6,155 research outputs found
Evidence for the droplet/scaling picture of spin glasses
We have studied the Parisi overlap distribution for the three dimensional
Ising spin glass in the Migdal-Kadanoff approximation. For temperatures T
around 0.7Tc and system sizes upto L=32, we found a P(q) as expected for the
full Parisi replica symmetry breaking, just as was also observed in recent
Monte Carlo simulations on a cubic lattice. However, for lower temperatures our
data agree with predictions from the droplet or scaling picture. The failure to
see droplet model behaviour in Monte Carlo simulations is due to the fact that
all existing simulations have been done at temperatures too close to the
transition temperature so that sytem sizes larger than the correlation length
have not been achieved.Comment: 4 pages, 6 figure
Numerical study of domain coarsening in anisotropic stripe patterns
We study the coarsening of two-dimensional smectic polycrystals characterized
by grains of oblique stripes with only two possible orientations. For this
purpose, an anisotropic Swift-Hohenberg equation is solved. For quenches close
enough to the onset of stripe formation, the average domain size increases with
time as . Further from onset, anisotropic pinning forces similar to
Peierls stresses in solid crystals slow down defects, and growth becomes
anisotropic. In a wide range of quench depths, dislocation arrays remain mobile
and dislocation density roughly decays as , while chevron boundaries
are totally pinned. We discuss some agreements and disagreements found with
recent experimental results on the coarsening of anisotropic electroconvection
patterns.Comment: 8 pages, 11 figures. Phys. Rev E, to appea
Sex distribution of offspring-parents obesity: Angel's hypothesis revisited
This study, which is based on two cross sectional surveys' data, aims to establish any effect of parental obesity sex distribution of offspring and to replicate the results that led to the hypothesis that obesity may be associated with sex-linked recessive lethal gene. A representative sample of 4,064 couples living in Renfrew/Paisley, Scotland was surveyed 1972-1976. A total of 2,338 offspring from 1,477 of the couples screened in 1972-1976, living in Paisley, were surveyed in 1996. In this study, males represented 47.7% among the total offspring of the couples screened in 1972-1976. In the first survey there was a higher male proportion of offspring (53%, p < 0.05) from parents who were both obese, yet this was not significant after adjustment for age of parents. Also, there were no other significant differences in sex distribution of offspring according to body mass index, age, or social class of parents. The conditions of the original 1949 study of Angel (1949) (which proposed a sex-linked lethal recessive gene) were simulated by selecting couples with at least one obese daughter. In this subset, (n = 409), obesity in fathers and mothers was associated with 26% of offspring being male compared with 19% of offspring from a non-obese father and obese mother. Finally we conclude that families with an obese father have a higher proportion of male offspring. These results do not support the long-established hypotheses of a sex-linked recessive lethal gene in the etiology of obesity
Phase-ordering of conserved vectorial systems with field-dependent mobility
The dynamics of phase-separation in conserved systems with an O(N) continuous
symmetry is investigated in the presence of an order parameter dependent
mobility M(\phi)=1-a \phi^2. The model is studied analytically in the framework
of the large-N approximation and by numerical simulations of the N=2, N=3 and
N=4 cases in d=2, for both critical and off-critical quenches. We show the
existence of a new universality class for a=1 characterized by a growth law of
the typical length L(t) ~ t^{1/z} with dynamical exponent z=6 as opposed to the
usual value z=4 which is recovered for a<1.Comment: RevTeX, 8 pages, 13 figures, to be published in Phys. Rev.
The Effects of Phase Separation in the Cuprate Superconductors
Phase separation has been observed by several different experiments and it is
believed to be closely related with the physics of cuprates but its exactly
role is not yet well known. We propose that the onset of pseudogap phenomenon
or the upper pseudogap temperature has its origin in a spontaneous phase
separation transition at the temperature . In order to perform
quantitative calculations, we use a Cahn-Hilliard (CH) differential equation
originally proposed to the studies of alloys and on a spinodal decomposition
mechanism. Solving numerically the CH equation it is possible to follow the
time evolution of a coarse-grained order parameter which satisfies a
Ginzburg-Landau free-energy functional commonly used to model superconductors.
