1,754 research outputs found
Spin diffusion of correlated two-spin states in a dielectric crystal
Reciprocal space measurements of spin diffusion in a single crystal of
calcium fluoride (CaF) have been extended to dipolar ordered states. The
experimental results for the component of the spin diffusion parallel with the
external field are cm/s for the
[001] direction and cm/s for the
[111] direction. The diffusion rates for dipolar order are significantly faster
than those for Zeeman order and are considerably faster than predicted by
simple theoretical models. It is suggested that constructive interference in
the transport of the two spin state is responsible for this enhancement. As
expected the anisotropy in the diffusion rates is observed to be significantly
less for dipolar order compared to the Zeeman case.Comment: 4 pages, 2 figures. Resubmitted to PRL - new figure added /
discussion expande
Hydrodynamic approach to coherent nuclear spin transport
We develop a linear response formalism for nuclear spin diffusion in a
dipolar coupled solid. The theory applies to the high-temperature,
long-wavelength regime studied in the recent experiments of Boutis et al.
[Phys. Rev. Lett. 92, 137201 (2004)], which provided direct measurement of
interspin energy diffusion in such a system. A systematic expansion of Kubo's
formula in the flip-flop term of the Hamiltonian is used to calculate the
diffusion coefficients. We show that this approach is equivalent to the method
of Lowe and Gade [Phys. Rev. 156, 817 (1967)] and Kaplan [Phys. Rev. B 2, 4578
(1970)], but has several calculational and conceptual advantages. Although the
lowest orders in this expansion agree with the experimental results for
magnetization diffusion, this is not the case for energy diffusion. Possible
reasons for this disparity are suggested.Comment: 7 pages, REVTeX4; Published Versio
Conditions for the Quantum to Classical Transition: Trajectories vs. Phase Space Distributions
We contrast two sets of conditions that govern the transition in which
classical dynamics emerges from the evolution of a quantum system. The first
was derived by considering the trajectories seen by an observer (dubbed the
``strong'' transition) [Bhattacharya, et al., Phys. Rev. Lett. 85: 4852
(2000)], and the second by considering phase-space densities (the ``weak''
transition) [Greenbaum, et al., Chaos 15, 033302 (2005)]. On the face of it
these conditions appear rather different. We show, however, that in the
semiclassical regime, in which the action of the system is large compared to
, and the measurement noise is small, they both offer an essentially
equivalent local picture. Within this regime, the weak conditions dominate
while in the opposite regime where the action is not much larger than Planck's
constant, the strong conditions dominate.Comment: 8 pages, 2 eps figure
Parameter scaling in the decoherent quantum-classical transition for chaotic systems
The quantum to classical transition has been shown to depend on a number of
parameters. Key among these are a scale length for the action, , a
measure of the coupling between a system and its environment, , and, for
chaotic systems, the classical Lyapunov exponent, . We propose
computing a measure, reflecting the proximity of quantum and classical
evolutions, as a multivariate function of and searching for
transformations that collapse this hyper-surface into a function of a composite
parameter . We report results
for the quantum Cat Map, showing extremely accurate scaling behavior over a
wide range of parameters and suggest that, in general, the technique may be
effective in constructing universality classes in this transition.Comment: Submitte
Seven exercises planned to stimulate the flow of ideas in creative composition
Thesis (Ed.M.)--Boston Universit
Suspension and Measurement of Graphene and Bi2Se3 Atomic Membranes
Coupling high quality, suspended atomic membranes to specialized electrodes
enables investigation of many novel phenomena, such as spin or Cooper pair
transport in these two dimensional systems. However, many electrode materials
are not stable in acids that are used to dissolve underlying substrates. Here
we present a versatile and powerful multi-level lithographical technique to
suspend atomic membranes, which can be applied to the vast majority of
substrate, membrane and electrode materials. Using this technique, we
fabricated suspended graphene devices with Al electrodes and mobility of 5500
cm^2/Vs. We also demonstrate, for the first time, fabrication and measurement
of a free-standing thin Bi2Se3 membrane, which has low contact resistance to
electrodes and a mobility of >~500 cm^2/Vs
Why highly expressed proteins evolve slowly
