9,689 research outputs found
HYDROGEN TRAPPING IN NIOBIUM VANADIUM ALLOYS
The authors have measured the localised vibrational modes of hydrogen in a Nd0.93V0.07 alloy. As one cools the sample for the first time to 10K one observes that hydrogen is trapped at octahedral sites associated with two substitutional vanadium atoms. The trap energy is small (10-20 meV) and less than the precipitation enthalpy. The phenomenon exhibits an interesting hysteresis. On repeated cooling cycles the hydrogen atoms tend to precipitate rather than stay in the trap sites. This is due to the formation of additional nucleation sites on the first cooling cycle
Catastrophic photometric redshift errors: weak lensing survey requirements
We study the sensitivity of weak lensing surveys to the effects of
catastrophic redshift errors - cases where the true redshift is misestimated by
a significant amount. To compute the biases in cosmological parameters, we
adopt an efficient linearized analysis where the redshift errors are directly
related to shifts in the weak lensing convergence power spectra. We estimate
the number Nspec of unbiased spectroscopic redshifts needed to determine the
catastrophic error rate well enough that biases in cosmological parameters are
below statistical errors of weak lensing tomography. While the straightforward
estimate of Nspec is ~10^6 we find that using only the photometric redshifts
with z<=2.5 leads to a drastic reduction in Nspec to ~30,000 while negligibly
increasing statistical errors in dark energy parameters. Therefore, the size of
spectroscopic survey needed to control catastrophic errors is similar to that
previously deemed necessary to constrain the core of the z_s-z_p distribution.
We also study the efficacy of the recent proposal to measure redshift errors by
cross-correlation between the photo-z and spectroscopic samples. We find that
this method requires ~10% a priori knowledge of the bias and stochasticity of
the outlier population, and is also easily confounded by lensing magnification
bias. The cross-correlation method is therefore unlikely to supplant the need
for a complete spectroscopic redshift survey of the source population.Comment: 14 pages, 3 figure
Five-dimensional PPN formalism and experimental test of Kaluza-Klein theory
The parametrized post Newtonian formalism for 5-dimensional metric theories
with a compact extra dimension is developed. The relation of the 5-dimensional
and 4-dimensional formulations is then analyzed, in order to compare the higher
dimensional theories of gravity with experiments. It turns out that the value
of post Newtonian parameter in the reduced 5-dimensional Kaluza-Klein
theory is two times smaller than that in 4-dimensional general relativity. The
departure is due to the existence of an extra dimension in the Kaluza-Klein
theory. Thus the confrontation between the reduced 4-dimensional formalism and
Solar system experiments raises a severe challenge to the classical
Kaluza-Klein theory.Comment: 4 pages, 1 table, accepted for publication in Physics Letters
Entanglement in holographic dark energy models
We study a process of equilibration of holographic dark energy (HDE) with the
cosmic horizon around the dark-energy dominated epoch. This process is
characterized by a huge amount of information conveyed across the horizon,
filling thereby a large gap in entropy between the system on the brink of
experiencing a sudden collapse to a black hole and the black hole itself. At
the same time, even in the absence of interaction between dark matter and dark
energy, such a process marks a strong jump in the entanglement entropy,
measuring the quantum-mechanical correlations between the horizon and its
interior. Although the effective quantum field theory (QFT) with a peculiar
relationship between the UV and IR cutoffs, a framework underlying all HDE
models, may formally account for such a huge shift in the number of distinct
quantum states, we show that the scope of such a framework becomes tremendously
restricted, devoiding it virtually any application in other cosmological epochs
or particle-physics phenomena. The problem of negative entropies for the
non-phantom stuff is also discussed.Comment: 10 pages, version to appear in PL
A high-speed tunable beam splitter for feed-forward photonic quantum information processing
We realize quantum gates for path qubits with a high-speed,
polarization-independent and tunable beam splitter. Two electro-optical
modulators act in a Mach-Zehnder interferometer as high-speed phase shifters
and rapidly tune its splitting ratio. We test its performance with heralded
single photons, observing a polarization-independent interference contrast
above 95%. The switching time is about 5.6 ns, and a maximal repetition rate is
2.5 MHz. We demonstrate tunable feed-forward operations of a single-qubit gate
of path-encoded qubits and a two-qubit gate via measurement-induced interaction
between two photons
On the 0-dimensional cusps of the Kahler moduli of a K3 surface
Let S be a projective K3 surface. It is proved that the 0-dimensional cusps
of the Kahler moduli of S are in one-to-one correspondence with the twisted
Fourier-Mukai partners of S. This leads to a counting formula for the
0-dimensional cusps of the Kahler moduli. Applications to rational maps between
K3 surfaces with large Picard numbers are given. When the Picard number of S is
1, the bijective correspondence is calculated explicitly.Comment: 24page
Size of spectroscopic calibration samples for cosmic shear photometric redshifts
Weak gravitational lensing surveys using photometric redshifts can have their
cosmological constraints severely degraded by errors in the photo-z scale. We
explore the cosmological degradation vs the size of the spectroscopic survey
required to calibrate the photo-z probability distribution. Previous work has
assumed a simple Gaussian distribution of photo-z errors; here we describe a
method for constraining an arbitrary parametric photo-z error model. As an
example we allow the photo-z probability distribution to be the sum of
Gaussians. To limit cosmological degradation to a fixed level, photo-z models
with multiple Gaussians require up to 5 times larger calibration sample than
one would estimate from assuming a single-Gaussian model. This degradation
saturates at . Assuming a single Gaussian when the photo-z
distribution has multiple parameters underestimates cosmological parameter
uncertainties by up to 35%. The size of required calibration sample also
depends upon the shape of the fiducial distribution, even when the RMS photo-z
error is held fixed. The required calibration sample size varies up to a factor
of 40 among the fiducial models studied, but this is reduced to a factor of a
few if the photo-z parameters are forced to be slowly varying with redshift.
Finally we show that the size of the required calibration sample can be
substantially reduced by optimizing its redshift distribution. We hope this
study will help stimulate work on better understanding of photo-z errors.Comment: 9 pages 4 figures, minor changes, match the published versio
Correlating AFM Probe Morphology to Image Resolution for Single-Wall Carbon Nanotube Tips
We report local-field-enhanced light emission from silicon nanocrystals close to a film of nanoporous gold. We resolve photoluminescence as the goldโSi nanocrystal separation distance is varied between 0 and 20 nm and observe a fourfold luminescence intensity enhancement concomitant with increases in the coupled silicon nanocrystal/nanoporous gold absorbance cross section and radiative decay rate. A detailed analysis of the luminescence data indicated a local-field-enhanced quantum efficiency of 58% for the Si nanocrystals coupled to the nanoporous gold layer
Directed Evolution of Protein-Based Neurotransmitter Sensors for MRI
The production of contrast agents sensitive to neuronal signaling events is a rate-limiting step in the development of molecular-level functional magnetic resonance imaging (molecular fMRI) approaches for studying the brain. High-throughput generation and evaluation of potential probes are possible using techniques for macromolecular engineering of protein-based contrast agents. In an initial exploration of this strategy, we used the method of directed evolution to identify mutants of a bacterial heme protein that allowed detection of the neurotransmitter dopamine in vitro and in living animals. The directed evolution method involves successive cycles of mutagenesis and screening that could be generalized to produce contrast agents sensitive to a variety of molecular targets in the nervous system
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