6,850 research outputs found
Nature of the low temperature ordering of Pr in PrBa_2Cu_3O_(6+x)
Theoretical model is presented to describe the anomalous ordered phase of Pr
ions in PrBa_2Cu_3O_(6+x) below T_Pr = 12-17 K. The model considers the Pr
multipole degrees of freedom and coupling between the Cu and Pr subsystems. We
identify the symmetry allowed coupling of Cu and Pr ions and conclude that only
an ab-plane Pr dipole ordering can explain the Cu spin rotation observed at
T_Pr by neutron diffraction by Boothroyd et al. [A. T. Boothroyd et al., Phys.
Rev. Lett. 78, 130 (1997)]. A substantial enhancement of the Pr ordering
temperature is shown to arise from the Cu-Pr coupling which is the key for the
anomalous magnetic behavior in PrBa_2Cu_3O_(6+x).Comment: 6 pages, 4 figure
Identification of criticality in neuronal avalanches: II. A theoretical and empirical investigation of the Driven case
The observation of apparent power laws in neuronal systems has led to the suggestion that the brain is at, or close to, a critical state and may be a self-organised critical system. Within the framework of self-organised criticality a separation of timescales is thought to be crucial for the observation of power-law dynamics and computational models are often constructed with this property. However, this is not necessarily a characteristic of physiological neural networks—external input does not only occur when the network is at rest/a steady state. In this paper we study a simple neuronal network model driven by a continuous external input (i.e. the model does not have an explicit separation of timescales from seeding the system only when in the quiescent state) and analytically tuned to operate in the region of a critical state (it reaches the critical regime exactly in the absence of input—the case studied in the companion paper to this article). The system displays avalanche dynamics in the form of cascades of neuronal firing separated by periods of silence. We observe partial scale-free behaviour in the distribution of avalanche size for low levels of external input. We analytically derive the distributions of waiting times and investigate their temporal behaviour in relation to different levels of external input, showing that the system’s dynamics can exhibit partial long-range temporal correlations. We further show that as the system approaches the critical state by two alternative ‘routes’, different markers of criticality (partial scale-free behaviour and long-range temporal correlations) are displayed. This suggests that signatures of criticality exhibited by a particular system in close proximity to a critical state are dependent on the region in parameter space at which the system (currently) resides
P\'olya number of continuous-time quantum walks
We propose a definition for the P\'olya number of continuous-time quantum
walks to characterize their recurrence properties. The definition involves a
series of measurements on the system, each carried out on a different member
from an ensemble in order to minimize the disturbance caused by it. We examine
various graphs, including the ring, the line, higher dimensional integer
lattices and a number of other graphs and calculate their P\'olya number. For
the timing of the measurements a Poisson process as well as regular timing are
discussed. We find that the speed of decay for the probability at the origin is
the key for recurrence.Comment: 8 pages, no figures. Accepted for publication in Physical Review
Beyond clustering: mean-field dynamics on networks with arbitrary subgraph composition
Clustering is the propensity of nodes that share a common neighbour to be connected. It is ubiquitous in many networks but poses many modelling challenges. Clustering typically manifests itself by a higher than expected frequency of triangles, and this has led to the principle of constructing networks from such building blocks. This approach has been generalised to networks being constructed from a set of more exotic subgraphs. As long as these are fully connected, it is then possible to derive mean-field models that approximate epidemic dynamics well. However, there are virtually no results for non-fully connected subgraphs. In this paper, we provide a general and automated approach to deriving a set of ordinary differential equations, or mean-field model, that describes, to a high degree of accuracy, the expected values of system-level quantities, such as the prevalence of infection. Our approach offers a previously unattainable degree of control over the arrangement of subgraphs and network characteristics such as classical node degree, variance and clustering. The combination of these features makes it possible to generate families of networks with different subgraph compositions while keeping classical network metrics constant. Using our approach, we show that higher-order structure realised either through the introduction of loops of different sizes or by generating networks based on different subgraphs but with identical degree distribution and clustering, leads to non-negligible differences in epidemic dynamics
chronic administration of quercetin induces biomechanical and pharmacological remodeling in the rat coronary arteries
Acute dilation brought about by the dietary flavonoid quercetin in coronary arterioles has been described earlier, but no information is available on its chronic effects. Male Wistar rats (body weight about 190 g) were divided to two groups: the quercetin-treated group (n=22) had quercetin supplementation of approximately 30 mg/kg/day, whereas the control group (n=20) had none. After eight weeks of treatment, intramural coronary arterioles with identical passive diameters (178±14 µm and 171±9 µm) were prepared and their biomechanics and pharmacological reactivities were tested using pressure arteriography ex vivo. The spontaneous tone of quercetin-treated arteries was higher (16.5±1.9 % vs. 12.9±0.9 %), which resulted in a reduced lumen size (144±9 μm vs. 167±12 μm), thicker vascular wall (22.6±1.8 μm vs. 17.4±1.6 μm) and decreased tangential wall stress (16.8±1.1 kPa vs. 20.5±1.6 kPa) in supplemented animals (in spontaneous tone at 50 mm Hg, p<0.01 in all these comparisons). Elevated basal NO release resulted in increased endothelial dilation in quercetin-treated animals, especially at higher intraluminal pressures (10.8±2.5 % vs. 5.7±1.3 % at 70 mm Hg, p<0.01). We found remodeling of the geometry of coronary arterioles to ensure higher dilatory reserve and nitrogen monoxide production, as well as lowered elastic stress of the vessel wall
Evidence for suppression of collective magnetism in Fe-Ag granular multilayers
Evidence for the suppression of collective magnetic behavior of dipolarly
interacting Fe nanoparticles is found in Fe-Ag granular multilayers.
Interaction of Fe particles located in neighboring Fe layers is studied as a
function of the nominal thickness of the Ag layer in between only two Fe
layers. The surprisingly increasing interaction with increasing Ag-layer
thickness, verified by memory-effect measurements, is explained by the
formation of pinholes in the Ag layer at small Ag thicknesses, allowing direct
ferromagnetic coupling between Fe particles in neighboring Fe layers which may
hinder the frustration of superspins favored by dipolar interactions. At larger
Ag thicknesses, the Ag layer is continuous without pinholes and frustration
leads to the appearance of the superspin-glass state. The effect of increasing
interactions correlates well with the growing deviation at low temperatures of
the measured field-cooled (FC) magnetization from the interaction-free FC curve
calculated by a model based on the relaxation of two-level systems. Similar
phenomenon is reported in a recently published paper (S\'anchez et al., Small
2022, 18, 2106762) where a dense nanoparticle system is studied. The collective
magnetic behavior of the particles due to dipolar interactions is suppressed
when the anisotropy energy of the individual particles exceeds a certain
threshold.Comment: 13 pages, 3 figure
The flux induced switch in the mechanism of catalytic decomposition of CH3OH on Ni foil as studied by MBRS and time resolved PES
Magnetic and Transport Properties of Fe-Ag granular multilayers
Results of magnetization, magnetotransport and Mossbauer spectroscopy
measurements of sequentially evaporated Fe-Ag granular composites are
presented. The strong magnetic scattering of the conduction electrons is
reflected in the sublinear temperature dependence of the resistance and in the
large negative magnetoresistance. The simultaneous analysis of the magnetic
properties and the transport behavior suggests a bimodal grain size
distribution. A detailed quantitative description of the unusual features
observed in the transport properties is given
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