bluuebunny/crossref_metadata_2025 huggingface dataset:
On the variational principle for dust shells in General Relativity
Authors: Gladush Valentin D.
Published: July 2002

The variational principle for a thin dust shell in General Relativity is constructed. The principle is compatible with the boundary-value problem of the corresponding Euler–Lagrange equations, and leads to “natural boundary conditions” on the shell. These conditions and the gravitational field equations which follow from an initial variational principle, are used for elimination of the gravitational degrees of freedom. The transformation of the variational formula for spherically-symmetric systems leads to two natural variants of the effective action. One of these variants describes the shell from a stationary interior observer’s point of view, another from the exterior one. The conditions of isometry of the exterior and interior faces of the shell lead to the momentum and Hamiltonian constraints. The canonical equivalence of the mentioned systems is shown in the extended phase space. Some particular cases are considered.
Optical transmission spectroscopy of semi-insulating GaAs substrate implanted by arsenic ions at different dosages
Authors: Lin Gong-Ru , Hsu Chin-Chia
Published: July 2002

The near infrared optical properties of arsenic ion-implanted GaAs (GaAs:As+) with different dosages are investigated using Fourier transform infrared spectroscopy. The band edge absorption coefficient and the band gap energy of GaAs: As+ increases from 6.2×103 to 2.2×104 cm−1 and redshift (shrink) from 1.40 to 1.36 eV as the dosage increases from 1013 to 1016 ions/cm2, respectively. Particularly, the reciprocal slope of absorption edge and the correlated defect concentration of the GaAs:As+ are determined for the first time to change from 6 to 23 meV and from <1017 cm−3 to 2–5×1019 cm−3, respectively. The category and origin of the damage-induced defects with their activation energies leveled at between 0.6 and 1.15 eV are identified.
Stereographic variables for multidimensional quantum group covariant q-oscillators
Authors: Arik M. , Arikan A. S.
Published: July 2002

In this paper, we study the unitary matrix representation of quantum group SUq(2) in terms of stereographic coordinates z and z* and a central unitary phase u. The fractional linear transformation of z and z* gives the action of SUq(2) on the quantum sphere SUq(2)/U(1) with coordinates z and z*. We then extend this action to SUq(2) and derive the transformation law of u. Finally, we construct the two-dimensional covariant q-oscillators in terms of stereographic variables and then we generalize it to d-dimensional case.
Refractive dispersion curve measurement of KTiOPO4 using periodically segmented waveguides and periodically poled crystals
Authors: Katz M. , Eger D. , Oron M. B. , Hardy A.
Published: July 2002

We introduce a technique for measuring the refractive indices of a crystal and apply it to measure the dispersion curve of KTiOPO4. The refractive index evaluation is based on measurement of the Bragg reflection wavelength of periodically segmented waveguides close to cut-off conditions. The measurement error is found to be less than 10−4. Second harmonic generation and optical parametric oscillation in periodically poled KTiOPO4 enable us to extend the measurement to a wide wavelength range. A room temperature Sellmeier type equation for the dispersion curve nz(λ) of flux-grown KTiOPO4 in the entire transparency range of the crystal is determined.
Surface energy of amorphous carbon films containing iron
Authors: Chen J. S. , Lau S. P. , Tay B. K. , Chen G. Y. , Sun Z. , Tan Y. Y. , Tan G. , Chai J. W.
Published: July 2002

Iron containing diamond-like amorphous carbon (a-C:Fe) films were deposited by filtered cathodic vacuum arc technique. The influences of Fe content and substrate bias on the surface energy of the films were investigated. The surface energy of a-C:Fe films was determined by the contact angle measurement. Atomic force microscopy, Raman spectroscopy, and x-ray induced photoelectron spectroscopy were employed to analyze the origin of the variation of surface energy with various Fe content and substrate bias. It is found that the contact angle for water increases significantly after incorporating Fe into the films and the films become hydrophobic. The roughness of these films has no effect on the contact angle. The surface energy is reduced from 42.8 to 25 dyne/cm after incorporating Fe into the a-C film (10% Fe in the target), which is due to the reduction of both dispersive and polar component. The reduction in dispersive component is ascribed to the decrease of atomic density of the a-C:Fe films due to the increase in sp2 bonded carbon. When sp2 content increases to some extent, the atomic density remains constant and hence dispersive component does not change. The absorption of oxygen on the surface plays an important role in the reduction of the polar component for the a-C:Fe films. It is proposed that such network as (Cn–O–Fe)–O–(Fe–O–Cn) may be formed and responsible for the reduction of polar component.
Correlation between electroresistance and magnetoresistance in La0.82Ca0.18MnO3 single crystal
Authors: Markovich V. , Rozenberg E. , Yuzhelevski Y. , Jung G. , Gorodetsky G. , Shulyatev D. A. , Mukovskii Ya. M.
Published: July 2002

The resistivity of La0.82Ca0.18MnO3 single crystal has been investigated as a function of external magnetic field and separately under an applied current flow. The measurements were carried out at various temperatures below and above the ferromagnetic transition temperature TC. It has been found that the dynamic electroresistance exhibits stunning similarities to the colossal magnetoresistance at the corresponding temperatures. The correlation observed between the electric- and magnetic-field effects is attributed to electrically induced magnetoresistance.
Generalized Goldstone theorem: Automatic imposition of the Higgs mechanism and application to scale and conformal symmetry breaking
Authors: Chodos A. , Gallatin G.
Published: July 2002

