396 research outputs found
An exact renormalization group approach to frustrated magnets
Frustrated magnets are a notorious example where usual perturbative methods
fail. Having recourse to an exact renormalization group approach, one gets a
coherent picture of the physics of Heisenberg frustrated magnets everywhere
between d=2 and d=4: all known perturbative results are recovered in a single
framework, their apparent conflict is explained while the description of the
phase transition in d=3 is found to be in good agreement with the experimental
context.Comment: 4 pages, Latex, invited talk at the Second Conference on the Exact
Renormalization Group, Rome, September 2000, for technical details see
http://www.lpthe.jussieu.fr/~tissie
Critical properties of a continuous family of XY noncollinear magnets
Monte Carlo methods are used to study a family of three dimensional XY
frustrated models interpolating continuously between the stacked triangular
antiferromagnets and a variant of this model for which a local rigidity
constraint is imposed. Our study leads us to conclude that generically weak
first order behavior occurs in this family of models in agreement with a recent
nonperturbative renormalization group description of frustrated magnets.Comment: 5 pages, 3 figures, minor changes, published versio
Frustrated magnets in three dimensions: a nonperturbative approach
Frustrated magnets exhibit unusual critical behaviors: they display scaling
laws accompanied by nonuniversal critical exponents. This suggests that these
systems generically undergo very weak first order phase transitions. Moreover,
the different perturbative approaches used to investigate them are in conflict
and fail to correctly reproduce their behavior. Using a nonperturbative
approach we explain the mismatch between the different perturbative approaches
and account for the nonuniversal scaling observed.Comment: 7 pages, 1 figure. IOP style files included. To appear in Journal of
Physics: Condensed Matter. Proceedings of the conference HFM 2003, Grenoble,
Franc
Characterisation of a mural cell network in the murine pituitary gland
The anterior and intermediate lobes of the pituitary are composed of endocrine cells, as well as vasculature and supporting cells, such as folliculostellate cells. Folliculostellate cells form a network with several postulated roles in the pituitary, including production of paracrine signalling molecules and cytokines, coordination of endocrine cell hormone release, phagocytosis, and structural support. Folliculostellate cells in rats are characterised by expression of S100B protein, and in humans by glial fibrillary acid protein. However, there is evidence for another network of supporting cells in the anterior pituitary that has properties of mural cells, such as vascular smooth muscle cells and pericytes. The present study aims to characterise the distribution of cells that express the mural cell marker platelet derived growth factor receptor beta (PDGFRβ) in the mouse pituitary and establish whether these cells are folliculostellate. By immunohistochemical localisation, we determine that approximately 80% of PDGFRβ+ cells in the mouse pituitary have a non‐perivascular location and 20% are pericytes. Investigation of gene expression in a magnetic cell sorted population of PDGFRβ+ cells shows that, despite a mostly non‐perivascular location, this population is enriched for mural cell markers but not enriched for rat or human folliculostellate cell markers. This is confirmed by immunohistochemistry. The present study concludes that a mural cell network is present throughout the anterior pituitary of the mouse and that this population does not express well‐characterised human or rat folliculostellate cell markers
Fixed points in frustrated magnets revisited
We analyze the validity of perturbative renormalization group estimates
obtained within the fixed dimension approach of frustrated magnets. We
reconsider the resummed five-loop beta-functions obtained within the minimal
subtraction scheme without epsilon-expansion for both frustrated magnets and
the well-controlled ferromagnetic systems with a cubic anisotropy. Analyzing
the convergence properties of the critical exponents in these two cases we find
that the fixed point supposed to control the second order phase transition of
frustrated magnets is very likely an unphysical one. This is supported by its
non-Gaussian character at the upper critical dimension d=4. Our work confirms
the weak first order nature of the phase transition occuring at three
dimensions and provides elements towards a unified picture of all existing
theoretical approaches to frustrated magnets.Comment: 18 pages, 8 figures. This article is an extended version of
arXiv:cond-mat/060928
Spin-stiffness and topological defects in two-dimensional frustrated spin systems
Using a {\it collective} Monte Carlo algorithm we study the low-temperature
and long-distance properties of two systems of two-dimensional classical tops.
Both systems have the same spin-wave dynamics (low-temperature behavior) as a
large class of Heisenberg frustrated spin systems. They are constructed so that
to differ only by their topological properties. The spin-stiffnesses for the
two systems of tops are calculated for different temperatures and different
sizes of the sample. This allows to investigate the role of topological defects
in frustrated spin systems. Comparisons with Renormalization Group results
based on a Non Linear Sigma model approach and with the predictions of some
simple phenomenological model taking into account the topological excitations
are done.Comment: RevTex, 25 pages, 14 figures, Minor changes, final version. To appear
in Phys.Rev.
A single cytochrome P450 oxidase from <i>Solanum habrochaites</i> sequentially oxidizes 7-<i>epi</i>-zingiberene to derivatives toxic to whiteflies and various microorganisms
On the leading OPE corrections to the ghost-gluon vertex and the Taylor theorem
This brief note is devoted to a study of genuine non-perturbative corrections
to the Landau gauge ghost-gluon vertex in terms of the non-vanishing
dimension-two gluon condensate. We pay special attention to the kinematical
limit which the bare vertex takes for its tree-level expression at any
perturbative order, according to the well-known Taylor theorem. Based on our
OPE analysis, we also present a simple model for the vertex, in acceptable
agreement with lattice data.Comment: Final version published in JHE
Spin Stiffness of Stacked Triangular Antiferromagnets
We study the spin stiffness of stacked triangular antiferromagnets using both
heat bath and broad histogram Monte Carlo methods. Our results are consistent
with a continuous transition belonging to the chiral universality class first
proposed by Kawamura.Comment: 5 pages, 7 figure
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