8,959 research outputs found
Subdiffusive motion in kinetically constrained models
We discuss a kinetically constrained model in which real-valued local
densities fluctuate in time, as introduced recently by Bertin, Bouchaud and
Lequeux. We show how the phenomenology of this model can be reproduced by an
effective theory of mobility excitations propagating in a disordered
environment. Both excitations and probe particles have subdiffusive motion,
characterised by different exponents and operating on different time scales. We
derive these exponents, showing that they depend continuously on one of the
parameters of the model.Comment: 12 pages, 5 figure
Large deviations and ensembles of trajectories in stochastic models
We consider ensembles of trajectories associated with large deviations of
time-integrated quantities in stochastic models. Motivated by proposals that
these ensembles are relevant for physical processes such as shearing and glassy
relaxation, we show how they can be generated directly using auxiliary
stochastic processes. We illustrate our results using the Glauber-Ising chain,
for which biased ensembles of trajectories can exhibit ferromagnetic ordering.
We discuss the relation between such biased ensembles and quantum phase
transitions.Comment: 14 pages, 1 fi
Duality symmetries in driven one-dimensional hopping models
We consider some duality relations for models of non-interacting particles
hopping on disordered one-dimensional chains. In particular, we discuss
symmetries of bulk-driven barrier and trap models, and relations between
boundary-driven and equilibrium models with related energy landscapes. We
discuss the relationships between these duality relations and similar results
for interacting many-body systems.Comment: 11 pages, 3 fig
Large deviations of the dynamical activity in the East model: analysing structure in biased trajectories
We consider large deviations of the dynamical activity in the East model. We
bias this system to larger than average activity and investigate the structure
that emerges. To best characterise this structure, we exploit the fact that
there are effective interactions that would reproduce the same behaviour in an
equilibrium system. We combine numerical results with linear response theory
and variational estimates of these effective interactions, giving the first
insights into such interactions in a many-body system, across a wide range of
biases. The system exhibits a hierarchy of responses to the bias, remaining
quasi-equilibrated on short length scales, but deviating far from equilibrium
on large length scales. We discuss the connection between this hierarchy and
the hierarchical aging behaviour of the system.Comment: Revised version, 29 pages, 9 fig
Effective interactions and large deviations in stochastic processes
We discuss the relationships between large deviations in stochastic systems,
and "effective interactions" that induce particular rare events. We focus on
the nature of these effective interactions in physical systems with many
interacting degrees of freedom, which we illustrate by reviewing several recent
studies. We describe the connections between effective interactions, large
deviations at "level 2.5", and the theory of optimal control. Finally, we
discuss possible physical applications of variational results associated with
those theories.Comment: 12 page
Algebraic and Topological Indices of Molecular Pathway Networks in Human Cancers
Protein-protein interaction networks associated with diseases have gained
prominence as an area of research. We investigate algebraic and topological
indices for protein-protein interaction networks of 11 human cancers derived
from the Kyoto Encyclopedia of Genes and Genomes (KEGG) database. We find a
strong correlation between relative automorphism group sizes and topological
network complexities on the one hand and five year survival probabilities on
the other hand. Moreover, we identify several protein families (e.g. PIK, ITG,
AKT families) that are repeated motifs in many of the cancer pathways.
Interestingly, these sources of symmetry are often central rather than
peripheral. Our results can aide in identification of promising targets for
anti-cancer drugs. Beyond that, we provide a unifying framework to study
protein-protein interaction networks of families of related diseases (e.g.
neurodegenerative diseases, viral diseases, substance abuse disorders).Comment: 15 pages, 4 figure
In situ fatigue loading stage inside scanning electron microscope
A fatigue loading stage inside a scanning electron microscopy (SEM) was developed. The stage allows dynamic and static high-magnification and high-resolution viewing of the fatigue crack initiation and crack propagation processes. The loading stage is controlled by a closed-loop servohydraulic system. Maximum load is 1000 lb (4450 N) with test frequencies ranging up to 30 Hz. The stage accommodates specimens up to 2 inches (50 mm) in length and tolerates substantial specimen translation to view the propagating crack. At room temperature, acceptable working resolution is obtainable for magnifications ranging up to 10,000X. The system is equipped with a high-temperature setup designed for temperatures up to 2000 F (1100 C). The signal can be videotaped for further analysis of the pertinent fatigue damage mechanisms. The design allows for quick and easy interchange and conversion of the SEM from a loading stage configuration to its normal operational configuration and vice versa. Tests are performed entirely in the in-situ mode. In contrast to other designs, the NASA design has greatly extended the life of the loading stage by not exposing the bellows to cyclic loading. The loading stage was used to investigate the fatigue crack growth mechanisms in the (100)-oriented PWA 1480 single-crystal, nickel-based supperalloy. The high-magnification observations revealed the details of the crack growth processes
Fatigue crack growth in a unidirectional SCS-6/Ti-15-3 composite
An investigation was conducted to characterize and model the fatigue crack growth (FCG) behavior of a SCS-6/Ti-15-3 metal matrix composite. Part of the study was conducted using a fatigue loading stage mounted inside a scanning electron microscope (SEM). This unique facility allowed high magnification viewing of the composite fatigue processes and measurement of the near crack tip displacements. The unidirectional composite was tested in the (0)8 (i.e., longitudinal) and (90)8 (i.e., transverse) orientations. For comparison purposes unreinforced matrix material produced by the identical process as the reinforced material was also tested. The results of the study reveal that the fatigue crack growth behavior of the composite is a function of specimen geometry, fiber orientation and the interaction of local stress fields with the highly anisotropic composite. In the case of (0)8 oriented single edge notch (SEN) specimens and (90)8 oriented compact tension (CT) specimens, the crack growth was normal to the loading direction. However, for the (0)8 CT specimens the crack grew mostly parallel to the loading and the fiber direction. The unusual fatigue behavior of the (0)8 CT specimens was attributed to the specimen geometry and the associated high tensile bending stresses perpendicular to the fiber direction. These stresses resulted in preferential cracking in the weak interface region perpendicular to the fiber direction. The interface region, and in particular the carbon coating surrounding the fiber proved to be the composites weakest link. In the (0)8 SEN the crack growth was confined to the matrix leaving behind unbroken fibers which bridged the cracked surfaces. As the crack grew longer, more fibers bridged the crack resulting in a progressive decrease in the crack growth rates and eventual crack arrest. The actual near crack tip displacement measurements were used in a proposed formulation for a bridging-corrected effective crack driving force, delta K(sub eff). This parameter was able to account for most of the experienced bridging and correlated the (0)8 SEN fatigue crack growth data reasonably well
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