8,855 research outputs found
Phase transitions in diluted negative-weight percolation models
We investigate the geometric properties of loops on two-dimensional lattice
graphs, where edge weights are drawn from a distribution that allows for
positive and negative weights. We are interested in the appearance of spanning
loops of total negative weight. The resulting percolation problem is
fundamentally different from conventional percolation, as we have seen in a
previous study of this model for the undiluted case.
Here, we investigate how the percolation transition is affected by additional
dilution. We consider two types of dilution: either a certain fraction of edges
exhibit zero weight, or a fraction of edges is even absent. We study these
systems numerically using exact combinatorial optimization techniques based on
suitable transformations of the graphs and applying matching algorithms. We
perform a finite-size scaling analysis to obtain the phase diagram and
determine the critical properties of the phase boundary.
We find that the first type of dilution does not change the universality
class compared to the undiluted case whereas the second type of dilution leads
to a change of the universality class.Comment: 8 pages, 7 figure
Magnetoresistance behavior of a ferromagnetic shape memory alloy: Ni_1.75Mn_1.25Ga
A negative-positive-negative switching behavior of magnetoresistance (MR)
with temperature is observed in a ferromagnetic shape memory alloy
Ni_1.75Mn_1.25Ga. In the austenitic phase between 300 and 120 K, MR is negative
due to s-d scattering. Curiously, below 120K MR is positive, while at still
lower temperatures in the martensitic phase, MR is negative again. The positive
MR cannot be explained by Lorentz contribution and is related to a magnetic
transition. Evidence for this is obtained from ab initio density functional
theory, a decrease in magnetization and resistivity upturn at 120 K. Theory
shows that a ferrimagnetic state with anti-ferromagnetic alignment between the
local magnetic moments of the Mn atoms is the energetically favoured ground
state. In the martensitic phase, there are two competing factors that govern
the MR behavior: a dominant negative trend up to the saturation field due to
the decrease of electron scattering at twin and domain boundaries; and a weaker
positive trend due to the ferrimagnetic nature of the magnetic state. MR
exhibits a hysteresis between heating and cooling that is related to the first
order nature of the martensitic phase transition.Comment: 17 pages, 5 figures. Accepted in Phys. Rev.
Tracking pulsar dispersion measures using the giant metrewave radio telescope
In this paper, we describe a novel experiment for the accurate estimation of pulsar dispersion measures (DMs) using the Giant Metrewave Radio Telescope. This experiment was carried out for a sample of 12 pulsars, over a period of more than one year (2001 January to 2002 May) with observations about once every fortnight. At each epoch, the pulsar DMs were obtained from simultaneous dual-frequency observations, without requiring any absolute timing information. The DM estimates were obtained from both the single-pulse data streams and from the average profiles. The accuracy of the DM estimates at each epoch is ~1 part in 104 or better, making the data set useful for many different kinds of studies. The time-series of DMs shows significant variations on time-scales of weeks to months for most of the pulsars. An analysis of the mean DM values from these data shows significant deviations from catalogue values (as well as from other estimates in the literature) for some of the pulsars, with PSR B1642-03 showing the most notable differences. From our analysis results it appears that the constancy of pulsar DMs (at the level of 1 in 103 or better) cannot be taken for granted. For PSR B2217+47, we see evidence of a large-scale DM gradient over a 1-yr period, which is modelled as being due to a blob of enhanced electron density sampled by the line of sight. For some pulsars, including pulsars with fairly simple profiles such as PSR B1642-03, we find evidence for small changes in DM values for different frequency pairs of measurement, a result that needs to be investigated in detail. Another interesting result is that we find significant differences in DM values obtained from average profiles and single-pulse data
Analysis of the loop length distribution for the negative weight percolation problem in dimensions d=2 through 6
We consider the negative weight percolation (NWP) problem on hypercubic
lattice graphs with fully periodic boundary conditions in all relevant
dimensions from d=2 to the upper critical dimension d=6. The problem exhibits
