1,402 research outputs found
Susceptibility of the 2D S=1/2 Heisenberg antiferromagnet with an impurity
We use a quantum Monte Carlo method (stochastic series expansion) to study
the effects of a magnetic or nonmagnetic impurity on the magnetic
susceptibility of the two-dimensional Heisenberg antiferromagnet. At low
temperatures, we find a log-divergent contribution to the transverse
susceptibility. We also introduce an effective few-spin model that can
quantitatively capture the differences between magnetic and nonmagnetic
impurities at high and intermediate temperatures.Comment: 5 pages, 4 figures, v2: Updated data in figures, minor changes in
text, v3: Final version, cosmetic change
Cerebrospinal fluid Aβ42/40 corresponds better than Aβ42 to amyloid PET in Alzheimer’s disease
Background: Decreased concentrations of amyloid-β 1-42 (Aβ(42)) in cerebrospinal fluid (CSF) and increased retention of Aβ tracers in the brain on positron emission tomography (PET) are considered the earliest biomarkers of Alzheimer’s disease (AD). However, a proportion of cases show discrepancies between the results of the two biomarker modalities which may reflect inter-individual differences in Aβ metabolism. The CSF Aβ(42/40) ratio seems to be a more accurate biomarker of clinical AD than CSF Aβ(42) alone. Objective: We tested whether CSF Aβ(42) alone or the Aβ(42/40) ratio corresponds better with amyloid PET status and analyzed the distribution of cases with discordant CSF-PET results. Methods: CSF obtained from a mixed cohort (n = 200) of cognitively normal and abnormal research participants who had undergone amyloid PET within 12 months (n = 150 PET-negative, n = 50 PET-positive according to a previously published cut-off) was assayed for Aβ(42) and Aβ(40) using two recently developed immunoassays. Optimal CSF cut-offs for amyloid positivity were calculated, and concordance was tested by comparison of the areas under receiver operating characteristic (ROC) curves (AUC) and McNemar’s test for paired proportions. Results: CSF Aβ(42/40) corresponded better than Aβ(42) with PET results, with a larger proportion of concordant cases (89.4% versus 74.9%, respectively, p < 0.0001) and a larger AUC (0.936 versus 0.814, respectively, p < 0.0001) associated with the ratio. For both CSF biomarkers, the percentage of CSF-abnormal/PET-normal cases was larger than that of CSF-normal/PET-abnormal cases. Conclusion: The CSF Aβ(42/40) ratio is superior to Aβ(42) alone as a marker of amyloid-positivity by PET. We hypothesize that this increase in performance reflects the ratio compensating for general between-individual variations in CSF total Aβ
Topological enslavement in evolutionary games on correlated multiplex networks
Governments and enterprises strongly rely on incentives to generate favorable
outcomes from social and strategic interactions between individuals. The
incentives are usually modeled by payoffs in evolutionary games, such as the
prisoner's dilemma or the harmony game, with imitation dynamics. Adjusting the
incentives by changing the payoff parameters can favor cooperation, as found in
the harmony game, over defection, which prevails in the prisoner's dilemma.
Here, we show that this is not always the case if individuals engage in
strategic interactions in multiple domains. In particular, we investigate
evolutionary games on multiplex networks where individuals obtain an aggregate
payoff. We explicitly control the strength of degree correlations between nodes
in the different layers of the multiplex. We find that if the multiplex is
composed of many layers and degree correlations are strong, the topology of the
system enslaves the dynamics and the final outcome, cooperation or defection,
becomes independent of the payoff parameters. The fate of the system is then
determined by the initial conditions
Evidence from accreted seamounts for a depleted component in the early Galapagos plume
The existence of an intrinsic depleted component in mantle plumes has previously been proposed for several hotspots in the Pacific, Atlantic, and Indian Oceans. However, formation of these depleted basalts is often associated with unusual tectonomagmatic processes such as plume-ridge interaction or multistage melting at plume initiation, where depleted basalts could reflect entrainment and melting of depleted upper mantle. Late Cretaceous to middle Eocene seamounts that accreted in Costa Rica and are part of the early Galapagos hotspot track provide new insights into the occurrence and nature of intrinsic depleted components. The Paleocene (ca. 62 Ma) seamounts include unusually depleted basalts that erupted on the Farallon plate far from a mid-ocean ridge. These basalts closely resemble Gorgona komatiites in terms of trace element and radiogenic isotope composition, suggesting formation from a similar, refractory mantle source. We suggest that this source may be common to plumes, but is only rarely sampled due to excessive extents of melting required to extract melts from the most refractory parts of a heterogeneous mantle plume
Virtual reality training to improve complex skills – concerns and remarks:A Letter to the Editor
Background: There has been limited literature on immersive virtual reality (VR) simulation in orthopaedic education. The purpose of this multicenter, blinded, randomized controlled trial was to determine the validity and efficacy of immersive VR training in orthopaedic resident education.Methods: Nineteen senior orthopaedic residents (resident group) and 7 consultant shoulder arthroplasty surgeons (expert group) participated in the trial comparing immersive VR with traditional learning using a technical journal article as a control. The examined task focused on achieving optimal glenoid exposure. Participants completed demographic qu`estionnaires, knowledge tests, and a glenoid exposure on fresh-frozen cadavers while being examined by blinded shoulder arthroplasty surgeons. Training superiority was determined by the outcome measures of the Objective Structured Assessment of Technical Skills (OSATS) score, a developed laboratory metric, verbal answers, and time to task completion.Results: Immersive VR had greater realism and was superior in teaching glenoid exposure than the control (p = 0.01). The expert group outperformed the resident group on knowledge testing (p = 0.04). The immersive VR group completed the learning activity and knowledge tests significantly faster (p < 0.001) at a mean time (and standard deviation) of 11 +/- 3 minutes than the control group at 20 +/- 4 minutes, performing 3 to 5 VR repeats for a reduction in learning time of 570%. The immersive VR group completed the glenoid exposure significantly faster ( p = 0.04) at a mean time of 14 +/- 7 minutes than the control group at 21 +/- 6 minutes, with superior OSATS instrument handling scores (p = 0.03). The immersive VR group scored equivalently in surprise verbal scores (p = 0.85) and written knowledge scores (p = 1.0).Conclusions: Immersive VR demonstrated substantially improved translational technical and nontechnical skills acquisition over traditional learning in senior orthopaedic residents. Additionally, the results demonstrate the face, content, construct, and transfer validity for immersive VR
Central American Subduction System
Workshop to Integrate Subduction Factory and Seismogenic Zone Studies in Central America, Heredia, Costa Rica, 18–22 June 2007 The driving force for great earthquakes and the cycling of water and climate-influencing volatiles (carbon dioxide, sulfur, halogens) across the convergent margin of Central America have been a focus of international efforts for over 8 years, as part of the MARGINS program of the U.S. National Science Foundation, the Collaborative Research Center (SFB 574) of the German Science Foundation, and the Central American science community. Over 120 scientists and students from 10 countries met in Costa Rica to synthesize this intense effort spanning from land to marine geological and geophysical studies
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