bluuebunny/crossref_metadata_2025 huggingface dataset:
Gap state formation during the initial oxidation of Si(100)-2×1
Authors: Bitzer T. , Rada T. , Richardson N. V. , Dittrich T. , Koch F.
Published: July 2002

High-resolution electron energy loss spectroscopy and photoluminescence measurements have been used to follow the formation of gap states during the initial oxidation of Si(100)-2×1 at room temperature. We find strong indications that gap states are already induced after the adsorption of 0.002 L of molecular oxygen on Si(100)-2×1. It is demonstrated that prolonged exposures of clean and dehydrogenated oxide films on Si(100) to H2O at room temperature decrease the density of gap states significantly.
Modeling of steady-state field distributions in blocked impurity band detectors
Authors: Haegel N. M. , Jacobs J. E. , White A. M.
Published: July 2002

Steady-state electric field and carrier distributions are calculated for blocked impurity band (BIB) detectors in the absence of external illumination. The results illustrate the role of space charge in the blocking layer. Calculations are presented for Ge:Ga, though the input is easily modified for other materials. The numerical model allows for observation of the effect of spatial doping variations and interface gradients. The BIB field distribution is highly dependent on the purity and compensation in the blocking layer, as well as interface sharpness. In some cases, space charge effects can cause a collapse of the field at the blocker/absorber interface or the contact and resultant low field regions in the blocking layer.
Enhanced response properties of a chromophore physisorbed on a metal particle
Authors: Corni S. , Tomasi J.
Published: July 2002

We present a theoretical methodology to study the electronic response properties of a chromophore physisorbed on a metal particle. In particular the chromophore is treated at ab initio level whereas the metal particle is described through its dielectric properties. The use of a boundary elements method for the resolution of the electrostatic problem allows the study of particles with complex shapes. The theoretical procedure is applied to the study of different quantities such as the chromophore effective polarizability, the UV-VIS absorption of the chromophore coated metal particle, and the surface enhanced Raman scattering (SERS) of the physisorbed molecule for two reliable chromophores (merocyanine dyes) physisorbed on copper and silver metal particles of various sizes and shapes.
Non-Born–Oppenheimer calculations on the LiH molecule with explicitly correlated Gaussian functions
Authors: Scheu C. E. , Kinghorn Donald B. , Adamowicz Ludwik
Published: July 2002

We report the first ever non-Born–Oppenheimer variational calculations on the ground state of a four electron molecular system. The basis set used in the calculations consists of explicitly correlated Gaussians multiplied by powers of the internuclear distance. To accelerate the optimization of the many nonlinear variational parameters involved in the variational wave function, we performed the calculations on a cluster of Linux workstations using MPI and a parallel implementation of the formulas. Results for the nonadiabatic ground state energy of LiH, as well as expectation values for the kinetic and potential energies, the internuclear and square of the internuclear distance, the virial coefficient, and the square of the energy gradient norm are reported.
Wavefunctions and eigenvalues of image potential states
Authors: Jo/rgensen Solvejg , Ratner Mark A. , Mikkelsen Kurt V.
Published: July 2002

Methods for calculating wavefunctions and eigenvalues of image potential states are presented. We consider cases corresponding to metal–vacuum interfaces, metal–dielectric medium interfaces and metal–dielectric medium–vacuum interfaces. We represent the metal as a perfect conductor or as a Fermi–Thomas metal. We present the dependence of the wavefunctions and eigenvalues of the image potential states on dielectric constants, electron affinity, work function of the metal, and inverse Fermi screening parameter.
Tuning colossal magnetoresistance response by Cr substitution in La0.67Sr0.33MnO3
Authors: Sun Young , Tong Wei , Xu Xiaojun , Zhang Yuheng
Published: July 2002

We have studied the effects of Cr substitution in the perovskite La0.67Sr0.33MnO3 on the magnetic, electrical transport, and magnetoresistance properties. Cluster glass behaviors have been observed in the La0.67Sr0.33Mn1−xCrxO3 system. The most interesting feature is that extraordinary transport and colossal magnetoresistance (CMR) behaviors, characterized by double peaks, were observed with Cr substitution. When 0.1⩽x⩽0.2, the temperature range of CMR response is greatly broadened, from the lowest temperature to above room temperature. These results suggest that Cr substitution can be a potent way in tuning CMR response and also imply a ferromagnetic interaction similar to double exchange occurs between Mn and Cr.
Formation and primary heating of the solar corona: Theory and simulation
Authors: Mahajan S. M. , Miklaszewski R. , Nikol’skaya K. I. , Shatashvili N. L.
Published: July 2002

An integrated magneto-fluid model, that accords full treatment to the velocity fields associated with the directed plasma motion, is developed to investigate the dynamics of coronal structures. It is suggested that the interaction of the fluid and the magnetic aspects of plasma may be a crucial element in creating so much diversity in the solar atmosphere. It is shown that the structures which comprise the solar corona can be created by particle (plasma) flows observed near the Sun’s surface—the primary heating of these structures is caused by the viscous dissipation of the flow kinetic energy.
The origin of the magnetic fields of the universe: The plasma astrophysics of the free energy of the universe
Authors: Colgate Stirling A. , Li Hui , Pariev Vladimir
Published: July 2002

