1,721,011 research outputs found
Kinetically Constrained Lattice Gases
No full textKinetically constrained lattice gases (KCLG) are interacting particle
systems which show some of the key features of the liquid/glass
transition and, more generally, of glassy dynamics. Their distintictive
signature is the following: i) reversibility w.r.t. product i.i.d.
Bernoulli measure at any particle density and ii) vanishing of the
exchange rate across any edge unless the particle configuration around
the edge satisfies a proper constraint besides hard core. Because of
degeneracy of the exchange rates the models can show anomalous time
decay in the relaxation process w.r.t. the usual high temperature
lattice gas models particularly in the so-called cooperative case, when
the vacancies have to collectively cooperate in order for the particles
to move through the systems. Here we focus on the Kob-Andersen (KA)
model, a cooperative example widely analyzed in the physics literature,
both in a finite box with particle reservoirs at the boundary and on the
infinite lattice. In two dimensions (but our techniques extend to any
dimension) we prove a diffusive scaling O(L (2)) (apart from logarithmic
corrections) of the relaxation time in a finite box of linear size L. We
then use the above result to prove a diffusive decay 1/t (again apart
from logarithmic corrections) of the density-density time
autocorrelation function at any particle density, a result that has been
sometimes questioned on the basis of numerical simulations. The
techniques that we devise, based on a novel combination of
renormalization and comparison with a long-range Glauber type
constrained model, are robust enough to easily cover other choices of
the kinetic constraints
Relaxation times of kinetically constrained spin models with glassy dynamics
No full textWe analyze the equilibrium dynamics of kinetically constrained spin
systems which have been proposed as models for strong or fragile glasses
and systems undergoing jamming transitions. We develop a novel
multi-scale approach which allows us to connect the timescales to the
typical size of the regions which have to be rearranged to
create/destruct a particle. Thus we obtain exact results for the
relaxation time tau as rho -> 1. For most of the models this regime was
previously investigated only by numerical simulations. Depending on the
choice of constraints, via our technique we obtain or we disprove the
previously conjectured scalings. In particular, for the Fredrickson
Andersen and East models at any rho < 1, we establish that the
persistence and the spin-spin time autocorrelation functions decay
exponentially. This excludes the stretched exponential relaxation which
was derived by numerical simulations. Moreover for the East model we
prove log tau = 1/(2 log 2)(log(1 - rho)) 2 as rho -> 1, getting a sharp
result for log t which differs by a factor of 2 from the one conjectured
in previous literature. For FA2f in d >= 2 and FA3f in d >= 3 we obtain
the super-Arrhenius bounds exp (1 - rho)(- 1) <= tau = exp (1 - rho)(-5)
and exp exp (1 - rho)(-1) <= tau <= exp exp (1 - rho)(-2), respectively.
For FA1f in d = 1, 2 we rigorously prove the power law scalings recently
derived in Jack et al (2006 Preprint cond-mat/0601529). Our techniques
are flexible enough to allow a variety of constraints. In particular we
can also deal with models displaying an ergodicity-breaking transition
at rho(c) < 1, e. g. the North-East model. In this case we prove
exponential decay to equilibrium in the whole ergodic regime
Kinetically constrained spin models
No full textWe analyze the density and size dependence of the relaxation time for
kinetically constrained spin models (KCSM) intensively studied in the
physics literature as simple models sharing some of the features of the
glass transition. KCSM are interacting particle systems on Z(d) with
Glauber-like dynamics, reversible w.r.t. a simple product i.i.d
Bernoulli(p) measure. The essential feature of a KCSM is that the
creation/destruction of a particle at a given site can occur only if the
current configuration around it satisfies certain constraints which
completely define each specific model. No other interaction is present
in the model. From the mathematical point of view, the basic issues
concerning positivity of the spectral gap inside the ergodicity region
and its scaling with the particle density p remained open for most KCSM
(with the notably exception of the East model in d = 1; Aldous and
Diaconis in J Stat Phys 107(5-6):945-975, 2002). Here for the first time
we: (i) identify the ergodicity region by establishing a connection with
an associated bootstrap percolation model; (ii) develop a novel
multi-scale approach which proves positivity of the spectral gap in the
whole ergodic region; (iii) establish, sometimes optimal, bounds on the
behavior of the spectral gap near the boundary of the ergodicity region
and (iv) establish pure exponential decay at equilibrium for the
persistence function, i.e. the probability that the occupation variable
at the origin does not change before time t. Our techniques are flexible
enough to allow a variety of constraints and our findings disprove
certain conjectures which appeared in the physical literature on the
basis of numerical simulations
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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