bluuebunny/crossref_metadata_2025 huggingface dataset:
Long-wavelength lasing from InAs self-assembled quantum dots on (311) B InP
Authors: Nishi Kenichi , Yamada Mitsuki , Anan Takayoshi , Gomyo Akiko , Sugou Shigeo
Published: July 2002

Nanometer-scale InAs quantum dots were grown on InP by self-assembly using gas-source molecular beam epitaxy. InAs depositions of 0.33 nm in nominal thickness were found to form quantum dots on (311) B InP with a lateral dimension of about 43 nm and a density of 2×1010 cm−2. A laser structure with seven periods of the quantum dot active layers lased in a wavelength range from 1.1 to 1.4 μm at 77 K under pulsed current injection. The lasing wavelength changed to a shorter wavelength as the cavity length decreased, indicating gain saturation due to state filling effect in discrete quantum levels, which is typical in quantum dot lasers. This phenomenon can be used to achieve wide-range multiwavelength lasers for optical communication, that can be adjusted merely by changing the effective cavity length.
Sonoluminescence initiated by laser irradiation of carbon suspensions
Authors: McGrath Thomas E. , Beveridge Andrew C. , Diebold Gerald J.
Published: July 2002

We report observation of sonoluminescence following irradiation of suspensions of carbon in water with a high power, pulsed laser. The mechanism for generation of a gas bubble is chemical in origin: the heating of the carbon particles by absorption of laser radiation causes the production of high pressure H2 and CO through reaction of carbon with the water surrounding the particles. Expansion of the bubble past its equilibrium diameter results in oscillation of the bubble diameter which, several hundred μs after the initial formation of the bubble, results in emission of optical and acoustic radiation. The effects of laser power, the concentration of carbon in the suspension, dissolved Ar, and temperature on the sonoluminescent intensity are reported.
Dependence of the silicon nanowire diameter on ambient pressure
Authors: Zhang H. Z. , Yu D. P. , Ding Y. , Bai Z. G. , Hang Q. L. , Feng S. Q.
Published: July 2002

Our present work provides a method to control the diameters of the silicon nanowires. As a dominant experimental parameter, the ambient pressure was controlled between 150 and 600 Torr. It is found that the average size of the silicon nanowires increases with increasing ambient pressure. The mean diameter of the silicon nanowires in our study is proportional to the 0.4 power of ambient pressure. Catalytic nanoparticles and the periodic instability of the nanowires suggest a vapor-liquid-solid growth mechanism. For the growth of nanowires, an explanation of the relationship between the mean diameter of the silicon nanowires and the ambient pressure has been proposed.
Analytic model for direct tunneling current in polycrystalline silicon-gate metal–oxide–semiconductor devices
Authors: Register Leonard F. , Rosenbaum Elyse , Yang Kevin
Published: July 2002

An analytic model of the direct tunneling current in metal–oxide–semiconductor devices as a function of oxide field is presented. Accurate modeling of the low-field roll-off in the current results from proper modeling of the field dependencies of the sheet charge, electron impact frequency on the interface, and tunneling probability. To obtain the latter dependence, a modified WKB approximation is used.
Intrinsic picosecond response times of Y–Ba–Cu–O superconducting photodetectors
Authors: Lindgren M. , Currie M. , Williams C. , Hsiang T. Y. , Fauchet P. M. , Sobolewski Roman , Moffat S. H. , Hughes R. A. , Preston J. S. , Hegmann F. A.
Published: July 2002

We report our femtosecond time-resolved measurements on the photoresponse of an epitaxial YBa2Cu3O7−x (YBCO) thin-film photodetector, patterned into a microbridge geometry. By varying the current–voltage biasing conditions between the superconducting and resistive (hot spot) states, we observed transients that correspond to the nonequilibrium kinetic-inductance and the nonequilibrium electron-heating response mechanisms, respectively. The two-temperature model and the Rothwarf–Taylor theory have been used to simulate the measured wave forms and to extract the temporal parameters. The electron thermalization time and the electron–phonon energy relaxation time were determined by the electron temperature rise and decay times, which were found to be 0.56 and 1.1 ps, respectively, in the resistive state. We have also measured the ratio between the phonon and electron specific heats to be 38, which corresponds to a phonon–electron scattering time of 42 ps. No phonon-trapping effect (typical for low-temperature superconductors) was observed in YBCO, in the superconducting state, so the quasiparticle lifetime was given by the quasiparticle recombination time, estimated from the Rothwarf–Taylor equations to be below 1 ps.
Antimonides with the half-Heusler structure: New thermoelectric materials
Authors: Mastronardi K. , Young D. , Wang C.-C. , Khalifah P. , Cava R. J. , Ramirez A. P.
Published: July 2002

The thermoelectric properties near ambient temperature of half-Heusler alloys based on LnPdSb, where Ln=Ho, Er, and Dy are reported. The Seebeck coefficients are large, between 60 and 250 μV/K, and the materials are p type. The resistivities are between 0.6 and 20 mΩ cm. Thermal conductivities are between approximately 5.0 and 3.5 W/mK at 300 K, and are smallest in intentionally disordered materials. The highest ambient temperature ZT obtained is 0.06. Band-structure calculations are presented for LuPdSb. It is suggested that half-Heusler alloys with 18 electrons per formula unit may represent a large class of thermoelectric materials.
Vacuum laser acceleration by an ultrashort, high-intensity laser pulse with a sharp rising edge
Authors: Cheng Ya , Xu Zhizhan
Published: July 2002