The interpretation of Faraday rotation measure maps of active galactic nuclei (AGNs) within galaxy clusters has revealed ordered or coherent regions, Lmag∼50−100 kpc (∼3×1023 cm), that are populated with large, ∼30 μG magnetic fields. The magnetic energy of these coherent regions is Lmag3(B2/8π)∼1059−60 ergs, and the total magnetic energy over the whole cluster (∼1 Mpc across) is expected to be even larger. Understanding the origin and role of these magnetic fields is a major challenge to plasma astrophysics. A sequence of physical processes that are responsible for the production, redistribution, and dissipation of these magnetic fields is proposed. These fields are associated with single AGNs within the cluster and therefore with all galaxies during their AGN (active galactic nucleus or quasar) phase, simply because only the central supermassive black holes (∼108M⊙) formed during the AGN phase have an accessible energy of formation, ∼1061 ergs, that can account for the magnetic field energy budget. An α–Ω dynamo process has been proposed that operates in an accretion disk around a black hole. The disk rotation naturally provides a large winding number, ∼1011 turns, sufficient to make both large gain and large flux. The helicity of the dynamo can be generated by the differential plume rotation derived from star-disk collisions. This helicity generation process has been demonstrated in the laboratory and the dynamo gain was simulated numerically. A liquid sodium analog of the dynamo is being built. Speculations are that the back reaction of the saturated dynamo will lead to the formation of a force-free magnetic helix, which will carry the energy and flux of the dynamo away from the accretion disk and redistribute the field within the clusters and galaxy walls. The magnetic reconnection of a small fraction of this energy logically is the source of the AGN (active galactic nucleus or quasar) luminosity, and the remainder of the field energy should then dominate the free energy of the present-day universe. The reconnection of this intergalactic field during a Hubble time is the only sufficient source of energy necessary to produce an extragalactic cosmic ray energy spectrum as observed in this galaxy, and at the same time allow this spectrum to escape to the galaxy voids faster than the GZK (blackbody radiation) loss.
Analysis of a multiple-pump Raman amplifier
Authors: Achtenhagen M. , Chang T. G. , Nyman B. , Hardy A.
Published: July 2002

Raman amplification is investigated both experimentally and numerically. It is demonstrated that, depending on the different pump wavelength combinations, the shape as well as the amount of the fiber gain can be altered accordingly. A numerical model is presented that includes the relevant physical effects to describe properly the different pump and signal interactions. Results of several pump combinations are compared to experimental data and used to derive numerical values for the local fiber gain.
High-precision ranging using a chaotic laser pulse train
Authors: Myneni Krishna , Barr Thomas A. , Reed Billy R. , Pethel Shawn D. , Corron Ned J.
Published: July 2002

We demonstrate the use of a chaotic laser pulse train for high-precision ranging. The pulse train is produced by inducing coherence collapse in an AlGaAs semiconductor laser. Measurements of optical spectra, intensity autocorrelation functions, and ladar ranging are presented.
Boundary defect in a spin ladder
Authors: Wang Yupeng , Schlottmann P.
Published: July 2002

The integrable su(1|3)-invariant spin-ladder model with boundary defect is studied using the Bethe ansatz method. The exact phase diagram for the ground state is obtained and the boundary quantum critical behavior is discussed. It consists of a gapped phase in which the rungs of the ladder form singlet states and a gapless Luttinger liquid phase. Depending on the boundary potential of the first rung a boundary bound state may occur. If populated at low temperatures this bound state represents a localized magnetic moment of spin one. In the Luttinger liquid phase the local moment is screened at low temperatures in analogy to the Kondo effect.
Exchange bias measurements of CoFe/IrMn
Authors: Brown H. , Dahlberg E. Dan , Hou C.
Published: July 2002

The exchange bias anisotropy field in CoFe/IrMn ferromagnetic/antiferromagnetic bilayers has been investigated by two different experimental probes. One was the traditional hysteresis loop shift technique and the other was a recently developed technique which monitors small reversible rotations of the magnetization with the anisotropic magnetoresistance (AMR). All the samples show approximately twice the exchange bias anisotropy field measured with the AMR technique compared to that measured with the traditional hysteresis loop method. Based on similar experiments in other materials, there is a portion of the exchange bias uniaxial anisotropy which rotates in a hysteresis loop measurement. It is surmised it is this energy which the hysteresis loop technique neglects and that the AMR technique is a better measure of the exchange bias anisotropy energy.
Experimental Energy Levels of the Water Molecule
Authors: Tennyson Jonathan , Zobov Nikolai F. , Williamson Ross , Polyansky Oleg L. , Bernath Peter F.
Published: July 2002

Experimentally derived energy levels are presented for 12248 vibration–rotation states of the H216O isotopomer of water, more than doubling the number in previous, disparate, compilations. For each level an error and reference to source data is given. The levels have been checked using energy levels derived from sophisticated variational calculations. These levels span 107 vibrational states including members of all polyads up to and including 8ν. Band origins, in some cases estimates, are presented for 101 vibrational modes.
Robustness of ultrathin aluminum oxide dielectrics on Si(001)
Authors: Copel M. , Cartier E. , Gusev E. P. , Guha S. , Bojarczuk N. , Poppeller M.
Published: July 2002

