High temperature susceptibility in electron doped Ca1-xYxMnO3: Double Exchange vs Superexchange

We present a study of the magnetic properties of the electron doped manganites Ca1-xYxMnO3 (for 0<=x<=0.25) in the paramagnetic regime. For the less doped samples (x<=0.1) the magnetic susceptibility, c(T), follows a Curie-Weiss (CW) law only for T > 450 K and, below this temperature, c^-1(T) shows a ferrimagnetic-like curvature. We approached the discussion of these results in terms of a simple mean-field model where double exchange, approximated by a ferromagnetic Heisenberg-like interaction between Mn3+ and Mn4+ ions, competes with classical superexchange. For higher levels of doping (x>=0.15), the CW behaviour is observed down to the magnetic ordering temperature (Tmo) and a better description of c(T) was obtained by assuming full delocalization of the eg electrons. In order to explore the degree of delocalization as a function of T and x, we analyzed the problem through Montecarlo simulations. Within this picture we found that at high T the electrons doped are completely delocalized but, when Tmo is approached, they form magnetic polarons of large spin that cause the observed curvature in c^-1(T) for x<=0.1.Comment: 15 pages, 10 figures, Submitted to J. Physics: Condensed Matter (06/28/02

On the magnetic properties of Gd implanted GaN

The wurzite type gallium nitride doped by gadolinium, Ga1-xGdxN (x similar to 0.01-0.07), was prepared by Gd ion implantation of the parent GaN thin films deposited on sapphire substrates. The material obtained exhibits a weak ferromagnetism (FM) persisting up to 700 K. At higher Gd concentrations, the minute FM component coexists with much more pronounced Curie-type paramagnetism. In a dilute limit (x <= 0.01), the latter part is substantially reduced and the saturated FM moment reaches the value M similar to 2 mu(B)/Gd atom

TiO 2 -catalyzed synthesis of sugars from formaldehyde in extraterrestrial impacts on the early Earth

Recent synthetic efforts aimed at reconstructing the beginning of life on our planet point at the plausibility of scenarios fueled by extraterrestrial energy sources. In the current work we show that beyond nucleobases the sugar components of the first informational polymers can be synthesized in this way. We demonstrate that a laser-induced high-energy chemistry combined with TiO 2 catalysis readily produces a mixture of pentoses, among them ribose, arabinose and xylose. This chemistry might be highly relevant to the Late Heavy Bombardment period of Earth s history about 4-3.85 billion years ago. In addition, we present an in-depth theoretical analysis of the most challenging step of the reaction pathway, i.e., the TiO 2 -catalyzed dimerization of formaldehyde leading to glycolaldehyde