734 research outputs found
Measurement of non-Gaussian shot noise: influence of the environment
We present the first measurements of the third moment of the voltage
fluctuations in a conductor. This technique can provide new and complementary
information on the electronic transport in conducting systems. The measurement
was performed on non-superconducting tunnel junctions as a function of voltage
bias, for various temperatures and bandwidths up to 1GHz. The data demonstrate
the significant effect of the electromagnetic environment of the sample.Comment: 13 pages, for the SPIE International Symposium on Fluctuations and
Noise, Maspalomas, Gran Canaria, Spain (May 2004
Noise Thermal Impedance of a Diffusive Wire
The current noise density S of a conductor in equilibrium, the Johnson noise,
is determined by its temperature T: S=4kTG with G the conductance. The sample's
noise temperature Tn=S/(4kG) generalizes T for a system out of equilibrium. We
introduce the "noise thermal impedance" of a sample as the amplitude of the
oscillation of Tn when heated by an oscillating power. For a macroscopic
sample, it is the usual thermal impedance. We show for a diffusive wire how
this (complex) frequency-dependent quantity gives access to the electron-phonon
interaction time in a long wire and to the diffusion time in a shorter one, and
how its real part may also give access to the electron-electron inelastic time.
These times are not simply accessible from the frequency dependence of S
itself.Comment: 4 pages, 2 figure
Performance-based inference of selection on stomatal length and specific leaf area varies with climate-of-origin of the forest tree, Eucalyptus ovata
Understanding how functional traits affect plant performance and fitness is a key step in unravelling the role of
natural selection in shaping the evolutionary trajectory of populations. We examined early-age selection acting
on leaf traits via their effects on growth performance and fitness, measured in Eucalyptus ovata trees planted in a
common-garden field trial embedded in a reforestation planting in Tasmania, Australia. We focused on two
important leaf traits - stomatal length and specific leaf area (SLA) - measured two years after planting, and
compared interplanted E. ovata groups originating from dry and wet home-site climates, with the trial site having
intermediate long-term mean annual rainfall. Two-year height growth was used as the performance attribute,
and the time-averaged tree survival over the subsequent six years as the fitness component. There was evidence
for performance-based selection on the leaf traits, with the strength and form of selection depending on the trait
and climate group being considered. In this sense, selection in the dry group operated mainly on stomatal length
where a combination of directional (favouring longer stomata) and stabilizing selection was detected, whereas
selection in the wet group acted only on SLA and was purely stabilizing. Estimates of performance-based
correlational selection were not statistically significant. For both climate groups, estimates of fitness-based selection
gradients provided evidence for significant directional (but not quadratic) selection on height performance,
favouring individuals with faster growth, but did not indicate statistical support for direct effects of the
leaf traits on tree survival, conditional on measured performance. These results validated qualitative inferences
of selection from the performance-based analysis, and suggested that selection on the leaf traits appeared to be
mediated by their effects on early-age height performance, which in turn directly influenced later-age survival.
We discuss the mechanisms by which the focal traits may have affected height performance, and likely factors
contributing to the different patterns of phenotypic selection observed in the two groups experiencing the same
environment. We also provide expressions of analytical derivatives that were developed for the estimation of
selection gradients based on a logistic regression model relating a binary fitness response to linear and nonlinear
covariate terms for the target regressor variables.info:eu-repo/semantics/publishedVersio
Energy resolution of terahertz single-photon-sensitive bolometric detectors
We report measurements of the energy resolution of ultra-sensitive
superconducting bolometric detectors. The device is a superconducting titanium
nanobridge with niobium contacts. A fast microwave pulse is used to simulate a
single higher-frequency photon, where the absorbed energy of the pulse is equal
to the photon energy. This technique allows precise calibration of the input
coupling and avoids problems with unwanted background photons. Present devices
have an intrinsic full-width at half-maximum energy resolution of approximately
23 terahertz, near the predicted value due to intrinsic thermal fluctuation
noise.Comment: 11 pages (double-spaced), 5 figures; minor revision
Linking leaf economic and hydraulic traits with early-age growth performance and survival of Eucalyptus pauciflora
Selection on plant functional traits may occur through their direct effects on
fitness (or a fitness component), or may be mediated by attributes of plant
performance which have a direct impact on fitness. Understanding this link is
particularly challenging for long-lived organisms, such as forest trees, where
lifetime fitness assessments are rarely achievable, and performance features
and fitness components are usually quantified from early-life history stages.