The stability of Al2O3 films during thermal processing will help determine their usefulness as an alternative gate dielectric for advanced complementary metal-oxide-semiconductor devices. We used medium energy ion scattering and atomic force microscopy to examine the degradation of ultrathin Al2O3 layers under ultrahigh vacuum annealing and the effects of low-temperature oxidation. No degradation is observed at 900 °C, but voids appear at higher temperatures. Growth of interfacial SiO2 takes place during low-pressure oxidation at 600 °C, which may limit the capacitance of extremely thin structures.
A general-purpose biasing scheme for Monte Carlo simulation of associating fluids
Authors: Wierzchowski Scott , Kofke David A.
Published: July 2002

We present a method for accelerating convergence of Monte Carlo simulations of associating fluids. Such fluids exhibit strong, short-ranged, orientation-specific intermolecular attractions which are difficult to sample via conventional molecular simulation. We propose a bias scheme that preferentially attempts Monte Carlo trials that lead to “unbonding” or “bonding” (UB) transitions of the associating molecules. The proposed method is most like the recently introduced aggregation volume bias Monte Carlo (AVBMC) algorithm of Chen and Siepmann. Both algorithms are much simpler, more efficient, and more generally applicable than previously proposed association-bias schemes. We study the UB algorithm via application to the simple ideal-association model of van Roij. Although unrealistic, the model contains the basic features of association that cause problems for simulation, and its simple nature facilitates analysis of the performance of the simulation algorithm. We find, at least in application to this model, that the UB algorithm exhibits better convergence properties when compared to AVBMC, and through analysis of the acceptance probability distributions we can develop an explanation for this difference. We also demonstrate the UB algorithm in the context of the Gibbs ensemble, reproducing the phase coexistence behavior of a dimerization model originally proposed by Tsangaris and de Pablo.
Unsteady compression waves in interferometer windows
Authors: Hayes Dennis
Published: July 2002

Optical windows for the velocity interferometer system for any reflector perturb the measurement of specimen/window interface velocity during compression wave experiments. The perturbation is caused by the variation of window index of refraction with density. The correction for shocks and steady waves is the ratio of apparent to actual interface velocity and is called 1+Δν/ν0 in the literature. We find 1+Δν/ν0 for unsteady waves also depends only upon interface velocity and not on details of the unsteady compression wave as previously reported. The correction 1+Δν/ν0 for unsteady waves is the same as for steady waves and shocks only if the index of refraction is linear in density in which case 1+Δν/ν0 is constant equaling the zero-density intercept of the index of refraction function. Differences in LiF window performance for shocks and for unsteady waves are shown to be small compared with other experimental uncertainties.
Influence of low intensity noise on assemblies of diffusively coupled chaotic cells
Authors: Lorenzo M. Nieves , Pérez-Muñuzuri Vicente
Published: July 2002

The effect of time-correlated and white Gaussian noises of low intensity on one-dimensional arrays consisting of diffusively coupled chaotic cells is analyzed. An improvement or worsening of the synchronization between cells of the array driven by low-intensity colored noise is observed for a resonant interval of time correlation values. A comparison between colored and white noise and additive and multiplicative contribution has been carried out investigating the nonlinear cooperative effects of noise strength, correlation time, and coupling strength to control spatiotemporal chaos in coupled arrays of chaotic cells. The possibility to distinguish highly correlated areas of a diffusively coupled network of cells by using low-intensity time correlated noise is discussed.
How the nonrandom distribution of nuclei affects the island density in thin-film growth
Authors: Fanfoni M. , Tomellini M. , Volpe M.
Published: July 2002

We propose a scheme of rate equations for stable dimers that permits the description quite satisfactorily of the evolution of the number of islands in the entire range of surface coverage. The characteristic time for coalescence and the capture number are computed through a stochastic approach for both random and nonrandom arrangements of nuclei. Rate equations are applied to describe kinetic Monte Carlo simulations previously published, in the whole range of surface coverages. It is found that to reproduce the simulation, the effect of the nonrandomicity of the nuclei distribution must be taken into account.
Optimal control of Tollmien–Schlichting waves in a developing boundary layer
Authors: Walther Steeve , Airiau Christophe , Bottaro Alessandro
Published: July 2002

Optimal control theory is used to determine the wall transpiration (unsteady blowing/suction) with zero net mass flux capable of attenuating Tollmien–Schlichting waves in a spatially developing boundary layer. The flow state is determined from the parabolized stability equations, in a linear setting. An appropriate cost functional is introduced and minimized iteratively by the numerical solution of the equations for the state and the dual state, coupled via transfer and optimality conditions. Central to the control is the determination of the wall Green’s function expressing the receptivity of the flow to wall inhomogeneities. The optimal wall velocity is obtained in few iterations and a reduction of several orders of magnitude in output disturbance energy is demonstrated, as compared to the uncontrolled case, for control laws operating both over the whole wall length and over a finite strip. Finally, white noise disturbances are applied to the optimal wall velocities already determined, to assess the influence of an imperfectly operating controller on the final result, and to decide on the practical feasibility of the approach.
Dynamical behaviors of GaCl on the GaAs(001) 4×6 surface by pulsed molecular beam scattering
Authors: Ohashi Masafumi , Ozeki Masashi
Published: July 2002

