Thermodynamics and Spin Tunneling Dynamics in Ferric Wheels with Excess Spin

We study theoretically the thermodynamic properties and spin dynamics of a class of magnetic rings closely related to ferric wheels, antiferromagnetic ring systems, in which one of the Fe (III) ions has been replaced by a dopant ion to create an excess spin. Using a coherent-state spin path integral formalism, we derive an effective action for the system in the presence of a magnetic field. We calculate the functional dependence of the magnetization and tunnel splitting on the magnetic field and show that the parameters of the spin Hamiltonian can be inferred from the magnetization curve. We study the spin dynamics in these systems and show that quantum tunneling of the Neel vector also results in tunneling of the total magnetization. Hence, the spin correlation function shows a signature of Neel vector tunneling, and electron spin resonance (ESR) techniques or AC susceptibility measurements can be used to measure both the tunneling and the decoherence rate. We compare our results with exact diagonalization studies on small ring systems. Our results can be easily generalized to a wide class of nanomagnets, such as ferritin.Comment: 15 pages, 5 figure

Online Learning of a Memory for Learning Rates

The promise of learning to learn for robotics rests on the hope that by extracting some information about the learning process itself we can speed up subsequent similar learning tasks. Here, we introduce a computationally efficient online meta-learning algorithm that builds and optimizes a memory model of the optimal learning rate landscape from previously observed gradient behaviors. While performing task specific optimization, this memory of learning rates predicts how to scale currently observed gradients. After applying the gradient scaling our meta-learner updates its internal memory based on the observed effect its prediction had. Our meta-learner can be combined with any gradient-based optimizer, learns on the fly and can be transferred to new optimization tasks. In our evaluations we show that our meta-learning algorithm speeds up learning of MNIST classification and a variety of learning control tasks, either in batch or online learning settings.Comment: accepted to ICRA 2018, code available: https://github.com/fmeier/online-meta-learning ; video pitch available: https://youtu.be/9PzQ25FPPO

Development and Application of a Statistically-Based Quality Control for Crowdsourced Air Temperature Data

In urban areas, dense atmospheric observational networks with high-quality data are still a challenge due to high costs for installation and maintenance over time. Citizen weather stations (CWS) could be one answer to that issue. Since more and more owners of CWS share their measurement data publicly, crowdsourcing, i.e., the automated collection of large amounts of data from an undefined crowd of citizens, opens new pathways for atmospheric research. However, the most critical issue is found to be the quality of data from such networks. In this study, a statistically-based quality control (QC) is developed to identify suspicious air temperature (T) measurements from crowdsourced data sets. The newly developed QC exploits the combined knowledge of the dense network of CWS to statistically identify implausible measurements, independent of external reference data. The evaluation of the QC is performed using data from Netatmo CWS in Toulouse, France, and Berlin, Germany, over a 1-year period (July 2016 to June 2017), comparing the quality-controlled data with data from two networks of reference stations. The new QC efficiently identifies erroneous data due to solar exposition and siting issues, which are common error sources of CWS. Estimation of T is improved when averaging data from a group of stations within a restricted area rather than relying on data of individual CWS. However, a positive deviation in CWS data compared to reference data is identified, particularly for daily minimum T. To illustrate the transferability of the newly developed QC and the applicability of CWS data, a mapping of T is performed over the city of Paris, France, where spatial density of CWS is especially high.DFG, 322579844, Hitzewellen in Berlin, Deutschland - StadtklimamodifkationenBMBF, 01LP1602A, Verbundprojekt Stadtklima: Evaluierung von Stadtklimamodellen (Modul B), 3DO Teilprojekt 1: Dreidimensionales Monitoring atmosphärischer Prozesse in Berli

Spin dynamics and coherent tunnelling in the molecular magnetic rings Fe_6 and Fe_8

We present detailed calculations of low-energy spin dynamics in the ``ferric wheel'' systems Na:Fe_6 and Cs:Fe_8 in a magnetic field. We compute by exact diagonalisation the low-energy spectra and matrix elements for total-spin and N'eel-vector components, and thus the time-dependent correlation functions of these operators. Comparison of our results with the semiclassical theory of coherent quantum tunnelling of the N'eel vector demonstrates the validity of a two-state description for the low-energy dynamics of ferric wheels. We discuss the implications of our results for mesoscopic quantum coherent phenomena, and for the experimental techniques to observe them, in molecular magnetic rings.Comment: 10 pages, 8 figures; considerably expanded discussion; to appear in Eur. Phys. J.

