16,866 research outputs found
Spin-independent v-representability of Wigner crystal oscillations in one-dimensional Hubbard chains: The role of spin-charge separation
Electrons in one-dimension display the unusual property of separating their
spin and charge into two independent entities: The first, which derive from
uncharged spin-1/2 electrons, can travel at different velocities when compared
with the second, built from charged spinless electrons. Predicted theoretically
in the early sixties, the spin-charge separation has attracted renewed
attention since the first evidences of experimental observation, with usual
mentions as a possible explanation for high-temperature superconductivity. In
one-dimensional (1D) model systems, the spin-charge separation leads the
frequencies of Friedel oscillations to suffer a 2k_F -- 4k_F crossover, mainly
when dealing with strong correlations, where they are referred to as Wigner
crystal oscillations. In non-magnetized systems, the current density
functionals which are applied to the 1D Hubbard model are not seen to reproduce
this crossover, referring to a more fundamental question: Are the Wigner
crystal oscillations in 1D systems non-interacting v-representable? Or, is
there a spin-independent Kohn-Sham potential which is able to yield spin-charge
separation? Finding an appropriate answer to both questions is our main task
here. By means of exact and DMRG solutions, as well as, a new approach of
exchange-correlation potential, we show the answer to be positive.
Specifically, the v-representable 4k_F oscillations emerge from attractive
interactions mediated by positively charged spinless holes -- the holons -- as
an additional contribution to the repulsive on-site Hubbard interaction
Information entropy of classical versus explosive percolation
We study the Shannon entropy of the cluster size distribution in classical as
well as explosive percolation, in order to estimate the uncertainty in the
sizes of randomly chosen clusters. At the critical point the cluster size
distribution is a power-law, i.e. there are clusters of all sizes, so one
expects the information entropy to attain a maximum. As expected, our results
show that the entropy attains a maximum at this point for classical
percolation. Surprisingly, for explosive percolation the maximum entropy does
not match the critical point. Moreover, we show that it is possible determine
the critical point without using the conventional order parameter, just
analysing the entropy's derivatives.Comment: 6 pages, 6 figure
A Method for Individual Source Brightness Estimation in Single- and Multi-band Data
We present a method of reliably extracting the flux of individual sources
from sky maps in the presence of noise and a source population in which number
counts are a steeply falling function of flux. The method is an extension of a
standard Bayesian procedure in the millimeter/submillimeter literature. As in
the standard method, the prior applied to source flux measurements is derived
from an estimate of the source counts as a function of flux, dN/dS. The key
feature of the new method is that it enables reliable extraction of properties
of individual sources, which previous methods in the literature do not. We
first present the method for extracting individual source fluxes from data in a
single observing band, then we extend the method to multiple bands, including
prior information about the spectral behavior of the source population(s). The
multi-band estimation technique is particularly relevant for classifying
individual sources into populations according to their spectral behavior. We
find that proper treatment of the correlated prior information between
observing bands is key to avoiding significant biases in estimations of
multi-band fluxes and spectral behavior, biases which lead to significant
numbers of misclassified sources. We test the single- and multi-band versions
of the method using simulated observations with observing parameters similar to
that of the South Pole Telescope data used in Vieira, et al. (2010).Comment: 11 emulateapj pages, 3 figures, revised to match published versio
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