We studied dynamical behaviors of gallium chloride (GaCl) on a GaAs(001) 4×6 surface using pulsed molecular beam scattering. The angular distribution of reflected GaCl consists of two parts; inelastic direct scattering contribution and thermal desorption of trapped molecules on the GaAs(001) 4×6 surface. We divided the time of flight spectra of thermally desorbed GaCl into two components with activation energies of 22.0 and 10.5 kcal/mol corresponding to GaCl trapping wells. These wells were associated with two comparatively large domains on the GaAs(001) 4×6 surface. The relatively large activation energy is responsible for the strong dipole-electrostatic interaction between GaCl, which has a large dipole moment, and the ionic GaAs crystal.
Near-transform-limited picosecond pulses from a gain-switched InGaAs diode laser with fiber Bragg gratings
Authors: Birkin D. J. L. , Rafailov E. U. , Sibbett W. , Zhang L. , Liu Y. , Bennion I.
Published: July 2002

We have generated near-transform-limited picosecond pulses (ΔτΔν≈0.45) from a gain-switched diode laser using periodic and chirped fiber Bragg gratings. This configuration reduced the spectral bandwidth from 11 to 0.08 nm and the pulse duration was reduced, from 30 to <18 ps. Average and peak powers of 27 and 770 mW, respectively, were obtained.
Stability of the ground state of a harmonic oscillator in a monochromatic wave
Authors: Berman Gennady P. , James Daniel F. V. , Kamenev Dmitry I.
Published: July 2002

The stability of the ground state of a harmonic oscillator in a monochromatic wave is studied. This model describes, in particular, the dynamics of a cold ion in a linear ion trap, interacting with two laser fields with close frequencies. The stability of the “classical ground state”—the vicinity of the point (x=0,p=0)—is analyzed analytically and numerically. For the quantum case, a method for studying a stability of the quantum ground state is developed, based on the quasienergy representation. It is demonstrated that stability of the ground state may be substantially improved by increasing the resonance number, l, where l=Ω/ω+δ, Ω and ω are, respectively, the wave frequency and the oscillator frequency, l=1,2,…, |δ|<1; or by detuning the system from exact resonance, so that δ≠0. The influence of a large-amplitude wave (in the presence of chaos) on the stability of the ground state is analyzed for different parameters of the model in both the quantum and classical cases.
Transport characteristics of electrons in weak short-period two-dimensional potential arrays
Authors: Kawaharazuka A. , Saku T. , Tokura Y. , Horikoshi Y. , Hirayama Y.
Published: July 2002

We study the transport characteristics of electrons in weak short-period two-dimensional potential arrays formed on a back-gated undoped GaAs/AlGaAs heterostructure with a shallow channel. The period of the potential is as short as 50 nm. We achieve the condition where the unit cell is filled by less than one electron. In addition to the conventional magnetoresistance characteristics, we have found a feature that is determined solely by electron density. This feature reflects the strong Coulomb interaction between the electrons confined in the potential arrays.
Greatly enhanced detection sensitivity for carbon, nitrogen, and oxygen in silicon by secondary-ion-mass spectrometry
Authors: Gnaser Hubert
Published: July 2002

The detection limits of secondary-ion-mass spectrometry for carbon, nitrogen, and oxygen in silicon are determined and are shown to be as low as about 1×1014 atoms/cm3 for nitrogen and 2×1015 atoms/cm3 for carbon and oxygen. This very high sensitivity is achieved by an analytical approach: to suppress the interference from residual-gas adsorption onto the sputtered surface, the elemental concentrations are extrapolated to a zero contribution from these adsorbed species by an incremental increase of the applied ion-beam erosion rate.
Terahertz field characterization using Fabry–Perot-like cantilevers
Authors: Dragoman D. , Dragoman M.
Published: July 2002

We show that a Fabry–Perot-like tunneling cantilever consisting of two parallel thin metallic cantilevers joined at their free ends is able to sense the power of an incoming terahertz (THz) field. An array of such microstructures, with linearly varying distances between the two parallel cantilevers, can measure simultaneously the power and frequency of the THz field in the bandwidth 1.2–58 THz.
On-the-fly depth profiling during ablation with ultrashort laser pulses: A tool for accurate micromachining and laser surgery
Authors: Lausten Rune , Balling Peter
Published: July 2002

A method for accurate depth profiling of a region subjected to ablation with ultrashort laser pulses is demonstrated. Time-gated imaging of the backscattered radiation from the ablation region is performed in a geometry, which allows the depth along a chosen axis on the sample to be determined with a single measurement. The profiling system has a spatial resolution of a few micrometers and applications are promoted by the fact that the measurement is performed with the same pulse that undertakes ablation. This also indicates that the method is inherently suited for in situ on-the-fly measurements.
Soft modes in order–disorder ferroelectrics
Authors: Liu Zhi-Rong , Duan Wenhui , Gu Bing-Lin , Zhang Xiao-Wen
Published: July 2002

We find that the conventional method to calculate the soft-mode frequency of the pseudospin model is incorrect, and present a valid modified method. It is demonstrated that the conventional method does not show the soft-mode frequency going to zero at a critical temperature in the presence of random internal fields, while the frequency calculated by our modified method goes to zero in random fields at a critical temperature. For the field-induced phase transition relaxor ferroelectrics, the soft-mode frequency decreases to zero at the first-order phase transition temperature though the symmetry has been destroyed at high temperatures under an external field.
Transport through a nine period silicon/oxygen superlattice
Authors: Seo Yong-Jin , Lofgrene John C. , Tsu Raphael
Published: July 2002

