100 research outputs found
Crossover from 2-dimensional to 1-dimensional collective pinning in NbSe3
We have fabricated NbSe structures with widths comparable to the
Fukuyama-Lee-Rice phase-coherence length. For samples already in the
2-dimensional pinning limit, we observe a crossover from 2-dimensional to
1-dimensional collective pinning when the crystal width is less than 1.6
m, corresponding to the phase-coherence length in this direction. Our
results show that surface pinning is negligible in our samples, and provide a
means to probe the dynamics of single domains giving access to a new regime in
charge-density wave physics.Comment: 4 pages, 2 figures, and 1 table. Accepted for publication in Physical
Review
The effects of cognitive impairment on the multi-scale dynamics of standing postural control during visual-search in older men
BackgroundCognitive impairment disrupts postural control, particularly when standing while performing an unrelated cognitive task (i.e., dual-tasking). The temporal dynamics of standing postural sway are “complex,” and such complexity may reflect the capacity of the postural control system to adapt to task demands. We aimed to characterize the impact of cognitive impairment on such sway complexity in older adults.MethodsForty-nine older adult males (Alzheimer’s disease (AD): n = 21; mild cognitive impairment (MCI): n = 13; cognitively-intact: n = 15) completed two 60-s standing trials in each of single-task and visual-search dual-task conditions. In the dual-task condition, participants were instructed to count the frequency of a designated letter in a block of letters projected on screen. The sway complexity of center-of-pressure fluctuations in anterior–posterior (AP) and medial-lateral (ML) direction was quantified using multiscale entropy. The dual-task cost to complexity was obtained by calculating the percent change of complexity from single- to dual-task condition.ResultsRepeated-measures ANOVAs revealed significant main effects of group (F > 4.8, p < 0.01) and condition (F = 7.7, p < 0.007) on both AP and ML sway complexity; and significant interaction between group and condition for ML sway complexity (F = 3.7, p = 0.03). The AD group had the lowest dual-task ML complexity, as well as greater dual-task cost to ML (p = 0.03) compared to the other two groups. Visual-search task accuracy was correlated with ML sway complexity in the dual-task condition (r = 0.42, p = 0.007), and the dual-task cost to ML sway complexity (r = 0.39, p = 0.01) across all participants.ConclusionAD-related cognitive impairment was associated with a greater relative reduction in postural sway complexity from single- to dual-tasking. Sway complexity appears to be sensitive to the impact of cognitive impairment on standing postural control
Temporally ordered collective creep and dynamic transition in the charge-density-wave conductor NbSe3
We have observed an unusual form of creep at low temperatures in the
charge-density-wave (CDW) conductor NbSe. This creep develops when CDW
motion becomes limited by thermally-activated phase advance past individual
impurities, demonstrating the importance of local pinning and related
short-length-scale dynamics. Unlike in vortex lattices, elastic collective
dynamics on longer length scales results in temporally ordered motion and a
finite threshold field. A first-order dynamic phase transition from creep to
high-velocity sliding produces "switching" in the velocity-field
characteristic.Comment: 4 pages, 4 eps figures; minor clarifications To be published in Phys.
Rev. Let
Contributions of spontaneous phase slippage to linear and non-linear conduction near the Peierls transition in thin samples of o-TaS_3
In the Peierls state very thin samples of TaS_3 (cross-section area \sim
10^{-3} mkm^2) are found to demonstrate smearing of the I-V curves near the
threshold field. With approaching the Peierls transition temperature, T_P, the
smearing evolves into smooth growth of conductance from zero voltage
interpreted by us as the contribution of fluctuations to the non--linear
conductance. We identify independently the fluctuation contribution to the
linear conductance near T_P. Both linear and non-linear contributions depend on
temperature with close activation energies \sim (2 - 4) x 10^3 K and apparently
reveal the same process. We reject creep of the {\it continuous} charge-density
waves (CDWs) as the origin of this effect and show that it is spontaneous phase
slippage that results in creep of the CDW. A model is proposed accounting for
both the linear and non-linear parts of the fluctuation conduction up to T_P.Comment: 6 pages, 5 Postscript figure, RevTeX, accepted for publication in PR
Using Cerebrospinal Fluid Biomarker Testing to Target Treatment to Patients with Mild Cognitive Impairment: A Cost-Effectiveness Analysis
Objective Cerebrospinal fluid (CSF) biomarkers are shown to facilitate a risk identification of patients with mild cognitive impairment (MCI) into different risk levels of progression to Alzheimer’s disease (AD). Knowing a patient’s risk level provides an opportunity for earlier interventions, which could result in potential greater benefits. We assessed the cost effectiveness of the use of CSF biomarkers in MCI patients where the treatment decision was based on patients’ risk level.
