Theory of proximity-induced exchange coupling in graphene on hBN/(Co, Ni)

We perform systematic first-principles calculations of the proximity exchange coupling, induced by cobalt (Co) and nickel (Ni) in graphene, via a few (up to three) layers of hexagonal boron nitride (hBN). We find that the induced spin splitting of the graphene bands is of the order of 10 meV for a monolayer of hBN, decreasing in magnitude but alternating in sign by adding each new insulating layer. We find that the proximity exchange can be giant if there is a resonant $d$ level of the transition metal close to the Dirac point. Our calculations suggest that this effect could be present in Co heterostructures, in which a $d$ level strongly hybridizes with the valence-band orbitals of graphene. Since this hybridization is spin dependent, the proximity spin splitting is unusually large, about 10 meV even for two layers of hBN. An external electric field can change the offset of the graphene and transition-metal orbitals and can lead to a reversal of the sign of the exchange parameter. This we predict to happen for the case of two monolayers of hBN, enabling electrical control of proximity spin polarization (but also spin injection) in graphene/hBN/Co structures. Nickel-based heterostructures show weaker proximity effects than cobalt heterostructures. We introduce two phenomenological models to describe the first-principles data. The minimal model comprises the graphene (effective) $p_z$ orbitals and can be used to study transport in graphene with proximity exchange, while the $p_z$-$d$ model also includes hybridization with $d$ orbitals, which is important to capture the giant proximity exchange. Crucial to both models is the pseudospin-dependent exchange coupling, needed to describe the different spin splittings of the valence and conduction bands.Comment: 14 pages, 17 figures, 2 table

Does participation lead to moderation? Understanding changes in Egyptian Islamist parties post-Arab Spring

Is it still important to consider the issue of participation and moderation in post-Spring Egypt? The question of inclusion and moderation was at the heart of debates about the prospect of the ‘taming’ of Islamist movements and parties throughout the early 2000s, that is at a time when democratisation seemed to be a possibility, albeit a distant one. In the post-Spring era, which saw the return of an authoritarian regime under President ‘Abd al-Fatah al-Sisi, there seems to be little taste for discussing whether there is still scope for the inclusion of Islamists in the political system. Yet, there is good reason to stipulate an ongoing relevance of this topic, particularly when studying Egypt’s Islamist parties in the post-Spring setting. The aim of this chapter is to critically examine the ‘participation-moderation’ thesis and, moreover, to engage in an original analysis of the political trajectories of Islamist parties in post-Spring Egypt. Focusing on those Islamist parties which demonstrated considerable influence in the period of democratic transition, in particular, the 2011/12 parliamentary elections as well as in the post-coup parliamentary elections of 2015, we can see that Salafi parties choose to participate for strategic reasons while reformist Islamist parties opted for non-participation. While the participation-moderation thesis remains a good starting point for analysing Islamist movements, the chapter reveals that the model has conceptual limitations with regards to the predicting ideological reform

The metamorphosis of social movements into political parties during democratic transition processes. A comparison of Egyptian and Tunisian movements and parties

Social Movements play a crucial role in the process of democratic transformation. They are the driving force in challenging authoritarianism. It is less recognized that SMs are also the fundamental building blocks for emerging political parties. While there is a significant body of work on the role of SMs in ending authoritarianism, there is little systematic research on either successful or indeed failed transitions into parties. Focusing on the transitions in Egypt and Tunisia following the Arab Spring, the paper studies newly established political parties with roots in SM activism, including Islamists and those on the secular spectrum. The aim is to observe whether and to what degree SMs develop formal boundaries which mark their institutionalization into independent parties. Although Goldstone pointed out that the differentiation between SMs and political parties is in any case ‘fuzzy’, the paper argues that the formation of formal boundaries is in fact essential to the success of the democratic transition process. The failure of democratic transition in Egypt in July 2013, to which pro-democratic movements and new political parties contributed, demonstrates this fact. The Tunisian case shows a more comprehensive metamorphosis of SMs into political parties, thus enabling a relatively structured process of transition. Tags: Social Movement Theory; social movements; political parties; democratic transition; authoritarianism; Arab Spring; Middle East; Egypt; Tunisi