A laser vacuum electron acceleration scheme is proposed in this letter. By the help of a one-dimensional model, we find that an ultrashort, high-intensity, plane-wave laser pulse with a sharp rising edge can be used to accelerate electrons to relativistic energy in vacuum.
Top-interface-controlled switching and fatigue endurance of (Pb,La)(Zr,Ti)O3 ferroelectric capacitors
Authors: Stolichnov I. , Tagantsev A. , Setter N. , Cross J. S. , Tsukada M.
Published: July 2002

Mechanisms of polarization switching and fatigue in (Pb,La)(Zr,Ti)O3 (PLZT) films are studied by comparative analysis of degradation and leakage conduction of PLZT capacitors with Pt, SrRuO3 (SRO), and layered Pt/SRO (80/5 nm) electrodes. It is found that the asymmetrical Pt/SRO/PLZT/Pt structure exhibits a good fatigue performance in combination with low leakage like that on identically processed Pt/PLZT/Pt capacitor. This asymmetrical structure exhibits very unusual dependence of the endurance of switching polarization on the driving alternating-current (ac) electric field amplitude. Specifically, for high ac electric field amplitude it shows good switching endurance similar to SRO/PLZT/SRO capacitors, whereas for amplitude lower than 80 kV/cm a pronounced polarization fatigue similar to that of the conventional Pt/PLZT/Pt capacitors is observed. Based on the analysis of our results on fatigue and leakage conduction we conclude that: (i) In asymmetrical Pt/SRO/PLZT/Pt system the physical mechanisms of polarization switching vary depending on the driving voltage amplitude. The polarization switching endurance under high-amplitude cycling is governed only by the top SRO interface, whereas the degradation properties of the bottom Pt interface do not play any role. On the contrary, for driving voltage amplitudes lower than 80 kV/cm the bottom Pt-interface degradation is a limiting factor for the fatigue performance. (ii) The fatigue in PZT films is controlled by a mechanism related to the inhibition of near-by-electrode nucleation of opposite domains, as suggested earlier.
Production and annealing of electron irradiation damage in ZnO
Authors: Look D. C. , Reynolds D. C. , Hemsky J. W. , Jones R. L. , Sizelove J. R.
Published: July 2002

High-energy (>1.6 MeV) electrons create acceptors and donors in single-crystal ZnO. Greater damage is observed for irradiation in the [0001] direction (Zn face) than in the [0001̄] direction (O face). The major annealing stage occurs at about 300–325 °C, and is much sharper for defects produced by Zn-face irradiation, than for those resulting from O-face irradiation. The defects appear to have a chain character, rather than being simple, near-neighbor vacancy/interstitial Frenkel pairs. These experiments suggest that ZnO is significantly more “radiation hard” than Si, GaAs, or GaN, and should be useful for applications in high-irradiation environments, such as electronics in space satellites.
Room-temperature photopumped InGaN/GaN/AlGaN vertical-cavity surface-emitting laser
Authors: Krestnikov I. L. , Lundin W. V. , Sakharov A. V. , Semenov V. A. , Usikov A. S. , Tsatsul’nikov A. F. , Alferov Zh. I. , Ledentsov N. N. , Hoffmann A. , Bimberg D.
Published: July 2002

We report photopumped room-temperature surface-mode lasing at 401 nm in a InGaAlN vertical-cavity surface-emitting laser grown on a sapphire substrate using metal–organic vapor-phase epitaxy. A 2λ cavity was formed by a quarter-wave Al0.15Ga0.85N/GaN distributed Bragg reflector on the one side of the active layer and a GaN–air interface on the other. A multilayer structure composed of 12-fold-stacked ultrathin InGaN insertions in a GaN matrix served as an active layer providing ultrahigh material gain and making possible vertical lasing without use of the upper Bragg reflector.
High spatial resolution thermal conductivity of lateral epitaxial overgrown GaN/sapphire (0001) using a scanning thermal microscope
Authors: Asnin V. M. , Pollak Fred H. , Ramer J. , Schurman M. , Ferguson I.
Published: July 2002

We have performed high spatial resolution thermal conductivity (κ) measurements at room temperature on different patterned sections of GaN/sapphire (0001) fabricated by lateral epitaxial overgrowth using a scanning thermal microscope. In a number of regions we find κ≈1.7–1.8 W/cm K, values that are substantially higher than κ≈1.3 W/cm K previously reported by Sichel and Pankove [J. Phys. Chem. Solids 38, 330 (1977)] on “bulk” material and comparable to the theoretical estimate of 1.7 W/cm K deduced by Slack [J. Phys. Chem. Solids 34, 321 (1973)]. The implications of these findings for device applications will be discussed.
Diameter grouping in bulk samples of single-walled carbon nanotubes from optical absorption spectroscopy
Authors: Jost O. , Gorbunov A. A. , Pompe W. , Pichler T. , Friedlein R. , Knupfer M. , Reibold M. , Bauer H.-D. , Dunsch L. , Golden M. S. , Fink J.
Published: July 2002

The influence of the synthesis parameters on the mean characteristics of single-wall carbon nanotubes in soot produced by the laser vaporization of graphite has been analyzed using optical absorption spectroscopy. The abundance and mean diameter of the nanotubes were found to be most influenced by the furnace temperature and the cobalt/nickel catalyst mixing ratio. Via an analysis of the fine structure in the optical spectra, the existence of preferred nanotube diameters has been established and their related fractional abundance could be determined. The results are consistent with nanotubes located mainly around the armchair axis.
Electron emission from films of carbon nanotubes and ta-C coated nanotubes
Authors: Dimitrijevic S. , Withers J. C. , Mammana V. P. , Monteiro O. R. , Ager J. W. , Brown I. G.
Published: July 2002