Methods We developed a state-transition model to project lifetime quality-adjusted life-years (QALYs) and costs for a cohort of 65-year-old MCI patients from a US societal perspective. We compared four test-and-treat strategies where the decision to treat was based on a patient’s risk level (low, intermediate, high) of progressing to AD with two strategies without testing, one where no patients were treated during the MCI phase and in the other all patients were treated. We performed deterministic and probabilistic sensitivity analyses to evaluate parameter uncertainty.
Results Testing and treating low-risk MCI patients was the most cost-effective strategy with an incremental cost-effectiveness ratio (ICER) of US18,900 and US$50,100 per QALY, respectively.
Conclusion Based on the best available evidence regarding the treatment effectiveness for MCI, this study suggests the potential value of performing CSF biomarker testing for early targeted treatments among MCI patients with a narrow range for the ICER
Understanding Cataract Risk in Aerospace Flight Crew And Review of Mechanisms of Cataract Formation
Induction of cataracts by occupational exposure in flight crew has been an important topic of interest in aerospace medicine in the past five years, in association with numerous reports of flight-associated disease incidences. Due to numerous confounding variables, it has been difficult to determine if there is increased cataract risk directly caused by interaction with the flight environment, specifically associated with added radiation exposure during flight. Military aviator records from the United States Air Force (USAF) and Navy (USN) and US astronauts at the National Aeronautics and Space Administration (NASA)/Lyndon B. Johnson Space Center (JSC) were evaluated for the presence, location and age of diagnosis of cataracts. Military aviators were found to have a statistically significant younger average age of onset of their cataracts compared with astronauts, however the incidence density of cataracts was found to be statistically higher in astronauts than in military aviators. USAF and USN aviator s cataracts were most commonly located in the posterior subcapsular region of the lens while astronauts cataracts were most likely to originate generally in the cortical zone. A prospective clinical trial which controls for confounding variables in examination technique, cataract classification, diet, exposure, and pharmacological intervention is needed to determine what percentage of the risk for cataracts are due to radiation, and how to best develop countermeasures to protect flight crews from radiation bioeffects in the future
NbSe3: Effect of Uniaxial Stress on the Threshold Field and Fermiology
We have measured the effect of uniaxial stress on the threshold field ET for
the motion of the upper CDW in NbSe3. ET exhibits a critical behavior, ET ~ (1
- e/ec)^g, wher e is the strain, and ec is about 2.6% and g ~ 1.2. This
ecpression remains valid over more than two decades of ET, up to the highest
fields of about 1.5keV/m. Neither g nor ec is very sensitive to the impurity
concentraction. The CDW transition temperature Tp decreases linearly with e at
a rate dTp/de = -10K/%, and it does not show any anomaly near ec. Shubnikov
de-Haas measurements show that the extremal area of the Fermi surface decreases
with increasing strain. The results suggest that there is an intimate
relationship between pinning of the upper CDW and the Fermiology of NbSe3.Comment: 4 pages, 5 figure
Thermal Rounding of the Charge Density Wave Depinning Transition
The rounding of the charge density wave depinning transition by thermal noise
is examined. Hops by localized modes over small barriers trigger
``avalanches'', resulting in a creep velocity much larger than that expected
from comparing thermal energies with typical barriers. For a field equal to the
depinning field, the creep velocity is predicted to have a {\em
power-law} dependence on the temperature ; numerical computations confirm
this result. The predicted order of magnitude of the thermal rounding of the
depinning transition is consistent with rounding seen in experiment.Comment: 12 pages + 3 Postscript figure
X-Ray Scattering Measurements of the Transient Structure of a Driven Charge-Density-Wave
We report time-resolved x-ray scattering measurements of the transient
structural response of the sliding {\bf Q} charge-density-wave (CDW) in
NbSe to a reversal of the driving electric field. The observed time scale
characterizing this response at 70K varies from 15 msec for driving
fields near threshold to 2 msec for fields well above threshold. The
position and time-dependent strain of the CDW is analyzed in terms of a
phenomenological equation of motion for the phase of the CDW order parameter.
The value of the damping constant, eV
seconds \AA, is in excellent agreement with the value
determined from transport measurements. As the driving field approaches
threshold from above, the line shape becomes bimodal, suggesting that the CDW
does not depin throughout the entire sample at one well-defined voltage.Comment: revtex 3.0, 7 figure
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