Standard discussions of Goldstone’s theorem based on a symmetry of the action assume constant fields and global transformations, i.e., transformations which are independent of spacetime coordinates. By allowing for arbitrary field distributions in a general representation of the symmetry we derive a generalization of the standard Goldstone’s theorem. When applied to gauge bosons coupled to scalars with a spontaneously broken symmetry the generalized theorem automatically imposes the Higgs mechanism, i.e., if the expectation value of the scalar field is nonzero then the gauge bosons must be massive. The other aspect of the Higgs mechanism, the disappearance of the “would be” Goldstone boson, follows directly from the generalized symmetry condition itself. We also use our generalized Goldstone’s theorem to analyze the case of a system in which scale and conformal symmetries are both spontaneously broken.
Friedmann limits of rotating hypersurface–homogeneous dust models
Authors: Krasiński Andrzej
Published: July 2002

The existence of Friedmann limits is systematically investigated for all the hypersurface–homogeneous rotating dust models, presented in previous papers by this author. Limiting transitions that involve a change of the Bianchi type are included. Except for stationary models that obviously do not allow it, the Friedmann limit expected for a given Bianchi type exists in all cases. Each of the three Friedmann models has parents in the rotating class; the k=+1 model has just one parent class, the other two each have several parent classes. The type IX class is the one investigated in 1951 by Gödel. For each model, the consecutive limits of zero rotation, zero tilt, zero shear, and spatial isotropy are explicitly calculated.
Approximate accelerated stochastic simulation of chemically reacting systems
Authors: Gillespie Daniel T.
Published: July 2002

The stochastic simulation algorithm (SSA) is an essentially exact procedure for numerically simulating the time evolution of a well-stirred chemically reacting system. Despite recent major improvements in the efficiency of the SSA, its drawback remains the great amount of computer time that is often required to simulate a desired amount of system time. Presented here is the “τ-leap” method, an approximate procedure that in some circumstances can produce significant gains in simulation speed with acceptable losses in accuracy. Some primitive strategies for control parameter selection and error mitigation for the τ-leap method are described, and simulation results for two simple model systems are exhibited. With further refinement, the τ-leap method should provide a viable way of segueing from the exact SSA to the approximate chemical Langevin equation, and thence to the conventional deterministic reaction rate equation, as the system size becomes larger.
Solitons and almost-intertwining matrices
Authors: Kasman Alex , Gekhtman Michael
Published: July 2002

We define the algebraic variety of almost intertwining matrices to be the set of triples (X,Y,Z) of n×n matrices for which XZ=YX+T for a rank one matrix T. A surprisingly simple formula is given for tau functions of the KP hierarchy in terms of such triples. The tau functions produced in this way include the soliton and vanishing rational solutions. The induced dynamics of the eigenvalues of the matrix X are considered, leading in special cases to the Ruijsenaars–Schneider particle system.
Hartree approximation I: The fixed point approach
Authors: Prodan E. , Nordlander P.
Published: July 2002

We consider the Hartree approximation at finite temperature. We give a justification of this approximation by using the methods of functional integration. For finite temperatures, a fixed point approach to solving the Hartree problem is proposed. For a class of two-body interactions and background potentials that includes the Coulomb interaction, we prove that the Hartree equation has a unique solution provided the coupling constant is small. We also prove a similar result for local density theory.
Study of the interaction of a 10 TW femtosecond laser with a high-density long-scale pulsed gas jet
Authors: Monot P. , D’Oliveira P. , Hulin S. , Faenov A. Ya. , Dobosz S. , Auguste T. , Pikuz T. A. , Magunov A. I. , Skobelev I. Yu. , Rosmej F. , Andreev N. E. , Lefebvre E.
Published: July 2002

A study on the interaction of a 10 TW, 60 fs, Ti–Sapphire laser with a high-density long-scale pulsed nitrogen gas jet is reported. Experimental data on the laser propagation are analyzed with the help of a ray-tracing model. The plasma dynamics is investigated by means of time-resolved shadowgraphy and time-integrated high-resolution x-ray spectroscopy. Shadowgrams show that the plasma does not expand during the first 55 ps, while x-ray spectra exhibit an unusual continuum-like structure attributed to hollow atoms produced by charge exchange process between bare nuclei expelled from the plasma and molecules of the surrounding gas. The interpretation of the results is supported by particle-in-cell simulations. The question of x-ray lasing is also examined using a hydrodynamic code to simulate the long lasting regime of recombination.
Effects of electron-beam exposure on a ruthenium nanocluster polymer
Authors: Thomas M. D. R. , Ahmed H. , Sanderson K. M. , Shephard D. S. , Johnson B. F. G. , Ozkaya D. , Sharma N. , Humphreys C.
Published: July 2002

The properties of the ruthenium nanocluster polymer of formula [Ru6C(CO)15P(C6H5)2CCP(C6H5)2]n are studied by atomic-force microscopy, Fourier-transform infrared spectroscopy, and electron microscopy as a function of electron-beam exposure. These properties are related to the electrical conductivity, which is measured at various temperatures and electric fields. A hopping correlation length of 240 nm is deduced and explained on the basis of the formation of ruthenium superclusters in an insulating matrix.
Low-temperature specific heat of isotopically modified boron single crystals
Authors: Hu Quanli , Noda Tetsuji , Suzuki Hiroshi , Numazawa Takenori , Arai Osamu , Hirano Toshiyuki , Nogi Naoyuki , Tanaka Satoru
Published: July 2002

