bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic band structures of GaInAsP/InP vertically stacked multiple quantum wires with strain-compensating barriers
Authors: Haque Anisul , Yagi Hideki , Sano Takuya , Maruyama Takeo , Arai Shigehisa
Published: July 2003

Energy-band structures of compressively strained GaInAsP/InP quantum wires fabricated by etching and regrowth method have been calculated using an 8 band k⋅p theory including strain relaxation. The effects of strain-compensating barriers and vertically stacking multiple wire layers on band structures are investigated. It is found that due to the dependence of strain relaxation on the amount of strain compensation in barrier regions and on the number of wire layers in the vertical stack, unlike strained quantum films, the energy-band structures of strained quantum wires are dependent on these factors. Experimentally observed wire-width dependence of the large energy blueshift in vertically stacked multiple quantum-wire structures is accurately explained using our calculations without any fitting parameter. Additional broadening in the emission spectra due to vertically stacking multiple quantum wires is found to be negligible. Our results show that strain compensation in barrier layers may be used effectively as an additional parameter to optimize practical device design.
Theoretical investigation of ion conduction in three-layered ion-conductor systems: Derivation of the isothermal transient ionic current and frequency-dependent impedance
Authors: Frenning G. , Jonsson A. K. , Larsson A. L. , Strømme M.
Published: July 2003

Ion conduction in three-layered ion-conductor systems is considered. Explicit analytic expressions for the isothermal transient ionic current and frequency-dependent impedance are derived. The analysis includes migration, diffusion and space-charge effects as contributors to the ionic motion. The resulting model allows conduction parameters such as diffusion coefficients and ion concentrations in three different layers to be obtained from one single measurement, either in the time or in the frequency domain. The implications of one or more of the layers being mixed ionic/electronic conductors are discussed, and it is argued that the proposed model provides a useful description of the coupled ionic/electronic motion occurring in such a mixed-conductor system.
Tunneling criteria and breakdown for low resistive magnetic tunnel junctions
Authors: Oliver Bryan , He Qing , Tang Xuefei , Nowak Janusz
Published: July 2003

The tunneling criteria are evaluated using magnetic tunnel junctions having ultrathin alumina barrier with and without pinholes. It is shown that the tunneling criteria formulated by Rowell [J. Appl. Phys. 41, 1915 (1970)] clearly do not rule out the presence of pinholes in an ultrathin insulating barrier. In particular, the third criterion, a downward temperature dependence of resistance, cannot be used to decisively rule out the presence of pinholes. Examination of the breakdown mechanism will reveal the true nature of the barrier quality, and thus should be applied alongside the tunneling criteria to identify tunneling and the presence of pinholes.
Influence of boron-doped Si cap layer on the photoluminescence of β-FeSi2 particles embedded in Si matrix
Authors: Li Cheng , Ohtsuka T. , Ozawa Y. , Suemasu T. , Hasegawa F.
Published: July 2003

The influence of a boron-doped silicon cap layer on the photoluminescence (PL) of β-FeSi2 particles embedded in a silicon p–n junction is investigated. PL is found to improve significantly by optimizing silicon growth temperature and boron concentration. Surface morphology is also analyzed by atomic force microscopy. Dislocations and point defects are found to be generated by oxygen incorporated into the heavily boron-doped silicon layer during the 14 h of thermal annealing at 900 °C, and are suggested to be responsible for the quenching of the 1.53 μm PL.
Helical twisting power and scaled chiral indices
Authors: Neal Maureen P. , Solymosi Miklos , Wilson Mark R. , Earl David J.
Published: July 2003

Chirality of optically active liquid crystal molecules has become an important research topic and the subject of a number of theoretical and experimental studies. We present here the results of the application of a newly developed scaling method of a chiral index to a range of chiral molecules. Good agreement is found between the scaled chiral index and the helical twisting power for relatively rigid molecules. Two flexible TADDOL (α,α,α′,α′-tetraaryl-1,3-dioxolan-4,5dimethanol) molecules are studied to determine which conformations may give rise to their high experimental helical twisting powers. A variety of links between the moment of inertia tensor, the dihedral angles, the scaled chiral indices, the minimum energy of the optimized geometry and the experimental helical twisting power are discussed. The scaled chiral and steric indices and dihedral angles are promising as predictors of experimental helical twisting power, in particular for relatively rigid molecules, in cases where all the relevant interactions are determined by the molecular structure.
Reorientation dynamics in a nematic liquid crystal studied by fast field cycling nuclear magnetic resonance
Authors: Acosta R. H. , Pusiol D. J.
Published: July 2003

We propose a novel approach for studying the reorientation process of nematic liquid crystals in the presence of a variable external magnetic field based on the fast field cycling nuclear magnetic resonance (NMR) technique. Thermotropic 4-cyano-4′-octylalkylbiphenyl was aligned as a nematic monodomain with an external magnetic field that is suddenly turned off, inspection of the NMR line shape shows that the nematic monodomain disorders in nematic domains that have a continuous distribution of local orientations. The dynamics of the orientation from a mesoscopic-disordered system were determined by the application of a constant magnetic field. Both experiments were done as a function of temperature for the whole nematic phase and as a function of the intensity of the external magnetic field. The technique allows a clear identification of two regimes, a high frequency one where the magnetic torque governs the dynamics of reorientation and a low frequency one where thermal fluctuations are responsible for the dynamics.
Free energy calculation from steered molecular dynamics simulations using Jarzynski’s equality
Authors: Park Sanghyun , Khalili-Araghi Fatemeh , Tajkhorshid Emad , Schulten Klaus
Published: July 2003