The silicon/adsorbed oxygen superlattice was introduced as an epitaxial barrier for silicon quantum devices. A barrier height of ∼0.5 eV with a thin layer of silicon sandwiched between two adsorbed oxygen layers has been achieved. This work describes a similar structure with nine periods which may serve as a replacement of the present amorphous silicon dioxide gate for metal oxide semiconductor field effect transistor. The insulating property derived from our current–voltage (I–V) and conductance–voltage (G–V) measurements indicates that the scheme may serve as a replacement of silicon on insulator.
Wave propagation in distributed media
Authors: Yang Jianbo , Kalliadasis Serafim , Merkin John H. , Scott Stephen K.
Published: July 2002

The problem of chemical reaction-diffusion wave propagation through a random, heterogeneous medium is considered using a model based on cubic autocatalysis with decay. The autocatalyst is taken to diffuse and react through a reactant loaded at constant initial concentration in a reaction domain except that there may be gaps of arbitrary width in which the reactant concentration is zero. We first study the propagation of a permanent-form wave across a single gap and determine the critical width of the gap in terms of the kinetic parameters in the system. The numerical results are compared with an analytical estimate. Next, the critical conditions for propagation across two gaps separated by a domain are determined numerically, and this is extended to a series of three gaps. From these results, a series of “rules” is established to allow us to predict whether a wave will pass through an arbitrary random array of gaps of a given size subject to some imposed total void fraction for the material.
Temperature dependence of line mixing effects in the stimulated Raman Q-branch of CO in He: A further test of close coupling calculations
Authors: Boissoles J. , Thibault F. , Domenech J. L. , Bermejo D. , Boulet C. , Hartmann J. M.
Published: July 2002

The fundamental isotropic Raman Q-branch of CO has been recorded using Raman loss spectroscopy for mixtures with helium at low temperatures (77–195 K). Accurate close coupling calculations of both diagonal and off diagonal (line coupling) cross sections have been performed starting from one of the best available potential energy surfaces [T. G. A. Heijmen, R. Moszynski, P. E. S. Wormer, and Ad van der Avoird, J. Chem. Phys. 107, 9921 (1997)]. Predictions are in good agreement with Q-branch Raman data. These results, which corroborate previous analysis based on IR absorption data, are a further test of the adequacy of the close coupling formalism for the prediction of line shapes in a rather wide range of physical situations (pressure, temperature, type of spectroscopy…), when an accurate potential exists.
On the accuracy of the fixed-node diffusion quantum Monte Carlo method
Authors: Manten Sebastian , Lüchow Arne
Published: July 2002

The accuracy of the fixed-node diffusion quantum Monte Carlo (FN-DQMC) method is compared to the coupled cluster method CCSD(T). For a test set of 20 small molecules and 17 reactions the electronic contribution to the reaction enthalpy is calculated with the FN-DQMC method using the nodes of a Slater determinant calculated at the HF/cc-pVTZ level. By comparison with reference reaction enthalpies the FN-DQMC method is shown to be more accurate than the CCSD(T)/cc-pVDZ method and almost as accurate as CCSD(T)/cc-pVTZ. The deviation from the reference data is comparable to the CCSD(T)/cc-pVTZ deviation, but, with only two exceptions, of opposite sign.
Berry phase in homogeneous Kähler manifolds with linear Hamiltonians
Authors: Boya Luis J. , Perelomov Askold M. , Santander Mariano
Published: July 2002

We study the total [dynamical plus geometrical (Berry)] phase of cyclic quantum motion for coherent states over homogeneous Kähler manifolds X=G/H, which can be considered as the phase spaces of classical systems and which are, in particular cases, coadjoint orbits of some Lie groups G. When the Hamiltonian is linear in the generators of a Lie group, both phases can be calculated exactly in terms of classical objects. In particular, the geometric phase is given by the symplectic area enclosed by the (purely classical) motion in the space of coherent states.
Stokes and Reynolds number dependence of preferential particle concentration in simulated three-dimensional turbulence
Authors: Hogan Robert C. , Cuzzi Jeffrey N.
Published: July 2002

An analysis of particle concentrations formed in direct numerical simulations of forced three-dimensional (3-D) turbulence is described. Up to 48 million particles responding passively to the flow with response times ranging from 0.2 to 6 times the dissipation time of the fluid were evolved together until the concentration field reached a statistical stationary state. The Stokes number (St), defined at the dissipation time scale, was the sole parameter used to characterize the particle–fluid coupling in the regime where particles preferentially concentrate. Concentrations resulting from three simulations equilibrating at the Taylor microscale Reynolds numbers (Reλ) 40, 80, and 140 were studied. We present several new results for concentration measures utilized in previous studies as well as measures introduced in this paper. The measures are compared and contrasted on a finer St grid than presented in previous work and are analyzed as functions of Reλ and spatial binning scale. The measures are based on (a) deviations of the concentration PDF (probability density function) from the PDF of a uniform particle field, (b) the correlation dimension (D2) for both 3-D and two-dimensional concentrations, and (c) the relative St-dependent concentrations contained in a localized region of space. Measure (c) is motivated by the observation that the total and St-dependent concentrations are linearly correlated. The concentration measures reveal St dependencies that are insensitive to the Reynolds number of the flow with each measure having its own characteristic shape. The widths and maxima of St functions for measures explicitly constructed on a single spatial binning scale showed a very weak dependence on bin sizes ranging from 2 to 6 times the Kolmogorov length scale. We conclude that the measures studied in this paper reveal a universality that may persist to much higher Reynolds numbers.
Generation of short millimeter-wave radio pulses using solitons in ferrite-based active rings
Authors: Kalinikos B. A. , Slavin A. N.
Published: July 2002

