463 research outputs found
What has NMR taught us about stripes and inhomogeneity?
The purpose of this brief invited paper is to summarize what we have (not)
learned from NMR on stripes and inhomogeneity in La{2-x}Sr{x}CuO{4}. We explain
that the reality is far more complicated than generally accepted.Comment: Accepted for publication in the Proceedings of the LT-23 Conference
(invited
Role of Internal Motions and Molecular Geometry on the NMR Relaxation of Hydrocarbons
The role of internal motions and molecular geometry on H NMR relaxation
times in hydrocarbons is investigated using MD (molecular dynamics)
simulations of the autocorrelation functions for in{\it tra}molecular
and in{\it ter}molecular H-H dipole-dipole interactions
arising from rotational () and translational () diffusion, respectively.
We show that molecules with increased molecular symmetry such as neopentane,
benzene, and isooctane show better agreement with traditional hard-sphere
models than their corresponding straight-chain -alkane, and furthermore that
spherically-symmetric neopentane agrees well with the Stokes-Einstein theory.
The influence of internal motions on the dynamics and relaxation of
-alkanes are investigated by simulating rigid -alkanes and comparing with
flexible (i.e. non-rigid) -alkanes. Internal motions cause the rotational
and translational correlation-times to get significantly shorter
and the relaxation times to get significantly longer, especially for
longer-chain -alkanes. Site-by-site simulations of H's along the chains
indicate significant variations in and across the chain,
especially for longer-chain -alkanes. The extent of the stretched (i.e.
multi-exponential) decay in the autocorrelation functions are
quantified using inverse Laplace transforms, for both rigid and flexible
molecules, and on a site-by-site bases. Comparison of measurements
with the site-by-site simulations indicate that cross-relaxation (partially)
averages-out the variations in and across the chain of
long-chain -alkanes. This work also has implications on the role of
nano-pore confinement on the NMR relaxation of fluids in the organic-matter
pores of kerogen and bitumen
NMR Spin-Rotation Relaxation and Diffusion of Methane
The translational-diffusion coefficient and the spin-rotation
contribution to the H NMR relaxation time for methane (CH) are
investigated using MD (molecular dynamics) simulations, over a wide range of
densities and temperatures , spanning the liquid, supercritical, and
gas phases. The simulated agree well with measurements, without any
adjustable parameters in the interpretation of the simulations. A minimization
technique is developed to compute the angular-velocity for non-rigid spherical
molecules, which is used to simulate the autocorrelation function
for spin-rotation interactions. With increasing (i.e. decreasing ),
shows increasing deviations from the single-exponential decay
predicted by the Langevin theory for hard spheres, and the deviations are
quantified using inverse Laplace transforms of . is
derived from using the kinetic model "km" for gases
(), and the diffusion model "dm" for liquids ().
shows better agreement with measurements at higher ,
while shows better agreement with measurements at lower
. is shown to dominate over the MD simulated H-H
dipole-dipole relaxation at high , while the opposite is found
at low . At high , the simulated spin-rotation correlation-time
agrees with the kinetic collision time for gases, from which
a new relation is inferred, without any adjustable
parameters
Influence of local fullerene orientation on the electronic properties of A3C60 compounds
We have investigated sodium containing fullerene superconductors Na2AC60, A =
Cs, Rb, and K, by Na-23 nuclear magnetic resonance (NMR) spectroscopy at 7.5 T
in the temperature range of 10 to 400 K. Despite the structural differences
from the Rb3C60 class of fullerene superconductors, in these compounds the NMR
line of the tetrahedrally coordinated alkali nuclei also splits into two lines
(T and T') at low temperature. In Na2CsC60 the splitting occurs at 170 K; in
the quenched cubic phase of Na2RbC60 and Na2KC60 we observe split lines at 80
K. Detailed investigations of the spectrum, spin-spin and spin-lattice
relaxation as well as spin-echo double resonance (SEDOR) in Na2CsC60 we show
that these two different tetrahedral sites are mixed on a microscopic scale.
The T and T' sites differ in the orientation of first-neighbor C60 molecules.
