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.

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A general methodology is presented for the dispersion of an inorganic compound within an organic polymer host through the use of solubilizing and polymerizable ligands. The dispersion of metal cluster cations 10 angstroms in diameter within a polymer host is achieved by free-radical polymerization of the hexafunctional metal cluster [Mo(6)Cl(8)(NVI)(6)](triflate)(4) with bound polymerizable ligands in N-vinylimidazole (NVI) solutions. Copolymerization of the activated cluster-bound ligands with the surrounding medium probably plays a key role in preventing aggregation and produces near monodisperse molecular clusters within the polymer matrix.

The preparation and analysis of inorganic-organic polymer nanocomposites consisting of inorganic nanowires and multiwire "cables" in a random-coil organic polymer host is reported. Dissolution of inorganic (LiMo3Se3)n wires in a strongly coordinating monomer, vinylene carbonate, and the use of a rapid polymerization in the presence of a cross-linking agent produce nanocomposites without phase separation. Polymerization of dilute solutions yields a material containing mostly (Mo3Se3(-))n mono- and biwires, 6 to 20 angstroms in diameter and 50 to 100 nanometers long. Polymerization of more concentrated liquid crystalline solutions yields a nanocomposite containing oriented multiwire cables, 20 to 40 angstroms in diameter and up to 1500 nanometers long, that display optical anisotropy and electrical conductivity.