370 research outputs found
Arctic East Siberia had a lower latitude in the Pleistocene
In Arctic East Siberia many remains of mammoths have been found. In this
region there is not sufficient sunlight over the year to allow for the growth
of the plants on which these animals feed. Consequently the latitude of these
regions must have been lower before the end of the Pleistocene than at present.
It is a challenge to reconstruct this geographic shift of the poles in a manner
compa- tible with known facts. A possible sequence of events is described here.
It as- sumes an additional planet, which must since have disappeared. This is
possible, if it moved in an extremely eccentric orbit and was hot as a result
of tidal work and solar radiation. During a few million years evaporation of
this planet led to a disk-shaped cloud of ions moving around the Sun. This
cloud partially shielded the Earth from the solar radiation, producing the
alteration of cold and warm periods characterizing the Pleistocene. The degree
of shielding is sensitive to the inclination of Earth's orbit, which has a
period of 100000 years. Two cloud structures are discussed. The first is small
and steady. The other builds up to a point where inelastic collisions between
particles induce its collapse The resulting near-periodic time dependence of
the shielding re- sembles that of Dansgaard-Oeschger events. The Pleistocene
came to an end when the additional planet had a close encounter with the Earth,
whereby the Earth suffered a one permil extensional deformation. While this
deformation relaxed to an equilibrium shape in a time of one to several years,
the globe turned relative to the rotation axis: The North Pole moved from
Greenland to the Arctic Sea. The additional planet split into fragments, which
subsequently evaporated. Simple estimates are used here for the
characterization of the complex processes; more elaborate studies are required.Comment: 10 pages, LaTex, Typing error corrected in list of author
Magnetic-induced phonon anisotropy in ZnCrO from first principles
We have studied the influence of magnetic order on the optical phonons of the
geometrically frustrated spinel ZnCrO from first-principles. By mapping
the first-principles phonon calculations onto a Heisenberg-like model, we
developed a method to calculate exchange derivatives and subsequently the
spin-phonon couping parameter from first-principles. All calculations were
performed within LSDA+U
Raman scattering investigation across the magnetic and MI transition in rare earth nickelate RNiO3 (R = Sm, Nd) thin films
We report a temperature-dependent Raman scattering investigation of thin film
rare earth nickelates SmNiO3, NdNiO3 and Sm0.60Nd0.40NiO3, which present a
metal-to-insulator (MI) transition at TMI and an antiferromagnetic-paramagnetic
Neel transition at TN. Our results provide evidence that all investigated
samples present a structural phase transition at TMI but the Raman signature
across TMI is significantly different for NdNiO3 (TMI = TN) compared to SmNiO3
and Sm0.60Nd0.40NiO3 (TMI =/ TN). It is namely observed that the
paramagnetic-insulator phase (TN < T < TMI) in SmNiO3 and Sm0.60Nd0.40NiO3 is
characterized by a pronounced softening of one particular phonon band around
420 cm-1. This signature is unusual and points to an important and continuous
change in the distortion of NiO6 octahedra (thus the Ni-O bonding) which
stabilizes upon cooling at the magnetic transition. The observed behaviour
might well be a general feature for all rare earth nickelates with TMI =/ TN
and illustrates intriguing coupling mechanism in the TMI > T > TN regime.Comment: Revised & published versio
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