Mitigation and adaptation to climate change

Climate change produces significant social and economic impacts in most parts of the world, thus global action is needed to address climate change. In this chapter, the different possibilities of mitigation are explored from different points of view, and analyse the possibilities of adaptation to climate change. First, substantial reduction of GHG emission is needed, on the other hand adaptation action must deal with the inevitable impacts. According to the assessment of the chapter, it is essential that coordinated actions be taken at an EU level. In our argumentation, a macroeconomic model is used for the cost- benefit analysis of GHG gas emissions reduction. The GHG emission structure is analysed on European and global level. Even in the case of a successful mitigation strategy there rest the long-term effects of climate change which will need a coherent adaptation strategy to be dealt with. Although certain adaptation measures already have been taken, these initiatives are still very modest, and insufficient to deal with the economic effects of climate change properly

Galois actions on generalized Macbeath-Hurwitz curves

Gegenstand dieser Arbeit sind Galoisoperationen auf quasiplatonischen Riemannschen Flächen mit einer Automorphismengruppe isomorph zu PSL(2,F(q)). Quasiplatonische Riemannsche Flächen werden durch torsionsfreie Normalteiler N in einer Dreiecksgruppe D uniformisiert, d.h. N ist die universelle Überlagerungsgruppe und die Flächen, die man auch als algebraische Kurven beschreiben kann, sind isomorph zu N\U, wenn U die obere Halbebene bezeichnet. Bzgl. der Größe der Automorphismengruppen bilden die quasiplatonischen Kurven die lokalen Maxima im Modulraum. Die absoluten Maxima liegen bei den Hurwitz-Kurven; hier hat die Automorphismengruppe die maximale Größe von 84(g-1), wenn g>1 das Geschlecht der Kurve ist. Der Normalisator in PSL(2,R) der Überlagerungsgruppe N ist dann die Dreiecksgruppe mit Signatur (2,3,7). Macbeath hat die Bedingungen dafür gefunden, wann PSL(2,F(q)) eine Hurwitz-Gruppe ist. Von besonderem Interesse ist dabei der Fall, dass q=p eine Primzahl kongruent +-1 mod 7 ist. Hier hat man drei nicht-isomorphe Kurven, die jedoch alle galoiskonjugiert zueinander sind. In der Arbeit werden Bedingungen angegeben, unter denen sich dieses Resultat auf Dreiecksgruppen D mit einer Signatur der Form (2,m_1,m_2) verallgemeinern lässt. Dabei gehen einerseits Ergebnisse von Frye ein, der die Anzahl der verschiedenen torsionsfreien Normalteiler N>PSL(2,F(q)) auch über die Quaternionenalgebra, die die Dreiecksgruppe über ihrem Spurkörper erzeugt. Die Normalteiler erweisen sich dann als Schnitt der Dreiecksgruppe mit einer Hauptkongruenzuntergruppe nach einem Primideal P|char(F(q)) in der Norm-1-Gruppe einer Ordnung der Quaternionenalgebra. Dabei ist das Spurtripel in PSL(2,F(q)) gerade das Spurtripel aus D modulo P. Ändert man P, so erhält man ein anderes Spurtripel in PSL(2,F(q)), also auch einen anderen Normalteiler. Bilden die zugehörigen Kurven eine Bahn unter der Galoisoperation, dann ergeben sich alle Normalteiler auf diese Weise. Die Galoisoperation auf den Tripeln der Multiplikatoren, also die Galoisoperation auf den Kurven, ist verträglich mit der Operation, die die Primideale P|char(F(q)) permutiert. Wir erhalten also eine natürliche Korrespondenz zwischen der Galoisoperation auf den Kurven einerseits und der Operation auf den Primidealen andererseits.In this thesis we look at Galois-actions on quasiplatonic Riemann surfaces with automorphism group isomorphic to PSL(2,F(q)). These surfaces are uniformized by torsion free normal subgroups in a triangle group D, i.e. N is the universal covering group and the surfaces, which can be described as algebraic curves, are isomorphic to N\U, U denoting the upper halfplane. Concerning the size of the automorphism group quasiplatonic curves are local maxima in moduli space. The absolute maxima are the so called Hurwitz curves. For Hurwitz curves the automorphism group has size 84(g-1) where g>1 denotes the genus. In this case, the normalizer in PSL(2,R) of the covering group N is the triangle group with signature (2,3,7). Macbeath proved conditions such that PSL(2,F(q)) is a Hurwitz group. The case where q is a prime congruent +-1 mod 7 is especially interesting, because one gets three non-isomorphic curves forming one orbit under the Galois-action. We show how to generalize this result to triangle groups D with a signature of type (2,m_1,m_2). We use results by Frye counting the number of different torsion free normal subgroups N>PSL(2,F(q)) via the quaternion algebra generated by the triangle group over its trace field. In this case the normal subgroups are the intersections of the triangle group with the principal congruence subgroups by prime ideals P|char(F(q)) in the norm-1-group of an order in the quaternion algebra. The trace tuple in PSL(2,F(q)) is equal to the trace tuple in D mod P. Changing P one gets a different trace tuple in PSL(2,F(q)) and thus a different normal subgroup. If the corresponding curves are all in one Galois-orbit, then all normal subgroups can be obtained in this way. The Galois-action on the tuples of multipliers, that is the action on the curves, is compatible with the action permuting prime ideals P|char(F(q)). We therefore have a correspondence between the Galois-action on the curves and the action on the prime ideals

