7,210 research outputs found
Exotic Mott phases of the extended t--J model on the checkerboard lattice at commensurate densities
Coulomb repulsion between electrons moving on a frustrated lattice can give
rise, at simple commensurate electronic densities, to exotic insulating phases
of matter. Such a phenomenon is illustrated using an extended t--J model on a
planar pyrochlore lattice for which the work on the quarter-filled case
[cond-mat/0702367] is complemented and extended to 1/8- and 3/8-fillings. The
location of the metal-insulator transition as a function of the Coulomb
repulsion is shown to depend strongly on the sign of the hopping. Quite
generally, the metal-insulator transition is characterized by lattice symmetry
breaking but the nature of the insulating Mott state is more complex than a
simple Charge Density Wave. Indeed, in the limit of large Coulomb repulsion,
the physics can be described in the framework of (extended) quantum
fully-packed loop or dimer models carrying extra spin degrees of freedom.
Various diagonal and off-diagonal plaquette correlation functions are computed
and the low-energy spectra are analyzed in details in order to characterize the
nature of the insulating phases. We provide evidence that, as for an electronic
density of n=1/2 (quarter-filling), the system at or exhibits
also plaquette order by forming a (lattice rotationally-invariant)
Resonating-Singlet-Pair Crystal, although with a quadrupling of the lattice
unit cell (instead of a doubling for ) and a 4-fold degenerate ground
state. Interestingly, qualitative differences with the bosonic analog (e.g.
known to exhibit columnar order at n=1/4) emphasize the important role of the
spin degrees of freedom in e.g. stabilizing plaquette phases w.r.t. rotational
symmetry-breaking phases.Comment: 7 pages, 7 figures Follow-up of cond-mat/070236
Spatio-Temporal Multiway Data Decomposition Using Principal Tensor Analysis on k-Modes: The R Package PTAk
The purpose of this paper is to describe the R package {PTAk and how the spatio-temporal context can be taken into account in the analyses. Essentially PTAk() is a multiway multidimensional method to decompose a multi-entries data-array, seen mathematically as a tensor of any order. This PTAk-modes method proposes a way of generalizing SVD (singular value decomposition), as well as some other well known methods included in the R package, such as PARAFAC or CANDECOMP and the PCAn-modes or Tucker-n model. The example datasets cover different domains with various spatio-temporal characteristics and issues: (i)~medical imaging in neuropsychology with a functional MRI (magnetic resonance imaging) study, (ii)~pharmaceutical research with a pharmacodynamic study with EEG (electro-encephaloegraphic) data for a central nervous system (CNS) drug, and (iii)~geographical information system (GIS) with a climatic dataset that characterizes arid and semi-arid variations. All the methods implemented in the R package PTAk also support non-identity metrics, as well as penalizations during the optimization process. As a result of these flexibilities, together with pre-processing facilities, PTAk constitutes a framework for devising extensions of multidimensional methods such ascorrespondence analysis, discriminant analysis, and multidimensional scaling, also enabling spatio-temporal constraints.
Infinite average lifetime of an unstable bright state in the green fluorescent protein
The time evolution of the fluorescence intensity emitted by well-defined
ensembles of Green Fluorescent Proteins has been studied by using a standard
confocal microscope. In contrast with previous results obtained in single
molecule experiments, the photo-bleaching of the ensemble is well described by
a model based on Levy statistics. Moreover, this simple theoretical model
allows us to obtain information about the energy-scales involved in the aging
process.Comment: 4 pages, 4 figure
How Geometry Controls the Tearing of Adhesive Thin Films on Curved Surfaces
Flaps can be detached from a thin film glued on a solid substrate by tearing
and peeling. For flat substrates, it has been shown that these flaps
spontaneously narrow and collapse in pointy triangular shapes. Here we show
that various shapes, triangular, elliptic, acuminate or spatulate, can be
observed for the tears by adjusting the curvature of the substrate. From
combined experiments and theoretical models, we show that the flap morphology
is governed by simple geometric rules.Comment: 6 pages, 5 figure
Relation between respiratory variations in pulse oximetry plethysmographic waveform amplitude and arterial pulse pressure in ventilated patients.
