bluuebunny/crossref_metadata_2025 huggingface dataset:
Room-temperature Ohmic contact on n-type GaN with surface treatment using Cl2 inductively coupled plasma
Authors: Jang Ho Won , Jeon Chang Min , Kim Jong Kyu , Lee Jong-Lam
Published: July 2002

A room-temperature Ti/Al Ohmic contact on n-type GaN was obtained by surface treatment using Cl2 inductively coupled plasma treatment. The specific contact resistivity was dramatically decreased from the Schottky behavior to 9.4×10−6 Ω cm2 by the treatment. The binding energy of the Ga–N bond and the atomic ratio of Ga/N were simultaneously increased after the plasma treatment. This provides evidence that N vacancies, acting as donors for electrons, were produced at the etched surface, resulting in a shift of the Fermi level near to the conduction band. This leads to the reduction in contact resistivity through the decrease of the Schottky barrier for the conduction of electrons.
Effect of phosphorus on Ge/Si(001) island formation
Authors: Kamins T. I. , Ohlberg D. A. A. , Williams R. Stanley
Published: July 2002

Adding PH3 during chemical vapor deposition of Ge on Si(001) partially suppresses island formation and changes the shape of the islands that do form. A shape not previously seen in undoped layers grown by chemical vapor deposition is a large pyramid, with base edges aligned along the 〈110〉 directions and sides bounded by {111} planes near the base and {113} planes near the top. This suggests that phosphorus changes the thermodynamics of island formation. During annealing in H2, the shape of the large pyramids changes toward a multifaceted structure. The presence of PH3 during annealing of undoped islands retards coarsening, probably by decreasing surface diffusion.
Aluminum single-electron transistors studied at 0.3 K in different transport regimes
Authors: Buonomo B. , Castellano M. G. , Leoni R. , Mattioli F. , Torrioli G. , Carelli P. , Chiarello F. , Cosmelli C.
Published: July 2002

We measured the transport properties and charge noise of superconducting single-electron transistors at the temperature of 0.3 K. The devices were fabricated with different ratios between charging and Josephson energies in order to explore their behavior in different experimental situations. We show that, in spite of the substantial thermal fluctuation, it is possible to extract from the data the values of the device parameters and information on the various tunneling mechanisms. Furthermore, we measured the device noise at different bias points, corresponding to regions with different dominant tunneling mechanisms, namely in the supercurrent, in the Josephson quasiparticle, and in the sequential tunneling regions.
High resolution resonance enhanced two photon ionization spectroscopy of RbCs in a cold molecular beam
Authors: Yoon Youngjee , Lee Yonghoon , Kim Taeshick , Ahn Jeung Sun , Jung Younghee , Kim Bongsoo , Lee Sungyul
Published: July 2002

The first rotationally resolved high resolution excitation spectrum of RbCs is obtained near 520 nm by resonance enhanced two photon ionization study. The excited electronic state is assigned to the 5 1Σ+ state by the spectrum analysis and comparison with ab initio calculations. The spectroscopic constants of the 5 1Σ+ state are determined to be Te=18 564.560(76) cm−1, ωe=39.193(25) cm−1, Re=4.951(5) Å. The Rydberg–Klein–Rees potential curve is constructed and compared with theoretical calculations. Avoided crossings between the 5 1Σ+ and 6 1Σ+ states of RbCs are observed.
Nonlinear propagation of two-dimensional gravity wave trains in magnetohydrodynamics
Authors: Rollins David K. , Shivamoggi Bhimsen K.
Published: July 2002

Nonlinear evolution of modulated two-dimensional gravity wave trains in a conducting fluid subject to a tangential applied magnetic field are considered. The effect of the applied magnetic field on the stability of the modulation and on the saturation amplitude in the long-time evolution are examined.
Photoelastic effects in tetragonal Pb(Zn1/3Nb2/3)O3–PbTiO3 single crystals near the morphotropic phase boundary
Authors: Lu Y. , Cheng Z.-Y. , Barad Y. , Zhang Q. M.
Published: July 2002

In order to explore the photoelastic effects of the ferroelectric single crystal Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN–PT), the piezo-optical coefficients π were characterized for 0.9PZN–0.1PT and 0.88PZN–0.12PT under uniaxial stress using an interferometer method. The results show that the crystal exhibits very large π values (for example, π33 can reach 19.8×10−12 m2/N), indicating that this material is a good candidate for stress sensors and acousto-optic modulators.
Enhancement of direct (type-I) excitonic transitions in ZnTe/CdSe-based type-II Bragg confining structures
Authors: Um Youngho , Lee S. , Liu X. , Furdyna J. K.
Published: July 2002

We have used distributed Bragg reflectors (DBRs) designed for electron de Broglie wavelengths to achieve enhanced localization of above-barrier states in the conduction band of a ZnTe/CdSe-based type-II heterostructure. We studied the effect of such DBRs by observing excitonic transitions between above-barrier electron states which are Bragg confined in the barrier layer of the conduction band and heavy holes localized in the quantum well of the valence band. In a type-II system, both carriers are thus confined in the same layer, leading to a direct (type-I-like) excitonic transition. In order to identify the physical region where this transition actually occurs, we have used a diluted magnetic semiconductor (DMS) alloy Zn0.94Mn0.06Te as the confining layer. In such a structure, the giant Zeeman splitting of the band edges in the DMS layer serves to identify the location where the states participating in the transition are confined. To enhance the intensity of such direct transitions, we have sandwiched the central DMS layer between two DBR stacks, each consisting of 10 alternating quarter-wave layers of ZnTe and CdSe tailored to the de Broglie wavelength of the above-barrier state in the conduction band of the central layer. As expected, the transition of interest was observed at an energy slightly higher than the band gap of the DMS layer. We have observed this direct transition in absorption and, more importantly, also in photoluminescence, attesting to the very significant confinement of the above-barrier layer in such DBR-based structures.
Organic nanocrystals grown in gel glasses for optical-power-limiting applications
Authors: Sanz N. , Ibanez A. , Morel Y. , Baldeck P. L.
Published: July 2002

