Group-Theoretical Analysis of Second Harmonic Generation at (110) and (111) Surfaces of Antiferromagnets

Extending our previous work we classify the nonlinear magneto-optical response at low index surfaces of fcc antiferromagnets, such as NiO. Antiferromagnetic bilayers are discussed here as models for the termination of bulk antiferromagnets.Comment: 7 pages, 4 figures (figures no. 1 and 3 updated

On Killing vectors in initial value problems for asymptotically flat space-times

The existence of symmetries in asymptotically flat space-times are studied from the point of view of initial value problems. General necessary and sufficient (implicit) conditions are given for the existence of Killing vector fields in the asymptotic characteristic and in the hyperboloidal initial value problem (both of them are formulated on the conformally compactified space-time manifold)

On the Effect of Constraint Enforcement on the Quality of Numerical Solutions in General Relativity

In Brodbeck et al 1999 it has been shown that the linearised time evolution equations of general relativity can be extended to a system whose solutions asymptotically approach solutions of the constraints. In this paper we extend the non-linear equations in similar ways and investigate the effect of various possibilities by numerical means. Although we were not able to make the constraint submanifold an attractor for all solutions of the extended system, we were able to significantly reduce the growth of the numerical violation of the constraints. Contrary to our expectations this improvement did not imply a numerical solution closer to the exact solution, and therefore did not improve the quality of the numerical solution.Comment: 14 pages, 9 figures, accepted for publication in Phys. Rev.

General scheme for stable single and multiatom nanomagnets according to symmetry selection rules

At low temperature, information can be stored in the orientation of the localized magnetic moment of an adatom. However, scattering of electrons and phonons with the nanomagnet leads its state to have incoherent classical dynamics and might cause fast loss of the encoded information. Recently, it has been understood that such scattering obeys certain selection rules due to the symmetries of the system. By analyzing the point-group symmetry of the surface, the time-reversal symmetry and the magnitude of the adatom effective spin, we identify which nanomagnets configurations are to be avoided and which are promising to encode a stable bit. A new tool of investigation is introduced and exploited: the quasi-spin quantum number. By means of this tool, our results are easily generalized to a broad class of bipartite cluster configurations where adatoms are coupled through Heisenberg-like interactions. Finally, to make contact with the experiments, numerical simulations have been performed to show how such stable configurations respond to typical scanning tunneling microscopy measurements.Comment: 15 pages, 7 figures. Published versio

Closing the gap between spatial and spin dynamics of electrons at the metal-to-insulator transition

We combine extensive precision measurements of the optically detected spin dynamics and magneto-transport measurements in a contiguous set of n-doped bulk GaAs structures in order to unambiguously unravel the intriguing but complex contributions to the spin relaxation at the metal-to-insulator transition (MIT). Just below the MIT, the interplay between hopping induced loss of spin coherence and hyperfine interaction yields a maximum spin lifetime exceeding 800~ns. At slightly higher doping concentrations, however, the spin relaxation deviates from the expected Dyakonov-Perel mechanism which is consistently explained by a reduction of the effective motional narrowing with increasing doping concentration. The reduction is attributed to the change of the dominant momentum scattering mechanism in the metallic impurity band where scattering by local conductivity domain boundaries due to the intrinsic random distribution of donors becomes significant. Here, we fully identify and model all intricate contributions of the relevant microscopic scattering mechanisms which allows the complete quantitative modeling of the electron spin relaxation in the entire regime from weakly interacting up to fully delocalized electrons

Darwins Erben : Evolutionstheorie heute

Rezensionen zu: Axel Meyer : Evolution ist überall : Gesammelte Kolumne ‚Quantensprung’ des Handelsblattes, Böhlau 2008, 158 Seiten gebunden, ISBN 978-3205777717, 19,90 Euro. Volker Storch, Ulrich Welsch & Michael Wink : Evolutionsbiologie, Springer, 2. Auflage 2007, 518 Seiten gebunden, ISBN 978-3540360728, 39,95 Euro. Andreas Sentger & Frank Wigger (Hg.) : Triebkraft Evolution : Vielfalt, Wandel, Menschwerdung, Spektrum 2008, 294 Seiten, Gebunden, ISBN 978-3827420008, 24,95 Euro. Sean B. Carroll : Die Darwin-DNA : Wie die neueste Forschung die Evolutionstheorie bestätigt, S. Fischer 2008, 320 Seiten, gebunden, ISBN 978-3100102317, 19,90 Euro. Manfred Grasshoff : Die Evolution, 2 Audio-CDs, Gesamtspielzeit 127 Minuten Vocalbar 2007, ISBN 978-3939696018, 25 Euro

Von Rüppells Blausteißpapagei bis zum Fuß des Archaeopteryx : ornithologische Notizen aus Frankfurts Geschichte

Die Vogelkunde besitzt in Frankfurt eine weitreichende Tradition. So zum Beispiel engagierten sich Naturforscher und -liebhaber schon lange vor Gründung der Universität im Jahre 1914 in Vereinigungen wie der Senckenberg Gesellschaft für Naturforschung (SGN, gegründet 1817) oder der Zoologischen Gesellschaft Frankfurt (ZGF, gegründet 1858). Biografi sche Skizzen zeichnen den Weg von den Pionierzeiten der Frankfurter Ornithologie bis heute nach