In this approach, we follow the process of charge segregation into two main
equilibrium hole density branches and the energy gap normally attributed to the
upper pseudogap arises as the free-energy potential barrier between these two
equilibrium densities below . This simulation provides quantitative
results %on the hole doping and temperature %dependence of the degree of the
charge inhomogeneity in agreement with %some experiments and the simulations
reproduce the observed stripe and granular pattern of segregation. Furthermore,
with a Bogoliubov-deGennes (BdG) local superconducting critical temperature
calculation for the lower pseudogap or the onset of local superconductivity, it
yields novel interpretation of several non-conventional measurements on
cuprates.Comment: Published versio
Absence of aging in the remanent magnetization in Migdal-Kadanoff spin glasses
We study the non-equilibrium behavior of three-dimensional spin glasses in
the Migdal-Kadanoff approximation, that is on a hierarchical lattice. In this
approximation the model has an unique ground state and equilibrium properties
correctly described by the droplet model. Extensive numerical simulations show
that this model lacks aging in the remanent magnetization as well as a maximum
in the magnetic viscosity in disagreement with experiments as well as with
numerical studies of the Edwards-Anderson model. This result strongly limits
the validity of the droplet model (at least in its simplest form) as a good
model for real spin glasses.Comment: 4 pages and 3 figures. References update
Replica field theory and renormalization group for the Ising spin glass in an external magnetic field
We use the generic replica symmetric cubic field-theory to study the
transition of short range Ising spin glasses in a magnetic field around the
upper critical dimension, d=6. A novel fixed-point is found, in addition to the
well-known zero magnetic field fixed-point, from the application of the
renormalization group. In the spin glass limit, n going to 0, this fixed-point
governs the critical behaviour of a class of systems characterised by a single
cubic interaction parameter. For this universality class, the spin glass
susceptibility diverges at criticality, whereas the longitudinal mode remains
massive. The third mode, the so-called anomalous one, however, behaves
unusually, having a jump at criticality. The physical consequences of this
unusual behaviour are discussed, and a comparison with the conventional de
Almeida-Thouless scenario presented.Comment: 5 pages written in revtex4. Accepted for publication in Phys. Rev.
Let
The Red-Naped Sapsucker in Nebraska
The Red-naped Sapsucker (Sphyrapicus nuchalis) was for many years accorded subspecific status within Yellow-bellied Sapsucker (Sphyapicus varius), but the American Ornithologists’ Union recently revised this opinion, and now regards nuchalis as a “good” species (Auk 102:680). Red-naped Sapsucker can be added to the Nebraska (species) list on the basis of three specimens in the collection of the University of Nebraska State Museum (UNSM).
The breeding range of Red-naped Sapsucker includes the Rocky Mountains east of the Cascades from southwest Canada south to central Arizona and it winters in much of northern Mexico, north as far as the southern part of its breeding range (The NOU Checklist of North American Birds, Sixth Edition, 1983)
The Red-Naped Sapsucker in Nebraska
The Red-naped Sapsucker (Sphyrapicus nuchalis) was for many years accorded subspecific status within Yellow-bellied Sapsucker (Sphyapicus varius), but the American Ornithologists’ Union recently revised this opinion, and now regards nuchalis as a “good” species (Auk 102:680). Red-naped Sapsucker can be added to the Nebraska (species) list on the basis of three specimens in the collection of the University of Nebraska State Museum (UNSM).
The breeding range of Red-naped Sapsucker includes the Rocky Mountains east of the Cascades from southwest Canada south to central Arizona and it winters in much of northern Mexico, north as far as the southern part of its breeding range (The NOU Checklist of North American Birds, Sixth Edition, 1983)
Singlet-Triplet Excitations in the Unconventional Spin-Peierls System TiOBr
We have performed time-of-flight neutron scattering measurements on powder
samples of the unconventional spin-Peierls compound TiOBr using the
fine-resolution Fermi chopper spectrometer (SEQUOIA) at the SNS. These
measurements reveal two branches of magnetic excitations within the
commensurate and incommensurate spin-Peierls phases, which we associate with n
= 1 and n = 2 triplet excitations out of the singlet ground state. These
measurements represent the first direct measure of the singlet-triplet energy
gap in TiOBr, which is determined to be Eg = 21.2 +/- 1.0 meV.Comment: 5 pages, 4 figures, submitted for publicatio
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