A method of generation of short high-power radio pulses at millimeter-wave frequencies is proposed. It uses pulsed self-generation in an active ring structure consisting of a wide-band millimeter-wave amplifier and a delay line, based on a highly anisotropic hexagonal ferrite film. The ring structure is operated in the nonlinear regime allowing self-generation of a periodic train of short soliton-like millimeter-wave radio pulses. The pulse power (P∼1.7 kW) and duration (τ∼2 ns) in the train are determined by the amplifier gain and the parameters of the hexaferrite film, while the period between pulses is determined by the time delay in the active ring.
Growth of crack-free hexagonal GaN films on Si(100)
Authors: Wan J. , Venugopal R. , Melloch M. R. , Liaw H. M. , Rummel W. J.
Published: July 2002

Hexagonal GaN films have been grown on Si(100) substrates by employing a sputtered AlN buffer layer followed by another high-temperature metalorganic chemical vapor deposition (MOCVD) grown AlN buffer layer. The highly oriented structure of sputtered AlN provides a hexagonal template for subsequent AlN and GaN growth. The GaN films are evaluated by transmission electron microscopy, selected area electron diffraction, x-ray diffraction, and photoluminescence and exhibit a purely hexagonal columnar structure. The orientation of the GaN columns depends on the thickness of both the sputtered AlN buffer layer and the MOCVD grown AlN buffer layer. The surface of GaN films is shiny and crack free up to a thickness of 2 μm studied in this work.
Enhanced luminescence from the disordered quantum-wire superlattice
Authors: Sasaki Akio , Okanishi Ryohei , Liu Xinquan , Wang Xuelun , Ogura Mutsuo
Published: July 2002

The disordered quantum-wire superlattice (d-QWRSL), in which individual layer thicknesses are randomly disordered, is grown. Luminescence characteristics are investigated with changing temperature and compared with those of a conventionally ordered quantum-wire superlattice (o-QWRSL). We observe the luminescence capability enhanced by disordering the atomic arrangement. The luminescence intensity of the d-QWRSL decays gently with increasing temperature and then becomes stronger above ∼100 K as compared with that of the o-QWRSL.
Upper bounds on the density of states of single Landau levels broadened by Gaussian random potentials
Authors: Hupfer Thomas , Leschke Hajo , Warzel Simone
Published: July 2002

We study a nonrelativistic charged particle on the Euclidean plane R2 subject to a perpendicular constant magnetic field and an R2-homogeneous random potential in the approximation that the corresponding random Landau Hamiltonian on the Hilbert space L2(R2) is restricted to the eigenspace of a single but arbitrary Landau level. For a wide class of R2-homogeneous Gaussian random potentials we rigorously prove that the associated restricted integrated density of states is absolutely continuous with respect to the Lebesgue measure. We construct explicit upper bounds on the resulting derivative, the restricted density of states. As a consequence, any given energy is seen to be almost surely not an eigenvalue of the restricted random Landau Hamiltonian.
Nitrogen effect on self-interstitial generation in Czochralski silicon revealed by gold diffusion experiments
Authors: Parakhonsky A. L. , Yakimov E. B. , Yang Deren
Published: July 2002

Gold diffusion in usual Czochralski (Cz) grown Si and in Cz Si doped with nitrogen during growth has been studied. The results presented can be explained under the assumption that the substitutional Aus concentration in the Cz Si samples is effected by the competition between the generation of self-interstitials and their annihilation at some defects. It is found that the Aus concentration after diffusion at 750 °C in Cz Si doped with nitrogen is always less than that in usual Cz Si independent of the thermal history of the wafers. A decrease in the Aus concentration in nitrogen doped crystals can be explained under the assumptions that nitrogen or nitrogen related centers stimulate oxygen precipitation or vacancy annihilation.
Crystal orientation dependence of optical gain in InGaN/GaN multiple-quantum-well structures
Authors: Chen Chii-Chang , Hsieh Kun-Long , Sheu Jinn-Kong , Chi Gou-Chung , Jou Ming-Juinn , Lee Chih-Hao , Lin Ming-Zhe
Published: July 2002

The net modal gain of the InGaN/GaN multiple quantum well has been measured by the variable excitation stripe length method for an optically pumped cavity along each crystal orientation on the (0001) plane. These results demonstrated the theoretical prediction of the fact that the maximum optical gain can be obtained at a [1̄21̄0]-oriented edge-emitting laser cavity, which has been reported in the literature. “Crystal orientation” is confirmed to be a related parameter to the optical gain for a GaN-based strained structure.
Misorientation-angle dependence of GaN layers grown on a-plane sapphire substrates by metalorganic chemical vapor deposition
Authors: Someya T. , Hoshino K. , Arakawa Y.
Published: July 2002

High-quality GaN with smooth surface morphology has been grown on vicinal a-plane sapphire substrates by metalorganic chemical vapor deposition. The misorientation angles of vicinal a-plane sapphire substrates were changed systematically and the results were compared with the growth on exact a- and c-plane sapphire substrates. Surface morphology and crystalline qualities are found to be very sensitive to misorientation angles of a-plane sapphire substrates and the misorientation angle was optimized to be 0.25°.
Continuous-wave operation of a 5.2 μm quantum-cascade laser up to 210 K
Authors: Ishaug Brian , Hwang Wen-Yen , Um Jae , Guo Bujin , Lee Hao , Lin Chih-Hsiang
Published: July 2002

