499 research outputs found
Power-law correlations and orientational glass in random-field Heisenberg models
Monte Carlo simulations have been used to study a discretized Heisenberg
ferromagnet (FM) in a random field on simple cubic lattices. The spin variable
on each site is chosen from the twelve [110] directions. The random field has
infinite strength and a random direction on a fraction x of the sites of the
lattice, and is zero on the remaining sites. For x = 0 there are two phase
transitions. At low temperatures there is a [110] FM phase, and at intermediate
temperature there is a [111] FM phase. For x > 0 there is an intermediate phase
between the paramagnet and the ferromagnet, which is characterized by a
|k|^(-3) decay of two-spin correlations, but no true FM order. The [111] FM
phase becomes unstable at a small value of x. At x = 1/8 the [110] FM phase has
disappeared, but the power-law correlated phase survives.Comment: 8 pages, 12 Postscript figure
Random Field Models for Relaxor Ferroelectric Behavior
Heat bath Monte Carlo simulations have been used to study a four-state clock
model with a type of random field on simple cubic lattices. The model has the
standard nonrandom two-spin exchange term with coupling energy and a random
field which consists of adding an energy to one of the four spin states,
chosen randomly at each site. This Ashkin-Teller-like model does not separate;
the two random-field Ising model components are coupled. When , the
ground states of the model remain fully aligned. When , a
different type of ground state is found, in which the occupation of two of the
four spin states is close to 50%, and the other two are nearly absent. This
means that one of the Ising components is almost completely ordered, while the
other one has only short-range correlations. A large peak in the structure
factor appears at small for temperatures well above the transition
to long-range order, and the appearance of this peak is associated with slow,
"glassy" dynamics. The phase transition into the state where one Ising
component is long-range ordered appears to be first order, but the latent heat
is very small.Comment: 7 pages + 12 eps figures, to appear in Phys Rev
An airborne regional carbon balance for central amazonia
We obtained regional estimates of surface CO2 exchange rates using atmospheric boundary layer budgeting techniques above tropical forest near Manaus, Brazil. Comparisons were made with simultaneous measurements from two eddy covariance towers below. Although there was good agreement for daytime measurements, large differences emerged for integrating periods dominated by the night-time fluxes. These results suggest that a systematic underestimation of night time respiratory effluxes may be responsible for the high Amazonian carbon sink suggested by several previous eddy covariance studies. Large CO2 fluxes from riverine sources or high respiratory losses from recently disturbed forests do not need to be invoked in order to balance the carbon budget of the Amazon. Our results do not, however, discount some contribution of these processes to the overall Amazon carbon budget
Quasi-long-range order in the random anisotropy Heisenberg model: functional renormalization group in 4-\epsilon dimensions
The large distance behaviors of the random field and random anisotropy O(N)
models are studied with the functional renormalization group in 4-\epsilon
dimensions. The random anisotropy Heisenberg (N=3) model is found to have a
phase with the infinite correlation radius at low temperatures and weak
disorder. The correlation function of the magnetization obeys a power law <
m(x) m(y) >\sim |x-y|^{-0.62\epsilon}. The magnetic susceptibility diverges at
low fields as \chi \sim H^{-1+0.15\epsilon}. In the random field O(N) model the
correlation radius is found to be finite at the arbitrarily weak disorder for
any N>3. The random field case is studied with a new simple method, based on a
rigorous inequality. This approach allows one to avoid the integration of the
functional renormalization group equations.Comment: 12 pages, RevTeX; a minor change in the list of reference
Aperiodicity-Induced Second-Order Phase Transition in the 8-State Potts Model
We investigate the critical behavior of the two-dimensional 8-state Potts
model with an aperiodic distribution of the exchange interactions between
nearest-neighbor rows. The model is studied numerically through intensive Monte
Carlo simulations using the Swendsen-Wang cluster algorithm. The transition
point is located through duality relations, and the critical behavior is
investigated using FSS techniques at criticality. For strong enough
fluctuations of the aperiodic sequence under consideration, a second order
phase transition is found. The exponents and are
obtained at the new fixed point.Comment: LaTeX file with Revtex, 4 pages, 5 eps figures, to appear in Phys.
Rev. Let
Critical behaviour of the Random--Bond Ashkin--Teller Model, a Monte-Carlo study
The critical behaviour of a bond-disordered Ashkin-Teller model on a square
lattice is investigated by intensive Monte-Carlo simulations. A duality
transformation is used to locate a critical plane of the disordered model. This
critical plane corresponds to the line of critical points of the pure model,
along which critical exponents vary continuously. Along this line the scaling
exponent corresponding to randomness varies continuously
and is positive so that randomness is relevant and different critical behaviour
is expected for the disordered model. We use a cluster algorithm for the Monte
Carlo simulations based on the Wolff embedding idea, and perform a finite size
scaling study of several critical models, extrapolating between the critical
bond-disordered Ising and bond-disordered four state Potts models. The critical
behaviour of the disordered model is compared with the critical behaviour of an
anisotropic Ashkin-Teller model which is used as a refference pure model. We
find no essential change in the order parameters' critical exponents with
respect to those of the pure model. The divergence of the specific heat is
changed dramatically. Our results favor a logarithmic type divergence at
, for the random bond Ashkin-Teller and four state Potts
models and for the random bond Ising model.Comment: RevTex, 14 figures in tar compressed form included, Submitted to
Phys. Rev.