The field emission properties of multiwall carbon nanotube films with and without a coating of tetrahedrally bonded amorphous carbon (ta-C) were investigated. Voltage thresholds of 2.4 V/μm for uncoated films and 1.5 V/μm for ta-C coated films were found. The results for the uncoated films are in good agreement with previous measurements of field emission from carbon nanotubes. The effect of the ta-C coating on the emission properties is discussed in light of current field emission models.
Polymerized carbon nanobells and their field-emission properties
Authors: Ma Xucun , Wang Enge , Zhou Wuzong , Jefferson David A. , Chen Jun , Deng Shaozhi , Xu Ningsheng , Yuan Jun
Published: July 2002

Aligned nitrogen-containing carbon nanofibers consisting of polymerized “nanobells” have been grown on a large scale using microwave plasma-assisted chemical-vapor deposition with a mixture of methane and nitrogen. A greater part of the fiber surface consists of open ends of the graphitic sheets. A side-emission mechanism is proposed. A low-threshold field of 1.0 V/μm and a high-emission current density of 200 mA/cm2 for an applied field of 5–6 V/μm were achieved, implying that the materials have a high potential for future application as electron field emitters, especially in flat-panel displays.
Beaded carbon tubes
Authors: Ting Jyh-Ming , Lan Jones B. C.
Published: July 2002

Unusual tubular carbon structures were observed when preparing carbon tubes using a vapor phase process. Regular, single-diametered carbon tubes were obtained at temperatures below 1300 °C. However, at temperatures greater than 1300 °C, in addition to regular carbon tubes, beaded carbon tubes appeared. The occurrence and appearance of such unusual beaded structure is thought to be related to the growth temperature, and is explained in terms of a three-dimensional nucleation model.
Efficient route to large arrays of CNx nanofibers by pyrolysis of ferrocene/melamine mixtures
Authors: Terrones M. , Terrones H. , Grobert N. , Hsu W. K. , Zhu Y. Q. , Hare J. P. , Kroto H. W. , Walton D. R. M. , Kohler-Redlich Ph. , Rühle M. , Zhang J. P. , Cheetham A. K.
Published: July 2002

We report a high-yield route to two-dimensional arrays (<400×400 μm2) of aligned C49Nx (x⩽1) nanofibers (<100 nm o.d.; <60 μm length), by pyrolyzing mixtures of ferrocene and melamine at 950–1050 °C under an Ar flow. The fibers exhibit unusual interlinked stacked-cone morphologies, ascribed to the presence of nitrogen. High-resolution electron energy-loss spectroscopy of the individual fibers reveals a 2% nitrogen content with ionization energies mainly at ∼400.9 eV, corresponding to N bonded to three C atoms within a hexagonal framework. The nanofibers may be useful for the economic fabrication of field emission sources and robust composites.
Two-stage superconducting-quantum-interference-device amplifier in a high-Q gravitational wave transducer
Authors: Harry Gregory M. , Jin Insik , Paik Ho Jung , Stevenson Thomas R. , Wellstood Frederick C.
Published: July 2002

We report on the total noise from an inductive motion transducer for a gravitational-wave antenna. The transducer uses a two-stage superconducting quantum interference device (SQUID) amplifier and has a noise temperature of 1.1 mK, of which 0.70 mK is due to back action noise from the SQUID chip. The total noise includes thermal noise from the transducer mass, which has a measured Q of 2.60×106. The noise temperature exceeds the expected value of 3.5 μK by a factor of 200, primarily due to voltage noise at the input of the SQUID. Noise from flux trapped on the chip is found to be the most likely cause.
Controlling the diameter, growth rate, and density of vertically aligned carbon nanotubes synthesized by microwave plasma-enhanced chemical vapor deposition
Authors: Choi Young Chul , Shin Young Min , Lee Young Hee , Lee Byung Soo , Park Gyeong-Su , Choi Won Bong , Lee Nae Sung , Kim Jong Min
Published: July 2002

Vertically aligned carbon nanotubes were synthesized on Ni-deposited Si substrates using microwave plasma-enhanced chemical vapor deposition. The grain size of Ni thin films varied with the rf power density during the rf magnetron sputtering process. We found that the diameter, growth rate, and density of carbon nanotubes could be controlled systematically by the grain size of Ni thin films. With decreasing the grain size of Ni thin films, the diameter of the nanotubes decreased, whereas the growth rate and density increased. High-resolution transmission electron microscope images clearly demonstrated synthesized nanotubes to be multiwalled.
Analysis of x-ray reflectivity data from low-contrast polymer bilayer systems using a Fourier method
Authors: Seeck O. H. , Kaendler I. D. , Tolan M. , Shin K. , Rafailovich M. H. , Sokolov J. , Kolb R.
Published: July 2002

X-ray reflectivity data of polymer bilayer systems have been analyzed using a Fourier method which takes into account different limits of integration in q-space. It is demonstrated that the interfacial parameters can be determined with high accuracy although the difference in the electron density (the contrast) of the two polymers is extremely small. This method is not restricted to soft-matter thin films. It can be applied to any reflectivity data from low-contrast layer systems.
Electron energy-loss spectrometry on lithiated graphite
Authors: Hightower A. , Ahn C. C. , Fultz B. , Rez P.
Published: July 2002

