Magnetic interactions in iron superconductors: A review

High temperature superconductivity in iron pnictides and chalcogenides emerges when a magnetic phase is suppressed. The multi-orbital character and the strength of correlations underlie this complex phenomenology, involving magnetic softness and anisotropies, with Hund's coupling playing an important role. We review here the different theoretical approaches used to describe the magnetic interactions in these systems. We show that taking into account the orbital degree of freedom allows us to unify in a single phase diagram the main mechanisms proposed to explain the (\pi,0) order in iron pnictides: the nesting-driven, the exchange between localized spins, and the Hund induced magnetic state with orbital differentiation. Comparison of theoretical estimates and experimental results helps locate the Fe superconductors in the phase diagram. In addition, orbital physics is crucial to address the magnetic softness, the doping dependent properties, and the anisotropies.Comment: Invited review article for a focus issue of Comptes Rendus Physique: 26 pages, 10 figures. Revised version, as accepted. Small changes throughout the text plus new subsection (Sec. IIIE

Rebels without a conscience: the evolution of the rogue states narrative in US security policy

This article examines how the foundations of the ‘rogue states’ security narrative in the United States developed prior to the declaration of the George W. Bush administration’s ‘Global War on Terror’ and President Bush’s representation of Iraq, Iran and North Korea as an ‘axis of evil’. The article argues that the puzzle of how US post-Cold War foreign and defence policy came to be focused on ‘irrational’ — but militarily inferior — adversaries can be understood through analysing how actors within the US defence community discursively constructed discrete international crises as the trigger for a major shift in US threat scenarios. This is developed through an examination of two crucial episodes in the construction of post-Cold War US national security interests: the crisis in the Persian Gulf in 1990–1 and the North Korean nuclear crisis in 1993–4. The article suggests the importance of historicizing contests over the interpretation of international crises in order to both better understand the process through which a country’s national security interests are defined and to gain greater analytical purchase on how security narratives are reconstructed during processes of systemic change

Controllable synthesis of graphene supported MnO2 nanowires via self-assembly for enhanced water oxidation in both alkaline and neutral solutions

The sluggish water oxidation reaction represents a significant challenge in water splitting for energy storage using hydrogen. We herein report the synthesis of MnO2 nanowires with the ultrasmall diameter and aspect ratio as high as 125 on graphene using a novel in situ polymer-mediated self-assembly approach in aqueous solution and under ambient conditions. The self-assembly process is simple and controllable by the concentration and pH of the polymer solution, in which the polymer serves as a soft template to direct the growth of MnO2 nanowires and also stabilize the structure, forming a unique graphene supported MnO2 nanowire, G@MnO2 NW. This nanostructure shows the most significant improvement of the catalytic activity compared to the graphene supported MnO2 nanoparticle and commercial Pt/C toward water oxidation under both alkaline and neutral conditions, and demonstrates for the first time a remarkable effect of the shape of MnO2 nanostructures on water oxidation catalysis. For example, at 0.7 V, it produces a current density of 5.9 mA cm−2, 14.8 times that of the graphene supported MnO2 nanoparticle (4.0 mA cm−2) and 8.4 times that of Pt/C (0.7 mA cm−2) in alkaline solution. Furthermore, it displays the highest turnover frequency reported among all the Mn oxides used for water oxidation catalysis. The G@MnO2 NW shows great potential as a water oxidation catalyst for energy storage applications