3,988 research outputs found
Identification and mechanical control of ferroelastic domain structure in rhombohedral CaMnO
We report on observation of ferroelastic domain structure in single crystals
of multiferroic CaMnO at room temperature. Two types of ferroelastic
domain wall are found, consistent with the material's rhombohedral symmetry
that is reduced from cubic symmetry at higher temperatures. Using Raman
spectroscopy along with other measurements, we develop a systematic method to
determine the microscopic domain orientation. Moreover, we find a switching
behavior of the domains, which allows us to detwin the crystals conveniently at
room temperature using a moderate uniaxial compression. Our result paves the
way for further spectroscopic study and domain engineering in CaMnO.Comment: 7 pages, 4 figure
3D mixing in hot Jupiter atmospheres. I. application to the day/night cold trap in HD 209458b
Hot Jupiters exhibit atmospheric temperatures ranging from hundreds to
thousands of Kelvin. Because of their large day-night temperature differences,
condensable species that are stable in the gas phase on the dayside, such as
TiO and silicates, may condense and gravitationally settle on the nightside.
Atmospheric circulation may counterbalance this tendency to gravitationally
settle. This three dimensional (3D) mixing of chemical species has not
previously been studied for hot Jupiters, yet it is crucial to assess the
existence and distribution of TiO and silicates in the atmospheres of these
planets. We perform 3D global circulation models of HD209458b including passive
tracers that advect with the 3D flow, including a source/sink on the nightside
to represent condensation and gravitational settling of haze particles. We show
that global advection patterns produce strong vertical mixing that can keep
condensable species lofted as long as they are trapped in particles of sizes of
a few microns or less on the night side. We show that vertical mixing results
not from small-scale convection but from the large-scale circulation driven by
the day-night heating contrast. Although this vertical mixing is not diffusive
in any rigorous sense, a comparison of our results with idealized diffusion
models allows a rough estimate of the vertical diffusion coefficient.
Kzz=5x10**4/Sqrt(Pbar) m2/s can be used in 1D models of HD 209458b. Moreover,
our models exhibit strong spatial and temporal variability in the tracer
concentration that could result in observable variations during transit or
secondary eclipse measurements. Finally, we apply our model to the case of TiO
in HD209458b and show that the day-night cold trap would deplete TiO if it
condenses into particles bigger than a few microns on the planet's night side,
making it unable to create the observed stratosphere of the planet.Comment: Accepted in A&A in August 2013
http://dx.doi.org/10.1051/0004-6361/20132113
Soft vibrational mode associated with incommensurate orbital order in multiferroic CaMnO
We report inelastic light scattering measurements of lattice dynamics related
to the incommensurate orbital order in . Below the
ordering temperature , we observe extra
phonon peaks as a result of Brillouin-zone folding, as well as a soft
vibrational mode with a power-law -dependent energy, . This temperature dependence demonstrates the
second-order nature of the transition at , and it indicates that
the soft mode can be regarded as the amplitude excitation of the composite
order parameter. Our result strongly suggests that the lattice degrees of
freedom are actively involved in the orbital-ordering mechanism.Comment: 7 pages, 8 figure
Rational spectral methods for PDEs involving fractional Laplacian in unbounded domains
Many PDEs involving fractional Laplacian are naturally set in unbounded
domains with underlying solutions decay very slowly, subject to certain power
laws. Their numerical solutions are under-explored. This paper aims at
developing accurate spectral methods using rational basis (or modified mapped
Gegenbauer functions) for such models in unbounded domains. The main building
block of the spectral algorithms is the explicit representations for the
Fourier transform and fractional Laplacian of the rational basis, derived from
some useful integral identites related to modified Bessel functions. With these
at our disposal, we can construct rational spectral-Galerkin and direct
collocation schemes by pre-computing the associated fractional differentiation
matrices. We obtain optimal error estimates of rational spectral approximation
in the fractional Sobolev spaces, and analyze the optimal convergence of the
proposed Galerkin scheme. We also provide ample numerical results to show that
the rational method outperforms the Hermite function approach
Dark matter coupling to electroweak gauge and Higgs bosons: an effective field theory approach
If dark matter is a new species of particle produced in the early universe as
a cold thermal relic (a weakly-interacting massive particle-WIMP), its present
abundance, its scattering with matter in direct-detection experiments, its
present-day annihilation signature in indirect-detection experiments, and its
production and detection at colliders, depend crucially on the WIMP coupling to
standard-model (SM) particles. It is usually assumed that the WIMP couples to
the SM sector through its interactions with quarks and leptons. In this paper
we explore the possibility that the WIMP coupling to the SM sector is via
electroweak gauge and Higgs bosons. In the absence of an ultraviolet-complete
particle-physics model, we employ effective field theory to describe the
WIMP--SM coupling. We consider both scalars and Dirac fermions as possible
dark-matter candidates. Starting with an exhaustive list of operators up to
dimension 8, we present detailed calculation of dark-matter annihilations to
all possible final states, including gamma gamma, gamma Z, gamma h, ZZ, Zh, W+
W-, hh, and f fbar, and demonstrate the correlations among them. We compute the
mass scale of the effective field theory necessary to obtain the correct
dark-matter mass density, and well as the resulting photon line signals
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