778 research outputs found
On the Computational Complexity of the Reticulate Cophylogeny Reconstruction Problem
The cophylogeny reconstruction problem is that of finding minimal cost explanations of differences between evolutionary histories of ecologically linked groups of biological organisms. We present a proof that shows that the general problem of reconciling evolutionary histories is NP-complete and provide a sharp boundary where this intractability begins. We also show that a related problem, that of finding Pareto optimal solutions, is NP-hard. As a byproduct of our results, we give a framework by which meta-heuristics can be applied to find good solutions to this problem
Cotunneling drag effect in Coulomb-coupled quantum dots
In Coulomb drag, a current flowing in one conductor can induce a voltage
across an adjacent conductor via the Coulomb interaction. The mechanisms
yielding drag effects are not always understood, even though drag effects are
sufficiently general to be seen in many low-dimensional systems. In this
Letter, we observe Coulomb drag in a Coulomb-coupled double quantum dot
(CC-DQD) and, through both experimental and theoretical arguments, identify
cotunneling as essential to obtaining a correct qualitative understanding of
the drag behavior.Comment: Main text: 5 pages, 5 figures; SM: 11 pages, 5 figures, 1 tabl
Pseudospin-Resolved Transport Spectroscopy of the Kondo Effect in a Double Quantum Dot
We report measurements of the Kondo effect in a double quantum dot (DQD),
where the orbital states act as pseudospin states whose degeneracy contributes
to Kondo screening. Standard transport spectroscopy as a function of the bias
voltage on both dots shows a zero-bias peak in conductance, analogous to that
observed for spin Kondo in single dots. Breaking the orbital degeneracy splits
the Kondo resonance in the tunneling density of states above and below the
Fermi energy of the leads, with the resonances having different pseudospin
character. Using pseudospin-resolved spectroscopy, we demonstrate the
pseudospin character by observing a Kondo peak at only one sign of the bias
voltage. We show that even when the pseudospin states have very different
tunnel rates to the leads, a Kondo temperature can be consistently defined for
the DQD system.Comment: Text and supplementary information. Text: 4 pages, 5 figures.
Supplementary information: 4 pages, 4 figure
Probabilistic Fragmentation and Effective Power Law
A simple fragmentation model is introduced and analysed. We show that, under
very general conditions, an effective power law for the mass distribution
arises with realistic exponent. This exponent has a universal limit, but in
practice the effective exponent depends on the detailed breaking mechanism and
the initial conditions. This dependence is in good agreement with experimental
results of fragmentation.Comment: 4 pages Revtex, 2 figures, zipped and uuencode
Age-dependent effects of microglial inhibition in vivo on Alzheimer’s disease neuropathology using bioactive-conjugated iron oxide nanoparticles
Background: Tau dysfunction is believed to be the primary cause of neurodegenerative disorders referred to as tauopathies, including Alzheimer’s disease, Pick’s disease, frontotemporal dementia and Parkinsonism. The role of microglial cells in the pathogenesis of tauopathies is still unclear. The activation of microglial cells has been correlated with neuroprotective effects through the release of neurotrophic factors and through clearance of cell debris and phagocytosis of cells with intracellular inclusions. In contrast, microglial activation has also been linked with chronic neuroinflammation contributing to the development of neurodegenerative diseases such as tauopathies. Microglial activation has been recently reported to precede tangle formation and the attenuation of tau pathology occurs after immunosuppression of transgenic mice. Methods: Here we report the specific inhibition of microglial cells in rTg4510 tau-mutant mice by using fibrin γ377-395 peptide conjugated to iron oxide (γ-Fe2O3) nanoparticles of 21 ± 3.5 nm diameter. Results: Stabilization of the peptide by its covalent conjugation to the γ-Fe2O3 nanoparticles significantly decreased the number of the microglial cells compared to the same concentration of the free peptide. The specific microglial inhibition induces different effects on tau pathology in an age dependent manner. The reduction of activation of microglial cells at an early age increases the number of neurons with hyperphosphorylated tau in transgenic mice. In contrast, reduction of activation of microglial cells reduced the severity of the tau pathology in older mice. The number of neurons with hyperphosphorylated tau and the number of neurons with tangles are reduced than those in animals not receiving the fibrin γ377-395 peptide-nanoparticle conjugate. Conclusions: These results demonstrate a differential effect of microglial activity on tau pathology using the fibrin γ377-395 peptide-nanoparticle conjugate, depending on age and/or stage of the neuropathological accumulation and aggregation
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