5,007 research outputs found
AdS4 backgrounds with N>16 supersymmetries in 10 and 11 dimensions
We explore all warped backgrounds with the most
general allowed fluxes that preserve more than 16 supersymmetries in -
and -dimensional supergravities. After imposing the assumption that either
the internal space is compact without boundary or the isometry
algebra of the background decomposes into that of AdS and that of
, we find that there are no such backgrounds in IIB supergravity.
Similarly in IIA supergravity, there is a unique such background with 24
supersymmetries locally isometric to , and in
supergravity all such backgrounds are locally isometric to the maximally
supersymmetric solution.Comment: 53 pages. v2: minor changes and references added. v3: typos corrected
and minor footnote added, published versio
Topological Invariants and Fibration Structure of Complete Intersection Calabi-Yau Four-Folds
We investigate the mathematical properties of the class of Calabi-Yau
four-folds recently found in [arXiv:1303.1832]. This class consists of 921,497
configuration matrices which correspond to manifolds that are described as
complete intersections in products of projective spaces. For each manifold in
the list, we compute the full Hodge diamond as well as additional topological
invariants such as Chern classes and intersection numbers. Using this data, we
conclude that there are at least 36,779 topologically distinct manifolds in our
list. We also study the fibration structure of these manifolds and find that
99.95 percent can be described as elliptic fibrations. In total, we find
50,114,908 elliptic fibrations, demonstrating the multitude of ways in which
many manifolds are fibered. A sub-class of 26,088,498 fibrations satisfy
necessary conditions for admitting sections. The complete data set can be
downloaded at
http://www-thphys.physics.ox.ac.uk/projects/CalabiYau/Cicy4folds/index.html .Comment: 25 pages, 7 figures, 1 table. v2: references added and minor changes.
Final version accepted for publicatio
Yang-Mills solutions and Spin(7)-instantons on cylinders over coset spaces with -structure
We study -valued Yang-Mills fields on cylinders
, where G/H is a compact seven-dimensional coset
space with -structure, is the Lie algebra of G, and Z(G/H)
inherits a Spin(7)-structure. After imposing a general G-invariance condition,
Yang-Mills theory with torsion on Z(G/H) reduces to Newtonian mechanics of a
point particle moving in under the influence of some quartic
potential and possibly additional constraints. The kinematics and dynamics
depends on the chosen coset space. We consider in detail three coset spaces
with nearly parallel -structure and four coset spaces with
SU(3)-structure. For each case, we analyze the critical points of the potential
and present a range of finite-energy solutions. We also study a
higher-dimensional analog of the instanton equation. Its solutions yield
G-invariant Spin(7)-instanton configurations on Z(G/H), which are special cases
of Yang-Mills configurations with torsion.Comment: 1+52 pages, 3 figures, 4 tables. v2: minor changes to match published
versio
Chern-Simons flows on Aloff-Wallach spaces and Spin(7)-instantons
Due to their explicit construction, Aloff-Wallach spaces are prominent in
flux compactifications. They carry G_2-structures and admit the G_2-instanton
equations, which are natural BPS equations for Yang-Mills instantons on
seven-manifolds and extremize a Chern-Simons-type functional. We consider the
Chern-Simons flow between different G_2-instantons on Aloff-Wallach spaces,
which is equivalent to Spin(7)-instantons on a cylinder over them. For a
general SU(3)-equivariant gauge connection, the generalized instanton equations
turn into gradient-flow equations on C^3 x R^2, with a particular cubic
superpotential. For the simplest member of the Aloff-Wallach family (with
3-Sasakian structure) we present an explicit instanton solution of tanh-like
shape.Comment: 1+17 pages, 1 figur
Kontexte, Theorien und Inhalte der Themen - Ergebnisse eine Expertenbefragung
In diesem Beitrag wird ein Überblick über die aktuellen Forschungsbereiche zu
den Themenfeldern Granulare Materie, Dissipative Strukturbildung und
Selbstorganisation gegeben. Dabei wird unterschieden in Forschungsbereiche,
die Modelle und Theorien umfassen, sowie in konkrete Kontexte, die sich auf
Anwendungen und Phänomene beziehen. Diese Ergebnisse basieren auf einer
Expertenbefragung
experimentelle Untersuchung von adhäsiven Haftpads
Der Beitrag beschreibt, wie ein Haftpad (auch bekannt als Anti-Rutsch-Pad oder
Klebepad) in Bezug auf seine Haftkraft sowie die Mechanismen seiner Haftung
untersucht werden kann. Dabei werden verschiedener Experimente und
(Rasterkraft-) mikroskopische Untersuchungen vorgestellt. Es zeigt sich, dass
eine experimentelle Bestimmung der Haftkraft schwierig ist, da sie von sehr
vielen Parametern abhängt. Die Messunsicherheiten sind daher groß. Die
maximale Haftkraft liegt im Bereich von 20 N/cm² (Plexiglas) bzw. 10 N/cm²
(Aluminium). Die Untersuchungen legen den Schluss nahe, dass für die Haftung
einerseits der Luftdruck und andererseits die van-der-Waals-Kräfte
ursächlich sind und beide in vergleichbarer Größenordnung liegen. Über die
experimentellen Erkenntnisse hinausgehend wird auch eine mögliche Eignung des
Haft-pads als Thema für den Physikunterricht diskutiert. Dafür spricht, dass
das Thema authentisch und anspruchsvoll ist und Raum für unterschiedliche
Lösungswege bietet. Bei der unterrichtlichen Umsetzungen sollten aber die
großen Messunsicherheiten und die Schwierigkeit eines stabilen experimentellen
Aufbaus mit bedacht werden
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