Transmission electron energy-loss spectrometry was used to investigate the electronic states of metallic Li and LiC6, which is the Li-intercalated graphite used in Li-ion batteries. The Li K edges of metallic Li and LiC6 were nearly identical, and the C K edges were only weakly affected by the presence of Li. These results suggest only a small charge transfer from Li to C in LiC6, contrary to prior results from surface spectra obtained by x-ray photoelectron spectroscopy. Effects of radiation damage and sample oxidation in the transmission electron microscopy are also reported.
Spin polarization of tunneling current from ferromagnet/Al2O3 interfaces using copper-doped aluminum superconducting films
Authors: Monsma D. J. , Parkin S. S. P.
Published: July 2002

Superconducting Cu-doped Al films sputtered at room temperature (RT) show improved characteristics for superconductor–insulator–ferromagnet spin-polarization measurements as compared to dirty Al films evaporated at 77 K. Detailed analysis including temperature, depairing and spin-orbit effects reveal spin-polarization values for a large variety of ferromagnetic transition-metal alloys. The high-spin polarization for these RT-sputtered junctions is consistent with large tunnel magnetoresistance values observed in similarly sputtered magnetic tunnel junctions.
Translational and rotational motion of isolated water molecules in nitromethane studied using O17 NMR
Authors: Price William S. , Ide Hiroyuki , Arata Yoji
Published: July 2002

Hydrogen bonding plays an enormous role in determining the solution properties of liquid water. In the present study, the translational diffusion and the reorientational correlation times of isolated water (H2 17O) molecules dissolved in nitromethane were studied using O17 NMR measurements in the temperature range of 260–314 K. It was found that the water diffusion coefficient was considerably faster than in pure water at the same temperature. Further, the activation energy for the translational motion of the water was about 10 kJ mol−1, which was the same as that of the (solvent) nitromethane. However, the activation energy for the reorientational motion of the water was significantly less at 7.7 kJ mol−1. In this study we show that the motions of the isolated water molecules behave significantly different than water molecules in pure water due to the absence of hydrogen bonding to nearby water molecules.
Microstructural, electrical, and optical properties of SnO2 nanocrystalline thin films grown on InP (100) substrates for applications as gas sensor devices
Authors: Kim T. W. , Lee D. U. , Yoon Y. S.
Published: July 2002

SnO 2 thin films were grown on p-InP (100) substrates by using radio-frequency magnetron sputtering at low temperature. Transmission electron microscopy (TEM) and electron diffraction pattern measurements showed that these SnO2 thin films were nanocrystalline. The capacitance–voltage measurements at room temperature showed that the type and the carrier concentration of the nominally undoped SnO2 film were n type and approximately 1.62×1016 cm−3, respectively. Raman scattering measurements showed that the grain sizes of the nanocrystalline films were below 10 nm, which was in reasonable agreement with the result obtained from the high-resolution TEM measurements. Photoluminescence measurements showed a broad peak below the band-to-band emission. These results can help improve the understanding of SnO2 nanocrystalline films grown on p-InP (100) substrates for applications in high-sensitivity gas sensors.
Reversible optical structuring of polymer waveguides doped with photochromic molecules
Authors: Lecomte S. , Gubler U. , Jäger M. , Bosshard Ch. , Montemezzani G. , Günter P. , Gobbi L. , Diederich F.
Published: July 2002

We show that polymeric films doped with the photochromic molecule 1,8a-dihydro-2(4-iodophenyl)-1,1-azulenedicarbonitrile can be reversibly structured by light. We discuss the relevant material properties of the photochromic molecule in solution as well as in polymer films and demonstrate light-induced waveguides at the telecommunication wavelength of 1.313 μm.
High spatial resolution thermal conductivity and Raman spectroscopy investigation of hydride vapor phase epitaxy grown n-GaN/sapphire (0001): Doping dependence
Authors: Florescu D. I. , Asnin V. M. , Pollak Fred H. , Molnar R. J. , Wood C. E. C.
Published: July 2002

We have measured high resolution thermal conductivity (κ) and Raman spectra {E2 mode [high frequency], A1 mode [longitudinal optical (LO)], and high frequency LO-plasmon coupled mode [LPP+]} at 300 K of three series of n-GaN/sapphire (0001) samples fabricated by hydride vapor phase epitaxy (HVPE). The former was determined with a scanning thermal microscope while the latter was obtained using a micro-Raman system, both having a spatial resolution of ≈2–3 μm. For all three sets of samples the thermal conductivity decreased linearly with log n, about a factor of two decrease in κ for every decade increase in n. Also, we found a correlation between film thickness and improved thermal conductivity. Furthermore, κ≈1.95 W/cm K for one of the most lightly doped samples (≈6.9×1016 cm−3), higher than previously reported κ≈1.7–1.8 W/cm K on lateral epitaxial overgrown (LEO) material with n≈(1–2)×1017 cm−3 [V. M. Asnin et al., Appl. Phys. Lett. 75, 1240 (1999)], κ=1.55 W/cm K on LEO samples using a third-harmonic technique [C. Y. Luo et al., Appl. Phys. Lett. 75, 4151 (1999)], and κ≈1.3 W/cm K on a HVPE sample [E. K. Sichel and J. I. Pankove, J. Phys. Chem. Solids 38, 330 (1977)]. The carrier concentration dependence of κ is similar to that of other semiconductors in a comparable temperature range. On a log–log scale the linewidth of the observed E2 Raman mode remained constant up to n≈1×1018 cm−3 and then increased linearly. The carrier concentration obtained from the LPP+ mode is less than the Hall effect determination. This is probably due to the fact that the latter measures n in both the epilayer and GaN/sapphire interfacial region [D. C. Look and R. J. Molnar, Appl. Phys. Lett. 70, 3377 (1997); W. Götz et al., Appl. Phys. Lett. 72, 1214 (1998)] while the Raman signal is primarily from the epilayer.
Dynamics of capillary evaporation. I. Effect of morphology of hydrophobic surfaces
Authors: Luzar Alenka , Leung Kevin
Published: July 2002

