4,802 research outputs found
Phase diagram of a dilute ferromagnet model with antiferromagnetic next-nearest-neighbor interactions
We have studied the spin ordering of a dilute classical Heisenberg model with
spin concentration , and with ferromagnetic nearest-neighbor interaction
and antiferromagnetic next-nearest-neighbor interaction . Magnetic
phases at absolute zero temperature are determined examining the
stiffness of the ground state, and those at finite temperatures are
determined calculating the Binder parameter and the spin correlation
length . Three ordered phases appear in the phase diagram: (i) the
ferromagnetic (FM) phase; (ii) the spin glass (SG) phase; and (iii) the mixed
(M) phase of the FM and the SG. Near below the ferromagnetic threshold , a reentrant SG transition occurs. That is, as the temperature is decreased
from a high temperature, the FM phase, the M phase and the SG phase appear
successively. The magnetization which grows in the FM phase disappears in the
SG phase. The SG phase is suggested to be characterized by ferromagnetic
clusters. We conclude, hence, that this model could reproduce experimental
phase diagrams of dilute ferromagnets FeAu and EuSrS.Comment: 9 pages, 23 figure
A Test of the Particle Paradigm in N-Body Simulations
We present results of tests of the evolution of small ``fluid elements'' in
cosmological N--body simulations, to examine the validity of their treatment as
particles. We find that even very small elements typically collapse along one
axis while expanding along another, often to twice or more their initial
comoving diameter. This represents a possible problem for high--resolution uses
of such simulations.Comment: Uses aasms4.sty; accepted for publication in ApJ Letters. Files
available also at ftp://kusmos.phsx.ukans.edu/preprints/ates
A low CMB variance in the WMAP data
We have estimated the CMB variance from the three-year WMAP data, finding a
value which is significantly lower than the one expected from Gaussian
simulations using the WMAP best-fit cosmological model, at a significance level
of 98.7 per cent. This result is even more prominent if we consider only the
north ecliptic hemisphere (99.8 per cent). Different analyses have been
performed in order to identify a possible origin for this anomaly. In
particular we have studied the behaviour of single radiometers and single year
data as well as the effect of residual foregrounds and 1/f noise, finding that
none of these possibilities can explain the low value of the variance. We have
also tested the effect of varying the cosmological parameters, finding that the
estimated CMB variance tends to favour higher values of than the one of
the WMAP best-fit model. In addition, we have also tested the consistency
between the estimated CMB variance and the actual measured CMB power spectrum
of the WMAP data, finding a strong discrepancy. A possible interpretation of
this result could be a deviation from Gaussianity and/or isotropy of the CMB.Comment: 13 pages, 5 figures. Some new tests added. Section 5 largely
modified. Accepted for publication in MNRA
Trust economics feasibility study
We believe that enterprises and other organisations currently lack sophisticated methods and tools to determine if and how IT changes should be introduced in an organisation, such that objective, measurable goals are met. This is especially true when dealing with security-related IT decisions. We report on a feasibility study, Trust Economics, conducted to demonstrate that such methodology can be developed. Assuming a deep understanding of the IT involved, the main components of our trust economics approach are: (i) assess the economic or financial impact of IT security solutions; (ii) determine how humans interact with or respond to IT security solutions; (iii) based on above, use probabilistic and stochastic modelling tools to analyse the consequences of IT security decisions. In the feasibility study we apply the trust economics methodology to address how enterprises should protect themselves against accidental or malicious misuse of USB memory sticks, an acute problem in many industries
Evolution of the Pairwise Peculiar Velocity Distribution Function in Lagrangian Perturbation Theory
The statistical distribution of the radial pairwise peculiar velocity of
galaxies is known to have an exponential form as implied by observations and
explicitly shown in N-body simulations. Here we calculate its statistical
distribution function using the Zel'dovich approximation assuming that the
primordial density fluctuations are Gaussian distributed. We show that the
exponential distribution is realized as a transient phenomena on megaparsec
scales in the standard cold-dark-matter model.Comment: 19 pages, 8 Postscript figures, AAS LaTe
Theory of Parabolic Arcs in Interstellar Scintillation Spectra
Our theory relates the secondary spectrum, the 2D power spectrum of the radio
dynamic spectrum, to the scattered pulsar image in a thin scattering screen
geometry. Recently discovered parabolic arcs in secondary spectra are generic
features for media that scatter radiation at angles much larger than the rms
scattering angle. Each point in the secondary spectrum maps particular values
of differential arrival-time delay and fringe rate (or differential Doppler
frequency) between pairs of components in the scattered image. Arcs correspond
to a parabolic relation between these quantities through their common
dependence on the angle of arrival of scattered components. Arcs appear even
without consideration of the dispersive nature of the plasma. Arcs are more
prominent in media with negligible inner scale and with shallow wavenumber
spectra, such as the Kolmogorov spectrum, and when the scattered image is
elongated along the velocity direction. The arc phenomenon can be used,
therefore, to constrain the inner scale and the anisotropy of scattering
irregularities for directions to nearby pulsars. Arcs are truncated by finite
source size and thus provide sub micro arc sec resolution for probing emission
regions in pulsars and compact active galactic nuclei. Multiple arcs sometimes
seen signify two or more discrete scattering screens along the propagation
path, and small arclets oriented oppositely to the main arc persisting for long
durations indicate the occurrence of long-term multiple images from the
scattering screen.Comment: 22 pages, 11 figures, submitted to the Astrophysical Journa
The progression of primary bud necrosis in the grapevine cv. Shiraz (Vitis vinifera L.): A histological analysis
Primary Bud Necrosis (PBN) is a physiological disorder occurring in the compound axillary buds of grapevines. PBN causes the axillary bud to senesce and in some cases secondary buds can also abort. Since PBN is common in the grapevine variety Shiraz the aim of this study was to characterise anatomical changes at different stages of PBN development in this cultivar. Grapevine buds were collected from a vineyard located at Charleston, South Australia, Australia. Buds were dissected, assessed for the presence of PBN and rated on severity of the disorder. Buds at various stages of PBN were fixed for light microscopy. Cell breakdown was observed in all buds where PBN was visible. Collapse and thickening of cell walls was observed in a region of necrotic tissue and severity of PBN appeared to increase over time. The location of cell breakdown due to PBN appeared to be random and was not isolated to one region within the primary bud. PBN appeared to stop primordial growth, with cells differentiating further and maturing more rapidly without forming whole leaves. This cell region then breaks down and the necrosis can extend into the secondary buds.
A comparison of mean density and microscale density fluctuations in a CME at 10 R ⊙
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94990/1/grl15395.pd
Extreme value statistics and return intervals in long-range correlated uniform deviates
We study extremal statistics and return intervals in stationary long-range
correlated sequences for which the underlying probability density function is
bounded and uniform. The extremal statistics we consider e.g., maximum relative
to minimum are such that the reference point from which the maximum is measured
is itself a random quantity. We analytically calculate the limiting
distributions for independent and identically distributed random variables, and
use these as a reference point for correlated cases. The distributions are
different from that of the maximum itself i.e., a Weibull distribution,
reflecting the fact that the distribution of the reference point either
dominates over or convolves with the distribution of the maximum. The
functional form of the limiting distributions is unaffected by correlations,
although the convergence is slower. We show that our findings can be directly
generalized to a wide class of stochastic processes. We also analyze return
interval distributions, and compare them to recent conjectures of their
functional form
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