392 research outputs found
Universal scaling of nonlocal and local resistance fluctuations in small wires
Resistance fluctuations in small metal samples result from coherent transport of the carriers. The wave functions of the carriers extend into regions which are not accessible classically. We have directly measured the length dependence of the nonlocal magnetoresistance fluctuations in Sb and Au wires by studying regions of our samples separated from the classical current path by a distance L which varied from 3Lcphi down to 0.2Lcphi (where Lcphi is the phase coherence length for the carriers in the metal). These fluctuations decay exponentially with L/Lcphi. Measurements along the classical current paths scale more slowly with L/Lcphi than predicted by the analytical theory but are in agreement with numerical simulations. We have also studied the length dependence of the magnetic field correlation scale BC, and we find that it is in qualitative agreement with a recent model calculation that accounts for the voltage probes
Asymmetry in the Magnetoconductance of Metal Wires and Loops
Universal conductance fluctuations in wires and Aharonov-Bohm oscillations in loops are not symmetric about H=0. The observation of asymmetry in the periodic oscillations is possible when the phase-coherence length of the wave function is comparable to the separation of the voltage probes. In both cases, four-probe measurements yield resistances which depend on lead configuration. The asymmetries appear like Hall voltages, and are consistent with Onsager\u27s relations
Probe-Configuration-Dependent Decoherence in an Aharonov-Bohm Ring
We have measured transport through mesoscopic Aharonov-Bohm (AB) rings with
two different four-terminal configurations. While the amplitude and the phase
of the AB oscillations are well explained within the framework of the
Landaur-B\"uttiker formalism, it is found that the probe configuration strongly
affects the coherence time of the electrons, i.e., the decoherence is much
reduced in the configuration of so-called nonlocal resistance. This result
should provide an important clue in clarifying the mechanism of quantum
decoherence in solids.Comment: 4 pages, 4 figures, RevTe
Anomalous Conductance Distribution in Quasi-One Dimension: Possible Violation of One-Parameter Scaling Hypothesis
We report measurements of conductance distribution in a set of
quasi-one-dimensional gold wires. The distribution includes the second cumulant
or the variance which describes the universal conductance fluctuations, and the
third cumulant which denotes the leading deviation. We have observed an
asymmetric contribution--or, a nonvanishing third cumulant--contrary to the
expectation for quasi-one-dimensional systems in the noninteracting theories in
the one-parameter scaling framework, which include the perturbative
diagrammatic calculations and the random matrix theory.Comment: 5 PAGE
Body Mass Index and Amyotrophic Lateral Sclerosis: A Study of US Military Veterans
Abstract Amyotrophic lateral sclerosis (ALS) may be associated with low body mass index (BMI) at the time of diagnosis. However, the role of premorbid BMI in the development of ALS and survival after diagnosis remains unclear. In 2005–2010, we interviewed 467 patients with ALS from the US National Registry of Veterans with ALS and 975 frequency-matched veteran controls. In this sample, we evaluated the association of BMI and BMI change at different ages with ALS risk using unconditional logistic models and with survival after ALS diagnosis using Cox proportional hazards models. After adjustment for confounders, compared with a moderate increase in BMI between ages 25 and 40 years, stable or decreasing BMI was positively associated with ALS risk (odds ratio (OR) = 1.61, 95% confidence interval (CI): 1.20, 2.16). A 1-unit increase in BMI at age 40 years (OR = 0.95, 95% CI: 0.91, 0.98) but not at age 25 years (OR = 0.99, 95% CI: 0.95, 1.03) was inversely associated with ALS. These associations were similar for bulbar and spinal ALS but stronger for those with a delay of less than 1 year between symptom onset and diagnosis. We found no association between prediagnosis BMI and survival. A decreasing BMI from early to middle age and a low BMI in middle age may be positively associated with ALS risk
Military service, deployments, and exposures in relation to amyotrophic lateral sclerosis etiology
Factors underlying a possible excess of amyotrophic lateral sclerosis (ALS) among military veterans remain unidentified. Limitations of previous studies on this topic include reliance on ALS mortality as a surrogate for ALS incidence, low statistical power, and sparse information on military-related factors
Decay of one dimensional surface modulations
The relaxation process of one dimensional surface modulations is re-examined.
Surface evolution is described in terms of a standard step flow model.
Numerical evidence that the surface slope, D(x,t), obeys the scaling ansatz
D(x,t)=alpha(t)F(x) is provided. We use the scaling ansatz to transform the
discrete step model into a continuum model for surface dynamics. The model
consists of differential equations for the functions alpha(t) and F(x). The
solutions of these equations agree with simulation results of the discrete step
model. We identify two types of possible scaling solutions. Solutions of the
first type have facets at the extremum points, while in solutions of the second
type the facets are replaced by cusps. Interactions between steps of opposite
signs determine whether a system is of the first or second type. Finally, we
relate our model to an actual experiment and find good agreement between a
measured AFM snapshot and a solution of our continuum model.Comment: 18 pages, 6 figures in 9 eps file
Military service, deployments, and exposures in relation to amyotrophic lateral sclerosis survival
Military veterans may have higher rates of amyotrophic lateral sclerosis (ALS) mortality than non-veterans. Few studies, with sparse exposure information and mixed results, have studied relationships between military-related factors and ALS survival. We evaluated associations between military-related factors and ALS survival among U.S. military veteran cases
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