5,761 research outputs found
Fractals in the Nervous System: conceptual Implications for Theoretical Neuroscience
This essay is presented with two principal objectives in mind: first, to
document the prevalence of fractals at all levels of the nervous system, giving
credence to the notion of their functional relevance; and second, to draw
attention to the as yet still unresolved issues of the detailed relationships
among power law scaling, self-similarity, and self-organized criticality. As
regards criticality, I will document that it has become a pivotal reference
point in Neurodynamics. Furthermore, I will emphasize the not yet fully
appreciated significance of allometric control processes. For dynamic fractals,
I will assemble reasons for attributing to them the capacity to adapt task
execution to contextual changes across a range of scales. The final Section
consists of general reflections on the implications of the reviewed data, and
identifies what appear to be issues of fundamental importance for future
research in the rapidly evolving topic of this review
Effect of dark matter halo substructures on galaxy rotation curves
The effect of halo substructures on galaxy rotation curves is investigated in
this paper using a simple model of dark matter clustering. A dark matter halo
density profile is developed based only on the scale free nature of clustering
that leads to a statistically self-similar distribution of the substructures at
galactic scale. Semi-analytical method is used to derive rotation curves for
such a clumpy dark matter density profile. It is found that the halo
substructures significantly affect the galaxy velocity field. Based on the
fractal geometry of the halo, this self-consistent model predicts an NFW-like
rotation curve and a scale free power spectrum of the rotation velocity
fluctuations.Comment: 6 pages, 3 figures. Accepted for publication in ApJ. The definitive
version will be available at http://iopscience.iop.org
Practical applications of small-angle neutron scattering.
Recent improvements in beam-line accessibility and technology have led to small-angle neutron scattering (SANS) becoming more frequently applied to materials problems. SANS has been used to study the assembly, dispersion, alignment and mixing of nanoscale condensed matter, as well as to characterise the internal structure of organic thin films, porous structures and inclusions within steel. Using time-resolved SANS, growth mechanisms in materials systems and soft matter phase transitions can also be explored. This review is intended for newcomers to SANS as well as experts. Therefore, the basic knowledge required for its use is first summarised. After this introduction, various examples are given of the types of soft and hard matter that have been studied by SANS. The information that can be extracted from the data is highlighted, alongside the methods used to obtain it. In addition to presenting the findings, explanations are provided on how the SANS measurements were optimised, such as the use of contrast variation to highlight specific parts of a structure. Emphasis is placed on the use of complementary techniques to improve data quality (e.g. using other scattering methods) and the accuracy of data analysis (e.g. using microscopy to separately determine shape and size). This is done with a view to providing guidance on how best to design and analyse future SANS measurements on materials not listed below
Fracton pairing mechanism for "strange" superconductors: Self-assembling organic polymers and copper-oxide compounds
Self-assembling organic polymers and copper-oxide compounds are two classes
of "strange" superconductors, whose challenging behavior does not comply with
the traditional picture of Bardeen, Cooper, and Schrieffer (BCS)
superconductivity in regular crystals. In this paper, we propose a theoretical
model that accounts for the strange superconducting properties of either class
of the materials. These properties are considered as interconnected
manifestations of the same phenomenon: We argue that superconductivity occurs
in the both cases because the charge carriers (i.e., electrons or holes)
exchange {\it fracton excitations}, quantum oscillations of fractal lattices
that mimic the complex microscopic organization of the strange superconductors.
For the copper oxides, the superconducting transition temperature as
predicted by the fracton mechanism is of the order of K. We suggest
that the marginal ingredient of the high-temperature superconducting phase is
provided by fracton coupled holes that condensate in the conducting
copper-oxygen planes owing to the intrinsic field-effect-transistor
configuration of the cuprate compounds. For the gate-induced superconducting
phase in the electron-doped polymers, we simultaneously find a rather modest
transition temperature of K owing to the limitations imposed by
the electron tunneling processes on a fractal geometry. We speculate that
hole-type superconductivity observes larger onset temperatures when compared to
its electron-type counterpart. This promises an intriguing possibility of the
high-temperature superconducting states in hole-doped complex materials. A
specific prediction of the present study is universality of ac conduction for
.Comment: 12 pages (including separate abstract page), no figure
Complexity Phenomena and ROMA of the Magnetospheric Cusp, Hydrodynamic Turbulence, and the Cosmic Web
Dynamic Complexity is a phenomenon exhibited by a nonlinearly interacting
system within which multitudes of different sizes of large scale coherent
structures emerge, resulting in a globally nonlinear stochastic behavior vastly
different from that could be surmised from the underlying equations of
interaction. The hallmark of such nonlinear, complex phenomena is the
appearance of intermittent fluctuating events with the mixing and distributions
of correlated structures at all scales. We briefly review here a relatively
recent method, ROMA (rank-ordered multifractal analysis), explicitly
constructed to analyze the intricate details of the distribution and scaling of
such types of intermittent structures. This method is then applied to the
analyses of selected examples related to the dynamical plasmas of the cusp
region of the magnetosphere, velocity fluctuations of classical hydrodynamic
turbulence, and the distribution of the structures of the cosmic gas obtained
through large scale, moving mesh simulations. Differences and similarities of
the analyzed results among these complex systems will be contrasted and
highlighted. The first two examples have direct relevance to the geospace
environment and are summaries of previously reported findings. The third
example on the cosmic gas, though involving phenomena much larger in
spatiotemporal scales, with its highly compressible turbulent behavior and the
unique simulation technique employed in generating the data, provides direct
motivations of applying such analysis to studies of similar multifractal
processes in various extreme environments. These new results are both exciting
and intriguing.Comment: 36 page
Practical applications of small-angle neutron scattering.
