201 research outputs found
Histogram Monte Carlo study of multicritical behavior in the hexagonal easy-axis Heisenberg antiferromagnet
The results of a detailed histogram Monte-Carlo study of critical-fluctuation
effects on the magnetic-field temperature phase diagram associated with the
hexagonal Heisenberg antiferromagnet with weak axial anisotropy are reported.
The multiphase point where three lines of continuous transitions merge at the
spin-flop boundary exhibits a structure consistent with scaling theory but
without the usual umbilicus as found in the case of a bicritical point.Comment: 7 pages (RevTex 3.0), 1 figure available upon request, CRPS-93-1
Tricritical behavior of the frustrated XY antiferromagnet
Extensive histogram Monte-Carlo simulations of the XY antiferromagnet on a
stacked triangular lattice reveal exponent estimates which strongly favor a
scenario of mean-field tricritical behavior for the spin-order transition. The
corresponding chiral-order transition occurs at the same temperature but
appears to be decoupled from the spin-order. These results are relevant to a
wide class of frustrated systems with planar-type order and serve to resolve a
long-standing controversy regarding their criticality.Comment: J1K 2R1 4 pages (RevTex 3.0), 4 figures available upon request,
Report# CRPS-94-0
Chiral exponents in O(N) x O(m) spin models at O(1/N^2)
The critical exponents corresponding to chirality are computed at O(1/N^2) in
d-dimensions at the stable chiral fixed point of a scalar field theory with an
O(N) x O(m) symmetry. Pade-Borel estimates for the exponents are given in three
dimensions for the Landau-Ginzburg-Wilson model at m = 2.Comment: 8 latex page
Spin Stiffness of Stacked Triangular Antiferromagnets
We study the spin stiffness of stacked triangular antiferromagnets using both
heat bath and broad histogram Monte Carlo methods. Our results are consistent
with a continuous transition belonging to the chiral universality class first
proposed by Kawamura.Comment: 5 pages, 7 figure
Hearing loss and cognitive decline among older adults with atrial fibrillation: the SAGE-AF study
Objective: To examine the association between hearing loss and cognitive function cross-sectionally and prospectively among older adults with atrial fibrillation (AF).
Methods: Patients with AF \u3e /= 65-year-old (n = 1244) in the SAGE (Systematic Assessment of Geriatric Elements)-AF study were recruited from five internal medicine or cardiology clinics in Massachusetts and Georgia. Hearing was assessed by a structured questionnaire at baseline. Cognitive function was assessed by Montreal Cognitive Assessment (MoCA) at baseline and one year. Cognitive impairment was defined as score \u3c /= 23 on the MoCA. The associations between hearing loss and cognitive function were examined by multivariable adjusted logistic regression.
Results: Participants with hearing loss (n = 451, 36%) were older, more likely to be male, and have depressive symptoms than patients without hearing loss. At baseline, 528 (42%) participants were cognitively impaired. Individuals with hearing loss were significantly more likely to have cognitive impairment at baseline [adjusted odds ratio (OR) = 1.37, 95% confidence interval (CI): 1.05-1.81]. Among the 662 participants who did not have cognitive impairment at baseline and attended the one-year follow-up visit, 106 (16%) developed incident cognitive impairment. Individuals with, versus those without, hearing loss were significantly more likely to develop incident cognitive impairment at one year (adjusted OR = 1.68, 95% CI: 1.07-2.64).
