3,591 research outputs found
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
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