A New Data Source for Inverse Dynamics Learning

Modern robotics is gravitating toward increasingly collaborative human robot interaction. Tools such as acceleration policies can naturally support the realization of reactive, adaptive, and compliant robots. These tools require us to model the system dynamics accurately -- a difficult task. The fundamental problem remains that simulation and reality diverge--we do not know how to accurately change a robot's state. Thus, recent research on improving inverse dynamics models has been focused on making use of machine learning techniques. Traditional learning techniques train on the actual realized accelerations, instead of the policy's desired accelerations, which is an indirect data source. Here we show how an additional training signal -- measured at the desired accelerations -- can be derived from a feedback control signal. This effectively creates a second data source for learning inverse dynamics models. Furthermore, we show how both the traditional and this new data source, can be used to train task-specific models of the inverse dynamics, when used independently or combined. We analyze the use of both data sources in simulation and demonstrate its effectiveness on a real-world robotic platform. We show that our system incrementally improves the learned inverse dynamics model, and when using both data sources combined converges more consistently and faster.Comment: IROS 201

Popular mobilisations in Lebanon: from anti-­system to sectarian claims

International audienceIn Lebanon, the Arab uprising is often analyzed through the lens of the side effects the Syrian uprising had on the country thanks to massive refugees influx and the involvement of Sunni and Shia Lebanese in Syria ' s battleground. In fact , popular mobilizations happened prior and during the current crisis as two different type of effects of the Arab uprisings at large. First , in late 2010 emerged an anti-­sectarian movement that brought up an anti-­system claim in line with other Arab social movements targeting authoritarian regimes and corruption. Second , the Sunni radical mobilization that started in Saida in 2011 around the popular Imam Sheikh Assir , gathered resentments toward the Shia leading party Hizbullah as involved alongside the Syrian regime and surprisingly adopting an active minority mobilization strategy. Both movement conducted demonstrations of different type (from classic march to on roads sit-­in) starting from a completely opposite perspective and in a very different environment : when the anti-­sectarian mobilization faded , the sectarian one rose. This paper will try to address the historical process of both mobilizations and assess their differences in light of several mobilization theories. It intends to raise questions about the type of actors involved , their discourses and justifications , and the contextual local and political environments

From Frontline to Borderscape: The Hizbullah Memorial Museum in South of Lebanon

International audienceAmong the cultural production on borderlines in the Middle East, the Hizbullah Museum on the site of “Mleeta” drawn on several aspects that inscribes it as a borderscape as it links politics with aesthetics. Built in 2010 on the former frontline of the Israeli occupied zone in South Lebanon, Mleeta articulate heritage, memory, and leisure with politics, education and morality. As part of the resistance society building, the blurring of the boundaries between tourism, architecture and ideology is done through the specific using that is ordering the landscape and the natural environment of this southern borderland as a vantage ground for its vision of the world. This major transformation of the borderland into a borderscape was render possible because of the transformation of South Lebanon into a Hizbullah’s military stronghold since the end of the 1980s and thanks to a new political trade-off after the Syrian withdrawal of Lebanon (2005) that confirmed its influence over the Lebanese political game. The memorial museum of Mleeta is part of a broader cultural policy adopted by Hizbullah that intends to transform the previous occupied zone of South Lebanon into a touristic landscape including several other sites and a “resistance trail”. In this framework, the site of Mleeta provide with a powerful narrative about resistance and memory due to its location on the former borderline set up during the Israeli occupation of South of Lebanon (1978-2000)

Le Kurdistan d'Irak : les disputed territories comme enjeu de définition nationale

International audienceCet article entend questionner l'apparition d'une frontière interne séparant la région du Gouvernement kurde régional (KRG) de celle de l'autorité centrale de Bagdad. Que signifie au-jourd'hui la « Green line » qui maté-rialisait en 1991 le retrait des troupes de Saddam Hussein? Comment l'évo-lution de la frontière interne matérialise-t-elle les ambitions politiques fédéralistes (voire indépendantiste) des Kurdes d'Irak ? Que se passe-t-il sur le plan de la souveraineté dans les territoires en discussion (disputed territories) qui bordent la « Green line » ? Quels rôles jouent les ressources en hy-drocarbures dans cette équation entre espace et pouvoir ? A partir du cadre théorique fourni par les border studies, cet article tentera d'abord de comprendre l'interaction qu'il y a entre la construction d'une identité collective et la territorialisation des Kurdes dans l'espace irakien. Nous verrons ensuite comment s'est développée la question des territoires en discussion (disputed territories) en lien avec la question des ressources en hydrocarbures et dans la perspective de la redéfinition de l'entité irakienne en fédération et/ou de la sécession du Kurdistan