The measurement of specific heat at low temperature for isotopically modified boron single crystals was performed between 0.5 and 100 K using a calorimetric technique known as the time-constant or relaxation method. The results indicate that the specific heat has obvious divergencies at 1.3 K<T<5 K and the divergencies for four kinds of isotopically modified boron samples vary with different B10 contents. The influence of lattice vibration modes and the isotopic effect on specific heat and thermal conductivity for isotopically modified boron are discussed.
Consequences of ’t Hooft’s equivalence class theory and symmetry by coarse graining
Authors: Liu X. F. , Sun C. P.
Published: July 2002

According to ’t Hooft [Class. Quantum. Grav. 16, 3263 (1999)], quantum gravity can be postulated as a dissipative deterministic system, where quantum states at the “atomic scale” can be understood as equivalence classes of primordial states governed by a dissipative deterministic dynamics law at the “Planck scale.” In this paper, it is shown that for a quantum system to have an underlying deterministic dissipative dynamics, the time variable should be discrete if the continuity of its temporal evolution is required. Besides, the underlying deterministic theory also imposes restrictions on the energy spectrum of the quantum system. It is also found that quantum symmetry at the “atomic scale” can be induced from ’t Hooft’s coarse graining classification of primordial states at the “Planck scale.”
Characterization of process-induced lattice distortion in silicon by double-crystal x-ray topography using a curved collimator
Authors: Kudo Y. , Liu K.-Y. , Kawado S. , Xiaowei Z. , Hirano K.
Published: July 2002

Instead of the conventional flat collimator a curved collimator was used in double-crystal x-ray topography. The curvature of the collimator was adjusted so that the Bragg condition for x-ray diffraction was uniformly satisfied over a wide area of the silicon wafer. The image area of the wafer was wide enough to characterize the local lattice distortion induced by test element groups of metal–oxide–semiconductor capacitors formed on the wafer. The lattice distortion was measured as variations in lattice plane spacing and in lattice plane orientation using local angular deviations from the Bragg condition. These angular deviations were determined by fitting the x-ray intensities measured at the same point on a series of topographs taken around the Bragg peak to the rocking curve of the sample. The lattice plane spacing changed abruptly by 10−6 at the boundary between the areas of gate oxide (11 nm thick) and the areas of field oxide (400 nm thick), and showed less variation within these areas. The lattice plane orientation changed monotonically in each area, with an inclination of the order of 10−5 rad within the largest gate oxide area (5×5 mm2).
Ricci-flat warped products and Painlevé analysis
Authors: Dancer Andrew , Wang McKenzie Y.
Published: July 2002

We apply Painlevé analysis to the Ricci-flat Einstein equations for a warped product with an arbitrary number of factors. We find that, as in the situation of the two factors examined [J. Geom. Phys. 38, 183–206 (2001)], the cases when the total dimension is 10 or 11 are singled out by the analysis.
Energetics of native defects in ZnO
Authors: Oba Fumiyasu , Nishitani Shigeto R. , Isotani Seiji , Adachi Hirohiko , Tanaka Isao
Published: July 2002

We have investigated the formation energies and electronic structure of native defects in ZnO by a first-principles plane-wave pseudopotential method. When p-type conditions are assumed, the formation energies of donor-type defects can be quite low. The effect of self-compensation by the donor-type defects should be significant in p-type doping. Under n-type conditions, the oxygen vacancy exhibits the lowest formation energy among the donor-type defects. The electronic structure, however, implies that only the zinc interstitial or the zinc antisite can explain the n-type conductivity of undoped ZnO.
A theorem for inhomogeneous systems: The generalization of the nucleation theorem
Authors: Bowles R. K. , Reguera D. , Djikaev Y. , Reiss H.
Published: July 2002

We show that the validity of the nucleation theorem transcends the phenomenon of nucleation and extends to all equilibrium systems containing local nonuniform density distributions stabilized by external fields, and that it remains valid down to the molecular level. This result is tested by the application of exact theory at the molecular level and is shown to be valid in all the cases for which we have been able to complete such an exact analysis. These cases include cavities and clusters in hard rod fluids, as well as the molecular excesses associated with the “atmospheres” of molecules in single and multicomponent fluids. We show that, at the molecular level, the theorem can be associated with the compressibility equation of state and, at the macroscopic level, with the Gibbs adsorption equation. It is thus a relation of great power and should be useful in many contexts.
Trefoil symmetry II. Another clover extension
Authors: Wills-Toro L. A. , Sánchez L. A. , Osorio J. M. , Jaramillo D. E.
Published: July 2002

We construct a novel graded extension of the Poincaré group with integer spin multiplets as novel generators. The extension involves Z4×Z4 graded parameters and produce space–time translations through the composition of novel symmetric vector and scalar multiplets.
Trefoil symmetry III. The full clover extension
Authors: Wills-Toro L. A.
Published: July 2002

We aim to approach a trefoil symmetry involving (Z4×Z4;q)-graded parameters, multiplets of generators of integer spin <2 with naive dimensions in the interval [0,1]. The resulting full clover extension involves as particular subalgebras the minimal clover extensions previously obtained. We address the introduction of further scalar, dimensionless multiplets, and central charges. They could provide connections among external and internal symmetries, perhaps after some graded symmetry breaking.
A long-range correction scheme for generalized-gradient-approximation exchange functionals
Authors: Iikura Hisayoshi , Tsuneda Takao , Yanai Takeshi , Hirao Kimihiko
Published: July 2002

