47,806 research outputs found
Whole-brain patterns of 1H-magnetic resonance spectroscopy imaging in Alzheimer's disease and dementia with Lewy bodies
Acknowledgements We thank Craig Lambert for his help in processing the MRS data. The study was funded by the Sir Jules Thorn Charitable Trust (grant ref: 05/JTA) and was supported by the National Institute for Health Research (NIHR) Newcastle Biomedical Research Centre and the Biomedical Research Unit in Lewy Body Dementia based at Newcastle upon Tyne Hospitals National Health Service (NHS) Foundation Trust and Newcastle University and the NIHR Biomedical Research Centre and Biomedical Research Unit in Dementia based at Cambridge University Hospitals NHS Foundation Trust and the University of Cambridge.Peer reviewedPublisher PD
The Decay of Debris Disks around Solar-Type Stars
We present a Spitzer MIPS study of the decay of debris disk excesses at 24
and 70 m for 255 stars of types F4 - K2. We have used multiple tests,
including consistency between chromospheric and X-ray activity and placement on
the HR diagram, to assign accurate stellar ages. Within this spectral type
range, at 24 m, of the stars younger than 5 Gyr have
excesses at the 3 level or more, while none of the older stars do,
confirming previous work. At 70 m, of the younger stars
have excesses at 3 significance, while only
% of the older stars do. To characterize the far infrared
behavior of debris disks more robustly, we double the sample by including stars
from the DEBRIS and DUNES surveys. For the F4 - K4 stars in this combined
sample, there is only a weak (statistically not significant) trend in the
incidence of far infrared excess with spectral type (detected fractions of
21.9, late F; 16.5, G; and
16.9, early K). Taking this spectral type range together,
there is a significant decline between 3 and 4.5 Gyr in the incidence of
excesses with fractional luminosities just under . There is an
indication that the timescale for decay of infrared excesses varies roughly
inversely with the fractional luminosity. This behavior is consistent with
theoretical expectations for passive evolution. However, more excesses are
detected around the oldest stars than is expected from passive evolution,
suggesting that there is late-phase dynamical activity around these stars.Comment: 46 pages. 7 figures. Accepted to Ap
Topological Gauge Structure and Phase Diagram for Weakly Doped Antiferromagnets
We show that the topological gauge structure in the phase string theory of
the {\rm t-J} model gives rise to a global phase diagram of antiferromagnetic
(AF) and superconducting (SC) phases in a weakly doped regime. Dual confinement
and deconfinement of holons and spinons play essential roles here, with a
quantum critical point at a doping concentration . The complex
experimental phase diagram at low doping is well described within such a
framework.Comment: 4 pages, 2 figures, modified version, to appear in Phys. Rev. Let
Estimation of soil and vegetation temperatures with multiangular thermal infrared observations: IMGRASS, HEIFE, and SGP 1997 experiments
The potential of directional observations in the thermal infrared region for land surface studies is a largely uncharted area of research. The availability of the dual-view Along Track Scanning Radiometer (ATSR) observations led to explore new opportunities in this direction. In the context of studies on heat transfer at heterogeneous land surfaces, multiangular thermal infrared (TIR) observations offer the opportunity of overcoming fundamental difficulties in modeling sparse canopies. Three case studies were performed on the estimation of the component temperatures of foliage and soil. The first one included the use of multi-temporal field measurements at view angles of 0°, 23° and 52°. The second and third one were done with directional ATSR observations at view angles of 0° and 53° only. The first one was a contribution to the Inner-Mongolia Grassland Atmosphere Surface Study (IMGRASS) experiment in China, the second to the Hei He International Field Experiment (HEIFE) in China and the third one to the Southern Great Plains 1997 (SGP 1997) experiment in Oklahoma, United States. The IMGRASS experiment provided useful insights on the applicability of a simple linear mixture model to the analysis of observed radiance. The HEIFE case study was focused on the large oasis of Zhang-Ye and led to useful estimates of soil and vegetation temperatures. The SGP 1997 contributed a better understanding of the impact of spatial heterogeneity on the accuracy of retrieved foliage and soil temperatures. Limitations in the approach due to varying radiative and boundary layer forcing and to the difference in spatial resolution between the forward and the nadir view are evaluated through a combination of modeling studies and analysis of field data
Bose-Einstein condensation of trapped interacting spin-1 atoms
We investigate Bose-Einstein condensation of trapped spin-1 atoms with
ferromagnetic or antiferromagnetic two-body contact interactions. We adopt the
mean field theory and develop a Hartree-Fock-Popov type approximation in terms
of a semiclassical two-fluid model. For antiferromagnetic interactions, our
study reveals double condensations as atoms in the state never seem
to condense under the constraints of both the conservation of total atom number
and magnetization . For ferromagnetic interactions, however, triple
condensations can occur. Our results can be conveniently understood in terms of
the interplay of three factors: (anti) ferromagnetic atom-atom interactions,
conservation, and the miscibilities between and among different condensed
components.Comment: RevTex 4, 9 pages, 5 eps figures, to appear in Phys. Rev. A, vol 70,
p
Voltage-controlled wavelength conversion by terahertz electro-optic modulation in double quantum wells
An undoped double quantum well (DQW) was driven with a terahertz (THz)
electric field of frequency \omega_{THz} polarized in the growth direction,
while simultaneously illuminated with a near-infrared (NIR) laser at frequency
\omega_{NIR}. The intensity of NIR upconverted sidebands
\omega_{sideband}=\omega_{NIR} + \omega_{THz} was maximized when a dc voltage
applied in the growth direction tuned the excitonic states into resonance with
both the THz and NIR fields. There was no detectable upconversion far from
resonance. The results demonstrate the possibility of using gated DQW devices
for all-optical wavelength shifting between optical communication channels
separated by up to a few THz.Comment: 3 pages, 6 figures. Figures 5 and 6 are JPEG files, figures/fig5.jpg
and fig6.jp
Isotopic difference in the heteronuclear loss rate in a two-species surface trap
We have realized a two-species mirror-magneto-optical trap containing a
mixture of Rb (Rb) and Cs atoms. Using this trap, we have
measured the heteronuclear collisional loss rate due to
intra-species cold collisions. We find a distinct difference in the magnitude
and intensity dependence of for the two isotopes Rb and
Rb which we attribute to the different ground-state hyperfine splitting
energies of the two isotopes.Comment: 4 pages, 2 figure
Eigenvalue variance bounds for Wigner and covariance random matrices
This work is concerned with finite range bounds on the variance of individual
eigenvalues of Wigner random matrices, in the bulk and at the edge of the
spectrum, as well as for some intermediate eigenvalues. Relying on the GUE
example, which needs to be investigated first, the main bounds are extended to
families of Hermitian Wigner matrices by means of the Tao and Vu Four Moment
Theorem and recent localization results by Erd\"os, Yau and Yin. The case of
real Wigner matrices is obtained from interlacing formulas. As an application,
bounds on the expected 2-Wasserstein distance between the empirical spectral
measure and the semicircle law are derived. Similar results are available for
random covariance matrices
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