Recent improvements in beam-line accessibility and technology have led to small-angle neutron scattering (SANS) becoming more frequently applied to materials problems. SANS has been used to study the assembly, dispersion, alignment and mixing of nanoscale condensed matter, as well as to characterise the internal structure of organic thin films, porous structures and inclusions within steel. Using time-resolved SANS, growth mechanisms in materials systems and soft matter phase transitions can also be explored. This review is intended for newcomers to SANS as well as experts. Therefore, the basic knowledge required for its use is first summarised. After this introduction, various examples are given of the types of soft and hard matter that have been studied by SANS. The information that can be extracted from the data is highlighted, alongside the methods used to obtain it. In addition to presenting the findings, explanations are provided on how the SANS measurements were optimised, such as the use of contrast variation to highlight specific parts of a structure. Emphasis is placed on the use of complementary techniques to improve data quality (e.g. using other scattering methods) and the accuracy of data analysis (e.g. using microscopy to separately determine shape and size). This is done with a view to providing guidance on how best to design and analyse future SANS measurements on materials not listed below
Phase separation and self-assembly in vitrimers: hierarchical morphology of molten and semi-crystalline polyethylene/dioxaborolane maleimide systems
Vitrimers - a class of polymer networks which are covalently crosslinked and
insoluble like thermosets, but flow when heated like thermoplastics - contain
dynamic links and/or crosslinks that undergo an associative exchange reaction.
These dynamic crosslinks enable vitrimers to have interesting
mechanical/rheological behavior, self-healing, adhesive, and shape memory
properties. We demonstrate that vitrimers can self-assemble into complex meso-
and nanostructures when crosslinks and backbone monomers strongly interact.
Vitrimers featuring polyethylene (PE) as the backbone and dioxaborolane
maleimide as the crosslinkable moiety were studied in both the molten and
semi-crystalline states. We observed that PE vitrimers macroscopically phase
separated into dioxaborolane maleimide rich and poor regions, and characterized
the extent of phase separation by optical transmission measurements. This phase
separation can explain the relatively low insoluble fractions and overall
crystallinities of PE vitrimers. Using synchrotron-sourced small-angle X-ray
scattering (SAXS), we discovered that PE vitrimers and their linear precursors
micro-phase separated into hierarchical nanostructures. Fitting of the SAXS
patterns to a scattering model strongly suggests that the nanostructures -
which persist in both the melt and amorphous fraction of the semi-crystalline
state - may be described as dioxaborolane maleimide rich aggregates packed in a
mass fractal arrangement. These findings of hierarchical meso- and
nanostructures point out that incompatibility effects between network
components and resulting self-assembly must be considered for understanding
behavior and the rational design of vitrimer materials
Universal Scaling Relations in Scale-Free Structure Formation
A large number of astronomical phenomena exhibit remarkably similar scaling
relations. The most well-known of these is the mass distribution which (to first order) describes stars,
protostellar cores, clumps, giant molecular clouds, star clusters and even dark
matter halos. In this paper we propose that this ubiquity is not a coincidence
and that it is the generic result of scale-free structure formation where the
different scales are uncorrelated. We show that all such systems produce a mass
function proportional to and a column density distribution with a
power law tail of . In the
case where structure formation is controlled by gravity the two-point
correlation becomes . Furthermore, structures formed by
such processes (e.g. young star clusters, DM halos) tend to a density profile. We compare these predictions with observations,
analytical fragmentation cascade models, semi-analytical models of
gravito-turbulent fragmentation and detailed "full physics" hydrodynamical
simulations. We find that these power-laws are good first order descriptions in
all cases.Comment: 12 pages, 6 figures, 2 tables, submitted to MNRA
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