bluuebunny/crossref_metadata_2025 huggingface dataset:
Coupling of framework modes and adsorbate vibrations for CO2 molecularly adsorbed on alkali ZSM-5 zeolites: Mid- and far-infrared spectroscopy and ab initio modeling
Authors: Garrone E. , Bonelli B. , Lamberti C. , Civalleri B. , Rocchia M. , Roy P. , Otero Areán C.
Published: November 2002

A thorough vibrational characterization of CO2 molecules adsorbed at room temperature on alkali metal exchanged M–ZSM-5 zeolites (M=Li+, Na+, K+, and Cs+) has been obtained: All three fundamental modes (ν1, ν2, and ν3) have been measured in the mid-IR, together with combination bands of the ν1 and ν3 modes with modes at 18, 45, and 60 cm−1. The nature of these low-lying modes is discussed, and it is proposed that these are framework vibrations. Ab initio calculations on molecular models mimicking the building blocks of zeolites revealed the systematic presence of such low-lying modes, and far-IR measurements using synchrotron radiation showed a couple of bands at 55–45 cm−1 in the spectrum of the bare samples, which undergo a slight perturbation upon CO2 adsorption. The presence of combination modes between molecular and framework vibrations lends support to the concept already advanced [Bonelli et al., J. Phys. Chem. B 104, 10978 (2000)] that the CO2 molecule has a primary interaction with the cation and a secondary one with an adjacent oxygen anion; the latter not strong enough to lead to a carbonate species. The occurrence of similar combination bands for CO2 molecularly adsorbed on other systems is also discussed.
Molecular dynamics simulation of heterogeneous nucleation at a structureless solid surface
Authors: Toxvaerd S.
Published: November 2002

The nucleation at a planar surface with attraction is investigated by molecular dynamics and compared with the corresponding homogeneous nucleation [J. Chem. Phys. 115, 8913 (2001)]. The attraction caused an increased vapor density near the surface which favors nucleation even for a weak attraction. The contact angle (Young’s angle) for the critical nucleus cannot be obtained from Young’s equation by using the equilibrium values for the tensions, due to the overlap of the solid–liquid interface with the liquid gas interface near the line of contact. After onset of nucleation the droplets grow algebraically with an increase of particles with a time exponent ≈1.25, and different from the Lifshitz–Slyozov growth. For strong attractions the system undergoes a first-order prewetting transition to a thin two-dimensional-like layer of a thickness of only a few particle diameters. This happens at densities less than the density of saturated bulk vapor. The dynamics of prewetting behaves qualitatively as the homogeneous and heterogeneous nucleation and with the onset of prewetting by crossing an activation free energy barrier.
Effects of anharmonicity and electronic coupling on photoinduced electron transfer in mixed valence compounds
Authors: Lockwood Daren M. , Ratner Mark A. , Kosloff Ronnie
Published: November 2002

We develop a semigroup model of electron transfer (ET) dynamics in mixed valence compounds. This model is useful for investigating the effects of anharmonicity in inner sphere nuclear modes, as well as the dependence of the electronic dynamics on the nature of the electronic coupling. Two effective “subsystem” nuclear vibrations are treated explicitly in the model, to account for the rapid electronic energy gap fluctuations induced by the inner sphere vibrations. The essentially Markovian effects of the remaining “bath” modes are approximated by semigroups. We find that including the anharmonicity in inner sphere vibrations leads to a very small increase in the rate of ET. This effect is due to the change in reactant and product vibronic states when anharmonicity is included, as well as the rapid electronic dephasing induced by the bath. An assumption of strong electronic coupling is found to be sufficient to explain experimentally observed ET rates, but the possible role of conical intersections in ultrafast ET reactions is also noted.
Exciton–exciton interaction engineering in coupled GaN quantum dots
Authors: De Rinaldis Sergio , D’Amico Irene , Rossi Fausto
Published: November 2002

We present a fully three-dimensional study of the multiexciton optical response of vertically coupled GaN-based quantum dots via a direct-diagonalization approach. The proposed analysis is crucial in understanding the fundamental properties of few-particle/exciton interactions and, more important, may play an essential role in the design/optimization of semiconductor-based quantum information processing schemes. In particular, we focus on interdot exciton–exciton coupling, the key ingredient in recently proposed all-optical quantum processors. Our analysis demonstrates that there is a large window of realistic parameters for which both biexcitonic shift and oscillator strength are compatible with such implementation schemes.
Theory of T junctions and symmetric tilt grain boundaries in pure and mixed polymer systems
Authors: Duque Daniel , Katsov K. , Schick M.
Published: November 2002

We apply self-consistent-field theory to T junctions and symmetric tilt grain boundaries in block copolymer systems with and without the addition of homopolymer. We find that, in the absence of homopolymer, T junctions have a larger free energy per unit area than that of the symmetric tilt junctions with which they compete except for a range of angles between about 100° and 130°. With the addition of homopolymer, this range increases. These results are quite consistent with experiment. As the angle between grains increases towards 180°, the T junction undergoes a morphological change somewhat similar to that which occurs in symmetric tilt grain boundaries. At the onset of this change, the free energy per unit area decreases markedly.
Computer simulation of copolymer phase behavior
Authors: Schultz Andrew J. , Hall Carol K. , Genzer Jan
Published: November 2002

