202 research outputs found
MgB2 superconducting thin films with a transition temperature of 39 Kelvin
We report the growth of high-quality c-axis-oriented epitaxial MgB2 thin
films by using a pulsed laser deposition technique. The thin films grown on
(1`1 0 2) Al2O3 substrates show a Tc of 39 K. The critical current density in
zero field is ~ 6 x 10^6 A/cm2 at 5 K and ~ 3 x 10^5 A/cm^2 at 35 K, suggesting
that this compound has great potential for electronic device applications, such
as microwave devices and superconducting quantum interference devices. For the
films deposited on Al2O3, X-ray diffraction patterns indicate a highly
c-axis-oriented crystal structure perpendicular to the substrate surface.Comment: 3 pages and 3 figure
Direct Visualization of Laser-Driven Focusing Shock Waves
Cylindrically or spherically focusing shock waves have been of keen interest
for the past several decades. In addition to fundamental study of materials
under extreme conditions, cavitation, and sonoluminescence, focusing shock
waves enable myriad applications including hypervelocity launchers, synthesis
of new materials, production of high-temperature and high-density plasma
fields, and a variety of medical therapies. Applications in controlled
thermonuclear fusion and in the study of the conditions reached in laser fusion
are also of current interest. Here we report on a method for direct real-time
visualization and measurement of laser-driven shock generation, propagation,
and 2D focusing in a sample. The 2D focusing of the shock front is the
consequence of spatial shaping of the laser shock generation pulse into a ring
pattern. A substantial increase of the pressure at the convergence of the
acoustic shock front is observed experimentally and simulated numerically.
Single-shot acquisitions using a streak camera reveal that at the convergence
of the shock wave in liquid water the supersonic speed reaches Mach 6,
corresponding to the multiple gigapascal pressure range 30 GPa
Optimization of drift gases for accuracy in pressurized drift tubes
Modern detectors such as ATLAS use pressurized drift tubes to minimize diffusion and achieve high coordinate accuracy. However, the coordinate accuracy depends on the exact knowledge of converting measured times into coordinates. Linear space-time relationships are best for reconstruction, but difficult to achieve in the field. Previous mixtures, which contained methane or other organic quenchers, are disfavored because of ageing problems. From our studies of nitrogen and carbon dioxide, two mixtures with only small deviations from linearity were determined and measured. Scaling laws for different pressures and magnetic fields are also given
Real time response on dS_3: the Topological AdS Black Hole and the Bubble
We study real time correlators in strongly coupled N=4 supersymmetric
Yang-Mills theory on dS_3 x S^1, with antiperiodic boundary conditions for
fermions on the circle. When the circle radius is larger than a critical value,
the dual geometry is the so-called "topological AdS_5 black hole". Applying the
Son- Starinets recipe in this background we compute retarded glueball
propagators which exhibit an infinite set of poles yielding the quasinormal
frequencies of the topological black hole. The imaginary parts of the
propagators exhibit thermal effects associated with the Gibbons-Hawking
temperature due to the cosmological horizon of the de Sitter boundary. We also
obtain R-current correlators and find that after accounting for a small
subtlety, the Son-Starinets prescription yields the retarded Green's functions.
The correlators do not display diffusive behaviour at late times. Below the
critical value of the circle radius, the topological black hole decays to the
AdS_5 "bubble of nothing". Using a high frequency WKB approximation, we show
that glueball correlators in this phase exhibit poles on the real axis. The
tunnelling from the black hole to the bubble is interpreted as a hadronization
transition.Comment: 52 pages, 11 figures, typos corrected, references adde
Numerical Evaluation of P-Multigrid Method for the Solution of Discontinuous Galerkin Discretizations of Diffusive Equations
This paper describes numerical experiments with P-multigrid to corroborate analysis, validate the present implementation, and to examine issues that arise in the implementations of the various combinations of relaxation schemes, discretizations and P-multigrid methods. The two approaches to implement P-multigrid presented here are equivalent for most high-order discretization methods such as spectral element, SUPG, and discontinuous Galerkin applied to advection; however it is discovered that the approach that mimics the common geometric multigrid implementation is less robust, and frequently unstable when applied to discontinuous Galerkin discretizations of di usion. Gauss-Seidel relaxation converges 40% faster than block Jacobi, as predicted by analysis; however, the implementation of Gauss-Seidel is considerably more expensive that one would expect because gradients in most neighboring elements must be updated. A compromise quasi Gauss-Seidel relaxation method that evaluates the gradient in each element twice per iteration converges at rates similar to those predicted for true Gauss-Seidel
Keck Adaptive Optics Observations of the Radio Galaxy 3C294: A Merging System at z = 1.786?
We present seeing-limited and adaptive optics (AO) images of the z = 1.786
radio galaxy 3C294 in the H and K' infrared bands obtained at Keck Observatory.
The infrared emission of 3C294 is dominated by two distinct components
separated by ~1" (9 kpc). The eastern knot contains an unresolved core that
contributes ~4% of the K'-band light; we identify this core with the active
nucleus. The western component is about 2.5 times brighter. The most plausible
interpretation of the near-infrared morphology is an ongoing merger event, with
the active nucleus located in the less massive of the two galaxies.Comment: Accepted for Publication in Astrophysical Journa
String Theory on Lorentzian AdS_3 in Minisuperspace
We investigate string theory on Lorentzian AdS_3 in the minisuperspace
approximation. The minisuperspace model reduces to the worldline theory of a
scalar particle in the Lorentzian AdS_3. The Hilbert space consists of
normalizable wave functions, and we see that the unitarity of the theory (or
the self-adjointness of the Hamiltonian) restricts the possible sets of wave
functions. The restricted wave functions have the property of probability
conservation (or current conservation) across the horizons. Two and three point
functions are also computed. In the Euclidean model functional forms of these
quantities are restricted by the SL(2,R) symmetry almost uniquely, however, in
the Lorentzian model there are several ambiguities left. The ambiguities are
fixed by the direct computation of overlaps of wave functions.Comment: 32 pages, no figures, minor changes, references adde
Scattering of Long Folded Strings and Mixed Correlators in the Two-Matrix Model
We study the interactions of Maldacena's long folded strings in
two-dimensional string theory. We find the amplitude for a state containing two
long folded strings to come and go back to infinity. We calculate this
amplitude both in the worldsheet theory and in the dual matrix model, the
Matrix Quantum Mechanics. The matrix model description allows to evaluate the
amplitudes involving any number of long strings, which are given by the mixed
trace correlators in an effective two-matrix model.Comment: 39 pages, 6 figure
Drag Prediction Using Adaptive Discontinuous Finite Elements
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106502/1/AIAA2013-51.pd
Einstein black holes, free scalars and AdS/CFT correspondence
We investigate AdS/CFT correspondence for two families of Einstein black
holes in d > 3 dimensions, modelling the boundary CFT by a free conformal
scalar field and evaluating the boundary two-point function in the bulk
geodesic approximation. For the d > 3 counterpart of the nonrotating BTZ hole
and for its Z_2 quotient, the boundary state is thermal in the expected sense,
and its stress-energy reflects the properties of the bulk geometry and suggests
a novel definition for the mass of the hole. For the generalised
Schwarzschild-AdS hole with a flat horizon of topology R^{d-2}, the boundary
stress-energy has a thermal form with energy density proportional to the hole
ADM mass, but stress-energy corrections from compactified horizon dimensions
cannot be consistently included at least for d=5.Comment: 32 pages. LaTeX with amsfonts, amsmath, amssymb. (v2: References
added. v3: Geodesic horizon-crossing clarified in section 2; comparison with
quasilocal energy-momentum included in section 4.
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