1,229 research outputs found
System among the corticosteroids: specificity and molecular dynamics.
Fil: Brookes, Jennifer C.. University College London; Estados UnidosFil: Galigniana, Mario Daniel. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Instituto de BiologĂa y Medicina Experimental (i); ArgentinaFil: Harker, Anthony H.. University College London; Estados UnidosFil: Stoneham, A. Marshall. University College London; Estados UnidosFil: Vinson, Gavin P.. Queen Mary University of London; Reino Unid
Importance of quantum tunneling in vacancy-hydrogen complexes in diamond
Our ab initio calculations of the hyperfine parameters for negatively charged vacancy-hydrogen and nitrogen-vacancy-hydrogen complexes in diamond compare static defect models and models which account for the quantum tunneling behavior of hydrogen. The static models give rise to hyperfine splittings that are inconsistent with the experimental electron paramagnetic resonance data. In contrast, the hyperfine parameters for the quantum dynamical models are in agreement with the experimental observations. We show that the quantum motion of the proton is crucial to the prediction of symmetry and hyperfine constants for two simple defect centers in diamond. Static a priori methods fail for these systems
Prospects for detecting the 21cm forest from the diffuse intergalactic medium with LOFAR
We discuss the feasibility of the detection of the 21cm forest in the diffuse
IGM with the radio telescope LOFAR. The optical depth to the 21cm line has been
derived using simulations of reionization which include detailed radiative
transfer of ionizing photons. We find that the spectra from reionization models
with similar total comoving hydrogen ionizing emissivity but different
frequency distribution look remarkably similar. Thus, unless the reionization
histories are very different from each other (e.g. a predominance of UV vs.
x-ray heating) we do not expect to distinguish them by means of observations of
the 21cm forest. Because the presence of a strong x-ray background would make
the detection of 21cm line absorption impossible, the lack of absorption could
be used as a probe of the presence/intensity of the x-ray background and the
thermal history of the universe. Along a random line of sight LOFAR could
detect a global suppression of the spectrum from z>12, when the IGM is still
mostly neutral and cold, in contrast with the more well-defined, albeit broad,
absorption features visible at lower redshift. Sharp, strong absorption
features associated with rare, high density pockets of gas could be detected
also at z~7 along preferential lines of sight.Comment: 12 pages, 13 figures. MNRAS, in pres
An isolated Dirac cone on the surface of ternary tetradymite-like topological insulators
We have extended the search for topological insulators to the ternary
tetradymite-like compounds M2X2Y (M = Bi or Sb; X and Y = S, Se or Te), which
are variations of the well-known binary compounds Bi2Se3 and Bi2Te3. Our
first-principles computations suggest that five existing compounds are strong
topological insulators with a single Dirac cone on the surface. In particular,
stoichiometric Bi2Se2S, Sb2Te2Se and Sb2Te2S are predicted to have an isolated
Dirac cone on their naturally cleaved surface. This finding paves the way for
the realization of the topological transport regime.Comment: 4 pages, 3 figure
Fast Large-Scale Reionization Simulations
We present an efficient method to generate large simulations of the Epoch of
Reionization (EoR) without the need for a full 3-dimensional radiative transfer
code. Large dark-matter-only simulations are post-processed to produce maps of
the redshifted 21cm emission from neutral hydrogen. Dark matter haloes are
embedded with sources of radiation whose properties are either based on
semi-analytical prescriptions or derived from hydrodynamical simulations. These
sources could either be stars or power-law sources with varying spectral
indices. Assuming spherical symmetry, ionized bubbles are created around these
sources, whose radial ionized fraction and temperature profiles are derived
from a catalogue of 1-D radiative transfer experiments. In case of overlap of
these spheres, photons are conserved by redistributing them around the
connected ionized regions corresponding to the spheres. The efficiency with
which these maps are created allows us to span the large parameter space
typically encountered in reionization simulations. We compare our results with
other, more accurate, 3-D radiative transfer simulations and find excellent
agreement for the redshifts and the spatial scales of interest to upcoming 21cm
experiments. We generate a contiguous observational cube spanning redshift 6 to
12 and use these simulations to study the differences in the reionization
histories between stars and quasars. Finally, the signal is convolved with the
LOFAR beam response and its effects are analyzed and quantified. Statistics
performed on this mock data set shed light on possible observational strategies
for LOFAR.Comment: 18 pages, 21 figures, submitted to MNRAS For high-resolution images
follow "http://www.astro.rug.nl/~thomas/eormap.pdf
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