Discontinuous molecular dynamics simulation is used to study the phase behavior of diblock copolymers modeled as chains of tangent hard spheres with square shoulder repulsions between unlike species as a function of chain length, volume fraction and interaction strength (χ). The location of the order–disorder transition for a symmetric copolymer is close to the predictions of Fredrickson and Helfand. Our simulation results for packing fractions of 0.35, 0.40, and 0.45 and chain lengths 10 and 20 are summarized in phase diagrams which display disordered, lamellae, perforated lamellae (PL), cylindrical, and BCC spherical (S) phases in the χN versus f plane. These phase diagrams are consistent with phase diagrams from other simulation studies. Contrary to theoretical predictions we observe the PL phase near regions of predicted gyroid stability, and the S phase only in the systems with high packing fraction and long chain length. These discrepancies may be due to the short chain lengths considered, as they are less evident in the 20-bead chains than the 10-bead chains. We examine the structural spacing of the microphases and the variation of that spacing with χN. We also examine the internal energy and entropy and their variation with χN. Our results are consistent with self-consistent field theory results for the strong segregation limit.
Dielectric functions of the double layer polarization theory of a charged colloidal suspension
Authors: Lu C.-Y. D.
Published: November 2002

The double layer polarization theory is analyzed, where the dielectric storage ε′(ω) and the dielectric loss ε″(ω) are expressed in terms of the free energy stored and the free energy dissipation. The dielectric functions are further put into the canonical form of the sum of the contributions from the elementary excitations. The excitations obey the extremal dissipation principle, subjected to the appropriate constraint. We calculate the dielectric function of the suspension of charged spheres as an example, where a simple algebraic formula is obtained.
Diffuse interface analysis of crystal nucleation in hard-sphere liquid
Authors: Gránásy László , Pusztai Tamás
Published: November 2002

We show that the increase of the interface free energy with deviation from equilibrium seen in recent Monte Carlo simulations [S. Auer and D. Frenkel, Nature (London) 413, 711 (2001)] can be recovered if the molecular scale diffuseness of the crystal–liquid interface is considered. We compare two models, Gránásy’s phenomenological diffuse interface theory, and a density functional theory that relies on the type of Ginzburg–Landau expansion for fcc nucleation, that Shih et al. introduced for bcc crystal. It is shown that, in the range of Monte Carlo simulations, the nucleation rate of the stable fcc phase is by several orders of magnitude higher than for the metastable bcc phase, seen to nucleate first in other fcc systems. The nucleation barrier that the diffuse interface theories predict for small deviations from equilibrium is in far better agreement with the simulations than the classical droplet model. The behavior expected at high densities is model dependent. Gránásy’s phenomenological diffuse interface theory indicates a spinodal point close to glass transition, while a nonsingular behavior is predicted by the density functional theory with constant Ginzburg–Landau coefficients. Remarkably, a minimum of the nucleation barrier, similar to the one seen in polydisperse systems, occurs if the known density dependence of the Ginzburg–Landau coefficients is considered.
Formation of face-centered-cubic zirconium by mechanical attrition
Authors: Manna I. , Chattopadhyay P. P. , Banhart F. , Fecht H.-J.
Published: November 2002

This study reports a hcp→fcc polymorphic transformation in elemental zirconium induced by mechanical attrition in a planetary ball mill. The transformation is monitored and verified by x-ray and electron diffraction and high-resolution transmission electron microscopy. Grain size decreases and lattice parameter increases with continued milling. The phase change is gradual and is accompanied by about 9% increase in volume per atom. It is suggested that structural instability due to plastic strain, increasing lattice expansion, and negative (from core to boundary) hydrostatic pressure is responsible for this hcp→fcc polymorphic transformation in zirconium.
Study of the stretching vibrational band intensities of XH4 molecules employing four-dimensional ab initio (X=C and Sn) and effective (X=C and Si) dipole moment surfaces
Authors: He Sheng-Gui , Liu An-Wen , Lin Hai , Hu Shui-Ming , Zheng Jing-Jing , Hao Lu-Yuan , Zhu Qing-Shi
Published: November 2002

Stretching vibrational band intensities of XH4 molecules were investigated employing four-dimensional ab initio (X=C and Sn) and effective (X=C and Si) dipole moment surfaces (DMS) in combination with the local mode potential energy surfaces. The ab initio DMS of CH4 and SnH4 calculated at the coupled cluster CCSD(T) level of theory reproduced most of the observed intensities within a factor of 1.5. The effective DMS of CH4 and SiH4 were obtained by adjusting some selected high-order terms in the ab initio DMS to fit the observed intensities. They were applied to the corresponding deuterated isotopomers yielding better results than the ab initio DMS. The intensities of the combination bands are mainly due to the interbond cross terms in the DMS for SiH4, GeH4, and SnH4, while for CH4, both diagonal and cross terms are important. The relatively strong combination band that has comparable intensity with the pure overtone was predicted at the fourth local mode manifold for SnH4.
An extended Heisenberg model for conjugated hydrocarbons
Authors: Wu Jian , Schmalz T. G. , Klein D. J.
Published: November 2002