Accordingly, we studied a cohort of trees from multiple populations of
Eucalyptus pauciflora grown in a common-garden field trial established at
the hot and dry end of the species distribution on the island of Tasmania,
Australia. We related the within-population variation in leaf economic (leaf
thickness, leaf area and leaf density) and hydraulic (stomatal density, stomatal
length and vein density) traits, measured from two-year-old plants, to two-year
growth performance (height and stem diameter) and to a fitness component
(seven-year survival). When performance-trait relationships were modelled for
all traits simultaneously, statistical support for direct effects on growth
performance was only observed for leaf thickness and leaf density.
Performance-based estimators of directional selection indicated that
individuals with reduced leaf thickness and increased leaf density were
favoured. Survival-performance relationships were consistent with size-
dependent mortality, with fitness-based selection gradients estimated for
performance measures providing evidence for directional selection favouring
individuals with faster growth. There was no statistical support for an effect
associated with the fitness-based quadratic selection gradient estimated for
growth performance. Conditional on a performance measure, fitness-based
directional selection gradients estimated for the leaf traits did not provide
statistical support for direct effects of the focal traits on tree survival. This
suggested that, under the environmental conditions of the trial site and time
period covered in the current study, early-stage selection on the studied leaf
traits may be mediated by their effects on growth performance, which in turn has a positive direct influence on later-age survival. We discuss the potential
mechanistic basis of the direct effects of the focal leaf traits on tree growth, and
the relevance of a putative causal pathway of trait effects on fitness through
mediation by growth performance in the studied hot and dry environmentinfo:eu-repo/semantics/publishedVersio
Directional Selection on Tree Seedling Traits Driven by Experimental Drought Differs Between Mesic and Dry Populations
Original ResearchWe evaluated population differences and drought-induced phenotypic selection on four
seedling traits of the Australian forest tree Eucalyptus pauciflora using a glasshouse drydown
experiment. We compared dry and mesic populations and tested for directional
selection on lamina length (reflecting leaf size), leaf shape, the node of ontogenetic
transition to the petiolate leaf (reflecting the loss of vegetative juvenility), and lignotuber
size (reflecting a recovery trait). On average, the dry population had smaller and broader
leaves, greater retention of the juvenile leaf state and larger lignotubers than the mesic
population, but the populations did not differ in seedling survival. While there was
statistical support for directional selection acting on the focal traits in one or other
population, and for differences between populations in selection gradient estimates
for two traits, only one trait—lamina length—exhibited a pattern of directional selection
consistent with the observed population differences being a result of past adaptation to
reduce seedling susceptibility to acute drought. The observed directional selection for
lamina length in the mesic population suggests that future increases in drought risk in the
wild will shift the mean of the mesic population toward that of the dry population. Further,
we provide evidence suggesting an early age trade-off between drought damage and
recovery traits, with phenotypes which develop larger lignotubers early being more
susceptible to drought death. Such trade-offs could have contributed to the absence of
population mean differences in survival, despite marked differentiation in seedling traitsinfo:eu-repo/semantics/publishedVersio
Probability of the resistive state formation caused by absorption of a single-photon in current-carrying superconducting nano-strips
We have studied supercurrent-assisted formation of the resistive state in
nano-structured Nb and NbN superconducting films after absorption of a single
photon. In amorphous narrow NbN strips the probability of the resistive state
formation has a pronounced spectral cut-off. The corresponding threshold photon
energy decreases with the bias current. Analysis of the experimental data in
the framework of the generalized hot-spot model suggests that the quantum yield
for near-infrared photons increases faster than the photon nergy. Relaxation of
the resistive state depends on the photon energy making the phenomenon feasible
for the development of energy resolving single-photon detectors.Comment: 9 pages, 9 figures, submitted to Eur. Phys. Journa
Reset dynamics and latching in niobium superconducting nanowire single-photon detectors
We study the reset dynamics of niobium (Nb) superconducting nanowire
single-photon detectors (SNSPDs) using experimental measurements and numerical
simulations. The numerical simulations of the detection dynamics agree well
with experimental measurements, using independently determined parameters in
the simulations. We find that if the photon-induced hotspot cools too slowly,
the device will latch into a dc resistive state. To avoid latching, the time
for the hotspot to cool must be short compared to the inductive time constant
that governs the resetting of the current in the device after hotspot
formation. From simulations of the energy relaxation process, we find that the
hotspot cooling time is determined primarily by the temperature-dependent
electron-phonon inelastic time. Latching prevents reset and precludes
subsequent photon detection. Fast resetting to the superconducting state is
therefore essential, and we demonstrate experimentally how this is achieved
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