We present evidence that the orientations of neighboring molecules are
uncorrelated. Thermally activated molecular reorientations cause an exchange
between the T and T' sites and motional narrowing at high temperature. We infer
the same activation energy, 3300 K, in the temperature range 125 to 300 K. The
spin lattice relaxation rate is the same for T and T' down to 125 K but
different below. Both the spin-lattice relaxation rate and Knight shift are
strongly temperature dependent in the whole range investigated. We interpret
this temperature variation by the effect of phonon excitations involving the
rigid librational motion of the C60 molecules. By extending the understanding
of the structure and molecular dynamics of C60 superconductors, these results
may help in clarifying the effects of the structure on the superconducting
properties.Comment: 13 pages, 10 figures, submitted to PR
Emergence of the Spin Polarized Domains in the Kagome Lattice Heisenberg Antiferromagnet Zn-barlowite (ZnCu)Cu(OD)FBr
Kagome lattice Heisenberg antiferromagnets are known to be highly sensitive
to perturbations caused by structural disorder. NMR is a local probe ideally
suited for investigating such disorder-induced effects, but in practice large
distributions in the conventional one-dimensional NMR data make it difficult to
distinguish the intrinsic behavior expected for pristine kagome quantum spin
liquids from extrinsic effects induced by disorder. Here we report a novel
two-dimensional NMR data acquisition scheme applied to Zn-barlowite
(ZnCu)Cu(OD)FBr kagome lattice, and successfully
correlate the distribution of the low energy spin excitations with that of the
local spin susceptibility. We present evidence for the gradual growth of
domains with a local spin polarization induced by 5\% Cu defect spins
occupying the interlayer non-magnetic Zn sites. These spin polarized
domains account for \% of the sample volume at 2~K, where extrinsic
gapless excitations dominate the low energy sector of spin excitations within
the kagome planes.Comment: 10 figure
Theory and modeling of molecular modes in the NMR relaxation of fluids
Traditional theories of the NMR autocorrelation function for intramolecular
dipole pairs assume single-exponential decay, yet the calculated
autocorrelation of realistic systems display a rich, multi-exponential behavior
resulting in anomalous NMR relaxation dispersion (i.e., frequency dependence).
We develop an approach to model and interpret the multi-exponential
autocorrelation using simple, physical models within a rigorous statistical
mechanical development that encompasses both rotational and translational
diffusion in the same framework. We recast the problem of evaluating the
autocorrelation in terms of averaging over a diffusion propagator whose
evolution is described by a Fokker-Planck equation. The time-independent part
admits an eigenfunction expansion, allowing us to write the propagator as a sum
over modes. Each mode has a spatial part that depends on the specified
eigenfunction, and a temporal part that depends on the corresponding eigenvalue
(i.e., correlation time) with a simple, exponential decay. The spatial part is
a probability distribution of the dipole-pair, analogous to the stationary
states of a quantum harmonic oscillator. Drawing inspiration from the idea of
inherent structures in liquids, we interpret each of the spatial contributions
as a specific molecular mode. These modes can be used to model and predict NMR
dipole-dipole relaxation dispersion of fluids by incorporating phenomena on the
molecular level. We validate our statistical mechanical description of the
distribution in molecular modes with molecular dynamics simulations interpreted
without any relaxation models or adjustable parameters: the most important
poles in the Pad{\'e}-Laplace transform of the simulated autocorrelation agree
with the eigenvalues predicted by the theory
The First Two Years of Electromagnetic Follow-Up with Advanced LIGO and Virgo
We anticipate the first direct detections of gravitational waves (GWs) with
Advanced LIGO and Virgo later this decade. Though this groundbreaking technical
achievement will be its own reward, a still greater prize could be observations
of compact binary mergers in both gravitational and electromagnetic channels
simultaneously. During Advanced LIGO and Virgo's first two years of operation,
2015 through 2016, we expect the global GW detector array to improve in
sensitivity and livetime and expand from two to three detectors. We model the
detection rate and the sky localization accuracy for binary neutron star (BNS)
mergers across this transition. We have analyzed a large, astrophysically
motivated source population using real-time detection and sky localization
codes and higher-latency parameter estimation codes that have been expressly
built for operation in the Advanced LIGO/Virgo era. We show that for most BNS
events the rapid sky localization, available about a minute after a detection,
is as accurate as the full parameter estimation. We demonstrate that Advanced
Virgo will play an important role in sky localization, even though it is
anticipated to come online with only one-third as much sensitivity as the
Advanced LIGO detectors. We find that the median 90% confidence region shrinks
from ~500 square degrees in 2015 to ~200 square degrees in 2016. A few distinct
scenarios for the first LIGO/Virgo detections emerge from our simulations.Comment: 17 pages, 11 figures, 5 tables. For accompanying data, see
http://www.ligo.org/scientists/first2year
Health, Nutrition and Healthcare Availability (Survey 2, Report 2)
With support and collaboration from the W.K. Kellogg Foundation through the America Healing initiative, researchers at the University of Michigan are leading the National Voices Project (NVP) from 2011-2015. The central goals of the NVP are to examine the sources of racial/ethnic inequity and other disparities for children in the United States today, identify interventions that address disparities effectively, and inform the public dialogue about racial healing and racial equality. The NVP offers a fresh perspective on community-level opportunities for children throughout the country, in the domains of health and nutrition, education, and economic security -- through the eyes of adults whose work and volunteer efforts affect such opportunities. In other words, the NVP reflects the perceptions of individuals throughout the United States who are in a position to improve children's opportunities in the future. The questionnaire for NVP Survey 2 was developed by the National Voices Project team at the University of Michigan, with input from WKKF collaborators. We examined how individuals who work or volunteer with children view opportunities for education, health and healthcare, and economic well-being related to children and adolescents. Many of the questions were identical to questions fielded for NVP Survey 1 in 2011, to facilitate comparisons of responses across these different samples and over time. New questions in NVP Survey 2 centered on respondents' perceptions of segregation and inequities in the communities they know best, and on respondents' awareness about efforts to bridge racial/ethnic inequities in those communities
- …