1,501 research outputs found
Multiresolution analysis in statistical mechanics. II. The wavelet transform as a basis for Monte Carlo simulations on lattices
In this paper, we extend our analysis of lattice systems using the wavelet
transform to systems for which exact enumeration is impractical. For such
systems, we illustrate a wavelet-accelerated Monte Carlo (WAMC) algorithm,
which hierarchically coarse-grains a lattice model by computing the probability
distribution for successively larger block spins. We demonstrate that although
the method perturbs the system by changing its Hamiltonian and by allowing
block spins to take on values not permitted for individual spins, the results
obtained agree with the analytical results in the preceding paper, and
``converge'' to exact results obtained in the absence of coarse-graining.
Additionally, we show that the decorrelation time for the WAMC is no worse than
that of Metropolis Monte Carlo (MMC), and that scaling laws can be constructed
from data performed in several short simulations to estimate the results that
would be obtained from the original simulation. Although the algorithm is not
asymptotically faster than traditional MMC, because of its hierarchical design,
the new algorithm executes several orders of magnitude faster than a full
simulation of the original problem. Consequently, the new method allows for
rapid analysis of a phase diagram, allowing computational time to be focused on
regions near phase transitions.Comment: 11 pages plus 7 figures in PNG format (downloadable separately
Multiresolution analysis in statistical mechanics. I. Using wavelets to calculate thermodynamic properties
The wavelet transform, a family of orthonormal bases, is introduced as a
technique for performing multiresolution analysis in statistical mechanics. The
wavelet transform is a hierarchical technique designed to separate data sets
into sets representing local averages and local differences. Although
one-to-one transformations of data sets are possible, the advantage of the
wavelet transform is as an approximation scheme for the efficient calculation
of thermodynamic and ensemble properties. Even under the most drastic of
approximations, the resulting errors in the values obtained for average
absolute magnetization, free energy, and heat capacity are on the order of 10%,
with a corresponding computational efficiency gain of two orders of magnitude
for a system such as a Ising lattice. In addition, the errors in
the results tend toward zero in the neighborhood of fixed points, as determined
by renormalization group theory.Comment: 13 pages plus 7 figures (PNG
Physiologic Specialization of \u3ci\u3ePuccinia recondita\u3c/i\u3e f. sp. \u3ci\u3etritici\u3c/i\u3e in Nebraska During 1995 and 1996
Field samples of Puccinia recondita f. sp. tritici, collected from four wheat-growing regions in Nebraska in 1995 and from three in 1996, were characterized for virulence. Twenty virulence phenotypes were identified in 1995 and 18 in 1996. Virulence phenotypes MBR-10,18 (virulent on Lr genes, 1, 3, 3ka, 10, 11, 18, and 30) and MDR-10,18 (virulent on Lr genes 1, 3, 3ka, 10, 11, 18, 24, and 30) were the most prevalent, with each phenotype comprising 21.6% of the isolates characterized in 1995. Of the 1995 isolates, 24% were virulent on 10 or more host genes. No virulence to Lr16 and Lr17 was detected. In 1996, virulence phenotype MBR-10,18 was the most prevalent and comprised 20.5% of the isolates characterized. Of the 1996 isolates, 33% were virulent on 10 or more host genes. All isolates in both years were virulent on Lr1, Lr3, and Lr10. New virulence phenotypes were detected in 1996 that were not detected in 1995. In 1996, virulence was more frequent on Lr2a, Lr16, and Lr17 and less frequent on Lr3ka, Lr18, Lr24, Lr26, and Lr30. The number of isolates virulent on Lr24 and Lr26 has decreased from 83 and 53%, respectively, in 1992, to 34 and 1%, respectively, in 1996
Millimeter-wave diode-grid phase shifters
Monolithic diode grids have been fabricated on 2-cm square gallium-arsenide wafers with 1600 Schottky-barrier varactor diodes. Shorted diodes are detected with a liquid-crystal technique, and the bad diodes are removed with an ultrasonic probe. A small-aperture reflectometer that uses wavefront division interference was developed to measure the reflection coefficient of the grids. A Phase shift of 70° with a 7-dB loss was obtained at 93 GHz when the bias on the diode grid was changed from -3 V to 1 V. A simple transmission-line grid model, together with the measured low-frequency parameters for the diodes, was shown to predict the measured performance over the entire capacitive bias range of the diodes, as well as over the complete reactive tuning range provided by a reflector behind the grid, and over a wide range of frequencies form 33 GHz to 141 GHz. This shows that the transmission-line model and the measured low-frequency diode parameters can be used to design an electronic beam-steering array and to predict its performance. An electronic beam-steering array made of a pair of grids using state-of-the-art diodes with 5-Ω series resistances would have a loss of 1.4 dB at 90 GHz
