molecule (H2) plasmas has been established. The H-atom concentration was evaluated by vacuum ultraviolet absorption spectroscopy (VUVAS) using a high-pressure H2 microdischarge hollow cathode lamp (H2-MHCL) as the Lyman α (Lα 121.6 nm) light emission source. A measurement technique of the background absorption caused by species other than H atoms at the Lα line was developed by using the VUVAS technique with the MHCL employing nitrogen molecules (N2-MHCL). The lines around Lα used for the background absorption measurements are 2p23s P5/24–2p3 S3/204 at 119.955 nm, 2p23s P3/24–2p3 S3/204 at 120.022 nm, and 2p23s P1/24–2p3 S3/204 at 120.071 nm of the N atom. By using the VUVAS technique with the MHCLs and subtracting the background absorption from the absorption of H atoms at Lα, we have achieved the measurement of the H-atom concentration in an inductively coupled plasma operated in CH4 and/or H2. The H-atom concentration increased from 2×1011 to 3×1012 cm−3 when increasing the CH4 flow rate ratios from 0% to 50% in the CH4–H2 mixture and was almost constant in its range between 50% and 100% at a pressure of 1.33 Pa, a radio frequency power of 200 W, and a total flow rate of 100 sccm. The behavior of the H-atom concentration was compared with that of the Balmer α emission intensity. The decay of the H-atom concentration in the H2 plasma afterglow was investigated to clarify the loss kinetics of H atoms. The dependence of the decay time constant on the pressure showed that H atoms were dominantly lost through diffusion to the wall surface. The diffusion constant of H atoms in H2 plasmas was determined to be 3.0×105 cm2 Pa s−1 at 400 K. The surface loss probability of H atoms on the stainless-steel and the hydrocarbon walls were estimated to be 0.15 and 0.07, respectively.
Vibrational polarization beats in femtosecond coherent anti-Stokes Raman spectroscopy: A signature of dissociative pump–dump–pump wave packet dynamics
Authors: Faeder J. , Pinkas Iddo , Knopp G. , Prior Yehiam , Tannor D. J.
Published: July 2002

Knopp et al. [J. Raman Spectrosc. 31, 51 (2000)] have recently used resonant femtosecond coherent anti-Stokes Raman spectroscopy (CARS) to prepare and probe highly excited vibrational wave packets on the ground electronic potential surface of molecular iodine. The experiment uses a sequence of three resonant femtosecond pulses with two independently variable time delays. The first two pulses act as a pump and dump sequence to create a predefined, highly excited wave packet on the ground electronic state, whose amplitude is optimized by selecting the proper pump–dump (Raman) frequency difference and varying the time delay. The third pulse promotes the pump–dump wave packet to an excited electronic state, resulting in subsequent coherent emission of light at the anti-Stokes frequency. This fully-resonant CARS signal, measured as a function of time delay between the second and third pulses, oscillates at a frequency characteristic of the pump–dump wave packet. Due to anharmonicity, this frequency is a sensitive measure of the amount of vibrational excitation. Knopp et al. observed that under certain conditions the signal exhibits pronounced beating between the pump–dump wave packet frequency and the frequency characteristic of the bottom of the ground state well. In this paper we show that these beats arise only when the final pump–dump–pump wave packet is above the excited state dissociation threshold of the molecule. We derive analytical expressions showing that under these conditions, where the polarization is short-lived, there may be strong interferences between the contributions from molecules originally in different vibrational states of the thermal ensemble. In contrast, the CARS polarization in the below threshold case is long-lived, and these interferences cancel. Numerical evaluation of the CARS signal through vibrational wave packet propagation confirms the predictions of the analytical theory and reproduces the distinctive beating pattern observed in the experiments. Additional experiments and simulations demonstrate that these interferences can be turned on or off by carefully selecting the pulse frequencies. The experiments can also be viewed from a different perspective, as an extension of the pump–dump mechanism for selective bond breaking on the ground electronic state, to a pump–dump–pump sequence for selective bond breaking on the excited electronic state.
Accuracy assessment of a linear birefringence measurement system using a Soleil–Babinet compensator
Authors: Wang Baoliang , Hellman Walter
Published: July 2002

