14,947 research outputs found
Quantum spin Hall phase in multilayer graphene
The so called quantum spin Hall phase is a topologically non trivial
insulating phase that is predicted to appear in graphene and graphene-like
systems. In this work we address the question of whether this topological
property persists in multilayered systems. We consider two situations: purely
multilayer graphene and heterostructures where graphene is encapsulated by
trivial insulators with a strong spin-orbit coupling. We use a four orbital
tight-binding model that includes the full atomic spin-orbit coupling and we
calculate the topological invariant of the bulk states as well as the
edge states of semi-infinite crystals with armchair termination. For
homogeneous multilayers we find that even when the spin-orbit interaction opens
a gap for all the possible stackings, only those with odd number of layers host
gapless edge states while those with even number of layers are trivial
insulators. For the heterostructures where graphene is encapsulated by trivial
insulators, it turns out that the interlayer coupling is able to induce a
topological gap whose size is controlled by the spin-orbit coupling of the
encapsulating materials, indicating that the quantum spin Hall phase can be
induced by proximity to trivial insulators.Comment: 7 pages, 6 figure
Real space mapping of topological invariants using artificial neural networks
Topological invariants allow to characterize Hamiltonians, predicting the
existence of topologically protected in-gap modes. Those invariants can be
computed by tracing the evolution of the occupied wavefunctions under twisted
boundary conditions. However, those procedures do not allow to calculate a
topological invariant by evaluating the system locally, and thus require
information about the wavefunctions in the whole system. Here we show that
artificial neural networks can be trained to identify the topological order by
evaluating a local projection of the density matrix. We demonstrate this for
two different models, a 1-D topological superconductor and a 2-D quantum
anomalous Hall state, both with spatially modulated parameters. Our neural
network correctly identifies the different topological domains in real space,
predicting the location of in-gap states. By combining a neural network with a
calculation of the electronic states that uses the Kernel Polynomial Method, we
show that the local evaluation of the invariant can be carried out by
evaluating a local quantity, in particular for systems without translational
symmetry consisting of tens of thousands of atoms. Our results show that
supervised learning is an efficient methodology to characterize the local
topology of a system.Comment: 9 pages, 6 figure
Surface magnetism in ZnO/Co3O4 mixtures
We recently reported the observation of room temperature ferromagnetism in
mixtures of ZnO and Co3O4 despite the diamagnetic and antiferromagnetic
character of these oxides respectively. Here we present a detailed study on the
electronic structure of this material in order to account for this unexpected
ferromagnetism. Electrostatic interactions between both oxides lead to a
dispersion of Co3O4 particles over the surface of ZnO larger ones. As a
consequence, the reduction of Co+3 to Co2+ at the particle surface takes place
as evidenced by XAS measurements and optical spectrocopy. This reduction allows
to xplain the observed ferromagnetic signal within the well established
theories of magnetism.Comment: Accepted in Journal of Applied Physic
Upconversion cooling of Er-doped low-phonon fluorescent solids
We report on a novel mechanism for laser cooling of fluorescent solids based
on infrared-to-visible upconversion often found in rare-earth-doped low-phonon
materials. This type of optical cooling presents some advantages with regards
to conventional anti-Stokes cooling. Among them, it allows to obtain cooling in
a broader range of frequencies around the barycenter of the infrared emitting
band.Comment: 4 pages, 1 figur
COLLABORATIVE NAVIGATION SIMULATION TOOL USING KALMAN FILTER WITH IMPLICIT CONSTRAINTS
Collaborative Positioning (CP) is a networked positioning technique in which different multi-sensor systems (nodes) enhance the accuracy and precision of these navigation solutions by performing measurements or by sharing information (links) between each other. The wide spectrum of available sensors that are used in these complex scenarios bring the necessity to analyze the sensibility of the system to different configurations in order to find optimal solutions. In this paper, we discuss the implementation and evaluation of a simulation tool that allows us to study these questions. The simulation tool is successfully implemented as a plane based localization problem, in which the sensor measurements are fused in a Collaborative Extended Kalman Filter (C-EKF) algorithm with implicit constraints. Using a real urban scenario with three vehicles equipped with various positioning sensors, the impact of the sensor configuration is investigated and discussed by intensive Monte Carlo simulations. The results show the influence of the laser scanner measurements on the accuracy and precision of the estimation, and the increased performance of the collaborative navigation techniques with respect to the single vehicle method