Nonlinear absorption and optical-limiting properties of sol–gel glasses doped with nanocrystals of stilbene 3 have been studied. These organic–inorganic nanocomposite materials can be highly doped (up to 30 wt %) with active molecules through the control of nanometer-scale crystallization. In the nanosecond time regime, the nonlinear absorption occurs in a two-step process, which is characterized by two-photon absorption and excited-state absorption spectroscopies. In a F/5 optical system, the output energy is damped below 20 μJ for all visible wavelengths, 6 μJ at λ=600 nm. Thus, these nanocomposite materials are a good alternative both to concentrated solutions, due to their stability, and to crystals, due to their isotropic response with polarization.
The electron bounce resonance in a low-pressure solenoidal inductive discharge
Authors: Chung Chin Wook , You K.-I. , Seo S. H. , Kim S. S. , Chang H. Y.
Published: July 2002

The electron energy distribution functions (EEDFs) are measured by a rf compensated Langmuir probe in a solenoidal inductive reactor at various frequencies and at 2 mTorr of argon. The frequency dependence of the electron energy distribution function is clearly observed. The energy diffusion coefficients against the applied frequencies are calculated from the nonlocal heating theory. It is found that the bounce resonance electrons determine the energy diffusion coefficient shape and the electron energy distribution function at the bounce resonance energy in the energy diffusion coefficient begins to flatten. The exact bounce resonance condition in solenoidal inductive discharge is presented. It is reported that the frequency dependence of the EEDFs is mainly due to the electron bounce resonance in a finite-size solenoidal inductive discharge.
A full description of the potential curve of the B 1Πu state of Li27
Authors: Bouloufa N. , Cacciani P. , Vetter R. , Yiannopoulou A. , Martin F. , Ross A. J.
Published: July 2002

An accurate potential curve for the bound and quasibound region of the B 1Πu state of Li27 is produced by analyzing high resolution sub-Doppler B 1Πu←X 1Σg+ excitation spectra in conjunction with lower resolution data of Hessel and Vidal [J. Chem. Phys. 70, 4439 (1979)] and of Russier et al. [J. Mol. Spectrosc. 168, 39 (1994)]. The bound and quasibound part of the curve is generated by the direct fit of molecular energies to a numerical potential; the outermost (repulsive) part of the curve is calculated by an asymptotic method using Coulombic and exchange parameters determined from the 1 1Πg and A 1Σu+ states of the lithium dimer. The full potential energy curve of the B 1Πu state reproduces all measured energies of f parity, and the widths of the predissociated lines, to within the experimental accuracy.
Evidence for edge gradients as control parameters of the spontaneous high-mode transition
Authors: Groebner R. J. , Thomas D. M. , Deranian R. D.
Published: July 2002

Measurements in the DIII-D tokamak [J. L. Luxon et al., Plasma Physics and Controlled Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] show that edge gradients of ion and electron temperature and pressure, ∇Ti, ∇Te, ∇Pi, and ∇Pe, are good candidates for parameters which respond to the heating power to control the fundamental physics of the spontaneous Low-mode (L-mode) to High-mode (H-mode) transition. These gradients are measured in the region where the H-mode transport barrier forms and they are found to consistently increase in time during the L-phase of discharges which make a transition to H-mode. Moreover, for a fixed magnetic configuration, there is a well-defined boundary between the L-mode and H-mode states in the ∇Te−∇Pe operational space. The values of Ti, Te, Pi, and Pe, measured at this location, show smaller relative changes during the L-phase of these discharges, indicating that the scale lengths of these quantities are decreasing as the L–H transition is approached in time. These results are consistent with several theoretical models in which gradients of pressure or temperature cause the transition.
Valence evaluation of cerium ions in Ce-doped α-sialon by electron energy loss spectra
Authors: Xu F. F. , Bando Y.
Published: July 2002

For a long time cerium ions, both Ce3+ and Ce4+, have not been considered to be able to enter the interstitial sites in an α-sialon structure until recent successes in the preparation of cerium-doped α-sialon materials using CeO2 as the starting powder. It is then of great importance and necessity to know the chemical valence of these incorporated cerium ions. Meanwhile, the specific structure of the interstices in α-sialon provides quite a different chemical environment upon which the electronic structures of cerium ions could be different from those ever reported in, for instance, metallic compounds and oxides. Electron energy loss spectra (EELS) in the M-edge region of rare-earth elements carries information on the initial state 4f occupancy. We have acquired and examined the M4,5-edge spectra of cerium at different locations in the microstructure. It has been found that all the Ce(IV) ions in the intergranular glass and the majority inside the α-sialon crystals have been reduced to Ce(III) while there exist inside the crystal cerium ions showing 4f0 initial-state peaks on the EELS spectrum. The 4f0 peak locates at about 0.6 eV higher in energy than that for CeO2. An increased and possibly by far the highest weight of f0 state is derived for Ce(IV) ions in α-sialon. The spectral evidences confirm the general suggestion of the unstability of Ce(IV) ions with silicon nitride.
Reversed field pinch plasma equilibria with shear flow
Authors: Sugimoto Hisaya , Ashida Hisao , Kurasawa Takasi
Published: July 2002

