16,661 research outputs found
Weak ferromagnetism and spiral spin structures in honeycomb Hubbard planes
Within the Hartree Fock- RPA analysis, we derive the spin wave spectrum for
the weak ferromagnetic phase of the Hubbard model on the honeycomb lattice.
Assuming a uniform magnetization, the polar (optical) and acoustic branches of
the spin wave excitations are determined. The bipartite lattice geometry
produces a q-dependent phase difference between the spin wave amplitudes on the
two sub-lattices. We also find an instability of the uniform weakly magnetized
configuration to a weak antiferromagnetic spiraling spin structure, in the
lattice plane, with wave vector Q along the Gamma-K direction, for electron
densities n>0.6. We discuss the effect of diagonal disorder on both the
creation of electron bound states, enhancement of the density of states, and
the possible relevance of these effects to disorder induced ferromagnetism, as
observed in proton irradiated graphite.Comment: 13 pages, 7 figure
Stable retrograde orbits around the triple system 2001 SN263
The NEA 2001 SN263 is the target of the ASTER MISSION - First Brazilian Deep
Space Mission. Araujo et al. (2012), characterized the stable regions around
the components of the triple system for the planar and prograde cases. Knowing
that the retrograde orbits are expected to be more stable, here we present a
complementary study. We now considered particles orbiting the components of the
system, in the internal and external regions, with relative inclinations
between , i.e., particles with retrograde
orbits. Our goal is to characterize the stable regions of the system for
retrograde orbits, and then detach a preferred region to place the space probe.
For a space mission, the most interesting regions would be those that are
unstable for the prograde cases, but stable for the retrograde cases. Such
configuration provide a stable region to place the mission probe with a
relative retrograde orbit, and, at the same time, guarantees a region free of
debris since they are expected to have prograde orbits. We found that in fact
the internal and external stable regions significantly increase when compared
to the prograde case. For particles with and , we found
that nearly the whole region around Alpha and Beta remain stable. We then
identified three internal regions and one external region that are very
interesting to place the space probe. We present the stable regions found for
the retrograde case and a discussion on those preferred regions. We also
discuss the effects of resonances of the particles with Beta and Gamma, and the
role of the Kozai mechanism in this scenario. These results help us understand
and characterize the stability of the triple system 2001 SN263 when retrograde
orbits are considered, and provide important parameters to the design of the
ASTER mission.Comment: 11 pages, 8 figures. Accepted for publication in MNRAS - 2015 March
1
Antisymmetric tensor propagator with spontaneous Lorentz violation
In this work, we study the spontaneous Lorentz symmetry breaking due to an
antisymmetric 2-tensor field in Minkowski spacetime. For a smooth quadratic
potential, the spectrum of the theory exhibits massless and massive
excitations. We show that the equations of motion for the free field obey some
constraints which lead to the massive mode be non-propagating at leading order.
Besides, there exists a massless mode in the theory which can be identified
with the usual Kalb-Ramond field, carrying only one on-shell degree of freedom.
The same conclusion holds when one analyses the pole structure of its Feynman
propagator. A new complete set of spin-type operators is found, which was the
requirement to evaluate the propagator of the Kalb-Ramond field modified by the
presence of a nonzero vacuum expectation value responsible for the Lorentz
violation.Comment: 13 pages. Some modifications to match published version in
EuroPhysics Letter
Irreversibility line and low-field grain-boundary pinning in electron-doped superconducting thin films
AC magnetic susceptibilities of electron-doped Pr_{1.85}Ce_{0.15}CuO_4 (PCCO)
and Sm_{1.85}Ce_{0.15}CuO_4 (SCCO) granular thin films have been measured as a
function of temperature and magnetic-field strength. Depending on the level of
homogeneity of our films, two different types of the irreversibility line (IL)
defined as the intergrain-loss peak temperature in the imaginary part of
susceptibility have been found. The obtained results are described via the
critical-state model taking into account the low-field grain-boundary pinning.
The extracted pinning-force densities in more granular SCCO films turn out to
be four times larger than their counterparts in less granular PCCO films
Early Aspects at ICSE 2007: Workshop on Aspect-Oriented Requirements Engineering and Architecture Design
The “Early Aspects @ ICSE’07” is the 11th workshop in the series of Early Aspects workshops [1] which focuses on aspect identification during the requirements engineering and architecture derivation activities. The specific aim of the present workshop is twofold: (a) to initiate creation of an Early Aspects application demonstration and comparisons benchmark; and (b) to solicit submission of new
research
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