This article describes a method for assessing the accuracy of a new linear birefringence measurement system based on photoelastic modulator technology. We used a Soleil–Babinet compensator in the experiment and observed that its retardation varies significantly across the optical aperture. To use this compensator as a meaningful retardation standard, we fixed the beam position relative to the Soleil–Babinet compensator before and after its calibration. Our results show that the birefringence measurement system is capable of providing accurate measurements for linear retardation below 125 nm with a relative uncertainty below 1%. Experimental results support the fact that this birefringence measurement system is self-calibrating for measuring linear retardation.
Two-dimensional simulation of polymer field-effect transistor
Authors: Tessler N. , Roichman Y.
Published: July 2002

A two-dimensional simulation of intrinsic top-contact field-effect transistor is presented. The simulated structure is unique to organic transistors and hence is most relevant. By time resolving the operation of such a transistor, the mechanisms underlying its operation are resolved. The effect of this device configuration on the measured “intrinsic” material properties is also discussed and shown to explain previously reported features.
Efficient green organic light-emitting diodes with stericly hindered coumarin dopants
Authors: Chen Chin H. , Tang Ching W.
Published: July 2002

We have discovered two stericly hindered green coumarin derivatives which showed significantly better thermal stability and overall electroluminescence (EL) performance than those of the corresponding 3-(2-benzothiazolyl)-7-(diethylamino)-2H-1-benzopyran-2-one (C-6) and 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl- 2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]pyrano[6,7, 8-ij] quinolizin-11-one (C-545T). The strategic incorporation of bulky spacer t-butyl groups in these dopants considerably delays the onset of concentration quenching. In particular, 10-[2(5,7-di-t-butyl)benzothiazolyl]-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo-[l]pyrano[6,7,8-ij]quinolizin-11-one (C-545TB) with its superb thermal stability (Tg=142 °C; Td=327 °C), high luminance efficiency (∼13 cd/A), good Commission Interationale d’Eclairage (CIE) coordinates (x=0.30; y=0.64) and a nearly flat EL efficiency dependence on drive current stands out as one of the best choice of green dopants for use in the passive-matrix organic light-emitting diode displays.
Power gain and dissipation in quantum-dot cellular automata
Authors: Timler John , Lent Craig S.
Published: July 2002

Quantum-dot cellular automata (QCA) may provide a novel way to bypass the transistor paradigm to form molecular-scale computing elements. In the QCA paradigm information is represented by the charge configuration of a QCA cell. We develop a theoretical approach, based on the density matrix formalism, which permits examination of energy flow in QCA devices. Using a simple two-state model to describe the cell, and an energy relaxation time to describe the coupling to the environment, we arrive at an equation of motion well suited to the quasi-adiabatically switched regime. We use this to examine the role of power gain and power dissipation in QCA cells. We show that QCA cells can exhibit true signal power gain. The energy lost to dissipative processes is restored by the clock. We calculate the power dissipated to the environment in QCA circuits and show that it is possible to achieve the ultralow levels of power dissipation required at molecular densities.
Sharp cutoff versus smooth filtering in large eddy simulation
Authors: De Stefano Giuliano , Vasilyev Oleg V.
Published: July 2002

The large eddy simulation (LES) equations of turbulent flows are formally derived by applying a low-pass filter to the Navier–Stokes equations. As a result the subgrid-scale (SGS) stress tensor strongly depends on the assumed filter shape, which causes a SGS model to be filter dependent. In particular, depending on the choice of the filter the corresponding SGS model should satisfy very different requirements in terms of large scale dynamics and kinetic energy budget. This paper is an attempt to systematically study the effect of the filter shape on the subgrid scale model and its subsequent effect on LES. For the sake of simplicity, we consider numerical simulation of a one-dimensional homogeneous flow, governed by the viscous Burgers equation. Large eddy simulations of the solution of the Burgers problem are performed using subgrid scale models obtained by filtering data from direct numerical simulations. Diagnostics include temporal evolution of energy and dissipation as well as energy spectra. It is demonstrated both theoretically and numerically that the assumed filter shape can have a significant effect on LES in terms of spectral content and physical interpretation of the solution. The results are generalized for LES of three-dimensional turbulent flows and specific recommendations for the use of filters and corresponding SGS models are made.
The retraction of the edge of a planar liquid sheet
Authors: Sünderhauf Gerhard , Raszillier Hans , Durst Franz
Published: July 2002

