Theory of coherent acoustic phonons in InGaN/GaN multi-quantum wells

A microscopic theory for the generation and propagation of coherent LA phonons in pseudomorphically strained wurzite (0001) InGaN/GaN multi-quantum well (MQW) p-i-n diodes is presented. The generation of coherent LA phonons is driven by photoexcitation of electron-hole pairs by an ultrafast Gaussian pump laser and is treated theoretically using the density matrix formalism. We use realistic wurzite bandstructures taking valence-band mixing and strain-induced piezo- electric fields into account. In addition, the many-body Coulomb ineraction is treated in the screened time-dependent Hartree-Fock approximation. We find that under typical experimental conditions, our microscopic theory can be simplified and mapped onto a loaded string problem which can be easily solved.Comment: 20 pages, 17 figure

The Contested Politics of Corporate Governance: The Case of the Global Reporting Initiative

The Global Reporting Initiative (GRI) has successfully become institutionalized as the preeminent global framework for voluntary corporate environmental and social reporting. Its success can be attributed to the “institutional entrepreneurs” who analyzed the reporting field and deployed discursive, material, and organizational strategies to change it. GRI has, however, fallen short of the aspirations of its founders to use disclosure to empower nongovernmental organizations (NGOs). The authors argue that its trajectory reflects the power relations between members of the field, their strategic choices and compromises, their ability to mobilize alliances and resources, and constraints imposed by the broader institutions of financial and capital markets. The authors draw three notable implications from this study. First, institutional theory needs to pay more attention to economic structures, strategies, and resources. Second, institutional entrepreneurship by relatively weak societal groups such as NGOs is inherently constrained by the structural power of wider institutions and by the compromises required to initiate change. Third, the strategies of NGOs represent a form of power capable of shifting, if not transforming, the field of corporate governance

The scope of Baker's law.

Baker's law refers to the tendency for species that establish on islands by long-distance dispersal to show an increased capacity for self-fertilization because of the advantage of self-compatibility when colonizing new habitat. Despite its intuitive appeal and broad empirical support, it has received substantial criticism over the years since it was proclaimed in the 1950s, not least because it seemed to be contradicted by the high frequency of dioecy on islands. Recent theoretical work has again questioned the generality and scope of Baker's law. Here, we attempt to discern where the idea is useful to apply and where it is not. We conclude that several of the perceived problems with Baker's law fall away when a narrower perspective is adopted on how it should be circumscribed. We emphasize that Baker's law should be read in terms of an enrichment of a capacity for uniparental reproduction in colonizing situations, rather than of high selfing rates. We suggest that Baker's law might be tested in four different contexts, which set the breadth of its scope: the colonization of oceanic islands, metapopulation dynamics with recurrent colonization, range expansions with recurrent colonization, and colonization through species invasions