Spanish study of anticoagulation in haemodialysis
This study's objectives were to determine which anticoagulation
methods are commonly used in patients who are undergoing haemodialysis (HD) in
Spain, on what criteria do they depend, and the consequences arising from their
use. MATERIAL AND METHOD: Ours was a cross-sectional study based on two types of
surveys: a "HD Centre Survey" and a "Patient Survey". The first survey was
answered by 87 adult HD units serving a total of 6093 patients, as well as 2
paediatric units. Among these units, 48.3% were part of the public health system
and the remaining 51.7% units were part of the private health system. The patient
survey analysed 758 patients who were chosen at random from among the
aforementioned 78 HD units. RESULTs: A) HD Centre Survey: The majority of adult
HD units (n=61, 70.2%) used both kinds of heparin, 19 of them (21.8%) only used
LMWH and 7 of them (8%) only used UFH. The most frequently applied criteria for
the use of LMWH were medical indications (83.3% of HD units) and ease of
administration (29.5%). The most frequently used methods for adjusting the dosage
were clotting of the circuit (88.2% of units), bleeding of the vascular access
after disconnection (75.3%), and patient weight (57.6%). B) Patient Survey: The
distribution of the types of heparin used was: UFH: 44.1%, LMWH: 51.5%, and
dialysis without heparin in 4.4% of patients. LMWH was more frequently used in
public medical centres (64.2% of patients) than in private medical centres
(46.1%) (P<.001). LMWH was more frequently used in on-line haemodiafiltration
(HF) than in high-flux HD (P<.001). Antiplatelet agents were given to 45.5% of
patients, oral anticoagulants to 18.4% of patients, and both to 5% of patients.
Additionally, 4.4% of patients had suffered bleeding complications during the
previous week, and 1.9% of patients suffered thrombotic complications. Bleeding
complications were more frequent in patients with oral anticoagulants (P=.001),
although there was no association between the type of heparin and the occurrence
of bleeding or thrombotic complications. CONCLUSIONS: We are able to conclude
that there is a great amount of disparity in the criteria used for the medical
prescription of anticoagulation in HD. It is advisable that each HD unit revise
their own results as well as those from other centres, and possibly to create an
Anticoagulation Guide in Haemodialysis
Topological phase transitions between chiral and helical spin textures in a lattice with spin-orbit coupling and a magnetic field
We consider the combined effects of large spin-orbit couplings and a
perpendicular magnetic field in a 2D honeycomb fermionic lattice. This system
provides an elegant setup to generate versatile spin textures propagating along
the edge of a sample. The spin-orbit coupling is shown to induce topological
phase transitions between a helical quantum spin Hall phase and a chiral
spin-imbalanced quantum Hall state. Besides, we find that the spin orientation
of a single topological edge state can be tuned by a Rashba spin-orbit
coupling, opening an interesting route towards quantum spin manipulation. We
discuss the possible realization of our results using cold atoms trapped in
optical lattices, where large synthetic magnetic fields and spin-orbit
couplings can be engineered and finely tuned. In particular, this system would
lead to the observation of a time-reversal-symmetry-broken quantum spin Hall
phase.Comment: 8 pages, 3 figures, Accepted in Europhys. Lett. (Dec 2011
Neuronal Metabolism and Neuroprotection: Neuroprotective Effect of Fingolimod on Menadione-Induced Mitochondrial Damage