An improved valence bond model, in the form of an extended Heisenberg model including next-nearest-neighbor transpositions and six-cyclic permutations, is developed to describe the low-lying “covalent” portion of the Hubbard model eigenspectrum (at half filling). The use of a cluster expansion including all diameter 1 and 2 subgraphs plus the diameter 3 six-membered ring allows the coefficients in the extended Heisenberg model to be determined from the results of accurate calculations of the Hubbard spectrum of benzene. The model is found to predict the lower energy states of two test molecules, styrene and naphthalene, rather accurately, and is thus expected to be generally applicable to benzenoid aromatic hydrocarbons.
A time dependent density functional treatment of superfluid dynamics: Equilibration of the electron bubble in superfluid He4
Authors: Eloranta J. , Apkarian V. A.
Published: November 2002

Time dependent density functionals are formulated and implemented in numerical simulations of the equilibration dynamics of an excess electron in superfluid helium. Previously developed density functionals that incorporate nonlocal potential and kinetic correlations and reproduce the dispersion curve of liquid He4, are used. The electron–helium interaction is treated using pseudopotentials, after testing their accuracy in reproducing the static properties of the solvated electron bubble through its known spectroscopy. The dynamics initiated by the sudden compression of the bubble is dissected, and the results are favorably compared to classical hydrodynamics. In the near-field, the fast motion corresponds to interfacial compressional waves, followed by the slow breathing of the cavity. The far-field motion consists of a shock wave, followed by radiating sound waves. The solitonic shock wave propagates at speeds as high as 580 m/s, determined by the amplitude of excitation. The energy carried by the shock front ensures that the subsequent bubble dynamics occurs in the linear response regime. Dissipation occurs through radiation of sound during the acceleration stages of the bubble, carried by driven phonons of λ=cτ=300 Å, where c=240 m/s is the speed of sound, and τ=130 ps is the breathing period of the bubble. The interfacial waves generate traveling excitations at k=2 Å−1, high on the positive roton branch. Excitations in the roton well are not observed. The time dependent spectroscopy of the trapped electron is shown to provide a sensitive probe of the evolving dynamics by tracking the damped oscillations of the bubble, which is damped in two periods. The results are consistent with the related time-resolved experiments on He2* Rydberg electrons, and significantly different from prior estimates of the electron-bubble relaxation dynamics.
Rotation of O2 molecules in solid D2 and HD: An electron spin resonance study
Authors: Kumada Takayuki
Published: November 2002

X-band electron spin resonance (ESR) spectroscopy has been applied to the study of molecular rotation of O2 molecules in isotopic solid hydrogen, D2 and HD. ESR signal of the O2 molecules in hindered rotational states has been observed, and its pressure dependence has been measured up to 19 MPa. Although molar volume of solid hydrogen decreases, the O2 rotation has become less hindered with the increase in pressure and isotope substitution from HD to D2. These effects of pressure and isotope substitution suggest that the potential barrier for the O2 rotation is mainly produced by distortion of O2 cages in D2 and HD. Since solid hydrogen becomes less compressible with the increase in pressure and the isotope substitution from HD to D2, the O2 rotation becomes less hindered in the less distorted O2 cages.
Ion soliton observation with laser-induced fluorescence
Authors: Claire N. , Bachet G. , Skiff F.
Published: November 2002

A laser-induced fluorescence observation of ion-acoustic waves in a collisionless unmagnetized double plasma multipolar device is presented. The optical pumping effect is found to be critical for the interpretation of fast changes of the ion velocity distribution functions induced by the propagation of a soliton. To take this effect into account, the continuity equation is used. The laser-induced fluorescence diagnostic shows that the separation of solitons requires a small plasma drift in the backward direction (reverse direction of the soliton propagation) and that the precursor ions are in fact a precursor wave.
Some cross-field instabilities in magnetized dusty plasmas
Authors: Shukla P. K. , Salimullah M. , Sorasio G.
Published: November 2002

It is shown that the resonant streaming energy associated with cross-field motions (due to the cross product of dc electric and magnetic fields) of the plasma particles can be coupled to low-frequency electrostatic waves. Conditions under which unstable waves appear are obtained for both collisionless and collisional dusty magnetoplasmas. The relevance of the present investigation to understanding the origin of nonthermal fluctuations in laboratory and space plasmas is pointed out.
Nucleation temperature of elements
Authors: Jiang Q. , Zhou X. H. , Zhao M.
Published: November 2002

A general equation, supported by two reasonable asymptotic limits and being free of any adjustable parameters, for the size-dependent and temperature-dependent solid–liquid interface free energy is developed. The prediction of the equation for homogeneous nucleation temperature of metallic elements corresponds well to Turnbull’s experiments. Moreover, the heterogeneous nucleation temperatures of Al nanowires and Pb thin films are predicted, which is in agreement with experimental results.
Structures of hard-sphere fluids from a modified fundamental-measure theory
Authors: Yu Yang-Xin , Wu Jianzhong
Published: November 2002

We reformulate Rosenfeld’s fundamental-measure theory using the excess Helmholtz energy density from the Boublik–Mansoori–Carnahan–Starling–Leland equation of state instead of that from the scaled-particle theory. The new density functional theory yields improved density distributions, especially the contact densities, of inhomogeneous hard-sphere fluids as well as more accurate direct and pair correlation functions of homogeneous hard spheres including those of highly asymmetric mixtures. This new density functional theory will provide an accurate reference for the further development of a statistical-thermodynamic theory of complex fluids at uniform and at inhomogeneous conditions.
Practical implementation of the instanton theory. II. Decay of metastable state through tunneling
Authors: Mil’nikov Gennady V. , Nakamura Hiroki
Published: November 2002