Capillary evaporation (cavitation) has been suggested to be a possible source of long range interactions between mesoscopic hydrophobic surfaces. While evaporation is predicted by thermodynamics, little is known about its kinetics. Glauber dynamics Monte Carlo simulations of a lattice gas close to liquid–gas coexistence and confined between partially drying surfaces are used to model the effect of water confinement on the dynamics of surface-induced phase transition. Specifically, we examine how kinetics of induced evaporation changes as the texture of hydrophobic surfaces is varied. Our results provide guidelines for efficient manipulation of surface properties. We find that evaporation rates can be considerably slowed upon deposition of relatively small amount of hydrophilic coverage. The distribution of hydrophilic patches is however crucial, with the regularly spaced distribution being much more effective in slowing the formation of vapor tubes that trigger the evaporation process. To relate simulation rates to experimental ones, we also perform calculations using the mass-conserving Kawasaki algorithm. We predict evaporation time scales that range from hundreds of picoseconds in the case of mesoscopic surfaces ∼104 nm2 to tens of nanoseconds for smaller surfaces ∼2×102 nm2, when the two surfaces are ∼10 solvent layers apart. The present study demonstrates that cavitation is kinetically viable in real systems and should be considered in studies of processes at confined geometry.
Absolute entropies from molecular dynamics simulation trajectories
Authors: Schäfer Heiko , Mark Alan E. , van Gunsteren Wilfred F.
Published: July 2002

A heuristic formula for calculating absolute entropies from the covariance matrix of atom-positional fluctuations was extensively tested. Because of its heuristic nature, the results obtained are compared to analytical expressions for an ensemble of harmonic oscillators, for the ideal gas, and to numerical results obtained from the equation of state for the Lennard-Jones fluid as a means of validation of the approximate formula for the entropy. The formula yields rather accurate results. The removal of translational and rotational rigid body motion and the effect of the various fitting procedures involved are discussed for the more realistic system of a β-heptapeptide in solution.
Lattice and spin polarons in two dimensions
Authors: Zhao Y. , Chen G. H. , Yu L.
Published: July 2002

A variational approach is employed to compute the wave function of a single polaron for a two-dimensional Holstein Hamiltonian with arbitrary forms of linear particle–boson interactions and boson dispersion relations. The Toyozawa ansatz is utilized, and generalizations to multiple polarons are outlined. Applications are made to model superradiance in pseudoisocyanine bromide J-aggregates, and to calculate quasiparticle dispersion of an itinerant hole in a two-dimensional antiferromagnet.
A giant magnetoresistance sensor for high magnetic field measurements
Authors: Mancoff F. B. , Dunn J. Hunter , Clemens B. M. , White R. L.
Published: July 2002

We used giant magnetoresistance in a magnetic field sensor able to measure large fields of up to several kG. We deposit spin valves with a magnetic multilayer with perpendicular anisotropy as one ferromagnet and a material with in-plane anisotropy as the other ferromagnet. For magnetic fields along the film normal, the multilayer’s magnetization is fixed perpendicular while the magnetic layer with in-plane anisotropy is rotated towards out-of-plane magnetization. The device response is nearly linear with applied field and determines both the magnitude and sign of the field, making it attractive for measuring large magnetic fields.
Longitudinal and transverse polarizability of the conducting double sphere
Authors: Felderhof B. U. , Palaniappan D.
Published: July 2002

Exact expressions are derived for the longitudinal and transverse polarizability of two overlapping conducting spheres of arbitrary radii and with arbitrary angle of intersection. The transverse polarizability is expressed as a single integral, which can be performed if the angle of intersection is a rational fraction of π, i.e., the angle of intersection can be expressed as mπ/n, with m and n integers. The longitudinal polarizability can be expressed as a single integral if the two spheres are equal. For unequal spheres it involves two integrals, as well as the capacity, which itself was expressed as a single integral earlier. For equal spheres the second integral vanishes by symmetry, and the capacity is not needed. Both integrals can be performed if the angle of intersection is a rational fraction of π. In earlier work by the authors the longitudinal and transverse polarizability were found only for discrete angles of intersection π/n with n integer. Our result for the longitudinal polarizability of two equal overlapping conducting spheres shows that an earlier result of Radchik et al. [J. Appl. Phys. 76, 4827 (1994)] for overlapping dielectric spheres is incorrect.
Efficient radioluminescence of the Ce3+-doped Na–Gd phosphate glasses
Authors: Nikl M. , Nitsch K. , Mihokova E. , Solovieva N. , Mares J. A. , Fabeni P. , Pazzi G. P. , Martini M. , Vedda A. , Baccaro S.
Published: July 2002

Photo- and radioluminescence, decay kinetics, and thermoluminescence are reported for a set of Ce3+-doped phosphate glasses. The presence of Gd3+ ions in the glass host matrix at concentrations above 20% enhances the energy transfer towards the Ce3+ emission centers, which results in a remarkable enhancement of the radioluminescence light output.
New τ-dependent correlation functional combined with a modified Becke exchange
Authors: Proynov E. , Chermette H. , Salahub D. R.
Published: July 2002

