11,442 research outputs found
Dynamics of fingering convection II: The formation of thermohaline staircases
Regions of the ocean's thermocline unstable to salt fingering are often
observed to host thermohaline staircases, stacks of deep well-mixed convective
layers separated by thin stably-stratified interfaces. Decades after their
discovery, however, their origin remains controversial. In this paper we use 3D
direct numerical simulations to shed light on the problem. We study the
evolution of an analogous double-diffusive system, starting from an initial
statistically homogeneous fingering state and find that it spontaneously
transforms into a layered state. By analysing our results in the light of the
mean-field theory developed in Paper I, a clear picture of the sequence of
events resulting in the staircase formation emerges. A collective instability
of homogeneous fingering convection first excites a field of gravity waves,
with a well-defined vertical wavelength. However, the waves saturate early
through regular but localized breaking events, and are not directly responsible
for the formation of the staircase. Meanwhile, slower-growing, horizontally
invariant but vertically quasi-periodic gamma-modes are also excited and grow
according to the gamma-instability mechanism. Our results suggest that the
nonlinear interaction between these various mean-field modes of instability
leads to the selection of one particular gamma-mode as the staircase
progenitor. Upon reaching a critical amplitude, this progenitor overturns into
a fully-formed staircase. We conclude by extending the results of our
simulations to real oceanic parameter values, and find that the progenitor
gamma-mode is expected to grow on a timescale of a few hours, and leads to the
formation of a thermohaline staircase in about one day with an initial spacing
of the order of one to two metres.Comment: 18 pages, 9 figures, associated mpeg file at
http://earth.uni-muenster.de/~stellma/movie_small.mp4, submitted to JF
Antiferromagnetic s-d exchange coupling in GaMnAs
Measurements of coherent electron spin dynamics in
Ga(1-x)Mn(x)As/Al(0.4)Ga(0.6)As quantum wells with 0.0006% < x < 0.03% show an
antiferromagnetic (negative) exchange bewteen s-like conduction band electrons
and electrons localized in the d-shell of the Mn2+ impurities. The magnitude of
the s-d exchange parameter, N0 alpha, varies as a function of well width
indicative of a large and negative contribution due to kinetic exchange. In the
limit of no quantum confinement, N0 alpha extrapolates to -0.09 +/- 0.03 eV
indicating that antiferromagnetic s-d exchange is a bulk property of GaMnAs.
Measurements of the polarization-resolved photoluminescence show strong
discrepancy from a simple model of the exchange enhanced Zeeman splitting,
indicative of additional complexity in the exchange split valence band.Comment: 5 pages, 4 figures and one action figur
Drift and Diffusion of Spins Generated by the Spin Hall Effect
Electrically generated spin accumulation due to the spin Hall effect is
imaged in n-GaAs channels using Kerr rotation microscopy, focusing on its
spatial distribution and time-averaged behavior in a magnetic field.
Spatially-resolved imaging reveals that spin accumulation observed in
transverse arms develops due to longitudinal drift of spin polarization
produced at the sample boundaries. One- and two-dimensional drift-diffusion
modeling is used to explain these features, providing a more complete
understanding of observations of spin accumulation and the spin Hall effect.Comment: 9 pages, 3 figure
Current-Induced Polarization and the Spin Hall Effect at Room Temperature
Electrically-induced electron spin polarization is imaged in n-type ZnSe
epilayers using Kerr rotation spectroscopy. Despite no evidence for an
electrically-induced internal magnetic field, current-induced in-plane spin
polarization is observed with characteristic spin lifetimes that decrease with
doping density. The spin Hall effect is also observed, indicated by an
electrically-induced out-of-plane spin polarization with opposite sign for
spins accumulating on opposite edges of the sample. The spin Hall conductivity
is estimated as 3 +/- 1.5 Ohms**-1 m**-1/|e| at 20 K, which is consistent with
the extrinsic mechanism. Both the current-induced spin polarization and the
spin Hall effect are observed at temperatures from 10 K to 295 K.Comment: 5 pages, 4 figure
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