A new instanton theory for decay rate problem at zero temperature is presented. The canonically invariant expression for the lifetime of metastable state is derived. The theory is fully implemented by an effective numerical recipe to find the instanton trajectory and is applicable to any high dimensional systems.
Catching the collision complex through a femtosecond coherently controlled pump/probe process
Authors: Grégoire G. , Kang H. , Dimicoli I. , Dedonder-Lardeux C. , Martrenchard S. , Jouvet C.
Published: November 2002

We propose a very simple and efficient way to stabilize ions issued from a collision complex through a femtosecond coherently controlled pump/probe process. Starting from a van der Waals complex, one can initiate a collision at a well-defined time and with a restricted impact parameter. Formation of stable ionic complex can be achieved by ionizing the collision complex at the “right time.” We present in this paper its application to the NaI–(CH3CN)1–2 system. Na+–CH3CN ion formation is coherently controlled by ionization of colliding Na atom on CH3CN molecules issued from the dissociation of NaI within NaI–(CH3CN)1–2. Classical mechanic calculations using simple ionization/dissociation conditions can reproduce the experimental data and give an insight into the control of such a reaction.
Complex hopping dynamics in the deuteron conductors K3D(SO4)2 and Rb3D(SO4)2
Authors: Qi F. , Winterich M. , Titze A. , Böhmer R.
Published: November 2002

The translational deuteron motions in the paraelectric phases of K3D(SO4)2 and Rb3D(SO4)2 were studied using two-dimensional exchange nuclear magnetic resonance, performed in the frequency as well as in the time domains. These experiments reveal that in Rb3D(SO4)2 the ion hopping is about one order of magnitude slower than in K3D(SO4)2. Additional measurements of the spinlattice relaxation times were carried out at the proton, deuteron, and rubidium sites. It was found that the relaxation times determined for deuterated as well as for protonated tri-rubidium-hydrogen-disulfates are a factor of ten longer than those for the corresponding tri-potassium-hydrogen-disulfates. The results reveal that the deuteron hopping is to be regarded as a two-step process involving a coupling to rotational or librational motions of the sulfate tetrahedra.
Adsorption configurations and thermal chemistry of acetylene on the Ge(100) surface
Authors: Kim Ansoon , Maeng Jae Yeol , Lee Jun Young , Kim Sehun
Published: November 2002

The adsorption structures and thermal desorption behavior of C2H2 on Ge(100) were studied in ultrahigh vacuum by scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). The STM investigation revealed that, at low coverage, C2H2 initially adsorbs onto the Ge(100) surface with two types of configurations: (i) a di-σ configuration on top of a single Ge–Ge dimer (on-top) and (ii) a tetra-σ configuration parallel to the dimer axes, bridging two neighboring Ge dimers (p-bridge). TPD measurements show that chemisorbed C2H2 desorbs from Ge(100) nondissociatively with two different desorption features, denoted as α (520 K) and β (560 K). In addition, it was found that the desorption of C2H2 follows first order kinetics for both states and that the desorption energies of the α (520 K) and β (560 K) states are 1.3 and 1.4 eV, respectively. STM studies of the adsorption of C2H2 at various Ge surface temperatures indicate that the α and β features correspond to the on-top and p-bridge configurations, respectively.
The Alfvén resonance in a dusty plasma with a distribution of grain sizes
Authors: Cramer N. F. , Verheest F. , Vladimirov S. V.
Published: November 2002

The resonance absorption of low-frequency electromagnetic waves in a dusty magnetized plasma is considered. The dust grains are assumed to carry a proportion of the negative charge of the plasma, and there is assumed to be a distribution of dust sizes, typical of that deduced to be present in interstellar molecular clouds. The resonances and cutoffs of the waves in a homogeneous plasma are discussed. The waves are shown to be damped due to Alfvén resonance, and it is shown that the effects of the distributed dust sizes are to produce a broad spatial region of resonance absorption due to dust cyclotron damping.
Hall magnetohydrodynamics model of double discontinuities
Authors: Whang Y. C.
Published: November 2002

A double discontinuity is a compound structure composed of a slow shock layer and an adjoining rotational discontinuity layer on the postshock side. Since the Hall current effects become important as the magnetic field rotates by tens of degree over a thin rotational layer, steady state solutions based on the Hall magnetohydrodynamics (MHD) theory can show the merging of a rotational layer and a slow shock layer to form the observed compound structure. This model uses a set of modified Rankine–Hugoniot relations to calculate the jump conditions between the upstream region and the interface between the two layers, and uses the Hall-MHD equations to calculate the variations of the plasma and magnetic field in the rotational layer in the downstream of the interface. Four series of solutions are presented to show the effect for each of four governing parameters on the structure of double discontinuities.
The stabilizing effect of external gas flows on the arc helical instability
Authors: Gong Ye , Gong Jiquan , Liu Jinyuan , Wang Xiaogang , Zhang Pengyun , Ma Tengcai
Published: November 2002

The effect of gas flow on the helical instability of arcs is studied by using a linear time dependent perturbation theory in this paper. Based on a set of electrostatic magnetohydrodynamic equations, the corresponding equations and the boundary conditions are derived in the presence of a vortical gas flow, combined by a rotation and an axial component (the rotating flow and the axial flow). The effects of rotating flow, axial flow, and both of them on the arc stability are discussed, respectively. The marginal Maecker’s number and the growth rate of the helical instability are given. Computational results show that the gas flow stabilizes the arc helical instability. In particular, the axial flow (the axial component) plays a more important role in practical applications.
Force dependent transition rates in chemical kinetics models for motor proteins
Authors: Lattanzi Gianluca , Maritan Amos
Published: November 2002

