643 research outputs found
Associative-algebraic approach to logarithmic conformal field theories
We set up a strategy for studying large families of logarithmic conformal
field theories by using the enlarged symmetries and non--semi-simple
associative algebras appearing in their lattice regularizations (as discussed
in a companion paper). Here we work out in detail two examples of theories
derived as the continuum limit of XXZ spin-1/2 chains, which are related to
spin chains with supersymmetry algebras gl() and gl(),
respectively, with open (or free) boundary conditions in all cases. These
theories can also be viewed as vertex models, or as loop models. Their
continuum limits are boundary conformal field theories (CFTs) with central
charge and respectively, and in the loop interpretation they
describe dense polymers and the boundaries of critical percolation clusters,
respectively. We also discuss the case of dilute (critical) polymers as another
boundary CFT with . Within the supersymmetric formulations, these boundary
CFTs describe the fixed points of certain nonlinear sigma models that have a
supercoset space as the target manifold, and of Landau-Ginzburg field theories.
The submodule structures of indecomposable representations of the Virasoro
algebra appearing in the boundary CFT, representing local fields, are derived
from the lattice. A central result is the derivation of the fusion rules for
these fields
Edge states and conformal boundary conditions in super spin chains and super sigma models
The sigma models on projective superspaces CP^{N+M-1|N} with topological
angle theta=pi mod 2pi flow to non-unitary, logarithmic conformal field
theories in the low-energy limit. In this paper, we determine the exact
spectrum of these theories for all open boundary conditions preserving the full
global symmetry of the model, generalizing recent work on the particular case
M=0 [C. Candu et al, JHEP02(2010)015]. In the sigma model setting, these
boundary conditions are associated with complex line bundles, and are labelled
by an integer, related with the exact value of theta. Our approach relies on a
spin chain regularization, where the boundary conditions now correspond to the
introduction of additional edge states. The exact values of the exponents then
follow from a lengthy algebraic analysis, a reformulation of the spin chain in
terms of crossing and non-crossing loops (represented as a certain subalgebra
of the Brauer algebra), and earlier results on the so-called one- and
two-boundary Temperley Lieb algebras (also known as blob algebras). A
remarkable result is that the exponents, in general, turn out to be irrational.
The case M=1 has direct applications to the spin quantum Hall effect, which
will be discussed in a sequel.Comment: 50 pages, 18 figure
Fourier factorization with complex polarization bases in the plane-wave expansion method applied to two-dimensional photonic crystals
We demonstrate an enhancement of the plane wave expansion method treating
two-dimensional photonic crystals by applying Fourier factorization with
generally elliptic polarization bases. By studying three examples of
periodically arranged cylindrical elements, we compare our approach to the
classical Ho method in which the permittivity function is simply expanded
without changing coordinates, and to the normal vector method using a
normal-tangential polarization transform. The compared calculations clearly
show that our approach yields the best convergence properties owing to the
complete continuity of our distribution of polarization bases. The presented
methodology enables us to study more general systems such as periodic elements
with an arbitrary cross-section or devices such as photonic crystal waveguides
The Large-scale Bipolar Wind in the Galactic Center
During a 9-month campaign (1996--1997), the Midcourse Space Experiment (MSX)
satellite mapped the Galactic Plane at mid-infrared wavelengths (4.3--21.3um).
Here we report evidence for a spectacular limb- brightened, bipolar structure
at the Galactic Center extending more than a degree (170 pc at 8.0 kpc) on
either side of the plane. The 8.3um emission shows a tight correlation with the
3, 6 and 11 cm continuum structure over the same scales. Dense gas and dust are
being entrained in a large-scale bipolar wind powered by a central starburst.
The inferred energy injection at the source is ~10^54/kappa erg for which
\kappa is the covering fraction of the dusty shell (kappa <= 0.1).
There is observational evidence for a galactic wind on much larger scales,
presumably from the same central source which produced the bipolar shell seen
by MSX. Sofue has argued that the North Polar Spur -- a thermal x-ray/radio
loop which extends from the Galactic Plane to b = +80 deg -- was powered by a
nuclear explosion (1-30 x 10^55 erg) roughly 15 Myr ago. We demonstrate that an
open-ended bipolar wind (~10^55 erg), when viewed in near-field projection,
provides the most natural explanation for the observed loop structure. The
ROSAT 1.5 keV diffuse x-ray map over the inner 45 deg provides compelling
evidence for this interpretation. Since the faint bipolar emission would be
very difficult to detect beyond the Galaxy, the phenomenon of large-scale
galactic winds may be far more common than has been observed to date.Comment: 24 pages, 6 figures, aastex. High resolution figures are available at
ftp://www.aao.gov.au/pub/local/jbh/astro-ph/GC/. Astrophysical Journal,
accepte
State of green infrastructure in the GCR
July 2013This State of Green Infrastructure report is both an assessment of the set of natural and manmade landscape features in the Gauteng City-Region (GCR) and an interrogation into how the services provided by these assets are perceived, understood and valued. Inspiration is drawn from the conceptual and planning framework of ‘green infrastructure’, through which ecological systems, green spaces and other landscape features are regarded as providing services to society in the same way as those offered by traditional ‘hard’ infrastructure.written by Alexis Schäffler et al. for GCR
Tau-Mediated Nuclear Depletion and Cytoplasmic Accumulation of SFPQ in Alzheimer's and Pick's Disease
Tau dysfunction characterizes neurodegenerative diseases such as Alzheimer's disease (AD) and frontotemporal lobar degeneration (FTLD). Here, we performed an unbiased SAGE (serial analysis of gene expression) of differentially expressed mRNAs in the amygdala of transgenic pR5 mice that express human tau carrying the P301L mutation previously identified in familial cases of FTLD. SAGE identified 29 deregulated transcripts including Sfpq that encodes a nuclear factor implicated in the splicing and regulation of gene expression. To assess the relevance for human disease we analyzed brains from AD, Pick's disease (PiD, a form of FTLD), and control cases. Strikingly, in AD and PiD, both dementias with a tau pathology, affected brain areas showed a virtually complete nuclear depletion of SFPQ in both neurons and astrocytes, along with cytoplasmic accumulation. Accordingly, neurons harboring either AD tangles or Pick bodies were also depleted of SFPQ. Immunoblot analysis of human entorhinal cortex samples revealed reduced SFPQ levels with advanced Braak stages suggesting that the SFPQ pathology may progress together with the tau pathology in AD. To determine a causal role for tau, we stably expressed both wild-type and P301L human tau in human SH-SY5Y neuroblastoma cells, an established cell culture model of tau pathology. The cells were differentiated by two independent methods, mitomycin C-mediated cell cycle arrest or neuronal differentiation with retinoic acid. Confocal microscopy revealed that SFPQ was confined to nuclei in non-transfected wild-type cells, whereas in wild-type and P301L tau over-expressing cells, irrespective of the differentiation method, it formed aggregates in the cytoplasm, suggesting that pathogenic tau drives SFPQ pathology in post-mitotic cells. Our findings add SFPQ to a growing list of transcription factors with an altered nucleo-cytoplasmic distribution under neurodegenerative conditions
Membrane-associated molecules regulate the formation of layer-specific cortical circuits
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