4,349 research outputs found
Spin melting and refreezing driven by uniaxial compression on a dipolar hexagonal plate
We investigate freezing characteristics of a finite dipolar hexagonal plate
by the Monte Carlo simulation. The hexagonal plate is cut out from a piled
triangular lattice of three layers with FCC-like (ABCABC) stacking structure.
In the present study an annealing simulation is performed for the dipolar plate
uniaxially compressed in the direction of layer-piling. We find spin melting
and refreezing driven by the uniaxial compression. Each of the melting and
refreezing corresponds one-to-one with a change of the ground states induced by
compression. The freezing temperatures of the ground-state orders differ
significantly from each other, which gives rise to the spin melting and
refreezing of the present interest. We argue that these phenomena are
originated by a finite size effect combined with peculiar anisotropic nature of
the dipole-dipole interaction.Comment: Proceedings of the Highly Frustrated Magnetism (HFM2006) conference.
To appear in a special issue of J. Phys. Condens. Matte
Oscillatory convective modes in red giants: a possible explanation of the long secondary periods
We discuss properties of oscillatory convective modes in low-mass red giants,
and compare them with observed properties of the long secondary periods (LSPs)
of semi-regular red giant variables. Oscillatory convective modes are very
nonadiabatic g modes and they are present in luminous stars, such as red
giants with \log L/{\rm L}_\odot \ga 3. Finite amplitudes for these modes are
confined to the outermost nonadiabatic layers, where the radiative energy flux
is more important than the convective energy flux. The periods of oscillatory
convection modes increase with luminosity, and the growth times are comparable
to the oscillation periods. The LSPs of red giants in the Large Magellanic
Cloud (LMC) are observed to lie on a distinct period-luminosity sequence called
sequence D. This sequence D period-luminosity relation is roughly consistent
with the predictions for dipole oscillatory convective modes in AGB models if
we adopt a mixing length of 1.2 pressure scale height ().
However, the effective temperature of the red-giant sequence of the LMC is
consistent to models with , which predict periods too short by a
factor of two.Comment: 7 pages, 6 figures, accepted for publication in MNRA
Gravitino and Axino SuperWIMPs
Gravitinos and axinos produced in the late decays of other supersymmetric
particles are well-motivated dark matter (DM) candidates, whose experimental
evidences are very distinctive and different from other standard candidates, as
thermal produced neutralinos in similar supersymmetric models. In particular,
charged sleptons could appear stable because of the length of its lifetime. The
direct production of such particles at both the Large Hadron Collider (LHC) and
a future International Linear Collider (ILC) can give not only a clear
signature of supersymmetry but also the first non-gravitational evidence of
dark matter.Comment: 4 pages, LaTeX, 1 figure, Updated references. To appear in
Proceedings of SUSY06, the 14th International Conference on Supersymmetry and
the Unification of Fundamental Interactions, UC Irvine, California, 12-17
June 200
SuperWIMP Solutions to Small Scale Structure Problems
Collisionless, cold dark matter in the form of weakly-interacting massive
particles (WIMPs) is well-motivated in particle physics, naturally yields the
observed relic density, and successfully explains structure formation on large
scales. On small scales, however, it predicts too much power, leading to cuspy
halos, dense cores, and large numbers of subhalos, in apparent conflict with
observations. We consider superWIMP dark matter, produced with large velocity
in late decays at times 10^5 s - 10^8 s. As analyzed by Kaplinghat in a more
general setting, we find that superWIMPs have sufficiently large free-streaming
lengths and low phase space densities to help resolve small scale structure
problems while preserving all of the above-mentioned WIMP virtues.Comment: 4 pages, 2 figures, published versio
Automatic normal orientation in point clouds of building interiors
Orienting surface normals correctly and consistently is a fundamental problem
in geometry processing. Applications such as visualization, feature detection,
and geometry reconstruction often rely on the availability of correctly
oriented normals. Many existing approaches for automatic orientation of normals
on meshes or point clouds make severe assumptions on the input data or the
topology of the underlying object which are not applicable to real-world
measurements of urban scenes. In contrast, our approach is specifically
tailored to the challenging case of unstructured indoor point cloud scans of
multi-story, multi-room buildings. We evaluate the correctness and speed of our
approach on multiple real-world point cloud datasets
Evolution of Pliocene climate cyclicity at Hole 806B (5-2 Ma); oxygen isotope record
A detailed Pliocene oxygen isotope record from the Ontong Java Plateau, based on measurements of the surface-dwelling planktonic foraminifer Globigerinoides sacculifer, was produced for the period from 5 to 2 Ma. The record documents major long and short-term climate changes. The results show periods of enhanced ice volume at 4.6 to 4.3 Ma and after 2.85 Ma, a long-term warming trend from 4.1 to 3.7 Ma, and a distinct cooling trend that was initiated at 3.5 Ma and progressed through the initiation of large-scale Northern Hemisphere glaciation after 2.85 Ma (according to the time scale of Shackleton and others proposed in 1990). Periods of high average ice volumes also show the highest δ 1 8 amplitudes. The pattern of climate cyclicity changed markedly at about 2.85 Ma. Earlier times were marked by high-frequency variability at the precessional frequencies or even higher frequencies, pointing to low-latitude processes as a main controlling factor driving planktonic δ 1 8 variability in this period. The high-frequency variability is not coherent with insolation and points to strong nonlinearity in the way the climate system responded to orbital forcing before the onset of large scale Northern Hemisphere glaciation. After 3 Ma, stronger 41-k.y. cyclicity appears in the record. The shift in pattern is clearest around 2.85 Ma (according to the time scale proposed by Shackleton and others in 1990), 100-200 k.y. before the most dramatic spread of Northern Hemisphere ice sheets. This indicates that high-latitude processes from this point on began to take over and influence most strongly the δ 1 8 record, which now reflects ice-volume fluctuations related to the climatic effects of obliquity forcing on the seasonality of high-latitude areas, most probably in the Northern Hemisphere. The general Pliocene trend is that high-latitude climate sensitivity and instability was increasing, and the causal factors producing the intensified glacial cyclicity during the Pliocene must be factors that enhance cooling and climate sensitivity in the subarctic areas
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