Half Semimetallic Antiferromagnetism in the Sr2_2CrTO6_6 System, T=Os, Ru

Double perovskite Sr$_2$CrOsO$_6$ is (or is very close to) a realization of a spin-asymmetric semimetallic compensated ferrimagnet, according to first principles calculations. This type of near-half metallic antiferromagnet is an unusual occurrence, and more so in this compound because the zero gap is accidental rather than being symmetry determined. The large spin-orbit coupling (SOC) of osmium upsets the spin balance (no net spin moment without SOC): it reduces the Os spin moment by 0.27 $\mu_B$ and induces an Os orbital moment of 0.17 $\mu_B$ in the opposite direction. The effects combine (with small oxygen contributions) to give a net total moment of 0.54 $\mu_B$ per cell in \scoo, reflecting a large impact of SOC in this compound. This value is in moderately good agreement with the measured saturation moment of 0.75 $\mu_B$. The value of the net moment on the Os ion obtained from neutron diffraction (0.73 $\mu_B$ at low temperature) differs from the calculated value (1.14 $\mu_B$). Rather surprisingly, in isovalent Sr$_2$CrRuO$_6$ the smaller SOC-induced spin changes and orbital moments (mostly on Ru) almost exactly cancel. This makes Sr$_2$CrRuO$_6$ a "half (semi)metallic antiferromagnet" (practically vanishing net total moment) even when SOC is included, with the metallic channel being a small-band-overlap semimetal. Fixed spin moment (FSM) calculations are presented for each compound, illustrating how they provide different information than in the case of a nonmagnetic material. These FSM results indicate that the Cr moment is an order of magnitude stiffer against longitudinal fluctuations than is the Os moment.Comment: 6 page

Prediction of force coefficients for labyrinth seals

The development of a linear model for the prediction of labyrinth seal forces and on its comparison to available stiffness data is presented. A discussion of the relevance of fluid damping forces and the preliminary stages of a program to obtain data on these forces are examined. Fluid-dynamic forces arising from nonuniform pressure patterns in labyrinth seal glands are known to be potentially destablizing in high power turbomachinery. A well documented case in point is that of the space Shuttle Main Engine turbopumps. Seal forces are also an important factor for the stability of shrouded turbines, acting in that case in conjunction with the effects of blade-tip clearance variations

Reconstructing Three-dimensional Structure of Underlying Triaxial Dark Halos From Xray and Sunyaev-Zel'dovich Effect Observations of Galaxy Clusters

While the use of galaxy clusters as {\it tools} to probe cosmology is established, their conventional description still relies on the spherical and/or isothermal models that were proposed more than 20 years ago. We present, instead, a deprojection method to extract their intrinsic properties from X-ray and Sunyaev--Zel'dovich effect observations in order to improve our understanding of cluster physics. First we develop a theoretical model for the intra-cluster gas in hydrostatic equilibrium in a triaxial dark matter halo with a constant axis ratio. In this theoretical model, the gas density profiles are expressed in terms of the intrinsic properties of the dark matter halos. Then, we incorporate the projection effect into the gas profiles, and show that the gas surface brightness profiles are expressed in terms of the eccentricities and the orientation angles of the dark halos. For the practical purpose of our theoretical model, we provide several empirical fitting formulae for the gas density and temperature profiles, and also for the surface brightness profiles relevant to X-ray and Sunyaev--Zel'dovich effect observations. Finally, we construct a numerical algorithm to determine the halo eccentricities and orientation angles using our model, and demonstrate that it is possible in principle to reconstruct the 3D structures of the dark halos from the X-ray and/or Sunyaev-Zel'dovich effect cluster data alone without requiring priors such as weak lensing informations and without relying on such restrictive assumptions as the halo axial symmetry about the line-of-sight.Comment: Accepted version, new discussions added, typos and minor mistakes corrected, ApJ in press (2004, Feb. 1 scheduled, Vol. 601, No. 2 issue),26 pages, 7 postscript figure