Continuous-wave operation of a 5.2 μm-type I quantum-cascade laser with more than 5 mW of output power is reported at a heat sink temperature of 210 K (−63 °C). This temperature is within the range obtainable with thermal-electric coolers. The device was mounted epi-side down on a copper submount and exhibited a thermal resistance of ∼10 K/W at 210 K. Using the experimentally determined values for T0=136 K, J0=535 A/cm2 and Vop=8.1 V and the above thermal resistance, the maximum theoretical operating temperature was found to be 212 K, in close agreement with experiment. Thermal simulations show that by improving the device design and heat sinking, thermal resistance can be reduced to 8.8 K/W and the maximum cw operating temperature can be increased to 230 K.
Atomic-scale surface control and second-harmonic generation in GdxY1−xCa4O(BO3)3 thin films grown by combinatorial laser molecular-beam epitaxy
Authors: Kim T.-W. , Arai N. , Koinuma H. , Matsumoto Y. , Yoshimura M. , Furuya H. , Nakao H. , Mori Y. , Sasaki T.
Published: July 2002

We studied the atomic-scale growth control and second-harmonic generation (SHG) properties of GdxY1−xCa4O(BO3)3 thin films. Combinatorial laser molecular-beam epitaxy was employed to optimize the growth conditions of the film. Ultrasmooth GdxY1−xCa4O(BO3)3 thin films with step-and-terrace structure were obtained by using atomically controlled GdCa4O(BO3)3 substrates. The dependence of SHG intensity on the fundamental wave polarization angle was studied for the epitaxial film on (010) GdCa4O(BO3)3 substrate in comparison for the randomly oriented film on (0001) sapphire substrate. It was found that only the epitaxially grown film clearly exhibited a fourfold SHG intensity showing type-II phase-matching direction.
Asymmetry diffraction magneto-optical phenomenon of NiFe grating
Authors: Shen Y. T. , Wu Y. H. , Chong T. C. , Xie H. , Guo Z. B. , Li K. B. , Qiu J. J.
Published: July 2002

We report the asymmetry of longitudinal magneto-optical Kerr loops obtained from diffracted spots of a NiFe grating. The analysis of several series of diffracted Kerr hysteresis loops reveals that the unsaturated magnetic structure not only contributes to the shape change of Kerr loops, but also results in asymmetric feature of Kerr loops because of the nonzero value of second-order magnetic response. A suitable pattern dimension, which defines the form factor, is helpful to observe asymmetry Kerr loops. Our experiment indicates that out-of-incidence-plane diffraction may offer an alternative method to observe lateral edge domain movements in patterns.
Efficiency of GaN/InGaN light-emitting diodes with interdigitated mesa geometry
Authors: Guo X. , Li Y.-L. , Schubert E. F.
Published: July 2002

GaN/InGaN light-emitting diodes (LEDs) with different mesa structures are studied. The optical emission power as well as current–voltage characteristics of different mesa patterns are measured. The results show that the optical emission of the device with interdigitated patterns is higher than devices with traditional square-shaped patterns. The leakage current is found to increase as the mesa sidewall perimeter increases. Based on the analysis, it is concluded that a surface leakage current flows across the mesa sidewall and the leakage current is directly proportional to the mesa perimeter. The implications of the results for large-area scalable LED structures are discussed.
Influence of ultraviolet-light exposure on electron-beam written gratings in amorphous As–Se thin films coated with different metals
Authors: Nordman Olli , Nordman Nina , Pashkevich Valfrid
Published: July 2002

Gratings were written in amorphous As–Se thin films using electron-beam lithography. Films were coated with different metals. Electrons caused the change in the refractive index, which could be analyzed through the diffraction pattern of the grating reading laser beam. The refractive index change was found to be dependent on the overcoated metal layer. Ultraviolet (UV) light exposure was applied on investigated films before and after the grating writing. In some cases the exposure given after the grating writing enhanced greatly the diffraction efficiency. Results suggest the use of these thin films as UV-light detectors.
Self-assembly of photoluminescent silicon films: Influence of doping on the physical properties
Authors: Di Francia G. , La Ferrara V. , Morvillo P. , Lettieri S. , Maddalena P.
Published: July 2002

Thin photoluminescent silicon films are fabricated by means of a purely wet-chemical process using, as a starting material, a fine powder obtained by ball milling p- and n-type silicon wafers. The reaction is characterized by a coalescence phenomenon and produces photoluminescent films whose physical properties depend on the material type. Samples fabricated by processing a mixture of p- and n-type powders exhibit different photoluminescent spectra, have lower reactivity towards oxidating environments, and show the longest emission lifetimes. In order to explain those properties, we propose that, as long as the reaction proceeds and consumes the silicon powders, nanostructures containing both p- and n-type silicon form. Suppression of the Auger recombination in such structures can account for the experimental findings.
Spectroscopic analysis and persistent photon-gated spectral hole burning in LiF:F2− color center crystal
Authors: Fedorov V. V. , Mirov S. B. , Ashenafi M. , Xie L.
Published: July 2002

