88 research outputs found
Model for Anisotropic Directed Percolation
We propose a simulation model to study the properties of directed percolation
in two-dimensional (2D) anisotropic random media. The degree of anisotropy in
the model is given by the ratio between the axes of a semi-ellipse
enclosing the bonds that promote percolation in one direction. At percolation,
this simple model shows that the average number of bonds per site in 2D is an
invariant equal to 2.8 independently of . This result suggests that
Sinai's theorem proposed originally for isotropic percolation is also valid for
anisotropic directed percolation problems. The new invariant also yields a
constant fractal dimension for all , which is the same
value found in isotropic directed percolation (i.e., ).Comment: RevTeX, 9 pages, 3 figures. To appear in Phys.Rev.
Deference Done Better
There are many thingsâcall them âexpertsââthat you should defer to in forming
your opinions. The trouble is, many experts are modest: theyâre less than certain
that they are worthy of deference. When this happens, the standard theories of
deference break down: the most popular (âReflectionâ-style) principles collapse to
inconsistency, while their most popular (âNew-Reflectionâ-style) variants allow you
to defer to someone while regarding them as an anti-expert. We propose a middle
way: deferring to someone involves preferring to make any decision using their
opinions instead of your own. In a slogan, deferring opinions is deferring decisions.
Generalizing the proposal of Dorst (2020a), we first formulate a new principle that
shows exactly how your opinions must relate to an expertâs for this to be so. We then
build off the results of Levinstein (2019) and Campbell-Moore (2020) to show that
this principle is also equivalent to the constraint that you must always expect the
expertâs estimates to be more accurate than your own. Finally, we characterize the
conditions an expertâs opinions must meet to be worthy of deference in this sense,
showing how they sit naturally between the too-strong constraints of Reflection
and the too-weak constraints of New Reflection
Theory of superfast fronts of impact ionization in semiconductor structures
We present an analytical theory for impact ionization fronts in reversely
biased p^{+}-n-n^{+} structures. The front propagates into a depleted n base
with a velocity that exceeds the saturated drift velocity. The front passage
generates a dense electron-hole plasma and in this way switches the structure
from low to high conductivity. For a planar front we determine the
concentration of the generated plasma, the maximum electric field, the front
width and the voltage over the n base as functions of front velocity and doping
of the n base. Theory takes into account that drift velocities and impact
ionization coefficients differ between electrons and holes, and it makes
quantitative predictions for any semiconductor material possible.Comment: 18 pagers, 10 figure
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In vivo photopharmacology with light-activated opioid drugs
Traditional methods for site-specific drug delivery in the brain are slow, invasive, and difficult to interface with recordings of neural activity. Here, we demonstrate the feasibility and experimental advantages of in vivo photopharmacology using "caged" opioid drugs that are activated in the brain with light after systemic administration in an inactive form. To enable bidirectional manipulations of endogenous opioid receptors in vivo, we developed photoactivatable oxymorphone (PhOX) and photoactivatable naloxone (PhNX), photoactivatable variants of the mu opioid receptor agonist oxymorphone and the antagonist naloxone. Photoactivation of PhOX in multiple brain areas produced local changes in receptor occupancy, brain metabolic activity, neuronal calcium activity, neurochemical signaling, and multiple pain- and reward-related behaviors. Combining PhOX photoactivation with optical recording of extracellular dopamine revealed adaptations in the opioid sensitivity of mesolimbic dopamine circuitry in response to chronic morphine administration. This work establishes a general experimental framework for using in vivo photopharmacology to study the neural basis of drug action
On the Chemical Origin of the Gap Bowing in (GaAs)1âxGe2x Alloys: A Combined DFTâQSGW Study
Motivated by the research and analysis of new materials for photovoltaics and by the possibility of tailoring their optical properties for improved solar energy conversion, we have focused our attention on the (GaAs)1âxGe2x series of alloys. We have investigated the structural properties of some (GaAs)1âxGe2x compounds within the local-density approximation to density-functional theory, and their optical properties within the Quasiparticle Self-consistent GW approximation. The QSGW results confirm the experimental evidence of asymmetric bandgap bowing. It is explained in terms of violations of the octet rule, as well as in terms of the orderâdisorder phase transition
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