Jarzynski’s equality is applied to free energy calculations from steered molecular dynamics simulations of biomolecules. The helix-coil transition of deca-alanine in vacuum is used as an example. With about ten trajectories sampled, the second order cumulant expansion, among the various averaging schemes examined, yields the most accurate estimates. We compare umbrella sampling and the present method, and find that their efficiencies are comparable.
Partition thermodynamics of ionic surfactants between phosphatidylcholine vesicle and water phases
Authors: Chu Shin-Chi , Hung Chia-Hui , Wang Shun-Cheng , Tsao Heng-Kwong
Published: July 2003

The partition of ionic surfactants (sodium alkyl sulfate and alkyl trimethyl ammonium bromide) between phosphatidylcholine vesicles and aqueous phase is investigated by simple conductometry under different temperatures. The experimental results can be well represented by the proposed regular solution theory and the thermodynamic parameters satisfy the thermodynamic consistency. The deviation from ideal partition is manifested through the effective interaction energy between lipid and surfactant wb, which is O(kT) large. It is found that wb rises as the alkyl chain is decreased for a specified head group. This is attributed to significant mismatch of chain lengths between surfactant and lipid molecules. The partition coefficient K declines with increasing temperature. The energy barrier from bilayer to aqueous phase, Δμ/kT∝ln K, is in the range of 16–26 kJ/mol. As the alkyl chain length is decreased for a given head group, Δμ is lowered by 1.3–1.5 kJ/mol per methylene group. Two independent analyses are employed to confirm this result. Using the thermodynamic parameters determined from experiments, the internal energy, entropy, and free energy of the partition process can be derived. Partition is essentially driven by the internal energy gain. The solubilizing ability, which is represented by the maximum surfactant–lipid ratio in the bilayer, Reb also decreases in accord with the K parameter. It is because the change in temperature influences the surfactant incorporation into the bilayer more than the formation of micelles.
Surface tension and vapor–liquid phase coexistence of the square-well fluid
Authors: Singh Jayant K. , Kofke David A. , Errington Jeffrey R.
Published: July 2003

Vapor–liquid interfacial tension of square-well (SW) fluids is calculated using three different methods viz., molecular dynamics (MD) with collision-based virial evaluation, Monte Carlo with virial computed by volume perturbation, and Binder’s density-distribution method in conjunction with grand-canonical transition-matrix Monte Carlo (GC-TMMC). Three values of the SW attractive well range parameter were studied: λ=1.5, 1.75, and 2.0, respectively. The results from MD and GC-TMMC methods are in very good mutual agreement, while the volume-perturbation method yields data of unacceptable quality. The results are compared with predictions from the statistical associating fluid theory (SAFT), and SAFT is shown to give a good estimate for the systems studied. Liquid and vapor coexistence densities and saturation pressure are determined from analysis of GC-TMMC data and the results are found to agree very well with the established literature data.
Shear Alfvén continua in stellarators
Authors: Spong D. A. , Sanchez R. , Weller A.
Published: July 2003

Shear Alfvén continua have been calculated for stellarators over a range of shapes and aspect ratios as a first step toward understanding Alfvén instability induced fast ion losses in such systems and possible means for minimizing these losses. Stellarators introduce strong poloidal/toroidal couplings in both |B| and the gρρ metric coefficient that can induce new continuum gap structures not present in axisymmetric tokamaks. Low field period (Nfp=2–3), low aspect ratio devices result in strongly coupled toroidal mode families (n=±n0,±n0±Nfp,±n0±2Nfp, etc.) that lead to helically coupled Alfvén gaps at lower frequencies and with wider gap structures than are the case for larger aspect ratio, higher field period stellarator devices.
Mechanism for anomalously high voltages in high-pressure dc microdischarge mixtures of He, Ne, and Xe
Authors: Ganter R. , Cappelli M.
Published: July 2003

Electrical characteristics in the microdischarge experiments of Postel and Cappelli [J. Appl. Phys. 89, 4719 (2001)], show that voltages are higher in mixtures of helium and xenon than in pure xenon in the abnormal glow discharge regime. While originally attributed to the possible formation of heterodimer ions which have lower secondary emission coefficients, we show here that we can reproduce the general trends seen experimentally with a one-dimensional fluid model by taking into account the charge exchange process, He++Xe→He+Xe+. The reaction rate coefficient used (10−9 cm3 s−1) corresponds to that for ions with energy of around 1 eV, which is not an uncommon energy for ions in the cathode sheath of strongly collisional microdischarges. Experimental results are also presented for mixtures of He, Xe, and Ne, at 50 Torr and 250 Torr.
Ferromagnetic resonance in FeCoNi electroplated wires
Authors: Garcı́a-Miquel H. , Bhagat S. M. , Lofland S. E. , Kurlyandskaya G. V. , Svalov A. V.
Published: July 2003

