Spiral Structure in IP Peg: Confronting Theory and Observations

The first convincing piece of evidence of spiral structure in the accretion disc in IP Pegasi was found by Steeghs et al. (1997). We performed two kinds of 2D hydrodynamic simulations, a SFS finite volume scheme and a SPH scheme, with a mass ratio of 0.5. Both results agreed well with each other. We constructed Doppler maps and line flux-binary phase relations based on density distributions, the results agreeing well with those obtained by observation.Comment: 4 pages, LaTeX with 2 ps figures using crckapb.sty. To appear in the Proceedings of Numerical Astrophysics 1998, Tokyo, Japan, 10-13 March, 1998, eds. S. M. Miyama, K. Tomisaka and T. Hanawa (Kluwer Academic Publishers

Three-dimensional turbopump flowfield analysis

A program was conducted to develop a flow prediction method applicable to rocket turbopumps. The complex nature of a flowfield in turbopumps is described and examples of flowfields are discussed to illustrate that physics based models and analytical calculation procedures based on computational fluid dynamics (CFD) are needed to develop reliable design procedures for turbopumps. A CFD code developed at NASA ARC was used as the base code. The turbulence model and boundary conditions in the base code were modified, respectively, to: (1) compute transitional flows and account for extra rates of strain, e.g., rotation; and (2) compute surface heat transfer coefficients and allow computation through multistage turbomachines. Benchmark quality data from two and three-dimensional cascades were used to verify the code. The predictive capabilities of the present CFD code were demonstrated by computing the flow through a radial impeller and a multistage axial flow turbine. Results of the program indicate that the present code operated in a two-dimensional mode is a cost effective alternative to full three-dimensional calculations, and that it permits realistic predictions of unsteady loadings and losses for multistage machines

Single Crystal Growth and Characterization of the Iron-Based Superconductor KFe2As2 Synthesized by KAs Flux Method

Centimeter sized platelet single crystals of KFe2As2 were grown using a self-flux method. An encapsulation technique using commercial stainless steel container allowed the stable crystal growth lasting for more than 2 weeks. Ternary K-Fe-As systems with various starting compositions were examined to determine the optimal growth conditions. Employment of KAs flux led to the growth of large single crystals with the typical size of as large as 15 mm x 10 mm x 0.4 mm. The grown crystals exhibit sharp superconducting transition at 3.4 K with the transition width 0.2 K, as well as the very large residual resistivity ratio exceeding 450, evidencing the good sample quality.Comment: 4 pages, 6 Postscript figure

An optimal gap theorem

By solving the Cauchy problem for the Hodge-Laplace heat equation for $d$-closed, positive $(1, 1)$-forms, we prove an optimal gap theorem for K\"ahler manifolds with nonnegative bisectional curvature which asserts that the manifold is flat if the average of the scalar curvature over balls of radius $r$ centered at any fixed point $o$ is a function of $o(r^{-2})$. Furthermore via a relative monotonicity estimate we obtain a stronger statement, namely a `positive mass' type result, asserting that if $(M, g)$ is not flat, then $\liminf_{r\to \infty} \frac{r^2}{V_o(r)}\int_{B_o(r)}\mathcal{S}(y)\, d\mu(y)>0$ for any $o\in M$

Knot Floer homology detects fibred knots

Ozsv\'ath and Szab\'o conjectured that knot Floer homology detects fibred knots in $S^3$. We will prove this conjecture for null-homologous knots in arbitrary closed 3--manifolds. Namely, if $K$ is a knot in a closed 3--manifold $Y$, $Y-K$ is irreducible, and $\hat{HFK}(Y,K)$ is monic, then $K$ is fibred. The proof relies on previous works due to Gabai, Ozsv\'ath--Szab\'o, Ghiggini and the author. A corollary is that if a knot in $S^3$ admits a lens space surgery, then the knot is fibred.Comment: version 4: incorporates referee's suggestions, to appear in Inventiones Mathematica

Current versus voltage characteristics of GaN/AlGaN/GaN double heterostructures with varying AlGaN thickness and composition under hydrostatic pressure

We have studied current versus voltage characteristics of n-GaN∕u-AlGaN∕n-GaN double heterostructure devices under hydrostatic pressure up to 500MPa. Devices were grown on c-plane sapphire substrates by organometallic vapor phase epitaxy using epitaxial layer overgrowth. The effect of AlGaN layer thickness and composition on the pressure sensitivity was investigated. For a fixed applied bias, we found that the current decreases approximately linearly in magnitude with increasing hydrostatic pressure over the range of voltages and pressures applied. The decrease in current magnitude can be attributed to piezoelectric effects and is consistent with model calculations. The polarizationcharge densities at the GaN∕AlGaN interfaces change with hydrostatic pressure, which in turn modifies the internal potential barrier. Changes in the AlGaN layer thickness and composition also modify the interfacial polarization, with thicker AlGaN layers and higher AlN content increasing the effect of pressure on the observed current versus voltage characteristics. The strain gauge factors obtained for these devices range from ∼200 to 800

Premarital Sex Behaviors of Teenagers: a Case in Bali, Indonesia

This study aims at describing premarital sex behavior as well as to analyze peers impact on premarital sexual behavior of teenagers. The study was conducted in Bali, Indonesia. This is an analytic observational research that applied cross-sectional approach. The subject was 566 teenagers who were studying at Senior High Schools in Buleleng, Bali. Data was collected using a questionnaire. We examined the validity and reliability of the data before analyzed it. We also collected data through interviews. After being analyzed by logistic regression analysis, the data shows that 47% of teenagers have  never watched porn, 35.9% of teenagers have never conducted kissing activity (with their partner), 24.7% of teenagers have never conducted sexuality rub toward their sexual organs, 22.8% of teenagers have ever masturbated, and 13.1% of teenagers have never experienced sexual intercourse. Teenagers who have got the correct information from peers are possible to have better premarital sex behaviors, 2 times greater than those who have not got correct information (OR = 2; p = 0.003). It is also found that teenagers who have got a good emotional relationship with peers are more possible to have better premarital sex behavior life, 3 times greater, than teenagers who have got a less emotional relationship with peers (OR = 3; p = 0.000). It is concluded that, statistically, there is significant influence between peers and premarital sex behaviors of teenagers in Indonesia

On resonant scatterers as a factor limiting carrier mobility in graphene

We show that graphene deposited on a substrate has a non-negligible density of atomic scale defects. This is evidenced by a previously unnoticed D peak in the Raman spectra with intensity of about 1% with respect to the G peak. We evaluated the effect of such impurities on electron transport by mimicking them with hydrogen adsorbates and measuring the induced changes in both mobility and Raman intensity. If the intervalley scatterers responsible for the D peak are monovalent, their concentration is sufficient to account for the limited mobilities achievable in graphene on a substrate.Comment: version 2: several comments are taken into account and refs adde