380 research outputs found
Revealing the impact of organic spacers and cavity cations on quasi-2D perovskites via computational simulations
Two-dimensional hybrid lead iodide perovskites based on methylammonium (MA) cation and butylammonium (BA) organic spacerâsuch as BA2MAnâ1PbnI3n+1âare one of the most explored 2D hybrid perovskites in recent years. Correlating the atomistic profile of these systems with their optoelectronic properties is a challenge for theoretical approaches. Here, we employed first-principles calculations via density functional theory to show how the cation partially canceled dipole moments through the NH3+ terminal impact the structural/electronic properties of the PbnI3n+1 sublattices. Even though it is known that at high temperatures, the organic cation assumes a spherical-like configuration due to the rotation of the cations inside the cage, our results discuss the correct relative orientation according to the dipole moments for ab initio simulations at 0 K, correlating well structural and electronic properties with experiments. Based on the combination of relativistic quasiparticle correction and spin-orbit coupling, we found that the MA horizontal-like configuration concerning the inorganic sublattice surface leads to the best relationship between calculated and experimental gap energy throughout n = 1, 2, 3, 4, and 5 number of layers. Conversely, the dipole moments cancellation (as in BA-MA aligned-like configuration) promotes the closing of the gap energies through an electron depletion mechanism. We found that the anisotropy â isotropy optical absorption conversion (as a bulk convergence) is achieved only for the MA horizontal-like configuration, which suggests that this configuration contribution is the majority in a scenario under temperature effects
Renormalization of the Inverse Square Potential
The quantum-mechanical D-dimensional inverse square potential is analyzed
using field-theoretic renormalization techniques. A solution is presented for
both the bound-state and scattering sectors of the theory using cutoff and
dimensional regularization. In the renormalized version of the theory, there is
a strong-coupling regime where quantum-mechanical breaking of scale symmetry
takes place through dimensional transmutation, with the creation of a single
bound state and of an energy-dependent s-wave scattering matrix element.Comment: 5 page
Inside the Outbreak of the 2009 Influenza A (H1N1)v Virus in Mexico
Influenza viruses pose a threat to human health because of their potential to cause global disease. Between mid March and mid April a pandemic influenza A virus emerged in Mexico. This report details 202 cases of infection of humans with the 2009 influenza A virus (H1N1)v which occurred in Mexico City as well as the spread of the virus throughout the entire country.From May 1st to May 5th nasopharyngeal swabs, derived from 751 patients, were collected at 220 outpatient clinics and 28 hospitals distributed throughout Mexico City. Analysis of samples using real time RT-PCR revealed that 202 patients out of the 751 subjects (26.9%) were confirmed to be infected with the new virus. All confirmed cases of human infection with the strain influenza (H1N1)v suffered respiratory symptoms. The greatest number of confirmed cases during the outbreak of the 2009 influenza A (H1N1)v were seen in neighbourhoods on the northeast side of Mexico City including Iztapalapa, Gustavo A. Madero, Iztacalco, and Tlahuac which are the most populated areas in Mexico City. Using these data, together with data reported by the Mexican Secretariat of Health (MSH) to date, we plot the course of influenza (H1N1)v activity throughout Mexico.Our data, which is backed up by MSH data, show that the greatest numbers of the 2009 influenza A (H1N1) cases were seen in the most populated areas. We speculate on conditions in Mexico which may have sparked this flu pandemic, the first in 41 years. We accept the hypothesis that high population density and a mass gathering which took in Iztapalapa contributed to the rapid spread of the disease which developed in three peaks of activity throughout the Country
Searching for TeV Dark Matter in Irregular dwarf galaxies with HAWC Observatory
We present the results of dark matter (DM) searches in a sample of 31 dwarf
irregular (dIrr) galaxies within the field of view of the HAWC Observatory.