We propose a new long-range correction scheme that combines generalized-gradient-approximation (GGA) exchange functionals in density-functional theory (DFT) with the ab initio Hartree–Fock exchange integral by using the standard error function. To develop this scheme, we suggest a new technique that constructs an approximate first-order density matrix that corresponds to a GGA exchange functional. The calculated results of the long-range correction scheme are found to support a previous argument that the lack of the long-range interactions in conventional exchange functionals may be responsible for the underestimation of 4s−3d interconfigurational energies of the first-row transition metals and for the overestimation of the longitudinal polarizabilities of π-conjugated polyenes in DFT calculations.
Fine gradings of o(5, C), sp(4, C) and of their real forms
Authors: Patera Jiřı́ , Pelantová Edita , Svobodová Milena
Published: July 2002

There are three fine gradings of the simple Lie algebra of type B2 over the complex number field. They provide a basic information about the structure of the algebra. In the paper an explicit description of all fine gradings is given in terms of the four-dimensional symplectic [sp(4, ℂ)] and five-dimensional orthogonal [o(5, ℂ)] representations of the algebra. In addition, the real forms of B2 are considered. It is shown which of the fine gradings survive the restriction to each of the real forms. These results should be useful in defining various sets of additive quantum numbers for systems with such symmetries, for systematic study of grading preserving contractions of this Lie algebra, and generally for choosing bases which reflect structural properties of the Lie algebra.
Investigation of noise in a spectrometer system using a short-arc source
Authors: Ebert M. , Aspnes D. E.
Published: July 2002

We show that noise can be reduced significantly at certain frequencies in spectrometer systems that use arc-lamp sources if the image of the lamp is oriented perpendicular to the entrance slit of the spectrometer. This appears to be a result of aligning the direction of arc wander along, rather than across, the slit. Data on a specific system show a reduction in the appropriate Fourier component of the power density spectrum by a factor of 4.
Atomic streak camera operating in the extreme ultraviolet
Authors: Warntjes J. B. M. , Gürtler A. , Osterwalder A. , Rosca-Pruna F. , Vrakking M. J. J. , Noordam L. D.
Published: July 2002

We present a new type of streak camera to measure the temporal shape of pulses in the extreme ultraviolet (XUV) region: A cloud of noble gas atoms is used as a photocathode. The first results at 57 nm show a time resolution of 10 ps. Modifications to improve the time resolution are discussed. Using a noble gas as a photocathode has the potential to decrease the temporal resolution of the streak camera in the XUV to less than 0.5 ps.
Lateral current spreading in unipolar semiconductor lasers
Authors: Becker Cyrille , Sirtori Carlo
Published: July 2002

Lateral current spreading in shallow ridge processed unipolar semiconductor lasers is described using a two-dimensional flow model. In these devices, contrary to bipolar diode lasers, the density of carriers can be considered constant also in the active region. Therefore electron diffusion is a negligible effect and the spatial distribution of the current can be obtained by solving a two-dimensional differential equation for the electric potential. Our calculations prove that the major contribution to the current spreading takes place right before electrons enter the active region and is caused by the discontinuity of the conductivity at the cladding–active region interface.
Nonsymmetric gravitation theory in noncommutative geometry
Authors: Khelili F. , Mimouni J. , Mebarki N.
Published: July 2002

We propose a reformulation of the nonsymmetric gravitation theory (NGT) in the context of noncommutative geometry. It is shown that all of the Moffat’s added terms in his new version of the NGT have a geometric origin.
Formation of periodic structures in a laser spark
Authors: Ping Yuan , Geltner Iddo , Suckewer Szymon
Published: July 2002

Periodic plasma density modulations have been observed in a laser spark created by focusing a 5–20 MW laser pulse into nitrogen at 1 to 2 atmospheres. The formation of such structures has been studied by measuring the period as a function of the wavelength λ and the incident angle γ of the laser pulse. The results show good agreement with a surface-instability model. The intensity of the scattered laser light has the same periodicity and is consistent with the measured plasma density profile.
Low-energy dynamics of CO and NO chemisorbed on Rh(111)
Authors: Witte Gregor
Published: July 2002

The structure and the low-energy vibrational dynamics of CO and NO chemisorbed on a Rh(111) surface have been investigated by means of He-atom scattering (HAS). For a dilute phase of isolated CO molecules a characteristic frequency of ℏω=5.7 meV is observed which shifts to 5.45 meV for the (3×3)R30° phase (θ=1/3). Based on isotope exchange measurements this mode is assigned to the parallel frustrated translational mode (T-mode) of CO at on-top sites. For the (2×2)3CO saturation structure a further molecular vibrational mode with a frequency of ℏω=11.7 meV is obtained and is assigned to the T-mode of CO adsorbed at hollow sites which are predominantly populated in this phase. The He-atom diffraction patterns indicate a successive appearance of various NO superstructures upon increasing NO exposure including a low coverage c(4×2), an intermediate (3×3) and finally the (2×2)3NO saturation structure. The corresponding inelastic HAS measurements reveal again two characteristic molecular vibrational modes at 7.5 and 11.5 meV which are identified as the T-modes of NO adsorbed at on-top and hollow sites, respectively. Moreover, for the NO saturation phase two additional phonon modes appear at energies below 14 meV. Based on a lattice dynamical analysis of their dispersion curves they are identified as a back-folded Rayleigh mode and a further perpendicular polarized phonon mode caused by the reduced Brillouin zone of the NO adlayer. The different T-mode frequencies for CO or NO at on-top and hollow sites suggest that this mode is a sensitive signature of adsorption sites rather than the commonly used internal stretch mode which had led to wrong adsorption site assignments.
Capacitance thermometer for use at low temperatures and high magnetic fields
Authors: Murphy T. P. , Palm E. C. , Peabody L. , Tozer S. W.
Published: July 2002

