1,027 research outputs found
Excitation Spectrum and Superexchange Pathways in the Spin Dimer VODPO_4 . 1/2 D_2O
Magnetic excitations have been investigated in the spin dimer material
VODPO_4 \cdot 1/2 D_2O using inelastic neutron scattering. A dispersionless
magnetic mode was observed at an energy of 7.81(4) meV. The wavevector
dependence of the scattering intensityfrom this mode is consistent with the
excitation of isolated V^{4+} spin dimers with a V-V separation of 4.43(7) \AA.
This result is unexpected since the V-V pair previously thought to constitute
themagnetic dimer has a separation of 3.09 \AA. We identify an alternative V-V
pair as the likely magnetic dimer, which involves superexchange pathways
through a covalently bonded PO_4 group. This surprising result casts doubt on
the interpretation of (VO)_2P_2O_7 as a spin ladder.Comment: 4 pages, 4 postscript figures - identical to previous paper but
figure 2 and 3 hopefully more compatible .p
Antiferromagnetic Order in MnO Spherical Nanoparticles
We have performed unpolarized and polarized neutron diffraction experiments
on monodisperse 8 nm and 13 nm antiferromagnetic MnO nanoparticles. For the 8
nm sample, the antiferromagnetic transition temperature (114 K) is
suppressed compared to the bulk material (119 K) while for the 13 nm sample
(120 K) is comparable to the bulk. The neutron diffraction data of the
nanoparticles is well described using the bulk MnO magnetic structure but with
a substantially reduced average magnetic moment of 4.20.3 /Mn for
the 8 nm sample and 3.90.2 /Mn for the 13 nm sample. An analysis of
the polarized neutron data on both samples shows that in an individual MnO
nanoparticle about 80 of Mn ions order. These results can be explained by a
structure in which the monodisperse nanoparticles studied here have a core that
behaves similar to the bulk with a surface layer which does not contribute
significantly to the magnetic order.Comment: 7 pages, 5 figure
Remote sensing vegetation index methods to evaluate changes in greenness and evapotranspiration in riparian vegetation in response to the Minute 319 environmental pulse flow to Mexico
During the spring of 2014, 130Â million m3 of water were
released from the United States' Morelos Dam on the lower Colorado River to
Mexico, allowing water to reach the Gulf of California for the first time in
13Â years. Our study assessed the effects of water transfer or ecological
environmental flows from one nation to another, using remote sensing.
Spatial applications for water resource evaluation are important for
binational, integrated water resources management and planning for the
Colorado River, which includes seven basin states in the US plus two
states in Mexico. Our study examined the effects of the historic binational
experiment (the Minute 319 agreement) on vegetative response along the
riparian corridor. We used 250 m Moderate Resolution Imaging
Spectroradiometer (MODIS), Enhanced Vegetation Index (EVI) and 30 m Landsat 8 satellite imagery to track
evapotranspiration (ET) and the normalized difference vegetation index
(NDVI). Our analysis showed an overall increase in NDVI and
evapotranspiration (ET) in the year following the 2014 pulse, which reversed
a decline in those metrics since the last major flood in 2000. NDVI and ET
levels decreased in 2015, but were still significantly higher (P < 0.001) than pre-pulse (2013) levels. Preliminary findings show that the
decline in 2015 persisted into 2016 and 2017. We continue to analyse results
for 2018 in comparison to short-term (2013–2018) and long-term (2000–2018)
trends. Our results support the conclusion that these environmental flows
from the US to Mexico via the Minute 319 “pulse” had a positive, but
short-lived (1Â year), impact on vegetation growth in the delta.</p
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