426 research outputs found
Spin Gaps in a Frustrated Heisenberg model for CaVO
I report results of a density matrix renormalization group (DMRG) study of a
model for the two dimensional spin-gapped system CaVO. This study
represents the first time that DMRG has been used to study a two dimensional
system on large lattices, in this case as large as , allowing
extrapolation to the thermodynamic limit. I present a substantial improvement
to the DMRG algorithms which makes these calculations feasible.Comment: 10 pages, with 4 Postscript figure
West Nile virus outbreak among horses in New York State, 1999 and 2000.
West Nile (WN) virus was identified in the Western Hemisphere in 1999. Along with human encephalitis cases, 20 equine cases of WN virus were detected in 1999 and 23 equine cases in 2000 in New York. During both years, the equine cases occurred after human cases in New York had been identified
Anything You Can Do, You Can Do Better: Neural Substrates of Incentive-Based Performance Enhancement
Performance-based pay schemes in many organizations share the fundamental assumption that the performance level for a given task will increase as a function of the amount of incentive provided. Consistent with this notion, psychological studies have demonstrated that expectations of reward can improve performance on a plethora of different cognitive and physical tasks, ranging from problem solving to the voluntary regulation of heart rate. However, much less is understood about the neural mechanisms of incentivized performance enhancement. In particular, it is still an open question how brain areas that encode expectations about reward are able to translate incentives into improved performance across fundamentally different cognitive and physical task requirements
Infinitesimal incommensurate stripe phase in an axial next-nearest-neighbor Ising model in two dimensions
An axial next-nearest-neighbor Ising (ANNNI) model is studied by using the
non-equilibrium relaxation method. We find that the incommensurate stripe phase
between the ordered phase and the paramagnetic phase is negligibly narrow or
may vanish in the thermodynamic limit. The phase transition is the second-order
transition if approached from the ordered phase, and it is of the
Kosterlitz-Thouless type if approached from the paramagnetic phase. Both
transition temperatures coincide with each other within the numerical errors.
The incommensurate phase which has been observed previously is a paramagnetic
phase with a very long correlation length (typically ). We could
resolve this phase by treating very large systems (),
which is first made possible by employing the present method.Comment: 12 pages, 10 figures. To appear in Phys.Rev.
Solar Jet Hunter: a citizen science initiative to identify coronal jets in EUV data sets
Context. Solar coronal jets seen in EUV are ubiquitous on the Sun, have been
found in and at the edges of active regions, at the boundaries of coronal
holes, and in the quiet Sun. Jets have various shapes, sizes, brightness,
velocities and duration in time, which complicates their detection by automated
algorithms. So far, solar jets reported in the Heliophysics Event Knowledgebase
(HEK) have been mostly reported by humans looking for them in the data, with
different levels of precision regarding their timing and positions. Aims. We
create a catalogue of solar jets observed in EUV at 304 {\AA} containing
precise and consistent information on the jet timing, position and extent.
Methods. We designed a citizen science project, "Solar Jet Hunter", on the
Zooniverse platform, to analyze EUV observations at 304 {\AA} from the Solar
Dynamic Observatory/Atmospheric Imaging Assembly (SDO/AIA). We created movie
strips for regions of the Sun in which jets have been reported in HEK and ask
the volunteers to 1) confirm the presence of at least one jet in the data and
2) report the timing, position and extent of the jet. Results. We report here
the design of the project and the results obtained after the analysis of data
from 2011 to 2016. 365 "coronal jet" events from HEK served as input for the
citizen science project, equivalent to more than 120,000 images distributed
into 9,689 "movie strips". Classification by the citizen scientists resulted
with only 21% of the data containing a jet, and 883 individual jets being
identified. Conclusions. We demonstrate how citizen science can enhance the
analysis of solar data with the example of Solar Jet Hunter. The catalogue of
jets thus created is publicly available and will enable statistical studies of
jets and related phenomena. This catalogue will also be used as a training set
for machines to learn to recognize jets in further data sets
Cisternal Organization of the Endoplasmic Reticulum during Mitosis
The endoplasmic reticulum (ER) of animal cells is a single, dynamic, and continuous membrane network of interconnected cisternae and tubules spread out throughout the cytosol in direct contact with the nuclear envelope. During mitosis, the nuclear envelope undergoes a major rearrangement, as it rapidly partitions its membrane-bound contents into the ER. It is therefore of great interest to determine whether any major transformation in the architecture of the ER also occurs during cell division. We present structural evidence, from rapid, live-cell, three-dimensional imaging with confirmation from high-resolution electron microscopy tomography of samples preserved by high-pressure freezing and freeze substitution, unambiguously showing that from prometaphase to telophase of mammalian cells, most of the ER is organized as extended cisternae, with a very small fraction remaining organized as tubules. In contrast, during interphase, the ER displays the familiar reticular network of convolved cisternae linked to tubules
Critical quantum chaos and the one dimensional Harper model
We study the quasiperiodic Harper's model in order to give further support
for a possible universality of the critical spectral statistics. At the
mobility edge we numerically obtain a scale-invariant distribution of the bands
, which is closely described by a semi-Poisson curve.
