554 research outputs found
Produção de plantas medicinais em sistemas agroflorestais: resultados preliminares de pesquisas participativas com agricultores familiares.
bitstream/item/41998/1/CT0070.pd
Anomalous Breaking of Anisotropic Scaling Symmetry in the Quantum Lifshitz Model
In this note we investigate the anomalous breaking of anisotropic scaling
symmetry in a non-relativistic field theory with dynamical exponent z=2. On
general grounds, one can show that there exist two possible "central charges"
which characterize the breaking of scale invariance. Using heat kernel methods,
we compute these two central charges in the quantum Lifshitz model, a free
field theory which is second order in time and fourth order in spatial
derivatives. We find that one of the two central charges vanishes.
Interestingly, this is also true for strongly coupled non-relativistic field
theories with a geometric dual described by a metric and a massive vector
field.Comment: 26 pages; major revision (results were unaffected), published versio
Holographic Renormalization for z=2 Lifshitz Space-Times from AdS
Lifshitz space-times with critical exponent z=2 can be obtained by
dimensional reduction of Schroedinger space-times with critical exponent z=0.
The latter space-times are asymptotically AdS solutions of AdS gravity coupled
to an axion-dilaton system and can be uplifted to solutions of type IIB
supergravity. This basic observation is used to perform holographic
renormalization for 4-dimensional asymptotically z=2 locally Lifshitz
space-times by Scherk-Schwarz dimensional reduction of the corresponding
problem of holographic renormalization for 5-dimensional asymptotically locally
AdS space-times coupled to an axion-dilaton system. We can thus define and
characterize a 4-dimensional asymptotically locally z=2 Lifshitz space-time in
terms of 5-dimensional AdS boundary data. In this setup the 4-dimensional
structure of the Fefferman-Graham expansion and the structure of the
counterterm action, including the scale anomaly, will be discussed. We find
that for asymptotically locally z=2 Lifshitz space-times obtained in this way
there are two anomalies each with their own associated nonzero central charge.
Both anomalies follow from the Scherk--Schwarz dimensional reduction of the
5-dimensional conformal anomaly of AdS gravity coupled to an axion-dilaton
system. Together they make up an action that is of the Horava-Lifshitz type
with nonzero potential term for z=2 conformal gravity.Comment: 32 pages, v2: modified discussion of the central charge
Neuroanatomical correlates of olfactory loss in normal aged subjects
In non-demented older persons, smell dysfunction, measured premortem, has been associated with postmortem brain degeneration similar to that of Alzheimer's disease. We hypothesized that distinct measures of gray and white matter integrity evaluated through magnetic resonance imaging (MRI) techniques could detect degenerative changes associated with age-related olfactory dysfunction. High-resolution T1-weighted images and diffusion-tensor images (DTI) of 30 clinically healthy subjects aged 51 to 77 were acquired with a 3-Tesla MRI scanner. Odor identification performance was assessed by means of the University of Pennsylvania Smell Identification Test (UPSIT). UPSIT scores correlated with right amygdalar volume and bilateral perirhinal and entorhinal cortices gray matter volume. Olfactory performance also correlated with postcentral gyrus cortical thickness and with fractional anisotropy and mean diffusivity levels in the splenium of the corpus callosum and the superior longitudinal fasciculi. Our results suggest that age-related olfactory loss is accompanied by diffuse degenerative changes that might correspond to the preclinical stages of neurodegenerative processes
Tuning the square-lattice antiferromagnet SrCu(TeW)O from N\'eel order to quantum disorder to columnar order
The spin-1/2 square-lattice Heisenberg model is predicted to have a quantum
disordered ground state when magnetic frustration is maximized by competing
nearest-neighbor and next-nearest-neighbor interactions (). The double perovskites SrCuTeO and SrCuWO are
isostructural spin-1/2 square-lattice antiferromagnets with N\'eel (
dominates) and columnar ( dominates) magnetic order, respectively. Here we
characterize the full isostructural solid solution series
SrCu(TeW)O () tunable from N\'eel order to
quantum disorder to columnar order. A spin-liquid-like ground state was
previously observed for the = 0.5 phase, but we show that the magnetic
order is suppressed below 1.5 K in a much wider region of 0.1-0.6.
This coincides with significant -linear terms in the low-temperature
specific heat. However, density functional theory calculations predict most of
the materials are not in the highly frustrated region
square-lattice Heisenberg model. Thus, a combination of both magnetic
frustration and quenched disorder is the likely origin of the spin-liquid-like
state in = 0.5.Comment: 20+5 pages, 6+4 figures. Accepted for publication in PR
Pressure Studies on a High- Superconductor Pseudogap and Critical Temperatures
We report simultaneous hydrostatic pressure studies on the critical
temperature and on the pseudogap temperature performed through
resistivity measurements on an optimally doped high- oxide
. The resistivity is measured as
function of the temperature for several different applied pressure below 1GPa.
We find that both and increases linearly with the pressure. This
result demonstrate that the well known intrinsic pressure effect on is
also present at and both temperatures are originated by the same
superconducting mechanism.Comment: 4 pages and 2 figures in eps, final versio
Cerebellar resting-state functional connectivity in Parkinson's disease and multiple system atrophy: Characterization of abnormalities and potential for differential diagnosis at the single-patient level
Background: Recent studies using resting-state functional connectivity and machine-learning to distinguish patients with neurodegenerative diseases from other groups of subjects show promising results. This approach has not been tested to discriminate between Parkinson's disease (PD) and multiple system atrophy (MSA) patients. Objectives: Our first aim is to characterize possible abnormalities in resting-state functional connectivity between the cerebellum and a set of intrinsic-connectivity brain networks and between the cerebellum and different regions of the striatum in PD and MSA. The second objective of this study is to assess the potential of cerebellar connectivity measures to distinguish between PD and MSA patients at the single-patient level. Methods: Fifty-nine healthy controls, 62 PD patients, and 30 MSA patients underwent resting-state functional MRI with a 3T scanner. Independent component analysis and dual regression were used to define seven restingstate networks of interest. To assess striatal connectivity, a seed-to-voxel approach was used after dividing the striatum into six regions bilaterally. Measures of cerebellar-brain network and cerebellar-striatal connectivity were then used as features in a support vector machine to discriminate between PD and MSA patients. Results: MSA patients displayed reduced cerebellar connectivity with different brain networks and with the striatum compared with PD patients and with controls. The classification procedure achieved an overall accuracy of 77.17% with 83.33% of the MSA subjects and 74.19% of the PD patients correctly classified. Conclusion: Our findings suggest that measures of cerebellar functional connectivity have the potential to distinguish between PD and MSA patients
Initial operation of the International Gravitational Event Collaboration
The International Gravitational Event Collaboration, IGEC, is a coordinated
effort by research groups operating gravitational wave detectors working
towards the detection of millisecond bursts of gravitational waves. Here we
report on the current IGEC resonant bar observatory, its data analysis
procedures, the main properties of the first exchanged data set. Even though
the available data set is not complete, in the years 1997 and 1998 up to four
detectors were operating simultaneously. Preliminary results are mentioned.Comment: 8 pages, 2 figures, 3 tables; Proceeding of the GWDAW'99. Submitted
to the International Journal of Modern Physic
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