The Grad–Shafranov equation with flow, which is derived by a variational method, involves unknown functions such as the dynamic pressure, P(Ψ). These functions are specified by minimizations of free energies under the constraints of constant P′(Ψ), magnetic helicity, flow helicity, and cross helicity in reversed field pinch (RFP) plasmas. New flow and cross helicities are introduced based on the analogy of the magnetic helicity, which are different from those used in fluid mechanics. The constraint of constant flow helicity provides flow with profiles from high- to low-shear flow. The Suydam-stable RFP equilibria obtained with flows are extremely different in β and βp from RFP equilibria without flow. The high-shear flow can extend the Suydam limit, allowing higher β and βp.
Effects of rapid thermal annealing and SiO2 encapsulation on GaNAs/GaAs single quantum wells grown by plasma-assisted molecular-beam epitaxy
Authors: Li L. H. , Pan Z. , Xu Y. Q. , Du Y. , Lin Y. W. , Wu R. H.
Published: July 2002

Effects of rapid thermal annealing and SiO2 encapsulation on GaNAs/GaAs single quantum wells grown by plasma-assisted molecular-beam epitaxy were studied. Photoluminescence measurements on a series of samples with different well widths and N compositions were used to evaluate the effects. The intermixing of GaNAs and GaAs layers was clearly enhanced by the presence of a SiO2-cap layer. However, it was strongly dependent on the N composition. After annealing at 900 °C for 30 s, a blueshift up to 62 meV was observed for the SiO2-capped region of the sample with N composition of 1.5%, whereas only a small blueshift of 26 meV was exhibited for the bare region. For the sample with the N composition of 3.1%, nearly identical photoluminescence peak energy shift for both the SiO2-capped region and the bare region was observed. It is suggested that the enhanced intermixing is mainly dominated by SiO2-capped layer induced defects-assisted diffusion for the sample with smaller N composition, while with increasing N composition, the diffusion assisted by interior defects become predominant.
Ballooning instabilities in a Heliotron J plasma
Authors: Yamagishi Osamu , Nakamura Yuji , Kondo Katsumi
Published: July 2002

Ideal magnetohydrodynamic stability analysis of local pressure-driven modes in an L=1 heliotron, Heliotron J [M. Wakatani et al., Nucl. Fusion 40, 569 (2000)], is investigated by means of three-dimensional (3D) ballooning formalism and the Mercier criterion. In 3D systems such as heliotrons, the ballooning modes are separated into two categories: One is tokamak-like ballooning modes which are localized only in the poloidal direction, and the other is modes inherent to 3D systems which are localized on the specific flux tubes. The tokamak-like ballooning modes change to the Mercier modes in the limit that the mode is sufficiently extended along the field line, but the nonaxisymmetric ballooning mode does not so. The L=1 Heliotron J equilibrium investigated here has weak global shear and the dominant Fourier amplitudes of magnetic-field strength is rather different from the conventional helical systems with L=2 helical coils. Since the weak global shear causes the reduction of integrated local shear along the field lines easily, which combines with strongly modulated destabilizing effects on the flux surface, the nonaxisymmetric ballooning modes localized on the specific flux tubes can become unstable. On the other hand, the Mercier modes are suppressed due to the deep magnetic well. The results obtained from the model equilibrium of L=2 Large Helical Device (LHD), for which several reports have already published [N. Nakajima, Phys. Plasmas 3, 4556 (1996), for example], are also shown and compared with the results of Heliotron J. The LHD equilibrium employed here has a magnetic hill region at the outer radius, and this tends to make the Mercier modes unstable. It is found that this difference of the Mercier stability property in two equilibria is concerned in the local ballooning stability, and the notable difference of local dispersion relations appears. It is also found from the comparison of two systems that the nonaxisymmetric ballooning modes have a similar property to the tokamak-like ballooning modes, in the sense of the s̄-ᾱ diagram where s̄ and ᾱ are shear and pressure gradient parameter.
Spiral vortices in compressible turbulent flows
Authors: Gomez T. , Politano H. , Pouquet A. , Larchevêque M.
Published: July 2002

We extend the spiral vortex solution of Lundgren [Phys. Fluids 25, 2193 (1982)] to compressible turbulent flows with a perfect gas. This model links the dynamical and the spectral properties of incompressible flows, providing a k−5/3 Kolmogorov energy spectrum. In so doing, a compressible spatiotemporal transformation is derived, reducing the dynamics of three-dimensional vortices, stretched by an axisymmetric incompressible strain, into a two-dimensional compressible vortex dynamics. It enables us to write the three-dimensional spectra of the incompressible and compressible square velocities in terms of, respectively, the two-dimensional spectra of the enstrophy and of the square velocity divergence, by the use of a temporal integration. Numerical results are presented from decaying direct simulations performed with 5122 grid points; initially, the rms Mach number is 0.23, with local values up to 0.9, the Reynolds number is 700, and the ratio between compressible and incompressible square velocities is 0.1. A k−5/3 inertial behavior is seen to result from the dynamical evolution for both the compressible and incompressible three-dimensional spectra.
Rapid crystallization of silicon films using electrical-current-induced joule heating
Authors: Sameshima T. , Andoh N. , Takahashi H.
Published: July 2002

