552 research outputs found
Ultraslow Electron Spin Dynamics in GaAs Quantum Wells Probed by Optically Pumped NMR
Optically pumped nuclear magnetic resonance (OPNMR) measurements were
performed in two different electron-doped multiple quantum well samples near
the fractional quantum Hall effect ground state nu=1/3. Below 0.5K, the spectra
provide evidence that spin-reversed charged excitations of the nu=1/3 ground
state are localized over the NMR time scale of ~40 microseconds. Furthermore,
by varying NMR pulse parameters, the electron spin temperature (as measured by
the Knight shift) could be driven above the lattice temperature, which shows
that the value of the electron spin-lattice relaxation time lies between 100
microseconds and 500 milliseconds at nu=1/3.Comment: 6 pages (REVTEX), 6 eps figures embedded in text; published version;
minor changes to match published versio
Palomar 5 and its Tidal Tails::A Search for New Members in the Tidal Stream
In this paper we present the results of a search for members of the globular
cluster Palomar 5 and its associated tidal tails. The analysis has been
performed using intermediate and low resolution spectroscopy with the AAOmega
spectrograph on the Anglo-Australian Telescope. Based on kinematics, line
strength and photometric information, we identify 39 new red giant branch stars
along 20 of the tails, a larger angular extent than has been
previously studied. We also recover eight previously known tidal tail members.
Within the cluster, we find seven new red giant and one blue horizontal branch
members and confirm a further twelve known red giant members. In total, we
provide velocity data for 67 stars in the cluster and the tidal tails. Using a
maximum likelihood technique, we derive a radial velocity for Pal 5 of km s and a velocity dispersion of km s. We
confirm and extend the linear velocity gradient along the tails of km s deg, with an associated intrinsic velocity dispersion
of km s. Neither the velocity gradient nor the dispersion
change in any significant way with angular distance from the cluster, although
there is some indication that the gradient may be smaller at greater angular
distances in the trailing tail. Our results verify the tails as kinematically
cold structures and will allow further constraints to be placed on the orbit of
Pal 5, ultimately permitting a greater understanding of the shape and extent of
the Galaxy's dark matter halo.Comment: 14 pages, 9 figures, 4 tables; Accepted for publication in MNRA
Association of Lifestyle and Genetic Risk With Incidence of Dementia
This is the final version. Available from the American Medical Association via the DOI in this recordImportance: Genetic factors increase risk of dementia, but the extent to which this can be offset by lifestyle factors is unknown. Objective: To investigate whether a healthy lifestyle is associated with lower risk of dementia regardless of genetic risk. Design, Setting, and Participants: A retrospective cohort study that included adults of European ancestry aged at least 60 years without cognitive impairment or dementia at baseline. Participants joined the UK Biobank study from 2006 to 2010 and were followed up until 2016 or 2017. Exposures: A polygenic risk score for dementia with low (lowest quintile), intermediate (quintiles 2 to 4), and high (highest quintile) risk categories and a weighted healthy lifestyle score, including no current smoking, regular physical activity, healthy diet, and moderate alcohol consumption, categorized into favorable, intermediate, and unfavorable lifestyles. Main Outcomes and Measures: Incident all-cause dementia, ascertained through hospital inpatient and death records. Results: A total of 196 383 individuals (mean [SD] age, 64.1 [2.9] years; 52.7% were women) were followed up for 1 545 433 person-years (median [interquartile range] follow-up, 8.0 [7.4-8.6] years). Overall, 68.1% of participants followed a favorable lifestyle, 23.6% followed an intermediate lifestyle, and 8.2% followed an unfavorable lifestyle. Twenty percent had high polygenic risk scores, 60% had intermediate risk scores, and 20% had low risk scores. Of the participants with high genetic risk, 1.23% (95% CI, 1.13%-1.35%) developed dementia compared with 0.63% (95% CI, 0.56%-0.71%) of the participants with low genetic risk (adjusted hazard ratio, 1.91 [95% CI, 1.64-2.23]). Of the participants with a high genetic risk and unfavorable lifestyle, 