20,106 research outputs found
The Monterey event in the Mediterranean: A record from shelf sediments of Malta
Oligo-Miocene carbonate platform and shelf sediments outcropping on the Maltese Islands provide an excellent archive of the paleoceanography of the central Mediterranean. A sequence of shallow water limestones, than shelf limestones, and marls, followed again by shallow water limestones, reflects drowning of a carbonate platform, the establishment of a shelf environment and, in the late Miocene, renewed progradation and aggradation of shallow water carbonates. The sequence recording the deepening of the Maltese platform contains several phosphorite hardgrounds and phosphorite pebble beds. These phosphorites were dated with strontium isotopes. Major episodes of phosphogenesis occurred between 25 and 16 Ma, and they are coeval with those phosphorite events reported from Florida and North Carolina. A Miocene carbon isotope and oxygen isotope stratigraphy was established on planktic and benthic foraminifera and on bulk samples. A major carbon isotope excursion with an amplitude of up to +l‰ between 18 and 12.5 Ma can be correlated with the globally recognized Monterey carbon isotope excursion. This is the first record of this event both in shallow water sediments and in the Mediterranean. The carbon isotope excursion precedes an oxygen isotope excursion which also was recognized in deep-sea records. Major episodes of phosphogenesis and platform drowning preceded the carbon isotope excursion by up to millions of years
Spin transport in a unitary Fermi gas close to the BCS transition
We consider spin transport in a two-component ultracold Fermi gas with
attractive interspecies interactions close to the BCS pairing transition. In
particular, we consider the spin-transport relaxation rate and the
spin-diffusion constant. Upon approaching the transition, the scattering
amplitude is enhanced by pairing fluctuations. However, as the system
approaches the transition, the spectral weight for excitations close to the
Fermi level is decreased by the formation of a pseudogap. To study the
consequence of these two competing effects, we determine the spin-transport
relaxation rate and the spin-diffusion constant using both a Boltzmann approach
and a diagrammatic approach. The former ignores pseudogap physics and finite
lifetime effects. In the latter, we incorporate the full pseudogap physics and
lifetime effects, but we ignore vertex corrections, so that we effectively
calculate single-particle relaxation rates instead of transport relaxation
rates. We find that there is qualitative agreement between these two approaches
although the results for the transport coefficients differ quantitatively.Comment: 9 pages, 10 figure
Entropic particle transport: higher order corrections to the Fick-Jacobs diffusion equation
Transport of point-size Brownian particles under the influence of a constant
and uniform force field through a three-dimensional channel with smoothly
varying periodic cross-section is investigated. Here, we employ an asymptotic
analysis in the ratio between the difference of the widest and the most narrow
constriction divided through the period length of the channel geometry. We
demonstrate that the leading order term is equivalent to the Fick-Jacobs
approximation. By use of the higher order corrections to the probability
density we derive an expression for the spatially dependent diffusion
coefficient D(x) which substitutes the constant diffusion coefficient present
in the common Fick-Jacobs equation. In addition, we show that in the diffusion
dominated regime the average transport velocity is obtained as the product of
the zeroth-order Fick-Jacobs result and the expectation value of the spatially
dependent diffusion coefficient . The analytic findings are corroborated
with the precise numerical results of a finite element calculation of the
Smoluchowski diffusive particle dynamics occurring in a reflection symmetric
sinusoidal-shaped channel.Comment: 9 pages, 3 figure
Probabilistic Search for Object Segmentation and Recognition
The problem of searching for a model-based scene interpretation is analyzed
within a probabilistic framework. Object models are formulated as generative
models for range data of the scene. A new statistical criterion, the truncated
object probability, is introduced to infer an optimal sequence of object
hypotheses to be evaluated for their match to the data. The truncated
probability is partly determined by prior knowledge of the objects and partly
learned from data. Some experiments on sequence quality and object segmentation
and recognition from stereo data are presented. The article recovers classic
concepts from object recognition (grouping, geometric hashing, alignment) from
the probabilistic perspective and adds insight into the optimal ordering of
object hypotheses for evaluation. Moreover, it introduces point-relation
densities, a key component of the truncated probability, as statistical models
of local surface shape.Comment: 18 pages, 5 figure
Gravitational Waves in Bianchi Type-I Universes I: The Classical Theory
The propagation of classical gravitational waves in Bianchi Type-I universes
is studied. We find that gravitational waves in Bianchi Type-I universes are
not equivalent to two minimally coupled massless scalar fields as it is for the
Robertson-Walker universe. Due to its tensorial nature, the gravitational wave
is much more sensitive to the anisotropy of the spacetime than the scalar field
is and it gains an effective mass term. Moreover, we find a coupling between
the two polarization states of the gravitational wave which is also not present
in the Robertson-Walker universe.Comment: 34 papers, written in ReVTeX, submitted to Physical Review
Driven Brownian transport through arrays of symmetric obstacles
We numerically investigate the transport of a suspended overdamped Brownian
particle which is driven through a two-dimensional rectangular array of
circular obstacles with finite radius. Two limiting cases are considered in
detail, namely, when the constant drive is parallel to the principal or the
diagonal array axes. This corresponds to studying the Brownian transport in
periodic channels with reflecting walls of different topologies. The mobility
and diffusivity of the transported particles in such channels are determined as
functions of the drive and the array geometric parameters. Prominent transport
features, like negative differential mobilities, excess diffusion peaks, and
unconventional asymptotic behaviors, are explained in terms of two distinct
lengths, the size of single obstacles (trapping length) and the lattice
constant of the array (local correlation length). Local correlation effects are
further analyzed by continuously rotating the drive between the two limiting
orientations.Comment: 10 pages 13 figure
Brownian transport in corrugated channels with inertia
The transport of suspended Brownian particles dc-driven along corrugated
narrow channels is numerically investigated in the regime of finite damping. We
show that inertial corrections cannot be neglected as long as the width of the
channel bottlenecks is smaller than an appropriate particle diffusion length,
which depends on the the channel corrugation and the drive intensity. Being
such a diffusion length inversely proportional to the damping constant,
transport through sufficiently narrow obstructions turns out to be always
sensitive to the viscosity of the suspension fluid. The inertia corrections to
the transport quantifiers, mobility and diffusivity, markedly differ for
smoothly and sharply corrugated channels.Comment: 9 pages including figures. arXiv admin note: substantial text overlap
with arXiv:1202.436
Conditional control of quantum beats in a cavity QED system
We probe a ground-state superposition that produces a quantum beat in the
intensity correlation of a two-mode cavity QED system. We mix drive with
scattered light from an atomic beam traversing the cavity, and effectively
measure the interference between the drive and the light from the atom. When a
photon escapes the cavity, and upon detection, it triggers our feedback which
modulates the drive at the same beat frequency but opposite phase for a given
time window. This results in a partial interruption of the beat oscillation in
the correlation function, that then returns to oscillate.Comment: 9 pages, 5 figures, XVII Reuni\'on Iberoamericana de \'Optica, X
Encuentro de \'Optica, L\'aseres y Aplicaciones (RIAO-OPTILAS-2010
Optimal Unravellings for Feedback Control in Linear Quantum Systems
For quantum systems with linear dynamics in phase space much of classical
feedback control theory applies. However, there are some questions that are
sensible only for the quantum case, such as: given a fixed interaction between
the system and the environment what is the optimal measurement on the
environment for a particular control problem? We show that for a broad class of
optimal (state-based) control problems (the stationary
Linear-Quadratic-Gaussian class), this question is a semi-definite program.
Moreover, the answer also applies to Markovian (current-based) feedback.Comment: 5 pages. Version published by Phys. Rev. Let
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