Conclusions: Hearing loss is a prevalent but under-recognized factor associated with cognitive impairment in patients with AF. Assessment for hearing loss may be indicated among these patients to identify individuals at high-risk for adverse outcomes
The critical behavior of frustrated spin models with noncollinear order
We study the critical behavior of frustrated spin models with noncollinear
order, including stacked triangular antiferromagnets and helimagnets. For this
purpose we compute the field-theoretic expansions at fixed dimension to six
loops and determine their large-order behavior. For the physically relevant
cases of two and three components, we show the existence of a new stable fixed
point that corresponds to the conjectured chiral universality class. This
contradicts previous three-loop field-theoretical results but is in agreement
with experiments.Comment: 4 pages, RevTe
Histogram Monte Carlo study of next-nearest-neighbor Ising antiferromagnet on a stacked triangular lattice
Critical properties of the Ising model on a stacked triangular lattice, with
antiferromagnetic first and second-neighbor in-plane interactions, are studied
by extensive histogram Monte Carlo simulations. The results, in conjunction
with the recently determined phase diagram, strongly suggest that the
transition from the period-3 ordered state to the paramagnetic phase remains in
the xy universality class. This conclusion is in contrast with a previous
suggestion of mean-field tricritical behavior.Comment: 13 pages (RevTex 3.0), 10 figures available upon request, CRPS-93-0
Magnetic-Field Induced First-Order Transition in the Frustrated XY Model on a Stacked Triangular Lattice
The results of extensive Monte Carlo simulations of magnetic-field induced
transitions in the xy model on a stacked triangular lattice with
antiferromagnetic intraplane and ferromagnetic interplane interactions are
discussed. A low-field transition from the paramagnetic to a 3-state (Potts)
phase is found to be very weakly first order with behavior suggesting
tricriticality at zero field. In addition to clarifying some long-standing
ambiguity concerning the nature of this Potts-like transition, the present work
also serves to further our understanding of the critical behavior at ,
about which there has been much controversy.Comment: 10 pages (RevTex 3.0), 4 figures available upon request, CRPS-93-0
Transition to Long Range Magnetic Order in the Highly Frustrated Insulating Pyrochlore Antiferromagnet Gd_2Ti_2O_7
Experimental evidence from measurements of the a.c. and d.c. susceptibility,
and heat capacity data show that the pyrochlore structure oxide, Gd_2Ti_2O_7,
exhibits short range order that starts developing at 30K, as well as long range
magnetic order at K. The Curie-Weiss temperature, =
-9.6K, is largely due to exchange interactions. Deviations from the Curie-Weiss
law occur below 10K while magnetic heat capacity contributions are found
at temperatures above 20K. A sharp maximum in the heat capacity at K
signals a transition to a long range ordered state, with the magnetic specific
accounting for only 50% of the magnetic entropy. The heat capacity above
the phase transition can be modeled by assuming that a distribution of random
fields acts on the ground state for Gd. There is no
frequency dependence to the a.c. susceptibility in either the short range or
long range ordered regimes, hence suggesting the absence of any spin-glassy
behavior. Mean field theoretical calculations show that no long range ordered
ground state exists for the conditions of nearest-neighbor antiferromagnetic
exchange and long range dipolar couplings. At the mean-field level, long range
order at various commensurate or incommensurate wave vectors is found only upon
inclusion of exchange interactions beyond nearest-neighbor exchange and dipolar
coupling. The properties of Gd$_2Ti_2O_7 are compared with other geometrically
frustrated antiferromagnets such as the Gd_3Ga_5O_{12} gadolinium gallium
garnet, RE_2Ti_2O_7 pyrochlores where RE = Tb, Ho and Tm, and Heisenberg-type
pyrochlore such as Y_2Mo_2O_7, Tb_2Mo_2O_7, and spinels such as ZnFe_2O_4Comment: Letter, 6 POSTSCRIPT figures included. (NOTE: Figure 5 is not
included --) To appear in Physical Review B. Contact:
[email protected]
Vitamin D in the prevention of exacerbations of asthma in preschoolers (DIVA): Protocol for a multicentre randomised placebo-controlled triple-blind trial
Introduction Preschoolers have the highest rate of emergency visits and hospitalisations for asthma exacerbations of all age groups, with most triggered by upper respiratory tract infections (URTIs) and occurring in the fall or winter. Vitamin D insufficiency is highly prevalent in Canadian preschoolers with recurrent asthma exacerbations, particularly in winter. It is associated with more URTIs and, in patients with asthma, more oral corticosteroid (OCS) use. Although evidence suggests that vitamin D supplements significantly decrease URTIs and asthma exacerbations requiring OCS, there is insufficient data in preschoolers. This study aims to determine the impact of vitamin D 3 supplementation on exacerbations requiring OCS, in preschoolers with recurrent URTI-induced asthma exacerbations. Methods and analysis This is a phase III, randomised, triple-blind, placebo-controlled, parallel-group multicentre trial of vitamin D 3 supplementation in children aged 1-5 years, with asthma triggered by URTIs and a recent history of frequent URTIs and OCS use. Children (n=865) will be recruited in the fall and early winter and followed for 7 months. They will be randomised to either the (1) intervention: two oral boluses of 100 000 international unit (IU) vitamin D 3 (3.5 months apart) with 400 IU vitamin D 3 daily; or (2) control: identical placebo boluses with daily placebo. The primary outcome is the number of exacerbations requiring OCS per child, documented by medical and pharmacy records. Secondary outcomes include number of laboratory-confirmed viral URTIs, exacerbation duration and severity, parent functional status, healthcare use, treatment deintensification, cost and safety. Ethics and dissemination This study has received ethical approval from all sites. Results will be disseminated via international conferences and manuscripts targeting paediatricians and respirologists, and to families of asthmatic children via our Quebec parents-partners outreach programme. If proven effective, findings may markedly influence the management of URTI-induced asthma in high-morbidity preschoolers and could be directly implemented into practice with an update to clinical guidelines. Trial registration number NCT03365687
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