532 research outputs found
Scale Mixture Models with Applications to Bayesian Inference
Scale mixtures of uniform distributions are used to model non‐normal data in time series and econometrics in a Bayesian framework. Heteroscedastic and skewed data models are also tackled using scale mixture of uniform distributions. © 2003 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87496/2/394_1.pd
Ground-State Spaces of Frustration-Free Hamiltonians
We study the ground-state space properties for frustration-free Hamiltonians.
We introduce a concept of `reduced spaces' to characterize local structures of
ground-state spaces. For a many-body system, we characterize mathematical
structures for the set of all the -particle reduced spaces, which
with a binary operation called join forms a semilattice that can be interpreted
as an abstract convex structure. The smallest nonzero elements in ,
called atoms, are analogs of extreme points. We study the properties of atoms
in and discuss its relationship with ground states of -local
frustration-free Hamiltonians. For spin-1/2 systems, we show that all the atoms
in are unique ground states of some 2-local frustration-free
Hamiltonians. Moreover, we show that the elements in may not be the
join of atoms, indicating a richer structure for beyond the convex
structure. Our study of deepens the understanding of ground-state
space properties for frustration-free Hamiltonians, from a new angle of reduced
spaces.Comment: 23 pages, no figur
Quantum Capacity Approaching Codes for the Detected-Jump Channel
The quantum channel capacity gives the ultimate limit for the rate at which
quantum data can be reliably transmitted through a noisy quantum channel.
Degradable quantum channels are among the few channels whose quantum capacities
are known. Given the quantum capacity of a degradable channel, it remains
challenging to find a practical coding scheme which approaches capacity. Here
we discuss code designs for the detected-jump channel, a degradable channel
with practical relevance describing the physics of spontaneous decay of atoms
with detected photon emission. We show that this channel can be used to
simulate a binary classical channel with both erasures and bit-flips. The
capacity of the simulated classical channel gives a lower bound on the quantum
capacity of the detected-jump channel. When the jump probability is small, it
almost equals the quantum capacity. Hence using a classical capacity
approaching code for the simulated classical channel yields a quantum code
which approaches the quantum capacity of the detected-jump channel
Surface Ocean pCO2 Seasonality and Sea-Air CO2 Flux Estimates for the North American East Coast
Underway and in situ observations of surface ocean pCO2, combined with satellite data, were used to develop pCO2 regional algorithms to analyze the seasonal and interannual variability of surface ocean pCO2 and sea-air CO2 flux for five physically and biologically distinct regions of the eastern North American continental shelf: the South Atlantic Bight (SAB), the Mid-Atlantic Bight (MAB), the Gulf of Maine (GoM), Nantucket Shoals and Georges Bank (NS+GB), and the Scotian Shelf (SS). Temperature and dissolved inorganic carbon variability are the most influential factors driving the seasonality of pCO2. Estimates of the sea-air CO2 flux were derived from the available pCO2 data, as well as from the pCO2 reconstructed by the algorithm. Two different gas exchange parameterizations were used. The SS, GB+NS, MAB, and SAB regions are net sinks of atmospheric CO2 while the GoM is a weak source. The estimates vary depending on the use of surface ocean pCO2 from the data or algorithm, as well as with the use of the two different gas exchange parameterizations. Most of the regional estimates are in general agreement with previous studies when the range of uncertainty and interannual variability are taken into account. According to the algorithm, the average annual uptake of atmospheric CO2 by eastern North American continental shelf waters is found to be between 3.4 and 5.4 Tg C/yr (areal average of 0.7 to 1.0 mol CO2 /sq m/yr) over the period 2003-2010
Capacitive Spring Softening in Single-Walled Carbon Nanotube Nanoelectromechanical Resonators
We report the capacitive spring softening effect observed in single-walled
carbon nanotube (SWNT) nanoelectromechanical (NEM) resonators. The nanotube
resonators adopt dual-gate configuration with both bottom-gate and side-gate
capable of tuning the resonance frequency through capacitive coupling.