We report on the development of capacitance thermometers that are independent of magnetic field, can be made with large capacitances (500–150 000 pf), have a small thermal mass, are temperature sensitive from 300 K to 20 mK, and are relatively easy to fabricate from thin sheets of insulator and metallic foils. Furthermore, we demonstrate that we can move the position of the minimum in capacitance with respect to temperature and vary the sensitivity of the device by changing its construction parameters. Measurements have demonstrated that the thermometers are insensitive to high magnetic fields up to 18 T in the temperature range 20–420 mK.
Conduction noise in La0.67Ca0.33MnO3 film
Authors: Khare Neeraj , Moharil U. P. , Gupta A. K.
Published: July 2002

The temperature and frequency dependence of conduction noise in La0.67Ca0.33MnO3 film are reported. The film was prepared by a screen printing technique and had a metal–insulator transition temperature (Tp) and ferromagnetic transition temperature (TC) at 140 and 230 K, respectively. The magnetoresistance ratio (MRR) of the film at 1 kOe was found to be mainly due to grain boundaries. The observed voltage noise spectral density (Sv) shows 1/fα type behavior for all the temperatures ranging from 77 to 300 K. Sv shows an increase as the temperature is decreased below TC and reaches a peak value at Tp. The increase in Sv below TC is attributed to extra noise arising due to fluctuations in spin alignment in the ferromagnetic state. Application of a 1 kOe magnetic field reduces noise and the reduction in the noise is observed up to TC. A comparison of the temperature dependence of MRR and noise reduction ratio due to the magnetic field indicates that the magnetoresistance in the polycrystalline film of La0.67Ca0.33MnO3 is strongly influenced by the grain boundaries but the noise due to fluctuations in spin alignment is intrinsic and comes mainly from the grains.
On the correlation functions of the vector bundle generalization of the bc-system
Authors: Schork Matthias
Published: July 2002

It is shown that the determinants of the correlation functions of the generalized bc-system introduced recently are given as pullbacks of the non-Abelian theta divisor.
Diamond nanocrystals formed by direct implantation of fused silica with carbon
Authors: Orwa J. O. , Prawer S. , Jamieson D. N. , Peng J. L. , McCallum J. C. , Nugent K. W. , Li Y. J. , Bursill L. A. , Withrow S. P.
Published: July 2002

We report synthesis of diamond nanocrystals directly from carbon atoms embedded into fused silica by ion implantation followed by thermal annealing. The production of the diamond nanocrystals and other carbon phases is investigated as a function of ion dose, annealing time, and annealing environment. We observe that the diamond nanocrystals are formed only when the samples are annealed in forming gas (4% H in Ar). Transmission electron microscopy studies show that the nanocrystals range in size from 5 to 40 nm, depending on dose, and are embedded at a depth of only 140 nm below the implanted surface, whereas the original implantation depth was 1450 nm. The bonding in these nanocrystals depends strongly on cluster size, with the smaller clusters predominantly aggregating into cubic diamond structure. The larger clusters, on the other hand, consist of other forms of carbon such as i-carbon and n-diamond and tend to be more defective. This leads to a model for the formation of these clusters which is based on the size dependent stability of the hydrogen-terminated diamond phase compared to other forms of carbon. Additional studies using visible and ultraviolet Raman Spectroscopy, optical absorption, and electron energy loss spectroscopy reveal that most samples contain a mixture of sp2 and sp3 hybridized carbon phases.
Extraordinarily wide optical gain spectrum in 2.2–2.5 μm In(Al)GaAsSb/GaSb quantum-well ridge-waveguide lasers
Authors: Donetsky D. V. , Westerfeld D. , Belenky G. L. , Martinelli R. U. , Garbuzov D. Z. , Connolly J. C.
Published: July 2002

A wide optical gain spectrum with full width at half maximum Δλ≈350 nm has been demonstrated in midinfrared (2.2–2.5 μm) room-temperature-operated InGa(As)Sb/GaSb quantum-well (QW) ridge-waveguide diode lasers. A QW width of 200 Å ensured a small energy separation between the first and second quantized states in the QW. Transitions from both quantized states contributed equally to the overall gain spectrum when the optical loss was optimized.
Cross field diffusion and transport in a nonrelativistic beam-driven magnetoplasma: Collisional kinetics
Authors: Mohanty J. N. , Baral K. C.
Published: July 2002

Streaming or beam-driven kinetic theory is formulated for a nonrelativistic and collisional plasma diffusing across magnetic field lines, including small density and temperature gradient. Explicit formulas for modified transport coefficients are presented and their dependence on streaming parameter (V0) is discussed both qualitatively and quantitatively, while retaining the exponent in normalization intact. The dramatic results concerning the beam feature reveal that the electrical resistivity (η⊥) and thermal conductivity (K) diminish sharply with the increase in temperature for varying V0 in the limit of T+=T−=108 K; it approaches minimal range to resemble a rectangular hyperbola, while only the thermal conductivity (K) shows a diminishing trend with increasing (V0). The thermoelectric coefficient (λ) is almost independent of temperature, though it increases with increasing streaming.
Erratum: “Thermochemistry of disulfur decafluoride, S2F10” [J. Chem. Phys. 103, 10162 (1995)]
Authors: Irikura Karl K.
Published: July 2002