Impact of strain on the optical fingerprint of monolayer transition metal dichalcogenides

Strain presents a straightforward tool to tune electronic properties of atomically thin nanomaterials that are highly sensitive to lattice deformations. While the influence of strain on the electronic band structure has been intensively studied, there are only few works on its impact on optical properties of monolayer transition metal dichalcogenides (TMDs). Combining microscopic theory based on Wannier and Bloch equations with nearestneighbor tight-binding approximation, we present an analytical view on how uni- and biaxial strain influences the optical fingerprint of TMDs including their excitonic binding energy, oscillator strength, optical selection rules, and the radiative broadening of excitonic resonances. We show that the impact of strain can be reduced to changes in the lattice structure (geometric effect) and in the orbital functions (overlap effect). In particular, we demonstrate that the valley-selective optical selection rule is softened in the case of uniaxial strain due to the introduced asymmetry in the lattice structure. Furthermore, we reveal a considerable increase of the radiative dephasing due to strain-induced changes in the optical matrix element and the excitonic wave functions

Molecule signatures in photoluminescence spectra of transition metal dichalcogenides

Monolayer transition metal dichalcogenides (TMDs) show an optimal surface-to-volume ratio and are thus promising candidates for novel molecule sensor devices. It was recently predicted that a certain class of molecules exhibiting a large dipole moment can be detected through the activation of optically inaccessible (dark) excitonic states in absorption spectra of tungsten-based TMDs. In this work, we investigate the molecule signatures in photoluminescence spectra in dependence of a number of different experimentally accessible quantities, such as excitation density, temperature as well as molecular characteristics including the dipole moment and its orientation, molecule-TMD distance, molecular coverage and distribution. We show that under certain optimal conditions, even room temperature detection of molecules can be achieved

Impact of strain on the excitonic linewidth in transition metal dichalcogenides

Monolayer transition metal dichalcogenides (TMDs) are known to be highly sensitive to externally applied tensile or compressive strain. In particular, strain can be exploited as a tool to control the optical response of TMDs. However, the role of excitonic effects under strain has not been fully understood yet. Utilizing the strain-induced modification of electron and phonon dispersion obtained by first principle calculations, we present in this work microscopic insights into the strain-dependent optical response of various TMD materials. In particular, we explain recent experiments on the change of excitonic linewidths in strained TMDs and predict their behavior for tensile and compressive strain at low temperatures.Comment: 7 pages, 7 figure

Dark excitons in transition metal dichalcogenides

Monolayer transition metal dichalcogenides (TMDs) exhibit a remarkably strong Coulomb interaction that manifests in tightly bound excitons. Due to the complex electronic band structure exhibiting several spin-split valleys in the conduction and valence band, dark excitonic states can be formed. They are inaccessibly by light due to the required spin-flip and/or momentum transfer. The relative position of these dark states with respect to the optically accessible bright excitons has a crucial impact on the emission efficiency of these materials and thus on their technological potential. Based on the solution of the Wannier equation, we present the excitonic landscape of the most studied TMD materials including the spectral position of momentum- and spin-forbidden excitonic states. We show that the knowledge of the electronic dispersion does not allow to conclude about the nature of the material's band gap, since excitonic effects can give rise to significant changes. Furthermore, we reveal that an exponentially reduced photoluminescence yield does not necessarily reflect a transition from a direct to a non-direct gap material, but can be ascribed in most cases to a change of the relative spectral distance between bright and dark excitonic states

First sequence-confirmed case of infection with the new influenza A(H1N1) strain in Germany

Here, we report on the first sequence-confirmed case of infection with the new influenza A(H1N1) virus in Germany. Two direct contacts of the patient were laboratory-confirmed as cases and demonstrate a chain of direct human-to-human transmission

Microscopic Theory of Externally Tunable Exciton Signatures of Two-Dimensional Materials

Atomically thin transition metal dichalcogenides (TMDs) are in the focus of current research due to their efficient light-matter interaction and the remarkably strong Coulomb interaction that leads to tightly bound excitons. Due to their unique band structure, TMDs show a variety of bright and optically inaccessible dark excitonic states. Moreover, the optimal surface-to-volume ratio makes these materials very sensitive to changes in their surroundings, which opens up the possibility of tailoring their optical properties via adsorption of molecules, application of strain, and deposition of defects.The aim of this thesis is to use a microscopic many-particle theory to predict different strategies to externally control the optical fingerprint of TMDs.\ua0We show that specific molecules can activate dark excitons leading to new pronounced resonances in optical spectra. We also find that these dark states are very sensitive to strain, leading to significant energy shifts and intensity changes. This renders 2D materials suitable for optical sensing of molecules and strain. Moreover, we investigate how local defects due to single molecules or local strain can trap excitons. We show direct signatures of localized bright excitonic states as well as indirect phonon-assisted side bands of localized momentum-dark excitons. We find that the visibility of these localized states is determined by the interplay between defect-induced exciton capture and intervalley exciton–phonon scattering. Finally, we investigate the formation dynamics and optical signatures of spatially separated interlayer excitons at interfaces of acene-based molecular crystals and 2D materials, which play a crucial role for conversion of light to electricity in photodetecting devices.\ua0Overall, the work contributes to a better microscopic understanding of exciton optics and its control via strain, molecules, magnetic fields and impurities in atomically thin semiconductors