Thermopower of the Correlated Narrow Gap Semiconductor FeSi and Comparison to RuSi

Iron based narrow gap semiconductors such as FeSi, FeSb2, or FeGa3 have received a lot of attention because they exhibit a large thermopower, as well as striking similarities to heavy fermion Kondo insulators. Many proposals have been advanced, however, lacking quantitative methodologies applied to this problem, a consensus remained elusive to date. Here, we employ realistic many-body calculations to elucidate the impact of electronic correlation effects on FeSi. Our methodology accounts for all substantial anomalies observed in FeSi: the metallization, the lack of conservation of spectral weight in optical spectroscopy, and the Curie susceptibility. In particular we find a very good agreement for the anomalous thermoelectric power. Validated by this congruence with experiment, we further discuss a new physical picture of the microscopic nature of the insulator-to-metal crossover. Indeed, we find the suppression of the Seebeck coefficient to be driven by correlation induced incoherence. Finally, we compare FeSi to its iso-structural and iso-electronic homologue RuSi, and predict that partially substituted Fe(1-x)Ru(x)Si will exhibit an increased thermopower at intermediate temperatures.Comment: 14 pages. Proceedings of the Hvar 2011 Workshop on 'New materials for thermoelectric applications: theory and experiment

Temperature dependence of RF losses in HR SOI substrates

High Resistivity (HR) Si substrates with resistivity values higher than 3 kΩ.cm have been demonstrated to be suitable for High Frequency applications due to their negligible ohmic losses [1, 2]. However, oxide-passivated HR Si substrates usually suffer from resistivity degradation near the SiO2/Si interface due to the presence of fixed charges (Qox) in the oxide. These charges attract free carriers near the substrate surface, forming a low conductivity accumulation or inversion layer [3]. Coplanar structures made on such substrates are very sensitive to the presence of this layer, which can lead to substantial RF loss increases [4]. As shown in [4] and [5], an efficient way to (partially or entirely) remove this conductive layer is to introduce a large density of traps (Dit) at the SiO2/Si interface. In this work we investigate for the first time the suitability of HR SOI substrates for high temperature RF applications. The proposed analysis combines the effects of high temperature (T) and interface traps on resistivity degradation in HR SOI substrates. The temperature range of interest is selected from 20 to 200°C. It is shown that substrates without traps exhibit poorer high temperature performance

Informal Governance of Non-State Armed Groups in the Sahel

Key Findings: Non-state armed groups, including violent extremist organizations, self-defense militias, and criminal gangs, are governance providers to local populations in the Sahel to a much greater degree than previously reported in the so-called “ungoverned spaces.” NSAGs provide four key forms of governance: security; justice; political and economic administration; social support and enforcement of social rules, across large sections of Mali, but also in Burkina Faso and Niger. Through this informal governance, NSAGs garner support and legitimacy from local leaders and communities at large. Current Responses: State and international responses have overall ignored this NSAG informal governance and have pursued: 1. a counter-terrorism strategy that is hampered by its failure to take into account the deeply embedded nature of NSAGs in local communities; and 2. a central statebuilding strategy that has so far largely failed to replace or even undermine NSAG legitimacy as governance providers. Relevance to NATO: Any involvement of international organizations in the Sahel needs to go beyond a strict counter-terrorism understanding of NSAGs, including VEOs. Furthermore, local and international actors on the ground warned that any additional involvement should avoid duplicating and undermining existing activities. Thus, any international involvement in the Sahel needs to take a broader vision of NSAG activities – beyond simply seeing them as “terrorists” or “criminal gangs” – and needs to be carefully coordinated with national and international partners

Organic and inorganic low-dimensional crystalline materials /

Proceeding

Finding minimum cost directed trees with demands and capacities

Given a directed graph, the authors consider the problem of finding a rooted directed tree (or branching) satisfying given demands at all the nodes and capacity constraints on the arcs. Various integer programming formulations are compared, including flow and multicommodity flow formulations and two partitioning-type formulations involving directed subtrees. Computational results concerning an application to the design of a low voltage electricity network are given. For the class of problems considered, one of the partitioning formulations allows one to solve problems with up to 100 nodes and several hundred arcs.Anglai