Due to an error in the computation of thermodynamic functions, a few of the originally reported values were in error by up to 0.8%. A revised version of Table V is presented, as well as a new table listing the principal thermodynamic functions for a range of temperatures.
Cold and ultracold ion-neutral inelastic collisions: Spin–orbit relaxation in He+Ne+
Authors: Siska P. E.
Published: July 2002

Electronic close-coupling scattering calculations are reported on the relaxation process Ne+(2p5 2Pj1=1/2,m1)+He→Ne+(2p5 2Pj2=3/2,m2)+He using a new interaction potential derived by combining spectroscopy and ab initio theory. Collision energies range from E/kB=300 K to less than 1 μK. As E is reduced, the total inelastic cross section σj1→j2=σ1/2→3/2 passes through a resonance-dominated regime and apparently approaches the Wigner limit below 10 μK, where the cross section is found to diverge in accord with the predicted E−1/2 dependence. A complex scattering length a=−53.0−0.04145i Å is derived from the low-energy behavior. m-dependent alignment and orientation cross sections, σj1|m1|→j2|m2| and σj1m1→j2m2, undergo rapid changes near resonances, and approach limiting behavior below 10 μK as well, while their ratios become asymptotically constant below 100 μK. Analysis of the scattering amplitude in the s-wave limit shows that the latter behavior is expected in general, with the ratios depending only on the geometry of angular momentum coupling. This geometrical effect disentangles s-wave dominance from the Wigner regime, and provides a new criterion for assessing the approach to the ultracold limit. Boltzmann averaging to simulate ultracold trapping conditions shows that a remnant of the resonance region and the characteristic limiting behavior remain observable.
Photoluminescence from quantum dots in cubic GaN/InGaN/GaN double heterostructures
Authors: Husberg O. , Khartchenko A. , As D. J. , Vogelsang H. , Frey T. , Schikora D. , Lischka K. , Noriega O. C. , Tabata A. , Leite J. R.
Published: July 2002

We have measured photoluminescence spectra of molecular-beam-epitaxy-grown cubic GaN/InxGa1−xN/GaN double heterostructures with x between 0.09 and 0.33. We observe a luminescence peak at about 2.3–2.4 eV which is almost independent of the InGaN layer composition. High-resolution x-ray diffraction measurements revealed a pseudomorphic In-rich phase with x=0.56±0.02 embedded in the InGaN layers. Including strain effects we calculate a gap energy Eg=2.13 eV of this phase. In cubic InGaN, spontaneous polarization and strain-induced piezoelectric fields are negligible. Therefore, the observed difference between the luminescence energy and the gap of the In-rich phase is assumed to be due to the localization of excitons at quantum-dot-like structures with a size of about 15 nm.
Origin of improved luminescence efficiency after annealing of Ga(In)NAs materials grown by molecular-beam epitaxy
Authors: Li Wei , Pessa Markus , Ahlgren Tommy , Decker James
Published: July 2002

Positron-annihilation measurements and nuclear reaction analysis [utilizing the N14(d,p)N15 and N14(d,He)C12 reactions] in conjunction with Rutherford backscattering spectrometry in the channeling geometry were used to study the defects in as-grown Ga(In)NAs materials grown by molecular beam epitaxy using a radio-frequency plasma nitrogen source. Our data unambiguously show the existence of vacancy-type defects, which we attribute to Ga vacancies, and nitrogen interstitials in the as-grown nitride–arsenide epilayers. These point defects, we believe, are responsible for the low luminescence efficiency of as-grown Ga(In)NAs materials and the enhanced diffusion process during annealing.
Reliability and current carrying capacity of carbon nanotubes
Authors: Wei B. Q. , Vajtai R. , Ajayan P. M.
Published: July 2002

The current-carrying capacity and reliability studies of multiwalled carbon nanotubes under high current densities (>109 A/cm2) show that no observable failure in the nanotube structure and no measurable change in the resistance are detected at temperatures up to 250 °C and for time scales up to 2 weeks. Our results suggest that nanotubes are potential candidates as interconnects in future large-scale integrated nanoelectronic devices.
Band structures and thermoelectric properties of the clathrates Ba8Ga16Ge30, Sr8Ga16Ge30, Ba8Ga16Si30, and Ba8In16Sn30
Authors: Blake Nick P. , Latturner Susan , Bryan J. Daniel , Stucky Galen D. , Metiu Horia
Published: July 2002