A new correlation functional is derived within the Kohn–Sham (KS) Density Functional Theory (DFT) involving the electron kinetic energy density τ and the Laplacian of the electron density as key nonlocal variables. The derivation is based on a direct resolution of the adiabatic connection formula and using an analogy with the local thermodynamic approach in DFT, following the Lap3 theory developed previously. Compared to the latter, the new functional involves higher order τ-dependent energy terms in a form suggesting a possible resummation procedure that could be used for further development. It is combined with the nonlocal exchange functional of Becke, by modifying the latter in an empirical fashion to achieve better synchronization between the two energy components. The resulting exchange-correlation scheme (named “Bmτ1”) is validated on several test systems known as difficult for DFT, at least at the Local Spin Density and Generalized Gradient Approximation levels. The recent nonempirical hybrid scheme PBE1PBE (“PBE0”) is included in the comparative tests as a parameter-free benchmark for the hybrid HF-KS DFT approach. Improved results for relative energies, activation barriers and equilibrium geometries are obtained with the Bmτ1 functional, particularly concerning aromatic compounds, systems with weak hydrogen bonds, proton transfer processes and transition-metal carbonyls.
Relaxation and leakage current characteristics of Pb1−xLax(ZryTi1−y)1−x/4O3 thin films with various Ir-based top electrodes
Authors: Yoon Soon-Gil , Kingon A. I. , Kim Seung-Hyun
Published: July 2002

The dielectric relaxation and leakage current characteristics were studied for Pb1−xLax(ZryTi1−y)1−x/4O3 (PLZT) capacitors with various iridium-based top electrodes. The dielectric relaxation current behavior of PLZT capacitors obeys the well-known Curie–von Schweidler law independent of various Ir-based top electrodes including Pt and shows surprisingly little impact of various atmospheres such as Ar, O2, and H2. Electrical charge hopping, bulk effect, is the dominant mechanism of ac electric conduction which exhibits a linear relationship with frequency at room temperature. The true leakage current was separated definitively from the dielectric relaxation contributions. The PLZT capacitors with Pt or IrO2 top electrodes contacted with PLZT films show strong time dependence of true leakage current, resulting in consistence with space-charge influenced injection model. On the other hand, true leakage current of capacitors with Ir or IrO2/Ir top electrodes is independent of time, resulting in contradiction to the space-charge injection model. The IrPb, conducting phase, at interface between Ir top electrode and PLZT induces a steady state current behavior without the contribution of relaxation current. The second phase formed at interface modified the Schottky barrier height and increases the leakage current density.
Inverse scattering for a Schrödinger equation with energy dependent potential
Authors: van der Mee Cornelis , Pivovarchik Vjacheslav
Published: July 2002

In this article the inverse scattering problem of reconstructing the energy dependent potential iE2−m2 P(x)+Q(x) of a Schrödinger equation on the line from its reflection coefficients and bound state data (i.e., poles of the transmission coefficients and associated norming constants) is solved using the Marchenko integral equation approach.
Energy landscapes of conformationally constrained peptides
Authors: Levy Yaakov , Becker Oren M.
Published: July 2002

Conformation constraints are known to affect the flexibility and bioactivity of peptides. In this study we analyzed the effect of conformation constraints on the topography of the energy landscapes of three analogous hexapeptides. The three analogs vary in the degree of constraint imposed on their conformational motion: linear alanine hexapeptide with neutral terminals (Ala6), linear alanine hexapeptide with charged terminals (chrg-Ala6), and cyclic alanine hexapeptide (cyc-Ala6). It was found that significantly different energy landscapes characterize each of the three peptides, leading to different folding behaviors. Since all three analogs would be encoded by the same gene, these results suggest that nongenomic post-translational modifications may play an important role in determining the properties of proteins as well as of their folding pathways. In addition, the present study indicates that the complexity of those energy landscapes that are dominated by funnel topography can be captured by one or two reaction coordinates, such as conformational similarity to the native state. However, for more complex landscapes characterized by multiple basins such a description is insufficient. This study also shows that similar views of the landscape topography were obtained by principal component analysis (based only on local minima) and by topological mapping analysis (based on minima and barrier information). Both methods were able to resolve the complex landscape topographies for all three peptides.
Models of the current–voltage dependence of BaTiO3 with positive temperature coefficient of resistivity
Authors: Kolodiazhnyi T. , Petric A. , Johari G. P.
Published: July 2002

Current–voltage j(V) characteristics of barium titanate based positive temperature coefficient of resistivity thermistors were analyzed in terms of space charge limited current, trap-filled limited current, Frenkel–Poole, double-Schottky barrier, and small radii polaron models. It is shown that for the double-Schottky barrier model, the j(V) dependence does not obey the relation j∝exp (eV/2kT) at temperatures near or above the resistivity maximum. Instead, the j(V) characteristics show significant dependence on the interface charge. With increasing temperature, the region of nonlinear j(V) dependence is expected to shift towards higher voltages due to an increase in the equilibrium concentration of unoccupied interface states.
Nuclear Fukui function from coupled perturbed Hartree–Fock equations
Authors: Balawender Robert , Geerlings Paul
Published: July 2002