Spin-orbit coupling in methyl functionalized graphene

We present first-principles calculations of the electronic band structure and spin-orbit effects in graphene functionalized with methyl molecules in dense and dilute limits. The dense limit is represented by a 2$\times$2 graphene supercell functionalized with one methyl admolecule. The calculated spin-orbit splittings are up to $0.6$ meV. The dilute limit is deduced by investigating a large, 7$\times$7, supercell with one methyl admolecule. The electronic band structure of this supercell is fitted to a symmetry-derived effective Hamiltonian, allowing us to extract specific hopping parameters including intrinsic, Rashba, and PIA (pseudospin inversion asymmetry) spin-orbit terms. These proximity-induced spin-orbit parameters have magnitudes of about 1 meV, giant compared to pristine graphene whose intrinsic spin-orbit coupling is about 10 $\mu$eV. We find that the origin of this giant local enhancement is the $sp^3$ corrugation and the breaking of local pseudospin inversion symmetry, as in the case of hydrogen adatoms. Also similar to hydrogen, methyl acts as a resonant scatterer, with a narrow resonance peak near the charge neutrality point. We also calculate STM-like images showing the local charge densities at different energies around methyl on graphene.Comment: 9 pages, 10 figure

Hyperostotic tympanic bone spicules in domestic and wild animal species

Hyperostotic tympanic bone spicules (HTBS), or "mucoperiosteal exostoses" (ME, syn.) are small, globular (>= 1 mm in diameter), mostly stalked and drumstick-like, bony structures, which arise from the inner wall of the tympanic bulla and project into the middle ear cavity. HTBS present as mineral densities inside the tympanic bulla on radiographs or computed tomographic (CT) images. They have previously been referred to as "otoliths" and were thought to represent mineral concretions secondary to otitis media. Recently, it was shown that HTBS actually consist of regularly composed bone tissue, covered by normal middle ear mucosa. So far, HTBS have only extensively been described in dogs, where they occur with a prevalence of up to >45%. A recent study detected ME, most likely representing HTBS, in the tympanic cavities of skeletonised skull bones of African lions. To estimate the occurrence of HTBS in other mammal species, the middle ears of adult animals of 78 different domestic, wild, and zoo species undergoing routine necropsy at the Institute of Veterinary Pathology of the LMU Munich, Germany were examined in the present study. HTBS were found in the tympanic bullae of carnivorous species, such as canids (wolf, fox), and in several large felid species (lion, tiger, leopard, cheetah). In contrast, HTBS were not present in domestic cats (more than to 200 cases), small carnivorous species such as mustelids, nor in any primate, ungulate, ruminant, pig, insectivore, or rodent species. The detectability of HTBS by CT of the tympanic bullae of large felids was demonstrated in an African lion. Histologically, HTBS consisted of mature lamellar bone, covered by periosteum and a partially ciliated, flat epithelium, regularly without any apparent inflammatory alterations. The present study demonstrates that HTBS may frequently occur in large felids and in different canid species. These findings should be taken into account when examining the middle ear, or interpreting bulla radiographs/CT-images of the respective species. However, the factors triggering the development of HTBS remain to be identified

Foray search: An effective systematic dispersal strategy in fragmented landscapes

In the absence of evidence to the contrary, population models generally assume that the dispersal trajectories of animals are random, but systematic dispersal could be more efficient at detecting new habitat and may therefore constitute a more realistic assumption. Here, we investigate, by means of simulations, the properties of a potentially widespread systematic dispersal strategy termed "foray search." Foray search was more efficient in detecting suitable habitat than was random dispersal in most landscapes and was less subject to energetic constraints. However, it also resulted in considerably shorter net dispersed distances and higher mortality per net dispersed distance than did random dispersal, and it would therefore be likely to lead to lower dispersal rates toward the margins of population networks. Consequently, the use of foray search by dispersers could crucially affect the extinction-colonization balance of metapopulations and the evolution of dispersal rates. We conclude that population models need to take the dispersal trajectories of individuals into account in order to make reliable predictions