We analyze the role of external forces (both chemical and mechanical) in the kinetics of motor proteins. Based on a generalized detailed balance condition, simple exponential force dependent transition rates are widely used to interpret the available data. Yet, the use of Fokker–Planck equations in continuous models allows for a direct insertion of the force. We describe an analytical approach, based on a renormalization group scheme, to calculate the force dependence of transition rates in a generic model. Our analysis shows that the simple exponential is a good approximation to the correct force dependence only at low values of forces and provided that the step sizes are very small. The law for the force dependent transition rates is tested on a set of data on kinesin, obtaining a good agreement with existing results and predictions for future experiments.
On tokamak equilibria with a zero current or negative current central region
Authors: Chu M. S. , Parks P. B.
Published: November 2002

Several tokamak experiments have reported the development of a central region with vanishing currents (the current hole). The straightforward application of results from the work of Greene, Johnson and Weimer [Phys. Fluids 14, 671 (1971)] on a tokamak equilibrium to these plasmas leads to the apparent singularities in several physical quantities including the Shafranov shift and casts doubts on the existence of this type of equilibria. In this paper, the above quoted equilibrium theory is re-examined and extended to include equilibria with a current hole. It is shown that singularities can be circumvented and that equilibria with a central current hole do satisfy the magnetohydrodynamic equilibrium condition with regular behavior for all the physical quantities and do not lead to infinitely large Shafranov shifts. Isolated equilibria with negative current in the central region could exist. But equilibria with negative currents in general do not have neighboring equilibria and thus cannot have experimental realization, i.e., no negative currents can be driven in the central region.
An exact solution for externally driven surface plasma waves
Authors: Stenflo L. , Yu M. Y.
Published: November 2002

An exact solution is obtained for large amplitude volume and surface waves driven by an external electric field oscillating at double the surface-wave frequency in a cold cylindrical plasma bounded by a rigid dielectric.
Moment equation description of Weibel instability
Authors: Basu B.
Published: November 2002

A macroscopic description of the linear Weibel instability, based on a closed set of linear moment equations, is presented. The moment equations are derived from the linearized Vlasov equation by taking the appropriate velocity moments of it and the closure is achieved by means of an assumption, which is justified when the temperature anisotropy is strong. The macroscopic description is manifestly more informative of the physical mechanism of the instability than the kinetic description. It is hoped that the researchers will find such a description analytically more convenient to use in solving plasma physics problems where Weibel instability due to strong temperature anisotropy plays a role.
Observation and analysis of a resistive mode with interchange parity in negative central shear plasmas in the DIII-D Tokamak
Authors: Jayakumar R. , Luce T. C. , Taylor T. S. , Turnbull A. D. , Wade M. R. , Austin M. E. , Casper T. A. , Makowski M. A. , Chu M. S. , Lao L. L. , Strait E. J. , Brennan D. P.
Published: November 2002

A magnetohydrodynamic (MHD) instability which has the characteristics that the displacement phase is in the same direction at all the affected flux surfaces (an interchange-like structure with no phase reversal across a flux surface), has been observed during the L mode in several negative central shear discharges of DIII-D tokamak [J. L. Luxon, P. Anderson, F. Baity et al., Plasma Physics and Controlled Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159]. The instability occurs when the minimum safety factor is around 2.0 and the profile of the safety factor q is deeply reversed in the center. Detailed stability analyses were carried out using standard numerical codes and the high quality magnetic probe, motional Stark effect and electron cyclotron emission (ECE) data. Analysis of the data after the onset shows that the instability has an n=1 mode number and a growth time of about 400 μs. The electron temperature fluctuations obtained from ECE measurements indicate a localized interchange-like structure early in time, the resistive interchange criterion indicates marginal stability, and ideal mode analyses indicate robust stability with an ideal beta limit of about a factor of 2 higher than the βN value at the time of onset. Therefore this interchange-parity mode is not an ideal MHD mode. The marginal value of the resistive interchange criterion observed only in discharges with the instability, indicates that this is probably a resistive interchange mode. However, some observed characteristics of the instability may require models beyond the linear resistive interchange theory.
Dynamic response of a one-component plasma at moderate coupling
Authors: Wierling A. , Pschiwul T. , Zwicknagel G.
Published: November 2002

The dynamic structure factor of one-component plasmas at moderate coupling (Γ=0.5–8) is studied analytically and by molecular dynamics simulations. The recurrence relation technique is used to derive a parameterization of the dynamic structure factor taking into account analytically accessible moments. These moments are calculated from the pair distribution function obtained within the hypernetted chain approximation. A truncation of the recurrence relations on the level of the second-order memory function is proposed. It is shown, that this presents an efficient way to incorporate collisions.
Fabrication of dot matrix, comb, and nanowire structures using laser ablation by interfered femtosecond laser beams
Authors: Nakata Yoshiki , Okada Tatsuo , Maeda Mitsuo
Published: November 2002