We have investigated the microwave properties (ferromagnetic resonance and ferromagnetic antiresonance) of FeCoNi magnetic tubes created by electroplating on CuBe wire. Important parameters such as the g factor, magnetization, anisotropy field, and damping parameter were obtained from the measurements. One sample, prepared by a method which entails rf-sputtering deposition of an additional FeNi layer, shows a clear ferromagnetic antiresonance.
Symmetry-related conditions for interface localized state existence in coupled superlattices
Authors: Kłos Jarosław
Published: July 2003

Localized electronic states in coupled superlattices are investigated. A special case of two identical semi-infinite superlattices coupled by a barrier spacer in their symmetry point (i.e., in the barrier/well center) is considered. Conditions for the existence of interface localized states, compatible with the system symmetry, are found. The theory is applied to numerical calculations of the interface localized states in AlxGa1−xAs coupled superlattices.
Physical characteristics and infrared fluorescence properties of sol–gel derived Er3+–Yb3+ codoped TiO2
Authors: Ting Chu-Chi , Chen San-Yuan , Lee Hsin-Yi
Published: July 2003

Er 3+ – Yb 3+ codoped TiO2 films were prepared on fused silica by sol–gel processes. The Yb3+ codoping effect on the physical characteristics and ∼1.54 μm photoluminescence (PL) properties of Er3+-doped TiO2 films was investigated. Maximum ∼1.54 μm PL intensity occurs in Er3+ (5 mol %)–Yb3+(30 mol %) codoped TiO2 samples annealed at 700 °C. However, when the concentration of Yb3+ ions is more than 30 mol %, the back energy transfer effect from Er3+ to Yb3+ will deteriorate the ∼1.54 μm PL efficiency. Extended x-ray absorption fine structure measurements show that the average spatial distance between Er3+ ions is slightly decreased due to the partial substitution of Yb3+ for Er3+ ions in the local structure. The Yb3+ ion in the Er3+–Yb3+ codoped TiO2 samples not only plays the role of disperser but is also a sensitizer of the Er3+ ion. This dual effect leads to larger PL intensity in the Er3+–Yb3+ codoped TiO2 system in comparison with Er3+–Y3+ codoped TiO2 samples. Compared with SiO2 films with Er3+ (5 mol %)–Yb3+ (30 mol %) codoped and annealed at optimal temperature of 985 °C, the Er3+–Yb3+ codoped TiO2 film obtains better PL properties at lower annealing temperature.
Characterization of selective quantum well intermixing in 1.3 μm GaInNAs/GaAs structures
Authors: Sun H. D. , Macaluso R. , Dawson M. D. , Robert F. , Bryce A. C. , Marsh J. H. , Riechert H.
Published: July 2003

Rapid thermal annealing combined with SiO2 caps deposited on the surface of samples by different techniques is used to selectively disorder 1.3 μm GaInNAs/GaAs multiquantum wells which have been preannealed in situ to the stage of blueshift saturation. After thermal annealing under specific conditions, a shift in band gap of over 170 meV has been obtained in sputtered SiO2-capped samples, while uncapped and plasma enhanced chemical vapor deposited SiO2-capped samples demonstrated a negligible shift. Quantum well intermixing in sputtered SiO2-capped samples originates from enhanced compositional interdiffusion due to the generation of point defects by ion bombardment during the sputtering process. Secondary ion mass spectrometry has confirmed that the enhanced blueshift was caused by the interdiffusion of group III atoms (In and Ga) between the quantum wells and barriers. Detailed photoluminescence and excitation spectroscopy were performed to study the optical properties of both intermixed and nonintermixed samples.
Medium permittivity bismuth zinc niobate thin film capacitors
Authors: Thayer R. L. , Randall C. A. , Trolier-McKinstry S.
Published: July 2003

Thin films were fabricated via metalorganic decomposition methods with three compositions: Bi1.5Zn1.0Nb1.5O7, Bi1.5Zn0.5Nb1.5O6.5, and Bi2Zn2/3Nb4/3O7. The Bi1.5Zn0.5Nb1.5O6.5 composition is a low temperature phase with the cubic pyrochlore structure. This phase may undergo a peritectoid decomposition at 700 °C to the high temperature phases of Bi1.5Zn1.0Nb1.5O7 and BiNbO4. Both the Bi1.5Zn1.0Nb1.5O7 and Bi1.5Zn0.5Nb1.5O6.5 cubic pyrochlores show medium room temperature permittivities (150 and 180, respectively) with negative temperature coefficients of capacitance and a low temperature dielectric relaxation. Both Bi1.5Zn1.0Nb1.5O7 and Bi1.5Zn0.5Nb1.5O6.5 films showed substantial dielectric tunability with electric field (>30%). The field dependence of the dielectric permittivity of the zirconolite structured Bi2Zn2/3Nb4/3O7 thin films demonstrates an unusual field induced transition at high field ∼2 MV/cm, with a maximum tunability of 20%. It is hypothesized that this is associated with field forced ordering of the Zn atoms between two closely spaced, partially occupied sites.
Subpeak structure of the lowest-frequency E-symmetry transverse optical phonon in Ba-doped PbTiO3 single crystals
Authors: Cho Seong M. , Park Jung H. , Jang Hyun M.
Published: July 2003

