23,266 research outputs found
Spin Decoherence from Hamiltonian dynamics in Quantum Dots
The dynamics of a spin-1/2 particle coupled to a nuclear spin bath through an
isotropic Heisenberg interaction is studied, as a model for the spin
decoherence in quantum dots. The time-dependent polarization of the central
spin is calculated as a function of the bath-spin distribution and the
polarizations of the initial bath state. For short times, the polarization of
the central spin shows a gaussian decay, and at later times it revives
displaying nonmonotonic time dependence. The decoherence time scale dep ends on
moments of the bath-spin distribuition, and also on the polarization strengths
in various bath-spin channels. The bath polarizations have a tendency to
increase the decoherence time scale. The effective dynamics of the central spin
polarization is shown to be describ ed by a master equation with non-markovian
features.Comment: 11 pages, 6 figures Accepted for publication in Phys.Rev
Magnetic and electron transport properties of the rare-earth cobaltates, La0.7-xLnxCa0.3CoO3 (Ln = Pr, Nd, Gd and Dy) : A case of phase separation
Magnetic and electrical properties of four series of rare earth cobaltates of
the formula La0.7-xLnxCa0.3CoO3 with Ln = Pr, Nd, Gd and Dy have been
investigated. Compositions close to x = 0.0 contain large ferromagnetic
clusters or domains, and show Brillouin-like behaviour of the field-cooled DC
magnetization data with fairly high ferromagnetic Tc values, besides low
electrical resistivities with near-zero temperature coefficients. The
zero-field-cooled data generally show a non-monotonic behaviour with a peak at
a temperatures slightly lower than Tc. The near x = 0.0 compositions show a
prominent peak corresponding to the Tc in the AC-susceptibility data. The
ferromagnetic Tc varies linearly with x or the average radius of the A-site
cations, (rA). With increase in x or decrease in (rA), the magnetization value
at any given temperature decreases markedly and the AC-susceptibility
measurements show a prominent transition arising from small magnetic clusters
with some characteristics of a spin-glass. Electrical resistivity increases
with increase in x, showed a significant increase around a critical value of x
or (rA), at which composition the small clusters also begin to dominate. These
properties can be understood in terms of a phase separation scenario wherein
large magnetic clusters give way to smaller ones with increase in x, with both
types of clusters being present in certain compositions. The changes in
magnetic and electrical properties occur parallely since the large
ferromagnetic clusters are hole-rich and the small clusters are hole-poor.
Variable-range hopping seems to occur at low temperatures in these cobaltates.Comment: 23 pages including figure
High Resolution Millimeter-Wave Mapping of Linearly Polarized Dust Emission: Magnetic Field Structure in Orion
We present 1.3 and 3.3 mm polarization maps of Orion-KL obtained with the
BIMA array at approximately 4 arcsec resolution. Thermal emission from
magnetically aligned dust grains produces the polarization. Along the Orion
``ridge'' the polarization position angle varies smoothly from about 10 degrees
to 40 degrees, in agreement with previous lower resolution maps. In a small
region south of the Orion ``hot core,'' however, the position angle changes by
90 degrees. This abrupt change in polarization direction is not necessarily the
signpost of a twisted magnetic field. Rather, in this localized region
processes other than the usual Davis-Greenstein mechanism might align the dust
grains with their long axes parallel with the field, orthogonal to their normal
orientation.Comment: AAS preprint:14 pages, 2 figures (3mm.eps and 1mm.eps); requires
aaspp4.sty To be published in Astrophysical Journal Letter
Hatchery production of Black-lip pearl oyster spat in the Andaman and Nicobar Islands: a success story
The lustrous black pearls produced by
the black-lip pearl oysler Pinctada
margaritifera in the inner surface its two
shells have become one of the most
valuable pearls on earth with prices
ranging from a minimum of 100 US per pearl, in
the case of those at exceptional and rare
quality. Through the production of black
pearls from Pinctada margaritifera, the
island country of Tahili became the global
leader in this business controlling 28.8%
of the world market
Probing The Dust-To-Gas Ratio of z > 0 Galaxies Through Gravitational Lenses
We report the detection of differential gas column densities in three
gravitational lenses, MG0414+0534, HE1104-1805, and PKS1830-211. Combined with
the previous differential column density measurements in B1600+434 and
Q2237+0305 and the differential extinction measurements of these lenses, we
probe the dust-to-gas ratio of a small sample of cosmologically distant normal
galaxies. We obtain an average dust-to-gas ratio of E(B-V)/NH =(1.4\pm0.5) e-22
mag cm^2/atoms with an estimated intrinsic dispersion in the ratio of ~40%.
This average dust-to-gas ratio is consistent with the average Galactic value of
1.7e-22 mag cm^2/atoms and the estimated intrinsic dispersion is also
consistent with the 30% observed in the Galaxy.Comment: 14 pages, 4 figures, Accepted by Ap
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