87 research outputs found
Phase diagrams of soluble multi-spin glass models
We include p-spin interactions in a spherical version of a soluble mean-field
spin-glass model proposed by van Hemmen. Due to the simplicity of the
solutions, which do not require the use of the replica trick, we are able to
carry out a detailed investigation of a number of special situations. For p
larger or equal to 3, there appear first-order transitions between the
paramagnetic and the ordered phases. In the presence of additional
ferromagnetic interactions, we show that there is no stable mixed phase, with
both ferromagnetic and spin-glass properties.Comment: To appear in Physica
Quantum spherical model with competing interactions
We analyze the phase diagram of a quantum mean spherical model in terms of
the temperature , a quantum parameter , and the ratio ,
where refers to ferromagnetic interactions between first-neighbor
sites along the directions of a hypercubic lattice, and is
associated with competing antiferromagnetic interactions between second
neighbors along directions. We regain a number of known results for
the classical version of this model, including the topology of the critical
line in the space, with a Lifshitz point at , for , and
closed-form expressions for the decay of the pair correlations in one
dimension. In the T=0 phase diagram, there is a critical border,
for , with a singularity at the Lifshitz point if
. We also establish upper and lower critical dimensions, and analyze
the quantum critical behavior in the neighborhood of .Comment: 18 pages, 3 figures, refs added, minor modifications to match
published versio
Moderate deviations for random field Curie-Weiss models
The random field Curie-Weiss model is derived from the classical Curie-Weiss
model by replacing the deterministic global magnetic field by random local
magnetic fields. This opens up a new and interestingly rich phase structure. In
this setting, we derive moderate deviations principles for the random total
magnetization , which is the partial sum of (dependent) spins. A typical
result is that under appropriate assumptions on the distribution of the local
external fields there exist a real number , a positive real number
, and a positive integer such that satisfies
a moderate deviations principle with speed and rate
function , where .Comment: 21 page
Low-temperature dynamics of the Curie-Weiss Model: Periodic orbits, multiple histories, and loss of Gibbsianness
We consider the Curie-Weiss model at a given initial temperature in vanishing
external field evolving under a Glauber spin-flip dynamics corresponding to a
possibly different temperature. We study the limiting conditional probabilities
and their continuity properties and discuss their set of points of
discontinuity (bad points). We provide a complete analysis of the transition
between Gibbsian and non-Gibbsian behavior as a function of time, extending
earlier work for the case of independent spin-flip dynamics. For initial
temperature bigger than one we prove that the time-evolved measure stays Gibbs
forever, for any (possibly low) temperature of the dynamics. In the regime of
heating to low-temperatures from even lower temperatures, when the initial
temperature is smaller than the temperature of the dynamics, and smaller than
1, we prove that the time-evolved measure is Gibbs initially and becomes
non-Gibbs after a sharp transition time. We find this regime is further divided
into a region where only symmetric bad configurations exist, and a region where
this symmetry is broken. In the regime of further cooling from low-temperatures
there is always symmetry-breaking in the set of bad configurations. These bad
configurations are created by a new mechanism which is related to the
occurrence of periodic orbits for the vector field which describes the dynamics
of Euler-Lagrange equations for the path large deviation functional for the
order parameter. To our knowledge this is the first example of the rigorous
study of non-Gibbsian phenomena related to cooling, albeit in a mean-field
setup.Comment: 31 pages, 24 figure
Thermodynamics of the incommensurate state in Rb_2WO_4: on the Lifshitz point in A`A``BX_4 compounds
We consider the evolution of the phase transition from the parent hexagonal
phase to the orthorhombic phase that occurs in several
compounds of family as a function of the hcp lattice parameter
. For compounds of type with larger than the threshold
value 1.26 the direct first-order transition is characterized
by the large entropy jump . For compounds , ,
with this transition occurs via an intermediate
incommensurate phase. DSC measurements were performed in
to characterize the thermodynamics of the transitions. It
was found that both transitions are again of the first order with entropy jumps
0.3Rln2c/a ~ 1.26A'A''BX_{4}BX_{4}$ tetrahedra
orientation as a possible source of the transitions discontinuity.Comment: 13 pages,1 Postscript figure. Submitted as Brief Report to Phys. Rev.