Polarized Raman scattering study of Ba-doped PbTiO3 single crystals indicated that, for a limited temperature range between 650 and 750 K, the line shape of the E(1TO) “soft” mode could not be described by a single damped harmonic oscillator but was adequately fitted by two independent oscillators. The line-shape fitting of the E(1TO) spectrum further indicated a gradual intensity inversion between the two subpeaks with increasing temperature. For temperatures below the crossover point, the subpeak with a higher mode frequency dominated the line shape. Contrary to this, the line shape was predominantly described by the subpeak with a lower frequency for temperatures above the crossover point. The subpeak structure becomes progressively evident with increased content of barium (Ba) dopant. We examined possible origins of the observed subpeak structure with the crossover in the integrated intensity and attributed this anomalous behavior to localized defects caused by the Ba impurity.
Effects of plasma nitridation of Al2O3 interlayer on thermal stability, fixed charge density, and interfacial trap states of HfO2 gate dielectric films grown by atomic layer deposition
Authors: Park Hong Bae , Cho Moonju , Park Jaehoo , Hwang Cheol Seong , Lee Jong-Cheol , Oh Se-Jung
Published: July 2003

HfO 2 / Al 2 O 3 gate dielectric thin-film stacks were deposited on Si wafers using an atomic-layer-deposition technique. A 2.3-nm-thick Al2O3 interlayer was grown at 450 °C using Al(CH3)3 and O3, and (4–5)-nm-thick HfO2 films were grown at 400 °C using HfCl4 and H2O as precursors. Because the Al2O3 interlayer was not an effective Si-diffusion barrier layer on its own, the Al2O3 interlayer was plasma treated under an NH3 atmosphere at 790 °C for 40 s. The plasma treatment increased the Al2O3 interlayer thickness by approximately 1 nm and decreased the overall capacitance density of the film stack. However, the thermal stability of the capacitance density was greatly improved as a result of the treatment resulting in a similar capacitance density after postannealing at 800 °C in a N2 atmosphere to that observed in nontreated samples after similar annealing conditions. The signs and amount of the fixed charges of the Al2O3/Si and HfO2/Al2O3 interfaces greatly depend on the nitridation of the Al2O3 layer. Furthermore, the interface trap density of the sample with the plasma-treated Al2O3 interlayer decreased from that of the nontreated sample by almost one order of magnitude.
Progress in relativistic gravitational theory using the inverse scattering method
Authors: Neugebauer G. , Meinel R.
Published: July 2003

The increasing interest in compact astrophysical objects (neutron stars, binaries, galactic black holes) has stimulated the search for rigorous methods, which allow a systematic general relativistic description of such objects. This article is meant to demonstrate the use of the inverse scattering method, which allows, in particular cases, the treatment of rotating body problems. The idea is to replace the investigation of the matter region of a rotating body by the formulation of boundary values along the surface of the body. In this way we construct solutions describing rotating black holes and disks of dust (“galaxies”). Physical properties of the solutions and consequences of the approach are discussed. Among other things, the balance problem for two black holes can be tackled.
Sphalerons, spectral flow, and anomalies
Authors: Klinkhamer Frans R. , Rupp Christian
Published: July 2003

The topology of configuration space may be responsible in part for the existence of sphalerons. Here, sphalerons are defined to be static but unstable finite-energy solutions of the classical field equations. Another manifestation of the nontrivial topology of configuration space is the phenomenon of spectral flow for the eigenvalues of the Dirac Hamiltonian. The spectral flow, in turn, is related to the possible existence of anomalies. In this review, the interconnection of these topics is illustrated for three particular sphalerons of SU(2) Yang–Mills–Higgs theory.
Density-functional theory calculations with correct long-range potentials
Authors: Wu Qin , Ayers Paul W. , Yang Weitao
Published: July 2003

A variational method for forcing the exchange-correlation potential in density-functional theory (DFT) to have the correct asymptotic decay is developed. The resulting exchange-correlation potentials are much improved while the total energies remain essentially the same, compared with conventional density-functional theory calculations. The highest occupied orbital energies from the asymptotically corrected exchange-correlation potentials are found to provide significantly more accurate approximations to the ionization potential (for a neutral molecule) and the electron affinity (for an anion) than those from conventional calculations, although the results are usually inferior to direct methods by computing energy differences. Extending recent results from exchange-only DFT, we show that exact exchange-correlation potential is nonuniform asymptotically. Correcting the asymptotic decay of approximate exchange-correlation potentials towards the exact functional form binds the highest occupied orbitals for atomic and molecular anions, which supports the use of DFT calculations for negatively charged molecular species. With this technique, even hybrid functionals have local exchange-correlation potentials, effectively removing the largest objection to including these functionals in the panoply of Kohn–Sham DFT methods.
The exchange-correlation potential in Kohn–Sham nuclear magnetic resonance shielding calculations
Authors: Keal Thomas W. , Tozer David J.
Published: July 2003

