1,590 research outputs found
Historical sociology and sociological history : theory and practice
Digitised version produced by the EUI Library and made available online in 2020
One-Flavour Hybrid Monte Carlo with Wilson Fermions
The Wilson fermion determinant can be written as product of the determinants
of two hermitian positive definite matrices. This formulation allows to
simulate non-degenerate quark flavors by means of the hybrid Monte Carlo
algorithm. A major numerical difficulty is the occurrence of nested inversions.
We construct a Uzawa iteration scheme which treats the nested system within one
iterative process.Comment: 11 pages, to appear in proceedings of the workshop "Numerical
Challenges in Lattice QCD", Springer Verla
Electric-field switchable magnetization via the Dzyaloshinskii-Moriya interaction: FeTiO_3 versus BiFeO_3
In this article we review and discuss a mechanism for coupling between
electric polarization and magnetization that can ultimately lead to
electric-field switchable magnetization. The basic idea is that a ferroelectric
distortion in an antiferromagnetic material can "switch on" the
Dzyaloshinskii-Moriya interaction which leads to a canting of the
antiferromagnetic sublattice magnetizations, and thus to a net magnetization.
This magnetization M is coupled to the polarization P via a trilinear free
energy contribution of the form P(M x L), where L is the antiferromagnetic
order parameter. In particular, we discuss why such an invariant is present in
R3c FeTiO_3 but not in the isostructural multiferroic BiFeO_3. Finally, we
construct symmetry groups that in general allow for this kind of
ferroelectrically-induced weak ferromagnetism.Comment: 15 pages, 3 images, to appear in J. Phys: Condens. Matter Focus Issue
on Multiferroic
Polarization state of the optical near-field
The polarization state of the optical electromagnetic field lying several
nanometers above complex dielectric structures reveals the intricate
light-matter interaction that occurs in this near-field zone. This information
can only be extracted from an analysis of the polarization state of the
detected light in the near-field. These polarization states can be calculated
by different numerical methods well-suited to near--field optics. In this
paper, we apply two different techniques (Localized Green Function Method and
Differential Theory of Gratings) to separate each polarisation component
associated with both electric and magnetic optical near-fields produced by
nanometer sized objects. The analysis is carried out in two stages: in the
first stage, we use a simple dipolar model to achieve insight into the physical
origin of the near-field polarization state. In the second stage, we calculate
accurate numerical field maps, simulating experimental near-field light
detection, to supplement the data produced by analytical models. We conclude
this study by demonstrating the role played by the near-field polarization in
the formation of the local density of states.Comment: 9 pages, 11 figures, accepted for publication in Phys. Rev.
Local Convergence and Global Diversity: From Interpersonal to Social Influence
Axelrod (1997) showed how local convergence in cultural influence can
preserve cultural diversity. We argue that central implications of Axelrod's
model may change profoundly, if his model is integrated with the assumption of
social influence as assumed by an earlier generation of modelers. Axelrod and
all follow up studies employed instead the assumption that influence is
interpersonal (dyadic). We show how the combination of social influence with
homophily allows solving two important problems. Our integration of social
influence yields monoculture in small societies and diversity increasing in
population size, consistently with empirical evidence but contrary to earlier
models. The second problem was identified by Klemm et al.(2003a,b), an
extremely narrow window of noise levels in which diversity with local
convergence can be obtained at all. Our model with social influence generates
stable diversity with local convergence across a much broader interval of noise
levels than models based on interpersonal influence.Comment: 20 pages, 3 figures, Paper presented at American Sociological
Association 103rd Annual Meeting, August 1-4, 2008, Boston, MA. Session on
Mathematical Sociolog
Discrete Breathers
Nonlinear classical Hamiltonian lattices exhibit generic solutions in the
form of discrete breathers. These solutions are time-periodic and (typically
exponentially) localized in space. The lattices exhibit discrete translational
symmetry. Discrete breathers are not confined to certain lattice dimensions.
