9,334 research outputs found
Ultra High Resolution Transmission Electron Microscopy of Matrix Mineral Grains in CM Chondrites: Preaccretionary or Parent Body Aqueous Processing?
CM chondrites are highly hydrated
meteorites associated with a parent asteroid that has
experienced significant aqueous processing. The meteoritic
evidence indicates that these non-differentiated
asteroids are formed by fine-grained minerals embedded
in a nanometric matrix that preserves chemical
clues of the forming environment. So far there are two
hypothesis to explain the presence of hydrated minerals
in the content of CM chondrites: one is based on textural
features in chondrule-rim boundaries [1-3], and
the other ‘preaccretionary’ hypothesis proposes the
incorporation of hydrated phases from the protoplanetary
disk [4-6]. The highly porous structure of these
chondrites is inherited from the diverse materials present
in the protoplanetary disk environment. These
bodies were presumably formed by low relative velocity
encounters that led to the accretion of silicate-rich
chondrules, refractory Ca- and Al-rich inclusions
(CAIs), metal grains, and the fine-grained materials
forming the matrix. Owing to the presence of significant
terrestrial water in meteorite finds [7], here we
have focused on two CM chondrite falls with minimal
terrestrial processing: Murchison and Cold Bokkeveld.
Anhydrous carbonaceous chondrite matrices are usually
represented by highly chemically unequilibrated
samples that contain distinguishable stellar grains.
Other chondrites have experienced hydration and
chemical homogeneization that reveal parent body
processes. We have studied CM chondrites because
these meteorites have experienced variable hydration
levels [8-10]. It is important to study the textural effects
of aqueous alteration in the main minerals to
decipher which steps and environments promote bulk
chemistry changes, and create the distinctive alteration
products. It is thought that aqueous alteration has particularly
played a key role in modifying primordial
bulk chemistry, and homogenizing the isotopic content
of fine-grained matrix materials [7, 11, 12]. Fortunately,
the mineralogy produced by parent-body and terrestrial
aqueous alteration processes is distinctive [5, 11]
Ancient Martian Floods in a Plausible Variable Climatic Environment as Revealed from the Sequential Growth of Allan Hills 84001 Carbonate Globules
No abstract available
Smoking, physical activity and healthy aging in India
__Abstract__
Background: To identify levels of physical inactivity and smoking and examine their relationships to health among older people in India. Methods. I
One-Watt level mid-IR output, singly resonant, continuous-wave optical parametric oscillator pumped by a monolithic diode laser
We report more than 1.1 Watt of idler power at 3373 nm in a singly resonant optical parametric oscillator (SRO), directly pumped by a single-frequency monolithic tapered diode laser. The SRO is based on a periodically poled MgO:LiNbO3 crystal in a four mirror cavity and is excited by 8.05 W of 1062 nm radiation. The SRO pump power at threshold is 4 W. The internal slope-efficiency and conversion efficiency reach 89% and 44% respectively. The signal and idler waves are temperature tuned in the range of 1541 to 1600 nm and 3154 to 3415 nm respectively. To the best of our knowledge, this is the highest output obtained for a diode pumped optical parametric oscillator (OPO), and the first time a SRO is directly pumped by a monolithic tapered diode laser
Charged stripes from alternating static magnetic field
We motivate and perform a calculation of the energy of a cold fluid of
charged fermions in the presence of a striped magnetic background. We find that
a non-trivial value for the doping density on the walls is preferredComment: RevTeX, 3 pages, 3 encapsulated PostScript figure
Charge ordering and chemical potential shift in LaSrNiO studied by photoemission spectroscopy
We have studied the chemical potential shift in LaSrNiO and
the charge ordering transition in LaSrNiO by
photoemission spectroscopy. The result shows a large ( 1 eV/hole)
downward shift of the chemical potential with hole doping in the high-doping
regime ( 0.33) while the shift is suppressed in the low-doping
regime ( 0.33). This suppression is attributed to a
segregation of doped holes on a microscopic scale when the hole concentration
is lower than . In the sample, the
photoemission intensity at the chemical potential vanishes below the charge
ordering transition temperature 240 K.Comment: 5 pages, 4 figure
Spin dynamics in stripe-ordered La5/3Sr1/3NiO4
Polarized and unpolarized neutron inelastic scattering has been used to
measure the spin excitations in the spin-charge-ordered stripe phase of
La5/3Sr1/3NiO4. At high energies, sharp magnetic modes are observed
characteristic of a static stripe lattice. The energy spectrum is described
well by a linear spin wave model with intra- and inter-stripe exchange
interactions between neighbouring Ni spins given by J = 15 +/- 1.5 meV and J' =
7.5 +/- 1.5 meV respectively. A pronounced broadening of the magnetic
fluctuations in a band between 10 meV and 25 meV is suggestive of coupling to
collective motions of the stripe domain walls.Comment: ReVTeX 4, 4 pages inc. 4 Fig
Color confinement and dual superconductivity in full QCD
We report on evidence that confinement is related to dual superconductivity
of the vacuum in full QCD, as in quenched QCD. The vacuum is a dual
superconductor in the confining phase, whilst the U(1) magnetic symmetry is
realized a la Wigner in the deconfined phase.Comment: 4 pages, 4 eps figure
Topological censorship for Kaluza-Klein space-times
The standard topological censorship theorems require asymptotic hypotheses
which are too restrictive for several situations of interest. In this paper we
prove a version of topological censorship under significantly weaker
conditions, compatible e.g. with solutions with Kaluza-Klein asymptotic
behavior. In particular we prove simple connectedness of the quotient of the
domain of outer communications by the group of symmetries for models which are
asymptotically flat, or asymptotically anti-de Sitter, in a Kaluza-Klein sense.
This allows one, e.g., to define the twist potentials needed for the reduction
of the field equations in uniqueness theorems. Finally, the methods used to
prove the above are used to show that weakly trapped compact surfaces cannot be
seen from Scri.Comment: minor correction
Positive mass theorem for the Paneitz-Branson operator
We prove that under suitable assumptions, the constant term in the Green
function of the Paneitz-Branson operator on a compact Riemannian manifold
is positive unless is conformally diffeomophic to the standard
sphere. The proof is inspired by the positive mass theorem on spin manifolds by
Ammann-Humbert.Comment: 7 page
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