Melt-regrowth properties of 60-nm-thick silicon films were characterized in the case of electrical-current-induced joule heating. The electrical energy accumulated at a capacitance caused melting of the silicon films via joule heating with a maximum intensity at 1.5×106 W/cm2. The melt-regrowth duration increased from 6 to 75 μs as the capacitance increased to 0.05–1.5 μF. Crystalline properties of the silicon films were also investigated. 7 μm long crystalline grains with the (110) preferential crystalline orientation were observed using a transmission electron microscope. The tensile stress at 3.4×108 Pa remained in the films. The analysis of electrical conductivity resulted in a density of defect states of 3.5×1016 cm−3 in the films. The product of the generation efficiency, the carrier mobility and the average carrier lifetime was estimated to be ∼10−3 cm2/V.
Inelastic magnon and phonon excitations in Al1−xCox/Al1−xCox-oxide/Al tunnel junctions
Authors: Han Xiu-Feng , Murai Junichirou , Ando Yasuo , Kubota Hitoshi , Miyazaki Terunobu
Published: July 2002

Magnetoelectric properties of the tunnel junctions, Al(50 nm)/Al2O3(1.2 nm)/Al(50 nm) (x=0), Al(55 nm)/Al2O3(1.0 nm)/Co(55 nm) (x=0 and top electrode is Co), and Al1−xCox(55 nm)/Al1−xCox-oxide(d nm)/Al(55 nm) (x=0.25, 0.50, 0.75, and 1.0), were investigated. Oxides of Al1−xCox (x=0, 0.25, 0.50, 0.75, and 1.0) were chosen as barrier materials in order to modulate the magnon and phonon excitations in the barrier layer and the interfaces. It was shown that the magnon and phonon excitations were the main sources of inelastic scattering in the tunneling processes for the conduction electrons in these tunnel junctions at nonzero bias voltages. The magnon effects were enhanced in the Co-rich barrier junctions. The Al–O–Co phonon energy decreased with increasing Co composition between the Al–O and Co–O phonon energies based on an Al–O–Co stretching mode in the Al1−xCox-oxide barrier as vibrational frequency of crystal lattice decreased with increasing Co composition.
Finding the collapse-inducing nucleus in a folding protein
Authors: Fernández Ariel , Appignanesi Gustavo A. , Colubri Andrés
Published: July 2002

We detect and characterize the structure and time evolution of a collapse-inducing nucleus in a small folding protein. The analysis requires a combination of kinetic and time-dependent thermodynamic data obtained from a coarse grained simulation of dominant folding pathways resolved at a topological level. A new sequence-dependent dynamic invariant, the set of F values, is introduced to infer which portions of the protein are involved in the formation of the nucleus and thus identify the site mutations that significantly affect the folding process. The theoretical results are corroborated by experimental findings which also support the topological nucleation scenario. Furthermore, the structural evolution of the nucleus is investigated well beyond the submillisecond range.
Symmetries-imposed configurational limit and configurational limit-induced change of symmetry in an inductively driven, dissipative tokamak plasma
Authors: Chu T. K.
Published: July 2002

Hastie showed that, in a force-free tokamak configuration that encloses the magnetic axis, there exists a critical on-axis safety factor, q0, above which the q profile monotonically increases with radius and below which decreases with radius [J. Hastie, Nucl. Fusion 29, 96 (1989)]. This critical q0, when interpreted under the requirement of maximum poloidal magnetic field energy for an inductively driven, dissipative tokamak plasma, which does not allow dq/dr<0 for the toroidally symmetric configuration, becomes the onset condition of helical symmetry during current peaking in a sawtooth cycle. The value is a result of the opposing demands placed on the sign of the on-axis curvature of the safety factor, q″(0), by the two rotational symmetries (m=0 and n=0) of the configuration.
Point defect equilibrium in strontium titanate thin films
Authors: McIntyre Paul C.
Published: July 2002

Equilibrium calculations of point defect and electronic carrier distributions in acceptor impurity-containing SrTiO3 thin films were performed. The simulation methodology was based on previous calculations of near-grain boundary point defect equilibrium in acceptor-doped SrTiO3 ceramics. Attention was focused on the effects of temperature and the presence of Pt electrode interfaces on the conditions for defect equilibrium. The local electrostatic potential and the distribution of oxygen vacancies across films of varying thickness were predicted for various simulated equilibration conditions. The effects of hydrogen doping were analyzed, and simulated equilibrium potential distributions were compared to the results of reported photoelectron spectroscopy studies of Pt/SrTiO3 and Pt/(Ba, Sr)TiO3 thin film structures.
Thinshell symmetry surrogates for the National Ignition Facility: A rocket equation analysis
Authors: Amendt Peter , Shestakov A. I. , Landen O. L. , Bradley D. K. , Pollaine S. M. , Suter L. J. , Turner R. E.
Published: July 2002

