Vibrations and diverging length scales near the unjamming transition

We numerically study the vibrations of jammed packings of particles interacting with finite-range, repulsive potentials at zero temperature. As the packing fraction $\phi$ is lowered towards the onset of unjamming at $\phi_{c}$, the density of vibrational states approaches a non-zero value in the limit of zero frequency. For $\phi>\phi_{c}$, there is a crossover frequency, $\omega^{*}$ below which the density of states drops towards zero. This crossover frequency obeys power-law scaling with $\phi-\phi_{c}$. Characteristic length scales, determined from the dominant wavevector contributing to the eigenmode at $\omega^{*}$, diverge as power-laws at the unjamming transition.Comment: Submitted to PRL, 4 pages + 7 .eps figure

Structural signatures of the unjamming transition at zero temperature

We study the pair correlation function $g(r)$ for zero-temperature, disordered, soft-sphere packings just above the onset of jamming. We find distinct signatures of the transition in both the first and split second peaks of this function. As the transition is approached from the jammed side (at higher packing fraction) the first peak diverges and narrows on the small-$r$ side to a delta-function. On the high-$r$ side of this peak, $g(r)$ decays as a power-law. In the split second peak, the two subpeaks are both singular at the transition, with power-law behavior on their low-$r$ sides and step-function drop-offs on their high-$r$ sides. These singularities at the transition are reminiscent of empirical criteria that have previously been used to distinguish glassy structures from liquid ones.Comment: 8 pages, 13 figure

Reply to "Comment on `Jamming at zero temperature and zero applied stress: The epitome of disorder' "

We answer the questions raised by Donev, Torquato, Stillinger, and Connelly in their "Comment on "Jamming at zero temperature and zero applied stress: The epitome of disorder.' " We emphasize that we follow a fundamentally different approach than they have done to reinterpret random close packing in terms of the "maximally random jammed" framework. We define the "maximally random jammed packing fraction" to be where the largest number of initial states, chosen completely randomly, have relaxed final states at the jamming threshold in the thermodynamic limit. Thus, we focus on an ensemble of states at the jamming threshold, while DTSC are interested in determining the amount of order and degree of jamming for a particular configuration. We also argue that soft-particle systems are as "clean" as those using hard spheres for studying jammed packings and point out the benefits of using soft potentials

Regulation and social practice online

© 2016 Taylor & Francis. This article argues that everyday media practices are foundational to regulation on social media platforms. Beginning from a practice theory perspective, supported by qualitative research conducted on Facebook and Reddit, this paper shows how individual interactions with the platform and with other people on the site shape central regulatory norms on these sites. We suggest that our focus on practice complements existing studies that consider how regulation operates on social media platforms and shows how both practices and algorithms operate in conversation with one another in order to govern these sites. This research sets out an alternative trajectory of regulation, which is not based in law or privately established processes (such as EULAs, ToS or flags) but instead one grounded in the everyday practices of sociality, reciprocity, and perhaps even the maintenance of a particular community ideal

Generalized Intelligent States for an Arbitrary Quantum System

Generalized Intelligent States (coherent and squeezed states) are derived for an arbitrary quantum system by using the minimization of the so-called Robertson-Schr\"odinger uncertainty relation. The Fock-Bargmann representation is also considered. As a direct illustration of our construction, the P\"oschl-Teller potentials of trigonometric type will be shosen. We will show the advantage of the Fock-Bargmann representation in obtaining the generalized intelligent states in an analytical way. Many properties of these states are studied

Do wildflower strips enhance pest control in organic cabbage?

Within this project we assess whether wildflower strips and companion plants increase the control of cabbage pests Plutella xylostella L. (Lepidoptera: Plutellidae), Mamestra brassicae L. (Lepidoptera: Noctuidae) and Pieris rapae L. (Lepidoptera: Pieridae) by (1) naturally occurring parasitoids and predators and (2) mass‐releasedn Trichogramma brassciae (Bezdenko) (Hymenoptera: Trichogrammatidae) parasitoids. Two organic cabbage fields were used for this study: adjacent to each field a wildflower strip was sown and companion plants (Centaurea cyanus L. (Asteraceae)) intermixed within the crop. Within each field ~15,000 M. brassicae eggs were placed out to determine the parasitism rates by mass‐released T. brassicae and to assess the levels of egg predation. Over 1,000 lepidopteran larvae were collected and screened for hymenopteran and tachinid parasitoid DNA using a multiplex PCR assay. Invertebrate generalist predators (n=1,063) were collected for DNA‐based gut content analysis. The wildflower strip had a significant positive effect on M. brassicae egg parasitism rates as rates increased 5‐fold in the vicinity to the strip. Moreover, companion plants enhanced invertebrate predation on M. brassicae eggs. Both, the release of T. brassicae and the use of companion plants, however, did not significantly increase egg parasitism rates. The infestation of plants by caterpillars increased with distance to the wildflower strip and there was a trend of decreasing larval parasitism rates with distance to the strip. Currently the invertebrate predators are being molecularly analysed to assess predation on unparasitized and parasitized lepidopteran pests

Two-lane traffic rules for cellular automata: A systematic approach

Microscopic modeling of multi-lane traffic is usually done by applying heuristic lane changing rules, and often with unsatisfying results. Recently, a cellular automaton model for two-lane traffic was able to overcome some of these problems and to produce a correct density inversion at densities somewhat below the maximum flow density. In this paper, we summarize different approaches to lane changing and their results, and propose a general scheme, according to which realistic lane changing rules can be developed. We test this scheme by applying it to several different lane changing rules, which, in spite of their differences, generate similar and realistic results. We thus conclude that, for producing realistic results, the logical structure of the lane changing rules, as proposed here, is at least as important as the microscopic details of the rules

Discrete stochastic models for traffic flow

We investigate a probabilistic cellular automaton model which has been introduced recently. This model describes single-lane traffic flow on a ring and generalizes the asymmetric exclusion process models. We study the equilibrium properties and calculate the so-called fundamental diagrams (flow vs.\ density) for parallel dynamics. This is done numerically by computer simulations of the model and by means of an improved mean-field approximation which takes into account short-range correlations. For cars with maximum velocity 1 the simplest non-trivial approximation gives the exact result. For higher velocities the analytical results, obtained by iterated application of the approximation scheme, are in excellent agreement with the numerical simulations.Comment: Revtex, 30 pages, full postscript version (including figures) available by anonymous ftp from "fileserv1.mi.uni-koeln.de" in the directory "pub/incoming/" paper accepted for publication in Phys.Rev.