Luminescence and absorption spectra of LiF crystals with F2− color centers (CCs) were studied in the 14–300 K temperature range. The Huang–Rhys factors measured from absorption and emission spectra were equal in magnitude and calculated to be 3.2 and 2.72 at 77 and 300 K, respectively. It was shown that below 40 K inhomogeneous broadening of the zero phonon line of the F2− CC prevails over homogeneous broadening. Inhomogeneous broadening of the zero phonon line was observed to vary from 5 to 15 cm−1 for different crystal preparations. Persistent spectral hole burning stable at room temperature was realized in LiF:F2− crystals.
Role of particle size distribution on the temperature dependence of coercive field in sputtered Co/Cu granular films
Authors: Vavassori P. , Angeli E. , Bisero D. , Spizzo F. , Ronconi F.
Published: July 2002

The temperature dependence of coercive field and of the ratio between the remanent and saturation magnetization of granular Co/Cu films grown by sequential sputtering has been studied with magneto-optic Kerr effect measurements in the temperature range 8.5–60 K. The observed temperature dependence of coercive field does not fit any of the plausible Tx laws commonly used to describe systems of single-domain ferromagnetic particles. We worked out a generalized model, which also includes temperature effects related to nonuniform single-domain particle size distribution. The model predictions account well for the observed temperature behavior of both coercive field and ratio between the remanent and saturation magnetization.
Nitrogen pairs, triplets, and clusters in GaAs and GaP
Authors: Kent P. R. C. , Zunger Alex
Published: July 2002

The electronic and atomic structure of substitutional nitrogen pairs, triplets, and clusters in GaP and GaAs is studied using the multiband empirical pseudopotential method with atomistically relaxed supercells. A single nitrogen impurity creates a localized a1(N) gap state in GaP, but in GaAs, the state is resonant above the conduction-band minimum. We show how the interaction of multiple a1 impurity levels, for more than one nitrogen, results in a nonmonotonic relationship between energy level and impurity separation. We assign the lowest (NN1) line in GaP to a [2,2,0] oriented pair, the second (NN2) line to a triplet of nitrogen atoms, and identify the origin of a deeper observed level as an [1,1,0] oriented triplet. We also demonstrate that small nitrogen clusters readily create very deep levels in both GaP and GaAs.
Recombination efficiency in organic single-layer light-emitting diodes at high fields
Authors: Yang Shengyi , Xu Zheng , Wang Zhenjia , Xu Xurong
Published: July 2002

By considering the interaction between charge carriers recombination and transport through the bulk at high fields, the expression of recombination efficiency for single-layer diodes is presented. By parameters calculation, a maximum recombination efficiency is promised for device ITO/MEH-PPV(120 nm)/Ca, and the same experimental fact for ITO/BEH-PPV(100 nm)/Ca [Y. Kawabe, et al., J. Appl. Phys. 84, 5306 (1998)] proved our theoretical prediction.
Thermal stability and diffusion in gadolinium silicate gate dielectric films
Authors: Landheer D. , Wu X. , Morais J. , Baumvol I. J. R. , Pezzi R. P. , Miotti L. , Lennard W. N. , Kim J. K.
Published: July 2002

Gadolinium silicate films on Si(100) annealed in oxygen and vacuum at temperatures up to 800 °C were analyzed by Rutherford backscattering and narrow resonance nuclear profiling. Oxygen diffused into the film eliminating oxygen vacancies, but Si diffusion, previously observed in Al and Y oxides and in La and Zr silicate films, was absent. Higher-temperature annealing in oxygen resulted in the formation of an interfacial layer observable in high-resolution electron micrographs. Gd0.23Si0.14O0.63 films crystallize at temperatures between 1000 and 1050 °C. These observations combined with recent electrical measurements show that gadolinium silicate films may be a good candidate for the replacement of SiO2 in deep submicron metal–oxide–semiconductor gates.
Characterization of the electrical properties and thickness of thin epitaxial semiconductor layers by THz reflection spectroscopy
Authors: Hashimshony D. , Geltner I. , Cohen G. , Avitzour Y. , Zigler A. , Smith C.
Published: July 2002

We have measured the dielectric properties and thickness of thin semiconductor epitaxy layers by the reflection of THz radiation from the surface of a two-layered semiconductor wafer. When reflecting from two interfaces the electromagnetic pulse has a destructive interference at a specific wavelength dependent on the thickness of the outer layer and its dielectric function. Near that frequency the reflection coefficient has a significant drop. By extending the incident pulse spectrum to include this interference frequency, a measurement of the thickness can be obtained together with a direct measurement of the carrier number density. By this technique epitaxy layers of thickness down to 15 μm are characterized.
Fingerprints of two distinct defects causing light-induced photoconductivity degradation in hydrogenated amorphous silicon
Authors: Heck Stephan , Branz Howard M.
Published: July 2002

We find distinct experimental fingerprints of two metastable defects created during illumination of hydrogenated amorphous silicon. The well-studied threefold-coordinated silicon dangling bond defect has an anneal activation energy near 1.1 eV and dominates annealing experiments above about 110 °C. The second defect created by illumination is the “primary recombination” (pr) center, which causes most of the light-induced photoconductivity decrease and dominates annealing experiments below about 110 °C. Because the pr centers are created in linear proportion to the dangling bond defects, they are difficult to distinguish during degradation. However, we observe clear fingerprints of the pr center during their low T annealing: (1) an anneal activation energy of 0.85 eV; (2) a sharp increase in photoconductivity; and (3) a surprising increase in hν⩽1.1 eV optical absorption.