Several techniques for inferring the degree of flux symmetry in indirectly driven cylindrical hohlraums have been developed over the past several years for eventual application to the National Ignition Facility (NIF) [Paisner et al., Laser Focus World 30, 75 (1994)]. These methods use various ignition capsule surrogates, including non-cryogenic imploded capsules [Hauer et al., Phys. Plasmas 2, 2488 (1995)], backlit aerogel foamballs [Amendt et al., Rev. Sci. Instrum. 66, 785 (1995)], reemission balls [Delamater, Magelssen, and Hauer, Phys. Rev. E 53, 5240 (1996)], and backlit thinshells [Pollaine et al., Phys. Plasmas 8, 2357 (2001)]. Recent attention has focussed on the backlit thinshells as a promising means for detecting higher-order Legendre flux asymmetries, e.g., P6 and P8, which are predicted to be important sources of target performance degradation on the NIF for levels greater than 1% [Haan et al., Phys. Plasmas 2, 2490 (1995)]. A key property of backlit thinshells is the strong amplification of modal flux asymmetry imprinting with shell convergence. A simple single-parameter analytic description based on a rocket model is presented which explores the degree of linearity of the shell response to an imposed flux asymmetry. Convergence and mass ablation effects introduce a modest level of nonlinearity in the shell response. The effect of target fabrication irregularities on shell distortion is assessed with the rocket model and particular sensitivity to shell thickness variations is shown. The model can be used to relate an observed or simulated backlit implosion trajectory to an ablation pressure asymmetry history. Ascertaining this history is an important element for readily establishing the degree of surrogacy of a symmetry target for a NIF ignition capsule.
Coherent measurement of the dispersive curves of an elastic waveguide
Authors: Zhang Hailan , Dai Shuwu , Zhang Chengyu
Published: July 2002

A coherent method is proposed to measure the dispersive curves of elastic waveguides. The use of the phase information makes the measurement more precise and stable than traditional noncoherent methods. The principle of the method should work well for other kinds of waveguides.
Periodic light coupler gratings in amorphous thin film solar cells
Authors: Eisele C. , Nebel C. E. , Stutzmann M.
Published: July 2002

Efficient light trapping structures for amorphous hydrogenated silicon (a-Si:H) solar cells have been realized using periodically structured aluminum doped zinc oxide (ZnO:Al) with periods between 390 and 980 nm as a transparent front contact. Atomic force microscopy, optical reflection, and diffraction efficiency measurements were applied to characterize solar cells deposited on such gratings. A simple formula for the threshold wavelength of total internal reflection is derived. Periodic light coupler gratings reduce the reflectance to a value below 10% in the wavelength range of 400–800 nm which is comparable to cells with an optimized statistical texture. Diffraction efficiency measurements and theoretical considerations indicate that a combination of transmission and reflection gratings contribute to the observed reduction of the reflectance.
Topographic study of sputter-deposited film with different process parameters
Authors: Ju Shin-Pon , Weng Cheng-I , Chang Jee-Gong , Hwang Chi-Chuan
Published: July 2002

In this study, molecular dynamics simulation is employed to investigate the surface topography of thin films produced by the sputtering process for different parameters such as substrate temperature, incident energy, and incident angle. Interface width is used to quantify the quality of the deposited film. The Morse potential is used to model the atomic interaction between atoms. From the results of this study, it is found that for lower substrate temperature, lower incident energy, and larger incident angle, the growing film structure tends toward a three-dimensional columnar structure, and a rougher film is produced. Conversely, for higher substrate temperature, higher incident energy, and smaller incident angle, the growing film structure tends toward a two-dimensional (Frank–van der Merwe) quasi-layer-by-layer structure, and a smoother film is produced. Finally, average surface kinetic energy is found to be an important factor in determining the surface properties produced in the process. Generally, the produced film is smoother when the average surface kinetic energy is larger.
Comment on “Dust ion-acoustic shocks and holes” [Phys. Plasmas 7, 1044 (2000)]
Authors: Gao Yi-Tian , Tian Bo
Published: July 2002

The dust ion-acoustic shocks in an unmagnetized dusty plasma have recently been observed, and their dynamics have been shown to be governed by a Korteweg–de Vries–Burgers (KdVB) equation, which is briefly mentioned to admit both oscillatory and monotonic shocks [Phys. Plasmas 7, 1044 (2000)]. However, we point out that the analytic expressions are a must to help us to a better understanding of the plasma system under investigation, but the aforementioned paper does not provide us with such information for the dust ion-acoustic shocks. To make the story complete, we hereby present some exact and approximate analytic solutions to the KdVB equation along with the original papers, which agree well with the recent observations.
Slow solitary waves in multi-layered magnetic structures
Authors: Ruderman M. S. , Roberts B. , Pelinovsky E. N. , Petrukhin N. S.
Published: July 2002

The propagation of slow sausage surface waves in a multi-layered magnetic configuration is considered. The magnetic configuration consists of a central magnetic slab sandwiched between two identical magnetic slabs (with equilibrium quantities different from those in the central slab) which in turn are embedded between two identical semi-infinite regions. The dispersion equation is obtained in the linear approximation. The nonlinear governing equation describing waves with a characteristic wavelength along the central slab much larger than the slab thickness is derived. Solitary wave solutions to this equation are obtained in the case where these solutions deviate only slightly from the algebraic soliton of the Benjamin–Ono equation.
Measurement of gas yields and flow rates using a custom flowmeter
Authors: Circone S. , Kirby S. H. , Pinkston J. C. , Stern L. A.
Published: July 2002

A simple gas collection apparatus based on the principles of a Torricelli tube has been designed and built to measure gas volume yields and flow rates. This instrument is routinely used to monitor and collect methane gas released during methane hydrate dissociation experiments. It is easily and inexpensively built, operates at ambient pressures and temperatures, and measures gas volumes of up to 7 L to a precision of about 15 ml (about 0.0025 mol). It is capable of measuring gas flow rates varying from more than 103 to less than 10−1 ml/min during gas evolution events that span minutes to several days. We have obtained a highly reproducible hydrate number of n=5.891 with a propagated uncertainty of ±0.020 for synthetic methane hydrate.
Vertex operators for AdS3 with Ramond background
Authors: Dolan L.
Published: July 2002