A simple gradient correction to the local density approximation functional is proposed, which improves the structure of the exchange-correlation potential. The optimized generalized gradient approximation (GGA) functional provides uncoupled isotropic and anisotropic nuclear magnetic resonance shielding constants that are 2–3 times more accurate than those of commonly used GGAs, for a series of challenging molecules involving first- and second-row atoms; the results are competitive with those of ab initio wave function methods. A correlation is observed between the lowest occupied-virtual eigenvalue difference and the shielding accuracy. Magnetizabilities are also improved. The performance of the functional for structural and energetic predictions is investigated. These properties can be improved by relaxing the uniform electron gas condition, with no degradation in shielding quality. Atomization energies, ionization potentials, and molecular bond lengths are then comparable to those of other GGA functionals, although total energies are very poor.
The generalized active space concept for the relativistic treatment of electron correlation. II. Large-scale configuration interaction implementation based on relativistic 2- and 4-spinors and its application
Authors: Fleig Timo , Olsen Jeppe , Visscher Lucas
Published: July 2003

The extension of a relativistic double group configuration interaction (CI) formalism to the use of 2- and 4-spinors is presented. We first elucidate the theoretical aspects of the formalism that is needed to work with spinors that are optimized with a Hartree–Fock scheme that includes spin–orbit coupling. We then describe a new general implementation for the computation of sigma vectors and n-particle density matrices that occur in direct CI algorithms. Sample calculations of the spin–orbit splitting in atoms with one particle or hole in an otherwise closed shell configuration l1 (l=1,2,3) and molecules containing such atoms illustrate the advantage of treating this effect in a basis of true spinors rather than in a basis of scalar relativistic orbitals as is conventionally done.
Homogeneous nucleation rates for D2O in a supersonic Laval nozzle
Authors: Khan Amjad , Heath Christopher H. , Dieregsweiler Uta M. , Wyslouzil Barbara E. , Strey Reinhard
Published: July 2003

We measured the homogeneous nucleation rates of D2O in a supersonic nozzle. Small angle neutron scattering (SANS) experiments characterized the nanodroplet aerosols and yielded the number density N of particles formed. In these new SANS measurements the number densities were in the range of 4×1011<N/cm−3<2×1012. We then determined the characteristic time interval corresponding to the peak nucleation rate ΔtJmax from conventional pressure profile measurements in the nozzle. The sensitive time is typically (11±3) μs. Combining these two pieces of information we obtained the nucleation rate J=NNZ/ΔtJmax, where NNZ is the number density of the aerosol in the nucleation zone. In this nozzle, the peak nucleation rate ranges between 6×1016 and 1.2×1017 cm−3 s−1 and is quite insensitive to the initial conditions of the expansion.
Alfvén waves in gyrokinetic plasmas
Authors: Lee W. W. , Qin H.
Published: July 2003

A brief comparison of the properties of Alfvén waves that are based on the gyrokinetic description with those derived from the magnetohydrodynamics (MHD) equations is presented. The critical differences between these two approaches are the treatment of the ion polarization effects. As such, the compressional Alfvén waves in a gyrokinetic plasma can be eliminated through frequency ordering, whereas geometric simplifications are needed to decouple the shear Alfvén waves from the compressional Alfvén waves within the context of MHD. Theoretical and numerical procedures of using gyrokinetic particle simulation for studying microturbulence and kinetic-MHD physics including finite Larmor radius effects are also presented.
Operating regime for a backward Raman laser amplifier in preformed plasma
Authors: Clark Daniel S. , Fisch Nathaniel J.
Published: July 2003

A critical issue in the generation of ultraintense, ultrashort laser pulses by backward Raman scattering in plasma is the stability of the pumping pulse to premature backscatter from thermal fluctuations in the preformed plasma. Malkin et al. [Phys. Rev. Lett. 84, 1208 (2000)] demonstrated that density gradients may be used to detune the Raman resonance in such a way that backscatter of the pump from thermal noise can be stabilized while useful Raman amplification persists. Here plasma conditions for which the pump is stable to thermal Raman backscatter in a homogeneous plasma and the density gradients necessary to stabilize the pump for other plasma conditions are quantified. Other ancillary constraints on a Raman amplifier are also considered to determine a specific region in the Te–ne plane where Raman amplification is feasible. By determining an operability region, the degree of uncertainty in density or temperature tolerable for an experimental Raman amplifier is thus also identified. The fluid code F3D [R. L. Berger et al., Phys. Plasmas 5, 4337 (1998)], which includes the effects of thermal fluctuations, is used to verify these analytic estimates.
Effect of the radial thickness of electron beams on mode coupling and stability in gyrotrons
Authors: Nusinovich G. S. , Sinitsyn O. V. , Yeddulla M. , Velikovich L. , Antonsen T. M. , Vlasov A. N. , Cauffman S. , Felch K.
Published: July 2003

In the development of gyrotrons for controlled fusion experiments, two of the main goals are the increase of the radiated power and the wavelength shortening. Both tendencies lead to tubes in which the radial thickness of an electron beam may play a significant role not only in the efficiency degradation but also in the mode interaction. The second of those effects is analyzed in the present paper. The paper starts with a simplified analytical consideration of the effect of beam thickness on the mode coupling and stability in gyrotrons with soft and hard self-excitation. Then, this analytical consideration is illustrated by some examples which represent the results obtained with the use of the time-dependent, self-consistent code MAGY [M. Botton et al., IEEE Trans. Plasma Sci. 26, 882 (1998)].
Compact room-temperature induction magnetometer with superconducting quantum interference device level field sensitivity
Authors: Prance R. J. , Clark T. D. , Prance H.
Published: July 2003