The tail appears when the mobility edge is approached from the
metal while is asymptotically log-normal for the insulator. The
multifractal critical density of states also leads to a sub-Poisson linear
number variance .Comment: 4 pages, 4 eps figure
Holography for chiral scale-invariant models
Deformation of any d-dimensional conformal field theory by a constant null
source for a vector operator of dimension (d + z -1) is exactly marginal with
respect to anisotropic scale invariance, of dynamical exponent z. The
holographic duals to such deformations are AdS plane waves, with z=2 being the
Schrodinger geometry. In this paper we explore holography for such chiral
scale-invariant models. The special case of z=0 can be realized with gravity
coupled to a scalar, and is of particular interest since it is related to a
Lifshitz theory with dynamical exponent two upon dimensional reduction. We show
however that the corresponding reduction of the dual field theory is along a
null circle, and thus the Lifshitz theory arises upon discrete light cone
quantization of an anisotropic scale invariant field theory.Comment: 62 pages; v2, published version, minor improvements and references
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The Short Range RVB State of Even Spin Ladders: A Recurrent Variational Approach
Using a recursive method we construct dimer and nondimer variational ansatzs
of the ground state for the two-legged ladder, and compute the number of dimer
coverings, the energy density and the spin correlation functions. The number of
dimer coverings are given by the Fibonacci numbers for the dimer-RVB state and
their generalization for the nondimer ones. Our method relies on the recurrent
relations satisfied by the overlaps of the states with different lengths, which
can be solved using generating functions. The recurrent relation method is
applicable to other short range systems. Based on our results we make a
conjecture about the bond amplitudes of the 2-leg ladder.Comment: REVTEX file, 32 pages, 10 EPS figures inserted in text with epsf.st
Mapping the spatiotemporal dynamics of calcium signaling in cellular neural networks using optical flow
An optical flow gradient algorithm was applied to spontaneously forming net-
works of neurons and glia in culture imaged by fluorescence optical microscopy
in order to map functional calcium signaling with single pixel resolution.
Optical flow estimates the direction and speed of motion of objects in an image
between subsequent frames in a recorded digital sequence of images (i.e. a
movie). Computed vector field outputs by the algorithm were able to track the
spatiotemporal dynamics of calcium signaling pat- terns. We begin by briefly
reviewing the mathematics of the optical flow algorithm, and then describe how
to solve for the displacement vectors and how to measure their reliability. We
then compare computed flow vectors with manually estimated vectors for the
progression of a calcium signal recorded from representative astrocyte
cultures. Finally, we applied the algorithm to preparations of primary
astrocytes and hippocampal neurons and to the rMC-1 Muller glial cell line in
order to illustrate the capability of the algorithm for capturing different
types of spatiotemporal calcium activity. We discuss the imaging requirements,
parameter selection and threshold selection for reliable measurements, and
offer perspectives on uses of the vector data.Comment: 23 pages, 5 figures. Peer reviewed accepted version in press in
Annals of Biomedical Engineerin
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