2,244 research outputs found
IrSr_2Sm_{1.15}Ce_{0.85}Cu_{2.175}O_{10}: A Novel Reentrant Spin-Glass Material
A new iridium containing layered cuprate material,
IrSr_2Sm_{1.15}Ce_{0.85}Cu_{2.175}O_{10, has been synthesized by conventional
ambient-pressure solid-state techniques. The material's structure has been
fully characterized by Rietveld refinement of high resolution synchrotron X-ray
diffraction data; tilts and rotations of the IrO_6 octahedra are observed as a
result of a bond mismatch between in-plane Ir-O and Cu-O bond lengths.
DC-susceptibility measurements evidence a complex set of magnetic transitions
upon cooling that are characteristic of a reentrant spin-glass ground-state.
The glassy character of the lowest temperature, Tg=10 K, transition is further
confirmed by AC-susceptibility measurements, showing a characteristic frequency
dependence that can be well fitted by the Vogel-Fulcher law and yields a value
of \Delta_(T_f)/[T_f \Delta log({\omega})] =0.015(1), typical of dilute
magnetic systems. Electronic transport measurements show the material to be
semiconducting at all temperatures with no transition to a superconducting
state. Negative magnetoresistance is observed when the material is cooled below
25 K, and the magnitude of this magnetoresistance is seen to increase upon
cooling to a value of MR = -9 % at 8 K
Slow Conduction in the Border Zones of Patchy Fibrosis Stabilizes the Drivers for Atrial Fibrillation: Insights from Multi-Scale Human Atrial Modeling
Introduction: The genesis of atrial fibrillation (AF) and success of AF ablation therapy have been strongly linked with atrial fibrosis. Increasing evidence suggests that patient-specific distributions of fibrosis may determine the locations of electrical drivers (rotors) sustaining AF, but the underlying mechanisms are incompletely understood. This study aims to elucidate a missing mechanistic link between patient-specific fibrosis distributions and AF drivers. Methods: 3D atrial models integrated human atrial geometry, rule-based fiber orientation, region-specific electrophysiology, and AF-induced ionic remodeling. A novel detailed model for an atrial fibroblast was developed, and effects of myocyte-fibroblast (M-F) coupling were explored at single-cell, 1D tissue and 3D atria levels. Left atrial LGE MRI datasets from 3 chronic AF patients were segmented to provide the patient-specific distributions of fibrosis. The data was non-linearly registered and mapped to the 3D atria model. Six distinctive fibrosis levels (0–healthy tissue, 5–dense fibrosis) were identified based on LGE MRI intensity and modeled as progressively increasing M-F coupling and decreasing atrial tissue coupling. Uniform 3D atrial model with diffuse (level 2) fibrosis was considered for comparison. Results: In single cells and tissue, the largest effect of atrial M-F coupling was on the myocyte resting membrane potential, leading to partial inactivation of sodium current and reduction of conduction velocity (CV). In the 3D atria, further to the M-F coupling, effects of fibrosis on tissue coupling greatly reduce atrial CV. AF was initiated by fast pacing in each 3D model with either uniform or patient-specific fibrosis. High variation in fibrosis distributions between the models resulted in varying complexity of AF, with several drivers emerging. In the diffuse fibrosis models, waves randomly meandered through the atria, whereas in each the patient-specific models, rotors stabilized in fibrotic regions. The rotors propagated slowly around the border zones of patchy fibrosis (levels 3–4), failing to spread into inner areas of dense fibrosis. Conclusion: Rotors stabilize in the border zones of patchy fibrosis in 3D atria, where slow conduction enable the development of circuits within relatively small regions. Our results can provide a mechanistic explanation for the clinical efficacy of ablation around fibrotic regions
Coupled model simulations of current Australian surface climate and its changes under greenhouse warming: an analysis of 18 CMIP2 models
Coupled climate models have been extensively used to further our understanding of the dynamics and physics of the Earth's climate system and the potential changes of regional and global climates in the future, especially due to human activities such as fossil fuel burning and land-use activities. Nevertheless, there are still large uncertainties in our knowledge of the global climate system and in our representations of such a complex system. The confidence of our projected future climate change, therefore, inevitably depends on how well the current climate is simulated by coupled climate models and how large the scatter is among the model simulations of current and future climates. As one of the diagnostic subprojects within the Coupled Model Intercomparison Project phase II (CMIP2), we present an evaluation of 18 CMIP2 coupled model simulations over the Australian region.Monthly rainfall and surface air temperature climatologies over the Australian region have been derived from the 18 CMIP2 control simulations and compared with observations from the Australian Bureau of Meteorology. The gross spatial patterns of austral summer rainfall (DJF) are reasonably simulated by the majority of the models. However, there are significant model errors in simulating the intensity and location of the heavy Australian monsoon rainfall in the north and eastern parts of the continent, with about half of the models showing more than 100 mm/month biases and a number of models simulating wrong locations of the monsoon rainfall. The seasonal cycle of the surface temperature is reasonably reproduced in the models although there are biases of around 2-4 degrees C present in the model simulated surface air temperature climatology.Based on the 80-year model simulations of perturbed climate, with 1% per year increase of atmospheric CO2 concentration, the changes of surface air temperature and precipitation have also been analysed. The average annual surface temperature change in the last 20-year period of the model simulations against the model control simulations over the Australian region varies from 1.00 degrees C to 2.18 degrees C, with an ensemble average of 1.59 degrees C and 0.33 degrees C scatter measured by one standard deviation. The models give a mixed signal in predicting averaged