Density functional calculations in the generalized gradient approximation are used to study the transport properties of the clathrates Ba8Ga16Ge30, Sr8Ga16Ge30, Ba8Ga16Si30, and Ba8In16Sn30. The band structures of these clathrates indicate that they are all semiconductors. Seebeck coefficients, conductivities and Hall coefficients are calculated, to assess the effects of carrier concentration on the quantity S2σ/τ (where S is the Seebeck coefficient, σ is the conductivity, and τ the electron relaxation time) which is proportional to the thermoelectric power factor. In each compound we find that both p- and n-doping will significantly enhance the thermoelectric capabilities of these compounds. For p-doping, the power factors of all four clathrates are of comparable magnitude and have similar temperature dependence, while for n-doping we see significant variations from compound to compound. We estimate that room-temperature ZT values of 0.5 may be possible for optimally n-doped Sr8Ga16Ge30 or Ba8In16Sn30; at 800 K ZT values as large as 1.7 may be possible. For single crystals of high quality, with substantially increased scattering times, the power factor of these materials will be significantly higher. Recent experiments are reviewed in the light of these calculations.
Symmetry classification of diatomic molecular chains
Authors: Lafortune S. , Tremblay S. , Winternitz P.
Published: July 2002

A symmetry classification of possible interactions in a diatomic molecular chain is provided. For nonlinear interactions the group of Lie point transformations, leaving the lattice invariant and taking solutions into solutions, is at most five-dimensional. An example is considered in which subgroups of the symmetry group are used to reduce the dynamical differential-difference equations to purely difference ones.
Monte Carlo simulation studies of the correlation between global size and helical structures in biopolymers
Authors: Zifferer Gerhard , Hofstetter Mark , Olaj Oskar Friedrich
Published: July 2002

There are several secondary structures in biological macromolecules such as beta sheets, loops, and helices. The subject of the present paper is to adopt different methods of inducing certain quotas of helical regions in macromolecules and to examine how the quota of helices influences global properties such as the radius of gyration. Two different methods were used to produce the chains on a tetrahedral lattice; a static step-by-step method with deliberately chosen probabilities of bond vectors in order to favor helices, and a dynamic method with intrachain potentials which induced the formation of helices, the latter approach being a simple model of self organization. Comparison of the results of the different methods revealed correspondence in many points but—not unexpectedly—also some differences, because of the use of different effective potentials in the polymer formation processes.
Field-induced Coulomb coupling in semiconductor macroatoms: Application to single-electron quantum devices
Authors: D’Amico Irene , Rossi Fausto
Published: July 2002

An original approach for the control of exciton–exciton Coulomb coupling in semiconductor macroatoms/molecules is proposed. We show that by applying properly tailored external fields, we can induce—or significantly reinforce—excitonic dipoles, which in turn allows one to control and magnify intra- as well as interdot few-exciton effects. Such a dipole–dipole interaction mechanism will be accounted for within a simple analytical model, which is found to be in good agreement with fully three-dimensional calculations. The proposed approach may play an important role for the design and realization of fully optical quantum gates as well as ultrafast optical switches.
Endothermic energy transfer: A mechanism for generating very efficient high-energy phosphorescent emission in organic materials
Authors: Adachi Chihaya , Kwong Raymond C. , Djurovich Peter , Adamovich Vadim , Baldo Marc A. , Thompson Mark E. , Forrest Stephen R.
Published: July 2002

Intermolecular energy transfer processes typically involve an exothermic transfer of energy from a donor site to a molecule with a substantially lower-energy excited state (trap). Here, we demonstrate that an endothermic energy transfer from a molecular organic host (donor) to an organometallic phosphor (trap) can lead to highly efficient blue electroluminescence. This demonstration of endothermic transfer employs iridium(III)bis(4,6-di-fluorophenyl)-pyridinato-N,C2′)picolinate as the phosphor. Due to the comparable energy of the phosphor triplet state relative to that of the 4,4′-N,N′-dicarbazole-biphenyl conductive host molecule into which it is doped, the rapid exothermic transfer of energy from phosphor to host, and subsequent slow endothermic transfer from host back to phosphor, is clearly observed. Using this unique triplet energy transfer process, we force emission from the higher-energy, blue triplet state of the phosphor (peak wavelength of 470 nm), obtaining a very high maximum organic light-emitting device external quantum efficiency of (5.7±0.3)% and a luminous power efficiency of (6.3±0.3)lm/W.
λ
and γ reversal: The dissociation-limit region of the B 3Σu− state of O2
Authors: Waring K. , Lewis B. R. , Baldwin K. G. H. , Gibson S. T.
Published: July 2002

Using narrow-bandwidth vacuum-ultraviolet radiation generated by difference-frequency four-wave mixing in Xe, the O2 photoabsorption cross section in the Schumann–Runge system B 3Σu−←X 3Σg−, near the dissociation limit of the B state, is measured with a resolution of ∼0.1 cm−1 full-width at half-maximum. The rotational structure of B−X(22,0) is assigned for the first time, and evidence is found for transitions into the F1 levels of B(v=23). Experimental values for the triplet-splitting constants λ and γ for B(v=22) are found to be inconsistent with the rising trend observed for v⩽19, due to a change in the sense of the perturbation of the B-state levels by the C′ 3Πu levels near B(v=20). A new value for the dissociation energy of the B state, D(B)=57 136.2±0.3 cm−1, determined from an analysis of the observed and missing lines near the dissociation limit, is in excellent agreement with previous values arising from the observation of rotational thresholds in the Schumann–Runge continuum. © 2001 American Institute of Physics.
Direct losses of fast particles in multiple-toroidicity stellarators
Authors: Smirnova M. S.
Published: July 2002