A line of periodic dot structure with an interval of 6.25 μm on a gold thin film was fabricated with a single shot of interfered femtosecond laser beams split by a diffraction beam splitter. The total length of the structure was 6 mm. In addition, dot matrix and comb structures were fabricated with transportation of samples at an arbitrary speed during the process. The samples worked as transmission and reflection gratings. In addition, nanowires were fabricated by peeling the comb structure, of which the thickness was 200 nm.
High-temperature centrifugation: a tool for finding eutectic compositions in multicomponent alloys
Authors: Löffler Jörg F. , Bossuyt Sven , Peker Atakan , Johnson William L.
Published: November 2002

A metallic melt of composition Al52.6Cu13.4Ge28Si6 was processed for 2 h in a centrifuge at a temperature of 530 °C and inertial acceleration of 60 000 g (g=gravitational acceleration), and then slowly cooled to room temperature during continuous centrifugation. Primary phases forming in the melt during cooling were sequentially separated from the remaining liquid, changing the composition of the liquid, until it solidified in a ternary eutectic microstructure of composition Al64Cu3Ge33. Presenting scanning electron microscope images, we demonstrate that this method of high-temperature centrifugation is a efficient tool to isolate and identify multiphase eutectic compositions and the sequence of crystallization in multicomponent alloys. The latter is particularly useful for the discovery of new bulk metallic glasses.
Suppressed crystallization of Hf-based gate dielectrics by controlled addition of Al2O3 using atomic layer deposition
Authors: Ho M.-Y. , Gong H. , Wilk G. D. , Busch B. W. , Green M. L. , Lin W. H. , See A. , Lahiri S. K. , Loomans M. E. , Räisänen Petri I. , Gustafsson T.
Published: November 2002

We demonstrate significantly improved thermal stability of the amorphous phase for hafnium-based gate dielectrics through the controlled addition of Al2O3. The (HfO2)x(Al2O3)1−x films, deposited using atomic layer deposition, exhibit excellent control over a wide range of composition by a suitable choice of the ratio between the Al and Hf precursor pulses. By this method, extremely predictable hafnium aluminate compositions are obtained, with Hf cation fractions ranging from 20% up to 100%, as measured by medium energy ion scattering. Using x-ray diffraction, we show that (HfO2)x(Al2O3)1−x films with Hf:Al∼3:1 (25% Al) remain amorphous up to 900 °C, while films with Hf:Al∼1:3 (75% Al) remain amorphous after a 1050 °C spike anneal.
Zonal flows and transport in ion temperature gradient turbulence
Authors: Dastgeer Sheikh , Mahajan Sangeeta , Weiland Jan
Published: November 2002

The nonlinear “Dimits” upshift found in gyrokinetic Cyclone simulations has been recovered in simulations using a reactive fluid model. Its magnitude is in good agreement with the gyrokinetic simulations. The upshift is caused by resonant excitation of zonal flows which also provide an absorbing boundary for long wavelengths in the regime above the nonlinear upshift. In agreement with analytical predictions, magnetic shear has a destabilizing effect in combination with zonal flows.
Simultaneous generation of coherent light in the three fundamental colors by quasicylindrical ferroelectric domains in Sr0.6Ba0.4(NbO3)2
Authors: Romero J. J. , Jaque D. , Garcı́a Solé J. , Kaminskii A. A.
Published: November 2002

Continuous-wave red, green, and blue coherent light has been simultaneously generated by quasiphase matching in a strontium barium niobate crystal used as an intracavity frequency converter of a Nd:YVO4 laser operating at 1.34 μm. The broad distribution of ferroelectric domain sizes (1–8 μm) provides the possibility of different simultaneous frequency conversion processes without requiring angle or temperature tuning. Blue and red colors were generated by second-harmonic generation of pump (0.88 μm) and infrared (1.34 μm) laser radiation, respectively. Green light was produced by sum frequency mixing of pump and laser radiations.
Vacuum thermionic refrigeration with a semiconductor heterojunction structure
Authors: Hishinuma Yoshikazu , Moyzhes Boris Y. , Geballe Theodore H. , Kenny Thomas W.
Published: November 2002

We consider possibilities for refrigeration by emission of electrons into vacuum using a semiconductor layered heterostructure and applying electric field in the order of 106 V/cm. Under the influence of a strong electric field, the height of the vacuum potential barrier is significantly reduced due to the Schottky effect and penetration of electric field into the semiconductor layer allowing high emission current. Joule heating inside the semiconductor layer can be minimized by creating a heterostructure with decreasing electron affinity from the metal–semiconductor boundary to the semiconductor–vacuum boundary. We find it is possible to obtain large Peltier currents while minimizing joule heating in the semiconductor. We find it is realistic to expect cooling of 10–100 W/cm2 at room temperature and down to 100 K by adjusting the thickness, and electron affinities of the semiconductor within practical ranges.
Band gap ion mass filter
Authors: Ohkawa T. , Miller R. L.
Published: November 2002

Ions in a plasma may be radially separated according to mass using a combination of an axial magnetic field and either a radial or azimuthal electric field. The separation is qualitatively different from that obtained by a plasma centrifuge and the characteristics of confined and unconfined ion orbits are analogous to the phenomenon of band gaps in semiconductors.
Photopyroelectric structural and thermal characterization of first-order phase transition in liquid crystals
Authors: Mercuri F. , Pizzoferrato R. , Zammit U. , Marinelli M.
Published: November 2002