Australian rainfall changes, with some models simulating more than 3 mm/month increase while others show more than 4 mm/month decrease with on average no change. The spatial distributions of the model-simulated surface temperature and precipitation changes have also been analysed. Surface temperature is increased over the whole continent in all models, while the changes in precipitation show large spatial variations. The ensemble mean model shows decreases in winter rainfall across southern Australia and over northwestern Australia during summer. Increased rainfall is simulated over parts of eastern Australia during winter, extending further north during summer. Besides the analysis of changes in mean climate, the potential impacts of global warming on Australian climate variability is explored in a preliminary way by analysing the changes in tropical Australian precipitation correlations with surface temperature variations over four key oceanic regions. Results suggest that the influence of tropical and subtropical sea-surface temperature (SST) forcing on the Australian climate may change under greenhouse warmin
Promotion of cooperation induced by nonlinear attractive effect in spatial Prisoner's Dilemma game
We introduce nonlinear attractive effects into a spatial Prisoner's Dilemma
game where the players located on a square lattice can either cooperate with
their nearest neighbors or defect. In every generation, each player updates its
strategy by firstly choosing one of the neighbors with a probability
proportional to denoting the attractiveness of the
neighbor, where is the payoff collected by it and
(0) is a free parameter characterizing the extent of the nonlinear
effect; and then adopting its strategy with a probability dependent on their
payoff difference. Using Monte Carlo simulations, we investigate the density
of cooperators in the stationary state for different values of
. It is shown that the introduction of such attractive effect
remarkably promotes the emergence and persistence of cooperation over a wide
range of the temptation to defect. In particular, for large values of ,
i.e., strong nonlinear attractive effects, the system exhibits two absorbing
states (all cooperators or all defectors) separated by an active state
(coexistence of cooperators and defectors) when varying the temptation to
defect. In the critical region where goes to zero, the extinction
behavior is power law-like , where the
exponent accords approximatively with the critical exponent
() of the two-dimensional directed percolation and depends
weakly on the value of .Comment: 7 pages, 4 figure
Spin dynamics and disorder effects in the S=1/2 kagome Heisenberg spin liquid phase of kapellasite
We report Cl NMR, ESR, SR and specific heat measurements on the
frustrated kagom\'e magnet kapellasite,
CuZn(OH)Cl, where a gapless spin liquid phase is
stabilized by a set of competing exchange interactions. Our measurements
confirm the ferromagnetic character of the nearest-neighbour exchange
interaction and give an energy scale for the competing interactions K. The study of the temperature-dependent ESR lineshift reveals a
moderate symmetric exchange anisotropy term , with %. These
findings validate a posteriori the use of the Heisenberg
model to describe the magnetic properties of kapellasite [Bernu et al., Phys.
Rev. B 87, 155107 (2013)]. We further confirm that the main deviation from this
model is the severe random depletion of the magnetic kagom\'e lattice by 27%,
due to Cu/Zn site mixing, and specifically address the effect of this disorder
by Cl NMR, performed on an oriented polycrystalline sample.
Surprisingly, while being very sensitive to local structural deformations, our
NMR measurements demonstrate that the system remains homogeneous with a unique
spin susceptibility at high temperature, despite a variety of magnetic
environments. Unconventional spin dynamics is further revealed by NMR and
SR in the low-, correlated, spin liquid regime, where a broad
distribution of spin-lattice relaxation times is observed. We ascribe this to
the presence of local low-energy modes.Comment: 15 pages, 11 figures. To appear in Phys. Rev.
Phase transitions and volunteering in spatial public goods games
Cooperative behavior among unrelated individuals in human and animal
societies represents a most intriguing puzzle to scientists in various
disciplines. Here we present a simple yet effective mechanism promoting
cooperation under full anonymity by allowing for voluntary participation in
public goods games. This natural extension leads to rock--scissors--paper type
cyclic dominance of the three strategies cooperate, defect and loner i.e. those
unwilling to participate in the public enterprise. In spatial settings with
players arranged on a regular lattice this results in interesting dynamical
properties and intriguing spatio-temporal patterns. In particular, variations
of the value of the public good leads to transitions between one-, two- and
three-strategy states which are either in the class of directed percolation or
show interesting analogies to Ising-type models. Although volunteering is
incapable of stabilizing cooperation, it efficiently prevents successful
spreading of selfish behavior and enables cooperators to persist at substantial
levels.Comment: 4 pages, 5 figure
Ground State and Intrinsic Susceptibility of the Kagome Antiferromagnet Vesignieite as seen by 51V NMR
The intrinsic magnetic susceptibility and local magnetization of the
near-kagome quantum magnet vesignieite, Cu3BaV2O8(OH)2, are presented as
measured using 51V NMR. The NMR line shift gives an accurate measurement of the
intrinsic susceptibility of the kagome sites which closely resembles that of
the quantum spin liquid herbertsmithite [A. Olariu et al. Phys. Rev. Lett. 100,
087202 (2008)]. It is therefore surprising that, at Tc ~ 9 K, a transition to a
heterogeneous ground state is observed. A gradual wipeout of half the NMR
intensity indicates a slowly fluctuating spin liquid component and a detailed
analysis of the linewidth reveals the onset of static magnetism at the
remaining half of the sites. It is proposed that this transition and unusual
ground state originate from a nearby quantum critical point induced by the
Dzyaloshinskii-Moriya interaction.Comment: 5 pages, 4 figure
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