470 research outputs found
Field Theory And Second Renormalization Group For Multifractals In Percolation
The field-theory for multifractals in percolation is reformulated in such a
way that multifractal exponents clearly appear as eigenvalues of a second
renormalization group. The first renormalization group describes geometrical
properties of percolation clusters, while the second-one describes electrical
properties, including noise cumulants. In this context, multifractal exponents
are associated with symmetry-breaking fields in replica space. This provides an
explanation for their observability. It is suggested that multifractal
exponents are ''dominant'' instead of ''relevant'' since there exists an
arbitrary scale factor which can change their sign from positive to negative
without changing the Physics of the problem.Comment: RevTex, 10 page
Hypocycloid-shaped hollow-core photonic crystal fiber Part I: Arc curvature effect on confinement loss
We report on numerical and experimental studies showing the influence of arc curvature on the confinement loss in hypocycloid-core Kagome hollow-core photonic crystal fiber. The results prove that with such a design the optical performances are strongly driven by the contour negative curvature of the core-cladding interface. They show that the increase in arc curvature results in a strong decrease in both the confinement loss and the optical power overlap between the core mode and the silica core-surround, including a modal content approaching true single-mode guidance. Fibers with enhanced negative curvature were then fabricated with a record loss-level of 17 dB/km at 1064 nm
Evaluating the ecological realism of plant species distribution models with ecological indicator values
Species distribution models (SDMs) are routinely applied to assess current as well as future species distributions, for example to assess impacts of future environmental change on biodiversity or to underpin conservation planning. It has been repeatedly emphasized that SDMs should be evaluated based not only on their goodness of fit to the data, but also on the realism of the modelled ecological responses. However, possibilities for the latter are hampered by limited knowledge on the true responses as well as a lack of quantitative evaluation methods. Here we compared modelled niche optima obtained from European-scale SDMs of 1,476 terrestrial vascular plant species with empirical ecological indicator values indicating the preferences of plant species for key environmental conditions. For each plant species we first fitted an ensemble SDM including three modeling techniques (GLM, GAM and BRT) and extracted niche optima for climate, soil, land use and nitrogen deposition variables with a large explanatory power for the occurrence of that species. We then compared these SDM-derived niche optima with the ecological indicator values by means of bivariate correlation analysis. We found weak to moderate correlations in the expected direction between the SDM-derived niche optima and ecological indicator values. The strongest correlation occurred between the modelled optima for growing degree days and the ecological indicator values for temperature. Correlations were weaker for SDM-derived niche optima with a more distal relationship to ecological indicator values (notably precipitation and soil moisture). Further, correlations were consistently highest for BRT, followed by GLM and GAM. Our method gives insight into the ecological realism of modelled niche optima and projected core habitats and can be used to improve SDMs by making a more informed selection of environmental variables and modeling techniques
Bond-charge Interaction in the extended Hubbard chain
We study the effects of bond-charge interaction (or correlated hopping) on
the properties of the extended ({\it i.e.,} with both on-site () and
nearest-neighbor () repulsions) Hubbard model in one dimension at
half-filling. Energy gaps and correlation functions are calculated by Lanczos
diagonalization on finite systems. We find that, irrespective of the sign of
the bond-charge interaction, , the charge--density-wave (CDW) state is more
robust than the spin--density-wave (SDW) state. A small bond-charge interaction
term is enough to make the differences between the CDW and SDW correlation
functions much less dramatic than when . For and fixed (
is the uncorrelated hopping integral), there is an intermediate phase between a
charge ordered phase and a phase corresponding to singly-occupied sites, the
nature of which we clarify: it is characterized by a succession of critical
points, each of which corresponding to a different density of doubly-occupied
sites. We also find an unusual slowly decaying staggered spin-density
correlation function, which is suggestive of some degree of ordering. No
enhancement of pairing correlations was found for any in the range
examined.Comment: 10 pages, 7 PostScript figures, RevTeX 3; to appear in Phys Rev
Impermeability effects in three-dimensional vesicles
We analyse the effects that the impermeability constraint induces on the
equilibrium shapes of a three-dimensional vesicle hosting a rigid inclusion. A
given alteration of the inclusion and/or vesicle parameters leads to shape
modifications of different orders of magnitude, when applied to permeable or
impermeable vesicles. Moreover, the enclosed-volume constraint wrecks the
uniqueness of stationary equilibrium shapes, and gives rise to pear-shaped or
