Evidence of several dipolar quasi-invariants in Liquid Crystals

In a closed quantum system of N coupled spins with magnetic quantum number I, there are about (2I + 1)^N constants of motion. However, the possibility of observing such quasi-invariant (QI) states in solid-like spin systems in Nuclear Magnetic Resonance (NMR) is not a strictly exact prediction. The aim of this work is to provide experimental evidence of several QI, in the proton NMR of small spin clusters, besides those already known Zeeman, and dipolar orders (strong and weak). We explore the spin states prepared with the Jeener-Broekaert pulse sequence by analyzing the time-domain signals yielded by this sequence as a function of the preparation times, in a variety of dipolar networks. We observe that the signals can be explained with two dipolar QIs only within a range of short preparation times. At longer times the time-domain signals have an echo-like behaviour. We study their multiple quantum coherence content on a basis orthogonal to the z-basis and see that such states involve a significant number of correlated spins. Then we show that the whole preparation time-scale can only be reconstructed by assuming the occurrence of multiple QI which we isolate experimentally

To Win Friends and Influence People: Regulation and Enforcement of Influencer Marketing After Ten Years of the Endorsement Guides

For the last ten years, social media influencer marketing has been regulated by the Federal Trade Commission (FTC) under the FTC’s Section 5 “unfair practices” authority, guided by the Endorsement Guides, a “best practices” document published by the FTC. This is a fairly “light” regulatory scheme where violators typically enter no-money, no-fault consent decrees and generally undertake to do a better job following the Endorsement Guides in the future. During this time, the practice has flourished, and companies are spending significant portions of their marketing budgets on social media influencer advertising. Recently, the FTC has submitted proposals for increased enforcement and penalties in this space based on a belief that misinformation is plaguing the internet. This Article reviews the history of influencer marketing and examines the current regulatory regime. This Article contends that the current regime, while not perfect, is preferable to a strict rule-based approach with substantial penalties due to the evolving nature of social media and consumer perception in this area

The excluded 3-minors for vf-safe delta-matroids

Vf-safe delta-matroids have the desirable property of behaving well under certain duality operations. Several important classes of delta-matroids are known to be vf-safe, including the class of ribbon-graphic delta-matroids, which is related to the class of ribbon graphs or embedded graphs in the same way that graphic matroids correspond to graphs. In this paper, we characterize vf-safe delta-matroids and ribbon-graphic deltamatroids by finding the minimal obstructions, called excluded 3-minors, to membership in the class. We find the unique (up to twisted duality) excluded 3-minor within the class of set systems for the class of vf-safe delta-matroids. In the literature, binary delta-matroids appear in many different guises, with appropriate notions of minor operations equivalent to that of 3-minors, perhaps most notably as graphs with vertex minors. We give a direct explanation of this equivalence and show that some well-known results may be expressed in terms of 3-minors

Quasi-equilibrium states in thermotropic liquid crystals studied by multiple quantum NMR

We study the nature of the quasiinvariants in nematic 5CB and measure their relaxation times by encoding the multiple quantum coherences of the states following the JB pulse pair on two orthogonal bases, Z and X. The experiments were also performed in powder adamantane at 301 K which is used as a reference compound having only one dipolar quasiinvariant. We show that the evolution of the quantum states during the build up of the quasi-equilibrium state in 5CB prepared under the S condition is similar to the case of adamantane and that their quasi-equilibrium density operators have the same tensor structure. In contrast, the second constant of motion, whose explicit operator form is not known, involves a richer composition of multiple quantum coherences on the X basis of even order, in consistency with the truncation inherent in its definition. We exploited the exclusive presence coherences 4, 6, 8, besides 0 and 2 under the W condition to measure the spin-lattice relaxation time T_{W} accurately, so avoiding experimental difficulties that usually impair dipolar order relaxation measurement such as Zeeman contamination at high fields, and also superposition of the different quasiinvariants. This procedure opens the possibility of measuring the spin-lattice relaxation of a quasiinvariant independent of the Zeeman and S reservoirs, so incorporating a new relaxation parameter useful for studying the complex molecular dynamics in mesophases. In fact, we report the first measurement of T_{W} in a liquid crystal at high magnetic fields. The comparison of the obtained value with the one corresponding to a lower field (16 MHz) points out that the relaxation of the W-order strongly depends on the intensity of the external magnetic field, similarly to the case of the S reservoir, indicating that the relaxation of the W-quasiinvariant is also governed by the cooperative molecular motions.Comment: 7 figures. http://www.famaf.unc.edu.ar/series/AFis2005.ht

State-insensitive trapping of Rb atoms: linearly versus circularly polarized lights

We study the cancellation of differential ac Stark shifts in the 5s and 5p states of rubidium atom using the linearly and circularly polarized lights by calculating their dynamic polarizabilities. Matrix elements were calculated using a relativistic coupled-cluster method at the single, double and important valence triple excitations approximation including all possible non-linear correlation terms. Some of the important matrix elements were further optimized using the experimental results available for the lifetimes and static polarizabilities of atomic states. "Magic wavelengths" are determined from the differential Stark shifts and results for the linearly polarized light are compared with the previously available results. Possible scope of facilitating state-insensitive optical trapping schemes using the magic wavelengths for circularly polarized light are discussed. Using the optimized matrix elements, the lifetimes of the 4d and 6s states of this atom are ameliorated.Comment: 13 pages, 13 tables and 4 figure

Forêt méditerranéenne : espace naturel ? quelles situations ?

Introduction au groupe de travail de Foresterranée 2002 : "Forêt méditerranéenne : espace naturel ? quelles situations ?

Reading ecosystem services at the local scale through a territorial approach: The case of peri-urban agriculture in the thau Lagoon, Southern France

© 2015 by the author(s). In recent years, the ecosystem services (ES) concept has become a major paradigm for natural resource management. While policy-makers demand “hard” monetary evidence that nature conservation would be worth investing in, ongoing attempts are being made to formalize the concept as a scientifically robust “one size fits all” analytical framework. These attempts have highlighted several major limitations of the ES concept. First, to date, the concept has paid little attention to the role of humans in the production of ES. Second, the ongoing formalization of the ES concept is turning it into a “technology of globalization,” thereby increasingly ignoring the socio-cultural context and history within which ecosystems emerge. Third, economic valuation has been shown to limit local stakeholders in expressing their daily and immediate ways of interacting with their environment over and beyond extrinsic motivation provided by financial gains. We address these three limitations by analyzing a social evaluation of the roles of peri-urban farmland from a territorial perspective. Our case study is the Thau lagoon in southern France. We conducted in-depth interviews with a broad range of stakeholders and ran two participatory workshops. Using a territorial meta-model that distinguishes three levels— physical, logical, and existential—stakeholder data were analyzed to unravel the interplay of territorial elements at these three levels that gives rise to ES in two broad categories: food production and aesthetic landscape. The coupling of ES and territory concepts opens up several novel analytical perspectives. It allows partitioning of ES in a manner that “re-contextualizes” them and gives insight about both their physical constituents and their meaning at the territorial level. Additional research should incorporate the dynamics of service demand and supply, and further investigate options for implementation