A calculation scheme of the nuclear Fukui function via a coupled perturbed Hartree–Fock approach is proposed avoiding the finite difference approach in DFT-based descriptors. Nucleophilic and electrophilic nuclear Fukui functions are compared with the numerical approximation for the nuclear Fukui function (FF) as the negative derivative of the chemical potential with respect to the atomic coordinates and as the derivative of the Helman–Feynman force with respect to the total number of electrons. The results for a set of diatomic molecules are shown. Analytical and numerical techniques do show a high correlation. Overall, values from both numerical methods are larger than those from the analytical one. The analytical results can be interpreted in terms of the character of the orbital involved during ionization or adding of electrons; the change in the equilibrium bond length upon ionization, which is positive for bonding orbitals and negative for antibinding orbitals is connected with the negative or positive values of the left-hand-side nuclear FF, respectively. The nucleophilic nuclear FF is positive for all cases except CO indicating a systematical increase of the bond length after addition of an electron.
Discrete aqueous solvent effects and possible attractive forces
Authors: Burak Y. , Andelman D.
Published: July 2002

We study discrete solvent effects on the interaction of two parallel charged surfaces in ionic aqueous solution. These effects are taken into account by adding a bilinear nonlocal term to the free energy of Poisson–Boltzmann theory. We study numerically the density profile of ions between the two plates, and the resulting interplate pressure. At large plate separations the two plates are decoupled and the ion distribution can be characterized by an effective Poisson–Boltzmann charge that is smaller than the nominal charge. The pressure is thus reduced relative to Poisson–Boltzmann predictions. At plate separations below ∼20 Å the pressure is modified considerably, due to the solvent mediated short-range attraction between ions in the system. For high surface charges this contribution can overcome the mean-field repulsion giving rise to a net attraction between the plates.
Simultaneous dual-color and dual-polarization imaging of single molecules
Authors: Cognet Laurent , Harms Gregory S. , Blab Gerhard A. , Lommerse Piet H. M. , Schmidt Thomas
Published: July 2002

We report the observation of single-molecule colocalization and quantitative fluorescence resonant energy transfer by simultaneously imaging the emission and polarization characteristics of two colocalized fluorophores using a simple optical design. The methodology was tested using the ligand-receptor system streptavidin, fluorescence labeled with the dye Cy5, and biotin labeled with tetramethylrhodamine. Discrimination of the two dyes permitted the observation of single-pair fluorescence resonant energy transfer with an efficiency of 89%. The multidimensional character of our fluorescence microscopy combined with the robustness of our design provides a simple method suitable to study biomolecular interactions on the single molecule level.
A first principle effective Hamiltonian for including nonadiabatic effects for H2+ and HD+
Authors: Schwenke David W.
Published: July 2002

We compute nonadiabatic corrections for all bound and long-lived quasi-bound vibrational levels of H2+ and HD+ for selected rotational levels. This is done using the Bunker and Moss formalism with the correction factors computed from ab initio wave functions. The electronic wave functions are expanded in terms of nuclear centered Gaussian basis functions. The agreement with accurate calculations is very good: for H2+, the root-mean-square error in the computed dissociation energies is 0.0006 cm−1, and, furthermore, most transition frequencies are predicted to within about 0.0001 cm−1. For HD+, the results are not quite as good due to the uncertainties in the adiabatic correction. This paves the way for using these techniques to accurately predict the nonadiabatic effects for more complicated molecules.
Electronic spectra of the chains HC2nH (n=8–13) in the gas phase
Authors: Pino Thomas , Ding Hongbin , Güthe Felix , Maier John P.
Published: July 2002

The intense Σu+1←X1Σg+ electronic transition of the chains HC2nH (n=8–13) has been recorded in the gas phase using resonant two color two photon ionization spectroscopy. The gas phase frequencies lie ∼1300 cm−1 to the blue of the values in a neon matrix. These polyynes appear to be linear in both the ground and the excited state and, as for the polyynes, spectral and theoretical evidence for large bond alternation in the ground state is found indicating localized π orbitals on the C≡C units. These species are the longest carbon chains observed spectroscopically in the gas phase up to now.
Domain patterns in ternary mixtures with different interfacial properties
Authors: Ma Yu-qiang
Published: July 2002

Numerical simulation of the phase separation kinetics in ternary mixtures with different interfacial properties is performed by means of the cell dynamics approach. We discuss the effects of the competition between the phase separation and the wetting phenomena on the system morphology and growth dynamics. Morphological changes and domain growth due to different interfacial properties including wetting and interface tension difference between two interfaces are observed in the late stage of spinodal decomposition.
Spectroscopy of the electron–phonon interaction in the layered two-dimensional dichalcogenide 1T–VSe2
Authors: Kamarchuk G. V. , Khotkevich A. V. , Bagatskiı̆ V. M. , Molinie P. , Leblanc A. , Faulques E.
Published: July 2002

The vibrational spectra of the layered two-dimensional transition-metal dichalcogenide 1T–VSe2 are investigated by the methods of point-contact and Raman spectroscopy. The measured point-contact spectra and the Raman scattering spectra contain features that coincide in position and relative intensity. Various regimes of current passage (ballistic, diffusive, and thermal) in 1T–VSe2/Cu point contacts are investigated. Both direct and inverse point-contact spectra are recorded, the former demonstrating the effect of the electron–phonon interaction in 1T–VSe2 and the latter reflecting the destruction of localization of the electronic states in the region near the contact, which leads to growth of the contact conductance. A study of the point contacts in the regime of small energy and momentum mean free paths of the electrons reveals nonlinearities caused by a phase transition of the material to a state with a charge density wave. The point-contact electron–phonon interaction function, previously unknown for this compound, is recovered from the data, and the values of the mean and rms phonon frequencies in 1T–VSe2 are calculated. An estimate of the Debye temperature is made.
Analysis of nonlinear dynamics on arbitrary geometries with the Virtual Cell
Authors: Schaff James C. , Slepchenko Boris M. , Choi Yung-Sze , Wagner John , Resasco Diana , Loew Leslie M.
Published: July 2002

