6,994 research outputs found
In-gap impurity states as the hallmark of the Quantum Spin Hall phase
We study the different response to an impurity of the two topologically
different phases shown by a two dimensional insulator with time reversal
symmetry, namely, the Quantum Spin Hall and the normal phase. We consider the
case of graphene as a toy model that features the two phases driven,
respectively, by intrinsic spin-orbit coupling and inversion symmetry breaking.
We find that strictly normalizable in-gap impurity states only occur in the
Quantum Spin Hall phase and carry dissipationless current whose quirality is
determined by the spin and pseudospin of the residing electron. Our results
imply that topological order can be unveiled by local probes of defect states.Comment: 5 pages, 3 figure
On the void explanation of the Cold Spot
The integrated Sachs-Wolfe (ISW) contribution induced on the cosmic microwave
background by the presence of a supervoid as the one detected by Szapudi et al.
(2015) is reviewed in this letter in order to check whether it could explain
the Cold Spot (CS) anomaly. Two different models, previously used for the same
purpose, are considered to describe the matter density profile of the void: a
top hat function and a compensated profile produced by a Gaussian potential.
The analysis shows that, even enabling ellipticity changes or different values
for the dark-energy equation of state parameter , the ISW contribution
due to the presence of the void does not reproduce the properties of the CS.
Finally, the probability of alignment between the void and the CS is also
questioned as an argument in favor of a physical connection between these two
phenomena
A wide family of singularity-free cosmological models
In this paper a family of non-singular cylindrical perfect fluid cosmologies
is derived. The equation of state corresponds to a stiff fluid. The family
depends on two independent functions under very simple conditions. A sufficient
condition for geodesic completeness is provided.Comment: 7 pages, RevTeX
Exploring two-spin internal linear combinations for the recovery of the CMB polarization
We present a methodology to recover cosmic microwave background (CMB)
polarization in which the quantity is linearly combined at
different frequencies using complex coefficients. This is the most general
linear combination of the and Stokes parameters which preserves the
physical coherence of the residual contribution on the CMB estimation. The
approach is applied to the internal linear combination (ILC) and the internal
template fitting (ITF) methodologies. The variance of of the resulting map
is minimized to compute the coefficients of the linear combination. One of the
key aspects of this procedure is that it serves to account for a global
frequency-dependent shift of the polarization phase. Although in the standard
case, in which no global E-B transference depending on frequency is expected in
the foreground components, minimizing is
similar to minimizing and separately (as previous methodologies proceed), multiplying
and by different coefficients induces arbitrary changes in the
polarization angle and it does not preserve the coherence between the spinorial
components. The approach is tested on simulations, obtaining a similar residual
level with respect to the one obtained with other implementations of the ILC,
and perceiving the polarization rotation of a toy model with the frequency
dependence of the Faraday rotation.Comment: 14 pages, 8 figures, 2 tables. Accepted for publication in MNRA
Searching for a dipole modulation in the large-scale structure of the Universe
Several statistical anomalies in the CMB temperature anisotropies seem to
defy the assumption of a homogeneous and isotropic universe. In particular, a
dipole modulation has been detected both in WMAP and Planck data. We adapt the
methodology proposed by Eriksen et al. (2007) on CMB data to galaxy surveys,
tracing the large-scale structure. We analyse the NRAO VLA Sky Survey (NVSS)
data at a resolution of ~2 degrees for three different flux thresholds: 2.5,
5.0 and 10.0 mJy respectively. No evidence of a dipole modulation is found.
This result suggests that the origin of the dipole asymmetry found in the CMB
cannot be assigned to secondary anisotropies produced at redshifts around z =
1. However, it could still have been generated at redshifts higher or lower,
such as the integrated Sachs-Wolfe effect produced by the local structures.
Other all-sky surveys, like the infrared WISE catalogue, could help to explore
with a high sensitivity a redshift interval closer than the one probed with
NVSS.Comment: 6 pages, 2 figures. Some minor changes have been done from the
original manuscript. This paper is accepted by MNRA
Analysis of the start to the first hurdle in 110m hurdles at the IAAF World Athletics Championships Beijing 2015
The purpose of this study was to use observational analysis to evaluate the relationships between variables
measured at the start of the men’s 110 hurdles event and race performance itself. Data were obtained for
competitors in 2015 IAAF World Athletics Ch
ampionships, in Beijing, China. The athletes’ start was quantified
by reaction time and time to the first hurdle; their action over the first hurdle was quantified by the take
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off
distance (i.e., the distance from the last step to the first hurdle), the la
nding distance, and the total distance
in the air over the first hurdle. Regression analyses revealed that the combination of one measure of the start
(either reaction time or time to the first hurdle) and the measure of propulsion over the first hurdle (d
istance
in air over the first hurdle) predicted performance (SEE = 0.23 s in the heats, SEE = 0.16 s in the semi
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finals,
SEE = 0.09 s in the finals). In addition, looking at performances in the finals, where all athletes with available
data used a seven
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st
ep approach to the first hurdle, inclusion of stride length data improved the prediction
somewhat (SEE = 0.07 s). The results demonstrate that a combination of a fast start, rapid acceleration, and
strong drive over the first hurdle quantifiably explains a
nd contributes to performance in the men’s 110 m hurdles at the highest level
Spatial and temporal variability of CO2 emisions in soils under conventional tillage and no-till farming
Agricultural soils can act as a carbon sink depending on the soil management practices employed. As a result of this functional duality, soil management systems are present in international documents relating to climate change mitigation. Agricultural practices are responsible for 14% of total greenhouse gas emissions (GHG’s) (MMA, 2009)(1). Conservation agriculture (CA) is one of the most effective agricultural systems for reducing CO2 emissions, as it increases the sequestration of atmospheric carbon in the soil.
In order to assess the performance of CA in terms of CO2 emissions, a field trial was conducted comparing soil derived CO2 fluxes under No-till (NT) farming and under conventional tillage. Three pilot farms were selected in the cereal-growing area of southern Spain, located in Las Cabezas de San Juan (Seville), Carmona (Seville) and Cordoba. Each pilot farm comprises six experimental plots with an approximate area of five hectares; three of the six plots implement CA practices, while the other three use conventional tillage techniques. The subdivision of each tillage system into 3 plots allowed the simultaneous cropping of the three crops of the wheat-sunflower-legume rotation each year.
Results showed that carbon dioxide emissions were 31 to 91% higher in tilled soils than in untilled soils, and that there was a great seasonal variability of CO2 emissions, as weather conditions also differed considerably for the different sampling periods. In all cases, the CO2 fluxes emitted into the atmosphere were always higher when soil was subject to conventional tillage
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