69,563 research outputs found
Frustration effects in magnetic molecules
By means of exact diagonalization we study the ground-state and the
low-temperature physics of the Heisenberg antiferromagnet on the cuboctahedron
and the icosidodecahedron. Both are frustrated magnetic polytopes and
correspond to the arrangement of magnetic atoms in the magnetic molecules
Cu12La8 and Mo72Fe30. The interplay of strong quantum fluctuations and
frustration influences the ground state spin correlations drastically and leads
to an interesting magnetization process at low temperatures. Furthermore the
frustration yields low-lying non-magnetic excitations resulting in an extra
low-temperature peak in the specific heat.Comment: 4 pages, 7 figure
Kinematics of the South Atlantic rift
The South Atlantic rift basin evolved as branch of a large
Jurassic-Cretaceous intraplate rift zone between the African and South American
plates during the final breakup of western Gondwana. By quantitatively
accounting for crustal deformation in the Central and West African rift zone,
we indirectly construct the kinematic history of the pre-breakup evolution of
the conjugate West African-Brazilian margins. Our model suggests a causal link
between changes in extension direction and velocity during continental
extension and the generation of marginal structures such as the enigmatic
Pre-salt sag basin and the S\~ao Paulo High. We model an initial E-W directed
extension between South America and Africa (fixed in present-day position) at
very low extensional velocities until Upper Hauterivian times (126 Ma)
when rift activity along in the equatorial Atlantic domain started to increase
significantly. During this initial 17 Myr-long stretching episode the
Pre-salt basin width on the conjugate Brazilian and West African margins is
generated. An intermediate stage between 126.57 Ma and Base Aptian is
characterised by strain localisation, rapid lithospheric weakening in the
equatorial Atlantic domain, resulting in both progressively increasing
extensional velocities as well as a significant rotation of the extension
direction to NE-SW. Final breakup between South America and Africa occurred in
the conjugate Santos--Benguela margin segment at around 113 Ma and in the
Equatorial Atlantic domain between the Ghanaian Ridge and the Piau\'i-Cear\'a
margin at 103 Ma. We conclude that such a multi-velocity, multi-directional
rift history exerts primary control on the evolution of this conjugate passive
margins systems and can explain the first order tectonic structures along the
South Atlantic and possibly other passive margins.Comment: 46 Pages, 22 figures. Submitted to Solid Earth
(http://www.solid-earth.net). Abstract shortened due to arXiv restrictions.
New version contains revisions and amendments as per reviewers requests.
Supplementary data is available at
http://datahub.io/en/dataset/southatlanticrif
Sonic Mach Cones Induced by Fast Partons in a Perturbative Quark-Gluon Plasma
We derive the space-time distribution of energy and momentum deposited by a
fast parton traversing a weakly coupled quark-gluon plasma by treating the fast
part on as the source of an external color field perturbing the medium. We then
use our result as a source term for the linearized hydrodynamical equations of
the medium. We show that the solution contains a sonic Mach cone and a
dissipative wake if the parton moves at a supersonic speed.Comment: Final version accepted for publicatio
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