Multi-moment advection scheme for Vlasov simulations

We present a new numerical scheme for solving the advection equation and its application to Vlasov simulations. The scheme treats not only point values of a profile but also its zeroth to second order piecewise moments as dependent variables, for better conservation of the information entropy. We have developed one-and two-dimensional schemes and show that they provide quite accurate solutions within reasonable usage of computational resources compared to other existing schemes. The two-dimensional scheme can accurately solve the solid body rotation problem of a gaussian profile for more than hundred rotation periods with little numerical diffusion. This is crucially important for Vlasov simulations of magnetized plasmas. Applications of the one- and two-dimensional schemes to electrostatic and electromagnetic Vlasov simulations are presented with some benchmark tests.     

Origin of the stability of Ge(105) on si: a new structure model and surface strain relaxation

The structure of Ge(105)-(1 x 2) grown on Si(105) is examined by scanning tunneling microscopy (STM) and first-principles calculations. The morphology evolution with an increasing amount of Ge deposited documents the existence of a tensile surface strain in Si(105) and its relaxation with increasing coverage of Ge. A detailed analysis of high-resolution STM images and first-principles calculations produce a new stable model for the Ge(105)-(1 x 2) structure formed on the Si(105) surface that includes the existence of surface strain. It corrects the model developed from early observations of the facets of "hut" clusters grown on Si(001).     

Synthesis, complexation, and fluorescence behavior of armed crown ethers carrying naphthyl group

Armed crown ethers (1-4) bonding through an amine, amide, ether, or ester linkage to naphthyl group were found to display unique photophysical properties in the presence of guest salts. Complexation of PET fluoroionophores (1a and 1b) with Zn(2+) increased the fluorescence intensities of the host by a factor of 2.4 and 2.7, respectively. (1)H and (13)C NMR analyses of this complexation behavior of 1a revealed that Zn(2+) strongly coordinates with the armed crown nitrogen to cause a dramatic decrease in an intramolecular charge-transfer character. The armed crowns (2 and 3), bonding through an ether or ester linkage to a naphthalene, gave fluorescence quenching with guest thiocyanates. While the amide derivative (4) exhibited high Ba(2+) fluorescence selectivity and in the presence of this cation the host fluorescence intensity was by a factor of 3.69.     

Convection in the presence of a first-order phase change

We report experimental and theoretical results for two-phase convection in a thin horizontal layer of a fluid with a first-order phase change and heated from below. A top layer of the nematic phase of a liquid crystal is located above the bottom layer of the isotropic phase of the same substance. A horizontal field of 1000 G is applied in order to align the director of the nematic phase. Over some ranges of the thickness of the isotropic phase, and in sufficiently large thermal gradients, the more dense nematic phase can be stably stratified above the less dense isotropic one, with a stable interface between them. Based on the equations of motion derived for this problem by Busse and Schubert [J. Fluid Mech. 46, 801 (1971)], we evaluate the bifurcation lines between the quiescent and convecting states and the corresponding critical wave vectors as a function of the interface position. We report experimental measurements based on Nusselt-number determinations for the locations of the bifurcation lines. They are in good agreement with the theoretical results. We also report approximate determinations of the critical wave numbers which are semiquantitatively consistent with the theory. A great diversity of patterns is observed in the convecting states, including normal and parallel rolls, rolls with defects and disorder, target patterns and spirals, and cellular flow with upflow or downflow at the cell center. These patterns are discussed in terms of the breaking of the mirror symmetry at the horizontal midplane by the interface, and in terms of the orienting effects of the magnetic field.     

