The size polydispersity of carbon nanotubes (CNTs) and their dispersion in the matrix are factors that strongly influence the conductivity characteristics of CNT-polymer nanocomposites. Our experiments on polydisperse CNT-SU8 materials and the theoretical modelling hint at a simple, yet comprehensive, understanding of these factors and of the role they have in the conductivity behaviour of the composite.
Anisotropy of transport in bulk Rashba metals
Valentina Brosco and Claudio Grimaldi
Physical Review B 95, 195164 (2017)
We predict that in three dimensional (3D) systems hosting a giant spin-orbit Rashba coupling, the dc electrical conductivity displays a strong anisotropic renormalization due exclusively to the Rashba interaction. We show that the electron velocity components orthogonal to the Rashba field are strongly renormalized, while the component parallel to the Rashba vector remains unaffected. Measurements of the conductivity anisotropy in bulk Rashba metals may therefore give a direct experimental assessment of the spin-orbit strength.
Signal coverage approach to the detection probability of hypothetical extraterrestrial emitters in the Milky Way
The lack of evidence for the existence of extraterrestrial life, even the simplest forms of microscopic life, makes it difficult to decide whether the search for extraterrestrial intelligence (SETI) is more a highrisk, high-payoff endeavor than a futile attempt.
The main unknown factor in SETI is the likelihood of detecting electromagnetic signals from possible galactic civilizations. Here, I derive the detection probability in terms of the probability that the Earth intersects a region of space covered by hypothetical extraterrestrial signals, without referring to particular hypothesis about the existence and the number of extraterrestrial emitters. I show that a universal bound sets an upper limit for detecting signals from hypothetical extraterrestrial civilizations in the galaxy. A surprising conclusion is that even if we assume that the Earth has a probability of 50% of being within a region covered by the signals, the mean number of potentially detectable emitters is less than one.
Sometimes, interactions between particles make things simpler. Here, I show that the percolating network of hard spherical particles with a short connectivity range has a dendritic, tree-like structure, which allows for a closed form solution for the percolation threshold. I derive an analytic expression of the percolation threshold which becomes increasingly accurate as the connectivity range diminishes. In principle, the tree-ansatz approach could be extended to describe percolation also in systems of anisotropic hard objects like, e.g., rod and platelet particles.
Area Coverage of Expanding E.T. Signals in the Galaxy: SETI and Drake's N
Claudio Grimaldi, Geoffrey W. Marcy, Nathalien K. Tellis, and Frank Drake
Publications of the Astronomical Society of the Pacific 130, 054101 (2018) arXiv:1802.09399
The famous Drake equation estimates the number N of currently emitting civilizations from a product of probabilities that events, which are necessary for the development ofcommunicative life in the galaxy, occur. Here we show that Drake's N coincides with the mean number of isotropic signals crossing Earth, at any moment. Our result implies therefore that N is, at least in principle, a quantifiable quantity, whose value could be assessed probabilistically by SETI experiments.