26 research outputs found
Localization in non-chiral network models for two-dimensional disordered wave mechanical systems
Scattering theoretical network models for general coherent wave mechanical
systems with quenched disorder are investigated. We focus on universality
classes for two dimensional systems with no preferred orientation: Systems of
spinless waves undergoing scattering events with broken or unbroken time
reversal symmetry and systems of spin 1/2 waves with time reversal symmetric
scattering. The phase diagram in the parameter space of scattering strengths is
determined. The model breaking time reversal symmetry contains the critical
point of quantum Hall systems but, like the model with unbroken time reversal
symmetry, only one attractive fixed point, namely that of strong localization.
Multifractal exponents and quasi-one-dimensional localization lengths are
calculated numerically and found to be related by conformal invariance.
Furthermore, they agree quantitatively with theoretical predictions. For
non-vanishing spin scattering strength the spin 1/2 systems show
localization-delocalization transitions.Comment: 4 pages, REVTeX, 4 figures (postscript
Renormalization group approach to energy level statistics at the integer quantum Hall transition
We extend the real-space renormalization group (RG) approach to the study of
the energy level statistics at the integer quantum Hall (QH) transition.
Previously it was demonstrated that the RG approach reproduces the critical
distribution of the {\em power} transmission coefficients, i.e., two-terminal
conductances, , with very high accuracy. The RG flow of
at energies away from the transition yielded the value of the critical
exponent, , that agreed with most accurate large-size lattice simulations.
To obtain the information about the level statistics from the RG approach, we
analyze the evolution of the distribution of {\em phases} of the {\em
amplitude} transmission coefficient upon a step of the RG transformation. From
the fixed point of this transformation we extract the critical level spacing
distribution (LSD). This distribution is close, but distinctively different
from the earlier large-scale simulations. We find that away from the transition
the LSD crosses over towards the Poisson distribution. Studying the change of
the LSD around the QH transition, we check that it indeed obeys scaling
behavior. This enables us to use the alternative approach to extracting the
critical exponent, based on the LSD, and to find very close
to the value established in the literature. This provides additional evidence
for the surprising fact that a small RG unit, containing only five nodes,
accurately captures most of the correlations responsible for the
localization-delocalization transition.Comment: 10 pages, 11 figure
Integer quantum Hall transition in the presence of a long-range-correlated quenched disorder
We theoretically study the effect of long-ranged inhomogeneities on the
critical properties of the integer quantum Hall transition. For this purpose we
employ the real-space renormalization-group (RG) approach to the network model
of the transition. We start by testing the accuracy of the RG approach in the
absence of inhomogeneities, and infer the correlation length exponent nu=2.39
from a broad conductance distribution. We then incorporate macroscopic
inhomogeneities into the RG procedure. Inhomogeneities are modeled by a smooth
random potential with a correlator which falls off with distance as a power
law, r^{-alpha}. Similar to the classical percolation, we observe an
enhancement of nu with decreasing alpha. Although the attainable system sizes
are large, they do not allow one to unambiguously identify a cusp in the
nu(alpha) dependence at alpha_c=2/nu, as might be expected from the extended
Harris criterion. We argue that the fundamental obstacle for the numerical
detection of a cusp in the quantum percolation is the implicit randomness in
the Aharonov-Bohm phases of the wave functions. This randomness emulates the
presence of a short-range disorder alongside the smooth potential.Comment: 10 pages including 6 figures, revised version as accepted for
publication in PR
Novos gênero e espécie de Satyrinae (Lepidoptera, Nymphalidae) do sudeste brasileiro New genus and species of Satyrinae (Lepidoptera, Nymphalidae) from Southern Brazil
<abstract language="eng">Euplychia paeon (Godart, 1824) (=Euptychia marmorata Butler, 1866) and Euptychia griseldis Weymer, 1911 are placed under Carminda gen.n. Carminda umuarama sp.n. is described. The studied specimens are from southem Brazil and adjacent areas of Paraguay and Argentina
Low mislabeling rates indicate marked improvements in European seafood market operations
Over the span of a decade, genetic identification methods have progressively exposed the inadequacies of the seafood supply chain, revealing previously unrecognized levels of seafood fraud, raising awareness among the public, and serving as a warning to industry that malpractice will be detected. Here we present the outcome of the latest and largest multi-species, transnational survey of fish labeling accuracy to date, which demonstrates an apparent sudden reduction of seafood mislabeling in Europe. We argue that recent efforts in legislation, governance, and outreach have had a positive impact on industry regulation. Coordinated, technology-based, policy-oriented actions can play a pivotal role in shaping a transparent, sustainable global seafood market and in bolstering healthier oceans