edge weights drawn from disorder distributions that allow for weights of either
sign. We are interested in in the full ensemble of loops with negative weight,
i.e. non-trivial (system spanning) loops as well as topologically trivial
("small") loops. The NWP phenomenon refers to the disorder driven proliferation
of system spanning loops of total negative weight. While previous studies where
focused on the latter loops, we here put under scrutiny the ensemble of small
loops. Our aim is to characterize -using this extensive and exhaustive
numerical study- the loop length distribution of the small loops right at and
below the critical point of the hypercubic setups by means of two independent
critical exponents. These can further be related to the results of previous
finite-size scaling analyses carried out for the system spanning loops. For the
numerical simulations we employed a mapping of the NWP model to a combinatorial
optimization problem that can be solved exactly by using sophisticated matching
algorithms. This allowed us to study here numerically exact very large systems
with high statistics.Comment: 7 pages, 4 figures, 2 tables, paper summary available at
http://www.papercore.org/Kajantie2000. arXiv admin note: substantial text
overlap with arXiv:1003.1591, arXiv:1005.5637, arXiv:1107.174
Maximum flow and topological structure of complex networks
The problem of sending the maximum amount of flow between two arbitrary
nodes and of complex networks along links with unit capacity is
studied, which is equivalent to determining the number of link-disjoint paths
between and . The average of over all node pairs with smaller degree
is for large with a constant implying that the statistics of is related to the
degree distribution of the network. The disjoint paths between hub nodes are
found to be distributed among the links belonging to the same edge-biconnected
component, and can be estimated by the number of pairs of edge-biconnected
links incident to the start and terminal node. The relative size of the giant
edge-biconnected component of a network approximates to the coefficient .
The applicability of our results to real world networks is tested for the
Internet at the autonomous system level.Comment: 7 pages, 4 figure
Taxi Planner Optimization: A Management Tool
This work introduces taxi planning optimization (TPO) as a methodology to guide airport surface management operations. The optimization model represents competing aircraft using limited ground resources. TPO improves aircraft taxiing routes and their schedule in situations of congestion, minimizing overall taxiing time (TT), and helping taxi planners to meet prespecified goals such as compliance with take-off windows, TT limits, and trajectory conflicts. By considering all simultaneous trajectories during a given planning horizon, TPO's estimation of TT from the stand to the runways improves over current planning methods. The operational optimization model is a large-scale space-time multi-commodity network with capacity constraints. In addition to its natural use as a real-time taxi planning tool, a number of TPO variants can be used for design purposes, such as expansion of new infrastructure. TPO is demonstrated using Madrid-Barajas as test airport
Modelling sorption thermodynamics and mass transport of n-hexane in a propylene-ethylene elastomer
Optimization of post polymerization processes of polyolefin elastomers (POE) involving solvents is of considerable industrial interest. To this aim, experimental determination and theoretical interpretation of the thermodynamics and mass transport properties of POE-solvent mixtures is relevant. Sorption behavior of n-hexane vapor in a commercial propylene-ethylene elastomer (V8880 Vistamaxx™ from ExxonMobil, Machelen, Belgium) is addressed here, determining experimentally the sorption isotherms at temperatures ranging from 115 to 140 °C and pressure values of n-hexane vapor up to 1 atm. Sorption isotherms have been interpreted using a Non Random Lattice Fluid (NRLF) Equation of State model retrieving, from data fitting, the value of the binary interaction parameter for the n-hexane/V8880 system. Both the cases of temperature-independent and of temperature-dependent binary interaction parameter have been considered. Sorption kinetics was also investigated at different pressures and has been interpreted using a Fick’s model determining values of the mutual diffusivity as a function of temperature and of n-hexane/V8880 mixture composition. From these values, n-hexane intra-diffusion coefficient has been calculated interpreting its dependence on mixture concentration and temperature by a semi-empiric model based on free volume arguments
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