This review gives results on vertex operators for the Type IIB superstring in an AdS3×S3 background with Ramond–Ramond flux, which were presented at Strings 2000. Constraint equations for these vertex operators are derived, and their components are shown to satisfy the supergravity linearized equations of motion for the six-dimensional (2, 0) theory of a supergravity and tensor multiplet expanded around AdS3×S3 space-time.
Phase transitions and microwave dielectric properties in the perovskite-like Ca(Al0.5Nb0.5)O3−CaTiO3 system
Authors: Levin I. , Chan J. Y. , Maslar J. E. , Vanderah T. A. , Bell S. M.
Published: July 2002

Phase transitions and microwave dielectric properties in the (1−x)Ca(Al0.5Nb0.5)O3–xCaTiO3 system were analyzed using x-ray and neutron powder diffraction, transmission electron microscopy, Raman spectroscopy, and dielectric measurements at microwave frequencies (2–8 GHz). Rietveld structural refinements demonstrated that both end compounds exhibit similar octahedral tilted frameworks, while in Ca(Al0.5Nb0.5)O3, tilting is superimposed onto NaCl-type ordering of Al and Nb on the B sites. Accordingly, the room-temperature structures of CaTiO3 and Ca(Al0.5Nb0.5)O3 are described by orthorhombic Pbnm and monoclinic P21/n symmetries, respectively, with similar lattice parameters, √2ac×√2ac×2ac (where ac is the lattice parameter of cubic perovskite). The (1−x)Ca(Al0.5Nb0.5)O3–xCaTiO3 system features both cation ordering and octahedral tilting phase transitions. The Ca(Al0.5Nb0.5)O3 structure remains ordered at least up to 1625 °C. However, the temperature of the order/disorder transition decreases rapidly with increasing Ti content, which correlates with a progressive increase of cation disorder in the specimens. A disordered structure is attained at x=0.5. For the “solid solutions,” the nonlinear dependence of both permittivity ε and the temperature coefficient of the resonant frequency τf on Ti content corresponds to a linear dependence of the macroscopic polarizability on composition; that is, the oxide additivity rule was closely obeyed. Therefore, this rule can be used to predict ε and τf for any intermediate composition from the permittivities and temperature coefficients of permittivity of the end compounds. A zero temperature coefficient of the resonant frequency occurs at the composition x≈0.5 with a relative permittivity of 50 and a Qf value of approximately 30 000 GHz (@4 GHz).
Structural transformation, amorphization, and fracture in nanowires: A multimillion-atom molecular dynamics study
Authors: Walsh Phillip , Li Wei , Kalia Rajiv K. , Nakano Aiichiro , Vashishta Priya , Saini Subhash
Published: July 2002

Multimillion-atom molecular dynamics simulations of silicon diselenide nanowires are used to study mechanical properties and changes in nanowire structure under strain. The nanowires transform from a body-centered orthorhombic structure to a body-centered tetragonal structure under uniaxial strain, which causes an unexpected elongation in one of the transverse directions. For larger strains, the nanowires undergo a process of local amorphization, followed by fracture at one of the resulting crystalline–amorphous interfaces. The critical strain for fracture is 15%. Local temperature and stress distributions after failure are interpreted in terms of the local amorphization.
Change of the superconducting, transport, and microscopic properties of transition metals upon introduction of interstitial impurities and deformation-induced defects
Authors: Sokolenko V. I. , Starodubov Ya. D. , Merisov B. A. , Khadzhay G. Ya.
Published: July 2002

For group-V transition metals (Nb, Ta) containing different concentrations of interstitial impurities (O, C, N, H) and deformation-induced defects, a numerical calculation of various effective microscopic characteristics averaged over the Fermi surface and of the band parameters in the framework of the Friedel model is carried out using the experimentally determined values of the superconducting transition temperature Tc and the temperature dependence of the resistivity in the interval Tc<T≲300 K. The causes of the polar character discerned in the influence of interstitial impurities and dislocations on the investigated physical characteristics are discussed.
Magneto-shape-memory effect in Co–Ni single crystals
Authors: Liu Y. , Zhou W. M. , Qi X. , Jiang B. H. , Wang W. H. , Chen J. L. , Wu G. H. , Wang J. C. , Feng C. D. , Xie H. Q.
Published: July 2002

A Co–33 wt % Ni single crystal was grown by an optical floating-zone furnace without crucible in a high-purity argon atmosphere. In the [001] direction, the as-grown single crystal exhibited a saturation magnetization of 124 A m2/kg, which is nearly twice as high as that of Ni2MnGa, a typical magneto-shape-memory material. A reversible strain of 3% was induced by an applied pulse magnetic field of 2 T in the [001] direction in the temperature range between Md and Ms. According to the martensitic transformation theory [T. Y. Hsu (Z. Y. Xu), Sci. China, Ser. E: Technol. Sci. 40, 561 (1997)] for fcc (γ)→hcp (ε), a possible mechanism based on the reversible motion of the Shockley partial dislocations is proposed in this letter.
Dynamical topology change in M theory
Authors: Greene Brian R. , Schalm Koenraad , Shiu Gary
Published: July 2002