IntroductionRespiratory variation in arterial pulse pressure is a reliable predictor of fluid responsiveness in mechanically ventilated patients with circulatory failure. The main limitation of this method is that it requires an invasive arterial catheter. Both arterial and pulse oximetry plethysmographic waveforms depend on stroke volume. We conducted a prospective study to evaluate the relationship between respiratory variation in arterial pulse pressure and respiratory variation in pulse oximetry plethysmographic (POP) waveform amplitude.MethodThis prospective clinical investigation was conducted in 22 mechanically ventilated patients. Respiratory variation in arterial pulse pressure and respiratory variation in POP waveform amplitude were recorded simultaneously in a beat-to-beat evaluation, and were compared using a Spearman correlation test and a Bland-Altman analysis.ResultsThere was a strong correlation (r2 = 0.83; P < 0.001) and a good agreement (bias = 0.8 +/- 3.5%) between respiratory variation in arterial pulse pressure and respiratory variation in POP waveform amplitude. A respiratory variation in POP waveform amplitude value above 15% allowed discrimination between patients with respiratory variation in arterial pulse pressure above 13% and those with variation of 13% or less (positive predictive value 100%).ConclusionRespiratory variation in arterial pulse pressure above 13% can be accurately predicted by a respiratory variation in POP waveform amplitude above 15%. This index has potential applications in patients who are not instrumented with an intra-arterial catheter
Josephson-Majorana cycle in topological single-electron hybrid transistors
Charge transport through a small topological superconducting island in
contact with a normal and a superconducting electrode occurs through a cycle
that involves coherent oscillations of Cooper pairs and tunneling in/out the
normal electrode through a Majorana bound state, the Josephson-Majorana cycle.
We illustrate this mechanism by studying the current-voltage characteristics of
a superconductor-topological superconductor-normal metal single-electron
transistor. At low bias and temperature the Josephson-Majorana cycle is the
dominant mechanism for transport. We discuss a three-terminal configuration
where the non-local character of the Majorana bound states is emergent.Comment: 6 pages, 4 figure
Role of the synthesis route on the properties of hybrid LDH-graphene as basic catalysts
Layered double hydroxides (LDH or HT) or their derived mixed oxides present marked acid-base properties useful in catalysis, but they lead to agglomerate inducing a weak accessibility to the active sites. In this study we report the preparation and characterization of HT/Graphene (HT/rGO) nanocomposites as active and selective basic catalysts for the acetone condensation reaction. The graphene high specific surface area and structural compatibility with the HT allowed increasing the number and accessibility of the active sites and activity of this later. Two series of HT/rGO nanocomposites with 0.5 = HT/rGO = 10 mass ratio were prepared by: i) direct HT coprecipitation in the presence of GO; ii) self-assembly of preformed HT with GO. The prepared HT/rGO nanocomposites were dried either in air at 80 °C or freeze-dried. A series of characterizations showed the great influence of the preparation method and HT/rGO mass ratio on both the nanocomposite structure and catalytic activity. An optimum activity was observed for a HT/rGO = 10 catalyst. Particularly, the highest catalytic activity was found in those nanocomposites obtained by coprecipitation and freeze dried (3 times more active than bulk HT) which can be connected to their structure with a better accessibility to the basic sites.Postprint (author's final draft
Spatio-Temporal Multiway Data Decomposition Using Principal Tensor Analysis on k-Modes: The R Package PTAk
The purpose of this paper is to describe the <b>R</b> package {<b>PTAk</b> and how the spatio-temporal context can be taken into account in the analyses. Essentially PTAk() is a multiway multidimensional method to decompose a multi-entries data-array, seen mathematically as a tensor of any order. This PTAk-modes method proposes a way of generalizing SVD (singular value decomposition), as well as some other well known methods included in the <b>R</b> package, such as PARAFAC or CANDECOMP and the PCAn-modes or Tucker-n model. The example datasets cover different domains with various spatio-temporal characteristics and issues: (i)~medical imaging in neuropsychology with a functional MRI (magnetic resonance imaging) study, (ii)~pharmaceutical research with a pharmacodynamic study with EEG (electro-encephaloegraphic) data for a central nervous system (CNS) drug, and (iii)~geographical information system (GIS) with a climatic dataset that characterizes arid and semi-arid variations. All the methods implemented in the <b>R</b> package <b>PTAk</b> also support non-identity metrics, as well as penalizations during the optimization process. As a result of these flexibilities, together with pre-processing facilities, <b>PTAk</b> constitutes a framework for devising extensions of multidimensional methods such ascorrespondence analysis, discriminant analysis, and multidimensional scaling, also enabling spatio-temporal constraints
The various manifestations of collisionless dissipation in wave propagation
The propagation of an electrostatic wave packet inside a collisionless and
initially Maxwellian plasma is always dissipative because of the irreversible
acceleration of the electrons by the wave. Then, in the linear regime, the wave
packet is Landau damped, so that in the reference frame moving at the group
velocity, the wave amplitude decays exponentially with time. In the nonlinear
regime, once phase mixing has occurred and when the electron motion is nearly
adiabatic, the damping rate is strongly reduced compared to the Landau one, so
that the wave amplitude remains nearly constant along the characteristics. Yet,
we show here that the electrons are still globally accelerated by the wave
packet, and, in one dimension, this leads to a non local amplitude dependence
of the group velocity. As a result, a freely propagating wave packet would
shrink, and, therefore, so would its total energy. In more than one dimension,
not only does the magnitude of the group velocity nonlinearly vary, but also
its direction. In the weakly nonlinear regime, when the collisionless damping
rate is still significant compared to its linear value, this leads to an
effective defocussing effect which we quantify, and which we compare to the
self-focussing induced by wave front bowing.Comment: 23 pages, 6 figure
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