The Virtual Cell is a modeling tool that allows biologists and theorists alike to specify and simulate cell-biophysical models on arbitrarily complex geometries. The framework combines an intuitive, front-end graphical user interface that runs in a web browser, sophisticated server-side numerical algorithms, a database for storage of models and simulation results, and flexible visualization capabilities. In this paper, we present an overview of the capabilities of the Virtual Cell, and, for the first time, the detailed mathematical formulation used as the basis for spatial computations. We also present summaries of two rather typical modeling projects, in order to illustrate the principal capabilities of the Virtual Cell.
Control of amorphous solid water morphology using molecular beams. II. Ballistic deposition simulations
Authors: Kimmel Greg A. , Dohnálek Z. , Stevenson K. P. , Smith R. Scott , Kay Bruce D.
Published: July 2002

Ballistic deposition simulations of thin film growth were performed. The results of the simulations are compared to experiments of N2 adsorption by porous amorphous solid water thin films. The simulations are in qualitative agreement with the experimental observations: The porosity of the thin films is controlled by using a collimated beam to vapor deposit the films. Films with normal or near normal growth angles (θ∼0°) are relatively dense and smooth. Films with larger growth angles are highly porous and the average pore size increases as the growth angle increases. The simulations indicate that for growth angles greater than ∼70°, adsorption into the largest pores is not possible leading to the experimentally observed maximum in N2 adsorption by porous amorphous solid water at θ=70°.
Enhanced Raman sensitivity using an actively stabilized external resonator
Authors: Taylor David J. , Glugla Manfred , Penzhorn Ralf-Dieter
Published: July 2002

An enhancement up to 250-fold in laser Raman signals for real-time gas analysis has been achieved within an actively stabilized external resonator (ASER), whose length is actively matched to the single-frequency excitation laser using the Pound–Drever technique. With the Raman cell present, enhancements up to 50-fold are achieved, and the resulting detection limit for hydrogen in ambient-pressure gas mixtures is about ten parts-per-million in a 1 min analysis period at unity signal-to-noise ratio. Based upon the recent development of a fiber-pumped Nd:YVO4 laser with single-frequency output exceeding 5 W at 532 nm, this highly sensitive instrument is applied to detection of tritiated gases, wherein the compactness and low heat of this laser head permit placing the entire optical system, including laser head, charge coupled Raman detector, and ASER, within the glove box necessary for secondary containment of tritium, thereby accomplishing a robust, highly sensitive Raman analytical system for hazardous substances.
Applications of the NRL tight-binding method to magnetic systems
Authors: Mehl M. J. , Papaconstantopoulos D. A. , Mazin I. I. , Bacalis N. C. , Pickett W. E.
Published: July 2002

The NRL developed tight-binding method has been very successful in describing the properties of nonmagnetic elemental metals and semiconductors with accuracy comparable to first-principles methods. In this article we discuss extensions of the method to magnetic systems. We first show that the method correctly predicts equilibrium ground state structures, elastic constants, and phonon frequencies in ferromagnetic iron. We also show how the magnetic calculations can be extended to noncollinear systems, focusing on the electronic behavior of iron.
Low-noise flux-gate magnetic-field sensors using ring- and rod-core geometries
Authors: Koch R. H. , Rozen J. R.
Published: July 2002

We have fabricated ring-core and single-domain rod-core flux-gate magnetic field sensors with 1/f noise levels at 1 Hz of 1.4 pT/Hz and 3.5 pT/Hz, respectively. These noise sensitivities were achieved by applying an electrical current through the core of the flux gate to magnetically bias the magnetic rotation of the core perpendicular to the easy-axis direction. We also found that in the rod-core sensor, the spatial correlation lengths of the magnetic fluctuations were 25 and 40 mm with and without the biasing current. The cross-power spectrum magnitude at 1 Hz was less than 200 fT/Hz.
Design and testing of high sensitivity microreceiver coil apparatus for nuclear magnetic resonance and imaging
Authors: Seeber D. A. , Cooper R. L. , Ciobanu L. , Pennington C. H.
Published: July 2002

We report the design and testing of a nuclear magnetic resonance (NMR) microcoil receiver apparatus, employing solenoidal microreceiver coils of dimensions of tens to hundreds of microns, using applied field of 9 T (proton resonance frequency 383 MHz). For the smallest receiver coils we attain sensitivity sufficient to observe proton NMR with signal to noise (S/N) one in a single scan applied to a ∼10 μm3 (10 fl) water sample, containing 7×1011 total proton spins. We also test the dependence of the S/N on important coil parameters, including coil composition and resistivity, turn spacing, and lead lengths.
Method for the determination of bulk and interface density of states in thin-film transistors
Authors: Lui O. K. B. , Tam S. W.-B. , Migliorato P. , Shimoda T.
Published: July 2002

In this article we present a method for the accurate determination of interface and bulk density of states (DOS) in thin-film transistors (TFTs), based on the combined analysis of transfer (ID–VGS) and capacitance–voltage characteristics. This analysis has achieved a number of results, eliminating sources of inaccuracies that are known to be present in other methods. A procedure for the determination of the electron and hole flatband conductances and bulk Fermi energy is demonstrated. A recursive procedure is employed to extract the bulk DOS directly from Poisson’s equation. The advantages of this method are the greater immunity to noise from the original data, the use of the complete Fermi function (no 0 K approximation), and the applicability to thin active layers. This method yields the interface state density spectrum as well as the bulk DOS. This information is very important for device design, process characterization, and modeling of TFTs.