We study the topology change in M theory compactifications on Calabi–Yau three-folds in the presence of G flux (the four form field strength). In particular, we discuss vacuum solutions in strongly coupled heterotic string theory in which the topology change is inevitable within a single space–time background. For rather generic choices of initial conditions, the field equations drive the Kähler moduli outside the classical moduli space of a Calabi–Yau manifold. Consistency of the solution suggests that degenerate flop curves—just as wrapped M theory fivebranes—carry magnetic charges under the four form field strength.
Brane-world Kaluza–Klein reductions and branes on the brane
Authors: Cvetič M. , Lü H. , Pope C. N.
Published: July 2002

We present a systematic study of a new type of consistent “brane-world Kaluza–Klein reduction,” which describes fully nonlinear deformations of codimension one objects that arise as solutions of a large class of gauged supergravity theories in diverse dimensions, and whose world-volume theories are described by ungauged supergravities with-one half of the original supersymmetry. In addition, we provide oxidations of these ansätz which are in general related to sphere compactified higher dimensional string theory or M-theory. Within each class we also provide explicit solutions of brane configurations localized on the world-brane. We show that at the Cauchy horizon (in the transverse dimension of the consistently Kaluza–Klein reduced world-brane) there is a curvature singularity for any configuration with a non-null Riemann curvature or a nonvanishing Ricci scalar that lives in the world-brane. Since the massive Kaluza–Klein modes can be consistently decoupled, they cannot participate in regulating these singularities.
The future of string theory
Authors: Schwarz John H.
Published: July 2002

Prophesy is just for fun. The more useful purpose of the exercise is to identify important issues and to stimulate thought about where they stand and how they might be resolved. The subject areas that are fair game include all of particle physics and cosmology.
Self-organized InGaAs quantum dots on GaAs (311)B studied by conductive atomic force microscope tip
Authors: Okada Yoshitaka , Miyagi Masashi , Akahane Kouichi , Iuchi Yoshimasa , Kawabe Mitsuo
Published: July 2002

We have used conductive atomic force microscope (AFM) tips in order to probe the local electronic properties of InGaAs quantum dots (QDs) grown on GaAs (311)B and (001) substrates by atomic H-assisted molecular beam epitaxy. Highly doped Si and Si3N4 AFM tips coated with a metal such as Au and Ti which warrant electrical conductivity were used to measure the current–voltage (I–V) characteristics of QDs of varying sizes and of any other arbitrary positions on the surface such as the wetting layer. In the case of QDs formed on (001) substrates, it was found that the local surface potentials of larger QDs were lower than the small QDs due to the effect of surface states. On the other hand, noticeable differences were not observed for the QDs formed on (311)B substrates. The local surface potential was similar on each QD and in fact over the whole (311)B surface, and a complex phase separation and strain-relief mechanism were thought to be responsible the observed QDs assembly on (311)B. Last, a resonant tunneling characteristics through the quantized energy levels was studied with a small QD with ∼45 nm in diameter and ∼4 nm in height.
Self-consistent calculations of inversion-layer mobility in highly doped silicon-on-insulator metal–oxide–semiconductor field-effect transistors
Authors: Iwata Hideyuki
Published: July 2002

The inversion-layer mobility in future highly doped silicon-on-insulator (SOI) n-channel metal–oxide–semiconductor field-effect transistors (MOSFETs) has been examined for various SOI layer thicknesses (tSOI) using self-consistent calculation. Not only phonon scattering but also surface roughness scattering and ionized impurity scattering have been taken into account. It has been found for SOI MOSFETs with a highly doped channel that, whenever tSOI (≳2 nm) is reduced under the full-depletion condition, the inversion-layer mobility in SOI MOSFETs becomes higher than that in bulk MOSFETs. The increase in mobility with the reduction of tSOI to about 10 nm is mainly caused by the suppression of surface roughness scattering. Independent of acceptor concentration (NA), the mobility reaches its peak at tSOI of about 3 nm and then decreases drastically with decreasing tSOI. For SOI MOSFETs with NA higher than 5×1017 cm−3, the mobility increases monotonously to the peak with decreasing tSOI under the full-depletion condition due to the large suppression of ionized impurity scattering for the tSOI range between 10 and 5 nm. These results are different from those in the previous works for SOI MOSFETs with low channel impurity concentration.
Electronic band structures of GaInNAs/GaAs compressive strained quantum wells
Authors: Fan W. J. , Yoon S. F.
Published: July 2002

The electronic structures of the Ga1−xInxNyAs1−y/GaAs compressive strained quantum wells are investigated using 6×6 k⋅p Hamiltonian including the heavy hole, light hole, and spin-orbit splitting band. By varying the well width and mole fraction of N in the well material, the effects of quantum confinement and compressive strain are examined. The curves of dependence of transition energy on well width and N mole fraction are obtained. The valence subband energy dispersion curves and TE and TM squared optical transition matrix elements of three possible quantum well structures for emitting 1.3 μm wavelength are given.
Effect of self-capacitance on the tunneling thresholds in linear arrays of nanoparticles
Authors: Berven C. A. , Wybourne M. N.
Published: July 2002

We consider the electron transport through gated one-dimensional chains of ligand stabilized metal nanoparticles. In such systems the self-capacitance of the core can be larger than the interparticle capacitance. In this regime we show that the self-capacitance cannot be neglected and must be included in the calculation of the free energy differences that govern the tunneling thresholds. We demonstrate the consequence of the self-capacitance on the tunneling thresholds of a single nanoparticle device and a chain of six nanoparticles.
Self-organized Cu–Ag nanocomposites synthesized by intermediate temperature ion-beam mixing
Authors: Enrique Raúl A. , Bellon Pascal
Published: July 2002