Three-dimensional MgB2_{2}-type superconductivity in hole-doped diamond

We substantiate by calculations that the recently discovered superconductivity below 4 K in 3% boron-doped diamond is caused by electron-phonon coupling of the same type as in MgB$_2$, albeit in 3 dimensions. Holes at the top of the zone-centered, degenerate $\sigma$-bonding valence band couple strongly to the optical bond-stretching modes. The increase from 2 to 3 dimensions reduces the mode-softening crucial for $T_{c}$ reaching 40 K in MgB$_{2}.$ Even if diamond had the same \emph{bare} coupling constant as MgB$_{2},$ which could be achieved with 10% doping, $T_{c}$ would only be 25 K. Superconductivity above 1 K in Si (Ge) requires hole-doping beyond 5% (10%).Comment: revised version, accepted by PR

SPH Simulations of Galactic Gaseous Disk with Bar: Distribution and Kinematic Structure of Molecular Clouds toward the Galactic Center

We have performed Smoothed Particle Hydrodynamic (SPH) simulations to study the response of molecular clouds in the Galactic disk to a rotating bar and their subsequent evolution in the Galactic Center (GC) region. The Galactic potential in our models is contributed by three axisymmetric components (massive halo, exponential disk, compact bulge) and a non-axisymmetric bar. These components are assumed to be invariant in time in the frame corotating with the bar. Some noticeable features such as an elliptical outer ring, spiral arms, a gas-depletion region, and a central concentration have been developed due to the influence of the bar. The rotating bar induces non-circular motions of the SPH particles, but hydrodynamic collisions tend to suppress the random components of the velocity. The velocity field of the SPH particles is consistent with the kinematics of molecular clouds observed in HCN (1-0) transition; these clouds are thought to be very dense clouds. However, the l-v diagram of the clouds traced by CO is quite different from that of our SPH simulation, being more similar to that obtained from simulations using collisionless particles. The $l-v$ diagram of a mixture of collisional and collisionless particles gives better reproduction of the kinematic structures of the GC clouds observed in the CO line. The fact that the kinematics of HCN clouds can be reproduced by the SPH particles suggests that the dense clouds in the GC are formed via cloud collisions induced by rotating bar.Comment: 31 pages, 10 pigures, accepted for publication in Ap

Dyons in N=4 Supersymmetric Theories and Three-Pronged Strings

We construct and explore BPS states that preserve 1/4 of supersymmetry in N=4 Yang-Mills theories. Such states are also realized as three-pronged strings ending on D3-branes. We correct the electric part of the BPS equation and relate its solutions to the unbroken abelian gauge group generators. Generic 1/4-BPS solitons are not spherically symmetric, but consist of two or more dyonic components held apart by a delicate balance between static electromagnetic force and scalar Higgs force. The instability previously found in three-pronged string configurations is due to excessive repulsion by one of these static forces. We also present an alternate construction of these 1/4-BPS states from quantum excitations around a magnetic monopole, and build up the supermultiplet for arbitrary (quantized) electric charge. The degeneracy and the highest spin of the supermultiplet increase linearly with a relative electric charge. We conclude with comments.Comment: 33 pages, two figures, LaTex, a footnote added, the figure caption of Fig.2 expanded, one more referenc

Quantum 1/4 BPS Dyons

Classical properties of 1/4 BPS dyons were previously well understood both in field theory context and in string theory context. Its quantum properties, however, have been more difficult to probe, although the elementary information of the supermultiplet structures is known from a perturbative construction. Recently, a low energy effective theory of monopoles was constructed and argued to contain these dyons as quantum bound states. In this paper, we find these dyonic bound states explicitly in the N=4 supersymmetric low energy effective theory. After identifying the correct angular momentum operators, we motivate an anti-self-dual ansatz for all BPS bound states. The wavefunctions are found explicitly, whose spin contents and degeneracies match exactly the expected results.Comment: 20 pages, no figure

Resonance NLS Solitons as Black Holes in Madelung Fluid

A new resonance version of NLS equation is found and embedded to the reaction-diffusion system, equivalent to the anti-de Sitter valued Heisenberg model, realizing a particular gauge fixing condition of the Jackiw-Teitelboim gravity. The space-time points where dispersion change the sign correspond to the event horizon, and the soliton solutions to the AdS black holes. The soliton with velocity bounded above describes evolution on the hyperboloid with nontrivial winding number and create under collisions the resonance states with a specific life time.Comment: Plain Tex, 12 pages, 6 figure