On weak vs. strong universality in the two-dimensional random-bond Ising ferromagnet
We address the issue of universality in two-dimensional disordered Ising
systems, by considering long, finite-width strips of ferromagnetic Ising spins
with randomly distributed couplings. We calculate the free energy and spin-spin
correlation functions (from which averaged correlation lengths, ,
are computed) by transfer-matrix methods. An {\it ansatz} for the
size-dependence of logarithmic corrections to is proposed. Data for
both random-bond and site-diluted systems show that pure system behaviour (with
) is recovered if these corrections are incorporated, discarding the
weak--universality scenario.Comment: RevTeX code, 4 pages plus 2 Postscript figures; to appear in Physical
Review B Rapid Communication
Risk of Stroke before Revascularisation in Patients with Symptomatic Carotid Stenosis: A Pooled Analysis of Randomised Controlled Trials.
OBJECTIVE: Current guidelines recommending rapid revascularisation of symptomatic carotid stenosis are largely based on data from clinical trials performed at a time when best medical therapy was potentially less effective than today. The risk of stroke and its predictors among patients with symptomatic carotid stenosis awaiting revascularisation in recent randomised controlled trials (RCTs) and in medical arms of earlier RCTs was assessed. METHODS: The pooled data of individual patients with symptomatic carotid stenosis randomised to stenting (CAS) or endarterectomy (CEA) in four recent RCTs, and of patients randomised to medical therapy in three earlier RCTs comparing CEA vs. medical therapy, were compared. The primary outcome event was any stroke occurring between randomisation and treatment by CAS or CEA, or within 120 days after randomisation. RESULTS: A total of 4 754 patients from recent trials and 1 227 from earlier trials were included. In recent trials, patients were randomised a median of 18 (IQR 7, 50) days after the qualifying event (QE). Twenty-three suffered a stroke while waiting for revascularisation (cumulative 120 day risk 1.97%, 95% confidence interval [CI] 0.75 - 3.17). Shorter time from QE until randomisation increased stroke risk after randomisation (Ï2 = 6.58, p = .011). Sixty-one patients had a stroke within 120 days of randomisation in the medical arms of earlier trials (cumulative risk 5%, 95% CI 3.8 - 6.2). Stroke risk was lower in recent than earlier trials when adjusted for time between QE and randomisation, age, severity of QE, and degree of carotid stenosis (HR 0.47, 95% CI 0.25 - 0.88, p = .019). CONCLUSION: Patients with symptomatic carotid stenosis enrolled in recent large RCTs had a lower risk of stroke after randomisation than historical controls. The added benefit of carotid revascularisation to modern medical care needs to be revisited in future studies. Until then, adhering to current recommendations for early revascularisation of patients with symptomatic carotid stenosis considered to require invasive treatment is advisable
State Hierarchy Induced by Correlated Spin Domains in short range spin glasses
We generate equilibrium configurations for the three and four dimensional
Ising spin glass with Gaussian distributed couplings at temperatures well below
the transition temperature T_c. These states are analyzed by a recently
proposed method using clustering. The analysis reveals a hierarchical state
space structure. At each level of the hierarchy states are labeled by the
orientations of a set of correlated macroscopic spin domains. Our picture of
the low temperature phase of short range spin glasses is that of a State
Hierarchy Induced by Correlated Spin domains (SHICS). The complexity of the low
temperature phase is manifest in the fact that the composition of such a spin
domain (i.e. its constituent spins), as well as its identifying label, are
defined and determined by the ``location'' in the state hierarchy at which it
appears. Mapping out the phase space structure by means of the orientations
assumed by these domains enhances our ability to investigate the overlap
distribution, which we find to be non-trivial. Evidence is also presented that
these states may have a non-ultrametric structure.Comment: 30 pages, 17 figure
T wave abnormalities, high body mass index, current smoking and high lipoprotein (a) levels predict the development of major abnormal Q/QS patterns 20 years later. A population-based study
BACKGROUND: Most studies on risk factors for development of coronary heart disease (CHD) have been based on the clinical outcome of CHD. Our aim was to identify factors that could predict the development of ECG markers of CHD, such as abnormal Q/QS patterns, ST segment depression and T wave abnormalities, in 70-year-old men, irrespective of clinical outcome. METHODS: Predictors for development of different ECG abnormalities were identified in a population-based study using stepwise logistic regression. Anthropometrical and metabolic factors, ECG abnormalities and vital signs from a health survey of men at age 50 were related to ECG abnormalities identified in the same cohort 20 years later. RESULTS: At the age of 70, 9% had developed a major abnormal Q/QS pattern, but 63% of these subjects had not been previously hospitalized due to MI, while 57% with symptomatic MI between age 50 and 70 had no major Q/QS pattern at age 70. T wave abnormalities (Odds ratio 3.11, 95% CI 1.18â8.17), high lipoprotein (a) levels, high body mass index (BMI) and smoking were identified as significant independent predictors for the development of abnormal major Q/QS patterns. T wave abnormalities and high fasting glucose levels were significant independent predictors for the development of ST segment depression without abnormal Q/QS pattern. CONCLUSION: T wave abnormalities on resting ECG should be given special attention and correlated with clinical information. Risk factors for major Q/QS patterns need not be the same as traditional risk factors for clinically recognized CHD. High lipoprotein (a) levels may be a stronger risk factor for silent myocardial infarction (MI) compared to clinically recognized MI
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