Our objective is to investigate the motion of a free two-dimensional edge of a liquid sheet, which recedes and accumulates fluid as it is pulled back toward the bulk of the sheet due to surface tension. In the long time limit, the velocity of this edge reaches a constant value given by Taylor [Proc. R. Soc. London, Ser. A 253, 313 (1959)] and Culick [J. Appl. Phys. 31, 1128 (1960)], independent of the fluid viscosity. The way this value is reached, however, depends on the viscosity. In order to follow quantitatively the acceleration process, time-dependent numerical simulations of the two-dimensional Navier–Stokes equations have been performed. For inertia-dominated flow situations, the results show that the motion of the rim is relatively well characterized by a global momentum balance. However, if viscous effects are dominant, the rim is shown to extend far out into the film. In this case, the decrease in film thickness toward the undisturbed film can be described quite precisely by another one-dimensional model. By comparison of the individual contributions to the energy balances, following from numerical computation and from the global momentum balance, a closer look has also been taken on the inner mechanism of the rim formation. Finally, on the basis of the performed simulations, a stability criterion for a moving liquid curtain of finite length is derived, which determines whether a newly formed free rim leads to a break-up of the curtain.
Quantitative evaluation of biaxial strain in epitaxial 3C-SiC layers on Si(100) substrates by Raman spectroscopy
Authors: Rohmfeld Stefan , Hundhausen Martin , Ley Lothar , Zorman Christian A. , Mehregany Mehran
Published: July 2002

We present experimentally determined biaxial strain coefficients for the longitudinal optical (LO) and transversal optical (TO) Raman lines in 3C-SiC. Suspended 3C-SiC membranes with a (100) texture are deflected by a variable pressure load on one side and the strain-induced shifts of the LO and TO Raman lines are measured while the strain is simultaneously calculated from the membrane deflection vs pressure. Using these results we measure the residual strain of 3C-SiC films grown on Si as a function of preparation conditions. The largest residual strain is found in thin samples which relaxes as film thickness increases to a value imposed by the different thermal expansion coefficients of 3C-SiC and Si. As the residual strain decreases and the film thickness increases, the Raman lines narrow indicating an improved crystalline quality. We also find a reduction of the residual strain with increasing growth rate.
Fabrication and characterization of different types of two-dimensional AlGaAs photonic crystal slabs
Authors: Sugimoto Yoshimasa , Ikeda Naoki , Carlsson Niclas , Asakawa Kiyoshi , Kawai Noriko , Inoue Kuon
Published: July 2002

Photonic crystals having two-dimensional periodicity on a length scale of 320–450 nm are fabricated by electron beam lithography in combination with high-aspect-ratio dry etching. To achieve three-dimensional control of the optical properties, three kinds of dielectric waveguide structures based on AlGaAs heterostructures, that is, semiconductor-clad, air-bridge, and oxide-clad structures, are investigated. The ability to guide light through such photonic crystal devices is demonstrated by optical transmission measurements. Clear photonic band gap effects resulting in 30 dB attenuation of the transmitted light can be observed in the stop-band regions. The measured results are in good agreement with theoretical band-structure calculations and with numerically computed transmission spectra obtained by the finite-difference time-domain method.
Donor and acceptor concentrations in degenerate InN
Authors: Look D. C. , Lu H. , Schaff W. J. , Jasinski J. , Liliental-Weber Z.
Published: July 2002

A formalism is presented to determine donor (ND) and acceptor (NA) concentrations in wurtzitic InN characterized by degenerate carrier concentration (n) and mobility (μ). The theory includes scattering not only by charged point defects and impurities, but also by charged threading dislocations, of concentration Ndis. For a 0.45-μm-thick InN layer grown on Al2O3 by molecular beam epitaxy, having Ndis=5×1010 cm−2, determined by transmission electron microscopy, n(20 K)=3.5×1018 cm−3 and μ(20 K)=1055 cm2/V s, determined by Hall effect measurements, the fitted values are ND=4.7×1018 cm−3 and NA=1.2×1018 cm−3. The identities of the donors and acceptors are not known, although a comparison of ND with analytical data, and also with calculations of defect formation energies, suggests that a potential candidate for the dominant donor is H.
Bubble formation due to electrical stress in organic light emitting devices
Authors: Ke Lin , Chua Soo-Jin , Zhang Keran , Chen Peng
Published: July 2002