In this article, we describe significant developments in the design and construction of robust and compact room-temperature induction magnetometers with magnetic field sensitivities comparable to those usually associated only with superconducting quantum interference device (SQUID) magnetometers operating at cryogenic temperatures. In this system, we make use of both temperature independent amorphous magnetic core materials and ultralow noise signal processing electronics to achieve a spot noise figure of 14 fT/Hz at 300 Hz measured in a specially constructed, very low field environment. We present results over an operating bandwidth of 30 mHz to 3 kHz and show that linearity is preserved over almost six decades in an applied magnetic field amplitude. We compare the spectral noise of our systems with recently published data on SQUID magnetometers.
Wavelength reference for optical telecommunications based on a temperature-tunable silicon etalon
Authors: Tuominen Jesse , Niemi Tapio , Ludvigsen Hanne
Published: July 2003

A wavelength reference based on a temperature-tunable silicon Fabry–Perot etalon is presented. The device provides a compact and cost-effective solution for calibration of the wavelength scale from 1.3 μm to 1.7 μm. The reference is fully automated, needs a single characterization, and operates without an absolute wavelength reference. The performance of the device was estimated by comparing its fringes against acetylene absorption lines. The wavelength differences measured were found to be less than 1 pm.
Development of a photoionization spectrometer for accurate ion yield measurements from gaseous fullerenes
Authors: Mori T. , Kou J. , Ono M. , Haruyama Y. , Kubozono Y. , Mistuke K.
Published: July 2003

A photoionization spectrometer has been developed for measuring the ion yields for fullerenes in the photon energy range of 23–200 eV. Gaseous fullerenes were supplied from a high-temperature oven, ionized by irradiation of monochromatized synchrotron radiation, and detected after analysis with a time-of-flight mass spectrometer. The fluxes of the synchrotron radiation and fullerene beams were monitored concurrently with the acquisition of the ion signal counts in order to obtain reliable photoionization efficiency curves. The performance of the apparatus was examined by measuring the efficiency curve of C60+ produced from C60. The spectrum demonstrated better statistics than the previous results in the same photon energy region. Three distinct features were newly observed in the higher-energy side of the prominent resonance at ∼20 eV.
Simultaneous atomic force microscopy measurement of topography and contact resistance of metal films and carbon nanotubes
Authors: Stadermann M. , Grube H. , Boland J. J. , Papadakis S. J. , Falvo M. R. , Superfine R. , Washburn S.
Published: July 2003

We present a quartz tuning-fork-based atomic force microscopy (AFM) setup that is capable of mapping the surface contact resistance while scanning topography. The tuning-fork setup allows us to use etched Pt/Ir tips, which have higher durability and better conductivity than probes used in earlier AFM conductance measurements. The performance of the method is demonstrated with contact resistance measurements of gold lines on silicon dioxide and carbon nanotubes on graphite.
Long-range charge-transfer excited states in time-dependent density functional theory require non-local exchange
Authors: Dreuw Andreas , Weisman Jennifer L. , Head-Gordon Martin
Published: July 2003

The electrostatic attraction between the separated charges in long-range excited charge-transfer states originates from the non-local Hartree-Fock exchange potential and is, thus, a non-local property. Present-day time-dependent density functional theory employing local exchange-correlation functionals does not capture this effect and therefore fails to describe charge-transfer excited states correctly. A hybrid method that is qualitatively correct is described.
The effect of the order of autocatalysis for reaction fronts in vertical slabs
Authors: Coroian Dan I. , Vasquez Desiderio A.
Published: July 2003

The coupling of molecular diffusion and chemical autocatalysis leads to propagating chemical fronts. The specific type of autocatalysis determines the thickness and speed of the front. Chemical fronts in liquids separate reacted from unreacted fluids with a corresponding mass density gradient. These differences may lead to convective flow which enhances the speed and determines the curvature of the front. The transition to convection is determined not only by the spatial geometry and density differences, but also by the type of chemical reaction. We determine the transition to convection for chemical fronts with different autocatalytic reactions of different order. We study fronts propagating in porous media, in viscous fluids, and fluids confined in a vertical slab. We compare these results with the results based on a thin front approximation using an eikonal relation.
Raman spectroscopy of template grown single wall carbon nanotubes in zeolite crystals
Authors: Hulman Martin , Kuzmany Hans , Dubay Orest , Kresse Georg , Li Ling , Tang Z. K.
Published: July 2003