Direct losses of fast ions are investigated numerically in current multiple-toroidicity stellarator configurations, in which magnetic field harmonic spectra can be described adequately by a sum of hundreds harmonics with different toroidal and poloidal mode numbers. For these configurations it is typical that amplitudes of distant harmonics with high-order N⩾2 toroidal mode numbers can exceed, near the plasma boundary, one-half of the amplitude of the main magnetic-field harmonic. This leads to the appearance of additional ripple wells along field lines which can trap charged particles (toroidally field ripple-trapped particles). An investigation of the role of these particles in plasma confinement in stellarators is the main goal of the paper. It is shown that a complication of the magnetic-field harmonic spectrum of a stellarator causes direct losses of transition and toroidally field ripple-trapped particles several times higher than losses of corresponding particles in rippled tokamaks.
Solvation of ion-pair states in nonpolar media: I2 in solid neon, argon and krypton
Authors: Helbing Jan , Chergui Majed
Published: July 2002

We report on the ion-pair state emission spectra of Iodine isolated in Neon, Argon and Krypton matrices in the spectral range from 300 nm to 700 nm, and present VUV-absorption spectra in the 50 000–60 000 cm−1 region. Spectroscopic analysis of the UV and visible emission bands of the lowest-lying ion-pair state D′(2g) in Ne and Ar provides unambiguous evidence for both vertical (energy) and horizontal shifts (equilibrium distance) due to solvation in a rare gas matrix, and suggests that the ion-pair states maintain their distinct character. In Krypton the previously reported broad emission band at 420 nm is shown to consist of two narrow bands, which have counterparts in the visible. They are assigned to emission from D′(2g) and a second ion-pair states of I2, most probably the δ(2u) state. Partial charge delocalization to the matrix can only play a minor role in Kr since very similar emission is also found for secondary sites in Ar. These findings should be highly relevant for a description of ion-pair states in nonpolar media. They are also useful for the interpretation of time-resolved experiments that use the ion-pair state emission as probe signal.
Bulk diffusion measurements to study the effectiveness of barrier layers: II. Exchange of sodium between liquid crystal display glass substrates with different barrier layers
Authors: Tian Lei , Dieckmann Rüdiger
Published: July 2002

Na-22 tracer diffusion experiments were performed to study the exchange of Na ions between liquid crystal display glass substrates (Corning Code 1737) separated by different types of layers. Different types of layers were generated (i) by RCA cleaning, (ii) by preannealing in wet air, and (iii) by low pressure chemical vapor deposition. A sandwich configuration was used to study the effect of such layers between two glass substrates on the exchange of Na ions between these substrates. The sandwiches were of the type substrate 1 (containing Na-22 tracer)/layer/substrate 2. Diffusion annealing of such sandwiches led to a redistribution of the sodium tracer. This redistribution was analyzed experimentally with regard to the sodium tracer diffusion coefficient in the bulk and the rate of the sodium tracer transfer across the layer. It was found that all three types of layers considered act as barrier layers, i.e., they suppress the exchange of Na ions between glass substrates.
X-ray photoelectron spectroscopy study of irradiation-induced amorphizaton of Gd2Ti2O7
Authors: Chen J. , Lian J. , Wang L. M. , Ewing R. C. , Boatner L. A.
Published: July 2002

The radiation-induced evolution of the microstructure of Gd2Ti2O7, an important pyrochlore phase in radioactive waste disposal ceramics and a potential solid electrolyte and oxygen gas sensor, has been characterized using transmission electron microscopy and x-ray photoelectron spectroscopy. Following the irradiation of a Gd2Ti2O7 single crystal with 1.5 MeV Xe+ ions at a fluence of 1.7×1014 Xe+/cm2, cross-sectional transmission electron microscopy revealed a 300-nm-thick amorphous layer at the specimen surface. X-ray photoelectron spectroscopy analysis of the Ti 2p and O 1s electron binding energy shifts of Gd2Ti2O7 before and after amorphization showed that the main results of ion-irradiation-induced disorder are a decrease in the coordination number of titanium and a transformation of the Gd–O bond. These features resemble those occurring in titanate glass formation, and they have implications for the chemical stability and electronic properties of pyrochlores subjected to displacive radiation damage.
Valence basis sets for relativistic energy-consistent small-core lanthanide pseudopotentials
Authors: Cao Xiaoyan , Dolg Michael
Published: July 2002

Gaussian (14s13p10d8f6g)/[6s6p5d4f3g] atomic natural orbital valence basis sets have been derived for relativistic energy-consistent small-core lanthanide pseudopotentials of the Stuttgart–Bonn variety. The existing set of lanthanide pseudopotentials has been supplemented by corresponding potentials for lanthanum and lutetium in order to arrive at a set analogous to the one available for the actinides. Multiconfiguration self-consistent field and subsequent multireference averaged coupled-pair functional calculations are presented for the first to fourth ionization potentials of all lanthanide elements. Molecular calibration studies using the coupled-cluster singles, doubles, and perturbative triples approach are reported for the monohydrides, monoxides, and monofluorides of lanthanum and lutetium.
Influence of microstructure on the carrier concentration of Mg-doped GaN films
Authors: Romano L. T. , Kneissl M. , Northrup J. E. , Van de Walle C. G. , Treat D. W.
Published: July 2002

Room-temperature Hall effect measurements of (0001) Mg-doped GaN films grown on sapphire substrates by metalorganic chemical vapor deposition show a reduction in hole concentration for Mg concentrations greater than 1020 cm−3. A combination of secondary ion mass spectrometry and transmission electron microscopy indicates a steadily increasing Mg incorporation during growth and the formation of inversion domains at these high concentrations. We discuss mechanisms that could give rise to a reduction of the hole concentration at high Mg doping levels.