9,463 research outputs found
Hypoxia in the Lower Rappahannock Estuary
There are several objectives to this study. The immediate objective is to collect a comprehensive and consistent set of field observations to better describe the hypoxic condition in the Rappahannock River. Other objectives can be classified as short and long term goals.
Long term: (1) Identify and quantify the processes contributing to the dissolved oxygen budget in deep waters, and explain the variabilities among the three Virginia major tributary estuaries. (2) Provide information for resource management so that the James River can be protected from hypoxic problems and, perhaps, the problems in the York and Rappahannock Rivers can be alleviated.
Short term: (1) Is the hypoxic condition in the Rappahannock River just an extension of that in Chesapeake Bay, or does it originate locally within the river
Coupled valence and spin state transition in (Pr0.7Sm0.3)0.7Ca0.3CoO3
The coupled valence and spin state transition (VSST) taking place in
(Pr0.7Sm0.3)0.7Ca0.3CoO3 was investigated by soft x-ray absorption spectroscopy
(XAS) experiments carried out at the Pr-M4,5, Co-L2,3, and O-1s edges. This
VSST is found to be composed of a sharp Pr/Co valence and Co spin state
transition centered at T*=89.3 K, followed by a smoother Co spin-state
evolution at higher temperatures. At T < T*, we found that the praseodymium
displays a mixed valence Pr3+/Pr4+ with about 0.13 Pr4+/f.u., while all the
Co3+ is in the low-spin (LS) state. At T around T*, the sharp valence
transition converts all the Pr4+ to Pr3+ with a corresponding Co3+ to Co4+
compensation. This is accompanied by an equally sharp spin state transition of
the Co3+ from the low to an incoherent mixture of low and high spin (HS)
states. An involvement of the intermediate spin (IS) state can be discarded for
the Co3+. While above T* and at high temperatures the system shares rather
similar properties as Sr-doped LaCoO3, at low temperatures it behaves much more
like EuCoO3 with its highly stable LS configuration for the Co3+. Apparently,
the mechanism responsible for the formation of Pr4+ at low temperatures also
helps to stabilize the Co3+ in the LS configuration despite the presence of
Co4+ ions. We also found out that that the Co4+ is in an IS state over the
entire temperature range investigated in this study (10-290 K). The presence of
Co3+ HS and Co4+ IS at elevated temperatures facilitates the conductivity of
the material.Comment: 19 pages, 7 figures, Accepted in PR
Incommensurate spin correlations in highly oxidized cobaltates LaSrCoO
We observe quasi-static incommensurate magnetic peaks in neutron scattering
experiments on layered cobalt oxides La2-xSrxCoO4 with high Co oxidation states
that have been reported to be paramagnetic. This enables us to measure the
magnetic excitations in this highly hole-doped incommensurate regime and
compare our results with those found in the low-doped incommensurate regime
that exhibit hourglass magnetic spectra. The hourglass shape of magnetic
excitations completely disappears given a high Sr doping. Moreover, broad
low-energy excitations are found, which are not centered at the incommensurate
magnetic peak positions but around the quarter-integer values that are
typically exhibited by excitations in the checkerboard charge ordered phase.
Our findings suggest that the strong inter-site exchange interactions in the
undoped islands are critical for the emergence of hourglass spectra in the
incommensurate magnetic phases of La2-xSrxCoO4.Comment: http://www.nature.com/articles/srep25117
Can the Lepton Flavor Mixing Matrix Be Symmetric?
Current neutrino oscillation data indicate that the 3x3 lepton flavor mixing
matrix V is likely to be symmetric about its V_{e3}-V_{\mu 2}-V_{\tau 1} axis.