Single wall carbon nanotubes with diameter 0.4 nm grown in the channels of AlPO4-5 crystals were studied by Raman spectroscopy and ab initio density functional calculations. In the experiment up to 19 different laser lines were used to characterize vibrational properties. Spectra depend strongly on the energy of the laser line used for excitation. Even though the observed Raman spectra were very rich on lines only two types of nanotubes with different chiralities, (5,0) and (4,2), were found to be responsible for the observed response. The frequencies of the radial breathing modes were reliably assigned. Even though the (5,0) is metallic, the A1g mode does not couple to the electronic continuum and the Peierls-type mechanism does not shift the mode toward lower frequencies. A strong response was also observed for frequencies around 1250 cm−1. The positions of two peaks assigned to the (5,0) do not depend on the laser energy whereas only one peak was observed for the (4,2) nanotube. Its frequency shifts with the laser energy like the D line of large diameter nanotubes, but the rate of the shift is only one half of the value known for the latter. These unexpected results could be traced back to the phonon dispersion of the narrow tubes.
Radiation absorption and reflection by a plasma with cold and hot electrons
Authors: Ferrante G. , Zarcone M. , Uryupina D. S. , Uryupin S. A.
Published: July 2003

Reflection and collisionless absorption of a test wave by a plasma, with a sharp boundary and containing a small amount of hot electrons, besides the bulk of more cold electrons, are investigated. It is established that, in the high-frequency skin-effect regime, the conditions are possible, when the absorption coefficient is basically determined by the hot electrons. On the contrary, in the transition to the anomalous skin-effect, the absorption by the cold electrons becomes dominant. In both the cases of high-frequency and anomalous skin-effect it is found that the reflected wave phase shift is determined by the cold electron bulk.
On the distribution of magnetic null points above the solar photosphere
Authors: Longcope D. W. , Brown D. S. , Priest E. R.
Published: July 2003

Many theories predict magnetic energy dissipation at locations, called null points, where the magnetic field vanishes. In several astrophysical contexts, most notably the solar corona, energy is released within a low-β magnetic field anchored to a lower boundary, the photosphere. A general expression is derived for the distribution of magnetic null points in potential magnetic fields anchored to a random, homogeneous distribution of field on the lower boundary. For all such fields the null point density decreases with height and scales with the inverse cube of the field’s characteristic length. For photospheric fields which appear unipolar at the largest scales the nulls are confined to a narrow layer. The results are applied to models of the quiet Sun whose photospheric field consists of discrete sources of mixed polarity. The number of coronal nulls depends on the degree of imbalance between positive and negative sources. Numerical experiments reveal that the greatest column density of null points occurs when ∼20% of the sources are of the minority sign. Were the coronal energy dissipation to occur at magnetic null points this result predicts an observable relationship between flux imbalance and the amplitude and distribution of dissipation.
Electrical characteristics of metal/semiconductor nanocontacts using light emission in a scanning tunneling microscope
Authors: Maurel C. , Coratger R. , Ajustron F. , Beauvillain J. , Gerard P.
Published: July 2003

Light emission from the tunneling junction of a scanning tunneling microscope (STM) has been used to obtain the electrical characteristics of small three-dimensional gold islands deposited on flat MoS2 surfaces. It is shown that these nanocontacts behave as poor rectifying junctions whose properties are altered by leakage currents. These properties are similar to those generally observed on point contacts where a sharp STM tip is brought in contact with a semiconductor.
Linewidth enhancement factor and chirp in quantum dot lasers
Authors: Oksanen Jani , Tulkki Jukka
Published: July 2003

We have made a comparative study of the linewidth enhancement factor (LEF) and chirp in quantum dot (QDL’s) and quantum well lasers (QWL’s). The simulations are based on the quasiequilibrium approximation and on semiempirical transition energies and amplitudes of InGaAs quantum pyramid structures. We have accounted for the carriers confined in the active material as well as for the carriers in all the other material layers. It is found that in the quasiequilibrium approximation inhomogeneous broadening leads to asymmetric population of the quantum dot ground state. If the QDL is operated at the gain maximum, the asymmetry leads to nonzero chirp even for a single bound resonance state located at a large distance from other resonances. Our calculations show that, by detuning the laser emission to ∼15 nm shorter wavelengths with a frequency selective cavity and by tailoring the resonance energies and inhomogeneous broadening, the LEF and chirp of a QDL can be made very small. This detuning does not add a substantial penalty to the efficiency of the laser. For QWL’s, a similar reduction of chirp is generally not feasible due to the fundamentally different density of states. Therefore QDL’s have an important advantage over QWL’s as directly modulated light sources in applications where the stability of the emission wavelength is critical.
Ultrabroad-band phase matching in two recently grown nonlinear optical crystals for the generation of tunable ultrafast laser radiation by type-I noncollinear optical parametric amplification
Authors: Kumbhakar P. , Kobayashi T.
Published: July 2003

