265 research outputs found
Theory of Disordered Itinerant Ferromagnets I: Metallic Phase
A comprehensive theory for electronic transport in itinerant ferromagnets is
developed. We first show that the Q-field theory used previously to describe a
disordered Fermi liquid also has a saddle-point solution that describes a
ferromagnet in a disordered Stoner approximation. We calculate transport
coefficients and thermodynamic susceptibilities by expanding about the saddle
point to Gaussian order. At this level, the theory generalizes previous
RPA-type theories by including quenched disorder. We then study soft-mode
effects in the ferromagnetic state in a one-loop approximation. In
three-dimensions, we find that the spin waves induce a square-root frequency
dependence of the conductivity, but not of the density of states, that is
qualitatively the same as the usual weak-localization effect induced by the
diffusive soft modes. In contrast to the weak-localization anomaly, this effect
persists also at nonzero temperatures. In two-dimensions, however, the spin
waves do not lead to a logarithmic frequency dependence. This explains
experimental observations in thin ferromagnetic films, and it provides a basis
for the construction of a simple effective field theory for the transition from
a ferromagnetic metal to a ferromagnetic insulator.Comment: 15pp., REVTeX, 2 eps figs, final version as publishe
Estimating the global conservation status of more than 15,000 Amazonian tree species
Estimates of extinction risk for Amazonian plant and animal species are rare and not often incorporated into land-use policy and conservation planning. We overlay spatial distribution models with historical and projected deforestation to show that at least 36% and up to 57% of all Amazonian tree species are likely to qualify as globally threatened under International Union for Conservation of Nature (IUCN) Red List criteria. If confirmed, these results would increase the number of threatened plant species on Earth by 22%. We show that the trends observed in Amazonia apply to trees throughout the tropics, and we predict thatmost of the world’s >40,000 tropical tree species now qualify as globally threatened. A gap analysis suggests that existing Amazonian protected areas and indigenous territories will protect viable populations of most threatened species if these areas suffer no further degradation, highlighting the key roles that protected areas, indigenous peoples, and improved governance can play in preventing large-scale extinctions in the tropics in this century
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