dIrr galaxies are DM dominated objects, which astrophysical gamma-ray emission
is estimated to be negligible with respect to the secondary gamma-ray flux
expected by annihilation or decay of Weakly Interacting Massive Particles
(WIMPs). While we do not see any statistically significant DM signal in dIrr
galaxies, we present the exclusion limits () for annihilation
cross-section and decay lifetime for WIMP candidates with masses between
and . Exclusion limits from dIrr galaxies are relevant and
complementary to benchmark dwarf Spheroidal (dSph) galaxies. In fact, dIrr
galaxies are targets kinematically different from benchmark dSph, preserving
the footprints of different evolution histories. We compare the limits from
dIrr galaxies to those from ultrafaint and classical dSph galaxies previously
observed with HAWC. We find that the contraints are comparable to the limits
from classical dSph galaxies and orders of magnitude weaker than
the ultrafaint dSph limits.Comment: 22 pages, 11 figures, 3 table
Gamma/Hadron Separation with the HAWC Observatory
The High Altitude Water Cherenkov (HAWC) gamma-ray observatory observesatmospheric showers produced by incident gamma rays and cosmic rays with energyfrom 300 GeV to more than 100 TeV. A crucial phase in analyzing gamma-raysources using ground-based gamma-ray detectors like HAWC is to identify theshowers produced by gamma rays or hadrons. The HAWC observatory records roughly25,000 events per second, with hadrons representing the vast majority() of these events. The standard gamma/hadron separation technique inHAWC uses a simple rectangular cut involving only two parameters. This workdescribes the implementation of more sophisticated gamma/hadron separationtechniques, via machine learning methods (boosted decision trees and neuralnetworks), and summarizes the resulting improvements in gamma/hadron separationobtained in HAWC.<br
The TeV Sun Rises: Discovery of Gamma rays from the Quiescent Sun with HAWC
We report the first detection of a TeV gamma-ray flux from the solar disk
(6.3), based on 6.1 years of data from the High Altitude Water
Cherenkov (HAWC) observatory. The 0.5--2.6 TeV spectrum is well fit by a power
law, dN/dE = , with TeV cm s and . The flux
shows a strong indication of anticorrelation with solar activity. These results
extend the bright, hard GeV emission from the disk observed with Fermi-LAT,
seemingly due to hadronic Galactic cosmic rays showering on nuclei in the solar
atmosphere. However, current theoretical models are unable to explain the
details of how solar magnetic fields shape these interactions. HAWC's TeV
detection thus deepens the mysteries of the solar-disk emission.Comment: 15 pages, 8 figures including supplementary material. Accepted for
publication in Physical Review Letter
Limits on the Diffuse Gamma-Ray Background above 10 TeV with HAWC
The high-energy Diffuse Gamma-Ray Background (DGRB) is expected to be
produced by unresolved isotropically distributed astrophysical objects,
potentially including dark matter annihilation or decay emissions in galactic
or extragalactic structures. The DGRB has only been observed below 1 TeV; above
this energy, upper limits have been reported. Observations or stringent limits
on the DGRB above this energy could have significant multi-messenger
implications, such as constraining the origin of TeV-PeV astrophysical
neutrinos detected by IceCube. The High Altitude Water Cherenkov (HAWC)
Observatory, located in central Mexico at 4100 m above sea level, is sensitive
to gamma rays from a few hundred GeV to several hundred TeV and continuously
observes a wide field-of-view (2 sr). With its high-energy reach and large area
coverage, HAWC is well-suited to notably improve searches for the DGRB at TeV
energies. In this work, strict cuts have been applied to the HAWC dataset to
better isolate gamma-ray air showers from background hadronic showers. The
sensitivity to the DGRB was then verified using 535 days of Crab data and Monte
Carlo simulations, leading to new limits above 10 TeV on the DGRB as well as
prospective implications for multi-messenger studies.Comment: 8 pages, 3 figure
Gamma-ray Emission from Classical Nova V392 Per: Measurements from Fermi and HAWC
This paper reports on the -ray properties of the 2018 Galactic novaV392 Per, spanning photon energies 0.1 GeV to 100 TeV by combiningobservations from the Fermi Gamma-ray Space Telescope and the HAWC Observatory.In one of the most rapidly evolving -ray signals yet observed for anova, GeV rays with a power law spectrum with index were detected over eight days following V392 Per's optical maximum. HAWCobservations constrain the TeV -ray signal during this time and alsobefore and after. We observe no statistically significant evidence of TeV-ray emission from V392 Per, but present flux limits. Tests of theextension of the Fermi/LAT spectrum to energies above 5 TeV are disfavored by 2standard deviations (95\%) or more. We fit V392 Per's GeV rays withhadronic acceleration models, incorporating optical observations, and comparethe calculations with HAWC limits.<br
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