53,210 research outputs found
LaFeAsOF: A low carrier density superconductor near itinerant magnetism
Density functional studies of 26K superconducting LaFeAs(O,F) are reported.
We find a low carrier density, high density of states, and modest
phonon frequencies relative to . The high leads to proximity to
itinerant magnetism, with competing ferromagnetic and antiferromagnetic
fluctuations and the balance between these controlled by doping level. Thus
LaFeAs(O,F) is in a unique class of high superconductors: high
ionic metals near magnetism.Comment: Shortened published form. Typos correcte
Finding the Origin of the Pioneer Anomaly
Analysis of radio-metric tracking data from the Pioneer 10/11 spacecraft at
distances between 20 - 70 astronomical units (AU) from the Sun has consistently
indicated the presence of an anomalous, small, constant Doppler frequency
drift. The drift can be interpreted as being due to a constant acceleration of
a_P= (8.74 \pm 1.33) x 10^{-8} cm/s^2 directed towards the Sun. Although it is
suspected that there is a systematic origin to the effect, none has been found.
As a result, the nature of this anomaly has become of growing interest. Here we
present a concept for a deep-space experiment that will reveal the origin of
the discovered anomaly and also will characterize its properties to an accuracy
of at least two orders of magnitude below the anomaly's size. The proposed
mission will not only provide a significant accuracy improvement in the search
for small anomalous accelerations, it will also determine if the anomaly is due
to some internal systematic or has an external origin. A number of critical
requirements and design considerations for the mission are outlined and
addressed. If only already existing technologies were used, the mission could
be flown as early as 2010.Comment: 21 SS pages, 4+1 figures. final changes for publicatio
Quantum games of asymmetric information
We investigate quantum games in which the information is asymmetrically
distributed among the players, and find the possibility of the quantum game
outperforming its classical counterpart depends strongly on not only the
entanglement, but also the informational asymmetry. What is more interesting,
when the information distribution is asymmetric, the contradictive impact of
the quantum entanglement on the profits is observed, which is not reported in
quantum games of symmetric information.Comment: 5 pages, 3 figure
Uniqueness of Nash equilibria in quantum Cournot duopoly game
A quantum Cournot game of which classical form game has multiple Nash
equilibria is examined. Although the classical equilibria fail to be Pareto
optimal, the quantum equilibrium exhibits the following two properties, (i) if
the measurement of entanglement between strategic variables chosen by the
competing firms is sufficiently large, the multiplicity of equilibria vanishes,
and, (ii) the more strongly the strategic variables are entangled, the more
closely the unique equilibrium approaches to the optimal one.Comment: 7 pages, 2 figure
Comment on "Self-Purification in Semiconductor Nanocrystals"
In a recent Letter [PRL 96, 226802 (2006)], Dalpian and Chelikowsky claimed
that formation energies of Mn impurities in CdSe nanocrystals increase as the
size of the nanocrystal decreases, and argued that this size dependence leads
to "self-purification" of small nanocrystals. They presented
density-functional-theory (DFT) calculations showing a strong size dependence
for Mn impurity formation energies, and proposed a general explanation. In this
Comment we show that several different DFT codes, pseudopotentials, and
exchange-correlation functionals give a markedly different result: We find no
such size dependence. More generally, we argue that formation energies are not
relevant to substitutional doping in most colloidally grown nanocrystals.Comment: 1 page, 1 figur
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Calibration of probabilistic quantitative precipitation forecasts with an artificial neural network
A feed-forward neural network is configured to calibrate the bias of a high-resolution probabilistic quantitative precipitation forecast (PQPF) produced by a 12-km version of the NCEP Regional Spectral Model (RSM) ensemble forecast system. Twice-daily forecasts during the 2002-2003 cool season (1 November-31 March, inclusive) are run over four U.S. Geological Survey (USGS) hydrologic unit regions of the southwest United States. Calibration is performed via a cross-validation procedure, where four months are used for training and the excluded month is used for testing. The PQPFs before and after the calibration over a hydrological unit region are evaluated by comparing the joint probability distribution of forecasts and observations. Verification is performed on the 4-km stage IV grid, which is used as "truth." The calibration procedure improves the Brier score (BrS), conditional bias (reliability) and forecast skill, such as the Brier skill score (BrSS) and the ranked probability skill score (RPSS), relative to the sample frequency for all geographic regions and most precipitation thresholds. However, the procedure degrades the resolution of the PQPFs by systematically producing more forecasts with low nonzero forecast probabilities that drive the forecast distribution closer to the climatology of the training sample. The problem of degrading the resolution is most severe over the Colorado River basin and the Great Basin for relatively high precipitation thresholds where the sample of observed events is relatively small. © 2007 American Meteorological Society
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Short-range probabilistic quantitative precipitation forecasts over the southwest United States by the RSM ensemble system
The National Centers for Environmental Prediction (NCEP) Regional Spectral Model (RSM) is used to produce twice-daily (0000 and 1200 UTC), high-resolution ensemble forecasts to 24 h. The forecasts are performed at an equivalent horizontal grid spacing of 12 km for the period 1 November 2002 to 31 March 2003 over the southwest United States. The performance of 6-h accumulated precipitation is assessed for 32 U.S. Geological Survey hydrologic catchments. Multiple accuracy and skill measures are used to evaluate probabilistic quantitative precipitation forecasts. NCEP stage-IV precipitation analyses are used as "truth," with verification performed on the stage-IV 4-km grid. The RSM ensemble exhibits a ubiquitous wet bias. The bias manifests itself in areal coverage, frequency of occurrence, and total accumulated precipitation over every region and during every 6-h period. The biases become particularly acute starting with the 1800-0000 UTC interval, which leads to a spurious diurnal cycle and the 1200 UTC cycle being more adversely affected than the 0000 UTC cycle. Forecast quality and value exhibit marked variability over different hydrologic regions. The forecasts are highly skillful along coastal California and the windward slopes of the Sierra Nevada Mountains, but they generally lack skill over the Great Basin and the Colorado basin except over mountain peaks. The RSM ensemble is able to discriminate precipitation events and provide useful guidance to a wide range of users over most regions of California, which suggests that mitigation of the conditional biases through statistical postprocessing would produce major improvements in skill. © 2007 American Meteorological Society
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