7,522 research outputs found
Numerical investigations of scaling at the Anderson transition
At low temperature T, a significant difference between the behavior of
crystals on the one hand and disordered solids on the other is seen:
sufficiently strong disorder can give rise to a transition of the transport
properties from conducting behavior with finite resistance R to insulating
behavior with R=infinity as T -> 0. This well-studied phenomenon is called the
disorder-driven metal-insulator transition and it is characteristic to
non-crystalline solids. In this review of recent advances, we have presented
results of transport studies in disordered systems, ranging from modifications
of the standard Anderson model of localization to effects of a two-body
interaction. Of paramount importance in these studies was always the highest
possible accuracy of the raw data combined with the careful subsequent
application of the finite-size scaling technique. In fact, it is this scaling
method that has allowed numerical studies to move beyond simple extrapolations
and reliably construct estimates of quantities as if one were studying an
infinite system.Comment: 18 pages, 6 figures, "The Anderson Transition and its
Ramifications-Localisation, Quantum Interference, and Interactions", 'Lecture
Notes in Physics' series, ed. T. Brandes and S. Kettemann, Springer Verlag,
to be publishe
Fir-dominated forests in Bavaria, Germany
The map of âRegional natural forest composition by main tree speciesâ (WALENTOWSKI et al. 2001) depicts Bavaria as a region largely predominated by the European beech (Fagus sylvatica). Analyses of climatope, hygrotope and trophotope of fir-dominated regional natural units make evident that the reasons for the preponderance of the European silver fir (Abies alba) are edaphic. In terms of regeneration vigour, growth and yield the fir particularly dominates in habitats with a combination of humus cover, acid-oligotrophic topsoils and clayey or waterlogged subsoils, where the beech usually exhibits stunted and malformed growth forms. This ecological preference has the effect that Bavarian Abies alba-forests are restricted to small patches within a matrix of potential natural vegetation formed by mixed deciduous-coniferous mountain forests. Within European Natura 2000 areas Abies- forests should be recorded carefully as special habitats. Their transitional character between temperate beech forests (habitat type 9130) and boreal spruce forests (habitat type 9410), the ecological preference of Abies alba as an endangered tree species and their sensitivity against environmental stressors, including changes in forest structure, air quality, and climate, make them important objects for nature conservation
Automatic Genre Classification in Web Pages Applied to Web Comments
Automatic Web comment detection could significantly facilitate information retrieval systems, e.g., a focused Web crawler. In this paper, we propose a text genre classifier for Web text segments as intermediate step for Web comment detection in Web pages. Different feature types and classifiers are analyzed for this purpose. We compare the two-level approach to state-of-the-art techniques operating on the whole Web page text and show that accuracy can be improved significantly. Finally, we illustrate the applicability for information retrieval systems by evaluating our approach on Web pages achieved by a Web crawler
The two-dimensional Anderson model of localization with random hopping
We examine the localization properties of the 2D Anderson Hamiltonian with
off-diagonal disorder. Investigating the behavior of the participation numbers
of eigenstates as well as studying their multifractal properties, we find
states in the center of the band which show critical behavior up to the system
size considered. This result is confirmed by an independent
analysis of the localization lengths in quasi-1D strips with the help of the
transfer-matrix method. Adding a very small additional onsite potential
disorder, the critical states become localized.Comment: 26 RevTeX 3.0 pages with 13 figures included via psfi
Interaction-dependent enhancement of the localisation length for two interacting particles in a one-dimensional random potential
We present calculations of the localisation length, , for two
interacting particles (TIP) in a one-dimensional random potential, presenting
its dependence on disorder, interaction strength and system size.
is computed by a decimation method from the decay of the Green
function along the diagonal of finite samples. Infinite sample size estimates
are obtained by finite-size scaling. For U=0 we reproduce
approximately the well-known dependence of the one-particle localisation length
on disorder while for finite , we find that with varying between and
. We test the validity of various other proposed fit
functions and also study the problem of TIP in two different random potentials
corresponding to interacting electron-hole pairs. As a check of our method and
data, we also reproduce well-known results for the two-dimensional Anderson
model without interaction.Comment: 34 RevTeX 3.0 pages with 16 figures include
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