Millimeter-Wave Diode-Grid Frequency Doubler
Monolithic diode grid were fabricated on 2-cm^2 gallium-arsenide wafers in a proof-of-principle test of a quasi-optical varactor millimeter-wave frequency multiplier array concept. An equivalent circuit model based on a transmission-line analysis of plane wave illumination was applied to predict the array performance. The doubler experiments were performed under far-field illumination conditions. A second-harmonic conversion efficiency of 9.5% and output powers of 0.5 W were achieved at 66 GHz when the diode grid was pumped with a pulsed source at 33 GHz. This grid had 760 Schottky-barrier varactor diodes. The average series resistance was 27 Ω, the minimum capacitance was 18 fF at a reverse breakdown voltage of -3 V. The measurements indicate that the diode grid is a feasible device for generating watt-level powers at millimeter frequencies and that substantial improvement is possible by improving the diode breakdown voltage
A terahertz grid frequency doubler
We present a 144-element terahertz quasi-optical grid frequency doubler. The grid is a planar structure with bow-tie antennas as a unit cell, each loaded with a planar Schottky diode. The maximum output power measured for this grid is 24 mW at 1 THz for 3.1-ÎĽs 500-GHz input pulses with a peak input power of 47 W. An efficiency of 0.17% for an input power of 6.3 W and output power of 10.8 mW is measured. To date, this is the largest recorded output power for a multiplier at terahertz frequencies. Input and output tuning curves are presented and an output pattern is measured and compared to theory
The X-ray Spectrum and Light Curve of Supernova 1995N
We report on multi-epoch X-ray observations of the Type IIn (narrow emission
line) supernova SN 1995N with the ROSAT and ASCA satellites. The January 1998
ASCA X-ray spectrum is well fitted by a thermal bremsstrahlung (kT~10 keV,
N_H~6e20 cm^-2) or power-law (alpha~1.7, N_H~1e21 cm^-2) model. The X-ray light
curve shows evidence for significant flux evolution between August 1996 and
January 1998: the count rate from the source decreased by 30% between our
August 1996 and August 1997 ROSAT observations, and the X-ray luminosity most
likely increased by a factor of ~2 between our August 1997 ROSAT and January
1998 ASCA observations, although evolution of the spectral shape over this
interval is not ruled out. The high X-ray luminosity, L_X~1e41 erg/sec, places
SN 1995N in a small group of Type IIn supernovae with strong circumstellar
interaction, and the evolving X-ray luminosity suggests that the circumstellar
medium is distributed inhomogeneously.Comment: MNRAS accepted. 6 pages, 2 figures; uses mn.sty and psfi
Magnetically Accreting Isolated Old Neutron Stars
Previous work on the emission from isolated old neutron stars (IONS)
accreting the inter-stellar medium (ISM) focussed on gravitational capture -
Bondi accretion. We propose a new class of sources which accrete via magnetic
interaction with the ISM. While for the Bondi mechanism, the accretion rate
decreases with increasing NS velocity, in magnetic accretors (MAGACs="magics")
the accretion rate increases with increasing NS velocity. MAGACs will be
produced among high velocity (~> 100 km s-1) high magnetic field (B> 1e14 G)
radio pulsars - the ``magnetars'' - after they have evolved first through
magnetic dipole spin-down, followed by a ``propeller'' phase (when the object
sheds angular momentum on a timescale ~< 1e10 yr). The properties of MAGACS may
be summarized thus: dipole magnetic fields of B~>1e14 G; minimum velocities
relative to the ISM of >25-100 km s-1, depending on B, well below the median in
the observed radio-pulsar population; spin-periods of >days to years; accretion
luminosities of 1e28- 1e31 ergs s-1 ; and effective temperatures kT=0.3 - 2.5
keV if they accrete onto the magnetic polar cap. We find no examples of MAGACs
among previously observed source classes (anomalous X-ray pulsars,
soft-gamma-ray repeaters or known IONS). However, MAGACs may be more prevelant
in flux-limited X-ray catalogs than their gravitationally accreting
counterparts.Comment: ApJ, accepte
- …