Luminescence and conduction charge in thin-film electroluminescent devices

The luminescence and conduction currents of a doubly-insulated thin-film electroluminescent device were studied under various excitation conditions. The conduction current waveform was calculated numerically from the luminescence waveform. It was composed of two components: a fast transient component j_fc, which rises and decays rapidly when a pulse is applied, and a dc-like component j_dc, which maintains a constant level during the duration of pulse. The ratio of these components varied considerably with applied voltage. Both the slow response of luminescence to voltage change and the change in j_fc when the excitation conditions were changed from repetitive pulses to a single pulse suggest an accumulation of charges inside the electroluminescence layer. This accumulation causes a space charge effect which explains the dominance of j_fc in the high-brightness region. The fact that luminescence intensity in this region is not related to the amplitude of individual pulses, but rather to the average amplitude of the pulse train also indicates that the space-charge-induced internal electric field is the main factor in accelerating the carriers which excite the luminescence centers. It was found that the two conduction current components showed similar characteristics in singly-insulated devices.     

Almost unique determination of neutral-current couplings in neutrino-hadron interactions

Simultaneous analyses of inclusive and semi-inclusive reactionns in neutrino-nucleon collisions and elastic neutrino-proton scattering uniquely determine the neutral-current couplings between neutrinos and hadrons up to a two-fold “vector-axial-vector ambiguity”. One of the preferred solutions coincides with the Salam-Weinberg model within errors while the other solution, also compatible with SU(2) ? U(1) models with the simplest Higgs mechanism, may be helpful in understanding the bismuth puzzle.     

A study of surface reproducibility and impurity segregation using ion neutralization and photoemission spectroscopies

We have studied the reproducibility of electron spectra obtained from a series of Si(111) surfaces on lightly doped crystals that were subjected to a common sputtering procedure but different subsequent heat treatments. Each of the surfaces displayed a sharp 7 × 7 LEED pattern and showed no impurities above the minimum detection limit of our Auger electron spectrograph. Ion neutralization spectroscopy (INS) and photoemission (UPS) at ?ω = 16.8 and 21.2 eV were used to obtain the electron spectra. From the observed differences in the electron spectra, the known characteristics of these spectroscopies, and a comparison with theory we conclude that these surfaces had small and differing amounts of impurity located principally in the selvedge or near-surface bulk rather than directly on or in the surface monolayer. The surface was cleaner than the near surface bulk. Longer heating of one sample to higher temperatures brought to the surface detectable amounts of Mo impurity that had diffused into the crystal from the Mo mounting clamps, changing the LEED pattern to √3 × √3(R 30°) and producing large modifications of both the INS and UPS spectra.     

The effect of disordering on the spectral properties of partial monolayers of H on Si(111)

Ultraviolet photoemission measurements are reported for a H covered Si(111) surface for which the H coverage ranged from a fraction to a full monolayer. These measurements reveal striking differences depending whether the Si(111) substrate is kept at room temperature (RT) or 150°C. In particular, the 150°C sample UPS spectral series shows monotonic growth with little line shape change while the RT series shows significant line shape modification with coverage. These results are interpreted as island growth at 150°C and disordered adsorption at RT. Theoretical model calculations are carried out of the electron density of states of a fractional monolayer of H chemisorbed to Si(111) that reproduce the essential features of the RT data and confirm the role of disordering there.     

Parallel vs. anti-parallel orientation in a curdlan/oligo(dA) complex as estimated by a FRET technique

We already found that beta-1,3-glucan polysaccharides form polymeric complexes with certain polynucleotides, but the parallel vs. anti-parallel orientation in those complexes had remained unsolved. In this paper, this controversial problem has been discussed for curdlan/oligo(dA) complexes utilizing two different energy transfer techniques. The first system consists of a combination of fluorescein-labeled curdlan and 3'-(or 5'-)tetramethyl-rhodamine (TAMRA)-labeled oligo(dA). The second system utilizes gold nanoparticles: that is, two curdlan chains were linked by a disulfide bond and after complexation with oligo(dA), the complex was immobilized on gold nanoparticles. In this system, TAMRA was attached to the 3'(or 5') end of oligo(dA) and the gold particle acted as a fluorescence quencher (energy acceptor). These experiments have led us to conclude that in the curdlan/oligo(dA) complex, parallel orientation is more favourable than anti-parallel orientation. These findings have enabled us to envision a clearer image for the complexation mode between beta-1,3-glucan polysaccharides and polynucleotides.