B, this paper reports a new work in Theory and Experiment, directed to
Structural Phase Transition
Approach to equilibrium for a class of random quantum models of infinite range
We consider random generalizations of a quantum model of infinite range
introduced by Emch and Radin. The generalization allows a neat extension from
the class of absolutely summable lattice potentials to the optimal class
of square summable potentials first considered by Khanin and Sinai and
generalised by van Enter and van Hemmen. The approach to equilibrium in the
case of a Gaussian distribution is proved to be faster than for a Bernoulli
distribution for both short-range and long-range lattice potentials. While
exponential decay to equilibrium is excluded in the nonrandom case, it is
proved to occur for both short and long range potentials for Gaussian
distributions, and for potentials of class in the Bernoulli case. Open
problems are discussed.Comment: 10 pages, no figures. This last version, to appear in J. Stat. Phys.,
corrects some minor errors and includes additional references and comments on
the relation to experiment
Particulate matter and episodic memory decline mediated by early neuroanatomic biomarkers of Alzheimer's disease
Evidence suggests exposure to particulate matter with aerodynamic diameter <2.5 μm (PM2.5) may increase the risk for Alzheimer's disease and related dementias. Whether PM2.5 alters brain structure and accelerates the preclinical neuropsychological processes remains unknown. Early decline of episodic memory is detectable in preclinical Alzheimer's disease. Therefore, we conducted a longitudinal study to examine whether PM2.5 affects the episodic memory decline, and also explored the potential mediating role of increased neuroanatomic risk of Alzheimer's disease associated with exposure. Participants included older females (n = 998; aged 73-87) enrolled in both the Women's Health Initiative Study of Cognitive Aging and the Women's Health Initiative Memory Study of Magnetic Resonance Imaging, with annual (1999-2010) episodic memory assessment by the California Verbal Learning Test, including measures of immediate free recall/new learning (List A Trials 1-3; List B) and delayed free recall (shortand long-delay), and up to two brain scans (MRI-1: 2005-06; MRI-2: 2009-10). Subjects were assigned Alzheimer's disease pattern similarity scores (a brain-MRI measured neuroanatomical risk for Alzheimer's disease), developed by supervised machine learning and validated with data from the Alzheimer's Disease Neuroimaging Initiative. Based on residential histories and environmental data on air monitoring and simulated atmospheric chemistry, we used a spatiotemporal model to estimate 3-year average PM2.5 exposure preceding MRI-1. In multilevel structural equation models, PM2.5 was associated with greater declines in immediate recall and new learning, but no association was found with decline in delayed-recall or composite scores. For each interquartile increment (2.81 μg/m3) of PM2.5, the annual decline rate was significantly accelerated by 19.3% [95% confidence interval (CI) = 1.9% to 36.2%] for Trials 1-3 and 14.8% (4.4% to 24.9%) for List B performance, adjusting for multiple potential confounders. Long-term PM2.5 exposure was associated with increased Alzheimer's disease pattern similarity scores, which accounted for 22.6% (95% CI: 1% to 68.9%) and 10.7% (95% CI: 1.0% to 30.3%) of the total adverse PM2.5 effects on Trials 1-3 and List B, respectively. The observed associations remained after excluding incident cases of dementia and stroke during the follow-up, or further adjusting for small-vessel ischaemic disease volumes. Our findings illustrate the continuum of PM2.5 neurotoxicity that contributes to early decline of immediate free recall/new learning at the preclinical stage, which is mediated by progressive atrophy of grey matter indicative of increased Alzheimer's disease risk, independent of cerebrovascular damage
Transitions of cardio-metabolic risk factors in the Americas between 1980 and 2014
Describing the prevalence and trends of cardiometabolic risk factors that are associated with non-communicable diseases (NCDs) is crucial for monitoring progress, planning prevention, and providing evidence to support policy efforts. We aimed to analyse the transition in body-mass index (BMI), obesity, blood pressure, raised blood pressure, and diabetes in the Americas, between 1980 and 2014
Associations of autozygosity with a broad range of human phenotypes
In many species, the offspring of related parents suffer reduced reproductive success, a phenomenon known as inbreeding depression. In humans, the importance of this effect has remained unclear, partly because reproduction between close relatives is both rare and frequently associated with confounding social factors. Here, using genomic inbreeding coefficients (F-ROH) for >1.4 million individuals, we show that F-ROH is significantly associated (p <0.0005) with apparently deleterious changes in 32 out of 100 traits analysed. These changes are associated with runs of homozygosity (ROH), but not with common variant homozygosity, suggesting that genetic variants associated with inbreeding depression are predominantly rare. The effect on fertility is striking: F-ROH equivalent to the offspring of first cousins is associated with a 55% decrease [95% CI 44-66%] in the odds of having children. Finally, the effects of F-ROH are confirmed within full-sibling pairs, where the variation in F-ROH is independent of all environmental confounding.Peer reviewe
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