Interestingly, downward resonance frequency shifting is observed with
increasing side-gate voltage, which can be attributed to the capacitive
softening of spring constant. Furthermore, in-plane vibrational modes exhibit
much stronger spring softening effect than out-of-plan modes. Our dual-gate
design should enable the differentiation between these two types of vibrational
modes, and open up new possibility for nonlinear operation of nanotube
resonators.Comment: 12 pages/ 3 figure
Photometry of Variable Stars from Dome A, Antarctica
Dome A on the Antarctic plateau is likely one of the best observing sites on
Earth thanks to the excellent atmospheric conditions present at the site during
the long polar winter night. We present high-cadence time-series aperture
photometry of 10,000 stars with i<14.5 mag located in a 23 square-degree region
centered on the south celestial pole. The photometry was obtained with one of
the CSTAR telescopes during 128 days of the 2008 Antarctic winter.
We used this photometric data set to derive site statistics for Dome A and to
search for variable stars. Thanks to the nearly-uninterrupted synoptic
coverage, we find 6 times as many variables as previous surveys with similar
magnitude limits. We detected 157 variable stars, of which 55% are
unclassified, 27% are likely binaries and 17% are likely pulsating stars. The
latter category includes delta Scuti, gamma Doradus and RR Lyrae variables. One
variable may be a transiting exoplanet.Comment: Accepted for publication in the Astronomical Journal. PDF version
with high-resolution figures available at
http://faculty.physics.tamu.edu/lmacri/papers/wang11.pd
The sky brightness and transparency in i-band at Dome A, Antarctica
The i-band observing conditions at Dome A on the Antarctic plateau have been
investigated using data acquired during 2008 with the Chinese Small Telescope
ARray. The sky brightness, variations in atmospheric transparency, cloud cover,
and the presence of aurorae are obtained from these images. The median sky
brightness of moonless clear nights is 20.5 mag arcsec^{-2} in the SDSS
band at the South Celestial Pole (which includes a contribution of about 0.06
mag from diffuse Galactic light). The median over all Moon phases in the
Antarctic winter is about 19.8 mag arcsec^{-2}. There were no thick clouds in
2008. We model contributions of the Sun and the Moon to the sky background to
obtain the relationship between the sky brightness and transparency. Aurorae
are identified by comparing the observed sky brightness to the sky brightness
expected from this model. About 2% of the images are affected by relatively
strong aurorae.Comment: There are 1 Latex file and 14 figures accepted by A
The biogeochemistry of carbon across a gradient of streams and rivers within the Congo Basin
Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Biogeosciences 119 (2014): 687–702, doi:10.1002/2013JG002442.Dissolved organic carbon (DOC) and inorganic carbon (DIC, pCO2), lignin biomarkers, and theoptical properties of dissolved organic matter (DOM) were measured in a gradient of streams and rivers within the Congo Basin, with the aim of examining how vegetation cover and hydrology influences the composition and concentration of fluvial carbon (C). Three sampling campaigns (February 2010, November 2010, and August 2011) spanning 56 sites are compared by subbasin watershed land cover type (savannah, tropical forest, and swamp) and hydrologic regime (high, intermediate, and low). Land cover properties predominately controlled the amount and quality of DOC, chromophoric DOM (CDOM) and lignin phenol concentrations (∑8) exported in streams and rivers throughout the Congo Basin. Higher DIC concentrations and changing DOM composition (lower molecular weight, less aromatic C) during periods of low hydrologic flow indicated shifting rapid overland supply pathways in wet conditions to deeper groundwater inputs during drier periods. Lower DOC concentrations in forest and swamp subbasins were apparent with increasing catchment area, indicating enhanced DOC loss with extended water residence time. Surface water pCO2 in savannah and tropical forest catchments ranged between 2,600 and 11,922 µatm, with swamp regions exhibiting extremely high pCO2 (10,598–15,802 µatm), highlighting their potential as significant pathways for water-air efflux. Our data suggest that the quantity and quality of DOM exported to streams and rivers are largely driven by terrestrial ecosystem structure and that anthropogenic land use or climate change may impact fluvial C composition and reactivity, with ramifications for regional C budgets and future climate scenarios.This work was
supported by the National Science
Foundation as part of the ETBC
Collaborative Research: Controls on the
Flux, Age, and Composition of Terrestrial
Organic Carbon Exported by Rivers to the
Ocean (0851101 and 0851015).2014-10-3
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