1.78% (95% CI, 1.38%-2.28%) developed dementia compared with 0.56% (95% CI, 0.48%-0.66%) of participants with low genetic risk and favorable lifestyle (hazard ratio, 2.83 [95% CI, 2.09-3.83]). There was no significant interaction between genetic risk and lifestyle factors (P = .99). Among participants with high genetic risk, 1.13% (95% CI, 1.01%-1.26%) of those with a favorable lifestyle developed dementia compared with 1.78% (95% CI, 1.38%-2.28%) with an unfavorable lifestyle (hazard ratio, 0.68 [95% CI, 0.51-0.90]). Conclusions and Relevance: Among older adults without cognitive impairment or dementia, both an unfavorable lifestyle and high genetic risk were significantly associated with higher dementia risk. A favorable lifestyle was associated with a lower dementia risk among participants with high genetic risk.James Tudor FoundationMary Kinross Charitable TrustHalpin TrustNational Institute for Health Research (NIHR)National Health and Medical Research Council, AustraliaNational Institute on Aging/National Institutes of HealthEngineering and Physical Sciences Research Council (EPSRC
Substructures and tidal distortions in the Magellanic stellar periphery
We use a new panoramic imaging survey, conducted with the Dark Energy Camera,
to map the stellar fringes of the Large and Small Magellanic Clouds to
extremely low surface brightness V 32 mag arcsec. Our results
starkly illustrate the closely interacting nature of the LMC-SMC pair. We show
that the outer LMC disk is strongly distorted, exhibiting an irregular shape,
evidence for warping, and significant truncation on the side facing the SMC.
Large diffuse stellar substructures are present both to the north and south of
the LMC, and in the inter-Cloud region. At least one of these features appears
co-spatial with the bridge of RR Lyrae stars that connects the Clouds. The SMC
is highly disturbed -- we confirm the presence of tidal tails, as well as a
large line-of-sight depth on the side closest to the LMC. Young,
intermediate-age, and ancient stellar populations in the SMC exhibit strikingly
different spatial distributions. In particular, those with ages 1.5-4
Gyr exhibit a spheroidal distribution with a centroid offset from that of the
oldest stars by several degrees towards the LMC. We speculate that the
gravitational influence of the LMC may already have been perturbing the gaseous
component of the SMC several Gyr ago. With careful modeling, the variety of
substructures and tidal distortions evident in the Magellanic periphery should
tightly constrain the interaction history of the Clouds.Comment: Submitted to ApJL; 5 page
Spectroscopic Evidence for the Localization of Skyrmions near Nu=1 as T->0
Optically pumped nuclear magnetic resonance measurements of Ga-71 spectra
were carried out in an n-doped GaAs/Al0.1Ga0.9As multiple quantum well sample
near the integer quantum Hall ground state Nu=1. As the temperature is lowered
(down to T~0.3 K), a ``tilted plateau'' emerges in the Knight shift data, which
is a novel experimental signature of quasiparticle localization. The dependence
of the spectra on both T and Nu suggests that the localization is a collective
process. The frozen limit spectra appear to rule out a 2D lattice of
conventional skyrmions.Comment: 4 pages (REVTEX), 5 eps figures embedded in text, published versio
Nuclear Spin Relaxation for Higher Spin
We study the relaxation of a spin I that is weakly coupled to a quantum
mechanical environment. Starting from the microscopic description, we derive a
system of coupled relaxation equations within the adiabatic approximation.
These are valid for arbitrary I and also for a general stationary
non--equilibrium state of the environment. In the case of equilibrium, the
stationary solution of the equations becomes the correct Boltzmannian
equilibrium distribution for given spin I. The relaxation towards the
stationary solution is characterized by a set of relaxation times, the longest
of which can be shorter, by a factor of up to 2I, than the relaxation time in
the corresponding Bloch equations calculated in the standard perturbative way.Comment: 4 pages, Latex, 2 figure
Fast Matrix Multiplication via Compiler-only Layered Data Reorganization and Intrinsic Lowering
The resurgence of machine learning has increased the demand for
high-performance basic linear algebra subroutines (BLAS), which have long
depended on libraries to achieve peak performance on commodity hardware.
