7,009 research outputs found
Glauber slow dynamics of the magnetization in a molecular Ising chain
The slow dynamics (10^-6 s - 10^4 s) of the magnetization in the paramagnetic
phase, predicted by Glauber for 1d Ising ferromagnets, has been observed with
ac susceptibility and SQUID magnetometry measurements in a molecular chain
comprising alternating Co{2+} spins and organic radical spins strongly
antiferromagnetically coupled. An Arrhenius behavior with activation energy
Delta=152 K has been observed for ten decades of relaxation time and found to
be consistent with the Glauber model. We have extended this model to take into
account the ferrimagnetic nature of the chain as well as its helicoidal
structure.Comment: 4 pages, 4 figures (low resolution), 16 references. Submitted to
Physical Review Letter
Sr2V3O9 and Ba2V3O9: quasi one-dimensional spin-systems with an anomalous low temperature susceptibility
The magnetic behaviour of the low-dimensional Vanadium-oxides Sr2V3O9 and
Ba2V3O9 was investigated by means of magnetic susceptibility and specific heat
measurements. In both compounds, the results can be very well described by an
S=1/2 Heisenberg antiferromagnetic chain with an intrachain exchange of J = 82
K and J = 94 K in Sr2V3O9 and Ba2V3O9, respectively. In Sr2V3O9,
antiferromagnetic ordering at T_N = 5.3 K indicate a weak interchain exchange
of the order of J_perp ~ 2 K. In contrast, no evidence for magnetic order was
found in Ba2V3O9 down to 0.5 K, pointing to an even smaller interchain
coupling. In both compounds, we observe a pronounced Curie-like increase of the
susceptibility below 30 K, which we tentatively attribute to a staggered field
effect induced by the applied magnetic field. Results of LDA calculations
support the quasi one-dimensional character and indicate that in Sr2V3O9, the
magnetic chain is perpendicular to the structural one with the magnetic
exchange being transferred through VO4 tetrahedra.Comment: Submitted to Phy. Rev.
Discriminant analysis of solar bright points and faculae I. Classification method and center-to-limb distribution
While photospheric magnetic elements appear mainly as Bright Points (BPs) at
the disk center and as faculae near the limb, high-resolution images reveal the
coexistence of BPs and faculae over a range of heliocentric angles. This is not
explained by a "hot wall" effect through vertical flux tubes, and suggests that
the transition from BPs to faculae needs to be quantitatively investigated. To
achieve this, we made the first recorded attempt to discriminate BPs and
faculae, using a statistical classification approach based on Linear
Discriminant Analysis(LDA). This paper gives a detailed description of our
method, and shows its application on high-resolution images of active regions
to retrieve a center-to-limb distribution of BPs and faculae. Bright "magnetic"
features were detected at various disk positions by a segmentation algorithm
using simultaneous G-band and continuum information. By using a selected sample
of those features to represent BPs and faculae, suitable photometric parameters
were identified in order to carry out LDA. We thus obtained a Center-to-Limb
Variation (CLV) of the relative number of BPs and faculae, revealing the
predominance of faculae at all disk positions except close to disk center (mu >
0.9). Although the present dataset suffers from limited statistics, our results
are consistent with other observations of BPs and faculae at various disk
positions. The retrieved CLV indicates that at high resolution, faculae are an
essential constituent of active regions all across the solar disk. We speculate
that the faculae near disk center as well as the BPs away from disk center are
associated with inclined fields
Wetting and Capillary Condensation in Symmetric Polymer Blends: A comparison between Monte Carlo Simulations and Self-Consistent Field Calculations
We present a quantitative comparison between extensive Monte Carlo
simulations and self-consistent field calculations on the phase diagram and
wetting behavior of a symmetric, binary (AB) polymer blend confined into a
film. The flat walls attract one component via a short range interaction. The
critical point of the confined blend is shifted to lower temperatures and
higher concentrations of the component with the lower surface free energy. The
binodals close the the critical point are flattened compared to the bulk and
exhibit a convex curvature at intermediate temperatures -- a signature of the
wetting transition in the semi-infinite system. Investigating the spectrum of
capillary fluctuation of the interface bound to the wall, we find evidence for
a position dependence of the interfacial tension. This goes along with a
distortion of the interfacial profile from its bulk shape. Using an extended
ensemble in which the monomer-wall interaction is a stochastic variable, we
accurately measure the difference between the surface energies of the
components, and determine the location of the wetting transition via the Young
equation. The Flory-Huggins parameter at which the strong first order wetting
transition occurs is independent of chain length and grows quadratically with
the integrated wall-monomer interaction strength. We estimate the location of
the prewetting line. The prewetting manifests itself in a triple point in the
phase diagram of very thick films and causes spinodal dewetting of ultrathin
layers slightly above the wetting transition. We investigate the early stage of
dewetting via dynamic Monte Carlo simulations.Comment: to appear in Macromolecule
Network design decisions in supply chain planning
Structuring global supply chain networks is a complex decision-making process. The typical inputs to such a process consist of a set of customer zones to serve, a set of products to be manufactured and distributed, demand projections for the different customer zones, and information about future conditions, costs (e.g. for production and transportation) and resources (e.g. capacities, available raw materials). Given the above inputs, companies have to decide where to locate new service facilities (e.g. plants, warehouses), how to allocate procurement and production activities to the variousmanufacturing facilities, and how to manage the transportation of products through the supply chain network in order to satisfy customer demands. We propose a mathematical modelling framework capturing many practical aspects of network design problems simultaneously. For problems of reasonable size we report on computational experience with standard mathematical programming software. The discussion is extended with other decisions required by many real-life applications in strategic supply chain planning. In particular, the multi-period nature of some decisions is addressed by a more comprehensivemodel, which is solved by a specially tailored heuristic approach. The numerical results suggest that the solution procedure can identify high quality solutions within reasonable computational time
Join-Reachability Problems in Directed Graphs
For a given collection G of directed graphs we define the join-reachability
graph of G, denoted by J(G), as the directed graph that, for any pair of
vertices a and b, contains a path from a to b if and only if such a path exists
in all graphs of G. Our goal is to compute an efficient representation of J(G).
In particular, we consider two versions of this problem. In the explicit
version we wish to construct the smallest join-reachability graph for G. In the
implicit version we wish to build an efficient data structure (in terms of
space and query time) such that we can report fast the set of vertices that
reach a query vertex in all graphs of G. This problem is related to the
well-studied reachability problem and is motivated by emerging applications of
graph-structured databases and graph algorithms. We consider the construction
of join-reachability structures for two graphs and develop techniques that can
be applied to both the explicit and the implicit problem. First we present
optimal and near-optimal structures for paths and trees. Then, based on these
results, we provide efficient structures for planar graphs and general directed
graphs
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