66 research outputs found
Electronic shell effects and the stability of alkali nanowires
Experimental conductance histograms for Na nanowires are analyzed in detail
and compared to recent theoretical results on the stability of cylindrical and
elliptical nanowires, using the free-electron model. We find a one-to-one
correspondence between the peaks in the histograms and the most stable nanowire
geometries, indicating that several of the commonly observed nanowires have
elliptical cross sections
On the Stability and Structural Dynamics of Metal Nanowires
This article presents a brief review of the nanoscale free-electron model,
which provides a continuum description of metal nanostructures. It is argued
that surface and quantum-size effects are the two dominant factors in the
energetics of metal nanowires, and that much of the phenomenology of nanowire
stability and structural dynamics can be understood based on the interplay of
these two competing factors. A linear stability analysis reveals that metal
nanocylinders with certain magic conductance values G=1, 3, 6, 12, 17, 23, 34,
42, 51, 67, 78, 96, ... times the conductance quantum are exceptionally stable.
A nonlinear dynamical simulation of nanowire structural evolution reveals a
universal equilibrium shape consisting of a magic cylinder suspended between
unduloidal contacts. The lifetimes of these metastable structures are also
computed.Comment: 8 pages, 6 figure
On the effects of the magnetic field and the isotopic substitution upon the infrared absorption of manganites
Employing a variational approach that takes into account electron-phonon and
magnetic interactions in perovskites with , the
effects of the magnetic field and the oxygen isotope substitution on the phase
diagram, the electron-phonon correlation function and the infrared absorption
at are studied. The lattice displacements show a strong correlation
with the conductivity and the magnetic properties of the system. Then the
conductivity spectra are characterized by a marked sensitivity to the external
parameters near the phase boundary.Comment: 10 figure
A mathematical framework for critical transitions: normal forms, variance and applications
Critical transitions occur in a wide variety of applications including
mathematical biology, climate change, human physiology and economics. Therefore
it is highly desirable to find early-warning signs. We show that it is possible
to classify critical transitions by using bifurcation theory and normal forms
in the singular limit. Based on this elementary classification, we analyze
stochastic fluctuations and calculate scaling laws of the variance of
stochastic sample paths near critical transitions for fast subsystem
bifurcations up to codimension two. The theory is applied to several models:
the Stommel-Cessi box model for the thermohaline circulation from geoscience,
an epidemic-spreading model on an adaptive network, an activator-inhibitor
switch from systems biology, a predator-prey system from ecology and to the
Euler buckling problem from classical mechanics. For the Stommel-Cessi model we
compare different detrending techniques to calculate early-warning signs. In
the epidemics model we show that link densities could be better variables for
prediction than population densities. The activator-inhibitor switch
demonstrates effects in three time-scale systems and points out that excitable
cells and molecular units have information for subthreshold prediction. In the
predator-prey model explosive population growth near a codimension two
bifurcation is investigated and we show that early-warnings from normal forms
can be misleading in this context. In the biomechanical model we demonstrate
that early-warning signs for buckling depend crucially on the control strategy
near the instability which illustrates the effect of multiplicative noise.Comment: minor corrections to previous versio
Active Brownian Particles. From Individual to Collective Stochastic Dynamics
We review theoretical models of individual motility as well as collective
dynamics and pattern formation of active particles. We focus on simple models
of active dynamics with a particular emphasis on nonlinear and stochastic
dynamics of such self-propelled entities in the framework of statistical
mechanics. Examples of such active units in complex physico-chemical and
biological systems are chemically powered nano-rods, localized patterns in
reaction-diffusion system, motile cells or macroscopic animals. Based on the
description of individual motion of point-like active particles by stochastic
differential equations, we discuss different velocity-dependent friction
functions, the impact of various types of fluctuations and calculate
characteristic observables such as stationary velocity distributions or
diffusion coefficients. Finally, we consider not only the free and confined
individual active dynamics but also different types of interaction between
active particles. The resulting collective dynamical behavior of large
assemblies and aggregates of active units is discussed and an overview over
some recent results on spatiotemporal pattern formation in such systems is
given.Comment: 161 pages, Review, Eur Phys J Special-Topics, accepte
Statistical Mechanics of Horizontal Gene Transfer in Evolutionary Ecology
The biological world, especially its majority microbial component, is
strongly interacting and may be dominated by collective effects. In this
review, we provide a brief introduction for statistical physicists of the way
in which living cells communicate genetically through transferred genes, as
well as the ways in which they can reorganize their genomes in response to
environmental pressure. We discuss how genome evolution can be thought of as
related to the physical phenomenon of annealing, and describe the sense in
which genomes can be said to exhibit an analogue of information entropy. As a
direct application of these ideas, we analyze the variation with ocean depth of
transposons in marine microbial genomes, predicting trends that are consistent
with recent observations using metagenomic surveys.Comment: Accepted by Journal of Statistical Physic
Size Doesn't Matter: Towards a More Inclusive Philosophy of Biology
notes: As the primary author, OâMalley drafted the paper, and gathered and analysed data (scientific papers and talks). Conceptual analysis was conducted by both authors.publication-status: Publishedtypes: ArticlePhilosophers of biology, along with everyone else, generally perceive life to fall into two broad categories, the microbes and macrobes, and then pay most of their attention to the latter. âMacrobeâ is the word we propose for larger life forms, and we use it as part of an argument for microbial equality. We suggest that taking more notice of microbes â the dominant life form on the planet, both now and throughout evolutionary history â will transform some of the philosophy of biologyâs standard ideas on ontology, evolution, taxonomy and biodiversity. We set out a number of recent developments in microbiology â including biofilm formation, chemotaxis, quorum sensing and gene transfer â that highlight microbial capacities for cooperation and communication and break down conventional thinking that microbes are solely or primarily single-celled organisms. These insights also bring new perspectives to the levels of selection debate, as well as to discussions of the evolution and nature of multicellularity, and to neo-Darwinian understandings of evolutionary mechanisms. We show how these revisions lead to further complications for microbial classification and the philosophies of systematics and biodiversity. Incorporating microbial insights into the philosophy of biology will challenge many of its assumptions, but also give greater scope and depth to its investigations
Factors Associated with Revision Surgery after Internal Fixation of Hip Fractures
Background: Femoral neck fractures are associated with high rates of revision surgery after management with internal fixation. Using data from the Fixation using Alternative Implants for the Treatment of Hip fractures (FAITH) trial evaluating methods of internal fixation in patients with femoral neck fractures, we investigated associations between baseline and surgical factors and the need for revision surgery to promote healing, relieve pain, treat infection or improve function over 24 months postsurgery. Additionally, we investigated factors associated with (1) hardware removal and (2) implant exchange from cancellous screws (CS) or sliding hip screw (SHS) to total hip arthroplasty, hemiarthroplasty, or another internal fixation device. Methods: We identified 15 potential factors a priori that may be associated with revision surgery, 7 with hardware removal, and 14 with implant exchange. We used multivariable Cox proportional hazards analyses in our investigation. Results: Factors associated with increased risk of revision surgery included: female sex, [hazard ratio (HR) 1.79, 95% confidence interval (CI) 1.25-2.50; P = 0.001], higher body mass index (fo
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