stomatocyte-like vesicles.Comment: 16 pages, 7 figure
Design and fabrication of hollow-core photonic crystal fibers for high-power ultrashort pulse transportation and pulse compression
Self-Dual Bending Theory for Vesicles
We present a self-dual bending theory that may enable a better understanding
of highly nonlinear global behavior observed in biological vesicles. Adopting
this topological approach for spherical vesicles of revolution allows us to
describe them as frustrated sine-Gordon kinks. Finally, to illustrate an
application of our results, we consider a spherical vesicle globally distorted
by two polar latex beads.Comment: 10 pages, 3 figures, LaTeX2e+IOPar
Activating cannabinoid receptor 2 preserves axonal health through GSK-3β/NRF2 axis in adrenoleukodystrophy
Aberrant endocannabinoid signaling accompanies several neurodegenerative disorders, including multiple sclerosis. Here, we report altered endocannabinoid signaling in X-linked adrenoleukodystrophy (X-ALD), a rare neurometabolic demyelinating syndrome caused by malfunction of the peroxisomal ABCD1 transporter, resulting in the accumulation of very long-chain fatty acids (VLCFAs). We found abnormal levels of cannabinoid receptor 2 (CB2r) and related endocannabinoid enzymes in the brain and peripheral blood mononuclear cells (PBMCs) of X-ALD patients and in the spinal cord of a murine model of X-ALD. Preclinical treatment with a selective agonist of CB2r (JWH133) halted axonal degeneration and associated locomotor deficits, along with normalization of microgliosis. Moreover, the drug improved the main metabolic disturbances underlying this model, particularly in redox and lipid homeostatic pathways, including increased lipid droplets in motor neurons, through the modulation of the GSK-3β/NRF2 axis. JWH133 inhibited Reactive Oxygen Species elicited by excess VLCFAs in primary microglial cultures of Abcd1-null mice. Furthermore, we uncovered intertwined redox and CB2r signaling in the murine spinal cords and in patient PBMC samples obtained from a phase II clinical trial with antioxidants (NCT01495260). These findings highlight CB2r signaling as a potential therapeutic target for X-ALD and perhaps other neurodegenerative disorders that present with dysregulated redox and lipid homeostasis.This study was funded by the Institute of Health Carlos III through projects [PI19/01008] to SF and [PI20/00759] to AP (co-funded by the European Regional Development Fund, ERDF, a way to build Europe), Miguel Servet program [CPII16/00016] to SF and [PFIS, FI18/00141] to LPS (co-funded by the European Social Fund, ESF investing in your future). This study was also funded by grants from the Spanish Ministry of Health, Social Services and Equality (EC10-137), the Autonomous Government of Catalonia [2017SGR1206], the Hesperia Foundation, CERTIS Obres i Serveis, and the Crowd funding Campaign Arnau’97 to AP. JP was a predoctoral fellow of IDIBELL. The Center for Biomedical Research on Rare Diseases (CIBERER), an initiative of the Institute of Health Carlos III, funded the position of MR. Locomotor experiments were performed by the SEFALer unit F5 led by AP, which belongs to the CIBERER structure. We thank the CERCA Program/Generalitat de Catalunya for institutional support
Nonconcave entropies in multifractals and the thermodynamic formalism
We discuss a subtlety involved in the calculation of multifractal spectra
when these are expressed as Legendre-Fenchel transforms of functions analogous
to free energy functions. We show that the Legendre-Fenchel transform of a free
energy function yields the correct multifractal spectrum only when the latter
is wholly concave. If the spectrum has no definite concavity, then the
transform yields the concave envelope of the spectrum rather than the spectrum
itself. Some mathematical and physical examples are given to illustrate this
result, which lies at the root of the nonequivalence of the microcanonical and
canonical ensembles. On a more positive note, we also show that the
impossibility of expressing nonconcave multifractal spectra through
Legendre-Fenchel transforms of free energies can be circumvented with the help
of a generalized free energy function, which relates to a recently introduced
generalized canonical ensemble. Analogies with the calculation of rate
functions in large deviation theory are finally discussed.Comment: 9 pages, revtex4, 3 figures. Changes in v2: sections added on
applications plus many new references; contains an addendum not contained in
published versio
Critical Exponents for Diluted Resistor Networks
An approach by Stephen is used to investigate the critical properties of
randomly diluted resistor networks near the percolation threshold by means of
renormalized field theory. We reformulate an existing field theory by Harris
and Lubensky. By a decomposition of the principal Feynman diagrams we obtain a
type of diagrams which again can be interpreted as resistor networks. This new
interpretation provides for an alternative way of evaluating the Feynman
diagrams for random resistor networks. We calculate the resistance crossover
exponent up to second order in , where is the spatial
dimension. Our result verifies a
previous calculation by Lubensky and Wang, which itself was based on the
Potts--model formulation of the random resistor network.Comment: 27 pages, 14 figure
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