1,721,016 research outputs found
Phase transitions of a two-dimensional periodic hydrophilic hydrophobic chain
No full textWe study a single self-avoiding hydrophilic hydrophobic polymer chain, through Monte Carlo lattice simulations. The affinity of monomer i for water is characterized by a (scalar) charge lambda(i), and the monomer-water interaction is short-ranged. Assuming incompressibility yields an effective short ranged interaction between monomer pairs (i, j), proportional to (lambda(i) + lambda(j)), In this article, we take lambda(i) = +1 (resp. (lambda(i) = -1)) for hydrophilic (resp, hydrophobic) monomers and consider a chain with (i) an equal number of hydrophilic and -phobic monomers (ii) a periodic distribution of the lambda(i) along the chain, with periodicity 2p. This model may be of interest in various situations (protein folding, polysoaps,...) The simulations are done on the square lattice (d = 2), for various chain lengths N. There is a critical value (p(c)(N) similar to 0.07N) of the periodicity, which distinguishes between different low temperature structures. For p > p(c), the ground state corresponds to a macroscopic phase separation between a dense hydrophobic core and hydrophilic loops. For p < p(c) (but not too small), one gets a microscopic (finite scale) phase separation, and the ground state corresponds to a chain or network of hydrophobic droplets, coated by hydrophilic monomers. These different cases will be explored through a Multiple Markov chain method. The results for the d = 3 case (where p(c)(N) similar to N-1/3) are similar
Collapse transitions of a periodic hydrophilic hydrophobic chain
No full textWe study a single self avoiding hydrophilic hydrophobic polymer chain, through Monte-Carlo lattice simulations. The affinity of monomer i for water is characterized by a (scalar) charge lambda(i), and the monomer-water interaction is short-ranged. Assuming incompressibility yields an effective short ranged interaction between monomer pairs (i, j), proportional to (lambda(i) + lambda(j)) In this article, we take lambda(i) = +1 (resp. (lambda(i) = -1)) for hydrophilic (resp. hydrophobic) monomers and consider a chain with (i) an equal number of hydro-philic and -phobic monomers (ii) a periodic distribution of the Xi along the chain, with periodicity 2p. The simulations are done for various chain lengths NI in d = 2 (square lattice) and d = 3 (cubic lattice). There is a critical value p(c)(d, N) of the periodicity, which distinguishes between different low temperature structures. For p > p(c), the ground state corresponds to a macroscopic phase separation between a dense hydrophobic core and hydrophilic loops. For p < p(c) (but not too small), one gets a microscopic (finite scale) phase separation, and the ground state corresponds to a chain or network of hydrophobic droplets, coated by hydrophilic monomers. We restrict our study to two extreme cases, p similar to O(N) and p similar to O(1) to illustrate the physics of the various phase transitions. A tentative variational approach is also presented
Phase diagram of magnetic polymers
No full textWe consider polymers made of magnetic monomers (Ising or Heisenberg-like) in a good solvent. These polymers are modeled as self-avoiding walks on a cubic lattice, and the ferromagnetic interaction between the spins carried by the monomers is short-ranged in space. At low temperature, these polymers undergo a magnetic induced first order collapse transition, that we study at the mean field level. Contrasting with an ordinary Theta point, there is a strong jump in the polymer density, as well as in its magnetization. In thr presence of a magnetic field, the collapse temperature increases: while the discontinuities decrease. Beyond a multicritical point, the transition becomes second order and Theta-like. Monte Carlo simulations for the Ising case are in qualitative agreement with these results
Overlap properties and adsorption transition of two Hamiltonian paths
No full textWe consider a model of two (fully) compact polymer chains, coupled through
an attractive interaction. These compact chains are represented
by Hamiltonian paths (HP), and the coupling favors the existence of common
bonds between the chains. We use a (n=0 component) spin representation for
these paths, and we evaluate the resulting partition function within
a homogeneous saddle point approximation.
For strong coupling (i.e. at low temperature), one finds a phase
transition towards a "frozen" phase where one chain is completely
adsorbed onto the other. By performing a Legendre transform,
we obtain the probability distribution of overlaps. The fraction of
common bonds between two HP, i.e. their overlap q, has both lower
(qm) and upper (qM) bounds. This means in particular that two HP
with overlap greater than qM coincide. These results may be of
interest in (bio)polymers and in optimization problems
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Full text linkWe 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
- …
