65 research outputs found
Optimizing Integrated Information with a Prior Guided Random Search Algorithm
Integrated information theory (IIT) is a theoretical framework that provides
a quantitative measure to estimate when a physical system is conscious, its
degree of consciousness, and the complexity of the qualia space that the system
is experiencing. Formally, IIT rests on the assumption that if a surrogate
physical system can fully embed the phenomenological properties of
consciousness, then the system properties must be constrained by the properties
of the qualia being experienced. Following this assumption, IIT represents the
physical system as a network of interconnected elements that can be thought of
as a probabilistic causal graph, , where each node has an
input-output function and all the graph is encoded in a transition probability
matrix. Consequently, IIT's quantitative measure of consciousness, , is
computed with respect to the transition probability matrix and the present
state of the graph. In this paper, we provide a random search algorithm that is
able to optimize in order to investigate, as the number of nodes
increases, the structure of the graphs that have higher . We also provide
arguments that show the difficulties of applying more complex black-box search
algorithms, such as Bayesian optimization or metaheuristics, in this particular
problem. Additionally, we suggest specific research lines for these techniques
to enhance the search algorithm that guarantees maximal
Self-consistent scattering description of transport in normal-superconductor structures
We present a scattering description of transport in several
normal-superconductor structures. We show that the related requirements of
self-consistency and current conservation introduce qualitative changes in the
transport behavior when the current in the superconductor is not negligible.
The energy thresholds for quasiparticle propagation in the superconductor are
sensitive to the existence of condensate flow (). This dependence is
responsible for a rich variety of transport regimes, including a voltage range
in which only Andreev transmission is possible at the interfaces, and a state
of gapless superconductivity which may survive up to high voltages if
temperature is low. The two main effects of current conservation are a shift
towards lower voltages of the first peak in the differential conductance and an
enhancement of current caused by the greater availability of charge
transmitting scattering channels.Comment: 31 pages, 10 PS figures, Latex file, psfig.sty file is added. To
appear in Phys. Rev. B (Jan 97
Giant Conductance Oscillations In Mesoscopic Andreev Interferometers
We analyze the electrical conductance of a two-dimensional, phase
coherent structure in contact with two superconductors, which is known to be an
oscillatory function of the phase difference between the
superconductors. It is predicted that for a metallic sample, the amplitude of
oscillation is enhanced by placing a normal barrier at the interface and that,
by tuning the strength of the barrier, can be orders of magnitude greater than
values observed in recent experiments. Giant oscillations can also be obtained
without a barrier, provided a crucial sum rule is broken. This can be achieved
by disorder induced normal scattering. In the absence of zero phase
inter-channel scattering, the conductance possesses a zero phase minimum.Comment: 4 pages of Revtex, 6 figures available on reques
Dynamics of conversion of supercurrents into normal currents, and vice versa
The generation and destruction of the supercurrent in a superconductor (S)
between two resistive normal (N) current leads connected to a current source is
computed from the source equation for the supercurrent density. This equation
relates the gradient of the pair potential's phase to electron and hole
wavepackets that create and destroy Cooper pairs in the N/S interfaces. Total
Andreev reflection and supercurrent transmission of electrons and holes are
coupled together by the phase rigidity of the non-bosonic Cooper-pair
condensate. The calculations are illustrated by snapshots from a computer film.Comment: 8 pages, 1 figure, accepted by Phys. Rev.
s-s*-d-wave superconductor on a square lattice and its BCs phase diagram
We study an extended Hubbard model with on-site repulsion and nearest
neighbors attraction which tries to mimic some of the experimental features of
doped cuprates in the superconducting state. We draw and discuss the phase
diagram as a function of the effective interactions among electrons for a wide
range of doping concentrations. We locate the region which is relevant for the
cuprates setting some constraints on the parameters which may be used in this
kind of effective models. We also study the effects of temperature and
orthorrombicity on the symmetry and magnitude of the gap function, and map the
model onto a simpler linearized hamiltonian, which produces similar phase
diagrams.Comment: 4 pages, 3 figures included. Accepted for publication in Phys. Rev.
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Policing the legume-Rhizobium symbiosis: a critical test of partner choice
In legume-Rhizobium symbioses, specialised soil bacteria fix atmospheric nitrogen in return for carbon. However, ineffective strains can arise, making discrimination essential. Discrimination can occur via partner choice, where legumes prevent ineffective strains from entering, or via sanctioning, where plants provide fewer resources. Several studies have inferred that legumes exercise partner choice, but the rhizobia compared were not otherwise isogenic. To test when and how plants discriminate ineffective strains we developed sets of fixing and non-fixing strains that differed only in the expression of nifH - essential for nitrogen fixation - and could be visualised using marker genes. We show that the plant is unable to select against the non-fixing strain at the point of entry, but that non-fixing nodules are sanctioned. We also used the technique to characterise mixed nodules (containing both a fixing and a non-fixing strain), whose frequency could be predicted using a simple diffusion model. We discuss that sanctioning is likely to evolve in preference to partner choice in any symbiosis where partner quality cannot be adequately assessed until goods or services are actively exchanged
CÓDIGOS DE CONDUCTA. COMERCIO ELECTRÓNICO DE FLORA Y FAUNA EXÓTICA INVASORA.
CÓDIGOS DE CONDUCTA. COMERCIO ELECTRÓNICO DE FLORA Y FAUNA EXÓTICA INVASORA
Código de conducta. Prevención de la introducción y dispersión de flora y fauna exótica invasora: agentes ambientales
Código de conducta. Prevención de la introducción y dispersión de flora y fauna exótica invasora: agentes ambientale
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