Much recent work has explored molecular and population-genetic constraints on
the rate of protein sequence evolution. The best predictor of evolutionary rate
is expression level, for reasons which have remained unexplained. Here, we
hypothesize that selection to reduce the burden of protein misfolding will
favor protein sequences with increased robustness to translational missense
errors. Pressure for translational robustness increases with expression level
and constrains sequence evolution. Using several sequenced yeast genomes,
global expression and protein abundance data, and sets of paralogs traceable to
an ancient whole-genome duplication in yeast, we rule out several confounding
effects and show that expression level explains roughly half the variation in
Saccharomyces cerevisiae protein evolutionary rates. We examine causes for
expression's dominant role and find that genome-wide tests favor the
translational robustness explanation over existing hypotheses that invoke
constraints on function or translational efficiency. Our results suggest that
proteins evolve at rates largely unrelated to their functions, and can explain
why highly expressed proteins evolve slowly across the tree of life.Comment: 40 pages, 3 figures, with supporting informatio
Xenopus fraseri: Mr. Fraser, where did your frog come from?
A comprehensive, accurate, and revisable alpha taxonomy is crucial for biodiversity studies, but is challenging when data from reference specimens are difficult to collect or observe. However, recent technological advances can overcome some of these challenges. To illustrate this, we used modern approaches to tackle a centuries-old taxonomic enigma presented by Fraser’s Clawed Frog, Xenopus fraseri, including whether X. fraseri is different from other species, and if so, where it is situated geographically and phylogenetically. To facilitate these inferences, we used high-resolution techniques to examine morphological variation, and we generated and analyzed complete mitochondrial genome sequences from all Xenopus species, including >150-year-old type specimens. Our results demonstrate that X. fraseri is indeed distinct from other species, firmly place this species within a phylogenetic context, and identify its minimal geographic distribution in northern Ghana and northern Cameroon. These data also permit novel phylogenetic resolution into this intensively studied and biomedically important group. Xenopus fraseri was formerly thought to be a rainforest endemic placed alongside species in the amieti species group; in fact this species occurs in arid habitat on the borderlands of the Sahel, and is the smallest member of the muelleri species group. This study illustrates that the taxonomic enigma of Fraser’s frog was a combined consequence of sparse collection records, interspecies conservation and intraspecific polymorphism in external anatomy, and type specimens with unusual morphology
Text Messaging for Disease Monitoring in Childhood Nephrotic Syndrome
© 2019 International Society of Nephrology Introduction: There is limited information on effective disease monitoring for prompt interventions in childhood nephrotic syndrome. We examined the feasibility and effectiveness of a novel text messaging system (SMS) for disease monitoring in a multicenter, prospective study. Methods: A total of 127 patients results, symptoms, and medication adherence were sent to a designated caregiver (n = 116) or adolescent patient (n = 3). Participants responded by texting. Feasibility of SMS was assessed by SMS adoption, retention, and engagement, and concordance between participant-reported results and laboratory/clinician assessments. The number of disease relapses and time-to-remission data captured by SMS were compared with data collected by conventional visits. Results: A total of 119 of 127 (94%) patients agreed to SMS monitoring. Retention rate was 94%, with a median follow-up of 360 days (interquartile range [IQR] 353–362). Overall engagement was high, with a median response rate of 87% (IQR, 68–97). Concordance between SMS-captured home urine protein results and edema status with same-day in-person study visit was excellent (kappa values 0.88 and 0.92, respectively). SMS detected a total of 108 relapse events compared with 41 events captured by scheduled visits. Median time to remission after enrollment was 22 days as captured by SMS versus 50 days as captured by scheduled visits. Conclusion: SMS was well accepted by caregivers and adolescent patients and reliably captured nephrotic syndrome disease activity between clinic visits. Additional studies are needed to explore the impact of SMS on disease outcomes
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