A photopyroelectric setup, which allows the study of the thermal behavior and the structural evolution of 8CB liquid crystal at the first-order nematic–isotropic phase transition, is presented. It is shown that the kinetics of the transition in liquid-crystal films are not the same on heating and cooling, and a quantitative determination of the width of the two-phase coexistence region is reported. A correlation between the photopyroelectric signal and the liquid-crystal morphology in the coexistence region has been found.
Melting and expansion behavior of indium in carbon nanotubes
Authors: Gao Yihua , Bando Yoshio , Golberg Dmitri
Published: November 2002

Indium-filled carbon nanotubes were synthesized via a simple chemical vapor deposition. The carbon nanotubes had diameters of 100–200 nm and length of ∼10 μm. The melting and expansion behavior of the indium in carbon nanotubes were investigated in an analytical transmission electron microscope. It was found that the melting and expansion behavior of indium were different from that in a macroscopic state. The melting behavior was explained using a particular equation developed for a nanoscale indium column. The analysis of the expansion behavior allows us to clarify the problems of indium filling usage in carbon nanotube-based nanothermometer.
Ultraviolet light emission from self-organized p–n domains in cubic boron nitride bulk single crystals grown under high pressure
Authors: Taniguchi T. , Watanabe K. , Koizumi S. , Sakaguchi I. , Sekiguchi T. , Yamaoka S.
Published: November 2002

Ultraviolet light emission at a wavelength of 250 nm was observed from Be-doped single crystals of cubic boron nitride (cBN) grown at high pressure and high temperatures. We found that dopants of Be were localized in the grown crystals when a small amount of Be was added to the crystals. The crystals had boundaries of blue-colored Be-rich regions and amber-colored ones. These colored regions were divided by the boundaries of growth sectors. When a direct current was induced across the boundaries, UV light emission was clearly observed. The crystals may include self-organized p–n regions along the growth sectors. Although some crystalline imperfections still prevent the crystals from exhibiting properties of materials that can emit UV light with a wide band gap (6.3 eV), it is noticeable that bulk single crystals have been synthesized that contain self-organized p–n boundaries and show UV light emission.
Characterization of silicate/Si(001) interfaces
Authors: Copel M. , Cartier E. , Narayanan V. , Reuter M. C. , Guha S. , Bojarczuk N.
Published: November 2002

Many of the proposed high permittivity gate dielectrics for silicon-based microelectronics rely on a stack configuration, with an SiO2 buffer layer to provide an interface. We describe a means for creating gate dielectrics with a direct yttrium silicate–silicon interface through the solid-state reaction of yttria and silicon oxynitride, avoiding the preparation of an oxide-free silicon surface. Characterization by medium-energy ion scattering indicates complete consumption of the underlying oxide through silicate formation during high-temperature annealing. Furthermore, the silicate dielectric exhibits small flat-band voltage shifts, indicating low quantities of charge, without passivation steps. Creation of a silicate–silicon interfaces by a simple route may enable the study of an alternate class of dielectrics.
Polarization effects in photoluminescence of C- and M-plane GaN/AlGaN multiple quantum wells
Authors: Kuokstis E. , Chen C. Q. , Gaevski M. E. , Sun W. H. , Yang J. W. , Simin G. , Asif Khan M. , Maruska H. P. , Hill D. W. , Chou M. C. , Gallagher J. J. , Chai B.
Published: November 2002

Polarization effects have been studied in GaN/AlGaN multiple quantum wells (MQWs) with different c-axis orientation by means of excitation-dependent photoluminescence (PL) analysis. Quantum structures were grown on [0001]-oriented sapphire substrates (C plane) and single-crystalline [11̄00]-oriented freestanding GaN (M plane) using the metalorganic chemical vapor deposition technique. Strong PL spectrum line blueshifts (up to 140 meV) which are correlated with the excitation intensity have been obtained for C-plane MQWs, whereas no shift has been observed for M-plane MQWs. Theoretical calculations and comparison with the PL data confirm that the built-in electric field for C-plane structures is much stronger than the field present for M-plane MQWs. In the former case, the excitation-induced blueshift of the PL line is due to the screening of the built-in electric field by photoinjected carriers, which is consistent with the field strength of 1.23 MV/cm in the absence of excitation.
Optical waveguides produced in LiF by MeV ion beam bombardment
Authors: Cremona M. , Pereira J. A. M. , Pelli S. , Righini G. C.
Published: November 2002

In this letter, the development of a simple method, based on high-energy light-ion beam irradiation, to create active waveguides in alkali halide materials is reported. A 1.1 MeV He+ beam at normal incidence was used to irradiate lithium fluoride crystals, with different ion doses varying from 1×1012 up to 6×1015 cm−2, producing thin colored strips. All strips showed several guided modes, confirming the effectiveness of this technique to create the conditions to allow guided propagation. Values of 1.5 dB/cm were found for the propagation losses.
Study of the relaxation process during InGaAs/GaAs (001) growth from in situ real-time stress measurements
Authors: González M. U. , González Y. , González L.
Published: November 2002

