968 research outputs found
Remarkably robust and correlated coherence and antiferromagnetism in (CeLa)CuGe
We present magnetic susceptibility, resistivity, specific heat, and
thermoelectric power measurements on (CeLa)CuGe single
crystals (0 1). With La substitution, the antiferromagnetic
temperature is suppressed in an almost linear fashion and moves below
0.36 K, the base temperature of our measurements for 0.8. Surprisingly, in
addition to robust antiferromagnetism, the system also shows low temperature
coherent scattering below up to 0.9 of La, indicating a small
percolation limit 9 of Ce that separates a coherent regime from a
single-ion Kondo impurity regime. as a function of magnetic field was
found to have different behavior for 0.9. Remarkably,
at = 0 was found to be linearly proportional to . The
jump in the magnetic specific heat at as a function of
for (CeLa)CuGe follows the theoretical prediction
based on the molecular field calculation for the = 1/2 resonant level
model
Effect of sintering temperature under high pressure in the uperconductivity for MgB2
We report the effect of the sintering temperature on the superconductivity of
MgB2 pellets prepared under a high pressure of 3 GPa. The superconducting
properties of the non-heated MgB2 in this high pressure were poor. However, as
the sintering temperature increased, the superconducting properties were vastly
enhanced, which was shown by the narrow transition width for the resistivity
and the low-field magnetizations. This shows that heat treatment under high
pressure is essential to improve superconducting properties. These changes were
found to be closely related to changes in the surface morphology observed using
scanning electron microscopy.Comment: 3 Pages including 3 figure
Managing Risk of Bidding in Display Advertising
In this paper, we deal with the uncertainty of bidding for display
advertising. Similar to the financial market trading, real-time bidding (RTB)
based display advertising employs an auction mechanism to automate the
impression level media buying; and running a campaign is no different than an
investment of acquiring new customers in return for obtaining additional
converted sales. Thus, how to optimally bid on an ad impression to drive the
profit and return-on-investment becomes essential. However, the large
randomness of the user behaviors and the cost uncertainty caused by the auction
competition may result in a significant risk from the campaign performance
estimation. In this paper, we explicitly model the uncertainty of user
click-through rate estimation and auction competition to capture the risk. We
borrow an idea from finance and derive the value at risk for each ad display
opportunity. Our formulation results in two risk-aware bidding strategies that
penalize risky ad impressions and focus more on the ones with higher expected
return and lower risk. The empirical study on real-world data demonstrates the
effectiveness of our proposed risk-aware bidding strategies: yielding profit
gains of 15.4% in offline experiments and up to 17.5% in an online A/B test on
a commercial RTB platform over the widely applied bidding strategies
The Infrared Einstein Ring in the Gravitational Lens MG1131+0456 and the Death of the Dusty Lens Hypothesis
We have obtained and modeled new NICMOS images of the lens system
MG1131+0456, which show that its lens galaxy is an H=18.6 mag, transparent,
early-type galaxy at a redshift of about z_l = 0.85; it has a major axis
effective radius R_e=0.68+/-0.05 arcsec, projected axis ratio b/a=0.77+/-0.02,
and major axis PA=60+/-2 degrees. The lens is the brightest member of a group
of seven galaxies with similar R-I and I-H colors, and the two closest group
members produce sufficient tidal perturbations to explain the ring morphology.
The host galaxy of the MG1131+0456 source is a z_s > 2 ERO (``extremely red
object'') which is lensed into optical and infrared rings of dramatically
different morphologies. These differences imply a strongly wavelength-dependent
source morphology that could be explained by embedding the host in a larger,
dusty disk. At 1.6 micron (H), the ring is spectacularly luminous, with a total
observed flux of H=17.4 mag and a de-magnified flux of 19.3 mag, corresponding
to a 1-2L_* galaxy at the probable source redshift of z_s > 2. Thus, it is
primarily the stellar emission of the radio source host galaxy that produces
the overall colors of two of the reddest radio lenses, MG1131+0456 and
B~1938+666, aided by the suppression of optical AGN emission by dust in the
source galaxy. The dusty lens hypothesis -- that many massive early-type
galaxies with 0.2 < z_l < 1.0 have large, uniform dust opacities -- is ruled
out.Comment: 27 pages, 8 COLOR figures, submitted to ApJ. Black and white version
available at http://cfa-www.harvard.edu/castle
Multiple regions of quantum criticality in YbAgGe
Dilation and thermopower measurements on YbAgGe, a heavy-fermion
antiferromagnet, clarify and refine the magnetic field-temperature (H-T) phase
diagram and reveal a field-induced phase with T-linear resistivity. On the
low-H side of this phase we find evidence for a first-order transition and
suggest that YbAgGe at 4.5 T may be close to a quantum critical end point. On
the high-H side our results are consistent with a second-order transition
suppressed to a quantum critical point near 7.2 T. We discuss these results in
light of global phase diagrams proposed for Kondo lattice systems
AC-induced superfluidity
We argue that a system of ultracold bosonic atoms in a tilted optical lattice
can become superfluid in response to resonant AC forcing. Among others, this
allows one to prepare a Bose-Einstein condensate in a state associated with a
negative effective mass. Our reasoning is backed by both exact numerical
simulations for systems consisting of few particles, and by a theoretical
approach based on Floquet-Fock states.Comment: Accepted for publication in Europhysics letters, 6 pages, 4 figures,
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