420 research outputs found
Shot-noise in transport and beam experiments
Consider two Fermi gases with the same {\it average} currents: a transport
gas, as in solid-state experiments where the chemical potentials of terminal 1
is and of terminal 2 and 3 is , and a beam, i.e., electrons
entering only from terminal 1 having energies between and . By
expressing the current noise as a sum over single-particle transitions we show
that the temporal current fluctuations are very different: The beam is noisier
due to allowed single-particle transitions into empty states below .
Surprisingly, the correlations between terminals 2 and 3 are the same.Comment: 4 pages, 2 figure
Applications of nonequilibrium Kubo formula to the detection of quantum noise
The Kubo fluctuation-dissipation theorem relates the current fluctuations of
a system in an equilibrium state with the linear AC-conductance. This theorem
holds also out of equilibrium provided that the system is in a stationary state
and that the linear conductance is replaced by the (dynamic) conductance with
respect to the non equilibrium state. We provide a simple proof for that
statement and then apply it in two cases. We first show that in an excess noise
measurement at zero temperature, in which the impedance matching is maintained
while driving a mesoscopic sample out of equilibrium, it is the nonsymmetrized
noise power spectrum which is measured, even if the bare measurement, i.e.
without extracting the excess part of the noise, obtains the symmetrized noise.
As a second application we derive a commutation relation for the two components
of fermionic or bosonic currents which holds in every stationary state and
which is a generalization of the one valid only for bosonic currents. As is
usually the case, such a commutation relation can be used e.g. to derive
Heisenberg uncertainty relationships among these current components.Comment: 10 pages, Invited talk to be given by Y. I. at the SPIE Noise
Conference, Grand Canary, June 2004. Added reference and 2 footnotes,
corrected typo in Eq.
Joint effect of habitat identity and spatial distance on spiders′ community similarity in a fragmented transition zone
Understanding the main processes that affect community similarity have been the focus of much ecological research. However, the relative effects of environmental and spatial aspects in structuring ecological communities is still unresolved and is probably scale-dependent. Here, we examine the effect of habitat identity and spatial distance on finegrained community similarity within a biogeographic transition zone. We compared four hypotheses: i) habitat identity alone, ii) spatial proximity alone, iii) non-interactive effects of both habitat identity and spatial proximity, and iv) interactive effect of habitat identity and spatial proximity. We explored these hypotheses for spiders in three fragmented landscapes located along the sharp climatic gradient of Southern Judea Lowlands (SJL), Israel. We sampled 14,854 spiders (from 199 species or morphospecies) in 644 samples, taken in 35 patches and stratified to nine different habitats. We calculated the Bray-Curtis similarity between all samples-pairs. We divided the pairwise values to four functional distance categories (same patch, different patches from the same landscape, adjacent landscapes and distant landscapes) and two habitat categories (same or different habitats) and compared them using non-parametric MANOVA. A significant interaction between habitat identity and spatial distance was found, such that the difference in mean similarity between same-habitat pairs and different-habitat pairs decreases with spatial distance. Additionally, community similarity decayed with spatial distance. Furthermore, at all distances, same-habitat pairs had higher similarity than different-habitats pairs. Our results support the fourth hypothesis of interactive effect of habitat identity and spatial proximity. We suggest that the environmental complexity of habitats or increased habitat specificity of species near the edge of their distribution range may explain this pattern. Thus, in transitions zones care should be taken when using habitats as surrogate of community composition in conservation planning since similar habitats in different locations are more likely to support different communities
AC-Conductance through an Interacting Quantum Dot
We investigate the linear ac-conductance for tunneling through an arbitrary
interacting quantum dot in the presence of a finite dc-bias. In analogy to the
well-known Meir-Wingreen formula for the dc case, we are able to derive a
general formula for the ac-conductance. It can be expressed entirely in terms
of local correlations on the quantum dot, in the form of a Keldysh block
diagram with four external legs. We illustrate the use of this formula as a
starting point for diagrammatic calculations by considering the ac-conductance
of the noninteracting resonant level model and deriving the result for the
lowest order of electron-phonon coupling. We show how known results are
recovered in the appropriate limits.Comment: 4+ pages, 4 figure
Generalized constraints on quantum amplification
We derive quantum constraints on the minimal amount of noise added in linear
amplification involving input or output signals whose component operators do
not necessarily have c-number commutators, as is the case for fermion currents.
This is a generalization of constraints derived for the amplification of
bosonic fields whose components posses c-number commutators.Comment: 4 pages, 1 figure, submitted to Physical Review Letter
Correction: Joint Effect of Habitat Identity and Spatial Distance on Spiders' Community Similarity in a Fragmented Transition Zone
[This corrects the article DOI: 10.1371/journal.pone.0168417.]
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