It was found that the generation of ultrabroad-band femtosecond pulsed laser radiation is possible in two recently grown nonlinear optical crystals, K2Al2B2O7 (KABO) and CsLiB6O10 (CLBO), by employing a type-I noncollinear optical parametric amplification (NOPA) scheme, pumped by Ti:sapphire second harmonic (395 nm) radiation. The phase-matching bandwidths of the process are as large as 152 and 164 THz in KABO and CLBO crystals, with the values of the noncollinear angles (α) between pump input (considered monochromatic) and seed pulses input are 3.3° and 3.0°, respectively. The phase-matching bandwidth calculated for the same type-I NOPA in BBO crystal is 157 THz with α=3.7°. In addition to the large phase-matching bandwidth in CLBO, the major advantages of this crystal are that it can easily be grown to large sizes and it also has a higher threshold against laser damage. The most attractive properties of KABO crystal are that it is nonhygroscopic as well as mechanically robust and easy to cut and polish. Having moderate birefringence the angular and spectral bandwidths of both CLBO and KABO crystals are larger than that of BBO crystal and the deleterious walk-off angles are smaller than that of BBO. The walk-off angles at the pump wavelength of NOPA with CLBO, KABO, and BBO crystals are about 2.1°, 2.7°, and 4.0°, respectively. Here we present the nonlinear optical parametric properties of CLBO and KABO crystals for the generation of ultrabroad band femtosecond laser radiation by the NOPA technique, since we know of no such report so far in the literature.
Thermal poling and ultraviolet erasure characteristics of type-III ultraviolet-grade fused silica and application to periodic poling on planar substrates
Authors: Chen Huai-Yi , Sue Jin-Sheng , Lin Yi-Hsuan , Tsai Chin-Shun , Wu Pao-Tung , Chao Shiuh
Published: July 2003

A periodically poled structure on type-III ultraviolet (UV)-grade planar fused silica was realized by periodic UV erasure of second-order nonlinearity of the thermally poled fused silica plate. Poling and UV-erasure characteristics are given. The second-order nonlinearity profile of thermally poled fused silica was found to correlate with a buried Gaussian function. The nonlinear depth, calculated by curve fitting Maker fringes assuming a step-like nonlinear profile, corresponded to the location of the 1/e peak in the buried Gaussian function. The etching rate in hydrofluoric acid of the UV-bleached thermally poled fused silica was found to be the same as that of unpoled fused silica along the direction of the poling field but was larger along the direction perpendicular to the poling field.
Influence of indium on the electronic states in GaInNAs/GaAs quantum well structures
Authors: Hetterich M. , Grau A. , Egorov A. Yu. , Riechert H.
Published: July 2003

We use room-temperature photoreflectance spectroscopy to investigate the influence of indium on the electronic structure of Ga1−xInxNyAs1−y/GaAs multiple quantum wells. To fit our experimental data, a semiempirical theoretical model based on the band anticrossing Hamiltonian is successfully applied. Thus, we can extract some information about the Hamiltonian, in particular, the dependence of the coupling parameter CNM on the In concentration in GaInNAs. CNM is shown to decrease with increasing indium mole fraction, confirming theoretical predictions.
Temporal development of the composition of Zr and Cr cathodic arc plasma streams in a N2 environment
Authors: Rosén Johanna , Anders André , Hultman Lars , Schneider Jochen M.
Published: July 2003

We describe the temporal development of the plasma composition in a pulsed plasma stream generated by cathodic arc. Cathodes of Zr and Cr were operated at various nitrogen pressures. The time-resolved plasma composition for the cathode materials was analyzed with time-of-flight charge-to-mass spectrometry, and was found to be a strong function of the nitrogen pressure. Large plasma composition gradients were detected within the first 60 μs of the pulse, the nitrogen ion concentration increasing with increasing pressure. The results are explained by the formation and erosion of a compound layer formed at the cathode surface in the presence of a reactive gas. The average charge state was also found to be affected by the reactive gas pressure as well as by the time after ignition. The charge states were highest in the beginning of the pulse at low nitrogen pressure, decreasing to a steady-state value at higher pressure. These results are of importance for reactive plasma processing and for controlling the evolution of thin film composition and microstructure.
Molecular dynamics simulation of anomalous self-diffusion for single-file fluids
Authors: Mon K. K. , Percus J. K.
Published: July 2003

Anomalous self-diffusion in a single-file fluid under the influence of random background forces has been predicted theoretically for decades, but is far from well understood. In particular, one does not fully comprehend the nature of the random force that is necessary to produce the anomalous self-diffusion. For this reason, a molecular dynamics simulation method that can produce anomalous diffusion will be of interest. We propose, in this paper, a fluid scattering mechanism for particles against the confining wall that will be sufficient to induce anomalous self-diffusion. It is efficient and simple to implement. Molecular dynamics simulations of hard sphere fluids in narrow cylindrical pores are used to demonstrate the new method.
Magnetohydrodynamic modeling of two-dimensional reconnection in the Magnetic Reconnection Experiment
Authors: Lukin V. S. , Jardin S. C.
Published: July 2003

A two-dimensional magnetohydrodynamics (MHD) code is used to investigate the dynamical evolution of driven reconnection in the Magnetic Reconnection Experiment (MRX) [M. Yamada et al., Phys. Plasmas 7, 1781 (2000)]. The initial conditions and dimensionless parameters of the simulation are set to be similar to the experimental values. Many features of the time-evolution of magnetic configurations for both co- and counter-helicity reconnection in MRX are successfully reproduced in the framework of resistive MHD. The resistive MHD model is then augmented by the addition of a “model Hall” term to begin to assess the importance of two-fluid physics in the experiment. The effective decoupling of the ion fluid from the reconnecting magnetic field due to the model Hall term is shown to be important during the early dynamic X-phase of MRX reconnection, while effectively negligible during the late “steady-state” Y-phase, when plasma heating takes place. These results are consistent with the available experimental evidence. Based on simple symmetry considerations, an experiment to directly measure the Hall effect in MRX configuration is proposed and numerical evidence for the expected outcome is given.