7,637 research outputs found
Real trajectories in the semiclassical coherent state propagator
The semiclassical approximation to the coherent state propagator requires
complex classical trajectories in order to satisfy the associated boundary
conditions, but finding these trajectories in practice is a difficult task that
may compromise the applicability of the approximation. In this work several
approximations to the coherent state propagator are derived that make use only
of real trajectories, which are easier to handle and have a more direct
physical interpretation. It is verified in a particular example that these real
trajectories approximations may have excellent accuracy.Comment: 19 pages, 6 figure
Quenching of magnetic excitations in single adsorbates at surfaces: Mn on CuN/Cu(100)
The lifetimes of spin excitations of Mn adsorbates on CuN/Cu(100) are
computed from first-principles. The theory is based on a strong-coupling
T-matrix approach that evaluates the decay of a spin excitation due to
electron-hole pair creation. Using a previously developed theory [Phys. Rev.
Lett. {\bf 103}, 176601 (2009) and Phys. Rev. B {\bf 81}, 165423 (2010)], we
compute the excitation rates by a tunneling current for all the Mn spin states.
A rate equation approach permits us to simulate the experimental results by
Loth and co-workers [Nat. Phys. {\bf 6}, 340 (2010)] for large tunnelling
currents, taking into account the finite population of excited states. Our
simulations give us insight into the spin dynamics, in particular in the way
polarized electrons can reveal the existence of an excited state population. In
addition, it reveals that the excitation process occurs in a way very different
from the deexcitation one. Indeed, while excitation by tunnelling electrons
proceeds via the s and p electrons of the adsorbate, deexcitation mainly
involves the d electrons
Fermion Helicity Flip in Weak Gravitational Fields
The helicity flip of a spin- Dirac particle
interacting gravitationally with a scalar field is analyzed in the context of
linearized quantum gravity. It is shown that massive fermions may have their
helicity flipped by gravity, in opposition to massless fermions which preserve
their helicity.Comment: RevTeX 3.0, 8 pages, 3 figures (available upon request), Preprint
IFT-P.013/9
A Bayesian estimate of the CMB-large-scale structure cross-correlation
Evidences for late-time acceleration of the Universe are provided by multiple
probes, such as Type Ia supernovae, the cosmic microwave background (CMB) and
large-scale structure (LSS). In this work, we focus on the integrated
Sachs--Wolfe (ISW) effect, i.e., secondary CMB fluctuations generated by
evolving gravitational potentials due to the transition between, e.g., the
matter and dark energy (DE) dominated phases. Therefore, assuming a flat
universe, DE properties can be inferred from ISW detections. We present a
Bayesian approach to compute the CMB--LSS cross-correlation signal. The method
is based on the estimate of the likelihood for measuring a combined set
consisting of a CMB temperature and a galaxy contrast maps, provided that we
have some information on the statistical properties of the fluctuations
affecting these maps. The likelihood is estimated by a sampling algorithm,
therefore avoiding the computationally demanding techniques of direct
evaluation in either pixel or harmonic space. As local tracers of the matter
distribution at large scales, we used the Two Micron All Sky Survey (2MASS)
galaxy catalog and, for the CMB temperature fluctuations, the ninth-year data
release of the Wilkinson Microwave Anisotropy Probe (WMAP9). The results show a
dominance of cosmic variance over the weak recovered signal, due mainly to the
shallowness of the catalog used, with systematics associated with the sampling
algorithm playing a secondary role as sources of uncertainty. When combined
with other complementary probes, the method presented in this paper is expected
to be a useful tool to late-time acceleration studies in cosmology.Comment: 21 pages, 15 figures, 4 tables. We extended the previous analyses
including WMAP9 Q, V and W channels, besides the ILC map. Updated to match
accepted ApJ versio
A new correlator in quantum spin chains
We propose a new correlator in one-dimensional quantum spin chains, the
Emptiness Formation Probability (EFP). This is a natural generalization
of the Emptiness Formation Probability (EFP), which is the probability that the
first spins of the chain are all aligned downwards. In the EFP we let
the spins in question be separated by sites. The usual EFP corresponds to
the special case when , and taking allows us to quantify non-local
correlations. We express the EFP for the anisotropic XY model in a
transverse magnetic field, a system with both critical and non-critical
regimes, in terms of a Toeplitz determinant. For the isotropic XY model we find
that the magnetic field induces an interesting length scale.Comment: 6 pages, 1 figur
Oxygen clamps in gold nanowires
We investigate how the insertion of an oxygen atom in an atomically thin gold
nanowire can affect its rupture. We find, using ab initio total energy density
functional theory calculations, that O atoms when inserted in gold nanowires
form not only stable but also very strong bonds, in such a way that they can
extract atoms from a stable tip, serving in this way as a clamp that could be
used to pull a string of gold atoms.Comment: 4 pages; 4 figure
Effects of Different Levels of Energy Supplementation on Growth of Grazing Holstein Heifers
Growth of Holstein heifers was studied for 1987 grazing season to evaluate pasture-drylot systems for raising dairy replacements. Orchardgrass (Dactylis glomerata L), tall fescue (Festuca arundinacea Schreb), bluegrass (Poa pratensis L) and white (Trifolium repens L) and red clover (Trifolium pratense, L) were the principal components of the grass-legume mixture in six paddocks of 1.5 ha each. Carrying capacity was estimated at six animals/ha (3.1 animal units) and excess forage was harvested as hay to control forage growth and quality. Grazing days were 140 days. None or 2.0 kg/d of cracked corn were fed in addition to grazing in a changeover design with repeated measurements. Alternating supplementation between groups did not affect body weight gains, wither height or heart girth although average daily gain was greater when corn was fed (837 vs 581 g/d)
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