1,175 research outputs found
Species richness and soil properties in Pinus ponderosa forests: A structural equation modeling analysis
Question: How are the effects of mineral soil properties on understory plant species richness propagated through a network of processes involving the forest overstory, soil organic matter, soil nitrogen, and understory plant abundance? Location: North-central Arizona, USA. Methods: We sampled 75 0.05-ha plots across a broad soil gradient in a Pinus ponderosa (ponderosa pine) forest ecosystem. We evaluated multivariate models of plant species richness using structural equation modeling. Results: Richness was highest at intermediate levels of understory plant cover, suggesting that both colonization success and competitive exclusion can limit richness in this system. We did not detect a reciprocal positive effect of richness on plant cover. Richness was strongly related to soil nitrogen in the model, with evidence for both a direct negative effect and an indirect non-linear relationship mediated through understory plant cover. Soil organic matter appeared to have a positive influence on understory richness that was independent of soil nitrogen. Richness was lowest where the forest overstory was densest, which can be explained through indirect effects on soil organic matter, soil nitrogen and understory cover. Finally, model results suggest a variety of direct and indirect processes whereby mineral soil properties can influence richness. Conclusions: Understory plant species richness and plant cover in P. ponderosa forests appear to be significantly influenced by soil organic matter and nitrogen, which are, in turn, related to overstory density and composition and mineral soil properties. Thus, soil properties can impose direct and indirect constraints on local species diversity in ponderosa pine forests
Exact microscopic analysis of a thermal Brownian motor
We study a genuine Brownian motor by hard disk molecular dynamics and
calculate analytically its properties, including its drift speed and thermal
conductivity, from microscopic theory.Comment: 4 pages, 5 figure
Low-energy quasiparticle states near extended scatterers in d-wave superconductors and their connection with SUSY quantum mechanics
Low-energy quasiparticle states, arising from scattering by single-particle
potentials in d-wave superconductors, are addressed. Via a natural extension of
the Andreev approximation, the idea that sign-variations in the superconducting
pair-potential lead to such states is extended beyond its original setting of
boundary scattering to the broader context of scattering by general
single-particle potentials, such as those due to impurities. The
index-theoretic origin of these states is exhibited via a simple connection
with Witten's supersymmetric quantum-mechanical model.Comment: 5 page
Evidence for field-induced excitations in low-temperature thermal conductivity of Bi_2Sr_2CaCu_2O_8
The thermal conductivity ,, of Bi_2Sr_2CaCu_2O_8 was studied as a
function of magnetic field. Above 5 K, after an initial decrease,
presents a kink followed by a plateau, as recently reported by Krishana et al..
By contrast, below 1K, the thermal conductivity was found to \emph{increase}
with increasing field. This behavior is indicative of a finite density of
states and is not compatible with the existence of a field-induced fully gapped
state which was recently proposed to describe the
plateau regime. Our low-temperature results are in agreement with recent works
predicting a field-induced enhancement of thermal conductivity by Doppler shift
of quasi-particle spectrum.Comment: 4 pages including 4 eps figures, submitted to Phys. Rev. Let
Zero-energy Andreev surface bound states in the lattice model
The conditions for zero-energy Andreev surface bound states to exist are
found for the lattice model of d-wave superconductor with arbitrary surface
orientation. Both nearest neighbors and next nearest neighbors models are
considered. It is shown that the results are very sensitive to the surface
orientation. In particular, for half-filled -surface zero-energy Andreev
surface states only appear under the condition that and are odd
simultaneouslyComment: 9 pages, 1 figur
Discrete-Lattice Model for Surface Bound States and Tunneling in d-Wave Superconductors
Surface bound states in a discrete-lattice model of a cuprate
superconductor are shown to be, in general, coherent superpositions of an
incoming excitation and more than one outgoing excitation, and a simple
graphical construction based on a surface Brillouin zone is developed to
describe their nature. In addition, a momentum-dependent lifetime contribution
to the width of these bound states as observed in tunneling experiments is
derived and elucidated in physical terms.Comment: 4 pages, 1 figure, revte
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