High-performance BLAS implementations rely on a layered approach that consists
of tiling and packing layers, for data (re)organization, and micro kernels that
perform the actual computations. The creation of high-performance micro kernels
requires significant development effort to write tailored assembly code for
each architecture. This hand optimization task is complicated by the recent
introduction of matrix engines by IBM's POWER10 MMA, Intel AMX, and Arm ME to
deliver high-performance matrix operations. This paper presents a compiler-only
alternative to the use of high-performance libraries by incorporating, to the
best of our knowledge and for the first time, the automatic generation of the
layered approach into LLVM, a production compiler. Modular design of the
algorithm, such as the use of LLVM's matrix-multiply intrinsic for a clear
interface between the tiling and packing layers and the micro kernel, makes it
easy to retarget the code generation to multiple accelerators. The use of
intrinsics enables a comprehensive performance study. In processors without
hardware matrix engines, the tiling and packing delivers performance up to 22x
(Intel), for small matrices, and more than 6x (POWER9), for large matrices,
faster than PLuTo, a widely used polyhedral optimizer. The performance also
approaches high-performance libraries and is only 34% slower than OpenBLAS and
on-par with Eigen for large matrices. With MMA in POWER10 this solution is, for
large matrices, over 2.6x faster than the vector-extension solution, matches
Eigen performance, and achieves up to 96% of BLAS peak performance
Forward and Back: Kinematics of the Palomar 5 Tidal Tails
The tidal tails of Palomar 5 (Pal 5) have been the focus of many
spectroscopic studies in an attempt to identify individual stars lying along
the stream and characterise their kinematics. The well-studied trailing tail
has been explored out to a distance of 15^\text{o} from the cluster centre,
while less than four degrees have been examined along the leading tail. In this
paper, we present results of a spectroscopic study of two fields along the
leading tail that we have observed with the AAOmega spectrograph on the
Anglo-Australian telescope. One of these fields lies roughly 7^\text{o} along
the leading tail, beyond what has been previously been explored
spectroscopically. Combining our measurements of kinematics and line strengths
with Pan-STARRS1 photometric data and Gaia EDR3 astrometry, we adopt a
probabilistic approach to identify 16 stars with high probability of belonging
to the Pal 5 stream. Eight of these stars lie in the outermost field and their
sky positions confirm the presence of ``fanning'' in the leading arm. We also
revisit previously-published radial velocity studies and incorporate Gaia EDR3
astrometry to remove interloping field stars. With a final sample of 109 {\it
bona fide} Pal 5 cluster and tidal stream stars, we characterise the 3D
kinematics along the the full extent of the system. We provide this catalogue
for future modeling work.Comment: 13 pages, 8 figures, accepted for publication in MNRAS. The extended
version of Table 4 is available as an ancillery file, and will be
supplementary material in MNRA
Optically Pumped NMR Measurements of the Electron Spin Polarization in GaAs Quantum Wells near Landau Level Filling Factor nu=1/3
The Knight shift of Ga-71 nuclei is measured in two different electron-doped
multiple quantum well samples using optically pumped NMR. These data are the
first direct measurements of the electron spin polarization,
P(nu,T)=/max, near nu=1/3. The P(T) data at nu=1/3 probe the
neutral spin-flip excitations of a fractional quantum Hall ferromagnet. In
addition, the saturated P(nu) drops on either side of nu=1/3, even in a Btot=12
Tesla field. The observed depolarization is quite small, consistent with an
average of about 0.1 spin-flips per quasihole (or quasiparticle), a value which
does not appear to be explicable by the current theoretical understanding of
the FQHE near nu=1/3.Comment: 4 pages (REVTEX), 5 eps figures embedded in text; minor changes,
published versio
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