4,236 research outputs found
Graphene Ripples as a Realization of a Two-Dimensional Ising Model: A Scanning Tunneling Microscope Study
Ripples in pristine freestanding graphene naturally orient themselves in an
array that is alternately curved-up and curved-down; maintaining an average
height of zero. Using scanning tunneling microscopy (STM) to apply a local
force, the graphene sheet will reversibly rise and fall in height until the
height reaches 60-70 percent of its maximum at which point a sudden, permanent
jump occurs. We successfully model the ripples as a spin-half Ising magnetic
system, where the height of the graphene is the spin. The permanent jump in
height, controlled by the tunneling current, is found to be equivalent to an
antiferromagnetic-to-ferromagnetic phase transition. The thermal load
underneath the STM tip alters the local tension and is identified as the
responsible mechanism for the phase transition. Four universal critical
exponents are measured from our STM data, and the model provides insight into
the statistical role of graphenes unusual negative thermal expansion
coefficient.Comment: 12 pages, 5 figures, 1 tabl
Generalised Factorial Moments and QCD Jets
{ In this paper we present a natural and comprehensive generalisation of the
standard factorial moments (\clFq) analysis of a multiplicity distribution.
The Generalised Factorial Moments are defined for all in the complex plane
and, as far as the negative part of its spectrum is concerned, could be useful
for the study of infrared structure of the Strong Interactions Theory of high
energy interactions (LEP multiplicity distribution under the ). The
QCD calculation of the Generalised Factorial Moments for negative is
performed in the double leading log accuracy and is compared to OPAL
experimental data. The role played by the infrared cut-off of the model is
discussed and illustrated with a Monte Carlo calculation. }Comment: 11pages 4 figures uuencode, LATEC, INLN 94/
Nonlinear rheology of Laponite suspensions under an external drive
We investigate the nonlinear rheological behavior of colloidal suspensions of
Laponite, a synthetic clay, driven by a steady and homogeneous shear strain. We
show that the external drive leads to a drastic slowing down of the aging
dynamics or even, in some cases, in the rejuvenation of the system. Under
shear, after a surprisingly long time, the spontaneous aging process observed
at rest is suppressed. The system then reaches a non-equilibrium stationary
state, characterised by a complex viscosity depending on the applied shear
rate. In addition, the glass exhibits a non-Newtonian shear-thinning behavior.
These rheological behaviors confirm recent numerical and theoretical
predictions.Comment: 17 pages, Latex, 6 figures, figures included in the body of the text,
to appear in Journal of Rheolog
Image Restoration Using Functional and Anatomical Information Fusion with Application to SPECT-MRI Images
Image restoration is usually viewed as an ill-posed problem in image processing, since there is no unique solution associated with it. The quality of restored image closely depends on the constraints imposed of the characteristics of the solution. In this paper, we propose an original extension of the NAS-RIF restoration technique by using information fusion as prior information with application in SPECT medical imaging. That extension allows the restoration process to be constrained by efficiently incorporating, within the NAS-RIF method, a regularization term which stabilizes the inverse solution. Our restoration method is constrained by anatomical information extracted from a high resolution anatomical procedure such as magnetic resonance imaging (MRI). This structural anatomy-based regularization term uses the result of an unsupervised Markovian segmentation obtained after a preliminary registration step between the MRI and SPECT data volumes from each patient. This method was successfully tested on 30 pairs of brain MRI and SPECT acquisitions from different subjects and on Hoffman and Jaszczak SPECT phantoms. The experiments demonstrated that the method performs better, in terms of signal-to-noise ratio, than a classical supervised restoration approach using a Metz filter
Interpretation of Light-Quenching Factor Measurements
We observe that the pattern of the quenching factors for scintillation light
from various ions, recently studied in in connection with dark matter
detectors, can be understood as a saturation phenomenon in which the light
output is simply proportional to track length, independent of the ion and its
energy. This observation is in accord with the high dE/dx limit of Birks' law.
It suggests a simple model for the intrinsic resolution of light detectors for
low energy ions, which we briefly discuss.Comment: Seven pages, seven figures, some with colo
Rfx6 Maintains the Functional Identity of Adult Pancreatic ÎČ Cells.
SummaryIncreasing evidence suggests that loss of ÎČ cell characteristics may cause insulin secretory deficiency in diabetes, but the underlying mechanisms remain unclear. Here, we show that Rfx6, whose mutation leads to neonatal diabetes in humans, is essential to maintain key features of functionally mature ÎČ cells in mice. Rfx6 loss in adult ÎČ cells leads to glucose intolerance, impaired ÎČ cell glucose sensing, and defective insulin secretion. This is associated with reduced expression of core components of the insulin secretion pathway, including glucokinase, the Abcc8/SUR1 subunit of KATP channels and voltage-gated Ca2+ channels, which are direct targets of Rfx6. Moreover, Rfx6 contributes to the silencing of the vast majority of âdisallowedâ genes, a group usually specifically repressed in adult ÎČ cells, and thus to the maintenance of ÎČ cell maturity. These findings raise the possibility that changes in Rfx6 expression or activity may contribute to ÎČ cell failure in humans
Energy Conservation Constraints on Multiplicity Correlations in QCD Jets
We compute analytically the effects of energy conservation on the
self-similar structure of parton correlations in QCD jets. The calculations are
performed both in the constant and running coupling cases. It is shown that the
corrections are phenomenologically sizeable. On a theoretical ground, energy
conservation constraints preserve the scaling properties of correlations in QCD
jets beyond the leading log approximation.Comment: 11 pages, latex, 5 figures, .tar.gz version avaliable on
ftp://www.inln.unice.fr
Entanglement of a Mesoscopic Field with an Atom induced by Photon Graininess in a Cavity
We observe that a mesoscopic field made of several tens of microwave photons
exhibits quantum features when interacting with a single Rydberg atom in a
high-Q cavity. The field is split into two components whose phases differ by an
angle inversely proportional to the square root of the average photon number.
The field and the atomic dipole are phase-entangled. These manifestations of
photon graininess vanish at the classical limit. This experiment opens the way
to studies of large Schrodinger cat states at the quantum-classical boundary
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