This off-diagonal symmetry corresponds to three pairs of {\it congruent}
unitarity triangles in the complex plane. Terrestrial matter effects can
substantially modify the genuine CP-violating parameter and off-diagonal
asymmetries of V in realistic long-baseline experiments of neutrino
oscillations.Comment: RexTex 14 pages (4 PS figures). More discussions adde
Intricacies of the Co spin state in SrCoIrO: an x-ray absorption and magnetic circular dichroism study
We report on a combined soft x-ray absorption and magnetic circular dichroism
(XMCD) study at the Co- on the hybrid 3/5 solid state oxide
SrCoIrO with the KNiF structure. Our data
indicate unambiguously a pure high spin state for the Co
(3) ions with a significant unquenched orbital moment
despite the sizeable elongation of the CoO octahedra. Using quantitative
model calculations based on parameters consistent with our spectra, we have
investigated the stability of this high spin state with respect to the
competing low spin and intermediate spin states.Comment: 7 pages, 4 figure
Electronically highly cubic conditions for Ru in alpha-RuCl3
We studied the local Ru 4d electronic structure of alpha-RuCl3 by means of
polarization dependent x-ray absorption spectroscopy at the Ru-L2,3 edges. We
observed a vanishingly small linear dichroism indicating that electronically
the Ru 4d local symmetry is highly cubic. Using full multiplet cluster
calculations we were able to reproduce the spectra excellently and to extract
that the trigonal splitting of the t2g orbitals is -12 meV, i.e.
negligible as compared to the Ru 4d spin-orbit coupling constant. Consistent
with our magnetic circular dichroism measurements, we found that the ratio of
the orbital and spin moments is 2.0, the value expected for a Jeff = 1/2 ground
state. We have thus shown that as far as the Ru 4d local properties are
concerned, alpha-RuCl3 is an ideal candidate for the realization of Kitaev
physics
Resonant Enhancement of Charge Density Wave Diffraction in the Rare-Earth Tritellurides
We performed resonant soft X-ray diffraction on known charge density wave
(CDW) compounds, rare earth tri-tellurides. Near the (3d - 4f) absorption
edge of rare earth ions, an intense diffraction peak is detected at a
wavevector identical to that of CDW state hosted on Te planes, indicating a
CDW-induced modulation on the rare earth ions. Surprisingly, the temperature
dependence of the diffraction peak intensity demonstrates an exponential
increase at low temperatures, vastly different than that of the CDW order
parameter. Assuming 4f multiplet splitting due to the CDW states,we present a
model to calculate X-ray absorption spectrum and resonant profile of the
diffraction peak, agreeing well with experimental observations. Our results
demonstrate a situation where the temperature dependence of resonant X-ray
diffraction peak intensity is not directly related to the intrinsic behavior of
the order parameter associated with the electronic order, but is dominated by
the thermal occupancy of the valence states.Comment: 7 pages, 5 figure
The MSW Effect in Quantum Field Theory
We show in detail the general relationship between the Schr\"{o}dinger
equation approach to calculating the MSW effect and the quantum field
theoretical S-matrix approach. We show the precise form a generic neutrino
propagator must have to allow a physically meaningful ``oscillation
probability'' to be decoupled from neutrino production fluxes and detection
cross-sections, and explicitly list the conditions---not realized in cases of
current experimental interest---in which the field theory approach would be
useful.Comment: 20 page REVTeX file, submitted to Phys. Rev.
Classical Nambu-Goldstone fields
It is shown that a Nambu-Goldstone (NG) field may be coherently produced by a
large number of particles in spite of the fact that the NG bosons do not couple
to flavor conserving scalar densities like . If a flavor
oscillation process takes place the phases of the pseudo-scalar or flavor
violating densities of different particles do not necessarily cancel each
other. The NG boson gets a macroscopic source whenever the total (spontaneously
broken) quantum number carried by the source particles suffers a net increase
or decrease in time. If the lepton numbers are spontaneously broken such
classical NG (majoron) fields may significantly change the neutrino oscillation
processes in stars pushing the observational capabilities of neutrino-majoron
couplings down to GeV.Comment: 11 pages, updated, to appear in PR
Quantum interference with photon pairs created in spatially separated sources
We report on a quantum interference experiment to probe the coherence between
two photons coming from non degenerate photon pairs at telecom wavelength
created in spatially separated sources. The two photons are mixed on a beam
splitter and we observe a reduction of up to 84% in the coincidence count rate
when the photons are made indistinguishable. This experiment constitutes an
important step towards the realization of quantum teleportation and
entanglement swapping with independent sources.Comment: 5 pages, 2 figures, changes according to referee's comments,
discussions partly rewritte
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