The degradation in electroluminescence of poly(p-phenylene vinylene)-based organic light-emitting devices is studied using optical microscopy, scanning electron microscopy, and secondary ion mass spectroscopy. “Bubbles” are formed at the polymer and indium tin oxide interface or in the polymer layer within the nonemissive area. This formation, which occurs during device electrical stress, is accompanied by a fluctuation of the device current. The bubbles are formed by the degraded polymer and/or the gas released from disintegration of the polymer. High local current density flowing near the dark spot center and the resultant heating, decomposes the polymer layer. The resultant carbonized area causes either local short circuit and/or open circuit leading to the final light-emitting device failure.
Accurate core electron binding energy calculations using small 6-31G and TZV core hole optimized basis sets
Authors: Carniato Stéphane , Millié Philippe
Published: July 2002

A procedure for optimizing basis sets for core hole binding energies is described. Contracted Gaussian basis sets are optimized for ground state and core hole state atomic configurations, exponents and contraction coefficients being determined by a minimization of the atomic self-consistent field state within a simulated annealing procedure. The basis sets are used in connection with Δself-consistent field, ΔMøller–Plesset and ΔKohn–Sham theory calculations of core electron binding energies and chemical shifts of high accuracy. Whatever the method, the small basis sets optimized in this way give results with an accuracy comparable to that obtained using very extended normal basis sets close to the complete basis set limit. They provide an excellent alternative to treat large molecular systems and push the accuracy of the ΔKohn–Sham technique for binding energy computations even further, exhibiting only small (a few tenths of an electron volt) deviations from experimental data.
Upconversion luminescence in Yb3+ doped CsMnCl3: Spectroscopy, dynamics, and mechanisms
Authors: Valiente Rafael , Wenger Oliver S. , Güdel Hans U.
Published: July 2002

Single crystals of CsMnCl3 doped with 0.9% Yb3+ were grown from the melt by the Bridgman technique and studied by means of variable temperature optical spectroscopy. At cryogenic temperatures, near-infrared Yb3+-excitation around 1 μm leads to intense Mn2+ upconversion luminescence in the red spectral region. This very efficient upconversion process is possible because of magnetic Yb3+–Mn2+ exchange interactions, and a new type of upconversion mechanism is found to be active in this system. The upconversion properties of Yb3+:CsMnCl3 are compared to those of Yb3+:RbMnCl3 and Yb3+:CsMnBr3. The upconversion efficiencies at cryogenic temperatures differ by many orders of magnitude. The bridging geometry between Yb3+ and Mn2+ is found to be a key factor for the efficiency of the process. The highest efficiency is observed for the title compound, and this is correlated with the most likely linear Yb3+–Cl−–Mn2+ arrangement in this crystal. At 15 K the dominant upconversion mechanism in the title compound involves an energy transfer step. By increasing the temperature to 100 K a new and very efficient mechanism involving a sequence of ground state and excited state absorption steps becomes dominant.
Critically Evaluated Atomic Transition Probabilities for Ba I and Ba II
Authors: Klose J. Z. , Fuhr J. R. , Wiese W. L.
Published: July 2002

Atomic transition probabilities for allowed and forbidden lines of Ba I and Ba II are tabulated, based on a critical evaluation of recent literature sources. The data are presented in multiplet format and are ordered by increasing excitation energies.
Anisotropy of quadratic magneto-optic effects in reflection
Authors: Postava K. , Hrabovský D. , Pištora J. , Fert A. R. , Višňovský Š. , Yamaguchi T.
Published: July 2002

Quadratic or second-order magneto-optic effects in reflection significantly effect in-plane magnetization measurements. While the magneto-optic effects linear in magnetization are independent of orientation of cubic crystal axes, the amplitude and sign of the quadratic effects change significantly under crystal rotation. Theoretical formulas for the magneto-optic effects have been derived using a permittivity tensor including terms quadratic in magnetization. A method for separation of the diagonal and off-diagonal quadratic magneto-optic tensor components (G11−G12) and 2G44 is proposed. The theory was completed by an experimental observation of the quadratic effect anisotropy in an epitaxial Fe layer prepared on a MgO substrate. The influence of the magnetization components on the magneto-optic vector magnetometry is discussed for an interface, a single layer, and exchange coupled bilayer system for a general magnetization direction including the quadratic magneto-optic effect anisotropy.