Strain evolution during In0.2Ga0.8As/GaAs (001) growth by molecular beam epitaxy has been monitored in real time. We have detected that three main relaxation stages, related to different mechanisms, take place during growth, and we have obtained the thickness range where those mechanisms are active. The in situ measured relaxation behavior in the plastic stages has been described by means of a simple equilibrium model that takes into account dislocations generation and interaction between them. The excellent agreement between the experimental data and the model allows us to determine the value of the formation energy per unit length of a misfit dislocation and the extent of the interaction between dislocations in this material system.
Growth of GaN films on porous SiC substrate by molecular-beam epitaxy
Authors: Yun F. , Reshchikov M. A. , He L. , Morkoç H. , Inoki C. K. , Kuan T. S.
Published: November 2002

Porous SiC (PSiC) substrates were used for the growth of GaN by reactive molecular-beam epitaxy with ammonia as the nitrogen source. Improved quality of GaN films has been demonstrated for growth on PSiC substrates, as compared to that on standard 6H–SiC substrates. Cross-sectional transmission electron microscopy and electron diffraction showed a reduction in dislocation density and a higher degree of lattice and thermal relaxation in the GaN films grown on porous substrates. The submicron GaN films exhibit a rocking curve linewidth of 3.3 arcmin for (0002) diffraction and 13.7 arcmin for (101̄2) diffraction. Low-temperature photoluminescence showed an excitonic transition with a full width at half maximum of 9.5 meV at 15 K, as well as high quantum efficiency, on the GaN layer grown on PSiC when the thin skin layer on porous SiC was removed before growth.
Planar GaN n+–p photodetectors formed by Si implantation into p-GaN
Authors: Sheu J. K. , Lee M. L. , Yeh L. S. , Kao C. J. , Tun C. J. , Chen M. G. , Chi G. C. , Chang S. J. , Su Y. K. , Lee C. T.
Published: November 2002

GaN n+–p junction diodes were fabricated by implanting Si atoms into p-GaN. It was shown that we could use these diodes as GaN-based planar photodetectors. It was found that the dark current density of the diodes was around 1.5 μA/cm2 and 50 nA/cm2 at reverse biases of 3 and 1 V, respectively. Spectra response measurements revealed a cutoff wavelength at around 365 nm and a peak responsivity of 0.33 mA/W at 365 nm for the GaN planar n+–p photodetectors. It was also found that the visible rejection ratio was around 260. Furthermore, temporal response measurements revealed that the fall times of these GaN planar n+–p photodetectors were found to be shorter than 0.4 μs.
Higher efficiency InGaN laser diodes with an improved quantum well capping configuration
Authors: Hansen M. , Piprek J. , Pattison P. M. , Speck J. S. , Nakamura S. , DenBaars S. P.
Published: November 2002

A thin AlGaN cap is typically employed above the multiple quantum wells of InGaN-based lasers to prevent electron overflow from the active region and to protect the InGaN active region from the high temperature growth of subsequent p-type layers. The growth conditions and placement of this cap can significantly affect the efficiency and operating characteristics of laser diodes. A 200 Å Al0.2Ga0.8N:Mg cap was placed above the last barrier of a three quantum well (QW) laser diode, as well as directly above the last QW. Lasers with the cap above the last QW exhibit a lower threshold current and a higher internal quantum efficiency than lasers with the cap above the last barrier. The internal quantum efficiency nearly doubles from 16.6% for lasers with the cap above the last barrier to 34.7% for the cap above the last QW. The improvement in efficiency from moving the cap to the last QW is attributed to electrons falling into the last QW and participating in radiative recombination, instead of nonradiative recombination at dislocation sites of the last barrier, or recombining radiatively at the barrier wavelength, rather than at the laser emission wavelength.
Interfacial oxide formation and oxygen diffusion in rare earth oxide–silicon epitaxial heterostructures
Authors: Narayanan V. , Guha S. , Copel M. , Bojarczuk N. A. , Flaitz P. L. , Gribelyuk M.
Published: November 2002

We report on controlled interfacial oxide formation within epitaxial (LaxY1−x)2O3/Si(111) heterostructures under UHV environments. Results indicate that exposure of these epitaxial films to molecular oxygen right after deposition results in the formation of an amorphous interfacial layer thicker than that expected when a bare silicon surface is exposed to molecular oxygen under the same conditions. The results imply significant oxygen diffusion through the epitaxial dielectric and reaction at the silicon–oxide interface. Arguments have been developed to explain these observations.
Heterojunction diode fabrication from polyaniline and a ferroelectric polymer
Authors: Xu B. , Ovchenkov Y. , Bai M. , Caruso A. N. , Sorokin A. V. , Ducharme Stephen , Doudin B. , Dowben P. A.
Published: November 2002

We have fabricated a p–n heterojunction diode by vapor depositing a thin film of polyaniline on top of the crystalline copolymer: poly(vinylidene fluoride with trifluoroethylene). The formation of a diode is expected from the band offsets of the two polymers near the Fermi level. The interface between the two components was investigated, and an abrupt interface was found that is very different from the inorganic analog.
Study of ZnCdTe crystals as terahertz wave emitters and detectors
Authors: Liu Kai , Kang Hyun-Shik , Kim Tae-Kyu , Zhang X.-C.
Published: November 2002

We report a systematic study of ternary ZnCdTe crystals as terahertz wave emitters and sensors with a femtosecond Ti:sapphire laser. Experimental measurements of the generation and detection of terahertz waves from 〈110〉-oriented Zn1−xCdxTe crystals indicate the optimum composition x=0.05. We also report generation and detection of terahertz wave from doped crystals. For terahertz wave applications, the resistivity of these crystals should be greater than 100 Ω cm.