letters to nature 944 NATURE | VOL 389 | 30 OCTOBER 1997 pyrochlore, a large degree of spin-polarization is primarily due to the extremely small number of carriers (estimated at 0.0001-0.0005 carriers per formula unit from Hall effect measurements 6 ). Thus even moderate exchange splitting of Tl 6s-derived bands can fully polarize the conducting electrons.What, then, can account for the different temperature dependences of the transport process at the grain boundary between the two systems? In the perovskite manganites, the surface magnetism will in general be quite different from that of the bulk, owing to the lower effective magnetic coupling, the tendency of spin canting (an important feature in the perovskite manganites 21 ), as well as the disorder due to the surface termination of the crystal structure. Also important is the influence of the Jahn-Teller lattice distortions at the surface. One possible consequence of these effects would be a lower surface T C and surface spin polarization with respect to the bulk. The hopping electron is strongly coupled to these surface spins by the large on-site J H , and thus the TMR is susceptible to fast degradation with increasing temperature via the magnetically altered surface spins. In the pyrochlore, however, there is no strong J H coupling the conducting electrons to the Mn spins, and our results imply less sensitivity to the effects of the surface.Another distinction can be visualized by considering the temperature dependence of MR* in absolute units (r 0 MR* shown in the lower panel of Fig. 3. In the perovskite, the absolute magnitude of MR* diminishes rapidly with increasing temperature, much below T C . In contrast, the same data for the pyrochlore is virtually constant for all temperatures up to T C . This difference may indicate that for Tl 2 Mn 2 O 7 , the potential barrier ∆ B is less than the electron energy E ↑ (referring to Fig. 4). In this case the grain boundary transport would not be considered tunnelling, but rather a scattering process across a disordered region acting as a series resistance. This implies that the low-field MR can be made even larger by raising the relative resistance of the grain boundaries with respect to the bulk resistivity, much as a thin-film tunnel junction increases this ratio with respect to bulk polycrystalline samples in perovskite manganites. Ⅺ
An unusual luminescent inorganic oxide, Sr2CeO4, was identified by parallel screening techniques from within a combinatorial library of more than 25,000 members prepared by automated thin-film synthesis. A bulk sample of single-phase Sr2CeO4 was prepared, and its structure, determined from powder x-ray diffraction data, reveals one-dimensional chains of edge-sharing CeO6 octahedra, with two terminal oxygen atoms per cerium center, that are isolated from one another by Sr2+ cations. The emission maximum at 485 nanometers appears blue-white and has a quantum yield of 0.48 +/- 0.02. The excited-state lifetime, electron spin resonance, magnetic susceptibility, and structural data all suggest that luminescence originates from a ligand-to-metal Ce4+ charge transfer.
A biological photosynthetic reaction center contains a spatially ordered array of chromophores, electron donors, and electron acceptors that efficiently separate reductive and oxidative equivalents upon irradiation with visible light. This fundamental process of redox separation has been modeled in redox assemblies based on metal-bipyridyl complexes (1) or porphyrin-quinone systems (2-5). We have described the synthesis and photophysical characterization of lysine-based donorchromophore-acceptor triads that undergo light-induced redox separation (6-9). We describe here a general method for designing and constructing a helical oligoproline assembly having a spatially ordered array of redox or other functional groups protruding from a proline-II helical rod. Solid-phase peptide synthesis (10) is an efficient method for assembling a modular chain containing functional sites at specific positions. An a-helical peptide chain can serve as a molecular framework to display functional sites in a spatially ordered array (9). Alternatively, a chain of nine or more proline residues folds into a stable proline-II helix even when several of the proline residues have large functional sites on their side chains (11). The distance between two functional sites is determined by their helical location and orientation. The position of a modified proline residue in the proline chain determines its location on the helical surface. The configuration and conformation of the modified proline residue determine its orientation on the helical surface. To illustrate this general method, we have assembled an oligoproline assembly (triad 1) bearing three different redox sites (Fig. 1). Folding of the peptide triad 1 into a proline-II helix places the donor, chromophore, and acceptor sites in a linear array on one side of the helical rod.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.