We perform an investigation of ion-beam mixing in the immiscible system Ag–Cu based on cross-sectional transmission electron microscopy. Multilayered samples consisting of ten periods of (6.7 nm Cu/11.2 nm Ag) are irradiated at temperatures ranging from 25 to 225 °C with normally incident 1 MeV Kr ions to doses in the range 1–2×1016 ion/cm2, enough to reach a nonequilibrium dynamical steady state. Regardless of the irradiation temperature, extensive grain growth takes place. At intermediate temperatures, competition between thermal decomposition and irradiation mixing results in a nanometer-scale phase separation. This spontaneous decomposition demonstrates that ion-beam mixing can be used as a processing tool to synthesize nanocomposite materials.
Characterization of thin film electron emitters by scanning anode field emission microscopy
Authors: Nilsson L. , Groening O. , Groening P. , Kuettel O. , Schlapbach L.
Published: July 2002

Scanning anode field emission microscopy is used to map the electron emission current I(x,y) under constant anode voltage and the electron extraction voltage V(x,y) under constant emission current as a function of tip position on carbon based thin film emitters. The spatially resolved field enhancement factor β(x,y) is derived from V(x,y) maps. It is shown that large variations in the emission site density (ESD) and current density can be explained in terms of the spatial variation of the field enhancement β(x,y). Comparison of β(x,y) and I(x,y) shows that electron emission currents are correlated to the presence of high aspect ratio field enhancing structures. We introduce the concept of field enhancement distribution f(β), which is derived from β(x,y) maps to characterize the field emission properties of thin films. In this context f(β)dβ gives the number of emitters on a unit surface with field enhancement factors in the interval (β,β+dβ). It is shown experimentally for the carbon thin film emitters investigated that f(β) has an exponential dependence with regard to the field enhancement factor β. The field enhancement distribution function f(β) can be said to give a complete characterization of the thin film field emission properties. As a consequence, the emitted current density and ESD can be optimized by tuning f(β) of the emitting thin film.
Effects of substrate micropatterning on nonlinear optical properties of polar self-assembling films
Authors: Li Leiming , Stupp Samuel I.
Published: July 2002

Self-assembling triblock molecules were previously shown to form thin films with polar order. These films are also known to contain polar supramolecular aggregates that are layered in head-to-tail fashion. In this work, glass substrates were prepared using a focused-ion-beam system to generate 1 mm×1 mm areas containing periodic arrays of squares with lateral dimensions of 7 and 17 μm. The squares were defined by 3 μm wide and 10 nm deep troughs created by Ga ion beams. Using second-harmonic-generation measurements, the average molecular tilt angle was found to be smaller in patterned films, suggesting that micropatterning enhances overall polarization in these films.
Carrier diffusion in microcrystalline silicon studied by the picosecond laser induced grating technique
Authors: Kudrna J. , Trojánek F. , Malý P. , Pelant I.
Published: July 2002

We report on the picosecond laser induced grating technique applied to hydrogenated microcrystalline silicon (μc-Si:H) and aimed at studying the photocarrier diffusion coefficient D. We have studied a series of three samples having a distinctly different content of the crystalline phase, and the microstructure and morphology of which are known in detail. Our results show that the coefficient D scales with the degree of crystallinity of the samples, reaching values up to D=9 cm2 s−1 close to crystalline silicon. The obtained carrier lifetime (≈1 ns) is constant in all measured samples. The extracted diffusion lengths are much greater than the dimensions of the microcrystalline grains in the samples. We conclude that small grain boundaries do not limit substantially the carrier diffusion length in microcrystalline silicon.
Concentration of Li atoms in plasmas produced from laser ablation of LiNbO3
Authors: Gordillo-Vázquez F. J.
Published: July 2002

An analytic kinetic model capable to predict the spatial and temporal evolution of the population densities of ground and excited state Li atoms (up to the 32D level) in nonequilibrium laser-generated plasmas from LiNbO3 targets is presented. The model is especially useful as a nonequilibrium diagnostic tool for determining the concentrations of Li atoms from available measurements of electron density (Ne) and temperature (Te). In addition, the present approach is able to determine the electron kinetic mechanisms contributing to populate and depopulate the Li atom ground and excited states in laser-produced plasmas. A very reasonable qualitative agreement is found when comparing the model predicted Li atom densities with those obtained experimentally. Thus, the proposed approach can be used as a useful tool to optimize the processes involved in pulsed laser deposition of LiNbO3 thin films.
Electrical characteristics of two Coulomb blockade devices driven by a periodically oscillating potential
Authors: Altebaeumer Thomas , Ahmed Haroon
Published: July 2002

We describe the electrical characteristics of two multiple tunnel junction Coulomb blockade devices connected in series and separated by an island, which is large enough for Coulomb blockade phenomena in the island to be neglected. A sine wave rf signal applied to this island results in a current, which switches its polarity depending on the magnitude and sign of the gate voltages applied to the multiple tunnel junctions, even if the source drain voltage across the entire circuit is nonzero. We show that the transport mechanism responsible for this current is strongly related to the Coulomb blockade characteristics of the multiple tunnel junctions, which need to be asymmetric to achieve pump operation. The circuit characteristic can be explained theoretically by a bidirectional electron pump, which is therefore inherently different from conventional electron pumps driven by multiple ac signals.