Necessary ingredients for their occurence are the existence of upper bounds on
the phonon spectrum (of small fluctuations around the groundstate) of the
system as well as the nonlinearity in the differential equations. We will
present existence proofs, formulate necessary existence conditions, and discuss
structural stability of discrete breathers. The following results will be also
discussed: the creation of breathers through tangent bifurcation of band edge
plane waves; dynamical stability; details of the spatial decay; numerical
methods of obtaining breathers; interaction of breathers with phonons and
electrons; movability; influence of the lattice dimension on discrete breather
properties; quantum lattices - quantum breathers. Finally we will formulate a
new conceptual aproach capable of predicting whether discrete breather exist
for a given system or not, without actually solving for the breather. We
discuss potential applications in lattice dynamics of solids (especially
molecular crystals), selective bond excitations in large molecules, dynamical
properties of coupled arrays of Josephson junctions, and localization of
electromagnetic waves in photonic crystals with nonlinear response.Comment: 62 pages, LaTeX, 14 ps figures. Physics Reports, to be published; see
also at http://www.mpipks-dresden.mpg.de/~flach/html/preprints.htm
Tailoring r-index for Document Listing Towards Metagenomics Applications
A basic problem in metagenomics is to assign a sequenced read to the correct species in the reference collection. In typical applications in genomic epidemiology and viral metagenomics the reference collection consists of a set of species with each species represented by its highly similar strains. It has been recently shown that accurate read assignment can be achieved with k-mer hashing-based pseudoalignment: a read is assigned to species A if each of its k-mer hits to a reference collection is located only on strains of A. We study the underlying primitives required in pseudoalignment and related tasks. We propose three space-efficient solutions building upon the document listing with frequencies problem. All the solutions use an r-index (Gagie et al., SODA 2018) as an underlying index structure for the text obtained as concatenation of the set of species, as well as for each species. Given t species whose concatenation length is n, and whose Burrows-Wheeler transform contains r runs, our first solution, based on a grammar-compressed document array with precomputed queries at non terminal symbols, reports the frequencies for the distinct documents in which the pattern of length m occurs in time. Our second solution is also based on a grammar-compressed document array, but enhanced with bitvectors and reports the frequencies in time, over a machine with wordsize w. Our third solution, based on the interleaved LCP array, answers the same query in time. We implemented our solutions and tested them on real-world and synthetic datasets. The results show that all the solutions are fast on highly-repetitive data, and the size overhead introduced by the indexes are comparable with the size of the r-index.Peer reviewe
Homophily and Social Distance in the Choice of Multiple Friends an Analysis Based on Conditionally Symmetric Log-Bilinear Association Models
Genetic Variance in the Adiponutrin Gene Family and Childhood Obesity
AIM: The adiponutrin gene family consists of five genes (PNPLA1-5) coding for proteins with both lipolytic and lipogenic properties. PNPLA3 has previously been associated with adult obesity. Here we investigated the possible association between genetic variants in these genes and childhood and adolescent obesity. METHODS/RESULTS: Polymorphisms in the five genes of the adiponutrin gene family were selected and genotyped using the Sequenom platform in a childhood and adolescent obesity case-control study. Six variants in PNPLA1 showed association with obesity (rs9380559, rs12212459, rs1467912, rs4713951, rs10947600, and rs12199580, p0.05). When analyzing these SNPs in relation to phenotypes, two SNPs in the PNPLA3 gene showed association with insulin sensitivity (rs12483959: beta = -0.053, p = 0.016, and rs2072907: beta = -0.049, p = 0.024). No associations were seen for PNPLA2, PNPLA4, and PNPLA5. CONCLUSIONS: Genetic variation in the adiponutrin gene family does not seem to contribute strongly to obesity in children and adolescents. PNPLA1 exhibited a modest effect on obesity and PNPLA3 on insulin sensitivity. These data, however, require confirmation in other cohorts and ethnic groups
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