Imbalance in the oxidative status in neurons, along with mitochondrial damage, are common characteristics in some neurodegenerative diseases. The maintenance in energy production is crucial to face and recover from oxidative damage, and the preservation of different sources of energy production is essential to preserve neuronal function. Fingolimod phosphate is a drug with neuroprotective and antioxidant actions, used in the treatment of multiple sclerosis. This work was performed in a model of oxidative damage on neuronal cell cultures exposed to menadione in the presence or absence of fingolimod phosphate. We studied the mitochondrial function, antioxidant enzymes, protein nitrosylation, and several pathways related with glucose metabolism and glycolytic and pentose phosphate in neuronal cells cultures. Our results showed that menadione produces a decrease in mitochondrial function, an imbalance in antioxidant enzymes, and an increase in nitrosylated proteins with a decrease in glycolysis and glucose-6-phosphate dehydrogenase. All these effects were counteracted when fingolimod phosphate was present in the incubation media. These effects were mediated, at least in part, by the interaction of this drug with its specific S1P receptors. These actions would make this drug a potential tool in the treatment of neurodegenerative processes, either to slow progression or alleviate symptoms
A decision tree to assess short-term mortality after an emergency department visit for an exacerbation of COPD: A cohort study
Background: Creating an easy-to-use instrument to identify predictors of short-term (30/60-day) mortality after an exacerbation of chronic obstructive pulmonary disease (eCOPD) could help clinicians choose specific measures of medical care to decrease mortality in these patients. The objective of this study was to develop and validate a classification and regression tree (CART) to predict short term mortality among patients evaluated in an emergency department (ED) for an eCOPD. Methods: We conducted a prospective cohort study including participants from 16 hospitals in Spain. COPD patients with an exacerbation attending the emergency department (ED) of any of the hospitals between June 2008 and September 2010 were recruited. Patients were randomly divided into derivation (50 %) and validation samples (50 %). A CART based on a recursive partitioning algorithm was created in the derivation sample and applied to the validation sample. Results: Two thousand four hundred eighty-seven patients, 1252 patients in the derivation sample and 1235 in the validation sample, were enrolled in the study. Based on the results of the univariate analysis, five variables (baseline dyspnea, cardiac disease, the presence of paradoxical breathing or use of accessory inspiratory muscles, age, and Glasgow Coma Scale score) were used to build the CART. Mortality rates 30 days after discharge ranged from 0 % to 55 % in the five CART classes. The lowest mortality rate was for the branch composed of low baseline dyspnea and lack of cardiac disease. The highest mortality rate was in the branch with the highest baseline dyspnea level, use of accessory inspiratory muscles or paradoxical breathing upon ED arrival, and Glasgow score <15. The area under the receiver-operating curve (AUC) in the derivation sample was 0.835 (95 % CI: 0.783, 0.888) and 0.794 (95 % CI: 0.723, 0.865) in the validation sample. CART was improved to predict 60-days mortality risk by adding the Charlson Comorbidity Index, reaching an AUC in the derivation sample of 0.817 (95 % CI: 0.776, 0.859) and 0.770 (95 % CI: 0.716, 0.823) in the validation sample. Conclusions: We identified several easy-to-determine variables that allow clinicians to classify eCOPD patients by short term mortality risk, which can provide useful information for establishing appropriate clinical care. Trial registration: NCT02434536
The X-ray Spectra of Black Hole X-ray Novae in Quiescence as Measured by Chandra
We present Chandra observations of black hole X-ray novae V404 Cyg, A0620-00,
GRO J1655-40 and XTE J1550-564 in quiescence. Their quiescent spectra can be
well fitted by a power-law model with slope . While a coronal
(Raymond-Smith) model is also a statistically acceptable representation of the
spectra, the best fit temperatures of these models is times higher
than that seen in active stellar coronae. These four spectra of quiescent X-ray
novae are all consistent with that expected for accretion via an
advection-dominated accretion flow (ADAF) and inconsistent with that expected
from a stellar corona. This evidence for continued accretion in quiescence
further strengthens the case for the existence of event horizons in black
holes. Both A0620-00 and GRO J1655-40 were fainter than in previous
observations, while V404 Cyg was more luminous and varied by a factor of 2 in a
few ksec. A reanalysis of the X-ray data for XTE J1550-564 shows that (like
V404 Cyg and A0620-00) its luminosity exceeds the maximum prediction of the
coronal model by a large factor. The 0.3-7 keV luminosity of the four sources
studied ranges from erg/s.Comment: 9 pages, 6 figures, accepted for publication in Ap
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